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Inhomogeneous high temperature melting and decoupling of charge density waves in spin-triplet superconductor UTe 2. Nat Commun 2024; 15:4456. [PMID: 38796494 DOI: 10.1038/s41467-024-48844-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2024] [Accepted: 05/15/2024] [Indexed: 05/28/2024] Open
Abstract
Charge, spin and Cooper-pair density waves have now been widely detected in exotic superconductors. Understanding how these density waves emerge - and become suppressed by external parameters - is a key research direction in condensed matter physics. Here we study the temperature and magnetic-field evolution of charge density waves in the rare spin-triplet superconductor candidate UTe2 using scanning tunneling microscopy/spectroscopy. We reveal that charge modulations composed of three different wave vectors gradually weaken in a spatially inhomogeneous manner, while persisting to surprisingly high temperatures of 10-12 K. We also reveal an unexpected decoupling of the three-component charge density wave state. Our observations match closely to the temperature scale potentially related to short-range magnetic correlations, providing a possible connection between density waves observed by surface probes and intrinsic bulk features. Importantly, charge density wave modulations become suppressed with magnetic field both below and above superconducting Tc in a comparable manner. Our work points towards an intimate connection between hidden magnetic correlations and the origin of the unusual charge density waves in UTe2.
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2
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Skyrmion lattice formation and destruction mechanisms probed with TR-SANS. NANOSCALE 2024. [PMID: 38712993 DOI: 10.1039/d4nr00858h] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2024]
Abstract
Magnetic skyrmions are topologically protected, nanoscale whirls of the spin configuration that tend to form hexagonally ordered arrays. As a topologically non-trivial structure, the nucleation and annihilation of the skyrmion, as well as the interaction between skyrmions, varies from conventional magnetic systems. Recent works have suggested that the ordering kinetics in these materials occur over millisecond or longer timescales, which is unusually slow for magnetic dynamics. The current work investigates the skyrmion ordering kinetics, particularly during lattice formation and destruction, using time-resolved small angle neutron scattering (TR-SANS). Evaluating the time-resolved structure and intensity of the neutron diffraction pattern reveals the evolving real-space structure of the skyrmion lattice and the timeframe of the formation. Measurements were performed on three prototypical skyrmion materials: MnSi, (Fe,Co)Si, and Cu2OSeO3. To probe lattice formation and destruction kinetics, the systems were prepared in the stable skyrmion state, and then a square-wave magnetic field modulation was applied. The measurements show that the skyrmions quickly form ordered domains, with a significant distribution in lattice parameters, which then converge to the final structure; the results confirm the slow kinetics, with formation times between 10 ms and 99 ms. Comparisons are made between the measured formation times and the fundamental material properties, suggesting the ordering temperature, saturation magnetization and magnetocrystalline anisotropy may be driving the timeframes. Micromagnetic simulations were also performed and support a scaling of the kinetics with sample volume, a behavior which is caused by the reconciling of misaligned domains.
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3
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Orphan high field superconductivity in non-superconducting uranium ditelluride. Nat Commun 2024; 15:3378. [PMID: 38643147 PMCID: PMC11032386 DOI: 10.1038/s41467-024-47090-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2023] [Accepted: 03/15/2024] [Indexed: 04/22/2024] Open
Abstract
Reentrant superconductivity is an uncommon phenomenon in which the destructive effects of magnetic field on superconductivity are mitigated, allowing a zero-resistance state to survive under conditions that would otherwise destroy it. Typically, the reentrant superconducting region derives from a zero-field parent superconducting phase. Here, we show that in UTe2 crystals extreme applied magnetic fields give rise to an unprecedented high-field superconductor that lacks a zero-field antecedent. This high-field orphan superconductivity exists at angles offset between 29o and 42o from the crystallographic b to c axes with applied fields between 37 T and 52 T. The stability of field-induced orphan superconductivity presented in this work defies both empirical precedent and theoretical explanation and demonstrates that high-field superconductivity can exist in an otherwise non-superconducting material.
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4
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Resonant Ultrasound Spectroscopy for Irregularly Shaped Samples and Its Application to Uranium Ditelluride. PHYSICAL REVIEW LETTERS 2024; 132:066003. [PMID: 38394590 DOI: 10.1103/physrevlett.132.066003] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/16/2023] [Revised: 08/22/2023] [Accepted: 01/11/2024] [Indexed: 02/25/2024]
Abstract
Resonant ultrasound spectroscopy (RUS) is a powerful technique for measuring the full elastic tensor of a given material in a single experiment. Previously, this technique was practically limited to regularly shaped samples such as rectangular parallelepipeds, spheres, and cylinders [W. M. Visscher et al. J. Acoust. Soc. Am. 90, 2154 (1991)JASMAN0001-496610.1121/1.401643]. We demonstrate a new method for determining the elastic moduli of irregularly shaped samples, extending the applicability of RUS to a much larger set of materials. We apply this new approach to the recently discovered unconventional superconductor UTe_{2} and provide its elastic tensor at both 300 and 4 kelvin.
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A review of UTe 2at high magnetic fields. REPORTS ON PROGRESS IN PHYSICS. PHYSICAL SOCIETY (GREAT BRITAIN) 2023; 86:114501. [PMID: 37729901 DOI: 10.1088/1361-6633/acfb93] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/02/2022] [Accepted: 09/20/2023] [Indexed: 09/22/2023]
Abstract
Uranium ditelluride (UTe2) is recognized as a host material to unconventional spin-triplet superconductivity, but it also exhibits a wealth of additional unusual behavior at high magnetic fields. One of the most prominent signatures of the unconventional superconductivity is a large and anisotropic upper critical field that exceeds the paramagnetic limit. This superconductivity survives to 35 T and is bounded by a discontinuous magnetic transition, which itself is also field-direction-dependent. A different, reentrant superconducting phase emerges only on the high-field side of the magnetic transition, in a range of angles between the crystallographicbandcaxes. This review discusses the current state of knowledge of these high-field phases, the high-field behavior of the heavy fermion normal state, and other phases that are stabilized by applied pressure.
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Detection of a pair density wave state in UTe 2. Nature 2023; 618:921-927. [PMID: 37380691 DOI: 10.1038/s41586-023-05919-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2022] [Accepted: 03/03/2023] [Indexed: 06/30/2023]
Abstract
Spin-triplet topological superconductors should exhibit many unprecedented electronic properties, including fractionalized electronic states relevant to quantum information processing. Although UTe2 may embody such bulk topological superconductivity1-11, its superconductive order parameter Δ(k) remains unknown12. Many diverse forms for Δ(k) are physically possible12 in such heavy fermion materials13. Moreover, intertwined14,15 density waves of spin (SDW), charge (CDW) and pair (PDW) may interpose, with the latter exhibiting spatially modulating14,15 superconductive order parameter Δ(r), electron-pair density16-19 and pairing energy gap17,20-23. Hence, the newly discovered CDW state24 in UTe2 motivates the prospect that a PDW state may exist in this material24,25. To search for it, we visualize the pairing energy gap with μeV-scale energy resolution using superconductive scanning tunnelling microscopy (STM) tips26-31. We detect three PDWs, each with peak-to-peak gap modulations of around 10 μeV and at incommensurate wavevectors Pi=1,2,3 that are indistinguishable from the wavevectors Qi=1,2,3 of the prevenient24 CDW. Concurrent visualization of the UTe2 superconductive PDWs and the non-superconductive CDWs shows that every Pi:Qi pair exhibits a relative spatial phase δϕ ≈ π. From these observations, and given UTe2 as a spin-triplet superconductor12, this PDW state should be a spin-triplet PDW24,25. Although such states do exist32 in superfluid 3He, for superconductors, they are unprecedented.
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7
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Magnetic-field-sensitive charge density waves in the superconductor UTe 2. Nature 2023; 618:928-933. [PMID: 37380690 DOI: 10.1038/s41586-023-06005-8] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2022] [Accepted: 03/23/2023] [Indexed: 06/30/2023]
Abstract
The intense interest in triplet superconductivity partly stems from theoretical predictions of exotic excitations such as non-Abelian Majorana modes, chiral supercurrents and half-quantum vortices1-4. However, fundamentally new and unexpected states may emerge when triplet superconductivity appears in a strongly correlated system. Here we use scanning tunnelling microscopy to reveal an unusual charge-density-wave (CDW) order in the heavy-fermion triplet superconductor UTe2 (refs. 5-8). Our high-resolution maps reveal a multi-component incommensurate CDW whose intensity gets weaker with increasing field, with the CDW eventually disappearing at the superconducting critical field Hc2. To understand the phenomenology of this unusual CDW, we construct a Ginzburg-Landau theory for a uniform triplet superconductor coexisting with three triplet pair-density-wave states. This theory gives rise to daughter CDWs that would be sensitive to magnetic field owing to their origin in a pair-density-wave state and provides a possible explanation for our data. Our discovery of a CDW state that is sensitive to magnetic fields and strongly intertwined with superconductivity provides important information for understanding the order parameters of UTe2.
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8
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CHESS: The future direct geometry spectrometer at the second target station. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2022; 93:065109. [PMID: 35778024 DOI: 10.1063/5.0089740] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/28/2022] [Accepted: 05/25/2022] [Indexed: 06/15/2023]
Abstract
CHESS, chopper spectrometer examining small samples, is a planned direct geometry neutron chopper spectrometer designed to detect and analyze weak signals intrinsic to small cross sections (e.g., small mass, small magnetic moments, or neutron absorbing materials) in powders, liquids, and crystals. CHESS is optimized to enable transformative investigations of quantum materials, spin liquids, thermoelectrics, battery materials, and liquids. The broad dynamic range of the instrument is also well suited to study relaxation processes and excitations in soft and biological matter. The 15 Hz repetition rate of the Second Target Station at the Spallation Neutron Source enables the use of multiple incident energies within a single source pulse, greatly expanding the information gained in a single measurement. Furthermore, the high flux grants an enhanced capability for polarization analysis. This enables the separation of nuclear from magnetic scattering or coherent from incoherent scattering in hydrogenous materials over a large range of energy and momentum transfer. This paper presents optimizations and technical solutions to address the key requirements envisioned in the science case and the anticipated uses of this instrument.
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9
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Multicomponent superconducting order parameter in UTe 2. Science 2021; 373:797-801. [PMID: 34385397 DOI: 10.1126/science.abb0272] [Citation(s) in RCA: 26] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2020] [Revised: 06/30/2020] [Accepted: 06/30/2021] [Indexed: 11/02/2022]
Abstract
An unconventional superconducting state was recently discovered in uranium ditelluride (UTe2), in which spin-triplet superconductivity emerges from the paramagnetic normal state of a heavy-fermion material. The coexistence of magnetic fluctuations and superconductivity, together with the crystal structure of this material, suggests that a distinctive set of symmetries, magnetic properties, and topology underlie the superconducting state. Here, we report observations of a nonzero polar Kerr effect and of two transitions in the specific heat upon entering the superconducting state, which together suggest that the superconductivity in UTe2 is characterized by a two-component order parameter that breaks time-reversal symmetry. These data place constraints on the symmetries of the order parameter and inform the discussion on the presence of topological superconductivity in UTe2.
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10
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Comparison of Two Different Synthesis Methods of Single Crystals of Superconducting Uranium Ditelluride. J Vis Exp 2021:10.3791/62563. [PMID: 34309595 PMCID: PMC10505351 DOI: 10.3791/62563] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/31/2022] Open
Abstract
Single crystal specimens of the actinide compound uranium ditelluride, UTe2, are of great importance to the study and characterization of its dramatic unconventional superconductivity, believed to entail spin-triplet electron pairing. A variety in the superconducting properties of UTe2 reported in the literature indicates that discrepancies between synthesis methods yield crystals with different superconducting properties, including the absence of superconductivity entirely. This protocol describes a process to synthesize crystals that exhibit superconductivity via chemical vapor transport, which has consistently exhibited a superconducting critical temperature of 1.6 K and a double transition indicative of a multi-component order parameter. This is compared to a second protocol that is used to synthesize crystals via the molten metal flux growth technique, which produces samples that are not bulk superconductors. Differences in the crystal properties are revealed through a comparison of structural, chemical, and electronic property measurements, showing that the most dramatic disparity occurs in the low-temperature electrical resistance of the samples.
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11
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Magnetic ordering in the Ising antiferromagnetic pyrochlore Nd 2ScNbO 7. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2021; 33:245802. [PMID: 33827056 DOI: 10.1088/1361-648x/abf594] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/22/2021] [Accepted: 04/07/2021] [Indexed: 06/12/2023]
Abstract
The question of structural disorder and its effects on magnetism is relevant to a number of spin liquid candidate materials. Although commonly thought of as a route to spin glass behaviour, here we describe a system in which the structural disorder results in long-range antiferromagnetic order due to local symmetry breaking. Nd2ScNbO7is shown to have a dispersionless gapped excitation observed in other neodymium pyrochlores belowTN= 0.37 K through polarized and inelastic neutron scattering. However the dispersing spin waves are not observed. This excited mode is shown to occur in only 14(2)% of the neodymium ions through spectroscopy and is consistent with total scattering measurements as well as the magnitude of the dynamic moment 0.26(2)μB. The remaining magnetic species order completely into the all-in all-out Ising antiferromagnetic structure. This can be seen as a result of local symmetry breaking due disordered Sc+3and Nb+5ions about theA-site. From this work, it has been established thatB-site disorder restores the dipole-like behaviour of the Nd+3ions compared to the Nd2B2O7parent series.
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12
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Abstract
Chiral superconductors have been proposed as one pathway to realize Majorana normal fluid at its boundary. However, the long-sought 2D and 3D chiral superconductors with edge and surface Majorana normal fluid are yet to be conclusively found. Here, we report evidence for a chiral spin-triplet pairing state of UTe2 with surface normal fluid response. The microwave surface impedance of the UTe2 crystal was measured and converted to complex conductivity, which is sensitive to both normal and superfluid responses. The anomalous residual normal fluid conductivity supports the presence of a significant normal fluid response. The superfluid conductivity follows the temperature behavior predicted for an axial spin-triplet state, which is further narrowed down to a chiral spin-triplet state with evidence of broken time-reversal symmetry. Further analysis excludes trivial origins for the observed normal fluid response. Our findings suggest that UTe2 can be a new platform to study exotic topological excitations in higher dimension.
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13
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Effect of chemical substitution on the skyrmion phase in Cu 2OSeO 3. PHYSICAL REVIEW. B 2020; 102:10.1103/PhysRevB.102.134410. [PMID: 37731841 PMCID: PMC10510729 DOI: 10.1103/physrevb.102.134410] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 09/22/2023]
Abstract
Magnetic skyrmions have been the focus of intense research due to their unique qualities which result from their topological protections. Previous work on Cu2OSeO3, the only known insulating multiferroic skyrmion material, has shown that chemical substitution alters the skyrmion phase. We chemically substitute Zn, Ag, and S into powdered Cu2OSeO3 to study the effect on the magnetic phase diagram. In both the Ag and the S substitutions, we find that the skyrmion phase is stabilized over a larger temperature range, as determined via magnetometry and small-angle neutron scattering (SANS). Meanwhile, while previous magnetometry characterization suggests two high temperature skyrmion phases in the Zn-substituted sample, SANS reveals the high temperature phase to be skyrmionic while we are unable to distinguish the other from helical order. Overall, chemical substitution weakens helical and skyrmion order as inferred from neutron scattering of the q ≈ 0.01 Å - 1 magnetic peak.
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14
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Quantum Versus Classical Spin Fragmentation in Dipolar Kagome Ice Ho 3Mg 2Sb 3O 14. PHYSICAL REVIEW. X 2020; 10:10.1103/PhysRevX.10.031069. [PMID: 37731951 PMCID: PMC10510738 DOI: 10.1103/physrevx.10.031069] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 09/22/2023]
Abstract
A promising route to realize entangled magnetic states combines geometrical frustration with quantum-tunneling effects. Spin-ice materials are canonical examples of frustration, and Ising spins in a transverse magnetic field are the simplest many-body model of quantum tunneling. Here, we show that the tripod-kagome lattice material Ho3Mg2Sb3O14 unites an icelike magnetic degeneracy with quantum-tunneling terms generated by an intrinsic splitting of the Ho3+ ground-state doublet, which is further coupled to a nuclear spin bath. Using neutron scattering and thermodynamic experiments, we observe a symmetry-breaking transition at T * ≈ 0.32 K to a remarkable state with three peculiarities: a concurrent recovery of magnetic entropy associated with the strongly coupled electronic and nuclear degrees of freedom; a fragmentation of the spin into periodic and icelike components; and persistent inelastic magnetic excitations down to T ≈ 0.12 K . These observations deviate from expectations of classical spin fragmentation on a kagome lattice, but can be understood within a model of dipolar kagome ice under a homogeneous transverse magnetic field, which we survey with exact diagonalization on small clusters and mean-field calculations. In Ho3Mg2Sb3O14, hyperfine interactions dramatically alter the single-ion and collective properties, and suppress possible quantum correlations, rendering the fragmentation with predominantly single-ion quantum fluctuations. Our results highlight the crucial role played by hyperfine interactions in frustrated quantum magnets and motivate further investigations of the role of quantum fluctuations on partially ordered magnetic states.
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15
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Three-dimensional magnetism and the Dzyaloshinskii-Moriya interaction in S = 3/2 kagome staircase Co 3V 2O 8. SCIENCE ADVANCES 2020; 6:eaay9709. [PMID: 32426474 PMCID: PMC7195150 DOI: 10.1126/sciadv.aay9709] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 08/01/2019] [Accepted: 02/12/2020] [Indexed: 06/11/2023]
Abstract
Time-of-flight neutron data reveal spin waves in the ferromagnetic ground state of the kagome staircase material Co3V2O8. While previous work has treated this material as quasi-two-dimensional, we find that an inherently three-dimensional description is needed to describe the spin wave spectrum throughout reciprocal space. Moreover, spin wave branches show gaps that point to an unexpectedly large Dzyaloshinskii-Moriya interaction on the nearest-neighbor bond, with D 1 ≥ J 1/2. A better understanding of the Dzyaloshinskii-Moriya interaction in this material should shed light on the multiferroicity of the related Ni3V2O8. At a higher temperature where Co3V2O8 displays an antiferromagnetic spin density wave structure, there are no well-defined spin wave excitations, with most of the spectral weight observed in broad diffuse scattering centered at the (0, 0.5, 0) antiferromagnetic Bragg peak.
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16
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Enhancement and reentrance of spin triplet superconductivity in UTe 2 under pressure. PHYSICAL REVIEW. B 2020; 101:10.1103/physrevb.101.140503. [PMID: 34131608 PMCID: PMC8201659 DOI: 10.1103/physrevb.101.140503] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
Spin triplet superconductivity in the Kondo lattice UTe2 appears to be associated with spin fluctuations originating from incipient ferromagnetic order. Here we show clear evidence of twofold enhancement of superconductivity under pressure, which discontinuously transitions to magnetic order, likely of ferromagnetic nature, at higher pressures. The application of a magnetic field tunes the system back across a first-order phase boundary. Straddling this phase boundary, we find another example of reentrant superconductivity in UTe2. As the superconductivity and magnetism exist on two opposite sides of the first-order phase boundary, our results indicate other microscopic mechanisms may be playing a role in stabilizing spin triplet superconductivity in addition to spin fluctuations associated with magnetism.
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17
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Low Energy Band Structure and Symmetries of UTe_{2} from Angle-Resolved Photoemission Spectroscopy. PHYSICAL REVIEW LETTERS 2020; 124:076401. [PMID: 32142327 DOI: 10.1103/physrevlett.124.076401] [Citation(s) in RCA: 16] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/21/2019] [Accepted: 01/23/2020] [Indexed: 06/10/2023]
Abstract
The compound UTe_{2} has recently been shown to realize spin triplet superconductivity from a nonmagnetic normal state. This has sparked intense research activity, including theoretical analyses that suggest the superconducting order parameter to be topologically nontrivial. However, the underlying electronic band structure is a critical factor for these analyses, and remains poorly understood. Here, we present high resolution angle-resolved photoemission measurements covering multiple planes in the 3D Brillouin zone of UTe_{2}, revealing distinct Fermi-level features from two orthogonal quasi-one-dimensional light electron bands and one heavy band. The electronic symmetries are evaluated in comparison with numerical simulations, and the resulting picture is discussed as a platform for unconventional many-body order.
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18
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Low-temperature crystal structure of the unconventional spin-triplet superconductor UTe 2 from single-crystal neutron diffraction. ACTA CRYSTALLOGRAPHICA SECTION B, STRUCTURAL SCIENCE, CRYSTAL ENGINEERING AND MATERIALS 2020; 76:137-143. [PMID: 32831248 PMCID: PMC8202135 DOI: 10.1107/s2052520619016950] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/25/2019] [Accepted: 12/18/2019] [Indexed: 06/01/2023]
Abstract
The crystal structure of a new superconductor UTe2 has been investigated using single-crystal neutron diffraction for the first time at the low temperature (LT) of 2.7 K, just above the superconducting transition temperature of ∼1.6 K, in order to clarify whether the orthorhombic structure of type Immm (No. 71), reported for the room-temperature (RT) structure persists down to the superconducting phase and can be considered as a parent symmetry for the development of spin-triplet superconductivity. In contrast to the previously reported phase transition at about 100 K [Stöwe (1996). J. Solid State Chem. 127, 202-210], our high-precision LT neutron diffraction data show that the body-centred RT symmetry is indeed maintained down to 2.7 K. No sign of a structural change from RT down to 2.7 K was observed. The most significant change depending on temperature was observed for the U ion position and the U-U distance along the c direction, implying its potential importance as a magnetic interaction path. No magnetic order could be deduced from the neutron diffraction data refinement at 2.7 K, consistent with bulk magnetometry. Assuming normal thermal evolution of the lattice parameters, moderately large linear thermal expansion coefficients of about α = 2.8 (7) × 10-5 K-1 are estimated.
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Point-node gap structure of the spin-triplet superconductor UTe 2. PHYSICAL REVIEW. B 2019; 100:10.1103/PhysRevB.100.220504. [PMID: 34136735 PMCID: PMC8204512 DOI: 10.1103/physrevb.100.220504] [Citation(s) in RCA: 23] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
Low-temperature electrical and thermal transport, magnetic penetration depth, and heat capacity measurements were performed on single crystals of the actinide superconductor UTe2 to determine the structure of the superconducting energy gap. Heat transport measurements performed with currents directed along both crystallographic a and b axes reveal a vanishingly small residual fermionic component of the thermal conductivity. The magnetic field dependence of the residual term follows a rapid, quasilinear increase consistent with the presence of nodal quasiparticles, rising toward the a-axis upper critical field where the Wiedemann-Franz law is recovered. Together with a quadratic temperature dependence of the magnetic penetration depth up to T/T c = 0.3, these measurements provide evidence for an unconventional spin-triplet superconducting order parameter with point nodes. Millikelvin specific heat measurements performed on the same crystals used for thermal transport reveal an upturn below 300 mK that is well described by a divergent quantum-critical contribution to the density of states (DOS). Modeling this contribution with a T -1/3 power law allows restoration of the full entropy balance in the superconducting state and a resultant cubic power law for the electronic DOS below T c , consistent with the point-node gap structure determined by thermal conductivity and penetration depth measurements.
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Coexistence of ferromagnetic fluctuations and superconductivity in the actinide superconductor UTe 2. PHYSICAL REVIEW. B 2019; 100:10.1103/physrevb.100.140502. [PMID: 34131607 PMCID: PMC8201662 DOI: 10.1103/physrevb.100.140502] [Citation(s) in RCA: 30] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
We report low-temperature muon spin relaxation/rotation (μSR) measurements on single crystals of the actinide superconductor UTe2. Below 5 K we observe a continuous slowing down of magnetic fluctuations that persists through the superconducting transition temperature (T c = 1.6 K), but we find no evidence of long-range or local magnetic order down to 0.025 K. The temperature dependence of the dynamic relaxation rate down to 0.4 K agrees with the self-consistent renormalization theory of spin fluctuations for a three-dimensional weak itinerant ferromagnetic metal. Our μSR measurements also indicate that the superconductivity coexists with the magnetic fluctuations.
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Orbital-selective Kondo lattice and enigmatic f electrons emerging from inside the antiferromagnetic phase of a heavy fermion. SCIENCE ADVANCES 2019; 5:eaaw9061. [PMID: 31667341 PMCID: PMC6799987 DOI: 10.1126/sciadv.aaw9061] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/04/2019] [Accepted: 09/25/2019] [Indexed: 05/25/2023]
Abstract
Novel electronic phenomena frequently form in heavy-fermions because of the mutual localized and itinerant nature of f-electrons. On the magnetically ordered side of the heavy-fermion phase diagram, f-moments are expected to be localized and decoupled from the Fermi surface. It remains ambiguous whether Kondo lattice can develop inside the magnetically ordered phase. Using spectroscopic imaging with scanning tunneling microscope, complemented by neutron scattering, x-ray absorption spectroscopy, and dynamical mean field theory, we probe the electronic states in antiferromagnetic USb2. We visualize a large gap in the antiferromagnetic phase within which Kondo hybridization develops below ~80 K. Our calculations indicate the antiferromagnetism and Kondo lattice to reside predominantly on different f-orbitals, promoting orbital selectivity as a new conception into how these phenomena coexist in heavy-fermions. Finally, at 45 K, we find a novel first order-like transition through abrupt emergence of nontrivial 5f-electronic states that may resemble the "hidden-order" phase of URu2Si2.
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Nearly ferromagnetic spin-triplet superconductivity. Science 2019; 365:684-687. [DOI: 10.1126/science.aav8645] [Citation(s) in RCA: 216] [Impact Index Per Article: 43.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2018] [Accepted: 07/12/2019] [Indexed: 11/02/2022]
Abstract
Spin-triplet superconductors potentially host topological excitations that are of interest for quantum information processing. We report the discovery of spin-triplet superconductivity in UTe2, featuring a transition temperature of 1.6 kelvin and a very large and anisotropic upper critical field exceeding 40 teslas. This superconducting phase stability suggests that UTe2 is related to ferromagnetic superconductors such as UGe2, URhGe, and UCoGe. However, the lack of magnetic order and the observation of quantum critical scaling place UTe2 at the paramagnetic end of this ferromagnetic superconductor series. A large intrinsic zero-temperature reservoir of ungapped fermions indicates a highly unconventional type of superconducting pairing.
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Quantum Spin Ice Dynamics in the Dipole-Octupole Pyrochlore Magnet Ce_{2}Zr_{2}O_{7}. PHYSICAL REVIEW LETTERS 2019; 122:187201. [PMID: 31144900 DOI: 10.1103/physrevlett.122.187201] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/28/2018] [Revised: 03/15/2019] [Indexed: 06/09/2023]
Abstract
Neutron scattering measurements on the pyrochlore magnet Ce_{2}Zr_{2}O_{7} reveal an unusual crystal field splitting of its lowest J=5/2 multiplet, such that its ground-state doublet is composed of m_{J}=±3/2, giving these doublets a dipole-octupole (DO) character with local Ising anisotropy. Its magnetic susceptibility shows weak antiferromagnetic correlations with θ_{CW}=-0.4(2) K, leading to a naive expectation of an all-in, all-out ordered state at low temperatures. Instead, our low-energy inelastic neutron scattering measurements show a dynamic quantum spin ice state, with suppressed scattering near |Q|=0, and no long-range order at low temperatures. This is consistent with recent theory predicting symmetry-enriched U(1) quantum spin liquids for such DO doublets decorating the pyrochlore lattice. Finally, we show that disorder, especially oxidation of powder samples, is important in Ce_{2}Zr_{2}O_{7} and could play an important role in the low-temperature behavior of this material.
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High temperature singlet-based magnetism from Hund's rule correlations. Nat Commun 2019; 10:644. [PMID: 30733441 PMCID: PMC6367396 DOI: 10.1038/s41467-019-08497-3] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/04/2018] [Accepted: 01/10/2019] [Indexed: 11/24/2022] Open
Abstract
Uranium compounds can manifest a wide range of fascinating many-body phenomena, and are often thought to be poised at a crossover between localized and itinerant regimes for 5f electrons. The antiferromagnetic dipnictide USb2 has been of recent interest due to the discovery of rich proximate phase diagrams and unusual quantum coherence phenomena. Here, linear-dichroic X-ray absorption and elastic neutron scattering are used to characterize electronic symmetries on uranium in USb2 and isostructural UBi2. Of these two materials, only USb2 is found to enable strong Hund’s rule alignment of local magnetic degrees of freedom, and to undergo distinctive changes in local atomic multiplet symmetry across the magnetic phase transition. Theoretical analysis reveals that these and other anomalous properties of the material may be understood by attributing it as the first known high temperature realization of a singlet ground state magnet, in which magnetism occurs through a process that resembles exciton condensation. Electrons in uranium-based materials are often on the border between localised and itinerant behaviour, which can lead to unusual magnetic behaviour. Here the authors combine experiment and theory to show that USb2 may be an unusually high temperature example of a singlet-ground-state magnet.
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Extreme magnetic field-boosted superconductivity. NATURE PHYSICS 2019; 15:10.1038/s41567-019-0670-x. [PMID: 34131432 PMCID: PMC8201648 DOI: 10.1038/s41567-019-0670-x] [Citation(s) in RCA: 35] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/03/2019] [Accepted: 08/21/2019] [Indexed: 05/31/2023]
Abstract
Applied magnetic fields underlie exotic quantum states, such as the fractional quantum Hall effect1 and Bose-Einstein condensation of spin excitations2. Superconductivity, however, is inherently antagonistic towards magnetic fields. Only in rare cases3-5 can these effects be mitigated over limited fields, leading to re-entrant superconductivity. Here, we report the coexistence of multiple high-field re-entrant superconducting phases in the spin-triplet superconductor UTe2 (ref. 6). We observe superconductivity in the highest magnetic field range identified for any re-entrant superconductor, beyond 65 T. Although the stability of superconductivity in these high magnetic fields challenges current theoretical models, these extreme properties seem to reflect a new kind of exotic superconductivity rooted in magnetic fluctuations7 and boosted by a quantum dimensional crossover8.
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Phonon dispersion of Mo-stabilized γ-U measured using inelastic x-ray scattering. PHYSICAL REVIEW. B 2019; 100:10.1103/physrevb.100.094311. [PMID: 33553858 PMCID: PMC7860637 DOI: 10.1103/physrevb.100.094311] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
We have measured the room-temperature phonon spectrum of Mo-stabilized γ-U. The dispersion curves show unusual softening near the H point, q = [1/2, 1/2, 1/2], which may derive from the metastability of the γ-U phase or from strong electron-phonon coupling. Near the zone center, the dispersion curves agree well with theory, though significant differences are observed away from the zone center. The experimental phonon density of states is shifted to higher energy compared to theory and high-temperature neutron scattering. The elastic constants of γ-UMo are similar to those of body-centered cubic elemental metals.
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Behavior of the breathing pyrochlore lattice Ba 3Yb 2Zn 5O 11 in applied magnetic field. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2018; 30:455801. [PMID: 30256218 DOI: 10.1088/1361-648x/aae45a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
The breathing pyrochlore lattice material Ba3Yb2Zn5O11 exists in the nearly decoupled limit, in contrast to most other well-studied breathing pyrochlore compounds. As a result, it constitutes a useful platform to benchmark theoretical calculations of exchange interactions in insulating Yb3+ magnets. Here we study Ba3Yb2Zn5O11 at low temperatures in applied magnetic fields as a further probe of the physics of this model system. Experimentally, we consider the behavior of polycrystalline samples of Ba3Yb2Zn5O11 with a combination of inelastic neutron scattering and heat capacity measurements down to 75 mK and up to fields of 10 T. Consistent with previous work, inelastic neutron scattering finds a level crossing near 3 T, but no significant dispersion of the spin excitations is detected up to the highest applied fields. Refinement of the theoretical model previously determined at zero field can reproduce much of the inelastic neutron scattering spectra and specific heat data. A notable exception is a low temperature peak in the specific heat at ∼0.1 K. This may indicate the scale of interactions between tetrahedra or may reflect undetected disorder in Ba3Yb2Zn5O11.
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Phase Competition in the Palmer-Chalker XY Pyrochlore Er_{2}Pt_{2}O_{7}. PHYSICAL REVIEW LETTERS 2017; 119:187201. [PMID: 29219594 DOI: 10.1103/physrevlett.119.187201] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/18/2017] [Indexed: 06/07/2023]
Abstract
We report neutron scattering measurements on Er_{2}Pt_{2}O_{7}, a new addition to the XY family of frustrated pyrochlore magnets. Symmetry analysis of our elastic scattering data shows that Er_{2}Pt_{2}O_{7} orders into the k=0, Γ_{7} magnetic structure (the Palmer-Chalker state), at T_{N}=0.38 K. This contrasts with its sister XY pyrochlore antiferromagnets Er_{2}Ti_{2}O_{7} and Er_{2}Ge_{2}O_{7}, both of which order into Γ_{5} magnetic structures at much higher temperatures, T_{N}=1.2 and 1.4 K, respectively. In this temperature range, the magnetic heat capacity of Er_{2}Pt_{2}O_{7} contains a broad anomaly centered at T^{*}=1.5 K. Our inelastic neutron scattering measurements reveal that this broad heat capacity anomaly sets the temperature scale for strong short-range spin fluctuations. Below T_{N}=0.38 K, Er_{2}Pt_{2}O_{7} displays a gapped spin-wave spectrum with an intense, flat band of excitations at lower energy and a weak, diffusive band of excitations at higher energy. The flat band is well described by classical spin-wave calculations, but these calculations also predict sharp dispersive branches at higher energy, a striking discrepancy with the experimental data. This, in concert with the strong suppression of T_{N}, is attributable to enhanced quantum fluctuations due to phase competition between the Γ_{7} and Γ_{5} states that border each other within a classically predicted phase diagram.
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Nonreciprocal Magnons and Symmetry-Breaking in the Noncentrosymmetric Antiferromagnet. PHYSICAL REVIEW LETTERS 2017; 119:047201. [PMID: 29341758 DOI: 10.1103/physrevlett.119.047201] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/27/2017] [Indexed: 06/07/2023]
Abstract
Inelastic neutron scattering measurements were performed to study spin dynamics in the noncentrosymmetric antiferromagnet α-Cu_{2}V_{2}O_{7}. For the first time, nonreciprocal magnons were experimentally measured in an antiferromagnet. These nonreciprocal magnons are caused by the incompatibility between anisotropic exchange and antisymmetric Dzyaloshinskii-Moriya interactions, which arise from broken symmetry, resulting in a collinear ordered state but helical spin dynamics. The nonreciprocity introduces the difference in the phase velocity of the counterrotating modes, causing the opposite spontaneous magnonic Faraday rotation of the left- and right-propagating spin waves. The breaking of spatial inversion and time reversal symmetry is revealed as a magnetic-field-induced asymmetric energy shift, which provides a test for the detailed balance relation.
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Emergent order in the kagome Ising magnet Dy 3Mg 2Sb 3O 14. Nat Commun 2016; 7:13842. [PMID: 27996012 PMCID: PMC5187434 DOI: 10.1038/ncomms13842] [Citation(s) in RCA: 55] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2016] [Accepted: 11/04/2016] [Indexed: 01/27/2023] Open
Abstract
The Ising model—in which degrees of freedom (spins) are binary valued (up/down)—is a cornerstone of statistical physics that shows rich behaviour when spins occupy a highly frustrated lattice such as kagome. Here we show that the layered Ising magnet Dy3Mg2Sb3O14 hosts an emergent order predicted theoretically for individual kagome layers of in-plane Ising spins. Neutron-scattering and bulk thermomagnetic measurements reveal a phase transition at ∼0.3 K from a disordered spin-ice-like regime to an emergent charge ordered state, in which emergent magnetic charge degrees of freedom exhibit three-dimensional order while spins remain partially disordered. Monte Carlo simulations show that an interplay of inter-layer interactions, spin canting and chemical disorder stabilizes this state. Our results establish Dy3Mg2Sb3O14 as a tuneable system to study interacting emergent charges arising from kagome Ising frustration. Frustration in lattices of interacting spins can lead to rich and exotic physics, such as fractionalized excitations and emergent order. Here, the authors demonstrate a low-temperature transition from a disordered spin-ice-like phase to an emergent charge ordered phase in the bulk kagome Ising magnet Dy3Mg2Sb3O14.
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Pressure-Resistant Intermediate Valence in the Kondo Insulator SmB_{6}. PHYSICAL REVIEW LETTERS 2016; 116:156401. [PMID: 27127976 DOI: 10.1103/physrevlett.116.156401] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/27/2016] [Indexed: 06/05/2023]
Abstract
Resonant x-ray emission spectroscopy was used to determine the pressure dependence of the f-electron occupancy in the Kondo insulator SmB_{6}. Applied pressure reduces the f occupancy, but surprisingly, the material maintains a significant divalent character up to a pressure of at least 35 GPa. Thus, the closure of the resistive activation energy gap and onset of magnetic order are not driven by stabilization of an integer valent state. Over the entire pressure range, the material maintains a remarkably stable intermediate valence that can in principle support a nontrivial band structure.
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Electrostatic coupling between two surfaces of a topological insulator nanodevice. PHYSICAL REVIEW LETTERS 2014; 113:206801. [PMID: 25432050 DOI: 10.1103/physrevlett.113.206801] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/28/2014] [Indexed: 06/04/2023]
Abstract
We report on electronic transport measurements of dual-gated nanodevices of the low-carrier density topological insulator (TI) Bi_{1.5}Sb_{0.5}Te_{1.7}Se_{1.3}. In all devices, the upper and lower surface states are independently tunable to the Dirac point by the top and bottom gate electrodes. In thin devices, electric fields are found to penetrate through the bulk, indicating finite capacitive coupling between the surface states. A charging model allows us to use the penetrating electric field as a measurement of the intersurface capacitance C_{TI} and the surface state energy-density relationship μ(n), which is found to be consistent with independent angle-resolved photoemission spectroscopy measurements. At high magnetic fields, increased field penetration through the surface states is observed, strongly suggestive of the opening of a surface state band gap due to broken time-reversal symmetry.
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Air-stable electron depletion of Bi(2)Se(3) using molybdenum trioxide into the topological regime. ACS NANO 2014; 8:6400-6406. [PMID: 24911767 DOI: 10.1021/nn502031k] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
We perform high-resolution photoelectron spectroscopy on in situ cleaved topological insulator Bi2Se3 single crystals and in situ transport measurements on Bi2Se3 films grown by molecular beam epitaxy. We demonstrate efficient electron depletion of Bi2Se3 via vacuum deposition of molecular MoO3, lowering the surface Fermi energy to within ∼100 meV of the Dirac point, well into the topological regime. A 100 nm MoO3 film provides an air-stable doping and passivation layer.
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Ambipolar surface state thermoelectric power of topological insulator Bi2Se3. NANO LETTERS 2014; 14:1701-1706. [PMID: 24605897 DOI: 10.1021/nl4032154] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
We measure gate-tuned thermoelectric power of mechanically exfoliated Bi2Se3 thin films in the topological insulator regime. The sign of the thermoelectric power changes across the charge neutrality point as the majority carrier type switches from electron to hole, consistent with the ambipolar electric field effect observed in conductivity and Hall effect measurements. Near the charge neutrality point and at low temperatures, the gate-dependent thermoelectric power follows the semiclassical Mott relation using the expected surface state density of states but is larger than expected at high electron doping, possibly reflecting a large density of states in the bulk gap. The thermoelectric power factor shows significant enhancement near the electron-hole puddle carrier density ∼0.5 × 10(12) cm(-2) per surface at all temperatures. Together with the expected reduction of lattice thermal conductivity in low-dimensional structures, the results demonstrate that nanostructuring and Fermi level tuning of three-dimensional topological insulators can be promising routes to realize efficient thermoelectric devices.
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Polarity-driven surface metallicity in SmB6. PHYSICAL REVIEW LETTERS 2013; 111:216402. [PMID: 24313508 DOI: 10.1103/physrevlett.111.216402] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/04/2013] [Indexed: 06/02/2023]
Abstract
By a combined angle-resolved photoemission spectroscopy and density functional theory study, we discover that the surface metallicity is polarity driven in SmB6. Two surface states, not accounted for by the bulk band structure, are reproduced by slab calculations for coexisting B6 and Sm surface terminations. Our analysis reveals that a metallic surface state stems from an unusual property, generic to the (001) termination of all hexaborides: the presence of boron 2p dangling bonds, on a polar surface. The discovery of polarity-driven surface metallicity sheds new light on the 40-year old conundrum of the low-temperature residual conductivity of SmB6, and raises a fundamental question in the field of topological Kondo insulators regarding the interplay between polarity and nontrivial topological properties.
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Pressure-induced unconventional superconducting phase in the topological insulator Bi2Se3. PHYSICAL REVIEW LETTERS 2013; 111:087001. [PMID: 24010465 DOI: 10.1103/physrevlett.111.087001] [Citation(s) in RCA: 59] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/26/2013] [Indexed: 06/02/2023]
Abstract
Simultaneous low-temperature electrical resistivity and Hall effect measurements were performed on single-crystalline Bi2Se3 under applied pressures up to 50 GPa. As a function of pressure, superconductivity is observed to onset above 11 GPa with a transition temperature Tc and upper critical field Hc2 that both increase with pressure up to 30 GPa, where they reach maximum values of 7 K and 4 T, respectively. Upon further pressure increase, Tc remains anomalously constant up to the highest achieved pressure. Conversely, the carrier concentration increases continuously with pressure, including a tenfold increase over the pressure range where Tc remains constant. Together with a quasilinear temperature dependence of Hc2 that exceeds the orbital and Pauli limits, the anomalously stagnant pressure dependence of Tc points to an unconventional pressure-induced pairing state in Bi2Se3 that is unique among the superconducting topological insulators.
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Layer-by-layer entangled spin-orbital texture of the topological surface state in Bi2Se3. PHYSICAL REVIEW LETTERS 2013; 110:216401. [PMID: 23745898 DOI: 10.1103/physrevlett.110.216401] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/29/2013] [Indexed: 06/02/2023]
Abstract
We study Bi(2)Se(3) by polarization-dependent angle-resolved photoemission spectroscopy and density-functional theory slab calculations. We find that the surface state Dirac fermions are characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through quantum interference effects. This explains the discrepancy between the spin polarization obtained in spin and angle-resolved photoemission spectroscopy-ranging from 20% to 85%-and the 100% value assumed in phenomenological models. It also suggests a way to probe the intrinsic spin texture of topological insulators, and to continuously manipulate the spin polarization of photoelectrons and photocurrents all the way from 0 to ±100% by an appropriate choice of photon energy, linear polarization, and angle of incidence.
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Intrinsic electron-phonon resistivity of Bi2Se3 in the topological regime. PHYSICAL REVIEW LETTERS 2012; 109:166801. [PMID: 23215109 DOI: 10.1103/physrevlett.109.166801] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/25/2012] [Indexed: 06/01/2023]
Abstract
We measure the temperature-dependent carrier density and resistivity of the topological surface state of thin exfoliated Bi(2)Se(3) in the absence of bulk conduction. When the gate-tuned chemical potential is near or below the Dirac point, the carrier density is strongly temperature-dependent, reflecting thermal activation from the nearby bulk valence band, while, above the Dirac point, unipolar n-type surface conduction is observed with negligible thermal activation of bulk carriers. In this regime, linear resistivity vs temperature reflects intrinsic electron-acoustic phonon scattering. A quantitative comparison with a theoretical transport calculation including both phonon and disorder effects gives the ratio of deformation potential to Fermi velocity D/ħν(F)=4.7 Å(-1). This strong phonon scattering in the Bi(2)Se(3) surface state gives intrinsic limits for the conductivity and charge carrier mobility at room temperature of ~550 μS per surface and ~10,000 cm(2)/V s.
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High pressure transport properties of the topological insulator Bi2Se3. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2012; 24:035602. [PMID: 22183616 DOI: 10.1088/0953-8984/24/3/035602] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
We report x-ray diffraction, electrical resistivity, and magnetoresistance measurements on Bi2Se3 under high pressure and low temperature conditions. Pressure induces profound changes in both the room temperature value of the electrical resistivity as well as the temperature dependence of the resistivity. Initially, pressure drives Bi2Se3 toward increasingly insulating behavior and then, at higher pressures, the sample appears to enter a fully metallic state coincident with a change in the crystal structure. Within the low pressure phase, Bi2Se3 exhibits an unusual field dependence of the transverse magnetoresistance Δρ(xx) that is positive at low fields and becomes negative at higher fields. Our results demonstrate that pressures below 8 GPa provide a non-chemical means to controllably reduce the bulk conductivity of Bi2Se3.
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Abstract
Thin (6-7 quintuple layer) topological insulator Bi(2)Se(3) quantum dot devices are demonstrated using ultrathin (2-4 quintuple layer) Bi(2)Se(3) regions to realize semiconducting barriers which may be tuned from ohmic to tunneling conduction via gate voltage. Transport spectroscopy shows Coulomb blockade with large charging energy >5 meV and additional features implying excited states.
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Rashba spin-splitting control at the surface of the topological insulator Bi2Se3. PHYSICAL REVIEW LETTERS 2011; 107:186405. [PMID: 22107654 DOI: 10.1103/physrevlett.107.186405] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/02/2011] [Indexed: 05/31/2023]
Abstract
The electronic structure of Bi(2)Se(3) is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultrahigh-vacuum conditions, can be overcome via in situ potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. Ab initio slab calculations reveal that these Rashba states are derived from 5-quintuple-layer quantum-well states. While the K-induced potential gradient enhances the spin splitting, this may be present on pristine surfaces due to the symmetry breaking of the vacuum-solid interface.
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Sr adatoms on As bridge positions on SrFe2As2 observed by scanning tunneling microscopy at 4.2 K. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2011; 23:265702. [PMID: 21666300 DOI: 10.1088/0953-8984/23/26/265702] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
We used a scanning tunneling microscope to image the surface of SrFe(2)As(2) single crystals at 4.2 K. We found, besides the commonly reported row structures and some disordered areas, also maze-like regions. Atomically resolved images of the maze show that the atoms on the surface sit on As bridge positions of the underlying Fe(2)As(2) layer. Examination of the corner positions within the maze-like reconstruction reveals the presence of adatoms rather than As dimers. Hence, the surface atoms on these samples are most likely to be Sr atoms.
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Insulating behavior in ultrathin bismuth selenide field effect transistors. NANO LETTERS 2011; 11:1925-1927. [PMID: 21486055 DOI: 10.1021/nl200017f] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Ultrathin (approximately three quintuple layer) field-effect transistors (FETs) of topological insulator Bi(2)Se(3) are prepared by mechanical exfoliation on 300 nm SiO(2)/Si susbtrates. Temperature- and gate-voltage-dependent conductance measurements show that ultrathin Bi(2)Se(3) FETs are n-type and have a clear OFF state at negative gate voltage, with activated temperature-dependent conductance and energy barriers up to 250 meV.
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Chemical control of interstitial iron leading to superconductivity in Fe1+xTe0.7Se0.3. Chem Sci 2011. [DOI: 10.1039/c1sc00114k] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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Phase Separation and Suppression of the Structural and Magnetic Transitions in Superconducting Doped Iron Tellurides, Fe1+xTe1−ySy. J Am Chem Soc 2010; 132:13000-7. [PMID: 20806923 DOI: 10.1021/ja105279p] [Citation(s) in RCA: 58] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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The suppression of hidden order and the onset of ferromagnetism in URu2Si2 via Re substitution. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2010; 22:164204. [PMID: 21386410 DOI: 10.1088/0953-8984/22/16/164204] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Substitution of Re for Ru in the heavy fermion compound URu(2)Si(2) suppresses the hidden order transition and gives rise to ferromagnetism at higher concentrations. The hidden order transition of URu(2 - x)Re(x)Si(2), tracked via specific heat and electrical resistivity measurements, decreases in temperature and broadens, and is no longer observed for x > 0.1. A critical scaling analysis of the bulk magnetization indicates that the ferromagnetic ordering temperature and ordered moment are suppressed continuously towards zero at a critical concentration of x ≈ 0.15, and this is accompanied by the additional suppression of the critical exponents γ and δ - 1 towards zero. This unusual trend appears to reflect the underlying interplay between Kondo and ferromagnetic interactions, and perhaps the proximity of the hidden order phase.
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Superconductivity at 23 K in Pt doped BaFe₂As₂ single crystals. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2010; 22:072204. [PMID: 21386376 DOI: 10.1088/0953-8984/22/7/072204] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
We report superconductivity in single crystals of the new iron-pnictide system BaFe(1.90)Pt(0.10)As(2) grown by a self-flux solution method and characterized via x-ray, transport, magnetic and thermodynamic measurements. The magnetic ordering associated with a structural transition at 139 K present in BaFe(2)As(2) is completely suppressed by substitution of 5% Fe with Pt and superconductivity is induced at a critical temperature T(c) = 23 K. Full diamagnetic screening in the magnetic susceptibility and a jump in the specific heat at T(c) confirm the bulk nature of the superconducting phase. All properties of the superconducting state-including the transition temperature T(c), the lower critical field H(c1) = 200 mT, the upper critical field H(c2)≈ 65 T, and the slope ∂H(c2)/∂T-are comparable in value to those found in other transition metal-substituted BaFe(2)As(2) series, indicating the robust nature of superconductivity induced by substitution of Group VIII elements.
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Evolution of critical scaling behavior near a ferromagnetic quantum phase transition. PHYSICAL REVIEW LETTERS 2009; 103:076404. [PMID: 19792669 DOI: 10.1103/physrevlett.103.076404] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/24/2009] [Indexed: 05/28/2023]
Abstract
Magnetic critical scaling in URu(2-x)Re(x)Si(2) single crystals continuously evolves as the ferromagnetic critical temperature is tuned towards zero via chemical substitution. As the quantum phase transition is approached, the critical exponents gamma and (delta-1) decrease to zero in tandem with the critical temperature and ordered moment, while the exponent beta remains constant. This novel trend distinguishes URu(2-x)Re(x)Si(2) from stoichiometric quantum critical ferromagnets and appears to reflect an underlying competition between Kondo and ferromagnetic interactions.
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