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Wallace MK, LaBarre PG, Li J, Pi ST, Pickett WE, Dessau DS, Haskel D, Ramirez AP, Subramanian MA. Local Moment Instability of Os in Honeycomb Li 2.15Os 0.85O 3. Sci Rep 2018; 8:6605. [PMID: 29700362 PMCID: PMC5919924 DOI: 10.1038/s41598-018-25028-0] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2018] [Accepted: 04/13/2018] [Indexed: 11/10/2022] Open
Abstract
Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, Li2.15(3)Os0.85(3)O3 (C2/c, a = 5.09 Å, b = 8.81 Å, c = 9.83 Å, β = 99.3°). Neutron diffraction shows large site disorder in the honeycomb layer and X-ray absorption spectroscopy indicates a valence state of Os (4.7 ± 0.2), consistent with the nominal concentration. We observe a transport band gap of Δ = 243 ± 23 meV, a large van Vleck susceptibility, and an effective moment of 0.85 μB, much lower than expected from 70% Os(+5). No evidence of long range order is found above 0.10 K but a spin glass-like peak in ac-susceptibility is observed at 0.5 K. The specific heat displays an impurity spin contribution in addition to a power law ∝T(0.63±0.06). Applied density functional theory (DFT) leads to a reduced moment, suggesting incipient itineracy of the valence electrons, and finding evidence that Li over stoichiometry leads to Os(4+)−Os(5+) mixed valence. This local picture is discussed in light of the site disorder and a possible underlying quantum spin liquid state.
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Affiliation(s)
- M K Wallace
- Department of Chemistry, Oregon State University, Corvallis, OR, 97331, USA
| | - P G LaBarre
- Department of Physics, University of California Santa Cruz, Santa Cruz, CA, 95064, USA
| | - Jun Li
- Department of Chemistry, Oregon State University, Corvallis, OR, 97331, USA
| | - S-T Pi
- Department of Physics, University of California Davis, Davis, CA, 95616, USA
| | - W E Pickett
- Department of Physics, University of California Davis, Davis, CA, 95616, USA
| | - D S Dessau
- Department of Physics, University of Colorado, Denver, CO, 80309, USA
| | - D Haskel
- Advanced Photon Source, Argonne National Laboratory, Argonne, IL, 60439, USA
| | - A P Ramirez
- Department of Physics, University of California Santa Cruz, Santa Cruz, CA, 95064, USA
| | - M A Subramanian
- Department of Chemistry, Oregon State University, Corvallis, OR, 97331, USA.
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2
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Quan Y, Yin ZP, Pickett WE. Single Nodal Loop of Accidental Degeneracies in Minimal Symmetry: Triclinic CaAs_{3}. Phys Rev Lett 2017; 118:176402. [PMID: 28498697 DOI: 10.1103/physrevlett.118.176402] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/31/2016] [Indexed: 06/07/2023]
Abstract
The existence of closed loops of degeneracies in crystals has been intimately connected with associated crystal symmetries, raising the following question: What is the minimum symmetry required for topological character, and can one find an example? Triclinic CaAs_{3}, in the space group P1[over ¯] with only a center of inversion, has been found to display, without need for tuning, a nodal loop of accidental degeneracies with topological character, centered on one face of the Brillouin zone that is otherwise fully gapped. The small loop is very flat in energy, yet is cut four times by the Fermi energy, a condition that results in an intricate repeated touching of inversion related pairs of Fermi surfaces at Weyl points. Spin-orbit coupling lifts the fourfold degeneracy along the loop, leaving trivial Kramers pairs. With its single nodal loop that emerges without protection from any point group symmetry, CaAs_{3} represents the primal "hydrogen atom" of nodal loop systems.
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Affiliation(s)
- Y Quan
- Department of Physics and Center for Advanced Quantum Studies, Beijing Normal University, Beijing 100875, China
| | - Z P Yin
- Department of Physics and Center for Advanced Quantum Studies, Beijing Normal University, Beijing 100875, China
| | - W E Pickett
- Department of Physics, University of California Davis, Davis, California 95616, USA
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3
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Pentcheva R, Arras R, Otte K, Ruiz VG, Pickett WE. Termination control of electronic phases in oxide thin films and interfaces: LaAlO3/SrTiO3(001). Philos Trans A Math Phys Eng Sci 2012; 370:4904-4926. [PMID: 22987035 DOI: 10.1098/rsta.2012.0202] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
A wealth of intriguing properties emerge in the seemingly simple system composed of the band insulators LaAlO(3) and SrTiO(3) such as a two-dimensional electron gas, superconductivity and magnetism. In this paper, we review the current insight obtained from first principles calculations on the mechanisms governing the behaviour of thin LaAlO(3) films on SrTiO(3)(001). In particular, we explore the strong dependence of the electronic properties on the surface and interface termination, the finite film thickness, lattice polarization and defects. A further aspect that is addressed is how the electronic behaviour and functionality can be tuned by an SrTiO(3) capping layer, adsorbates and metallic contacts. Lastly, we discuss recent reports on the coexistence of magnetism and superconductivity in this system for what they might imply about the electronic structure of this system.
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Affiliation(s)
- R Pentcheva
- Department of Earth and Environmental Sciences, Section Crystallography and Center of Nanoscience, University of Munich, Germany.
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Ronning F, Zhu JX, Das T, Graf MJ, Albers RC, Rhee HB, Pickett WE. Superconducting gap structure of the 115s revisited. J Phys Condens Matter 2012; 24:294206. [PMID: 22773378 DOI: 10.1088/0953-8984/24/29/294206] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
Density functional theory calculations of the electronic structure of Ce- and Pu-based heavy fermion superconductors in the so-called 115 family are performed. The gap equation is used to consider which superconducting order parameters are most favorable assuming a pairing interaction that is peaked at (π, π, qz)—the wavevector for the antiferromagnetic ordering found in close proximity. In addition to the commonly accepted dx2−y2 order parameter, there is evidence that an extended s-wave order parameter with nodes is also plausible. We discuss whether these results are consistent with current observations and possible measurements that could help distinguish between these scenarios.
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Affiliation(s)
- F Ronning
- Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
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5
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Gray AX, Papp C, Ueda S, Balke B, Yamashita Y, Plucinski L, Minár J, Braun J, Ylvisaker ER, Schneider CM, Pickett WE, Ebert H, Kobayashi K, Fadley CS. Probing bulk electronic structure with hard X-ray angle-resolved photoemission. Nat Mater 2011; 10:759-764. [PMID: 21841798 DOI: 10.1038/nmat3089] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/25/2010] [Accepted: 07/05/2011] [Indexed: 05/31/2023]
Abstract
Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects.
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Affiliation(s)
- A X Gray
- Department of Physics, University of California Davis, Davis, California 95616, USA.
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6
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Abstract
Using a non-empirical ionic model, we find harmonically unstable, double well potential surfaces for both La2CuO4 and YBa2Cu3O7 which may be related to the high temperature superconductivity. We have applied the Potential Induced Breathing (PIB) Model to La2CuO4 and YBa2Cu3O7 and find that the ionic description gives semiquantitative agreement with structural and vibrational observations. Furthermore, comparison with self-consistent linearized augmented plane wave [1] (LAPW) calculations shows that ionic contributions to the bonding and lattice dynamics of oxide superconductors are large. For La2CuO4 we find that an ionic description predicts the observed tetragonal to orthorhombic distortion, predicts a lower symmetry ground state, and predicts a stable oxygen breathing mode. We find harmonically unstable phonon branches that we relate to phase transitions. Unstable (anharmonic) modes will couple via ionic forces to the charge density in the metallic planes. This ionic coupling has not been included in previous lattice dynamics calculations. For YBa2Cu3O7 we find similar unstable phonon branches that involve oxygen motions perpendicular to the Cu-O bonds. We find the crystalline charge density is best approximated by overlapping ions if Cu and O ions of charge +1.6 and -1.8, respectively, are used.
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Krakauer H, Cohen RE, Pickett WE. Electron-Phonon Coupling Strength of Specific Phonons from First Principles Lapw Calculations. ACTA ACUST UNITED AC 2011. [DOI: 10.1557/proc-141-165] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
AbstractElectron-phonon matrix elements, phonon linewidths and mode coupling strengths are being calculated for La2-xMxCuO4 (M-divalent cation, for paramagnetic x-0.0 and for x-0.15 in a rigid band picture) from first principles local density calculations. The change in potential due to a particular phonon mode is calculated from the difference of self-consistent one-electron potentials, and appropriate Fermi surface averages are carried out for selected modes, allowing us to obtain the phonon linewidth due to the electron-phonon interaction, and the corresponding coupling strength λQ. Here we establish the numerical accuracy within the dual representation of the potential used in the Linearized Augmented Plane Wave (LAPW) method. Evaluations of phonon linewidths and mode coupling strengths are presented for Al and Nb and compared with previous information on these modes. We present preliminary results for the full matrix elements and coupling of the La2CuO4 oxygen planar X-point breathing mode, and compare with a simpler approximation.
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Smith JC, Banerjee S, Pardo V, Pickett WE. Dirac point degenerate with massive bands at a topological quantum critical point. Phys Rev Lett 2011; 106:056401. [PMID: 21405413 DOI: 10.1103/physrevlett.106.056401] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/23/2010] [Revised: 01/10/2011] [Indexed: 05/30/2023]
Abstract
The quasilinear bands in the topologically trivial skutterudite insulator CoSb(3) are studied under adiabatic, symmetry-conserving displacement of the Sb sublattice. In this cubic, time-reversal and inversion symmetric system, a transition from trivial insulator to topological point Fermi surface system occurs through a critical point in which massless (Dirac) bands appear, and moreover are degenerate with massive bands. Spin-orbit coupling, while small due to the type of band character, coupled with tetragonal strain opens the gap required to give the topological insulator. The mineral skutterudite (CoSb(3)) is very near the critical point in its natural state.
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Affiliation(s)
- J C Smith
- Department of Physics, University of California Davis, Davis, California 95616, USA
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9
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Pentcheva R, Huijben M, Otte K, Pickett WE, Kleibeuker JE, Huijben J, Boschker H, Kockmann D, Siemons W, Koster G, Zandvliet HJW, Rijnders G, Blank DHA, Hilgenkamp H, Brinkman A. Parallel electron-hole bilayer conductivity from electronic interface reconstruction. Phys Rev Lett 2010; 104:166804. [PMID: 20482074 DOI: 10.1103/physrevlett.104.166804] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/22/2009] [Indexed: 05/29/2023]
Abstract
The perovskite SrTiO3-LaAlO3 structure has advanced to a model system to investigate the rich electronic phenomena arising at polar oxide interfaces. Using first principles calculations and transport measurements we demonstrate that an additional SrTiO3 capping layer prevents atomic reconstruction at the LaAlO3 surface and triggers the electronic reconstruction at a significantly lower LaAlO3 film thickness than for the uncapped systems. Combined theoretical and experimental evidence (from magnetotransport and ultraviolet photoelectron spectroscopy) suggests two spatially separated sheets with electron and hole carriers, that are as close as 1 nm.
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Affiliation(s)
- R Pentcheva
- Department of Earth and Environmental Sciences, University of Munich, D-80333 Munich, Germany
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10
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Banerjee S, Singh RRP, Pardo V, Pickett WE. Tight-binding modeling and low-energy behavior of the semi-Dirac point. Phys Rev Lett 2009; 103:016402. [PMID: 19659161 DOI: 10.1103/physrevlett.103.016402] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/09/2009] [Indexed: 05/28/2023]
Abstract
We develop a tight-binding model description of semi-Dirac electronic spectra, with highly anisotropic dispersion around point Fermi surfaces, recently discovered in electronic structure calculations of VO2-TiO2 nanoheterostructures. We contrast their spectral properties with the well-known Dirac points on the honeycomb lattice relevant to graphene layers and the spectra of bands touching each other in zero-gap semiconductors. We also consider the lowest order dispersion around one of the semi-Dirac points and calculate the resulting electronic energy levels in an external magnetic field. In spite of apparently similar electronic structures, Dirac and semi-Dirac systems support diverse low-energy physics.
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Affiliation(s)
- S Banerjee
- Department of Physics, University of California, Davis, California 95616, USA
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11
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Ylvisaker ER, Kunes J, McMahan AK, Pickett WE. Charge fluctuations and the valence transition in Yb under pressure. Phys Rev Lett 2009; 102:246401. [PMID: 19659033 DOI: 10.1103/physrevlett.102.246401] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/20/2009] [Indexed: 05/28/2023]
Abstract
We present a dynamical mean-field theory study of the valence transition (f;{14} --> f;{13}) in elemental, metallic Yb under pressure. Our calculations reproduce the observed valence transition as reflected in the volume dependence of the 4f occupation. The transition is advanced by heating, and suggests quasiparticle or Kondo-like structure in the spectra of the trivalent end state, consistent with the early lanthanides. Results for the local charge fluctuations and susceptibility, however, show novel signatures uniquely associated with the valence transition itself, indicating that Yb is a fluctuating valence material in contrast with the intermediate valence behavior seen in the early trivalent lanthanides Ce, Pr, and Nd.
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Affiliation(s)
- E R Ylvisaker
- Department of Physics, University of California, Davis, California 95616, USA
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12
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Yin ZP, Lebègue S, Han MJ, Neal BP, Savrasov SY, Pickett WE. Electron-hole symmetry and magnetic coupling in antiferromagnetic LaFeAsO. Phys Rev Lett 2008; 101:047001. [PMID: 18764356 DOI: 10.1103/physrevlett.101.047001] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/21/2008] [Indexed: 05/26/2023]
Abstract
When either electron or hole doped at concentrations x approximately 0.1, the LaFeAsO family displays remarkably high temperature superconductivity with Tc up to 55 K. In the most energetically stable Q-->M=(pi,pi,0) antiferromagnetic (AFM) phase comprised of tetragonal-symmetry breaking alternating chains of aligned spins, there is a deep pseudogap in the Fe 3d states centered at the Fermi energy arising from light carriers (m* approximately 0.25-0.33), and very strong magnetophonon coupling is uncovered. Doping (of either sign) beyond x approximately 0.08 results in heavy carriers per Fe (by roughly an order of magnitude) with a large Fermi surface. Calculated Fe-Fe transverse exchange couplings Jij(R) reveal that exchange coupling is strongly dependent on both the AFM symmetry and on the Fe-As distance.
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Affiliation(s)
- Z P Yin
- Department of Physics, University of California Davis, Davis, California 95616, USA
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13
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Maddox BR, Lazicki A, Yoo CS, Iota V, Chen M, McMahan AK, Hu MY, Chow P, Scalettar RT, Pickett WE. 4f delocalization in Gd: inelastic x-ray scattering at ultrahigh pressure. Phys Rev Lett 2006; 96:215701. [PMID: 16803251 DOI: 10.1103/physrevlett.96.215701] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/17/2005] [Indexed: 05/10/2023]
Abstract
We present resonant inelastic x-ray scattering and x-ray emission spectroscopy results on Gd metal to 113 GPa which suggest Kondo-like aspects in the delocalization of 4f electrons. Analysis of the resonant inelastic x-ray scattering data reveals a prolonged and continuous delocalization with volume throughout the entire pressure range, so that the volume-collapse transition at 59 GPa is only part of the phenomenon. Moreover, the Lgamma1 x-ray emission spectroscopy spectra indicate no apparent change in the bare 4f moment across the collapse, suggesting that Kondo screening is responsible for the expected Pauli-like behavior in magnetic susceptibility.
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Affiliation(s)
- B R Maddox
- Lawrence Livermore National Laboratory, Livermore, California 94551, USA
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14
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Lee KW, Pickett WE. Charge and spin ordering in insulating Na0.5CoO2: effects of correlation and symmetry. Phys Rev Lett 2006; 96:096403. [PMID: 16606289 DOI: 10.1103/physrevlett.96.096403] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/19/2005] [Indexed: 05/08/2023]
Abstract
Ab initio band theory including correlations due to intra-atomic repulsion is applied to study charge disproportionation and charge and spin ordering in insulating Na0.5CoO2. Various ordering patterns (zigzag and two striped) for four-Co supercells are analyzed before focusing on the observed "out-of-phase stripe" pattern of antiferromagnetic Co4+ spins along charge-ordered stripes. This pattern relieves frustration and shows distinct analogies with the cuprate layers: a bipartite lattice of antialigned spins, with axes at 90degrees angles. Substantial distinctions with cuprates are also discussed, including the tiny gap of a new variant of "charge-transfer" type within the Co 3d system.
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Affiliation(s)
- K-W Lee
- Department of Physics, University of California, Davis, California 95616, USA
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15
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Kasinathan D, Kunes J, Lazicki A, Rosner H, Yoo CS, Scalettar RT, Pickett WE. Superconductivity and lattice instability in compressed lithium from Fermi surface hot spots. Phys Rev Lett 2006; 96:047004. [PMID: 16486875 DOI: 10.1103/physrevlett.96.047004] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/05/2005] [Indexed: 05/06/2023]
Abstract
The highest superconducting temperature Tc observed in any elemental metal (Li with Tc approximately 18-20 K at pressure 35-48 GPa) is shown to arise from increasingly strong electron-phonon coupling concentrated along intersections of Kohn anomaly surfaces with the evolving Fermi surface. First-principles linear response calculations of the phonon spectrum and spectral function alpha2F(omega) reveal very strong Q- and phonon-polarization dependence of coupling strength, resulting in values of in the observed range. The sharp momentum dependence of the coupling even for the simple Li Fermi surface indicates more generally that a fine Q mesh is required for precise evaluation of lamda.
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Affiliation(s)
- Deepa Kasinathan
- Department of Physics, University of California-Davis, Davis, California 95616, USA
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16
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Qian MC, Fong CY, Liu K, Pickett WE, Pask JE, Yang LH. Half-metallic digital ferromagnetic heterostructure composed of a delta-doped layer of Mn in Si. Phys Rev Lett 2006; 96:027211. [PMID: 16486632 DOI: 10.1103/physrevlett.96.027211] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/31/2005] [Indexed: 05/06/2023]
Abstract
We propose and investigate the properties of a digital ferromagnetic heterostructure consisting of a delta-doped layer of Mn in Si, using ab initio electronic-structure methods. We find that (i) ferromagnetic order of the Mn layer is energetically favorable relative to antiferromagnetic, and (ii) the heterostructure is a two-dimensional half-metallic system. The metallic behavior is contributed by three majority-spin bands originating from hybridized Mn-d and nearest-neighbor Si-p states, and the corresponding carriers are responsible for the ferromagnetic order in the Mn layer. The minority-spin channel has a calculated semiconducting gap of 0.25 eV. The band lineup is found to be favorable for retaining the half-metal character to near the Curie temperature. This kind of heterostructure may be of special interest for integration into mature Si technologies for spintronic applications.
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Affiliation(s)
- M C Qian
- Department of Physics, University of California, Davis, Davis, California 95616-8677, USA
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17
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Lazicki A, Maddox B, Evans WJ, Yoo CS, McMahan AK, Pickett WE, Scalettar RT, Hu MY, Chow P. New cubic phase of Li3N: stability of the N3- ion to 200 GPa. Phys Rev Lett 2005; 95:165503. [PMID: 16241815 DOI: 10.1103/physrevlett.95.165503] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/29/2005] [Indexed: 05/05/2023]
Abstract
Diamond-anvil cell experiments augmented by first-principles calculations have found a remarkable stability of the N(3-) ion in Li3N to a sixfold volume reduction. A new (gamma) phase is discovered above 40(+/-5) GPa, with an 8% volume collapse and a band gap quadrupling at the transition determined by synchrotron x-ray diffraction and inelastic x-ray scattering. gamma-Li(3)N (Fm3m, Li(3)Bi-like structure) remains stable up to 200 GPa, and calculations do not predict metallization until approximately 8 TPa. The high structural stability, wide band gap, and simple electronic structure make this N(3-) based system analogous to lower valency compounds (MgO, NaCl, Ne), meriting its use as an internal pressure standard.
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Affiliation(s)
- A Lazicki
- Lawrence Livermore National Laboratory, California 94550, USA
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18
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Yoo CS, Maddox B, Klepeis JHP, Iota V, Evans W, McMahan A, Hu MY, Chow P, Somayazulu M, Häusermann D, Scalettar RT, Pickett WE. First-order isostructural Mott transition in highly compressed MnO. Phys Rev Lett 2005; 94:115502. [PMID: 15903870 DOI: 10.1103/physrevlett.94.115502] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/14/2004] [Indexed: 05/02/2023]
Abstract
We present evidence for an isostructural, first-order Mott transition in MnO at 105+/-5 GPa, based on high-resolution x-ray emission spectroscopy and angle-resolved x-ray diffraction data. The pressure-induced structural and spectral changes provide a coherent picture of MnO phase transitions from paramagnetic B1 to antiferromagnetic distorted B1 at 30 GPa, to paramagnetic B8 at 90 GPa, and to diamagnetic B8 at 105+/-5 GPa. The last is the Mott transition, accompanied by a significant loss of magnetic moment, an approximately 6.6% volume collapse and the insulator-metal transition as demonstrated by recent resistance measurements.
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Affiliation(s)
- C S Yoo
- Lawrence Livermore National Laboratory, Livermore, CA 94551, USA
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19
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Lee KW, Kunes J, Novak P, Pickett WE. Disproportionation, metal-insulator transition, and critical interaction strength in Na(1/2)CoO2. Phys Rev Lett 2005; 94:026403. [PMID: 15698202 DOI: 10.1103/physrevlett.94.026403] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/18/2004] [Indexed: 05/24/2023]
Abstract
Charge disproportionation (CD) and spin differentiation in Na(1/2)CoO2 are studied using the correlated band local-density approximation + Hubbard U (LDA+U) approach. The simultaneous CD and gap opening seen previously is followed in detail through a first-order charge disproportionation transition 2Co(3.5+)-->Co3++Co4+. Disproportionation in the Co a(g) orbital results in half of the ions (Co3+) becoming electronically and magnetically dead, transforming the quarter-filled a(g) system into a half-filled subsystem that subsequently undergoes the observed charge ordering or metal-insulator transition. Comparing with data in the x approximately 0.3 regime suggests the system has moved into the multiband regime where the effective Coulomb repulsion U-->U(eff)=U/sqrt[3] strongly lessens correlation effects.
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Affiliation(s)
- K-W Lee
- Department of Physics, University of California, Davis, California 95616, USA
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20
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Abstract
Superconductivity of boron-doped diamond, reported recently at T(c)=4 K, is investigated exploiting its electronic and vibrational analogies to MgB2. The deformation potential of the hole states arising from the C-C bond-stretch mode is 60% larger than the corresponding quantity in MgB2 that drives its high T(c), leading to very large electron-phonon matrix elements. The calculated coupling strength lambda approximately 0.5 leads to T(c) in the 5-10 K range and makes phonon coupling the likely mechanism. Higher doping should increase T(c) somewhat, but the effects of three dimensionality primarily on the density of states keep doped diamond from having a T(c) closer to that of MgB2.
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Affiliation(s)
- K-W Lee
- Department of Physics, University of California, Davis, CA 95616, USA
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Pogrebnyakov AV, Redwing JM, Raghavan S, Vaithyanathan V, Schlom DG, Xu SY, Li Q, Tenne DA, Soukiassian A, Xi XX, Johannes MD, Kasinathan D, Pickett WE, Wu JS, Spence JCH. Enhancement of the superconducting transition temperature of MgB2 by a strain-induced bond-stretching mode softening. Phys Rev Lett 2004; 93:147006. [PMID: 15524834 DOI: 10.1103/physrevlett.93.147006] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/05/2004] [Indexed: 05/24/2023]
Abstract
We report a systematic increase of the superconducting transition temperature T(c) with a biaxial tensile strain in MgB2 films to well beyond the bulk value. The tensile strain increases with the MgB2 film thickness, caused primarily by the coalescence of initially nucleated discrete islands (the Volmer-Weber growth mode.) The T(c) increase was observed in epitaxial films on SiC and sapphire substrates, although the T(c) values were different for the two substrates due to different lattice parameters and thermal expansion coefficients. We identified, by first-principles calculations, the underlying mechanism for the T(c) increase to be the softening of the bond-stretching E(2g) phonon mode, and we confirmed this conclusion by Raman scattering measurements. The result suggests that the E(2g) phonon softening is a possible avenue to achieve even higher T(c) in MgB2-related material systems.
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Affiliation(s)
- A V Pogrebnyakov
- Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
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22
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Ku W, Rosner H, Pickett WE, Scalettar RT. Insulating ferromagnetism in La4Ba2Cu2O10: an Ab initio wannier function analysis. Phys Rev Lett 2002; 89:167204. [PMID: 12398752 DOI: 10.1103/physrevlett.89.167204] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/14/2002] [Indexed: 05/24/2023]
Abstract
Microscopic mechanisms of the puzzling insulating ferromagnetism of half-filled La4Ba2Cu2O10 are elucidated with energy-resolved Wannier states. The dominant magnetic coupling, revealed through evaluated parameters (t, U, and J), turns out to be the intersite direct exchange, a currently ignored mechanism that overwhelms the antiferromagnetic superexchange. By contrast, the isostructural Nd4Ba2Cu2O10 develops the observed antiferromagnetic order via its characteristics of a 1D chain. Surprisingly, the in-plane order of both cases is not controlled by coupling between nearest neighbors. An intriguing pressure-induced ferromagnetic to antiferromagnetic transition is predicted.
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Affiliation(s)
- Wei Ku
- Department of Physics, University of California, Davis 95616-8677, USA
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Rosner H, Singh RRP, Zheng WH, Oitmaa J, Drechsler SL, Pickett WE. Realization of a large J(2) quasi-2D spin-half Heisenberg system: Li(2)VOSiO(4). Phys Rev Lett 2002; 88:186405. [PMID: 12005707 DOI: 10.1103/physrevlett.88.186405] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/17/2001] [Indexed: 05/23/2023]
Abstract
Exchange couplings are calculated for Li(2)VOSiO(4) using the local-density approximation (LDA). While the sum of in-plane couplings J(1)+J(2) = 9.5+/-1.5 K and the interplane coupling J( perpendicular) approximately 0.2- 0.3 K agree with recent experimental data, the ratio J(2)/J(1) approximately 12 exceeds the reported value by an order of magnitude. Using geometrical considerations, high temperature expansions and perturbative mean field theory, we show that the LDA-derived exchange constants lead to a remarkably accurate description of the properties of these materials including specific heat, susceptibility, Néel temperature, and NMR spectra.
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Affiliation(s)
- H Rosner
- Department of Physics, University of California, Davis, CA 95616, USA
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24
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Abstract
The layered lithium borocarbide LiBC, isovalent with and structurally similar to the superconductor MgB2, is an insulator due to the modulation within the hexagonal layers (BC vs B2). We show that hole doping of LiBC results in Fermi surfaces of B-C p sigma character that couple very strongly to B-C bond stretching modes, precisely the features that lead to superconductivity at T(c) approximately equal to 40 K in MgB2. Comparison of Li(0.5)BC with MgB2 indicates the former to be a prime candidate for electron-phonon coupled superconductivity at substantially higher temperature than in MgB2.
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Affiliation(s)
- H Rosner
- Department of Physics, University of California, Davis, California 95616, USA
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25
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Ku W, Pickett WE, Scalettar RT, Eguiluz AG. Ab initio investigation of collective charge excitations in MgB2. Phys Rev Lett 2002; 88:057001. [PMID: 11863767 DOI: 10.1103/physrevlett.88.057001] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/18/2001] [Indexed: 05/23/2023]
Abstract
A sharp collective charge excitation is predicted in MgB2 at approximately 2.5 eV for q perpendicular to the boron layers, based on an all-electron analysis of the dynamical density response within time-dependent density functional theory. This novel excitation, consisting of coherent charge fluctuation between Mg and B sheets, induces an abrupt plasma edge in the experimentally observable reflectivity. The existence of this mode reflects the unique electronic structure of MgB2 that is also responsible for strong electron-phonon coupling. By contrast, the acoustic plasmon, recently suggested to explain the high T(c), is not realized when realistic transition strengths are incorporated.
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Affiliation(s)
- Wei Ku
- Physics Department, University of California, Davis, California 95616, USA
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26
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Abstract
An unusual quasi-two-dimensional heavy band mass van Hove singularity (vHs) lies very near the Fermi energy in MgCNi3, recently reported to superconduct at 8.5 K. This compound is strongly exchange enhanced and unstable to ferromagnetism upon hole doping with approximately 12% Mg-->Na or Li (i.e., 0.04 hole/Ni). We identify an essentially infinite mass along the M-Gamma line, which accounts for the two dimensionality of this vHs. This compound provides new opportunities to probe the ferromagnetic critical point as well as introducing the novelties of 2D behavior into a 3D system.
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Affiliation(s)
- H Rosner
- Department of Physics, University of California, Davis, California 95616, USA
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27
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Abstract
A series of calculations on MgB2 and related isoelectronic systems indicates that the layer of Mg2+ ions lowers the nonbonding B pi ( p(z)) bands relative to the bonding sigma ( sp(x)p(y)) bands compared to graphite, causing sigma-->pi charge transfer and sigma band doping of 0.13 holes/cell. Because of their two dimensionality the sigma bands contribute strongly to the Fermi level density of states. Calculated deformation potentials of gamma point phonons identify the B bond stretching modes as dominating the electron-phonon coupling. Superconductivity driven by sigma band holes is consistent with the report of destruction of superconductivity by doping with Al.
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Affiliation(s)
- J M An
- Department of Physics, University of California, Davis, California 95616, USA
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Abstract
A consistent picture on the mean-field level of the magnetic properties and electronic structure of the superconducting itinerant ferromagnet UGe2 requires inclusion of correlation effects beyond the local density approximation (LDA). The " LDA+U" approach reproduces both the magnitude of the observed moment and the magnetocrystalline anisotropy. The largest Fermi surface sheet is composed primarily of spin majority states with orbital projection m(l) = 0, suggesting a much simpler picture of the pairing than is possible for general strong spin-orbit coupled materials. The quasi-two-dimensional geometry of the Fermi surface supports the likelihood of magnetically mediated p-wave triplet pairing.
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Affiliation(s)
- A B Shick
- Department of Physics, University of California, Davis, California 95616, USA
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30
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Pickett WE. Relationship between the electronic structure of coherent composition modulated alloys and the supermodulus effect. ACTA ACUST UNITED AC 2000. [DOI: 10.1088/0305-4608/12/10/014] [Citation(s) in RCA: 79] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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31
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Falter C, Ludwig W, Selmke M, Pickett WE. A quasi-ion approach to lattice dynamics and electron-phonon interaction with applications. ACTA ACUST UNITED AC 2000. [DOI: 10.1088/0022-3719/20/4/004] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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32
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Wang CS, Krakauer H, Pickett WE. Unconventional superconductivity and normal-state properties in the heavy fermion superconductor UPt3. ACTA ACUST UNITED AC 2000. [DOI: 10.1088/0305-4608/16/12/003] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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Abstract
Electron-hole mixing-induced fine structure in alkaline earth hexaborides leads to lower energy (temperature) scales, and thus a stronger tendency toward an excitonic instability than in their doped counterparts (viz. Ca1-xLaxB6, x approximately 0.005), which are high-Curie-temperature, small-moment ferromagnets. Comparison of Fermi surfaces and spectral distributions with de Haas-van Alphen, optical, transport, and tunneling data indicates that SrB6 remains a fermionic semimetal down to (at least) 5 K, rather than forming an excitonic condensate. For the doped system the Curie temperature is higher than the degeneracy temperature.
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Affiliation(s)
- CO Rodriguez
- Instituto de Fisica de Liquidos y Sistemas Biologicos, Grupo Fisica del Solido, Casilla de Correo 565, La Plata 1900, Argentina
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Engel GE, Pickett WE. Investigation of density functionals to predict both ground-state properties and band structures. Phys Rev B Condens Matter 1996; 54:8420-8429. [PMID: 9984514 DOI: 10.1103/physrevb.54.8420] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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36
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Ethridge EC, Erwin SC, Pickett WE. Nb4N3: Polymorphism in crystalline niobium nitrides. Phys Rev B Condens Matter 1996; 53:12563-12565. [PMID: 9982911 DOI: 10.1103/physrevb.53.12563] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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37
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Pickett WE, Singh DJ. Comment on "Density-functional description of the electronic structure of LaMO3 (M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni)". Phys Rev B Condens Matter 1996; 53:7550. [PMID: 9982207 DOI: 10.1103/physrevb.53.7550] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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38
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Pickett WE, Singh DJ. Electronic structure and half-metallic transport in the La1-xCaxMnO3 system. Phys Rev B Condens Matter 1996; 53:1146-1160. [PMID: 9983571 DOI: 10.1103/physrevb.53.1146] [Citation(s) in RCA: 104] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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39
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Ethridge EC, Erwin SC, Pickett WE. Metastability of NbN in the ordered vacancy NbO phase. Phys Rev B Condens Matter 1995; 52:R8589-R8592. [PMID: 9979901 DOI: 10.1103/physrevb.52.r8589] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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Singh DJ, Pickett WE. Structural modifications in bismuth cuprates: Effects on the electronic structure and Fermi surface. Phys Rev B Condens Matter 1995; 51:3128-3133. [PMID: 9979097 DOI: 10.1103/physrevb.51.3128] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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42
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Singh DJ, Pickett WE. Skutterudite antimonides: Quasilinear bands and unusual transport. Phys Rev B Condens Matter 1994; 50:11235-11238. [PMID: 9975246 DOI: 10.1103/physrevb.50.11235] [Citation(s) in RCA: 95] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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43
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Pickett WE. Negative electron affinity and low work function surface: Cesium on oxygenated diamond (100). Phys Rev Lett 1994; 73:1664-1667. [PMID: 10056852 DOI: 10.1103/physrevlett.73.1664] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
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Davidson BN, Pickett WE. Tight-binding study of hydrogen on the C(111), C(100), and C(110) diamond surfaces. Phys Rev B Condens Matter 1994; 49:11253-11267. [PMID: 10009976 DOI: 10.1103/physrevb.49.11253] [Citation(s) in RCA: 63] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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48
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Pederson MR, Pickett WE, Erwin SC. Electronic-structure investigations of siloxenic clusters and films. Phys Rev B Condens Matter 1993; 48:17400-17405. [PMID: 10008352 DOI: 10.1103/physrevb.48.17400] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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49
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Pickett WE, Broughton JQ. Variational Monte Carlo study of the partially polarized electron gas. Phys Rev B Condens Matter 1993; 48:14859-14867. [PMID: 10008017 DOI: 10.1103/physrevb.48.14859] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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50
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Krakauer H, Pickett WE, Cohen RE. Large calculated electron-phonon interactions in La2-xMxCuO4. Phys Rev B Condens Matter 1993; 47:1002-1015. [PMID: 10005579 DOI: 10.1103/physrevb.47.1002] [Citation(s) in RCA: 66] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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