Pressure- and composition-induced structural quantum phase transition in the cubic superconductor (Sr, Ca)3Ir4Sn13

Tiny Sc Allows the Chains to Rattle: Impact of Lu and Y Doping on the Charge-Density Wave in ScV6Sn6

The kagome metals display an intriguing variety of electronic and magnetic phases arising from the connectivity of atoms on a kagome lattice. A growing number of these materials with vanadium-kagome nets host charge-density waves (CDWs) at low temperatures, including ScV6Sn6, CsV3Sb5, and V3Sb2. Curiously, only the Sc version of the RV6Sn6 materials with a HfFe6Ge6-type structure hosts a CDW (R = Gd-Lu, Y, Sc). In this study, we investigate the role of rare earth size in CDW formation in the RV6Sn6 compounds. Magnetization measurements on our single crystals of (Sc,Lu)V6Sn6 and (Sc,Y)V6Sn6 establish that the CDW is suppressed by substituting Sc by larger Lu or Y. Single-crystal X-ray diffraction reveals that compressible Sn-Sn bonds accommodate the larger rare earth atoms within loosely packed R-Sn-Sn chains without significantly expanding the lattice. We propose that Sc provides extra room in these chains crucial to CDW formation in ScV6Sn6. Our rattling chain model explains why both physical pressure and substitution by larger rare earth atoms hinder CDW formation despite opposite impacts on lattice size. We emphasize the cooperative effect of pressure and rare earth size by demonstrating that pressure further suppresses the CDW in a Lu-doped ScV6Sn6 crystal. Our model not only addresses why a CDW only forms in the RV6Sn6 materials with tiny Sc but also advances our understanding of why unusual CDWs form in the kagome metals.

Band Structure Studies of the R5Rh6Sn18 (R = Sc, Y, Lu) Quasiskutteridite Superconductors

We report on X-ray photoelectron spectroscopy and ab initio electronic structure investigations of the skutterudite-related R5Rh6Sn18 superconductors, where R = Sc, Y, and Lu. These compounds crystallise with a tetragonal structure (space group I41/acd) and are characterised by a deficiency of R atoms in their formula unit (R5−δRh6Sn18, δ≪1). Recently, we documented that the vacancies δ and atomic local defects (often induced by doping) are a reason for the enhancement in the superconducting transition temperature Tc of these materials, as well as metallic (δ=0) or semimetallic (δ≠0) behaviours in their normal state. Our band structure calculations show the pseudogap at a binding energy of −0.3 eV for the stoichiometric compounds, which can be easily moved towards the Fermi level by vacancies δ. As a result, dychotomic nature in electric transport of R5Rh6Sn18 (metallic or semimetallic resistivity) depends on δ, which has not been interpreted before. We have shown that the densities of states are very similar for various R5Rh6Sn18 compounds, and they practically do not depend on the metal R, while they are determined by the Rh d-and Sn s- and p-electron states. The band structure calculations for Sc5Rh6Sn18 have not been reported yet. We also found that the electronic specific heat coefficients γ0 for the stoichiometric samples were always larger with respect to the γ0 of the respective samples with vacancies at the R sites, which correlates with the results of ab initio calculations.

Composition dependent polymorphism and superconductivity in Y3+x{Rh,Ir}4Ge13-x

Polymorphism is observed in the Y_3+xRh₄Ge_13-x series. The decrease of Y-content leads to the transformation of the primitive cubic Y_3.6Rh₄Ge_12.4 [x = 0.6, space group Pm3̄n, a = 8.96095(9) Å], revealing a strongly disordered structure of the Yb₃Rh₄Sn₁₃ Remeika prototype, into a body-centred cubic structure [La₃Rh₄Sn₁₃ structure type, space group I4₁32, a = 17.90876(6) Å] for x = 0.4 and further into a tetragonal arrangement (Lu₃Ir₄Ge₁₃ structure type, space group I4₁/amd, a = 17.86453(4) Å, a = 17.91076(6) Å) for the stoichiometric (i.e. x = 0) Y₃Rh₄Ge₁₃. Analogous symmetry lowering is found within the Y_3+xIr₄Ge_13-x series, where the compound with Y-content x = 0.6 is crystallizing with La₃Rh₄Sn₁₃ structure type [a = 17.90833(8) Å] and the stoichiometric Y₃Ir₄Ge₁₃ is isostructural with the Rh-analogue [a = 17.89411(9) Å, a = 17.9353(1) Å]. The structural relationships of these derivatives of the Remeika prototype are discussed. Compounds from the Y_3+xRh₄Ge_13-x series are found to be weakly-coupled BCS-like superconductors with T_c = 1.25, 0.43 and 0.6, for x = 0.6, 0.4 and 0, respectively. They also reveal low thermal conductivity (<1.5 W K^-1 m^-1 in the temperature range 1.8-350 K) and small Seebeck coefficients. The latter are common for metallic systems. Y₃Rh₄Ge₁₃ undergoes a first-order phase transition at T_f = 177 K, with signatures compatible to a charge density wave scenario. The electronic structure calculations confirm the instability of the idealized Yb₃Rh₄Sn₁₃-like structural arrangements for Y₃Rh₄Ge₁₃ and Y₃Ir₄Ge₁₃.

Order-Disorder Transitions in (Ca_{x}Sr_{1-x})_{3}Rh_{4}Sn_{13}

The classification of structural phase transitions as displacive or order-disorder in character is usually based on spectroscopic data above the transition. We use single crystal x-ray diffraction to investigate structural correlations in the quasiskutterudites, (Ca_{x}Sr_{1-x})_{3}Rh_{4}Sn_{13}, which have a quantum phase transition at x∼0.9. Three-dimensional pair distribution functions show that the amplitudes of local atomic displacements are temperature independent below the transition and persist to well above the transition, a signature of order-disorder behavior. The implications for the associated electronic transitions are discussed.

Pressure-induced enhancement of the superconducting transition temperature in La2O2Bi3AgS6

Charge density wave (CDW) instability is often found in phase diagrams of superconductors such as cuprates and certain transition-metal dichalcogenides. This proximity to superconductivity triggers the question on whether CDW instability is responsible for the pairing of electrons in these superconductors. However, this issue remains unclear and new systems are desired to provide a better picture. Here, we report the temperature-pressure phase diagram of a recently discovered BiS₂superconductor La₂O₂Bi₃AgS₆, which shows a possible CDW transition atT* ∼ 155 K and a superconducting transition atT_c∼ 1.0 K at ambient pressure, via electrical resistivity measurements. Upon applying pressure,T* decreases linearly and extrapolates to 0 K at 3.9 GPa. Meanwhile,T_cis enhanced and reaches maximum value of 4.1 K at 3.1 GPa, forming a superconducting dome in the temperature-pressure phase diagram.

Unusual Pressure-Induced Quantum Phase Transition from Superconducting to Charge-Density Wave State in Rare-Earth-Based Heusler LuPd_{2}In Compound

We investigate the pressure effects on the electronic structures and phonon properties of rare-earth-based cubic-Heusler compound LuPd_{2}In, on the basis of ab initio density functional theory. We find the occurrence of intriguing phase transition from the superconducting (SC) to charge-density wave (CDW) state under pressure (P), which is quite unusual in that the pressure is detrimental to the CDW state in usual systems. The SC transition temperature T_{C} of LuPd_{2}In increases first with increasing pressure, up to P_{C}≈28  GPa, above which a quantum phase transition into the CDW state takes place. This extraordinary transition originates from the occurrence of phonon softening instability at a special q=M in the Brillouin zone. We thus propose that LuPd_{2}In is a quite unique material, in which the CDW quantum critical point is realized under the SC dome by applying the pressure.

Strong coupling superconductivity in a quasiperiodic host-guest structure

We examine the low-temperature states supported by the quasiperiodic host-guest structure of elemental bismuth at high pressure, Bi-III. Our electronic transport and magnetization experiments establish Bi-III as a rare example of type II superconductivity in an element, with a record upper critical field of ~ 2.5 T, unusually strong electron-phonon coupling, and an anomalously large, linear temperature dependence of the electrical resistivity in the normal state. These properties may be attributed to the peculiar phonon spectrum of incommensurate host-guest structures, which exhibit additional quasi-acoustic sliding modes, suggesting a pathway toward strong coupling superconductivity with the potential for enhanced transition temperatures and high critical fields.

Magnetic correlations in the intermetallic antiferromagnet Nd3Co4Sn13

Specific heat, magnetic susceptibility, and neutron scattering have been used to investigate the nature of the spin system in the antiferromagnet Nd₃Co₄Sn₁₃. At room temperature Nd₃Co₄Sn₁₃ has a cubic, Pm-3n structure similar to Yb₃Rh₄Sn₁₃. Antiferromagnetic interactions between, Nd³⁺ ions dominate the magnetic character of this sample and at 2.4 K the Nd spins enter a long range order state with a magnetic propagation vector q  =  (0 0 0) with an ordered moment of 1.78(2) µ_B at 1.5 K. The magnetic Bragg intensity grows very slowly below 1 K, reaching ~2.4 µ_B at 350 mK. The average magnetic Nd³⁺ configuration corresponds to the 3D irreducible representation Γ₇. This magnetic structure can be viewed as three sublattices of antiferromagnetic spin chains coupled with each other in the 120°-configuration. A well-defined magnetic excitation was measured around the 1 1 1 zone centre and the resulting dispersion curve is appropriate for an antiferromagnet with a gap of 0.20(1) meV.

Revealing correlation effect of Co 3d electrons in La3Co4Sn13 and Ce3Co4Sn13 by infrared spectroscopy study

We report resistivity, specific heat and optical spectroscopy measurements on single crystal samples of [Formula: see text] [Formula: see text] [Formula: see text] and [Formula: see text] [Formula: see text] [Formula: see text]. We observed clear temperature-induced spectral weight suppression below 4000 [Formula: see text] for both compounds in the conductivity spectra [Formula: see text], indicating the progressive formation of gap-like features with decreasing temperature. The suppressed spectral weight transfers mostly to the higher energy region. This observation reflects the presence of the correlation effect in the compounds. We attribute the correlation effect to the Co 3d electrons.

Quasilinear quantum magnetoresistance in pressure-induced nonsymmorphic superconductor chromium arsenide

In conventional metals, modification of electron trajectories under magnetic field gives rise to a magnetoresistance that varies quadratically at low field, followed by a saturation at high field for closed orbits on the Fermi surface. Deviations from the conventional behaviour, for example, the observation of a linear magnetoresistance, or a non-saturating magnetoresistance, have been attributed to exotic electron scattering mechanisms. Recently, linear magnetoresistance has been observed in many Dirac materials, in which the electron–electron correlation is relatively weak. The strongly correlated helimagnet CrAs undergoes a quantum phase transition to a nonmagnetic superconductor under pressure. Here we observe, near the magnetic instability, a large and non-saturating quasilinear magnetoresistance from the upper critical field to 14 T at low temperatures. We show that the quasilinear magnetoresistance may arise from an intricate interplay between a nontrivial band crossing protected by nonsymmorphic crystal symmetry and strong magnetic fluctuations.

The electronic structure of the helimagnet CrAs is unusual due to its nonsymmorphic crystal symmetry. Here, the authors observe quasilinear magnetoresistance close to a pressure-driven superconducting transition, which may arise from the interaction of the band structure and magnetic fluctuations.

Electronic and atomic structures of the Sr3Ir4Sn13 single crystal: A possible charge density wave material

X-ray scattering (XRS), x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopic techniques were used to study the electronic and atomic structures of the high-quality Sr₃Ir₄Sn₁₃ (SIS) single crystal below and above the transition temperature (T^* ≈ 147 K). The evolution of a series of modulated satellite peaks below the transition temperature in the XRS experiment indicated the formation of a possible charge density wave (CDW) in the (110) plane. The EXAFS phase derivative analysis supports the CDW-like formation by revealing different bond distances [Sn₁₍₂₎-Sn₂] below and above T^* in the (110) plane. XANES spectra at the Ir L₃-edge and Sn K-edge demonstrated an increase (decrease) in the unoccupied (occupied) density of Ir 5d-derived states and a nearly constant density of Sn 5p-derived states at temperatures T < T^* in the (110) plane. These observations clearly suggest that the Ir 5d-derived states are closely related to the anomalous resistivity transition. Accordingly, a close relationship exists between local electronic and atomic structures and the CDW-like phase in the SIS single crystal.

Candidate Elastic Quantum Critical Point in LaCu_{6-x}Au_{x}

The structural properties of LaCu_{6-x}Au_{x} are studied using neutron diffraction, x-ray diffraction, and heat capacity measurements. The continuous orthorhombic-monoclinic structural phase transition in LaCu_{6} is suppressed linearly with Au substitution until a complete suppression of the structural phase transition occurs at the critical composition x_{c}=0.3. Heat capacity measurements at low temperatures indicate residual structural instability at x_{c}. The instability is ferroelastic in nature, with density functional theory calculations showing negligible coupling to electronic states near the Fermi level. The data and calculations presented here are consistent with the zero temperature termination of a continuous structural phase transition suggesting that the LaCu_{6-x}Au_{x} series hosts an elastic quantum critical point.

Charge density waves in strongly correlated electron systems

Strong electron correlations are at the heart of many physical phenomena of current interest to the condensed matter community. Here we present a survey of the mechanisms underlying such correlations in charge density wave (CDW) systems, including the current theoretical understanding and experimental evidence for CDW transitions. The focus is on emergent phenomena that result as CDWs interact with other charge or spin states, such as magnetism and superconductivity. In addition to reviewing the CDW mechanisms in 1D, 2D, and 3D systems, we pay particular attention to the prevalence of this state in two particular classes of compounds, the high temperature superconductors (cuprates) and the layered transition metal dichalcogenides. The possibilities for quantum criticality resulting from the competition between magnetic fluctuations and electronic instabilities (CDW, unconventional superconductivity) are also discussed.

Strong Coupling Superconductivity in the Vicinity of the Structural Quantum Critical Point in (Ca(x)Sr(1-x))₃Rh₄Sn₁₃

The family of the superconducting quasiskutterudites (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) features a structural quantum critical point at x(c)=0.9, around which a dome-shaped variation of the superconducting transition temperature T(c) is found. Using specific heat, we probe the normal and the superconducting states of the entire series straddling the quantum critical point. Our analysis indicates a significant lowering of the effective Debye temperature on approaching x(c), which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of 2Δ/k(B)T(c) and ΔC/γT(c) beyond the Bardeen-Cooper-Schrieffer values is found in the vicinity of the structural quantum critical point. The phase diagram of (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) thus provides a model system to study the interplay between structural quantum criticality and strong electron-phonon coupling superconductivity.

Distinct vortex-glass phases in Yb₃Rh₄Sn₁₃ at high and low magnetic fields

The vortex lattice (VL) in the mixed state of the stannide superconductor Yb3Rh4Sn13 has been studied using small-angle neutron scattering (SANS). The field dependences of the normalized longitudinal and transverse correlation lengths of the VL, ξ(L)/a0 and ξ(T)/a0, reveal two distinct anomalies that are associated with vortex-glass phases below μ0Hl ≈ 700 G and above μ0Hh ∼ 1.7 T (a0 is the intervortex distance). At high fields, around 1.7 T, the longitudinal correlation decreases abruptly with increasing fields indicating a weakening (but not a complete destruction) of the VL due to a phase transition into a glassy phase, below μ0Hc2 (1.8 K) ≈2.5 T. ξ(L)/a0 and ξ(T)/a0, gradually decrease for decreasing fields of strengths less than 1 T and tend towards zero. The shear elastic modulus c66 and the tilting elastic modulus c44 vanish at a critical field μ0Hl ≈ 700 G, providing evidence for a disorder-induced transition into a vortex-glass. A 'ring' of scattered intensity is observed for fields lower than 700 G, i.e. μ0Hc1 = 135 G < μ0H < 700 G. This low-field phenomenon is of different nature than the one observed at high fields, where ξ(L)/a0 but not ξ(T)/a0, decreases abruptly to an intermediate value.

Ambient pressure structural quantum critical point in the phase diagram of (Ca(x)Sr(1-x))(3)Rh(4)Sn(13)

The quasiskutterudite superconductor Sr_{3}Rh_{4}Sn_{13} features a pronounced anomaly in electrical resistivity at T^{*}∼138  K. We show that the anomaly is caused by a second-order structural transition, which can be tuned to 0 K by applying physical pressure and chemical pressure via the substitution of Ca for Sr. A broad superconducting dome is centered around the structural quantum critical point. Detailed analysis of the tuning parameter dependence of T^{*} as well as insights from lattice dynamics calculations strongly support the existence of a structural quantum critical point at ambient pressure when the fraction of Ca is 0.9 (i.e., x_{c}=0.9). This establishes the (Ca_{x}Sr_{1-x})_{3}Rh_{4}Sn_{13} series as an important system for exploring the physics of structural quantum criticality without the need of applying high pressures.