We report a study of isoelectronic chemical substitution in the recently discovered quantum critical ferromagnet CeRh$_6$Ge$_4$. Upon silicon-doping, the ferromagnetic ordering temperature of CeRh$_6$(Ge$_{1-x}$Si$_x$)$_4$ is continuously suppressed, and no transition is observed beyond $x_c$$\approx$0.125. Non-Fermi liquid behavior with $C/T \propto$log($T^*/T$) is observed close to $x_c$, indicating the existence of strong quantum fluctuations, while the $T$-linear behavior observed upon pressurizing the parent compound is absent in the resistivity, which appears to be a consequence of the disorder induced by silicon doping. Our findings provide evidence for the role played by disorder on the unusual ferromagnetic quantum criticality in CeRh$_6$Ge$_4$, and provides further evidence for understanding the origin of this behavior.

A central research topic in condensed matter physics is the understanding of the evolution of various phases and phase transitions under different tuning parameters such as temperature,magnetic field and pressure.To explore the pressure-induced evolution of the magnetism and Fermi surface of the heavy fermion antiferromagnet YbPtBi,we performed tunnel diode oscillator based measurements under pressure at low temperatures in high magnetic fields.Our results reveal that the magnetic order strengthens and the Fermi surface shrinks as the pressure increases,which are consistent with typical observations for Yb-based heavy fermion compounds.In addition,an anomalous change in the quantum oscillation amplitudes is observed above 1.5 GPa,and determining the origin requires further study.

The CuIr2-xRuxTe4 superconductors (with a Tc around 2.8 K) can host charge-density waves, whose onset and interplay with superconductivity are not well known at a microscopic level. Here, we report a comprehensive study of the x = 0 and 0.05 cases, whose superconductivity was characterized via electrical-resistivity, magnetization, and heat-capacity measurements, while their microscopic superconducting properties were studied via muon-spin rotation and relaxation (mu SR). In CuIr2-xRuxTe4, both the temperature-dependent electronic specific heat and the superfluid density (determined via transverse-field mu SR) are best described by a two-gap (s + d)-wave model, comprising a nodeless gap and a gap with nodes. The multigap superconductivity is also supported by the temperature dependence of the upper critical field Hc2(T ). However, under applied pressure, a charge-density-wave order starts to develop and, as a consequence, the superconductivity of CuIr2Te4 achieves a more conventional s-wave character. Our series of experiments provides ample evidence that the CuIr2-xRuxTe4 family belongs to the rare cases where an unconventional superconducting pairing is found near a charge-density-wave quantum critical point.

We report an investigation of the superconducting properties of the hexagonal noncentrosymmetric compound LaPdIn. Electrical resistivity, specific heat, and ac susceptibility measurements demonstrate the presence of bulk superconductivity below ${T}_{c}=1.6$ K. The specific heat, together with the penetration depth measured using transverse-field muon spin rotation and the tunnel-diode-oscillator-based method, are well described by single-gap $s$-wave superconductivity, with a gap magnitude of $1.8{k}_{B}{T}_{c}$. From zero-field muon spin relaxation results no evidence is found for the spontaneous emergence of magnetic fields in the superconducting state, indicating that time-reversal symmetry is preserved. Band structure calculations reveal that there is a relatively weak effect of antisymmetric spin-orbit coupling on the electronic bands near the Fermi level, which is consistent with there being negligible singlet-triplet mixing due to broken inversion symmetry. On the other hand, isostructural LuPdIn and LaPtIn do not exhibit superconductivity down to 0.4 K, which may be due to these systems having a smaller density of states at the Fermi level.

We report a comprehensive study of the centrosymmetric Re3B and noncentrosymmetric Re7B3 superconductors. At a macroscopic level, their bulk superconductivity (SC), with T-c = 5.1K (Re3B) and 3.3 K (Re7B3), was characterized via electrical-resistivity, magnetization, and heat-capacity measurements, while their microscopic superconducting properties were investigated by means of muon-spin rotation and relaxation (mu SR). In both Re3B and Re7B3 the low-T zero-field electronic specific heat and the superfluid density (determined via transverse-field mu SR) suggest a nodeless SC. Both compounds exhibit some features of multigap SC, as evidenced by the temperature-dependent upper critical fields Hc(2)(T), as well as by electronic band-structure calculations. The absence of spontaneous magnetic fields below the onset of SC, as determined from zero-field mu SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of both Re3B and Re7B3. Our results suggest that a lack of inversion symmetry and the accompanying antisymmetric spin-orbit coupling effects are not essential for the occurrence of multigap SC in these rhenium-boron compounds.

A. Wang, Z. Y. Nie, F. Du, G. M. Pang, N. Kase, J. Akimitsu, Y. Chen, M. J. Gutmann, D. T. Adroja, 5 R. S. Perry, 6 C. Cao, 1 M. Smidman, 8, ∗ and H. Q. Yuan 8, 9, 10, † Center for Correlated Matter and Department of Physics, Zhejiang University, Hangzhou 310058, China Department of Applied Physics, Tokyo University of Science, 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585, Japan Research Institute for Interdisciplinary Science, Okayama University, 3-1-1 Tsushima-naka, Kitaku, Okayama 7008530, Japan ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxon OX11 0QX, United Kingdom Highly Correlated Matter Research Group, Physics Department, University of Johannesburg, P. O. Box 524, Auckland Park 2006, South Africa Centre for Materials Discovery and London Centre for Nanotechnology, University College London, London WC1E 6BT, United Kingdom Condensed Matter Group, Department of Physics, Hangzhou Normal University, Hangzhou 311121, China Zhejiang Province Key Laboratory of Quantum Technology and Device, Department of Physics, Zhejiang University, Hangzhou 310058, China State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310058, China Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China (Dated: April 25, 2021)

Structural rejuvenation in metallic glasses (MGs) induced by cryogenic thermal cycling has been intensively studied. However, the effect of thermal cycling on the atomic dynamics in MGs is still missing. In this work, we present a systematic study on the atomic dynamics in a La-based MG affected by sub-Tg annealing and thermal cycling. We find that the thermal cycling has little effect on the atomic dynamics in both as-cast and annealed MGs although it could significantly increase the free volume and change mechanical properties of the as-cast MG sample. In contrast, both the structure and mechanical properties of the well-relaxed sample are almost unchanged by thermal cycling. The increased open volume, mainly in the form of atomic vacancies, does not greatly promote the atomic dynamics in the as-cast MG sample, indicating that the free volume induced by thermal cycling is not the key factor in stimulating the β-relaxation and the boson peak in the studied MG.

We report a study of the structural and magnetic properties of single crystals of Ce$_2$IrGa$_{12}$. Ce$_2$IrGa$_{12}$ crystallizes in a layered tetragonal structure, and undergoes an antiferromagnetic transition below 3.1 K. We characterize the temperature-field phase diagrams of Ce$_2$IrGa$_{12}$ for fields both within the $ab$-plane and along the $c$-axis, where the presence of a field-induced magnetic phase is found for in-plane fields. The ordering temperature is moderately enhanced upon the application of pressures up to 2.3~GPa, suggesting that Ce$_2$IrGa$_{12}$ corresponds to the well localized region of the Doniach phase diagram.

Author(s): Xie, W; Wu, Y; Du, F; Wang, A; Su, H; Chen, Y; Nie, ZY; Mo, SK; Smidman, M; Cao, C; Liu, Y; Takabatake, T; Yuan, HQ | Abstract: © 2020 American Physical Society. A number of rare-earth monopnictides have topologically nontrivial band structures together with magnetism and strong electronic correlations. In order to examine whether the antiferromagnetic (AFM) semimetal YbAs (TN=0.5 K) exhibits such a scenario, we have grown high-quality single crystals using a flux method, and characterized the magnetic properties and electronic structure using specific heat, magnetotransport, and angle-resolved photoemission spectroscopy (ARPES) measurements, together with density functional theory (DFT) calculations. Both ARPES and DFT calculations find no evidence for band inversions in YbAs, indicating a topologically trivial electronic structure. From low-temperature magnetotransport measurements, we map the field-temperature phase diagram, where we find the presence of a field stabilized phase distinct from the AFM phase at low temperatures. An extremely large magnetoresistance (XMR) for both YbAs and the nonmagnetic counterpart LuAs is also observed, which can consistently be accounted for by the presence of electron-hole compensation. Moreover, an angle-dependent study of the Shubnikov-de Haas effect oscillations reveals very similar Fermi surfaces between YbAs and LuAs, with light effective masses down to at least 0.5 K, indicating that the Yb-4f electrons are well localized, and do not contribute to the Fermi surface. However, the influence of the localized Yb-4f electrons on the magnetotransport of YbAs can be discerned from the distinct temperature dependence of the XMR compared to that of LuAs, which we attribute to the influence of short-ranged spin correlations that appear well above TN.

We report superconductivity below ${T}_{\mathrm{c}}=0.64$ K in the charge density wave (CDW) ordered material ${\mathrm{LaAuSb}}_{2}$, from measurements of the electrical resistivity, specific heat, and ac magnetic susceptibility. To investigate the interplay between superconductivity and CDW order in ${\mathrm{LaAuSb}}_{2}$, we measured the resistivity under pressures up to 2.0 GPa and constructed the temperature-pressure phase diagram. With the application of pressure, ${T}_{\mathrm{c}}$ increases gradually before exhibiting a sudden jump at around 0.64 GPa, while the CDW order is suppressed to lower temperatures before abruptly vanishing at the same pressure. We suggest that the jump of ${T}_{\mathrm{c}}$ may be due to the enhancement of the density of states with the closure of the CDW energy gap when CDW order is suppressed. On the other hand, the normalized upper critical field ${H}_{\mathrm{c}2}$ changes little with pressure, suggesting that orbital limiting is the dominant pair-breaking mechanism in ${\mathrm{LaAuSb}}_{2}$.

We present a detailed investigation of the physical properties of Ce$_2 $Sb and Ce$_2 $Bi single crystals, which undergo antiferromagnetic transitions at around 8.2 and 10 K respectively. When magnetic fields are applied parallel to the $c$ axis, metamagnetic transitions are observed at low temperatures, corresponding to a magnetic field-induced phase transition. It is found that the field-induced transition changes from second-order at higher temperatures, to first-order at low temperatures, suggesting the existence of tricritical points (TCPs) in both compounds. Since replacing Bi with Sb suppresses the TCP to lower temperatures and corresponds to a positive chemical pressure, these results suggest that applying pressure to Ce$_2$Sb may suppress the TCP to lower temperatures, potentially to zero temperature at a quantum tricritical point.

The noncentrosymmetric superconductor Re6Zr has attracted much interest due to the observation of broken time-reversal symmetry in the superconducting state. Here we report an investigation of the superconducting gap structure of Re6Zr single crystals by measuring the magnetic penetration depth shift Delta lambda(T) and electronic specific heat C-e(T). Delta lambda(T) exhibits an exponential temperature dependence behavior for T << T-c, which indicates a fully open superconducting gap. Our analysis shows that a single gap s-wave model is sufficient to describe both the superfluid density rho(s)(T) and C-e(T) results, with a fitted gap magnitude larger than the weak coupling BCS value, providing evidence for fully gapped superconductivity in Re6Zr with moderate coupling.

Superconductivity was recently observed in CrAs as the helimagnetic order is suppressed by applying pressure, suggesting possible unconventional superconductivity. To reveal the nature of the superconducting order parameter of CrAs, here we report the angular dependence of the upper critical field under pressure. Upon rotating the field by ${360}^{\ensuremath{\circ}}$ in the $bc$ plane, six maxima are observed in the upper critical field, where the oscillations have both sixfold- and twofold-symmetric components. Our analysis suggests the presence of an unconventional odd-parity spin-triplet state.

A combined resistivity and hard x-ray diffraction study of superconductivity and charge ordering in Ir Ir(1-x)PtxTe(2), as a function of Pt substitution and externally applied hydrostatic pressure, is presented. Experiments are focused on samples near the critical composition x(c)-0.045 where competition and switching between charge order and superconductivity is established. We show that charge order as a function of pressure in Ir0.95Pt0.05Te2 is preempted-and hence triggered - by a structural transition. Charge ordering appears uniaxially along the short crystallographic (1, 0, 1) domain axis with a (1/5, 0, 1/5) modulation. Based on these results we draw a charge-order phase diagram and discuss the relation between stripe ordering and superconductivity.

We report measurements of the London penetration depth [Delta lambda(T)] of the recently discovered iron-based superconductor (Li1- xFex) OHFeSe, in order to characterize the nature of the superconducting gap structure. At low temperatures, Delta lambda(T) displays nearly temperature-independent behavior, indicating a fully open superconducting gap. We also analyze the superfluid density rho(s)(T), which cannot be well accounted for by a single- gap isotropic s-wave model but is consistent with either a two-gap model, a model for the orbital selective s x tau(3) state, or anisotropic s-wave superconductivity.

The recently discovered BaPt2As2 shows a structural distortion at around 275 K, followed by the emergence of superconductivity at lower temperatures. Here we identify the presence of charge-density-wave order at room temperature and ambient pressure using single-crystal x-ray diffraction, with both a superlattice and an incommensurate modulation, where there is a change of the superlattice structure below similar or equal to 275 K. Upon applying pressure, BaPt2As2 shows a rich temperature-pressure phase diagram with multiple pressure-induced transitions at high temperatures, the emergence or disappearance of which are correlated with sudden changes in the superconducting transition temperature T-c. These findings demonstrate that BaPt2As2 is a promising system for studying competing interactions and the relationship between high-temperature electronic instabilities and superconductivity.

We report an investigation of the superconducting order parameter of the noncentrosymmetric compound PbTaSe$_2$, which is believed to have a topologically nontrivial band structure. Precise measurements of the London penetration depth $\Delta\lambda(T)$ obtained using a tunnel diode oscillator (TDO) based method show an exponential temperature dependence at $T\ll T_c$, suggesting a nodeless superconducting gap structure. A single band s-wave model well describes the corresponding normalized superfluid density, with a gap magnitude of $\Delta(0)=1.85T_c$. This is very close to the value of $1.76T_c$ for weak-coupling BCS superconductors, indicating conventional fully-gapped superconductivity in PbTaSe$_2$.

The superconducting pairing state of LaPt$_{4}$Ge$_{12}$ is studied by measuring the magnetic penetration depth $\lambda(T,B)$ and the superfluid density $\rho_s(T)$ using a tunnel-diode-oscillator (TDO)-based method and by transverse field muon-spin rotation ($\mu$SR) spectroscopy. $\lambda(T)$ follows an exponential-type temperature dependence at $T\ll T_{c}$, but its zero-temperature value $\lambda(0)$ increases linearly with magnetic field. Detailed analyses demonstrate that both $\lambda(T)$ and the corresponding $\rho_{s}(T)$, measured in the Meissner state by the TDO method are well described by a two-gap $\gamma$ model with gap sizes of $\Delta_1(0)=1.31k_{B}T_c$ and $\Delta_2(0)=1.80k_{B}T_c$ and a very weak interband coupling. In contrast, $\rho_s(T)$, derived from the $\mu \rm{SR}$ data taken in a small field, can be fitted by a single-gap BCS model with a gap close to $\Delta_2(0)$. We conclude that LaPt$_{4}$Ge$_{12}$ is a marginal two-gap superconductor and the small gap $\Delta_1$ seems to be destroyed by a small magnetic field. In comparison, in PrPt$_4$Ge$_{12}$ the $4f$-electrons may enhance the interband coupling and, therefore, give rise to more robust multiband superconductivity.

We present a pressure study of the electrical resistivity, AC magnetic susceptibility and powder x-ray diffraction (XRD) of the newly discovered BiS$_2$-based superconductor EuBiS$_2$F. At ambient pressure, EuBiS$_2$F shows an anomaly in the resistivity at around $T_0\approx 280$ K and a superconducting transition at $T_c\approx 0.3$ K. Upon applying hydrostatic pressure, there is little change in $T_0$ but the amplitude of the resistive anomaly is suppressed, whereas there is a dramatic enhancement of $T_c$ from 0.3 K to about 8.6 K at a critical pressure of $p_c$ $\approx{1.4}$ GPa. XRD measurements confirm that this enhancement of $T_c$ coincides with a structural phase transition from a tetragonal phase ($P4/nmm$) to a monoclinic phase ($P2_1$/m), which is similar to that observed in isostructural LaO$_{0.5}$F$_{0.5}$BiS$_2$. Our results suggest the presence of two different superconducting phases with distinct crystal structures in EuBiS$_2$F, which may be a general property of this family of BiS$_2$-based superconductors.

We report the synthesis of BaPt2As2 single crystals and the discovery of superconductivity and a structural phase transition in this compound by measuring the electrical resistivity, magnetic susceptibility and specific heat as well as the x-ray diffraction at low temperatures. BaPt2As2 crystallizes in the CaBe2Ge2-type tetragonal structure (P4/nmm) at room temperature and undergoes a first-order structural transition at TS ≃ 275 K, which is likely to be associated with a charge-density-wave (CDW) instability. BCS-like superconductivity with two subsequent transitions Tc1 ≃ 1.67 K and Tc2 ≃ 1.33 K is observed. Our results demonstrate that BaPt2As2 may serve as a new system for studying the interplay of superconductivity and the CDW order.