arXiv subjects

A. D. Hillier

Publications and source records attributed to A. D. Hillier.

At least 55 records · Page 3Linked to original sources

arXiv2021

Superconductivity in noncentrosymmetric NbReSi investigated by muon spin rotation and relaxation

Noncentrosymmetric materials are promising paradigm to explore unconventional superconductivity. In particular, several Re containing noncentrosymmetric materials have attracted considerable attention due to a superconducting state with a broken time reversal symmetry. A comprehensive study on the superconducting ground state of NbReSi was investigated using magnetization, resistivity, and muon spin rotation/relaxation measurements. Zero field muon spectroscopy results showed the absence of any spontaneous magnetic field below the superconducting transition temperature, T$ _{c} $ = 6.29 K, indicating the preserved time-reversal symmetry. Transverse field muon spin rotation measurements confirms a s-wave nature of the sample with $Δ(0)/k_{B}T_{c} $ = 1.726. This study urges further investigation on more noncentrosymmetric materials to elucidate the selective appearance of unconventional nature and unveil its dependence on antisymmetric spin-orbit coupling strength.

cond-mat.supr-con↗

arXiv2021

Magnetic order and crystalline electric field excitations of the quantum critical heavy fermion ferromagnet CeRh$_6$Ge$_4$

CeRh$_6$Ge$_4$ is an unusual example of a stoichiometric heavy fermion ferromagnet, which can be cleanly tuned by hydrostatic pressure to a quantum critical point. In order to understand the origin of this anomalous behavior, we have characterized the magnetic ordering and crystalline electric field (CEF) scheme of this system. While magnetic Bragg peaks are not resolved in neutron powder diffraction, coherent oscillations are observed in zero-field $μ$SR below $T_{\rm C}$, which are consistent with in-plane ferromagnetic ordering consisting of reduced Ce moments. From analyzing the magnetic susceptibility and inelastic neutron scattering, we propose a CEF-level scheme which accounts for the easy-plane magnetocrystalline anisotropy, where the low lying first excited CEF exhibits significantly stronger hybridization than the ground state. These results suggest that the orbital anisotropy of the ground state and low lying excited state doublets are important for realizing anisotropic electronic coupling between the $f$- and conduction electrons, which gives rise to the highly anisotropic hybridization observed in photoemission experiments.

cond-mat.str-el↗

arXiv2021

Probing the superconducting ground state of noncentrosymmetric high entropy alloys using muon-spin rotation and relaxation

Recently, high entropy alloys (HEAs) have emerged as a new platform for discovering superconducting materials and offer avenues to explore exotic superconductivity. The highly disordered nature of HEA suggests regular phonon required for BCS superconductivity may be unlikely to occur. Therefore understanding the microscopic properties of these superconducting HEA is important. We report the first detailed characterization of the superconducting properties of the noncentrosymmetric ($α$-Mn structure) HEA {(HfNb)}$_{0.10}${(MoReRu)}$_{0.90}$, and {(ZrNb)}$_{0.10}${(MoReRu)}$_{0.90}$ by using magnetization, specific heat, AC transport, and muon-spin relaxation/rotation ($μ$SR). Despite the disordered nature, low temperature specific heat and transverse-field muon spin rotation measurements suggest nodeless isotropic superconducting gap and Zero-field $μ$SR measurements confirm that time reversal symmetry is preserved in the superconducting ground state.

cond-mat.supr-con↗

arXiv2021

Microscopic investigation of superconducting properties of a strongly coupledsuperconductor IrGe via μSR

Exploring superconductors which can possess pairing mechanism other than the BCS predicted s-wave have continually attracted considerable interest. Superconductors with low-lying phonons may exhibit unconventional superconductivity as the coupling of electrons with these low-lying phonons can potentially affect the nature of the superconducting ground state, resulting in strongly coupled superconductivity. In this work, by using magnetization, AC transport, specific heat, and muon spin rotation/relaxation ($μ$SR) measurements, we report a detailed investigation on the superconducting ground state of the strongly coupled superconductor, IrGe, that has a transition temperature, T$_{C}$, at 4.7 K. Specific heat (SH), and transverse field $μ$SR is best described with an isotropic s-wave model with strong electron-phonon coupling, indicated by the values of both $Δ(0)/k_{B}T_{C}$ = 2.3, 2.1 (SH, $μ$SR), and $ΔC_{el}/γ_{n}T_{C}$ = 2.7. Zero-field $μ$SR measurements confirm the presence of time-reversal symmetry in the superconducting state of IrGe.

cond-mat.supr-con↗

arXiv2021

A crossover from Kondo semiconductor to metallic antiferromagnet with $5d$-electron doping in CeFe$_2$Al$_{10}$

We report a systematic study of the $5d$-electron-doped system Ce(Fe$_{1-x}$Ir$_x$)$_2$Al$_{10}$ ($0 \leq x \leq 0.15$). With increasing $x$, the orthorhombic $b$~axis decreases slightly while accompanying changes in $a$ and $c$ leave the unit cell volume almost unchanged. Inelastic neutron scattering, along with thermal and transport measurements, reveal that for the Kondo semiconductor CeFe$_2$Al$_{10}$, the low-temperature energy gap which is proposed to be a consequence of strong $c \mhyphen f$ hybridization, is suppressed by a small amount of Ir substitution for Fe, and that the system adopts a metallic ground state with an increase in the density of states at the Fermi level. The charge or transport gap collapses (at $x=$~0.04) faster than the spin gap with Ir substitution. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements demonstrate that the system undergoes long-range antiferromagnetic order below a Néel temperature, $T_{\mathrm{N}}$, of 3.1(2)~K for $x = 0.15$. The ordered moment is estimated to be smaller than 0.07(1)~$μ_\mathrm{B}$/Ce although the trivalent state of Ce is confirmed by Ce L$_3$-edge x-ray absorption near edge spectroscopy. It is suggested that the $c \mhyphen f$ hybridization gap, which plays an important role in the unusually high ordering temperatures observed in Ce$T_2$Al$_{10}$ ($T$ = Ru and Os), may not be necessary for the onset of magnetic order with a low $T_{\mathrm{N}}$ seen here in Ce(Fe$_{1-x}$Ir$_x$)$_2$Al$_{10}$.

cond-mat.str-el↗

arXiv2021

Superconducting ground state of nonsymmorphic superconducting compound Zr$_{2}$Ir

The nonsymmorphic Zr$_{2}$Ir alloy is a possible topological semimetal candidate material and as such may be part of an exotic class of superconductors. Zr$_{2}$Ir is a superconductor with a transition temperature of 7.4 K with critical fields of 19.6(3) mT and 3.79(3) T, as determined by heat capacity and magnetisation. Zero field muon spin relaxation measurements show that time-reversal symmetry is preserved in these materials. The specific heat and transverse field muon spin rotation measurements rule out any possibility to have a nodal or anisotropic superconducting gap, revealing a conventional s-wave nature in the superconducting ground state. Therefore, this system is found to be conventional nonsymmorphic superconductor, with time-reversal symmetry being preserved and an isotropic superconducting gap.

cond-mat.supr-con↗

arXiv2021

Antiferromagnetic correlations in strongly valence fluctuating CeIrSn

CeIrSn with a quasikagome Ce lattice in the hexagonal basal plane is a strongly valence fluctuating compound, as we confirm by hard x-ray photoelectron spectroscopy and inelastic neutron scattering, with a high Kondo temperature of $T_{\mathrm{K}}\sim 480$\,K. We report a negative in-plane thermal expansion $α/T$ below 2\,K, which passes through a broad minimum near 0.75\,K. Volume and $a$-axis magnetostriction for $B \parallel a$ are markedly negative at low fields and change sign before a sharp metamagnetic anomaly at 6\,T. These behaviors are unexpected for Ce-based intermediate valence systems, which should feature positive expansivity. Rather they point towards antiferromagnetic correlations at very low temperatures. This is supported by muon spin relaxation measurements down to 0.1\,K, which provide microscopic evidence for a broad distribution of internal magnetic fields. Comparison with isostructural CeRhSn suggests that these antiferromagnetic correlations emerging at $T\ll T_{\mathrm{K}}$ result from geometrical frustration.

cond-mat.str-el↗

arXiv2021

Fully gapped superconductivity in centrosymmetric and non-centrosymmetric Re-B compounds probed with $μ$SR

We present a comprehensive study on superconducting properties of Re$_7$B$_3$ and Re$_3$B through specific heat, magnetic susceptibility, resistivity, and transverse and zero-field muon spin rotation/relaxation ($μ$SR) experiments on polycrystalline samples. Re$_7$B$_3$ (T$_C$ = 3.2~K) is a non-centrosymmetric type-II ($κ$ $\approx$ 9.27) superconductor in the weak coupling ($λ_{e-ph}$ = 0.54) regime. On the other hand, Re$_3$B (T$_C$ = 5.19~K) is a centrosymmetric type-II ($κ$ $\approx$ 34.55) superconductor in the moderate coupling ($λ_{e-ph}$ = 0.64) regime. Our transverse-field $μ$SR measurements show evidence for isotropically gapped BCS type superconductivity with normalized gap ($Δ_0/k_BT_C$) values of 1.69 (Re$_7$B$_3$) and 1.75 (Re$_3$B).

cond-mat.supr-con↗

arXiv2021

Chiral singlet superconductivity in the weakly correlated metal LaPt3P

Topological superconductors (SCs) are novel phases of matter with nontrivial bulk topology. They host at their boundaries and vortex cores zero-energy Majorana bound states, potentially useful in fault-tolerant quantum computation. Chiral SCs are particular examples of topological SCs with finite angular momentum Cooper pairs circulating around a unique chiral axis, thus spontaneously breaking time-reversal symmetry (TRS). They are rather scarce and usually feature triplet pairing: best studied examples in bulk materials are UPt3 and Sr2RuO4 proposed to be f-wave and p-wave SCs respectively, although many open questions still remain. Chiral triplet SCs are, however, topologically fragile with the gapless Majorana modes weakly protected against symmetry preserving perturbations in contrast to chiral singlet SCs. Using muon spin relaxation (muSR) measurements, here we report that the weakly correlated pnictide compound LaPt3P has the two key features of a chiral SC: spontaneous magnetic fields inside the superconducting state indicating broken TRS and low temperature linear behaviour in the superfluid density indicating line nodes in the order parameter. Using symmetry analysis, first principles band structure calculation and mean-field theory, we unambiguously establish that the superconducting ground state of LaPt3P is chiral d-wave singlet.

cond-mat.supr-con↗

arXiv2020

Evidence for the coexistence of time-reversal symmetry breaking and Bardeen-Cooper-Schrieffer-like superconductivity in La$_{7}$Pd$_{3}$

Time-reversal symmetry breaking (TRSB) with a Bardeen-Cooper-Schrieffer (BCS) -like superconductivity occurs in a small, but growing number of noncentrosymmetric (NCS) materials, although the mechanism is poorly understood. We present heat capacity, magnetization, resistivity, and muon spin resonance/relaxation ($μ$SR) measurements on polycrystalline samples of NCS La$_{7}$Pd$_{3}$. Transverse-field $μ$SR and heat capacity data show La$_{7}$Pd$_{3}$ is a type-II superconductor with a BCS-like gap structure, while zero-field $μ$SR results provide evidence of TRSB. We discuss the implications of these results for both the La$_{7}X_{3}$ (where $X=$ Ni, Pd, Rh, Ir) group of superconductors and other CS and NCS superconductors for which TRSB has been observed.

cond-mat.supr-con↗

arXiv2020

Time-reversal symmetry breaking and multigap superconductivity in the noncentrosymmetric superconductor La$_{7}$Ni$_{3}$

The Th$_{7}$Fe$_{3}$ family of superconductors provides a rich playground for unconventional superconductivity. La$_7$Ni$_3$ is the latest member of this family, which we here investigate by means of thermodynamic and muon spin rotation and relaxation measurements. Our specific heat data provides evidence for two distinct and approximately isotropic superconducting gaps. The larger gap has a value slightly higher than that of weak-coupling BCS theory, indicating the presence of significant correlations. These observations are confirmed by transverse-field muon-rotation measurements. Furthermore, zero-field measurements reveal small internal fields in the superconducting state, which occur close to the onset of superconductivity and indicate that the superconducting order parameter breaks time-reversal symmetry. We discuss two possible microscopic scenarios -- an unconventional $E_{2}(1,i)$ state and an $s+i\,s$ superconductor, which is reached by two consecutive transitions -- and illustrate which interactions will favor these phases. Our results establish La$_{7}$Ni$_{3}$ as the first member of the Th$_{7}$Fe$_{3}$ family displaying both time-reversal-symmetry-breaking and multigap superconductivity.

cond-mat.supr-con↗

arXiv2020

Machine Learning approach to muon spectroscopy analysis

In recent years, Artificial Intelligence techniques have proved to be very successful when applied to problems in physical sciences. Here we apply an unsupervised Machine Learning (ML) algorithm called Principal Component Analysis (PCA) as a tool to analyse the data from muon spectroscopy experiments. Specifically, we apply the ML technique to detect phase transitions in various materials. The measured quantity in muon spectroscopy is an asymmetry function, which may hold information about the distribution of the intrinsic magnetic field in combination with the dynamics of the sample. Sharp changes of shape of asymmetry functions - measured at different temperatures - might indicate a phase transition. Existing methods of processing the muon spectroscopy data are based on regression analysis, but choosing the right fitting function requires knowledge about the underlying physics of the probed material. Conversely, Principal Component Analysis focuses on small differences in the asymmetry curves and works without any prior assumptions about the studied samples. We discovered that the PCA method works well in detecting phase transitions in muon spectroscopy experiments and can serve as an alternative to current analysis, especially if the physics of the studied material are not entirely known. Additionally, we found out that our ML technique seems to work best with large numbers of measurements, regardless of whether the algorithm takes data only for a single material or whether the analysis is performed simultaneously for many materials with different physical properties.

cond-mat.mtrl-sci↗

arXiv2020

Study of the superconducting ground state of topological superconducting candidates Ti$_{3}$X (X = Ir, Sb)

The topologically non-trivial band structure of A15 compounds has drawn attention owing to the possible realization of topological superconductivity. Here, we report a microscopic investigation of the superconducting ground state in A15 compound Ti$_{3}$X (X = Ir, Sb) by muon spectroscopy measurements. Zero field muon measurements have shown that time-reversal symmetry is preserved in these materials. Furthermore, specific heat and a transverse field muon spectroscopy measurement rule out any possibility to have a nodal or anisotropic superconducting gap, revealing a conventional s-wave nature in the superconducting ground state. This work classifies A15 compound Ti$_{3}$X (X = Ir, Sb) as a time-reversal preserved topological superconductor.

cond-mat.supr-con↗

arXiv2020

Time-reversal-symmetry breaking and unconventional pairing in the noncentrosymmetric superconductor La$_{7}$Rh$_{3}$

Noncentrosymmetric superconductors have sparked significant research interests due to their exciting properties, such as the admixture of spin-singlet and spin-triplet Cooper pairs. Here we report $μ$SR and thermodynamic measurements on the noncentrosymmetric superconductor La7Rh3 which indicate a fully established superconducting gap and spontaneous time-reversal-symmetry breaking at the onset of superconductivity. We show that our results pose severe constraints on any microscopic theory of superconductivity in the system. A symmetry analysis identifies ground states compatible with time-reversal-symmetry breaking and the resulting gap functions are discussed. Furthermore, general energetic considerations indicate the relevance of electron-electron interactions for the pairing mechanism, in accordance with hints of spin-fluctuations revealed in susceptibility measurements.

cond-mat.supr-con↗

arXiv2020

Coexistence of type-I and type-II superconductivity signatures in ZrB12 probed by muon spin rotation measurements

Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example at different temperatures is rare in nature. We the employed muon spin rotation (muSR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12 and obtained clear evidence of both type-I and type-II characteristics. Most importantly, we found a region showing unusual behavior where the usually mutually exclusive muSR signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modeling that required taking into account multiple bands and multiple coherence lengths, which suggests that material has one coherence length larger and another smaller than the magnetic field penetration length (the type-1.5 regime). At stronger fields, a footprint of the type-II mixed state showing square flux-line lattice was also obtained using neutron diffraction.

cond-mat.supr-con↗

arXiv2020

Investigation of the magnetic ground state of the ordered double perovskite Sr2YbRuO6: a tale of two transitions

Comprehensive muon spin rotation/relaxation (muSR) and neutron powder diffraction (NPD) studies supported via bulk measurements have been performed on the ordered double perovskite Sr2YbRuO6 to investigate the nature of the magnetic ground state. Two sharp transitions at TN1 ~ 42 K and TN2 ~ 36 K have been observed in the static and dynamic magnetization measurements, coinciding with the heat capacity data. In order to confirm the origin of the observed phase transitions and the magnetic ground state, microscopic evidences are presented here. An initial indication of long-range magnetic ordering comes from a sharp drop in the muon initial asymmetry and a peak in the relaxation rate near TN1. NPD confirms that the magnetic ground state of Sr2YbRuO6 consists of an antiferromagnetic (AFM) structure with interpenetrating lattices of parallel Yb3+ and Ru5+ moments lying in the ab-plane and adopting a A-type AFM structure. Intriguingly, a small but remarkable change is observed in the long-range ordering parameters at TN2 confirming the presence of a weak spin reorientation (i.e. change in spin configuration) transition of Ru and Yb moments, as well as a change in the magnetic moment evolution of the Yb3+ spins at TN2. The temperature dependent behaviour of the Yb3+ and Ru5+ moments suggests that the 4d-electrons of Ru5+ play a dominating role in stabilizing the long range ordered magnetic ground state in the double perovskite Sr2YbRuO6 whereas only the Yb3+ moments show an arrest at TN2. The observed magnetic structure and the presence of a ferromagnetic interaction between Ru- and Yb- ions are explained with use of the Goodenough-Kanamori-Anderson (GKA) rules. Possible reasons for the presence of the second magnetic phase transition and of a compensation point in the magnetization data are linked to competing mechanisms of magnetic anisotropy.

cond-mat.str-el↗

arXiv2020

Multigap superconductivity in the new BiCh$_{2}$-based layered superconductor La$_\mathrm{0.7}$Ce$_\mathrm{0.3}$OBiSSe

The layered bismuth oxy-sulfide materials, which are structurally related to the Fe-pnictides/chalcogenides and cuprates superconductors, have brought substantial attention for understanding the physics of reduced dimensional superconductors. We have examined the pairing symmetry of recently discovered BiCh$_2$-based superconductor, La$_\mathrm{1-x}$Ce$_\mathrm{x}$OBiSSe with $x$ = 0.3, through transverse field (TF) muon spin rotation measurement, in addition we present the results of magnetization, resistivity and zero field (ZF) muon spin relaxation measurements. Bulk superconductivity has been observed below 2.7 K for $x$ = 0.3, verified by resistivity and magnetization data. The temperature dependence of the magnetic penetration depth has been determined from TF-$μ$SR data can be described by an isotropic two-gap $s+s$ wave model compared to a single gap $s$- or anisotropic $s$-wave models, the resemblance with Fe-pnictides/chalcogenides and MgB$_2$. Furthermore, from the TF-$μ$SR data, we have determined the London's penetration depth $λ_\mathrm{L}(0)$ = 452(3) nm, superconducting carrier's density $n_\mathrm{s}$ = 2.18(1) $\times$10$^{26}$ carriers/m$^{3}$ and effective mass enhancement $m^{*}$ = 1.66(1) $m_\mathrm{e}$, respectively. No signature of spontaneous internal field is found down to 100 mK in ZF-$μ$SR measurement suggest that time-reversal symmetry is preserved in this system.

cond-mat.supr-con↗

arXiv2020

Probing nodeless superconductivity in LaMSi (M = Ni, Pt) using muon-spin rotation and relaxation

Systems with strong spin-orbit coupling have been a topic of fundamental interest in condensed matter physics due to the exotic topological phases and the unconventional phenomenon they exhibit. In this particular study, we have investigated the superconductivity in the transition-metal ternary noncentrosymmetric compounds LaMSi (M = Ni, Pt) with different spin-orbit coupling strength, using muon-spin rotation and relaxation measurements. Transverse-field measurements made in the vortex state indicate that the superconductivity in both materials is fully gapped, with a conventional s-wave pairing symmetry and BCS-like magnitudes for the zero-temperature gap energies. Zero field measurements suggest a time-reversal symmetry preserved superconductivity in both the systems, though a small increase in muon depolarization is observed upon decreasing temperature. However, this has been attributed to quasi-static magnetic fluctuations.

cond-mat.supr-con↗