arXiv subjects

Rhea Stewart

Publications and source records attributed to Rhea Stewart.

10 recordsLinked to original sources

arXiv2026

Superconducting ground state study of Cr-based equiatomic high-entropy alloy through $\mu\text{SR}$

High-entropy alloy superconductors, characterized by extreme chemical disorder and complex electronic environments, have attracted significant attention as model systems for exploring superconductivity in disordered materials. Here, we investigate a Cr-based equiatomic HEA, Cr-V-Ti-Nb-Ta, which contains a magnetic 3d element, providing an opportunity to examine the influence of magnetic elements on superconductivity in highly disordered systems. Despite expected magnetic pair-breaking, this alloy exhibits bulk type-II superconductivity with a transition temperature of $T_c = 2.33(3)$ K and a high upper critical field. Transverse-field $\mu$SR measurements reveal an s-wave superconducting gap close to the BCS value, while zero-field $\mu$SR suggests preserved time-reversal symmetry. These results establish Cr-V-Ti-Nb-Ta as a promising platform for exploring the interplay between disorder, magnetism and superconductivity in high entropy alloys.

cond-mat.supr-con↗

arXiv2026

Future perspective of muons; a quantum particle measuring quantum processes

Although considered a niche technique, muon spectroscopy provides a unique and complementary insight into a range of different materials from hard condensed matter to biological samples and everything in between. In matter, the muon has a mass of $\frac{1}{9}~m_p$ or $207~m_e$, and is a local probe of quantum states that can provide a focus on the bulk properties of materials. While often interpreted in a classical framework, the muon is itself a quantum particle and it is increasingly common for researchers to take account of this when thinking about muon spectroscopy experiments. In this perspective, we focus on the power of using this quantum treatment of muon spectroscopy, which is a key future direction for the technique.

cond-mat.other↗

arXiv2026

Nonunitary triplet superconductivity in the Z2 topological metal SrPd2As2

In Z2 topological metals, nontrivial band topology and strong spin-orbit coupling (SOC) impose symmetry constraints that can stabilize unconventional superconducting states, even when thermodynamic probes indicate an isotropic gap. Here, we investigate the superconducting ground state of such a material, SrPd2As2, using muon spin rotation and relaxation (muSR), first-principles calculations, and Ginzburg-Landau analysis. Transverse-field muSR indicates a fully gapped superconducting state below Tc = 0.94 K, while zero-field muSR detects spontaneous internal magnetic fields below Tc, establishing time-reversal symmetry (TRS) breaking. Electronic structure calculations identify SrPd2As2 as a Z2 topological metal with surface states crossing the Fermi level. Standard anisotropic Migdal-Eliashberg calculations predict a nodal gap and overestimate Tc, indicating that a purely phonon-mediated pairing mechanism is insufficient. We resolve this apparent contradiction by showing that the interplay of SOC, tetragonal symmetry, and an open Fermi surface topology stabilizes a nonunitary triplet superconducting state whose symmetry-imposed nodes lie in momentum-space regions devoid of electronic states. This yields a fully gapped thermodynamic response while naturally breaking TRS. Our results establish SrPd2As2 as a clean platform for bulk nonunitary triplet pairing and a promising candidate for intrinsic topological superconductivity.

cond-mat.supr-con↗

arXiv2026

Discovery of Quasi One Dimensional Superconductivity in PtPb3Bi

Quasi one dimensional materials provide a compelling platform where reduced dimensionality stabilizes intertwined topological and superconducting phases. Here we report superconductivity in a new Bi based quasi 1D compound, PtPb3Bi, which hosts a nontrivial electronic structure. It exhibits type II superconductivity below 3.01(1) K. Heat capacity and transverse field muon spin rotation relaxation (muSR) measurements demonstrate a fully gapped isotropic s wave state with moderate electron phonon coupling, while zero field muSR confirms the preservation of time reversal symmetry (TRS). Transport measurements reveal low carrier mobility with diffusive normal state transport. Electronic structure calculations show strong dispersion along the quasi 1D direction and relatively flatter bands in the transverse plane, giving rise to pronounced Fermi surface nesting in the kx-ky plane. Consistent with this, the compound undergoes a charge density wave transition at 280(1) K. The flow of Wannier charge centers, together with surface state dispersion, establishes nontrivial band topology. These results identify PtPb3Bi as a new quasi 1D superconductor with nontrivial electronic structure and a promising candidate for topological superconductivity.

cond-mat.supr-con↗

arXiv2026

Hourglass Dirac chains enable intrinsic topological superconductivity in nonsymmorphic silicides

Nonsymmorphic crystalline symmetries provide a robust route to symmetry-protected electronic topology, yet their role in stabilizing intrinsic topological superconductivity remains largely unexplored. Here, we report \ch{TaPtSi} as a new member of the superconducting nonsymmorphic silicide family, characterized via AC transport, magnetization, heat capacity, and muon spin rotation/relaxation ($\mu$SR) measurements. Zero field $\mu$SR reveals spontaneous internal magnetic fields below $T_{\rm c}$, establishing time reversal symmetry breaking in \ch{TaPtSi}. First principles calculations on \ch{TaPtSi} and its isostructural nonsymmorphic superconducting analogues reveal the presence of symmetry-protected hourglass dispersions. The "necks" of these dispersions form Dirac nodal rings and chains that reside near or intersect the Fermi level. Guided by Ginzburg Landau symmetry analysis, we identify an internally antisymmetric non unitary triplet pairing state as the unique ground state consistent with the experimental phenomenology. Based on Bogoliubov de Gennes calculations, we further demonstrate that this state supports Majorana surface modes, establishing its intrinsically topological nature. These results reveal a systematic route by which nonsymmorphic symmetry drives the interplay between hourglass Dirac chain topology and unconventional triplet pairing, positioning equiatomic silicides as a unified materials platform for intrinsic topological superconductivity.

cond-mat.supr-con↗

arXiv2025

Probing the intermediate state of type-I superconductor SnAs using Muon Spin Spectroscopy

Superconductivity with non-trivial band topology provides a novel platform for exploring topological superconductivity and its quantum applications. A detailed microscopic understanding of the superconducting ground state in such materials is crucial. Here, we report the results of a muon spin rotation/relaxation study ($\mu$SR) of the topologically non-trivial superconductor SnAs, which exhibits superconductivity below 3.74(1) \si{K}. Zero-field (ZF) $\mu$SR data reveal that this system is a time-reversal invariant superconductor, and systematic transverse-field (TF) $\mu$SR measurements unveil the type-I nature of the SnAs superconductor. We have established the superconducting phase diagram to understand the intermediate state of type-I superconductors. Moreover, ab \textit{initio} band structure and phonon calculations are performed, which correlate with the experimental characterization.

cond-mat.supr-con↗

arXiv2025

Test of LGAD as Potential Next-Generation {\mu}SR Spectrometer Detectors

Muon Spin Rotation/Relaxation/Resonance ($\mu$SR) is a versatile and powerful non-destructive technology for investigating the magnetic properties of materials at the microscopic level. The $\mu$SR technique typically utilizes fully spin polarized beams of positive muons generated at particle accelerator facilities and measures the evolution of the muon spin polarization inside a sample to extract information about the local magnetic environment in materials. With the development of accelerator technologies, intensities of muon beams are being continuously improved, which will cause a pile-up problem to the $\mu$SR spectrometer. The first muon source in China, named MELODY, is currently under construction and will be a pulsed source of muons operated at a repetition frequency of only 1 Hz due to limitations of the accelerator system at CSNS. Consequently, there is a strong motivation to operate MELODY at significantly higher muon intensities. This necessitates an upgrade of the detector system inside the spectrometer, which should be smaller and faster to accommodate the increased intensity per pulse of muons. The Low Gain Avalanche Diode (LGAD), characterized by a typical pulse width of 2 ns and a segmentation size in the centimeters range, has the potential to significantly improve the counting rates of $\mu$SR spectrometers that utilize a high intensity pulsed muon source. Thus, it is expected that the LGAD detector is a promising candidate to enhance the performance of $\mu$SR spectrometers at the new MELODY muon source.To validate this, tests on the LGAD were conducted at the ISIS pulsed muon source at the Rutherford Appleton Laboratory, UK. This paper will describe the setup of the candidate LGAD devices and the subsequent analysis of the experiment data.

physics.ins-det↗

arXiv2025

Unveiling superconducting properties of an equiatomic hexagonal high entropy alloy via muon spin relaxation and rotation measurement

Superconducting high-entropy alloys (HEAs) present a unique platform for studying the effect of disorder, composition, and crystal structure on superconducting pairing. In this study, we present a comprehensive bulk and microscopic investigation of the rarely observed equiatomic hexagonal HEA Nb-Mo-Ru-Re-Ir using magnetization, resistivity, heat capacity, and muon spin relaxation and rotation ($\mu$SR) measurements. Our findings confirm bulk type-II superconductivity with a transition temperature of 4.63(2) K and a high upper critical field. Heat capacity and transverse-field $\mu$SR data reveal conventional s-wave superconductivity, while zero-field $\mu$SR results suggest the preservation of time-reversal symmetry in the superconducting state. These findings provide valuable insights into the superconducting pairing mechanism in disordered multicomponent systems.

cond-mat.supr-con↗

arXiv2025

Topological superconductivity in hourglass Dirac chain metals (Ti, Hf)IrGe

Realizing topological superconductivity in stoichiometric materials is a key challenge in condensed matter physics. Here, we report the discovery of ternary germanide superconductors, $M$IrGe ($M$ = Ti, Hf), as prime candidates for topological superconductivity, predicted to exhibit nonsymmorphic symmetry-protected hourglass Dirac chains. Using comprehensive thermodynamic and muon-spin rotation/relaxation ($\mu$SR) measurements, we establish these materials as conventional bulk type-II superconductors with transition temperatures of 2.24(5) K for TiIrGe and 5.64(4) K for HfIrGe, featuring a full gap and preserved time-reversal symmetry. First-principles calculations reveal striking topological features in $M$IrGe, including hourglass-shaped bulk dispersions and a Dirac chain -- a ring of fourfold-degenerate Dirac points protected by nonsymmorphic symmetry. Each Dirac point corresponds to the neck of the hourglass dispersion, while the Dirac chain gives rise to drumhead-like surface states near the Fermi level. Additionally, nontrivial $\mathbb{Z}_2$ topology leads to isolated Dirac surface states with helical spin textures that disperse across the Fermi level, forming an ideal platform for proximity-induced topological superconductivity. The coexistence of conventional bulk superconductivity, symmetry-protected hourglass topology, and helical spin-textured surface states establishes $M$IrGe as a rare and robust platform to realize topological superconductivity, opening new avenues for next-generation quantum technologies.

cond-mat.supr-con↗

arXiv2024

Observation of superconductivity in a nontrivial $\mathcal{Z}_2$ approximant quasicrystal

Superconductivity and nontrivial topology are highly sought-after phenomena in quantum materials. While many topological crystalline materials have been found to exhibit superconductivity, their presence in quasicrystals - materials with a unique aperiodic yet ordered structure - has remained largely unexplored. In this work, we report the discovery of superconductivity in a monoclinic approximant to the decagonal quasicrystal Al$_{13}$Os$_{4}$, that exhibits a high superconducting transition temperature and a nontrivial electronic structure. The resistivity, magnetization, specific heat, and $\mu$SR measurements confirm superconductivity with a critical temperature of $\sim5.47$ K. Detailed electronic structure and symmetry analysis reveal nontrivial state with $\mathcal{Z}_{2}=1$ and spin-polarized conducting surface states. Importantly, we identify three-dimensional saddle point van Hove singularities with substantial flat energy dispersion at the Fermi level, which can enhance superconductivity. Our results highlight a rich interplay between superconductivity and nontrivial electronic states in Al$_{13}$Os$_{4}$, demonstrating it as a unique platform for exploring unconventional superconducting states in quasicrystalline materials.

cond-mat.supr-con↗