arXiv subjects

D. Singh

Publications and source records attributed to D. Singh.

At least 37 records · Page 2Linked to original sources

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

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

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↗

arXiv2020

Unconventional superconducting properties of noncentrosymmetric Re5.5Ta

Rhenium based noncentrosymmetric superconductors crystallizing in $α$-Mn structure have become potential candidates to exhibit an unconventional superconducting ground-state. Here we report a detailed investigation on the superconducting and normal state properties of Re$_{5.5}$Ta, that also has the $α$-Mn structure. Magnetization, specific heat, and transport measurements confirm the bulk superconducting transition \textit{T}$_{C}$ at 8.0 K. Upper critical field value (H$_{C2}$(0)) calculated from magnetization, specific heat and AC transport measurements exceed the Pauli paramagnetic limit (14.7 T), indicating that the superconducting properties of Re$_{5.5}$Ta are probably unconventional in nature. However, low-temperature specific heat and transverse-field muon spin rotation measurements suggest a surprising nodeless isotropic superconducting gap, although with strong electron-phonon coupling.

cond-mat.supr-con↗

arXiv2020

Heat radiation reduction in the cryostat with multilayer insulation technique

Multilayer insulation (MLI) is an important technique for the reduction of radiation heat load in cryostats. The present work is focused on investigation for the selection of suitable reflective layer and spacer material in MLI systems. In our analysis, we have selected perforated double-Aluminized Mylar (DAM) with Dacron, unperforated DAM with Silk-net and perforated DAM with Glass-tissue for their evaluation as the reflective layer as well as spacer materials in MLI technique. Current work would discuss the calculation of the effect of layer density and the number of layers on the heat load. Knowing the key parameters of MLI, we have compared the heat load generation in spherical as well as cylindrical cryostats and the effect of layering near and outer surface on the heat load.

physics.ins-det↗

arXiv2020

An empirical metallicity tracer in CEMP and C-normal stars

Deriving the metallicity, [Fe/H], in low-resolution spectra of carbon-enhanced metal-poor (CEMP) stars is a tedious task that, owing to the large number of line blends, often leads to uncertainties on [Fe/H] exceeding 0.25dex. The CEMP stars increase in number with decreasing [Fe/H] and some of these are known to be bona fide second generation halo stars. Hence, knowing their [Fe/H] is important for tracing the formation and chemical evolution of the Galaxy. Here, we aim to improve the [Fe/H] measurements in low-resolution spectra by avoiding issues related to blends. We improve our chemical tagging in such spectra at low metallicities. We developed an empirical way of deriving [Fe/H] in CEMP (and C-normal) stars that relates the equivalent width (EW) of strong lines, which remain detectable in lower-resolution, metal-poor spectra. The best [Fe/H] tracers are found to be Cr I and Ni I, which both show strong transitions in spectral regions that are free of molecular bands (between ~5200-6800A, a region accessible to most surveys). We derive different relations for dwarfs and giants. The relations are valid in the ranges ~-3<[Fe/H]<-0.5 and 10 -3.2 and EW>5mA (Ni), depending on the element and line as well as the stellar evolutionary stage. The empirical relations are valid for both CEMP and C-normal stars and have been proven to be accurate tracers in a sample of ~400 stars (mainly giants). The metallicities are accurate to within ~0.2 depending on the sample and resolution, and the empirical relations are robust to within 0.05-0.1dex. Our relations will improve the metallicity determination in future surveys, which will encounter a large number of CEMP stars, and will greatly speed up the process of determining [Fe/H] as the EWs only need to be measured in two or three lines in relatively clean regions compared to dealing with numerous blended Fe lines. Abrigded.

astro-ph.SR↗

arXiv2019

Coexistence of type-I and type-II superconductivity in topological superconductor PdTe$_2$

The type-II Dirac semimetal PdTe2 was recently reported to be a type-I superconductor with a superconducting transition temperature Tc = 1.65 K. However, the recent results from tunneling and point contact spectroscopy suggested the unusual state of a mixture of type-I and type-II superconductivity. These contradictory results mean that there is no clear picture of the superconducting phase diagram and warrants a detailed investigation of the superconducting phase. We report here the muon spin rotation and relaxation ($μ$SR) measurements on the superconducting state of the topological Dirac semimetal PdTe2. From $μ$SR measurements, we find that PdTe2 exhibits mixed type-I/type-II superconductivity. Using these results a phase diagram has been determined. In contrast to previous results where local type-II superconductivity persists up to Hc2 = 600 G, we observed that bulk superconductivity is destroyed above 225 G.

cond-mat.supr-con↗

arXiv2019

Investigations of the superconducting ground state of Zr$_{3} $Ir: Introducing a new noncentrosymmetric superconductor

Superconductivity in materials whose crystal structure lacks inversion symmetry is a prime candidate for unconventional superconductivity. A new noncentrosymmetric compound Zr$_{3}$Ir crystallizes in a tetragonal $ α$-V$_{3} $S structure. The magnetization, specific heat and muon spin rotation confirm s-wave superconductivity, having a transition temperature T$_{c}$ = 2.3 K. Muon spin relaxation confirms the preservation of time reversal symmetry in the superconducting ground state.

cond-mat.supr-con↗

arXiv2019

Magnetization reversal, giant exchange bias effect and magnetoresistance in oxygen vacancy ordered Sr$_{4}$Fe$_{3}$CoO$_{11}$

We report the structural, magnetic, exchange bias and magnetotransport effect in Sr$_{4}$Fe$_{3}$CoO$_{11}$. The material crystallizes in the orthorhombic $\textit{Cmmm}$ space group. It shows antiferromagnetic (G-type) transition (T$_{N}$ = 255 K) along with interesting temperature induced magnetization reversal (T$_{Comp.}$= 47 K measured at 100 Oe). The magnetic reversal can be elucidated considering the increased magnetocrystalline anisotropy with Co substitution. Magnetoresistance measurements shows an interesting crossover from negative to positive side at $\sim$ 100 K. The negative magnetoresistance reaches 80 $\%$ at 25 K in 7 T magnetic field. Giant exchange bias effect is observed below T$_{N}$ under field cooling condition. The origin of the negative magnetoresistance and giant exchange bias in this sample can be attributed to the magnetic frustration.

cond-mat.str-el↗

arXiv2019

Nodeless s-wave superconductivity in the α-Mn structure type noncentrosymmetric superconductor TaOs: A μSR study

Noncentrosymmetric superconductors can lead to a variety of exotic properties in the superconducting state such as line nodes, multigap behavior, and time-reversal symmetry breaking. In this paper, we report the properties of the new noncentrosymmetric superconductor TaOs, using muon spin relaxation and rotation measurements. It is shown using the zero-field muon experiment that TaOs preserve the time-reversal symmetry in the superconducting state. From the transverse field muon measurements, we extract the temperature dependence of $λ(T)$ which is proportional to the superfluid density. This data can be fit with a fully gapped s-wave model for $α$ = $Δ(0)/k_{B}T_{c}$ = 2.01 $\pm$ 0.02. Furthermore, the value of magnetic penetration depth is found to be 5919 $\pm$ 45 \textÅ, which is consistent with the value obtained from the bulk measurements.

cond-mat.supr-con↗

arXiv2019

Type-I superconductivity in noncentrosymmetric superconductor AuBe

The noncentrosymmetric superconductor AuBe have been investigated using the magnetization, resistivity, specific heat, and muon-spin relaxation/rotation measurements. AuBe crystallizes in the cubic FeSi-type B20 structure with superconducting transition temperature observed at $T_{c}$ = 3.2 $\pm$ 0.1 K. The low-temperature specific heat data, $C_{el}$(T), indicate a weakly-coupled fully gapped BCS superconductivity with an isotropic energy gap 2$Δ(0)/k_{B}T_{c}$ = 3.76, which is close to the BCS value of 3.52. Interestingly, type-I superconductivity is inferred from the $μ$SR measurements, which is in contrast with the earlier reports of type-II superconductivity in AuBe. The Ginzburg-Landau parameter is $κ_{GL}$ = 0.4 $<$ 1/$\sqrt{2}$. The transverse-field $μ$SR data transformed in the maximum entropy spectra depicting the internal magnetic field probability distribution, P(H), also confirms the absence of the mixed state in AuBe. The thermodynamic critical field, $H_{c}$, calculated to be around 259 Oe. The zero-field $μ$SR results indicate that time-reversal symmetry is preserved and supports a spin-singlet pairing in the superconducting ground state.

cond-mat.supr-con↗

arXiv2019

Superconducting and normal state properties of noncentrosymmetric superconductor NbOs$_{2}$ investigated by muon spin relaxation and rotation

Noncentrosymmetric superconductors with $α$-manganese structure has attracted much attention recently, after the discovery of time-reversal symmetry breaking in all the members of Re$_{6}$X (X = Ti, Hf, Zr) family. Similar to Re$_{6}$X, NbOs$_{2}$ also adopts $α$-$Mn$ structure and found to be superconducting with critical temperature $T_{c}$ $\approx$ 2.7 K. The results of the resistivity, magnetization, specific heat and muon-spin relaxation/rotation measurements show that NbOs$_{2}$ is a weakly coupled type-II superconductor. Interestingly, the zero-field muon experiments indicate that the time-reversal symmetry is preserved in the superconducting state. The low-temperature transverse-field muon measurements and the specific heat data evidence a conventional isotropic fully gapped superconductivity. However, the calculated electronic properties in this material show that the NbOs$_{2}$ is positioned close to the band of unconventionality of the Uemura plot, indicating that NbOs$_{2}$ potentially borders an unconventional superconducting ground state.

cond-mat.supr-con↗

arXiv2018

Superconducting properties of the noncentrosymmetric superconductor LaPtGe

We report superconducting properties of noncentrosymmetric superconductor LaPtGe which crystallizes in noncentrosymmetric $ α$-ThSi$ _{2} $ structure. The magnetization, resistivity and specific heat measurements confirm that LaPtGe is a type-II bulk superconductor with a transition temperature T$ _{c} $ = 3.05 $ \pm $ 0.05 K. Muon-spin relaxation/rotation measurements confirms that time reversal preserved in the superconducting ground state.

cond-mat.supr-con↗

arXiv2018

Fully-gapped superconductivity in single crystals of noncentrosymmetric Re$_6$Zr with broken time-reversal symmetry

The noncentrosymmetric superconductor Re$_6$Zr 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 Re$_6$Zr single crystals by measuring the magnetic penetration depth shift $Δλ(T)$ and electronic specific heat $C_e(T)$. $Δλ(T)$ exhibits an exponential temperature dependence behavior for $T~\ll~T_c$, 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 $ρ_s(T)$ and $C_e(T)$ results, with a fitted gap magnitude larger than the weak coupling BCS value, providing evidence for fully-gapped superconductivity in Re$_6$Zr with moderate coupling.

cond-mat.supr-con↗

arXiv2018

Time Reversal Symmetry Breaking in noncentrosymmetric superconductor Re6Ti

We have investigated the superconducting state of the noncentrosymmetric superconductor Re6Ti (Tc = 6.0 K) using muon-spin rotation/relaxation (muSR) technique. The zero-field muon experiment shows the presence of spontaneous magnetic fields in the superconducting state, indicating time-reversal symmetry breaking (TRSB). However, the low-temperature transverse field muon measurements suggest nodeless s-wave superconductivity. The time reversal symmetry breaking further confirmed in the stoichiometric composition Re24Ti5. These results indicate that the pairing symmetry is not affected by spin-orbital coupling Re6X family of compounds. Altogether these studies suggest unconventional nature (TRSB) of superconductivity is intrinsic to Re6X family of compounds and paves the way for further studies of this family of materials.

cond-mat.supr-con↗

arXiv2017

Time-reversal symmetry breaking in noncentrosymmetric superconductor Re6Hf:further evidence for unconventional behaviour in the alpha-Mn family of materials

The discovery of new families of unconventional superconductors is important both experimentally and theoretically, especially if it challenges current models and thinking. By using muon spin relaxation in zero-field, time-reversal symmetry breaking has been observed in Re6Hf. Moreover, the temperature dependence of the superfluid density exhibits s-wave superconductivity with an enhanced electron-phonon coupling. This, coupled with the results from isostructural Re6Zr, shows that the Re6X family are indeed a new and important group of unconventional superconductors.

cond-mat.supr-con↗

arXiv2017

Superconducting properties of the noncentrosymmetric superconductor TaOs

The noncentrosymmetric superconductor TaOs has been characterized using x-ray diffraction, resistivity, magnetization, and specific heat measurements. Magnetization and specific heat measurements show a bulk superconducting transition at 2.07 K. These measurements suggest that TaOs is a weakly coupled type-II superconductor. The electronic specific heat in the superconducting state can be explained by the single-gap BCS model, suggesting s-wave superconductivity in TaOs.

cond-mat.supr-con↗