arXiv subjects

R. P. Singh

Publications and source records attributed to R. P. Singh.

At least 19 recordsLinked to original sources

arXiv2026

Superconductivity in Pb2-xBixPd Single Crystals

Chemical substitution provides an effective route to tune the structural and superconducting properties and to explore the evolution of superconductivity across related materials. In this work, we investigate the effect of Pb/Bi mixing on superconductivity in possible topological superconductors \ch{Pb2Pd} and $β$-\ch{Bi2Pd}, by synthesizing and characterizing single crystals of \ch{Pb_{2-x}Bi_{x}Pd} ($0.2\le x\le1.8$). Increasing Bi content triggers a structural phase transition from non-symmorphic ($I4/mcm$, $x<1$) to layered ($I4/mmm$, $x\ge1$) and induces a monotonic expansion of the lattice parameters. Across the doped range, all superconducting samples behave as weakly-coupled type-II superconductors exhibiting isotropic s-wave superconducting gaps.

cond-mat.supr-con↗

arXiv2026

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

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 $μ$SR measurements reveal an s-wave superconducting gap close to the BCS value, while zero-field $μ$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

Evolution of the electronic and superconducting properties of Re-based quinary high-entropy alloys under chemical and physical pressure

We report a comparative study of chemical- and physical-pressure effects on the electronic and superconducting properties of the Re-based quinary high-entropy alloys (HEAs) [Nb0.67-xRex][TiZrHf]0.33 (x = 0.10, 0.20, and 0.56). Increasing Re concentration suppresses the superconducting transition temperature Tc from 5.4 to 3.9 K while producing positive, composition-dependent cocktail-effect ratios. Field-dependent transport and magnetization establish all three compositions as strongly type-II superconductors and identify x = 0.20 as the most distinctive composition, with the largest upper critical field and Ginzburg-Landau parameter and a Maki parameter close to unity. Most importantly, the pressure coefficient of Tc changes sign across the bcc-hcp structural change: dTc/dP is positive for the bcc x = 0.10 and 0.20 samples, with values of approximately 0.018 and 0.053 K/GPa, respectively, but negative for the hcp x = 0.56 sample, with a value of approximately -0.033 K/GPa. This systematic contrast within a single chemically related alloy series establishes a robust structure-associated superconducting response and identifies crystal structure as a key organizing variable under compression. Metallic transport and superconductivity remain robust up to approximately 8 GPa in all three compositions. These results reveal that chemical substitution and hydrostatic compression are complementary but nonequivalent routes for tuning superconductivity in Re-based HEAs.

cond-mat.supr-con↗

arXiv2026

Emergence of Quasi-two-dimensional Superconductivity in W-doped Bulk Noncentrosymmetric 3$R$-TaSe$_2$

Noncentrosymmetric transition-metal dichalcogenides offer a rich environment for the study of unconventional superconducting phenomena. Here, we present a comprehensive analysis of single-crystalline W-doped 3$R$-TaSe$_2$, revealing weakly coupled anisotropic unconventional superconductivity at $T_c$ = 2.82(2) K, with an in-plane upper critical field exceeding the Pauli limit by 1.7 times. The angular dependence of the upper critical field, along with the observation of a Berezinskii-Kosterlitz-Thouless transition, reveals quasi-two-dimensional superconductivity. Crucially, magnetotransport reveals a distinct two-fold rotational symmetry within the superconducting state under in-plane fields, breaking the underlying three-fold lattice symmetry. These findings establish W-doped $3R\text{-TaSe}_2$ as a bulk model system for exploring intrinsic low-dimensional superconductivity and broken rotational symmetry, thus opening new directions for future quantum technologies.

cond-mat.supr-con↗

arXiv2026

Experimental Demonstration of Free-Space Unidimensional Continuous-Variable Quantum Key Distribution Under High Detector Noise

Continuous-variable quantum key distribution (CV-QKD), which uses quadratures of the electromagnetic field, enables practical quantum communication using standard telecommunication technologies. Unidimensional CV-QKD (UD-CVQKD) simplifies the implementation by restricting modulation to a single quadrature. In this work, we experimentally demonstrate a free-space Gaussian-modulated UD-CVQKD system operating under a high detector electronic-noise regime (1.4 shot-noise units). The system employs polarized coherent states with signal and local oscillator co-propagating in the same spatial mode in orthogonal polarizations, ensuring stable interference. System security is analyzed under both untrusted (UTD) and trusted (TD) detector noise models. While no positive secret key rate is obtained under the UTD model, the TD model enables secure key generation over a finite range of modulation variances, highlighting the critical role of detector trust in high-noise conditions. A maximum secret key rate of 270 kbps is achieved at an optimal modulation variance of 11.57. Furthermore, secure operation requires high-transmittance (low-loss) channels under such noise conditions. This study demonstrates the practical feasibility of free-space UD-CVQKD in realistic high electronic-noise detection constraints and highlights detector electronic noise as a key limiting factor in practical systems.

quant-ph↗

arXiv2026

Evidence of anisotropic bulk superconductivity in disorder-induced ZrTe$_{3-x}$

Transition-metal trichalcogenides distinguish themselves from other two-dimensional materials in nanoscience and materials science due to their remarkable range of intrinsic properties, including various electronic, optical, and magnetic behaviors. Here, we report a comprehensive study of superconductivity in disordered ZrTe$_{3-x}$ ($x$ = 0.2) with suppressed charge density wave. We observe a type-II bulk anisotropic superconductivity with a superconducting transition at $T_c$ = 3.59(4) \si{K}. Angle-dependent upper critical field measurements and Berezinskii-Kosterlitz-Thouless transition confirm the inherent quasi-two-dimensional nature of superconductivity in this disordered system.

cond-mat.supr-con↗

arXiv2025

High Critical Temperature and Field Superconductivity in Nb$_{0.85}$X$_{0.15}$, (X = Ti, Zr, Hf) Alloys: Promising Candidates for Superconducting Devices

Niobium and its alloys with early transition metals have been extensively studied for their excellent superconducting properties. They have high transition temperatures, strong upper critical fields, and high critical current densities, making them ideal for superconducting applications such as SQUIDs, MRI, NMR, particle accelerators, and Qubits. Here we report a systematic investigation of as-cast Nb-rich alloys, Nb$_{0.85}$X$_{0.15}$ (X = Ti, Zr, Hf), using magnetization, electrical transport, and specific heat measurements. They exhibit strong type-II bulk superconductivity with moderate superconducting transition temperatures and upper critical fields. The estimated magnetic field-dependent critical current density lies in the range of 10$^5$--10$^6$~A/cm$^2$ across various temperatures, while the corresponding flux-pinning force density is on the order of GNm$^{-3}$, suggesting the potential of these materials for practical applications. Electronic-specific heat data reveal a strongly coupled, single, isotropic, nodeless superconducting gap. These Nb-rich alloys, characterized by robust superconducting properties, hold significant potential for applications in superconducting device technologies.

cond-mat.supr-con↗

arXiv2025

Quasi-two-dimensional superconductivity in 1$T$-Ti$_{1-x}$Ta$_x$Se$_2$

The emergence of two-dimensional (2D) superconductivity in bulk transition metal dichalcogenides (TMDs) is a fascinating area of research, as their weak interlayer coupling leads to novel superconducting behavior and offers a rich platform to host nontrivial gap structures and interactions with other electronic orders. In this work, we present a comprehensive study of the superconducting properties of bulk single-crystalline $1T$-Ti$_{1-x}$Ta$_x$Se$_2$ for x = 0.2. Our results confirm the weakly coupled anisotropic superconductivity. Angle-dependent upper critical field measurements and observation of a Berezinskii-Kosterlitz-Thouless transition confirm the quasi-2D nature of the superconducting state. These results position $1T$-Ti$_{1-x}$Ta$_x$Se$_2$ as a promising platform for exploring low-dimensional superconducting physics and highlight bulk TMD crystals as a promising platform for realizing intrinsic 2D superconductivity, opening avenues for future quantum applications.

cond-mat.supr-con↗

arXiv2025

Aspects of Single Particle Excitations and Collectivity in $^{69}$Ga

The excitation scheme of the $^{69}$Ga ($Z = 31, N = 38$) nucleus has been studied following its population in $^{59}$Co($^{13}$C,2pn) reaction at $E_{lab} = 45, 50$ MeV, and using an array of Compton suppressed HPGe clover detectors as the detection system. The existing level scheme has been considerably extended with identification of new $γ$-ray transitions and their multipolarity assignments. The level energies have been calculated in the framework of the large basis shell model and their overlap with the experimental values has been satisfactory, subject to the choice of the interaction. The band structures identified in the nucleus have been characterized with the Moment of Inertia (MOI) and aligned angular momentum, and compared to those of the similar structures in the neighboring isotopes. The shapes corresponding to these bands have been probed in the light of their Total Routhian Surface (TRS) that exhibited varied deformation characteristics, such as prolate and $γ$-softness, associated with the individual sequences. Further, evidence of strong octupole correlation has been identified from an E3 transition between bands of opposite parities. The study comprehensively brings forth multiple aspects of single particle and collective characteristics in the level structure of $^{69}$Ga.

nucl-ex↗

arXiv2025

Unconventional superconductivity in a non-centrosymmetric $α$-Mn alloy NbTaOs$_{2}$

Non-centrosymmetric superconductors have emerged as a fascinating avenue for exploring unconventional superconductivity. Their broken inversion and time-reversal symmetries make them prime candidates for realizing the intrinsic superconducting diode effect (SDE). In this work, we synthesize the ternary non-centrosymmetric $α$-Mn alloy NbTaOs$_{2}$ and conduct a comprehensive investigation of its superconducting properties through resistivity, magnetization, specific heat and muon spin rotation/relaxation ($μ$SR) techniques. Our transverse field-$μ$SR and specific heat results provide evidence of a moderately coupled, fully-gaped superconducting state. Zero field-$μ$SR measurements reveal a subtle increase in the relaxation rate below the transition temperature, suggesting time reversal symmetry breaking in the superconducting ground state of NbTaOs$_{2}$.

cond-mat.supr-con↗

arXiv2025

Strengthening the No-Go Theorem for QRNGs

Quantum random numbers are essential for security against quantum algorithms. Randomness as a beacon is a service being provided for companies and governments to upgrade their security standards from RSA to PQC-QKD or PQC-RSA protocols. Both security mechanisms assume trust in the service provider unless one aims for device-independent protocols. How does an entity ensure that the beacon service has a quantum signature other than relying on faith? Specifically, given a bit-stream, can a user verify a quantum signature in it? Researchers claim this is indecipherable and have stated a no-go theorem for post-processed bit-streams [Physical Review A \textbf{109}, 022243 (2024)]. In this article, we corroborate the results of the no-go theorem while discussing its nuances using two different random number generators and four test methods. These include the NIST statistical test suite and machine learning algorithms that strengthen the theorem. This work is relevant for companies and governments using QRNG, provided to enhance security against quantum threats.

quant-ph↗

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 ($μ$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 $μ$SR data reveal conventional s-wave superconductivity, while zero-field $μ$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

High critical field superconductivity in a 3d dominated lightweight equiatomic high entropy alloy

The lightweight high entropy alloy represents an innovative class of multicomponent systems that combine low density with the exceptional mechanical properties of high-entropy alloys. We present a detailed synthesis and investigation of a 3d rich equiatomic high entropy alloy superconductor Sc-Ti-V-Nb-Cu, which crystallizes in a body-centered cubic structure. Magnetization, electrical resistivity, and heat capacity measurements confirm weakly coupled bulk type II superconductivity with a 7.21(3) K transition temperature and an upper critical field of 12.9(1) T. The upper critical field approaches the Pauli paramagnetic limit, suggesting potential unconventional behavior. The low density, moderate transition temperature, and high upper critical field stand out Sc-Ti-V-Nb-Cu as a promising candidate for next-generation superconducting device applications.

cond-mat.supr-con↗

arXiv2024

Enhancing key rates of QKD protocol by Coincidence Detection

In theory, quantum key distribution (QKD) provides unconditional security; however, its practical implementations are susceptible to exploitable vulnerabilities. This investigation tackles the constraints in practical QKD implementations using weak coherent pulses. We improve on the conventional approach of using decoy pulses by integrating it with the coincidence detection (CD) protocol. Additionally, we introduce an easy-to-implement algorithm to compute asymptotic key rates for the protocol. Furthermore, we have carried out an experimental implementation of the protocol, where we demonstrate that monitoring coincidences in the decoy state protocol leads to enhanced key rates under realistic experimental conditions.

quant-ph↗

arXiv2024

Passive polarization-encoded BB84 protocol using a heralded single-photon source

The BB84 quantum key distribution protocol set the foundation for achieving secure quantum communication. Since its inception, significant advancements have aimed to overcome experimental challenges and enhance security. In this paper, we report the implementation of a passive polarization-encoded BB84 protocol using a heralded single-photon source. By passively and randomly encoding polarization states with beam splitters and half-wave plates, the setup avoids active modulation, simplifying design and enhancing security against side-channel attacks. The heralded single-photon source ensures a low probability of multi-photon emissions, eliminating the need for decoy states and mitigating photon number splitting vulnerabilities. The quality of the single-photon source is certified by measuring the second-order correlation function at zero delay, $g^{2}(0)=0.0408\pm0.0008$, confirming a very low probability of multi-photon events. Compared to conventional BB84 or BBM92 protocols, our protocol provides optimized resource trade-offs, with fewer detectors (compared to BBM92) and no reliance on external quantum random number generators (compared to typical BB84) to drive Alice's encoding scheme. Our implementation achieved a quantum bit error rate of 7% and a secure key rate of 5 kbps. These results underscore the practical, secure, and resource-efficient framework our protocol offers for scalable quantum communication technologies.

quant-ph↗

arXiv2024

Time-Reversal Symmetry Breaking in Re-Based Kagome Lattice Superconductor

We investigated the Re-based kagome superconductor Re$_2$Zr through various measurements, including resistivity, magnetization, specific heat, and muon spin rotation and relaxation spectroscopy. These results suggest that Re$_2$Zr is a moderately coupled potential two-gap superconductor. Zero-field muon relaxation data indicate the possible presence of a time-reversal symmetry-breaking state in the superconducting ground state. Our investigation identifies Re$_{2}$Zr as a new unconventional superconductor with a potential complex order parameter that warrants considerable experimental and theoretical interest.

cond-mat.supr-con↗

arXiv2024

Stabilization of Ambient Pressure Rocksalt Crystal Structure and High Critical Field Superconductivity in ReC via Mo and W Substitution

Transition-metal-based carbides (TMCs), renowned for their exceptional hardness, mechanical strength, and thermal properties, have recently emerged as promising candidates for topological superconductivity. In this study, we synthesized ReC in the NaCl structure at ambient pressure by substituting Mo or W at the Re-site. We investigated the superconducting properties of Re$_{1-x}$T$_{x}$C (where T = Mo, W) for $x = 0.5$ using magnetization, resistivity and specific heat measurements. These compounds display type-II, fully gapped, weakly coupled superconductivity with high critical fields, establishing them as new members of superconducting ultra-hard materials at ambient pressure and paving the way for superconducting device applications under extreme conditions.

cond-mat.supr-con↗

arXiv2024

Investigating a Device Independence Quantum Random Number Generation

Quantum random number generation (QRNG) is a resource that is a necessity in the field of cryptography. However, its certification has been challenging. In this article, we certify randomness with the aid of quantum entanglement in a device independent setting, where we choose two-photon interference for source characterisation. The CHSH inequality violation and quantum state tomography are used as independent checks on the measurement devices. These measures ensure the unpredictability of quantum random number generation. This work can be easily extended to faster randomness expansion protocols.

quant-ph↗