arXiv subjects

Samar Layek

Publications and source records attributed to Samar Layek.

18 recordsLinked to original sources

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↗

arXiv2025

Rapid reduction of nitrophenols using reusable magnetic \textit{h}-BN/Ni--NiO nanocomposites

The efficient and cost-effective conversion of nitro compounds to amines is crucial for industrial processes and environmental remediation, highlighting the growing demand for earth-abundant metal-based catalysts. In this study, magnetic Ni--NiO nanostructures and their composites with two-dimensional hexagonal boron nitride (\textit{h}-BN) were synthesized via a simple and scalable combustion method. The structural, morphological, and compositional properties of the synthesized materials were systematically investigated using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV--Vis spectroscopy. The catalytic activity of both Ni--NiO and \textit{h}-BN/Ni--NiO nanostructures was evaluated using nitrophenol reduction as a model reaction. The \textit{h}-BN/Ni--NiO nanocomposite exhibited significantly enhanced catalytic performance compared to pristine Ni--NiO, highlighting the synergistic interaction between \textit{h}-BN and Ni--NiO nanoparticles. Notably, the magnetic nature of the Ni--NiO core enabled facile recovery of the catalyst using an external magnetic field, and the composite demonstrated excellent stability and reusability for up to six catalytic cycles with minimal loss of activity. The combination of high catalytic efficiency, magnetic separability, and structural stability positions the \textit{h}-BN/Ni--NiO nanocomposite as a promising candidate for green and sustainable catalytic applications, particularly in environmental remediation.

physics.chem-ph↗

arXiv2025

High-Entropy Oxide Nanostructures for Rapid and Sustainable Nitrophenol Reduction

High-entropy materials have emerged as a promising class of catalysts, driven by their high configurational entropy originating from structural disorder in single-phase multicomponent systems. Despite their potential, the catalytic performance of high-entropy oxides (HEOs) remains relatively underexplored. In this study, we present a simple solution-based combustion route to synthesize two low-cost, transition metal-rich multicationic oxides positioned in the medium-entropy (HEO-4) and high-entropy (HEO-5) regimes. Rietveld refinement of powder X-ray diffraction data confirmed single-phase formation with a face-centered cubic (fcc) crystal structure for both nanostructures. The morphology, particle size, and multicationic elemental distribution were investigated using scanning and transmission electron microscopy. The catalytic performance of the synthesized HEOs was evaluated in the hydrogenation of a series of nitrophenol derivatives. Notably, HEO-5 exhibited significantly enhanced catalytic activity ($k_{\mathrm{app}} \approx 0.5~\mathrm{min^{-1}}$, TOF $= 2.1 \times 10^{-3}~\mathrm{mol\,g^{-1}\,s^{-1}}$), achieving rapid conversion of \emph{p}-nitrophenol compared to the medium-entropy oxide nanostructures ($k_{\mathrm{app}} \approx 0.02~\mathrm{min^{-1}}$, TOF $= 7.2 \times 10^{-4}~\mathrm{mol\,g^{-1}\,s^{-1}}$). Furthermore, the kinetic and thermodynamic parameters of the reaction, including the activation energy ($E_a$), enthalpy of activation ($\Delta H^{\ddagger}$), Gibbs free energy of activation ($\Delta G^{\ddagger}$), and entropy of activation ($\Delta S^{\ddagger}$), were determined to gain mechanistic insight into the reduction process. This study opens new avenues for the rational design and facile synthesis of high-entropy oxide catalysts, highlighting their potential for efficient and sustainable large-scale amine production.

cond-mat.mtrl-sci↗

arXiv2025

MOF-derived Fe-doped $\delta$-MnO$_2$ nanoflowers as oxidase mimics: Chromogenic sensing of Hg$^{2+}$ and hydroquinone in aqueous media

Structure and morphology play a crucial role in enhancing the biomimetic oxidase activity of nanozymes. In this study, a facile \emph{in situ} chemical oxidation strategy was employed to synthesize MOF-derived MnO$_x$, utilizing the structural features of the parent MOF to enhance oxidase-mimicking activity. We systematically investigated the effects of phase evolution, structural modulation, and morphology on the oxidase activity of MnO$_x$ with Fe substitution. The oxidase-like activity was evaluated using the chromogenic substrate 3,3$'$,5,5$'$-tetramethylbenzidine (TMB), which produced a blue-colored oxidized TMB (ox-TMB) with an absorption peak at 652~nm upon oxidation. While all Fe-doped MnO$_x$ nanostructures exhibited oxidase-like activity, the 10\% Fe-doped sample (10Fe-MnO$_x$) demonstrated the highest performance, likely due to a synergistic effect of structure, morphology, and the presence of oxygen vacancies. The underlying oxidase mechanism was investigated using steady-state kinetics and electron paramagnetic resonance (EPR) analysis. In addition, a colorimetric assay was developed for the detection of Hg$^{2+}$ and hydroquinone (HQ) in real water samples collected from industrial and natural sources. The calculated detection limits of the 10Fe-MnO$_x$ colorimetric probe for HQ (1.74~$\mu$M) and Hg$^{2+}$ (0.47~$\mu$M) outperformed those of conventional metal oxide-based nanozymes. These findings pave the way for the development of easily synthesizable, scalable, and highly sensitive oxidase-based MOF-derived metal oxide nanomaterials with significant potential in biological and environmental applications.

cond-mat.mes-hall↗

arXiv2025

Design principles for metal-organic receptors targeting optical recognition of Pd(II) in environmental matrices

A precise detection of palladium (Pd) ions is a critical challenge with significant socio-economic implications across various industrial and chemical sectors. Due to its widespread use and poor biodegradability, Pd2+ accumulates in environmental ecosystems, posing severe risks to both the environment and living organisms. Consequently, there is a strong demand for selective, sensitive, and user-friendly detection methods. Among emerging strategies, optical detection techniques (both luminescent and colorimetric) using metal-based receptors have gained considerable attention. These sensors offer distinct advantages over traditional organic probes, including large Stokes shifts, long emission lifetimes, exceptional photostability, enhanced water solubility, recyclability, and remarkable chemical versatility. These attributes make them highly suitable for diverse applications in sensing and bioanalytical fields. This review provides a comprehensive overview of recent advancements in luminescent and colorimetric metal-based probes, including metal complexes and metal-organic frameworks (MOFs), for the selective detection of Pd2+. It discusses key design strategies, critical performance factors, and future prospects, offering valuable insights for researchers working on next-generation sensing platform.

cond-mat.mtrl-sci↗

arXiv2025

Nanoparticles and Quantum Dots as Emerging Optical Sensing Platforms for $\mathrm{Ni}^{2+}$ Detection: Recent Approaches and Perspectives

Over the preceding years, nickel (Ni) and its compounds have been increasingly employed in various aspects of human social life, metallurgical/industrial manufactures, healthcare, and chemical processes. Although Ni is considered an essential trace element in biological systems, excessive intake or metabolic deficiency of $\mathrm{Ni}^{2+}$ ions may cause detrimental health effects to living organisms. Therefore, a facile and accurate detection of $\mathrm{Ni}^{2+}$, especially in environmental and biological settings, is of huge significance. As an efficient detection method, assaying $\mathrm{Ni}^{2+}$ using optical (colorimetric and/or fluorogenic) sensors has experienced quite a vigorous growth period, with a large number of excellent research contributions. Nanomaterial-based optical sensors, including metal nanoparticles (MNPs), quantum dots (QDs), and carbon dots (CDs), offer distinct advantages over conventional small-molecule organic and inorganic sensors. This study mainly provides an overview of the recent advancements and challenges related to the design strategies of various optical nanosensors to selectively detect the $\mathrm{Ni}^{2+}$ ion. Emphasis has also been placed on comparing the sensing performance of various nanosensors, along with exploring future perspectives.

physics.app-ph↗

arXiv2025

A Pyridyl-Benzimidazole Based Ruthenium(II) Complex as Optical Sensor: Targeted Cyanide Detection and Live Cell Imaging Applications

The extreme toxicity of cyanide (\ce{CN^-}) ions in diverse environmental media has garnered significant attention toward the design of well-organized molecular probes for their selective and sensitive detection. In this context, we present a monometallic Ru(II) complex (Ru-1), based on the 2-(pyridin-2-yl)-1H-benzo[d]imidazole moiety, acting as a highly selective luminescent probe for \ce{CN^-} recognition in pure water. Additionally, Ru-1 also functions as an efficient sensor for \ce{F^-}, \ce{AcO^-}, and \ce{H2PO4^-} ions, along with \ce{CN^-}, when acetonitrile is used as the solvent system. The binding constant ($K_b$) and detection limit (LoD) for \ce{CN^-} were determined to be $3.05 \times 10^6$~M$^{-1}$ and 12.8~nM, respectively, in water. The close proximity of the N--H site to the Ru(II) center, along with its notable acidity, were identified as the primary factors responsible for the high selectivity of Ru-1 toward \ce{CN^-} in aqueous media. Job's plots and density functional theory (DFT) analyses were conducted to support the anion binding mechanism. Furthermore, time-resolved fluorescence (TRF) spectroscopy was employed to evaluate the \ce{CN^-}-induced emission lifetime change of Ru-1 in water. To explore practical applicability, the Ru-1 probe was developed into paper-based strips capable of detecting \ce{CN^-} ions in the millimolar range via the naked eye under 365~nm UV illumination. It was also effectively applied for the detection of \ce{CN^-} in human breast cancer MCF-7 cell lines and natural food sources, such as apple seeds and sprouting potatoes.

cond-mat.mtrl-sci↗

arXiv2023

Lattice dynamics in the FeSi-based family of superconductors

The lattice dynamics of the superconducting materials LaFeSiH and LaFeSiO as well as their intermetallic precursor LaFeSi are investigated by polarized Raman spectroscopy and first-principles calculations, together with X-ray and advanced electron diffraction techniques for their structural analysis. We find that the Fe-dominated Raman-active modes reflect the chemical peculiarities of these silicides compared to their pnictide counterparts, with enhanced structural couplings between the FeSi layer and the spacer that can be related to the ionic vs covalent character of the latter. In addition, we find signatures of enhanced electron-phonon coupling for some of the Raman-active modes. Beyond that, our study reveals intriguing Fe-based Raman features as well as structural subtleties in LaFeSiH suggesting that this superconductor may formally be non-centrosymmetric.

cond-mat.supr-con↗

arXiv2022

Hidden Granular Superconductivity Above 500K in off-the-shelf graphite materials

It has been reported that graphite hosts room temperature superconductivity. Here we provide new results that confirm these claims on different samples of highly oriented pyrolytic graphite (HOPG) and commercial flexible graphite gaskets (FGG). After subtraction of the intrinsic graphite diamagnetism, magnetization measurements show convoluted ferromagnetism and superconducting-like hysteresis loops. The ferromagnetism is deconvoluted by fitting with a sigmoidal function and subtracting it from the data. The obtained superconducting-like hysteresis loops are followed to the highest available temperature, 400K. The extrapolation of the decrease of its moment width with temperature indicates a transition temperature T$_{c}$$\sim$ 550K$\pm$50K for all samples. Electrical resistance measurements confirm the existence at these temperatures of a transition in HOPG samples, albeit without percolation. Besides, the FGG show transitions at temperatures (70K, 270K) near to those reported previously on intercalated-deintercalated graphite, confirming the general character of these superconducting transitions. These results are the first steps in the unveiling of the above room temperature superconductivity of graphite.

cond-mat.supr-con↗

arXiv2022

Possible high temperature superconducting transitions in disordered graphite obtained from room temperature deintercalated KC$_8$

Although progress with twisted graphene nano-devices is boosting the superconductivity that is the consequence of their Moir\'e flat electronic bands, the immense choice for future development is an obstacle for their optimisation. We report here that soft-chemistry deintercalation of KC$_8$ breaks down graphite stacking generating a strong disorder that includes stacking twists and variable local doping. We obtain a bulk graphite whose individual crystallites have different stackings with arbitrary twists and doping, scanning in the same sample a huge number of stacking configurations. We perform magnetisation measurements on batches with different synthesis conditions. The disorder weakens the huge diamagnetism of graphite, revealing several phase transitions. A "ferromagnetic-like" magnetisation appears with Curie temperatures T$_0$$\sim$450K, that has to be subtracted from the measured magnetisation. Depending on sample synthesis, anomalies towards diamagnetic states appear at T$_c$$\sim$110K (3 samples), $\sim$240K (4 samples), $\sim$320K (2 samples). Electrical resistivity measurements yield anomalies for the T$_c\sim$240K transition, with one sample showing a 90% drop. We discuss the possibility that these (diamagnetic and resistitive) anomalies could be due to superconductivity.

cond-mat.supr-con↗

arXiv2018

Pressure-driven collapse of the relativistic electronic ground state in a honeycomb iridate

The electronic ground state in many iridate materials is described by a complex wave-function in which spin and orbital angular momenta are entangled due to relativistic spin-orbit coupling (SOC). Such a localized electronic state carries an effective total angular momentum of $J_{eff}=1/2$. In materials with an edge-sharing octahedral crystal structure, such as the honeycomb iridates Li2IrO3 and Na2IrO3, these $J_{eff}=1/2$ moments are expected to be coupled through a special bond-dependent magnetic interaction, which is a necessary condition for the realization of a Kitaev quantum spin liquid. However, this relativistic electron picture is challenged by an alternate description, in which itinerant electrons are confined to a benzene-like hexagon, keeping the system insulating despite the delocalized nature of the electrons. In this quasi-molecular orbital (QMO) picture, the honeycomb iridates are an unlikely choice for a Kitaev spin liquid. Here we show that the honeycomb iridate Li2IrO3 is best described by a $J_{eff}=1/2$ state at ambient pressure, but crosses over into a QMO state under the application of small (~ 0.1 GPa) hydrostatic pressure. This result illustrates that the physics of iridates is extremely rich due to a delicate balance between electronic bandwidth, spin-orbit coupling, crystal field, and electron correlation.

cond-mat.str-el↗

arXiv2017

Pressure induced spin crossover in disordered {\alpha}-LiFeO2

Structural, magnetic and electrical-transport properties of {\alpha}-LiFeO2, crystallizing in the rock salt structure with random distribution of Li and Fe ions, have been studied by synchrotron X-ray diffraction, 57Fe M\"ossbauer spectroscopy and electrical resistance measurements at pressures up to 100 GPa using diamond anvil cells. It was found that the crystal structure is stable at least to 82 GPa, though a significant change in compressibility has been observed above 50 GPa. The changes in the structural properties are found to be on a par with a sluggish Fe3+ high- to low-spin (HS-LS) transition (S=5/2 to S=1/2) starting at 50 GPa and not completed even at ~100 GPa. The HS-LS transition is accompanied by an appreciable resistance decrease remaining a semiconductor up to 115 GPa and is not expected to be metallic even at about 200 GPa. The observed feature of the pressure-induced HS-LS transition is not an ordinary behavior of ferric oxides at high pressures. The effect of Fe3+ nearest and next nearest neighbors on the features of the spin crossover is discussed.

cond-mat.str-el↗

arXiv2017

Superconductor-insulator transition in fcc-GeSb2Te4 at elevated pressures

We show that polycrystalline GeSb2Te4 in the fcc phase (f-GST), which is an insulator at low temperature at ambient pressure, becomes a superconductor at elevated pressures. Our study of the superconductor to insulator transition versus pressure at low temperatures reveals a second order quantum phase transition with linear scaling (critical exponent close to unity) of the transition temperature with the pressure above the critical zero-temperature pressure. In addition, we demonstrate that at higher pressures the f-GST goes through a structural phase transition via amorphization to bcc GST (b-GST), which also become superconducting. We also find that the pressure regime where an inhomogeneous mixture of amorphous and b-GST exists, there is an anomalous peak in magnetoresistance, and suggest an explanation for this anomaly.

cond-mat.supr-con↗

arXiv2016

Superconductivity in multiple phases of compressed GeSb2Te4

Here we report the discovery of superconductivity in multiple phases of the compressed GeSb2Te4 (GST) phase change memory alloy, which has attracted considerable attention for the last decade due to its unusual physical properties with many potential applications. Superconductivity is observed through electrical transport measurements, both for the amorphous (a-GST) and for the crystalline (c-GST) phases. The superconducting critical temperature, TC, continuously increases with the applied pressure reaching a maximum Tc =6K at P=20 GPa for a-GST, whereas the critical temperature of the cubic phase reaches a maximum Tc =8 K at 30 GPa. This new material system, exhibiting a superconductor-insulator quantum phase transition (SIT) has an advantage over disordered metals since it has a continuous control of the crystal structure and the electronic properties using pressure as an external stimulus, which was lacking in SIT studies until today.

cond-mat.supr-con↗

arXiv2015

M\"ossbauer and Magnetic Studies of Surfactant Mediated Ca-Mg Doped Ferrihydrite Nanoparticles

Ultrafine (2-5 nm) particles of amorphous Ca-Mg co-doped ferrihydrite have been synthesized by surfactant mediated co-precipitation method. The evolution of the amorphous ferrihydrite by Ca-Mg co-doping is quite different from our earlier investigations on individual Ca and Mg doping studies. Amorphous phase of ferrihydrite for the present study has been confirmed by X-ray diffraction (XRD) and M\"ossbauer spectroscopy at room temperature and low temperatures (40 K and 20 K). Hematite nanoparticles with crystallite size about 8, 38 and 70 nm were obtained after annealing the as-prepared samples at 400, 600 and 800$^0$C respectively in air atmosphere. Superparamagnetism has been found in 8 nm sized hematite nanoparticles which has been confirmed from the magnetic hysteresis loop with zero remanent magnetization and coercive field and also from the superparamagnetic doublet of its room temperature M\"ossbauer spectrum. The magnetic properties of the 38 and 70 nm sized particles have been studied by room temperature magnetic hysteresis loop measurements and M\"ossbauer spectroscopy. The coercive field in these hematite nanoparticles increases with increasing particle size. Small amount of spinel MgFe$_{2}$O$_{4}$ phase has been detected in the 800$^0$C annealed sample.

cond-mat.mtrl-sci↗

arXiv2015

Magnetic and Dielectric Properties of Multiferroic BiFeO3 Nanoparticles Synthesized by a Novel Citrate Combustion Method

Single phase BiFeO3 nanoparticles have been successfully synthesized for the first time by a novel citrate combustion method without using any solvent. Well mixed metal nitrates along with citric acid which is used as fuel combust to give BiFeO3 nanoparticles after annealing. These particles are single phase in nature and crystallize in the rhombohedral distorted perovskite structure (space group-R3c) which has been confirmed by the Rietveld refinement of the room temperature powder x-ray diffraction data. Nearly spherical particles of average particle size 47 nm have been seen from transmission electron micrograph. Room temperature magnetic hysteresis measurement shows weak ferromagnetism though the magnetization does not saturate upto 1.75 T applied field. The coercive field value is calculated to be 180 Oe which is 3 times higher than that prepared by solvent free combustion method using Glycine. 57Fe M\"ossbauer spectrum can be fitted with a sextet corresponding to single magnetic state of hyperfine field about 49.5 T corresponding to Fe3+ state of the iron atom. The dielectric relaxation and ac conductivity as a function of frequency have been discussed. High dielectric permittivity has not been found in these nanoparticles like other reported BiFeO3 ceramics.

cond-mat.mtrl-sci↗

arXiv2015

Preparation, structural and magnetic studies on BiFe1-xCrxO3 (x=0.0, 0.05 and 0.1) multiferroic nanoparticles

BiFe1-xCrxO3 (x=0.0, 0.05 and 0.1) nanoparticles are prepared by a combustion method without using any solvent. All the synthesized nanoparticles are single phase in nature, nearly spherical in shape and crystallize in distorted perovskite structure and space group R3c with an average crystallite size of the order of 40 nm. The room temperature magnetization observed in BiFeO3 nanoparticles is larger than that in the bulk. Increasing Cr doping leads to increase in the magnetization and coercivity. Strong superexchange interaction between Fe3+ and Cr3+ atoms is likely to give rise to such increase in magnetization with Cr-doping. M\"ossbauer data of these nanoparticles show ordered magnetic state in which Fe atoms are in 3+ oxidation states.

cond-mat.mtrl-sci↗

arXiv2009

Valence fluctuation in Ce$_{2}$Co$_{3}$Ge$_{5}$ and crystal field effect in Pr$_{2}$Co$_{3}$Ge$_{5}$

Polycrystalline samples of ternary rare earth germanides R$_{2}$Co$_{3}$Ge$_{5}$(R = La, Ce and Pr) have been prepared and investigated by means of magnetic susceptibility, isothermal magnetization, electrical resistivity and specific heat measurements. All these compounds crystallize in orthorhombic U$_{2}$Co$_{3}$Si$_{5}$ structure (space group \textit{Ibam}). No evidence of magnetic or superconducting transition is observed in any of these compounds down to 2 K. The unit cell volume of Ce$_{2}$Co$_{3}$Ge$_{5}$ deviates from the expected lanthanide contraction, indicating non trivalent state of Ce ions in this compound. The reduced value of effective moment ($\mu_{eff}$ $\approx$ 0.95 $\mu_{B}$) compared to that expected for trivalent Ce ions further supports valence fluctuating nature of Ce in Ce$_{2}$Co$_{3}$Ge$_{5}$. The observed temperature dependence of magnetic susceptibility is consistent with ionic interconfiguration fluctuation (ICF) model. Although no sharp anomaly due to a phase transition is seen, a broad Schottky-type anomaly is observed in the magnetic part of specific heat of Pr$_{2}$Co$_{3}$Ge$_{5}$. An analysis of $C_{mag}$ data suggests a singlet ground state in Pr$_{2}$Co$_{3}$Ge$_{5}$ separated from the singlet first excited state by 22 K and a doublet second excited state at 73 K.

cond-mat.mtrl-sci↗