arXiv subjects

N. K. Karn

Publications and source records attributed to N. K. Karn.

At least 19 recordsLinked to original sources

arXiv2026

Tunable Magnetic Frustration in the Cu-Ru-based Double Perovskite La$_{2-x}$Sm$_x$CuRuO$_6$ (x = 0, 1, 2) Oxides

In this study, we investigate structural, magnetic, and electronic properties of the copper-ruthenate based oxide double perovskite La$_{2-x}$Sm$_x$CuRuO$_6$ (x = 0, 1, 2), synthesized through the solid-state reaction method. X-ray diffraction analysis reveals that all compounds crystallize in a monoclinic symmetry, with varying degree of structural distortion that increases in moving from La$^{3+}$ to smaller size cation Sm$^{3+}$. Electrical resistivity studies indicate insulating behaviour in all compounds, with variable-range-hopping domination at low temperatures, due to presence of anti-site disorder. AC susceptibility and heat capacity measurements suggest suppression of frustration in Sm-bearing compounds, affecting the magnetic behavior. Our first-principles calculations suggest that the combined effects of lattice distortion and Sm magnetism play a crucial role in weakening magnetic frustration, thereby rationalizing the experimental observations. These findings shed light on the complex interplay of crystal structure and magnetism in Cu-Ru double perovskites, and open up an avenue for tuning of magnetic properties through rare-earth-ion substitution.

cond-mat.mtrl-sci↗

arXiv2026

Field driven Metal-Insulator transition in rhombohedral Bismuth and Arsenic crystals

The metal to insulator (MIT) transition is accompanied by huge changes in physical responses by the control and tuning of experimental parameters like doping, pressure, chemical composition, and magnetic field. Here, we study the magnetic field-driven MIT for two pnictides in their elemental form, namely Arsenic and Bismuth. At low temperatures, Bismuth shows an unusual behaviour of a re-entrant IMT at high fields in addition to a higher temperature MIT at smaller fields. However, Arsenic shows the commonly observed single MIT. The Shubnikov de Haas (SdH) oscillations are observed for both As and Bi below 10 K. Giant magneto-resistance of the order of ~105 (MR%) is observed for both crystals at 2 K and 14 Tesla transverse magnetic field. The unusual Kohler scaling behaviour of MR at low temperature indicate the presence of increased carrier density attributed to the melting of excitons. Based on a microscopic model, the microscopic processes underpinning the unusual features of a field-driven MIT and re-entrant IMT, along with the relevance of both excitonic and Bose metal correlations near these incipient instabilities, are qualitatively described in the framework of field-driven excitonic condensate and Das-Doniach preformed pair scenarios in one single picture.

cond-mat.mtrl-sci↗

arXiv2025

Crystal Growth & Physical Property Characterization of Mixed Topological Insulator BiSbTe$_3$

This article reports the synthesis of a single crystalline mixed topological insulator (TI) BiSbTe$_3$ and its detailed structural and magneto-transport properties. The single crystalline samples of BiSbTe$_3$ are grown by the melt-growth process and characterized by X-ray diffraction (XRD), Energy dispersive X-ray analysis (EDAX) and Raman spectroscopy. The single crystal XRD peaks dictated the growth direction along the c-axis. The Raman spectrum elucidated the characteristic peaks of the mixed topological insulator. The broadening of Raman peaks exhibited the formation of Te-Bi-Te and Te-Sb-Te bonds and associated vibrational modes. The single crystals are characterized by magneto-transport measurements down to 2 K and up to 14 Tesla transverse magnetic field. The residual resistance ratio (R200 K/R0 K) is found to be 3.64, which endorses the metallic nature of the synthesized crystal. The relative resistance turns out to be higher for the mixed TI than the pure TIs i.e., Bi$_2$Te$_3$ or Sb$_2$Te$_3$. The lower Debye temperature (82.64 K) of BiSbTe$_3$ connotes the presence of effective electron-phonon interaction at quite low temperatures in comparison to pure TI, which explains the observed suppression in magnetoresistance (MR) for the mixed TI. At 2 K, an MR of 150 percent is observed for BiSbTe$_3$, which is suppressed in contrast to the pure TIs i.e., Bi$_2$Te$_3$ or Sb$_2$Te$_3$. Though the MR% is suppressed significantly, its non-saturating linear behavior indicates the topological nature of the studied mixed TI. The modified Hikami-Larkin-Nagaoka (HLN) equation analysis of magneto-conductivity of mixed TI revealed that the conductivity has not only a surface states driven 2D component but also contributions from the bulk charge carriers and quantum scattering.

cond-mat.mtrl-sci↗

arXiv2025

Magneto transport and first principle study of strong topological insulator gray Arsenic

This article reports the synthesis of a single crystalline gray Arsenic (As) via the Bismuth flux method. The X-ray Diffraction (XRD) pattern revealed the single phase of the grown crystal, which crystallized in the rhombohedral structure with the space group R3m. The sharp XRD peaks observed on mechanically exfoliated thin flakes of the same ensured high crystallinity of the same with growth direction along the c-axis. The resistivity measurements illustrated its metallic nature throughout, right from 300K down to 2K. The measured residual resistivity ratio of the sample is 180, which endorses the high metallic nature of the as-synthesized As single crystal. The transverse magnetic field-dependent resistivity (RH) measurements elucidated huge magneto-resistance (MR) at 2K and 14Tesla transverse magnetic fields. Also seen are the SDH oscillations, indicating the presence of topological surface states. The non-trivial band topology and edge states in As are confirmed by first principle calculations.

cond-mat.mtrl-sci↗

arXiv2024

Normal state magneto transport properties of FeSe$_{0.5}$Te$_{0.5}$ superconductor: The role of topological surface states

Doped Iron Chalcogenide (FeCh) superconductors are extensively studied in the context of topological superconductivity. However, the evidence of topological surface states in electrical transport measurements of the doped FeCh system is yet warranted. In the present letter, we performed angle-dependent magneto transport measurements on a single crystal of a doped FeCh system, i.e., FeSe$_{0.5}$Te$_{0.5}$. A non-saturating linear magnetoresistance (MR) has been observed under the magnetic field up to 14 T in the normal state of FeSe$_{0.5}$Te$_{0.5}$. The MR is shown to possess anisotropy, which indicates the presence of topological surface states in FeSe$_{0.5}$Te$_{0.5}$. Angle-dependent Magneto-conductivity (MC) at low magnetic fields has been modelled by Hikami Larkin Nagaoka (HLN) formalism, which shows the presence of weak antilocalization (WAL) effect in FeSe$_{0.5}$Te$_{0.5}$. The observed WAL effect is found to be 2D in nature through angle-dependent magneto transport measurements. Theoretical calculations based on Density Functional Theory (DFT) are also performed to get more confidence on the presence of topological surface states in FeSe$_{0.5}$Te$_{0.5}$.

cond-mat.supr-con↗

arXiv2024

Investigation of Angle dependent SdH oscillations in Topological Insulator Bismuth

The current article investigated the band structure in the presence and absence of spin-orbit coupling (SOC), examined the Z2 invariants, and investigated the detailed angle-dependent magneto-transport of up to 10 T (Tesla) and down to 2 K for the Bismuth crystal. The out-of-plane field-dependent magnetoresistance (MR) is positive and is huge to the order of ~104% at 2 K and 10 T. On the other hand, the longitudinal (in-plane) field-dependent MR is relatively small and is negative. The thermal activation energy is also estimated by using the Boltzmann formula from resistivity vs temperature measurement under applied transverse magnetic fields. The topological nature of Bi is confirmed by Z2 invariant calculation using Density functional theory. PBESol bands show trivial but Hybrid functional (HSE) bands show non-trivial topology being present in Bismuth. This article comprehensively studies the dependence of MR oscillations upon the angle between the applied field and the current. The observed oscillations fade away as the angle is increased. This article is an extension of our previous work on Bismuth [1], in which we conducted a comprehensive analysis of its structural and micro-structural properties along with its transport behavior in an applied transverse magnetic field.

cond-mat.mtrl-sci↗

arXiv2024

Superconductivity at 9 K in Pb-Bi Alloy

In the present work, we report the synthesis of Pb-Bi alloy with enhanced Tc of up to 9K, which is higher than that of Pb. The alloy is synthesized via a solid-state reaction route in the vacuum-encapsulated quartz tube at 7000C in an automated furnace. The synthesized sample is characterized by X-ray Diffraction(XRD) and Energy dispersive X-ray analysis(EDAX) for its phase purity and elemental composition. Rietveld refinement of XRD reveals that the end product is a majority hexagonal Pb7Bi3, with minor rhombohedral Bi. The electronic transport measurement shows metallic behavior with the Debye temperature of 108K and a superconductivity transition temperature (Tc) below 9K, which is the maximum to date for any reported Pb-Bi alloy, Pb or Bi at ambient pressure. Partial substitution of Bi at the Pb site may modify the free density of electronic states within the BCS model to attain the optimum Tc, which is higher by around 2K from the reported Tc of Pb. The superconductor phase diagram derived from magneto-transport measurements reveals that the synthesized alloy is a conventional superconductor with an upper critical field (Hc2) of 3.9 Tesla, which lies well within the Pauli paramagnetic limit. The magnetization measurements carried out following ZFC(Zero Field Cool) protocols infer that the synthesized alloy is a bulk superconductor below 9K. The isothermal M-H(Magnetization vs. Field) measurements performed below Tc establish it as a type-II superconductor. The specific heat capacity measurements show that the Pb-Bi alloy is a strongly coupled bulk superconductor below around 9K with possibly two superconducting gaps.

cond-mat.supr-con↗

arXiv2024

Anisotropy in Electronic and Magneto-transport of 2D superconductor NbSe$_2$

This article reports the successful synthesis of single crystalline two-dimensional thin flakes of NbSe$_2$. The XRD pattern of the grown crystal ensured its crystallization in a single phase with a hexagonal structure. The EDAX endorsed the stoichiometry of the as grown sample. To study the vibrational modes, the Raman spectra were recorded, which exhibited the expected four Raman active modes. The resistance vs temperature measurement showed a well-established superconducting transition (Tc) at 7.3 K. The ZFC (Zero-Field Cooled) & FC (Field Cooled) magnetization curves, as well as the isothermal MH (Magnetization vs. field) measurements, have been performed for both in plane and out-of-plane H directions. Distinct anisotropy is observed in both magnetization and magneto-transport measurements with field direction, leading to different critical fields (Hc). Out-of-plane magneto-transport data hints towards the existence of a filamentary state. The density functional theory (DFT) has been used to study the band structure of NbSe$_2$. Although the bulk band structure confirmed metallic behavior, the same for mono-layers of NbSe$_2$ within the GGA+U framework showed a band gap of 1.17eV. The article addresses the anisotropy in the electronic and magneto-transport of 2D superconductor NbSe$_2$.

cond-mat.supr-con↗

arXiv2023

Synthesis of possible room temperature superconductor LK-99:Pb$_9$Cu(PO$_4$)$_6$O

The quest for room-temperature superconductors has been teasing scientists and physicists, since its inception in 1911 itself. Several assertions have already been made about room temperature superconductivity but were never verified or reproduced across the labs. The cuprates were the earliest high transition temperature superconductors, and it seems that copper has done the magic once again. Last week, a Korean group synthesized a Lead Apatite-based compound LK-99, showing a T$_c$ of above 400$^\circ$K. The signatures of superconductivity in the compound are very promising, in terms of resistivity (R = 0) and diamagnetism at T$_c$. Although, the heat capacity (C$_p$) did not show the obvious transition at T$_c$. Inspired by the interesting claims of above room temperature superconductivity in LK-99, in this article, we report the synthesis of polycrystalline samples of LK-99, by following the same heat treatment as reported in [1,2] by the two-step precursor method. The phase is confirmed through X-ray diffraction (XRD) measurements, performed after each heat treatment. The room temperature diamagnetism is not evidenced by the levitation of a permanent magnet over the sample or vice versa. Further measurements for the confirmation of bulk superconductivity on variously synthesized samples are underway. Our results on the present LK-99 sample, being synthesized at 925$^\circ$C, as of now do not approve the appearance of bulk superconductivity at room temperature. Further studies with different heat treatments are though, yet underway.

cond-mat.supr-con↗

arXiv2023

Absence of superconductivity in LK-99 at ambient conditions

The report of synthesis of modified Lead apatite (LK-99) with evidence of superconductivity at more than boiling water temperature has steered the whole scientific community. There have been several failures to reproduce superconductivity in LK-99 including partial successes. Here, we have continued our efforts to synthesize phase pure LK-99 with improved precursors. The process has been followed as suggested by Sukbae Lee et. al., [1,2]. The phase purity of each precursor is evidenced by Powder X-ray diffraction (PXRD) and well fitted by Rietveld refinement. The PXRD confirms the synthesis of phase pure polycrystalline LK-99 with apatite structure. The freshly synthesized sample does not show any signature of superconductivity levitation on a magnet (diamagnetism). The magnetization measurements on SQUID also show that LK-99 is diamagnetic at 280 K, there is no sign of superconductivity in LK-99 at room temperature. Moreover, we have also performed first principle calculations to investigate the electronic band structure of the LK-99 near Fermi level. Our study verifies that the Cu doped lead apatite (LK-99) has bands crossing at Fermi level, indicating generation of strong correlation in the system.

cond-mat.supr-con↗

arXiv2023

Weak Antilocalization and topological edge states in PdSn$_4$

Here we report, the successful synthesis of single crystals of topological semimetal (TSM) candidate, PdSn$_4$ using the self-grown route. The synthesized crystal is well characterized through X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS). Detailed Rietveld analysis of the powder XRD pattern of PdSn$_4$ confirmed the same to crystallize in the Aea2 space group instead of reported Ccce. A large magnetoresistance (MR) along with Subnikov-de Haas oscillations have been observed in magnetotransport measurements at 2K. The presence of a weak antilocalization (WAL) effect in synthesized PdSn$_4$ crystal is confirmed and analyzed using Hikami Larkin Nagaoka (HLN) formalism, being applied on magnetoconductivity of the same at the low magnetic field. An extended Kohler rule is implemented on MR data, to determine the role of the scattering process and temperature-dependent carrier density on transport phenomenon in PdSn$_4$. Further, the non-trivial band topology and presence of edge states are shown through density functional theory (DFT) based theoretical calculations. All calculations are performed considering the Aea2 space group symmetry. The calculated Z2 invariants suggest the presence of weak topological insulating properties in PdSn$_4$. Clear evidence of topological edge states at the $Γ$ point is visible in calculated edge state spectra. This is the first report on PdSn$_4$, showing the presence of SdH oscillation in magneto transport measurement.

cond-mat.mtrl-sci↗

arXiv2022

Detailed structural and topological analysis of SnBi2Te4 single crystal

We report herein the successful synthesis of the topological material SnBi2Te4 in single-crystal form. Phase purity and unidirectional growth are evident from X-ray diffraction (XRD) patterns acquired from a powdered sample and a crystal flake. The crystalline morphology has also been visualized by acquiring a field-emission scanning electron microscope (FESEM) image. The crystal has been thoroughly characterized by means of Raman spectroscopy and X ray photoelectron spectroscopy (XPS) measurements. The topological properties of SnBi2Te4 have been probed through magneto-transport measurements. SnBi2Te4 has been found to exhibit a small but non-saturating magneto-resistance (MR) up to 12 T. The low-field magnetoconductivity (MC) of SnBi2Te4 at 2 K can be well explained through the Hikami Larkin Nagaoka (HLN) formalism, which confirms the presence of a weak anti-localization (WAL) effect in its crystal. Moreover, the non-trivial topological character has been evidenced through first-principles calculations using density functional theory (DFT), with and without spin-orbit coupling (SOC) protocols. A significant change in the bulk electronic band structure is observed upon the inclusion of SOC parameters, signifying the topological properties of SnBi2Te4. Its topological non-trivial character has also been verified through the calculation of Z2 invariants and the surface states spectrum in the (111) plane.

cond-mat.mtrl-sci↗

arXiv2022

Drastic suppression of CDW (charge density wave) by Pd addition in TiSe2

TiSe2 is a known Topological semimetal (TSM) having both the semi-metallic and topological characters simultaneously and the Charge density wave (CDW) at below 200K. In the current short article, we study the impact of Pd addition on the CDW character of TiSe2 and the possible induction of superconductivity at low temperatures. Bulk samples of TiSe2 and Pd0.1TiSe2 are synthesized by solid-state reaction route, which is further characterized by powder X-ray diffraction (PXRD) and field emission scanning electron microscopy (FESEM), respectively, for their structural and micro-structural details. The vibrational modes of both samples are being analyzed by Raman spectroscopy, showing the occurrence of both A1g and Eg modes. CDW of pure TiSe2 seen at around 200K in electrical transport measurements, in terms of sharp semi-metallic to metallic transition peak with hysteresis in cooling/warming cycles is not seen in Pd0.1TiSe2, and rather a near metallic transport is seen down to 2K. Although superconductivity is not seen down to 2K, the CDW transition is seemingly completely suppressed in Pd0.1TiSe2. Pd addition in TiSe2 suppresses CDW drastically. Trials are underway to induce superconductivity in Pd-added TiSe2. Density functional theory (DFT) calculations show primary evidence of suppression of CDW by adding Pd in TiSe2 due to an increase in the density of states.

cond-mat.mtrl-sci↗

arXiv2022

Shubnikov de Haas (SdH) Oscillation in Self-Flux Grown Rhombohedral Single Crystalline Bismuth

The historic de Haas van Alphen effect observed in the late 1950s in CSIR-NPL by J.S. Dhillon and D. Shoenberg in pure bismuth and zinc metal is revisited in this article hrough a single crystalline phase of bismuth crystal, which is observed in terms of resistivity as predicted by Shubnikov de Haas oscillations. The occurrence of oscillations in the transverse magnetic field confirms the presence of the Fermi surface. The single crystal of bismuth is grown through solid-state reaction under an optimized heat treatment whose purity and structural phase are confirmed through XRD, SEM, and EDAX.

cond-mat.mtrl-sci↗

arXiv2022

Topological Superconductor AuSn4: A first principle study

Topological semimetals such as Weyl or Dirac semimetals with superconductivity have emerged as a new class of topological materials to realize and study Majorana Fermion. This article reports the Density Functional Theory (DFT) calculated bulk electronic band structure of recently discovered topological superconductor candidate AuSn4. The study has been performed on AuSn4 considering two space groups symmetries viz. Aea2 and Ccce, as reported earlier. This study is further extended to the calculation of Z2 invariants. The Fermi surfaces corresponding to the bands, which are responsible for non-trivial band topology, along with the surface states, are also mapped. The complete study suggests that AuSn4 is a topological semimetal. On AuSn4, it is the first report in the literature showing the non-trivial band topology based on first-principle calculations.

cond-mat.supr-con↗

arXiv2022

Bulk superconductivity and non-trivial band topology analysis of Pb2Pd

A perfect topological superconductor can be realized by the simultaneous existence of bulk superconductivity along with topological non-trivial surface states. In this article, we report single crystal growth of superconducting binary compound Pb2Pd. The crystal is well characterized through X-Ray Diffraction (XRD), Selected Area electron diffraction (SAED), Transmission Electron Microscopy (TEM), Field emission Scanning electron Microscopy (FESEM) and X-Ray Photoelectron spectroscopy (XPS). The bulk superconducting nature of the studied crystal is determined through AC susceptibility and magneto-heat capacity measurements. The specific heat jump at superconducting transition suggests Pb2Pd to be a moderately coupled s-wave superconductor. The topological non-trivial character of Pb2Pd is evidenced through bulk electronic band structure and Z2 invariants, which are calculated under the protocols of Density Functional Theory (DFT). This is the first report on topological invariant of Pb2Pd, showing Pb2Pd to possess non-trivial topological band structure

cond-mat.supr-con↗

arXiv2022

A comprehensive review on topological superconducting materials and interfaces

Superconductivity in topological materials has drawn a significant interest of the scientific community as these materials provide a hint of the existence of Majorana fermions conceived from the quantized thermal conductivity, a zero-biased conduction peak and the anomalous Josephson effect. In this review, we make a systematic study of recent advances in the field of topological superconductivity. The article comprises of both bulk systems as well as heterostructures. A brief description of Majorana fermions and their relationship with topological superconductors and heterostructures is also carried out. Also, this review consists of details of key experimental techniques to characterize candidates of topological superconductivity. Moreover, we summarize the potential material candidate that may demonstrate topological superconductivity. We also consider some intrinsic odd-parity superconductors, non-centrosymmetric, centrosymmetric superconductors, doped topological insulators, doped topological crystalline insulators and some other materials that are expected to show superconductivity along with topological non-trivial states in bulk from. The effect of pressure, emergence of superconductivity in topological materials and Muon Spin Rotation studies are also summarized in this article.

cond-mat.supr-con↗

arXiv2021

SnAs: a 4K weak type-II superconductor with non-trivial band topology

Superconductors with non-trivial band topology are emerging as one of the best avenues to study quantum anomalies and experimental realization of Majorana Fermions. This article reports the successful crystal growth of superconducting SnAs, which can have topologically non-trivial states, as evidenced in DFT (Density Functional Theory) calculations, Z2 invariants, and topologically surface state. Here, we followed a two-step method to grow SnAs crystal. The powder XRD (X-ray Diffractometry) pattern of synthesized crystal ensures that the crystal is grown in a single phase with a NaCl type cubic structure and the EDAX (Energy Dispersive X-ray Analysis) endorses the stoichiometry of the as-grown sample. The DFT calculations performed with and without the inclusion of spin-orbit coupling (SOC) show band inversion at various K symmetry points near the Fermi level. The recorded Raman spectra show two different modes, which are assigned as A1 and ETO vibrations. The ZFC (Zero-Field Cooled) & FC (Field Cooled), as well as the isothermal M-H (Magnetization vs. field) measurements, are also performed for the topological non-trivial SnAs superconductor, which eventually confirms the weak type-II superconductivity at 4K. Various other superconductivity parameters viz. kappa parameter, coherence length, and penetration depth are also calculated to probe the as-grown sample's characteristics.

cond-mat.supr-con↗