arXiv subjects

Arumugam Sonachalam

Publications and source records attributed to Arumugam Sonachalam.

4 recordsLinked to original sources

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↗

arXiv2024

Pressure-induced enhancement of superconductivity in a non-centrosymmetric compound LaPtGe

We report a pressure-induced enhancement of the superconducting transition temperature (Tc) in a non-centrosymmetric (NCS) compound, LaPtGe. With pressure, till 3 GPa, we observed a modest enhancement of the Tc with a rate of 0.071 K/GPa. However, above this pressure, the rate showed a ~2.5 times increase, 0.183 K/GPa. We observed a Tc of 3.94 K at 6 GPa, the highest pressure value used in our transport study. Synchrotron high-pressure x-ray powder diffraction (HP-XRPD) measurements do not reveal any structural phase transition in this system in this pressure range. However, it showed an apparent change of slope in the pressure dependence of lattice parameters above and below 3 GPa. Pressure dependence of the unit-cell volume also followed a distinct trend below and above 3 GPa, with the Birch-Murnaghan equation of state fit providing a bulk modulus (B0) value of ~144 and ~162 GPa, respectively, for two pressure regions. Further, the magnetotransport measurement under pressure up to 2.45 GPa reveals the enhancement of the upper critical field (Hc2(0)) from 0.7 T (0 GPa) to 0.92 T (2.45 GPa). In addition, the upward curvature in Hc2(T) becomes stronger with increasing pressure, suggesting a change of the underlying Fermi surface topology with pressure. The signature for the inducible Hc2(T) with pressure, the distinct enhancement of Tc around 3 GPa, and the noticeable change in lattice parameters around 3 GPa suggests the possibility of multi-gap superconductivity in LaPtGe similar to identical structure NCS compound, LaPtSi. The enhancement of Tc by pressure can be correlated with the possible underlying lattice modulation by compression and the change in the density of states at the Fermi level. Also, the bare change in pressure-dependent activation energy U0 up to 2.45 GPa calculated using the Arrhenius relation clearly shows that the shift in Tc does not arise from grain boundaries.

cond-mat.supr-con↗

arXiv2024

Hydrostatic pressure effect on structural and transport properties of co-existing layered and disordered rock-salt phase of LixCoO2

It is widely believed that the origin of a significant cause for the voltage and capacity fading observed in lithium (Li)-ion batteries is related to structural modifications occurring in the cathode material during the Li-ion insertion/de-insertion process. The Li-ion insertion/de-insertion mechanism and the resulting structural changes are known to exert a severe strain on the lattice, and consequently leading to performance degradation. Here, with a view to shed more light on the effect of such strain on the structural properties of the cathode material, we have systematically investigated the pressure dependence of structural and transport properties of an LixCoO2 single crystal, grown using 5% excess Li in the precursors. Ambient pressure synchrotron diffraction on these crystals reveals that, the excess Li during the growth, has facilitated the stabilization of a layered rhombohedral phase (space group R3m) as well as a disordered rock-salt phase (space group Fm3m). The volume fraction of the rhombohedral and cubic phase is 60:40, respectively, which remains unchanged up to 10.6 GPa. No structural phase transition has been observed up to 10.6 GPa. An increase in resistance with a decrease in temperature has revealed the semi-metallic nature of the sample. Further, the application of hydrostatic pressure up to 2.8 GPa shows the enhancement of semi-metallic nature. The obtained experimental results can be qualitatively explained via density functional theory (DFT) and thermodynamics modelling. The calculated density of states was reduced, and the activation energy was increased by applied pressure. Our investigations indicate a significant phase stability of the mixed phase crystals under externally applied high pressure and thus suggest the possible use of such mixed phase materials as a cathode in lithium-ion batteries.

cond-mat.mtrl-sci↗

arXiv2022

Superconducting and structural properties of non-centrosymmetric Re6Hf superconductor under high pressure

We report the effect of high pressure on the superconducting, vortex pinning, and structural properties of a polycrystalline non-centrosymmetric superconductor Re6Hf. The superconducting transition temperature, Tc, reveals a modest decrease as pressure P increases with a slope -0.046 K/GPa (-0.065 K/GPa) estimated from resistivity measurements up to 8 GPa (magnetization measurement ~ 1.1 GPa). Structural analysis up to ~18 GPa reveals monotonic decreases of lattice constant without undergoing any structural transition and a high value of bulk modulus B0= 333.63 GPa, indicating the stability of the structure. Furthermore, the upper critical field and lower critical field at absolute temperature (Hc2(0) & Hc1(0)) decreases slightly from the ambient pressure value as pressure increases up to 2.5 GPa. In addition, up to P ~ 2.5 GPa using thermally activated flux flow of vortices revealed a double linearity field dependence of activation energy of vortices, confirming the coexistence of single and collective pinning vortex states. Moreover, analysis of critical current density using the collective pinning theory showed the transformation of δTc to δl pinning as pressure increases, possibly due to migration of grain boundaries. Besides, the band structure calculations using density functional theory show that density of states decreases modestly with pressure, which may be a possible reason for such a small decrease in Tc by pressure.

cond-mat.supr-con↗