arXiv subjects

Velaga Srihari

Publications and source records attributed to Velaga Srihari.

9 recordsLinked to original sources

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↗

arXiv2023

Significantly increased magnetic anisotropy in Co nano-columnar multilayer structure via a unique sequential oblique-normal deposition approach

Oblique/normal sequential deposition technique is used to create Co based unique multilayer structure [Co-oblique(4.4nm)/Co-normal (4.2 nm)]x10, where each Co-oblique layer is deposited at an oblique angle of 75deg, to induce large in-plane uniaxial magnetic anisotropy (UMA). Compared to the previous ripple, stress and oblique angle deposition (OAD) related studies on Cobalt in literature, one-order higher UMA with the easy axis of magnetization along the projection of the tilted nano-columns in the multilayer plane is observed. The multilayer retains magnetic anisotropy even after annealing at 450C. The in-plane UMA in this multilayer is found to be the combination of shape, and magneto-crystalline anisotropy (MCA) confirmed by the temperature-dependent grazing incidence small angle X-ray scattering (GISAXS), in situ reflection high energy electron diffraction (RHEED) and grazing incidence X-ray diffraction (GIXRD) measurements. The crystalline texturing of hcp Co in the multilayer minimizes spin-orbit coupling energy along the column direction, which couples with the shape anisotropy energies and results in preferential orientation of the easy magnetic axis along the projection of the columns in the multilayer plane. Reduction in UMA after annealing is attributed to diffusion/merging of columns and annihilating crystallographic texturing. The obtained one-order high UMA demonstrates the potential application of the unique structure engineering technique, which may have far-reaching advantages in magnetic thin films/multilayers and spintronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Effect of magnetic phase coexistence on spin-phonon coupling and magnetoelectric effect in polycrystalline Sm0.5Y0.5Fe0.58Mn0.42O3

The polycrystalline co-doped samples of Sm0.5Y0.5Fe0.58Mn0.42O3 were prepared by solid-state reaction route and its various physical properties with their correlations have been investigated. The dc magnetization measurements on the sample revealed a weak ferromagnetic (WFM) transition at TN=361 K that is followed by an incomplete spin reorientation (SR) transition at TSR1= 348 K. A first order magnetic transition (FOMT) around 292 K completes the spin reorientation transition and the material enters into a nearly collinear antiferromagnetic (AFM) state for T < 260 K. The compound exhibited magnetization reversal below the compensation temperature (Tcomp) = 92 K at low measured field of 100 Oe. At further low temperature below 71 K, the compound also exhibited Zero-field cooled memory effects confirming a reentrant spinglass state formation. Robust magnetodielectric (MD) magnetoelectric coupling has been established in the present material through field dependent dielectric and resistivity measurements. True ferroelectric transition with a considerable value of saturation polarization (= 0.06 micro C/cm2 at 15 K) have been found in the specimen below TFE= 108 K. We observed an intense spin-phonon coupling (SPC) across TSR and TN from the temperature dependent Raman spectroscopy and is responsible for the intrinsic magnetoelectric effect. This SPC also stabilizes the ferroelectric state below TFE in the material. The delicate interplay of the lattice (Phonons), charge and spins governs the observed features in the investigated physical properties of the material that makes the specimen a promising multifunctional material.

cond-mat.mtrl-sci↗

arXiv2020

Pressure controlled trimerization for switching of anomalous Hall effect in triangular antiferromagnet Mn$_3$Sn

Here, we present a detailed theoretical and experimental study on the pressure induced switching of anomalous Hall effect (AHE) in the triangular antiferromagnetic (AFM) compound Mn$_3$Sn. Our theoretical model suggests pressure driven significant splitting of the in-plane Mn bond lengths $i.e.$ an effective trimerization, which in turn stabilizes a helical AFM ground state by modifying the inter-plane exchange parameters in the system. We experimentally demonstrate that the AHE in Mn$_3$Sn reduces from 5$μΩ$ cm at ambient pressure to zero at an applied pressure of about 1.5 GPa. Furthermore, our pressure dependent magnetization study reveals that the conventional triangular AFM ground state of Mn$_3$Sn systematically transforms into the helical AFM phase where the symmetry does not support a non-vanishing Berry curvature required for the realization of a finite AHE. The pressure dependent x-ray diffraction (XRD) study rules out any role of structural phase transition in the observed phenomenon. In addition, the temperature dependent in-plane lattice parameter at ambient pressure is found to deviate from the monotonic behavior when the system enters into the helical AFM phase, thereby, supporting the proposed impact of trimerization in controlling the AHE. We believe that the present study makes an important contribution towards understanding the stabilization mechanism of different magnetic ground states in Mn$_3$Sn and related materials for their potential applications pertaining to AHE switching.

cond-mat.mtrl-sci↗

arXiv2018

Structural, dielectric and ferroelectric studies of thermally stable and efficient energy storage ceramic material: (Na0.5-xKxBi0.5-xLax)TiO3

The structural, dielectric and ferroelectric properties of lead-free (Na0.5-xKxBi0.5-xLax)TiO3 powders synthesized by sol-gel self-combustion method were investigated. Rietveld refinement of Synchrotron x-ray diffraction data confirms pure single phase rhombohedral crystal structure with R3c space group for all the compositions and anti-phase octahedral tilting angle decreased with increase in composition x. Homogeneity and elemental proportions were confirmed by Energy dispersive x-ray spectrometry. The temperature-dependent dielectric study has shown two diffuse type of dielectric anomaly for all the samples, due to A-site disorder in the lattice, which has been assigned to two-has transitions: ferroelectric to anti-ferroelectric and anti-ferroelectric to the paraelectric phase transition. The transition temperature of these phase transitions is found to decrease as a function of composition. Thermal stability range of dielectric constant increases from ~100C to 220C as a function of composition. Stable dielectric constant first increases, from 1557 10 % for parent compound, with the composition, highest for 6 % composition with emid ~ 2508 10 % for the temperature range ~180 C to 340C and after that decreases to 1608 10 % for 12 % but remain higher than the parent compound Na0.5Bi0.5TiO3. Ferroelectric measurements have shown monotonously decreasing coercive field as a function of the composition due to a decrease in grain size, confirmed by microstructural studies using Field Emission Scanning Electron Microscope. Exponential increases in the energy storage efficiency from ~ 17 % to 87 % as a function of composition have also observed. These types of materials, with stable high dielectric constant and low tan delta, have a vast scope in the field of the thermally stable dielectric constant materials and energy storage applications.

cond-mat.mtrl-sci↗

arXiv2018

(Pb1-xBix)(Ti1-xMnx)O3: Competing mechanism of Tetragonal-Cubic phase on A/B site modifications

Structural, vibrational and dielectric properties of (Pb1-xBix)(Ti1-xMnx)O3 (PBTM) (0 x 0.50) polycrystalline ceramics have been examined as a function of temperature. Synchrotron-based powder x-ray diffraction was employed to confirm phase purity and crystal structure of samples. Tetragonality (c/a ratio) of the PBTM system exhibit an increase 1.065, 1.066 for x = 0.06 and 0.09 compositions respectively, compare to 1.064 for x = 0 sample. Curie point was found ~763 K and 773 K for x = 0.06 and 0.09 samples respectively. Though, tetragonality start to decreasing from samples with x 0.18, Curie point of the samples started decreasing for x 0.12. Temperature-dependent x-ray diffraction reveals the structural change from tetragonal to cubic structure. Unit cell volume was found to decrease with increasing temperature; indicate these materials are negative thermal expansion type until phase transition temperature, with negative thermal coefficients are -1.571*10-5 /K, -2.44*10-5 /K, and -5.025*10-6 /K for x = 0, 0.06, and 0.09 samples respectively. Raman spectra showed softening of the transverse optical phonon modes and increase in the full width at half maxima with the increase in composition. Field-emission scanning electron microscope equipped with energy dispersive x-ray spectrometer (EDS) confirmed compositional homogeneity and dense type microstructure.

cond-mat.mtrl-sci↗

arXiv2018

Investigation of La and Al substitution on the spontaneous polarization and lattice dynamics of the Pb(1-x)LaxTi(1-x)AlxO3 ceramics

The phase purity and crystal structure of PLTA samples (synthesized via sol-gel process) were confirmed using synchrotron x-ray powder diffraction (wavelength, lmbda= 0.44573 A. Rietveld analyses of powder x-ray diffraction data confirmed the tetragonal structure for compositions with more than 0.18 and cubic structure for the sample with 0.25 composition. Temperature-dependent XRD was performed to investigate the structural change from tetragonal to cubic structure phase transition. Raman spectroscopy at room temperature also confirmed this phase transition with composition. Field emission scanning electron provided information about surface morphology while an energy dispersive x-ray spectrometer attached with FESEM confirmed the chemical compositions of samples. Temperature and frequency dependent dielectric studies showed that the tetragonal to cubic phase transition decreased from 680 K to 175 K with the increase in the x from 0.03 to 0.25, respectively. This is correlated with the structural studies. Electric field dependent spontaneous polarization showed proper ferroelectric loop for 0.06 to 0.18 belonging to a tetragonal phase while after 0.25 composition the spontaneous polarization vanishes.Bipolar strain versus electric field revealed a butterfly loop for 0.06 to 0.18 compositions. Energy storage efficiency initially increases nominally with substitution but beyond 0.18 composition enhances considerably.

cond-mat.mtrl-sci↗

arXiv2017

High pressure studies on properties of FeGa3: role of on-site coulomb correlation

High pressure X-ray diffraction measurements have been carried out on the intermetallic semiconductor FeGa$_3$ and the equation of state for FeGa$_3$ has been determined. First principles based DFT calculations within the GGA approximation indicate that although the unit cell volume matches well with the experimentally obtained value at ambient pressure, it is significantly underestimated at high pressures and the difference between them increases as pressure increases. GGA + U calculations with increasing values of U$_{Fe(3d)}$ (on-site Coulomb repulsion between the Fe 3d electrons) at high pressures, correct this discrepancy. Further, the GGA+U calculations also show that along with U$_{Fe(3d)}$, the Fe 3d band width also increases with pressure and around a pressure of 4 GPa, a small density of states appear at the Fermi level. High pressure resistance measurements carried out on FeGa$_3$ also clearly show a signature of an electronic transition. Beyond the pressure of 19.7 GPa, the diffraction peaks reduce in intensity and are not observable beyond $\sim$ 26 GPa, leading to an amorphous state.

cond-mat.mtrl-sci↗

arXiv2013

Temperature-dependent structural property and power factor of n type thermoelectric Bi0.90Sb0.10 and Bi0.86Sb0.14 alloys

Thermal variation of structural property, linear thermal expansion coefficient, resistivity, thermopower and power factor of polycrystalline Bi1-xSbx (x=0.10, 0.14) samples are reported. Temperature-dependent powder diffraction experiments indicate that samples do not undergo any structural phase transition. Rietveld refinement technique has been used to perform detailed structural analysis. Temperature dependence of thermal expansion coefficient is found to be stronger for Bi0.90Sb0.10. Also, power factor for direct band gap Bi0.90Sb0.10 is higher as compared to that for indirect band gap Bi0.86Sb0.14. Role of electron-electron and electron-phonon scattering on resistivity, thermopower and power factor have been discussed.

cond-mat.mtrl-sci↗