arXiv subjects

Brajesh Tiwari

Publications and source records attributed to Brajesh Tiwari.

At least 19 recordsLinked to original sources

arXiv2026

Symmetry-Adapted Physical and Vibrational Properties of Ferroelectric Perovskite Oxides: Application to PbZrxTi1-xO3

Crystal symmetry governs macroscopic physical properties and lattice dynamics in functional materials. We present a systematic application of tensor analysis and group theory to determine allowed physical-property tensors, vibrational-mode symmetries, and Raman selection rules directly from crystallographic point-group symmetry. The approach is applied to the prototypical ferroelectric PbZrxTi1-xO3 (PZT). Symmetry lowering across the PZT phase diagram increases the number of independent pyroelectric, dielectric, and piezoelectric tensor components and modifies the symmetry classification of Raman-active vibrations. These mode classifications enable symmetry-based decomposition of reported room-temperature powder Raman spectra as a function of composition (x) across the morphotropic phase boundary, revealing the tetragonal-to-rhombohedral transition as a continuous redistribution of spectral intensity rather than emergence of new Raman modes. Persistent subpeak structure in selected modes indicates local symmetry breaking due to cation disorder and lattice anharmonicity, underscoring the importance of crystallographic symmetry analysis for interpreting functional and vibrational properties of ferroelectric perovskite oxides.

cond-mat.mtrl-sci↗

arXiv2026

Emergence of polar monoclinic phase in heterohalogen substituted CsGeX$_3$

The occurrence of ferroelectricity in inorganic germanium-based halide perovskites has provided an alternative to oxide counterparts. Using first-principles methods, we have studied CsGeX$_3$ materials with heterohalogen substitution at the X site in a 2:1 configuration. Structurally, such variation alters the octahedral environment more strongly than in pristine materials, giving rise to a polar monoclinic phase at room temperature. The occurrence of the monoclinic phase is also confirmed through the energetics of structures generated by the displacements of atoms in accordance with soft mode eigenvectors of the dynamical matrix along various directions. In the chemically tuned phase, the polarization is along [101] and increases by 10-15\% compared to pristine ones. The electronic structure analysis reveals that spin-splitting energy ranges from 25 to 250 meV in the valence band and from 9 to 80 meV in the conduction band in chemically tuned structures. In addition, these structures exhibit Rashba and persistent spin textures, which are coupled to the polarization direction. The parameters of the symmetry-dependent \textbf{k.p } Hamiltonian provide insights into the strength of spin-splitting and the nature of spin-texture. The semiconducting and spin-polarized nature of CsGeX$_3$ materials makes them strong candidates for Datta-Das spin transistors.

cond-mat.mtrl-sci↗

arXiv2023

Impact of Cr-O hyrbidization in ACrO3 (A=La, Y): A Theoretical Investigation

Electronic properties of spin polarized antiferromagnetic ACrO3 (A = La, Y) are explored with Hubbard Model using Density Functional Theory (DFT). These two isostructural systems are investigated using the different Hubbard energy and analyzed the hybridization of chromium 3d orbitals and oxygen 2p orbitals and the change in energy band gaps against the Hubbard energy. The bond length and bond angle affect significantly the orbital contributions of Cr-3d and O-2p electrons for both the system. We noticed that the Cr-O hybridization affects the orbital degeneracy and is substantiated with partial density of states. These results emphasize the contribution of Hubbard energy in correlated electron systems.

cond-mat.mtrl-sci↗

arXiv2019

Anomalous magnetic behavior and complex magnetic structure of proximate LaCrO3 LaFeO3 system

We investigated complex magnetic properties of multifunctional LaCrO3-LaFeO3 system. The magnetic measurements substantiate the presence of competing complex magnetic ordering against temperature, showing paramagnetic to ferrimagnetic transition at 300 K, followed by antiferromagnetic (AFM) transition near 250 K superimposed on ferrimagnetic phase. The onset of weak ferrimagnetic ordering is attributed to the competing complex interaction between two AFM LaCrO3-LaFeO3 sublattices. The low-temperature AFM ordering is also substantiated by temperature-dependent Raman measurements, where the intensity ratio of 700 cm-1 Raman active mode showed the clear enhancement with lowering the temperature. The non-saturating nature of magnetic moments in LaCrO3-LaFeO6 suggests the predominating AFM ordering in conjunction with ferrimagnetic ordering between 250 K to 300 K up to 5 T magnetic field. A complex magnetic structure of LaCrO3-LaFeO3 is constructed, emphasizing the metastable magnetic phase near room temperature and low temperature antiferromagnetic state.

cond-mat.str-el↗

arXiv2019

Magnetic Entropy in a Non-Collinear Weak Ferromagnetic YCrO3

We carried out temperature and field dependent magnetic measurements to understand the evolution of magnetic non-collinearity near antiferromagnetic phase in conjunction with the evolution of magnetic entropy near phase transition. We observed the maximum change in entropy just before magnetic ordering of Cr3+ in YCrO3 with the maximum change in magnetic entropy of -0.38 Jkg-1K-1 at 8 Tesla external field. The data is linear in higher fields 3 T - 8 T , whereas it showed deviations in the lower field region. The maximum entropy change fits well with mean field approximation at higher fields, while the observed deviation in lower field substantiates the onset of weak ferromagnetism in YCrO3.

cond-mat.str-el↗

arXiv2017

High Field (up to 140kOe) Angle Dependent Magneto Transport of Bi2Te3 Single Crystals

We report the angle dependent high field (up to 140kOe) magneto transport of Bi2Te3 single crystals, a well-known topological insulator. The crystals were grown from melt of constituent elements via solid state reaction route by self-flux method. Details of crystal growth along with their brief characterization up to 5 Tesla applied field was reported by some of us recently [J. Magn. Mag. Mater. 428, 213 (2017)]. The angle dependence of the magneto-resistance (MR) of Bi2Te3 follows the cos Theta function i.e., MR is responsive, when the applied field is perpendicular (tilt angle Theta = o and/or 180) to the transport current. The low field (10 kOe) MR showed the signatures of weak anti localization (WAL) character with typical cusp near origin at 5 K. Further, the MR is linear right up to highest applied field of 140 kOe. The large positive MR are observed up to high temperatures and are above 250 and 150 percent at 140 kOe in perpendicular fields at 50 K and 100 K respectively. Heat capacity CP(T) measurements revealed the value of Debye temperature to be 135 K. ARPES (angle resolved photoemission spectroscopy) data clearly showed that the bulk Bi2Te3 single crystal consists of a single Dirac cone.

cond-mat.mtrl-sci↗

arXiv2016

Influence of Ni doping on critical parameters of PdTe superconductor

We report the effect of Ni doping on superconductivity of PdTe. The superconducting parameters like critical temperature (Tc), upper critical field (Hc2) and normalized specific-heat jump are reported for Ni doped Pd1-xNixTe. The samples of series Pd1-xNixTe with nominal compositions x=0, 0.01, 0.05, 0.07, 0.1, 0.15, 0.2, 0.3 and 1.0 are synthesized via vacuum shield solid state reaction route. All the studied samples of Pd1-xNixTe series are crystallized in hexagonal crystal structure as refined by Rietveld method to space group P63/mmc. Both the electrical resistivity and magnetic measurements revealed that Tc decreases with increase of Ni concentration in Pd1-xNixTe. The magneto-transport measurements suggest that flux is better pinned for 20% Ni doped PdTe as compared to other compositions of Pd1-xNixTe. The effect and contribution of Ni 3d electron to electronic structure and density of states near Fermi level in Pd1-xNixTe are also studied using first-principle calculations within spin polarized local density approximation. The overlap of bands at Fermi level for NiTe is larger as compared to PdTe. Also the density of states just below Fermi level (in conduction band) drops much lower for PdTe than as for NiTe. Summarily, Ni doping in Pd1-xNixTe superconductor suppresses superconductivity moderately and also Ni is of non magnetic character in these compounds.

cond-mat.supr-con↗

arXiv2015

Impact of Ni doping on critical parameters of PdTe type-II BCS superconductor

We report the impact of Ni doping on superconductivity of PdTe superconductor. The superconducting parameters like critical temperature (Tc), upper critical field (Hc2) and normalized specific-heat jump are reported for Ni doped Pd1-xNixTe. The samples of series Pd1-xNixTe with nominal compositions x=0, .01, 0.05, 0.07, 0.1, 0.15, 0.2, 0.3 and 1.0 are synthesized via solid state reaction route. All the studied samples of series Pd1-xNixTe (x = 0.0 to 1.0) are crystallized in hexagonal crystal structure within the space group P63/mmc. Unit cell volume shrinks almost linearly upon Ni doping in Pd1-xNixTe. The normal state residual resistivity increases with Ni substitution on Pd site. Both the electrical resistivity and magnetic measurements revealed that Tc decreases with increase of Ni concentration in Pd1-xNixTe and is not observed down to 2K for x=0.30 i.e., 30% of Ni doping at Pd site. Interestingly, this is unusual for magnetic Ni doping in a known type-II BCS type superconductor. Magnetic Ni must suppress the superconductivity much faster. Interestingly, the isothermal magnetization measurements for NiTe revealed that Ni is non-magnetic in Pd1-xNixTe structure and hence the Tc depression is mainly due to disorder. The magneto-transport measurements revealed that flux is better pinned for 20% Ni doped PdTe as compared to other compositions of Pd1-xNixTe. The magnetic field dependence of specific heat of Pd1-xNixTe for x=0.01 was studied and the estimated value of the normalized specific-heat jump,is found to be 1.42, which is under BCS weak-coupling limit. Summarily, we report the impact of Ni doping in Pd1-xNixTe superconductor and conclude that Ni substitutes at Pd site, suppress superconductivity moderately and is of non magnetic nature in this system. To best of our knowledge this is the first study on Ni substitution in PdTe superconductor.

cond-mat.supr-con↗

arXiv2015

Specific heat of robust Nb2PdS5 superconductor

We report specific heat under different magnetic fields for recently discovered quasi-one dimensional Nb2PdS5 superconductor. The studied compound is superconducting below 6 K. Nb2PdS5 is quite robust against magnetic field with dHc/dT of -42 kOe/K. The estimated upper critical field [Hc2(0)] is 190 kOe, clearly surpassing the Pauli-paramagnetic limit of 1.84Tc. Low temperature heat capacity in superconducting state of Nb2PdS5 under different magnetic fields showed s-wave superconductivity with two different gaps. Two quasi-linear slopes in Somerfield-coefficient as a function of applied magnetic field and two band behavior of the electronic heat capacity demonstrate that Nb2PdS5 is a multiband su-perconductor in weak coupling limit with deltagamma/deltaTc=0.9.

cond-mat.supr-con↗

arXiv2015

Superconductivity at 4.4K in PdTe2-chains of a Ta Based Compound

Superconductivity in PdTe2 heavy transition metal based compounds is rapidly developing field in condensed matter physics community. Here, we report superconductivity in a nominal ternary telluride Ta2Pd0.97Te6 compound, which is synthesized in sealed evacuated quartz ampoule. The resultant compound is crystallized in layered monoclinic Ta4Pd3Te16 phase with space group C2/m, having lattice parameters a=21.304(6)A, b=3.7365(7)A, and c=17.7330A with Beta =120.65(1)degree. Both transport and magnetic measurements demonstrated bulk superconductivity at 4.4K in studied polycrystalline sample. The metallic normal state conductivity can be well ascribed to Fermi-liquid behaviour. The superconducting upper critical field of the compound is determined to be 5.5Tesla based on magneto-restively measurements.

cond-mat.supr-con↗

arXiv2015

PdTe a 4.5K Type II BCS Superconductor

We report on the structure and physical properties of bulk Palladium Tellurium superconductor, which is synthesized via quartz vacuum encapsulation technique at 750 C. The as synthesized compound is crystallized in hexagonal crystal structure. Magnetization and Magneto-transport measurements provided the values of lower and upper critical field to be 250 and 1200 Gauss respectively at 2 Kelvin. The Coherence length and GL parameter are estimated from the experimentally determined upper and lower critical fields, which are 45 nm and 1.48 respectively. The jump in Cp(T) at Tc is found to be 1.33 and the Debye temperature and electronic specific heat constant are 203 Kelvin and 6.01mJ/mole-K2 respectively.

cond-mat.supr-con↗

arXiv2015

Appearance of bulk Superconductivity under Hydrostatic Pressure in Sr0.5RE0.5FBiS2 (RE = Ce, Nd, Pr and Sm) compounds

We report the appearance of superconductivity under hydrostatic pressure (0.35 to 2.5GPa) in Sr0.5RE0.5FBiS2 with RE = Ce, Nd, Pr and Sm. The studied compounds, synthesized by solid state reaction route, are crystallized in tetragonal P4/nmm space group. At ambient pressure though the RE = Ce exhibit the onset of superconductivity below 2.5K, the Nd, Pr and Sm samples are not superconducting down to 2K. With application of hydrostatic pressure (up to 2.5GPa), superconducting transition temperature is increased to around 10K for all the studied samples. The magneto-transport measurements are carried out on all the samples with maximum Tc i.e., at under 2.5GPa pressure and their upper critical fields are determined. The new superconducting compounds appear to be quite robust against magnetic field but within Pauli paramagnetic limit. The new superconducting compounds with various RE (Ce, Nd, Pr and Sm) belonging to Sr0.5La0.5FBiS2 family are successfully synthesized for the first time and superconductivity is induced in them under hydrostatic pressure.

cond-mat.supr-con↗

arXiv2014

Role of MgO impurity on the superconducting properties of MgB2

We address the effect of MgO impurity on the superconducting properties of MgB2. The synthesis of MgB2 is very crucial because of sensitivity of Mg to oxidation which may lead to MgO as a secondary phase. Rietveld refinement was performed to determine the quantitative volume fraction of MgO in the samples synthesized by two different techniques. Both the samples were subjected to magnetization measurements under dc and ac applied magnetic fields and the observed results were compared as a function of temperature. Paramagnetic Meissner effect has been observed in a sample of MgB2 having more amount of MgO (with Tc = 37.1K) whereas the pure sample MgB2 having minor quantity of MgO shows diamagnetic Meissner effect with Tc = 38.8K. M-H measurements at 10K reveal a slight difference in irreversibility field which is due to MgO impurity along with wide transition observed from ac magnetic susceptibility measurements. The magnetotransport measurements R(T)H using RN = 90%, 50% and 10% criterion on pure sample of MgB2 has been used to determine the upper critical field whereas the sample having large quantity of MgO does not allow these measurements due to its high resistance.

cond-mat.supr-con↗

arXiv2014

Hydrostatic Pressure Studies on Parent Phase SrFBiS2 of BiS2-based Superconducting Family

In the present work, we measure the temperature dependent electrical resistivity from 300K down to 2K under applied hydrostatic pressure of upto 2.5GPa for SrFBiS2, which is the parent compound for the BiS2 based superconductors. Though the normal state resistivity of the compound decreases with pressure, the same is not superconducting down to 2K under applied pressure of up to 2.5Gpa.

cond-mat.supr-con↗

arXiv2014

Superconductivity at 4K in Pd deficient layered Ta_2Pd_xS_6

Here in this short note, we report on the low dimensional 4d and 5d transition metals-chalcogenide based compounds i.e., Ta2PdxS6, showing semiconducting to superconducting transition around 4K with upper critical field outside Pauli paramagnetic limit. It seems couple of different superconducting phases do exist in these new set of compounds. Our short note in this regards is thought provoking, asking to explore various unearthed possible superconducting phases in (Nb/Ta)2Pdx(S/Se/Te)y systems.

cond-mat.supr-con↗

arXiv2014

Magnetotransport studies of FeSe under hydrostatic pressure

The discoveries of iron-based superconductors with relatively high transition temperature are under intense experimental and theoretical investigation. Here we present magnetotransport measurements on FeSe superconductor under hydrostatic pressure. We show that in Fe-deficient tetragonal FeSe binary compound, the onset of superconducting transition is almost doubled under 1.98GPa pressure and the estimated upper critical field of 26.7Tesla is increased to 47.5Tesla.

cond-mat.supr-con↗

arXiv2014

Robust superconductivity with large upper critical field in Nb2PdS5

We report synthesis, structural details and complete superconducting characterization of very recently discovered Nb2PdS5 new superconductor. The synthesized compound is crystallized in mono-clinic structure. Bulk superconductivity is seen in both magnetic susceptibility and electrical resistivity measurements with superconducting transition temperature (Tc) at 6K. The upper critical field (Hc2), being estimated from high field magneto-transport measure-ments is above 240kOe. The estimated Hc2(0) is clearly above the Pauli paramagnetic limit. Heat capacity measurements show clear transition with well defined peak at Tc, but with lower jump than as expected for a BCS type superconductor. The Sommerfield constant and Debye temperature as determined from low temperature fitting of heat capacity data are 32mJ/moleK2 and 263K respectively. Hall coefficients and resistivity in conjugation with electronic heat capacity indicates multiple gap superconductivity signatures in Nb2PdS5. We also studied the impact of hydrostatic pressure on superconductivity of Nb2PdS5 and found nearly no change in Tc for the given pressure range.

cond-mat.supr-con↗

arXiv2014

Impact of hydrostatic pressure on superconductivity of Sr0.5La0.5FBiS2

We report the impact of hydrostatic pressure (0-1.97GPa) on superconductivity of recently discovered 2K superconductor Sr0.5La0.5FBiS2. Resistivity under pressure measurements are performed by using HPC-33 Piston type pressure cell with Quantum design DC resistivity Option. The superconducting transition temperature (Tc) is increased by 5 fold to around 10K with just above 1GPa pressure, which remains nearly unaltered for studied higher pressures of up to 1.97GPa. The fivefold increase in Tc of Sr0.5La0.5FBiS2 with just above 1GPa pressure suggests very strong electron correlations in these systems and the same calls for the attention of solid state physics community.

cond-mat.supr-con↗