arXiv subjects

V. Kanchana

Publications and source records attributed to V. Kanchana.

32 records · Page 2Linked to original sources

arXiv2014

Thermoelectric properties of chalcopyrite type CuGaTe$_2$ and chalcostibite CuSbS$_2$

Electronic and transport properties of CuGaTe$_2$, a hole-doped ternary copper based chalcopyrite type semiconductor, are studied using calculations within the Density Functional Theory and solving the Boltzmann transport equation within the constant relaxation time approximation. The electronic bandstructures are calculated by means of the full-potential linear augmented plane wave method, using the Tran-Blaha modified Becke-Johnson potential. The calculated band gap of 1.23 eV is in agreement with the experimental value of 1.2 eV. The carrier concentration- and temperature dependent thermoelectric properties of CuGaTe$_2$ are derived, and a figure of merit of $zT= 1.69$ is obtained at 950 K for a hole concentration of $3.7\cdot10^{19}$ cm$^{-3}$, in agreement with a recent experimental finding of $zT= 1.4$, confirming that CuGaTe$_2$ is a promising material for high temperature thermoelectric applications. The good thermoelectric performance of p-type CuGaTe$_2$ is associated with anisotropic transport from a combination of heavy and light bands. Also for CuSbS$_2$ (chalcostibite) a better performance is obtained for p-type than for n-type doping. The variation of the thermopower as a function of temperature and concentration suggests that CuSbS$_2$ will be a good thermoelectric material at low temperatures, similarly to the isostructural CuBiS$_2$ compound.

cond-mat.mtrl-sci↗

arXiv2013

Interplay of electron correlation and covalency in FeTe(0.6)Se(0.4)

We investigate the electronic structure of FeTe(0.6)Se(0.4) employing high resolution photoemission spectroscopy and ab initio band structure calculations. Fe 2p core level and the valence band spectra exhibit signature of strong electron correlation in the electronic structure. The electronic states near the Fermi level reduces in intensity with the decrease in temperature in conformity with the insulating transport observed near 300 K. The observation of an insulator to metal transition around 150 K in the transport properties may be related to the spectral lineshape change in the vicinity of the Fermi level observed in this study. The spectral features near Fermi level exhibit significant p orbital character due to the correlation induced Fe d spectral weight transfer. The experimental spectra reveal dominant temperature dependence of the spectral functions possessing large p-character. These results demonstrate significant renormalization of the character of the conduction electrons due to electron correlation and emphasizes the importance of ligand states in the superconductivity of these materials.

cond-mat.str-el↗

arXiv2013

Phase Stability and Thermoelectric Properties of the Mineral FeS2: An Ab Initio Study

First principles calculations were carried out to study the phase stability and thermoelectric properties of the naturally occurring marcasite phase of FeS$_2$ at ambient condition as well as under pressure. Two distinct density functional approaches has been used to investigate the above mentioned properties. The plane wave pseudopotential approach was used to study the phase stability and structural, elastic, and vibrational properties. The full potential linear augment plane wave method has been used to study the electronic structure and thermoelectric properties. From the total energy calculations, it is clearly seen that marcasite FeS$_2$ is stable at ambient conditions, and it undergoes a first order phase transition to pyrite FeS$_2$ at around 3.7 GPa with a volume collapse of about 3$\%$. The calculated ground state properties such as lattice parameters, bond lengths and bulk modulus of marcasite FeS$_2$ agree quite well with the experiment. Apart from the above studies, phonon dispersion curves unambiguously indicate that marcasite phase is stable under ambient conditions. Further, we do not observe any phonon softening across the marcasite to pyrite transition and the possible reason driving the transition is also analyzed in the present study, which has not been attempted earlier. In addition, we have also calculated the electronic structure and thermoelectric properties of the both marcasite and pyrite FeS$_2$. We find a high thermopower for both the phases, especially with p-type doping, which enables us to predict that FeS$_2$ might find promising applications as good thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2013

Electronic topological transition in LaSn$_3$ under pressure

The electronic structure, Fermi surface and elastic properties of the iso-structural and iso-electronic LaSn$_3$ and YSn$_3$ intermetallic compounds are studied under pressure within the framework of density functional theory including spin-orbit coupling. The LaSn$_3$ Fermi surface consists of two sheets, of which the second is very complex. Under pressure a third sheet appears around compression $V/V_0=0.94$, while a small topology change in the second sheet is seen at compression $V/V_0=0.90$. This may be in accordance with the anomalous behaviour in the superconducting transition temperature observed in LaSn$_3$, which has been suggested to reflect a Fermi surface topological transition, along with a non-monotonic pressure dependence of the density of states at the Fermi level. The same behavior is not observed in YSn$_3$, the Fermi surface of which already includes three sheets at ambient conditions, and the topology remains unchanged under pressure. The reason for the difference in behaviour between LaSn$_3$ and YSn$_3$ is the role of spin-orbit coupling and the hybridization of La - $4f$ states with the Sn - $p$ states in the vicinity of the Fermi level, which is well explained using the band structure calculation. The elastic constants and related mechanical properties are calculated at ambient as well as at elevated pressures. The elastic constants increase with pressure for both compounds and satisfy the conditions for mechanical stability under pressure.

cond-mat.mtrl-sci↗

arXiv2009

Magnetism in Re-based ferrimagnetic double perovskites

We have investigated spin and orbital magnetic moments of the Re 5d ion in the double perovskites A2FeReO6 (A = Ba, Sr, Ca) by X-ray magnetic circular dichroism (XMCD) at the Re L(2,3) edges. In these ferrimagnetic compounds an unusually large negative spin and positive orbital magnetic moment at the Re atoms was detected. The presence of a finite spin magnetic moment in a 'non-magnetic' double perovskite as observed in the double perovskite Sr2ScReO6 proves that Re has also a small, but finite intrinsic magnetic moment. We further show for the examples of Ba and Ca that the usually neglected alkaline earth ions undoubtedly also contribute to the magnetism in the ferrimagnetic double perovskites.

cond-mat.mtrl-sci↗

arXiv2009

High pressure structural study of fluoro perovskite CsCdF3 upto 60 GPa: A combined experimental and theoretical study

The structural behaviour of CsCdF3 under pressure is investigated by means of theory and experiment. High-pressure powder x-ray diffraction experiments were performed up to a maximum pressure of 60 GPa using synchrotron radiation. The cubic $Pm\bar{3}m$ crystal symmetry persists throughout this pressure range. Theoretical calculations were carried out using the full-potential linear muffin-tin orbital method within the local density approximation and the generalized gradient approximation for exchange and correlation effects. The calculated ground state properties -- the equilibrium lattice constant, bulk modulus and elastic constants -- are in good agreement with experimental results. Under ambient conditions, CsCdF3 is an indirect gap insulator with the gap increasing under pressure.

cond-mat.mtrl-sci↗

arXiv2009

Density functional study of elastic and vibrational properties of the Heusler-type alloys Fe$_2$VAl and Fe$_2$VGa

The structural and elastic properties as well as phonon-dispersion relations of the Heusler-type alloys Fe$_2$VAl and Fe$_2$VGa are computed using density-functional and density-functional perturbation theory within the generalized-gradient approximation. The calculated equilibrium lattice constants agree well with the experimental values. The elastic constants of Fe$_2$VAl and Fe$_2$VGa are predicted for the first time. From the elastic constants the shear modulus, Young's modulus, Poisson's ratio, sound velocities and Debye temperatures are obtained. By analyzing the ratio between the bulk and shear modulii, we conclude that both Fe$_2$VAl and Fe$_2$VGa are brittle in nature. The computed phonon-dispersion relation shows that both compounds are dynamically stable in the L1$_2$ structure without any imaginary phonon frequencies. The isomer shifts of Fe in the two compounds are discussed in terms of the Fe s partial density of states, which reveal larger ionicity/less hybridization in Fe$_2$VGa than in Fe$_2$VAl. For the same reason the Cauchy pressure is negative in Fe$_2$VAl but positive in Fe$_2$VGa

cond-mat.mtrl-sci↗

arXiv2007

High-pressure structural, elastic and electronic properties of the scintillator host material, KMgF_3

The high-pressure structural behaviour of the fluoroperovskite KMgF_3 is investigated by theory and experiment. Density functional calculations were performed within the local density approximation and the generalized gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. In situ high-pressure powder x-ray diffraction experiments were performed up to a maximum pressure of 40 GPa using synchrotron radiation. We find that the cubic Pm\bar{3}m crystal symmetry persists throughout the pressure range studied. The calculated ground state properties -- the equilibrium lattice constant, bulk modulus and elastic constants -- are in good agreement with experimental results. By analyzing the ratio between the bulk and shear modulii, we conclude that KMgF_3 is brittle in nature. Under ambient conditions, KMgF_3 is found to be an indirect gap insulator with the gap increasing under pressure.

cond-mat.mtrl-sci↗

arXiv2007

Elastic constants and high-pressure structural transitions in lanthanum monochalcogenides from experiment and theory

The high-pressure structural behavior of lanthanum monochalcogenides is investigated by theory and experiment. Theory comprises density functional calculations of LaS, LaSe and LaTe with the general gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. The experimental studies consist of high-pressure angle dispersive x-ray diffraction investigations of LaS and LaSe up to a maximum pressure of 41 GPa. A structural phase transition from the NaCl type to CsCl type crystal structure is found to occur in all cases. The experimental transition pressures are 27-28 GPa and 19 GPa, for LaS and LaSe, respectively, while the calculated transition pressures are 29 GPa, 21 GPa and 10 GPa for LaS, LaSe and LaTe, respectively. The calculated ground state properties such as equilibrium lattice constant, bulk modulus and its pressure derivative, and Debye temperatures are in good agreement with experimental results. Elastic constants are predicted from the calculations.

cond-mat.mtrl-sci↗

arXiv2007

Sr2CrOsO6: Endpoint of a spin polarized metal-insulator transition by 5d band filling

In the search for new spintronic materials with high spin-polarization at room-temperature, we have synthesized an osmium based double perovskite with a Curie-temperature of 725 K. Our combined experimental results confirm the existence of a sizable induced magnetic moment at the Os site, supported by band-structure calculations in agreement with a proposed kinetic energy driven mechanism of ferrimagnetism in these compounds. The intriguing property of Sr2CrOsO6 is that it is at the endpoint of a metal-insulator transition due to 5d band filling, and at the same time ferrimagnetism and high-spin polarization is preserved.

cond-mat.str-el↗

arXiv2005

Magnetic moments of W 5d in Ca2CrWO6 and Sr2CrWO6 double perovskites

We have investigated the magnetic moment of the W ion in the ferrimagnetic double perovskites Sr2CrWO6 and Ca2CrWO6 by X-ray magnetic circular dichroism (XMCD) at the W L(2,3) edges. In both compounds a finite negative spin and positive orbital magnetic moment was detected. The experimental results are in good agreement with band-structure calculations for (Sr/Ca)2CrWO6 using the full-potential linear muffin-tin orbital method. It is remarkable, that the magnetic ordering temperature, TC, is correlated with the magnetic moment at the 'non-magnetic' W atom.

cond-mat.mtrl-sci↗

arXiv2005

Electronic structure and X-ray magnetic circular dichroism of CrO$_2$

A detailed theoretical study on the electronic structure and magnetic properties of half-metallic ferromagnet CrO$_2$ was carried out by means of relativistic full-potential linear muffin-tin orbital method within the generalized gradient approximation (GGA) to the exchange correlation potential. Our calculation favours the [001] magnetization axis to be the easy axis of magnetization when compared to the [100] axis which is in agreement with the experiments. The calculated spin and orbital magnetic moments of Cr agrees well with the experimental and other theoretical works. The Cr L$_{2,3}$ x-ray absorption and x-ray magnetic circular dichroism (XMCD) spectra were calculated for both the quantization axis and compared with the experiment. In addition the oxygen K edge and XMCD spectra were also calculated which compares well with the experiments. The XMCD sum rules were used to compute the spin and orbital magnetic moments and results agree quite well with the direct calculation.

cond-mat.mtrl-sci↗

arXiv2004

Pseudo-half-metalicity in the double perovskite Sr$_2$CrReO$_6$ from density-functional calculations

The electronic structure of the spintronic material Sr$_2$CrReO$_6$ is studied by means of full-potential linear muffin-tin orbital method. Scalar relativistic calculations predict Sr$_2$CrReO$_6$ to be half-metallic with a magnetic moment of 1 $μ_B$. When spin-orbit coupling is included, the half-metallic gap closes into a pseudo-gap, and an unquenched rhenium orbital moment appears, resulting in a significant increase of the total magnetic moment to 1.28 $μ_B$. This moment is significantly larger than the experimental moment of 0.9 $μ_B$. A possible explanation of this discrepancy is that the anti-site disorder in Sr$_2$CrReO$_6$ is significantly larger than hitherto assumed.

cond-mat.mtrl-sci↗

arXiv2004

Electronic Structure of Samarium Monopnictides and Monochalcogenides

The electronic structures of SmX (X=N, P, As, Sb, Bi, O, S, Se, Te, Po)compounds are calculated using the self-interaction corrected local-spin density approximation. The Sm ion is described with either five or six localized $f$-electrons while the remaining electrons form bands, and the total energies of these scenarios are compared. With five localized $f$-electrons a narrow $f$-band is formed in the vicinity of the Fermi level leading to an effective intermediate valence. This scenario is the ground state of all the pnictides as well as SmO. With six localized $f$-electrons, the chalcogenides are semiconductors, which is the ground state of SmS, SmSe and SmTe. Under compression the Sm chalcogenides undergo first order transitions with destabilization of the $f$ states into the intermediate valence state, the bonding properties of which are well reproduced by the present theory.

cond-mat.str-el↗