arXiv subjects

Sudhir K. Pandey

Publications and source records attributed to Sudhir K. Pandey.

88 records · Page 5Linked to original sources

arXiv2016

Investigation of the Thermoelectric Properties of ZnV$_{2}$O$_{4}$ Compound in High Temperature Region

In the present work, we report the experimental thermopower ($α$) data for ZnV$_{2}$O$_{4}$ compound in the high temperature range 300-600 K. The value of $α$ is found to be $\sim$184 and $\sim$126 $μ$V/K at $\sim$300 and $\sim$600 K, respectively. The temperature dependent behavior of $α$ is almost linear in the measured temperature range. To understand the large and positive $α$ value observed in this compound, we have also investigated the electronic and thermoelectric properties by combining the \textit{ab-initio} electronic structures calculations with Boltzmann transport theory. Within the local spin density approximation plus Hubbard U, the anti-ferromagnetic ground state calculation gives an energy gap $\sim$0.33 eV for U=3.7 eV, which is in accordance with the experimental results. The effective mass for holes in the valance band is found nearly four times that of electrons in conduction band. The large effective mass of holes are mainly responsible for the observed positive and large $α$ value in this compound. There is reasonably good matching between calculated and experimental $α$ data in the temperature range 300-410 K. The power factor calculation shows that thermoelectric properties in high temperature region can be enhanced by tuning the sample synthesis conditions and suitable doping. The estimated value of \textit{figure-of-merit}, ZT, at different absolute temperature suggest that ZnV$_{2}$O$_{4}$ compound can be a good thermoelectric material in high temperature range.

cond-mat.mtrl-sci↗

arXiv2016

Constrained DFT+$U$ approach for understanding the magnetic behaviour of ACr$_{2}$O$_{4}$ (A=Zn, Mg, Cd and Hg) compounds

In this work, we try to understand the inconsistency reported by [Yaresko, Phys. Rev. B. {\bf 77}, 115106 (2008)] in the theoretically estimated nature and the variation of magnitude of nearest neighbour exchange coupling constant ($\arrowvert${\it J$_{1}$}$\arrowvert$) with increasing $U$ in ACr$_{2}$O$_{4}$ (A=Zn, Cd, Mg and Hg) compounds by using density functional theory. In unconstrained calculations, the nature and variation of $\arrowvert${\it J$_{1}$}$\arrowvert$ as a function of $U$ in the present study are not consistent with the experimental data and not according to the relation, {\it J$_{1}$}$\propto$$\frac{t^{2}}{U}$ especially for CdCr$_{2}$O$_{4}$ and HgCr$_{2}$O$_{4}$ for $U >$3 eV and U=2-6 eV, respectively. Such an inconsistent behavior of $\arrowvert${\it J$_{1}$}$\arrowvert$ is almost similar to that of Yaresko for these two compounds for $U$=2-4 eV. For ZnCr$_{2}$O$_{4}$ and MgCr$_{2}$O$_{4}$, the nature and the variation of $\arrowvert${\it J$_{1}$}$\arrowvert$ in the present work are in accordance with the experimental data and above mentioned relation for $U$=2-6 eV and are similar to that of Yaresko for ZnCr$_{2}$O$_{4}$ for $U$=2-4 eV. However, in constrained calculations the nature and variation of $\arrowvert${\it J$_{1}$}$_{c}$$\arrowvert$ in the present work are according to experimental data and above above mentioned relation for all four compounds. Hence, the present study shows the importance of constrained calculations in understanding the magnetic behaviour of these spinels. The values of magnitude of Curie-Weiss temperature [$\arrowvert$($\varTheta$$_{CW}$)$_{c}$$\arrowvert$] for ZnCr$_{2}$O$_{4}$$>$MgCr$_{2}$O$_{4}$$>$CdCr$_{2}$O$_{4}$$>$HgCr$_{2}$O$_{4}$ for $U$=2-5 eV, which are according to the order of experimentally observed values for these spinels.

cond-mat.str-el↗

arXiv2016

Density matrix approach to the orbital ordering in the spinel vanadates: A case study

In this work we apply the density matrices approach to orbital ordering (OO) in order to study the OO of the spinel vanadates AV$_{2}$O$_{4}$ (A $\equiv$ Zn, Cd and Mg), which is normally believed to be responsible for the structural transition from cubic to tetragonal phase observed in these compounds. The density matrices of vanadium atoms are obtained by using {\it state-of-the-art} full-potential linearized augmented plane wave method based GGA+U calculations. In the absence of spin-orbit coupling, the present study shows the existence of anti-ferro OO in the global (local octahedral) coordinate system where $d_{xz}$ and $d_{yz}$ ($d_{xz}$+$d_{yz}$ and $d_{xz}$-$d_{yz}$) orbitals are mainly occupied at the neighboring V sites for all the compounds.

cond-mat.str-el↗

arXiv2016

Limitations of unconstrained LSDA+$U$ calculations in predicting the electronic and magnetic ground state of a geometrically frustrated ZnV$_{2}$O$_{4}$ compound

In the present work, we investigate the applicability of the LSDA+$U$ method in understanding the electronic and magnetic properties of a geometrically frustrated ZnV$_2$O$_4$ compound, where the delicate balance of electrons, lattice, orbital and spin interactions play an important role in deciding its physical properties. In the ferromagnetic solution of the compound, only one type of orbital solution is found to exist in all ranges of $U$ studied here. However, in antiferromagnetic (AFM) phase, two types of orbital solutions, AFM(OS1) and AFM(OS2), exist for $U >$3 eV. If the difference of the electronic occupancy of $d_{xz}$ and $d_{yz}$ orbitals is less than 0.25, then AFM(OS1) solution is stabilized, whereas for higher values AFM(OS2) solution is stabilized. The use of unconstrained calculations within the fully localized double counting scheme is unable to predict the AFM ground state for $U \leqslant$3 eV. Our results clearly suggest the importance of constrained calculations in understanding the electronic and magnetic properties of a compound, where various competing interactions are present. In the AFM solution, the orbital ground state of the compound changes with varying $U$, where AFM(OS1) is found to be the ground state for $U \leqslant$3 eV and for higher values of $U$, AFM(OS2) is the ground state. The analysis of the band gap suggests that the AFM(OS2) is the real ground state of the compound.

cond-mat.str-el↗

arXiv2014

Importance of two current model in understanding the electronic transport behavior of inverse Heusler alloy: Fe 2 CoSi

Here we explore the applicability of the two current model in understanding the transport behavior of Fe 2 CoSi compound by using the first principles calculations in combination with the Boltzmann transport theory. The spin-unpolarized calculation shows large density of states (DOS) at Fermi level (E F) and is unable to provide the correct temperature dependence of transport coefficients. The spin-polarised calculation shows reduced DOS at the E F in the spin-up channel, whereas spin-dn channel have almost zero DOS at the E F . The absolute value of Seebeck coefficient in the spin-up channel shows linear increment with the temperature and in the spin-dn channel it varies non-linearly. The electrical conductivity also shows non-linear temperature dependence in both the spin channels whereas, the electronic thermal conductivity shows linear temperature dependence. The values of transport coefficients and their temperature dependence obtained by using the two current model are found to be in fairly good agreement with the experimental data. Present work clearly suggests the importance of two current model in understanding the transport properties of the compound.

cond-mat.mtrl-sci↗

arXiv2014

{Effect of on-site Coulomb interaction (\textit{U}) on the electronic and magnetic properties of Fe$_{2}$MnSi, Fe$_{2}$MnAl and Co$_{2}$MnGe

The electronic band structures, density of states plots and magnetic moments of Fe$_{2 }$MnSi, Fe$_{2 }$MnAl, and Co$_{2 }$MnGe are studied by using the first principles calculations. The FM solutions using LSDA without \textit{U} show the presence of half-metallic ferromagnetic (HFM) ground state in Fe$_{2 }$MnSi, whereas the ground state of Fe$_{2 }$MnAl is found to be metallic. In both compounds the maximum contribution to the total magnetic moment is from the Mn atom, while the Fe atom contributes very less. The electronic structures and magnetic moments of Fe-based compounds affected significantly by \textit{U}, whereas its effect is very less on Co$_{2}$MnGe. The magnetic moment of Fe atom in Fe$_{2 }$MnSi (Fe$_{2 }$MnAl), increased by $\sim$ 70 % ($\sim$ 75 %) and in Mn atom it decreases by $\sim$ 50 % ($\sim$ 70 %) when the value of \textit{U} is increased from 1 to 5 eV. The Hund's like exchange interactions are increasing in Fe atom while decreasing in Mn atom with increase in \textit{U}. The Fe and Mn moments are ferromagnetically coupled in Fe$_{2 }$MnSi for all values of \textit{U}, whereas in Fe$_{2 }$MnAl they coupled antiferromagnetically below \textit{U} = 2 eV and ferromagnetically above it. Above \textit{U} = 2 eV the metallic ground state of Fe$_{2 }$MnAl changes to semiconducting ground state and the ferromagnetic coupling between Fe and Mn atoms appears to be responsible for this.

cond-mat.mtrl-sci↗

arXiv2014

Investigation of the electronic and thermoelectric properties of Fe2 ScX (X = P, As and Sb) full Heusler alloys by using first principles calculations

By using ab initio electronic structure calculations here we report the three new full Heusler alloys which are possessing very good thermoelectric behavior and expected to be synthesized in the laboratories. These are Fe2 ScP, Fe2 ScAs and Fe2 ScSb compound. First two compounds are indirect band gap semiconductors and the last one shows semimetallic ground state. The value of band gap of Fe2 ScP and Fe2 ScAs is 0.3 and 0.09 eV, respectively. These compounds show the presence of flat conduction bands along Γ - X-direction suggesting for the large electron like effective mass and promising for very good thermoelectric behavior of the compounds. At 200 K, the Seebeck coefficients of Fe2 ScP, Fe2 ScAs and Fe2 ScSb compounds are -770, -386 and -192μV/K, respectively. The maximum power factor (P F ) is expected for the n-type doping in these materials. The heavily doped Fe2 ScP and Fe2 ScAs have P F >60 for a wide temperature range, which is comparable to the PF of Bi2 Te3 - a well known and one of the best commercially used thermoelectric material. Present work suggests the possible thermoelectric applicability of these materials in a wide temperature range.

cond-mat.mtrl-sci↗

arXiv2014

Investigation of thermoelectric properties of half-metallic Co2 MnGe by using first principles calculations

By combining the electronic structures obtained from first principles calculations with Boltzmann transport theory we have investigated the electronic, magnetic and transport properties of Co$_{2}$MnGe Heusler compound. The density of states plots, dispersion curves and total energy of paramagnetic and ferromagnetic (FM) phases clearly show the half-metallic FM ground state for the compound with an indirect band gap of about 400 meV in the minority spin channel. It has integer value of the magnetic moment equal to 5 $μ_{B}$. In the FM phase a very large value ($\sim$550 $μ$V/K) of Seebeck coefficient (S) is obtained for down-spin electrons due to the existence of almost flat conduction band along X to $Γ$ direction. The two current model has been used to find the total S and the obtained value is about 10 $μ$V/K. The calculated values of Seebeck coefficient, resistivity and electronic thermal conductivity show nice agreement with the experimental results.

cond-mat.mtrl-sci↗

arXiv2012

Effect of pressure on the electronic and magnetic properties of CdV$_2$O$_4$: Density functional theory studies

We investigate the effect of pressure on the electronic and magnetic states of CdV$_2$O$_4$ by using ab initio electronic structure calculations. The Coulomb correlation and spin-orbit coupling play important role in deciding the structural, electronic and magnetic properties of the compound. The total magnetic moment of V ion is found to be $\sim$1.3 $μ_B$ and making an angle of $\sim$9.5 degree with the z-axis. In the tetragonal phase, the ground state is the orbital ordered state where V $d_{xz}$ and $d_{yz}$ obtitals are mainly occupied at the neighbouring sites. This work predicts the electronic phase transition from orbital-ordered-insulator to orbital-ordered-metal to orbital-disordered-metal with increasing pressure. The pressure induced broadening of lower and upper Hubbard bands gives rise to metal-insulator transition above 35 GPa. The simple mean-field theory used in the present work is able to describe the pressure dependent variation of the antiferromagnetic transition temperature suggesting the applicability of the method in the study of the magnetic behaviour of similar geometrically frustrated systems.

cond-mat.str-el↗

arXiv2012

Nitrogen and fluorine doped ZrO$_2$: A promising $p$-$n$ junction for ultraviolet light-emitting diode

In the present work we study the effect of nitrogen (N) and fluorine (F) doping in the electronic properties of ZrO$_2$ by using \emph{ab initio} electronic structure calculations. Our calculations show the importance of on-site Coulomb correlation in estimating the correct band gap of ZrO$_2$. The N and F doping provide hole and electron type impurity states in the band gap closer to the top of the valance band and bottom of the conduction band, respectively. The formation of such impurity states may be exploited in fabricating a $p$-$n$ junction expected to be useful in making an ultraviolet light-emitting diode.

cond-mat.mtrl-sci↗

arXiv2012

Electronic structure near quantum critical point

We studied the evolution of the electronic structure across the quantum critical point in V doped Cr employing high resolution photoemission spectroscopy. Experimental results exhibit signatures of pseudogap and orbital Kondo resonance peak at low temperatures for all the compositions studied suggesting a scenario of spin density wave quantum criticality corresponding to orbital Kondo effect. The pseudogap and the Kondo peak gradually reduces with V doping but remains finite at the quantum critical point indicating their relevance in widely discussed quantum phases in correlated electron systems. The spectral lineshape near the Fermi level exhibit (E_F - E)^{0.25} dependence evidencing deviation from Fermi liquid behavior.

cond-mat.str-el↗

arXiv2011

Orbital ordering in the geometrically frustrated MgV$_2$O$_4$: \emph{Ab initio} electronic structure calculations

In the light of recent interesting experimental work on MgV$_2$O$_4$ we employ the density functional theory to investigate the crucial role played by different interaction parameters in deciding its electronic and magnetic properties. The strong Coulomb correlation in presence of antiferromagnetic (AFM) coupling is responsible for the insulating ground state. In the ground state the $d_{xz}$ and $d_{yz}$ orbitals are ordered and intra-chain vanadium ions are antiferromagnetically coupled. The calculation gives small spin-orbit coupling (SOC), which provides a tilt of $\sim11.3^0$ to the magnetic moment from the z-axis. In the presence of weak SOC and strong exchange coupling, the experimentally observed small magnetic moment and low AFM transition temperature appear to arise from spin fluctuation due to activeness of geometrical frustration.

cond-mat.str-el↗

arXiv2011

Effect of non-magnetic impurities on the magnetic states of anatase TiO$_2$

The electronic and magnetic properties of TiO$_2$, TiO$_{1.75}$, TiO$_{1.75}$N$_{0.25}$, and TiO$_{1.75}$F$_{0.25}$ compounds have been studied by using \emph{ab initio} electronic structure calculations. TiO$_2$ is found to evolve from a wide-band-gap semiconductor to a narrow-band-gap semiconductor to a half-metallic state and finally to a metallic state with oxygen vacancy, N-doping and F-doping, respectively. Present work clearly shows the robust magnetic ground state for N- and F-doped TiO$_2$. The N-doping gives rise to magnetic moment of $\sim$0.4 $μ_B$ at N-site and $\sim$0.1 $μ_B$ each at two neighboring O-sites, whereas F-doping creates a magnetic moment of $\sim$0.3 $μ_B$ at the nearest Ti atom. Here we also discuss the possible cause of the observed magnetic states in terms of the spatial electronic charge distribution of Ti, N and F atoms responsible for bond formation.

cond-mat.str-el↗

arXiv2010

Correlation induced half-metallicity in a ferromagnetic single-layered compound: Sr$_2$CoO$_4$

The electronic and magnetic properties of Sr$_2$CoO$_4$ compound have been studied using $\emph{ab initio}$ electronic structure calculations. As opposed to GGA calculation, which gives ferromagnetic metallic solution, GGA+$U$ calculations provide two kind of ferromagnetic solutions: (i) half-metallic and (ii) metallic. The half-metallic solution is a ground state of the system and the metallic one is a metastable state. The strong hybridization between Co 3$d$ and O 2$p$ orbitals decides the electronic and magnetic properties of the compound. The total magnetic moment per formula unit is found to be $\sim$ 3 $μ_B$ ($S$ = 3/2). Our calculations give the magnetocrystalline anisotropy energy of $\sim$ 2.7 meV, which provides a good description of experimentally observed large magnetocrystalline anisotropy. The Heisenberg exchange parameters up to fourth nearest neighbours are also calculated. The mean-field theory gives the $T_C$ = 887 K. The possible physical implications of the ferromagnetic half-metallic ground state are also discussed.

cond-mat.str-el↗

arXiv2009

Spectral evolution in an insulator exhibiting linear specific heat

We investigate the spectral evolution of an antiferromagnetic insulator, La$_{0.2}$Sr$_{0.8}$MnO$_3$ exhibiting linear specific heat using state-of-the-art high resolution photoemission spectroscopy. Experimental spectral functions exhibit Fermi liquid like energy dependence at all the temperatures studied. Room temperature spectrum possess finite density of states at the Fermi level that vanishes generating a soft gap at about 260 K (the magnetic transition temperature). High resolution spectra reveal a hard gap in the magnetically ordered phase (C-type antiferromagnet). These results indicate signature of an amorphous phase coexisting with the long range ordered phase in these materials.

cond-mat.str-el↗

arXiv2009

Study of magnetic interactions in a spin liquid, Sr3NiPtO6 using density functional approach

We investigate the magnetic interactions in Sr3NiPtO6, characterized to be a spin liquid using ab initio calculations. The results reveal a novel metal to insulator transition due to finite exchange interaction strength; the magnetic solutions (independent of magnetic ordering) are large band gap insulators, while the non-magnetic solution is metallic. The Ni moment is found to be large and the coupling among intra-chain Ni moments is antiferromagnetic unlike other compounds in this family. These results, thus, reveal the importance of intra-chain antiferromagnetic interaction in addition to geometrical frustration to derive spin liquid phase.

cond-mat.str-el↗