Effect of pressure on the electronic and magnetic properties of CdV2_2O4_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 $\mu_B$ 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.Comment: 12 pages, 7 figures, to appear in Phys. Rev.

Efficiency calculation of thermoelectric generator using temperature dependent material's properties

Accurate measurement of efficiency for thermoelectric generator (TEG) is of great importance for materials research and development. Approximately all the parameters of a material are temperature dependent, so we can't directly apply the $\eta_\text{max}$ formula for efficiency calculation in the large temperature range. To overcome that problem, we tried to calculate the efficiency of TEG by dividing large working temperature range into a number of small temperature difference. The aim is to make temperature dependent parameter to be constant for that small temperature range. Using maximum individual efficiency of each segment obtained by $\eta_\text{max}$ in the equation of $\eta_\text{overall}$, which gives overall efficiency. The $\eta_\text{overall}$ of TEG using $Bi_2Te_3$ and $TAGS$ as thermoelectric materials come out to be $7.1\%$ and $8.94\%$, respectively, which is close to experimental results. For the high-temperature region, we have used $SiGe$ material in TEG and found out $\eta_\text{overall}=3.5\%$. The cumulative efficiency obtained by keeping one end temperature fixed with another end varying can be applied in real life application, i.e. automobile sector. The present work provides a simple way for the design engineers to calculate the efficiency of TEG by using the temperature dependent materials parameters like thermal conductivity, electrical conductivity, and Seebeck coefficient on which $z\bar{T}$ depends.Comment: 4 pages, 3 figure

Fabrication of Simple Apparatus for Resistivity Measurement in High Temperature Range 300-620 K

A simple and low cost apparatus has been designed and built to measure the electrical resistivity, ($\rho$), of metal and semiconductors in 300-620 K temperature range. The present design is suitable to do measurement on rectangular bar sample by using conventional four-probe dc method. A small heater is made on the sample mounting copper block to achieve the desired temperature. Heat loss from sample holder is minimize by using very low thermal conductive insulator block. This unique design of heater and minimized heat loss from sample platform provide uniform sample temperature and also have very good thermal stability during the measurement. The electrical contacts of current leads and potential probes on the sample are done by using very thin (42 SWG) copper wires and high temperature silver paste. The use of limited components and small heater design make present instrument very simple, light weight, easy to sample mount, small in size, and low cost. To calibrate the instrument pure nickel sample was used, and two other materials La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and LaCoO$_{3}$ (LCO) were also characterized to demonstrate the accuracy of this set-up. $\rho$(T) behavior on these samples were found to be in good agreement with the reported data. The metal-insulator transition for LSMO (T$_{MI}$ = $\sim$358 K) and the insulator-metal transition for LCO (T$_{IM}$ = $\sim$540 K) were clearly observed and these transitions temperature were also consistent with those reported in literature.Comment: 6 pages, 5 figure

Role of orbital degrees of freedom in investigating the magnetic properties of geometrically frustrated vanadium spinels

The inconsistency about the degree of geometrical frustration has been a long issue in AV$_{2}$O$_{4}$ (A $\equiv$ Zn, Cd and Mg) compounds, which arises from the two experimental results: (i) frustration indices and (ii) magnetic moments. In the present study, we try to understand such inconsistency by using {\it ab initio} electronic structure calculations. The orbital degrees of freedom are found to play an important role in understanding the geometrically frustrated magnetic behaviour of these compounds. The inclusion of the orbital and spin angular momenta for calculating the frustration indices improves the understanding about the degree of geometrical frustration in these compounds. The calculated values of the frustration indices ($f$$_{\it J}$) are largest for MgV$_{2}$O$_{4}$ and smallest for CdV$_{2}$O$_{4}$ for 3.3$\leq$ $U \leq$5.3 eV. In this range of $U$, the calculated values of $\Delta$M$_{2}$=M$_{\rm total}$-M$_{\rm exp}$ are largest for MgV$_{2}$O$_{4}$ and smallest for CdV$_{2}$O$_{4}$. Hence, the consistency about the degree of geometrical frustration is achieved. The absolute values of the nearest neighbour exchange coupling constant ({\it J$_{nn}$}) between V spins are found to be largest for MgV$_{2}$O$_{4}$ and smallest for CdV$_{2}$O$_{4}$, which indicate that the calculated absolute values of the Curie-Weiss temperature ($\varTheta$$_{CW}$)$_{\it J}$ are highest for MgV$_{2}$O$_{4}$ and smallest for CdV$_{2}$O$_{4}$ for 3.3$\leq$ $U \leq$5.3 eV. In this range of $U$, the magnetic transition temperature ($T$$_{N}$)$_{\it J}$ is found to be $\sim$150 K, $\sim$60 K and $\sim$22 K for MgV$_{2}$O$_{4}$, ZnV$_{2}$O$_{4}$ and CdV$_{2}$O$_{4}$, respectively, which shows that the order of ($T$$_{N}$)$_{\it J}$ is similar to that of ($T$$_{N}$)$_{\rm exp}$ for these compounds.Comment: 27 pages, 9 figures, 1 tabl

{Effect of on-site Coulomb interaction (\textit{U}) on the electronic and magnetic properties of Fe2_{2}MnSi, Fe2_{2}MnAl and Co2_{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

Studying the applicability of different thermoelectric materials for efficiency calculation in hybrid thermoelectric generator for waste heat recovery from automobile and steel industry

In this work, we study the suitability of different thermoelectric materials like $Bi_2Te_3$, $Sb_{2}Te_{3}$, $PbTe$, $TAGS$, $CeFe_{4}Sb_{12}$, $SiGe$ and $TiO_{1.1}$ in estimation of thermoelectric generator's (TEG) efficiency. The efficiency of TEG made up of ${Be_{2}Te_{3}}$ or $Sb_{2}Te_{3}$ gives $\sim$7\% in temperature range of 310 K - 500 K. $PbTe$ or $TAGS$ or $CeFe_{4}Sb_{12}$ gives $\sim$6\% in temperature range of 500 K - 900 K and $SiGe$ or $TiO_{1.1}$ also have remarkable efficiency in higher temperature range i.e $\sim$1200 K. Here, we report the enhancement of efficiency by using hybridization technique for different combination of above-mentioned materials. Hybridization of two different materials of TEG module is done by considering compatibility factor aspect. To this end, the proposed values of overall efficiency of TEG by hybridizing ${Be_{2}Te_{3}}$ and $PbTe$; ${Be_{2}Te_{3}}$ and $TAGS$; $Bi_{2}Te_{3}$ and $CeFe_{4}Sb_{12}$ are 12\%, 14\% and 11.88\%, respectively, for temperature range of 310 K to 900 K, which can be installed in an automobile. For steel industry and spacecraft application (till 1200 K) hybridization of $Bi_{2}Te_{3}$, $PbTe$ and $SiGe$; ${Be_{2}Te_{3}}$ and $TiO_{1.1}$ yields efficiency of $\sim$15.2\% and $\sim$17.2\%, respectively. The proposed results can be treated as a viable option for engineers, who are looking for fabricating TEG in real life applications such as automobile, spacecraft and steel industry

Fabrication of a simple apparatus for the Seebeck coefficient measurement in high temperature region

A simple apparatus for the measurement of Seebeck coefficient ($\alpha$) in the temperature range 300-620 K has been fabricated. Our design is appropriate for the characterization of samples with different geometries like disk and rod shaped. The sample holder assembly of the apparatus has been designed in such a way that, single heater used for sample heating purpose is enough to provide a self maintain temperature gradient (1-10 K) across the sample. The value of $\alpha$ is obtained without explicit measurement of temperature gradient. The whole apparatus is fabricated from the materials, which are commonly available, so that any part can be replaced in case of any damage. Commercially available standard Nickel (Ni) metal sample has been used as a reference material for calibration of the instrument. The experimentally observed value of $\alpha$ by our apparatus gives the similar temperature dependent behavior as reported in the literature. In order to study the thermoelectric behavior of oxide materials, we have synthesized polycrystalline LaCoO$_{3}$ powder samples by using solution combustion method and investigated the thermoelectric properties. Temperature dependent thermoelectric behavior of these samples were characterized in the temperature range 300-600 K.Comment: 8 pages, 6 figure

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 $\mu_{B}$. In the FM phase a very large value ($\sim$550 $\mu$V/K) of Seebeck coefficient (S) is obtained for down-spin electrons due to the existence of almost flat conduction band along X to $\Gamma$ direction. The two current model has been used to find the total S and the obtained value is about 10 $\mu$V/K. The calculated values of Seebeck coefficient, resistivity and electronic thermal conductivity show nice agreement with the experimental results

Understanding the Thermoelectric Properties of LaCoO3_{3} Compound

We present the thermoelectric (TE) properties of LaCoO$_{3}$ compound in the temperature range 300-600 K. The experimental value of Seebeck coefficient ($\alpha$) at 300 K is found to be $\sim$635 $\mu$V/K. The value of $\alpha$ decreases continuously with increase in temperature and reaches to $\sim$46 $\mu$V/K at $\sim$600 K. The electronic and TE properties of the compound have also been investigated by combining the \textit{ab-initio} electronic structures and Boltzmann transport calculations. LSDA plus Hubbard U (U= 2.75 eV) calculation on low spin configuration of the compound gives an energy gap of $\sim$0.5 eV, which is close to the experimentally reported energy gap. The effective mass of holes (\textit{m$^{*}_h$}) at $\Gamma$ point is nearly two times larger than the value of effective mass of electrons (\textit{m$^{*}_e$}) at FB point along the L and T directions, whereas the \textit{m$^{*}_e$} at FB point along the $\Gamma$ direction is nearly eight times larger than the value of \textit{m$^{*}_h$} at $\Gamma$ point along the FB direction. The large effective mass at FB point along the $\Gamma$ direction suggests that the TE property of this compound is mainly decided by the effective mass of the charge carriers in this direction. The calculated temperature dependent values of $\alpha$ are in fairly good agreement with experimental data in the temperature range 300-360 K, and above this temperature slight deviation is observed. The value of power factor (PF) for \textit{n}-type is $\sim$1.3 times larger the value of \textit{p}-type doped compound at 1100 K. The value of \textit{figure-of-merit} (\textit{ZT}) for \textit{n}-type doped compound is obtained $\sim$0.35 in the temperature range 600-1100 K, which suggests that with appropriate \textit{n}-type doping this compound can be used as a good TE material in the high temperature region.Comment: 8 Pages, 7 figures, Philosophical Magazine (In Press

Limitations of unconstrained LSDA+UU calculations in predicting the electronic and magnetic ground state of a geometrically frustrated ZnV2_{2}O4_{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.Comment: 23 pages, 7 figures, 2 table