Ab initio studies of electronic structure of defects in PbTe

Understanding the detailed electronic structure of deep defect states in narrow band-gap semiconductors has been a challenging problem. Recently, self-consistent ab initio calculations within density functional theory (DFT) using supercell models have been successful in tackling this problem. In this paper, we carry out such calculations in PbTe, a well-known narrow band-gap semiconductor, for a large class of defects: cationic and anionic substitutional impurities of different valence, and cationic and anionic vacancies. For the cationic defects, we study a series of compounds RPb2n-1Te2n, where R is vacancy or monovalent, divalent, or trivalent atom; for the anionic defects, we study compounds MPb2nTe2n-1, where M is vacancy, S, Se or I. We find that the density of states (DOS) near the top of the valence band and the bottom of the conduction band get significantly modified for most of these defects. This suggests that the transport properties of PbTe in the presence of impurities can not be interpreted by simple carrier doping concepts, confirming such ideas developed from qualitative and semi-quantitative arguments

Layer-dependent electronic structures and magnetic ground states of polar-polar LaVO3/KTaO3\rm{LaVO_3/KTaO_3} (001) interfaces

Using first-principles and model Hamiltonian approach, we explore the electronic properties of polar-polar LaVO$_3$/KTaO$_3$ (LVO/KTO, 001) hetero-interfaces of up to six and five layers of KTO and LVO, respectively. Our calculations suggest the presence of multiple Lifshitz transitions (LT) in the $t_{2g}$ bands which may show up in high thermal conductivity and Seebeck coefficient. The LT can be tuned by the number of LaVO$_3$ layers or gate voltage. The spin-orbit coupling is found to be negligible, coming only from the Ta $5d_{xy}$-derived band, 5$d_{xz}$ and 5$d_{yz}$ bands being far away from the Fermi level. The magnetic properties of the interfaces, due to Vanadium ions, turn out to be intriguing. The magnetic states are highly sensitive to the number of layers of LaVO$_3$ and KTaO$_3$: the interfaces with equal number of LVO and KTO layers always favor an antiferromagnetic (AFM) ordering. Moreover, the combination of even-even and odd-odd layers shows an AFM order for more than two LaVO$_3$ layers. The spin-polarized density of states reveals that all the interfaces with ferromagnetic (FM) ground states are \textit{half-metallic}. The small energy differences between AFM and FM configurations indicate a possible coexistence of competing AFM and FM ground states in these interfaces. In addition, the interface requires different number of LVO layers for it to be metallic: half-metallic FM for three and above, and metallic AFM for four and above.Comment: 11 pages, 10 figure

Resonant States in the Electronic Structure of the High Performance Thermoelectrics AgPbmSbTe_{m}SbTe_{2+m}$ ; The Role of Ag-Sb Microstructures

Ab initio electronic structure calculations based on gradient corrected density functional theory were performed on a class of novel quaternary compounds AgPb$_{m}SbTe$_{2+m}$, which were found to be excellent high temperature thermoelctrics with large figure of merit ZT ~2.2 at 800K. We find that resonant states appear near the top of the valence and bottom of the conduction bands of bulk PbTe when Ag and Sb replace Pb. These states can be understood in terms of modified Te-Ag(Sb) bonds. Electronic structure near the gap depends sensitively on the microstructural arrangements of Ag-Sb atoms, suggesting that large ZT values may originate from the nature of these ordering arrangements.Comment: Accepted in Physical Review Letter

Hartree-Fock variational bounds for ground state energy of chargeless fermions with finite magnetic moment in presence of a hard core potential:A stable ferromagnetic state

We use different types of determinantal Hartree-Fock (HF) wave functions to calculate variational bounds for the ground state energy of spin-half fermions in volume V_0, with mass m, electric charge zero, and magnetic moment mu, which are interacting through long range magnetic dipole-dipole interaction. We find that at high densities when the average inter particle distance r_0 becomes small compared to the magnetic length r_m, a ferromagnetic state with spheroidal occupation function, involving quadrupolar deformation, gives a lower energy compared to the variational energy for the uniform paramagnetic state. This HF variational bound to the ground state energy turns out to have a lower energy than our earlier calculation in which instead of a determinantal wavefunction we had used a positive semi-definite single particle density matrix operator whose eigenvalues, having quadrupolar deformation, were allowed to take any value from 0 to 1. This system is of course still unstable towards infinite density collapse, but we show here explicitly that a suitable short range repulsive (hard core) interaction of strength U_0 and range a can stop this collapse.The existence of a stable high density ferromagnetic state with spheroidal occupation function is possible as long as the ratio of hard-core and magnetic dipole coupling constants is not very small compared to 1.Comment: A shorter version of this paper will appear in Pramana - Journal of Physic

Design of Reconfigurable Array Antennas With Minimum Variation of Active Impedances

[Abstract] In this letter, the authors propose an optimization method based on the genetic algorithm (GA) to reconfigure a linear array of vertical half-wavelength dipole antennas to generate two patterns with minimum active impedance variation when the antenna switches from one pattern to other in the presence or absence of a ground plane behind the array. The problem is to find a fixed voltage amplitude distribution that generates two broadside symmetrical beams in the horizontal plane: a pencil beam with zero phases and a flat-top beam with phases in the range from -180deg to +180deg. Mutual coupling effect is taken into account via open circuit mutual impedance matri

Canted-spin-caused electric dipoles: a local symmetry theory

A pair of magnetic atoms with canted spins Sa, Sb can give rise to an electric dipole moment P. Several forms for the behavior of such a moment have appeared in the theoretical literature, some of which have been invoked to explain experimental results found in various multiferroic materials. The forms specifically are P1 ~ R x (Sa x Sb); P2 ~ Sa x Sb, and P3 ~ Sa (R . Sa) - Sb (R . Sb), where R is the relative position of the atoms and Sa, Sb are unit vectors. To unify and generalize these various forms we consider P as the most general quadratic function of the spin components that vanishes whenever Sa and Sb are collinear, i.e. we consider the most general expressions that require spin canting. The study reveals new forms. We generalize to the vector P, Moriya's symmetry considerations regarding the (scalar) Dzyaloshinskii-Moriya energy D. Sa x Sb (which led to restrictions on D). This provides a rigorous symmetry argument which shows that P1 is allowed no matter how high the symmetry of the atoms plus environment, and gives restrictions for all other contributions. The analysis leads to the suggestion of terms omitted in the existing microscopic models, suggests a new mechanism behind the ferroelectricity found in the 'proper screw structure' of CuXO2, X=Fe, Cr, and predicts an unusual antiferroelectric ordering in the antiferromagnetically and ferroelectrically ordered phase of RbFe(MoO4)2.Comment: 11 pages, 2 figures. The present work is a considerable generalization of the earlier version. It corrects the statement in the abstract as to the generality of the expression for Delta. Clarification of the term 'canted-spin-caused' is given, and adds application to additional experimental example