arXiv subjects

Deepa Kasinathan

Publications and source records attributed to Deepa Kasinathan.

29 records · Page 2Linked to original sources

arXiv2008

High-Temperature Superconductivity in Eu0.5K0.5Fe2As2

Subsequent to our recent report of SDW type transition at 190 K and antiferromagnetic order below 20 K in EuFe2As2, we have studied the effect of K-doping on the SDW transition at high temperature and AF order at low temperature. 50% K doping suppresses the SDW transition and in turn gives rise to high-temperature superconductivity below T_c = 32 K, as observed in the electrical resistivity, AC susceptibility as well as magnetization. A well defined anomaly in the specific heat provides additional evidence for bulk superconductivity.

cond-mat.supr-con↗

arXiv2008

Electrical Resistivity and Specific Heat of EuFe2As2 Single Crystals: Magnetic homologue of SrFe2As2

We have grown single crystals of EuFe2As2 and investigated its electrical transport and thermodynamic properties. Electrical resistivity and specific heat measurements clearly establish the intrinsic nature of magnetic phase transitions at 20 K and 195 K. While the high temperature phase transition is associated with the itinerant moment of Fe, the low temperature phase transition is due to magnetic order of localized Eu-moments. Band structure calculations point out a very close similarity of the electronic structure with SrFe2As2. Magnetically, the Eu and Fe2As2 sublattice are nearly de-coupled.

cond-mat.supr-con↗

arXiv2008

Quasi one-dimensional magnetism driven by unusual orbital ordering in CuSb2O6

Essentially all undoped cuprates exhibit a quasi-planar, fourfold Cu-O coordination responsible for the magnetically active antibonding 3d-(x^2-y^2)-like state. Here, we present an electronic structure study for CuSb2O6 that reveals, in contrast, a half-filled 3d-(3z^2-r^2) orbital. This hitherto unobserved ground state originates from a competition of in- and out-of-plaquette orbitals where the strong Coulomb repulsion drives the surprising and unique orbital ordering. This, in turn, gives rise to an unexpected quasi one-dimensional magnetic behavior. Our results provide a consistent explanation of recent thermodynamical and neutron diffraction measurements.

cond-mat.str-el↗

arXiv2008

Electronic and magnetic properties of K$_{2}$CuP$_{2}$O$_{7}$ - a model S=1/2 Heisenberg chain system

The electronic and magnetic properties of K$_{2}$CuP$_{2}$O$_{7}$ were investigated by means of susceptibility, specific heat and $^{31}$P nuclear magnetic resonance (NMR) measurements and by LDA band structure calculations. The temperature dependence of the NMR shift $K(T)$ is well described by the $S=1/2$ Heisenberg antiferromagnetic chain (HAF) model with nearest neighbor exchange $J_{1}$ $\simeq $ $(141\pm 5)$ K. The corresponding mapping of an LDA-derived tight-binding model leads to $J_{1}^{LDA}$ $\simeq$ 196 K. The spin lattice relaxation rate $1/T_{1}$ decreases with temperature below 300 K but becomes nearly temperature independent between 30 K and 2 K as theoretically expected for an $S = {1/2}$ HAF chain. None of the investigated properties give any evidence for long range magnetic order above 2K, in agreement with the results of the band structure calculation, which yield extremely weak exchange to the next nearest neighbor (NNN) and a very small and frustrated inter-chain exchange. Thus, K$_{2}$CuP$_{2}$O$_{7}$ seems to be a better realization of a nearest neighbor $S = {1/2}$ HAF chain than the compounds reported so far.

cond-mat.str-el↗

arXiv2008

Pressure-driven magnetic moment collapse in the ground state of MnO

The zero temperature Mott transition region in antiferromagnetic, spin S=5/2 MnO is probed using the correlated band theory LSDA+U method. The first transition encountered is an insulator-insulator volume collapse within the rocksalt structure that is characterized by an unexpected Hund's rule violating `spin-flip' moment collapse. This spin-flip to S=1/2 takes fullest advantage of the anisotropy of the Coulomb repulsion, allowing gain in the kinetic energy (which increases with decreasing volume) while retaining a sizable amount of the magnetic exchange energy. While transition pressures vary with the interaction strength, the spin-flip state is robust over a range of interaction strengths and for both B1 and B8 structures.

cond-mat.str-el↗

arXiv2006

Mott Transition of MnO under Pressure: Comparison of Correlated Band Theories

The electronic structure, magnetic moment, and volume collapse of MnO under pressure are obtained from four different correlated band theory methods; local density approximation + Hubbard U (LDA+U), pseudopotential self-interaction correction (pseudo-SIC), the hybrid functional (combined local exchange plus Hartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each method treats correlation among the five Mn 3d orbitals (per spin), including their hybridization with three O $2p$ orbitals in the valence bands and their changes with pressure. The focus is on comparison of the methods for rocksalt MnO (neglecting the observed transition to the NiAs structure in the 90-100 GPa range). Each method predicts a first-order volume collapse, but with variation in the predicted volume and critical pressure. Accompanying the volume collapse is a moment collapse, which for all methods is from high-spin to low-spin (5/2 to 1/2), not to nonmagnetic as the simplest scenario would have. The specific manner in which the transition occurs varies considerably among the methods: pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives insulator-to-insulator and the hybrid method gives an insulator-to-semimetal transition. Projected densities of states above and below the transition are presented for each of the methods and used to analyze the character of each transition. In some cases the rhombohedral symmetry of the antiferromagnetically ordered phase clearly influences the character of the transition.

cond-mat.str-el↗

arXiv2006

Origin of Strong Coupling in Lithium under Pressure

In an attempt to provide a clearer understanding of the impressive increase in T_c under pressure in elemental Li, linear response calculation of the phonon dispersion curves, electron-phonon matrix elements, phonon linewidths and mode lambda's have been carried out on a finer mesh (24^3 in the Brillouin zone) than done previously, for the volume corresponding to 20 GPa pressure. The result illustrates the great need for a fine mesh (even finer than this) for converged results of lambda and the spectral function alpha^2 F. Although the initial pressure-induced transverse T_1 phonon instability (in harmonic approximation) near the symmetry point K has dominated attention, the current results show that the high value of T_c gets strong contributions from elsewhere in the zone, particularly from the longitudinal mode along (100).

cond-mat.supr-con↗

arXiv2006

Origin of ferromagnetism in Cs$_2$AgF$_4$: importance of Ag - F covalency

The magnetic nature of Cs$_{2}$AgF$_{4}$, an isoelectronic and isostructural analogue of La$_2$CuO$_4$, is analyzed using density functional calculations. The ground state is found to be ferromagnetic and nearly half metallic. We find strong hybridization of Ag-$d$ and F-$p$ states. Substantial moments reside on the F atoms, which is unusual for the halides and reflects the chemistry of the Ag(II) ions in this compound. This provides the mechanism for ferromagnetism, which we find to be itinerant in character, a result of a Stoner instability enhanced by Hund's coupling on the F.

cond-mat.mtrl-sci↗

arXiv2006

Destruction of the small Fermi surfaces in Na$_x$CoO$_2$ by Na disorder

We show using density functional calculations that the small $e_g'$ Fermi surfaces in Na$_x$CoO$_2$ are destroyed by Na disorder. This provides a means to resolve the prediction of these sections in band structure calculations with their non-observation in angle resolved photoemission experiments.

cond-mat.supr-con↗

arXiv2005

Superconductivity and Lattice Instability in Compressed Lithium from Fermi Surface Hot Spots

The highest superconducting temperature T$_c$ observed in any elemental metal (Li with T$_c$ ~ 20 K at pressure P ~ 40 GPa) is shown to arise from critical (formally divergent) electron-phonon coupling to the transverse T$_1$ phonon branch along intersections of Kohn anomaly surfaces with the Fermi surface. First principles linear response calculations of the phonon spectrum and spectral function $α^2 F(ω)$ reveal (harmonic) instability already at 25 GPa. Our results imply that the fcc phase is anharmonically stabilized in the 25-38 GPa range.

cond-mat.supr-con↗

arXiv2004

On Heavy Carbon Doping of MgB$_2$

Heavy carbon doping of MgB$_2$ is studied by first principles electronic structure studies of two types, an ordered supercell (Mg(B$_{1-x}$C$_{x}$)$_{2}$,x=0.0833) and also the coherent potential approximation method that incorporates effects of B-C disorder. For the ordered model, the twofold degenerate $σ$-bands that are the basis of the high temperature superconductivity are split by 60 meV (i.e.7 meV/%C) and the $σ$ Fermi cylinders contain 0.070 holes/cell, compared to 0.11 for MgB$_2$. A virtual crystal treatment tends to overestimate the rate at which $σ$ holes are filled by substitutional carbon. The coherent potential approximation (CPA) calculations give the same rate of band filling as the supercell method. The occupied local density of states of C is almost identical to that of B in the upper 2 eV of the valence bands, but in the range -8 eV to -2 eV, C has a considerably larger density of states. The calculations indicate that the $σ$ Fermi surface cylinders pinch off at the zone center only above the maximum C concentration x ~ 0.10. These results indicate that Mg(B$_{1-x}$C$_{x}$)$_{2}$ as well as Mg$_{1-x}$Al$_x$B$_2$ is a good system in which to study the evolution of the unusual electron-phonon coupling character and strength as the crucial $σ$ hole states are filled.

cond-mat.supr-con↗