arXiv subjects

C. D. Pemmaraju

Publications and source records attributed to C. D. Pemmaraju.

24 records · Page 2Linked to original sources

arXiv2009

Polaronic distortion and vacancy-induced magnetism in MgO

The electronic structure of the neutral and singly charged Mg vacancy in MgO is investigated using density functional theory. For both defects, semilocal exchange correlation functionals such as the local spin density approximation incorrectly predict a delocalized degenerate ground state. In contrast functionals that take strong correlation effects into account predict a localized solution, in agreement with spin resonance experiments. Our results, obtained with the HSE hybrid, atomic self-interaction corrected and LDA+U functionals, provide a number of constraints to the possibility of ferromagnetism in hole doped MgO.

cond-mat.mtrl-sci↗

arXiv2009

Magnetic state electrical readout of Mn12 molecules

We demonstrate that the different magnetic states of a Mn12 molecule can be distinguished in a two-probe transport experiment from a complete knowledge of the current-voltage curve. Our results, obtained with state-of-the-art non-equilibrium transport methods combined with density functional theory, demonstrate that spin configuration-specific negative differential resistances (NDRs) appear in the I-V curves. These originate from the interplay between electron localization and the re-hybridization of the molecular levels in an external electric field and allow the detection of the molecule's spin-state.

cond-mat.mtrl-sci↗

arXiv2008

Predicting d$^0$ magnetism

Predicting magnetism originating from 2$p$ orbitals is a delicate problem, which depends on the subtle interplay between covalency and Hund's coupling. Calculations based on density functional theory and the local spin density approximation fail in two remarkably different ways. On the one hand the excessive delocalization of spin-polarized holes leads to half-metallic ground states and the expectation of room temperature ferromagnetism. On the other hand, in some cases a magnetic ground state may not be predicted at all. We demonstrate that a simple self-interaction correction scheme modifies both these situations via an enhanced localization of the holes responsible for the magnetism and possibly Jahn-Teller distortion. In both cases the ground state becomes insulating and the magnetic coupling between the impurities weak.

cond-mat.mtrl-sci↗

arXiv2008

Impurity-Ion pair induced high-temperature ferromagnetism in Co-doped ZnO

Magnetic 3d-ions doped into wide-gap oxides show signatures of room temperature ferromagnetism, although their concentration is two orders of magnitude smaller than that in conventional magnets. The prototype of these exceptional materials is Co-doped ZnO, for which an explanation of the room temperature ferromagnetism is still elusive. Here we demonstrate that magnetism originates from Co2+ oxygen-vacancy pairs with a partially filled level close to the ZnO conduction band minimum. The magnetic interaction between these pairs is sufficiently long-ranged to cause percolation at moderate concentrations. However, magnetically correlated clusters large enough to show hysteresis at room temperature already form below the percolation threshold and explain the current experimental findings. Our work demonstrates that the magnetism in ZnO:Co is entirely governed by intrinsic defects and a phase diagram is presented. This suggests a recipe for tailoring the magnetic properties of spintronics materials by controlling their intrinsic defects.

cond-mat.mtrl-sci↗

arXiv2007

Polarizability of molecular chains: does one need exact exchange?

Standard density functional approximations greatly over-estimate the static polarizability of longchain polymers, but Hartree-Fock or exact exchange calculations do not. Simple self-interaction corrected (SIC) approximations can be even better than exact exchange, while their computational cost can scale only linearly with the number of occupied orbitals.

physics.atm-clus↗

arXiv2006

Atomic self-interaction correction for molecules and solids

We present an atomic orbital based approximate scheme for self-interaction correction (SIC) to the local density approximation of density functional theory. The method, based on the idea of Filippetti and Spaldin [Phys. Rev. B 67, 125109 (2003)], is implemented in a code using localized numerical atomic orbital basis sets and is now suitable for both molecules and extended solids. After deriving the fundamental equations as a non-variational approximation of the self-consistent SIC theory, we present results for a wide range of molecules and insulators. In particular, we investigate the effect of re-scaling the self-interaction correction and we establish a link with the existing atomic-like corrective scheme LDA+U. We find that when no re-scaling is applied, i.e. when we consider the entire atomic correction, the Kohn-Sham HOMO eigenvalue is a rather good approximation to the experimental ionization potential for molecules. Similarly the HOMO eigenvalues of negatively charged molecules reproduce closely the molecular affinities. In contrast a re-scaling of about 50% is necessary to reproduce insulator bandgaps in solids, which otherwise are largely overestimated. The method therefore represents a Kohn-Sham based single-particle theory and offers good prospects for applications where the actual position of the Kohn-Sham eigenvalues is important, such as quantum transport.

cond-mat.str-el↗