arXiv subjects

D. D. Sarma

Publications and source records attributed to D. D. Sarma.

102 records · Page 6Linked to original sources

arXiv2000

Electronic Structure of Sr_2FeMoO_6

We have analysed the unusual electronic structure of Sr_2FeMoO_6 combining ab-initio and model Hamiltonian approaches. Our results indicate that there are strong enhancements of the intraatomic exchange strength at the Mo site as well as the antiferromagnetic coupling strength between Fe and Mo sites. We discuss the possibility of a negative effective Coulomb correlation strength (U_{eff}) at the Mo site due to these renormalised interaction strengths.

cond-mat.str-el↗

arXiv2000

Electronic structure of NiS_{1-x}Se_x

We investigate the electronic structure of the metallic NiS$_{1-x}$Se$_x$ system using various electron spectroscopic techniques. The band structure results do not describe the details of the spectral features in the experimental spectrum, even for this paramagnetic metallic phase. However, a parameterized many-body multi-band model is found to be successful in describing the Ni~2$p$ core level and valence band, within the same model. The asymmetric line shape as well as the weak intensity feature in the Ni~2$p$ core level spectrum has been ascribed to extrinsic loss processes in the system. The presence of satellite features in the valence band spectrum shows the existence of the lower Hubbard band, deep inside the $pd$ metallic regime, consistent with the predictions of the dynamical mean field theory.

cond-mat.str-el↗

arXiv2000

Magnetoresistance in ordered and disordered double perovskite oxide, Sr$_2$FeMoO$_6$

We have prepared crystallographically ordered and disorder specimens of the double perovskite, Sr$_2$FeMoO$_6$ and investigated their magnetoresistance behaviour. The extent of ordering between the Fe and Mo sites in the two samples is determined by Rietveld analysis of powder x-ray diffraction patterns and reconfirmed by Mössbauer studies. While the ordered sample exhibits the sharp low-field response, followed by moderate changes in the magnetoresistance at higher fields, the disordered sample is characterised by the absence of the spectacular low-field response. We argue that the low field response depends crucially on the half-metallic ferromagnetism, while the high-field response follows from the overall magnetic nature of the sample, even in absence of the half-metallic state.

cond-mat.str-el↗

arXiv1998

Auger transition from orbitally degenerate systems: Effects of screening and multielectron excitations

We calculate Auger spectra given by the two-hole Green's function from orbitally degenerate Hubbard-like models as a function of correlation strength and band filling. The resulting spectra are qualitatively different from those obtained from fully-filled singly degenerate models due to the presence of screening dynamics and multielectron excitations. Application to a real system shows remarkable agreement with experimental results leading to reinterpretation of spectral features.

cond-mat.str-el↗

arXiv1998

Disorder effects in electronic structure of substituted transition metal compounds

Investigating LaNi(1-x)M(x)O3 (M = Mn and Fe), we identify a characteristic evolution of the spectral function with increasing disorder in presence of strong interaction effects across the metal-insulator transition. We discuss these results vis-a-vis existing theories of electronic structure in simultaneous presence of disorder and interaction.

cond-mat.str-el↗

arXiv1998

Evolution of spectral function in a doped Mott insulator : surface vs. bulk contributions

We study the evolution of the spectral function with progressive hole doping in a Mott insulator, $La_{1-x}Ca_xVO_3$ with $x$ = 0.0 - 0.5. The spectral features indicate a bulk-to-surface metal-insulator transition in this system. Doping dependent changes in the bulk electronic structure are shown to be incompatible with existing theoretical predictions. An empirical description based on the single parameter, $U/W$, is shown to describe consistently the spectral evolution.

cond-mat.str-el↗

arXiv1998

Electronic structure of NiS$_{1-x}$Se$_x$ across the phase transition

We report very highly resolved photoemission spectra of NiS(1-x)Se(x) across the so-called metal-insulator transition as a function of temperature as well as composition. The present results convincingly demonstrate that the low temperature, antiferromagnetic phase is metallic, with a reduced density of states at E$_F$. This decrease is possibly due to the opening of gaps along specific directions in the Brillouin zone caused by the antiferromagnetic ordering.

cond-mat.str-el↗

arXiv1996

Electronic excitation spectra from ab-initio band structure results for LaMO_3 (M = Cr-Ni)

We present calculated electron excitation spectra for the LaMO_3 series (M=Cr-Ni) obtained within ab-initio band structure calculations for the real geometric and magnetic structures. The calculated results show good agreement with the experimentally obtained spectra. This suggests that the transition metal-transition metal interactions via the oxygen atom play an important role in determining the spectroscopic features and indicates a smaller value of U/t than has been believed so far. The present approach undermines the importance of multiplet interactions which otherwise play an important role within various single impurity models.

cond-mat.str-el↗

arXiv1996

Estimates of electronic interaction parameters for La$M$O$_3$ compounds ($M$=Ti-Ni) from ab-initio approaches

We have analyzed the ab-initio local density approximation band structure calculations for the family of perovskite oxides, La$M$O$_3$ with $M$=Ti-Ni within a parametrized nearest neighbor tight-binding model and extracted various interaction strengths. We study the systematics in these interaction parameters across the transition metal series and discuss the relevance of these in a many-body description of these oxides. The results obtained here compare well with estimates of these parameters obtained via analysis of electron spectroscopic results in conjunction with the Anderson impurity model. The dependence of the hopping interaction strength, t, is found to be approximately $r^{-3}$.

cond-mat↗

arXiv1996

Transport and magnetic properties of La$_{1-x}$Ca$_x$VO$_3$

We report the temperature dependence of transport and magnetic properties of La$_{1-x}$Ca$_x$VO$_3$ for x = 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5. The system exhibits an insulator-to-metal transition concomitant with an antiferromagnetic-to-paramagnetic transition near x = 0.2 with increasing substitution. Disorder effects are found to influence the low temperature transport properties of both insulating and metallic compositions near the critical concentration. At higher temperatures, the resistivity of the metallic samples is found to exhibit either a $T^{1.5}$ or a $T^2$ dependence depending on the composition. The molar susceptibility for the metallic samples indicate substantial enhancements due to electron correlation.

cond-mat.str-el↗

arXiv1996

Spectral functions in doped transition metal oxides

We present experimental photoemission and inverse photoemission spectra of SrTiO$_{3- δ}$ representing electron doped $d^0$ systems. Photoemission spectra in presence of electron doping exhibit prominent features arising from electron correlation effects, while the inverse photoemssion spectra are dominated by spectral features explainable within single-particle approaches. We show that such a spectral evolution in chemically doped correlated systems is not compatible with expectations based on Hubbard or any other similar model. We present a new theoretical approach taking into account the inhomogeneity of the `real' system which gives qualitatively different results compared to standard `homogeneous' models and is in quantitative agreement with experiments.

cond-mat↗

arXiv1996

Evolution of photoemission spectral functions in doped transition metal oxides

We discuss the experimental photoemission and inverse photoemission of early transition metal oxides, in the light of the dynamical mean field theory of correlated electrons which becomes exact in the limit of infinite dimensions. We argue that a comprehensive description of the experimental data requires spatial inhomogeneities and present a calculation of the evolution of the spectral function in an inhomogenous system with various degrees of inhomogeneity. We also point out that comparaison of experimental results and large d calculations require that the degree of correlation and disorder is larger in the surface than in the bulk.

cond-mat↗