arXiv subjects

D. D. Sarma

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

At least 73 records · Page 4Linked to original sources

arXiv2007

Growth mechanism of nanocrystals in solution: ZnO, a case study

We investigate the mechanism of growth of nanocrystals from solution using the case of ZnO. Spanning a wide range of values of the parameters, such as the temperature and the reactant concentration, that control the growth, our results establish a qualitative departure from the widely accepted diffusion controlled coarsening (Ostwald ripening) process quantified in terms of the Lifshitz-Slyozov-Wagner theory. Further, we show that these experimental observations can be qualitatively and quantitatively understood within a growth mechanism that is intermediate between the two well-defined limits of diffusion control and kinetic control.

cond-mat.mtrl-sci↗

arXiv2007

Electronic structure of NaWO$_3$: Role of the impurity potential

We have performed {\it ab-initio} electronic structure calculations to determine the evolution of the electronic structure of WO$_3$ with Na doping. Na doping introduces an additional electron when introduced into WO$_3$. The ensuing electronic structure of Na doped WO$_3$, we find, is very similar to the electronic structure of an electron introduced into WO$_3$, thus clarifying the role of the impurity potential due to Na. While the electronic structure of NaWO$_3$ allows a rigid band like description over a certain energy range, modifications introduced in the electronic structure can be related back to the electron due to Na and not the impurity potential that one generally believes to be responsible.

cond-mat.mtrl-sci↗

arXiv2007

Unusual doping and temperature dependence of photoemission spectra from manganites

A recent, major, puzzle in the core-level photoemission spectra of doped manganites is the observation of a 1-2 eV wide shoulder with intensity varying with temperature T as the square of the magnetization over a T scale of order 200K, an order of magnitude less than electronic energies. This is addressed and resolved here, by extending a recently proposed two electron fluid l-b model for these systems to include core-hole effects. The shoulder arises from a rapid redistribution of e_g electron density, as a function of T, between the highly localized (l) and band-like (b) states. Furthermore, our theory leads to a correspondence between spectral changes due to increasing doping and decreasing T, as experimentally observed.

cond-mat.str-el↗

arXiv2007

Critical test for Altshuler-Aronov theory: Evolution of the density of states singularity in double perovskite Sr$_2$FeMoO$_6$ with controlled disorder

With high-resolution photoemission spectroscopy measurements, the density of states (DOS) near the Fermi level ($E_\mathrm{F}$) of double perovskite Sr$_2$FeMoO$_6$ having different degrees of Fe/Mo antisite disorder has been investigated with varying temperature. The DOS near $E_\mathrm{F}$ showed a systematic depletion with increasing degree of disorder, and recovered with increasing temperature. Altshuler-Aronov (AA) theory of disordered metals well explains the dependences of the experimental results. Scaling analysis of the spectra provides experimental indication for the functional form of the AA DOS singularity.

cond-mat.mtrl-sci↗

arXiv2007

Theoretical study of doped-Tl$_{2}$Mn$_{2}$O$_{7}$ and Tl$_{2}$Mn$_{2}$O$_{7}$ under pressure

Using first-principles density functional based calculations, we study the effect of doping and pressure in manganese based pyrochlore compound,Tl$_{2}$Mn$_{2}$O$_{7}$ that exhibits colossal magneto-resistive behavior. The theoretical study is motivated by the counter-intuitive experimental observation of suppression of ferromagnetic transition temperature upon application of pressure and its enhancement upon substitution of Mn by moderate amount of nonmagnetic Sb ion. We also attempt to resolve the issue related to crystal structure changes that may occur upon application of pressure.

cond-mat.mtrl-sci↗

arXiv2006

Surface criticality and multifractality at localization transitions

We develop the concept of surface multifractality for localization-delocalization (LD) transitions in disordered electronic systems. We point out that the critical behavior of various observables related to wave functions near a boundary at a LD transition is different from that in the bulk. We illustrate this point with a calculation of boundary critical and multifractal behavior at the 2D spin quantum Hall transition and in a 2D metal at scales below the localization length.

cond-mat.mes-hall↗

arXiv2006

Electron-spectroscopic investigation of metal-insulator transition in Sr2Ru1-xTixO4 (x=0.0-0.6)

We investigate the nature and origin of the metal-insulator transition in Sr2Ru1-xTixO4 as a function of increasing Ti content (x). Employing detailed core, valence, and conduction band studies with x-ray and ultraviolet photoelectron spectroscopies along with Bremsstrahlung isochromat spectroscopy, it is shown that a hard gap opens up for Ti content greater than equal to 0.2, while compositions with x<0.2 exhibit finite intensity at the Fermi energy. This establishes that the metal-insulator transition in this homovalent substituted series of compounds is driven by Coulomb interaction leading to the formation of a Mott gap, in contrast to transitions driven by disorder effects or band flling.

cond-mat.str-el↗

arXiv2006

A microspectroscopic study of the electronic homogeneity of ordered and disordered Sr2FeMoO6

Besides a drastic reduction in saturation magnetization of disordered Sr2FeMoO6 compared to highly ordered samples, magnetizations as a function of the temperature for different disordered samples may also show qualitatively different behaviors. We investigate the origin of such diversity by performing spatially resolved photoemission spectroscopy on various disordered samples. Our results establish that extensive electronic inhomogeneity, arising most probably from an underlying chemical inhomogeneity in disordered samples is responsible for the observed magnetic inhomogeneity. It is further pointed out that these inhomogeneities are connected with composition fluctuations of the type Sr2Fe1+xMo1-xO6 with Fe-rich (x>0) and Mo-rich (x<0) regions.

cond-mat.mtrl-sci↗

arXiv2006

Origin of Ferromagnetism and its pressure and doping dependence in Tl$_{2}$Mn$_{2}$O$_{7}$

Using NMTO-{\it downfolding} technique, we explore and establish the origin of ferromagnetism in the pyrochlore system, Tl$_{2}$Mn$_{2}$O$_{7}$. It is found to be driven by hybridization induced spin-polarization of the delocalized charge carriers derived from Tl-$s$ and O-$p$ states. The mean-field estimate of the ferromagnetic transition temperature, T$_c$, estimated using computed exchange integrals are found to be in good agreement with the measurements. We find an enhancement of T$_{c}$ for moderate doping with nonmagnetic Sb and a suppression of T$_{c}$ upon application of pressure, both in agreement with experimental findings.

cond-mat.mtrl-sci↗

arXiv2005

Understanding the bulk electronic structure of Ca1-xSrxVO3

We investigate the electronic structure of Ca1-xSrxVO3 using careful state-of-the-art experiments and calculations. Photoemission spectra using synchrotron radiation reveal a hitherto unnoticed polarization dependence of the photoemission matrix elements for the surface component leading to a substantial suppression of its intensity. Bulk spectra extracted with the help of experimentally determined electron escape depth and estimated suppression of surface contributions resolve outstanding puzzles concerning the electronic structure in Ca1-xSrxVO3.

cond-mat.str-el↗

arXiv2005

Electronic structure of and Quantum size effect in III-V and II-VI semiconducting nanocrystals using a realistic tight binding approach

We analyze the electronic structure of group III-V semiconductors obtained within full potential linearized augmented plane wave (FP-LAPW) method and arrive at a realistic and minimal tight-binding model, parameterized to provide an accurate description of both valence and conduction bands. It is shown that cation sp3 - anion sp3d5 basis along with the next nearest neighbor model for hopping interactions is sufficient to describe the electronic structure of these systems over a wide energy range, obviating the use of any fictitious s* orbital, employed previously. Similar analyses were also performed for the II-VI semiconductors, using the more accurate FP-LAPW method compared to previous approaches, in order to enhance reliability of the parameter values. Using these parameters, we calculate the electronic structure of III-V and II-VI nanocrystals in real space with sizes ranging upto about 7 nm in diameter, establishing a quantitatively accurate description of the band-gap variation with sizes for the various nanocrystals by comparing with available experimental results from the literature.

cond-mat.mtrl-sci↗

arXiv2004

Strong correlation effects of the Re 5$d$ electrons on the metal-insulator transition in Ca$_2$FeReO$_6$

We have investigated the electronic structure of polycrystalline Ca$_2$FeReO$_6$ using photoemission spectroscopy and band-structure calculations within the local-density approximation+$U$ (LDA+$U$) scheme. In valence-band photoemission spectra, a double-peak structure which is characteristic of the metallic double perovskite series has been observed near the Fermi level ($E_{\rm F}$), although it is less distinct compared to the Sr$_2$FeMoO$_6$ case. The leading near-$E_{\rm F}$ structure has a very weak spectral weight at $E_{\rm F}$ above the metal-insulator transition (MIT) temperature $T_{\rm MI}$ of $\sim$140 K, and it loses the $E_{\rm F}$ weight below $T_{\rm MI}$, forming a small energy gap. To reproduce this small energy gap in the calculation, we require a very large effective $U$ ($U_{\rm eff}$) for Re (4 eV) in addition to a relatively large $U_{\rm eff}$ for Fe (4 eV). Although the most of the experimental features can be interpreted with the help of the band theory, the overall agreement between the theory and the experiment was not satisfactory. We demonstrate that the effective transfer integral between Fe and Re is actually smaller than that between Fe and Mo in Ca$_2$FeMoO$_6$, which can explain both MIT and very high ferrimagnetic transition temperature.

cond-mat.str-el↗

arXiv2004

An Interesting Magnetoresistive System: Sr2FeMoO6

Ordered double perovskite oxides of the general formula, A2BB'O6, have been known for several decades to have interesting electronic and magnetic properties. However, a recent report of a spectacular negative magnetoresistance effect in a specific member of this family, namely Sr2FeMoO6, has brought this class of compounds under intense scrutiny. In this small review, we present few theoretical and experimental results, describing mainly the effects of Fe/Mo antisite defects on the properties of this compound and also briefly discuss few other puzzling facts about this fascinating compound

cond-mat.mtrl-sci↗

arXiv2004

Direct observation of electronic domains in manganites by spatially resolved spectroscopy

We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional sensitivity to chemical compositions to investigate high-quality single crystal samples of La_{1/4}Pr_{3/8}Ca_{3/8}MnO_{3}, establishing the formation of distinct insulating domains embedded in the metallic host at low temperatures. These domains are found to be at least an order of magnitude larger in size compared to previous estimates and exhibit memory effects on temperature cycling in the absence of any perceptible chemical inhomogeneity, suggesting long-range strains as the probable origin.

cond-mat.mtrl-sci↗

arXiv2004

Unusual direction dependence of exchange energies in GaAs:Mn - Is the RKKY description relevant

Ferromagnetism in Mn-doped GaAs, the prototypical dilute magnetic semiconductor, has so far been attributed to hole mediated RKKY-type interactions. First-principles calculations reveal a strong direction dependence of the ferromagnetic (FM) stabilization energy of two magnetic ions, a dependence that cannot be explained within RKKY. In the limit of host-like hole (engineered here by an GGA+U approach with large U) where the RKKY model is applicable, we find that the exchange energies are strongly reduced, suggesting that this limit cannot explain the observed ferromagnetism. The dominant contribution stabilizing the FM state is found to be maximal for <110>-oriented pairs and minimal for <100> oriented pairs, providing an alternate explanation for magnetism in such materials in terms of energy lowering due to p-d hopping interactions, and offering a new design degree of freedom to enhance FM.

cond-mat.other↗

arXiv2003

Evolution of the electronic structure with size in II-VI semiconductor nanocrystals

In order to provide a quantitatively accurate description of the band gap variation with sizes in various II-VI semiconductor nanocrystals, we make use of the recently reported tight-binding parametrization of the corresponding bulk systems. Using the same tight-binding scheme and parameters, we calculate the electronic structure of II-VI nanocrystals in real space with sizes ranging between 5 and 80 Å in diameter. A comparison with available experimental results from the literature shows an excellent agreement over the entire range of sizes.

cond-mat.mtrl-sci↗

arXiv2003

On the origin of magnetoresistance in Sr$_2$FeMoO$_6$

We report detailed magnetization ($M$) and magnetoresistance ($MR$) studies on a series of Sr$_2$FeMoO$_6$ samples with independent control on anti-site defect and grain boundary densities. These results, exhibiting a switching-like behavior of $MR$ with $M$, establish that the $MR$ is controlled by the magnetic polarization of grain boundary regions, rather than of the grains within a resonant tunnelling mechanism.

cond-mat.str-el↗

arXiv2003

X-ray photoemission study of NiS_{2-x}Se_x (x = 0.0 - 1.2)

Electronic structure of NiS_{2-x}Se_x system has been investigated for various compositions (x) using x-ray photoemission spectroscopy. An analysis of the core level as well as the valence band spectra of NiS_2 in conjunction with many-body cluster calculations provides a quantitative description of the electronic structure of this compound. With increasing Se content, the on-site Coulomb correlation strength (U) does not change, while the band width W of the system increases, driving the system from a covalent insulating state to a pd-metallic state.

cond-mat.str-el↗