Subhendra D. Mahanti

STRUCTURE OF NANOCRYSTALLINE MATERIALS USING ATOMIC PAIR DISTRIBUTION FUNCTION ANALYSIS: STUDY OF LIMOS2

The structure of

Scanning tunneling microscopy of defect states in the semiconductor Bi2Se3

{\mathrm{LiMoS}}_{2}

Percolation and diffusion in two‐dimensional microporous media: Pillared clays

has been experimentally determined. The approach of atomic pair distribution function analysis was used because of the lack of well-defined Bragg peaks due to the short structural coherence (\ensuremath{\sim}50 \AA{}) in this intercalation compound. The reduction of Mo by Li results in Mo-Mo bonding with the formation of chains of distorted

Localized-electron mechanism for configuration mixing in Sm compounds

\mathrm{Mo}\ensuremath{-}{\mathrm{S}}_{6}

Physics of bandgap formation in Cu–Sb–Se based novel thermoelectrics: the role of Sb valency and Cu d levels

octahedra. Using refined structural parameters the electronic band structure for this material has been calculated and is in good agreement with observed material properties.

Exploration of metastability and hidden phases in correlated electron crystals visualized by femtosecond optical doping and electron crystallography

Scanning tunneling spectroscopy images of Bi2Se3 doped with excess Bi reveal electronic defect states with a striking shape resembling clover leaves. With a simple tight-binding model, we show that the geometry of the defect states in Bi2Se3 can be directly related to the position of the originating impurities. Only the Bi defects at the Se sites five atomic layers below the surface are experimentally observed. We show that this effect can be explained by the interplay of defect and surface electronic structure. Understanding the electronic properties of defects and the ability to control them will be crucial for the performance of the future microelectronic devices. 1 Scanning tunneling microscopy ~STM! represents a unique tool for the studies of defects as it combines atomic scale resolution with local spectroscopic capability. However, STM observation and analysis of defect states in semiconductors are complicated by surface effects such as in-gap surface states and reconstruction. These effects are avoided at the ~110! surfaces of a number of III-V semiconducting systems, 2 attracting extensive research. 3‐ 8 A number of point defect types have been observed. However, the positions of these defects with respect to the surface plane could be inferred only from indirect observations. The interpretation of such observations is complicated by the drastic effect the surface proximity may have on the defect states. 9 Modeling STM measurements of defects in semiconductors is not straightforward: Approximation of the STM images by maps of the local surface electronic density of states 10 is justified only if the charge relaxation rates of defect states significantly exceed the tunneling rate of electrons between the tip and the sample. 11 Tip-induced effects also need to be taken into account. These may include both local band bending, 3 and charging of the defect states by the tunneling current, resulting in bias voltage-dependent lattice relaxation in the vicinity of the defect atoms. 8 Careful analysis is necessary to clearly separate these effects from the intrinsic defect properties, and the bulk features of the observed defect states from the surface effects.

Temperature dependent total scattering structural study of CaCu3Ti4O12

We have investigated the adsorptive and diffusive properties of N2, H2O, and rare gas atoms (Ar and He) in the pillared layered silicate clay systems [Cr(en)3+3]x[Co(en)3+2−(en)]1−x−L, where L is vermiculite (V), fluorohectorite (FHT), or montmorillonite (M), and (en) is an ethylenediamine ligand. In these mixed ion intercalates the intercalated [Cr(en)3+3] cation, where all three en ligands are coordinated to chromium, represents a laterally small pillaring agent, whereas [Co(en)3+2−en] represents a laterally large, ligand‐dissociated species. Such systems are excellent models for two‐dimensional microporous media. Adsorption measurements were carried out for N2, H2O, and Ar and diffusion studies were performed using simulation methods for both Ar and He. We find that the adsorptive and diffusive properties depend sensitively on the size of the diffusing species and the concentrations x and (1−x) of the intercalants. For Ar adsorption in the FHT system we observe a percolative response when x reaches 0.7...

Collective electron effects of O2− in potassium superoxide

Abstract A new mechanism for (4f)5 - (4f)5 5d configuration mixing in SmB6 and SmS is described. It involves the simultaneous intra-atomic excitation of two Sm sites.

Self-assembly of ionic surfactants and formation of mesostructures

In this paper we discuss the results of ab initio electronic structure calculations for Cu(3)SbSe(4) (Se4) and Cu(3)SbSe(3) (Se3), two narrow bandgap semiconductors of thermoelectric interest. We find that Sb is trivalent in both the compounds, in contrast to a simple nominal valence (ionic) picture which suggests that Sb should be 5 + in Se4. The gap formation in Se4 is quite subtle, with hybridization between Sb 5s and the neighboring Se 4s, 4p orbitals, position of Cu d states, and non-local exchange interaction, each playing significant roles. Thermopower calculations show that Se4 is a better p-type system. Our theoretical results for Se4 agree very well with recent experimental results obtained by Skoug et al (2011 Sci. Adv. Mater. 3 602).

Spin splitting in 2D monochalcogenide semiconductors.

Using femtosecond photodoping and crystallography to explore metastable and hidden quantum phases in tantalum disulfide. Characterizing and understanding the emergence of multiple macroscopically ordered electronic phases through subtle tuning of temperature, pressure, and chemical doping has been a long-standing central issue for complex materials research. We report the first comprehensive studies of optical doping–induced emergence of stable phases and metastable hidden phases visualized in situ by femtosecond electron crystallography. The electronic phase transitions are triggered by femtosecond infrared pulses, and a temperature–optical density phase diagram is constructed and substantiated with the dynamics of metastable states, highlighting the cooperation and competition through which the macroscopic quantum orders emerge. These results elucidate key pathways of femtosecond electronic switching phenomena and provide an important new avenue to comprehensively investigate optical doping–induced transition states and phase diagrams of complex materials with wide-ranging applications.