arXiv subjects

S. Kumar

Publications and source records attributed to S. Kumar.

211 records · Page 12Linked to original sources

arXiv2009

Electronic structure studies of Fe- ZnO nanorods by x-ray absorption fine structure

We report the electronic structure studies of well characterized polycrystalline Zn_{1-x}Fe_xO (x = 0.0, 0.01, 0.03, and 0.05) nanorods synthesized by a co-precipitation method through x-ray absorption fine structure (XAFS). X-ray diffraction (XRD) reveals that Fe doped ZnO crystallizes in a single phase wurtzite structure without any secondary phase. From the XRD pattern, it is observed that peak positions shift towards lower 2θvalue with Fe doping. The change in the peak positions with increase in Fe contents clearly indicates that Fe ions are replacing Zn ions in the ZnO matrix. Linear combination fittings (LCF) at Fe K-edge demonstrate that Fe is in mixed valent state (Fe3+/Fe2+) with a ratio of ~ 7:3 (Fe3+:Fe2+). XAFS data is successfully fitted to wurtzite structure using IFEFFIT and Artemis. The results indicate that Fe substitutes Zn site in the ZnO matrix in tetrahedral symmetry.

cond-mat.mtrl-sci↗

arXiv2009

Study of nuclei in the vicinity of the "Island of Inversion" through fusion-evaporation reaction

We report the first observation of high-spin states in nuclei in the vicinity of the "island of inversion", populated via the 18O+18O fusion reaction at an incident beam energy of 34 MeV. The fusion reaction mechanism circumvents the limitations of non-equilibrated reactions used to populate these nuclei. Detailed spin-parity measurements in these difficult to populate nuclei have been possible from the observed coincidence anisotropy and the linear polarization measurements. The spectroscopy of 33,34P and 33S is presented in detail along with the results of calculations within the shell model framework.

nucl-ex↗

arXiv2008

Pulling self-interacting polymers in two-dimensions

We investigate a two-dimensional problem of an isolated self-interacting end-grafted polymer, pulled by one end. In the thermodynamic limit, we find that the model has only two different phases, namely a collapsed phase and a stretched phase. We show that the phase diagram obtained by Kumar {\it at al.\} [Phys. Rev. Lett. {\bf 98}, 128101 (2007)] for small systems, where differences between various statistical ensembles play an important role, differ from the phase diagram obtained here in the thermodynamic limit.

cond-mat.stat-mech↗

arXiv2008

Spin-orbital frustrations and anomalous metallic state in iron-pnictide superconductors

We develop an understanding of the anomalous metal state of the parent compounds of recently discovered iron based superconductors starting from a strong coupling viewpoint, including orbital degrees of freedom. On the basis of an intermediate-spin (S=1) state for the Fe^{2+} ions, we derive a Kugel-Khomskii spin-orbital Hamiltonian for the active t_{2g} orbitals. It turns out to be a highly complex model with frustrated spin and orbital interactions. We compute its classical phase diagrams and provide an understanding for the stability of the various phases by investigating its spin-only and orbital-only limits. The experimentally observed spin-stripe state is found to be stable over a wide regime of physical parameters and can be accompanied by three different types of orbital orders. Of these the orbital-ferro and orbital-stripe orders are particularly interesting since they break the in-plane lattice symmetry -- a robust feature of the undoped compounds. We compute the magnetic excitation spectra for the effective spin Hamiltonian, observing a strong reduction of the ordered moment, and point out that the proposed orbital ordering pattern can be measured in resonant X-ray diffraction.

cond-mat.supr-con↗

arXiv2008

Role of pulling direction in understanding the energy landscape of proteins

Single molecule force spectroscopy provide details of the underlying energy surfaces of proteins which are essential to the understanding of their unfolding process. Recently, it has been observed experimentally that by pulling proteins in different directions relative to their secondary structure, one can gain a better understanding of the shape of the energy landscape. We consider simple lattice models which are anisotropic in nature to study the response of a force in unfolding of a polymer. Our analytical solution of the model, supported by extensive numerical calculations, reveal that the force temperature diagrams are very different depending on the direction of the applied force. We find that either unzipping or shearing kind transitions dominate the dynamics of the unfolding process depending solely on the direction of the applied force.

cond-mat.stat-mech↗

arXiv2008

Correlation between site preference of ternary Mn addition in LaAg and superconductivity

The results of an extensive investigation of structure, surface morphology, composition and the superconducting-normal phase diagram of a new unconventional superconductor LaAg1-cMnc with nominal composition c = 0.0, 0.025, 0.05, 0.1, 0.2 and 0.3, reveal the following. The alloys with c = 0, 0.025 and 0.05 are essentially single phase alloys with the actual Mn concentration, x, same as the nominal one, i.e., c = x, whereas in the alloys with c = 0.1, 0.2 and 0.3, the actual Mn concentration of the majority phase (crystalline grains) is x = 0.050(1), 0.080(1) and 0.100(1), respectively. The ternary Mn addition does not alter the CsCl structure of the parent compound LaAg. Neither a structural phase transition occurs nor a long-range antiferromagnetic order exists at any temperature within the range 1.8K < = T < = 50K in any of the Mn containing alloys. Mn has exclusive La (Ag) site preference in the alloy (alloys) with x = c = 0.025 (x < = 0.05 or c < = 0.1) whereas in the alloy with x = c = 0.05, Mn has essentially no site preference in that all the Mn atoms either occupy the La sites or the Ag sites. In the alloys (alloy) with x < = 0.05 (x = c = 0.025), substitution of Ag (La) by Mn at the Ag (La) sub-lattice sites in LaAg host gives rise to unconventional superconductivity (destroys the conventional phonon-mediated superconductivity prevalent in the parent LaAg compound).

cond-mat.mtrl-sci↗

arXiv2006

Current-Voltage Characteristics of Long-Channel Nanobundle Thin-Film Transistors: A Bottom-up Perspective

By generalizing the classical linear response theory of stick percolation to nonlinear regime, we find that the drain current of a Nanobundle Thin Film Transistor (NB-TFT) is described under a rather general set of conditions by a universal scaling formula ID = A/LS g(LS/LC, rho_S * LS * LS) f(VG, VD), where A is a technology-specific constant, g is function of geometrical factors like stick length (LS), channel length (LC), and stick density (rho_S) and f is a function of drain (VD) and gate (VG) biasing conditions. This scaling formula implies that the measurement of full I-V characteristics of a single NB-TFT is sufficient to predict the performance characteristics of any other transistor with arbitrary geometrical parameters and biasing conditions.

cond-mat.mes-hall↗

arXiv2006

Computationally-efficient Expressions for the Collision Efficiency Between Electrically Charged Aerosol Particles and Cloud Droplets

A multifactor parameterization is described to permit the efficient calculation of collision efficiency (E) between electrically charged aerosol particles and neutral cloud droplets in numerical cloud and climate models. The four parameter representation summarizes the results obtained from a detailed microphysical model of collision efficiency, which accounts for the different forces acting on the aerosol in the path of falling cloud droplets. The parameterizations range of validity is for aerosol particle radius 0.4 to 10 micron, aerosol particle density 1 to 2.0 g.cm-3, aerosol particle charge from neutral to 100 elementary charges and drop radii 18.55-142 micron. It yields collision efficiencies well within an order of magnitude of the detailed models values, from a data set of of 3978 E values. 95 percent of these values have modeled to parameterized ratios between 0.5 and 1.5 for aerosol particle size range 0.4 to 2 micron and about 96 percent in the second size range. This parameterization speeds up the collision efficiency calculation by a factor of about 103, as compared with the original microphysical model, permitting the inclusion of electric charge effects in numerical cloud and climate models. In the following pages parameterization code in C language is provided for readymade use.

physics.ao-ph↗

arXiv2004

Circuit Quantum Electrodynamics: Coherent Coupling of a Single Photon to a Cooper Pair Box

Under appropriate conditions, superconducting electronic circuits behave quantum mechanically, with properties that can be designed and controlled at will. We have realized an experiment in which a superconducting two-level system, playing the role of an artificial atom, is strongly coupled to a single photon stored in an on-chip cavity. We show that the atom-photon coupling in this circuit can be made strong enough for coherent effects to dominate over dissipation, even in a solid state environment. This new regime of matter light interaction in a circuit can be exploited for quantum information processing and quantum communication. It may also lead to new approaches for single photon generation and detection.

cond-mat.mes-hall↗

arXiv2004

Extensions of Algebraic Groups

Let $G$ be a connected complex algebraic group and $A$ a connected abelian algebraic group endowed with an algebraic action of $G$ by group automorphisms. In the present note we describe the abelian group $\Ext_{alg}(G,A)$ of algebraic group extensions of $G$ by $A$ in terms of a short exact sequence relating the ext-group to a relative second Lie algebra cohomology space and the fundamental group of the commutator group. Our second main result is an analog of the Van-Est Theorem for algebraic group cohomology, saying that for an algebraic $G$ module $\a$ and $p \geq 0$ the algebraic group cohomology $H^p_{alg}(G,\a)$ is given by the relative cohomology of the Lie algebra $\g$ with respect to the Lie algebra of a maximal reductive subgroup.

math.AG↗

arXiv2002

A simple and exactly solvable model for a semiflexible polymer chain interacting with a surface

We use the lattice model of directed walks to investigate the conformational as well as the adsorption properties of a semiflexible homopolymer chain immersed in a good solvent in two and three dimensions. To account for the stiffness in the chain we have introduced energy barrier for each bend in the walk and have calculated the persistent length as a function of this energy. For the adsorption on an impenetrable surface perpendicular to the preferred direction of the walk we have solved the model exactly and have found the critical value of the surface attractions for the adsorption in both two and three dimensions. We have also enumerated all the possible walks on square and cubic lattices for the number of steps N <= 30 for two-dimensions and N <= 20 for three dimensions and have used ratio method for extrapolation. The transition located using this method is in excellent agreement with the results found from the analytical method.

cond-mat.stat-mech↗

arXiv2002

Does a surface attached globule phase exist ?

A long flexible neutral polymer chain immersed in a poor solvent and interacting with an impenetrable attractive surface exhibits a phase known as surface attached globule ({\bf SAG}) in addition to other adsorbed and desorbed phases. In the thermodynamic limit, the {\bf SAG} phase has the same free energy per monomer as the globular phase, and the transition between them is a surface transition. We have investigated the phase diagrams of such a chain in both two- and three- dimensions and calculated the distribution of monomers in different domains of the phase diagram.

cond-mat.stat-mech↗

arXiv1999

Self-duality and soldering in odd dimensions

Using the recently developed soldering formalism we highlight certain features of quantum mechanical models. The complete correspondence between these models and self dual field theoretical models in odd dimensions is established. The distinction between self duality and self dual factorisation in these dimensions is clarified.

hep-th↗