arXiv subjects

Anil Kumar

Publications and source records attributed to Anil Kumar.

At least 109 records · Page 6Linked to original sources

arXiv2016

Charge-order-induced ferroelectricity in LaVO$_{3}$/SrVO$_{3}$ superlattices

The structure and properties of the 1:1 superlattice of LaVO$_{3}$ and SrVO$_{3}$ are investigated with a first-principles density-functional-theory-plus-$U$ (DFT+$U$) method. The lowest energy states are antiferromagnetic charge-ordered Mott-insulating phases. In one of these insulating phases, layered charge ordering combines with the layered cation ordering to produce a polar structure with nonzero spontaneous polarization normal to the interfaces. This polarization is produced by electron transfer between the V$^{3+}$ and V$^{4+}$ layers, and is comparable to that of conventional ferroelectrics. The energy of this polar state relative to the nonpolar ground state is only 3 meV per vanadium. Under tensile strain, this energy difference can be further reduced, suggesting that the polar phase can be induced by applied electric field, yielding an antiferroelectric double-hysteresis loop. If the system does not switch back to the nonpolar state on removal of the field, a ferroelectric-type hysteresis loop could be observed.

cond-mat.mtrl-sci↗

arXiv2016

Experimental evidence for bulk superconductivity in pure Bismuth single crystal at ambient pressure

Bulk rhombohedral Bismuth (Bi) at ambient pressure is a semimetal and it remains in the normal state down to 10~mK. The superconductivity (SC) in bulk Bi is thought to be very unlikely due to extremely low carrier density. The question of SC in Bi has remained unsolved both theoretically and experimentally. Here, we report first ever observation of bulk SC in highly pure Bi single crystals (99.9999\%) below 0.53~mK under ambient pressure with an estimated critical magnetic field of 5.2~$μ$T at 0~K. The conventional Bardeen-Cooper-Schrieffer (BCS) theory cannot explain the observed SC in Bi, since the adiabatic approximation of the BCS theory, $ω_D/E_F<< 1$, does not hold true for Bi. Bi has a multi-valley type electronic band structure and SC in Bi could be brought about by the inter-valley electron-phonon coupling. Such a scenario calls for new theoretical ideas to understand SC in such low carrier systems with unusual band structure in the non-adiabatic limit, $ω_D/E_F \gtrsim 1$. The observation of SC in Bi makes it the lowest carrier density superconductor surpassing the record held by doped SrTiO$_3$ for nearly 50 years.

cond-mat.supr-con↗

arXiv2016

Approximate Controllability of a Class of Partial Integro-Differential Equations of Parabolic Type

In this paper, we discuss the distributed control problem governed by the following parabolic integro-differential equation (PIDE) in the abstract form \begin{eqnarray*} \frac{\partial y}{\partial t} + A y &=& \int_0^t B(t, s) y(s) ds + Gu, \;\, t \in [0, T], \;\;\;\;\;\;\;\;\;\;\;\;\;\;\, \hfill{(\ast)}\\ y(0) &=& y_0 \, \in X, \nonumber \end{eqnarray*} where, $y$ denotes the state space variable, $u$ is the control variable, $A$ is a self adjoint, positive definite linear (not necessarily bounded) operator in a Hilbert space $X$ with dense domain $D(A) \subset X,$ $B(t,s)$ is an unbounded operator, smooth with respect to $t$ and $s$ with $D(A) \subset D(B(t,s)) \subset X$ for $0 \leq s \leq t \leq T$ and $G$ is a bounded linear operator from the control space to $X.$ Assuming that the corresponding evolution equation ($B \equiv 0$ in ($\ast$)) is approximately controllable, it is shown that the set of approximate controls of the distributed control problem ($\ast$) is nonempty. The problem is first viewed as constrained optimal control problem and then it is approximated by unconstrained problem with a suitable penalty function. The optimal pair of the constrained problem is obtained as the limit of optimal pair sequence of the unconstrained problem. The approximation theorems, which guarantee the convergence of the numerical scheme to the optimal pair sequence, are also proved.

math.OC↗

arXiv2016

First-principles study of $\langle c+a \rangle$ dislocations in Mg

We use first-principles density functional theory to study the generalized stacking fault energy surfaces for pyramidal-I and pyramidal-II slip systems in Mg. We demonstrate that the additional relaxation of atomic motions normal to the slip direction allows for the appropriate local minimum in the generalized stacking fault energy (GSFE) curve to be found. The fault energy calculations suggest that formation of pyramidal-I dislocations would be slightly more energetically favorable than that for pyramidal-II dislocations. The calculated pyramidal-II GSFE curves also indicate that the full pyramidal II dislocations would dissociate into the Stohr and Poirier (SP) configuration, consisting of two $\frac{1}{2}\langle c+a \rangle$ partials, $\frac{1}{6}[11{\bar2}3] + \frac{1}{6}[11{\bar2}3]$ , but the pyramidal-I GSFE curves, while also possessing a local minimum, would not dissociate into the same SP configuration. We report observation of these partials here emanating from a $\{10{\bar1}2 \}$ twin boundary. Using MD simulations with MEAM potential for Mg, we find that the full pyramidal-II $\langle c+a \rangle $ dislocation splits into two equal value partials $\frac{1}{6}[11{\bar2}3] + \frac{1}{6}[11{\bar2}3]$ separated by ~22.6 $Å$. We reveal that the full pyramidal-I $\langle c+a \rangle$ dislocation dissociates also into two equal value partials but onto alternating $(30{\bar3}4)$ and $(30{\bar3}2)$ planes with $\frac{1}{6} [20{\bar2}3]$ and $\frac{1}{6} [02{\bar2}3]$ Burgers vectors separated by a 30.4 $Å$ wide stacking fault. When a stress is applied, edge and mixed dislocations of the extended pyramidal-II dislocation can move on their glide plane; however, pyramidal-I dislocations of similar character cannot.

cond-mat.mtrl-sci↗

arXiv2014

Discovery of an isostructural phase transition within orthorhombic phase field of CaTiO3

Earths lower mantle extending from 670 to 2,990 km deep is predominantly composed of a perovskite-type (Mg,Fe)SiO3 phase1,2. The perovskite phase undergoes a structural phase transition to a post-perovskite phase responsible for D" layer seismic discontinuity2,3 at about 2690 km depth in the lowermost region of the lower mantle. However, structural basis of other seismic discontinuities occurring in the upper region of the lower mantle (700 km to 1,200 km deep) remains unexplained4-7, as no apparent change in the crystal symmetry of the orthorhombic perovskite phase has been reported5. We present here unambiguous evidence for a non-apparent isostructural phase transition8 in the stable orthorhombic perovskite phase of CaTiO3 which may have relevance to phase transitions in the perovskite phase of (Mg,Fe)SiO3 also, as both the compounds have similar structure, tolerance factor and thermochemical properties9-11. Our results are based on the analysis of neutron powder diffraction patterns using Rietveld and mode crystallography techniques and are supported by density functional and Landau theory calculations. The present results on CaTiO3 would encourage search for isostructural phase transition in the perovskite phase of (Mg,Fe)SiO3 that may provide clue to the unexplained geophysical phenomena in the upper part of the earths lower mantle.

cond-mat.mtrl-sci↗

arXiv2013

Monogamy of quantum correlations reveals frustration in a quantum Ising spin system: Experimental demonstration

We report a nuclear magnetic resonance experiment, which simulates the quantum transverse Ising spin system in a triangular configuration and further show that the monogamy of quantum correlations can be used to distinguish between the frustrated and non-frustrated regimes in the ground state of this system. Adiabatic state preparation methods are used to prepare the ground states of the spin system. We employ two different multipartite quantum correlation measures to analyze the experimental ground state of the system in both the frustrated and non-frustrated regimes. In particular, we use multipartite quantum correlation measures generated by monogamy considerations of negativity, a bipartite entanglement measure, and that of quantum discord, an information-theoretic quantum correlation measure. As expected from theoretical predictions, the experimental data confirm that the non-frustrated regime shows higher multipartite quantum correlations compared to the frustrated one.

quant-ph↗

arXiv2013

Simulation of mirror inversion of quantum states in an XY spin chain using NMR

We report an experimental quantum simulation of unitary dynamics of an XY spin chain with pre-engineered couplings. Using this simulation, we demonstrate the mirror inversion of quantum states, proposed by Albanese et al. [Phys. Rev. Lett. 93, 230502 (2004)]. The experiment is performed with a 5-qubit dipolar coupled spin system using nuclear magnetic resonance techniques. To perform quantum simulation we make use of the recently proposed unitary operator decomposition algorithm of Ajoy et al. [Phys. Rev. A 85, 030303 (2012)] along with numerical pulse optimization techniques. Further, using mirror inversion, we demonstrate that entangled states can be transferred from one end of the chain to the other end. The simulations are implemented with high experimental fidelity, which implies that these kind of simulations may be possible in larger systems.

quant-ph↗

arXiv2013

Quantum Simulation of Dzyaloshinsky-Moriya Interaction

Quantum simulation of a Hamiltonian H requires unitary operator decomposition (UOD) of its evolution operator, ($U=exp(-i H t)$) in terms of experimentally preferable unitaries. Here, using Genetic Algorithm optimization, we numerically evaluate the most generic UOD for the Hamiltonian, DM interaction in the presence of Heisenberg XY interaction, $H_{DH}$. Using these decompositions, we studied the entanglement dynamics of Bell state in the Hamiltonian $H_{DH}$ and verified the entanglement preservation procedure by Hou et al. [Annals of Physics 327, 292 (2012)].

quant-ph↗

arXiv2013

Electron-photon scattering mediated by localized plasmons: A quantitative analysis by eigen-response theory

We show that the scattering interaction between a high energy electron and a photon can be strongly enhanced by different types of localized plasmons in a non-trivial way. The scattering interaction is predicted by an eigen-response theory, numerically verified by finite-difference-time-domain simulation, and experimentally verified by cathodoluminescence spectroscopy. We find that the scattering interaction associated with dark plasmons can be as strong as that of bright plasmons. Such a strong interaction may offer new opportunities to improve single-plasmon detection and high-resolution characterization techniques for high quality plasmonic materials.

physics.optics↗

arXiv2012

Singlet state creation and Universal quantum computation in NMR using Genetic Algorithm

Experimental implementation of a quantum algorithm requires unitary operator decomposition. Here we treat the unitary operator decomposition as an optimization problem and use Genetic Algorithm, a global optimization method inspired by nature's evolutionary process for operator decomposition. As an application, we apply this to NMR Quantum Information Processing and find a probabilistic way of doing universal quantum computation using global hard pulses. We also demonstrate efficient creation of singlet state (as a special case of Bell state) directly from thermal equilibrium using an optimum sequence of pulses.

quant-ph↗

arXiv2012

Algorithmic approach to simulate Hamiltonian dynamics and an NMR simulation of Quantum State Transfer

We propose an iterative algorithm to simulate the dynamics generated by any $n$-qubit Hamiltonian. The simulation entails decomposing the unitary time evolution operator $U$ (unitary) into a product of different time-step unitaries. The algorithm product-decomposes $U$ in a chosen operator basis by identifying a certain symmetry of $U$ that is intimately related to the number of gates in the decomposition. We illustrate the algorithm by first obtaining a polynomial decomposition in the Pauli basis of the $n$-qubit Quantum State Transfer unitary by Di Franco et. al. (Phys. Rev. Lett. 101, 230502 (2008)) that transports quantum information from one end of a spin chain to the other; and then implement it in Nuclear Magnetic Resonance to demonstrate that the decomposition is experimentally viable and well-scaled. We furthur experimentally test the resilience of the state transfer to static errors in the coupling parameters of the simulated Hamiltonian. This is done by decomposing and simulating the corresponding imperfect unitaries.

quant-ph↗

arXiv2012

90-degree Ferroelectric Domains in PbTiO3: Experimental Observation and Molecular Dynamics Simulations

We report observation of 90-degree ferroelectric domain structures in transmission electron microscopy (TEM) of epitaxially-grown films of PbTiO3. Using molecular dynamics (MD) simulations based on first-principles effective Hamiltonian of bulk PbTiO3, we corroborate the occurance of such domains showing that it arises as metastable states only in cooling simulations (as the temperature is lowered) and establish characteristic stability of 90-degree domain structures in PbTiO3. In contrast, such domains do not manifest in similar simulations of BaTiO3. Through a detailed analysis based on energetics and comparison between PbTiO3 and BaTiO3, we find that 90-degree domain structures are energetically favorable only in the former, and the origin of their stability lies in the polarization-strain coupling. Our analysis suggests that they may form in BaTiO3 due to special boundary condition and/or defect-related inhomogeneities.

cond-mat.mtrl-sci↗

arXiv2012

Coupling between quantum Hall state and electromechanics in suspended graphene resonator

Using graphene resonator, we perform electromechanical measurements in quantum Hall regime to probe the coupling between a quantum Hall (QH) system and its mechanical motion. Mechanically perturbing the QH state through resonance modifies the DC resistance of the system and results in a Fano-lineshape due to electronic interference. Magnetization of the system modifies the resonator's equilibrium position and effective stiffness leading to changes in resonant frequency. Our experiments show that there is an intimate coupling between the quantum Hall state and mechanics - electron transport is affected by physical motion and in turn the magnetization modifies the electromechanical response.

cond-mat.mes-hall↗

arXiv2011

Temperature-dependent Raman study of CeFeAsO0.9F0.1 Superconductor: Crystal field excitations, phonons and their coupling

We report temperature-dependent Raman spectra of CeFeAsO0.9F0.1 from 4 K to 300 K in spectral range of 60 to 1800 cm-1 and interpret them using estimates of phonon frequencies obtained from first-principles density functional calculations. We find evidence for a strong coupling between the phonons and crystal field excitations; in particular Ce3+ crystal field excitation at 432 cm-1 couples strongly with Eg oxygen vibration at 389 cm-1 . Below the superconducting transition temperature, the phonon mode near 280 cm-1 shows softening, signaling its coupling with the superconducting gap. The ratio of the superconducting gap to Tc thus estimated to be ~ 10 suggests CeFeAsO0.9F0.1 as a strong coupling superconductor. In addition, two high frequency modes observed at 1342 cm-1 and 1600 cm-1

cond-mat.supr-con↗

arXiv2011

Raman Evidence for Superconducting Gap and Spin-Phonon Coupling in Superconductor Ca(Fe0.95Co0.05)2As2

Inelastic light scattering studies on single crystal of electron-doped Ca(Fe0.95Co0.05)2As2 superconductor, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at TSM ~ 140 K and superconducting transition temperature Tc ~ 23 K, reveal evidence for superconductivity-induced phonon renormalization; in particular the phonon mode near 260 cm-1 shows hardening below Tc, signaling its coupling with the superconducting gap. All the three Raman active phonon modes show anomalous temperature dependence between room temperature and Tc i.e phonon frequency decreases with lowering temperature. Further, frequency of one of the modes shows a sudden change in temperature dependence at TSM. Using first-principles density functional theory-based calculations, we show that the low temperature phase (Tc < T < TSM) exhibits short-ranged stripe anti-ferromagnetic ordering, and estimate the spin-phonon couplings that are responsible for these phonon anomalies.

cond-mat.supr-con↗

arXiv2011

Entanglement in a 3-spin Heisenberg-XY chain with nearest-neighbor interactions, simulated in an NMR quantum simulator

The evolution of entanglement in a 3-spin chain with nearest-neighbor Heisenberg-XY interactions for different initial states is investigated here. In an NMR experimental implementation, we generate multipartite entangled states starting from initial separable pseudo-pure states by simulating nearest-neighbor XY interactions in a 3-spin linear chain of nuclear spin qubits. For simulating XY interactions, we follow algebraic method of Zhang et al. [Phys. Rev. A 72, 012331 (2005)]. Bell state between end qubits has been generated by using only the unitary evolution of the XY Hamiltonian. For generating W-state and GHZ-state a single qubit rotation is applied on second and all the three qubits respectively after the unitary evolution of the XY Hamiltonian.

quant-ph↗

arXiv2011

High Q electromechanics with InAs nanowire quantum dots

In this report, we study electromechanical properties of a suspended InAs nanowire (NW) resonator. At low temperatures, the NW acts as the island of a single electron transistor (SET) and we observe a strong coupling between electrons and mechanical modes at resonance; the rate of electron tunneling is approximately 10 times the resonant frequency. Above and below the mechanical resonance, the magnitude of Coulomb peaks is different and we observe Fano resonance in conductance due to the interference between two contributions to potential of the SET. The quality factor ($Q$) of these devices is observed $\sim10^5$ at 100 mK.

cond-mat.mes-hall↗

arXiv2011

A Thin Film Broadband Absorber Based on Multi-sized Nanoantennas

We experimentally demonstrate an infrared broadband absorber for TM polarized light based on an array of nanostrip antennas of several different sizes. The broadband property is due to the collective effect of magnetic responses excited by these nano-antennas at distinct wavelengths. By manipulating the differences of the nanostrip widths, the measured spectra clearly validate our design for the purpose of broadening the absorption band. The present broadband absorber works very well in a wide angular range.

physics.optics↗