arXiv subjects

Pramod Kumar

Publications and source records attributed to Pramod Kumar.

At least 37 records · Page 2Linked to original sources

arXiv2017

Temperature and doping induced instabilities of the repulsive Hubbard model on Lieb lattice

The properties of a phase at finite interactions can be significantly influenced by the underlying dispersion of the non-interacting Hamiltonian. We demonstrate this by studying the repulsive Hubbard model on the $2$D Lieb lattice, which has a flat band for vanishing interaction $U$. We perform real-space dynamical mean-field theory calculations at different temperatures and dopings using a continuous time quantum Monte Carlo impurity solver. Studying the frequency dependence of the self-energy, we show that a finite temperature non-magnetic non-Fermi liquid behavior is a concomitant of the flat band singularity. At half-filling we also find a magnetically ordered region, where the order parameter varies linearly with the interaction strength, and a strongly correlated Mott insulating phase. The double occupancy decreases sharply for small $U$, highlighting the flat band contribution. Away from half-filling, we observe the stripe order, i.e. an inhomogeneous spin and charge density wave of finite wavelength which turns into a sub-lattice ordering at higher temperatures.

cond-mat.str-el↗

arXiv2017

Optimum stabilization of self-mode-locked quantum dash lasers using dual optical feedback with improved tolerance against phase delay mismatch

We experimentally investigate the RF linewidth and timing jitter over a wide range of delay tuning in a self-mode-locked two-section quantum dash lasers emitting at ~ 1.55 micron and operating at ~ 21 GHz repetition rate subject to single and dual optical feedback into gain section. Various feedback conditions are investigated and optimum levels determined for narrowest linewidth and reduced timing jitter for both single and dual loop configurations. We demonstrate that dual loop feedback, with the shorter feedback cavity tuned to be fully resonant, followed by fine tuning of the phase of the longer feedback cavity, gives stable narrow RF spectra across the widest delay range, unlike single loop feedback. In addition, for dual loop configurations, under fully resonant conditions, integrated timing jitter is reduced from 3.9 ps to 295 fs [10 kHz-100 MHz], the RF linewidth narrows from 100 kHz to < 1 kHz, with more than 30 dB fundamental side-mode suppression. We show that dual loop optical feedback with separate fine tuning of both external cavities is far superior to single loop feedback, with increased system tolerance against phase delay mismatch, making it a robust and cost-effective technique for developing practical, reliable and low-noise mode-locked lasers, optoelectronic oscillators and pulsed photonic circuits.

physics.optics↗

arXiv2017

Observation of transient superconductivity at the LaAlO$_3$/SrTiO$_3$ interface

We report the observation of a magnetic field assisted transient superconducting state in the two dimensional electron gas existing at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. This metastable state depends critically on the density of charge carriers in the system. It appears concomitantly with a Lifshitz transition as a consequence of the interplay between ferromagnetism and superconductivity and the finite relaxation time of the in-plane magnetization. Our results clearly demonstrate the inherently metastable nature of the superconducting state competing with a magnetic order in these systems. The co-existence of superconductivity and ferromagnetism in the conducting electronic layer formed at the interface of insulating oxides has thrown up several intriguing and as yet unanswered questions. An open question in this field is the energetics of the interplay between these two competing orders and the present observation goes a long way in understanding the underlying mechanism.

cond-mat.str-el↗

arXiv2017

Interaction-Induced Topological and Magnetic Phases in the Hofstadter-Hubbard Model

Interaction effects have been a subject of contemporary interest in topological phases of matter. But in the presence of interactions, the accurate determination of topological invariants in their general form is difficult due to their dependence on multiple integrals containing Green's functions and their derivatives. Here we employ the recently proposed "effective topological Hamiltonian" approach to explore interaction-induced topological phases in the time-reversal-invariant Hofstadter-Hubbard model. Within this approach, the zero-frequency part of the self-energy is sufficient to determine the correct topological invariant. We combine the topological Hamiltonian approach with the local self-energy approximation, both for the static and the full dynamical self-energy evaluated using dynamical mean field theory (DMFT), and present the resulting phase diagram in the presence of many-body interactions. We investigate the emergence of quantum spin Hall (QSH) states for different interaction strengths by calculating the $\mathbb{Z}_2$ invariant. The interplay of strong correlations and a staggered potential also induces magnetic long-range order with an associated first order transition. We present results for the staggered magnetization ( $m_s$ ), staggered occupancy ( $n_s$ ), and double occupancy across the transition.

cond-mat.str-el↗

arXiv2017

Unraveling the Phase-amplitude Coupling Modulation in a Delay-Coupled Diode Lasers Functionality

We numerically investigate the effect of phase-amplitude coupling modulation on power spectra in semiconductor lasers subject to optical injection in a face to face configuration, when a non-negligible injection delay time is taken into account. We find that as phase-amplitude coupling factor alpha varies, the system goes through a sequence of phase transactions between In-phase locking states to anti-phase locking states via phase-flip bifurcation. Moreover, we observed the signature of frequency discretization (Frequency Island) and uncovered the physical mechanism for the existence of multi-stability near the phase transitions regimes. Within the windows between successive anti-phases to in-phase locking regions, optical injection induced modulation in alpha, unveiling a remarkable universal feature in the various dynamics of coupled lasers system which could be useful in controlling the chirp or pulse repetition rate of a photonic integrated compact device with the aid of phase control.

physics.optics↗

arXiv2016

An algorithm based on a new DQM with modified exponential cubic B-splines for solving hyperbolic telegraph equation in $(2+1)$ dimension

This paper developed a method called "modified exponential cubic B-Spline differential quadrature (mExp-DQM) for space discretization together with a time integration algorithm" for the numerical computation of hyperbolic telegraph equation in $(2+1)$ dimension. The mExp-DQM is a new differential quadrature method based on modified exponential cubic B-splines as basis which reduces the problem into an amenable system of ordinary differential equations. The resulting system is solved using a time integration algorithm. The stability of the method is also studied by computing the eigenvalues of the coefficients matrices, it is found that the scheme is conditionally stable. The accuracy of the method is illustrated by computing the error between analytical solutions and numerical solutions is measured by using $L_2$ and $L_{\infty}$ error norms for each problem. A comparison of mExp-DQM solutions with the results of the other numerical methods has been carried out for various space sizes and time step sizes.

math.NA↗

arXiv2016

Algorithms based on DQM with new sets of base functions for solving parabolic partial differential equations in $(2+1)$ dimension

This paper deals with the numerical computations of two space dimensional time dependent parabolic partial differential equations by adopting adopting an optimal five stage fourth-order strong stability preserving Runge Kutta (SSP-RK54) scheme for time discretization, and three methods of differential quadrature with different sets of modified B-splines as base functions, for space discretization: namely i) mECDQM: (DQM with modified extended cubic B-splines); ii) mExp-DQM: DQM with modified exponential cubic B-splines, and iii) MTB-DQM: DQM with modified trigonometric cubic B-splines. Specially, we implement these methods on convection-diffusion equation to convert them into a system of first order ordinary differential equations,in time which can be solved using any time integration method, while we prefer SSP-RK54 scheme. All the three methods are found stable for two space convection-diffusion equation by employing matrix stability analysis method. The accuracy and validity of the methods are confirmed by three test problems of two dimensional convection-diffusion equation, which shows that the proposed approximate solutions by any of the method are in good agreement with the exact solutions.

math.NA↗

arXiv2016

A New Approximate Solution of Time-Fractional, Non-linear Schrodinger Equations Using Fractional Reduced Differential Transformation

This paper is concerned with an alternative analytical solution of time-fractional nonlinear Schrodinger equation and nonlinear coupled Schrodinger equation obtained by employing fractional reduced differential transform method. The proposed solutions are obtained in series form, converges to the exact solution very rapidly. These results are agreed well with the results obtained by using differential transform method, homotopy perturbation method, homotopy analysis method and Adomian decomposition method. However, the computations shows that the described method is easy to apply, and it needs small size of computation contrary to the existing above said methods.

math.NA↗

arXiv2016

Homotopy perturbation transform method for solving fractional partial differential equations with proportional delay

This paper deals the implementation of \emph{homotopy perturbation transform method} (HPTM) for numerical computation of initial valued autonomous system of time-fractional partial differential equations (TFPDEs) with proportional delay, including generalized Burgers equations with proportional delay. The HPTM is a hybrid of Laplace transform and homotopy perturbation method. To confirm the efficiency and validity of the method, the computation of three test problems of TFPDEs with proportional delay presented. The proposed solutions are obtained in series form, converges very fast. The scheme seems very reliable, effective and efficient powerful technique for solving various type of physical models arising in sciences and engineering.

math.NA↗

arXiv2016

An algorithm based on DQM with modified trigonometric cubic Bsplines for solving coupled viscous Burgers equations

This paper deals with a new algorithm called modified trigonometric cubic B-spline differential quadrature method for numerical computation of the time dependent partial differential equations. Specially the numerical computation of the Burgers equation is obtained by using modified trigonometric cubic B spline differential quadrature method with time integration algorithm. The modified trigonometric cubic B-spline differential quadrature method is differential quadrature method with modified trigonometric cubic B-splines as new basis. The initial boundary value system of Burgers equation is first transformed into an initial value system of ordinary differential equations by means of modified trigonometric cubic B-spline differential quadrature method. The resulting system of ordinary differential equations is solved by using an optimal five stage four order strong stability preserving RK method. The accuracy and efficiency of the method is illustrated by five test problems in terms of and error norms and their comparisons with existing results. The modified trigonometric cubic B-spline differential quadrature method produces better results than the results due to almost all the existing schemes. The modified trigonometric cubic B-spline differential quadrature method is shown conditionally stable using the matrix stability analysis method for various grid points.

math.NA↗

arXiv2016

Correlated Non-Gaussian phase fluctuations in LaAlO$_3$/SrTiO$_3$ heterointerface

We probe the existence of large correlated non-Gaussian phase fluctuations in the vicinity of the superconducting phase transition in the conducting layer residing at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. The non-Gaussian fluctuations appear between the Berezinskii-Kosterlitz-Thouless transition temperature $T_{BKT}$ and the mean field transition temperature $T_C$. Subsequent theoretical analysis reveals that non-Gaussianity arises predominantly due to the percolative transition of a Josephson coupled network of superconductors. Our results confirm that the superconductivity in this system is confined to two-dimensions. Our study of the non-Gaussian resistance fluctuation spectrum provides a novel means to explore the BKT-transition in two-dimensional inhomogeneous superconductors.

cond-mat.supr-con↗

arXiv2015

Magnetocaloric effect in Gd3Ir compound

Here we report magneto-thermal properties of Gd3Ir compounds using magnetic and heat capacity data. We observed large isothermal entropy change, adiabatic temperature change and relative cooling power are 4.1J/Kg, 4K and 437J/Kg on 50KOe respectively.

cond-mat.mtrl-sci↗

arXiv2014

Kondo-hole substitution in heavy fermions: dynamics and transport

Kondo-hole substitution is a unique probe for exploring the interplay of interactions, f-electron dilution and disorder in heavy fermion materials. Within the diluted periodic Anderson model, we investigate the changes in single-particle dynamics as well as response functions, as a function of Kondo hole concentration ($x$) and temperature. We show that the spectral weight transfers due to Kondo hole substitution has characteristics that are different from those induced by temperature; The dc resistivity crosses over from a highly non-monotonic form with a coherence peak in the $x\rightarrow 0$ limit to a monotonic single-impurity like form that saturates at low temperature. The thermopower exhibits a characteristic maximum as a function of temperature, the value of which changes sign with increasing $x$, and its location is shown to correspond to a low energy scale of the system. The Hall coefficient also changes sign with increasing $x$ at zero temperature and is highly temperature dependent for all $x$. As $x$ is increased beyond a certain $x_c$, the Drude peak and the mid-infrared peak in the optical conductivity vanish almost completely; A peak in the optical scattering rate melts and disappears eventually. We discuss the above-mentioned changes in the properties in terms of a crossover from coherent, Kondo lattice behaviour to single impurity like, incoherent behaviour with increasing $x$. A comparison of theory with experiments carried out for the dc resistivity and the thermopower of Ce$_{1-x}$La$_x$B$_6$ yields good agreement.

cond-mat.str-el↗

arXiv2014

Enhanced spin-orbit coupling and charge carrier density suppression in LaAl1-xCrxO3/SrTiO3 heterointerfaces

We report a gradual suppression of the two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface on substitution of chromium at the Al sites. The sheet carrier density at the interface (ns) drops monotonically from 2.2x10(14) cm-2 to 2.5x10(13) cm-2 on replacing nearly 60 percent of Al sites by Cr and the sheet resistance (Rs) exceeds the quantum limit for localization (h/2e2) in the concentrating range 40 to 60 percent of Cr.

cond-mat.str-el↗

arXiv2014

Chemical potential shift and gap-state formation in SrTiO$_{3-δ}$ revealed by photoemission spectroscopy

In this study, we report on investigations of the electronic structure of SrTiO$_3$ annealed at temperature ranging between 550 and 840$^\circ$C in an ultrahigh vacuum. Annealing induced oxygen vacancies (O$_{vac}$) impart considerable changes in the electronic structure of SrTiO$_3$. Using core-level photoemission spectroscopy, we have studied the chemical potential shift ($Δμ$) as a function of annealing temperature. The result shows that the chemical potential monotonously increases with electron doping in SrTiO$_{3-δ}$. The monotonous increase of the chemical potential rules out the existence of electronic phase separation in the sample. Using valence band photoemission, we have demonstrated the formation of a low density of states at the near Fermi level electronic spectrum of SrTiO$_{3-δ}$. The gap-states were observed by spectral weight transfer over a large energy scale of the stoichiometric band gap of SrTiO$_3$ system leading finally to an insulator - metal transition. We have interpreted our results from the point of structural distortions induced by oxygen vacancies.

cond-mat.mtrl-sci↗

arXiv2014

Unveiling spatial correlations in biophotonic architecture of transparent insect wings

We probe the natural complex structures in the transparent insect wings by a simple, non-invasive, real time optical technique using both monochromatic and broadband femtosecond lasers. A stable, reproducible and novel diffraction pattern is observed unveiling long range spatial correlations and structural-symmetry at various length scales for a large variety of wings. While matching the sensitivity of SEM for such microstructures, it is highly efficient for extracting long range structural organization with potentially broad applicability.

physics.optics↗

arXiv2014

Optical probing of long range spatial correlation and symmetry in complex biophotonic architectures on the transparent insect wings

We experimentally probe complex bio-photonic architecture of microstructures on the transparent insect wings by a simple, non-invasive, real time optical technique. A stable and reproducible far-field diffraction pattern in transmission was observed using collimated cw and broadband fs pulses. A quantitative analysis of the observed diffraction pattern unveiled a new form of long-range semi-periodic order of the microstructures over mm scale. These observations agree well with Fourier analysis of SEM images of the wing taken at various length scales. We propose a simple quantitative model based on optical diffraction by an array of non-overlapping microstructures with minimal disorder which supports our experimental observations. Two different applications of our techniques are demonstrated. First, by scanning the laser beam across the wing sample we observed a rotation of the original diffraction profile which gives direct signature of organizational symmetry of microstructures. Second, we report the first optical detection of reorganization in the photonic architecture on the Drosophila wings by various genetic mutations. These results have potentials for design and development of diffractive optical components for applications and identifying routes to genetic control of biomemetic devices.

physics.optics↗

arXiv2014

Non-Fermi liquid behaviour from dynamical effects of impurity scattering in correlated Fermi liquids

The interplay of disorder and interactions is a subject of perennial interest. In this work, we have investigated the effect of disorder due to chemical substitution on the dynamics and transport properties of correlated Fermi liquids. A low frequency analysis in the concentrated and dilute limits shows that the dynamical local potentials arising through disorder averaging generate a linear (in frequency) term in the scattering rate. Such non-Fermi liquid behaviour (nFL) is investigated in detail for Kondo hole substitution in heavy fermions within dynamical mean field theory. We find closed form expressions for the dependence of the static and linear terms in the scattering rate on substitutional disorder and model parameters. We argue that the low temperature resistivity will acquire a linear in temperature term, and show that the Drude peak structure in the optical conductivity will disappear beyond a certain disorder $p_c$, that marks the crossover from lattice coherent to single-impurity behaviour. A full numerical solution of the DMFT equations reveals that the nFL term will show up significantly only in certain regimes, although it is present for any non-zero disorder concentration in principle. We highlight the dramatic changes that occur in the quasiparticle scattering rate in the proximity of $p_c$. Remarkably, we find that the nFL behaviour due to dynamical effects of impurity scattering has features that are distinct from those arising through Griffiths singularities or distribution of Kondo scales. Relevance of our findings to experiments on alloyed correlated systems is pointed out.

cond-mat.str-el↗