Self-similar spectrum in effective time independent Hamiltonians for kicked systems

We study multifractal properties in the spectrum of effective time-independent Hamiltonians obtained using a perturbative method for a class of delta-kicked systems. The evolution operator in the time-dependent problem is factorized into an initial kick, an evolution dictated by a time-independent Hamiltonian, and a final kick. We have used the double kicked $SU(2)$ system and the kicked Harper model to study butterfly spectrum in the corresponding effective Hamiltonians. We have obtained a generic class of $SU(2)$ Hamiltonians showing self-similar spectrum. The statistics of the generalized fractal dimension is studied for a quantitative characterization of the spectra.

Floquet analysis of pulsed Dirac systems: A way to simulate rippled graphene

The low energy continuum limit of graphene is effectively known to be modeled using Dirac equation in (2+1) dimensions. We consider the possibility of using modulated high frequency periodic driving of a two-dimension system (optical lattice) to simulate properties of rippled graphene. We suggest that the Dirac Hamiltonian in a curved background space can also be effectively simulated by a suitable driving scheme in optical lattice. The time dependent system yields, in the approximate limit of high frequency pulsing, an effective time independent Hamiltonian that governs the time evolution, except for an initial and a final kick. We use a specific form of 4-phase pulsed forcing with suitably tuned choice of modulating operators to mimic the effects of curvature. The extent of curvature is found to be directly related to $omega^{-1}$ the time period of the driving field at the leading order. We apply the method to engineer the effects of curved background space. We find that the imprint of curvilinear geometry modifies the electronic properties, such as LDOS, significantly. We suggest that this method shall be useful in studying the response of various properties of such systems to non-trivial geometry without requiring any actual physical deformations.

Effective time-independent analysis for quantum kicked systems

We present a mapping of potentially chaotic time-dependent quantum kicked systems to an equivalent effective time-independent scenario, whereby the system is rendered integrable. The time-evolution is factorized into an initial kick, followed by an evolution dictated by a time-independent Hamiltonian and a final kick. This method is applied to the kicked top model. The effective time-independent Hamiltonian thus obtained, does not suffer from spurious divergences encountered if the traditional Baker-Cambell-Hausdorff treatment is used. The quasienergy spectrum of the Floquet operator is found to be in excellent agreement with the energy levels of the effective Hamiltonian for a wide range of system parameters. The density of states for the effective system exhibits sharp peak-like features, pointing towards quantum criticality. The dynamics in the classical limit of the integrable effective Hamiltonian shows remarkable agreement with the non-integrable map corresponding to the actual time-dependent system in the non-chaotic regime. This suggests that the effective Hamiltonian serves as a substitute for the actual system in the non-chaotic regime at both the quantum and classical level.

Thermal Properties of a Particle Confined to a Parabolic Quantum Well in 2D Space with Conical Disclination

The thermal properties of a system, comprising of a spinless non-interacting charged particle in the presence of a constant external magnetic field and confined in a parabolic quantum well are studied. The focus has been on the effects of a topological defect, of the form of conical disclination, with regard to the thermodynamic properties of the system. We have obtained the modifications to the traditional Landau-Fock-Darwin spectrum in the presence of conical disclination. The effect of the conical kink on the degeneracy structure of the energy levels is investigated. The canonical formalism is used to compute various thermodynamic variables. The study shows an interplay between magnetic field, temperature and the degree of conicity by setting two scales for temperature corresponding to the frequency of the confining potential and the cyclotron frequency of external magnetic field. The kink parameter is found to affect the quantitative behaviour of the thermodynamic quantities. It plays a crucial role in the competition between the external magnetic field and temperature in fixing the values of the thermal response functions. This study provides an important motivation for studying similar systems, however with non trivial interactions in the presence of topological defects.

How much random a random network is : a random matrix analysis

We analyze complex networks under random matrix theory framework. Particularly, we show that $Delta_3$ statistic, which gives information about the long range correlations among eigenvalues, provides a qualitative measure of randomness in networks. As networks deviate from the regular structure, $Delta_3$ follows random matrix prediction of linear behavior, in semi-logarithmic scale with the slope of $1/pi^2$, for the longer scale.