arXiv subjects

Tsampikos Kottos

Publications and source records attributed to Tsampikos Kottos.

139 records · Page 8Linked to original sources

arXiv2001

Failure of random matrix theory to correctly describe quantum dynamics

Consider a classically chaotic system which is described by a Hamiltonian H_0. At t=0 the Hamiltonian undergoes a sudden-change H_0 -> H. We consider the quantum-mechanical spreading of the evolving energy distribution, and argue that it cannot be analyzed using a random-matrix theory (RMT) approach. RMT can be trusted only to the extend that it gives trivial results that are implied by first-order perturbation theory. Non-perturbative effects are sensitive to the underlying classical dynamics, and therefore the hbar-->0 behavior for effective RMT models is strikingly different from the correct semiclassical limit.

cond-mat↗

arXiv2001

Quantum mechanical relaxation of open quasiperiodic systems

We study the time evolution of the survival probability $P(t)$ in open one-dimensional quasiperiodic tight-binding samples of size $L$, at critical conditions. We show that it decays algebraically as $P(t)\sim t^{-α}$ up to times $t^*\sim L^γ$, where $α= 1-D_0^E$, $γ=1/D_0^E$ and $D_0^E$ is the fractal dimension of the spectrum of the closed system. We verified these results for the Harper model at the metal-insulator transition and for Fibonacci lattices. Our predictions should be observable in propagation experiments with electrons or classical waves in quasiperiodic superlattices or dielectric multilayers.

cond-mat.mes-hall↗

arXiv2000

Parametric dependent Hamiltonians, wavefunctions, random-matrix-theory, and quantal-classical correspondence

We study a classically chaotic system which is described by a Hamiltonian $H(Q,P;x)$ where $(Q,P)$ are the canonical coordinates of a particle in a 2D well, and $x$ is a parameter. By changing $x$ we can deform the `shape' of the well. The quantum-eigenstates of the system are $|n(x)>$. We analyze numerically how the parametric kernel $P(n|m)= | |^2$ evolves as a function of $x-x0$. This kernel, regarded as a function of $n-m$, characterizes the shape of the wavefunctions, and it also can be interpreted as the local density of states (LDOS). The kernel $P(n|m)$ has a well defined classical limit, and the study addresses the issue of quantum-classical correspondence (QCC). We distinguish between restricted QCC and detailed QCC. Both the perturbative and the non-perturbative regimes are explored. The limitations of the random-matrix-theory (RMT) approach are demonstrated.

nlin.CD↗

arXiv2000

Quantum-Mechanical Non-Perturbative Response of Driven Chaotic Mesoscopic Systems

Consider a time-dependent Hamiltonian $H(Q,P;x(t))$ with periodic driving $x(t)=A\sin(Ωt)$. It is assumed that the classical dynamics is chaotic, and that its power-spectrum extends over some frequency range $|ω|<ω_{cl}$. Both classical and quantum-mechanical (QM) linear response theory (LRT) predict a relatively large response for $Ω<ω_{cl}$, and a relatively small response otherwise, independently of the driving amplitude $A$. We define a non-perturbative regime in the $(Ω,A)$ space, where LRT fails, and demonstrate this failure numerically. For $A>A_{prt}$, where $A_{prt}\propto\hbar$, the system may have a relatively strong response for $Ω>ω_{cl}$, and the shape of the response function becomes $A$ dependent.

cond-mat↗

arXiv2000

Effect of resonances on the transport properties of two-dimensional disordered systems

We study both analytically and numerically how the electronic structure and the transport properties of a two-dimensional disordered system are modified in the presence of resonances. The energy dependence of the density of states and the localization length at different resonance energies and strengths of coupling between resonances and random states are determined. The results show, that at energy equals to the resonance energy there is an enhancement in the density of states. In contrast, the localization length remains unaffected from the presence of the resonances and is similar to the one of the standard Anderson model. Finally, we calculate the diffusion constant as a function of energy and we reveal interesting analogies with experimental results on light scattering in the presence of Mie resonances.

cond-mat.dis-nn↗

arXiv2000

Statistics of resonances and of delay times in quasiperiodic Schr"odinger equations

We study the statistical distributions of the resonance widths ${\cal P} (Γ)$, and of delay times ${\cal P} (τ)$ in one dimensional quasi-periodic tight-binding systems with one open channel. Both quantities are found to decay algebraically as $Γ^{-α}$, and $τ^{-γ}$ on small and large scales respectively. The exponents $α$, and $γ$ are related to the fractal dimension $D_0^E$ of the spectrum of the closed system as $α=1+D_0^E$ and $γ=2-D_0^E$. Our results are verified for the Harper model at the metal-insulator transition and for Fibonacci lattices.

cond-mat.mes-hall↗

arXiv2000

Chaotic Scattering on Graphs

Quantized, compact graphs were shown to be excellent paradigms for quantum chaos in bounded systems. Connecting them with leads to infinity we show that they display all the features which characterize scattering systems with an underlying classical chaotic dynamics. We derive exact expressions for the scattering matrix, and an exact trace formula for the density of resonances, in terms of classical orbits, analogous to the semiclassical theory of chaotic scattering. A statistical analysis of the cross sections and resonance parameters compares well with the predictions of Random Matrix Theory. Hence, this system is proposed as a convenient tool to study the generic behavior of chaotic scattering systems, and their semiclassical description.

chao-dyn↗

arXiv2000

Spreading and localization of wavepackets in disordered wires in a magnetic field

We study the diffusive and localization properties of wavepackets in disordered wires in a magnetic field. In contrast to a recent supersymmetry approach our numerical results show that the decay rate of the steady state changes {\em smoothly} at the crossover from preserved to broken time-reversal symmetry. Scaling and fluctuation properties are also analyzed and a formula, which was derived analytically only in the pure symmetry cases is shown to describe also the steady state wavefunction at the crossover regime. Finally, we present a scaling for the variance of the packet which shows again a smooth transition due to the magnetic field.

cond-mat.mes-hall↗

arXiv2000

Scaling properties of one-dimensional Anderson models in an electric field: Exponential vs. factorial localization

We investigate the scaling properties of eigenstates of a one-dimensional (1D) Anderson model in the presence of a constant electric field. The states show a transition from exponential to factorial localization. For infinite systems this transition can be described by a simple scaling law based on a single parameter $λ_{\infty} = l_{\infty}/l_{\rm el}$, the ratio between the Anderson localization length $l_{\infty}$ and the Stark localization length~$l_{\rm el}$. For finite samples, however, the system size $N$ enters the problem as a third parameter. In that case the global structure of eigenstates is uniquely determined by two scaling parameters $λ_N=l_\infty/N$ and $λ_\infty=l_\infty/l_{\rm el}$.

cond-mat.dis-nn↗

arXiv2000

Quantum Graphs: A model for Quantum Chaos

We study the statistical properties of the scattering matrix associated with generic quantum graphs. The scattering matrix is the quantum analogue of the classical evolution operator on the graph. For the energy-averaged spectral form factor of the scattering matrix we have recently derived an exact combinatorial expression. It is based on a sum over families of periodic orbits which so far could only be performed in special graphs. Here we present a simple algorithm implementing this summation for any graph. Our results are in excellent agreement with direct numerical simulations for various graphs. Moreover we extend our previous notion of an ensemble of graphs by considering ensemble averages over random boundary conditions imposed at the vertices. We show numerically that the corresponding form factor follows the predictions of random-matrix theory when the number of vertices is large---even when all bond lengths are degenerate. The corresponding combinatorial sum has a structure similar to the one obtained previously by performing an energy average under the assumption of incommensurate bond lengths.

nlin.CD↗

arXiv2000

Wavepacket dynamics in energy space, RMT and quantum-classical correspondence

We apply random-matrix-theory (RMT) to the analysis of evolution of wavepackets in energy space. We study the crossover from ballistic behavior to saturation, the possibility of having an intermediate diffusive behavior, and the feasibility of strong localization effect. Both theoretical considerations and numerical results are presented. Using quantal-classical correspondence (QCC) considerations we question the validity of the emerging dynamical picture. In particular we claim that the appearance of the intermediate diffusive behavior is possibly an artifact of the RMT strategy.

chao-dyn↗

arXiv1999

Statistical properties of phases and delay times of the one-dimensional Anderson model with one open channel

We study the distribution of phases and of Wigner delay times for a one-dimensional Anderson model with one open channel. Our approach, based on classical Hamiltonian maps, allows us an analytical treatment. We find that the distribution of phases depends drastically on the parameter $σ_A = σ/sin k$ where $σ^2$ is the variance of the disorder distribution and $k$ the wavevector. It undergoes a transition from uniformity to singular behaviour as $σ_A$ increases. The distribution of delay times shows universal power law tails $~ 1/τ^2$, while the short time behaviour is $σ_A$- dependent.

cond-mat.dis-nn↗

arXiv1998

Periodic Orbit Theory and Spectral Statistics for Quantum Graphs

We quantize graphs (networks) which consist of a finite number of bonds and vertices. We show that the spectral statistics of fully connected graphs is well reproduced by random matrix theory. We also define a classical phase space for the graphs, where the dynamics is mixing and the periodic orbits proliferate exponentially. An exact trace formula for the quantum spectrum is developed in terms of the same periodic orbits, and it is used to investigate the origin of the connection between random matrix theory and the underlying chaotic classical dynamics. Being an exact theory, and due to its relative simplicity, it offers new insights into this problem which is at the fore-front of the research in Quantum Chaos and related fields.

chao-dyn↗