Large Frequency Limit Research Articles

Reaction–Diffusion Problems on Time-Periodic Domains

AbstractReaction–diffusion equations are studied on bounded, time-periodic domains with zero Dirichlet boundary conditions. The long-time behaviour is shown to depend on the principal periodic eigenvalue of a transformed periodic-parabolic problem. We prove upper and lower bounds on this eigenvalue under a range of different assumptions on the domain, and apply them to examples. The principal eigenvalue is considered as a function of the frequency, and results are given regarding its behaviour in the small and large frequency limits. A monotonicity property with respect to frequency is also proven. A reaction–diffusion problem with a class of monostable nonlinearity is then studied on a periodic domain, and we prove convergence to either zero or a unique positive periodic solution.

Magneto-transport signatures in periodically-driven Weyl and multi-Weyl semimetals

We investigate the influence of a time-periodic driving (for example, by shining circularly polarized light) on three-dimensional Weyl and multi-Weyl semimetals, in the planar Hall and planar thermal Hall set-ups. We incorporate the effects of the drive by using the Floquet formalism in the large frequency limit. We evaluate the longitudinal magneto-conductivity, planar Hall conductivity, longitudinal thermo-electric coefficient, and transverse thermo-electric coefficient, using the semi-classical Boltzmann transport equations. We demonstrate the explicit expressions of these transport coefficients in certain limits of the system parameters, where it is possible to perform the integrals analytically. We cross-check our analytical approximations by comparing the physical values with the numerical results, obtained directly from the numerical integration of the integrals. The answers obtained show that the topological charges of the corresponding semimetals have profound signatures in these transport properties, which can be observed in experiments.

Functional integral approach to the transfer function of a stochastic scattering channel

ABSTRACT We apply the formalism of functional integration to the calculation of the transfer function of a stochastic scattering channel formed by stationary, non-interacting point scatterers. The channel is described through a scattering amplitude density, defined over space, whose random component is characterized by a functional probability distribution. This random component induces in turn a probability distribution for the scattering transfer function, which we compute by means of functional integration in the case of Gaussian distributions. Some geometric configurations relevant to radar operation are worked out, as well as the statistical properties of the transfer function in the large-frequency limit. Extensions of the formulation in order to include scattering phase shifts, i.e. complex scattering amplitudes, and to consider non-Gaussian probability distributions are outlined.

Holographic pump probe spectroscopy

We study the non-linear response of a 2+1 dimensional holographic model with weak momentum relaxation and finite charge density to an oscillatory electric field pump pulse. Following the time evolution of one point functions after the pumping has ended, we find that deviations from thermality are well captured within the linear response theory. For electric pulses with a negligible zero frequency component the response approaches the instantaneously thermalizing form typical of holographic Vaidya models. We link this to the suppression of the amplitude of the quasinormal mode that governs the approach to equilibrium. In the large frequency limit, we are also able to show analytically that the holographic geometry takes the Vaidya form. A simple toy model captures these features of our holographic setup. Computing the out-of-equilibrium probe optical conductivity after the pump pulse, we similarly find that for high-frequency pulses the optical conductivity reaches its final equilibrium value effectively instantaneously. Pulses with significant DC components show exponential relaxation governed by twice the frequency of the vector quasinormal mode that governs the approach to equilibrium for the background solution. We explain this numerical factor in terms of a simple symmetry argument.

Gauge independence of the eikonal equation in Yang-Mills gravity

When one takes the geometric-optics limit of Maxwell’s wave equations in Yang-Mills gravity, one arrives at a new eikonal equation. Although we cannot perform the usual electromagnetic gauge transformation to the eikonal equation, we have the same equation for a wide class of gauge conditions, including the non-covariant temporal and axial gauges. We also show the presence of a small correction term if the large frequency limit is not taken. This small term suggests a frequency dependence in the deflection of light experiment. Moreover, there is a small violation of the electromagnetic U 1 gauge symmetry by gravity. This violation is crucial for the emergence of an effective Riemannian metric tensor associated with the new eikonal equation in Yang-Mills gravity. The results are consistent with known experiments and more accurate experiments in the future are required to test them.

Floquet resonant states and validity of the Floquet-Magnus expansion in the periodically driven Friedrichs models

The Floquet eigenvalue problem is analyzed for periodically driven Friedrichs models on discrete and continuous space. In the high-frequency regime, there exists a Floquet bound state consistent with the Floquet-Magnus expansion in the discrete Friedrichs model, while it is not the case in the continuous model. In the latter case, however, the bound state predicted by the Floquet-Magnus expansion appears as a metastable state whose lifetime diverges in the limit of large frequencies. We obtain the lifetime by evaluating the imaginary part of the quasi-energy of the Floquet resonant state. In the low-frequency regime, there is no Floquet bound state and instead the Floquet resonant state with exponentially small imaginary part of the quasi-energy appears, which is understood as the quantum tunneling in the energy space.

Quasiparticle localisation via frequent measurements

Topological quantum memories in two dimensions are not thermally stable, since once a quasiparticle excitation is created, it will delocalise at no energy cost. This places an upper bound on the lifetime of quantum information stored in them. We address this issue for a topological subsystem code introduced in [Bravyi S.{\em et al.}, Quant.~Inf.~Comp.~13(11):0963]. By frequently measuring the gauge operators of the code, a technique known as Operator Quantum Zeno Effect [Li Y. {\em et al.}, preprint arXiv:1305.2464], the dynamics responsible for quasiparticle motion is suppressed, and can eventually be ``frozen" in the large frequency limit. A feature of this method is that the density operator of the code does not commute with the measurement operators, so the density matrix will be randomised after a few measurements. However the logical operators commute with the measurement operators and are thus protected.

Dynamical localization in a chain of hard core bosons under periodic driving

We study the dynamics of a one-dimensional lattice model of hard core bosons which is initially in a superfluid phase with a current being induced by applying a twist at the boundary. Subsequently, the twist is removed, and the system is subjected to periodic $\ensuremath{\delta}$-function kicks in the staggered on-site potential. We present analytical expressions for the current and work done in the limit of an infinite number of kicks. Using these, we show that the current (work done) exhibits a number of dips (peaks) as a function of the driving frequency and eventually saturates to zero (a finite value) in the limit of large frequency. The vanishing of the current (and the saturation of the work done) can be attributed to a dynamic localization of the hard core bosons occurring as a consequence of the periodic driving. Remarkably, we show that for some specific values of the driving amplitude, the localization occurs for any value of the driving frequency. Moreover, starting from a half-filled lattice of hard core bosons with the particles localized in the central region, we show that the spreading of the particles occurs in a light-cone-like region with a group velocity that vanishes when the system is dynamically localized.

Holographic charge transport in 2+1 dimensions at finite N

We study holographic charge transport in (2+1) dimensions at finite N, whose dual gravity background is given by perturbative black hole solution in Einstein theory plus cubic terms of Weyl tensor. We consider the higher derivative corrections to the standard Maxwell action, given by the interacting terms between the Weyl tensor and the field strength. We calculate the DC conductivity by using both the membrane paradigm and the Kubo's formula and find precise agreement. We compute the AC conductivity and find an analog of the crossover from "metal" to "bad metal" in the low frequency limit. Moreover, the conductivity becomes a constant in the large frequency limit. We derive two universal relations for the Green's functions and observe that they are exactly the same as the infinite N counterparts.

Langevin Diffusion in Holographic Backgrounds with Hyperscaling Violation

We consider a relativistic heavy quark which moves in the quark-gluon plasmas. By using the holographic methods, we analyze the Langevin diffusion process of this relativistic heavy quark. This heavy quark is described by a trailing string attached to a flavor brane and moving at constant velocity. The fluctuations of this string are related to the thermal correlators and the correlation functions are precisely the kinds of objects that we compute in the gravity dual picture. We obtain the action of the trailing string in hyperscaling violation backgrounds and we then find the equations of motion. These equations lead us to constructing the Langevin correlator which helps us to obtain the Langevin constants. Using the Langevin correlators we derive the spectral densities and simple analytic expressions in the small- and large-frequency limits. We examine our works for planar andR-charged black holes with hyperscaling violation and find new constraints onθin the presence of velocityv.

Periodically driven holographic superconductor

As a first step towards our holographic investigation of the far-from-equilibrium physics of periodically driven systems at strong coupling, we explore the real time dynamics of holographic superconductor driven by a monochromatically alternating electric field with various frequencies. As a result, our holographic superconductor is driven to the final oscillating state, where the condensate is suppressed and the oscillation frequency is controlled by twice of the driving frequency. In particular, in the large frequency limit, the three distinct channels towards the final steady state are found, namely under damped to superconducting phase, over damped to superconducting and normal phase, which can be captured essentially by the low lying spectrum of quasi-normal modes in the time averaged approximation, reminiscent of the effective field theory perspective.

1+1-dimensionalp-wave superconductors from intersecting D-branes

In this work we explore 1+1 dimensional p-wave superconductors using the probe D-brane construction. Specifically, we choose three intersecting D-brane models: D1/D5, D2/D4 and D3/D3 systems. According to the dilaton running behavior, we denote the former two systems as nonconformal models and the last system as conformal. We find that all three models are qualitatively similar in describing superconducting condensate as well as some basic features (such as the gap formation and DC superconductivity) of superconducting conductivity. There also exist some differences among the three models as far as the AC conductivity is concerned. Specifically, for D3/D3 model there is no peak at nonzero frequency for the imaginary part of the conductivity, which is present in the nonconformal models; their asymptotic behaviors are different-for D1/D5 the real part of the AC conductivity approaches one at large frequency limit, for D2/D4 it slowly goes to a certain nonzero constant smaller than one and for D3/D3 it goes to zero. We find the profile of the AC conductivity for the D1/D5 system is very similar to that of higher dimensional p-wave superconductors.

Production of prompt photons: Holographic duality and thermalization

We study the production of prompt photons in strongly coupled out-of-equilibrium Super Yang-Mills plasma using the AdS/CFT correspondence. Our goal is to determine the photon emission spectrum at different stages of a thermalization process, which is modeled via the gravitational collapse of a thin spherical shell in AdS$_5$ space. Particular emphasis is placed on the limit of large frequencies, which we are able to treat analytically.

Langevin diffusion of heavy quarks in non-conformal holographic backgrounds

The Langevin diffusion process of a relativistic heavy quark in a non-conformal holographic setup is discussed. The bulk geometry is a general, five-dimensional asymptotically AdS black hole. The heavy quark is described by a trailing string attached to a flavor brane, moving at constant velocity. From the equations describing linearized fluctuations of the string world-sheet, the correlation functions defining a generalized Langevin process are constructed via the AdS/CFT prescription. In the local limit, analytic expressions for the Langevin diffusion and friction coefficients are obtained in terms of the bulk string metric. Modified Einstein relations between these quantities are also derived. The spectral densities associated to the Langevin correlators are analyzed, and simple analytic expressions are obtained in the small and large frequency limits. Finally, a numerical analysis of the jet-quenching parameter, and a comparison to RHIC phenomenology are performed in the case of Improved Holographic QCD.

Dynamical spin-spin correlation functions in the Kondo model out of equilibrium

We calculate the dynamical spin-spin correlation functions of a Kondo dot coupled to two noninteracting leads held at different chemical potentials. To this end we generalize a recently developed real-time renormalization group method in frequency space (RTRG-FS) to allow the calculation of dynamical correlation functions of arbitrary dot operators in systems describing spin and/or orbital fluctuations. The resulting two-loop RG equations are analytically solved in the weak-coupling regime. This implies that the method can be applied provided either the voltage $V$ through the dot or the external magnetic field $h_0$ are sufficiently large, $\max\{V,h_0\}\gg T_K$, where the Kondo temperature $T_K$ is the scale where the system enters the strong-coupling regime. Explicitly, we calculate the longitudinal and transverse spin-spin correlation and response functions as well as the resulting fluctuation-dissipation ratios. The correlation functions in real-frequency space can be calculated in Matsubara space without the need of any analytical continuation. We obtain analytic results for the line-shape, the small- and large-frequency limits and several other features like the height and width of the peak in the transverse susceptibility at $\Omega\approx\tilde{h}$, where $\tilde{h}$ denotes the reduced magnetic field. Furthermore, we discuss how the developed method can be generalized to calculate dynamical correlation functions of other operators involving reservoir degrees of freedom as well.

Dynamics of the Instantaneous Firing Rate in Response to Changes in Input Statistics

We review and extend recent results on the instantaneous firing rate dynamics of simplified models of spiking neurons in response to noisy current inputs. It has been shown recently that the response of the instantaneous firing rate to small amplitude oscillations in the mean inputs depends in the large frequency limit f on the spike initiation dynamics. A particular simplified model, the exponential integrate-and-fire (EIF) model, has a response that decays as 1/f in the large frequency limit and describes very well the response of conductance-based models with a Hodgkin-Huxley type fast sodium current. Here, we show that the response of the EIF instantaneous firing rate also decays as 1/f in the case of an oscillation in the variance of the inputs for both white and colored noise. We then compute the initial transient response of the firing rate of the EIF model to a step change in its mean inputs and/or in the variance of its inputs. We show that in both cases the response speed is proportional to the neuron stationary firing rate and inversely proportional to a 'spike slope factor' Delta(T) that controls the sharpness of spike initiation: as 1/Delta(T) for a step change in mean inputs, and as 1/Delta(T) (2) for a step change in the variance in the inputs.

Generalized Ornstein Uhlenbeck processes and associated self-similar processes

We consider three types of generalized Ornstein–Uhlenbeck processes: the stationary process obtained from the Lamperti transformation of fractional Brownian motion, the process with stretched exponential covariance and the process obtained from the solution of the fractional Langevin equation. These stationary Gaussian processes have many common properties, such as the fact that their local covariances share a similar structure and they exhibit identical spectral densities at large frequency limit. In addition, the generalized Ornstein–Uhlenbeck processes can be shown to be local stationary representations of fractional Brownian motion. Two new self-similar Gaussian processes, in addition to fractional Brownian motion, are obtained by applying the (inverse) Lamperti transformation to the generalized Ornstein–Uhlenbeck processes. We study some of the properties of these self-similar processes such as the long-range dependence. We give a simulation of their sample paths based on numerical Karhunan–Loeve expansion.

Oscillatory Flow Near a Stagnation Point

The classical Hiemenz solution describes incompressible two-dimensional stagnation point flow at a solid wall. We consider an unsteady version of this problem, examining particularly the response close to the wall when the solution at infinity is modulated in time by a periodic factor of specified amplitude and frequency. While this problem has already been tackled in the literature for general frequency in cases when the amplitude of the time-periodic factor is either large or small, we compute the flow for arbitrary values of both these parameters. For any given amplitude, we find that there exists a threshold frequency above which the flow is regular and periodic, with the same period as the modulation factor, and beneath which the solution terminates in a finite time singularity. The dividing line in parameter space between these two possibilities is identified and favorably compared with the predictions of asymptotic analyses in the small and large frequency limits.

Shock waves in porous saturated media

This work is concerned with deriving macroscopic models for describing the propagation of shock waves in porous elastic media saturated by a viscous Newtonian fluid. This is performed using the results that are obtained by upscaling the porous medium behaviour from the pore scale description. This upscaling shows four different macroscopic models. Porous media are classified according to two separated categories that are related to the elastic and viscous properties of the solid and the fluid. In each category, shock waves are described by the large frequency limit behaviour. That yields a monophasic elastic model for category 1 media of low fluid viscosity or large solid rigidity and a monophasic viscoelastic model for large fluid viscosity or low solid rigidity for category 2 media.

Nonisotropic effective-medium approximation for diffusion problems in random media

We present the nonisotropic effective-medium approximation to solve diffusion problems in a two-dimensional anisotropic random media. The problem has been worked out by introducing a generalization of the well-known effective-medium approximation. A set of coupled nonlinear self-consistent equations must be solved to find the effective rates in each direction. We have considered (analytically) some particular models in short and large frequency limits. The dc conductivity is also compared against the isotropic case. The ac conductivity and Cole-Cole diagrams for the nonisotropic random bond percolation model have been analyzed in terms of the physical parameters that characterize the anisotropy and the disorder in the media. A monoparametric nonisotropic bond disordered model $(\ensuremath{\alpha}$ model) has also been worked out to show the applicability of the present approach in the context of weak or strong disorder. Such a model of disorder leads the system to show a quasi-one-dimensional behavior, proper of nonisotropic materials.