arXiv subjects

A. P. Itin

Publications and source records attributed to A. P. Itin.

16 recordsLinked to original sources

arXiv2017

Efficient excitation of nonlinear phonons via chirped mid-infrared pulses: induced structural phase transitions

Nonlinear phononics play important role in strong laser-solid interactions. We discuss nonlinear dynamical protocols which allow for efficient excitation and control of nonlinear phonons. We consider recent inspiring proposals: inducing ferroelectricity in paraelectric material such as KTaO$_3$ and inducing structural deformations in cuprates like La$_2$CuO$_4$ [A. Subedi et.al, Phys. Rev. B 89,220301 (2014), A.Subedi, Phys. Rev. B 95, 134113 (2017)]. High-frequency phonon modes are driven by mid-infrared pulses, and coupled to lower-frequency modes those indirect excitation causes structural deformations. Such proposals are in line with a series of recent experiments on light-induced phase transitions. We study in a more detail the case of KTaO$_3$ without strain, where (at first glance) it was not possible to excite the needed low frequency phonon mode by resonant driving of the higher frequency one. Behaviour of the phonon system is explained using a reduced model of coupled driven nonlinear oscillators. We find a dynamical mechanism which prevents effective excitation at resonance driving, and show that for certain detunings of driving frequency from the exact resonance the system response is, counterintuitively, greatly amplified. In order to induce ferroelectricity in KTaO$_3$ without a strain we employ driving with sweeping frequency, realizing so called capture into resonance. The method works for realistic femtosecond pulses. Our approach can be applied to other related systems, e.g. laser driven orthorombic perovskites like ErFeO$_3$.

cond-mat.other↗

arXiv2015

Effective Hamiltonians for fastly driven many-body lattice systems

We consider 1D lattices described by Hubbard or Bose-Hubbard models, in the presence of periodic high-frequency perturbations, such as uniform ac force or modulation of hopping coefficients. Effective Hamiltonians for interacting particles are derived using an averaging method resembling classical canonical perturbation theory. As is known, a high-frequency force may renormalize hopping coefficients, causing interesting phenomena such as coherent destruction of tunnelling and creation of artificial gauge fields. We find explicitly additional corrections to the effective Hamiltonians due to interactions, corresponding to non-trivial processes such as single-particle density-dependent tunnelling, correlated pair hoppings, nearest neighbour interactions, etc. Some of these processes arise also in multiband lattice models, and are capable to give rise to a rich variety of quantum phases. The apparent contradiction with other methods, e.g. Floquet-Magnus expansion, is explained. The results may be useful for designing effective Hamiltonian models in experiments with ultracold atoms, as well as in the field of ultrafast nonequilibrium magnetism. An example of manipulating exchange interaction in a Mott-Hubbard insulator is considered, where our corrections play an essential role.

cond-mat.other↗

arXiv2014

Effective Hamiltonians for fastly driven tight-binding chains

We consider a single particle tunnelling in a tight-binding model with nearest-neighbour couplings, in the presence of a periodic high-frequency force. An effective Hamiltonian for the particle is derived using an averaging method resembling classical canonical perturbation theory. Three cases are considered: uniform lattice with periodic and open boundary conditions, and lattice with a parabolic potential. We find that in the latter case, interplay of the potential and driving leads to appearence of the effective next-nearest neighbour couplings. In the uniform case with periodic boundary conditions the second- and third-order corrections to the averaged Hamiltonian are completely absent, while in the case with open boundary conditions they have a very simple form, found before in some particular cases by S.Longhi [Phys. Rev. B 77, 195326 (2008)]. These general results may found applications in designing effective Hamiltonian models in experiments with ultracold atoms in optical lattices, e.g. for simulating solid-state phenomena.

cond-mat.other↗

arXiv2012

Intrinsic Photoconductivity of Ultracold Fermions in Optical Lattices

We report on the experimental observation of an analog to a persistent alternating photocurrent in an ultracold gas of fermionic atoms in an optical lattice. The dynamics is induced and sustained by an external harmonic confinement. While particles in the excited band exhibit long-lived oscillations with a momentum dependent frequency a strikingly different behavior is observed for holes in the lowest band. An initial fast collapse is followed by subsequent periodic revivals. Both observations are fully explained by mapping the system onto a nonlinear pendulum.

cond-mat.quant-gas↗

arXiv2012

Fermi acceleration in time-dependent rectangular billiards due to multiple passages through resonances

We consider a slowly rotating rectangular billiard with moving boundaries and use the canonical perturbation theory to describe the dynamics of a billiard particle. In the process of slow evolution certain resonance conditions can be satisfied. Correspondingly, phenomena of scattering on a resonance and capture into a resonance happen in the system. These phenomena lead to destruction of adiabatic invariance and to unlimited acceleration of the particle.

nlin.CD↗

arXiv2012

Directed transport in a classical lattice with a high-frequency driving

We analyze the dynamics of a classical particle in a spatially periodic potential under the influence of a periodic in time uniform force. It was shown in [S.Flach, O.Yevtushenko, Y. Zolotaryuk, Phys. Rev. Lett. 84, 2358 (2000)] that despite zero average force, directed transport is possible in the system. Asymptotic description of this phenomenon for the case of slow driving was developed in [X. Leoncini, A. Neishtadt, A. Vasiliev, Phys. Rev. E 79, 026213 (2009)]. Here we consider the case of fast driving using canonical perturbation theory. An asymptotic formula is derived for the average drift velocity as a function of the system parameters and the driving law. We show that directed transport arises in an effective Hamiltonian that does not possess chaotic dynamics, thereby clarifying the relation between chaos and transport in the system. Sufficient conditions for transport are derived.

nlin.CD↗

arXiv2011

Semiclassical Spectrum of Small Bose-Hubbard Chains: A Normal Form Approach

We analyze the spectrum of the 3-site Bose-Hubbard model with periodic boundary conditions using a semiclassical method. The Bohr-Sommerfeld quantization is applied to an effective classical Hamiltonian which we derive using resonance normal form theory. The derivation takes into account the 1:1 resonance between frequencies of a linearized classical system, and brings nonlinear terms into a corresponding normal form. The obtained expressions reproduce the exact low-energy spectrum of the system remarkably well even for a small number of particles N corresponding to fillings of just two particles per site. Such small fillings are often used in current experiments, and it is inspiring to get insight into this quantum regime using essentially classical calculations.

cond-mat.stat-mech↗

arXiv2010

Dynamics of quantum phase transitions in Dicke and Lipkin-Meshkov-Glick models

We consider dynamics of Dicke models, with and without counterrotating terms, under slow variations of parameters which drive the system through a quantum phase transition. The model without counterrotating terms and sweeped detuning is seen in the contexts of a many-body generalization of the Landau-Zener model and the dynamical passage through a second-order quantum phase transition (QPT). Adiabaticity is destroyed when the parameter crosses a critical value. Applying semiclassical analysis based on concepts of classical adiabatic invariants and mapping to the second Painleve equation (PII), we derive a formula which accurately describes particle distributions in the Hilbert space at wide range of parameters and initial conditions of the system. We find striking universal features in the particle distributions which can be probed in an experiment on Feshbach resonance passage or a cavity QED experiment. The dynamics is found to be crucially dependent on the direction of the sweep. The model with counterrotating terms has been realized recently in an experiment with ultracold atomic gases in a cavity. Its semiclassical dynamics is described by a Hamiltonian system with two degrees of freedom. Passage through a QPT corresponds to passage through a bifurcation, and can also be described by PII (after averaging over fast variables), leading to similar universal distributions. Under certain conditions, the Dicke model is reduced to the Lipkin-Meshkov-Glick model.

cond-mat.stat-mech↗

arXiv2009

Dynamics of a many-particle Landau-Zener model: inverse sweep

We consider dynamics of a slowly time-dependent Dicke model, which represents a many-body generalization of the Landau-Zener model. In particular, the model describes narrow Feshbach resonance passage in an ultracold gas of Fermi atoms. Adiabaticity is destroyed when a parameter crosses a critical value, even at very slow sweeping rates of a parameter. The dynamics crucially depends on direction of the sweep. We apply our recent analysis [A.P. Itin, P. Torma, arXiv:0901.4778v1] to the "inverse" sweep through the resonance, corresponding (in a context of Feshbach resonance passage) to dissociation of molecules. On a level of the mean-field approximation, the dynamics is equivalent to a molecular condensate formation from Bose atoms within a two-mode model. Mapping the system to a Painlevé equation allows us to calculate deviation from adiabaticity at very slow sweeps analytically.

cond-mat.other↗

arXiv2009

Stationary wave patterns generated by an impurity moving with supersonic velocity through a Bose-Einstein condensate

Formation of stationary 3D wave patterns generated by a small point-like impurity moving through a Bose-Einstein condensate with supersonic velocity is studied. Asymptotic formulae for a stationary far-field density distribution are obtained. Comparison with three-dimensional numerical simulations demonstrates that these formulae are accurate enough already at distances from the obstacle equal to a few wavelengths.

cond-mat.other↗

arXiv2007

Integrability, stability, and adiabaticity in nonlinear stimulated Raman adiabatic passage

We study dynamics of a two-color photoassociation of atoms into diatomic molecules via nonlinear Stimulated Raman adiabatic passage (STIRAP) process. This system has a famous counterpart in (linear) quantum mechanics, and been discussed recently in the context of generalizing quantum adiabatic theorem to nonlinear systems. Here we use another approach to study adiabaticity and stability in the system: we apply methods of classical Hamiltonian dynamics. We found nonlinear dynamical instabilities, cases of complete integrability, and improved conditions of adiabaticity.

cond-mat.other↗

arXiv2007

Tunneling of Bose-Einstein condensates and nonlinear models with separatrix crossings

This paper has been withdrawn by the author. Instead, more comprehensive and accurate works are placed in: 1) cond-mat/0610767: Universality in nonadiabatic behaviour of classical actions in nonlinear models with separatrix crossings, to appear in Phys.Rev.E; and 2) nlin/0611007: Change in the adiabatic invariant in a nonlinear two-mode model of Feshbach resonance passage, to appear in Physica D

nlin.CD↗

arXiv2007

Change in the adiabatic invariant in a nonlinear two-mode model of Feshbach resonance passage

Mean-field approach has recently been used to model coupled atom-molecular Bose-Einstein condensates (BEC) and coupled Fermi-Bose condensates near Feshbach resonance. Sweeping of magnetic field across the resonance gives a new (nonlinear) version of Landau-Zener problem. We investigate the structure of the corresponding classical phase space and calculate change in the action which corresponds to finite-rate efficiency of the sweep. We consider the case of non-zero initial action, which corresponds to some finite initial molecular fraction.

nlin.CD↗

arXiv2007

Universality in nonadiabatic behaviour of classical actions in nonlinear models with separatrix crossings

We discuss dynamics of approximate adiabatic invariants in several nonlinear models being related to physics of Bose-Einstein condensates (BEC). We show that nonadiabatic dynamics in Feshbach resonance passage, nonlinear Landau-Zener (NLZ) tunnelling, and BEC tunnelling oscillations in a double-well can be considered within a unifying approach based on the theory of separatrix crossings. The separatrix crossing theory was applied previously to some problems of classical mechanics, plasma physics and hydrodynamics, but has not been used in the rapidly growing BEC-related field yet. We derive explicit formulas for the change in the action in several models. Extensive numerical calculations support the theory and demonstrate its universal character. We also discovered a qualitatively new nonlinear phenomenon in a NLZ model which we propose to call {\em separated adiabatic tunnelling}

cond-mat.other↗

arXiv2006

Vortex creation during magnetic trap manipulations of spinor Bose-Einstein condensates

We investigate several mechanisms of vortex creation during splitting of a spinor BEC in a magnetic trap controlled by a pair of current carrying wires and bias magnetic fields. Our study is motivated by a recent MIT experiment on splitting BECs with a similar trap, where unexpected fork-like structure appeared in the interference fringes corresponding to interference of two condensates, one with and the other without a singly quantized vortex. It is well-known that in a spin-1 BEC in a quadrupole trap a doubly quantized vortex is produced topologically by a ``slow'' reversal of bias magnetic field $B_z$. We find that in the magnetic trap considered it is also possible to produce a 4- and 1-quantized vortex in a spin-1 BEC. The latter is possible, for example, during the magnetic field switching-off process. We therefore provide a possible explanation for the unexpected interference patterns in the experiment. We also give an example of the creation of singly quantized vortices due to ``fast'' splitting, which is a possible alternative mechanism of the interference pattern.

cond-mat.other↗

arXiv2005

Resonant phenomena in slowly perturbed elliptic billiards

We consider an elliptic billiard whose shape slowly changes. During slow evolution of the billiard certain resonance conditions can be fulfilled. We study the phenomena of capture into a resonance and scattering on resonances which lead to the destruction of the adiabatic invariance in the system.

nlin.CD↗