Self-trapping Research Articles

Effects of quantum fluctuations on macroscopic quantum tunneling and self-trapping of a BEC in a double-well trap

In this paper, we study the influence of quantum fluctuations (QFs) on the macroscopic quantum tunneling and self-trapping (ST) of a two-component Bose–Einstein condensate in a double-well trap. QFs are described by the Lee–Huang–Yang (LHY) term in the modified Gross–Pitaevskii (GP) equation. Employing the modified GP equation in a scalar approximation, we derive a dimer model using a two-mode approximation. The frequencies of Josephson oscillations and ST conditions under QFs are found analytically and proven by numerical simulations of the modified GP equation. The tunneling and localization phenomena are also investigated for the case of the LHY fluid loaded in a double-well potential.

Exciton Self-Trapping Dynamics in 1D Perovskite Single Crystals: Effect of Quantum Tunnelling.

We present experimental and theoretical investigations of the photophysics in the one-dimensional (1D) hybrid organic-inorganic perovskite (HOIP) white-light emitter, [DMEDA]PbBr4. It is found that the broadband-emission nature of the 1D perovskite is similar to the case of two-dimensional (2D) HOIP materials, exciton self-trapping (ST) is the dominant mechanism. By comprehensive spectroscopic investigations, we observed direct evidence of exciton crossing the energy barrier separating free and ST states through quantum tunnelling. Moreover, we consider the lattice shrinking mechanisms at low temperatures and interpret the ST exciton formation process using a configuration coordinate diagram. We propose that the energy barrier separating free and ST excitons is temperature-dependent, and consequently, the manner of excitons crossing it is highly dependent on the exciting energy and temperature. For excitons located at the bottom of the free excitonic states, the quantum tunnelling is the dominant channel to the ST states.

Nature of extrinsic and intrinsic self-trapping of charge carriers in underdoped cuprate high-Tc superconductors

Nature of extrinsic and intrinsic self-trapping (ST) of charge carriers in cuprates have been studied theoretically. The binding energies and radii of the extrinsic and intrinsic large polarons and bipolarons in cuprates are calculated variationally using the continuum model and adiabatic approximation. We have shown that the extrinsic and intrinsic three-dimensional (3D) large bipolarons exist in underdoped cuprates at $\eta=\varepsilon_{\infty}/\varepsilon_0<0.127$ and $\eta<0.138$, respectively [where $\varepsilon_{\infty}$ ($\varepsilon_0$) is the optic (static) dielectric constant].

The effect of s-wave scattering length on self-trapping and tunneling phenomena of Fermi gases in one-dimensional accelerating optical lattices**Project supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA01020304) and the National Natural Science Foundation of China (Grant Nos. 11275156, 91026005, 11365020, and 11047010).

We investigate the tunneling dynamics of the Fermi gases in an optical lattice in the Bose–Einstein condensation (BEC) regime. The three critical scattering lengths and the system energies are found in different cases of Josephson oscillation (JO), oscillating-phase-type self-trapping (OPTST), running-phase-type self-trapping (RPTST), and self-trapping (ST). It is found that the s-wave scattering lengths have a crucial role on the tunneling dynamics. By adjusting the scattering length in the adiabatic condition, the transition probability changes with the adiabatic periodicity and a rectangular periodic pattern emerges. The periodicity of the rectangular wave depends on the system parameters such as the periodicity of the adjustable parameter, the s-wave scattering length.

Self-trapping and its periodic modulation of superfluid Fermi gases in one-dimensional accelerating optical lattice from BEC to unitarity regime

We study the tunneling dynamics of superfluid Fermi gases in an optical lattice from BEC to unitarity regime. Four different cases are found, namely Josephson oscillations (JO), oscillating-phase-type self-trapping (OST), running-phase-type self-trapping (RST) and self-trapping (ST). We find that the s-wave scattering length has a crucial role on the tunneling dynamics. Periodic modulation can also affect the tunneling phenomena, for example, in the high frequency modulation, the optical lattice is splitted at the zero points of the zeroth order Bessel function J0 and strong suppression of tunneling by the modulation is observed at these points.

Landau-Zener tunneling of Bose-Fermi mixture in double-well

The nonlinear Landau-Zener tunneling of a Bose-Fermi mixture in a double-well potential is studied in the present paper. The effect of interaction parameters on bosonic and fermionic tunneling probability is studied for the mixture of 40K-87Rb. The tunneling phenomena of the system can be controled by adjusting sweeping rate, intraspecies interaction, interspecies interaction and the numbers of bosons and fermions. It is noted that there are three different regions in phase diagram: self-trapping (ST), complete tunneling (CT) and incomplete tunneling (ICT).

On the relevance of self-trapping as the mechanism for charge and energy transfer in biological systems

The concept that polarons and solitons may play a crucial role as the underlying transport agents in biological structures has been critically re-examined on the basis of the general theory of self-trapping (ST) phenomena. The conditions determining the existence and stability of polarons and solitons, and their character as a function of the values of the basic physical parameters of the system, were formulated. It was found that Davydov's soliton model cannot explain the intramolecular energy transfer in the α-helix and acetanilide, and an alternative formulation in terms of the so-called Takeno model is more realistic. However, Davydov's theory is sufficiently flexible to describe electron transfer in the α-helix and acetanilide.

Exciton dynamics and trapping in J-aggregates of carbocyanine dyes

Exciton relaxation processes in J-aggregates of two thiacarbocyanine dyes (TDC and THIATS) in water/ethylene glycol low temperature glass have been studied by means of steady-state, site-selective and time-resolved spectroscopy. The results of different experiments can be rationalized within the framework of exciton self-trapping (ST) in a 1D molecular chain. The measured Stokes shift is found to be proportional to the exciton - phonon coupling strength squared that is in accordance with the model of large radius ST excitons. A negative fluorescence anisotropy was observed for J-aggregates of both investigated dyes upon excitation outside the J-band. The observed monotonic dependence of the fluorescence anisotropy on the excitation wavelength is interpreted quantitatively within the framework of intraband absorption and Davydov splitting.

Self-Trapping in Closed Systems

Abstract We investigate the self-trapping (ST) in a closed one-dimensional (1d) system and show that there exists a critical size for the onset of the ST, l cr(g), depending on the coupling constant, g.

Dimensionality Dependence in Self-Trapping of Excitons

Self-trapping (ST) of excitons (or electrons) interacting with phonons via short-range potentials depends strongly on the degree of freedom of their motion on the lattice. When excitons can move three-dimensionally, the self-trapped (S) state appears suddenly as a strongly-localized one when the coupling constant ( g ) exceeds a certain critical value. Free (F) states do not become unstable however large g is. When exciton motion is limited only in one dimension, excitons are always self-trapped and F states are unstable irrespective of the magnitude of g (≠0). The S state appears as a strongly-extended one in the limit of g →0, and its spatial extension decreases as g increases. For excitons mobile in two dimensions, there exist two critical values g c1 and g c2 (> g c1 ) of g : The S state appears suddenly as a strongly-localized one when g exceeds g c1 , but F states become unstable when g exceeds g c2 although they are metastable for g c1 < g < g c2 .

Theoretical analysis of hole self-trapping in ionic solids: Application to the KCl crystal.

A method for the calculation of the hole self-trapping (ST) energy in ionic crystals is proposed. It combines model-Hamiltonian and quantum-chemical approaches. An artificial path for the ST process has been suggested containing (a) a free hole not interacting with the lattice vibrations; (b) a free-hole wave packet localized in a small crystal volume in the form of the real ST state, all crystal ions being in their perfect lattice positions; (c) the final ST state of the hole, accompanied with a corresponding lattice relaxation, including strong displacements of ions belonging to the hole region. Some intermediate states might be adopted between (a) and (b) in order to simplify the calculations. The first step (a\ensuremath{\rightarrow}b) is calculated with the use of a simple model Hamiltonian taking into account inertial-free crystal polarization; the latter is calculated by means of Toyazawa's electronic polaron model. Quantum-chemical calculations are used for the last (b\ensuremath{\rightarrow}c) and all intermediate (if any) steps, and are made by means of the embedded-molecular-cluster model combined wtih a self-consistent treatment of both the crystal polarization and the electronic structure. In order to illustrate the method, a ST hole in the form of the ${\mathit{V}}_{\mathit{k}}$ center (${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$ quasimolecular ion) in the KCl crystal is considered and the ST energy is calculated as carefully as possible. In particular, the semiempirical intermediate neglect of the differential overlap modification of the unrestricted Hartree-Fock-Roothaan method is employed for actual calculations. The hole ST energy in the KCl crystal is found to be near -2.4 eV.

THEORY OF THE MULTIPHONON TRAPPING RATE

A theory of the self‐trapping (ST) rate w(T), and also of the rate of the multi‐phonon trapping of charge carriers and excitons by defects is developed in the semiclassical approximation. The preexponential factor in w(T) is estimated in both low‐temperature (instanton) and high‐frequency (Arrhenius) limits.

Self-trapping model of desorption

Desorption induced by self-trapping (ST) of a surface excitation is studied for a model in which the localization of the surface excitation eliminates the surface bond of the desorbing particles. The adiabatic energy of the system is calculated with the use of discrete and continuum models. The free (F) state is meta- or unstable against the desorptive state for any small force to expel the adsorbate at the site of the localization. The absence of the elastic deformation energy due to bond scission causes decrease of the adiabatic energy with increase in the degree of localization, which induces a large bond dilation leading to desorption. The barrier height separating F and ST-induced desorptive states is evaluated from the adiabatic energy. The desorption probability is discussed in conjunction with its temperature and coverage dependence, where the possibility of tunneling from the metastable F state to the desorptive one is predicted.

Self-trapping model of desorption

Desorption induced by self-trapping (ST) of a surface excitation is studied for a model in which the localization of the surface excitation eliminates the surface bond of the desorbing particles. The adiabatic energy of the system is calculated with the use of discrete and continuum models. The free (F) state is meta- or unstable against the desorptive state for any small force to expel the adsorbate at the site of the localization. The absence of the elastic deformation energy due to bond scission causes decrease of the adiabatic energy with increase in the degree of localization, which induces a large bond dilation leading to desorption. The barrier height separating F and ST-induced desorptive states is evaluated from the adiabatic energy. The desorption probability is discussed in conjunction with its temperature and coverage dependence, where the possibility of tunneling from the metastable F state to the desorptive one is predicted.

Dynamical model of desorption on rare gas solids stimulated by exciton self-trapping

A single model describing the desorption of electronically excited rare gas solids (RGS) is proposed. It is based on the fact that the work function of electrons is negative in light RGS. As a result, in the course of the self-trapping (ST) an exciton produces a force acting on the surrounding atoms. This force pushes them away and results in their emission from the surface within a few picoseconds of the self-trapping.

New approach to the theory of non-radiative transitions: Passing the self-trapping barrier

The adiabatic theory of temperature dependence of the self-trapping (ST) rate is developed. It is shown that there exist two types of solutions. They correspond to the classical activational regime and to thermoactivated tunneling. The first solution dominates at high temperatures and results in the Arrhenius law. The second one dominates at low temperatures and is described mathematically in terms of instantons. These general results of the theory are not restricted by any specific models. The theory is developed in an exponential approximation, however, for the low-temperature region the temperature dependence and magnitude of the pre-exponential factor are found as well. The dependence of the ST rate on the energy of the exciton, as well as a mechanism of the defect production in the course of ST are discussed. In conclusion a comparison with experimental data has been outlined briefly.

Self-trapping of hot particles

Abstract The theory of self-trapping (ST) of hot particles having non-equilibrium distribution function is presented. It is shown, that for fast particles the trapping at the adiabatic level produced by the lattice deformation is a bottleneck controlling the ST rate. As a result, the ST rate reaches its maximum for particles having a moderate energy which does not exceed the height of self-trapping barrier. This energy decreases with increasing the lattice temperature.

Is the Barrier Height of Exciton Self-Trapping Evaluated So far Correct?–Quantum Mechanical Reevaluation–

The barrier height E B for self-trapping (ST) has been evaluated so far by analyzing the data on thermal quenching of free-exciton luminescence with the use of the classical formula (∝ exp [- E B / k B T ]) of the ST rate even at temperature T considerably lower than the Debye one. The rate of multiphonon nonradiative transition such as the ST rate cannot be described by the classical formula at such low temperatures. Analysis of the data of RbI below 30 K with the quantum mechanical formula gives E B ≃38–39 meV much larger than 17–18 meV obtained so far. The analysis shows simultaneously that concerning n =1 free excitons ST into a state of one-center type determines the rate in RbI while in KI it is overcome by ST into a state of two-center type with increasing temperature. This enables us to predict that the so called E x luminescence should be emitted from a still unknown self-trapped state of one-center type.

Self-Trapping of Excitons Beginning with Tunneling Nucleation

The rate of self-trapping (ST) is calculated, considering that ST begins with quantum-mechanical tunneling of a free exciton to a state of nucleation less localized than the relaxed self-trapped one. Three cases appear depending on the site-diagonal and off-diagonal exciton-phonon interaction energies S 1 and S 2 relative to the half width B of the exciton band. When \(S_{2}/B \gtrsim 0.21\), ST through the nucleation state of two-center type predominates. When \(S_{2}/B \lesssim 0.21\) and \(S_{1}/B \lesssim 1.25\), only ST through that of one-center type occurs. When \(S_{2}/B \lesssim 0.21\) but \(S_{1}/B \gtrsim 1.25\), the latter channel predominates at low temperatures but it is over-come by the former one with increasing temperature. Alkali halides except iodides are classified into the first case, RbI into the second one, and KI into the third one. The so called E x luminescence observed only in alkali iodides should be emitted from a still unknown self-trapped state of one-center type.