Evidence for an excitonic insulator phase in 1T-TiSe2

We present a new high-resolution angle-resolved photoemission study of 1T-TiSe2 in both its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalizations at high-symmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A calculation of the theoretical spectral function for an excitonic insulator phase reproduces the experimental features with very good agreement. This gives strong evidence in favor of the excitonic insulator scenario as a driving force for the charge-density wave transition in 1T-TiSe2.     

Time of a semiconductor-metal phase transition induced by an ultrashort light pulse in vanadium dioxide

A relationship is obtained for a time τ of a photoinduced semiconductor-metal phase transition of the Peierls type under exposure to an ultrashort laser pulse. It is demonstrated that (i) the time τ increases as the pulse energy density W decreases, and (ii) the phase transition is initiated when the pulse energy density W exceeds a critical value W _c . The results obtained are compared with the experiment on irradiation of a vanadium dioxide film in a light field.     

Ultrafast semiconductor-metal phase transition in vanadium dioxide induced by a femtosecond laser pulse

A model is proposed for a photoinduced Peierls-type semiconductor-metal phase transition that makes it possible to determine the time dependence of the bandgap width in the electronic spectrum of vanadium dioxide subjected to a light field and the dependence of the time at which a photoinduced semiconductor-metal phase transition occurs on the laser pulse duration. The theoretical results obtained are consistent with experimental data on the illumination of a VO₂ film with an intense laser pulse.     

转动诱发原字核量子相变的一种可能途径

基于微观IBM理论，提出转动诱导出玻色子量子相变的一种可能途径：一旦原子核在受到高能激发或作快速旋转时，假如外界提供的能量足以使玻色子完成拆对顺排，则核处于集体相与单粒子态的共存相，其特征是出现较密集的能谱；假如能量不足以完成拆对或顺排，可能发生两种情况之一，当核旋转达到某个临界转动频率ωc时，或者一个高角动量的玻色子脱离“集体”而“游离”出来，或者发生一个高角动量的玻色子转变为一个低角动量的玻色子，核仍旧处于集体相；均会伴随出现光辐射，产生基态带的一条能级——相变信号．正是这类玻色子相变导致了原子核的量子相变．本物理图像统一了玻色子拆对顺排相变和退耦释放光子相变的描述．以100^Pd核的141^＋，142^＋和143^＋态的产生机理为例，对模型作了仔细说明．     

Ultrasonic attenuation near the phase transition for the magnetic field induced excitonic phase in bismuth

The fluctuation contribution to the ultrasonic attenuation above T_c is shown to have a log T form similar to the log T components in properties of Kondo systems. The recent results by Mase et al. show this behaviour in bismuth.     

Coherent transport in a homojunction between an excitonic insulator and semimetal

From the solution of a two-band model, we predict that the thermal and electrical transport across the junction of a semimetal and an excitonic insulator will exhibit high resistance behavior and low entropy production at low temperatures, distinct from a junction of a semimetal and a normal semiconductor. This phenomenon, ascribed to the dissipationless exciton flow which dominates over the charge transport, is based on the much longer length scale of the change of the effective interface potential for electron scattering due to the coherence of the condensate than in the normal state.     

Ultrashort optical pulse in a thin film of a topological insulator

We consider the propagation of ultrashort optical pulses in a thin film of a topological insulator within the framework of an effective long-wave Hamiltonian for low-temperature media. The electromagnetic field is taken as classical Maxwellian. We reveal the dependence on the maximum amplitude of ultrashort optical pulses.     

Noised-induced phase transition in an oscillatory system with dynamical traps

A new type of noised induced phase transitions is proposed. It occurs in noisy systems with dynamical traps. Dynamical traps are regions in the phase space where the regular forces are depressed substantially. By way of an example, a simple oscillatory system with additive white noise is considered and its dynamics is analyzed numerically. The dynamical trap region is assumed to be located near the x-axis where the velocity v of the system becomes sufficiently low. The meaning of this assumption is discussed. The observed phase transition is caused by the asymmetry in the residence time distribution in the vicinity of zero value velocity. This asymmetry is due to a cooperative effect of the random Langevin force in the trap region and the regular force not changing the direction of action when crossing the trap region.Received: 25 April 2003, Published online: 19 November 2003PACS: 05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion - 05.45.-a Nonlinear dynamics and nonlinear dynamical systems - 05.70.Fh Phase transitions: general studies     

First-order phase transition through an intermediate state

The theory of first-order phase transitions in systems where the direct formation of nuclei of a new phase is inhibited for any reason, for example, because of the extremely high elastic energy, has been constructed using the example of the silicon-silicon carbide phase transition due to the chemical reaction with carbon monoxide. It has been shown that, in this case, the phase transition occurs through an intermediate state, which significantly promotes the formation of new-phase nuclei. For the silicon-silicon carbide phase transition, such an intermediate state is the “pre-carbide” state of silicon saturated with dilatation dipoles, i.e., pairs formed by a carbon atom and a silicon vacancy that are strongly attracted to each other. The model dependence of the potential energy of systems with an intermediate phase on the reaction coordinates has been investigated. The kinetics of transformation of the intermediate state into a new phase has been described.     

The pressure induced insulator to metal transition in FeSb2

The evolution of the ground state properties of FeSb(2) has been investigated via temperature (4.2-300 K), magnetic field (0-12 T) and pressure (0-8.8 GPa) dependent electrical resistivity studies. The temperature dependence of the resistivity follows activated behavior in the high temperature (HT) regime (T > 60 K), while variable range hopping (VRH) dictates the transport in the intermediate temperature (IT) regime (10 K > T > 45 K) and power law behavior is observed in the low temperature (LT) regime (T < 10 K). The pressure profoundly affects the resistivity in all the temperature regimes. The energy gap (Δ) extracted in the HT regime initially increases with pressure and then decreases, while the VRH parameter T(0) deduced in the IT regime is seen to decrease monotonically and vanish beyond 5 GPa leading to an insulator to metal transition (MIT) on account of delocalization of the electronic states in the gap. The analysis of the logarithmic derivative of the conductivity indicates the MIT to occur at ~6 GPa. The magnetoresistivity is found to be positive. The analysis of the resistivity behavior under pressure and magnetic field indicates that the former induces delocalization, while the latter tends to assist localization of the defect states inside the gap of FeSb(2).     

Extremely short optical pulse in a thin-film topological insulator with a hexagonal lattice

The propagation of an extremely short optical pulse in a thin-film topological insulator has been considered. The electrons are described by a low-temperature long-wavelength effective Hamiltonian, whereas the electromagnetic field is treated classically within the Maxwell??s equations. The dependence on the velocity and maximum amplitude of the extremely short pulse has been revealed.     

Distortion in optical pulse equalization through phase modulation and dispersive transmission

By combining phase modulation and dispersive transmission we propose an adjustable optical device to compress a time-varying input signal for producing ultrashort light pulses of high optical power. The relationship between the required properties of the input signal and the device parameters to get well-conformed sharp pulses with large energy concentration was established. High-order aberrations effects of the electro-optical modulator acting as the time lens of the employed setup are also considered. We found a mathematical analogy between the output signal of the analyzed device and the signal which would be obtained by using an equivalent first-order dispersive line. Some numerical simulations are shown which illustrate the feasibility of the method.     

Metal to insulator transition in epitaxial graphene induced by molecular doping

The capability to control the type and amount of charge carriers in a material and, in the extreme case, the transition from metal to insulator, is one of the key challenges of modern electronics. By employing angle-resolved photoemission spectroscopy we find that a reversible metal to insulator transition and a fine-tuning of the charge carriers from electrons to holes can be achieved in epitaxial bilayer and single layer graphene by molecular doping. The effects of electron screening and disorder are also discussed. These results demonstrate that epitaxial graphene is suitable for electronics applications, as well as provide new opportunities for studying the hole doping regime of the Dirac cone in graphene.     

Scaling function near a transition to an insulator state

Analysis of experimental results yields a scaling function of the form β(g)=121/g near the metal-insulator transition in three-dimensional systems. In two-dimensional electronic systems demonstrating a transition to an insulator state, the same relation holds for the function νβ, where ν is the critical exponent characterizing the divergence of the correlation length. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 11, 807–811 (10 December 1998)     

A dynamical phase transition in a caricature of a spin glass

This paper studies the rate of convergence to equilibrium of Glauber dynamics (Gibbs Sampler) for a system ofN Ising spins with random energy (at inverse temperature >0). For each of the 2 ^N spin configurations the energy is drawn independently from the values 0 and-logN with probabilities 1-N ^–7, resp.N ^– (>0), and is kept fixed during the evolution. The main result is an estimate of the coupling time of two Glauber dynamics starting from different configurations and coupled via the same updating noise. AsN the system exhibits two dynamical phase transitions: (1) at =1 the coupling time changes from polynomial (>1) to stretched exponential (<1) inN; (2) if <1, then at = the almost coupling time [i.e., the first time that the two dynamics are within distanceo(N)] changes from polynomial (<) to stretched exponential (>) inN. The techniques used to control the randomness in the coupling are static and dynamic large-deviation estimates and stochastic domination arguments.     

Quantum dynamical phase transition in a system with many-body interactions

Recent experiments, [G.A. Álvarez, E.P. Danieli, P.R. Levstein, H.M. Pastawski, J. Chem. Phys. 124 (2006) 194507], have reported the observation of a quantum dynamical phase transition in the dynamics of a spin swapping gate. In order to explain this result from a microscopic perspective, we introduce a Hamiltonian model of a two level system with many-body interactions with an environment whose excitation dynamics is fully solved within the Keldysh formalism. If a particle starts in one of the states of the isolated system, the return probability oscillates with the Rabi frequency ω₀. For weak interactions with the environment , we find a slower oscillation whose amplitude decays with a rate . However, beyond a finite critical interaction with the environment, , the decay rate becomes . The oscillation period diverges showing a quantum dynamical phase transition to a Quantum Zeno phase consistent with the experimental observations.     

Loschmidt echo near a dynamical phase transition in a Bose-Einstein condensate

We show that the quantum Loschmidt echo can be employed to characterize the dynamical phase transition, from a tunnelling phase to a self-trapping phase, of a Bose-Einstein condensate in a double-well potential. The echo is found to have a relatively fast decay in the transition region, with a Gaussian decay in the self-trapping phase and a stretched exponential decay in the tunnelling phase.