Photoinduced instability and semiconductor-metal phase transition in a Peierls system

A microscopic theory of the appearance of electron-phonon instability and a semiconductor-metal phase transition away from thermodynamic equilibrium in a Peierls system upon the optical excitation of electron-hole pairs is devised. An equation which specifies the dependence of the order parameter of the phase transition and the uniquely related gap width in the electron spectrum, on the concentration n of conduction-band electrons is obtained. The critical concentration n=n _c, above which the semiconductor phase of the system is unstable toward the transition to the metallic state, is calculated. A comparison with an experiment on the irradiation of a substrate-supported vanadium dioxide film by a laser pulse is made. Fiz. Tverd. Tela (St. Petersburg) 40, 2113–2118 (November 1998)     

Photoinduced semiconductor-metal phase transition in the surface layer of vanadium dioxide

The photoinduced semiconductor-metal phase transition occurring for a time Δt < 1 ps in the surface layer of vanadium dioxide is studied theoretically. A nonthermal mechanism of instability development is considered. An equation for the order parameter ξ of the photoinduced semiconductor-metal phase transition is derived. It is shown that the transition of the surface layer of VO₂ to the metallic state requires irradiation by a laser pulse whose energy density W exceeds a critical value W _c. The phase transition is initiated at the surface, after which the interface propagates deep into the sample. The critical energy density W _c, the velocity of propagation of the metal-semiconductor interface, the thickness z ₀, and the characteristic time Δt of formation of the metal layer are calculated. The theoretical results obtained are in agreement with the experimental data on irradiation of vanadium dioxide single crystals by high-intensity laser pulses.     

Photoinduced semiconductor-metal phase transition and switching wave in the vanadium dioxide film

A switching wave has been studied in the VO₂ film during the photoinduced semiconductor-metal phase transition. The dependences of the critical radiation intensities corresponding to the direct and reverse phase transitions on the ambient temperature have been obtained in the framework of the thermal model. The profile and velocity of the switching wave have been calculated. The calculated data are compared with the experimental data.     

Nonequilibrium semiconductor-metal phase transition due to self-heating

A model of the nonequilibrium semiconductor-metal phase transition in narrow-gap transition metals due to self-heating is considered. It is shown that self-heating may diminish the energy gap and, as a consequence, increase the current. Parameters resposible for the bistable behavior of the system are found.     

Photoinduced spin-Peierls phase transition

An equation is obtained for the order parameter ξ of the spin-Peierls phase transition in a light field. This equation is used to derive the expressions for the frequency ν of the photogenerated longitudinal optical phonon mode and the time τ of the photoinduced phase transition. Numerical values of the quantities ν and τ calculated in terms of the proposed model agree well with the experimental data for the quasi-one-dimensional compound potassium tetracyanoquinodimethane (K TCNQ) irradiated by a laser pulse. Original Russian Text ? A.L. Semenov, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 1, pp. 164–167.     

Photoinduced phase transition to a new macroscopic spin-crossover-complex phase

We clarified for the first time that the photoinduced phase is quite different in structure from the thermally induced phase by the resonant Raman spectroscopy in the spin-crossover complex, [Fe(2-pic)(3)]-Cl2EtOH. In the photoinduced phase we observed a number of additional lines assigned to infra-active vibrational modes which are strongly prohibited by selection rules in the thermally induced phase. These results indicate that a dramatic symmetry lowering should take place in the photoinduced phase. The cooperative Jahn-Teller transformation is a plausible candidate for the symmetry lowering in the photoinduced phase.     

Photoinduced phase transitions in a Peierls system

A theory is constructed that explains photoinduced phase transitions in a Peierls system being irradiated by light with a finite width of the optical spectrum and a central frequency close to the upper van Hove singularity of the first kind in the combined density of electron states. The electron spectrum and the matrix elements of the dipole-moment operator are calculated by Bogolyubov’s method of canonical transformations. The interaction with the light is described by the Liouville equation for the density matrix of the electron subsystem in the dipole approximation. The light field is considered a quasimonochromatic time-independent random process with a Lorentzian spectrum. The derived equations are analyzed for two limits: (1) when the width of the optical spectrum tends to zero (a monochromatic light field), and (2) when the width of the optical spectrum is close to the upper limit (a bifurcation point) at which a photoinduced phase transition can still be observed. An existence criterion for such a transition is obtained, and the main parameters of the transition (the critical points and the size of the hysteresis loop) are calculated. The broadening of the optical spectrum of the incident light is shown to narrow the range of values of the central frequency of the light field and to reduce the size of the hysteresis loop. Finally, near the phase transition point, cavityless optical bistability sets in in the system, with light absorption increasing in the process. Zh. éksp. Teor. Fiz. 114, 1407–1420 (October 1998)     

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.     

Magnetic breakdown induced peierls transition

We predict a new type of phase transition in a quasi-one-dimensional system of interacting electrons at high magnetic fields, the stabilization of a density wave which transforms a two-dimensional open Fermi surface into a periodic chain of large pockets with small distances between them. We show that quantum tunneling of electrons between the neighboring closed orbits enveloping these pockets transforms the electron spectrum into a set of extremely narrow energy bands and gaps that decreases the total electron energy, thus leading to a magnetic breakdown induced density wave ground state analogous to the well-known instability of the Peierls type.     

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.     

Photoinduced phase transition in Ag3AsS3 crystals

It is shown that the illumination of samples leads to the appearance of anomalies in the temperature dependences of the propagation velocity of longitudinal ultrasonic waves along the principal crystallographic directions of proustite (Ag₃AsS₃) at T∼150 K. The features discovered are associated with a photoinduced phase transition caused by restructuring of the cation sublattice of proustite. Fiz. Tverd. Tela (St. Petersburg) 41, 702–704 (April 1999)     

Destruction of SmS single crystals during the semiconductor-metal phase transition under hydrostatic compression

The behavior of internal microstresses, the size of the X-ray coherent scattering region, and the residual amount of the metal phase during the cyclic loading of SmS single crystals at hydrostatic pressure above the critical pressure of the semiconductor-metal phase transition has been investigated. It has been shown that the samples are destroyed as the microstresses reach the values corresponding to the ultimate stress of SmS single crystals. As the number of loading cycles increases, the coherent scattering region gradually decreases, which is accompanied by a decrease in the amount of the metal phase in the samples.     

Photoinduced phase transitions in a system with a transformable electron spectrum

An electron theory of photoinduced phase transitions and cavityless optical bistability in a light field with a finite optical-spectrum width is constructed. The Liouville equation for the density matrix is used to derive the existence criteria and calculate the main characteristics of the given phenomena. Broadening of the optical spectrum is shown to reduce the possibility of observing critical features (the existence criterion becomes more stringent and the hysteresis-loop area becomes smaller). Finally, results are compared with experimental data for CdS and amorphous GeS₂. Zh. éksp. Teor. Fiz. 111, 2147–2157 (June 1997)     

Photoinduced phase transition accompanied with the changes in magnetic properties

Recently, many studies have been started in search for materials which show a photoinduced phase transition (PIPT). In this work, we review two systems as typical examples of PIPT accompanied with changes in magnetic characteristics; (1) organo-metal complex [Fe(2-pic)₃]Cl₂ EtOH (2-pic = 2-amino-methyl-pyridine) and (2) III-V based magnetic semiconductors (In_1-x , Mn _x )As. In the former case, we show several nonlinear characteristics in dynamical process of photoinduced spin state transition from low-spin to high-spin states. In the latter one, photocarrier-induced ferromagnetic order has been observed by both magnetic and transport measurements.     

Fluctuation conductivity above the peierls transition

Reasons are given why the calculation by Strässler and Toombs of the phonon drag contribution to the electrical conductivity above the Peierls transition temperature is wrong.     

Photoinduced superstructure in the low-temperature phase of a Peierls system

This paper is a theoretical study of the properties of the low-temperature phase of a Peierls system when nonequilibrium electron-hole pairs are excited in the phase. A microscopic theory is developed to show that at low temperatures a spatially nonuniform periodic structure with a modulated band gap forms in the thermodynamically nonequilibrium system considered. The critical temperature of formation of such a superstructure, the critical electron-hole pair concentration, the spatial period, and the percentage modulation are calculated. Zh. éksp. Teor. Fiz. 115, 1297–1314 (April 1999)     

Soliton excitations in a commensurability 2 mixed peierls system

We consider a one-dimensional half-filled band system with the electrons coupled to both internal mode and bond modulation. For weak and intermediate coupling strength the site dimerization are mutually exclusive in the uniform ground states and inside neutral solitons. They coexist however inside charged solitons. The solitons carry charge $\text{±}\text{e}\text{2}$ per spin. Creation energy decreases as the two couplings become less different. The position of the gap state depends on the occupation number.     

Melt-crystal phase transition in a two-component system

The melt-crystal phase transition in a two-component system is described by a generalized Ornstein-Zernicke equation for one- and two-particle distribution functions. The jump in the density δn under melt crystallization is a small parameter. The desired distribution functions are found as δn power series. The two-particle distribution function for the melt along its crystallization line is chosen as a zeroth approximation. Institute of Applied Physics at the Irkutsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 54–61, February 2000.     

Order-disorder phase transition in a chaotic system

For soft-mode turbulence, which is essentially the spatiotemporal chaos caused by the nonlinear interaction between convective modes and Goldstone modes in electroconvection of homeotropic nematics, a type of order-disorder phase transition was revealed, in which a new order parameter was introduced as pattern ordering. We calculated the spatial correlation function and the anisotropy of the convective patterns as a 2D XY system because the convective wave vector could freely rotate in the homeotropic system. We found the hidden order in the chaotic patterns observed beyond the Lifshitz frequency f(L), and a transition from a disordered to a hidden ordered state occurred at the f(L) with the increase of the frequency of the applied voltages.