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.     

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 semiconductor-metal phase transition in a peierls system

The dynamical equation for the order parameter of the metal-semiconductor phase transition, as well as the kinetic equation for the density of nonequilibrium electron-hole pairs of a Peierls system in a light field, has been derived. An expression for the time τ of the nonthermal photoinduced semiconductor-metal phase transition has been obtained from these equations for the case of an ultrashort light pulse. It has been shown that, to initiate the phase transition, the energy density W of the light pulse must be higher than the critical value W _c. The W _c, τ, and optical absorption coefficient γ₀ that are calculated in the framework of the proposed model are in agreement with the experimental data (W _c ≈ 12 mJ/cm², τ ≈ 75 fs, and γ₀ ≈ 10⁵ cm^?1) on the irradiation of a vanadium dioxide film by a laser pulse with a duration of τ_p ≈ 15 fs, a photon energy of ?θ₀ = 1.6 eV, and an energy density of W = 50 mJ/cm².     

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 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.     

Capturing one-dimensional precursors of a photoinduced transformation in a material

Achieving control of photoinduced phase transitions requires understanding how materials work during transformation induced by a laser pulse. Here we investigate the precursors of a photoinduced phase transition in the highly cooperative charge-transfer molecular crystal tetrathiafulvalene-p-chloranil and provide key insights. The photogeneration of one-dimensional nanoscale clusters was detected by time-resolved diffuse x-ray scattering with 50-ps time resolution. Such clustering of structurally relaxed electronic excitations is expected to be a common process in many materials presenting photoinduced transformations.     

Photoinduced phase transition in an organic radical crystal with room-temperature optical and magnetic bistability

A phase control by photoirradiation is successfully achieved in a spin-Peierls system of the organic radical crystal, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which exhibits optical and magnetic bistability around room temperature with a large hysteresis loop. A nanosecond laser pulse is found to induce a transition from a diamagnetic low-temperature phase to a paramagnetic high-temperature phase both inside (296 K) and outside (11 K) the hysteresis loop. Comparison of the excitation energy dependence between transition efficiency and photoconductivity suggests that the photoinduced transition is driven by suppression of the spin-Peierls instability by the accumulation of photocarriers.     

Time resolved spectra of optical and electrical transient response induced by nanosecond laser pulses in amorphous AgInSbTe films

The phase transition dynamics of amorphous Ag₈In₁₄Sb₅₅Te₂₃ (AIST) thin films induced by single nanosecond laser pulses were studied by transient optical reflectivity and electrical resistance measurements with nanosecond resolution. Phase transition driven by nanosecond laser pulses can be achieved in a proper fluence range on AIST thin films. The results show that phase transition dynamics driven by nanosecond laser pulses was a multi-stage optical evolution process involving melt, solidification, recalescence, and recrystallion. However, it was found that the real-time responses of optical and electrical signals were quite different under the same irradiated condition. The recalescence process reflected by the second rising of optical reflectivity will not result in obvious changes in electrical resistance. The dependence of saturated time determined by optical and electrical evolution curve on laser pulse fluence was compared and analyzed. A two-dimensional percolation model was employed to explain the difference between electrical and optical transient responses.     

Photoinduced coherent phonons in bismuth

The equation of the photoinduced dynamics for atoms of the crystal lattice of Bi has been obtained. The time dependence of atomic shifts under the effect of an ultrashort laser pulse on the crystal has been calculated based on this equation. It has been shown that the frequency of the oscillating part of the shifts depends on the energy density of the laser pulse and time. The intensity of the Fourier spectrum of photoinduced coherent vibrations of the crystal lattice (coherent phonons) has been calculated. The obtained theoretical results have been compared with the experimental data.     

Ultrafast Faraday effect and the dynamics of the antiferromagnet-paramagnet phase transition in FeBO3

The optical pump-probe technique using ultrashort laser pulses with a photon energy of 1.55 eV was used to study the dynamics of the antiferromagnet-paramagnet phase transition in FeBO₃. The Faraday magneto-optical effect was measured with a time resolution of 100 fs, and signal transients were observed as functions of sample temperature. The rate of photoinduced phase transition was shown to be limited by the phonon-magnon relaxation rate with a characteristic time of 700 ps. The subpicosecond dynamics of Faraday rotation is not associated with the destruction of magnetic order but is caused by electron photoexcitation and recombination.     

Features of the semiconductor-metal transition in GaAs at ultrahigh pressures: New intermediate phases

Using the thermopower method (Seebeck effect), the semiconductor-metal transition that occurs in gallium arsenide single crystals of n and p types at ultrahigh pressures P above ～11–18 GPa has been studied. It has been found that the transition in n-type samples begins at lower pressures. In the region of the semiconductor-metal phase transition, features have been observed on the thermopower dependences S(P). These features indicate that lattices intermediate between the initial semiconductor structure of zinc blende and the Cmcm high-pressure orthorhombic metallic phase are formed. By analogy with ZnTe, one intermediate phase (semiconductor with hole conductivity) is suggested to have the cinnabar structure and the second intermediate phase (semimetallic with electron conductivity) possibly has the SC16 structure. A model of the semiconductor-metal transition is discussed. The behavior of the thermoelectric properties in GaAs under pressure is compared with the behavior of these properties in other A_NB_8?N semiconductors, which also undergo the transition to the metallic state.     

Electrical and magnetic properties of manganites La1 ? xAgxMnO3 (x = 0.05, 0.1, and 0.2) at 77 K < T < 300 K

The electrical conductivity, magnetization, and magnetoresistance of manganites La_{1 − x} Ag _x MnO₃ have been investigated in the temperature range 78–300 K. The samples have been synthesized by the sol-gel method. At room temperature, the magnetic field of 0.6 T has no effect on the electrical conductivity. As the temperature decreases, an abrupt jump is observed in the magnetization curve due to the semiconductor-metal phase transition. This transition hardly affects the temperature dependence of the resistance.     

Semiconductor-metal transition in selenium under shock compression

Phase transitions in selenium are studied by time-resolved measurements of the electrical conductivity under shock compression at a pressure of up to 32 GPa. The pressure dependence of the electrical conductivity (σ(P)) has two portions: a sharp increase at P < 21 GPa and a plateau at P > 21 GPa. The experimental data and the temperature estimates indicate that, at P < 21 GPa, selenium is in the semiconductor state. The energy gap of semiconducting selenium decreases substantially under compression. At P > 21 GPa, the electrical conductivity saturates at ～10⁴ Ω^?1 cm^?1. Such a high value of the electrical conductivity shows the effective semiconductor-metal transition taking place in shock-compressed selenium. Experiments with samples having different initial densities demonstrate the effect of temperature on the phase transition. For example, powdered selenium experiences the transition at a lower shock pressure than solid selenium. Comparison of the temperature estimates with the phase diagram of selenium shows that powdered selenium metallizes in a shock wave as a result of melting. The most plausible mechanism behind the shock-induced semiconductor-metal transition in solid selenium is melting or the transition in the solid phase. Under shock compression, the metallic phase arises without a noticeable time delay. After relief, the metallic phase persists for a time, delaying the reverse transition.     

Laser-induced pattern formation in liquid sulfur

Liquid sulfur is a well-known liquid which exhibits a polymerization transition at T_p=159 °C. Recently, it was found from our experiments that such a transition can be induced below T_p through laser illumination and that an iridescent pattern appears under strong illumination with a pulsed laser of more than 60 mJ/cm² pulse. It is proposed that the visible change in iridescence is due to a macroscopic reconstruction of laser-generated polymers and that a laser-induced phase transition takes place from a freely expanded polymer phase to an ordered polymer phase when increasing the laser illumination. To further examine this possibility, the time variation of the iridescent pattern has been fully investigated using a macro lens, a polarized microscope and an optical microscope. In an analysis of the iridescent pattern, a rapid decrease in the area was observed after an initial slow decrease, suggesting a type of phase transition. Results from the observation of a quenched sulfur sample with a polarized microscope gave evidence that the iridescent region consists of polymers. Through observation of the liquid with a microscope, a striped pattern with micrometer sized spacing was noted in the iridescent pattern. A drastic color change was observed in the pattern from its generation to its disappearance. Sample thickness dependence of the pattern was also observed. These results were well explained by assuming the self-arrangement of laser-generated colloidal polymers.     

Photoinduced trion absorption in monolayer WSe2

Photoinduced trion absorption in the time- and spectrum-resolved circular dichroism (CD) of a pristine WSe₂ monolayer is manifested by a pronounced trion transition when the circular polarization of a probe pulse is opposite to that of the preceding pump pulse. Valley-polarized trion absorption enables visualization of the inter-valley scattering dynamics of spin-polarized carriers, revealing a large excitation dependence that supports the valley depolarization through carrier-carrier scattering. Moreover, the CD and the Faraday rotation, which are both dominated by the spin-polarized shift of the A-exciton absorbance and the photoinduced trion transition, are confirmed to satisfy the Kramers-Kronig relationship between each other.     

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.     

Formation of resistive properties of two-phase semiconductor-metal systems based on FeSi1+x with small deviations from stoichiometry

Using the method of finite elements it has been found that in a two-phase system a semiconductor-metal concentrational transition takes place. Specific features of the dependence on concentration and temperature of the effective electrical conductivity in a FeSi-FeSi₂ system are studied in the vicinity of critical concentrations.     

Conductivity of Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> manganites in a magnetic field within the temperature range 78–300 K

The electrical conductivity of the family of Ce _x Sr_1?x MnO₃ (x = 0.50, 0.67) alloys is studied in magnetic fields of up to 0.6 T in the temperature range 78–300 K. The semiconductor-metal phase transition is observed in unannealed samples with x = 0.5 and in both annealed and unannealed samples with x = 0.67. All samples exhibit giant negative magnetoresistance. The temperature dependence of the giant negative magnetoresistance effect, the dependence of the electrical resistivity on the magnetic field at 78 K, and the time dependence of the magnetoresistance at 78 K are measured for the first time. Some samples reveal the properties of spin glass and strong ferromagnets. The reproducibility of the data obtained for these samples depends on the prehistory of the samples, specifically on the conditions of annealing and exposure to a magnetic field.     

Optically pumped laser action in dideuteroacetylene

Optically pumped laser action has been observed in dideuteroacetylene. A total of fifteen output wavelengths in the 17.4 to 20.5 μm region are produced, pumped by seven CO₂ laser lines. The laser shows a number of unusual features, in particular pumping on hot bands and formally forbidden transitions and operation at high pressure (65 torr). Output energies were ～ 2 mJ from an unoptimised system. The temperature and pressure dependence of the output energy has been measured for typical lines. The time dependence of the output pulse varies markedly between the various output lines and suggests that one transition is Raman in origin.     

Pulse evolution and mode selection characteristics in a TEA-CO2 laser perturbed by injection of external radiation

A grating-tunable TEA-CO₂ laser with an unstable resonator cavity, modified to allow injection of cw CO₂ laser radiation at the resonant transition line by means of an intracavity NaCl window, has been used to study the coupling requirements for generation of single frequency pulses. The width and shape of the mode selection region, and the dependence of the gain-switched spike buildup time and the pulse shapes on the intensity and detuning frequency of the injected radiation are reported. Comparisons of the experimental results with previously reported mode selection behavior are discussed.