Time-resolved photoexcitation of the superconducting two-gap state in MgB2 thin films

Femtosecond pump-probe studies show that carrier dynamics in MgB2 films is governed by the sub-ps electron-phonon (e-ph) relaxation present at all temperatures, the few-ps e-ph process well pronounced below 70 K, and the sub-ns superconducting relaxation below T(c). The amplitude of the superconducting component versus temperature follows the superposition of the isotropic dirty gap and the three-dimensional pi gap dependences, closing at two different T(c) values. The time constant of the few-ps relaxation exhibits a double divergence at temperatures corresponding to the T(c)'s of the two gaps.     

Femtosecond carrier relaxation dynamics and photoinduced phase separation in κ-(BEDT-TTF)2Cu[N(CN)2]X (X=Br,Cl)

We investigate the relaxation dynamics of nonequilibrium carriers in organic conductors κ-(BEDT-TTF)(2)Cu[N(CN)(2)]X (X=Br and Cl) using ultrafast time-resolved optical spectroscopy. The dynamics for both salts show similar temperature dependences, which is well characterized by the carrier relaxation across the pseudogap (PG) of the magnitude Δ(PG) ≈ 16 meV for Br salt and 7.0 meV for Cl salt. On the other hand, only the Br salt shows an abrupt increase of the decay time at low temperature, indicating an additional decay component associated with the superconducting (SC) gap below T(c). The fluence dependent dynamics at low temperature evidences the superposition of the SC component onto the PG component. These results indicate a metallic-insulating phase separation in the Br salt triggered by photoexcited nonequilibrium carriers.     

Spin gap and Luttinger liquid description of the NMR relaxation in carbon nanotubes

Recent NMR experiments by Singer et al. [Singer, Phys. Rev. Lett. 95, 236403 (2005).] showed a deviation from Fermi-liquid behavior in carbon nanotubes with an energy gap evident at low temperatures. Here, a comprehensive theory for the magnetic field and temperature dependent NMR 13C spin-lattice relaxation is given in the framework of the Tomonaga-Luttinger liquid. The low temperature properties are governed by a gapped relaxation due to a spin gap ( approximately 30 K), which crosses over smoothly to the Luttinger liquid behavior with increasing temperature.     

Characteristic features of the pseudogap and superconducting states of YBa2Cu3O7−x

The relaxation dynamics of the lattice and low-energy quasiparticles in a YBa₂Cu₃O_7−x superconductor are studied by the light reflection technique with femtosecond temporal resolution in a wide temperature range. It is shown that, for T>T _c, there exist two temperature regions with qualitatively and quantitatively different excitation dynamics, and the transition between these regions is of a hysteretic nature. It is also found that the character of changes observed in the charge carrier relaxation dynamics in the superconducting state testifies to the presence of an anisotropic gap with nodes at the Fermi surface.     

多晶碲化锌薄膜载能子超快动力学实验研究

利用双波长飞秒激光抽运-探测实验方法测量了掺氮多晶ZnTe薄膜在飞秒激光加热情况下载能子超快动力学过程. 采用包含电子弛豫过程和晶格加热过程的理论模型拟合实验数据, 二者符合得很好. 拟合得到10 ps以内影响掺氮多晶ZnTe薄膜表面超快反射率变化的三个弛豫过程的时间常数均为亚皮秒量级. 其中, 正振幅电子弛豫过程是由电子-光子相互作用引起的载流子扩散和带间载流子冷却过程, 负振幅电子弛豫过程是由缺陷造成的光激载能子的俘获效应引起的, 晶格加热过程主要通过电子-声子耦合过程进行的.     

自组织量子点光致荧光的温度依赖性

自组织量子点光致荧光的温度依赖性研究,对于实现室温下高效的量子点光电器件有着非常重要的意义。而量子点中的载流子动力学过程将决定其光致荧光的温度依赖性。采用稳态速率方程模型模拟了自组织量子点中载流子动力学过程,并且考虑了量子点材料带隙随温度的变化;模拟了不同温度下自组织量子点的光致荧光光谱,并获得了光谱的积分强度、峰值能量及半峰全宽随温度的变化曲线。研究表明:模拟结果与已报道的实验数据符合得很好。由此可知,所采用的模型能够很好地反映自组织量子点中的载流子动力学过程。     

基于分子束外延生长的1.05 eV InGaAsP的超快光学特性研究

利用分子束外延方法制备了应用于四结光伏电池的1.05 eV InGaAsP薄膜, 并对其超快光学特性进行了研究. 温度和激发功率有关的发光特性表明: InGaAsP材料以自由激子发光为主. 室温下InGaAsP材料的载流子发光弛豫时间达到10.4 ns, 且随激发功率增大而增大. 发光弛豫时间随温度升高呈现S形变化, 在低于50 K时随温度升高而增大, 在50–150 K之间时减小, 而温度高于150 K时再次增大. 基于载流子弛豫动力学, 分析并解释了温度及非辐射复合中心浓度对样品材料载流子发光弛豫时间S形变化的影响.     

Polarization-dependent carrier dynamics in a passive InAs/InP quantum dot waveguide

We investigate carrier dynamics in a passive InAs/InP quantum dot (QD) waveguide using 255 fs optical pulses at a central wavelength of 1568 nm. We observe strong anisotropy of absorption saturation for different polarizations. Pump-probe measurements indicate the presence of carrier relaxation dynamics on a timescale in the order of tens of picoseconds due to cascaded relaxation of carriers generated by two-photon absorption (TPA) from the bulk region to the QDs via the wetting layer. These relaxation timescales are much longer than in QD amplifiers. Our observations are supported by a rate-equation model which includes TPA, showing good agreement with the pump-probe measurements.     

Investigation of MWS Relaxation in Nylon 1010 using Dielectric Relaxation Spectroscopy

Maxwell–Wagner–Sillars (MWS) relaxation in nylon 1010, arising from charge carriers accumulated at the interphase between amorphous and crystalline regions, has been investigated by means of dielectric relaxation spectra. In the frequency spectra of nylon 1010, dielectric permittivity showed high values at low frequencies originating from charge carrier movement. For the MWS relaxation, the dielectric strength was independent of temperature. The results revealed that there is a transition temperature, located between 110 and 120°C, resulting in the separation of two different charge carrier movement mechanisms. Below and above this transition temperature, the temperature dependence of the MWS relaxation time follows the Vogel–Tammann–Fulcher type, showing that the charge carrier transport is governed by the motion of the polymer chains. The change of charge carrier movement mechanisms is due to the onset of polymer chain motion in the interphase.     

Temperature dependent fluorescence in disordered Frenkel chains: interplay of equilibration and local band-edge level structure

We model the optical dynamics in linear Frenkel exciton systems governed by scattering on static disorder and lattice vibrations and calculate the temperature dependent fluorescence spectrum and lifetime. The fluorescence Stokes shift shows a nonmonotonic behavior with temperature, which derives from the interplay of the local band-edge level structure and thermal equilibration. The model yields excellent fits to experiments performed on linear dye aggregates.     

Electron spin relaxation in pseudo-Jahn-Teller low-symmetry Cu(II) complexes in diaqua(L-aspartate)Zn(II).H(2)O crystals.

Low-temperature (4-55 K) pulsed EPR measurements were performed with the magnetic field directed along the z-axis of the g-factor of the low-symmetry octahedral complex [(63)Cu(L-aspartate)(2)(H2O)2] undergoing dynamic Jahn-Teller effect in diaqua(L-aspartate)Zn(II) hydrate single crystals. Spin-lattice relaxation time T(1) and phase memory time T(M) were determined by the electron spin echo (ESE) method. The relaxation rate 1/T(1) increases strongly over 5 decades in the temperature range 4-55 K. Various processes and mechanisms of T(1)-relaxation are discussed, and it is shown that the relaxation is governed mainly by Raman relaxation processes with the Debye temperature Theta(D)=204 K, with a detectable contribution from disorder in the doped Cu(2+) ions system below 12 K. An analytical approximation of the transport integral I(8) is given in temperature range T=0.025-10Theta(D) and applied for computer fitting procedures. Since the Jahn-Teller distorted configurations differ strongly in energy (delta(12)=240 cm(-1)), there is no influence of the classical vibronic dynamics mechanism on T(1). Dephasing of the ESE (phase relaxation) is governed by instantaneous diffusion and spectral diffusion below 20 K with resulting rigid lattice value 1/T(0)(M)=1.88 MHz. Above this temperature the relaxation rate 1/T(M) increases upon heating due to two mechanisms. The first is the phonon-controlled excitation to the first excited vibronic level of energy Delta=243 cm(-1), with subsequent tunneling to the neighbor potential well. This vibronic-type dynamics also produces a temperature-dependent broadening of lines in the ESEEM spectra. The second mechanism is produced by the spin-lattice relaxation. The increase in T(M) is described in terms of the spin packets forming inhomogeneously broadened EPR lines.     

Investigation of ultrafast dynamics of CdTe quantum dots by femtosecond fluorescence up-conversion spectroscopy

The ultrafast carrier relaxation processes in CdTe quantum dots are investigated by femtosecond fluorescence upconversion spectroscopy.Photo-excited hole relaxing to the edge of the forbidden gap takes a maximal time of ～ 1.6 ps with exciting at 400 nm,depending on the state of the photo-excited hole.The shallow trapped states and deep trap states in the forbidden gap are confirmed for CdTe quantum dots.In addition,Auger relaxation of trapped carriers is observed to occur with a time constant of ～ 5 ps.A schematic model of photodynamics is established based on the results of the spectroscopy studies.Our work demonstrates that femtosecond fluorescence up-conversion spectroscopy is a suitable and effective tool in studying the transportation and conversion dynamics of photon energy in a nanosystem.     

Band-structure-dependent demagnetization in the Heusler alloy Co₂Mn(1-x)FexSi

We investigate the ultrafast demagnetization for two Heusler alloys (Co?Mn(1-x)FexSi) with a different lineup of the minority band gap and the Fermi level. Even though electronic spin-flip transitions are partially blocked by the band gap in one compound, the respective magnetization dynamics, as measured by the time-resolved Kerr effect, are remarkably similar. Based on a dynamical model that includes momentum and spin-dependent carrier scattering, we show that the magnetization dynamics are dominated by hole spin-flip processes, which are not influenced by the gap.     

Dynamics of an Optically Switched Bistable Semiconductor Laser

The switching time of an optically switched laser diode between a locked mode and free oscillation mode is studied. By starting with linear stability analysis, analytical expressions are developed to predict the switching time. It is shown that the switching of optical output between the locked and unlocked modes is governed by two relaxation processes with different time constants, and the change in the first relaxation process, dependent on the carrier density in the initial state, gives a strong effect on the total switching time, in contrast to the dynamic behavior of the carrier density dominated by a single relaxation process. Our analytical results show that larger detuning and power of optical input are effective in shortening the switching time and are believed useful to estimate the switching time of an optically switched bistable laser diode.     

Modification of carrier dynamics in CdSe nanocrystals by excess Cd in deposition bath

Photoexcited carrier dynamics in thin CdSe nanocrystalline films prepared by chemical bath deposition are strongly dependent on the deposition parameters. In this paper, we show how an increase in concentration of CdSO₄ in deposition bath affects the photoexcited carrier dynamics in nanocrystals. We used ultrafast absorption and photoluminescence laser spectroscopy to compare the carrier dynamics in samples prepared with and without increased CdSO₄ concentration. We have found that in the Cd-rich samples the spectral dependences of both photoluminescence and absorption dynamics are considerably less pronounced and the dynamics are slower. The observed effects were explained by the suppression of surface mediated carrier relaxation due to the passivation of individual nanocrystals by cadmium hydroxide and/or by sulphatocomplexes of Cd²⁺.     

Optical spin injection and spin lifetime in Ge heterostructures

We demonstrate optical orientation in Ge/SiGe quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin polarizations and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct gap photoluminescence exceeds the theoretical bulk limit, yielding ～37% and ～85% for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is estimated to be ～0.5 ps and it is governed by transitions between light and heavy hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5?ns below 150?K. Theoretical analysis of the spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime of electrons.     

Effect of two-body collisions on heavy ion dynamics

Two-body collisions are included in TDHF by the time-relaxation method. Calculations are made in a one-dimensional slab-geometry at $\text{E}{}_{\mathrm{c.m}}\text{A}\text{= 20}\text{MeV}$. The relaxation time is assumed to be temperature dependent. Results arc shown for two functions of the temperature as well as for ordinary TDHF (infinite relaxation time). Temperature and the two components of the pressuretensor (perpendicular and parallel to the beam direction) are shown. The approach towards the hydrostatic limit (isototropic pressure) as the relaxation time decreases is shown. Relaxation time is expected to decrease with temperature, i.e. beam energy. The energy at which the pressure becomes isotropic is important for understanding the dynamics of heavy ion collisions.     

Water dynamics as affected by interaction with biomolecules and change of thermodynamic state: a neutron scattering study

The dynamics of water as subtly perturbed by both the interaction with biomolecules and the variation of temperature and pressure has been investigated via neutron scattering spectroscopy. A measurement of inelastic neutron scattering devoted to the study of the coherent THz dynamics of water in a water-rich mixture with DNA (hydration level of 1 g DNA/15 g D(2)O) at room temperature is reported. The DNA hydration water coherent dynamics is characterised by the presence of collective modes, whose dispersion relations are similar to those observed in bulk water. These dispersion relations are well described by the interaction model developed in the case of bulk water, and the existence of a fast sound is experimentally demonstrated. The behaviour of the collective water dynamics was complemented by studying the single-particle dynamics of bulk water along the isotherm T = 298 K in the pressure range 0.1-350 MPa by means of incoherent scattering. This experiment is an attempt to simulate the change of the water molecular arrangement due to the interaction with DNA, by increasing the pressure as the presence of the biomolecule produces an increase in the density. An anomaly is found in the behaviour of the relaxation time derived from the quasi-elastic scattering signal, which can be related to the hypothetical second critical point in water. This anomaly and the transition from slow to fast sound take place in the same Q range, thus suggesting that the two phenomena could be related at some microscopic level.     

Dynamics of money

We present a dynamical many-body theory of money in which the value of money is a time dependent "strategic variable" that is chosen by the individual agents. The value of money in equilibrium is not fixed by the equations, and thus represents a continuous symmetry. The dynamics breaks this continuous symmetry by fixating the value of money at a level which depends on initial conditions. The fluctuations around the equilibrium, for instance in the presence of noise, are governed by the "Goldstone modes" associated with the broken symmetry. The idea is illustrated by a simple network model of monopolistic vendors and buyers.     

Femtosecond multicolor transient absorption spectroscopy of colloidal silver nanoparticles

Time-resolved femtosecond multicolor absorption spectroscopy of silver nanoparticle (NP) colloids with particle diameter in range of 10–30 nm is presented. The amplified femtosecond excitation of the surface plasmon resonance band resulted in transient absorption spectra reflecting the electron-phonon relaxation dynamics, which takes place on the early picosecond time scale. The monitored band with enhanced absorption in the 490–540 nm spectral range exhibited red-shift with increasing pump fluency from 0.4 mJ/cm² to the 1.5 mJ/cm² level. The growth of the relaxation time with increasing pump fluency reveals the temperature dependent relaxation dynamics caused by the nanometer sized electron confinement in the case of silver. This effect was confirmed also by identification of the relaxation time dependence on the particle diameter at constant pump fluency. The complex experimental results revealed nonlinearities both in the laser excitation and electron relaxation processes.