Ultrafast relaxation of excited Dirac fermions in epitaxial graphene using optical differential transmission spectroscopy

We investigate the ultrafast relaxation dynamics of hot Dirac fermionic quasiparticles in multilayer epitaxial graphene using ultrafast optical differential transmission spectroscopy. We observe differential transmission spectra which are well described by interband transitions with no electron-hole interaction. Following the initial thermalization and emission of high-energy phonons, the electron cooling is determined by electron-acoustic phonon scattering, found to occur on the time scale of 1 ps for highly doped layers, and 4-11 ps in undoped layers. The spectra also provide strong evidence for the multilayer structure and doping profile of thermally grown epitaxial graphene on SiC.     

Electron and phonon effects in superconducting tunnel detectors of x-radiation

Nb/Al/AlO_x/Nb superconducting tunnel junctions were investigated in the role of x-ray detectors. Amplitude spectra of pulses arising upon irradiation of tunnel junctions of different sizes by ⁵⁵Mn x-radiation were recorded at a temperature T=1.4 K. We also analyzed the temporal shape of the pulses. We considered the influence of diffuse motion of nonequilibrium quasiparticles, the inverse tunneling effect, and exchange of 2Δ phonons between electrodes, on the characteristics of the tunnel detectors. It is shown that phonon processes can bring about changes in the amplitude, duration, and polarity of the signal. Fiz. Tverd. Tela (St. Petersburg) 41, 1168–1175 (July 1999)     

Properties of optical and acoustic phonons in diamond crystals

The dependence of the energy on the quasi-momentum of optical and acoustic phonons in diamond-type crystals is analyzed. It is shown that the dispersion relation of optical branches near the Brillouin zone center in such crystals can be derived based on the modified Klein-Gordon equation corresponding to quasiparticles (optical phonons) with negative effective mass. Analytical expressions are derived for dispersion curves of acoustic and optical branches in the entire Brillouin zone, based on the sinusoidal approximation. The obtained analytical dependences of dispersion relations are compared to the experimental results of the study of inelastic neutron scattering spectra in diamond crystals. The law of the coordinate and time transformation for optical phonons with negative effective rest mass when going to the new inertial reference frame, similar to Lorentz transformations in relativistic mechanics, is determined.     

Inelastic tunneling in Al-AlN-Pb junctions

The technique of the prepagation of the tunnel junctions with a barrier made of AlN has been presented. To produce a barrier layer a freshly evaporated Al film was exposed to a glow discharge in atmosphere of pure ammonia. The features shown in the tunnel spectra of these junctions have been interpreted as AlN optic phonons and a vibration spectrum of NH₃ molecules adsorbed in the junctions.     

Growth and optical properties of semiconductor nanocrystals in a glass matrix

Over the last 15 years nanocrystals embedded in a glass matrix have been a subject for studies of fundamental phenomena of quasiparticles (electrons, holes, excitons, phonons) quantum confinement in the nanosize semiconductor materials. Growth of the nanocrystals in a glass matrix is based on the thermodynamic process of the diffusion-controlled phase decomposition of oversaturated solid solutions. Three stages of the process in solutions prepared by co-melting, co-sputtering and ion-implantation techniques are discussed. It is shown that the growth technique makes it possible to vary the mean size of the particles, their size distribution and crystalline structure.

The optical properties of nanocrystals of various semiconductor compounds grown in different glass matrices are discussed. Attention is given to studies of a fine structure of optical spectra at resonant size-selective spectroscopy for both “strong” and “weak” confinement regimes. Energy spectra of confined acoustic and optical phonons in a “strong” confinement regime, studied by resonant Raman scattering, are discussed.     

Self-consistent calculations within the quasiparticle time blocking approximation with exact account taken of the single-particle continuum

A self-consistent version of the model based on the method of Green’s functions which takes into account conventional phonons of the random phase approximation, complex configurations like 2 quasiparticles?phonon ones, and exact single-particle continuum is used to describe many discrete natural-parity states and giant resonances in the ¹²³Sn and ²⁰⁸Pb nuclei. The quasiparticle-phonon interaction is shown to be important not only for low-lying and high-lying collective states but also for low-lying noncollective states.     

Rotational quantum friction in superfluids: Radiation from object rotating in superfluid vacuum

The friction experienced by a body rotating in a superfluid liquid at T=0 is discussed. The effect is analogous to the amplification of electromagnetic radiation and spontaneous emission by a body or black hole rotating in the quantum vacuum, first discussed by Zel’dovich and Starobinsky. The friction is caused by the interaction of the part of the liquid which is rigidly connected with the rotating body and thus represents a comoving detector, with the “Minkowski” superfluid vacuum outside the body. The emission process is the quantum tunneling of quasiparticles from the detector to the ergoregion, where the energy of quasiparticles is negative in the rotating frame. This quantum rotational friction caused by the emission of quasiparticles is estimated for phonons and rotons in superfluid ⁴He and for Bogoliubov fermions in superfluid ³He. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 257–262 (25 February 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Tunneling characteristics of trilayer tunnel junctions

We report here the tunneling characteristics (d²V/dI²) of trilayer junctions of the form AlOxAgCuPb and AlOxCuAgPb. The Cu and Ag thicknesses both being 300 Å and the Pb thickness being 3000 Å. The two contributions to the tunneling density of states seen are: (i) the interference effect in the two normal layers showing structures associated with the Pb phonons [1]. This structure is very similar to the one of AlOxAgPb (or AlOxCuPb where the Ag (or Cu) thickness is 600 Å thus showing that the interference effect takes place in both normal metals, (ii) the induced pair potential in the normal layers produces a structure associated with the normal metal phonons. However, only the normal phonons of the metal next to the oxide are seen thus showing that the electron tunneling is a very local probe.     

Topological surface states in lead-based ternary telluride Pb(Bi(1-x)Sb(x))2Te4

We have performed angle-resolved photoemission spectroscopy on Pb(Bi(1-x)Sb(x))2Te4, which is a member of lead-based ternary tellurides and has been theoretically proposed as a candidate for a new class of three-dimensional topological insulators. In PbBi2Te4, we found a topological surface state with a hexagonally deformed Dirac-cone band dispersion, indicating that this material is a strong topological insulator with a single topological surface state at the Brillouin-zone center. Partial replacement of Bi with Sb causes a marked change in the Dirac carrier concentration, leading to the sign change of Dirac carriers from n type to p type. The Pb(Bi(1-x)Sb(x))2Te4 system with tunable Dirac carriers thus provides a new platform for investigating exotic topological phenomena.     

Absorption and electroabsorption near the indirect edge of GaSe

Absorption and electroabsorption spectra have been measured near the indirect edge of the layer semiconductor GaSe. The experimental structures observed at 1.5 K are ascribed to the indirect excitonic transition with emission of different phonons. The determination of the threshold energy of the no-phonon indirect absorption process affords the energies of the phonons involved in the indirect transition to be derived.     

Brillouin-Mandelstam scattering from thermal and excited magnons

Brillouin-Mandelstam scattering (BMS) is the scattering of light from acoustical quasiparticles (phonons, magnons and others). The frequency shift under BMS is 10–100 GHz. The observation of BMS from magnons became possible only after J. Sandercock had designed a multi-pass Fabry-Perot interferometer with a high contrast (1971). BMS from magnons has, by now, been observed in CrBr₃ (Sandercock), YIG, FeBO₃ (Jantz, Sandercock, Wettling), CoCO₃ (Borovik-Romanov, Jotikov, Kreines), EuO, EuS (Grünberg, Metawe), Ni, Fe (Sandercock, Wettling), metglasses (Chang, Malozemoff, Grimsditch, Senn, Winterling). In this review the main results of the above works are presented.The dispersion laws of magnons were studied by BMS in the energy range (inaccessible for neutron diffraction) where the contributions due to three types of interaction: magnetic, dipole-dipole, and exchange can be separated.Investigation of BMS in EuO and metals has led to the discovery of surface magnons. BMS from standing spin waves has been observed in thin films of metglasses.By observing BMS, it is possible to study quasiparticles pumped by microwave power. It was found that under ferro- or antiferromagnetic resonance an excess of quasiparticles arises, these quasiparticles being magnons with the frequency equal to that of microwave power and phonons with half the microwave frequency. Scattering of light from parametrically excited magnons has also been observed. This opens new possibilities for studying relaxation processes in magnetic materials.     

CdTe/CdZnTe多量子阱激子复合动力学性质的研究

报道了分子束外延制备的高质量CdTe/Cd_0.64Zn_0.36Te多量子阱结构的光学性质,由变温光致发光光谱讨论了随温度升高辐射线展宽和辐射复合效率降低的机理.在变密度激发的皮秒时间分辨光谱中,发现不同激发密度下发光衰减时间不同,并研究了它的机理.在高激发密度下观测到n=2的重空穴激子发光. 关键词：     

Investigation of the low temperature behaviour of the Anderson lattice

A recently developed new perturbational approach to the nonlocal correlations in the Anderson lattice model is used to study low temperature properties. These include the one particle excitation spectra and possible instabilities towards magnetic or superconducting states. The method rests upon a systematic and in principle exact incorporation of the large interaction energy, the Coulomb repulsionU between local electrons on the same lattice site, into the low energy dynamics of quasiparticles and phonons. The resulting dressing of quasiparticles and their resulting effective interactions are treated in the frame of established approximations: The former in NCA, self-consistently adapted to the lattice (LNCA), and the interacting low energy problem with help of ladder summations for the two-particle scattering. Numerical results for the one particle properties are presented, which show an interesting collective effect in the formation of a Kondo lattice state. Stoner-like expressions for various susceptibilitites are derived. Criteria for the occurrence of magnetic and different superconducting phases are given which clearly exhibit the role of quasiparticle band structure, electron-electron interaction and electronphonon interaction.This work was performed within the research program of SFB 252 Elektronisch hochkorrelierte metallische Materialien     

Nonequilibrium gap instability under tunnel injection of quasiparticles utilizing superconducting junctions with negative tunnel resistance

Nonequilibrium superconductivity under tunnel injection of quasiparticles in the low-voltage injection regime is investigated utilizing Sn double-tunnel junctions, particularly when the injector tunnel resistance is so low that the negative resistance behavior may be observed in the I–V characteristic. The experiments are performed under the condition that the sample is immersed in superfluid helium in order to avoid simple heating effect. No sharp transition to the inhomogeneous gap state is observed in the injector characteristic. A new interesting feature in the detector characteristics is observed for the transition to the inhomogeneous state, which may be compared with the previous works utilizing Al double-tunnel junctions.     

IR Emission of Single-Crystal Silicon Excited by Broadband Light

Room-temperature mid-IR emission from single-crystal silicon exposed to broadband (320 to 700 nm) light with power up to 150 m W has been revealed by Fourier transform IR emission spectroscopy. It is shown that broadband optical excitation significantly enhances IR emission in comparison with thermal emission. Spectral lines of lattice vibrations caused by interaction between combinations of longitudinal and transverse optical or acoustic phonons are identified in experimental IR emission spectra.     

Spectral diffusion of quasi localized excitons in single silicon nanocrystals

Evolution in time of photoluminescence spectra of SiO_x capped single silicon nanocrystals has been investigated by means of confocal optical spectroscopy at room temperature. Large spectral jumps between subsequent spectra of up to 40 meV have been detected leading to noticeable line broadening and variation in the electron–phonon coupling. Further, a correlation between emission energy and emission intensity has been found and discussed in terms of an intrinsic Stark effect. Anti-correlated variations of the electron–phonon coupling to Si and SiO₂ phonons as a function of photoluminescence energy indicate that the nearly localized excition is to some extent coupled to phonons in the shell covering the silicon nanocrystal. However, coupling is reduced upon increasing Stark effect, while at the same time coupling to phonons of the Si core increases.     

Resonance exciton-phonon spectra in open ZnCdSe/ZnSe nanowires: Raman scattering and hot luminescence,extended and localized exciton states

Low-temperature (T ～ 8 K) secondary emission spectra of open Zn_0.87Cd_0.13Se/ZnSe nanowires were studied comprehensively in the region of the ground excitonic state under tunable laser excitation. The spectra revealed a fine structure produced in the interaction of excitons with optical phonons. The observed resonance exciton-phonon (REP) lines are shown to be formed through two different mechanisms. The strongest component is due to Raman scattering via free excitonic states. The other REP lines are generated in the hot luminescence of localized excitons. The spectrum of the optical phonons involved in the formation of the REP lines in the biaxially strained Zn_0.87Cd_0.13Se/ZnSe structures was analyzed.     

Phonon focussing patterns: Calculation of response of finite area detectors to pulsed ballistic beams of dispersive and dispersionless phonons

The phonon images of crystals are described in the frame of the Boltzmann kinetic equation. Monochromatic heat pulses of the dispersive and dispersionless acoustic phonons are considered. Exact expressions for the energy and quasimomentum carried by a pulsed beam of monochromatic dispersionless acoustic phonons falling onto a detector of the finite area are derived. These formulae provide us with a convenient starting point for numerical calculations of phonon images. For the example of long wave-length acoustic phonons and a point as well as extended sources, an algorithm for numerical calculations of phonon images of anisotropic crystalline media is presented. However, it is quite general and can be easily adapted for dispersive phonons and to quasiparticles with an arbitrary dispersion low.     

Nonequilibrium Josephson effect in systems of tunnel superconducting junctions and in layered superconductors

This paper examines the time-dependent Josephson effect in systems of tunnel superconducting junctions and in layered superconductors (the intrinsic Josephson effect) with allowance for nonequilibrium superconductivity effects. Kinetic and quasihydrodynamic equations are derived that describe self-consistently the dynamics of Josephson phases and nonequilibrium quasiparticles. It is found that the state of nonequilibrium between the layers leads to an effective mechanism of the interaction between interlayer Josephson junctions, which can be used to synchronize the junctions. Current-voltage characteristics of chains of intrinsic junctions are obtained for different values of the parameters. Zh. éksp. Teor. Fiz. 116, 1798–1816 (November 1999)