A model of hopping and band DC photoconduction in doped crystals

Expressions for the screening length and the ambipolar diffusion length are derived, for the first time, for the case where hopping conduction and band conduction coexist in semiconductors with hydrogen-like impurities. A method is proposed for calculating the diffusion coefficient of electrons (holes) hopping between impurity atoms from data on the Hall effect, in the case where the hopping and band conductivities are equal. An interpretation is given of available experimental data on hopping photoconduction between acceptors (Ga) and donors (As) in p-Ge at T=4.2 K doped by a transmutation method. It is shown that the relative magnitude of the mobilities of electrons hopping between donors and holes hopping between acceptors can be found from the hopping photoconductivity measured as a function of the intensity of band-to-band optical carrier excitation.     

Geometry and thermodynamics of smeared Reissner–Nordstrm black holes in d-dimensional AdS spacetime

We construct a family of d-dimensional Reissner–Nordstr o¨m-Ad S black holes inspired by noncommutative geometry. The density distribution of the gravitational source is determined by the dimension of space, the minimum length of spacetime l, and other parameters(e.g., n relating to the central matter density). The curvature of the center and some thermodynamic properties of these black holes are investigated. We find that the center of the source is nonsingular for n 0(under certain conditions it is also nonsingular for-2 n 0), and the properties at the event horizon, including the Hawking temperature, entropy, and heat capacity, are regular for n -2. Due to the presence of l, there is an exponentially small correction to the usual entropy.     

Stationary hopping photoconduction among multiply charged impurity atoms in crystals

A derivation of the equation for the hopping current of electrons and holes (electron vacancies) among impurities of a single species in three charge states [(−1), (0), and (+1)] is given in the continuum approximation. The screening length of an electrostatic field and the diffusion length of charge carriers are calculated. The dependence of the effective lifetime of electrons hopping among impurities relative to (−1)→(+1) transitions [and of holes relative to (+1)→(−1) transitions] on the degree of compensation and the rate of interimpurity photoexcitation, which stimulates the formation of ions, is obtained. The calculations of the dependence of the hopping photoconductivity on photoexcitation rate are consistent with known experimental data, which have not previously found a theoretical explanation. Fiz. Tverd. Tela (St. Petersburg) 40, 1805–1809 (October 1998)     

Dynamic instability of thin viscoelastic films under lateral stress

We study the influence of lateral stress on the stability of thin viscoelastic films. The free surface of a deposited film under stress is shown to undergo an instability initiated by an anisotropic diffusion of the polymer molecules. This instability ultimately results in the formation of holes provided that the initial surface fluctuations are larger than a critical value. The latter is found to decrease when increasing the stress. An increase of the holes number density with the stress is therefore predicted. Most interestingly, the holes number density is also predicted to increase when increasing the molecular weight of the polymers. Additionally, we demonstrate that the friction of the substrate suppresses any spatial coherence between holes on large length scales. These predictions explain recent experimental observations made on thin spin-coated polystyrene films [G. Reiter, Nat. Mater. 4, 754 (2005)10.1038/nmat1484].     

Local density theory of X-ray and photoemission from YBa2Cu3O7−δ: The high Tc superconductor

Photoemission, inverse photoemission and X-ray emission spectra have been calculated using results of a highly precise local density energy band study. Compared to experiment, we find a shift of ∼2 eV to higher binding energy for the Cu-d and the O-pπ bonding states. This shift is attributed to the lack of metallic screening for the Cu-d holes which leaves only charge transfer screening by O ligand electrons. The larger shift of the O-pπ states close to E_F compared to those lying lower in energy is attributed to different degrees of localization. By contrast, the lower lying O-pσ bonding states, which show greater itinerant character than the higher lying pπ states, are less affected by relaxation effects.     

Response of colloidal liquids containing magnetic holes of different volume densities to magnetic field characterized by transmission measurement

This paper systematically investigates the response of colloidal liquids containing magnetic holes of different volume densities to magnetic field by conventional transmission measurements. It finds that the enhancement in the transmission of such a colloidal liquid under a magnetic field exhibits a strong dependence on the volume density of magnetic holes. A linear increase in the maximum enhancement factor is observed when the volume density of magnetic holes is below a critical level at which a maximum enhancement factor of ～150 is achieved in the near infrared region. Once the volume density of magnetic holes exceeds the critical level, a sharp drop of the maximum enhancement factor to ～2 is observed. After that, the maximum enhancement factor increases gradually till a large volume density of ～9%. By monitoring the arrangement of magnetic holes under a magnetic field, it reveals that the colloidal liquids can be classified into three different phases, i.e., the gas-like, liquid-like and solid-like phases, depending on the volume density of magnetic holes. The response behaviour of colloidal liquids to magnetic field is determined by the interaction between magnetic holes which is governed mainly by their volume density. A phase transition, which is manifested in the dramatic reduction in the maximum enhancement factor, is clearly observed between the liquid-like and solid-like phases. The optical switching operations for colloidal liquids in different phases are compared and the underlying physical mechanisms are discussed.     

Photoluminescence of germanium near the screening ionization limit of excitons

Photoluminescence spectra from highly excited germanium at 25–30 K are presented. The density of free electrons and holes is determined from induced free carrier absorption and achieved values up to 10¹⁷ cm^-3. At such densities the electron-hole system is pumped to well above the screening ionization limit of excitons. The band-to-band luminescence. however, is strongly Coulomb enhanced. The spectral shape of the measured luminescence fits well with calculations based on a Yukawa interaction potential.     

Improved photonic crystal directional coupler with short length

We investigate a photonic crystal (PC) waveguide coupler which is formed by two closely spaced linear waveguides in a two-dimensional triangular lattice of air holes. Our study shows that shifting one row of the air holes between the waveguides affects the dispersion curves of the guided modes and if the triangular lattice of air holes between the waveguides is replaced by a rectangular lattice, this modification results in an ultra-short coupling structure with coupling length less than 3a, where a is the lattice constant. Also, we investigate the effect of changing the radii of air holes that are adjacent to or between the waveguides on the coupling length and show that increasing the radius of air holes between the waveguides decreases the coupling length. We analyze the output spectrum of an ultra-short channel drop filter designed based on this structure.     

Laser drilling of high aspect ratio holes in copper with femtosecond,picosecond and nanosecond pulses

Deep laser holes were drilled in copper sheets using various pulse lengths and environments. By recording the intensity on a photodiode placed under the sample while drilling the holes, we obtained the number of pulses to drill through the sheet as a function of pulse length and energy. The entrance diameter of the holes was successfully predicted using a Gaussian approximation and a material removal fluence threshold of 0.39 J/cm² for a pulse length of 150 fs. From cross sections of the holes, the morphology of the inside walls was observed and shows an increase in the amount of molten material with pulse length. A transition pulse length is defined as the point at which the laser affected material goes from being mainly vaporized to mainly melted. This transition occurs near ∼10 ps, which corresponds approximately to the electron–phonon relaxation time for copper. PACS 62.20.Mk; 62.25.+g; 79.20.Ds     

Photorefractive effect in the relaxor ferroelectric material 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3

The photorefractive effect in a nominally undoped 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 single crystal is measured. We report what is, to our knowledge, the first observation of the photorefractive effect in Pb-based relaxor ferroelectric crystals. The crystal is grown by the flux solution method. Then it is cut into a 2 mm x 4 mm x 8 mm piece and electrically poled along the [111] direction. The coupling constant of the two-wave mixing is 17 cm(-1), and the normalized time constant under 1-W/cm(2) illumination is 12 s at a wave-length of 476 nm. The effective trap density is calculated as 5 x 10(16) cm(-3) from the Debye screening length under the assumption of Kukhtarev's band-transport model. The dominant carrier is identified to be holes from the direction of two-wave mixing energy transfer.     

Holographic screening length in a hot plasma of two sphere

We study the screening length \(L_{\mathrm{max}}\) of a moving quark–antiquark pair in a hot plasma, which lives in a two sphere, \(S^2\), using the AdS/CFT correspondence in which the corresponding background metric is the four-dimensional Schwarzschild–AdS black hole. The geodesic of both ends of the string at the boundary, interpreted as the quark–antiquark pair, is given by a stationary motion in the equatorial plane by which the separation length L of both ends of the string is parallel to the angular velocity \(\omega \). The screening length and total energy H of the quark–antiquark pair are computed numerically and show that the plots are bounded from below by some functions related to the momentum transfer \(P_c\) of the drag force configuration. We compare the result by computing the screening length in the reference frame of the moving quark–antiquark pair, in which the background metrics are “Boost-AdS” and Kerr–AdS black holes. Comparing both black holes, we argue that the mass parameters \(M_{\mathrm{Sch}}\) of the Schwarzschild–AdS black hole and \(M_{\mathrm{Kerr}}\) of the Kerr–AdS black hole are related at high temperature by \(M_{\mathrm{Kerr}}=M_{\mathrm{Sch}}(1-a^2l^2)^{3/2}\), where a is the angular momentum parameter and l is the AdS curvature.     

Thermodynamic density of states of two-dimensional GaAs systems near the apparent metal-insulator transition

We perform combined resistivity and compressibility studies of two-dimensional hole and electron systems which show the apparent metal-insulator transition--a crossover in the sign of deltaR/deltaT with changing density. No thermodynamic anomalies have been detected in the crossover region. Instead, despite a tenfold difference in r(s), the compressibility of both electrons and holes is well described by the theory of nonlinear screening of the random potential. We show that the resistivity exhibits a scaling behavior near the percolation threshold found from analysis of the compressibility. Notably, the percolation transition occurs at a much lower density than the crossover.     

一种新型光子晶体波导定向耦合型超微偏振光分束器

将两个二维空气孔光子晶体波导平行放置,两波导之间由三排空气孔相隔,构成一个定向耦合器.数值分析了TE(磁场平行于空气孔)和TM(电场平行于空气孔)偏振态光波在该定向耦合器中的传播行为.结果表明,减小耦合区两波导间的一排介质柱的半径,TE模的耦合长度减小,而TM模的耦合长度不变.基于此结构,设计了超微偏振光分束器,整个器件的尺寸为10.1μm,与已报道的24.2μm的结果相比,该器件具有更小的器件尺寸和更高的输出效率.     

Entropy of Four-Dimensional Spherically Symmetric Black Holes with Planck Length

Considering corrections to all orders in Planck length on the quantum state density from a generalized uncertainty principle (GUP), we calculate the statistical entropy of the Bose field and Fermi field on the background of the four-dimensional spherically symmetric black holes without any cutoff. It is obtained that the statistical entropy is directly proportional to the area of horizon.     

Effect of an electric field on the carrier collection efficiency of InAs quantum dots

Individual and multiquantum dots of InAs are studied by means of microphotoluminescence in the case where, in addition to the principal laser exciting photoluminescence, second infrared laser is used. It is demonstrated that the absorption of the infrared photons effectively creates free holes in the sample, which leads to both a change in the charge state of a quantum dot and to a considerable reduction of their photoluminescence signal. The latter effect is explained in terms of effective screening of the internal electric field, facilitating carrier transport along the plane of a wetting layer, by the surplus holes from the infrared laser. It is shown that the effect of quenching of quantum dot photoluminescence gradually disappears at increased sample temperature (T) and/or dot density. This fact is due to the essentially increased value of quantum dot collection efficiency, which could be achieved at elevated sample temperatures for individual quantum dots or even at low T for the case of multiquantum dots. It is suggested that the observed phenomena can be widely used in practice to effectively manipulate the collection efficiency and the charge state of quantum-dot-based optical devices.     

The role of macroinstrument and microinstrument and of observable quantities in the new conception of thermodynamics

In the first part of the paper, we introduce the concept of observable quantities associated with a macroinstrument measuring the density and temperature and with a microinstrument determining the radius of a molecule and its free path length, and also the relationship between these observable quantities. The concept of the number of degrees of freedom, which relates the observable quantities listed above, is generalized to the case of low temperatures. An analogy between the creation and annihilation operators for pairs (dimers) and the creation and annihilation operators for particles (molecules) is carried out. A generalization of the concept of a Bose condensate is introduced for classical molecules as an analog of an ideal liquid (without attraction). The negative pressure in the liquid is treated as holes (of exciton type) in the density of the Bose condensate. The phase transition gas-liquid is calculated for an ideal gas (without attraction). A comparison with experimental data is carried out. In the other part of the paper, we introduce the concept of new observable quantity, namely, of a pair (a dimer), as a result of attraction between nearest neighbors. We treat in a new way the concepts of Boyle temperature T _B (as the temperature above which the dimers disappear) and of the critical temperature T _c (below which the trimers and clusters are formed). The equation for the Zeno line is interpreted as the relation describing the dependence of the temperature on the density at which the dimers disappear. We calculate the maximal density of the liquid and also the maximal density of the holes. The law of corresponding states is derived as a result of an observation by a macrodevice which cannot distinguish between molecules of distinct gases, and a comparison of theoretical and experimental data is carried out.     

THE SCREENING BEHAVIOR IN DIFFUSION-LIMITED AGGREGATION

The measure of the screening behavior of the diffusion-Limited aggregation in 100 clusteis by computer simulations shows that the screening length is not inversely proportional to the number density of particles [97] or to the square root of the number density of particles[10] quantitatively.     

Density correlations and analog dynamical Casimir emission of Bogoliubov phonons in modulated atomic Bose-Einstein condensates

We present a theory of the density correlations that appear in an atomic Bose-Einstein condensate as a consequence of the emission of correlated pairs of Bogoliubov phonons by a time-dependent atom-atom scattering length. This effect can be considered as a condensed matter analog of the dynamical Casimir effect of quantum field theory. Different regimes as a function of the temporal shape of the modulation are identified and a simple physical picture of the phenomenon is discussed. Analytical expressions for the density correlation function are provided for the most significant limiting cases. This theory is able to explain some unexpected features recently observed in numerical studies of analog Hawking radiation from acoustic black holes.