V. G. Kytin

Effect of humidity on the ac conductivity of nanoporous TiO2

Impedance spectroscopy measurements are performed on a sintered network of TiO2 nanoparticles at different humidity. A transition from nearly normal to dispersive transport is observed at a certain frequency depending on the humidity. The low-frequency conductivity depends on the humidity by power law with an exponent of 1.73, while the low-frequency dielectric constant remains constant. The results can be explained by hopping proton conduction over the percolation threshold. We propose that chemisorbed water molecules mainly supply the protons and the hopping sites are provided by physisorbed water molecules.

Relaxation of Photogenerated Carriers in P3HT:PCBM Organic Blends

The after-pulse time-resolved microwave conductivity (TRMC) decays observed in P3HT:PCBM blends display a dependence on time close to t(-beta), independent of excitation intensity, in the 10 ns-1 micros range. This is explained in terms of the relaxation of carriers in a Gaussian density of states (DOS). The model is based on a demarcation level that moves with time by thermal release and retrapping of initially trapped carriers. The model shows that when the disorder is large the after-pulse decay of the type t(-beta) is obtained, while at low disorder and large temperature the carrier distribution becomes independent of time. In the measurements different beta values were observed depending on the solvent used for spin-coating: 0.4-0.6 for chlorobenzene and 0.3-0.4 for toluene. The model was applied to extract the shape of the DOS from the TRMC decays, giving a dispersion parameter of about 120 meV for blends with high P3HT content.

Injection currents and effect of negative capacitance in porous TiO2

Transient dark and photocurrents and the frequency-dependent capacitance were investigated in porous (por-) TiO2 (anatase) layers. The current transients consist of a fast part being characteristic for a transit of charge and of a slow part which increases strongly in time for high applied potentials up to a saturation level. The drift mobility of injected electrons is 5×10−5 cm2/V s. The saturation current is described by space-charge-limited current (SCLC) in the presence of traps with an exponential energy distribution (exponential parameter, 80 meV). At high injection currents, SCLC causes slow polarization of the medium, which leads to a time-dependent capacitance and strong inductive behavior of the dark current transients.

Transport and optical properties of tin-delta-doped GaAs structures

The transport and optical properties of tin δ layers in GaAs are investigated as functions of the Sn concentration. The Shubnikov-de Haas and Hall effects are measured in the temperature range 0.4–12 K in magnetic fields up to 38 T. The band diagrams and quantum mobilities of electrons in the quantum-well subbands are calculated. Features associated with electronic transitions from quantum-well levels are found in the photoluminescence spectra of the structures. Oscillations of the resistance are observed in a magnetic field parallel to the δ layer and are attributed to features in the density of states at the Fermi level.

Superhard superconducting materials based on diamond and cubic boron nitride

Superhard superconducting samples with a critical temperature of Tc = 12.6 K are obtained when synthetic diamond powders that were preliminarily coated with a niobium film are sintered at a pressure of 7.7 GPa and a temperature of 1973 K. Superhard superconductors with Tc = 9.3 K are obtained when diamond and molybdenum powders are sintered at a pressure of 7.7 GPa and a temperature of 2173 K. Superconducting samples with Tc = 36.1–37.5 K have been obtained in the systems diamond-MgB2 and cubic boron nitride-MgB2.

Galvanomagnetic and Thermoelectric Properties of BiTeBr and BiTeI Single Crystals and their Electronic Structure

BiTeI and BiTeBr single crystals are synthesized by the Bridgman method, and their galvanomagnetic and thermoelectrical properties are investigated. Both semiconductors have n-type conductivity. The thermoelectric efficiency of BiTeBr is much higher than that of BiTeI, which is related mainly to a larger See-beck coefficient for the former compound. For both crystals, the band structure is calculated from the density-functional theory. It is shown that both compounds are semiconductors with an indirect energy band gap.

Anomalous increase of the thermopower and thermoelectric figure of merit in Ga-doped p-(Bi0.5Sb0.5)2Te3 single crystals

The effect of Ga doping on the temperature dependences (5 K ≤ T ≤ 300 K) of the Seebeck coefficient α, electrical conductivity σ, thermal conductivity coefficient κ, and thermoelectric figure of merit Z of p-(Bi0.5Sb0.5)2Te3 single crystals has been investigated. It has been shown that, upon Ga doping, the hole concentration decreases, the Seebeck coefficient increases, the electrical conductivity decreases, and the thermoelectric figure of merit increases. The observed variations in the Seebeck coefficient cannot be completely explained by the decrease in the hole concentration and indicate a noticeable variation in the density of states due to the Ga doping.

High power polymer dye laser with improved stability

A polymer dye laser was developed suitable for high-power and high-repetition-rate pump lasers. A maximum output power of the dye laser of 1.5 W was achieved. The output power of the polymer gain medium is demonstrated to be absolutely stable for 4 h within 3% accuracy in a cyclic operation mode with a duty cycle of 83%. The enhanced stability of the gain medium stems from a partial recovery of photodegradation due to the dissociation of intermediate chemically active products which are detected with electron spin resonance and are shown to be produced much faster at elevated temperature.

The conductivity and magnetic properties of zinc oxide thin films doped with cobalt

The magnetic properties, temperature dependences of the resistivity, Hall constant, and magnetoresistance of epitaxial zinc oxide films doped with cobalt are studied. The ferromagnetism of the films is observed at room temperature. The conduction and magnetoresistance of the films are attributed to transport of electrons in the conduction band at high temperatures and to hopping transport at low temperatures. With increasing concentration of cobalt dopants, the resistivity of the films increases and the concentration of electrons decreases. This is due to the increase in the ionization energy of donor states because of the increase in the energy of exchange interaction between electrons at donor states and electrons of the d shell of cobaltions.

Electronic properties of single-crystal diamonds heavily doped with boron

Single-crystal diamonds with characteristic sizes of 2–7 mm doped with boron in the concentration range 1019–1020 cm−3 have been grown by the temperature gradient method at high static pressures. The temperature dependence of the resistance R of the synthesized single crystals has been measured in the range 0.5 K < T < 297 K. An activated dependence R(T) with an activation energy of about 50 meV is observed in the range from room temperature to T ≈ 200 K. At temperatures below approximately 50 K, the temperature dependence of the conductivity for heavily doped crystals is proportional to T1/2, which is characteristic of degenerate semiconductors with a high number of defects.