Infrared spectroscopy of the intermediate-valence semiconductor YbB12

The dynamic conductivity and permittivity spectra of the intermediate-valence compound YbB₁₂ are measured in the frequency range (6–10⁴) cm^?1 (quantum energy 0.75 meV-1.24 eV) at temperatures of 5–300 K. Analysis of the spectral singularities associated with the response of free charge carriers has made it possible for the first time to determine the temperature dependences of their microscopic parameters, viz., concentration, effective mass, relaxation frequency and time, mobility, and plasma frequency. It is shown that the relaxation frequency decreases upon cooling from 300 K to the coherence temperature T ^* = 70 K for YbB₁₂, which is mainly associated with the phonon mechanism of scattering of charge carriers. For cooling below the coherence temperature T ^* = 70 K, the temperature dependence of the relaxation frequency for charge carriers of the Fermi-liquid type is found to be γ ～ γ₀ + T ², while their effective mass and relaxation time increase, respectively, to m ^*(20 K) = 34m ₀ (m ₀ is the free electron mass) and τ(20 K) = 4 × 10^?13 s, indicating the establishment of coherent scattering of carriers from localized magnetic moments of the f centers. At a temperature of T = 5 K, the conductivity spectrum contains an absorption line at a frequency of 22 cm^?1 (2.7 meV); the origin of this line can be associated with the exciton-polaron bound state. Since such a state was observed earlier in other intermediate-valence semiconductors (such as SmB₆, TmSe_1?x Te, and (Sm, Y)S), it is probably typical of this class of compounds.     

Relaxation phenomena in TlGa<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript>Se<Subscript>2</Subscript> single crystals

The relaxation electronic phenomena occurring in TlGa_0.99Fe_0.01Se₂ single crystals in an external dc electric field are investigated. It is established that these phenomena are caused by electric charges accumulated in the single crystals. The charge relaxation at different electric field strengths and temperatures, the hysteresis of the current-voltage characteristic, and the electric charge accumulated in the TlGa_0.99Fe_0.01Se₂ single crystals are consistent with the relay-race mechanism of transfer of a charge generated at deep-lying energy levels in the band gap due to the injection of charge carriers from the electric contact into the crystal. The parameters characterizing the electronic phenomena observed in the TlGa_0.99Fe_0.01Se₂ single crystals are determined to be as follows: the effective mobility of charge carriers transferred by deep-lying centers μ_f=5.6×10^?2 cm²/(V s) at 300 K and the activation energy of charge transfer ΔE=0.54 eV, the contact capacitance of the sample C _c =5×10^?8 F, the localization length of charge carriers in the crystal d _c =1.17×10^?6 cm, the electric charge time constant of the contact τ=15 s, the time a charge carrier takes to travel through the sample t _t =1.8×10^?3 s, and the activation energy of traps responsible for charge relaxation ΔE _σ = ΔE _Q = 0.58 eV.     

Variations of ionic binding energies and the distribution of charge carriers in orthorhombic La2−x Sr x CuO4

The significance of heterovalent, substitutional disorder for the distribution of charge carriers in La_2?x Sr _x CuO₄ has been investigated. Disorder is shown to cause strong variations of binding energies of the ions ranging to some eV for Sr contentsx=0.1. Balancing the energy for a hole transport, Cu³⁺+O^2?→Cu²⁺+O^?, and taking binding energy variations into account, the process is realized to become possible without consuming energy for a subset Θ for allx Cu³⁺ in one formula unit of La_2?x Sr _x CuO₄. The functions Θ(x) are presented for hole transports to apex and in-plane oxygens, respectively. The delocalization of charge carriers is interpreted to be caused by valency disorder on metal lattice sites.     

Photoconductivity and photodielectric effect in LiY1 − x Lu x F4 crystals doped with Ce3+ and Yb3+ ions

Time and spectral dependences of the dielectric permittivity of the LiY_{1 ? x} Lu _x F₄ (x = 0, 0.5, and 1) crystals doped with Ce³⁺ and co-doped with Yb³⁺ ions under UV laser excitation were studied by the 8-mm microwave resonant technique at room temperature. The obtained photoconductivity spectrum in 240–310 nm spectral range was interpreted as a stepwise photoionization spectrum of the Ce³⁺ ions due to sequential 4f–5d and 5d–6s transitions. Average lifetimes of free and defect trapped (color centers) charge carriers were estimated.     

Infrared spectroscopy of Bi2Te2Se

The infrared reflection and transmission spectra of Bi₂Te₂Se single crystals grown by the modified Bridgeman method have been studied in the spectral range of 30–10000 cm^?1 at temperatures of 5–300 K. The bandgap and its temperature dependence, optical function spectra, and concentration of free charge carriers in Bi₂Te₂Se have been determined.     

Superposition structure of crystal field spectra in high-T c superconductors: Overdoped regime

Experiments on inelastic scattering of neutrons show that the crystal field spectra for high-T _c super-conductors R_1?y Ca_yBa₂Cu₃O _x≈7 (R=Ho, Er; 0 > y > 0.25) have two spectral components associated with optimally doped and overdoped clusters, respectively. An increase in the calcium concentration does not affect the local density of charge carriers in clusters, but changes the concentration of clusters themselves and, hence, the spectral weights of the spectral components. In light of such a “two-phase” pattern observed earlier for cuprate-based superconductors with a doping level below optimal, an increase in the charge carrier concentration leads to a smooth transition (crossover) from the underdoped regime to the overdoped one. The obtained results show, however, that these two regions of the phase diagram differ qualitatively in the form of charge distribution in CuO₂ planes responsible for superconductivity.     

Jump rate and linear distance of vacancy on the structure and electrical conductivity of Ni0.65Zn0.35CuxFe2−xO4 ferrites

Ferrite compositions of Ni_0.65Zn_0.35Cu_xFe_2−xO₄ (0⩽x<1) were examined using X-ray analysis. The effect of the linear distance of vacancy jumping on the lattice parameter was studied. The jump rate of vacancy increased with increasing Cu concentration. The increase of jump rate of vacancy enhanced the linear distance which increased the conductivity and mobility of the charge carriers. The majority of charge carriers of our systems are holes. The estimated linear distance of each jump was 2.86×10⁻⁷ m. The decrease of thermal conductivity was attributed to the increase of the jump rate and also the linear distance. The formation of oxygen vacancies during the substitution of Cu²⁺ ions for Fe³⁺ ions helped the internal stress to decrease the lattice parameter. Because the ionic radius of O²⁻ (0.136 nm) is larger than that of Fe³⁺ (0.067 nm) ion.     

Screening of excitonic states by low-density 2D charge carriers in GaAs/AlGaAs quantum wells

Screening of excitonic states by a system of 2D electrons (or holes) in GaAs/AlGaAs single quantum wells is studied. With increasing concentration of 2D charge carriers, a threshold-type disappearance of excitonic states is observed in both luminescence and reflectance spectra. The higher the quality of the 2D system, the lower the corresponding threshold concentration. In the best systems, the collapse of excitonic states occurs at unexpectedly low electron densities n _e =5×10⁹ cm^?2, which correspond to the mean dimensionless distance between the particles r _s =8. This value far exceeds the threshold values observed for 3D systems (r _s ≈2), as well as the values obtained for quantum wells in previous studies. The problem of measuring the concentration of low-density 2D charge carriers in photoexcitation conditions is solved by applying the method of optical detection of the dimensional magnetoplasma resonance. This method provides reliable measurements of the density of a 2D system to the values about 10⁹ cm^?2.     

Kinetic effects in manganites La1 − x Ag y MnO3 (y ≤ x)

We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La_{1 ? x} Ag _y MnO₃ (y ≤ x) doped with silver as functions of temperature (4.2–350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression ρ_FM(T) = ρ₀ + AT ² + BT ^4.5 presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons.     

Absorption of Ni-centres in alkali chlorides

Absorption spectra of Ni²⁺ doped NaCl, KCl, and RbCl were measured in the spectral range from 55,000 to 5,000 cm^?1. The bands in the UV region are ascribed to the transition 3t _1u(σ, π)→3e _g(σ) of NiCl₆ complex ion. The connection of the intensity of charge transfer andd-d transitions has been discussed.     

New mechanisms of localization of charge carriers in nanosilicon

This paper reports on the results of investigations into the spectral dependences of thermally stimulated luminescence and the temperature dependences of tunneling luminescence in highly oxidized porous silicon samples. Two new mechanisms of localization of charge carriers are considered in terms of the specific features revealed in the spectral dependences of the thermally stimulated luminescence and nonmonotonic temperature dependences of the Becquerel index of tunneling luminescence decay. The proposed mechanisms of charge carrier localization are associated with structure heterogeneities inherent in these objects, namely, superficial SiO_x oxide shells (0<x≤2) enclosing silicon particles and an undulating structure of silicon wires.     

Magnetic structure and transport properties of atomically disordered crystals of two-dimensional manganites La2−2x Sr1+2x Mn2O7

The magnetic structure and transport properties of partially disordered crystals of two-dimensional manganites La_2?2x Sr_1+2x Mn₂O₇ (x = 0.3, 0.4) are studied over a wide range of temperatures. The crystals are transformed into an atomically disordered state under irradiation with fast neutrons at a dose of 2 × 10¹⁹ cm^?2. The average concentration of substitutional defects in the crystal is ≈4%. It is found that substitutional defects are responsible for the transition of these manganites from the ferromagnetic metal state to the insulator state with a spin glass structure. The results obtained are discussed in terms of the ratio between the kinetic energy of charge carriers and the exchange energy of localized spins.     

Charge carriers and mobility of Cu-Ti ferrite

The concentration and drift mobility of charge carriers in Cu_1–x Ti _x Fe₂O₄ ferrite are calculated, over a wide range of temperatures (300–773 K), employing d.c. conductivity and thermoelectric power data. With increasing temperature the concentration of charge carriers decreases whilst the drift mobility exhibits an exponential increase. Over the above-mentioned temperature range, the obtained density of charge carriers varies between 10²¹ and 10²² cm^–3 whereas the drift mobility has values between 10^–8 and 10^–4 cm²/V s. The results are discussed on the basis of a small-polaron hopping conduction. The activation of the d.c. conductivity has been attributed to the thermal activation of the mobility.     

PhotoleitfÄhigkeit von CaWO4-Einkristallen

Photoconductivity has been detected in CaWO₄-single crystals. By means of a pulse method the mobility of the charge carriers could be determined. The very small mobilities (Μ ⁺=0.1 cm²/Vs,Μ ^?≈ 5 cm²/Vs) are explained in the picture of “small polarons”.     

Untersuchung des „Auroral Afterglow“ und seiner Präparationsstadien

The intensity decay of negative bands of N ₂ ⁺ and of first positive bands of N₂ in the auroral afterglow has been measured. An explanation, seeming possible for the decay curves of the first ones is proposed. By this the intensity decay in the beginning of the afterglow at high ionic densities is determined by the recombination of N ₂ ⁺ with electrons. Later, the ambipolar diffusion of the charge carriers to the wall causes the time dependance. A value of the ambipolar diffusion coefficient in nitrogen, derived from the measurements is given by the relationD _a ·p ≈ 124cm² sec^?1 Torr. The temperature of the charge carriers is estimated to be 313? K. The intensity decay of the pos. group in the afterglow seems mainly to be due to the diffusion of metastable particles to the wall.     

Specific features of the behavior of optical properties upon the metal-insulator transition in EuBaCo<Subscript>2</Subscript>O<Subscript>5+δ</Subscript>

The temperature dependences of the optical properties and the electrical resistivity for EuBaCo₂O_5+δ single crystals are investigated. At temperatures below the metal-insulator transition (T _MI = 340 K), the electrical resistivity is well approximated by the relationship ρ = ρ₀exp(T/T ₀)^1/4. The optical band gap E _g = 0.05 eV for the insulating phase is underestimated as compared to the theoretical value. The specific features in the dispersion of the optical conductivity and the real part of the complex permittivity upon the metal-insulator transition are determined. It is demonstrated that the optical response from charge carriers on the metal side of the metal-insulator transition is caused by the redistribution of the spectral weight of the optical conductivity from the high-energy range to the low-energy range and exhibits a strongly incoherent character. The revealed features are associated with the manifestation of the strongly correlated metallic state.     

Interaction of nitrogen dioxide molecules with the surface of silicon nanocrystals in porous silicon layers

The methods of infrared absorption spectroscopy and electron paramagnetic resonance are used for studying the effect of adsorption of NO₂ molecules, which are strong acceptors of electrons, on the electronic and optical properties of silicon nanocrystals in mesoporous silicon layers. It is found that the concentration of free charge carriers (holes) in silicon nanocrystals, which exhibits a nonmonotonic dependence on the NO₂ pressure, sharply increases in the presence of these molecules. At the same time, a monotonic increase in the concentration of dangling silicon bonds (P_b1 centers) is observed. A microscopic model proposed for explaining this effect presumes the formation of donor-acceptor pairs P ₊ ^b1 -(NO₂)^? on the surface of nanocrystals, which ensure an increase in the hole concentration in nanocrystals, as well as P_b1 centers, which are hole-trapping centers. The proposed model successfully explains a substantial increase in photoconductivity (by two or three orders of magnitude) in the layers of porous silicon in the presence of NO₂ molecules; the increment in the concentration of free charge carriers is detected within an order of magnitude of this quantity. The results can be used in designing electronic and luminescence devices based on silicon nanocrystals.     

Capture cross-section and density of deep gap states in a−SiHx schottky barrier structures

Recombination of charge carriers in a?SiH_x Schottky barriers with density of states near mid-gap ranging from 2.8×10¹⁵?7×10¹⁶cm^-1eV^-1 is attributed to recombination centers with hole capture cross-section of 1.3×10^-15cm².     

Proton conduction in (NH4)3 H(SO4)2 single crystals

D.C. electrical conductivity, DTA and coulometric studies on (NH₄)₃ H(SO₄)₂ single crystals are made. Conductivity is markedly anisotropic with maximum along c¹ direction. A sudden jump in the conductivity plot along c¹ direction at 413 K is supported by a large endothermic peak in DTA, confirming the presence of transition at this temperature. The values of activation energy calculated from conductivity measurements indicated that the charge carriers are protons. This was further confirmed by coulometric experiment where the gas evolved was hydrogen, as established by a gas chromatograph and the volume of H₂ released agreed with that expected from electrolysis. The mechanism of protonic conduction in this crystal is discussed.