Optical and electrical properties of thermally evaporated In49Se48Sn3 films

Nearly stoichiometric thin films of In₄₉Se₄₈Sn₃ were deposited at room temperature, by conventional thermal evaporation of the presynthesized materials, onto precleaned glass substrates. The microstructural studies on the as-deposited and annealed films, using transmission electron microscopy and diffraction (TEMD), revealed that the as-deposited films are amorphous in nature, while those annealed at 498 K are crystalline. The optical properties of the investigated films were determined from the transmittance and reflectance data, in the spectral range 650-2500 nm. An analysis of the optical absorption spectra revealed a non-direct energy gap characterizing the amorphous films, while both allowed and forbidden direct energy gaps characterized the crystalline films. The electrical resistance of the deposited films was carried out during heating and cooling cycles in the temperature range 300-600 K. The results show an irreproducible behavior, while after crystallization the results become reproducible. The analysis of the temperature dependence of the resistance (ln(R) vs. 1000/T) for crystalline films shows two straight lines corresponding to both extrinsic and intrinsic conduction. The room temperature I-V characteristics of the as-deposited films sandwiched between similar Ag metal electrodes shows an ohmic behavior, while non-ohmic behavior attributed to space charge limited conduction has been observed when the films are sandwiched between dissimilar Ag/Al metal electrodes.     

Superconductivity in a new NdBa2Ca3Sr4Cu5Ox system

A new compound, NdBa₂Ca₃Sr₄Cu₅O_x, has been synthesised using the conventional solid state reaction technique. The material was characterised by powder XRD, electrical resistivity, ac susceptibility and dc magnetisation studies. The results of powder X-ray diffraction show that the structure is pseudo orthorhombic, having unit cell dimensions a = 5.47 Å, b = 5.46 Å and c = 14.58 Å. Magnetisation studies on a SQUID magnetometer showed a superconducting transition at 52 K. This was confirmed by the measurements of electrical resistance and ac susceptibility of this sample, which also showed superconductivity at 52 K. Details of preparation and characterisation are discussed.     

Optical properties of new organic conductors based on the BEDT-TSeF molecule (the κ-(BETS)4Hg2.84Br8 superconductor and κ-(BETS)4Hg3Cl8 Metal) in the range 300–15 K

Polarized reflectance and optical conductivity spectra of single crystals of two new isostructural organic conductors based on the BEDT-TSeF molecule, namely, the κ-(BETS)₄Hg_2.84Br₈ superconductor (T _c =2 K) and the κ-(BETS)₄Hg₃Cl₈ metal, which undergoes a smooth transition to the dielectric state near 35 K, have been obtained in the spectral region 700–6500 cm⁻¹ at temperatures of 300–15 K. At 300 K, the spectra of both compounds are nearly identical and differ from the Drude spectrum characteristic of metals. The nature of the observed difference is discussed, and the spectra are described in terms of a cluster approach with inclusion of electron-electron correlations in the Hubbard approximation combined with the Drude model. The parameters of the theory were determined, including the electron transfer integrals between molecules in a cluster. The spectra in the conducting plane of the crystals were found to be essentially anisotropic, which should be assigned to specific features of in-plane interaction between molecules. The spectra of the superconductor and the metal become increasingly different as the temperature is lowered. The spectra of the metal obtained for T<150 K exhibit splitting of the broad electronic maximum in the mid-IR region into two bands, which is accompanied by a splitting of a vibronic feature deriving from electron interaction with intramolecular BETS vibrations of ν₃(A _g ) symmetry. No such splitting is observed in the superconductor spectra with decreasing temperature. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 11, 2004, pp. 1921–1929. Original Russian Text Copyright ? 2004 by Vlasova, Drichko, Petrov, Semkin, Zhilyaeva, Lyubovskaya, Olejniczak, A. Kobayashi, H. Kobayashi.     

InAs/GaAs表面量子点的压电调制反射光谱研究

运用压电调制反射光谱(PzR)方法测量了在以GaAs(311)B为衬底的In_0.35Ga_0.65As模板上生长的InAs表面量子点结构的反射谱.在77K温度下，观察到了来自样品各个组成结构(包括表面量子点本身、被覆盖层覆盖的量子点、In_0.35Ga_0.65As模板以及GaAs衬底等)的调制信号.来自表面量子点本身的调制信号是多个清晰的调制峰.用一阶和三阶微分洛伦兹线形对PzR谱中对应结构的实验数据进行了拟合，精确确定了与样品的各个组成结构相对应的调制峰的能量位置.对不同样品PzR谱的差异进行了定性的说明. 关键词： 压电调制光谱 InAs/GaAs 表面量子点 洛伦兹线形拟合     

Magnetic behavior of amorphous Fe−Ni−Zr alloys and their response to radiation damage

The magnetic properties of two amorphous Fe?Ni?Zr alloys, Fe_89.7Ni_0.03Zr₁₀ and Fe₇₀Ni₂₀Zr₁₀, both in the “as cast” and neutron irradiated states were investigated by Mössbauer spectroscopy and dc magnetic susceptibility measurements. The upper magnetic ordering temperatures of Fe_89.7Ni_0.03Zr₁₀ are 232K and 246K for the “as cast” and irradiated samples, respectively. The magnetic ordering temperature for Fe₇₀Ni₂₀Zr₁₀ was about 478K for both the “as cast” and irradiated samples. Both compositions yield magnetic hyperfine spectra, which show a considerable relaxation effect that must be explicitly considered in the calculation of the average local Fe moments. When this is done, these values derived from Mössbauer spectra are in good agreement with the dc susceptibility values. The effects of neutron irradiation on the magnetic properties of these alloys are small.     

The photoemission study of NdNiO3/NdGaO3 thin films, through the metal-insulator transition

The electronic structure of thin films NdNiO₃/NdGaO₃ with various thicknesses (from 17 nm to 150 nm), have been studied by photoemission spectroscopy at 300 K and 169 K. The XPS results are consistent with the literature ab initio calculations of the NdNiO₃ electronic structure. A noticeable variation attributed to the metal-insulator (MI) transition has been found only for the films with relatively high thickness (150 nm). Furthermore, the photoemission spectra and their temperature dependence have been discussed with regard to the results of dc electrical resistivity measurements which also exhibit large thickness dependence. Finally, these new results support a possible large hetero-epitaxial effect on the thinnest sample (17 nm) which could stress the NdNiO₃ structure and consequently makes its electronic structure nearly stabilized.     

Far-infrared probe of energy gap in LaO1−xFxFeGe compound

We report on the electrical resistivity and far-infrared reflectance measurements of LaO_1?xF_xFeGe samples. Furthermore, we introduce a new method to probe the energy gap and determine its value. The onset transition temperature was 22.8 K for x = 0.13, and a clear anomaly was observed at 90 K in the ρ(T) curve for x = 0.11 with T_c = 20.6 K. We clearly observed the phonon-suppressed feature in reflectance spectra where F-doping caused a strong suppression of a peak at 200 cm^?1. The energy gap above T_c, 2Δ = 2.10 meV, was determined from the measured spectra based on the changes in reflectivity by F-doping.     

Optical investigations of the metal-insulator transition in a low-dimensional organic conductor (EDT-TTF)4[Hg3I8]

The polarized reflectance spectra of single crystals of the low-dimensional organic conductor (EDT-TTF)₄[Hg₃I₈] undergoing a metal-insulator phase transition at a temperature T < 35 K have been presented. The spectral region of the study is 700–6000 cm^?1 (0.087–0.74 eV), and the temperature range is 300–9 K. It has been shown that the reflectance spectra are determined by a system of quasi-free electrons of the upper half-occupied molecular π-orbitals, which form a half-filled metallic band in the crystals. A high anisotropy of the spectra and their temperature dependences have been found. For two polarizations, the quantitative analysis of the spectra at 100 and 25 K has been performed in the framework of the phenomenological Drude model, the effective mass and the width of the initial metallic π-electron band have been deter-mined, and it has been found that the conducting system in the crystals has a quasi-one-dimensional character. As temperature decreases, the spectra demonstrate substantial changes indicating the formation of the energy gap (or pseudogap) in the spectrum of electronic states in the range of ～1500–2500 cm^?1. In the low-frequency region (700–1600 cm^?1), a vibrational structure has been observed, and the most intense feature of the structure (ω = 1340 cm^?1) is caused by the interaction of electrons with intramolecular vibrations of the C=C bonds of the EDT-TTF molecule. For temperatures of 15 and 9 K, the analysis of the spectra has been performed in the framework of the theoretical “phase phonon” model taking into account the interaction of electrons with the intramolecular vibrations. It has been concluded that the metal-insulator transition observed in the reflectance spectra of the crystals is similar to the Peierls dielectric transition that occurs in a system of electrons coupled with the intramolecular vibrations of the molecules forming the crystal.     

Optical determination of the charge transfer in AsF5-graphite intercalation compounds

Optical reflectance experiments are performed on well-characterized compounds C_8nAsF₅ (n = 1, 2), in the temperature range 10–300 K. The theoretical analysis of the reflectivity spectra within the 2-D model of graphite independent subsystems [1] provides an optical determination of E_F and the charge transfer coefficient which is found independent of temperature.     

Refractive indices of SrTiO3 in the infrared region

We measured transmittance and reflectance spectra of SrTiO₃ crystals in the infrared region, at 200 and 100 K. These data and our previous 300 and 20 K data are analyzed in order to derive frequency and temperature dependence of the refractive indices n and k. We thus provide detailed n(v) and k(v) data at 300, 200, 100, and 20 K, in the frequency range 25–16000 cm⁻¹.     

Ferromagnetic resonance of magnetic field oriented Fe3O4 nanoparticles in frozen ferrofluids

Measurements of the orientational dependence of the ferromagnetic resonance (FMR) spectra are made on Fe₃O₄ nanoparticles in ferrofluids solidified in dc magnetic fields. The in field solidification locks the direction of magnetization parallel to the direction of the cooling field enabling measurements as a function of orientation with respect to the direction of magnetization in the frozen state. The g value of the FMR spectra at 77 K is 2.16 and the anisotropy constant is −1.23 J/m³. A marked reduction of the difference between the field position in the parallel and perpendicular orientation onsets on warming to 140 K well below the melting temperature of the fluid carrier and is attributed to the onset of fluctuations in the direction of the magnetization in the solid phase. The phase transition of the magnetic symmetry observed in bulk Fe₃O₄ occurs at much lower temperature in the nanoparticles.     

Hall breakdown in a modulation-doped GaAs/AlAs heterostructure

The effect of dc current I _dc on the electron transport in a GaAs quantum well with AlAs/GaAs superlattice barriers is studied experimentally at a temperature of 4.2 K in magnetic fields up to 2 T. A sharp increase in resistance R _xx is observed in magnetic fields higher than the critical field B _c. The value of B _c is found to decrease with increasing the current I _dc. The phenomenon observed in the experiment is qualitatively explained by the electric breakdown of superlattice barriers under the action of the Hall field.     

A study of the physical properties of FeCl3 filled PVA

Polyvinyl alcohol (PVA) films filled with various mass fractions of FeCl₃ were prepared using a casting method. The structural, electrical and magnetic properties were clarified. The filling level (FL) dependence of certain IR absorption peaks was correlated with the obtained physical parameter characterizing the other properties. The XRD scans revealed a semicrystalline feature of the virgin polymer and an existence of two halos. The FL dependence of the intensity of the two halos was studied. The dc electrical resistivity was measured in the temperature range of 300–400 K. An intrachain one-dimensional interpolaron hopping mechanism was assumed to interpret the electrical conduction. The temperature dependence of the dc magnetic susceptibility exhibited a Curie–Weiss behavior in the range of 90–270 K. The ESR studies of PVA filled with various mass fractions of FeCl₃ revealed very complicated spectra due to hyperfine and fine structure. The evolution of the ESR spectra with the FL suggested the distribution of Fe³⁺ ions in isolated and then aggregated modes within the PVA matrix.     

Low resistance state up to 210 K in a mixed compound Y-Ba-Cu-O

We have observed a low resistance state up to 210 K in a mixed system of the nominal composition Y_1.2Ba_0.8CuO_4-δ prepared under simultaneous action of a pulsed dc electric field up to 30 kV/cm and uniaxiakl pressure of ≈1.5 GPa. An onset of a decrease of the resistance at 281 K is observed in repeated cycles and is accompanied by diamagnetism in the temperature interval 150 K<T<280 K. We understand it as possible superconductivity related to the particular composition.     

天然FeS2(黄铁矿)的红外光谱研究

主要报道天然FeS₂黄铁矿,(以下称FeS₂(P))的室温、低温的远红外反射光谱和透射光谱,并对其光谱结构及物理起源作了较详细的讨论。通过我们的研究发现:室温反射光谱低频端等离子体反射边和样品中所含的PbS(方铅矿)杂技存在密切的联系。我们以FeS₂(P)矿物单晶和PbS矿物单晶的反射光谱作为基谱对不同样品的实验结果进行了复合,所得出的x值不仅可以反映PbS杂质的含量,而且间接地与贵金属元素Au等存在较好的正相关性。这对于研究矿物的成因等有 关键词：     

Ferromagnetic resonance observation of a phase transition in magnetic field-aligned Fe2O3 nanoparticles

Fe₂O₃ hematite (alpha) nanoparticles suspended in the liquid phase of the liquid crystal 4,4-azoxyanlsole (PAA) are cooled below the freezing temperature (397 K) in a 4000 G dc magnetic field. The in field solidification locks the direction of maximum magnetization of the particles parallel to the direction of the applied dc magnetic field removing the effects of dynamical fluctuations of the nanoparticles on the magnetic properties allowing a study of the intrinsic magnetic properties of the nanoparticles as well as the anisotropic behavior of the ferromagnetic resonance (FMR) signal. Freezing in PAA allows temperature-dependent measurements to be made at much higher temperature than previous measurements. The field position, line width and intensity of the FMR signal as a function of temperature as well as the magnetization show anomalies in the vicinity of 200 K indicative of a magnetic transition, likely the previously observed Morin transition shifted to lower temperature due to the small particle size. Weak ferromagnetism is observed below T_c in contrast to the bulk material where it is antiferromagnetic below T_c. The Raman spectrum above and below 200 K shows no evidence of a change in lattice symmetry associated with the magnetic transition.     

Infrared modulated reflectance spectra of n and p type gallium antimonide and n type indium antimonide

We have observed the modulated reflectance spectra of n and p type GaSb at 300, 80, and 5 K from 0.56 to 2 eV. The modulated reflectance of intrinsic n type InSb was measured at 80 K from 0.2 to 2 eV. The “dry sandwich” vapor deposition technique was used to make the electroreflectance (ER) samples. The low-temperature spectrum of the undoped p type GaSb sample shows three peaks at the band edge that could be associated with transitions from the top of the valence band, the light (0.903 eV) and heavy (1.014eV) hole state Fermi levels to the conduction band. The energies of the observed peaks are in agreement with the Fermi level determination from Hall effect and Faraday rotation measurements. This modulation mechanism is based on band population effects. The ER signal of InSb under flatband condition at 80 K has five half oscillations at the direct band gap. The contribution of piezoelectric strain to ER is present since the dc bias required to achieve flatband condition is different at the band gap than at E₁. The ER signal corresponding to the direct gap energy E₀ and to the spin-orbit energy E₀ + Δ₀ was determined in the n and p type samples of GaSb at different temperatures. We have measured the intrinsic energy gap in GaSb at room temperature. E_g = 0.74 eV. The corresponding spin-orbit splitting was found to be Δ₀ = 0.733 ± 0.002 eV.     

PREPARATION OF a-AXIS ORIENTED YBa2Cu3Ox THIN FILMS

The a-axis oriented YBa₂Cu₃O_x(YBCO) thin films could be grown on (100) SrTiO₃(STO) substrates with STO buffer layers by dc and rf magnetron sputtering either by low-ering the deposition temperature, or by using a self-template technique. For the latter, the resistivity of the thin film at 290K along the substrate [001] direction is about four times larger than that in the [010] direction. The zero resistance temperatures T_c0 are 89 K in both directions. So high quality a-axis oriented YBCO thin films can be prepared by the self-template technique. Also the T_c0 increase monotonously with the reduction of the thickness of the YBCO seed layer.     

Reflectance and absorption spectra near the band gap in CuInSe2

The reflectance and absorption spectra of CuInSe₂ near the band gap were obtained at 7 K. From the analysis of the data, an estimate of the exciton binding energy, Bohr radius, and low frequency dielectric constant has been made, which are 15 meV, 5.5 nm, and 8.5 respectively.     

Temperature dependence of the reflectance spectrum of (TMTSF)2ClO4

The near-infrared and visible reflectance spectrum was measured on (TMTSF)₂ClO₄ single crystal at various temperatures in the range of 30K–290K. The reflectance spectrum polarized parallel to the stacking axis was analyzed by Drude model, giving the results that the band width 4t is about 1 eV and m¹_p/m is about 1.3. The band parameters obtained here were almost the same as those of (TMTSF)₂PF₆ reported by Jacobsen et al.