Thermal expansion of thin C60 films

The thermal expansion coefficient a and structure of C₆₀ films with thickness t∼3–10 nm were investigated in the temperature interval from room to liquid-nitrogen temperature by electron-optical methods. The thermal expansion coefficient was determined from the temperature shift of the diffraction maxima in the electron diffraction patterns. The objects of investigation were epitaxial C₆₀ films condensed in vacuum on a (100) NaCl cleavage surface and oriented in the (111) plane. A surface-induced size effect in the thermal expansion coefficient was observed. It was established that as t decreases α _f increases and is described well by the relation α _f=17·10⁻⁶ K⁻¹+8.3·10⁻⁵ nm K⁻¹ t ⁻¹. This relation was used to estimate the linear expansion coefficient α _s of the C₆₀ surface in the (111) plane as α _s=60·10⁻⁶K⁻¹, which is several times larger than the bulk value. The experimental results agree satisfactorily with the theoretical calculations of the mean-square displacements of molecules located in a region near the surface. Zh. éksp. Teor. Fiz. 114, 1868–1875 (November 1998)     

Absorption spectra and magneto-optic figures of merit in the Bi x Sm3-x Fe5-y Ga y O12 system

The absorption (α) and Faraday rotation (θ) spectra of 14 garnets belonging to the series Bi _x Sm_3-x Fe_5-y Ga _y O₁₂ (0<x<1.05, 0.8<y<1.15) have been measured between 15 000 cm⁻¹ and 19 000 cm⁻¹. The figure of merit (θ/α) at 17 850 cm⁻¹ (560 nm) increases linearly with increasing bismuth concentration up tox∼0.6 where it begins to increase less rapidly. For operation of magneto-optic display devices at 17 850 cm⁻¹ there is no advantage in using garnets in this series withx>0.8. The Faraday rotation at 17 850 cm⁻¹ increases linearly with bismuth concentration whereas the absorption coefficient increases more rapidly. The presence of Bi³⁺ increases the intensity of all Fe³⁺ pair transitions in the garnet system as a result of the increased superexchange induced by Bi³⁺. This is in keeping with the observation that the intensity of the⁶A_1g (S)→⁴T_1g (G) transition in (RE)₃Fe₅O₁₂ (RE=Er, Y, Dy, Gd, Eu) increases on traversing the above RE series as do the Curie temperatures of these iron garnets.     

Incorporation of Sn into epitaxial GaAs grown from the liquid phase

The incorporation of Sn into LPE GaAs was studied as a function of the atomic fractionx _Sn ^l of Sn in the liquid (1.6×10⁻⁴≤x _Sn ^l ≤0.54), the growth temperatureT _K and the cooling rate α. The diffusion coefficient of As in Ga for moderate Sn-doping was deduced from the growth velocities to beD _As (760° C)=(3.3±1.0)×10⁻⁵ cm²/s. The epitaxial layers were analyzed after van der Pauw with special emphasis on the sources of experimental error. With the aid of current mobility theories the concentrations of the ionized donors and acceptors were derived. From their dependence onx _Sn ^l , on α and onT _K combined with the Schottky-barrier model of Sn incorporation it can be concluded that the melt and the growing crystal surface were in thermal equilibrium. The diffusion coefficient of Sn in GaAs is about 8×10⁻¹⁴ cm²/s at 760° C. The distribution coefficient for Sn increases from 4.4×10⁻⁵ to 12.3×10⁻⁵ in the temperature range from 690 to 800° C. The total Sn incorporationx _Sn ^s was measured using the atomic absorption spectroscopy for the first time down tox _Sn ^s =10¹⁷/cm³. From these data it can be concluded that up tox _Sn ^l =0.54 the dopant Sn is incorporated as donor and as acceptor only and that within the experimental scatter there is no indication of incorporation as a neutral species.     

An apparatus for the measurement of thermal expansion of solids at low temperatures

A dilatometer, using the three terminal capacitance technique, suitable for measurement of linear thermal expansion of solids in the temperature range 1.3–300 K is described. The dialtometer is designed such that the mounting system for the specimen does not undergo any significant changes in dimensions when the specimen is heated. The apparatus, therefore, yields in principle absolute values of α, the coefficient of linear thermal expansion. The performance of the apparatus has been checked by measurements on copper in the temperature range of 77–300 K. Some preliminary results on the behaviour of α for Y₁Ba₂Cu₃O_6.9 compound in the vicinity of superconducting transition temperature,T _c are also described. The system can detect relative changes in length Δl/l ₀ of about 10⁻⁸. Attempts are being made to improve the sensitivity.     

Galvanomagnetic properties of the Al1−x Six nonequilibrium substitutional solid solutions

Galvanomagnetic characteristics of the A_1−x Si_x solid solutions (x<12 at. %) have been studied within a broad range of temperatures (1.8–290 K) and magnetic fields (up to 15 T). An anomaly in the concentration dependence of the Hall coefficient R _H (x,T=290 K) has been revealed near the boundary of absolute instability (x<8.5 at. %) of compounds in the Al_1−x Si_x series. The variation of the Hall coefficient and of the magnetoresistance in the A_1−x Si_x series at low (T<77 K) temperatures is analyzed within models taking into account the anisotropy in conduction-band electron scattering. Fiz. Tverd. Tela (St. Petersburg) 41, 3–10 (January 1999)     

Abnormalities of the thermal expansion of Bi2Sr2-xLaxCuO6 + δ high-temperature superconductor in the dielectric phase (x ≥ 0.8) and charge ordering in an oxygen sublattice

Abnormal negative thermal expansion of single crystals of high-temperature Bi₂Sr_{2 − x} La _x CuO_{6 + ·} superconductor in the dielectric phase (x ≥ 0.8) was detected at low temperatures. It was established that material shrinkage upon heating occurs at higher temperatures than in superconducting compositions. The anisotropic and nonmonotonous effect of a magnetic field of 3–6 T on the coefficient of thermal expansion was revealed. We attribute these abnormalities to the existence of superstructure charge ordering in the oxygen sublattice.     

μ+SR studies on superconducting YBa2(Cu1−xFex)3Oy system

Spin-glass like magnetic ordering of iron moments was observed in both orthorhombic and tetragonal YBa₂(Cu_1−xFe_x)₃O_y (x=0.08) by μ⁺SR measurements. In a “Tetra” sample, all the muons sense the superconducting transition at 60 K and magnetic ordering at around 15 K, while in an “Ortho” sample they reveal that two magnetically different parts exist in the sample: about 40% of the sample is superconducting withT _c ≈90K and the remaining part is magnetic withT _M≈33K. These phenomena can be explained in terms of clustering of the Fe atoms in the “Ortho” sample.     

Superconductivity in the BaPb1−x Bi x O3 system

Superconductivity properties of the BaPb_1−x Bi _x O₃ system have been investigated by precise measurements of electrical resistivity and Hall coefficient in specimens prepared by the hot press technique. Sharp superconducting transitions have been observed in those specimens, indicating the homogeneities of the specimens thus prepared. superconductivity has been observed for 0.05<x≦0.35 and a maximum critical temperature of 11.7 K has been observed atx=0.25. The carrier concentration deduced from the Hall coefficient has been found to show nearly the samex dependence as does the critical temperature. The upper critical field has been determined from the magnetic field dependence of the lowcurrent-density resistivity. Discussions are given about the superconductivity in the BaPb_1−x Bi _x O₃ system on the basis of the experiment results.     

X-ray,electrical conductivity,magnetic and infrared studies of the system Co2−x Ge1−x Fe2x O4

X-ray, electrical conductivity, magnetic hysteresis and IR studies for the system Co_2−x Ge_1−x Fe_2x O₄ were carried out. All the compounds, 0⩽x⩽1, showed cubic symmetry. X-ray intensity calculations, magnetic hysteresis measurements and IR studies indicated the presence of Ge⁴⁺ at tetrahedral, Co²⁺ at octahedral and Fe³⁺ at both the sites. The activation energy and threshold frequency decreased with increasing value ofx. The compounds withx⩽0.5 arep-type and those withx⩾0.75 aren-type semiconductors. Magnetic hysteresis indicated that all the compounds are ferrimagnetic except forx=0 which is antiferromagnetic. The shapes of χ/χ _i vsT plots, highH _c values andJ _R/J_s ratios showed that all the compounds exceptx=0 exhibit single-domain behaviour. Curie temperature,T _c increased with increasing Fe³⁺ ions. The probable ionic configuration for the system is suggested as Ge _1−x ⁴⁺ Fe _x ³⁺ [co _2−x ²⁺ Fe _x ³⁺ ]O ₄ ²⁻ .     

Kramers-Kronig analysis of the reflectance spectra of Pb-doped Bi-4334 type glass and the corresponding glass-ceramic superconductor

The room temperature reflectance spectra in UV-VIS-NIR region (energy range of 0.6 to 6.2 eV) for glassy, partially crystalline and its fully crystalline superconducting ceramic phases of Bi_3.9Pb_0.1Sr₃Ca₃Cu₄O _x have been studied by Kramers-Kronig (KK) analysis. A comparative study of the energy loss function [− Im (1/ε)] and the absorption coefficient [α(E)] has been done. Excitions in the superconducting phase hitherto evidenced by the authors are located in the polarizable layers of the superconducting cuprate and their implications for superconductivity have been pointed out. An estimate of the optical band gap energy (E _g) has also been made from the linear fit ofα ² vs.E curve for the superconducting phase. Jezierski’s method ofR-extrapolations in the higher energy has been used to show that both methods yield results that agree quantitatively and can be relied upon.     

Measurement of the spindependent relative conversion coefficients in α-Fe and α-Fe2O3

The relative differences δ _ns (n=1, 2, 3) of the spindependent conversion coefficients were measured for α-Fe and α=Fe₂O₃. In contrast to theoretical predictions of δ_1s≃−10⁻⁵ we found δ_1s≃−1.0(4)x10⁻² for both α-Fe and α-Fe₂O₃. As a possible source for this difference we consider a dynamic coupling with the atomic spin during the conversion process.     

Anomalous effect of magnetic field on the low-temperature thermal expansion of the Bi2Sr2?xLaxCuO6+δ and Nd2?xCexCuO4?δ HTSC systems in the non-superconducting phase

The thermal expansion of single crystals of the Bi₂Sr_2−x La _x CuO_6+δ high-temperature superconducting (HTSC) system in the insulating phase with compositions having no superconducting transition to a temperature of 1.8 K (x ≥ 0.8) is measured in an arbitrary direction in the (ab) plane in the temperature range of 7–50 K. Temperature regions of material compression upon heating are found. The study of anomalies in magnetic fields of 3 and 6 T, parallel and perpendicular to the c-axis revealed an anisotropic and nonmonotonic effect of the field on thermal expansion. Such anomalies for the n-type Nd_2−x Ce _x CuO_4−δ HTSC sample also having no superconducting transition are detected for the first time. The results show that the anomaly nature is caused by anisotropic electronic ordering, probably, by the charge density wave in the CuO₂ plane and superconductivity fluctuations in the insulating phase.     

Universal behavior of CePd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Rh<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> ferromagnet at the quantum critical point

The heavy-fermion metal CePd_1−x Rh _x can be tuned from ferromagnetism at x = 0 to the nonmagnetic state at some critical concentration x _c . The non-Fermi liquid behavior (NFL) at x ≃ x _c is recognized by the power-law dependence of the specific heat C(T) given by the electronic contribution susceptibility X(T) and volume expansion coefficient α(T) at low temperatures: C/T ∝ X(T) ∝ α(T)/T∝ 1/ √T. We also demonstrate that the behavior of the normalized effective mass M _N ^* observed in CePd_1−x Rh _x at x ≃ 0.8 agrees with that of M _N ^* observed in paramagnetic CeRu₂Si₂ and conclude that these alloys exhibit the universal NFL thermodynamic behavior at their quantum critical points. We show that the NFL behavior of CePd_1−x Rh _x can be accounted for within the frameworks of the quasiparticle picture and fermion condensation quantum phase transition, while this alloy exhibits a universal thermodynamic NFL behavior that is independent of the characteristic features of the given alloy such as its lattice structure, magnetic ground state, dimension, etc. The text was submitted by the authors in English.     

Reduction in the rate of phonon scattering by a spatially correlated system of iron ions and low-temperature “anomaly” of the thermoelectric phenomena in HgSe:Fe crystals

A new effect of the reduction in the rate of phonon scattering by the spatially correlated system of iron ions in HgSe:Fe crystals is detected experimentally and calculated theoretically. The thermoelectric power is measured using HgSe:Fe samples with different iron content in the temperature range 7.5–60 K. It is found that the dependence of the thermoelectric power on iron content exhibits remarkable features at T<10 K: the quantity |α(N _Fe )| increases as the iron concentration increases to N _Fe =5×10¹⁸ cm⁻³, reaches a maximum at N _Fe ≈(1–2)×10¹⁹ cm⁻³, but then monotonically decreases with further increases in N _Fe . It is shown that the obseved increase in the thermoelectric power is due to a reduction in the rate of phonon scattering by the spatially correlated system of Fe³⁺ ions. This new effect is analyzed theoretically, and the theoretical results are compared with the experimental data. Zh. éksp. Teor. Fiz. 114, 191–207 (July 1998)     

Thermal expansion of orthorhombic and tetragonal phases of the highT c superconductors RE1Ba2Cu3O y (RE=Y,Gd, Dy)

Measurements of the coefficient of linear thermal expansion α have been carried out for the orthorhombic and tetragonal phases of RE₁Ba₂Cu₃O _y (RE=Y, Gd, Dy) compounds using a high resolution capacitance dilatometer in the temperature range 77–300 K. All the superconducting samples exhibit a jump Δα at their respective transition temperatures,T _c. Evidences of, sample-to-sample variation in α values and dependence of Δα on the sample preparation conditions, have been obtained. The non-superconducting samples, in general, exhibit lower values of α possibly because of lowering of oxygen content.     

Magnetic and electrical properties of Mg1−x CuxO solid solutions and magnetic shielding in Cu-Mg1−x CuxO structures

Mg_1−x Cu_xO solid solutions having an NaCl structure with 0⩽x⩽0.20 are synthesized and Cu-Mg_1−x Cu_xO structures are prepared for superconductivity studies. The magnetic susceptibility χ, electron paramagnetic resonance (EPR), and electrical conductivity of the solid solutions are studied at temperatures of 5–550 K. It is shown that χ ⁻¹(T) obeys the Curie-Weiss law with a paramagnetic Curie temperature Θ close to zero and an effective magnetic moment μ _eff=1.9 μ _B, close to the 1.73 μ _B of a Cu²⁺ ion with spin S=1/2. The width ΔH of the EPR line depends weakly on temperature and increases as x is raised. The volume narrowing of the EPR linewidth ΔH is used to estimate the exchange interaction parameter, 3×10⁻⁴ eV. The g-factor is close to 2 and is temperature independent. The electrical conductivity of Mg_1−x Cu_xO at T=300 K is ≈10⁻¹¹–10⁻¹² Ω⁻¹ cm⁻¹ for x=0 and increases to 10⁻⁵–10⁻⁶ Ω⁻¹ cm⁻¹ for x=0.15–0.20. The conductivity is p-type. Magnetic shielding is observed in Cu-Mg_1−x Cu_xO structures with x=0.15 and 0.20. The possible connection of this phenomenon with interference superconductivity in the contact layer of the structure is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 293–296 (February 1999)     

2p3/2?13x?1?3x?13d?1 X-ray satellites spectra in the Lα1 region of 4d transition elements

The X-ray satellite spectra arising due to 2p _3/2 ⁻¹3x ⁻¹−3x ⁻¹3d ⁻¹ (x ≡ s, p, d) transition array, in elements with Z = 40 to 48, have been calculated, using available Hartree-Fock-Slater (HFS) data on 1s ⁻¹−2p ⁻¹ 3x ⁻ and 2p _3/2 ⁻¹−3x ⁻¹,3x ⁻¹ Auger transition energies. The relative intensities of all the possible transitions have been estimated by considering cross — sections for the Auger transitions simultaneous to a hole creation and then distributing statistically the total cross sections for initial two hole states 2p _3/2 ⁻¹−3x ⁻¹ amongst various allowed transitions from these initial states to 3x ⁻¹ 3d ⁻¹ final states by Coster-Kronig (CK) and shake off processes. In both these processes initial single hole creation is the prime phenomenon. Each transition has been assumed to give rise to a Gaussian line and the overall spectrum has been computed as the sum of these Gaussian curves. The calculated spectra have been compared with the measured satellite energies in Lα₁ spectra. Their intense peaks have been identified as the observed satellite lines. The peaks in the theoretical satellite spectra were identified as the experimentally reported satellites α₃, α₄ and α₅, which lie on the high-energy side of the Lα₁ dipole line.     

Microstructure and magnetic properties in arrays of ac electrodeposited FexNi1?x nanowires induced by the continuous and pulse electrodeposition

FeNi nanowires were fabricated by ac and pulse electrodeposition into the alumina template matrix. The effects of continuous ac electrodeposition as well as pulse features on the structure and magnetic properties of the nanowire arrays were studied. The microstructures and magnetic properties of the Fe _x Ni_1−x nanowires are seen to be independent of the deposition frequency and off-time between the pulses. The ac electrodeposited Ni nanowires were not formed at more than 400  Hz deposition frequency, while the Fe _x Ni_1−x nanowires, containing a small amount of Fe, formed in the all frequencies. For x less than 50% the coercivity slowly increases but over 50% Fe added to the FeNi alloy increases the coercivity with a higher rate and maximum coercivity was seen for the Fe_0.97Ni_0.03. The Fe and Fe _x Ni_1−x nanowires containing less than 30 at.% Ni was seen to have a bcc structures with (110) preferential direction while Fe _x Ni_1−x nanowires with more than 30 at.% Ni showed (110) bcc (Fe) and/or (111) bcc (FeNi) plus (111) fcc (Ni). A preferential (111) fcc structure was obtained for the Ni nanowires.     

Anomalous millimeter-wave absorption in the superconducting phase of La2−x SrxCuO4

Direct measurements of the complex conductivity spectra of thin-film La_2−2x Sr_xCuO₄ are made at frequencies of 5–40 cm⁻¹. Narrow, intense Drude-type excitation is observed in the superconducting phase. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 406–409 (10 September 1998)     

The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa₂(Cu_1−xNi_x)₃O_7−y withx=0.1 and 0.2, and YBa₂(Cu_1−x Zn _x )₃O_7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs⁻¹.1.6μs⁻¹, and 1 μs⁻¹ forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs⁻¹, 0.75 μs⁻¹, 0.65 μs⁻¹, and 0.4 μs⁻¹ forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.