Multicomponent Order Parameter in the YBa2Cu3O7−δ

We have analyzed the temperature dependencies of the superfluid density in YBa₂Cu₃O₇ along a- and b- crystallographic axes using the multicomponent order parameter for the superconducting gap. Estimated values of the gap components for the d-wave and the isotropic s-wave are Δ _d =29 meV and Δ _s =5 meV, correspondingly. Band structure parameters were taken accordingly ARPES and neutron scattering data.     

Suppression of superconductivity in the Er1− x Pr x Ba2Cu3O7−δ

Measurements of electrical resistivity. X-ray diffraction patterns, magnetic susceptibility and thermoelectric power of the Er_1-x Pr _x Ba₂Cu₃O_7- system have been made. The superconducting transition temperature was found to decrease monotonically with praseodymium concentration, x. From the susceptibility data, it was determined that the valence of praseodymium lies between +3 and +4. The thermoelectric power was found to increase with x, and the slopes of dS/dT were negative except for the case x= 0. The. tendency of the thermopower to change with increasing praseodymium concentration has been qualitatively explained using the theory for strongly correlated systems.     

Dielectric properties of the Sr1−x La x Sn1−x Co x O3 system

The possibility of the formation of a solid solution in the Sr_1–x La _x Sn_1–x Co _x O₃ system has been explored. Single-phase solid solution forms in the compositions for x0.10. All single-phase solid solution compositions have a cubic structure similar to SrSnO₃. The dielectric behaviour of these solid solution compositions has been studied as a function of temperature and frequency. The frequency dependence of dielectric constant and dielectric loss in these materials indicates that space charge polarization contributes significantly to their observed dielectric parameters. Microstructural studies show the presence of well-faceted grains. The average grain size in these samples is small.     

Investigation of the lithium-rich boundary of the Li1+x Mn2−x O4 cubic spinel phase in air

Several compositions of the cubic spinel Li_1+x Mn_2?x O_4?δ phase in the lithium–manganese–oxygen (Li–Mn–O) system were synthesized at 700, 750, and 800 °C in air ( $ p_{{{\text{O}}_{2} }} $  = 0.2 atm) to investigate the Li-rich boundary of the cubic spinel phase at these temperatures. The lattice parameters of the several compositions were determined by Rietveld analysis of the measured X-ray patterns, and the Li and Mn contents of the samples were measured using inductively coupled plasma with optical emission spectroscopy (ICP-OES). A Vegard-like dependence of the measured lattice parameter of the cubic spinel phase with Li to Mn ratio exists in the homogeneity range of the cubic spinel. This dependence could be used to derive the boundary of the single phase cubic spinel field in the Li–Mn–O system at 700 and 750 °C at $ p_{{{\text{O}}_{2} }} $  = 0.2 atm and to estimate the Li-rich boundary at 800 °C. The results of the present study are compared with two other experimental studies on the homogeneity range of the cubic spinel phase in an attempt to resolve the contradiction between these two studies.     

Investigation of the gapless state induced by pressure in Hg1−x Cd x Te alloys

We report the results of an experimental study of the gapless state (GS) induced inp-type semimetallic alloys Hg _1–x Cd _x Te (0<x<0.15) by pressure. Galvanomagnetic effects in weak magnetic fields (2T<300 K) and the Shubnikov-de Haas effect have been investigated in the pressure interval 1p<15 kbar. Direct evidence for the existence inp-type semimetallic alloys of an impurity hole band overlapping with the conduction band by 3–4 me V is obtained. At liquid helium temperatures the Fermi level is located in the impurity band, so that two groups of carriers take part in transport effects: light electrons in the conduction band and heavy holes in the impurity band. The electron Fermi energyE _F is proved to be essentially constant during the transition to the GS. A linear dependence of the electron effective mass at the band edgem* (0) upon the gapE _g is obtained. A significant role of scattering of electrons into the impurity band at liquid helium temperatures is revealed.     

Phase Formation in the Course of Oxidation of In x Se1 − x Alloys (0.47 ≤ x ≤ 0.53)

We have determined the characteristic temperature ranges of heating of In _x Se_{1 – x} (0.50 x 0.52) alloys in air. We have established that the annealing of InSe powder in selenium vapors leads to the stabilization of the rhombohedral structure.     

In-Plane Anisotropy of the Upper Critical Field of La2−x Sr x CuO4 in the Underdoped and Overdoped Regimes

For La_2–x Sr _x CuO₄ single-crystals in the underdoped and overdoped regimes, we have measured the electrical resistivity along the c-axis, _c , under constant magnetic fields, changing the field direction in the ab-plane. At x=0.08, 0.12 and 0.16 in the underdoped and almost optimally-doped regimes, clear anisotropy of _c with the fourfold symmetry in the ab-plane has been observed. This symmetry is regarded as the fourfold symmetry of the upper critical field, H _c2, and explained as being mainly due to the anisotropy of the superconducting energy gap owing to the d_x ^{2-y 2} pairing. The magnitude of the fourfold component of H _c2, H _c2/H _c2, is largest in x=0.08 among the three crystals. At x=0.20 in the overdoped regime, no clear fourfold symmetry has been observed. It appears that the fourfold symmetry is easy to observe in the underdoped regime rather than in the overdoped regime.     

Point-Contact Andreev-Reflection Spectroscopy in the Fe-based Superconductor LaFeAsO1−x F x

Point-contact Andreev-reflection (PCAR) spectroscopy measurements have been performed in the Fe-based superconductor LaFeAsO_1−x F _x in order to investigate the energy gaps. The Andreev spectra show clear and reproducible features that could be related to two nodeless gaps. Their values, as determined by fitting the conductance curves measured in junctions with local T _c =27.3–28.6 K within the two-band BTK model, are Δ ₁=2.75–3.8 meV for the small gap and Δ ₂=7.9–10.2 meV for the larger one, respectively. The absence of zero-bias conductance peaks and considerations regarding the nonperfectly directional current injection in PCAR experiments and the polycrystalline nature of the samples rule out the possibility of nodal gaps in the superconductor. Moreover, the conductance curves show pseudogap-like features coexisting, at low temperature, with superconductivity and disappearing close to the Néel temperature of the parent compound, T _N≈140 K.     

Thermodynamic study of the magnetic ordering of Fe3−x Mn x Si alloys

The compounds Fe_3–x Mn _x Si exhibit a complex magnetic behavior for 0.6x1.75 involving ferromagnetic ordering followed by a reordering at lower temperature. Measurements of the thermal expansion and the specific heat C _p show only a weak anomaly at the Curie temperature, T _C, whereas a large, relatively sharp peak occurs at the magnetic reordering temperature, T _R. Thermal expansion measurements for the Fe_2.2Mn_0.8Si compound made about T _R are indicative of long-range ordering that can be suppressed in an applied field of 0.4T. Magnetization measurements show that low fields, 1–2 mT, strongly influence the bulk magnetization. Irreversible magnetization behavior, similar to that in spin-glass systems, is displayed.Paper presented at the Tenth International Thermal Expansion Symposium, June 6–7, 1989, Boulder, Colorado, U.S.A.     

Role of the proton tunneling in the phase transition of K3D1−x H x (SO4)2

Dielectric and thermodynamic properties have been investigated for isotopically mixed crystals of K₃D_1–x H_x(SO₄)₂, which are composed of structurally isolated dimeric units SO ₄ ^– ... H-SO ₄ ^– . The critical temperature T_c for the antiferroelectric phase transition decreases with proton concentration (x) and eventually the phase transition vanishes at x_c 0.66 ± 0.04. The x-dependent behavior of T_c, the dielectric constant (T) and the specific heat C(T) are well accounted for by the mean-field theory for the transverse Ising model, indicating an important role of the proton tunneling on the phase transition and related properties. These features are compared with conventional hydrogen-bonded dielectrics with infinite networks of the hydrogen bonds.     

Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La2−x Ce x CuO4−y and Pr2−x Ce x CuO4−y

From measurements of the magnetic penetration depth, (T), from 1.6 K to T _c in films of electron-doped cuprates La_2–x Ce _x CuO_4–y and Pr_2–x Ce _x CuO_4–y we obtain the normalized density of states, N _s(E) at T=0 by using a simple model. In this framework, the flat behavior of ^–2(T) at low T implies N _s(E) is small, possibly gapped, at low energies. The upward curvature in ^–2(T) near T _c seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3k _B T _c of the Fermi surface.     

Optical Properties of the Cu–O Plane in the Bi2Sr2−x La x CuO6 Family

The ab-plane optical conductivity of seven single crystals, belonging to the family Bi₂Sr_2?x La _x CuO₆, has been measured for hole concentrations per Cu site 0.03≤p≤0.18, and for 6 K≤T≤300 K (500 K for p=0.16). At low doping, ten phonon lines are detected, which are due to the removal of the degeneracy of five E _u modes (out of the predicted six). They are superimposed to a far-infrared band, which as doping increases, closes the insulating gap thus building up the Drude term. The insulator-to-metal transition occurs between p=0.7 and p=0.10 consistently with a Mott mechanism. In the metallic phase, a multiband analysis identifies a Drude term plus a mid-infrared band, which weakly depends on temperature and softens as p increases, like in other cuprates. The optical response of the crystal at optimum doping has been analyzed also in the superconducting phase. The Ferrel–Glover–Tinkham sum rule requires an integration up to Ω?6Δ and the penetration depth is 290 nm. The bosonic spectral function includes a strong peak around 50 meV, which survives up to 500 K and, therefore, might be assigned to an electron–phonon interaction.     

X-ray,electrical conductivity and infrared studies of the system Zn1−x CO x Mn1−x Fe x CrO4

The system Zn_1–x Co _x Mn_1–x Fe _x CrO₄ is tetragonal in the range 0.0x0.4 and cubic in the range 0.5x1. Electrical resistivity temperature behaviour obeys Wilson's law for all the compounds and thermoelectric coefficient values vary between 135 to 226V K^–1. All compounds exhibit P-type semiconductivity and possess low mobility values (10^–9 cm² V^–1 sec^–1). The infrared spectra show the presence of two strong absorption bands around 500 and 600 cm^–1. The probable ionic configuration for the system is suggested to be Zn _1–x ²⁺ Fe _x ^–y3+ -Co _x ²⁺ [Mn _1–x ³⁺ Fe _y ³⁺ CO _x ^–y2+ Cr³⁺]O₄.     

High-Pressure Studies on Superconductivity in LaFeAsO1−x F x and SmFeAsO1−x F x

The pressure dependence on the superconducting transition temperature (T _c ) was investigated for the iron-based superconductors LaFeAsO_1−x F _x and SmFeAsO_1−x F _x . The T _c ’s increase largely for LaFeAsO_1−x F _x with a small increase of pressure, while a sharp decrease of T _c was observed for SmFeAsO_1−x F _x . The electrical resistivity measurements reveal pressure-induced superconductivity for undoped LaFeAsO and SmFeAsO. These pressure effects seem to be related to an anisotropic decrease of the lattice constants under high pressure from the x-ray diffraction measurements up to 10 GPa for the LaFeAsO_1−x F _x system.     

Transport anomalies in the Zn-substituted La2−x Sr x Cu1−y Zn y O4 with x ∼ 0.115; Possibility of the Pinning of CDW and SDW by Zn

Anomalies in the thermoelectric power and the Hall coefficient, which are analogous to those observed in La _2–x Ba _x CuO ₄ with x 1/8, have been observed in the Zn-substituted La _2–x Sr _x Cu _1–y Zn _y O ₄ with x = 0.115 and y = 0 – 0.025 and with x = 0.15 and y 0.02. Based on the so-called stripe model, these are explained as being due to the formation of the static order of the stripe-patterned CDW and SDW on account of the pinning by Zn. Accordingly, the long-standing problem on the conspicuous suppression of superconductivity in the Zn-substituted La _2–x Sr _x Cu _1–y Zn _y O ₄ with x 0.115 is likely to be attributed to this static order.     

Antiferromagnetism and the Effect of Exchange Bias in LaCr1−x Fe x O3 (x=0.40 to 0.60)

LaCr_1?x Fe _x O₃ (x=0, 0.4, 0.45, 0.6 and 1.0) have been prepared in single phase form with Pbnm space group. The x=0 sample exhibits antiferromagnetic (AFM) transition at Néel temperature T _N =290 K and it is found to reduce with increase in doping concentration up to x=0.45. Magnetization reversal below a compensation temperature of 100 K was observed in the x=0.45 sample and it is explained in terms of competition between two AFM interactions and associated magnetic anisotropy. Exchange bias was observed in the x=0.45 sample and the temperature dependence of exchange bias field and vertical magnetization shift were analyzed.     

Crystallization kinetics of Zr-rich FexZr0−x0−x0−x(20 ⩽ x ⩽ 40) metallic glasses

The crystallization kinetics of the melt-spun Fe-Zr metallic glasses in the iron-rich region has been investigated by means of DSC and X-ray diffraction. The crystallization mode changes with iron concentration. In the lower iron region, 20 x 25, the Fe _x Zr100–x glasses crystallize into -Zr and Ti₂Ni-type FeZr₂ with an accompanying sharp and large exotherm at the first crystallization step and immediately after this step, they transform into orthorhombic FeZr₃. On the other hand, the alloys with 35 x 40 exhibit a gradual exotherm which initiates from a temperature far below the definite crystallization temperature (T _x). The Fe-Zr metallic glasses in this concentration region crystallize polymorphously into the oxygenstabilized Ti₂Ni-type FeZr₂ with accompanying relatively small and composite exotherms. The annealing at a temperature where the gradual exotherm occurs for the alloys with 30 x 40 does not cause any changes of X-ray halo pattern but results in the reduction of the heat of exotherm due to the crystallization.     

Nuclear ferromagnetic ordering of141Pr in the diluted Van Vleck paramagnets Pr1−x Y x Ni5

We report on the first investigation of the effect of magnetic dilution on nuclear magnetic spin interactions in metals; we studied this effect in the diluted Van Vleck paramagnets Pr_1–xY_xNi₅. In addition, we investigated the electronic magnetic properties of these intermetallic compounds. For this purpose, we have measured the nuclear and electronic susceptibilities of Pr_1–xY_xNi5 with x = 0 to 0.2 at 50K T 8 mK and at 2.2 K T 300 K. We observe a linear decrease of the electronic Van Vleck susceptibility and of the hyperfine enhancement factor by 13% (per mole Pr³⁺). The hyperfine-enhanced nuclear susceptibility decreases by about a factor of three when going fromx = 0 tox = 0.2. The samples show nuclear ferromagnetic transitions of¹⁴¹Pr with nuclear Curie temperatures T_c decreasing from 370 K to 100 K and Weiss temperatures decreasing from 218 K to 13 K in this concentration range. These data are compared with the results of mean-field calculations. In addition, we report on measurements of the nuclear spin relaxation time of these compounds, for which we find values of a few msec at millikelvin temperatures and a critical speeding-up at T_c. Our data give hints that for x > 0.2 the character of the nuclear magnetic transition may change, possibly to a nuclear spin glass freezing.     

Dielectric properties of the system Ba1−xLaxTi1−xCo x O3

The dielectric behaviour of compositions withx=0.01, 0.05, 0.10 and 0.20 in the system Ba_1–x La _x Ti-_1-x Co _x O₃ was studied in the temperature range 300–473 K. The compositions withx = 0.01 and 0.05 show a diffuse ferroelectric-paraelectric phase transition, while other compositions do not show this transition in this temperature range. The frequency dependence of dielectric constant and dielectric loss in the samples withx0.05 indicates that spacecharge polarization contributes significantly to their observed dielectric parameters.