Observation of multiple superconducting gaps in the infrared reflectivity spectra of Ba(Fe<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript>

The results of infrared reflectivity measurements for the iron-based high-temperature superconductor Ba(Fe_0.9Co_0.1)₂As₂ are reported. The reflectivity is found to be close to unity at frequencies ω lower than 2Δ/h (2Δ is the superconducting gap and h is Planck’s constant). This is evidence for the s ^+/− or s ^+/+ symmetry of the superconducting order parameter in the studied compound. The infrared reflectivity spectra of Ba(Fe_0.9Co_0.1)₂As₂ manifest opening of several superconducting gaps at temperatures lower than critical T _c .     

Study of ion–solvent interactions and activation energy of LiBr in DMSO,H<Subscript>2</Subscript>O and DMSO–H<Subscript>2</Subscript>O mixtures at various temperatures

The Jones–Dole B coefficients of the electrolyte Lithium bromide (LiBr), reference salts tetra butyl ammonium tetra phenyl borate (BU₄NBPh₄), tetra butyl ammonium bromide (BU₄NBr), and potassium chloride (KCl) in dimethylsulfoxide (DMSO), water, and DMSO–water mixtures were obtained at different temperatures range from 25 to 45 °C For this, the relative viscosities were measured for Lithium bromide (LiBr) and reference salts in DMSO, water, and DMSO–water mixtures at above-mentioned temperatures. The B coefficients of these electrolytes were behaved as structure makers in DMSO, while in H₂O and DMSO–H₂O mixtures, the B-coefficient values were less positive showing the weak structure-making effect. Ionic viscosity B coefficients allow us to assess the behavior of ions in the solvent mixtures. In this study it was observed that all the values of ionic B coefficient of (Li⁺) were positive and small showing the weak structure-making effects. It was also observed that Br⁻ ions maintain negative B coefficient values in all DMSO–H₂O mixtures, except in 60% DMSO mole fraction. From this it can be concluded that Br⁻ ion behaved as a structure breaker in water and in all DMSO–H₂O mixtures except in 60% DMSO mole fraction mixtures. The low B _± values of alkali metal ions and Br⁻ ions in water are due to the breakdown of the tetrahedral structural of water and the formation of strongly structured solvated ion. It is also observed that the values of the energy of activation of the flow for LiBr are greater in DMSO–water mixtures and in pure water than in DMSO. This may be due the presence of a network of hydrogen bonds which cause the hindrance in the flow of the solution of LiBr in DMSO–water mixtures and in pure water than in DMSO.     

Magnetocaloric effect in the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

The magnetocaloric effect ΔT has been studied by a direct method in two samples of the manganite Sm_0.55Sr_0.45MnO₃, namely, a single crystal (sample A) and a ceramic sample (sample C). The temperature dependences of the ΔT effect of both samples exhibit a maximum at T _max = 143.3 K for the sample A and T _max = 143 K for the sample C. In these maxima, the values of the ΔT effect are 0.8 and 0.4 K in the magnetic field H = 14.2 kOe for the samples A and C, respectively. In addition, the ΔT(T) curve of the sample A has a minimum at T _min = 120 K, in which ΔT = −0.1 K. The maximum value of the ΔT effect increases with an increase in the magnetic field H in the range of magnetic fields up to 14.2 kOe, and the rate of this increase at H > 8 kOe is higher than that at H < 8 kOe. These features of the ΔT effect are explained by the presence of ferromagnetic and antiferromagnetic A- and CE-type clusters in the samples.     

Formation and composition of the clathrate phase in the H<Subscript>2</Subscript>O-H<Subscript>2</Subscript> system at pressures to 1.8 kbar

The transition of the hexagonal ice phase I_h to the clathrate phase sII has been found in the H₂O-H₂ system at a pressure of about 1 kbar under conditions of an excess of gaseous hydrogen. The pressures of the I_h → sII and sII → I_h transitions have been determined over a temperature range from ?36 to ?18°C, and the pressure dependence of the synthesis temperature of the clathrate phase from a liquid at pressures from 1.0 to 1.8 kbar has been constructed. The solubility of hydrogen in the I_h and sII phases and in liquid water has been measured. The concentration of hydrogen in the clathrate phase sII is about 1.2 wt % (10 mol %) near the boundary of the sII → I_h transition, and it increases to 2 wt % (16 mol %) at a pressure of 1.8 kbar.     

Nonlinear properties and paramagnet-to-ferromagnet transition in Nd<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single crystal with metallic ground state

We present the results from studying the magnetic properties (linear and nonlinear susceptibilities and the depolarization of polarized neutrons) of Nd_{1 − x} Ba _x MnO₃ manganite, x = 0.3, with Curie temperature T _C ≈ 140 K and dielectric-to-metal transition temperature T _DM ≈ 129 K. Its critical behavior corresponds to that of an isotropic 3-D ferromagnet at temperatures above T*≈ 144 K, but a strong nonlinear response in weak magnetic fields and depolarization are observed at temperatures below T*. It is shown that this nontraditional behavior is related to the generation of ferromagnetic clusters in the paramagnetic matrix that form a conducting percolative network at temperatures near T _DM.     

Experimental estimation of the cooperative Jahn-Teller energy in orbitally ordered KCu<Subscript>0.8</Subscript>Mg<Subscript>0.2</Subscript>F<Subscript>3</Subscript> perovskite

High resolution synchrotron radiation X-ray powder diffraction was used to investigate the melting of the cooperative Jahn-Teller distortion (cJTd) in a perovskite of composition KCu_0.8Mg_0.2F₃. A first order phase transition relaxing the cJTd is observed at T ∼ 600 K. From the transition temperature, an estimation of kT is derived (kT = 0.05 eV) for the cJTd in the doped compound. This is the very first observation of cJTd melting in a compound of the series KCu_1−x Mg _x F₃. A structural phase diagram for the Cu rich zone of the series is proposed. In principle, the extension of this experimental investigation could be used to disentangle the orbital order (OO) and cJTd energy scales in the parent compound KCuF₃, a test system for OO theories.     

Charge density wave and superconductivity in 2H- and 4H-NbSe<Subscript>2</Subscript>: A revisit

Good-quality hexagonal NbSe₂ single crystals were prepared. In 2H-NbSe₂, superconducting and charge density wave (CDW) transitions were found at T _s = 7.4 K and T _c = 35 K respectively as reported previously. We have noticed that these two transitions are changed to T _c = 42 K and T _s = 6.5 K, in 4H-NbSe₂. Thermopower has shown clear anomaly at CDW transitions. The anisotropic upper critical field was calculated as ~3 and 6.3 for 2H- and 4H-single crystals around t = 0.81, where t = T/T _s, from resistivity and explained in terms of coherence length. From the relation, H_c2 (T)=H_c2 (0)[1-t²]H_{\rm c2} (T)=H_{\rm c2} (0)[1-t^2], H_c2^l (0)H_{\rm c2}^l (0) was calculated as ~8.15 T and 16.98 T at t = 0.84 in 2H-NbSe₂ and 4H-NbSe₂ respectively. However, H_c2^t (0) = 2.68H_{\rm c2}^t (0) = 2.68T for both single crystals.     

Electrophysical properties and the structure of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> compound thermally restored after low-temperature decomposition

The electrophysical properties and structure of the nonstoichiometric high-temperature superconductor YBa₂Cu₃O _y restored at T = 930–950°C after low-temperature decomposition (T = 200°C) into phases different in the oxygen content have been studied. It has been shown that, unlike heat treatments at T ≤ 900°C, the superconducting properties are almost completely restored for 3–5 h during grain recrystallization, which is impossible at lower temperatures. After short-term annealing at T = 930–950°C (for 1–2 h), the ceramic material still contains a significant number of structural defects, most likely, in cation sublattices. These defects can contribute to the pinning of magnetic vortices, which substantially increases the critical current density in magnetic fields up to 2 T as compared to ceramic materials produced by the conventional technology.     

Temperature-dependent characteristics of pulse-laser-deposited (Pb,Sr)TiO<Subscript>3</Subscript> films at low temperatures

Pulsed laser deposition (PLD) of (Pb,Sr)TiO₃ (PSrT) film on Pt/SiO₂/Si at low substrate temperatures (T_s), ranging from 300–450 °C, has been investigated. As T_s increases, the films reveal coarsening clusters, improved crystallization of the perovskite phase, distinct capacitance–electric field (C–E) hysteretic loops and a larger dielectric constant. The 350 °C-deposited film shows strong (100) preferred orientation and optimum dielectric properties with the dielectric constant of ∼620. The current density increases as the measurement temperature and the electric field increase. Moreover, PSrT films exhibit a strong negative temperature coefficient of resistance (NTCR) behavior at temperatures ranging from 100 to 390 °C. PACS 81.15.Fg; 77.22.Ch; 68.60.Dv     

Dielectric and polar order behaviors of BaTiO<Subscript>3</Subscript>-Bi(Mg<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics

New perovskite solid solution ceramics of (1−x)BaTiO₃-xBi(Mg_1/2Ti_1/2)O₃ ((1−x)BT-xBMT, x≤0.09) were synthesized by the solid-state reaction technique. X-ray diffraction studies have revealed a stable single perovskite structure for all samples. Dielectric measurements were carried out at different frequencies and temperatures. The polarization evolutions with temperatures were measured to investigate the ferroelectric properties. All the compositions show features of ferroelectrics with diffuse phase transition, though the temperature T _m of their dielectric constant maximum ε _m is frequency dependent. The dielectric constant peak ε(T) of (1−x)BT-xBMT ceramics become broad with increasing BMT content. During the temperature range of ε(T) peak summit, (1−x)BT-xBMT ceramics present quasi-linear dielectric phenomenon under high electric field with very high dielectric constant.     

Growth and sintering effects of hydrated polycrystalline Li<Subscript>2</Subscript>WO<Subscript>4</Subscript>

Polycrystalline Li₂WO₄ was sintered at temperatures, 400, 450, 500, 550, 600, 650, and 700 °C. After sintering at each particular temperature, the Li₂WO₄ was cooled to room temperature. The X-ray diffraction pattern of Li₂WO₄ exhibits dominant peaks attributable to 7Li₂WO₄.4H₂O (cubic) and Li₂WO₄ (hexagonal) and thus reveals the extent of hydration of the material. The composition varies on heating at several temperatures as shown by the presence of new peaks in the diffractogram. Thermogravimetric analysis is used to correlate respective structural and thermal properties in variation. The impedance spectra show the presence of a semicircle in the higher frequency regions and straight line behaviors at lower frequencies. SEM micrographs depict the image of sintered Li₂WO₄. Grain growth studies reveal the sensitiveness of grain toward temperature. The maximum grain size is observed to be ≈5.7 μm at 700 °C.     

Kinetic phenomena in zero-gap semiconductors CuFeS<Subscript>2</Subscript> and CuFeTe<Subscript>2</Subscript>: Effect of pressure and heat treatment

Electrical resistivity ρ and Hal coefficient R are measured as a function of the temperature (T = 1.7−310 K) and the magnetic field (up to H = 28 kOe) in zero-gap semiconductor CuFeS₂ samples subjected to hydrostatic compression and under various heat-treatment conditions. At low temperatures, anomalies are observed in the kinetic effects related to the presence of ferromagnetic clusters: the magnetoresistance at T = 4.2 K and T = 20.4 K acquires a hysteretic character and thermopower α changes its sign at T < 15 K. The temperature dependence of conduction-electron concentration n in CuFeS₂ has a power form in the temperature range T = 14−300 K, which is characteristic of the intrinsic conductivity in zero-gap semiconductors. In CuFeS₂, we have n(T) ∝ T ^1.2; in isoelectron compound Cu_1.13Fe_1.22Te₂, we have n(T) ∝ T ^1.93. Heat treatment is found to affect the intrinsic conductivity of CuFeS₂, as the action of hydrostatic compression (carrier concentration changes); that is, the carrier concentration changes. However, a power form of the n(T) and ρ(T) dependences is retained.     

Conversion of ortho-para H2O isomers in water and a jump in erythrocyte fluidity through a microcapillary at a temperature of 36.6±0.3°C

A mechanism is proposed for the previously observed [1] jump in erythrocyte fluidity through a microcapillary 1.3 μm in diameter at a temperature of 36.6±0.3°C. Our interpretation is based on the experimental evidence both for existence of ortho and para H₂O isomers in water and on spin-selective interaction of proteins with para H₂O isomers as hydration shells of biomolecules are being formed [2]. It is important that the formation of hydration shells of proteins and DNA in aqueous solutions is accompanied by an increase in the Brillouin shift to 0.4 cm⁻1 (≃0.25 cm⁻¹ in water), which points to the formation of icelike structures. We believe that the coincidence of the translational energy kT of the Brownian motion and the energy of the rotational quanta for the 3₁₃–2₀₂ transition of para H₂O isomers at the temperature 36.6°C increases the probability for excitation of para H₂O isomers in collisions. Collisions mix quantum states of closely spaced levels in para H₂O (3₁₃, 285.2 cm⁻¹) and ortho H₂O (3₃₀, 285.4 cm⁻¹) and induce conversion of para isomers to ortho H₂O. It is assumed that this conversion in the icelike hydration shell of hemoglobin (Hb) is accelerated under the catalyzing effect of oxygen and iron present in Hb and triggers a chain reaction: release of ortho H₂O isomers through the erythrocyte membrane→compaction of Hb molecules and increase in concentration of catalysts→acceleration of conversion→structural gel-sol transition. It is the sequence of these processes that provides a jump in fluidity of erythrocytes through a microcapillary and the anomalous increase in fluidity of the aqueous solution of hemoglobin by almost an order of magnitude at temperatures close to 36.6°C and an increase in the solution concentration by a factor of 1.7.     

Anomalies in the Young modulus at structural phase transitions in rare-earth cobaltites RBaCo<Subscript>4</Subscript>O<Subscript>7</Subscript> (R = Y,Tm-Lu)

The elastic properties of rare-earth cobaltites RBaCo₄O₇ (R = Y, Tm-Lu) have been experimentally studied in the temperature range of 80–300 K. The strong softening of the Young modulus ΔE(T)/E ₀ ≈ −(0.1–0.2) of cobaltites with Lu and Yb ions has been revealed, which is due to the instability of the crystal structure upon cooling and is accompanied by an inverse jump at the second-order structural phase transition. The softening of the Young modulus and the jump at the phase transition decrease by an order of magnitude and the transition temperature T _s and hysteresis ΔT _s increase from a compound with Lu to that with Tm. A large softening of the Young modulus at the structural transition in Lu- and Yb cobaltites indicates that the corresponding elastic constant goes to zero, whereas this constant in Tm cobaltite is not a “soft” mode of the phase transition. It has been found that the structural phase transition in Lu- and Yb cobaltites is accompanied by a large absorption maximum at the phase transition point and an additional maximum in the low-temperature phase and absorption anomalies in Tm cobaltite is an order of magnitude smaller.     

Two triple points in the H2O–H2 system

Using a volumetric technique, phase transitions in the H₂O–H₂ system were investigated in the vicinity of two points of an invariant equilibrium, L+I _h+sII and L+sII+C ₁, located at 1.07 kbar and?10 °C and at 3.6 kbar and 1 °C, respectively. Liquid water (L), low-pressure hexagonal ice (I _h) and high-pressure cubic (sII) and rhombohedral (C ₁) clathrate hydrates were in equilibrium with gaseous hydrogen taken in excess.     

Elastic properties of D<Subscript>2</Subscript>O ices in solid-state amorphization and transformations between amorphous phases

Nonequilibrium phase transformations in D₂O ices, including the solid-state amorphization of ice 1h (1h-hda) and the heating-induced transition cascade hda-lda-1c-1h from high-density amorphous (hda) ice to low-density amorphous (lda) ice followed by crystallization in cubic ice 1c and phase transition to ordinary hexagonal ice 1h, were studied using an ultrasonic technique. It has been shown that, as in H₂O ice, the softening of a crystal lattice or an amorphous network precedes nonequilibrium transformations. However, noticeable isotopic differences in the behavior of the elastic properties of H₂O and D₂O, in particular, their 1h and hda modifications, call for a more detailed study of the structural features of these H₂O and D₂O phases.     

Phase transitions in the CN<Subscript>x</Subscript>-Co system induced by changes in the film growth temperature

The structures of amorphous CN_x-Co films grown at different temperatures (T _s = 200–365°C) are studied by X-ray diffraction. As the growth temperature increases above T _s = 200°C, a concentration phase transition is found to occur in the amorphous state; this transition is related to a change in the major portion of carbon or cobalt in the structure of the cluster films. At T _s = 365°C, a disorder-order phase transition, which is accompanied by the transition from the amorphous to crystalline state, occurs in the films.     

Insulator-metal transition shift related to magnetic polarons in La<Subscript>0.67-x</Subscript>Y<Subscript>x</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

The magnetic transport properties have been measured for La_0.67-xY_xCa_0.33MnO₃ ( 0 ⩽ x ⩽ 0.14) system. It was found that the transition temperature T _p almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above T _p , there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of T _p vs. H was understood, and it was estimated that the size of the magnetic polaron is of 9.7 ∼ 15.4 ? which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The transport properties at temperatures higher than T _p conform to the variable-range hopping mechanism. Received 27 August 2002 / Received in final form 2 December 2002 Published online 14 March 2003     

Effect of hydrostatic compression on the indium-impurity-induced superconducting transition in Pb<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript>Te

This paper reports on a study of the low-temperature conductivity and parameters of the superconducting state, namely, the critical temperature T _c and the second critical magnetic field H_c2, in the (Pb_0.3Sn_0.7)_0.95In_0.05Te solid solution under hydrostatic pressure P ≤ 9 kbar at T = 4.2 K. The choice of this material has been motivated by the fact that, according to earlier observations, it undergoes a superconducting transition at T _c ∼ 2.3 K, i.e., close to the maximum value T _c ∼ 2.9 K found for the (Pb _z Sn_{1 − z} )_0.95In_0.05Te solid solutions with a lead content z ∼ 0.15–0.25. It has been demonstrated that an increase in the pressure to P ≤ 9 kbar leads to a bell-shaped dependence T _c (P). The observed dependences are assigned to the effect of hydrostatic compression on the band structure of the solid solution and indicate a shift in the position of the Fermi level E _F with increasing pressure within the impurity band of the In quasi-local states. In this case, E _F passes through a maximum in the density of impurity states at P = 3–5 kbar.     

Thulium doped monoclinic KLu(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> single crystals: growth and spectroscopy

This paper presents the crystal growth and optical characterization of thulium-doped KLu(WO₄)₂ (KLuW). Thulium-doped KLuW macrodefect-free monoclinic single crystals (a^*×b×c≈10×7×15 mm³) were grown by the top seeded solution growth slow cooling method with dopant concentrations of 0.5%, 1%, 3% and 5% atomic in solution. The evolution of unit cell parameters in relation with thulium doping was studied by X-ray powder patterns. Thulium energy levels in the KLuW host were determined by 6 K polarized optical absorption. The Judd–Ofelt parameters determined were Ω₂=9.01×10^-20 cm², Ω₄=1.36×10^-20 cm² and Ω₆=1.43×10^-20 cm². The maximum emission cross section for the 1.9 μm emission, calculated by Füchtbauer–Ladenburg method, is 1.75×10^-20 cm², at 1845 nm with E//N_m. The intensity decay time from the emitting levels ¹ G ₄ and ³ H ₄ levels in relation to the concentration were studied. For the lowest thulium concentration, the measured decay times from ¹ G ₄ and ³ H ₄ emitting levels are 140 μs and 230 μs, respectively. PACS 42.55.Rz; 78.20.-e; 78.55.-m