Kinetic study of non-isothermal crystallization of BixSe100−x chalcogenide glasses

Results of differential thermal analysis (DTA) under non-isothermal conditions on four glasses of Bi_xSe_100−x (x=5, 10, 15 and 25 at%) are reported and discussed. The glass transition temperatures (T_g), the onset crystallization temperatures (T_c) and the peak temperatures of crystallization (T_p) were found to be dependent on the compositions and the heating rates. From the dependence on the heating rates of (T_g) and (T_p), the activation energy for glass transition, E_g, and the activation energy for crystallization, E_c, are calculated and their composition dependence is discussed. The crystalline phases resulting from DTA and scanning electron microscopy (SEM) have been identified using X-ray diffraction. According to the Avrami exponent (n), the results show a one-dimensional growth for the composition Bi₅Se₉₅ and two-dimensional growth for the compositions Bi₁₀Se₉₀, Bi₁₅Se₈₅ and Bi₂₅Se₇₅. The kinetic parameters determined have made it possible to discuss the glass-forming ability.     

Gigantic uniaxial pressure effect in single crystals of iron-based superconductors

In order to elucidate the anisotropic pressure effect on superconductivity in an iron-based superconductor, magnetization measurements have been performed in Ba(Fe_0.92Co_0.08)₂As₂ single crystals under uniaxial pressures applied along the c-axis. Gigantic T_c suppression, dT_c/dP_//c = −15 K/GPa, was observed when the anisotropic deformation with the a-expansion and c-compression was induced by the c-pressure, which should be compared with dT_c/dP < +1 K/GPa in the isotropic pressure case. This suggests that the a-axis (c-axis) compression has a positive (negative) contribution to T_c.     

Magnetic properties and hydrostatic pressure effect on the curie temperature of (Co,Mn)2B amorphous alloys

The temperature dependence of magnetization and magnetic susceptibility and hydrostatic pressure effect on the Surie temperature (dT_c/dP) are measured for (Co_1-xMn_x)₂B (0?x?0.4) amorphous alloys and the results are compared with those of crystalline compounds with the same composition. The Curie temperature decreases linearly with an increasing Mn content but magnetization shows a maximum around x=0.15. The reciprocal magnetic susceptibility of all the prepared alloys obeys the Curie-Weiss law above T_c. The magnitude of the negative value of dT_c/dP decreases linearly with increasing x from about 1.1 K/kbar (x=0) to zero (x=0.4), the composition dependence of which is opposite to that of the crystalline compound. The composition dependence of the average magnetic moment per transition metal atom and the Curie temperature and dT_c/dP are analysed on the basis of the local environment and the pair order interaction mode, respectively.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Specific heat measurements in pure and in (Cu, Mn, Fe, Ni)-doped single-crystals of l-arginine phosphate monohydrate

The influence of highly diluted impurities (Cu, Mn, Fe, Ni) on the temperature (T) dependence of the specific heat (c_p) of l-arginine phosphate monohydrate (LAP) was investigated in the temperature range 1.8-300 K. The doped samples yielded values for c_p in excess to those obtained for a pure LAP sample. The melting temperatures (T_m≈420 K) obtained by differential scanning calorimetry for pure and doped LAP samples were found not to be significantly affected by the impurities. The T-dependence of c_p was fully accounted for by taking into consideration the Debye contribution, an Einstein term and a contribution due to both Frenkel and Schottky defects. The model fit all c_p versus T data using a single value for both the Debye (θ_D=160 K) and the Einstein (T_E=376.8 K) temperatures, and for the energy (ε_d=157.9 meV) required to create the defects.     

Temperature dependence of the energy separation between Γ and L minima in n-type GaSb

Energy separation ΔE_c between Λ and L minima of GaSb conduction band is deduced from temperature dependence of tunneling current in p−n junctions. ΔE_c is found to inceasing vx. temperature with a coefficient d(ΔE_c/dT) of about - 2.10^-4eV/dgK.     

Pressure dependence of the superconducting transition temperature of a (Mo0.6Ru0.4)86B14 metallic glass

We have studied the pressure dependence of the superconducting transition temperature of amorphous (Mo_0.6Ru_0.4)₈₆B₁₄ for hydrostatic pressures up to P ～ 9 kbar. The transition temperature T_c decreases with pressure at a rate dT_c/dP=-(9±1) mK kbar^-1. We estimate the Grüneisen parameter and the volume dependence of the electron-phonon coupling constant.     

The pressure influence on the incommensurate-commensurate ferroelectric phase transition in [4-NH2C5H4NH][SbCl4]

The dielectric properties of the [4-NH₂C₅H₄NH] SbCl₄ (abbreviated as 4-APCA) crystal were investigated under hydrostatic pressure up to 300 Mpa. The pressure-temperature phase diagram was given. The paraelectric-ferroelectric phase transition (II→III) temperature (T_c) increases linearly with increasing pressure with a slope dT_c/dp=21×10⁻² K/MPa. The pressure dependence of Curie-Weiss constants has been evaluated also. In the paraelectric phase (II) the Curie constant (C₊) was pressure dependent whereas the C₋ constant over the ferroelectric phase (III) was almost constant. The results are interpreted in terms of improper and displacive type phase transition model with a soft phonon at a zone boundary.     

Spin dependence of the inclusive reaction p + p (polarized) → π0 + X at 24 GeV/c for high-pTπ0 produced in the central region

The spin dependence of π⁰ inclusive production by 24 GeV/c protons has been measured using a polarized target for Fhe Feynman x near 0 in the transverse momentum range 1.0 < p_T < 2.5 GeV/c. The results indicate a negative updown asymmetry growing strongly with p_T and greater than 50% in absolute value for p_T greater than 2 GeV/c.     

Effects of Zn on the grain boundary properties of La2−xSrxCu1−yZnyO4 superconductors

The properties of the grain boundaries (GBs) are of significant importance in high-T_c cuprates. Most large scale applications of cuprate superconductors involve usage of sintered compounds. The critical current density and the ability to trap high magnetic flux inside the sample depend largely on the quality of the GBs. Zn has the ability to pin vortices but it also degrades superconductivity. In this study we have investigated the effect of Zn impurity on the intergrain coupling properties in high-quality La_2−xSr_xCu_1−yZn_yO₄ sintered samples with different hole concentrations, p (≡x), over a wide range of Zn contents (y) using field-dependent AC susceptibility (ACS) measurements. The ACS results enabled us to determine the superconducting transition temperature T_c, and the temperature T_gcp, at which the randomly oriented superconducting grains become coupled as a function of hole and disorder contents. We have analyzed the behavior of the GBs from the systematic evolution of the values of T_gcp(p, y), T_c(p, y), and from the contribution to the field-dependent ACS signal coming from the intergrain shielding current. Zn suppresses both T_c and T_gcp in a similar fashion. The hole content and the carrier localization due to Zn substitution seem to have significant effect on the coupling properties of the GBs. We have discussed the possible implications of these findings in detail in this article.     

Pressure effect on magnetization for (CoTm)90Zr10 (Tm = Cr,Mo) amorphous alloys

Temperature dependences of the forced volume magnetostriction dω/dH and the saturation magnetization σ_s for (CoTm)₉₀Zr₁₀ (Tm = Cr, Mo) amorphous alloys have recently been measured by the 3-terminal capacitance method and a vibrating sample magnetometer and magnetic balance at temperatures from 77 K to the Curie temperature T_c or the crystallization temperature. The pressure coefficient of σ_s0 at 0 K, d ln σ_s0/dp, is estimated from (dω/dH)₀ extrapolated to 0 K using the thermodynamical relation. The values of d ln σ_s0/dp increase in negative value with increasing Tm concentration. The relation between d ln σ_s0/dp and the pressure coefficient of T_c, d ln T_c/dp, estimated indirectly from dω/dH is discussed.     

Negative c-axis magnetoresistance in graphite

We have studied the c-axis interlayer magnetoresistance (ILMR), Rc(B) in graphite. The measurements have been performed on strongly anisotropic highly oriented pyrolytic graphite (HOPG) and single crystalline Kish graphite samples in magnetic field up to B=9 T, and the temperature interval 2 K?T?300 K. We have observed negative magnetoresistance, dRc/dB<0, for B‖c-axis for both samples above a certain field Bm(T)>5.4 T and 0.2 T for HOPG and Kish graphite, respectively. The results can be understood consistently by assuming that ILMR is related to a tunneling between zero-energy Landau levels of quasi-two-dimensional Dirac fermions, in a close analogy with the behavior reported for α-(BEDT-TTF)₂I₃ [N. Tajima, et al., Phys. Rev. Lett. 102 (2009) 176403], another multilayer Dirac electron system.     

Ferroelastic phase transition and anomalous elastic and thermal properties of betaine borate (CH3)3NCH2COO·H3BO3

The temperature and pressure derivatives of the elastic constants of orthorhombic betaine borate, (CH₃)₃NCH₂COO·H₃BO₃, have been determined by measuring temperature and stress induced shifts of resonance frequencies of thick plates at ca. 15 MHz in the range between 140 and 300 K and 0 and 3 kbar. The elastic ‘shear’ resistance c₄₄ exhibits a value as low as 0.0492×10¹⁰Nm^-2at 293 K. With decreasing temperature c₄₄ approaches zero at ca. 142.5 K, indicating an acoustic soft mode behaviour connected with a ferroelastic phase transition. The softening of c₄₄ is described in a good approximation by c₄₄(T)_p=0 =alogT/T₀ with a=0.0663×10¹⁰Nm^-2 and T₀ = 139.5 K. Further, c₄₄ decreases with increasing pressure according to the linear relation c₄₄(p)_{T=293 K} = 0.0492?0.184×10^-4p (p in bar, c₄₄ in 10¹⁰ Nm^-2). All other elastic constants show a quite normal temperature and pressure dependence. At 293 K the transition is induced by a pressure of 2.65 kbar. The transition temperature T_c depends linearly on pressure according to T_c = 142.5+0.0568 p (pinbar, T_cinK). Passing through the transition no discontinuous change of the lattice constants is observed. The three principal coefficients of thermal expansion and the pressure derivatives of the dielectric constants exhibit discontinuities at the transition. The transition is of strongly second order.     

Pressure dependence of elastic constants of orthorhombic Li2Ge7O15 above and below the ferroelectric transition

The pressure derivatives of the elastic constants c_ij of orthorhombic Li₂Ge₇O₁₅ have been determined at 293 K by the method of pressure-induced shifts of resonance frequencies of thick plates at ca. 15 MHz in the range between 0 and 1500 bar. Approaching the transition at ca. 630 bar, all P_ij = dc_ij/dp (i, j = 1, 2, 3; p pressure) develop strongly negative values. At higher pressures a similar behaviour with reversed sign is observed. The pressure derivatives of the pure “shear resistances” c₄₄, c₅₅, and c₆₆ depend only slightly on pressure even in the vicinity of the transition. The main interactions driving the transition are of the totally symmetric type. The values dK^?1/dp (K volume compressibility) deviate strongly from the quasi-invariant value of ca. 5 observed in almost all stable crystals (dK^?1/dp = ? 1750 at 620 bar and 1380 at 700 bar). The anomalous piezoelastic behaviour reflects the anomalous thermoelastic behaviour: negative P_ij in the low pressure (high temperature) phase correspond to positive T_ij = d log c_ij/dT (T temperature) and vice versa.     

High pressure behaviour of KHSO4 studied by electrical impedance spectroscopy

Measurements of the electrical conductivity were performed in KHSO₄ at pressures between 0.5 and 2.5 GPa and in the temperature range 120-350 °C by the use of the impedance spectroscopy. The temperatures of the α-β phase transition (T_Tr) and of the melting (T_m), determined from the Arrhenius plots ln(σT) vs. 1/T, increase with pressure up to 1.5 GPa having dT/dP∼+45 K/GPa. Above the pressure 1.5 GPa, the pressure dependencies of T_Tr and T_m are negative dT/dP∼−45 K/GPa. At pressures above 0.5 GPa, the reversible decomposition of KHSO₄ into K₃H(SO₄)₂+H₂SO₄ (and probably into K₅H₃(SO₄)₄+H₂SO₄) affects the electrical conductivity of KHSO₄, with the typical values of the protonic electrical conductivity, c. 10⁻¹ S/cm at 2.5 GPa.     

d-Wave superconductivity from correlated-hopping interactions determined by angle-resolved photoemission spectroscopy

Starting from a generalized Hubbard model with correlated-hopping interactions, we solve numerically two coupled integral equations within the Bardeen–Cooper–Schrieffer formalism, in order to study the doping effects on the critical temperature (_Tc$T_c$), d-wave superconducting gap, and the electronic specific heat. Within the mean-field approximation, we determine the single- and correlated-electron-hopping parameters for La_2 − xSr_xCuO₄ by using angle-resolved photoemission spectroscopy data. The resulting parametrized Hubbard model is able to explain the experimental _Tc$T_c$ variation with the doping level (x). Moreover, the observed power-law behavior of the superconducting specific heat is reproduced by this correlated-hopping Hubbard model without adjustable parameters.     

Effect of 3d metal ion doping on the structure and superconductivity of (Tl0.5Pb0.5)Sr2CaCu2O7

(Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ (M=Co, Ni and Zn) have been synthesized and investigated by means of X-ray diffraction, scanning electron microscope, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all studied samples contain the nearly single ‘1212’ phase. They crystallize in a tetragonal unit cell with a=3.8028-3.8040 Å and c=12.0748-12.1558 Å. In (Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ system (M=Co or Ni), the superconducting critical temperature T_c decreases linearly with both Co and Ni concentrations and the rate of T_c decrease is around −6.5 and −7.0 K/at%, respectively. For (Tl_0.5Pb_0.5)Sr₂Ca (Cu_2−xZn_x)O₇ system, the dependence of T_c on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses T_c much less (−2.5 K/at%) than the Co and Ni substitutions. The suppression in T_c in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of T_c in Zn doped samples is not caused by the reduction in carrier concentration which should remain constant, but rather due to nonmagnetic pair-breaking mechanism induced by disorder as well as the filling of the local Cu d_x²−y² state due to the full d band of Zn ions.     

Anisotropic thermopower of the organic metal, β-(BEDT-TTF)2I3

Thermopower of the ambient pressure organic superconductor β-(BEDT-TTF)₂I₃ has been studied. Measurements performed on, respectively, crystals of needle formed morphology and on flake-like crystals with hexagon shape showed equal thermopower results. S was measured along the a-axis as well as along the b′-axis. Marked anisotropy is observed in the entire temperature region studied. The temperature dependence, as represented by dS/dT, is, however, nearly isotropic. On the basis of an analysis of anisotropic thermopower we attribute the isotropic part of S to a term depending on the bond-properties and the anisotropic part of S to the scattering mechanism. The analysis yields transfer integrals of the order of 0.12 eV along both a- and b′-axes.     

Critical temperature of superconductor/ferromagnet bilayers

Superconductor/ferromagnet bilayers are known to exhibit nontrivial dependence of the critical temperature T _c on the thickness d _f of the ferromagnetic layer. We develop a general method for investigation of T _c as a function of the bilayer parameters. It is shown that interference of quasiparticles makes T _c (d _f ) a nonmonotonic function. The results are in good agreement with experiment. Our method also applies to multilayered structures.