LaAg under hydrostatic pressure: Superconductivity and phase transformation

The electrical resistivity of LaAg has been measured from 1–300 K at hydrostatic pressures to 12.4 kbar and the superconducting transition temperature T_c determined inductively to 23 kbar. For P ? 6.2 kbar a lattice transformation is observed at a temperature T_M which increases rapidly with pressure. T_c shows an oscillatory pressure dependence, increasing initially from T_c (0) = 1.062 K. There is no obvious correlation between the pressure dependence of T_M and T_c.     

Pressure-induced superconductivity in tetragonal NdBa2Cu3O6+x

The appearance of superconductivity and relaxation of the transition temperature to its equilibrium value T _c≈30 K over the course of five days have been observed in a tetragonal sample of NdBa₂Cu₃O_6.67 under 1 GPa pressure. The time dependence T _c(t) correlates with a decrease of the room-temperature electrical resistance R(t). The superconducting phase disappears 1.3 h after the pressure is removed. This behavior is explained by a redistribution of charge as a result of pressure-induced oxygen reordering in the CuO_x planes. A large effect of oxygen ordering on the transition temperature under pressure has been observed near the metal-insulator transition (the largest of all those measured in the 1-2-3 system), along with a nonlinear temperature dependence of T _c. Fiz. Tverd. Tela (St. Petersburg) 39, 1328–1334 (August 1997)     

Effect of strain-induced electronic topological transitions on the superconducting properties of La 2 - x Sr x CuO 4 thin films

We propose a Ginzburg-Landau phenomenological model for the dependence of the critical temperature on microscopic strain in tetragonal high-T _c cuprates. Such a model is in agreement with the experimental results for LSCO under epitaxial strain, as well as with the hydrostatic pressure dependence of T _c in most cuprates. In particular, a nonmonotonic dependence of T _c on hydrostatic pressure, as well as on in-plane or apical microstrain, is derived. From a microscopic point of view, such results can be understood as due to the proximity to an electronic topological transition (ETT). In the case of LSCO, we argue that such an ETT can be driven by a strain-induced modification of the band structure, at constant hole content, at variance with a doping-induced ETT, as is usually assumed. Received 1st October 2001 and Received in final form 5 December 2001     

An NMR Analysis of Transitions in FexNbxV2-2xO4

The first order semiconductor - metall phase transition with the lattice symmetry change from monoclinic to tetragonal and the conductivity jump of several orders of magnitude is observed in pure vanadium dioxide and the dioxide doped by varios impurities [I]. Transition temperature T_t for pure VO₂ is 68°C and depends on the impurity. For example, the impurities which enter the insulating phase as pentavalent ions Nb⁵⁺ [2] give rise to a decrease in metal-semiconductor transition temperature at low concentrations and the trivalent ions Cr³⁺, Fe³⁺ [3,4] lead to an increase in T_t. In the latter case three different monoclinic phases are stabilized at a temperature below T_t. It has been already shown [5] that for double doping of vanadium dioxide by pentavalent Nb⁵⁺ and trivalent Fe³⁺ or Cr³⁺ ions three semiconductor phases exist (M_I, M₂, M₃) but the transition temperature decreases as the impurity concentration increases.     

Ultrasonic attenuation anomalies at the two transitions in antiferromagnetic FeGe2

The attenuation of longitudinal and shear sound waves is measured through the Néel temperature, T_N≈286 K, and the lower transition temperature, T_K≈265 K, of FeGe₂. Longitudinal sound with wavevector q along the [100] axis of this tetragonal antiferromagnetic metal shows an attenuation peak at T_N, which is reversibly suppressed by compressive uniaxial stress σ along [010]. The estimated pressure dependence of T_N is dT_N/dp=(?2.8±0.3) mK bar^?1. The peak at T_N shows thermal and stress hysteresis, which suggests that it is associated with domain wall motion and that this transition is first order.     

Pressure-induced structural phase transition in ReO3

We have investigated the pressure-induced structural phase transition in ReO₃ by neutron diffraction on a single crystal. We collected neutron diffraction intensities from the ambient and high pressure phases at P=7 kbar and refined the crystal structures. We have determined the stability of the high pressure phase as a function temperature down to T=2 K and have constructed the (P-T) phase diagram. The critical pressure is P_c=5.2 kbar at T=300 K and decreases almost linearly with decreasing temperature to become P_c=2.5 kbar at T=50 K. The phase transition is driven by the softening of the M₃ phonon mode. The high pressure phase is formed by the rigid rotation of almost undistorted ReO₆ octahedra and the Re-O-Re angle deviates from 180°. We do not see any evidence for the existence of the tetragonal (P4/mbm) intermediate pressure phase reported earlier.     

Structural,magnetic and superconducting phase transitions in CaFe2As2 under ambient and applied pressure

At ambient pressure CaFe₂As₂ has been found to undergo a first order phase transition from a high temperature, tetragonal phase to a low-temperature orthorhombic/antiferromagnetic phase upon cooling through T ～ 170 K. With the application of pressure this phase transition is rapidly suppressed and by ～0.35 GPa it is replaced by a first order phase transition to a low-temperature collapsed tetragonal, non-magnetic phase. Further application of pressure leads to an increase of the tetragonal to collapsed tetragonal phase transition temperature, with it crossing room temperature by ～1.7 GPa. Given the exceptionally large and anisotropic change in unit cell dimensions associated with the collapsed tetragonal phase, the state of the pressure medium (liquid or solid) at the transition temperature has profound effects on the low-temperature state of the sample. For He-gas cells the pressure is as close to hydrostatic as possible and the transitions are sharp and the sample appears to be single phase at low temperatures. For liquid media cells at temperatures below media freezing, the CaFe₂As₂ transforms when it is encased by a frozen media and enters into a low-temperature multi-crystallographic-phase state, leading to what appears to be a strain stabilized superconducting state at low temperatures.     

Ferroelectric phase transitions of the 0.32PIN-0.345PMN-0.335PT single crystals studied by temperature-dependent Raman spectroscopy,dielectric and ferroelectric performance

The ferroelectric phase transition characteristics of the 0.32Pb(In_1/2Nb_1/2)O₃-0.345Pb(Mg_1/3Nb_2/3)O₃-0.335PbTiO₃ (0.32PIN-0.345PMN-0.335PT) single crystals were studied by the temperature-dependent Raman spectroscopy and some electrical properties. Ferroelectric monoclinic phase was confirmed at room temperature by the numbers of the Raman modes. Successive ferroelectric phase transitions, i.e. ferroelectric monoclinic phase to ferroelectric tetragonal phase transition (FE_M-FE_T) and ferroelectric tetragonal phase to paraelectric cubic phase transition (FE_T-P_C), are evidenced by the anomalies of Raman modes line width, peaks intensity and their ratios around T_M-T and T_C/T_m temperatures. The temperature dependent permittivity derivative ξ = d?/dT not only provides further evidence of the successive ferroelectric phase transitions, but also demonstrates the second-order transition characteristic of the FE_M-FE_T phase transition and the first-order transition feature of the FE_T-P_C phase transition. The FE_M-FE_T phase transition is also confirmed by the abnormal narrowing of the P-E loops, decrease of the P_r and E_c values, and extremums of the pyroelectric performance.     

Electrical properties and Tc of CaLaBaCu3Ox at pressures up to 9 GPa

Abstract

By means of a four-point resistivity method the critical temperature (T_c) of the tetragonal high temperature super-conductor CaLaBaCu₃O_x was studied for pressures up to 9 GPa. The pressure dependence of T_c is small and negative, dT_c/dp = - 0.77 K/GPa, and agrees with the general trend observed in previous data.     

Pressure dependence of shear elastic mode near structural transition in the band Jahn-teller model

The temperature dependence of the pressure derivative of the shear elastic constant $\text{dC}{}_{\mathrm{<mtext>S</mtext>}}\text{dp}$ for degenerate electronic system is worked out in the band Jahn Teller model and is shown to follow C_S linearly as the temperature decreases and approaches the transition temperature T_M. The constant of proportionality gives useful informations on the characteristics of the band Jahn-Teller transition. This analysis can explain well the recently observed experimental results on Nb₃Sn.     

Enhancement of the magnetic field dependence of the martensitic transition temperature in a degenerate electronic band by Coulomb and exchange interactions

An expression for the martensitic transition temperature T_M for electrons in a two-fold, degenerate band has been derived incorporating the effect of the intra-atomic Coulomb and exchange interactions in the Hartree—Fock approximation in the presence of an external magnetic field. As a result the field dependence of T_M can be considerably enhanced which has been found experimentally in the case of La₃S₄. It is also shown that for certain form of the density of states the onset of ferromagnetism can completely suppress the structural transition.     

The influence of domains on the structural phase transition of RbCaF3

The temperature dependence of the tetragonal rotational displacement below the phase transition of RbCaF₃ is described by (T₀ - T)^β with T₀ = 199 K and β = 0.25 measured with single crystal X-ray diffraction. A method was developed to study the variability of the domains as a function of the temperature. The influence of the domains on the critical behaviour of the order parameter is pointed out.     

The glass transition temperature of amorphous poly(ethylene terephthalate) by thermally stimulated currents

The relation between the temperature T_α of the dipolar relaxation, obtained by the technique of thermally stimulated currents (TSC) and the glass transition temperature T_g has been studied in amorphous poly(ethylene terephthalate) samples. The temperature T_α depends fundamentally on the polarization temperature T_p, the polarization time t_p, and the heating rate v. For each heating rate a maximum T_α, T_M, was obtained for an optimum polarization temperature T_po. The value of T_po is 70°C, independent of the heating rate, and very close to the glass transition temperature obtained by differential scanning calorimetry (69°C). The resulting value for T_M coincides with T_po in the limits of null heating rate and null isothermal polarization time, and, consequently, T_M gives the value of the glass transition temperature for each heating rate as a function of the isothermal dipolar contribution on polarizing at the temperature T_po.     

The magnetic field dependence of the structural transition temperature in La3S4 and La3Se4

The magnetic field dependence of the structural transition temperature T_m from the cubic to the tetragonal phase has been determined for single crystals of La₃S₄ and La₃Se₄. The observed field dependence of T_m can be accounted for by the band Jahn-Teller model of the coupling of an e_g-band to the shear mode of the cubic lattice without invoking any coupling to acoustic or optical phonons.     

Structural and magnetic phase transitions in Pr<Subscript>0.15</Subscript>Sr<Subscript>0.85</Subscript>MnO<Subscript>3</Subscript> at high pressure

The crystal and magnetic structures of Pr_0.15Sr_0.85MnO₃ manganite have been studied by means of powder X-ray and neutron diffraction in the temperature range 10–400 K at high external pressures up to 55 and 4 GPa, respectively. A structural phase transition from cubic to tetragonal phase upon compression was observed, with large positive pressure coefficient of transition temperature dT _ct /dP = 28(2) K/GPa. The C-type antiferromagnetic (AFM) ground state is formed below T _N ≈ 260 K at ambient pressure. While at ambient pressure the structural and magnetic transition temperatures are close, T _ct ~ T _N , upon compression they become decoupled with T _N ≪ T _ct due to much weaker T _N pressure dependence with coefficient dT _N /dP = 3.8(1) K/GPa.     

Study of magnetotransport in double-layered La1.4Ca1.6Mn2O7 manganite: Presence of nano-ferromagnetic domains in paramagnetic matrix

Double-layered manganite La_1.4Ca_1.6Mn₂O₇ has been synthesized using the solid-state reaction method. It had a metal-to-insulator transition at temperature T_M1≈127 K. The temperature dependence of ac susceptibility showed a broad ferromagnetic transition. The two-dimensional (2D)-ferromagnetic ordering temperature (T_C2) was observed as ≈245 K. The temperature dependence of its low-field magnetoresistance has been studied. The low-field magnetoresistance of double-layered manganite, in the temperature regions between T_M1 and T_C2, has been found to follow 1/T⁵. The observed behaviour of temperature dependence of resistivity and low-field magnetoresistance has been explained in terms of two-phase model where ferromagnetic domains exist in the matrix of paramagnetic regions in which spin-dependent tunneling of charge carriers occurs between the ferromagnetic correlated regions. Based on the two-phase model, the dimension of these ferromagnetic domains inside the paramagnetic matrix has been estimated as ∼12 Å.     

Metal-induced effects on the glass transition kinetics of glassy Se70Te30 alloy

The calorimetric measurements have been made in glassy Se₇₀Te₃₀ and Se₇₀Te₂₈M₂ (M?=?Ag, Cd, and Zn) alloys using non-isothermal differential scanning calorimetry technique to see the effects of Ag, Cd, and Zn additives on the glass transition kinetics of binary Se₇₀Te₃₀. From the heating rate dependence of glass transition temperature, T _g, different kinetic parameters of glass transition have been evaluated. The composition dependence of glass transition temperature T _g and the related activation energy (E_t ) is also discussed.     

Magnetic properties of the compounds N(CH3)4MnIIMIII(CN)6 ·8 H2O(MIII = Mn,Fe)

In the isostructural cyanobridged chain compounds N(CH₃)₄Mn^IIM^III(CN)₆ · 8H₂O high spin Mn(II) ions couple antiferromagnetically to low spin Mn(III) of Fe(III) ions. The Mn^II–Mn^III compound orders ferrimagnetically below T_N = 28.5 ± 1 K. The tetragonal a and b axes are easy ones for the magnetic moments. In the Mn^II–Fe^III compound antiferromagnetic order occurs below T_N = 9.3 K, with spins aligned along the tetragonal c axis. The compound undergoes a meta-magnetic transition from the antiferromagnetic to a ferrimagnetic phase. This occurs at 2 K for a field H_crit ≈ 1.2 T. The temperature dependence of H_crit, which vanishes at T_N, is followed. The tricritical temperature T¹ is ～ 5 K.     

Persistence of tetragonal raman lines in cubic KNbO3

The temperature dependence of the vibrationnal modes in KNbO₃ around the cubic-tetragonal phase transition is studied by Raman scattering measurements. An unexpected intense spectrum is observed above the transition. In addition to broad bands probably due to second order process, it consists of forbidden lines which are characteristic of the tetragonal phase. In particular the presence of the hard component of the soft ferroelectric cubic mode until 30° above T_c can be attributed to the existence of a precursor order near the transition.     

Observation of magnetization step at order–disorder transition in MgB2 single crystals

We have studied the order–disorder transition in high quality MgB₂ single crystals, using a torque magnetometry combined with a ‘vortex shaking’ technique. In the wide range of temperature T, field H and the H direction, we succeed in obtaining reversible magnetization curves M_rev(T, H) by shaking the pinned vortices. Especially at low temperatures below 25 K and high fields, where the irreversible magnetization curve exhibits the peak effect due to the order–disorder transition, it is found that the peak is transformed into the clear step in M_rev(H). Similar step-like behavior is also observed in the temperature dependence of magnetization M_rev(T). These results give direct evidence that the order–disorder transition, which is hidden by the large hysteresis of magnetization, has the nature of first-order transition.