[en] The authors report a sharp, dramatic enhancement of the superconducting transition temperature (T_c) from 3.8 K to 9.1 K for the compound Lu₅Tr₄Si₁₀ under hydrostatic pressure. This increase in T_c, which occurs at a critical pressure of rho_c = 21-+ 1 kbar is caused by the suppression of an electronic phase transition which results in the partial gapping of the Fermi surface. Using electrical resistivity at ambient and high pressure and static magnetic susceptibility measurements, they estimate that the density of electronic states at the Fermi level is reduced by 36% due to this phase transition. The transition occurs at 79 K at ambient pressure, and has the experimental characteristics indicative of charge- or spin-density wave formation

[en] The search for a new superconductor in an early transition metal oxide prompted the investigation of the La_1-xSr_xTiO₃ series of compounds. The solid solution La_1-xSr_xTiO₃ (0 ≤ x ≤ 1) has been prepared by arc melting stoichiometric amounts of SrTiO₃ and LaTiO₃. Single-phase samples can be made for the entire stoichiometry range. The polycrystalline samples have been characterized using microprobe, thermogravimetric analysis, powder X-ray and neutron diffraction, temperature-dependent magnetization, and dc resistivity. The compounds crystallize in the orthorhombic space groups Pbnm (x ≤ 0.30) and Ibmm (0.30 < x < 0.80), and the cubic space group Pm3m (x ≥ 0.80). The Pbnm structure exists in two forms, O'-orthorhombic (x < 0.10) and O-orthorhombic (0.10 ≤ x ≤ 0.30), which are a result of Jahn-Teller distortion. The samples x = 0.00, 0.05 are canted antiferromagnets which exhibit a net ferromagnetic moment below 150 K and temperature-independent magnetism from 150 to 300 K. All other compositions show temperature-independent magnetism from 160 to 300 K with some samples (x ≥ 0.50) displaying paramagnetism in the low-temperature range. The temperature dependent resistivity measurements show that the samples are metallic (x ≤ 0.70) and semiconducting (x = 0.90). The x = 0.80 sample undergoes a semiconductor-metal transition. The properties of these materials are discussed in reference to the phenomenological phase diagram for perovskites proposed by Goodenough and high-T_c superconductivity

[en] In this study, the authors investigate the superconducting properties under hydrostatic pressure of three high T/sub c/ oxides with the composition (La/sub 1.85/M/sub 0.15/CuO/sub 4/, where M = Ca, Sr, Ba. These materials, which crystallize in the K/sub 2/NiF/sub 4/-type body-centered tetragonal structure (I4/mmm) or a related orthorhombic modification (Fmmm), have been the focus of intense experimental and theoretical research since the initial report of superconductivity above 30K in a multiphase sample of these ternary oxides. The effect of high pressure on T/sub c/ is an important factor in the investigation of the occurrence of superconductivity in ternary compounds. With few exceptions, pressure effects on the elements and binary compounds are nearly linear and small (usually vertical bar dT/sub c//dp vertical bar --10/sup -5/K/bar). In contrast, there are numerous ternary superconductors that exhibit pronounced non-linearities in T/sub c/ versus pressure with values of dT/sub c//dp an order of magnitude higher than normally observed. Reference is made to a large pressure sensitivity of T/sub c/ for some compounds with this structure

[en] Stoichiometric LiBC has been synthesized using both arc melting and sealed tantalum ampoule methods. Hole-doped Li_1-xBC (0< x≤0.37) compounds have been realized through vacuum deintercalation from LiBC. The hexagonal crystal lattice has remained largely intact, with only slight decreases in lattice parameters upon hole-doping. The samples are intrinsically diamagnetic and are semiconducting in the 2-300 K range studied. Increased hole-doping leads to a color darkening as well as a decrease in resistivity

[en] In this paper, the time-dependent magnetic relaxation is measured in a full field penetrated YBa₂Cu₃O_7-δ detwinned crystal for long times up to 5 x 10⁵ sec. The relaxation data exhibit a power-law time dependence over a wide range of temperatures and magnetic fields indicative of flux creep with logarithmic dependence of the effective creep activation barrier U(J). A theoretical model is employed to analyze the data and to determine the activation energy

[en] The dependence of the Curie temperature on hydrostatic pressure has been determined to 21 kbar for the ferromagnetic molecular magnet (Mn(OC₁₄H₂₉)₄(TCNE), where Mn^III-ions are situated in ferrimagnetic porphyrin chains separated by 31.2 A. In the present experiments both He-gas and piston-cylinder pressure systems are used to 6 and 21 kbar, respectively. Following an initial rapid decrease of the Curie temperature T_c with applied pressure from 20 to 15 K at 1.5 kbar, T_c abruptly begins to increase with pressure at an equally rapid rate, ultimately reaching nearly 37 K at 21 kbar. The change in the sign in dT_c/dP at 1.5 kbar is likely the result of a phase transition in the spin and/or lattice system. The possible role of dipolar interactions in determining the magnetic state in a system of parallel ferrimagnetic chains is discussed

[en] Polycrystalline samples with superconducting transition temperatures higher than 130 K have been synthesized in the system Tl_1-xHg_xBa₂Ca₂Cu₃O_8+δ with x = 0.0, 0.1, 0.3, 0.5, 0.7, and 0.9. The magnetization data of these samples as a function of applied magnetic field up to 5.5 T are measured in a SQUID magnetometer over a wide temperature range. These hysteresis curves are analyzed with the Bean critical-state model to estimate the intragrain critical current density, J_c. The results show that the values of J_c are comparable to the values of YBCO and the early reported pure Hg-1223 sample. The maximum value of J_c occurs in the (Tl_0.7Hg_0.3)-1223 sample with 5.9 x 10⁶ A/cm² at 10 K and 1.1 X 10⁴ A/cm² at 77 K in 1 T. Also, an exponential decay of J_c with temperature has been found in these Tl_1-xHg_x-1223 compounds, indicating the presence of weak links

[en] Low-temperature heat-capacity, electrical resistivity, and magnetization measurements are performed to investigate the superconducting and magnetic behavior of ErBa₂Cu₃O/sub 7-//sub x/. This compound exhibits a superconducting transition at 84 K as determined from zero resistance and Meissner effect. A well-defined magnetic transition is seen at 0.87 K from heat-capacity measurements. We deduce the coexistence of superconductivity and antiferromagnetic order in this compound; however, magnetization data show that the paramagnetism from Er/sup 3+/ and superconductivity are independent, leading to indirect evidence of extremely anisotropic superconductivity in this class of compounds

[en] We have performed field- and temperature-dependent magnetization, resistivity, and heat-capacity measurements on polycrystalline samples of the Y(Ni_2-xCo_x)B₂C system with 0.0≤x≤0.4. Values of T_c,χ₀,H_c2,Θ_D, λ, and N(E_d) were determined for various samples. We observe that Θ_D increases with x, while all the other parameters decrease with x. The T_c vs x data can be described using the BCS theory and the measured values of N(E_f) and Θ_D. The results suggest that the decrease in T_c is due to the decrease in N(E_f), in agreement with the results from band structure calculations. copyright 1996 The American Physical Society

[en] The hysteresis loops and magnetic relaxation in (Bi,Pb)₂Sr₂CaCu₂O₈ crystals were measured at fields up to 5.5 T. The relaxation rate dM/d lnt exhibits a strong field dependence in the temperature range 6--20 K. By expanding Bean's model of the critical current density to include the field dependence of the flux-creep effect, we developed expressions for both M and dM/d lnt as a function of the field. These expressions were used to fit our experimental data of field-dependent magnetization and relaxation rate