James S. Schilling

Superconducting Phase Diagram of Li Metal in Nearly Hydrostatic Pressures up to 67 GPa

The dependence of the superconducting transition temperature T(c) on nearly hydrostatic pressure has been determined to 67 GPa in an ac susceptibility measurement for a Li sample embedded in helium pressure medium. With increasing pressure, superconductivity appears at 5.47 K for 20.3 GPa, T(c) rising rapidly to approximately 14 K at 30 GPa. The T(c)(P) dependence to 67 GPa differs significantly from that observed in previous studies where no pressure medium was used. Evidence is given that superconductivity in Li competes with symmetry breaking structural phase transitions which occur near 20, 30, and 62 GPa. In the pressure range 20-30 GPa, T(c) is found to decrease rapidly in a dc magnetic field, the first evidence that Li is a type I superconductor.

Pressure as a parameter in the study of dilute magnetic alloys

Abstract A large body of data is reviewed, which illustrates how the high pressure technique can be used to gain information about the magnetic and electronic state of dilute magnetic alloys. Values for the pressure dependence of the elementary effective exchange interaction between a magnetic impurity spin and the conduction electron sea are extracted from the data and tabulated for a number of extremely dilute alloys containing both transition metal and rare earth impurities. Results of experiments on host alloy series employing ‘lattice pressure’ are compared to the present ‘external pressure’ results and critically analysed; it is shown how such a comparison can be used to give information about the extent of the electronic screening around an impurity potential. The effect of pressure on impurity-impurity interactions in spin glasses is also examined; a comparison of the pressure studies on extremely dilute alloys to those on spin glasses allows an estimate of the nature of the interaction mechanisms...

Marked anomalies in the pressure dependence of the superconducting transition temperature in Tl2Ba2CuO6+y as a function of oxygen content

Abstract We report the results of experiments of the effect of hydrostatic pressure to 6 kbar on the superconducting transition temperature T c of a Tl 2 Ba 2 CuO 6+ y sinter for oxygen content y in the range 0.10≤ y ≤0.21. superconductivity is detected by an AC inductance technique and pure hydrostatic pressure is generated with a helium gas system. For experiments in which the pressure is changed at room temperature, T c falls rapidly under pressure; when the pressure is changed at low temperature, T c remains almost constant. A subsequent experiment on a YBa 2 Cu 3 O 7 single crystal revealed no such irreversible behavior.

The use of high pressure in basic and materials science

There are at least three important applications of the high pressure technique in basic and materials science: (1) to synthesize novel materials not accessible by other techniques, (2) to transform a given material from one phase to another, and (3) to uncover underlying systematics and critically test theoretical concepts. High pressure studies currently enjoy an increasing popularity which is fueled by advances in the notably difficult experimental techniques. In this paper I will attempt to capture some of the current excitement in this field by offering brief synopses of selected experiments. It is noted that the use of high pressures in materials synthesis is clearly underutilized.

The dependence of T{sub c} on hydrostatic pressure in superconducting MgB{sub 2}.

The dependence of Tc on hydrostatic (He-gas) pressure for superconducting MgB2 has been determined to 0.7 GPa. We find that Tc decreases linearly and reversibly under pressure at the rate dTc/dP ≃ −1.11 ± 0.02 K/GPa. These studies were carried out on the same sample used in earlier structural studies under He-gas pressure which yielded the bulk modulus B = 147.2 ±0.7 GPa. The value of the logarithmic volume derivative of Tc is thus accurately determined, dln Tc/dln V = +4.16 ± 0.08, allowing quantitative comparison with theory. The present results support the emerging picture that MgB2 is a BCS superconductor with electron-phonon pairing interaction.

The effect of alloying on Tc in superconducting YNi2−xMxB2C (M = Co, Cu)

Abstract In order to probe the influence of the density of states (DOS) at the Fermi level N ( E F ) on the superconducting transition temperature T c , we investigated the effect of alloying on T c in YNi 2− x M x B 2 C (M = Co, Cu). The lattice contracts (expands) as a result of substitution of Ni with Co (Cu). From the observed drop in T c for both substitutions, we infer that N ( E F ) may lie very close to a maximum in the DOS in the parent compound YNi 2 B 2 C, in agreement with published band-structure calculations. However, the much steeper drop in T c for Co substitution (d T c /d x = −45 K compared to −19 K for Cu) indicates that either the peak in the DOS is asymmetric in energy, or alloying affects both N ( E F ) and the electron-phonon coupling strength.

Influence of pressure on the ferromagnetic transition temperature of SrRuO3

Abstract Measurements of electrical resistivity and AC magnetic susceptibility on the ferromagnetic oxide SrRuO3 under hydrostatic pressure in the range 0⩽P⩽6.1 GPa yield the pressure derivative of the Curie temperature ∂Tc/∂P = -5.7 ± 0.2 K/GPa where T c (P = 0) ⋍165 K . This result is in qualitative agreement with the predictions of the Wohlfarth model. Our pressure results and the DC magnetization studies herein suggest the classification of this material as a moderately weak itinerant ferromagnet.

Dependence of the superconducting transition temperature of HgBa2CuO4+δ on hydrostatic pressure

Abstract The real χ′ and imaginary χ″ components of the AC susceptibility for a ceramic sample of superconducting HgBa2CuO4+δ have been measured at HAC=0.1 Oe over the temperature range 80–300 K under purely hydrostatic pressure conditions. Upon cooling, a slight but sharp decrease in slope dχ′/dT occurs at the “upper” transition temperature, T u c ⋍98 K , which we attribute to the onset of superconductivity in a small volume (≁3%) of the sample which is nearly optimally doped. Below the “lower” transition temperature, T 1 c ⋍95 K , χ′(T) drops steeply, approaching for T d T c / d P⋍+1.75±0.1 K/GPa which is independent of whether the pressure is changed at room temperature or 80 K. These experiments allow us to speculate that the superconducting transition for optimally doped HgBa2CuO4+δ at ambient pressure probably lies near 98 K.

Pressure dependence of the superconducting transition temperature in Bi2Sr2CaCu2O8+y as a function of oxygen content

Abstract In this paper we report the results of hydrostatic pressure experiments up to 6 kbar on the superconducting transition temperature T c of sintered Bi 2 Sr 2 CaCu 2 O 8+ y as a function of the oxygen content. The transition to superconductivity is detected by an AC inductance technique and pure hydrostatic pressure is generated with a helium gas system. Over the range of oxygen content studied (0.11 ≤ y ≤ 0.24), T c ( y ) passes through a maximum at y ≅0.15. The pressure derivative (d T c /d p ≅+0.15 K/k bar), however, is positive, nearly independent of the oxygen concentration, and, unlike for Tl 2 Ba 2 CuO 6+ y , not dependent on the temperature at which the pressure is changed.

High-pressure susceptibility studies on the ferromagnetic uranium monochalcogenides US, USe and UTe

Abstract The dependence of the Curie temperature T c of the ferromagnetic compounds UX (X  S, Se or Te) on hydrostatic pressure to 13 GPa has been determined using a diamond-anvil cell loaded with dense helium as pressure medium. A sensitive primary/secondary coil system allows the detection of the ferromagnetic transition in the ac susceptibility for tiny samples with mass ≤1 μ g. The Curie temperatures of USe (170 K) and UTe (102 K) initially increase with applied pressure at the rates +2.7 K/GPa and +13.3 K/GPa, passing through maxima at pressures of approximately 6 GPa and 7 GPa, respectively. T c for US (173 K) decreases monotonically under pressure at the rate −3.9 K/GPa. For all three chalcogenides the pressure dependence of T c is clearly influenced by structural phase transitions. The present results are compared to those from earlier resistivity studies and discussed within the framework of both a phase diagram proposed by Doniach for local-moment Kondo-lattice systems and pressure-dependent itinerant electron magnetism.