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The application of giga-Pascal scale pressures has been widely used as a tool to systematically tune the properties of materials in order to access such general questions as the driving mechanisms underlying phase transitions. While there is a large and growing set of experimental tools successfully applied to high-pressure environments, the compatibility between diamond anvil cells and optical probes offers further potential for examining lattice, magnetic, and electronic states, along with their excitations. Here, we describe the construction of a highly efficient optical Raman spectrometer that enables measurements of magnetic excitations in single crystals down to energies of 9 cm−1 (1.1 meV or 13 K) at cryogenic temperatures and under pressures of tens of GPa. [ABSTRACT FROM AUTHOR]

Presents a study which performed magnetic susceptibility, calorimetric and neutron scattering analyses of the randomly diluted dipolar-coupled Ising magnet LiHo[subx]Y[sub1-x]F[sub4]. Presence of dipolar interactions in paramagnets and paraelectrics; Characteristics of LiHo[subx]Y[sub1-x]F[sub4]; Experimental methods used; Discussion on quantum mechanics.

We explore the behavior of the nested bandstructure of chromium as a function of temperature and pressure to the point where magnetism disappears. X-ray diffraction measurements of the charge order parameter suggest that the nesting condition is maintained at high pressure, where the spin density wave ground state is destabilized by a continuous quantum phase transition. By comparing diffraction line-shapes measured throughout the temperature-pressure phase diagram we are able to identify and describe three regimes: thermal near-critical, weak coupling ground state, and quantum critical. [ABSTRACT FROM AUTHOR]

We discuss techniques for performing continuous measurements across a wide range of pressure-field-temperature phase space, combining the milli-Kelvin temperatures of a helium dilution refrigerator with the giga-Pascal pressures of a diamond anvil cell and the Tesla magnetic fields of a superconducting magnet. With a view towards minimizing remnant magnetic fields and background magnetic susceptibility, we characterize high-strength superalloy materials for the pressure cell assembly, which allows high fidelity measurements of low-field phenomena such as superconductivity below 100 mK at pressures above 10 GPa. In situ tunability and measurement of the pressure permit experiments over a wide range of pressure, while at the same time making possible precise steps across abrupt phase transitions such as those from insulator to metal. [ABSTRACT FROM AUTHOR]

We present the design of an efficient bellows-controlled diamond anvil cell that is optimized for use inside the bores of high-field superconducting magnets in helium-3 cryostats, dilution refrigerators, and commercial physical property measurement systems. Design of this non-magnetic pressure cell focuses on in situ pressure tuning and measurement by means of a helium-filled bellows actuator and fiber-coupled ruby fluorescence spectroscopy, respectively. We demonstrate the utility of this pressure cell with ac susceptibility measurements of superconducting, ferromagnetic, and antiferromagnetic phase transitions to pressures exceeding 8 GPa. This cell provides an opportunity to probe charge and magnetic order continuously and with high resolution in the three-dimensional Magnetic Field-Pressure-Temperature parameter space. [ABSTRACT FROM AUTHOR]

We describe a technique for making electrical transport measurements in a diamond anvil cell using an alcohol pressure medium, permitting acute sensitivity while preserving sample fidelity. The sample is suspended in the liquid medium by four gold leads that are electrically isolated by a composite gasket made of stainless steel and an alumina-loaded epoxy. We demonstrate the technique with four-probe resistivity measurements of chromium single crystals at temperatures down to 4 K and pressures above 10 GPa. Our assembly is optimized for making high precision measurements of the magnetic phase diagram and quantum critical regime of chromium, which require repeated temperature sweeps and fine pressure steps while maintaining high sample quality. The high sample quality enabled by the quasi-hydrostatic pressure medium is evidenced by the residual resistivity below 0.1 μΩ cm and the relative resistivity ratio ρ(120 K)/ρ(5 K) = 15.9 at 11.4 GPa. By studying the quality of Cr's antiferromagnetic transition over a range of pressures, we show that the pressure inhomogeneity experienced by the sample is always below 5%. Finally, we solve for the Debye temperature of Cr up to 11.4 GPa using the Bloch-Gruneisen formula and find it to be independent of pressure. [ABSTRACT FROM AUTHOR]

Condensed matter experiments at high pressure accentuate the need for accurate pressure scales over a broad range of temperatures, as well as placing a premium on a homogeneous pressure environment. However, challenges remain in diamond anvil cell technology, including both the quality of various pressure transmitting media and the accuracy of secondary pressure scales at low temperature. We directly calibrate the ruby fluorescence R1 line shift with pressure at T=4.5 K using high-resolution x-ray powder diffraction measurements of the silver lattice constant and its known equation of state up to P=16 GPa. Our results reveal a ruby pressure scale at low temperatures that differs by 6% from the best available ruby scale at room T. We also use ruby fluorescence to characterize the pressure inhomogeneity and anisotropy in two representative and commonly used pressure media, helium and methanol:ethanol 4:1, under the same preparation conditions for pressures up to 20 GPa at T=5 K. Contrary to the accepted wisdom, both media show equal levels of pressure inhomogeneity measured over the same area, with a consistent ΔP/P per unit area of ±1.8 %/(104 μm2) from 0 to 20 GPa. The helium medium shows an essentially constant deviatoric stress of 0.021±0.011 GPa up to 16 GPa, while the methanol:ethanol mixture shows a similar level of anisotropy up to 10 GPa, above which the anisotropy increases. The quality of both pressure media is further examined under the more stringent requirements of single crystal x-ray diffraction at cryogenic temperature. For such experiments we conclude that the ratio of sample-to-pressure chamber volume is a critical parameter in maintaining sample quality at high pressure, and may affect the choice of pressure medium. [ABSTRACT FROM AUTHOR]

Pressure tuning of phase transitions is a powerful tool in condensed matter physics, permitting high-resolution studies while preserving fundamental symmetries. At the highest pressures, energy dispersive x-ray diffraction (EDXD) has been a critical method for geometrically confined diamond anvil cell experiments. We develop a chemical filter technique complementary to EDXD that permits the study of satellite peaks as weak as 10-4 of the crystal Bragg diffraction. In particular, we map out the temperature dependence of the incommensurate charge density wave diffraction from single-crystal, elemental chromium. This technique provides the potential for future GPa pressure studies of many-body effects in a broad range of solid state systems. [ABSTRACT FROM AUTHOR]

A gas loading cell has been developed to load rare earth thin film samples with hydrogen at pressures up to 200 bars at room temperature. A miniature valve closes the gas inlet, after which the cell is suspended from the cold tail of a [sup 3]He flow cryostat into the bore of a 16 T superconducting magnet. An ultraviolet stroboscope outside the cryostat illuminates the sample by way of an optical fiber to a window in the cell. Electrical feedthroughs permit photoconductivity and magnetotransport measurements over three decades in temperature. Extension to other materials, different gas atmospheres, and helium dilution refrigerator temperatures is straightforward. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have probed the temperature and magnetic-field dependence of the thermopower and resistance of a p-type silver chalcogenide, Ag[sub 2-δ]Te. The application of a magnetic field causes not only a large magnetoresistance but also a giant magnetothermopower effect. The maximum change of thermopower is as high as 470 µV/K in a 7 T magnetic field. Both the magnetoresistance and the magnetothermopower show a pronounced peak and nearly linear behavior near the sign change of the thermopower. Bandcrossing and quantum confinement due to disorder appear to play key roles in the heightened response to field. [ABSTRACT FROM AUTHOR]

Designing an apparatus for experiments that operate deep into the milliKelvin regime requires a careful selection of materials and construction techniques, and often entails balancing mutually contradictory requirements. In the case of magnetic susceptibility measurements, this involves choosing materials that realize both high thermal conductivity for good heat sinking and low electrical conductivity for minimal eddy-current heating. A coil mounting system with a completely nonmetallic construction, achieved through the use of a sapphire skeleton and a machined carbon-fiber body, resolves this dilemma. This system permits the use of large ac magnetic field amplitudes at kHz frequencies and above, opening the door to studies of the nonlinear dynamics of a wide variety of magnetic systems that were previously inaccessible. [ABSTRACT FROM AUTHOR]

We compare the large magnetoresistive response of slightly nonstoichiometric Ag[sub 2±δ]Te for a wide range of hole (p<=8x10[sup 17] cm[sup -3]) and electron (n≤4x10[sup 18] cm[sup -3]) carrier densities. In the p-type material alone, a characteristic peak in the resistivity ρ(T,H) is dramatically enhanced and moves to higher temperature with increasing magnetic field, resulting in a high field (H∼5 T) magnetoresistance that is sizeable even at room temperature. By contrast, n-type specimens are geared for low-field (H<0.1 T) applications because of a striking linear field dependence of the magnetoresistance that appears to be restricted to the Ag-rich materials. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The application of giga-Pascal scale pressures has been widely used as a tool to systematically tune the properties of materials in order to access such general questions as the driving mechanisms underlying phase transitions. While there is a large and growing set of experimental tools successfully applied to high-pressure environments, the compatibility between diamond anvil cells and optical probes offers further potential for examining lattice, magnetic, and electronic states, along with their excitations. Here, we describe the construction of a highly efficient optical Raman spectrometer that enables measurements of magnetic excitations in single crystals down to energies of 9 cm -1 (1.1 meV or 13 K) at cryogenic temperatures and under pressures of tens of GPa.

Condensed matter experiments at high pressure accentuate the need for accurate pressure scales over a broad range of temperatures, as well as placing a premium on a homogeneous pressure environment. However, challenges remain in diamond anvil cell technology, including both the quality of various pressure transmitting media and the accuracy of secondary pressure scales at low temperature. We directly calibrate the ruby fluorescence R1 line shift with pressure at T=4.5 K using high-resolution x-ray powder diffraction measurements of the silver lattice constant and its known equation of state up to P=16 GPa. Our results reveal a ruby pressure scale at low temperatures that differs by 6% from the best available ruby scale at room T. We also use ruby fluorescence to characterize the pressure inhomogeneity and anisotropy in two representative and commonly used pressure media, helium and methanol:ethanol 4:1, under the same preparation conditions for pressures up to 20 GPa at T=5 K. Contrary to the accepted wisdom, both media show equal levels of pressure inhomogeneity measured over the same area, with a consistent DeltaP/P per unit area of +/-1.8 %/(10(4) microm(2)) from 0 to 20 GPa. The helium medium shows an essentially constant deviatoric stress of 0.021+/-0.011 GPa up to 16 GPa, while the methanol:ethanol mixture shows a similar level of anisotropy up to 10 GPa, above which the anisotropy increases. The quality of both pressure media is further examined under the more stringent requirements of single crystal x-ray diffraction at cryogenic temperature. For such experiments we conclude that the ratio of sample-to-pressure chamber volume is a critical parameter in maintaining sample quality at high pressure, and may affect the choice of pressure medium.