Jason Baker

Structural stability of WS2 under high pressure

Structural behavior of bulk WS2 under high pressure was investigated using synchrotron X-ray diffraction and diamond anvil cell up to 52 GPa along with high temperature X-ray diffraction and high pressure Raman spectroscopy analysis. The high pressure results obtained from X-ray diffraction and Raman analysis did not show any pressure induced structural phase transformations up to 52 GPa. The high temperature results show that the WS2 crystal structure is stable upon heating up to 600°C. Furthermore, the powder X-ray diffraction obtained on shock subjected WS2 to high pressures up to 10 GPa also did not reveal any structural changes. Our results suggest that even though WS2 is less compressible than the isostructural MoS2, its crystal structure is stable under static and dynamic compressions up to the experimental limit.

Pressure induced high spin-low spin transition in FeSe superconductor studied by x-ray emission spectroscopy and ab initio calculations

FeSe is a simple binary system in the iron based superconducting family and exhibits a significant pressure induced increase in the superconducting transition temperature (Tc). In addition to pressure effect, spin fluctuations, magnetic ordering, and crystal structure all play vital roles in altering Tc. Even though various experiments and theoretical simulations explain the connection among them and superconductivity, the interplay between these important parameters is still not clearly understood. Here, we report the pressure effect on the spin state of Fe in FeSe superconductor studied using synchrotron x-ray emission spectroscopy at ambient and low temperatures down to 8 K near Tc. Pressure induced high spin to low spin transition was observed at both ambient and low temperatures with continuous suppression of Fe magnetic moments under increasing pressure. The spin transition is closely related to the pressure induced tetragonal to orthorhombic structural transition.

Pressure-induced superconductivity in LaFeAsO: The role of anionic height and magnetic ordering

We have investigated the crystal structure and magnetic ordering of LaFeAsO at low temperature (∼10 K) and high pressures. The long range antiferromagnetic ordering is suppressed under pressure, and the structural parameters obtained show a close correlation between the anionic height (ha) and the transition temperature (Tc). An orthorhombic to tetragonal transition is observed above 30 GPa. Density functional theory calculations show that the shape of the hole surface becomes two dimensional under pressure. Our results demonstrate that the variation of ha and dimensionality play important roles in the evolution of pressure induced superconductivity in addition to magnetic ordering.

High-pressure Seebeck coefficients and thermoelectric behaviors of Bi and PbTe measured using a Paris-Edinburgh cell

A new sample cell assembly design for the Paris-Edinburgh type large-volume press for simultaneous measurements of X-ray diffraction, electrical resistance, Seebeck coefficient and relative changes in the thermal conductance at high pressures has been developed. The feasibility of performing in situ measurements of the Seebeck coefficient and thermal measurements is demonstrated by observing well known solid-solid phase transitions of bismuth (Bi) up to 3 GPa and 450 K. A reversible polarity flip has been observed in the Seebeck coefficient across the Bi-I to Bi-II phase boundary. Also, successful Seebeck coefficient measurements have been performed for the classical high-temperature thermoelectric material PbTe under high pressure and temperature conditions. In addition, the relative change in the thermal conductivity was measured and a relative change in ZT, the dimensionless figure of merit, is described. This new capability enables pressure-induced structural changes to be directly correlated to electrical and thermal properties.

In situ x-ray diffraction, electrical resistivity and thermal measurements using a Paris-Edinburgh cell at HPCAT 16BM-B beamline

We have established a new type of experimental set-up utilizing a Paris-Edinburgh (PE) type large volume press with a dedicated sample cell assembly for simultaneous x-ray diffraction, electrical resistance, and temperature gradient measurements at the High-Pressure Collaborative Access Team (HPCAT) at Advanced Photon Source (APS), Argonne National Laboratory 16BM-B beamline. We demonstrate the feasibility of performing in situ measurements and correlating the measured electrical-thermal-structural properties over a broad range of P-T conditions by observing the well-known solid-solid and solid-melt transitions of bismuth (Bi) up to 5 GPa and 600° C. The goal of developing this new multi-probe measurement capability is to further improve detection of the onset of solid-solid and solid-melt transitions, relate structural and electrical properties of materials, determine changes in thermal conductivity at high P-T, and ultimately extend the technique for investigating other parameters, such as the Seebeck coefficient of thermoelectric materials.

Giant Pressure-Induced Enhancement of Seebeck Coefficient and Thermoelectric Efficiency in SnTe

The thermoelectric properties of polycrystalline SnTe have been measured up to 4.5 GPa at 330 K. SnTe shows an enormous enhancement in Seebeck coefficient, greater than 200 % after 3 GPa, which correlates to a known pressure-induced structural phase transition that is observed through simultaneous in situ X-ray diffraction measurement. Electrical resistance and relative changes to the thermal conductivity were also measured, enabling the determination of relative changes in the dimensionless figure of merit (ZT), which increases dramatically after 3 GPa, reaching 350 % of the lowest pressure ZT value. The results demonstrate a fundamental relationship between structure and thermoelectric behaviours and suggest that pressure is an effective tool to control them.

High Pressure Materials Physics Research at UNLV

High-pressure studies on strongly correlated-electron systems allow the study of the relationship between structural, elastic, electronic, and magnetic properties of d-and f-band systems. The High Pressure Science and Engineering Center (HiPSEC) at UNLV performs interdisciplinary research on a wide variety of materials at high pressures. One such system, YbB2 displays antiferromagnet order at ambient pressure. We present heat capacity measurements at high magnetic fields to 9 T and structural measurement at pressures up to 5 GPa on YbB2.