Samuel T. Weir

Crystal Structures at Megabar Pressures Determined by Use of the Cornell Synchrotron Source

X-ray diffraction studies have been carried out on alkali halide samples 10 micrometers in diameter (volume 10-9 cubic centimeter) subjected to megabar pressures in the diamond anvil cell. Energy-dispersive techniques and a synchrotron source were used. These measurements can be used to detect crystallographic phase transitions. Cesium iodide was subjected to pressures of 95 gigapascals (fractional volume of 46 percent) and rubidium iodide to pressures of 89 gigapascals (fractional volume of 39 percent). Cesium iodide showed a transformation from the cubic B2 phase (cesium chloride structure) to a tetragonal phase and then to an orthorhombic phase, which was stable to 95 gigapascals. Rubidium iodide showed only a transition from the low-pressure cubic B1 phase (sodium chloride structure) to the B2 phase, which was stable up to 89 gigapascals.

High-temperature superconductivity stabilized by electron-hole interband coupling in collapsed tetragonal phase of KFe 2 As 2 under high pressure

We report a high-pressure study of simultaneous low-temperature electrical resistivity and Hall effect measurements on high quality single-crystalline KFe2As2 using designer diamond anvil cell techniques with applied pressures up to 33 GPa. In the low-pressure regime, we show that the superconducting transition temperature Tc finds a maximum onset value of 7 K near 2 GPa, in contrast to previous reports that find a minimum Tc and reversal of pressure dependence at this pressure. Upon applying higher pressures, this Tc is diminished until a sudden drastic enhancement occurs coincident with a first-order structural phase transition into a collapsed tetragonal phase. The appearance of a distinct superconducting phase above 13 GPa is also accompanied by a sudden reversal of dominant charge carrier sign, from hole- to electron-like, which agrees with our band structure calculations predicting the emergence of an electron pocket and diminishment of hole pockets upon Fermi surface reconstruction. Our results suggest the high-temperature superconducting phase in KFe2As2 is substantially enhanced by the presence of nested electron and hole pockets, providing the key ingredient of high-Tc superconductivity in iron pnictide superconductors.

Synthetic diamonds produce pressure of 125 GPa (1.25 Mbar)

Synthetic gray-blue diamonds were used as anvils in a diamond anvil cell to produce a pressure of 125 GPa (1.25 Mbar) in a gasketed sample. Pressure was measured by x-ray diffraction methods by using gold and iron as a calibrant and also by optical methods based on the shift of the fluorescence peaks of ruby with pressure. The future potential of synthetic diamonds for ultrapressure research is discussed.

Phase transitions near 1 Mbar

Abstract The nature of the high pressure phase transitions in InAs, BaTe and CsI are discussed. X-ray diffraction studies in the Cornell High Energy Synchrotron Source to 101 GPa show that CsI becomes tetragonal at about 38 GPa and orthorhombic at about 56 GPa. The separation of the nearest like neighbors is reduced dramatically when the orthorhombic phase forms; at 56 GPa this separation is 28% less than the zero pressure value. At 56 GPa the coordination is essentially ten-fold with eight unlike neighbors and two like neighbors.

Magnetic transitions in erbium at high pressures

Electrical resistance measurements have been carried out on polycrystalline erbium (Er) at temperatures down to 10 K and pressures up to 20 GPa. An abrupt change in the slope of the resistance is observed with decreasing temperature below 84 K which is associated with the c-axis modulated antiferromagnetic (AFM) ordering of the Er moments. With increasing pressure, the temperature of this resistance slope change and the corresponding AFM ordering temperature decrease until vanishing above 10.6 GPa. At higher pressures, a more gradual change in the slope of the resistance is found to occur around 45 K which disappears at pressures near 17 GPa. The transformation from the hexagonal-close-packed structural phase to a nine-layer α-Sm structural phase at a similar pressure of 11 GPa indicates (i) that the disappearance in the c-axis modulated antiferromagnetic ordering of Er moments above 10.6 GPa is correlated to the structural phase change and (ii) that the smaller resistance changes around 45 K result from ...

Superconducting Bi2Te: Pressure-induced universality in the (Bi2)m(Bi2Te3)n series

Using high-pressure magnetotransport techniques we have discovered superconductivity in Bi2Te, a member of the infinitely adaptive (Bi2)m(Bi2Te3)n series, whose end members, Bi and Bi2Te3, can be tuned to display topological surface states or superconductivity. Bi2Te has a maximum Tc = 8.6 K at P = 14.5 GPa and goes through multiple high pressure phase transitions, ultimately collapsing into a bcc structure that suggests a universal behavior across the series. High-pressure magnetoresistance and Hall measurements suggest a semi-metal to metal transition near 5.4 GPa, which accompanies the hexagonal to intermediate phase transition seen via x-ray diffraction measurements. In addition, the linearity of Hc2 (T) exceeds the Werthamer-Helfand-Hohenberg limit, even in the extreme spin-orbit scattering limit, yet is consistent with other strong spin-orbit materials. Furthermore, considering these results in combination with similar reports on strong spin-orbit scattering materials seen in the literature, we suggest the need for a new theory that can address the unconventional nature of their superconducting states.

Strongly coupled electronic, magnetic, and lattice degrees of freedom in LaCo5 under pressure

In this study, we have performed high-pressure magnetotransport and x-ray diffraction measurements on ferromagnetic LaCo5, confirming the theoretically predicted electronic topological transition driving the magnetoelastic collapse seen in the related compound YCo5. Our x-ray diffraction results show an anisotropic lattice collapse of the c axis near 10 GPa that is also commensurate with a change in the majority charge carriers evident from high-pressure Hall effect measurements. The coupling of the electronic, magnetic, and lattice degrees of freedom is further substantiated by the evolution of the anomalous Hall effect, which couples to the magnetization of the ordered state of LaCo5.

Pressure-induced superconductivity and structural transitions in Ba(Fe[subscript 0.9]Ru[subscript 0.1])[subscript 2]As[subscript 2]

Electrical transport and structural characterizations of isoelectronically substituted Ba(Fe0.9Ru0.1)2As2 have been performed as a function of pressure up to ∼ 30 GPa and temperature down to ∼ 10 K using designer diamond anvil cell. Similar to undoped members of the AFe2As2 (A = Ca, Sr, Ba) family, Ba(Fe0.9Ru0.1)2As2 shows anomalous a-lattice parameter expansion with increasing pressure and a concurrent ThCr2Si2 type isostructural (I4/mmm) phase transition from tetragonal (T) phase to a collapsed tetragonal (cT) phase occurring between 12 and 17 GPa where the a is maximum. Above 17 GPa, the material remains in the cT phase up to 30 GPa at 200 K. The resistance measurements show evidence of pressure-induced zero resistance that may be indicative of high-temperature superconductivity for pressures above 3.9 GPa. The onset of the resistive transition temperature decreases gradually with increasing pressure before completely disappearing for pressures above ∼ 10.6 GPa near the T-cT transition. We have determined the crystal structure of the high-Tc phase of Ru-doped BaFe2As2 to remain as tetragonal (I4/mmm) by analyzing the X-ray diffraction pattern obtained at 10 K and 9.7 ± 0.7 GPa, as opposed to inferring the structural transition from electrical resistance measurement, as in a previous report [S.K. Kim, M.S. Torikachvili, E. Colombier, A. Thaler, S.L. Bud’ko, P.C. Canfield, Phys. Rev. B 84, 134525 (2011)].