Shanti Deemyad

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.

Melting line of hydrogen at high pressures.

The insulator to metal transition in solid hydrogen was predicted over 70 years ago but the demonstration of this transition remains a scientific challenge. In this regard, a peak in the temperature versus pressure melting line of hydrogen may be a possible precursor for metallization. However, previous measurements of the fusion curve of hydrogen have been limited in pressure and temperature by diffusion of hydrogen into the gasket or diamonds. To overcome this limitation we have used an innovative technique of pulsed laser heating of the sample and find a peak in the melting line at P=64.7+/-4 GPa and T=1055+/-20 K.

Pulsed laser heating and temperature determination in a diamond anvil cell

Pulsed laser heating in a diamond anvil cell has several advantages over cw heating: it can suppress thermally activated diffusion of the pressurization medium into the gasket and diamond, suppress chemical reactions of impurities with the sample and environment, requires far less average power and thus less heating of the sample environment, and can attain very high temperatures. It was recently shown that it is possible to accurately measure the melting point of platinum and other materials at ambient conditions using a pulsed laser and a simple ungated CCD detector, measuring the time-averaged irradiance. We show that this same technique can be used for high-pressure samples in a diamond anvil cell. As a demonstration, the high-pressure melting curve of iron is reproduced and compared to literature values.

Dependence of the superconducting transition temperature of MgB2 on pressure to 20 GPa

Abstract The dependence of Tc on nearly hydrostatic pressure has been measured for an isotopically pure ( 11 B) MgB2 sample in a helium-loaded diamond-anvil-cell to nearly 20 GPa. Tc decreases monotonically with pressure from 39.1 K at ambient pressure to 20.9 K at 19.2 GPa. The initial dependence is the same as that obtained earlier (dTc/dP≃−1.11(2) K/GPa) on the same sample in a He-gas apparatus to 0.7 GPa. The observed pressure dependence Tc(P) to 20 GPa can be readily described in terms of simple lattice stiffening within standard phonon-mediated BCS superconductivity.

Dependence of the superconducting transition temperature of single and polycrystalline MgB2 on hydrostatic pressure

Abstract The dependence of T c for MgB 2 on purely hydrostatic or nearly hydrostatic pressure has been determined to 29 GPa for single-crystalline and to 32 GPa for polycrystalline samples, and found to be in good agreement. T c decreases from 39 K at ambient pressure to 15 K at 32 GPa with an initial slope d T c /d P ≃−1.11(2) K/GPa. Evidence is presented that the differing values of d T c /d P reported in the literature may result primarily from shear-stress effects in nonhydrostatic pressure media rather than from differences in the samples. Although comparison of these results with theory supports phonon-mediated superconductivity, a critical test of theory must await calculations based on the solution of the anisotropic Eliashberg equations as a function of the lattice parameters.

Temperature dependence of the emissivity of platinum in the IR.

The accuracy of temperature determination by fitting the spectral irradiance to a Planck curve depends on knowledge of the emissivity at all temperatures and pressures of interest within a spectral region. Here, the emissivity of platinum is measured in the near infrared as a function of temperature. In the wavelength range of study and the temperature range of 650-1100 K, we find the emissivity to be independent of temperature to within experimental error. This result should lead to improved accuracy of temperature measurement by optical pyrometry where platinum is used as a thermal emitter.

High-pressure superconducting phase diagram of 6Li: Isotope effects in dense lithium

Significance The emergence of exotic quantum states, such as fluid ground state and two-component superconductivity and superfluidity, in a compressed light metallic system has been entertained theoretically for metallic phases of hydrogen. The difficulty of compressing hydrogen to metallization densities has prevented experimental proof of these effects. Studying lithium, which is isovalent to hydrogen and the lightest metal, is considered as a route to studying the lattice quantum effects in a dense light metallic system. Here, by comparing the superconductivity of lithium isotopes under pressure, we present evidence that properties of lithium at low temperature may significantly be dominated by its lattice quantum dynamics. This study is the first experimental report on superconducting properties of 6Li, the lightest superconducting material. We measured the superconducting transition temperature of 6Li between 16 and 26 GPa, and report the lightest system to exhibit superconductivity to date. The superconducting phase diagram of 6Li is compared with that of 7Li through simultaneous measurement in a diamond anvil cell (DAC). Below 21 GPa, Li exhibits a direct (the superconducting coefficient, α, Tc∝M−α, is positive), but unusually large isotope effect, whereas between 21 and 26 GPa, lithium shows an inverse superconducting isotope effect. The unusual dependence of the superconducting phase diagram of lithium on its atomic mass opens up the question of whether the lattice quantum dynamic effects dominate the low-temperature properties of dense lithium.

Strategy and enhanced temperature determination in a laser heated diamond anvil cell

We show that a strategy for increased accuracy in temperature determination by optical pyrometry when the wavelength dependence of the emissivity is unknown is to measure the spectral irradiance at short wavelengths. We then introduce an improved method of determining the temperature in laser heated diamond anvil cells. In general a blackbody source is used to determine the optical transfer function required for determining the blackbody curve. By using the thermal radiation of a heated absorber at ambient pressure and known temperature, uncertainties in the temperature determination caused by the wavelength dependence of the emissivity of the heated absorber can be eliminated. Temperature determination reduces to a one-parameter fit to the blackbody curve rather than the usual two parameters (emissivity and temperature), leading to increased precision and accuracy.

Quantum and isotope effects in lithium metal

Lithium gets a new ground state For the past 70 years, the lowest-energy crystal structure of lithium was believed to be a relatively complex one called the 9R structure. Ackland et al. show that this is incorrect. The actual lowest-energy structure for lithium is the much simpler closest-packed face-centered cubic form. In addition, 6Li and 7Li isotopes have crystal phase transitions at slightly different pressures and temperatures. This difference is chalked up to large quantum mechanical effects between the isotopes. Lithium is the only metal that shows this type of quantum effect and presents a challenge for theoreticians to explain. Science, this issue p. 1254 Lithium’s ground state has a face-centered cubic structure, and quantum effects alter the phase diagram between the 6Li and 7Li isotopes. The crystal structure of elements at zero pressure and temperature is the most fundamental information in condensed matter physics. For decades it has been believed that lithium, the simplest metallic element, has a complicated ground-state crystal structure. Using synchrotron x-ray diffraction in diamond anvil cells and multiscale simulations with density functional theory and molecular dynamics, we show that the previously accepted martensitic ground state is metastable. The actual ground state is face-centered cubic (fcc). We find that isotopes of lithium, under similar thermal paths, exhibit a considerable difference in martensitic transition temperature. Lithium exhibits nuclear quantum mechanical effects, serving as a metallic intermediate between helium, with its quantum effect–dominated structures, and the higher-mass elements. By disentangling the quantum kinetic complexities, we prove that fcc lithium is the ground state, and we synthesize it by decompression.

Boundaries for martensitic transition of 7 Li under pressure

Physical properties of lithium under extreme pressures continuously reveal unexpected features. These include a sequence of structural transitions to lower symmetry phases, metal-insulator-metal transition, superconductivity with one of the highest elemental transition temperatures, and a maximum followed by a minimum in its melting line. The instability of the bcc structure of lithium is well established by the presence of a temperature-driven martensitic phase transition. The boundaries of this phase, however, have not been previously explored above 3 GPa. All higher pressure phase boundaries are either extrapolations or inferred based on indirect evidence. Here we explore the pressure dependence of the martensitic transition of lithium up to 7 GPa using a combination of neutron and X-ray scattering. We find a rather unexpected deviation from the extrapolated boundaries of the hR3 phase of lithium. Furthermore, there is evidence that, above ∼3 GPa, once in fcc phase, lithium does not undergo a martensitic transition.