August 18, 2015

Hydrogen sulfide at high pressures 200 GPa (2 million atmospheres) has superconductivity at -70 celsius has sparked renewed mainstream efforts for room temperature superconductivity

Hydrogen sulfide — the compound responsible for the smell of rotten eggs — conducts electricity with zero resistance at a record high temperature of 203 kelvin (–70 °C), reports a paper published in Nature.

The first results of the work, which represents a historic step towards finding a room-temperature superconductor, were released on the arXiv preprint server in December and followed up by more in June. They have already sparked a wave of excitement within the research community.

A superconductor that works at room-temperature would make everyday electricity generation and transmission vastly more efficient, as well as giving a massive boost to current uses of superconductivity such as the enormous magnets used in medical imaging machines.

Temperature dependence of resistance of sulfur hydride and sulfur deuteride measured at different pressures . The pressures did not change during the cooling (within ≈5 GPa). Resistance was measured with four electrodes deposited on a diamond anvil touched the sample (photo). Diameter of the samples was ~25 microns and the thickness ~1 microns.

(a) Sulfur hydride as measured at the growing pressures, the values are indicated near the corresponding plot. Plots at pressures less than 135 GPa were scaled (reduced in 5-10 times) for easier comparison with the higher pressure steps. The resistance was measured with current of 10 microA. Bottom: the resistance plots near zero. (b) Comparison of the superconducting steps of sulfur deuteride and hydride at similar pressures. Bottom: resistance measured near zero. Resistance was measured in four channels with van der Pauw method (SI Fig. 4) with current of 10 mA.

Arxiv - Conventional superconductivity at 190 K at high pressures (16 pages)

The highest critical temperature of superconductivity Tc has been achieved in cuprates: 133 K at ambient pressure and 164 K at high pressures. As the nature of superconductivity in these materials is still not disclosed, the prospects for a higher Tc are not clear. In contrast the Bardeen-Cooper-Schrieffer (BCS) theory gives a clear guide for achieving high Tc: it should be a favorable combination of high frequency phonons, strong coupling between electrons and phonons, and high density of states. These conditions can be fulfilled for metallic hydrogen and covalent hydrogen dominant compounds. Numerous followed calculations supported this idea and predicted Tc=100-235 K for many hydrides but only moderate Tc~17 K has been observed experimentally. Here we found that sulfur hydride transforms at P~90 GPa to metal and superconductor with Tc increasing with pressure to 150 K at ~200 GPa. This is in general agreement with recent calculations of Tc~80 K for H2S. Moreover we found superconductivity with Tc~190 K in a H2S sample pressurized to P over 150 GPa at T over 220 K. This superconductivity likely associates with the dissociation of H2S, and formation of SHn (n over 2) hydrides. We proved occurrence of superconductivity by the drop of the resistivity at least 50 times lower than the copper resistivity, the decrease of Tc with magnetic field, and the strong isotope shift of Tc in D2S which evidences a major role of phonons in the superconductivity. H2S is a substance with a moderate content of hydrogen therefore high Tc can be expected in a wide range of hydrogen-contain materials. Hydrogen atoms seem to be essential to provide the high frequency modes in the phonon spectrum and the strong electron-phonon coupling.

H2S is known as a typical molecule substance with a rich phase diagram. At about 96 GPa hydrogen sulfide transforms to metal. The transformation is complicated by the partial dissociation of H2S and the appearance of elemental sulfur at Pressure over 27 GPa at room temperature, and higher pressures for lower temperatures. Therefore, the metallization of hydrogen sulfide can be explained by elemental sulfur which is known to become metallic above 95 GPa23. No experimental studies on hydrogen sulfide are known above 100 GPa. Recent theoretical work revised the phase diagram of
H2S, and a number of new stable structures were found. At Pressure over 130 GPa higher pressures than experimentally observed hydrogen sulfide was predicted to become a metal and a superconductor with a maximal transition temperature of ∼80 K at 160 GPa. Precipitation of sulfur has been found to be very unlikely, in apparent contradiction to the experiments.
Pressure dependence of critical superconducting temperature Tc on pressure. Only four probe electrical measurements in van der Pauw geometry are presented in both panels. Critical superconducting temperature Tc was determined as a point where resistance starts to sharply decrease with cooling from the plateau at the R(T) plot (inset in (b)).

(a) Data were obtained when pressure was applied in the 100-190 GPa pressure range at 100-150 K, and higher temperatures at P~200 GPa when Tc sharply increased. Black points are data from Fig. 1a. Blue points - other runs. Red points are measurements of D2 S. Dark yellow points are Tc s of pure sulfur. Grey stars are calculations from Ref. 7.

Mikhail Eremets and two other physicists at the Max Planck Institute for Chemistry in Mainz reported that they had discovered hydrogen sulfide superconducting below 190 K. When they placed a 10 micrometre-wide sample of the material in a diamond-anvil cell and subjected it to a pressure of about 1.5 million atmospheres, they found that its electrical resistance dropped by more than a factor of 1,000 when cooled below the threshold, or 'critical', temperature.

In the latest work, the authors got together with two physicists from the University of Mainz to build a non-magnetic cell and acquire a very sensitive type of magnetometer known as a SQUID. They placed 50 micrometre-wide samples of hydrogen sulfide under pressures of up to 2 million atmospheres in an external magnetic field, and slowly warmed them, starting from a few degrees above absolute zero. They observed the tell-tale sign of the Meissner effect — a sudden increase in the sample's 'magnetization signal' — when the temperature rose past 203 K

Some groups in China and Japan have reproduced the results, including the drop in electrical resistance and the Meissner effect.

ther hydrogen compounds might then superconduct at even higher temperatures, and possibly even at room temperature, given that the BCS theory does not place any upper limit on the superconducting transition.

But theoretical physicist Jorge Hirsch at the University of California, San Diego, does not believe that lattice vibrations are the correct interpretation. "The question of where the high critical temperature comes from is still wide open in my opinion,” he says.
SOURCES - Nature, Arxiv

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