Katsuya Shimizu

Bio: Katsuya Shimizu is an academic researcher from Osaka University. The author has contributed to research in topics: Superconductivity & Electrical resistivity and conductivity. The author has an hindex of 35, co-authored 247 publications receiving 5156 citations. Previous affiliations of Katsuya Shimizu include Kanazawa Institute of Technology & Venezuelan Institute for Scientific Research.

Crystal structure of the superconducting phase of sulfur hydride

Abstract: A superconducting critical temperature above 200 K has recently been discovered in H2S (or D2S) under high hydrostatic pressure1, 2. These measurements were interpreted in terms of a decomposition of these materials into elemental sulfur and a hydrogen-rich hydride that is responsible for the superconductivity, although direct experimental evidence for this mechanism has so far been lacking. Here we report the crystal structure of the superconducting phase of hydrogen sulfide (and deuterium sulfide) in the normal and superconducting states obtained by means of synchrotron X-ray diffraction measurements, combined with electrical resistance measurements at both room and low temperatures. We find that the superconducting phase is mostly in good agreement with theoretically predicted body-centered cubic (bcc) structure for H3S (Ref.3). The presence of elemental sulfur is also manifest in the X-ray diffraction patterns, thus proving the decomposition mechanism of H2S to H3S + S under pressure4-6.

Superconductivity in compressed lithium at 20 K

Abstract: Superconductivity at high temperatures is expected in elements with low atomic numbers, based in part on conventional BCS (Bardeen-Cooper-Schrieffer) theory. For example, it has been predicted that when hydrogen is compressed to its dense metallic phase (at pressures exceeding 400 GPa), it will become superconducting with a transition temperature above room temperature. Such pressures are difficult to produce in a laboratory setting, so the predictions are not easily confirmed. Under normal conditions lithium is the lightest metal of all the elements, and may become superconducting at lower pressures; a tentative observation of a superconducting transition in Li has been previously reported. Here we show that Li becomes superconducting at pressures greater than 30 GPa, with a pressure-dependent transition temperature (T(c)) of 20 K at 48 GPa. This is the highest observed T(c) of any element; it confirms the expectation that elements with low atomic numbers will have high transition temperatures, and suggests that metallic hydrogen will have a very high T(c). Our results confirm that the earlier tentative claim of superconductivity in Li was correct.

Superconductivity in the non-magnetic state of iron under pressure.

Abstract: Ferromagnetism and superconductivity are thought to compete in conventional superconductors, although in principle it is possible for any metal to become a superconductor in its non-magnetic state at a sufficiently low temperature. At pressures above 10 GPa, iron is known to transform to a non-magnetic structure and the possibility of superconductivity in this state has been predicted. Here we report that iron does indeed become superconducting at temperatures below 2 K at pressures between 15 and 30 GPa. The transition to the superconducting state is confirmed by both a drop in resistivity and observation of the Meissner effect.

Experimental determination of the electrical resistivity of iron at Earth’s core conditions

Abstract: Using a laser-heated diamond-anvil cell to measure the electrical resistivity of iron under the high temperature and pressure conditions of the Earth’s core yields a value that means Earth’s core has high thermal conductivity, suggesting that its inner core is less than 0.7 billion years old, much younger than thought. The thermal conductivity of iron and its alloys at high pressure and temperature is a critical factor in the evolution and dynamics of Earth-like planets. Recently, increasing uncertainty in these values has produced dramatically variable predictions for Earth's history that challenge traditional geophysical theories. Two groups reporting in this issue of Nature use laser-heated diamond-anvil cells to study the properties of iron at the extreme temperatures and pressures relevant to Earth's core, but using different methodologies, and they arrive at contrasting results. Kenji Ohta and co-authors measured the electrical resistivity of iron at up to 4,500 kelvin and obtained an estimate that is even lower than the low values predicted from recent ab initio studies. They conclude that this suggests a high thermal conductivity for Earth's core, which would imply rapid core cooling by conduction and a relatively young inner core. Zuzana Konopkova and co-authors measured heat pulses propagating through solid iron after heating with a laser pulse at pressures and temperatures relevant to the cores of planets ranging in size from Mercury to Earth. Their measurements place the thermal conductivity of Earth's core near the low end of previous estimates, implying that thermal convection in Earth's core could have driven the geodynamo for billions of years, and allowing for an ancient inner core. In a linked News & Views, David Dobson discusses the interpretation of these two tours de force of experimental geophysics. Earth continuously generates a dipole magnetic field in its convecting liquid outer core by a self-sustained dynamo action. Metallic iron is a dominant component of the outer core, so its electrical and thermal conductivity controls the dynamics and thermal evolution of Earth’s core1. However, in spite of extensive research, the transport properties of iron under core conditions are still controversial2,3,4,5,6,7,8,9. Since free electrons are a primary carrier of both electric current and heat, the electron scattering mechanism in iron under high pressure and temperature holds the key to understanding the transport properties of planetary cores. Here we measure the electrical resistivity (the reciprocal of electrical conductivity) of iron at the high temperatures (up to 4,500 kelvin) and pressures (megabars) of Earth’s core in a laser-heated diamond-anvil cell. The value measured for the resistivity of iron is even lower than the value extrapolated from high-pressure, low-temperature data using the Bloch–Gruneisen law, which considers only the electron–phonon scattering. This shows that the iron resistivity is strongly suppressed by the resistivity saturation effect at high temperatures. The low electrical resistivity of iron indicates the high thermal conductivity of Earth’s core, suggesting rapid core cooling and a young inner core less than 0.7 billion years old10. Therefore, an abrupt increase in palaeomagnetic field intensity around 1.3 billion years ago11 may not be related to the birth of the inner core.

Superconductivity in oxygen

Abstract: Among the simple diatomic molecules, oxygen is of particular interest because it shows magnetism at low temperatures Moreover, at pressures exceeding 95 GPa (∼095 Mbar), solid molecular oxygen becomes metallic, accompanied by a structural transition1 The metallization process is characterized by an increase in optical reflectivity2, and a change in the slope of the resistance–temperature curve3 Here we report that at pressures of around 100 GPa, solid oxygen becomes superconducting, with a transition temperature of 06 K The transition is revealed by both resistivity measurements and a Meissner demagnetization signal

Crystal structure prediction using ab initio evolutionary techniques: principles and applications.

Abstract: We have developed an efficient and reliable methodology for crystal structure prediction, merging ab initio total-energy calculations and a specifically devised evolutionary algorithm. This method allows one to predict the most stable crystal structure and a number of low-energy metastable structures for a given compound at any P-T conditions without requiring any experimental input. Extremely high (nearly 100%) success rate has been observed in a few tens of tests done so far, including ionic, covalent, metallic, and molecular structures with up to 40 atoms in the unit cell. We have been able to resolve some important problems in high-pressure crystallography and report a number of new high-pressure crystal structures (stable phases: epsilon-oxygen, new phase of sulphur, new metastable phases of carbon, sulphur and nitrogen, stable and metastable phases of CaCO3). Physical reasons for the success of this methodology are discussed.

Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system

Abstract: A superconductor is a material that can conduct electricity without resistance below a superconducting transition temperature, Tc. The highest Tc that has been achieved to date is in the copper oxide system: 133 kelvin at ambient pressure and 164 kelvin at high pressures. As the nature of superconductivity in these materials is still not fully understood (they are not conventional superconductors), the prospects for achieving still higher transition temperatures by this route are not clear. In contrast, the Bardeen-Cooper-Schrieffer theory of conventional superconductivity gives a guide for achieving high Tc with no theoretical upper bound--all that is needed is a favourable combination of high-frequency phonons, strong electron-phonon coupling, and a high density of states. These conditions can in principle be fulfilled for metallic hydrogen and covalent compounds dominated by hydrogen, as hydrogen atoms provide the necessary high-frequency phonon modes as well as the strong electron-phonon coupling. Numerous calculations support this idea and have predicted transition temperatures in the range 50-235 kelvin for many hydrides, but only a moderate Tc of 17 kelvin has been observed experimentally. Here we investigate sulfur hydride, where a Tc of 80 kelvin has been predicted. We find that this system transforms to a metal at a pressure of approximately 90 gigapascals. On cooling, we see signatures of superconductivity: a sharp drop of the resistivity to zero and a decrease of the transition temperature with magnetic field, with magnetic susceptibility measurements confirming a Tc of 203 kelvin. Moreover, a pronounced isotope shift of Tc in sulfur deuteride is suggestive of an electron-phonon mechanism of superconductivity that is consistent with the Bardeen-Cooper-Schrieffer scenario. We argue that the phase responsible for high-Tc superconductivity in this system is likely to be H3S, formed from H2S by decomposition under pressure. These findings raise hope for the prospects for achieving room-temperature superconductivity in other hydrogen-based materials.

Crystal structure prediction using ab initio evolutionary techniques: principles and applications

Abstract: We have developed an efficient and reliable methodology for crystal structure prediction, merging ab initio total-energy calculations and a specifically devised evolutionary algorithm. This method allows one to predict the most stable crystal structure and a number of low-energy metastable structures for a given compound at any P-T conditions without requiring any experimental input. Extremely high success rate has been observed in a few tens of tests done so far, including ionic, covalent, metallic, and molecular structures with up to 40 atoms in the unit cell. We have been able to resolve some important problems in high-pressure crystallography and report a number of new high-pressure crystal structures. Physical reasons for the success of this methodology are discussed.