Session S23: Materials in Extremes: Towards Room-Temperature Superconductivity

Live

Room temperature superconductivity has been discovered recently in H-C-S compounds.
However, its crystal structure including chemical composition is poorly understood. The phase diagram
of H-C-S compounds with variable stoichiometry is investigated using first-principles crystal structure
prediction at pressures up to 300 GPa. Several candidate structures are investigated including their
atomic, electronic, vibrational, electronic and superconducting properties. Raman spectra of various
compounds at different temperature are calculated and compared to available experimental data to
understand chemical evolution upon increase of compression pressure.   

Live

We synthesized two C-S-H compounds from a mixture of amorphous nano-carbon and sulfur in fluid hydrogen and from sulfur in mixed methane-hydrogen fluids by gentle laser heating at 4 GPa (c.f. [1]). X-ray synchrotron single-crystal diffraction and Raman spectroscopy have been applied to these samples up to 57 and 143 GPa, respectively. While both samples show similar structural properties (the structure will be reported at the meeting) reflecting the lattice properties of the heavier elements, Raman spectroscopy probing the local hydrogen structure and ordering show a sequence of phase changes from a plastic to orientationally ordered molecular crystal and, finally, to an extended solid. The equations of state of these materials are slightly different reflecting either a different carbon composition or a difference in the hydrogen local structure detected also by Raman spectroscopy. The basic structure remains unchanged even after laser heating to 1400 K at 143 GPa suggesting that the structure of superconducting C-S-H compound [1] is different from a common cubic H₃S reported superconducting with T_c=203 K at 150 GPa [2].
[1] Sneider, E., et al., Nature (2020); https://doi.org/10.1038/s41586-020-2801-z
[2] Drozdov, A. P., et al., Nature (2015); doi:10.1038/nature14964   

Live

Thanks to recent breakthroughs, novel materials can be discovered on the computer and then made experimentally.
This breakthrough was enabled by advances in crystal structure prediction [1], where a special role was played by the evolutionary method/code USPEX (http://uspex-team.org), developed by my group since 2004 [2,3]. Today USPEX has over 7000 registered users worldwide and can deal with crystals, polymers, 2D-materials, surfaces, grain boundaries, nanoclusters. To discover technologically useful materials, USPEX utilizes multiobjective (Pareto) optimization of the desired physical properties. Here I will focus on the application of these methods to prediction of high-Tc superconductors. I will discuss binary systems explored by theory and experiment, the role of the electronic structure of the hydride-forming element and link to Mendeleev’s Periodic Table, and first results on ternary high-Tc superconducting hydrides.

[1] Oganov A.R., Pickard C.J., Zhu Q., Needs R.J. (2019). Structure prediction drives materials discovery. Nature Rev. Mater. 4, 331-348.
[2] Oganov A.R. (2018). Crystal structure prediction: reflections on present status and challenges. Faraday Discussions 211, 643-660.
[3] Oganov A.R., Glass C.W. (2006). Crystal structure prediction using ab initio evolutionary techniques: principles and applications. J. Chem. Phys. 124, 244704.   

Live

The more recent discoveries in the field of Hydrogen based BCS superconductivity show that T_c can span from few Kelvins as for AlH₃ and PrH₉ with respective T_cs being 4K and 7K to much higher values as in the case for H₃S, YH₉ and LaH₁₀ with respective T_cs of with 203K, 243K and 260K. In addition the latest finding for a Sulful-Carbon-Hydrogen based compound [1] with T_c above 280 K proves the possibility of room-temperature superconductivity. The greatest challenge is now to achieve ambient temperature-pressure superconductivity.
Moreover there are more then hundred predicted compounds [2,3] and there is the possibility to classify systems based on their properties in order to obtain prediction trends. Although the possibility to rely on simple impactful variables for predictions is still missing.
In this talk we investigate the chemical, structural and electronic properties for a wide set of Hydrogen based superconductors. We then propose a simple general observable based on the study of the ELF showing an incredible strong correlation with the T_c.
[1] DOI: https://doi.org/10.1038/s41586-020-2801-z
[2] DOI: https://doi.org/10.1016/B978-0-12-409547-2.11435-0
[3] DOI: https://doi.org/10.1016/j.physrep.2020.02.003   

Live

With T_c ∼9.6 K, Be₂₂Re exhibits one of the highest critical temperatures among Be-rich compounds. We have carried out a series of high pressure electrical resistivity measurements to 30 GPa. The data show that the critical temperature T_c is suppressed gradually at a rate of dT_c/dP = –0.05 K/GPa. Density functional theory calculations demonstrate a corresponding increase in the density of states at the Fermi level. Together, these results indicate that the electron-phonon coupling decreases with pressure. We discuss the relationship between low-Z Be-rich superconductors and the high-Tc superhydrides.   

Live

Binary metal superhydrides have been predicted to reach high critical superconducting temperatures. Recent experimental observations of LaH10±δ has shown them capable of Tc’s up to 250-260 kelvin. Predictions have shown yttrium superhydrides to be the most promising with an estimated Tc in excess of 300 kelvin for YH10. Here we report the synthesis of an yttrium superhydride that exhibits a superconducting critical temperature of 262 kelvin at 182 ± 8 gigapascal. An innovative synthesis procedure utilizing palladium assists the hydrogenation process by protecting sputtered yttrium from oxidation and allowing hydrogen passage to the yttrium film. Phonon-mediated superconductivity is established by the observation of zero resistance, an isotope effect and the reduction of Tc under an external magnetic field. The upper critical magnetic field is ~103 tesla at zero temperature. Using theoretical predictions, we suggest YH9±δ is the synthesized product based on comparison of the measured Raman spectra .   

Live

In this talk, results from density-functional theory and molecular dynamics to simulate dense metallic liquid hydrogen will be presented. Simulations were performed at 500 K over the pressure range 386.8(4)--883.7(3) GPa. Of particular interest is the possible superconductivity of this state. The superconducting transition temperature is calculated to be 308(6) K at 386.8 GPa and increasing to a maximum of 372(2) K at 783.7(4) GPa. The melting line of solid hydrogen decreases over this pressure range; the melting temperature is 291 K at 400 GPa. Given that the calculated critical temperature is above the melting one, it is concluded that hydrogen will be a liquid superconductor above room temperature at high pressures. These results will have important applications for future theoretical and experimental work, as well as several possible applications. These will be discussed.   

Live

Pressure can dramatically enhance the superconductivity in the materials. This originates from the modification of multiple interactions in the quantum states with respect to high pressures. Here we would like to introduce our recent progress on the topic of pressure generated superconductivity in the compressed configurations of materials [Reference: 1.PNAS 116, 12156 (2019); 2. Journal of Superconductivity and Novel Magnetism 33, 81(2020); 3. Physical Review B 99, 224515 (2019); 4. Physical Review B 101, 180509(R) (2020); 5. Physical Review Materials 3, 44802 (2019); 6. Phys. Rev. B 101, 35151(2020); 7. Physical Review Research 2, 33356 (2020); 8. arXiv: 2004.11651v1(2020)].   

Live

Diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 Nb₃Si material to trace T_c as a function of pressure. T_c is suppressed to ~ 5.2 K at 88 GPa. Using these T_c (P) data for A15 Nb₃Si, applying pressures up to 120 GPa at room temperature on tetragonal Nb₃Si and measuring superconductivity present via resistivity gave no indication of any transition to the A15 structure. This is in contrast to the explosive compression (up to P ~ 110 GPa) of tetragonal Nb₃Si, which produced 50-70% A15 material and T_c = 18 K at ambient pressure in a 1981 Los Alamos National Laboratory experiment. We believe that, despite theoretical calculations that A15 Nb₃Si has an enthalpy vs the tetragonal structure that is 0.07 eV/atom smaller at 100 GPa, it is the accompanying high temperature (1000 ^oC) caused by explosive compression that is necessary to successfully drive the reaction kinetics of the tetragonal to A15 Nb₃Si structural transformation. We performed annealing experiments on the A15 explosively compressed material that are consistent with this viewpoint.   

Live

One of the long-standing challenges in experimental physics is the observation of room-
temperature superconductivity (RTSC). More than a century of rigorous research has led
physicists to believe that the highest T c that can be achieved is 40 K for conventional
superconductors. However, the discovery of superconductivity in hydrogen sulfide (H 2 S) at 203
K changed the notion of what might be possible for phonon–mediated superconductors. As H 2 S
readily mixes with hydrogen to form guest-host structures at lower pressures, the comparable
size of methane (CH 4 ) to H 2 S should allow molecular exchange within a large assemblage of van
der Waals solids that are (highly) hydrogen-rich with H 2 inclusions that are then the building
blocks for novel superconducting compounds at extreme conditions. Here, we report
superconductivity in a photochemically transformed carbonaceous sulfur hydride system with a
maximum superconducting transition temperature of 287.7 ± 1.2 K (~15° C) achieved at 267 ±
10 GPa. Superconductivity is established by the observation of zero resistance, magnetic
susceptibility of up to 190 GPa, and reduction of the transition temperature under an external
magnetic field of up to 9 T, with an upper critical magnetic field of about 62 T according to the
Ginzburg–Landau model at zero temperature. The Raman spectroscopy is used to probe the
chemical and structural transformations before metallization. The discovery achieves the more
than a century long quest to find room temperature superconductivity, a phenomenon that was
first observed by Kamerlingh Onnes in 1911.   

On Demand

We investigate the superconducting properties of the non-centrosymmetric AuBe superconductor under pressure. First, we identify possible phase transformations under pressure using the Genetic Algorithm for Structure and Phase Prediction (GASP) coupled to density functional theory calculations. Our calculations reveal several possible stable phases above 50 GPa that exhibit enthalpies below that of the ambient pressure ground-state structure. Out of these predicted stable phases, the phase with the centrosymmetric Cmcm crystal structure shows the lowest enthalpy. Next, we measure the low-temperature resistivity of our sample as a function of pressure to study the effect of pressure on the superconducting properties. We also perform high-pressure X-ray diffraction to verify our predictions of phase transition under pressure.