Hydride Superconductors: Developments In Our Understanding And Uses For High Tc Applications

Hydride Superconductors: Developments in our Understanding and Uses for High T_c Applications

Paper by Rhys

The Story so Far

Superconductivity is a quantum mechanical phenomenon wherein conduction electrons in electrically conducting materials group together to form pairs known as cooper pairs, and in doing so the host conductor exhibits an effective zero resistance. Originally discovered by Dutch physicist Heike Onnes in 1911, superconductivity sparked international interest due to its desirable magnetic and conductive properties, finding applications in systems such as MRI imaging, NMR spectroscopy, particle accelerators and even standard electrical power transmission [1].

The greatest limitation on the applicability of superconductor technology is the associated cooling requirement for achieving a superconductive state. Classical low-temperature superconductors (LTS) operate in the sub-20K regime, thereby requiring liquid hydrogen cooling. &Given that the cheaper and more widely available coolant, liquid nitrogen, boils at 77 K, vast exhaustive research has been conducted in the field of high-temperature superconductivity (HTS), which specifically targets superconducting media with critical temperatures (T_c) exceeding the 77K threshold. Until 2008, only copper and oxygen based compounds, known as cuprates, were thought to have HTS properties [2], however the subsequent discovery of iron pnictide superconductors provided new instances of HTS, albeit only at a moderate 26K [3]. Several advances have been made since then, in attempt to push the upper temperature limit of the superconductive state, for example the copper-oxide system at 164K [4].

The mechanisms associated with copper-oxide superconductors are not well understood to this day, since they are not conventional superconductors, so the search for conventional HTSs superconductors continues to be a research area of interest in condensed matter physics. In 2014, a new record for the highest T_c was announced by A.P Drosdov and a team of scientists, achieving T_c =203K with a sulfur hydride (sulfate, H₂S) system under high pressure [5]. This unbeaten record raises hopes for possible hydrogen-based room-temperature superconductors emerging in the near future.

With references to the associated physics, the scope and limitations of high T_c hydride superconductors will hereby be reviewed in the context of potential applications and further work.

High T_c &Hydride Superconductors
The Bardeen Cooper Schrieffer (BCS) theory of conventional superconductivity hypothesizes that lattice distortions grant single conduction electrons a small excess positive charge, which, in turn, attracts adjacent conduction electrons to form cooper pairs. BCS also predicts that given a favourable combination of high-frequency phonons, strong electron phonon coupling, and high density of states, one can achieve a high T_c with no theoretical upper bound [6]. In its simplest form, BCS s relation for the critical temperature of a superconducting medium can be represented by Equation (1) below.

Here, N_o is the electronic density of states at the Fermi level, and the Boltzmann constant. The electron-phonon coupling potential, V, provides, in a fundamental way, an attractive electron-electron interaction [7] and the Debye cut-off energy, E_D, describes a lattice vibration energy_, above which the lattice is unable to exploit the phonon vibration, because the vibrational wavelength is dimensionally smaller than the basic unit size of the atomic arrangement.

The criteria set out by BCS theory to achieve high T_c can indeed be satisfied by metallic hydrogen as well as hydrogen-dominated covalent compounds. This is due to the fact that hydrogen atoms generally provide the necessary high-frequency phonon modes that facilitate a high E_D, as well as the strong electron phonon coupling required to reach larger critical temperatures. The hydrides strong coupling emerges due to the lack of an electronic core in H.

In order to be superconducting, any hydrogen-dominated compound must first be metallized. This has high pressure demands, and generally, higher metallizing pressures produce better conductors (in the normal conducting regime) which in turn exhibit lower critical transition temperatures. Lower pressures are required to metallize hydrides in comparison with pure hydrogen, which serves as an important experimental characteristic for industrial applicability. The sulfur hydride system researched by Drosdov had a predicted Tc of 80K for pressures of around 160GPa according to various previous calculations [8]. The experimental results found in the study matched the theoretical model very well. Samples were first annealed, then pressurized at medium-low temperatures (200K). Following this, samples were slowly cooled (to 4K) then subsequently re-heated. lt;/p>The sample was found to first conduct at pressures greater than 50 GPa for T~200k. Under these conditions, the temperature dependence of the resistance, as well as the pronounced photoconductivity were indicative of a semiconductor. At &P>&90GPa the resistance drops slightly, and the temperature dependence becomes classically metallic. No photoconductive response is observed in this metallic state. Sulfur hydride in its metallic state is a poor metal. At , 100 K and 110GPa its resistivity is still & = &3.0x10^-5 &m at 110 GPa (compared to e.g. 1.7x10^-8 &m in Copper at atmospheric pressure). Of course, in the superconductive state, this loses itsrelevance as R &0. Higher temperature pressure-onsets to those producing the data shown in Figure 1, resulted in even higher superconducting transitions temperatures, including that of the widely known 203K transition. Similar experiments were carried out using sulfur deuteride (D₂S), but sulfur hydride produced the most promising results. Despite this, the H₃S stoichiometry has been confirmed, through extensive structural research, to be the most stable configuration at high pressure [9].

Impact and Implications
Although this breakthrough resolved the application limitaitons concerned with near-kelvin regime cooling, it has introduced a new problem& the need for astronomically high pressures. Nevertheless the discovery of metallic hydride superconduction bands at 150 GPa might point towards even lower hydrogen metallization pressures and higher critical temperatures being produced in the near future.

Although current applications are limited, the implications associated with high-pressure hydride superconductors might be of interest in the field of astrophysics. Many planets and stars compositions are predominantly hydrogen or hydrogen-based, such as Jupiter s. In addition, Jupiter is known to have a temperature in the range of 100-200K and a substantial magnetic field. If indeed the bulk of the planet is composed of hydrogen in the metallic state, BCS theory predicts that it may also be in a superconducting state, and the association of magnetic fields with, for example, persistent currents may be of some significance [10].

References:

[1] H. Padamsee, J. Knobloch and T. Hays (2008), RF Superconductivity: Science, Technology, and Applications: RF superconductivity for accelerators

[2] J. G. Bednorz and K. A. M ller (1986), & Zeitschrift f r Physik B Condensed Matter, Volume 64, Issue 2,.Possible high T_c superconductivity in the Ba &La &Cu &O system, pp 189 193

[3] P.J. Ford, G.A. Saunders (2004), The Rise of the Superconductors

[4] L. Gao, Y. Y. Xue, F. Chen, Q. Xiong, R. L. Meng, D. Ramirez, C. W. Chu, J. H. Eggert, and H. K. Mao (1994), Superconductivity up to 164 K in HgBa₂Ca_{m &1}Cu_mO2m+2+ lt;/sub> (m=1, 2, and 3) under quasihydrostatic pressures

[5] A.P. Drozdov, M.I. Eremets, I.A. Troyan, (2014), Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system

[6] L.N. Cooper, D.Feldman. (2011), BCS: 50 Years. Singapore, World Scientific

[7] R. Heid (2017), Electron-Phonon Coupling, Institute for Solid State Physics Karlsruhe Institute of Technology

[8] Y. Li, , J. Hao, H. Liu, Y. Li, and Y. Ma (2014), The metallization and superconductivity of dense hydrogen sulphide

[9] NW. Ashcroft, (1968) Metallic Hydrogen, a high-temperature superconductor? Physical Review Letters

[10] J. A. Flores, L. A. Sannaa, and E.K.U. Gross (2016), High temperature superconductivity in sulfur and selenium hydrides at high pressures