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Interplay Between Magnetism and Superconductivity in Metallic Hydrogen and Hydrides at High Pressure

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Interplay Between Magnetism and Superconductivity in Metallic Hydrogen and Hydrides at High Pressure EPJ Web of Conferences 185, 08003 (2018) https://doi.org/10.1051/epjconf/201818508003 MISM 2017 Interplay between magnetism and superconductivity in metallic hydrogen and hydrides at high pressure Lev Mazov1,* 1Dept.of Phys.of Supercond., Inst.for Phys.of Microstruct. Russian Academy of Sciences, 603600 Nizhny Novgorod, Russia Abstract. The detailed analysis of resistive and magnetic measurements with sulfur hydrides at high pressure is performed. The hydrogen system at high pressure can exhibit ferromagnetic (FM) as well as superconducting (SC) properties with layered structure. The onset temperature of resistive transition at 200 K in metallic sulfur hydrides corresponds to magnetic (AF SDW) phase transition rather than SC one. SC transition in these sulfur hydrides occurs only when magnetic (AF SDW) phase transition is over (~ 40 K). The SC mechanism in metallic sulfur hydrides is not conventional but corresponds to Keldysh-Kopaev model characteristic for systems with coexistence of dielectric and SC pairing. 1 Introduction enclosed between two sapphire anvils. Multiple shock pulse was induced by impact on the cryogenic sample According to modern viewpoint, hydrogen at high holder by means of either an Al or Cu plate accelerated pressure must pass from the molecular to the metallic to a certain speed with two-stage light-and-gas gun. The phase, which can be SC at relatively high temperatures pressures generated were in the range of 90-180 GPa. [1]. However, the experimental implementation of such a transition in laboratory conditions still encounters a number of problems related to the stability of this state. On the other hand, as is known, such pressures are achievable for a number of stars and planets, a significant part of whose composition is hydrogen. Moreover, such celestial bodies, as follows from observational data, have a significant magnetic field. In particular, this is indicated by the polarization of the radio emission observed from "white dwarfs" (WD) [2]. Among the planets in our solar system, as is known, Jupiter and Saturn have the largest magnetic fields, having a large share of hydrogen in their structure (gas giants) (see, e.g. [3]). A recent comparison of theoretical and observational data for magnetic WD indicates that large magnetic field of astrophysical objects is due to Fig. 1. Phase T-P diagram of hydrogen (see, e.g. [5,6]). (Here, monoatomic hydrogen in metallic, FM-ordered state [4]. symbols are: S – solid, L – liquid, M – metallic, H – hydrogen) The electrical conductivity increased by almost four 2 Hydrogen phase change with orders of magnitude between 90 and 140 GPa, in which increasing pressure range liquid H at T > 1000 K is a semiconductor with conduction electrons thermally excited through the energy gap. It closes with a density of up to ~ kBT by ~ 2.1 Molecular and metallic hydrogen 0.64 mol/cm3. At 140 GPa to 180 GPa thermal excitation is quenched and liquid H is a bad metal [5,6]. Measured As known, liquid-metal hydrogen in laboratory was -1 -1 obtained under dynamic compression, at a certain conductivity 2000 Ohm cm characteristic for liquid density, P = 140 GPa and T ~ 2600 K (see, e.g. [5,6]). metals and highly disordered alloys is in good agreement These states were obtained repeating shock compression with the theoretical calculations (see, e.g. [5,6]). generated by a shock wave in liquid H at T = 20 K, It is interesting that later, at a static pressure P > 220 2 GPa, a layer-like lattice, called Phase IV, was detected. * Corresponding author: [email protected] © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 185, 08003 (2018) https://doi.org/10.1051/epjconf/201818508003 MISM 2017 Ph is sulating nd, ossibly, emi-metall up diagram, PM and FM phases correspond to the PM and 360 Pa, bov which, s redicted, can ecom FM fluids, respectively; S-phase corresponds to bcc metallic (see, g. 5,6]). ce layer-lik cc phas f crystalline solid. Points Tspf and Cmag correspond to the H2 is imilar raph rms f very w tron triple point and the critical point for the FM transition. charg density n ground tate," shcrof [1,7 considered e p properties f graphite btain understanding f natur of olid 2 at ver gh pressur I concluded th solid emimetal 2 at high ressures robably h layered, ystalline structur f th cc typ d that th aired- n n this structur wi suppor is semimeta has very high ressur thereby reventing transition m h ressur higher than th ressur n th lamellar hase. 2.2 Ferromagnetism in metallic hydrogen There were put forward two concepts indicating that metallic hydrogen can be FM in nature at high pressure [3,4]. The nature of magnetic transition from a paramagnetic (PM) to a FM phase is in fact in a Fig. 2. Magnetic phase H-T diagram of metallic hydrogen [4]. competition between the spin-orientation dependent The diamonds represent the data for magnetic white dwarfs exchange processes which favor a spin-aligned FM state estimated from observational data [4] (for legend, see text). and the contributions of kinetic energy which favor a PM state [4]. In the ground state a FM solid is formed due to Table 1. Physical parameters at the triple point Tspf and the critical the predominance of exchange effects between parallel point Cmag for the metallic hydrogen (m = mn is the mass density) spins. In one of above approaches [4], the phase diagram for dense metallic hydrogen can be essentially -- -- Tspf Cmag characterized by two dimensionless parameters -3 17 28 2 3 n (cm 3. x 6. x r me 2a 2 and 2 mk T 2 3 2 n , s B (g/cm3) 8.3 02 1. x -5 corresponding to density and temperature, respectively. m 4 7 Here m, n, and kB refer to the mass of a fermion, the BM(G) 8. x 1. x number density of the fermions, and the Boltzmann r R 183 43 1 3 s s constant, respectively; a 4 n / 3 . T(K) 3. x 3 5. x 4 In a FM state the Fermi liquid then induces a , 0.281 0.141 magnetic field with the strength BM 4M n , where M denotes the magnetic moment of a fermion, and for 2 2 In this Table, Rs mH e a exp(0.85Ks ) , an electron, = 9.28477 x 10-21 (erg / G). M 2 2 2 3 2m k T (3 n) , n and mH refer to the The free energy for the strongly coupled (rs > 20 F H B and > 10) PM electron liquids can been expressed in a number density and the mass of hydrogen (after [4]). parametrized form as [4] The data in Fig.2 elucidate physical mechanisms for the origin of strong magnetization in the magnetic f (r ) f () f (,,) , (1) PM s, 0 1 WD [4]. For the nuclear FM to be of relevance to the 2 stellar magnetism, the state of matter should fall in the where e akBT . First term f0 () corresponds to FM domain. Solidification of metallic hydrogen, on the the Helmholtz free energy per particle of the ideal-gas other hand, may not take place in such WD, since each electrons in the PM state in units of the thermal energy. of the T values exceeds the temperature at the triple The ideal-gas contribution to the free energy is expressed eff point [4]. The FM phase transition is a concept which as a balance between those of the chemical potential and may provide a relevant account for an origin of the of the pressure as strong magnetization e.g. in degenerate magnetic stars. f o () 0 () p0 () (2) The free-energy formulas for the strongly coupled FM 3 Analysis of experimental data electron fluids in the finite temperature regime, were constructed analogously to the way Eq. (1), where 3.1 Metallic hydrogen parameter should be then naturally replaced to 2 2 2 3 The abov work [5 was f grea teres nd excitation F 2mekBT (6 n) [4]. in h metallization f hydrogen an unprecedentedly The resulting magnetic phase diagram for metallic high ressure, eporting perimen aman pectra, hydrogen was constructed in [4] (see, Fig.2). In this 2 2 EPJ Web of Conferences 185, 08003 (2018) https://doi.org/10.1051/epjconf/201818508003 MISM 2017 ph ansition, metallization, d eflection coefficients bserved isually. fortunately, values of electr resistances btained rom heir measured electr resistances nd reduced imensions f he samp was in de agnitud larger an or e metal, nd was no lear to wh exten hydrogen caused es observations [6]. I he beginni f this ear, appears teresting paper 8 reporting the observation f the henomenon of phase ansition, which ndic h production f m hydrogen. Atom hydrogen rom h liquid ph assed solid pressur up 95 (see Fig.1 temperatur of .5 in iamond vil between h tips f tific iamonds. ydrogen rom s of ansparen lik glass, ansformed to s of shiny meta lik copper r old eflecting gh (see Fig.3). h transition solid h w confirmed by pectroscop alysis. he 8] was predicted th after emov of ressure, eta ydrogen he new periments would emain metastable. Fig. 4. T-dependence of resistance in H2S (see [9,10]). no directly elated SC. n r pe 10], as suggested h th upper ar of h resistiv transition s associated with magneti (A SDW-like) has transition n h system (shaded ea Fig.4) while th SC transition ccurs much wer mperatur The magn resistivity m (T) can estimated fro the to esistivity tot (T) via subtracting hono contribution ph (T ) (Bloch-Gruneisen (BG) curv Fig.4) d esidu par res (give n ig.4 y th oint o intersection f dashed n with rdin axis).
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