Akitaka Nakanishi

Superconducting H5S2 phase in sulfur-hydrogen system under high-pressure

Recently, hydrogen sulfide was experimentally found to show the high superconducting critical temperature (Tc) under high-pressure. The superconducting Tc shows 30–70 K in pressure range of 100–170 GPa (low-Tc phase) and increases to 203 K, which sets a record for the highest Tc in all materials, for the samples annealed by heating it to room temperature at pressures above 150 GPa (high-Tc phase). Here we present a solid H5S2 phase predicted as the low-Tc phase by the application of the genetic algorithm technique for crystal structure searching and first-principles calculations to sulfur-hydrogen system under high-pressure. The H5S2 phase is thermodynamically stabilized at 110 GPa, in which asymmetric hydrogen bonds are formed between H2S and H3S molecules. Calculated Tc values show 50–70 K in pressure range of 100–150 GPa within the harmonic approximation, which can reproduce the experimentally observed low-Tc phase. These findings give a new aspect of the excellent superconductivity in compressed sulfur-hydrogen system.

Pressure-Induced Structural Transition and Enhancement of Energy Gap of CuAlO2

By using first-principles calculations, we studied the stable crystal structures and energy gaps of CuAlO 2 under high pressure. Our simulation shows that CuAlO 2 transforms from a delafossite structure to a leaning delafossite structure. The critical pressure of the transition was determined to be 60 GPa. The energy gap of CuAlO 2 increases through the structural transition due to the enhanced covalency of Cu 3d and O 2p states. We found that a chalcopyrite structure does not appear as a stable structure under high pressure.

Computational materials design of negative effective U system in hole-doped chalcopyrite CuFeS2.

A general rule of negative effective U(U(eff)) system caused by (i) exchange correlation and (ii) charge excitation mechanisms is proposed. Based on the general rule, we perform ab initio electronic structure calculations by generalized gradient approximation (GGA) + U method for hole-doped chalcopyrite CuFeS2 [Cu(+)(d(10))Fe(3+)(d(5))S(2-)(s(2)p(6))2]. It is found from our calculations that the hole-doped CuFeS2 has the negative U(eff) = -0.44 eV, where U(eff) ≡ E(N + 1) + E(N - 1) - 2E(N) < 0 and E(N) is the total energy of the hole-doped CuFeS2. The negative U(eff) is caused by the charge-excitation in the hole-doped Cu(2+)(d(9)) and S(-)(s(2)p(5)), and also caused by the exchange-correlation in the hole-doped Fe(4+)(d(4)). The strong attractive electron-electron interaction (U(eff) = -0.44 eV ∼ -5000 K) originates from the purely electronic mechanism. The closed shell of the d(10) electronic configuration is more stable than the d(9) electronic configuration, since the first excited state with the d(9)s(1) electronic configuration and the ground state with the d(10) electronic configuration are very close, then these two states repel very strongly through the second order perturbation. Therefore, the spin-polarized total energy curve for the hole-doped CuFeS2 shows the strong upward convexity with N - 1, N and N + 1 electronic configurations leading to the negative U(eff). The hole-doped paramagnetic and metallic CuFeS2 with the negative U(eff) may cause a possible high-Tc superconductor (Tc ∼ 1000 K, if 2Δ/kBTc ≈ 10 by assuming a strong coupling regime) because of the strong attractive electron-electron interactions (superconducting gap Δ ≈ |U(eff|) ∼ 5000 K). Finally, we propose a new computational materials design methodology to design ultra high-Tc superconductors by using three steps starting from the atomic number only.

First-Principles Study of NaFeAs, NaCoAs, and NaNiAs

To investigate the material dependence of the electronic structure of arsenide superconductors, the chemical trend of the Kohn–Sham band structures of a series of compounds, i.e., NaFeAs, NaCoAs, and NaNiAs, is studied by first-principles calculation based on generalized gradient approximation. Hypothetical structures of NaCoAs and NaNiAs in P 4/ n m m are found to be stable by structural optimization simulation. Results on the electronic states suggest that a characteristic two-dimensional electronic structure appearing as rodlike Fermi pockets is clearly found only in NaFeAs, while three-dimensional electronic structures are found in NaCoAs and NaNiAs with larger density of states than NaFeAs at the Fermi level, when paramagnetic electronic states are assumed.

General Rule and Materials Design of Negative Effective U System for High-Tc Superconductivity

Based on the microscopic mechanisms of (1) charge-excitation-induced negative effective U in s1 or d9 electronic configurations, and (2) exchange-correlation-induced negative effective U in d4 or d6 electronic configurations, we propose a general rule and materials design of negative effective U system in itinerant (ionic and metallic) system for the realization of high-Tc superconductors. We design a Tc-enhancing layer (or clusters) of charge-excitation-induced negative effective U connecting the superconducting layers for the realistic systems.

Self-Interaction Corrected Electronic Structure and Energy Gap of CuAlO2 beyond Local Density Approximation

We implemented a self-interaction correction (SIC) into first-principles calculation code to go beyond local density approximation and applied it to CuAlO 2 . Our simulation shows that the valence band width calculated within the SIC is narrower than that calculated without the SIC because the SIC makes the d -band potential deeper. The energy gap calculated within the SIC expands and is close to experimental data.

Theoretical Evidences for Enhanced Superconducting Transition Temperature of CaSi2 in a High-Pressure AlB2 Phase

By means of first-principles calculations, we studied stable lattice structures and estimated superconducting transition temperature of CaSi2 at high pressure. Our simulation showed stability of the AlB2 structure in a pressure range above 17 GPa. In this structure, doubly degenerated optical phonon modes, in which the neighboring silicon atoms oscillate alternately in a silicon plane, show prominently strong interaction with the conduction electrons. In addition there exists a softened optical mode (out-of-plan motion of silicon atoms), whose strength of the electron-phonon interaction is nearly the same as the above mode. The density of states at the Fermi level in the AlB2 structure is higher than that in the trigonal structure. These findings and the estimation of the transition temperature strongly suggest that higher Tc is expected in the AlB2 structure than the trigonal structures which are known so far.By means of first-principles calculations, we studied stable lattice structures and estimated superconducting transition temperature of CaSi 2 at high pressure. Our simulation shows stability of the AlB 2 structure in a pressure range above 17 GPa. In this structure, doubly degenerated optical phonon modes, in which the neighboring silicon atoms oscillate alternately in a silicon plane, show prominently strong interaction with the conduction electrons. In addition there exists a softened optical mode (out-of-plan motion of silicon atoms), whose strength of the electron–phonon interaction is nearly the same as the above mode. The density of states at the Fermi level in the AlB 2 structure is higher than that in the trigonal structure. These findings and the estimation of the transition temperature strongly suggest that higher T c is expected in the AlB 2 structure than the trigonal structures which are known so far.

Chemical Trend of Superconducting Critical Temperatures in Hole-Doped CuBO2, CuAlO2, CuGaO2, and CuInO2

We calculated the superconducting critical temperature (Tc) for hole-doped CuXO2 (X = B, Al, Ga, and In) compounds using first-principles calculations based on rigid band model. The compounds with X = Al, Ga, and In have delafosite-type structures and take maximum Tc values at 0.2–0.3 with respect to the number of holes (Nh) in the unit-cell: 50 K for CuAlO2, 10 K for CuGaO2, and 1 K for CuInO2. The decrease of Tc for this change in X is involved by covalency reduction and lattice softening associated with the increase of ionic mass and radius. For CuBO2 which is a lighter compound than CuAlO2, the delafosite structure is unstable and a body-centered tetragonal structure emerges as the most stable structure. As the results, the electron–phonon interaction is decreased and Tc is lower by approximately 43 K than that of CuAlO2 at the hole-doping conditions of Nh = 0.2–0.3.

Origin of enhanced superconducting transition temperature through structural transformation in CaSi2

Using the first-principles lattice dynamics, we have studied physical origin of enhancement in the superconducting transition temperature Tc of CaSi2 with structural phase transition. Optimization results show that CaSi2 has the AlB2 structure as an optimized structure above 17GPa. The electron-phonon interaction is enhanced, when CaSi2 takes the AlB2 structure compared to phase III. Especially, an E2g Einstein mode and a softened optical B2g mode are important.

First-principles study on superconductivity of simple cubic, modulated and simple hexagonal phases in phosphorus

Superconducting properties of simple cubic (sc), modulated, and simple hexagonal (sh) phases in phosphorus are investigated from first-principles. The superconducting transition temperature decreases through the phase transition from the sc phase to the modulated phase. The superconductivity survives in the modulated phase, which indicates the coexistence of the superconducting state and a charge density wave (CDW) if the modulated structure in phosphorus is caused by CDW. In the modulated phase, phonon modes which show strong electron–phonon coupling lie along the direction of a modulation wave vector. By further compression through phase transition from the modulated phase to the sh phase, the transition temperature increases again. The variation in the transition temperature can be correlated to the variation of the density of states at the Fermi level.