arXiv subjects

Yinwei Li

Publications and source records attributed to Yinwei Li.

23 records · Page 2Linked to original sources

arXiv2015

Metallic Icosahedron Phase of Sodium at Terapascal Pressures

Alkali metals exhibit unexpected structures and electronic behavior at high pressures. Compression of metallic sodium (Na) to 200 GPa leads to the stability of a wide-band-gap insulator with the double hexagonal hP4 structure. Post-hP4 structures remain unexplored, but they are important for addressing the question of the pressure at which Na reverts to a metal. Here we report the reentrant metallicity of Na at the very high pressure of 15.5 terapascal (TPa), predicted using first-principles structure searching simulations. Na is therefore insulating over the large pressure range of 0.2-15.5 TPa. Unusually, Na adopts an oP8 structure at pressures of 117-125 GPa, and the same oP8 structure at 1.75-15.5 TPa. Metallization of Na occurs on formation of a stable and striking body-centered cubic cI24 electride structure consisting of Na12 icosahedra, each housing at its center about one electron which is not associated with any Na ions.

cond-mat.mtrl-sci↗

arXiv2015

High-energy-density and superhard nitrogen-rich B-N compounds

The pressure-induced transformation of diatomic nitrogen into non-molecular polymeric phases may produce potentially useful high-energy-density materials. We combine first-principles calculations with structure searching to predict a new class of nitrogen-rich boron nitrides with a stoichiometry of B3N5 that are stable or metastable relative to solid N2 and h-BN at ambient pressure. The most stable phase at ambient pressure has a layered structure (h-B3N5) containing hexagonal B3N3 layers sandwiched with intercalated freely rotating N2 molecules. At 15 GPa, a three-dimensional C2221 structure with single N-N bonds becomes the most stable. This pressure is much lower than that required for triple-to-single bond transformation in pure solid nitrogen (110 GPa). More importantly, C2221-B3N5 is metastable, and can be recovered under ambient conditions. Its energy density of 3.44 kJ/g makes it a potential high-energy-density material. In addition, stress-strain calculations estimate a Vickers hardness of 44 GPa. Structure searching reveals a new clathrate sodalite-like BN structure that is metastable under ambient conditions.

cond-mat.mtrl-sci↗

arXiv2015

Non-BCS thermodynamic properties of H2S superconductor

The present paper determines the thermodynamic properties of the superconducting state in the ${\rm H_{2}S}$ compound. The values of the pressure from $130$ GPa to $180$ GPa were taken into consideration. The calculations were performed in the framework of the Eliashberg formalism. In the first step, the experimental course of the dependence of the critical temperature on the pressure was reproduced: $T_{C}\in\left<31,88\right>$ K, whereas the Coulomb pseudopotential equal to $0.15$ was adopted. Next, the following quantities were calculated: the order parameter at the temperature of zero Kelvin ($Δ\left(0\right)$), the specific heat jump at the critical temperature ($ΔC\left(T_{C}\right)\equiv C^{S}\left(T_{C}\right)-C^{N}\left(T_{C}\right)$), and the thermodynamic critical field ($H_{C}\left(0\right)$). It was found that the values of the dimensionless ratios: $R_Δ\equiv 2Δ(0)/k_{B}T_{C}$, $R_{C}\equiv{ΔC\left(T_{C}\right)}/{C^{N}\left(T_{C}\right)}$, and $R_{H}\equiv{T_{C}C^{N}\left(T_{C}\right)}/{H_{C}^{2}\left(0\right)}$ deviate from the predictions of the BCS theory: $R_Δ\in\left<3.64,4.16\right>$, $R_{C}\in\left<1.59,2.24\right>$, and $R_{H}\in\left<0.144, 0.163\right>$. Generalizing the results on the whole family of the ${\rm H_{n}S}$-type compounds, it was shown that the maximum value of the critical temperature can be equal to $\sim 290$ K, while $R_Δ$, $R_{C}$ and $R_{H}$ adopt the following values: $6.53$, $3.99$, and $0.093$, respectively.

cond-mat.supr-con↗

arXiv2015

Hydrogen sulphide at high pressure: a strongly-anharmonic phonon-mediated superconductor

We use first principles calculations to study structural, vibrational and superconducting properties of H$_2$S at pressures $P\ge 200$ GPa. The inclusion of zero point energy leads to two different possible dissociations of H$_2$S, namely 3H$_2$S $\to$ 2H$_3$S + S and 5H$_2$S $\to$ 3H$_3$S + HS$_2$, where both H$_3$S and HS$_2$ are metallic. For H$_3$S, we perform non-perturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction ($λ\approx 2.64$ at 200 GPa) and T$_c$. Anharmonicity hardens HS bond-stretching modes and softens H--S bond-bending modes. As a result, the electron-phonon coupling is suppressed by $30\%$ ($λ\approx 1.84$ at 200 GPa). Moreover, while at the harmonic level T$_c$ decreases with increasing pressure, the inclusion of anharmonicity leads to a T$_c$ that is almost independent of pressure. High pressure hydrogen sulfide is a strongly anharmonic superconductor.

cond-mat.supr-con↗

arXiv2014

The metallization and superconductivity of dense hydrogen sulfide

Hydrogen sulfide (H2S) is a prototype molecular system and a sister molecule of water. The phase diagram of solid H2S at high pressures remains largely unexplored arising from the challenges in dealing with the looser S-H bond and larger atomic core difference between H and S. Metallization is yet achieved for water ice, but it was established for H2S above 96 GPa. However, the metallic structure of H2S remains elusive, greatly impeding the understanding of its metallicity and the potential superconductivity. We have performed an extensive structural study on solid H2S under high pressures through unbiased first-principles structure predictions based on swarm intelligence. Besides the findings of best-known candidate structures for nonmetallic phases IV and V, we are able to establish stable metallic structures violating an earlier proposal of elemental decomposition into sulfur and hydrogen [PRL 85, 1254 (2000)]. Our study unraveled a superconductive potential of metallic H2S with an estimated maximal transition temperature of ~ 80 K at 160 GPa, higher than those predicted for most archetypal hydrogen-containing compounds (e.g., SiH4 and GeH4, etc).

cond-mat.supr-con↗