Tuning the Flat Band in Bi2O2Se by Pressure to Induce Superconductivity.

The discovery of superconductivity in twisted bilayer graphene has reignited enthusiasm in the field of flat-band superconductivity. However, important challenges remain, such as constructing a flat-band structure and inducing a superconducting state in materials. Here, we successfully achieved superconductivity in Bi2O2Se by pressure-tuning the flat-band electronic structure. Experimental measurements combined with theoretical calculations reveal that the occurrence of pressure-induced superconductivity at 30 GPa is associated with a flat-band electronic structure near the Fermi level. Moreover, in Bi2O2Se, a van Hove singularity is observed at the Fermi level alongside pronounced Fermi surface nesting. These remarkable features play a crucial role in promoting strong electron-phonon interactions, thus potentially enhancing the superconducting properties of the material. These findings demonstrate that pressure offers a potential experimental strategy for precisely tuning the flat band and achieving superconductivity.

Topological Phase Transition and Phonon-Space Dirac Topology Surfaces in ZrTe_{5}.

We use first-principles methods to demonstrate that, in ZrTe_{5}, a layered van der Waals material like graphite, atomic displacements corresponding to five of the six zone-center A_{g} (symmetry-preserving) phonon modes can drive a topological transition from a strong to a weak topological insulator with a Dirac semimetal state emerging at the transition, giving rise to a Dirac topology surface in the multidimensional space formed by the A_{g} phonon modes. This implies that the topological transition in ZrTe_{5} can be realized with many different settings of external stimuli capable of penetrating through the phonon-space Dirac surface without breaking the crystallographic symmetry. Furthermore, we predict that domains with effective mass of opposite signs can be created by laser pumping and will host Weyl modes of opposite chirality propagating along the domain boundaries. Studying phonon-space topology surfaces provides a new route to understanding and utilizing the exotic physical properties of ZrTe_{5} and related quantum materials.

Crossover from metal to insulator in dense lithium-rich compound CLi4.

At room environment, all materials can be classified as insulators or metals or in-between semiconductors, by judging whether they are capable of conducting the flow of electrons. One can expect an insulator to convert into a metal and to remain in this state upon further compression, i.e., pressure-induced metallization. Some exceptions were reported recently in elementary metals such as all of the alkali metals and heavy alkaline earth metals (Ca, Sr, and Ba). Here we show that a compound of CLi4 becomes progressively less conductive and eventually insulating upon compression based on ab initio density-functional theory calculations. An unusual path with pressure is found for the phase transition from metal to semimetal, to semiconductor, and eventually to insulator. The Fermi surface filling parameter is used to describe such an antimetallization process.

Emergent property of high hardness for C-rich ruthenium carbides: partial covalent Ru-Ru bonds.

Hard materials are being investigated all the time by combining transition metals with light elements. Combining a structure search with first-principles functional calculations, we first discovered three stable stoichiometric C-rich ruthenium carbides in view of three synthesis routes, namely, the ambient phases of Ru2C3 and RuC, and two high pressure phases of RuC4. There is a phase transition of RuC4 from the P3[combining macron]m1 structure to the R3[combining macron]m structure above 98 GPa. The calculations of elastic constants and phonon dispersions show their mechanical and dynamical stability. The large elastic modulus, high Debye temperature and the estimated hardness values suggest that these hard ruthenium carbides have good mechanical properties. The analyses of electronic structure and chemical bonding indicate that chemical bonding, not carbon content, is the key factor for the hardness in these metallic C-rich ruthenium carbides. The partial covalent Ru-C bonds and strong covalent C-C bonds are responsible for the high hardness. Moreover, the emergence of partial covalent Ru-Ru bonds can enhance the hardness of RuC, while the ionic Ru-Ru bonds can weaken the hardness of Ru2C3.

Pressure-Stabilized Superconductive Ionic Tantalum Hydrides.

High-pressure structures of tantalum hydrides were investigated over a wide pressure range of 0-300 GPa by utilizing evolutionary structure searches. TaH and TaH2 were found to be thermodynamically stable over this entire pressure range, whereas TaH3, TaH4, and TaH6 become thermodynamically stable at pressures greater than 50 GPa. The dense Pnma (TaH2), R3̅m (TaH4), and Fdd2 (TaH6) compounds possess metallic character with a strong ionic feature. For the highly hydrogen-rich phase of Fdd2 (TaH6), a calculation of electron-phonon coupling reveals the potential high-Tc superconductivity with an estimated value of 124.2-135.8 K.

Investigation of superconductivity in compressed vanadium hydrides.

Aiming at finding new superconducting materials, we performed systematical simulations on phase diagrams, crystal structures, and electronic properties of vanadium hydrides under high pressures. The VH, VH2, VH3, and VH5 species were found to be stable under high pressures; among these, VH2 had previously been investigated. Moreover, all three novel stoichiometries showed a strong ionic character as a result of the charge transfer from V to H. The electron-phonon coupling calculations revealed the potentially superconductive nature of these vanadium hydrides, with estimated superconducting critical temperature (Tc) values of 6.5-10.7 K for R3[combining overline]m (VH), 8.0-1.6 K for Fm3[combining overline]m (VH3), and 30.6-22.2 K for P6/mmm (VH5) within the pressure range from 150 GPa to 250 GPa.

Investigation of stable germane structures under high-pressure.

The evolutionary structure-searching method discovers that the energetically preferred compounds of germane can be synthesized at a pressure of 190 GPa. New structures with the space groups Ama2 and C2/c proposed here contain semimolecular H2 and V-type H3 units, respectively. Electronic structure analysis shows the metallic character and charge transfer from Ge to H. The conductivity of the two structures originates from the electrons around the hydrogen atoms. Further electron-phonon coupling calculations predict that the two phases are superconductors with a high Tc of 47-57 K for Ama2 at 250 GPa and 70-84 K for C2/c at 500 GPa from quasi-harmonic approximation calculations, which may be higher than under actual conditions.

Pressure induced phase transition in MH2 (M = V, Nb).

High-pressure structures of MH2 (M = V, Nb) are explored through ab initio evolutionary methodology. As the same main group metal hydrides, VH2 and NbH2 adopt the same tetragonal structure with space group Fm-3m at low pressures. However, at high pressures VH2 and NbH2 possess Pnma and P63mc phases differently. The two phase transitions are both the first order phase transition identified by volume collapses. Our calculations suggest that two high-pressure structures have both dynamical and mechanical stability up to 100 GPa. Pnma VH2 and P63mc NbH2 are metallic phases demonstrated by the band structure and density of states. However, their superconducting temperatures are only several Kelvins.

Structure determination of ultra dense magnesium borohydride: a first-principles study.

Magnesium borohydride (Mg(BH4)2) is one of the potential hydrogen storage materials. Recently, two experiments [Y. Filinchuk, B. Richter, T. R. Jensen, V. Dmitriev, D. Chernyshov, and H. Hagemann, Angew. Chem., Int. Ed. 50, 11162 (2011); L. George, V. Drozd, and S. K. Saxena, J. Phys. Chem. C 113, 486 (2009)] found that α-Mg(BH4)2 can irreversibly be transformed to an ultra dense δ-Mg(BH4)2 under high pressure. Its volumetric hydrogen content at ambient pressure (147 g/cm(3)) exceeds twice of DOE's (U.S. Department of Energy) target (70 g/cm(3)) and that of α-Mg(BH4)2 (117 g/cm(3)) by 20%. In this study, the experimentally proposed P4(2)nm structure of δ-phase has been found to be dynamically unstable. A new Fddd structure has been reported as a good candidate of δ-phase instead. Its enthalpy from 0 to 12 GPa is much lower than P4(2)nm structure and the simulated X-ray diffraction spectrum is in satisfied agreement with previous experiments. In addition, the previously proposed P-3m1 structure, which is denser than Fddd, is found to be a candidate of ε-phase due to the agreement of Raman shifts.

Superconducting high-pressure phases of disilane.

High-pressure structures of disilane (Si(2)H(6)) are investigated extensively by means of first-principles density functional theory and a random structure-searching method. Three metallic structures with P-1, Pm-3m, and C2/c symmetries are found, which are more stable than those of XY(3)-type candidates under high pressure. Enthalpy calculations suggest a remarkably wide decomposition (Si and H(2)) pressure range below 135 GPa, above which three metallic structures are stable. Perturbative linear-response calculations for Pm-3m disilane at 275 GPa show a large electron-phonon coupling parameter lambda of 1.397 and the resulting superconducting critical temperature beyond the order of 10(2) K.

A first-principles study on crystal structures and metallization of sodium-rich sulfides under high pressure.

We performed systematical theoretical simulations on phase diagrams, crystal structures, electron properties, and phonon features of Na-S system under high pressures. NaS, Na2S, and Na4S, were found to be stable under pressures. The superconducting transition critical temperature was estimated to nearly 0 K at 100 GPa in Na3S due to the weak electron-phonon coupling. Furthermore, by the comparison on the structures, the electron features, and alkali metal ions of stoichiometric proportion, we found that not only the pressure but also the number of sodium atoms in the formula unit of alkali metal atoms can promote the insulator-metal transformation in the alkali metal sulfides, such as Li-S, Na-S, and K-S systems.

Revealing unusual rigid diamond net analogues in superhard titanium carbides.

Transition metal carbides (TMCs) are considered to be potential superhard materials and have attracted much attention. With respect to titanium and carbon atoms, we confirm the pressure-composition phase diagram of the Ti-C system using structure searches and first-principles calculations. We firstly discovered stable TiC4 which was expected to be synthesized at high pressure, as well as metastable TiC2 and TiC3. These layered titanium carbides are diamond net analogues due to the unusual C-layers in the form of puckered graphene-like, diamond-like and double diamond-like C-layers. The existence of diamond-like C-layers might help to understand the formation of diamond. All the studied titanium carbides could be recoverable at ambient pressure and exhibited great mechanical properties (strong ability to resist volume and shear deformations, small anisotropy, and high hardness). Moreover, we crystallized the structure of TiC4 in other transition metal carbides and obtained five superhard TMC4s (TM = V, Zr, Nb, Hf and Ta). Interactions between layers were revealed to be the source of the great mechanical properties and high hardness through combining detailed analyses of electronic structure and chemical bonding, namely, weak ionic interactions of neighboring Ti- and C-layers and the strong covalent interactions of C- and C-layers.

High-temperature superconductivity in compressed solid silane.

Crystal structures of silane have been extensively investigated using ab initio evolutionary simulation methods at high pressures. Two metallic structures with P21/c and C2/m symmetries are found stable above 383 GPa. The superconductivities of metallic phases are fully explored under BCS theory, including the reported C2/c one. Perturbative linear-response calculations for C2/m silane at 610 GPa reveal a high superconducting critical temperature that beyond the order of 10(2) K.

Pressure confinement effect in MoS2 monolayers.

With ever increasing interest in layered materials, molybdenum disulfide has been widely investigated due to its unique optoelectronic properties. Pressure is an effective technique to tune the lattice and electronic structure of materials such that high pressure studies can disclose new structural and optical phenomena. In this study, taking MoS2 as an example, we investigate the pressure confinement effect on monolayer MoS2 by in situ high pressure Raman and photoluminescence (PL) measurements. Our results reveal a structural deformation of monolayer MoS2 starting from 0.84 GPa, which is evidenced by the splitting of E(1)2g and A1g modes. A further compression leads to a transition from the 1H-MoS2 phase to a novel structure evidenced by the appearance of two new peaks located at 200 and 240 cm(-1). This is a distinct feature of monolayer MoS2 compared with bulk MoS2. The new structure is supposed to have a distorted unit with the S atoms slided within a single layer like that of metastable 1T'-MoS2. However, unlike the non-photoluminescent 1T'-MoS2 structure, our monolayer shows a remarkable PL peak and a pressure-induced blue shift up to 13.1 GPa. This pressure-dependent behavior might enable the development of novel devices with multiple phenomena involving the strong coupling of the mechanical, electrical and optical properties of layered nanomaterials.

Nitrogen concentration driving the hardness of rhenium nitrides.

The structures and properties of rhenium nitrides are studied with density function based first principle method. New candidate ground states or high-pressure phases at Re:N ratios of 3:2, 1:3, and 1:4 are identified via a series of evolutionary structure searches. We find that the 3D polyhedral stacking with strong covalent N-N and Re-N bonding could stabilize Re nitrides to form nitrogen rich phases, meanwhile, remarkably improve the mechanical performance than that of sub-nitrides, as Re3N, Re2N, and Re3N2. By evaluating the trends of the crystal configuration, electronic structure, elastic properties, and hardness as a function of the N concentration, we proves that the N content is the key factor affecting the metallicity and hardness of Re nitrides.

Crystal structure prediction and hydrogen-bond symmetrization of solid hydrazine under high pressure: a first-principles study.

In this article, the crystal structure of solid hydrazine under pressure has been extensively investigated using ab initio evolutionary simulation methods. Calculations indicate that hydrazine remains both insulating and stable up to at least 300 GPa at low temperatures. A structure with P21 symmetry is found for the first time through theoretical prediction in the pressure range 0-99 GPa and it is consistent with previous experimental results. Two novel structures are also proposed, in the space groups Cc and C2/c, postulated to be stable in the range 99-235 GPa and above 235 GPa, respectively. Below 3.5 GPa, C2 symmetry is found originally, but it becomes unstable after adding the van der Waals interactions. The P21→Cc transition is first order, with a volume discontinuity of 2.4%, while the Cc→C2/c transition is second order with a continuous volume change. Pressure-induced hydrogen-bond symmetrization occurs at 235 GPa during the Cc→C2/c transition. The underlying mechanism of hydrogen-bond symmetrization has also been investigated by analysis of electron localization functions and vibrational Raman/IR spectra.

A new high-pressure polar phase of crystalline bromoform: a first-principles study.

The high-pressure phases of bromoform at zero temperature have been investigated by first-principles pseudopotential plane-wave calculations based on the density functional theory. A new high-pressure polar phase, ε, with space group CC has been found after a series of simulated annealing and geometry optimizations. Our calculated enthalpies showed that the transition from ß phase to γ phase occurs at 1 GPa, then the γ phase transforms to the ε phase at 90 GPa. In addition, the Br···Br and C-H···Br interactions are the key factors for the polar aggregation in the ε phase. Further calculations show that the insulate-metal transition in ε phase due to band overlap happens at ~130 GPa.