Identification of Riluzole derivatives as novel calmodulin inhibitors with neuroprotective activity by a joint synthesis, biosensor, and computational guided strategy

The development of new molecules for the treatment of calmodulin related cardiovascular or neurodegenerative diseases is an interesting goal. In this work, we introduce a novel strategy with four main steps: (1) chemical synthesis of target molecules, (2) Förster Resonance Energy Transfer (FRET) biosensor development and in vitro biological assay of new derivatives, (3) Cheminformatics models development and in vivo activity prediction, and (4) Docking studies. This strategy is illustrated with a case study. Firstly, a series of 4-substituted Riluzole derivatives 1-3 were synthetized through a strategy that involves the construction of the 4-bromoriluzole framework and its further functionalization via palladium catalysis or organolithium chemistry. Next, a FRET biosensor for monitoring Ca2+-dependent CaM-ligands interactions has been developed and used for the in vitro assay of Riluzole derivatives. In particular, the best inhibition (80%) was observed for 4-methoxyphenylriluzole 2b. Besides, we trained and validated a new Networks Invariant, Information Fusion, Perturbation Theory, and Machine Learning (NIFPTML) model for predicting probability profiles of in vivo biological activity parameters in different regions of the brain. Next, we used this model to predict the in vivo activity of the compounds experimentally studied in vitro. Last, docking study conducted on Riluzole and its derivatives has provided valuable insights into their binding conformations with the target protein, involving calmodulin and the SK4 channel. This new combined strategy may be useful to reduce assay costs (animals, materials, time, and human resources) in the drug discovery process of calmodulin inhibitors.

MLe-KCNQ2: An Artificial Intelligence Model for the Prognosis of Missense KCNQ2 Gene Variants

Despite the increasing availability of genomic data and enhanced data analysis procedures, predicting the severity of associated diseases remains elusive in the absence of clinical descriptors. To address this challenge, we have focused on the KV7.2 voltage-gated potassium channel gene (KCNQ2), known for its link to developmental delays and various epilepsies, including self-limited benign familial neonatal epilepsy and epileptic encephalopathy. Genome-wide tools often exhibit a tendency to overestimate deleterious mutations, frequently overlooking tolerated variants, and lack the capacity to discriminate variant severity. This study introduces a novel approach by evaluating multiple machine learning (ML) protocols and descriptors. The combination of genomic information with a novel Variant Frequency Index (VFI) builds a robust foundation for constructing reliable gene-specific ML models. The ensemble model, MLe-KCNQ2, formed through logistic regression, support vector machine, random forest and gradient boosting algorithms, achieves specificity and sensitivity values surpassing 0.95 (AUC-ROC > 0.98). The ensemble MLe-KCNQ2 model also categorizes pathogenic mutations as benign or severe, with an area under the receiver operating characteristic curve (AUC-ROC) above 0.67. This study not only presents a transferable methodology for accurately classifying KCNQ2 missense variants, but also provides valuable insights for clinical counseling and aids in the determination of variant severity. The research context emphasizes the necessity of precise variant classification, especially for genes like KCNQ2, contributing to the broader understanding of gene-specific challenges in the field of genomic research. The MLe-KCNQ2 model stands as a promising tool for enhancing clinical decision making and prognosis in the realm of KCNQ2-related pathologies.

A Janus CrSSe monolayer with interesting ferromagnetism

The search for intrinsic half-metallic ferromagnetic (FM) monolayers with a high Curie temperature (TC), considerable magnetic anisotropy energy (MAE), and multiferroic coupling is key for the development of ultra-compact spintronics. Here, we have identified a new stable FM Janus monolayer, the tetrahedral CrSSe, through first-principles structural search calculations, which not only exhibits very interesting magnetoelectric properties with a high TC of 790 K, a large MAE of 0.622 meV per Cr, and robust half-metallicity, but also shows obvious ferroelasticity with a modest energy barrier of 0.31 eV per atom. Additionally, there appears to be interesting multiferroic coupling between in-plane magnetization and ferroelasticity. Furthermore, by replacing the Se/S atoms in the CrSSe monolayer with S/Se atoms, we obtained two new half-metallic FM CrS2 and CrSe2 monolayers, which also exhibit excellent magnetoelectric properties. Therefore, our findings provide a pathway to design novel multiferroic materials and enrich the understanding of 2D transition metal chalcogenides.

Emergent superconductivity in TaO3 at high pressures

Tantalum (Ta) is an interesting transition metal that exhibits superconductivity in its elemental states. Additionally, several Ta chalcogenides (S and Se) have also demonstrated superconducting properties. In this work, we propose the existence of five high-pressure metallic Ta-O compounds (e.g., TaO3, TaO2, TaO, Ta2O, and Ta3O), composed of polyhedra centered on Ta/O atoms. These compounds exhibit distinct characteristics compared to the well-known semiconducting Ta2O5. One particularly interesting finding is that TaO3 shows an estimated superconducting transition temperature (Tc) of 3.87 K at 200 GPa. This superconductivity is primarily driven by the coupling between the low-frequency phonons derived from Ta and the O 2p and Ta 5d electrons. Remarkably, its dynamically stabilized pressure can be as low as 50 GPa, resulting in an enhanced electron-phonon coupling and a higher Tc of up to 9.02 K. When compared to the superconductivity of isomorphic TaX3 (X = O, S, and Se) compounds, the highest Tc in TaO3 is associated with the highest NEF and phonon vibrational frequency. These characteristics arise from the strong electronegativity and small atomic mass of the O atom. Consequently, our findings offer valuable insights into the intrinsic physical mechanisms of high-pressure behaviors in Ta-O compounds.

Molecular dynamics simulations of the calmodulin-induced α-helix in the SK2 calcium-gated potassium ion channel

The family of small-conductance Ca²⁺-activated potassium ion channels (SK channels) is composed of four members (SK1, SK2, SK3, and SK4) involved in neuron-firing regulation. The gating of these channels depends on the intracellular Ca²⁺ concentration, and their sensitivity to this ion is provided by calmodulin (CaM). This protein binds to a specific region in SK channels known as the calmodulin-binding domain (CaMBD), an event which is essential for their gating. While CaMBDs are typically disordered in the absence of CaM, the SK2 channel subtype displays a small prefolded α-helical region in its CaMBD even if CaM is not present. This small helix is known to turn into a full α-helix upon CaM binding, although the molecular-level details for this conversion are not fully understood yet. In this work, we offer new insights on this physiologically relevant process by means of enhanced sampling, atomistic Hamiltonian replica exchange molecular dynamics simulations, providing a more detailed understanding of CaM binding to this target. Our results show that CaM is necessary for inducing a full α-helix along the SK2 CaMBD through hydrophobic interactions with V426 and L427. However, it is also necessary that W431 does not compete for these interactions; the role of the small prefolded α-helix in the SK2 CaMBD would be to stabilize W431 so that this is the case. In conclusion, our findings provide further insight into a key interaction between CaM and SK channels that is important for channel sensitivity to Ca²⁺.

Intrinsic Ferromagnetism in 2D Fe2H with a High Curie Temperature

The rational design of ferromagnetic materials is crucial for the development of spintronic devices. Using first-principles structural search calculations, we have identified 73 two-dimensional transition metal hydrides. Some of them show interesting magnetic properties, even when combined with the characteristics of the electrides. In particular, the P3̅m1 Fe₂H monolayer is stabilized in a 1T-MoS₂-type structure with a local magnetic moment of 3 μ_B per Fe atom, whose robust ferromagnetism is attributed to the exchange interaction between neighboring Fe atoms within and between sublayers, leading to a remarkably high Curie temperature of 340 K. On the other hand, it has a large magnetic anisotropic energy and spin-polarization ratio. Interestingly, the above room-temperature ferromagnetism of the Fe₂H monolayer is well preserved within a biaxial strain of 5%. The structure and electron property of surface-functionalized Fe₂H are also explored. All of these interesting properties make the Fe₂H monolayer an attractive candidate for spintronic nanodevices.

Superconducting Li10Se electride under pressure

Achieving a compound with interesting multiple coexisting states, such as electride, metallicity, and superconductivity, is of great interest in basic research and practical application. Pressure has become an effective way to realize high-temperature superconductivity in hydrides, whereas most electrides are semiconducting or insulating at high pressure. Here, we have applied swarm-intelligence structural search to identify a hitherto unknown C2/m Li₁₀Se electride that is superconducting at high pressure. More interestingly, Li₁₀Se is estimated to exhibit the highest T_c value of 16 K at 50 GPa, which is the lowest pressure among Li-based chalcogen electrides. This superconducting transition is dominated by Se-related low frequency vibration modes. The increasing electronic occupation of the Se 4d orbital and the decreasing amount of interstitial anion electrons with pressure heighten their coupling with low-frequency phonons, which is responsible for the enhancement of the T_c value. The finding of Li-based chalcogen superconducting electrides provides a reference for the realization of other superconducting electrides at lower pressures.

Wide Band Gap P3S Monolayer with Anisotropic and Ultrahigh Carrier Mobility

Phosphorene has offered an additional advantage for developing new optoelectronic devices due to its anisotropic and high carrier mobility. However, its instability in air causes a rapid degradation of the performance of the device. Thus, improving the stability of phosphorene while maintaining its original properties has become the key to the development of high-performance electronic devices. Herein, we propose that the formation of two-dimensional (2D) P-rich P-S compounds could achieve this goal. First-principles swarm-structural searches revealed two previously unkonwn P₃S and P₂S monolayers. The P₃S monolayer, consisting of n-bicyclo-P₆ units along the armchair direction, exhibits anisotropic and wide band gap characteristics. Interestingly, its carrier mobility reaches 1.11 × 10⁴ cm² V^-1 s^-1 and is much higher than in phosphorene. Its electronic band gap and optical absorption coefficients in the ultraviolet region reach 2.71 eV and 10⁵ cm^-1, respectively. Additionally, the P₃S monolayer has a high structural stability and resistance to air oxidation.

Anisotropic and High-Mobility C3S Monolayer as a Photocatalyst for Water Splitting

Taking into account the high conductivity and stability of carbon materials, such as graphene, and the strong polar covalent bonding character of main-group compounds, we explore potential 2D materials in the C-S binary system through first-principles structure search calculations. Herein, a hitherto unknown semiconducting C₃S monolayer is identified, consisting of well-known n-biphenyl and S atom linked benzenes, exhibiting an obvious direction-dependent atomic arrangement. Thus, it exhibits anisotropic mechanical properties and carrier mobility. Its electron mobility reaches 2.14 × 10⁴ cm² V^-1 s^-1 in the b direction, along which n-biphenyl units are arranged, and is much higher than that in the well-used MoS₂ monolayer and black phosphorus. Meanwhile, the C₃S monolayer has high optical absorption coefficients (10⁵ cm^-1), high thermal and dynamical stabilities, and a moderate ability to split water. All these desirable properties make the C₃S monolayer a promising candidate for applications in novel optoelectronic devices.

Do calmodulin binding IQ motifs have built-in capping domains?

Most calmodulin (CaM) targets are α-helices. It is not clear if CaM induces the adoption of an α-helix configuration to its targets or if those targets are selected as they spontaneously adopt an α-helical conformation. Other than an α-helix propensity, there is a great variety of CaM targets with little more in common. One exception to this rule is the IQ site that can be recognized in a number of targets, such as those ion channels belonging to the KCNQ family. Although there is negligible sequence similarity between the IQ motif and the docking site on SK2 channels, both adopt a similar three-dimensional disposition. The isolated SK2 target presents a pre-folded core region that becomes fully α-helical upon binding to CaM. The existence of this pre-folded state suggests the occurrence of capping within CaM targets. In this review, we examine the capping properties within the residues flanking this core domain, and relate known IQ motifs and capping.

An epilepsy-causing mutation leads to co-translational misfolding of the Kv7.2 channel

Background

The amino acid sequence of proteins generally carries all the necessary information for acquisition of native conformations, but the vectorial nature of translation can additionally determine the folding outcome. Such consideration is particularly relevant in human diseases associated to inherited mutations leading to structural instability, aggregation, and degradation. Mutations in the KCNQ2 gene associated with human epilepsy have been suggested to cause misfolding of the encoded Kv7.2 channel. Although the effect on folding of mutations in some domains has been studied, little is known of the way pathogenic variants located in the calcium responsive domain (CRD) affect folding. Here, we explore how a Kv7.2 mutation (W344R) located in helix A of the CRD and associated with hereditary epilepsy interferes with channel function.

Results

We report that the epilepsy W344R mutation within the IQ motif of CRD decreases channel function, but contrary to other mutations at this site, it does not impair the interaction with Calmodulin (CaM) in vitro, as monitored by multiple in vitro binding assays. We find negligible impact of the mutation on the structure of the complex by molecular dynamic computations. In silico studies revealed two orientations of the side chain, which are differentially populated by WT and W344R variants. Binding to CaM is impaired when the mutated protein is produced in cellulo but not in vitro, suggesting that this mutation impedes proper folding during translation within the cell by forcing the nascent chain to follow a folding route that leads to a non-native configuration, and thereby generating non-functional ion channels that fail to traffic to proper neuronal compartments.

Conclusions

Our data suggest that the key pathogenic mechanism of Kv7.2 W344R mutation involves the failure to adopt a configuration that can be recognized by CaM in vivo but not in vitro.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01040-1.

Ba with Unusual Oxidation States in Ba Chalcogenides under Pressure

The preparation of compounds with novel atomic oxidation states and emergent properties is of fundamental interest in chemistry. As s-block elements, alkali-earth metals invariably show a +2 formal oxidation state at normal conditions, and among them, barium (Ba) presents the strongest chemical reactivity. Herein, we propose that novel valence states of Ba can be achieved in pressure-induced chalcogenides, where it also shows a feature of 5d-elements. First-principles swarm-intelligence structural search calculations identify three novel stoichiometric compounds: BaCh₄ (Ch = O, S) containing Ba²⁺, Ba₃Ch₂ (Ch = S, Se, Te) with Ba⁺ and Ba²⁺, and Ba₂Ch (Ch = Se, Te) with Ba⁺ cations. The pressure-induced drop of the Ba 5d level relative to Ba 6s is responsible for this unusual oxidation state. These compounds display captivating structural characters, such as Ba-centered polyhedra and chain-shaped Ch units. More interestingly still, the interaction between two Ba⁺ ions ensures their structural stability.

Semiconducting MnB5monolayer as a potential photovoltaic material

Exploring new two-dimensional (2D) materials is of great significance for both basic research and practical applications. Although boron can form various 3D and 2D allotropes due to its ease of forming multi-center bonds, the coexistence of honeycomb and kagome boron structures has never been observed in any 2D material yet. In this article we apply first-principle swarm structural searches to predict the existence of a stable MnB₅structure, consisting of a sandwich of honeycomb and kagome borophenes. More interestingly, a MnB₅nanosheet is a semiconductor with a band gap of 1.07 eV and a high optical absorption in a broad band, which satisfies the requirements of a very good photovoltaic material. Upon moderate strain, MnB₅undergoes a conversion from an indirect to a direct band gap semiconductor. The power conversion efficiency of a heterostructure solar cell made of MnB₅is up to 18%. The MnB₅nanosheet shows a robust dynamical and thermal stability, stemming from the presence of intra- and interlayer multi-center σ and π bonds. These characteristics make MnB₅a promising photovoltaic material.

Anomalous High-Temperature Superconductivity in YH6

Pressure-stabilized hydrides are a new rapidly growing class of high-temperature superconductors, which is believed to be described within the conventional phonon-mediated mechanism of coupling. Here, the synthesis of one of the best-known high-T_C superconductors-yttrium hexahydride I m 3 ¯ m -YH₆ is reported, which displays a superconducting transition at ≈224 K at 166 GPa. The extrapolated upper critical magnetic field B_c2 (0) of YH₆ is surprisingly high: 116-158 T, which is 2-2.5 times larger than the calculated value. A pronounced shift of T_C in yttrium deuteride YD₆ with the isotope coefficient 0.4 supports the phonon-assisted superconductivity. Current-voltage measurements show that the critical current I_C and its density J_C may exceed 1.75 A and 3500 A mm^-2 at 4 K, respectively, which is higher than that of the commercial superconductors, such as NbTi and YBCO. The results of superconducting density functional theory (SCDFT) and anharmonic calculations, together with anomalously high critical magnetic field, suggest notable departures of the superconducting properties from the conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories, and presence of an additional mechanism of superconductivity.

Weyl fermions, Fermi arcs, and minority-spin carriers in ferromagnetic CoS2

Magnetic Weyl semimetals are a newly discovered class of topological materials that may serve as a platform for exotic phenomena, such as axion insulators or the quantum anomalous Hall effect. Here, we use angle-resolved photoelectron spectroscopy and ab initio calculations to discover Weyl cones in CoS₂, a ferromagnet with pyrite structure that has been long studied as a candidate for half-metallicity, which makes it an attractive material for spintronic devices. We directly observe the topological Fermi arc surface states that link the Weyl nodes, which will influence the performance of CoS₂ as a spin injector by modifying its spin polarization at interfaces. In addition, we directly observe a minority-spin bulk electron pocket in the corner of the Brillouin zone, which proves that CoS₂ cannot be a true half-metal.

The Subchalcogenides Ir2In8Q (Q = S, Se, Te): Dirac Semimetal Candidates with Re-entrant Structural Modulation

Subchalcogenides are uncommon compounds where the metal atoms are in unusually low formal oxidation states. They bridge the gap between intermetallics and semiconductors and can have unexpected structures and properties because of the exotic nature of their chemical bonding as they contain both metal-metal and metal-main group (e.g., halide, chalcogenide) interactions. Finding new members of this class of materials presents synthetic challenges as attempts to make them often result in phase separation into binary compounds. We overcome this difficulty by utilizing indium as a metal flux to synthesize large (millimeter scale) single crystals of novel subchalcogenide materials. Herein, we report two new compounds Ir₂In₈Q (Q = Se, Te) and compare their structural and electrical properties to the previously reported Ir₂In₈S analogue. Ir₂In₈Se and Ir₂In₈Te crystallize in the P4₂/mnm space group and are isostructural to Ir₂In₈S, but also have commensurately modulated (with q vectors q = 1/6a* + 1/6b* and q = 1/10a* + 1/10b* for Ir₂In₈Se and Ir₂In₈Te, respectively) low-temperature phase transitions, where the chalcogenide anions in the channels experience a distortion in the form of In-Q bond alternation along the ab plane. Both compounds display re-entrant structural behavior, where the supercells appear on cooling but revert to the original subcell below 100 K, suggesting competing structural and electronic interactions dictate the overall structure. Notably, these materials are topological semimetal candidates with symmetry-protected Dirac crossings near the Fermi level and exhibit high electron mobilities (∼1500 cm² V^-1 s^-1 at 1.8 K) and moderate carrier concentrations (∼10²⁰ cm^-3) from charge transport measurements. This work highlights metal flux as a synthetic route to high quality single crystals of novel intermetallic subchalcogenides with Dirac semimetal behavior.

Achieving high hydrogen evolution reaction activity of a Mo2C monolayer

Two-dimensional Mo2C materials (1T and 2H phases) have emerged as promising electrocatalysts for the hydrogen evolution reaction (HER) due to their low cost, inherent metallicity, and high stability. Unfortunately, the catalytic activity of Mo2C is lower than that of Pt, and it needs to be substantially improved for practical applications. It is necessary and urgent to consider the effect of synergetic interactions among defects, functions, and strain on the HER activity. In this study, the geometric structures, electronic properties, and the HER activity of the Mo2C monolayer, with vacancy defects (i.e. Mo and C), oxygen functionalization, and strain, are studied by using first-principles calculations. According to our results, the combination of Mo vacancies, which can be obtained under C-rich conditions, and oxygen functionalization is the most effective way to improve the HER activity of 1T- and 2H-Mo2C. Considering the abundant active sites and optimal Gibbs free energy of hydrogen adsorption, the 1T phase we obtained shows excellent HER activity even at high H coverage and improves the utilization of active sites, for which the HER activity is comparable to that of Pt. This can be attributed to the fact that oxygen atoms gain more electrons from Mo2C, which weakens the strength of the O-H bond. Our work provides not only an opportunity to better understand the catalytic mechanism, but also a guide to achieving high HER activity of a Mo2C monolayer.

Plasmons in Li under compression

We report the high-pressure behavior of plasmon in polycrystalline Li up to 15 GPa at room temperature studied by inelastic x-ray scattering and ab initio calculation. The plasmon energy ([Formula: see text]) increases with decreasing atomic volume ([Formula: see text]), and the [Formula: see text] slope exhibits a discontinuity at bcc  →  fcc structural phase boundary reflecting the electronic band structure change. The plasmon peak width ([Formula: see text]) versus momentum transfer (q) curve of bcc-Li below 6.5 GPa keeps similar parabola-like shape. Above 8.4 GPa, where Li is in fcc, it changes from that of bcc-Li and has a convex shape.

Predicted Pressure-Induced Superconducting Transition in Electride Li_{6}P

Electrides are unique compounds where most of the electrons reside at interstitial regions of the crystal behaving as anions, which strongly determines its physical properties. Interestingly, the magnitude and distribution of interstitial electrons can be effectively modified either by modulating its chemical composition or external conditions (e.g., pressure). Most of the electrides under high pressure are nonmetallic, and superconducting electrides are very rare. Here we report that a pressure-induced stable Li_{6}P electride, identified by first-principles swarm structure calculations, becomes a superconductor with a predicted superconducting transition temperature T_{c} of 39.3 K, which is the highest among the already known electrides. The interstitial electrons in Li_{6}P, with dumbbell-like connected electride states, play a dominant role in the superconducting transition. Other Li-rich phosphides, Li_{5}P, Li_{11}P_{2}, Li_{15}P_{2}, and Li_{8}P, are also predicted to be superconducting electrides, but with a lower T_{c}. Superconductivity in all these compounds can be attributed to a combination of a weak electronegativity of phosphorus (P) with a strong electropositivity of lithium (Li), and opens up the interest to explore high-temperature superconductivity in similar binary compounds.

Phonon Collapse and Second-Order Phase Transition in Thermoelectric SnSe

Since 2014 the layered semiconductor SnSe in the high-temperature Cmcm phase is known to be the most efficient intrinsic thermoelectric material. Making use of first-principles calculations we show that its vibrational and thermal transport properties are determined by huge nonperturbative anharmonic effects. We show that the transition from the Cmcm phase to the low-symmetry Pnma is a second-order phase transition driven by the collapse of a zone border phonon, whose frequency vanishes at the transition temperature. Our calculations show that the spectral function of the in-plane vibrational modes are strongly anomalous with shoulders and double-peak structures. We calculate the lattice thermal conductivity obtaining good agreement with experiments only when nonperturbative anharmonic scattering is included. Our results suggest that the good thermoelectric efficiency of SnSe is strongly affected by the nonperturbative anharmonicity.

Strong Electron-Phonon and Band Structure Effects in the Optical Properties of High Pressure Metallic Hydrogen

The recent claim of having produced metallic hydrogen in the laboratory relies on measurements of optical spectra. Here, we present first-principles calculations of the reflectivity of hydrogen between 400 and 600 GPa in the I4_{1}/amd crystal structure, the one predicted at these pressures, based on both time-dependent density functional and Eliashberg theories, thus, covering the optical properties from the infrared to the ultraviolet regimes. Our results show that atomic hydrogen displays an interband plasmon at around 6 eV that abruptly suppresses the reflectivity, while the large superconducting gap energy yields a sharp decrease of the reflectivity in the infrared region approximately at 120 meV. The experimentally estimated electronic scattering rates in the 0.7-3 eV range are in agreement with our theoretical estimations, which show that the huge electron-phonon interaction of the system dominates the electronic scattering in this energy range. The remarkable features in the optical spectra predicted here encourage extending the optical measurements to the infrared and ultraviolet regions as our results suggest optical measurements can potentially identify high-pressure phases of hydrogen.

Anharmonic enhancement of superconductivity in metallic molecular Cmca - 4 hydrogen at high pressure: a first-principles study

First-principles calculations based on density-functional theory including anharmonicity within the variational stochastic self-consistent harmonic approximation are applied to understand how the quantum character of the proton affects the candidate metallic molecular Cmca  -  4 structure of hydrogen in the 400-450 GPa pressure range, where metallization of hydrogen is expected to occur. Anharmonic effects, which become crucial due to the zero-point motion, have a large impact on the hydrogen molecules by increasing the intramolecular distance by approximately a 6%. This induces two new electron pockets at the Fermi surface opening new scattering channels for the electron-phonon interaction. Consequently, the electron-phonon coupling constant and the superconducting critical temperature are approximately doubled by anharmonicity and Cmca  -  4 hydrogen becomes a superconductor above 200 K in all the studied pressure range. Contrary to many superconducting hydrides, where anharmoncity tends to lower the superconducting critical temperature, our results show that it can enhance superconductivity in molecular hydrogen.

Structural characterization of slightly boron-deficient LiB, LiB0.9 and LiB0.8, under pressure

Results of computational investigations of two slightly boron-deficient lithium borides, LiB(0.9) and LiB(0.8), under pressure are reported. Structure predictions based on particle swarm optimization reveal that at low pressure both compositions adopt chain structures, as stoichiometric 1 : 1 LiB. With increasing pressure both undergo phase transitions to layered arrangements. The evolution of the structural parameters of these stoichiometries as a function of pressure and the results obtained from the enthalpies indicate that boron-deficient structures are more favoured than 1 : 1 LiB, even at zero pressure. Moreover, as pressure is increased a larger deficiency in B seems to be favoured.

Spin-flip transitions induced by time-dependent electric fields in surfaces with strong spin-orbit interaction

We present a comprehensive theoretical investigation of the light absorption rate at a Pb/Ge(111)-β√3 × √3R30° surface with strong spin-orbit coupling. Our calculations show that electron spin-flip transitions cause as much as 6% of the total light absorption, representing 1 order of magnitude enhancement over Rashba-like systems. Thus, we demonstrate that a substantial part of the light irradiating this nominally nonmagnetic surface is attenuated in spin-flip processes. Remarkably, the spin-flip transition probability is structured in well-defined hot spots within the Brillouin zone, where the electron spin experiences a sudden 90° rotation. This mechanism offers the possibility of an experimental approach to the spin-orbit phenomena by optical means.

Anharmonic stabilization of the high-pressure simple cubic phase of calcium

The phonon spectrum of the high-pressure simple cubic phase of calcium, in the harmonic approximation, shows imaginary branches that make it mechanically unstable. In this Letter, the phonon spectrum is recalculated by using density-functional theory ab initio methods fully including anharmonic effects up to fourth order at 50 GPa. Considering that the perturbation theory cannot be employed with imaginary harmonic frequencies, a variational procedure based on the Gibbs-Bogoliubov inequality is used to estimate the renormalized phonon frequencies. The results show that strong quantum anharmonic effects make the imaginary phonons become positive even at zero temperature so that the simple cubic phase becomes mechanically stable, as experiments suggest. Moreover, our calculations find a superconducting T(c) in agreement with experiments and predict an anomalous behavior of the specific heat.

First-principles simulations of lithium melting: stability of the bcc phase close to melting

We report large-scale first-principles simulations of melting of four different phases of Li at pressures ranging from 0 to 50 GPa. We find excellent agreement with existing experimental data at low pressures, and confirm that above 10 GPa the melting line develops a negative slope, in parallel to what occurs for Na at 30 GPa. Surprisingly, our results indicate that the melting temperature of the bcc phase is always higher than that of fcc Li, suggesting the intriguing possibility of the existence of a narrow field of bcc stability separating the fcc and liquid phases, as predicted by Alexander and McTague [Phys. Rev. Lett. 41, 702 (1978)].

Exotic behavior and crystal structures of calcium under pressure

Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc --> bcc --> simple cubic --> Ca-IV --> Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is similar to that for Sr. The beta-tin (I4(1)/amd) structure, rather than simple cubic, is predicted to be the theoretical ground state at 0 K and 33-71 GPa. This structure can be represented as a large distortion of the simple cubic structure, just as the higher-pressure phases stable between 71 and 134 GPa. The structure of Ca-V, stable above 134 GPa, is a complex host-guest structure. According to our calculations, the predicted phases are superconductors with Tc increasing under pressure and reaching approximately 20 K at 120 GPa, in good agreement with experiment.

Novel structures and superconductivity of silane under pressure

Following the suggestion that hydrogen-rich compounds, and, in particular, silane (SiH4), might be high-T_{c} superconductors at moderate pressures, very recent experiments have confirmed that silane metallises and even becomes superconducting at high pressure. In this article, we present a structural characterization of compressed silane obtained with an ab initio evolutionary algorithm for crystal structure prediction. Besides the earlier molecular and chainlike structures of P2_{1}/c and I4_{1}/a symmetries, respectively, we propose two novel structures with space groups Fdd2 and Pbcn, to be stable at 25-55 and 220-250 GPa, respectively. According to our calculations, silane becomes metallic and superconducting at 220 GPa in the layered Pbcn structure, with a theoretical T_{c} of 16 K. Our calculations also show that the imaginary phonons of the recently proposed P6_{3} generate the Pbcn structure.

Superconducting high pressure phase of germane

High-pressure structures of germane (GeH4) are explored through ab initio evolutionary methodology to reveal a metallic monoclinic structure of C2/c (4 molecules/cell). The C2/c structure consists of layerlike motifs containing novel "H2" units. Enthalpy calculations suggest a remarkably wide decomposition (Ge+H2) pressure range of 0-196 GPa, above which C2/c structure is stable. Perturbative linear-response calculations for C2/c GeH4 at 220 GPa predict a large electron-phonon coupling parameter lambda of 1.12 and the resulting superconducting critical temperature reaches 64 K.

Structures and potential superconductivity in at high pressure: en route to "metallic hydrogen"

A way to circumvent the high pressures needed to metallize hydrogen is to "precompress" it in hydrogen-rich molecules, a strategy probed theoretically for silane. We show that phases with tetrahedral SiH4 molecules should undergo phase transitions with sixfold- and eightfold-coordinate Si appearing above 25 GPa. The most stable structure found can be metallized at under a megabar and at a compression close to the prediction of Goldhammer-Herzfeld criterion. According to a BCS-like estimate, metallic silane should be a high-temperature superconductor.

Enhanced Friedel structure and proton pairing in dense solid hydrogen

The mechanism of proton pairing in dense solid hydrogen, and its progression with density, are both studied using effective potentials between protons which include electronic response up to quadratic terms. For high pressures nonlinear effects originating with different pairs are crucial in establishing the net attraction within a given pair, and in this picture Friedel oscillations are considerably enhanced by quadratic response, subsequently playing a very important role in the overall pairing. Calculated vibron frequencies also show substantial agreement with experiment, reflecting at the same time significant changes in the physical character of the pairing itself.