Superconductivity in Compression-Shear Deformed Diamond

Superconductivity in Diamond-Like BC15

Advancing the compositional space of a compound class can result in intriguing superconductors, such as LaH10. Herein, we performed a comprehensive first-principles structural search on a binary B-C system with various chemical compositions. The identified diamond-like BC15, named d-BC15, is thermodynamically superior to the synthesized BC3 and BC5. Interestingly, d-BC15 shows anisotropic superconductivity resulting from three distinct Fermi surfaces. Its predicted critical temperature (Tc) is 43.6 K at ambient pressure, beyond the McMillan limit. d-BC15 reaches a maximum of around 75 K at 0.43% hole doping due to the substantially enhanced density of states at the Fermi level. Additionally, d-BC15 demonstrates superhard characteristics with a Vickers hardness of 75 GPa. The calculated tensile and shear stresses are 72 and 73 GPa, respectively. The combination of high superconductivity and superhardness in d-BC15 offers new insights into the design of multifunctional materials.

Picometer-Scale Atomic Shifts Governing Subdisordered Structures in Diamond

Diamond is considered the most promising next-generation semiconductor material due to its excellent physical characteristics. It has been more than three decades since the discovery of a special structure named n-diamond. However, despite extensive efforts, its crystallographic structure and properties are still unclear. Here, we show that subdisordered structures in diamond provide an explanation for the structural feature of n-diamond. Monocrystalline diamond with subdisordered structures is synthesized via the chemical vapor deposition method. Atomic-resolution scanning transmission electron microscopy characterizations combined with the picometer-precision peak finder technology and diffraction simulations reveal that picometer-scale shifts of atoms within cells of diamond govern the subdisordered structures. First-principles calculations indicate that the bandgap of diamond decreases rapidly with increasing shifting distance, in accordance with experimental results. These findings clarify the crystallographic structure and electronic properties of n-diamond and provide new insights into the bandgap adjustment in diamond.

Mechanical quenching phenomenon in diamond

The structure of dislocation cores, the fundamental knowledge on crystal plasticity, remains largely unexplored in covalent crystals. Here, we conducted atomically resolved characterizations of dislocation core structures in a plastically deformed diamond anvil cell tip that was unloaded from an exceptionally high pressure of 360 GPa. Our observations unveiled a series of nonequilibrium dislocation cores that deviate from the commonly accepted "five-seven-membered ring" dislocation core model found in FCC-structured covalent crystals. The nonequilibrium dislocation cores were generated through a process known as "mechanical quenching," analogous to the quenching process where a high-energy state is rapidly frozen. The density functional theory-based molecular dynamic simulations reveal that the phenomenon of mechanical quenching in diamond arises from the challenging relaxation of the nonequilibrium configuration, necessitating a large critical strain of 25% that is difficult to maintain. Further electronic-scale analysis suggested that such large critical strain is spent on the excitation of valance electrons for bond breaking and rebonding during relaxation. These findings establish a foundation for the plasticity theory of covalent materials and provide insights into the design of electrical and luminescent properties in diamond, which are intimately linked to the dislocation core structure.

Theoretical Design of Strengthened Nanotwinned γ*-Boron

The distinctive electron deficiency and unusual multicenter bonding situations of boron give rise to fascinating chemical complexity and imaginative structural polymorphism. Herein, we employ an independently developed method to construct the new twinned γ*-boron based on the well-known hardest elemental boron, γ-B28. Notably, the newly propounded γ*-boron phases exhibit considerably close energy levels with γ-B28 under ambient conditions. The simulated X-ray diffraction patterns of stable twinned structure present excellent agreement with experimental data. First-principles calculations reveal a 7.5% increase in the ideal Vickers shear strength of γ*-boron compared to γ-B28, attributed to diverse bond responses within the twinned slabs. The evaluated hardness of nanotwinned γ*-B reaches 59 GPa in consideration of the size hardening effect. Our research presents an efficient strategy for constructing new polymorphs of boron with improved mechanical properties and expands the knowledge about twinning structures of boron.

Structural and Stress Response of Nanotwinned B13CN under Large Strains

Boron-rich carbides with icosahedral cages as pivotal structural units, which exhibit high hardness and low density, have promising industrial applications. However, the insufficient fracture toughness of these materials hinders their engineering applications. A recent first-principles study revealed that single-crystal B13CN (sc-B13CN) exhibits interesting structural deformation modes and superior mechanical properties to boron-rich carbides, prompting us to further explore this intriguing material. Herein, we adopted sc-B13CN as an archetypal system owing to its excellent structural and mechanical properties to construct nanotwinned B13CN (nt-B13CN) and explore its mechanical properties and structural deformation modes under large strains. We unraveled the specific stress-strain relationship of nt-B13CN and the considerable effect of twinning on its structural deformation modes under diverse loading conditions. Our results indicate that twinning leads to interesting structural deformation patterns and is extremely beneficial to improving the structural stability and mechanical properties of boron-rich materials. The current results provide an improved understanding of the theoretical design for various nanotwinned boron-rich materials with intricate bonding configurations.

Superconductivity Above 100 K Predicted in Carbon-Cage Network

To explore carbide superconductors with higher transition temperature, two novel carbon structures of cage-network are designed and their superconductivity is studied by doping metals. MC6 and MC10 are respectively identified as C24 and C32 cage-network structures. This study finds that both carbon structures drive strong electron-phonon interaction and can exhibit superconductivity above liquid nitrogen temperature. Importantly, the superconducting transition temperatures above 100 K are predicted to be achieved in C24 -cage-network systems doped by Na, Mg, Al, In, and Tl at ambient pressure, which is far higher than those in graphite, fullerene, and other carbides. Meanwhile, the superconductivity of cage-network carbides is also found to be sensitive to the electronegativity and concentration of dopant M. The result indicates that the higher transition temperatures can be obtained by optimizing the carbon-cage-network structures and the doping conditions. The study suggests that the carbon-cage-network structure is a direction to explore high-temperature superconducting carbides.

Observation of non-superconducting phase changes in nitrogen doped lutetium hydrides

The recent report of near-ambient superconductivity and associated color changes in pressurized nitrogen doped lutetium hydride has triggered worldwide interest and raised major questions about the nature and underlying physics of these latest claims. Here we report synthesis and characterization of high-purity nitrogen doped lutetium hydride LuH2±xNy. We find that pressure conditions have notable effects on Lu-N and Lu-NH chemical bonding and the color changes likely stem from pressure-induced electron redistribution of nitrogen/vacancies and interaction with the LuH2 framework. No superconducting transition is found in all the phases at temperatures 1.8-300 K and pressures 0-38 GPa. Instead, we identify a notable temperature-induced resistance anomaly of electronic origin in LuH2±xNy, which is most pronounced in the pink phase and may have been erroneously interpreted as a sign of superconducting transition. This work establishes key benchmarks for nitrogen doped lutetium hydrides, allowing an in-depth understanding of its novel pressure-induced phase changes.

Recent Progress in Strain Engineering on Van der Waals 2D Materials: Tunable Electrical, Electrochemical, Magnetic, and Optical Properties

Strain engineering is a promising way to tune the electrical, electrochemical, magnetic, and optical properties of 2D materials, with the potential to achieve high-performance 2D-material-based devices ultimately. This review discusses the experimental and theoretical results from recent advances in the strain engineering of 2D materials. Some novel methods to induce strain are summarized and then the tunable electrical and optical/optoelectronic properties of 2D materials via strain engineering are highlighted, including particularly the previously less-discussed strain tuning of superconducting, magnetic, and electrochemical properties. Also, future perspectives of strain engineering are given for its potential applications in functional devices. The state of the survey presents the ever-increasing advantages and popularity of strain engineering for tuning properties of 2D materials. Suggestions and insights for further research and applications in optical, electronic, and spintronic devices are provided.

Unexpected low thermal expansion coefficients of pentadiamond

A new carbon allotrope, pentadiamond, was recently reported in the literature. Herein, we investigate its thermal expansion and thermoelastic properties by first principles. It is observed that the bulk modulus and hardness of pentadiamond are far less than those of diamond, but the thermal expansion of pentadiamond is lower than that of diamond in the range of 0 K to 2000 K, and even negative in the temperature range of 0-190 K. The negative thermal expansion at low temperature originates from the transverse vibrations of the edge-shared atoms in the coplanar double-pentagon. The low thermal expansion at high temperature is contributed by the strong bonds in pentadiamond. Benefiting from the low thermal expansion, the elastic constants of pentadiamond decrease very slowly with respect to temperature compared with those of diamond. The low sensitivity of thermodynamic and thermoelastic properties to temperature makes pentadiamond a promising material for high anti-thermal-shock and accurate electronic device applications.

Shock-formed carbon materials with intergrown sp3- and sp2-bonded nanostructured units

Studies of dense carbon materials formed by bolide impacts or produced by laboratory compression provide key information on the high-pressure behavior of carbon and for identifying and designing unique structures for technological applications. However, a major obstacle to studying and designing these materials is an incomplete understanding of their fundamental structures. Here, we report the remarkable structural diversity of cubic/hexagonally (c/h) stacked diamond and their association with diamond-graphite nanocomposites containing sp³-/sp²-bonding patterns, i.e., diaphites, from hard carbon materials formed by shock impact of graphite in the Canyon Diablo iron meteorite. We show evidence for a range of intergrowth types and nanostructures containing unusually short (0.31 nm) graphene spacings and demonstrate that previously neglected or misinterpreted Raman bands can be associated with diaphite structures. Our study provides a structural understanding of the material known as lonsdaleite, previously described as hexagonal diamond, and extends this understanding to other natural and synthetic ultrahard carbon phases. The unique three-dimensional carbon architectures encountered in shock-formed samples can place constraints on the pressure-temperature conditions experienced during an impact and provide exceptional opportunities to engineer the properties of carbon nanocomposite materials and phase assemblages.

Strain Engineering: A Boosting Strategy for Photocatalysis

Whilst the photocatalytic technique is considered to be one of the most significant routes to address the energy crisis and global environmental challenges, the solar-to-chemical conversion efficiency is still far from satisfying practical industrial requirements, which can be traced to the suboptimal bandgap and electronic structure of photocatalysts. Strain engineering is a universal scheme that can finely tailor the bandgap and electronic structure of materials, hence supplying a novel avenue to boost their photocatalytic performance. Accordingly, to explore promising directions for certain breakthroughs in strained photocatalysts, an overview on the recent advances of strain engineering from the basics of strain effect, creations of strained materials, as well as characterizations and simulations of strain level is provided. Besides, the potential applications of strain engineering in photocatalysis are summarized, and a vision for the future controllable-electronic-structure photocatalysts by strain engineering is also given. Finally, perspectives on the challenges for future strain-promoted photocatalysis are discussed, placing emphasis on the creation and decoupling of strain effect, and the modification of theoretical frameworks.

Macroscale Robust Superlubricity on Metallic NbB2

Robust superlubricity (RSL), defined by concurrent superlow friction and wear, holds great promise for reducing material and energy loss in vast industrial and technological operations. Despite recent advances, challenges remain in finding materials that exhibit RSL on macrolength and time scales and possess vigorous electrical conduction ability. Here, the discovery of RSL is reported on hydrated NbB₂ films that exhibit vanishingly small coefficient of friction (0.001-0.006) and superlow wear rate (≈10^-17 m³ N^-1 m^-1 ) on large length scales reaching millimeter range and prolonged time scales lasting through extensive loading durations. Moreover, the measured low resistivity (≈10^-6 Ω m) of the synthesized NbB₂ film indicates ample capability for electrical conduction, extending macroscale RSL to hitherto largely untapped metallic materials. Pertinent microscopic mechanisms are elucidated by deciphering the intricate load-driven chemical reactions that generate and sustain the observed superlubricating state and assessing the strong stress responses under diverse strains that produce the superior durability.

Indentation Strengths of Zirconium Diboride: Intrinsic versus Extrinsic Mechanisms

Zirconium diboride (ZrB₂) is an important ultra-high-temperature ceramic, which exhibits outstanding mechanical properties and is widely used in extreme environments. Extensive experimental studies, however, have found that synthesized ZrB₂ specimens show widely scattered indentation hardness values ranging from 8.7 to 26 GPa. We have performed comprehensive stress-strain calculations of ZrB₂ to explore its structural and stress responses and found that ZrB₂ possesses an intrinsic indentation strength of 32.7 GPa, which is on par with those of other transition-metal borides that exhibit higher indentation hardness values of ∼30 GPa. This result suggests that large variations in measured hardness are driven by extrinsic factors, and an analysis of available experimental data indicates that the quality of the crystallinity of specimens holds the key to realizing improved hardness corresponding to the predicted intrinsic indentation strength. These findings offer insights into the origin of the previously reported lower hardness values of ZrB₂ and raise the prospects of achieving superior strengths in well-crystallized ZrB₂ that approach or match those of other ultrahard transition-metal compounds.

Insights into the Microstructures and Energy Levels of Pr3+-Doped YAlO3 Scintillating Crystals

Trivalent praseodymium (Pr3+)-doped materials have been extensively used in high-resolution laser spectroscopy, owing to their outstanding conversion efficiencies of plentiful transitions in the visible laser region. However, to clarify the microstructure and energy transfer mechanism of Pr3+-doped host crystals is a challenging topic. In this work, the stable structures of Pr3+-doped yttrium orthoaluminate (YAlO3) have been widely searched based on the CALYPSO method. A novel monoclinic structure with the Pm group symmetry is successfully identified. The Pr3+ impurity can precisely occupy the Y3+ position and get incorporated into the YAlO3 (YAP) host crystal with a Pr3+ concentration of 6.25%. The result of the electronic band structure reveals a 3.62 eV band gap, which suggests a semiconductor character of YAP:Pr. Using our developed well-established parametrization matrix diagonalization (WEPMD) method, we have systematically analyzed the energy level scheme and proposed a set of newly improved parameters. Additionally, the energy transfer mechanism of YAP:Pr is clarified by deciphering the numerical electric dipole and magnetic dipole transitions. The popular red emission at 653 nm is assigned to the transition 3P0 → 3F2, while the transition 3P0 → 3H4 with a large branching ratio is predicted to be a good laser channel. Many promising emission lines for laser actions are also obtained in the visible light region. Our results not only provide important insights into the energy transfer mechanisms of rare-earth ion-doped materials but also pave the way for the implementation of new types of laser devices.

Pressure-Induced Evolution of Crystal and Electronic Structure of Ammonia Borane

Ammonia borane (NH₃BH₃) has long attracted considerable interest for its high hydrogen content and easy dehydrogenation conditions which make it a promising hydrogen storage material. Here, we report on a computational study of the structural stability and phase transition sequence of NH₃BH₃ and associated lattice dynamics and electronic properties in a wide pressure range up to 300 GPa. The results confirm previously reported structures, including the experimentally observed orthorhombic Pmn2₁ structure at low temperature and ambient pressure, and predict the phase transition sequence Pmn2₁ → Pc → P2₁ → P1̅ for NH₃BH₃. Our calculations also reveal systematic trends of monotonically decreasing band gap with rising pressure in the three high-pressure NH₃BH₃ phases, which nevertheless all remain nonconducting up to the highest pressure of 300 GPa examined in this work. The present findings elucidate structural and electronic properties of NH₃BH₃ over an extensive pressure range, providing knowledge essential to further study of NH₃BH₃ in an expanded pressure-temperature phase space.

Pronounced Enhancement of Superconductivity in ZrN via Strain Engineering

Zirconium nitride (ZrN) exhibits excellent mechanical and electronic properties and hosts a superconducting transition temperature (T_c) of 10.0 K that is on the high end among transition-metal nitrides. Here, we report on a first-principles study of tuning superconductivity of ZrN via strain engineering under extensive tensile and shear deformation modes. Our results reveal strikingly effective strain-induced enhancement of T_c up to 17.1 K, which is achieved under tensile strains along the high-symmetry crystallographic [001] deformation path. A systematic analysis of the calculated results indicates that such pronounced strain modulation of superconductivity stems from simultaneous increase of electronic density of states and softening of lattice vibration in the strain-deformed ZrN crystal. The present findings show that strain engineering offers an effective tool for optimizing superconductivity in transition-metal compounds, opening a fresh avenue for improving a major functionality of this class of materials that may find applications in advanced devices.

Extended Falicov-Kimball model: Hartree-Fock vs DMFT approach

In this work, we study the extended Falicov-Kimball model at half-filling within the Hartree-Fock approach (HFA) (for various crystal lattices) and compare the results obtained with the rigorous ones derived within the dynamical mean field theory (DMFT). The model describes a system, where electrons with spin-↓ are itinerant (with hopping amplitude t), whereas those with spin-↑ are localized. The particles interact via on-site U and intersite V density-density Coulomb interactions. We show that the HFA description of the ground state properties of the model is equivalent to the exact DMFT solution and provides a qualitatively correct picture also for a range of small temperatures. It does capture the discontinuous transition between ordered phases at U = 2V for small temperatures as well as correct features of the continuous order-disorder transition. However, the HFA predicts that the discontinuous boundary ends at the isolated-critical point (of the liquid-gas type) and it does not merge with the continuous boundary. This approach cannot also describe properly a change of order of the continuous transition for large V as well as various metal-insulator transitions found within the DMFT.

Atomistic Mechanisms for Contrasting Stress-Strain Relations of B13CN and B13C2

Boron-rich compounds comprise intricate bonding structures and possess excellent mechanical properties. Here, we report on a comparative study of B₁₃CN and B₁₃C₂, which are isostructural but differ in electron fillings, with the former being electron-precise and the latter electron-deficient. Our results show that the different electron fillings in B₁₃CN and B₁₃C₂ have profound effects on the bonding features despite their shared crystal structure, generating distinct structural deformation modes and the accompanying stress responses under diverse loading strain conditions. The most striking phenomena include a creeplike stress response under a tensile strain and superior strength under the vast majority of loading conditions for B₁₃CN compared to B₁₃C₂. Such enhanced stability of the B₁₂ icosahedra in B₁₃CN by N-induced electron compensation may be effective for structural and mechanical enhancement of other boron-rich compounds and offers improved understanding of a broader class of covalent crystals with complex bonding networks.

Elucidating Stress-Strain Relations of ZrB12 from First-Principles Studies

Transition-metal boron-rich compounds exhibit favorable synthesis conditions and mechanical properties that hold great promise for wide-ranging applications. However, the complex bonding networks of these compounds produce diverse structural and mechanical behaviors that require in-depth studies. A notable case is ZrB₁₂, which has been reported to possess high Vickers hardness comparable to those of ReB₂ and WB₄. Surprisingly, first-principles calculations of stress-strain relations reveal unexpected low indentation strengths of ZrB₁₂ well below those of ReB₂ and WB₄. Such contrasting results are reconciled by noting that the additional presence of a boron-rich phase of ZrB₅₀ in the experimental synthesis process likely plays a key role in the extrinsic strengthening. These findings uncover mechanisms for the higher measured strength of ZrB₁₂ and offer insights for elucidating extrinsic hardening phenomena that may exist in other transition-metal compounds.

Metallization of diamond

Experimental discovery of ultralarge elastic deformation in nanoscale diamond and machine learning of its electronic and phonon structures have created opportunities to address new scientific questions. Can diamond, with an ultrawide bandgap of 5.6 eV, be completely metallized, solely under mechanical strain without phonon instability, so that its electronic bandgap fully vanishes? Through first-principles calculations, finite-element simulations validated by experiments, and neural network learning, we show here that metallization/demetallization as well as indirect-to-direct bandgap transitions can be achieved reversibly in diamond below threshold strain levels for phonon instability. We identify the pathway to metallization within six-dimensional strain space for different sample geometries. We also explore phonon-instability conditions that promote phase transition to graphite. These findings offer opportunities for tailoring properties of diamond via strain engineering for electronic, photonic, and quantum applications.