Anisotropic Full-Gap Superconductivity in 2M-WS2 Topological Metal with Intrinsic Proximity Effect.

Layered 2M-WS2 is recently observed to show Majorana bound states in vortices, but its superconducting pairing mechanism remains unknown, hindering the understanding of its topological superconducting nature. Using the ab initio Migdal-Eliashberg theory and electron-phonon Wannier interpolation, we demonstrate that both bulk and bilayer 2M-WS2 have a single anisotropic full-gap superconducting order of s-wave symmetry. We successfully reproduce the experimental superconducting critical temperature for the bulk and predict the bilayer 2M-WS2, a two-dimensional (2D) Z2 topological metal with nontrivial edge states right at the Fermi energy, to superconduct at 7 K, much higher than that in most 2D transition metal dichalcogenides (TMDs). A distinct proximity-enhanced surface superconductivity is further revealed by simulating quasiparticle density of states. This work unveils a universal electron-phonon full-gap pairing in 2M group VI TMDs and suggests a strong intrinsic surface-bulk proximity effect for 2M-WS2, paving the way to engineering topological superconductivity in TMD-based nanoscale devices.

Coexistence of Superconductivity with Enhanced Charge Density Wave Order in the Two-Dimensional Limit of TaSe2.

Bulk 2H-TaSe2 is a model charge density wave (CDW) metal with superconductivity emerging at extremely low temperature (Tc = 0.1 K). Here, by first-principles calculations including the explicit calculation of the screened Coulomb interaction, we demonstrate enhanced superconductivity in the CDW state of monolayer 1H-TaSe2 observed in recent experiments. Its ground-state 3 × 3 CDW phase features triangular clustering of Ta atoms and possesses a large electron-phonon coupling of λ = 0.74, yielding an order of magnitude higher superconducting Tc compared to the bulk. Upon lowering the thickness from bulk to monolayer TaSe2, the CDW intensifies with slightly decreased Fermi-level density of states, while superconductivity gets boosted via a largely increased intrinsic electron-phonon coupling strength, which overcomes both the CDW effect and naturally reinforced Coulomb repulsion. These results uncover the simultaneously enhanced CDW and superconducting orders in the two-dimensional limit for the first time and have key implications for other CDW metals like 2H-TaS2.

Continuous, Ultra-lightweight, and Multipurpose Super-aligned Carbon Nanotube Tapes Viable over a Wide Range of Temperatures.

In extreme environments, such as at ultrahigh or ultralow temperatures, the amount of tape used should be minimal so as to reduce system contamination and unwanted residues. However, tapes made from conventional materials typically lose their adhesiveness or leave residues difficult to remove under such conditions. Thus, the development of more versatile, lightweight, and easily removable tapes for applications in such extreme environments has received considerable attention. Here, we report that horizontally superaligned carbon nanotube (SACNT) tapes can be used to provide perfect van der Waals (vdW) interface contacts over a wide range of temperatures (from -196 to 1000 °C), yielding outstanding adhesiveness with specific adhesion strengths up to ∼1.1 N/µg. With a surface density of only 0.5-5 µg/cm2, hundreds of times lighter than the vertically aligned CNT adhesives, the SACNT tapes can be cost-effectively provided in hundreds of meters. They have multipurpose adhesive abilities for versatile materials and are also easily separated from samples even after exposure to extreme temperature regimes. First-principles calculations confirm the mechanism of vdW adhesion and reveal that ultraflat and nanometer-thick SACNT tapes may yield far greater adhesive abilities. These SACNT tapes show great potential for use in mechanical bonding, electrical bonding, and thermal dissipation in electronic devices.

Unveiling Charge-Density Wave, Superconductivity, and Their Competitive Nature in Two-Dimensional NbSe2.

Recently, charge-density wave (CDW) and superconductivity are observed to coexist in atomically thin metallic NbSe2. Lacking of knowledge on the structural details of CDW, however, prevents us to explore its interplay with superconductivity. Using first-principles calculations, we identify the ground state 3 × 3 CDW atomic structure of monolayer NbSe2, which is characterized by the formation of triangular Nb clusters and shows a scanning tunnelling microscopy (STM) image and Raman CDW modes in good agreement with experiments. We further demonstrate that from bulk to monolayer NbSe2, as the layer thickness decreases, the CDW order is gradually enhanced with rising energy gain and strengthened Fermi surface gapping, while superconductivity is weakened due to the increasingly reduced Fermi level density of states in the CDW state. These results well explain the observed opposite thickness dependencies of CDW and superconducting transition temperatures and uncover the nature of competitive interaction between the two collective orders in two-dimensional NbSe2.

Phonon-mediated high-T c superconductivity in hole-doped diamond-like crystalline hydrocarbon.

We here predict by ab initio calculations phonon-mediated high-T c superconductivity in hole-doped diamond-like cubic crystalline hydrocarbon K 4-CH (space group I21/3). This material possesses three key properties: (i) an all-sp 3 covalent carbon framework that produces high-frequency phonon modes, (ii) a steep-rising electronic density of states near the top of the valence band, and (iii) a Fermi level that lies in the σ-band, allowing for a strong coupling with the C-C bond-stretching modes. The simultaneous presence of these properties generates remarkably high superconducting transition temperatures above 80 K at an experimentally accessible hole doping level of only a few percent. These results identify a new extraordinary electron-phonon superconductor and pave the way for further exploration of this novel superconducting covalent metal.

Pseudospins and Topological Effects of Phonons in a Kekulé Lattice.

The search for exotic topological effects of phonons has attracted enormous interest for both fundamental science and practical applications. By studying phonons in a Kekulé lattice, we find a new type of pseudospin characterized by quantized Berry phases and pseudoangular momenta, which introduces various novel topological effects, including topologically protected pseudospin-polarized interface states and a phonon pseudospin Hall effect. We further demonstrate a pseudospin-contrasting optical selection rule and a pseudospin Zeeman effect, giving a complete generation-manipulation-detection paradigm of the phonon pseudospin. The pseudospin and topology-related physics revealed for phonons is general and applicable for electrons, photons, and other particles.

Crystalline structures of polymeric hydrocarbon with 3,4-fold helical chains.

Molecular hydrocarbons are well-known to polymerize under pressure to form covalently bonded frameworks. Here we predict by ab initio calculations two distinct three-dimensional hydrocarbon crystalline structures composed of 3-fold and 4-fold helical CH chains in rhombohedral (R3) and tetragonal (I41/a) symmetry, respectively. Both structures with 1:1 stoichiometry are found to be energetically more favorable than solid acetylene and cubane, and even more stable than benzene II solid at high pressure. The calculations on vibrational, electronic, and optical properties reveal that the new chiral hydrocarbons are dynamically stable with large bulk moduli around 200 GPa, and exhibit a transparent insulating behavior with indirect band gaps of 5.9 ~ 6.7 eV and anisotropic adsorption spectra. Such forms of hydrocarbon, once synthesized, would have wide applications in mechanical, optoelectronic, and biological materials.

Three-dimensional polymeric structures of single-wall carbon nanotubes.

We explore by ab initio calculations the possible crystalline phases of polymerized single-wall carbon nanotubes (P-SWNTs) and determine their structural, elastic, and electronic properties. Based on direct cross-linking and intertube sliding-assisted cross-linking mechanisms, we have identified a series of stable three-dimensional polymeric structures for the zigzag nanotubes up to (10,0). Among proposed P-SWNT phases, the structures with favorable diamond-like sp(3) intertube bonding configuration and small tube cross-section distortion are found to be the most energetically stable ones. These polymeric crystalline phases exhibit high bulk and shear moduli superior to SWNT bundles, and show metallic or semiconducting properties depending on the diameter of constituent tubes. We also propose by hydrostatic pressure simulations that the intertube sliding between van der Waals bonded nanotubes may be an effective route to promote the polymerization of SWNTs under pressure.

Hydrogenated K4 carbon: a new stable cubic gauche structure of carbon hydride.

The structural and electronic properties of hydrogenated K(4) carbon as a new cubic gauche structure in I2(1)3 symmetry are investigated using first-principles calculations. The total energy for this carbon hydride (labeled by K(4)-CH) is 0.47 eV per CH unit lower than that of solid molecular cubane, suggesting its energetic stability. Based on the calculated phonon dispersion curves and electronic band structure obtained by hybrid density functional method, we find that K(4)-CH is dynamically stable and exhibits as an insulator with an indirect band gap of 6.07 eV, which is close to 6.10 eV of cubic gauche nitrogen (cg-N). To study the doping effect of nitrogen, we have also investigated N-doped K(4)-CH with a composition of C(4)H(4)N(4) in P2(1)3 symmetry. The phonon and electronic band structures show that it is dynamically stable and behaves as an insulator with an indirect band gap of 5.39 eV, smaller than that of both K(4)-CH and cg-N. These results broaden our understanding of the cubic gauche structure.