Using first-principles calculations, we predict a new type of two-dimensional (2D) boride MB3 (M = Be, Ca, Sr), constituted by boron kagome monolayer and the metal atoms adsorbed above the center of the boron hexagons. The band structures show that the three MB3 compounds are metallic, thus the possible phonon-mediated superconductivity is explored. Based on the Eliashberg equation, for BeB3, CaB3, and SrB3, the calculated electron–phonon coupling constants λ are 0.46, 1.09, and 1.33, and the corresponding superconducting transition temperatures T c are 3.2, 22.4, and 20.9 K, respectively. To explore superconductivity with higher transition temperature, hydrogenation and charge doping are further considered. The hydrogenated CaB3, i.e., HCaB3, is stable, with the enhanced λ of 1.39 and a higher T c of 39.3 K. Moreover, with further hole doping at the concentration of 5.8 × 1011 hole/cm2, the T c of HCaB3 can be further increased to 44.2 K, exceeding the McMillan limit. The predicted MB3 and HCaB3 provide new platforms for investigating 2D superconductivity in boron kagome lattice since superconductivity based on monolayer boron kagome lattice has not been studied before.

Semihydrogenated graphene (C2H) and semifluorinated graphene (C2F), both in the chair conformation, were predicted by first-principles calculations to be ferromagnetic and antiferromagnetic. It is unclear what is the underlying mechanism leading to such distinct magnetic orders of the two materials. We show that a single-orbital tight-binding model of the graphene lattice up to the next-nearest-neighbor hopping term, with one carbon sublattice coupled to a single active orbital of the adatoms (hydrogen for C2H, and fluorine for C2F), supplemented by the Hubbard interactions on the carbon sites, correctly reproduces the distinct magnetic orders of C2H and C2F. In terms of a fairly good approximation to the low-energy band of the nonmagnetic state, we analytically elucidate how a finite next-nearest-neighbor hopping term makes the bandwidth of the low-energy band of C2H much smaller than that of C2F. Competition between the reduction of the interaction energy and the increase of the band energy then favors the ferromagnetic state for C2H and the antiferromagnetic state for C2F. Implications of the mechanism, including electric field tuning of the bandwidth and application to elemental analogs of graphene such as silicene, are analyzed.

Employing the density-functional theory with local density approximation, we show that the fully hydrogenated monolayer-hexagonal boron nitride (H2BN) has a direct-band gap of 2.96 eV in the blue-light region while the pristine h-BN has a wider indirect-band gap of 4.78 eV. The hole-doped H2BN is stable at low carrier density (n) but becomes dynamically unstable at higher n. We predict that it is a phonon-mediated superconductor with a transition temperature (Tc) which can reach ∼31 K at n of 1.5× 10 holes cm near the lattice instability. The Tc could be enhanced up to ∼82 K by applying a biaxial tensile strain at 6 % along with doping at n of 3.4× 10 holes cm close to a new lattice instability.

The Higgs mode associated with amplitude fluctuations of the superconducting gap in uniform superconductors usually is heavy, which makes its excitation and detection difficult. We report on the existence of a gapless Higgs mode in the Fulde-Ferrell-Larkin-Ovchinnikov states. This feature is originated from the Goldstone mode associated with the translation symmetry breaking. The existence of the gapless Higgs mode is demonstrated by using both a phenomenological model and microscopic Bardeen-Cooper-Schrieffer (BCS) theory. The gapless Higgs mode can avoid the decay into other low energy excitations, which renders it stable and detectable.

The Coulomb excitations of charge density oscillation are calculated for a double-layer heterostructure. Specifically, we consider two-dimensional (2D) layers of silicene and graphene on a substrate. From the obtained surface response function, we calculated the plasmon dispersion relations, which demonstrate how the Coulomb interaction renormalizes the plasmon frequencies. Most importantly, we have conducted a thorough investigation of how the decay rates of the plasmons in these heterostructures are affected by the Coulomb coupling between different types of two-dimensional materials whose separations could be varied. A novel effect of nullification of the silicene band gap is noticed when graphene is introduced into the system. To utilize these effects for experimental and industrial purposes, graphical results for the different parameters are presented.

The Coulomb excitations of charge density oscillation are calculated for a double-layer heterostructure. Specifically, we consider two-dimensional (2D) layers of silicene and graphene on a substrate. From the obtained surface response function, we calculated the plasmon dispersion relations which demonstrate the way in which the Coulomb coupling renormalizes the plasmon frequencies. Additionally, we present a novel result for the damping rates of the plasmons in this Coulomb coupled heterostructure and compare these results as the separation between layers is varied.

The frustrated XY model on the honeycomb lattice has drawn lots of attention because of the potential emergence of chiral spin liquid (CSL) with the increasing of frustrations or competing interactions. In this paper, we study the extended spin-$\frac{1}{2}$ XY model with nearest-neighbor $({J}_{1})$, and next-nearest-neighbor $({J}_{2})$ interactions in the presence of a three-spins chiral $({J}_{\ensuremath{\chi}})$ term using density matrix renormalization group methods. We obtain a quantum phase diagram with both conventionally ordered and topologically ordered phases. In particular, the long-sought Kalmeyer-Laughlin CSL is shown to emerge under a small ${J}_{\ensuremath{\chi}}$ perturbation due to the interplay of the magnetic frustration and chiral interactions. The CSL, which is a nonmagnetic phase, is identified by the scalar chiral order, the finite spin gap on a torus, and the chiral entanglement spectrum described by chiral $SU{(2)}_{1}$ conformal field theory.

Whether there is a minimal conductivity in the alpha-T-3 model or not is still in debate. By considering the evanescent modes contributing to the zero-energy conductivity, we showed that the conductivity of a large sample vanishes in the clean limit but it can rise up swiftly to the same magnitude of order of sigma(0) = 4e(2)/h pi when either a mass term of Dirac electrons or a weak disorder is introduced into the system. Both methods can render a nonzero minimal conductivity that is continuous upon the parameter a. The mass term can cause the conductivity smaller than sigma(0) depending on alpha, while the static disorder can make it approach to sigma(0) for alpha -> 0 and be a little larger than sigma(0) for alpha -> 1 in a mesoscopic sample. It is also found that the recovered minimal conductivity is quite stable against a moderate disorder strength.

High-temperature iron-based superconductivity develops in a structure with unusual lattice-orbital geometry, based on a planar layer of Fe atoms with 3d orbitals and tetrahedrally coordinated by anions. Here we elucidate the electronic role of anions in the iron-based superconductors utilizing state-of-the-art scanning tunneling microscopy. By measuring the local electronic structure, we find that As anion in Ba0.4K0.6Fe2As2 has a striking impact on the electron pairing. The superconducting electronic feature can be switched off/on by removing/restoring As atoms on Fe layer at the atomic scale. Our analysis shows that this remarkable atomic switch effect is related to the geometrical cooperation between anion mediated hopping and unconventional pairing interaction. Our results uncover that the local Fe-anion coupling is fundamental for the pairing interaction of iron-based superconductivity, and promise the potential of bottom-up engineering of electron pairing.

Motivated by the experiments on the organic compound $(Per)_{2}[Pt(mnt)_{2}]$, we study the ground state of the one-dimensional Kondo lattice model at quarter filling with the density matrix renormalization group method. We show a coupled dimer and bond-order-wave (BOW) state in the weak coupling regime for the localized spins and itinerant electrons, respectively. The quantum phase transitions for the dimer and the BOW orders occur at the same critical coupling parameter $J_{c}$, with the opening of a charge gap. The emergence of the combination of dimer and BOW order agrees with the experimental findings of the simultaneous Peierls and spin-Peierls transitions at low temperatures, which provides a theoretical understanding of such phase transition. We also show that the localized spins in this insulating state have quasi-long ranged spin correlations with collinear configurations, which resemble the classical dimer order in the absence of a magnetic order.

The electronic structure and possible electronic orders in monolayer NbF$_4$ are investigated by density functional theory and functional renormalization group. Because of the niobium-centered octahedra, the energy band near the Fermi level is found to derive from the $4d_{xy}$ orbital, well separated from the other bands. Local Coulomb interaction drives the undoped system into an antiferromagnetic insulator. Upon suitable electron/hole doping, the system is found to develop $d_{x^2-y^2}$-wave superconductivity with sizable transition temperature. Therefore, the monolayer NbF$_4$ may be an exciting $4d^1$ analogue of cuprates, providing a new two-dimensional platform for high-$T_c$ superconductivity.

It has been theoretically predicted and experimentally confirmed that graphene deposited with atoms of a univalent alkali metal such as Li (${\mathrm{LiC}}_{6}$) or divalent alkaline-earth metal such as Ca (${\mathrm{CaC}}_{6}$) can be a superconductor. For atoms of a trivalent metal such as Al, if deposited on graphene, it was predicted that ${\mathrm{AlC}}_{8}$ can be in a metallic state. Whether this compound is stable and can be made superconducting is an issue which has not been addressed and deserves further investigation. In this work, based on first-principles calculations, it is found that the phonon spectrum of ${\mathrm{AlC}}_{8}$ shows imaginary frequencies for the two lowest branches, indicating the structure is dynamically unstable. By hole doping, the imaginary frequencies basically disappear and the lattice is stabilized. Besides, biaxial tensile strain was applied to study its effect on phonon and electron-phonon coupling. With the increase of tensile strain, the high-energy phonon spectrum associated with the C-C stretching modes softens greatly and the electron-phonon coupling becomes stronger, resulting in the increase of superconducting transition temperature ${T}_{c}$ to a value of more than 22 K for a sample at the experimentally accessible hole doping ($7.5\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$) and tensile strain ($12%$) levels. This is above the liquid hydrogen temperature of 20.3 K. Thus, besides Li and Ca deposited graphene, ${\mathrm{AlC}}_{8}$ provides another platform for realizing superconductivity in graphene.

Motivated by the recent discovery of superconductivity in Nb2C MXene, we perform theoretical studies on both pristine and functionalized Nb2C using density functionals theory. First, the possible absorbing sites and structures for various functionalized groups are determined by calculating the binding energy. Second, electronic structures of Nb2C as well as all the functionalized systems are obtained which indicate that they are metallic. The band structures of both Nb2C and Nb2CS2 exhibit Dirac points near the Fermi level. Last but most importantly, the lattice dynamics and electron phonon coupling (EPC) are studied for pristine and functionalized Nb2C. Though the EPC in Nb2C is weak, the functional-groups significantly enhance the EPC and thus obtain much high superconducting transition temperature (Tc). Considering the fact that MXenes easily absorb the oxygen atoms in experiments, we suggest that the estimated Tc∼ 12 K for Nb2CO2 may explain the experimentally observed Tc∼ 12.5 K for Nb2C.

We use scanning tunneling microscopy/spectroscopy to elucidate the Cooper pairing of the iron pnictide superconductor Ba0.6K0.4Fe2As2. By a cold-cleaving technique, we obtain atomically resolved termination surfaces with different layer identities. Remarkably, we observe that the low-energy tunneling spectrum related to superconductivity has an unprecedented dependence on the layer identity. By cross referencing with the angle-revolved photoemission results and the tunneling data of LiFeAs, we find that tunneling on each termination surface probes superconductivity through selecting distinct Fe-3d orbitals. These findings imply the real-space orbital features of the Cooper pairing in the iron pnictide superconductors, and propose a general concept that, for complex multiorbital material, tunneling on different terminating layers can feature orbital selectivity.

We study the Kondo physics of a quantum magnetic impurity in two-dimensional topological superconductors (TSCs), either intrinsic or induced on the surface of a bulk topological insulator, using a numerical renormalization group technique. We show that, despite sharing the p + ip pairing symmetry, intrinsic and extrinsic TSCs host different physical processes that produce distinct Kondo signatures. Extrinsic TSCs harbor an unusual screening mechanism involving both electron and orbital degrees of freedom that produces rich and prominent Kondo phenomena, especially an intriguing pseudospin Kondo singlet state in the superconducting gap and a spatially anisotropic spin correlation. In sharp contrast, intrinsic TSCs support a robust impurity spin doublet ground state and an isotropic spin correlation. These findings advance fundamental knowledge of novel Kondo phenomena in TSCs and suggest experimental avenues for their detection and distinction.

The surface terminations of 122-type alkaline earth metal iron pnictides AEFe(2)As(2) (AE = Ca, Ba) are investigated with scanning tunneling microscopy/spectroscopy. Cleaving these crystals at a cryogenic temperature yields a large majority of terminations with an atomically resolved (root 2 x root 2)R45 or 1 x 2 lattice, as well as a very rare termination of 1 x 1 lattice symmetry. By analyzing the lattice registration and selective chemical marking, we identify these terminations as (root 2 x root 2)R45-reconstructed AE, 1 x 2-reconstructed As, and (root 2 x root 2)R45-reconstructed Fe surface layers, respectively. Layer-resolved tunneling spectroscopy on these terminating surfaces reveals a well-defined superconducting energy gap on the As terminations, while the gap features become weaker on the AE terminations and absent on the Fe terminations. The superconducting gap is hardly affected locally by the As or AE surface reconstructions. The definitive identification of the surface terminations and the associated spectroscopic signatures shed light on the essential roles of As and the pnictogen-iron-pnictogen trilayer building block in iron-based superconductivity.

We study the energy level structures of the defective graphane lattice, where a carbon dimer defect is created by removing the hydrogen atoms on two nearest-neighbor carbon sites. Robust defect states emerge inside the bulk insulating gap of graphane. While for the stoichiometric half-filled system there are two doubly degenerate defect levels, there are four nondegenerate and spin-polarized in-gap defect levels in the system with one electron less than half filling. A universal set of quantum gates can be realized in the defective graphane lattice, by triggering resonant transitions among the defect states via optical pulses and \emph{ac} magnetic fields. The sizable energy separation between the occupied and the empty in-gap states enables precise control at room temperature. The spatial locality of the in-gap states implies a qubit network of extremely high areal density. Based on these results, we propose that graphane as a unique platform could be used to construct the future all-purpose quantum computers.

We study a possible topological charge pump in a traditional two-parameter pump device, which is based on a one-dimensional (1D) semiconductor nanowire with Rashba spin-orbit interaction. The 1D nanowire has a superlattice structure through periodic folding and the pumping parameters are the two time-dependent Zeeman fields with a phase lag phi between them. It is shown that the Zeeman field can open an energy gap of the nanowire superlattice and the system enters into a topological state. There are two electron charges pumped out adiabatically in a pumping cycle if the Fermi energy resides in the energy gap and phi is not close to n pi (n, an integer). This originates from the topologically protected interface state forming between the two pumping sources, which evolves with time, resulting in electron charges from one end transported to the other end of the wire. The quantized current direction can be modulated by some system parameters such as the Fermi energy, pumping phase, and the local potential of the device via gate voltage.