Despite intense interest and considerable works, definitive experimental evidence for time-reversal-invariant topological superconductivity is still lacking. Hence searching for such superconductivity in real materials remains one of the main challenges in the field of topological material. Previously it has been shown that in the buckled honeycomb lattice structure, hydrogenated single bilayer Bi, namely BiH, is a topological insulator. Here we predict that upon $n$-type doping, BiH is a time-reversal-invariant topological superconductor. Interestingly the edge states of such a superconductor consist of both helical complex fermion modes and helical Majorana fermion modes.

The high pressure structural and electronic evolution of bulk ${\mathrm{MoS}}_2$, an important transition metal layered dichalchogenide, is currently under active investigation, particularly in search of superconductivity. Recent theoretical and experimental work predicted and verified a $2H_c\to 2H_a$ layer sliding structural transition at 20 GPa and a band overlap semiconductor-semimetal transition in the same pressure range. The poorly metallic and nonsuperconducting $2H_a$ structure is known to persist up to a pressure of 81 GPa, but properties at higher pressures remain experimentally unknown. Here we predict, with a first-principles evolutionary search, that major structural and electronic transformations should take place in equilibrium at higher pressures near 130–140 GPa. The main motif is a decomposition into MoS + S, also heralded in a small bimolecular cell by the appearance of a metastable nonlayered metallic ${\mathrm{MoS}}_2$ structure with space group $P4/mmm$. Unlike semimetallic $2H_a-{\mathrm{MoS}}_2$, the new high pressure phases are expected to be fully metallic and superconducting with higher critical temperatures than even alkali-intercalated ${\mathrm{MoS}}_2$.

We demonstrate that a combination of electronic structure modification and oxygen vacancy ordering can stabilize a long-range ferromagnetic ground state in epitaxial ${\mathrm{LaCoO}}_3$ thin films. Highest saturation magnetization values are found in the thin films in tension on ${\mathrm{SrTiO}}_3$ and $(\mathrm{La},\mathrm{Sr})(\mathrm{Al},\mathrm{Ta}){\mathrm{O}}_3$ substrates and the lowest values are found in thin films in compression on ${\mathrm{LaAlO}}_3$. Electron microscopy reveals oxygen vacancy ordering to varying degrees in all samples, although samples with the highest magnetization are the most defective. Element-specific x-ray absorption techniques reveal the presence of high spin ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$ as well as low spin ${\mathrm{Co}}^{3+}$ in different proportions depending on the strain state. The interactions among the high spin Co ions and the oxygen vacancy superstructure are correlated with the stabilization of the long-range ferromagnetic order.

We use cellular dynamical mean-field theory to study the phase diagram of the square lattice bilayer Hubbard model with an interlayer interaction. The layers are populated by two-component fermions, and the densities in both layers and the strength of the interactions are varied. We find that an attractive interlayer interaction can induce a checkerboard density-ordered phase and superfluid phases, with either interlayer or intralayer pairing. Remarkably, the latter phase does not require an intralayer interaction to be present: it can be attributed to an induced attractive interaction caused by density fluctuations in the other layer.

Motivated by the earlier experimental results and ab initio studies on the electronic structure of layered ruthenates (${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$) we introduce and investigate the multiband $d\text{−}p$ charge transfer model describing a single ${\mathrm{RuO}}_4$ layer, similar to the charge transfer model for a single ${\mathrm{CuO}}_2$ plane including apical oxygen orbitals in high $T_c$ cuprates. The present model takes into account nearest-neighbor anisotropic ruthenium-oxygen $d\text{−}p$ and oxygen-oxygen $p\text{−}p$ hopping elements, crystal-field splittings, and spin-orbit coupling. The intraorbital Coulomb repulsion and Hund's exchange are defined not only at ruthenium but also at oxygen ions. Our results demonstrate that the ${\mathrm{RuO}}_4$ layer cannot be regarded to be a pure ruthenium $t_{2g}$ system. We examine a different scenario in which ruthenium $e_g$ orbitals are partly occupied and highlight the significance of oxygen orbitals. We point out that the predictions of an idealized model based on ionic configuration (with $n_0=4+4\times 6=28$ electrons per ${\mathrm{RuO}}_4$ unit) do not agree with the experimental facts for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ which support our finding that the electron number in the $d\text{−}p$ states is significantly smaller. In fact, we find the electron occupation of $d$ and $p$ orbitals for a single ${\mathrm{RuO}}_4$ unit $n=28-x$, being smaller by at least 1–1.5 electrons from that in the ionic model and corresponding to self-doping with $x\simeq 1.5$.

Two-dimensional dilute magnetic semiconductors can provide fundamental insights into the very nature of magnetic order and their manipulation through electron and hole doping. Besides the fundamental interest, due to the possibility of control of charge density, they can be extremely important in spintronics applications such as spin valve and spin-based transistors. In this paper, we studied a two-dimensional dilute magnetic semiconductor consisting of a phosphorene monolayer doped with cobalt atoms in substitutional and interstitial defects. We show that these defects can be stabilized and are electrically active. Furthermore, by including holes or electrons by a potential gate, the exchange interaction and magnetic order can be engineered, and may even induce a ferromagnetic-to-antiferromagnetic phase transition in $p$-doped phosphorene. At a Co concentration of 2.7%, we estimate a Curie temperature of $T_{\text{C}}^{MFA}=466$ K in the mean-field approximation.

We have observed Shubnikov–de Haas and de Haas–van Alphen effects in the single crystals of the three-dimensional Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ up to 50 K, traceable at fields as low as 2 and 1 T, respectively. The values of the Fermi wave vector, the Fermi velocity, and the effective cyclotron mass of the charge carrier, calculated from both techniques, are close to each other and match well with earlier reports. However, the de Haas–van Alphen effect clearly reflects the existence of two different Fermi surface cross sections along certain directions and a nontrivial Berry's phase, which is the signature of a three-dimensional Dirac fermion in ${\mathrm{Cd}}_3{\mathrm{As}}_2$.

The problem of two magnetic impurities in a normal metal exposes the two opposite tendencies in the formation of a singlet ground state, driven respectively by the single-ion Kondo effect with conduction electrons to screen impurity spins or the Ruderman-Kittel-Kasuya-Yosida interaction between the two impurities to directly form impurity spin singlet. However, whether the competition between these two tendencies can lead to a quantum critical point has been debated over more than two decades. Here, we study this problem by applying the newly proposed natural orbitals renormalization group method to a lattice version of the two-impurity Kondo model with a direct exchange $K$ between the two impurity spins. The method allows for unbiased access to the ground state wave functions and low-lying excitations for sufficiently large system sizes. We demonstrate the existence of a quantum critical point, characterized by the power-law divergence of impurity staggered susceptibility with critical exponent $\gamma =0.60\left(1\right)$, on the antiferromagnetic side of $K$ when the interimpurity distance $R$ is even lattice spacing, while a crossover behavior is recovered when $R$ is odd lattice spacing. These results have ultimately resolved the long-standing discrepancy between the numerical renormalization group and quantum Monte Carlo studies, confirming a link of this two-impurity Kondo critical point to a hidden particle-hole symmetry predicted by the local Fermi liquid theory.

We investigate quasiparticle band structures of rutile and anatase phases of ${\mathrm{TiO}}_2$ and cubic ${\mathrm{SrTiO}}_3$ using various levels of $GW$ approximations such as $G_0{W}_0, G{W}_0, GW, QPG{W}_0$, and $QPGW$. It is found that the quasiparticle band gap increases as the self-consistency level is ratcheted up from $G_0{W}_0$ to $G{W}_0$ and $GW$, and the $GW$ results overestimate the band gap by $\sim 50\%$ in comparison with the experimental values. Interestingly, when the wave functions are updated in $QPG{W}_0$ and $QPGW$, the band gap is significantly reduced, which is opposite to the tendency known for other materials. The experimental band gap is most closely reproduced by $QPG{W}_0$ within $\sim 10\%$ error. We explain the improvement by the wave-function update in terms of the local change in the charge density, which is different at Ti and O sites and reduces the quasiparticle energy gap.

We discuss three-dimensional (3D) photonic crystals fabricated by simultaneous multidirectional plasma etching. First, we investigate a method for controlling the ion sheath used in reactive ion etching for obtaining multidirectional etching. We then discuss the fabrication tolerance from an analytical perspective. Based on our results, we demonstrate the fabrication of 3D photonic crystals with thicknesses of 1, 1.5, and 2 lattice periods in the surface-normal direction on single-crystalline silicon wafers, which show high reflectance $(\sim 100\%)$ and low transmittance $(-17\mathrm{dB})$ at optical communication wavelengths, suggesting the formation of a complete photonic band gap. We reveal that the shape of the etched holes limits the performance of 3D photonic crystals and suggest possible ways to improve the band-gap effect. Moreover, we show that 3D photonic crystals with short lattice constants show high reflectance $(\sim 80\%)$ at visible to near-infrared wavelengths. By investigating the influence of absorption on the characteristics of 3D photonic crystals, we reveal that the reflectance remains as high as 94% in the photonic band-gap range even when the absorption of silicon is taken into account. We find that a unique increase of absorption occurs at several discrete wavelengths below the photonic band gap, suggesting the possibility of manipulating light absorption. These results not only simplify the fabrication of 3D photonic crystals, but also provide a basis for realizing 3D photonic nanostructures that include other materials.

Conventional and spin-related thermoelectric effects in electronic transport through a nanoscopic system exhibiting magnetic anisotropy, with both uniaxial and transverse components, are studied theoretically in the linear-response regime. In particular, a magnetic tunnel junction with a large-spin impurity, either a magnetic atom or a magnetic molecule, embedded in the barrier is considered as an example. Owing to magnetic interaction with the impurity, conduction electrons traversing the junction can scatter on the impurity, which effectively can lead to angular momentum and energy exchange between the electrons and the impurity. As we show, such processes have a profound effect on the thermoelectric response of the system. Specifically, we present a detailed analysis of charge, spin, and thermal conductance, together with the Seebeck and spin Seebeck coefficients (thermopowers). Since the scattering mechanism also involves processes when electrons are inelastically scattered back to the same electrode, one can expect the flow of spin and energy also in the absence of charge transport through the junction. This, in turn, results in a finite spin thermopower, and the magnetic anisotropy plays a key role for this effect to occur.

In this Rapid Communication we study the Haldane model with on-site repulsive interactions at half-filling. We show that the mean-field Hamiltonian with magnetic order effectively modifies parameters in the Haldane Hamiltonian, such as sublattice energy difference and phase in next-nearest hopping. As interaction increases, increasing of magnetic order corresponds to varying these parameters and, consequently, drives topological transitions. At the mean-field level, one scenario is that the magnetic order continuously increases, and, inevitably, the fermion gap closes at the topological transition point. Beyond the mean field, fluctuation-induced interaction can further open up the gap, rendering a first-order transition. Another scenario is a first-order transition at the mean-field level across which a canted magnetic order develops discontinuously, avoiding the fermion gap closing. We find that both scenarios exist in the phase diagram of the Haldane-Hubbard model. Our predication is relevant to recent experimental realization of the Haldane model in cold-atom system.

Conventional Cooper pairing arises from attractive interaction of electrons in the metallic bands. A recent experiment on Co-doped LiFeAs shows superconductivity in the insulating valence band, which is evolved from a metallic hole band upon doping. Here we examine this phenomenon by studying superconductivity in a three-orbital Hamiltonian relevant to the doped LiFeAs. We show explicitly that Cooper pairing of the insulating hole band requires a finite pairing interaction strength. For strong coupling, the superconductivity in the hole band is robust against the sink of the hole band below the Fermi level. Our theory predicts a substantial upward shift of the chemical potential in the superconducting transition for Co-doped LiFeAs.

We use thermalization indicators and numerical linked cluster expansions to probe the onset of many-body localization in a disordered one-dimensional hard-core boson model in the thermodynamic limit. We show that after equilibration following a quench from a delocalized state, the momentum distribution indicates a freezing of one-particle correlations at higher values than in thermal equilibrium. The position of the delocalization to localization transition, identified by the breakdown of thermalization with increasing disorder strength, is found to be consistent with the value from the level statistics obtained via full exact diagonalization of finite chains. Our results strongly support the existence of a many-body localized phase in the thermodynamic limit.

We report that the special coupling between Dirac fermion and lattice vibrations, in other words, electron-phonon coupling (EPC), in silicene layers on an Ag(111) surface was probed by in situ Raman spectroscopy. We find the EPC is significantly modulated due to tensile strain, which results from the lattice mismatch between silicene and the substrate, and the charge doping from the substrate. The special phonon modes corresponding to two-dimensional electron gas scattering at the edge sites in the silicene were identified. Detecting the relationship between EPC and Dirac fermions through Raman scattering will provide a direct route to investigate the exotic property in buckled two-dimensional honeycomb materials.

We report layer-resolved measurements of the unoccupied electronic structure of ultrathin MgO films grown on Ag(001). The metal-induced gap states at the metal/oxide interface, the oxide band gap, and a surface core exciton involving an image-potential state of the vacuum are revealed through resonant Auger spectroscopy of the $\mathrm{Mg}K{L}_{23}{L}_{23}$ Auger transition. Our results demonstrate how to obtain new insights on empty states at interfaces of metal-supported ultrathin oxide films.

Based on its condensed-matter properties, crystal structure, and metallurgy, which includes a phase diagram with six allotropic phases, plutonium is one of the most complicated pure elements in its solid state. Its anomalous properties, which are indicative of a very strongly correlated state, are related to its special position in the periodic table, which is at the boundary between the light actinides that have itinerant $5f$ electrons and the heavy actinides that have localized $5f$ electrons. As a foundational study to probe the role of local electronic correlations in Pu, we use the local-density approximation together with a continuous-time quantum Monte Carlo simulation to investigate the electronic structure of a single Pu atom that is either substitutionally embedded in the bulk and or adsorbed on the surface of a Th host. This is a simpler case than the solid phases of Pu metal. For the Pu impurity atom we have found a Kondo resonance peak, which is an important signature of electronic correlations, in the local density of states around the Fermi energy. Furthermore, we show that the peak width of this resonance is narrower for Pu atoms at the surface of Th than for those in the bulk due to a weakened $\mathrm{Pu}-5f$ hybridization with the ligands at the surface.

We present electron and phonon spectral functions calculated from determinant quantum Monte Carlo simulations of the half-filled two-dimensional Hubbard-Holstein model on a square lattice. By tuning the relative electron-electron $(e-e)$ and electron-phonon $(e-ph)$ interaction strengths, we show the electron spectral function evolving between antiferromagnetic insulating, metallic, and charge-density-wave (CDW) insulating phases. The phonon spectra concurrently gain a strong momentum dependence and soften in energy upon approaching the CDW phase. In particular, we study how the $e-e$ and $e-ph$ interactions renormalize the spectra and find that the presence of both interactions suppresses the amount of renormalization at low energy, thus allowing the emergence of a metallic phase at intermediate coupling strengths. In addition, we find a modest enhancement of the $d$-wave pairing susceptibility in the metallic regime, although spin and charge correlations are still dominant at the temperatures considered in our study. These findings demonstrate the importance of considering the influence of multiple interactions in spectroscopically determining any one interaction strength in strongly correlated materials.

We study thermodynamic properties as well as the dynamical spin and quadrupolar structure factors of the O(3)-symmetric spin-1 Heisenberg model with bilinear-biquadratic exchange interactions on the triangular lattice. Based on a sign-problem-free quantum Monte Carlo approach, we access both the ferromagnetic and the ferroquadrupolar ordered spin nematic phase as well as the SU(3)-symmetric point that separates these phases. Signatures of Goldstone soft modes in the dynamical spin and the quadrupolar structure factors are identified, and the properties of the low-energy excitations are compared to the thermodynamic behavior observed at finite temperatures as well as to Schwinger-boson flavor-wave theory.

Fringe fields emanating from magnetic domain structures can give rise to magnetoresistance in organic semiconductors. In this article, we explain these magnetic-field effects in terms of a $\Delta B$ mechanism. This mechanism describes how variations in magnetic-field strength between two polaron hopping sites can induce a difference in precessional motion of the polaron spins, leading to mixing of their spin states. In order to experimentally explore the fringe-field effects, polymer thin-film devices on top of a rough in-plane magnetized cobalt layer are investigated. The cobalt layer can be described by a distribution of out-of-plane magnetic anisotropies, most likely induced by thickness variations in the cobalt. With a magnetic field perpendicular to the cobalt layer, fringe fields are created because some domains are magnetized out of plane whereas the magnetization of other domains remains approximately in plane. By varying the distance between the polymer layer and the cobalt layer, we find that the magnetoresistance arising from these fringe fields reduces with the gradient in the fringe fields, in agreement with the $\Delta B$ mechanism.