Accurate predictions of electronic band gaps are key to the computational design of functional materials with tailored optical and thermoelectric properties. While an accurate description of the underlying electronic structure is indispensable, electron-phonon interactions also play a prominent role in determining the band gap. The harmonic vibrational approximation is used almost universally to describe electron-phonon coupling and provides a satisfactory description in most systems. Here, we demonstrate that this is not the case for molecular crystals due to the presence of strongly anharmonic motions. We demonstrate the breakdown of the harmonic approximation in acene molecular crystals, and show that converged band gaps can be obtained by rigorously accounting for vibrational anharmonicity using path-integral molecular dynamics. Finally, we characterize the most anharmonic vibrational modes and their contributions to the band-gap renormalization.

The kagome lattice is a fertile platform to explore topological excitations with both Fermi-Dirac and Bose-Einstein statistics. While relativistic Dirac fermions and flat bands have been discovered in the electronic structure of kagome metals, the spin excitations have received less attention. Here, we report inelastic neutron scattering studies of the prototypical kagome magnetic metal FeSn. The spectra display well-defined spin waves extending to 120 meV. Above this energy, the spin waves become progressively broadened, reflecting interactions with the Stoner continuum. Using linear spin-wave theory, we determine an effective spin Hamiltonian that reproduces the measured dispersion. This analysis indicates that the Dirac magnon at the $K$ point remarkably occurs on the brink of a region where well-defined spin waves become unobservable. Our results emphasize the influential role of itinerant carriers on the topological spin excitations of metallic kagome magnets.

We numerically study a model of interacting spin-$1/2$ electrons with random exchange coupling on a fully connected lattice. This model hosts a quantum critical point separating two distinct metallic phases as a function of doping: a Fermi-liquid phase with a large Fermi-surface volume and a low-doping phase with local moments ordering into a spin glass. We show that this quantum critical point has non-Fermi-liquid properties characterized by $T$-linear Planckian behavior, $\omega /T$ scaling, and slow spin dynamics of the Sachdev-Ye-Kitaev type. The $\omega /T$ scaling function associated with the electronic self-energy is found to have an intrinsic particle-hole asymmetry, a hallmark of a “skewed” non-Fermi liquid.

We consider a bulk system supporting parity and time reversal ($PT$) symmetry, and investigate how the $PT$ phase transition of edge states is influenced by different truncations of the system. As an example, we study a two-dimensional $PT$-symmetric Su–Schrieffer–Heeger lattice with non-Hermitian onsite potentials. We find that when the truncation preserves certain symmetries of the bulk lattice, the edge states can remain in the $PT$-unbroken phase when the non-Hermitian onsite potentials are less than a nonzero critical value. On the other hand, when the truncation removes such symmetries, edge states with complex eigen-energies are observed for infinitesimal non-Hermitian onsite potentials. We develop an analytic theory to account for such behaviors. Our results are important in the manipulation of the gain and loss behaviors of edge states in non-Hermitian systems, with potential applications in the study of topological lasers, quantum sensors, and unidirectional invisibility.

Half-quantized Hall conductance is characteristic of quantum systems with the parity anomaly. Here, we investigate topological and transport properties of a class of parity anomalous semimetals, in which massive Dirac fermions coexist with massless Dirac fermions in momentum space or real space, and uncover a distinct bulk-edge correspondence in which the half-quantized Hall effect is realized via the bulk massless Dirac fermions while the nontrivial Berry curvature is provided by the massive Dirac fermions. The spatial distribution of the edge current decays away from the boundary according to a power law instead of an exponential law in the integer quantum Hall effect. We further address the physical relevance of parity anomalous semimetals to three-dimensional semimagnetic topological insulators and two-dimensional photonic crystals.

Motivated by recent experiments [H. Zhou et al., Science 375, 774 (2022) and S. C. de la Barrera et al., arXiv:2110.13907], here we propose a general mechanism for valley and/or spin degeneracy lifting of the electronic bands in doped Bernal bilayer graphene, subject to electric displacement ($D$) fields. A $D$-field induced layer polarization (LP), when accompanied by a Hubbard repulsion-driven layer antiferromagnet (LAF) and next-nearest-neighbor repulsion-driven quantum anomalous Hall (QAH) orders, lifts the fourfold degeneracy of electronic bands, yielding a quarter metal for small doping, as also observed in ABC trilayer graphene. With the disappearance of the QAH order, electronic bands recover twofold valley degeneracy, thereby forming a conventional or compensated (with majority and minority carriers) half metal at moderate doping, depending on the relative strength of LP and LAF. At even higher doping and for a weak $D$ field only LAF survives and the Fermi surface recovers fourfold degeneracy. We also show that a pure repulsive electronic interaction mediated triplet $f$-wave pairing emerges from a parent correlated nematic liquid or compensated half metal when an in-plane magnetic field is applied to the system.

Chemical pressure (or “chemical precompression”) has a similar effect on changing the properties of matter as physical pressure does. However, a suitable method to evaluate the effect of “precompression” on matter has not been yet proposed. Here, we propose a concept of generalized pressure $\left(P_{\mathrm{G}}\right)$ to achieve the unification of all similar pressure factors, including internal chemical pressure $\left(P_{\mathrm{I}}\right)$ and external physical pressure $\left(P_{\mathrm{E}}\right)$. The $P_{\mathrm{G}}$ is the ensemble of all kinds of pressure factors based on the equivalence principle in the pressure-driven volume strain of crystal lattice or any other measurable physical quantity dominated by pressure. The equation of state for generalized pressure can elucidate the equivalent relationship between the chemical and physical pressure. To verify the proposed concept, we employed a semiconductor alloy ${\mathrm{Al}}_{0.86}{\mathrm{Ga}}_{0.16}\mathrm{N}$ as the experimental model.

We propose a new drag phenomenon, an interband magnon drag, and report on interaction effects and multiband effects in magnon transport of ferrimagnetic insulators. We study a spin-Seebeck coefficient $S_{\text{m}}$, a magnon conductivity ${\sigma }_{\text{m}}$, and a magnon thermal conductivity ${\kappa }_{\text{m}}$ of interacting magnons for a minimal model of ferrimagnetic insulators using a $1/S$ expansion of the Holstein-Primakoff method, the linear-response theory, and a method of Green's functions. We show that the interband magnon drag enhances ${\sigma }_{\text{m}}$ and reduces ${\kappa }_{\text{m}}$, whereas its total effects on $S_{\text{m}}$ are small. This drag results from the interband momentum transfer induced by the magnon-magnon interactions. We also show that the higher-energy band magnons contribute to $S_{\text{m}}$, ${\sigma }_{\text{m}}$, and ${\kappa }_{\text{m}}$ even for temperatures smaller than the energy difference between the two bands.

When quantum flavor Hall insulator phases of itinerant fermions are disordered by strong quantum fluctuations, the condensation of skyrmion textures of order parameter fields can lead to superconductivity. In this work, we address the mechanism of skyrmion condensation by considering the scattering between $(2+1)$-dimensional Weyl fermions and hedgehog-type tunneling configurations of order parameters that violate the skyrmion-number conservation law. We show the quantized, flavor Hall conductivity $\left({\sigma }_{xy}^f\right)$ controls the degeneracy of topologically protected, fermion zero-modes, localized on hedgehogs. The overlap between zero-mode eigenfunctions or 't Hooft vertex is shown to control the nature of the paired states. Employing this formalism for the $N=2$ model of twisted bilayer graphene, we describe the competition among flavor Hall orders, charge $4e^{-}$ superconductivity, and various charge $2e^{-}$ paired states in BCS and paired-density-wave channels. At charge neutrality, we show that the competition between flavor Hall insulators and charge $2e^{-}$ states can be captured by $SO\left(9\right)$ nonlinear sigma models. If the topological pairing mechanism can dominate over the conventional pairing mechanism, our work predicts the flavor-symmetry-preserving charge $4e^{-}$ superconductivity as a natural candidate for the paired state in the vicinity of the charge neutral point.

We present a numerical study of competing orders in the 1D $t\text{−}J$ model with long-range RKKY-like staggered spin interactions. By circumventing the constraints imposed by Mermin-Wagner's theorem, this Hamiltonian can realize long-range Néel order at half filling. We determine the full phase diagram as a function of the exchange and particle density using the density matrix renormalization group (DMRG) method. We show that pairing is disfavored and the AFM insulator and metallic phases are separated by a broad regime with phase segregation, before spin-charge separation re-emerges at low densities. Upon doping, interactions induce a confining potential that binds holons and spinons into full fledged fermionic quasiparticles in a range of parameters and densities. We numerically calculate the photoemission spectrum of the model, showing the appearance of a coherent quasiparticle band splitting away from the holon-spinon continuum with a width determined by $J$ that survives at finite doping. Comparison with analytical results using the self-consistent Born approximation (SCBA) and by solving the spinon-holon problem offer insight into the internal structure of the quasiparticles and help us explain the different features in the spectrum. We discuss how this simple toy model can teach us about the phenomenology of its higher-dimensional counterpart.

Using density functional theory calculations we study atomic, electronic, and magnetic structures and their influence on the topological phase of ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ and ${\mathrm{Mn}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ van der Waals compounds. Our results show that the antiferromagnetic topological insulator (AFM TI) phase in ${\mathrm{Mn}}_2{\mathrm{Bi}}_2{\mathrm{Te}}_5$ is robust both to details of the magnetic ordering within its structural units, nonuple layer (NL) blocks, and the type of atomic layer stacking, NaCl-type ABC or NiAs-type ABAC, within the (${\mathrm{MnTe})}_2$ sublattice. The structure with the NiAs-type stacking is energetically more favorable for both compounds. However, for ${\mathrm{Mn}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ the AFM TI phase is realized in the unstable structure with ABC stacking while it is a Dirac semimetal in favorable structure with NiAs stacking within a (${\mathrm{MnTe})}_2$ sublattice. We also show that imposing the overall ferromagnetic state by applying an external magnetic field can drive the ${\mathrm{Mn}}_2\mathrm{Bi}{\left(\mathrm{Sb}\right)}_2{\mathrm{Te}}_5$ compounds into different topologically nontrivial phases like axion insulator or Weyl semimetal.

We uncover the transverse Majorana spin polarization of a class of higher-order topological superconductors. “Transverse” means that the spin polarization of the Majorana zero modes is perpendicular instead of parallel with the applied magnetic field. With this novel Majorana spin property, we predict the spin-selective Andreev reflection occurs at lead polarization perpendicular to the applied magnetic field, in sharp contrast to that in the Majorana nanowire and vortex system. The induced topological Josephson ${\varphi }_0$ junction shows the qualitative difference from the topologically trivial Josephson ${\varphi }_0$ junction and provides benefits in constructing all electronically controlled Majorana networks for braiding. We thus provide a comprehensive scheme for probing non-Abelian statistics in this class of higher-order topological superconductors with transverse Majorana spin texture.

We study bilayer manganese phthalocyanine (MnPc) molecules and MnPc polymeric sheets using first-principles simulations with a focus on the magnetic interactions between Mn atoms. We find that the most stable position of the upper layer with respect to the lower layer is shifted about $1/8$ of a lattice vector from the center of the bottom layer along the direction toward a nearest-neighbor N atom. The magnetic ground state is the Néel antiferromagnetic (AFM) configuration within a layer and ferromagnetic (FM) between Mn atoms in adjacent layers. In this state, the system becomes a semiconductor with an indirect band gap of $11\text{meV}$. The strongest interaction is the interlayer coupling between the closest Mn atoms. A maximally localized Wannier analysis suggests that the dominant coupling pathway is Mn-(N,C)-Mn rather than a direct Mn-Mn coupling. The maximum calculated magnetic anisotropy energy is found to be $1.0\text{meV}$ per Mn atom. We also find that the bilayer molecule shows a significant stacking angle change from FM to AFM configurations accompanied by a change of orbital filling ordering.

Motived by time-reversal symmetry breaking and giant anomalous Hall effect in kagome superconductor $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$ = Cs, K, Rb), we carried out the thermal transport measurements on $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$. In addition to the anomalous Hall effect, the anomalous Nernst effect and the anomalous thermal Hall effect emerge. Interestingly, the longitudinal thermal conductivity ${\kappa }_{xx}$ largely deviates from the electronic contribution obtained from the longitudinal conductivity ${\sigma }_{xx}$ by the Wiedemann-Franz law. In contrast, the thermal Hall conductivity ${\kappa }_{xy}$ is roughly consistent with the Wiedemann-Franz law from electronic contribution. All these results indicate the large phonon contribution in the longitudinal thermal conductivity. Moreover, the thermal Hall conductivity is also slightly greater than the theoretical electronic contribution, indicating other charge neutral contributions. More than that, the Nernst coefficient and Hall resistivity show the multiband behavior with possible additional contribution from Berry curvature at the low fields.

We describe here a quantum simulator of extended bipartite Hubbard model with broken sublattice symmetry. The simulator consists of a structured lateral gate confining two-dimensional electrons in a quantum well into artificial minima arranged in a hexagonal lattice. The sublattice symmetry breaking is generated by forming an artificial triangular graphene quantum dot (ATGQD) with zigzag edges. The resulting extended Hubbard model generates tunable ratio of tunneling strength to electron-electron interactions and of sublattice symmetry with control over shape. The validity of the simulator is confirmed for small systems using mean-field and exact diagonalization many-body approaches which show that the ground state changes from a metallic to an antiferromagnetic (AF) phase by varying the distance between sites or depth of the confining potential. The one-electron spectrum of these triangular dots contains a macroscopically degenerate shell at the Fermi level. The shell persists at the mean-field level for weak interactions (metallic phase) but disappears for strong interactions in the AF phase. We determine the effects of electron-electron interactions on the ground state, the total spin, and the excitation spectrum as a function of filling of the ATGQD. We find that the half-filled charge neutral shell leads to a partially spin-polarized state in both metallic and AF regimes in accordance with Lieb's theorem. In both regimes a relatively large gap separates the spin-polarized ground state to the first excited many-body state at half-filling of the degenerate shell. By adding or removing an electron, this gap drops dramatically, and alternate total spin states emerge with energies nearly degenerate to a spin-polarized ground state.

The conduction electrons in the topologically nontrivial surface states of three-dimensional topological insulators have been actively exploited to exhibit emergent exotic properties and functionalities. The existence of a trivial two-dimensional electron gas near the surface has also been widely established, stemming from surface carrier doping of the topological insulators. Here we investigate theoretically the coupling between the electrons in the topological surface states and two-dimensional electron gas, and reveal that the helical nature of the former can be effectively shared with the latter through proximity effect. Next we examine the collective modes of the combined system and predict an optical plasmon mode rooted in the enhanced interband transitions of the nontrivial bands with the same helicity. The emergence of the interband transitions also introduce pronounced variations in the Dirac plasmon mode of the topological surface states, characterized by its gapped feature. These findings may find important applications in plasmonic and spintronic devices.

Bilayer graphene has two nonequivalent sublattices and, therefore, the same adatom impurity can manifest in spectrally distinct ways—sharp versus broad resonances near the charge neutrality—depending on the sublattice it adsorbs at. Employing Green's function analytical methods and the numerical kwant package, we investigate the spectral and transport interplay between the resonances and superconducting coherence induced in bilayer graphene by proximity to an $s$-wave superconductor. Analyzing doping and temperature dependencies of quasiparticle spin-relaxation rates, energies of Yu-Shiba-Rusinov states, Andreev spectra, and the supercurrent characteristics of Josephson junctions, we find unique superconducting signatures discriminating between resonant and off-resonant regimes. Our findings are in certain aspects going beyond the superconducting bilayer graphene and hold for generic $s$-wave superconductors functionalized by the resonant magnetic impurities.

Non-Hermitian systems have provided a rich platform to study unconventional topological phases. These phases are usually robust against external perturbations that respect certain symmetries of the system. In this Letter, we provide a different method to analytically study the effect of disorder, using tools from quantum field theory applied to discrete models around the phase-transition points. We investigate two different one-dimensional models, the paradigmatic non-Hermitian Su-Schrieffer-Heeger model and an $s$-wave superconductor with imbalanced pairing. These analytic results are compared to numerical simulations in the discrete models. It is found that the systems are driven from a topological to a trivial phase in the same way.

We argue that negative critical currents arise generically in Josephson junctions formed by single-channel conductors. Specifically, we theoretically study the Josephson coupling between two superconducting leads connected by a one-dimensional conductor in the Coulomb blockade regime. We show that in the clean regime the sign of the critical current alternates with the number of electrons in the normal region. For odd occupancy the critical current is negative even when the number of electrons on the conductor is large.