Two-dimensional van der Waals (vdW) heterostructures with several possibilities of band edge alignment are promising candidates in the design of semiconductor junction devices with good electrostatic control. We predict a robust broken-gap band alignment for the ${\mathrm{MoTe}}_2/{\mathrm{ZrS}}_2$ vdW heterostructure and uncover the underlying mechanism. Our first-principles calculations reveal that upon formation of the vdW heterostructure a small amount of charge is transferred between the layers which sits on the central transition metal atoms and is responsible for the band edge alignment. Moreover, the lattice-mismatch-induced in-plane strain and the interfacial polarization are shown to play no role in band alignment. However, we find a weak sensitivity of the electronic structure and band edge alignment of the vdW heterostructures to an external normal-to-plane strain or electric field. Our findings are helpful in designing heterojunction devices with desired electronic structures.

We theoretically study the optical activity in a doped system and derive the optical activity tensor from a light wave vector dependent linear optical conductivity. Although the light-matter interaction is introduced through the velocity gauge from a minimal-coupling Hamiltonian, we find that the well-known “false divergences” problem can be avoided in practice if the electronic states are described by a finite-band effective Hamiltonian, such as a few-band tight-binding model. The expression we obtain for the optical activity tensor is in good numerical agreement with a recent theory derived for an undoped topologically trivial gapped system. We apply our theory to the optical activity of a gated twisted bilayer graphene, with a detailed discussion of the dependence of the results on twist angle, chemical potential, gate voltage, and location of rotation center forming the twisted bilayer graphene.

Thermal conductivity $\left(\kappa \right)$ of semiconducting and insulating solids generally show inverse linear temperature dependence above the Debye temperature $\left(T_{\mathrm{D}}\right)$ owing to dominant phonon-phonon scattering, i.e, $\kappa \propto T^{-1}$. Recently, in ultralow $\kappa$ materials, $\kappa$ is found to decrease sublinearly ($\kappa \propto T^{-\alpha }$ where $0<\alpha <1$) above $T_{\mathrm{D}}$ as interbranch wavelike tunneling contribution becomes significant. Here we show that the deviation from linearity can be unprecedently large in incommensurate (IC) phases as exemplified by archetypal Zintl-like semiconductor ${\mathrm{TlInTe}}_2$. This happens because two mutually incompatible translational symmetries allow spatially and temporally varying phase shifts, thus giving rise to two new heat-carrying modes: phasons and amplitudons. Using comprehensive transport and spectroscopy measurements combined with first-principles simulations of multichannel thermal transport, i.e., phonon-phonon scattering and tunneling contributions, we find that new modes contribute nearly 10-30% of the total $\kappa$ near the IC transition. The origin of this IC transition is rooted in the Tl lone pair and large polarizability of the electronic cloud, whose fingerprints are visible in the phonon linewidths. Our study paves the way for understanding ultralow $\kappa$ in IC phases, particularly for charge-density-wave materials where IC modulation is ubiquitous.

Recently, a colossal magnetoresistance (CMR) was observed in $\mathrm{Eu}{\mathrm{Cd}}_2{\mathrm{P}}_2$, a compound that does not fit the conventional mixed-valence paradigm. Instead, experimental evidence points to a resistance driven by strong magnetic fluctuations within the two-dimensional ferromagnetic planes of the layered antiferromagnetic structure. While the experimental results have not yet been fully understood, a recent theory relates the CMR to a topological vortex-antivortex unbinding, i.e., Berezinskii-Kosterlitz-Thouless (BKT), phase transition. Motivated by these observations, in this work we explore the magnetic phases hosted by a microscopic classical magnetic model for $\mathrm{Eu}{\mathrm{Cd}}_2{\mathrm{P}}_2$, which easily generalizes to other Eu A-type antiferromagnetic compounds. Using Monte Carlo techniques to probe the specific heat and the helicity modulus, we show that our model can exhibit a vortex-antivortex unbinding phase transition. We find that this phase transition displays the same sensitivity to in-plane magnetization, interlayer coupling, and easy-plane anisotropy that is observed experimentally in the CMR signal, providing qualitative numerical evidence that the effect is related to a magnetic BKT transition.

We employ quench dynamics as an effective tool to probe different universality classes of topological phase transitions. Specifically, we study a model encompassing both Dirac-like and nodal loop criticalities. Examining the Kibble-Zurek scaling of topological defect density, we discover that the scaling exponent is reduced in the presence of extended nodal loop gap closures. For a quench through a multicritical point, we also unveil a path-dependent crossover between two sets of critical exponents. Bloch state tomography finally reveals additional differences in the defect trajectories for sudden quenches. While the Dirac transition permits a static trajectory under specific initial conditions, we find that the underlying nodal loop leads to complex time-dependent trajectories in general. In the presence of a nodal loop, we generically find a mismatch between the momentum modes where topological defects are generated and where dynamical quantum phase transitions occur. We also find notable exceptions where this correspondence breaks down completely.

The recent discovery of extraordinary-log universality has generated intense interest in classical and quantum boundary critical phenomena. Despite tremendous efforts, the existence of quantum extraordinary-log universality remains extremely controversial. Here, by utilizing quantum Monte Carlo simulations, we study the quantum edge criticality of a two-dimensional Bose-Hubbard model featuring emergent bulk criticality. On top of an insulating bulk, the open edges experience a Kosterlitz-Thouless-like transition into the superfluid phase when the hopping strength is sufficiently enhanced on edges. At the bulk critical point, the open edges exhibit the special, ordinary, and extraordinary critical phases. In the extraordinary phase, logarithms are involved in the finite-size scaling of two-point correlation and superfluid stiffness, which admit a classical-quantum correspondence for the extraordinary-log universality. Thanks to modern quantum emulators for interacting bosons in lattices, the edge critical phases might be realized in experiments.

We introduce a family of quantum Rényi fidelities and discuss their symmetry resolution. We express the symmetry-resolved fidelities as Fourier transforms of charged fidelities, for which we derive exact formulas for fermionic Gaussian states. These results also yield a formula for the total fidelities of fermionic Gaussian states, which we expect to have applications beyond the scope of this paper. We investigate the total and symmetry-resolved fidelities in the XX spin chain and focus on (i) fidelities between thermal states and (ii) fidelities between reduced density matrices at zero temperature. Both thermal and reduced fidelities can detect the quantum phase transition of the XX spin chain. Moreover, we argue that symmetry-resolved fidelities are sensitive to the inner structure of the states. In particular, they can detect the phase transition through the reorganization of the charge sectors at the critical point. This a main feature of symmetry-resolved fidelities which we expect to be general. We also highlight that reduced fidelities can detect quantum phase transitions in the thermodynamic limit.

CeAlGe, a proposed type-II Weyl semimetal, orders antiferromagnetically below 5 K. At 2 K, spin-flop and spin-flip transitions to less than 1 ${\mu }_B$/Ce are observed in the $\mathit{M}\left(\mathit{H}\right)$ data below 30 kOe, H$\parallel$a and b, and 4.3 kOe, H$\parallel \langle 110\rangle$, respectively, indicating a fourfold symmetry of the $M\left(H\right)$ data along the principal directions in the tetragonal ab plane with $\langle 110\rangle$ set of easy directions. However, anomalously robust and complex twofold symmetry is observed in the angular dependence of resistivity and magnetic torque data in the magnetically ordered state once the field is swept in the ab plane. This twofold symmetry is independent of temperature and field hystereses and suggests a magnetic phase transition that separates two different magnetic structures in the ab plane. The boundary of this magnetic phase transition and possibly the type of low-field magnetic structure can be tuned by an Al deficiency.

Layered material ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ has bridged the fields of topology, magnetism and two-dimensional (2D) van der Waals materials, and attracted tremendous interest recently. Based on the first-principles calculation, we find that the topological, magnetic and ferroelectric properties of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ bilayer can be engineered by interlayer sliding. By sliding bilayer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, the interlayer exchange interaction can be tuned between ferromagnetic and antiferromagnetic, enabling the transition from ferromagnetic insulators to Chern insulators, and the charge transfer would lead to coexistence with antiferromagnetic and ferroelectric phases. Our work paves a different mechanical way to engineer quantum phases in 2D magnetic materials.

Dirac semimetals are topological materials with degenerated Dirac points in their band structures. In particular, the Dirac points play a central role in the topologically protected phases. Recently, it was reported that the peculiar type-II Dirac points could be realized in photonics by means of photonic crystals (PCs). Nevertheless, the practical implementation is rather challenging. In this paper, we demonstrate that the nonlocal effect in the interior of a homogeneous photonic metamaterial (HPM) enables the existence of type-II Dirac points in the HPM's band structure. With theoretical formulations and numerical evaluations, we discuss the evolution of the HPM's band type and present the interesting transition between type-I and type-II Dirac degeneracy of the HPMs. Remarkably, a pair of the type-II Dirac points can be generated at two different frequencies, which is supported by the mode overlapping in the HPMs with weak nonlocality. The Fermi arc surface states connect the projections of the type-II Dirac points at the boundary between the vacuum and HPMs. The topological protection of the Fermi arc surface states is verified by the reflectionless transmission with sharp corners. Our paper provides a fundamental understanding of topological photonics by type-II Dirac points in HPMs.

Recent studies have demonstrated that two-dimensional intrinsic magnetic materials with high Curie temperature $\left(T_{\text{c}}\right)$ and large magnetic anisotropy energy (MAE) are highly desirable for future spintronics. After careful investigations through density functional theory, bipolar ${\mathrm{VSi}}_2{\mathrm{As}}_4$ and ${\mathrm{VGe}}_2{\mathrm{As}}_4$ monolayers, with semiconductor valence and conduction band edges fully spin polarized in different spin directions, are demonstrated to be highly stable and have in-plane ferromagnetism (FM) with large MAE of $\sim 5.5$ meV. The FM interaction is found to be dominated by the superexchange between d orbitals of V atoms through p orbitals of anions. More interestingly, $T_{\text{c}}$ is estimated to be $\sim 900$ K through Monte Carlo simulation, which is significantly higher than room temperature. In addition, both MAE and $T_{\text{c}}$ can be substantially regulated and increased under biaxial strain and the transition from FM semiconductors to metals will occur. Our calculations and analyses indicate that ${\mathrm{VSi}}_2{\mathrm{As}}_4$ and ${\mathrm{VGe}}_2{\mathrm{As}}_4$ monolayers are ideal systems for the fundamental understanding of magnetic physics as well as building blocks for magnetoelastic applications, high-temperature, or/and gate-tunable spintronic nanodevices.

In spin-orbit coupled crystals, symmetries can protect multifold degeneracies with large Chern numbers and Brillouin zone spanning topological surface states. In this work, we explore the extent to which the nontrivial topology of chiral multifold fermions impacts the spin texture of bulk states. To do so, we formulate a definition of spin-momentum locking in terms of reduced density matrices. Using tools from the theory of topological quantum chemistry, we show how the reduced density matrix can be determined from the knowledge of the basis orbitals and band representation forming the multifold fermion. We show how onsite spin-orbit coupling, crystal-field splitting, and Wyckoff position multiplicity compete to determine the spin texture of states near chiral fermions. We compute the spin texture of multifold fermions in several representative examples from space groups $P432$ (207) and $P{2}_{1}3$ (198). We show that the winding number of the spin around the Fermi surface can take many different integer values, from zero all the way to $\pm 7$. Finally, we conclude by showing how to apply our theory to real materials using the example of PtGa in space group $P{2}_{1}3$.

An anisotropic photonic crystal containing two anisotropic defect layers is considered. It is demonstrated that the system can support a Fabry-Perot bound state in the continuum (FP-BIC). A fully analytic solution of the scattering problem as well as a condition for FP-BIC have been derived in the framework of the temporal coupled-mode theory.

We propose a theoretical approach based on (constrained) density functional theory and the Franck-Condon approximation for the calculation of the temperature dependent photoluminescence of nanostructures. The method is computationally advantageous and only slightly more demanding than a standard density functional theory calculation and includes transitions into multiphonon final states (higher class transitions). We use the approach for Si and CdSe colloidal nanoclusters with up to 693 atoms and obtain very good agreement with experiment which allows us to identify specific peaks and explain their origin. Generally, breathing type modes are shown to dominate the phonon replicas, while optical modes have significant contributions for CdSe nanoclusters (NCs) and play a lesser role in Si NCs. We obtain significant anti-Stokes peak starting at 140 K for Si NC explaining the broadening observed in the corresponding experiment. We also apply the method to small molecular-like carbon structures (diamondoids), where electron-phonon coupling is typically large, and find that multiphonon processes (up to class 4) are very relevant and necessary to compare favorably with experiment. While it is crucial to include these multiphonon states in the small diamondoids with few tens of atoms, neglecting them in only marginally larger ${\mathrm{Si}}_{87}{\mathrm{H}}_{76}$ and ${\mathrm{Cd}}_{43}{\mathrm{Se}}_{44}{\mathrm{H}}_{76}^{*}$ (and larger) quantum dots represents a good approximation.

We study Hilbert space fragmentation in the extended Fermi-Hubbard model with nearest- and next-nearest-neighbor interactions. Using a generalized spin/mover picture and saddle point methods, we derive lower bounds for the scaling of the number of frozen states and for the size of the largest block preserved under the dynamics. We find fragmentation for strong nearest- and next-nearest-neighbor repulsions as well as for the combined case. Our results suggest that the involvement of next-nearest-neighbor repulsions leads to an increased tendency for localization. We then model the dynamics for larger systems using Markov simulations to test these findings and unveil in which interaction regimes the dynamics becomes spatially localized. In particular, we show that for strong nearest- and next-nearest-neighbor interactions random initial states will localize provided that the density of initial movers is sufficiently low.

Isolated quantum systems typically follow the eigenstate thermalization hypothesis, but there are exceptions, such as many-body localized (MBL) systems and quantum many-body scars. Here, we present the study of a weak violation of MBL due to a special state embedded in a spectrum of MBL states. The special state is not MBL since it displays logarithmic scaling of the entanglement entropy and of the bipartite fluctuations of particle number with subsystem size. In contrast, the bulk of the spectrum becomes MBL as disorder is introduced. We establish this by studying the entropy as a function of disorder strength and by observing that the level spacing statistics undergoes a transition from Wigner-Dyson to Poisson statistics as the disorder strength is increased.