Although real materials are finite in size, electronic structure theory is built on the assumption of infinitely large solids, which led to a longstanding controversy: where is the vacuum level? Here, we introduce an analytic real-space potential-unfolding approach to uncover the vacuum level in infinitely large solids. First-principles calculations show that, in the absence of a physical surface, the bulk band structure, often measured with respect to an average bulk potential, is offset by a hereto unknown and orientation-dependent bulk quadrupole with respect to the vacuum level. By identifying intrinsic contributions of a bulk solid to its surface and interface properties, our theory eliminates the ambiguities surrounding the physical origin of the band alignment between matters.

We theoretically show that the three-dimensional (3D) topological insulator (TI)/thin-film ferromagnetic metal (FMM) heterostructure is possible to be a quantum anomalous Hall (QAH) insulator with a wide global band gap. Studying the band structure and the weight distributions of eigenstates, we demonstrate that the attachment of a metallic thin film on the 3DTI can shift the topologically nontrivial state into the metal layers due to the hybridization of bands around the original Dirac point. By introducing the magnetic exchange interaction in the thin-film layers, we compute the anomalous Hall conductivity and magnetic anisotropy of the heterostructure to suggest the appearance of a wider gap realizing QAH effect than usual materials, such as magnetically doped thin films of 3DTI and 3DTI/ferromagnetic insulator heterostructures. Our results indicate that the 3DTI/thin-film FMM heterostructure may implement the QAH effect even at room temperature, which will pave the way to the experimental realization of the other exotic topological quantum phenomena.

In this study, we investigate the cyclotron resonance effect in the first-order ac current, magnetic ratchet effect, and second harmonic generation in phosphorene in the presence of a steady tilted magnetic field and under THz laser radiation. We establish that various cyclotron resonances exist in the deduced currents based on the angular frequency of the incoming light. These resonances are dependent on the ${\omega }_{c_{+}}$ value, a function of the carrier charge, the perpendicular magnetic field, and the effective masses along the armchair or zigzag edges. We discuss the direction and the magnitude of the deduced currents for various radiation polarizations. We compare the results with a zero perpendicular magnetic field. Cyclotron resonance for the first-order ac current occurs at $\omega =\pm {\omega }_{c_{+}}$. The deduced current declines if the perpendicular magnetic field is zero. Meanwhile, for the ratchet current, cyclotron resonances occur at $\omega =\pm {\omega }_{c_{+}}$, $\omega =\pm 2{\omega }_{c_{+}}$, and radiation helicity affects the deduced current for circularly polarized light. Cyclotron resonance for the second harmonic generation current occurs at $\omega =\pm {\omega }_{c_{+}}$, $\omega =\pm 2{\omega }_{c_{+}}$, and $\omega =\pm {\omega }_{c_{+}}/2$, and the current is stronger compared to the case with no perpendicular magnetic field. As the magnetic field rotates in the plane of anisotropic phosphorene, separate directions are predicted for the second harmonic generation–related current. It is noteworthy that the magnitude of the ratchet and second harmonic generation current are within the same range and comparable to the magnetic ratchet current in monolayer graphene, $\mu \text{A/cm}$.

Strong coupling between quantum emitters and photonic modes, at which the level splitting (LS) of the mixed quantum states occurs, has aroused great interest due to diverse important quantum applications. The spectral Rabi-splitting (SRS) in photon emission or absorption has been extensively used to characterize strong coupling based on the equality assumption by assigning the SRS to LS. Here, we demonstrate that the equality assumption is usually invalid via a unified quantum treatment on the coupled plasmon-exciton system. It is revealed that the strong coupling characterized by the SRS is easily observed from the subsystem with larger decay, manifesting itself in relativity and diversity, which is highly correlated to the dissipative decays of the coupling subsystems. The plasmon-exciton interactions can be classified into the weak coupling, pseudo-, dark-, intermediate- and super-strong coupling regimes. Our work not only enriches the underlying physics of strong light-matter interactions, but also may stimulate further experimental researches in this field.

Nonlinear wave mixing in solids with ultrafast x-rays can provide insight into complex electronic dynamics of materials. Here, tabletop-based attosecond noncollinear four-wave mixing (FWM) spectroscopy using one extreme ultraviolet (XUV) pulse from high harmonic generation and two separately timed few-cycle near-infrared (NIR) pulses characterizes the dynamics of the ${\mathrm{Na}}^{+}$ $L_{2,3}$ edge core-excitons in NaCl around 33.5 eV. An inhomogeneous distribution of core-excitons underlying the well-known doublet absorption of the ${\mathrm{Na}}^{+}$ $\mathrm{\Gamma }$ point core-exciton spectrum is deconvoluted by the resonance-enhanced nonlinear wave mixing spectroscopy. In addition, other dark excitonic states that are coupled to the XUV-allowed levels by the NIR pulses are characterized spectrally and temporally. Approximately $<10\mathrm{fs}$ coherence lifetimes of the core-exciton states are observed. The core-excitonic properties are discussed in the context of strong electron-hole exchange interactions, electron-electron correlation, and electron-phonon broadening. This investigation successfully indicates that tabletop attosecond FWM spectroscopies represent a viable technique for time-resolved solid state measurements.

Nodal-point and nodal-line structures in the dispersion of electron energy bands are characterized by their high degeneracy in certain corners or lines in the Brillouin zone (BZ). These nodal structures can also exist in the dispersion of itinerant electrons in magnetically ordered materials whose symmetry groups are antiunitary groups called the magnetic space groups. In the present paper, we provide a complete list of magnetic space groups, which can host symmetry-protected nodal-point/line band structures for spin-1/2 fermionic particles, where the degeneracies at the nodal points/lines are guaranteed by irreducible projective representations (IPReps) of the little cogroups. Our discussion is restricted to the magnetic space groups whose magnetic point group contains the space-time inversion operation $\tilde{T}=\mathcal{I}T$, the combined operation of spacial inversion $\mathcal{I}$ and time reversal $T$, such that the energy bands are at least doubly degenerate at arbitrary points in the BZ. For these magnetic point groups we provide the invariants to label the classes of projective Reps, and for each class we calculate all the inequivalent IPReps. From the results we select out all the groups and the corresponding Rep classes, which support high-dimensional ($d\ge 4$) IPReps. We then list the magnetic space groups and their high-symmetry points/lines whose little cogroups have high-dimensional ($d\ge 4$) IPReps with the corresponding factor systems. Examples of candidate materials are discussed.

Probing the energy and spin electron properties of materials by means of photoemission spectroscopy gives insights into the low-energy phenomena of matter driven by spin orbit coupling or exchange interaction. The information that can be derived from complete photoelectron spectroscopy experiments, beyond $E$(k), is contained in the photoemission transition matrix elements that determine peak intensities. We present here a complete photoemission study of the spin-polarized bands of $2H\text{−}{\mathrm{NbSe}}_2$, a material that presents a surface spin-texture. Circular dichroism in angular-resolved photoemission spectroscopy (CD-ARPES) data are compared with spin-polarized angular-resolved spectra (SARPES) as measured with linearly polarized radiation in a well-characterized experimental chirality, at selected photon energy values. CD-ARPES is due to a matrix element effect that depends strongly on photon energy and experimental geometry: we show that it cannot be used to infer intrinsic spin properties in $2H\text{−}{\mathrm{NbSe}}_2$. On the other hand, SARPES data provide reliable direct information on the spin properties of the electron states. The results on $2H\text{−}{\mathrm{NbSe}}_2$ are discussed, and general methodological conclusions are drawn on the best experimental approach to the determination of the spin texture of quantum materials.

Various many-body perturbation theory techniques for calculating electron behavior rely on $W$, the screened Coulomb interaction. Computing $W$ requires complete knowledge of the dielectric response of the electronic system, and the fidelity of the calculated dielectric response limits the reliability of predicted electronic and structural properties. As a simplification, calculations often begin with the random-phase approximation (RPA). However, even RPA calculations are costly and scale poorly, typically as $N^4$ ($N$ representing the system size). A local approach has been shown to be efficient while maintaining accuracy for screening core-level excitations [Ultramicroscopy 106, 986 (2006)]. We extend this method to valence-level excitations. We present improvements to the accuracy and execution of this scheme, including reconstruction of the all-electron character of the pseudopotential-based wave functions, improved $N^{2}logN$ scaling, and a parallelized implementation. We discuss applications to Bethe-Salpeter equation calculations of core and valence spectroscopies.

In this work, based on the Fredkin spin chain, we introduce a family of spin-1/2 many-body Hamiltonians with a three-site interaction featuring a fragmented Hilbert space with coexisting quantum many-body scars. The fragmentation results from an emergent kinetic constraint resembling the conserved spin configuration in the one-dimensional Fermi-Hubbard model in the infinite on-site repulsion limit. To demonstrate the many-body scars, we construct an exact middle spectrum eigenstate within each fractured subsector displaying either logarithmic or area-law entanglement entropy. The interplay between fragmentation and scarring leads to rich tunable nonergodic dynamics by quenching different initial states shown through large-scale matrix product state simulations. In addition, we provide a Floquet quantum circuit that displays nonergodic dynamics due to sharing the same fragmentation structure and scarring as the Hamiltonian.

The formation of polarons due to the interaction between charge carriers and lattice ions has been proposed to have wide-ranging effects on charge carrier dynamics in lead halide perovskites. The hypothesis underlying many of those proposals is that charge carriers are ‘protected’ from scattering by their incorporation into large polarons. Following the approach of Kadanoff for scattering due to polar optical phonons, we derive expressions for the rates of scattering of polarons by acoustic phonons and ionized impurities, and compute the energy and angular dependent rates for electrons and holes in $\mathrm{M}\mathrm{A}\mathrm{Pb}{\mathrm{I}}_3, \mathrm{M}\mathrm{A}\mathrm{Pb}{\mathrm{Br}}_3$, and $\mathrm{Cs}\mathrm{Pb}{\mathrm{I}}_3$. We then use the ensemble Monte Carlo method to compute polaron distribution functions which satisfy a Boltzmann transport equation incorporating the same three scattering mechanisms, from which we extract mobilities for temperatures in the range 50–500 K. A comparison of the results with those of analogous calculations for bare band carriers indicates that polaronic effects on the scattering and mobilities of charge carriers in lead halide perovskites are more limited than has been suggested in some parts of the recent literature.

Owing to comparably strong spin-orbit coupling and Hubbard interaction, iridium based 227 pyrochlore oxides harbor a rich confluence of competing magnetic orders and emergent multipolar Weyl quasiparticles. Here we show that this family of materials is predominantly susceptible toward the nucleation of electronic noncoplanar all-in all-out (AIAO) and three-in one-out (3I1O) orders, respectively transforming under the singlet $A_{2u}$ and triplet $T_{1u}$ representations, supporting octupolar and dipolar Weyl fermions, and favored by strong on-site Hubbard and nearest-neighbor ferromagnetic interaction. Furthermore, a coplanar magnetic order generically appears as an intermediate phase between them. This order transforms under the triplet $T_{2u}$ representation and also hosts octupolar Weyl fermions. With the AIAO and 3I1O phases possibly being realized in ${\left({\mathrm{Nd}}_{1-x}{\mathrm{Pr}}_x\right)}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ when $x=0$ and 1, respectively, the intervening $T_{2u}$ order can in principle be found at an intermediate doping $(0<x<1)$ or in pressured (hydrostatic) ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$.

How to effectively regulate and preserve valley polarization is very important for the development of valleytronic devices in practical applications. Here, we propose a scheme of using ferroelectric polarization reversal to control valley polarization and, further, using ferroelectric nonvolatility to stably preserve the regulated valley state. We design a trilayer heterostructure, which is composed of vertically stacked two-dimensional (2D) valleytronic material ${\mathrm{HfN}}_2$, 2D ferromagnetic material ${\mathrm{CrI}}_3$, and 2D ferroelectric material ${\mathrm{In}}_2{\mathrm{Se}}_3$. Through first-principles simulation, we find that the polarization reversal of ${\mathrm{In}}_2{\mathrm{Se}}_3$ can change the orientation of the easy magnetization axis for ${\mathrm{CrI}}_3$, and the change of magnetic moment direction causes the change of valley polarization for ${\mathrm{HfN}}_2$. When ${\mathrm{In}}_2{\mathrm{Se}}_3$ is polarized upward, the easy magnetization axis of ${\mathrm{CrI}}_3$ is the $z$ axis, and the valley polarization is 15 meV, while when ${\mathrm{In}}_2{\mathrm{Se}}_3$ is polarized downward, the easy magnetization axis of ${\mathrm{CrI}}_3$ is changed to the $x\text{−}y$ plane, and the valley polarization is changed to zero. Then we study the ferroelectric bistability of the ${\mathrm{HfN}}_2/{\mathrm{CrI}}_3/{\mathrm{In}}_2{\mathrm{Se}}_3$ heterotrilayer and propose a method to preserve different valley states using ferroelectric nonvolatility.

We investigate the quantum dynamics of two-level systems (TLS) in glasses at low temperatures (1 K and below). We study an ensemble of TLSs coupled to phonons. By integrating out the phonons within the framework of the Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) master equation, we derive analytically the explicit form of the interactions among TLSs, and of the dissipation terms. We find that the unitary dynamics of the system shows clear signatures of many-body localization physics. We study numerically the time behavior of the concurrence, which measures pairwise entanglement also in nonisolated systems, and show that it presents a power-law decay both in the absence and in the presence of dissipation, if the latter is not too large. These features can be ascribed to the strong, long-tailed disorder characterizing the distributions of the model parameters. Our findings show that assuming ergodicity when discussing TLS physics might not be justified for all kinds of experiments on low-temperature glasses.

Many-body localization (MBL) is well characterized in Fock space. To quantify the degree of this Fock-space localization, the multifractal dimension $D_q$ is employed; it has been claimed that $D_q$ shows a jump from the delocalized value $D_q=1$ in the eigenstate thermalization hypothesis (ETH) phase to a smaller value $0<D_q<1$ at the ETH-MBL transition, yet exhibiting a conspicuous discrepancy from the fully localized value $D_q=0$, which indicates that multifractality remains inside the MBL phase. Here, to better quantify the situation, we employ, instead of the commonly used computational basis, the one-particle density matrix (OPDM) and use its eigenstates (natural orbitals) as a Fock state basis for representing many-body eigenstates $|\psi \rangle$ of the system. Using this basis, we compute $D_q$ and other indices quantifying the Fock-space localization, such as the local purity $S$, which is derived from the occupation spectrum $\left\{n_{\alpha }\right\}$ (eigenvalues of the OPDM). We highlight the statistical distribution of Hamming distance $x_{\mu \nu }$ occurring in the pairwise coefficients $|a_{\mu }{|}^2{\left|a_{\nu }\right|}^2$ in $S$, and compare this with a related quantity considered in the literature.

Recent acoustic and electrical-circuit experiments have reported the third-order (or octupole) topological insulating phase, while its counterpart in classical magnetic systems has yet to be realized. Here we explore the collective dynamics of magnetic vortices in three-dimensional breathing cuboids and find that the vortex lattice can support zero-dimensional corner states, one-dimensional hinge states, two-dimensional surface states, and three-dimensional bulk states when the ratio of alternating intralayer and interlayer bond lengths goes beyond a critical value. We show that only the corner states are stable against external frustrations because of the topological protection. Full micromagnetic simulations verify our theoretical predictions with good agreement.

We report a comprehensive investigation of the structural, magnetic, and electrical properties in a quasi-two-dimensional planar antiferromagnet ${\mathrm{BaNi}}_2{\left({\mathrm{PO}}_4\right)}_2$, which crystallizes in the rhombohedral structure (space group $R\overline{3}$) consisting of honeycomb layers of ${\mathrm{Ni}}^{2+}$ ions. Magnetic susceptibility and heat capacity data reveal a long-range antiferromagnetic ordering of ${\mathrm{Ni}}^{2+}$ ions at $T_N=24$ K. Interestingly, an applied magnetic field induces a dielectric anomaly and an electric polarization at $T_N$ with the polarization proportional to the applied magnetic fields, demonstrating the linear magnetoelectric effect in ${\mathrm{BaNi}}_2{\left({\mathrm{PO}}_4\right)}_2$ with a coupling coefficient of 1.67 ps/m. It is interesting to note that the magnetic symmetry associated with the magnetoelectric effect is ${\overline{1}}^{\prime }$, which allows all tensor elements of the magnetoelectric susceptibility.

Much has been learned regarding dynamical quantum phase transition (DQPT) due to sudden quenches across quantum critical points in traditional quantum systems. However, not much has been explored when a system undergoes a localization-delocalization transition. Here, we study one-dimensional fermionic systems in presence of a quasiperiodic potential, which induces delocalization-localization transition even in one dimension. We show signatures of DQPT in the many-body dynamics, when quenching is performed between phases belonging to different universality classes. We investigate how the nonanalyticity in the dynamical free energy gets affected with filling fractions in the bare system and, further, study the fate of DQPT under interaction. Strikingly, whenever quenching is performed from the low-entangled localized phase to the high-entangled delocalized phase, our studies suggest an intimate relationship between DQPT and the rate of the entanglement growth: Faster growths of entanglement entropy ensures quicker manifestation of the nonanalyticities in the many-body dynamical free energy.

Low-power spintronic devices based on the propagation of pure magnonic spin currents in antiferromagnetic insulator materials offer several distinct advantages over ferromagnetic components including higher-frequency magnons and a stability against disturbing external magnetic fields. In this work, we make use of the insulating antiferromagnetic phase of iron oxide, the mineral hematite $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ to investigate the long-distance transport of thermally generated magnonic spin currents. We report on the excitation of magnons generated by the spin Seebeck effect, transported both parallel and perpendicular to the antiferromagnetic easy-axis under an applied magnetic field. Making use of an atomistic hematite toy model, we calculate the transport characteristics from the deviation of the antiferromagnetic ordering from equilibrium under an applied field. We resolve the role of the magnetic order parameters in the transport, and experimentally we find significant thermal spin transport without the need for a net magnetization.

We consider a time reversal symmetry (TRS) broken Kane-Mele model superimposed with a Haldane model and chart out the phase diagram using spin Chern number to investigate the fate of the quantum anomalous Hall insulator (QAHI) and quantum spin Hall insulator (QSHI) phases. Interestingly, in addition to the QSHI and QAHI phase, the phase diagram unveils a quantum anomalous spin Hall insulator (QASHI) phase where only one spin sector is topological. We also find multicritical points where three or four topological phase boundaries coalesce. These topological phases are protected by an effective TRS and a composite antiunitary particle-hole symmetry leading to remarkable properties of edge modes. We find spin-selective, spin-polarized, and spin-neutral edge transport in the QASHI, QSHI, and QAHI phases, respectively. Our study indicates that the robustness of the topological phase mainly depends on the spin gap which does not necessarily vanish at the Dirac points across a topological phase transition. We believe that our proposals can be tested in the near future using recent experimental advancements in solid state and cold atomic systems.