The complex energy bands of non-Hermitian systems braid in momentum space even in one dimension. Here, we reveal that the non-Hermitian braiding underlies the Hermitian topological physics with chiral symmetry under a general framework that unifies Hermitian and non-Hermitian systems. Particularly, we derive an elegant identity that equates the linking number between the knots of braiding non-Hermitian bands and the zero-energy loop to the topological invariant of chiral-symmetric topological phases in one dimension. Moreover, we find an exotic class of phase transitions arising from the critical point transforming different knot structures of the non-Hermitian braiding, which are not included in the conventional Hermitian topological phase transition theory. Nevertheless, we show the bulk-boundary correspondence between the bulk non-Hermitian braiding and boundary zero modes of the Hermitian topological insulators. Finally, we construct typical topological phases with non-Hermitian braidings, which can be readily realized by artificial crystals.

We demonstrate the controlled generation and annihilation of (anti)skyrmions with tunable chirality in magnetic heterostructures by means of micromagnetic simulations. By making use of magnetic (anti)vortices in a patterned ferromagnetic layer, we stabilize (anti)skyrmions in an underlying skyrmionic thin film in a reproducible manner. The stability of the (anti)skyrmion depends on the polarization of the (anti)vortex, whereas their chirality is given by those of the (anti)vortices. We investigate the influence of geometric parameters such as nanodisk radius and film thickness on the stability of the (anti)skyrmions. By introducing the interlayer Dzyaloshinskii-Moriya interaction into our modeling, we predict that the same coupling mechanism works also for chiral skyrmions. Furthermore, we demonstrate that the core coupling between the (anti)vortices and (anti)skyrmions allows deleting and writing of spin objects in a controlled fashion by applying short pulses of in-plane external magnetic fields or charge currents, representing a new key paradigm in skyrmionic devices.

The robustness of the chiral surface current of chiral superconductors against surface roughness is studied utilizing the quasiclassical Eilenberger theory. We consider the general chiral superconductors where the pair potential is given by the spherical harmonics $Y_l^m$ such that the $(l,m)=(1,\pm 1)$ state corresponds to a $(p_x\pm i{p}_y)$-wave superconductor. The self-consistent calculations demonstrate that the robustness of the chiral current is determined by whether subdominant $s$-wave Cooper pairs are induced by disorder. The induced $s$-wave pairs act as an effective pair potential. As a result, the spontaneous chiral current of $(p_x+i{p}_y)$- and $(d_{x^2-y^2}+i{d}_{xy})$-wave superconductors is robust against the roughness because the subdominant $s$-wave Cooper pairs are present.

The strange metal phase of overdoped cuprate superconductors exhibits a linear in temperature resistivity in low temperatures; however, the origin of this remarkable anomaly is still not well understood. Here the linear temperature dependence of the resistivity in the strange metal phase of overdoped cuprate superconductors is investigated. The momentum dependence of the transport scattering rate is arisen from the electron umklapp scattering mediated by the spin excitation and is employed to calculate the resistivity by making use of the Boltzmann equation. It is shown that the resistivity is mainly dominated by the antinodal and nodal umklapp scattering. In particular, a very low temperature $T_{\mathrm{scale}}$ scales with ${\mathrm{\Delta }}_p^2$, where ${\mathrm{\Delta }}_p$ is the minimal umklapp vector at the antinode. In the low temperature above $T_{\mathrm{scale}}$, the resistivity is linear in temperature with a coefficient that decreases with the increase of doping; however, in far lower temperatures below $T_{\mathrm{scale}}$, the resistivity is instead quadratic in temperature. The theory also shows that the same spin excitation that acts like a bosonic glue to hold the electron pairs together also mediates scattering of electrons in the strange metal phase responsible for the linear in temperature resistivity in low temperatures.

Silicon anodes hold great promise for next-generation Li-ion batteries. The main obstacle to exploiting their high performance is the challenge of linking experimental observations to atomic structures due to the amorphous nature of Li-Si alloys. We unveil the atomistic-scale structures of amorphous Li-Si using our recently developed density functional tight-binding model. Our claim is supported by the successful reproduction of experimental x-ray pair distribution functions, NMR, and Mössbauer spectra using simple nearest-neighbor models. The predicted structures are publicly available.

The spin-flop transition in noncentrosymmetric ${\mathrm{Ca}}_2{\mathrm{Fe}}_{2-x}{\mathrm{Al}}_x{\mathrm{O}}_5$ induced by a magnetic field is accompanied by the appearance of a macroscopic electric polarization whose origin was ascribed to a noncollinear spin arrangement in the domain walls between two collinear magnetic phases and/or metal-ligand hybridization effects depending on the spin direction [Abe et al., Phys. Rev. B 89, 054437 (2014)]. Here, we present an extensive study combining experimental and theoretical efforts to propose an alternative mechanism that explains the observed electrical polarization. Our macroscopic magnetic and magnetoelectric as well as neutron diffraction data on a ${\mathrm{Ca}}_2{\mathrm{Fe}}_{1.1}{\mathrm{Al}}_{0.9}{\mathrm{O}}_5$ single-crystal sample are in agreement with an angular spin configuration, continuously rotating from being parallel to the $c$ axis at low temperatures and low magnetic fields to being parallel to the $a$ axis either by increasing the temperature or by applying a magnetic field along the $c$ axis. A phenomenological Landau free-energy model emphasizes that the crystal structure belongs to the pyroelectric family with electric polarization along the $c$ axis, which also depends on the spin orientation. It clarifies the origin of the spontaneous and field-induced phase transitions and especially the magnetoelectric features.

In magnetic materials with strong spin-lattice coupling, magnon-phonon interactions can change the sensitivity of the lattice thermal conductivity in an applied magnetic field. Applying an out-of-plane magnetic field to change ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ between antiferromagnetic (AFM), canted antiferromagnetic (CAFM), and ferromagnetic (FM) phases, we controlled the lattice thermal conductivity, generating both a positive and a negative magnetic field dependence. The in-plane thermal conductivity decreases with field in the AFM phase, remains approximately constant in the CAFM phase, and increases with field in the FM phase. We explain this in terms of the field-induced changes of the magnon gap which modifies magnon-phonon scattering. We also report thermal Hall data measured in the same configuration.

The $q$-state Potts chain with ferromagnetic couplings, $J=1$, in the presence of a transverse field $\mathrm{\Gamma }$ has a quantum phase transition at $\mathrm{\Gamma }/q=1$, which is continuous for $q\le 4$ and of first order for $q>4$. Here we introduce a $q$-periodic alternating longitudinal field of strength $h$ and study the phase diagram and the critical properties of the model. For $h<q/(q-1)$, there is a ferromagnetic ordered phase, for $\mathrm{\Gamma }<{\mathrm{\Gamma }}_c\left(h\right)$, and at $h=q/(q-1)$, there is a classical endpoint at $\mathrm{\Gamma }=0$, with finite entropy at $T=0$. We considered the $q=3$ model and using density matrix renormalization group techniques, we calculated the low-lying spectrum of the Hamiltonian, the transverse magnetization, and the spin-spin correlation function, all of which signal a diverging correlation length at the transition point with the exponent of the three-state Potts model. In the vicinity of the classical endpoint, the model is mapped to a quantum hard rod model, which also belongs to the universality class of the ferromagnetic three-state Potts model. Also, the spectrum of the critical Hamiltonian is found in agreement with conformal invariance. At the same time, the correlation function shows a jump at the transition point, and thus the transition is of mixed order for $h<q/(q-1)$.

We thoroughly analyze the divergences of the irreducible vertex functions occurring in the charge channel of the half-filled Hubbard model in close proximity to the Mott metal-insulator transition (MIT). In particular, by systematically performing dynamical mean-field theory (DMFT) calculations on the two-particle level, we determine the location and the number of the vertex divergences across the whole coexistence region adjacent to the first-order metal-to-insulator transition. We find that the lines in the parameter space, along which the vertex divergences occur, display a qualitatively different shape in the coexisting metallic and insulating phase, which is also associated to an abrupt jump of the number of divergences across the MIT. Physically, the systematically larger number of divergences on the insulating side of the transition reflects the sudden suppression of local charge fluctuation at the MIT. Further, a systematic analysis of the results demonstrates that the number of divergence lines increases as a function of the inverse temperature $\beta ={\left(k_{\mathrm{B}}T\right)}^{-1}$ by approaching the Mott transition in the zero-temperature limit. This makes it possible to identify the zero-temperature MIT as an accumulation point of an infinite number of vertex divergence lines, unveiling the highly nonperturbative nature of the underlying transition.

We introduce and study several classes of quantum spin models with multibody interactions that exhibit quantum many-body scars. The models are constructed by two different methods: one exploiting boundary states in integrable spin chains and the other based on a variant of existing methods such as restricted spectrum generating algebras. The first method allows us to construct deformations of the Majumdar-Ghosh and Affleck-Kennedy-Lieb-Tasaki models—prototypes of frustration-free systems. With the second method, we construct a large class of spin-1 models involving scalar spin chirality in both one and two dimensions. Interestingly, in some cases, the models so constructed have towers of scar states of different character. For each example, we show that the scar states behave differently from thermal states by comparing their spectral and dynamical properties with those of other states. We also show that a superposition of the scar states constructed by the second method exhibits perfectly periodic revivals in the dynamics.

We study radiative heat transfer between two nanoemitters placed inside different types of closed cavities by means of a fluctuational-electrodynamics approach. We highlight a very sharp dependence of this transfer on cavity width and connect this to the matching between the material-induced resonance and the resonant modes of the cavity. In resonant configurations, this allows for an energy-flux amplification of several orders of magnitude with respect to the one exchanged between two emitters in vacuum as well as between two blackbodies, even at separation distances much larger than the thermal wavelength. On the other hand, variations of the cavity width by a few percent allow a reduction of the flux by several orders of magnitude and even a transition to inhibition compared to the vacuum scenario. Our results pave the way to the design of thermal waveguides for the long-distance transport of super-Planckian heat flux and selective heat transfer in many-body systems.

We develop a unified treatment of the non-Abelian Aharonov-Bohm (AB) effect in isotropic multiband systems, namely, the scattering of particles on a gauge field corresponding to a noncommutative Lie group. We present a complex contour integral representation of the scattering states for such systems, and using their asymptotic form, we calculate the differential scattering cross section. The angular dependence of the cross section turns out to be the same as that obtained originally by Aharonov and Bohm in their seminal paper [Phys. Rev. 115, 485 (1959)], but this time, it depends on the polarization of the incoming plane wave. As an application of our theory, we perform the contour integrals for the wave functions explicitly and calculate the corresponding cross section for three nontrivial isotropic multiband systems relevant to condensed matter and particle physics. To have a deeper insight into the nature of the scattering, we plot the probability and current distributions for different incoming waves. This paper is a generalization of our recent results on the Abelian AB effect providing an extension of exactly solvable AB scattering problems.

Magnetite $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$ doped with earth-abundant metals has emerged as a promising catalyst material, with Ni-doped magnetite $(\mathrm{Ni}/{\mathrm{Fe}}_3{\mathrm{O}}_4)$ being a cost-effective, durable, and highly active material for photocatalytic and electrochemical water oxidation. While previous studies have investigated the incorporation of Ni atoms into ${\mathrm{Fe}}_3{\mathrm{O}}_4$ single-crystalline surfaces using surface science characterization methods and density functional theory calculations, an experimental study is still required to understand the impact of Ni incorporation on the electronic structure of $\mathrm{Ni}/{\mathrm{Fe}}_3{\mathrm{O}}_4$ systems. To address this, we employed angle-resolved photoemission spectroscopy, analyzed within the one-step model of photoemission by a real-space multiple scattering code to investigate the electronic structure of the reconstructed magnetite surface. Moreover, the half-metal to semiconductor phase transition upon Ni incorporation is reflected in an almost complete disappearance of states near the Fermi level. Finally, we report on the systematic changes in the unoccupied states observed with the increasing amount of Ni dopant. These findings offer insights into the influence of Ni incorporation on the electronic structure of $\mathrm{Ni}/{\mathrm{Fe}}_3{\mathrm{O}}_4$, which can link to an increased catalytic activity.

Coupling of excitations, arising from electronic correlation or electron-phonon interaction, leads to intriguing effects on the spectra of materials. Current approximations to calculate photoluminescence spectra most often describe this coupling insufficiently. Starting from basic equations of many-body perturbation theory, we derive a cumulant formulation for photoluminescence spectra that contains excitonic effects and the coupling between excitons and other degrees of freedom such as plasmons and phonons. The cumulant approach allows us to include dynamical effects arising from the electronic correlation and electron-phonon coupling in a simple and intuitive way. It can be implemented as a postprocessing of state-of-the-art out-of-equilibrium Bethe-Salpeter calculations of excitonic states.

We employ the determinant quantum Monte Carlo method to study the finite-temperature properties of the half-filled attractive $\mathrm{SU}\left(3\right)$ Hubbard model on a honeycomb lattice. We calculate the phase diagram in which the phase boundary separates the disordered phase and the charge density wave (CDW) phase and the transition temperature $T_{\text{tr}}\left(\right|U\left|\right)$ varies nonmonotonically with attractive Hubbard interaction $\left|U\right|$. As the Hubbard $\left|U\right|$ increases at constant temperature $T<\text{max}(T_{\text{tr}}\left(\right|U\left|\right))$, the system first undergoes a transition from thermal Dirac semimetal phase to CDW phase, and eventually the CDW state is thermally melted at a strong Hubbard $\left|U\right|$ where the system enters a trion liquid phase. In between the two transition points the nonmonotonic $\left|U\right|$ dependence of CDW order strength is strikingly different from the zero-temperature monotonic behavior. In the trion CDW state where off-site trions arise from quantum fluctuations (a fermion inside an on-site trion hops to a nearest-neighbor site), the simulated triple occupancy at constant Hubbard $\left|U\right|$ surprisingly increases with temperature, implying that the formation of off-site trions is suppressed by the thermal delocalization of on-site trions. We have also calculated the entropy-temperature relations for various attractive Hubbrad interactions, which exhibit the prominent characteristic of the Pomeranchuk effect. Our work has revealed that the formation of on-site and off-site trions has significant consequences for thermodynamic properties of SU(3) Dirac fermions.

We investigate the Rashba effect in thermodynamically stable nonpolar transition metal dichalcogenide (TMD) alloys of the form $\mathrm{Mo}{\mathrm{S}}_{2(1-x)}{\mathrm{Se}}_{2x}$ in the presence of an out-of-plane electric field using first-principles calculations. These alloys exhibit a nonlinear anisotropic Rashba effect, which is explained by considering symmetry-dependent higher-order terms in the Rashba Hamiltonian. Of the three stable alloys studied here, $\mathrm{Mo}{\mathrm{S}}_{0.67}{\mathrm{Se}}_{1.33}$ and $\mathrm{Mo}{\mathrm{S}}_{1.33}{\mathrm{Se}}_{0.67}$ belong to the same symmetry group as $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{Mo}{\mathrm{Se}}_2$, and the anisotropy in their Rashba splitting originates mainly from a single anisotropic third-order term in the Hamiltonian. On the other hand, the MoSSe alloy belongs to the $C_{2v}$ point group symmetry, where all the first- and third-order parameters are anisotropic and significantly contribute to the anisotropy of the Rashba effect. Furthermore, the Rashba energy splitting in these nonpolar alloys can be extensively tuned and enhanced significantly by applying a perpendicular external electric field and biaxial strain, making them promising candidates for spintronic applications.

Heterostructures composed of two-dimensional materials have gained significant attention due to their unique properties and potential applications in various fields. Covalent organic frameworks (COFs) represent a novel class of organic porous materials that have shown promise in gas storage, catalysis, and optoelectronics. In this paper, we present a first-principles study of the optoelectronic properties of bilayers and heterostructures of ${\mathrm{C}}_6{\mathrm{N}}_6$ and ${\mathrm{B}}_6{\mathrm{O}}_6$ COFs, including excitonic effects within the ${\mathrm{G}}_0{\mathrm{W}}_0$-BSE approach and the effects of van der Waals corrections. Our study shows that the energetically favorable configuration is AC-stacking, where the upper pore is covered by the C-N (B-O) ring of the bottom layer. We also found that the AC-stacking ${\mathrm{C}}_6{\mathrm{N}}_6/{\mathrm{B}}_6{\mathrm{O}}_6$ heterostructure exhibits a surface buckling of 0.55 Å and an intrinsic type II band alignment, resulting in a redshift in optical absorption. The calculated band edge alignments for ${\mathrm{C}}_6{\mathrm{N}}_6$ and ${\mathrm{B}}_6{\mathrm{O}}_6$ monolayers and heterostructures using the HSE06 hybrid functional affirm their suitability for the photocatalytic splitting of water. Furthermore, we investigated the impact of strain on the band gap and demonstrated that increasing tensile strain leads to a decrease in the distance between the two layers, causing an increase in the electronic band gap and optical gap. These results provide a solid theoretical foundation for future experimental investigations and suggest potential applications in optoelectronic devices. Overall, our study sheds light on the optoelectronic properties of COF heterostructures and highlights their potential for use in a range of important applications.

First-principles-based effective Hamiltonian simulations are used to reveal the hidden connection between various topological defects, namely point defects and skyrmions, in copper oxide selenite (${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$). Using this approach, we show that (i) ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ hosts different topological defects with a nonzero local topological charge in various regions of the magnetic-field–temperature phase diagram $(H,T)$, (ii) point defects exert external pressure on skyrmions leading to their gradual annihilation as temperature increases, and (iii) this annihilation of skyrmions is a result of the melting of the lattice into a glassylike state of spins that connects with the paramagnetic phase.

In this paper, we present a theoretical research on the lattice relaxations, phonon properties, and relaxed electronic structures in magic-angle twisted bilayer graphene (TBG). We construct a continuum elastic model in order to study the lattice dynamics of magic-angle TBG, where both in-plane and out-of-plane lattice displacements are taken into account. The fully relaxed lattice structure calculated using such a model is in quantitative agreement with experimental measurements. Furthermore, we investigate the phonon properties in magic-angle TBG using the continuum elastic model, where both the in-plane and out-of-plane phonon modes are included and treated on equal footing. We identify different types of moiré phonons including in-plane sliding modes, soft out-of-plane flexural modes, as well as out-of-plane breathing modes. The latter two types of phonon modes exhibit interesting monopolar, dipolar, quadrupolar, and octupolar-type out-of-plane vibration patterns. Additionally, we explore the impact of the relaxed moiré superlattice structure on the electronic band structures of magic-angle TBG using an effective continuum model, which shows nearly exact agreement with those calculated using a microscopic atomistic tight-binding approach. Our paper lays foundation for further studies on the electron-phonon coupling effects and their interplay with electron-electron interactions in magic-angle TBG.