Magnetoelectric (ME) effect triggered by spiral spin orders in various hexaferrites develops both the theory and application of ME coupling. However, the strength of such ME effect is restricted by the small spin-orbit coupling (SOC) of $3d$ electrons in M-type hexaferrite. Here, we introduce the $5d$ element Ir with large SOC into $\mathrm{Sr}{\mathrm{Fe}}_{12}{\mathrm{O}}_{19}$. It is found that a low ${\mathrm{Ir}}^{4+}\text{−}\mathrm{doping}$ level (about 4.2% of total ${\mathrm{Fe}}^{3+}$ ions) can induce conical spin order in $\mathrm{Sr}{\mathrm{Mg}}_x{\mathrm{Ir}}_x{\mathrm{Fe}}_{12-2x}{\mathrm{O}}_{19}$ ceramics, mainly because ${\mathrm{Ir}}^{4+}$ ions prefer to enter a crucial $4f_2$ magnetic-interacted site and thus strengthen the local Dzyaloshinskii-Moriya interaction. Spin-order-induced polarization $\left(P_{\mathrm{spin}}\right)$ is achieved and displays a synchronous temperature-dependent behavior with the displacive polarization $\left(P_{\mathrm{disp}}\right)$. This interesting ME effect is discussed via $d\text{−}p$ hybridization emerging at $\mathrm{Fe}{\mathrm{O}}_5$ trigonal bipyramids. Moreover, $P_{\mathrm{spin}}$ response decreases step by step during the periodical ME test, reflecting the controllable helicity and the possible multilevel states. These results demonstrate the impact of the $5d$ element with strong SOC on the formation of conical spin order and the enhancement of ME effect in M-type hexaferrite, and also reveal the interconnectedness between $P_{\mathrm{spin}}$ and $P_{\mathrm{disp}}$.

We analyze resonant magnetoelastic interactions between standing perpendicular spin wave modes (exchange magnons) and longitudinal acoustic phonon modes in free-standing hybrid metal-ferromagnet bilayer and trilayer structures. Whereas the ferromagnetic layer acts as a magnetic cavity, all metal layers control the frequencies and eigenmodes of acoustic vibrations. The design proposed here allows for achieving and tuning the spectral and spatial mode overlap between phonons and magnons that results in their strong resonant interaction. Realistic simulations for gold-nickel multilayers show that sweeping the external magnetic field should allow for observing resonantly enhanced interactions between individual magnon and phonon modes in a broad range of frequencies spanning from tens of gigahertz up to several hundreds of gigahertz, which can be finely tuned through the multilayer design. Our results would enable the systematic study and the deep understanding of resonantly enhanced magnetoelastic coupling between individual phonon and magnon modes up to frequencies of great contemporary fundamental and applied interest.

The deconfined quantum critical point (DQCP) between the Néel and valence bond solid order was originally proposed in quantum spin systems with a local Hamiltonian. In the last few years analogs of DQCPs with nonlocal interactions have been explored, which can lead to rich possibilities. The nonlocal interactions can either arise from an instantaneous long-range interaction in the Hamiltonian, or from gapless modes that reside in one higher spatial dimension. Here, we consider another mechanism of generating nonlocal interactions by coupling the DQCP to the “hot spots” of a Fermi surface. We demonstrate that at least within a substantial energy window, the physics of the DQCP is controlled by a new fixed point with a dynamical exponent $z>1$.

We study the $\nu =\pm 1/2$ fractional quantum Hall states in graphene observed by Zibrov et al. [A. A. Zibrov, E. M. Spanton, H. Zhou, C. Kometter, T. Taniguchi, K. Watanabe, and A. F. Young, Nat. Phys. 14, 930 (2018)]. The parton construction is employed to provide a valley unpolarized trial wave function for these states. The lattice scale valley anisotropic terms in the Hamiltonian soften the repulsion between electrons in different valleys to favor the valley unpolarized state. The validity of our proposal is corroborated by numerical calculations.

We present an implementation of the Bethe-Salpeter equation (BSE) for core-conduction band pairs within the framework of the projector augmented-wave method. For validation, the method is applied to the $K$ edges of diamond, graphite and hexagonal boron-nitride, as well as four lithium-halides (LiF, LiCl, LiI, LiBr). We compare our results with experiment, previous theoretical BSE results, and the density functional theory-based supercell core-hole method. In all considered cases, the agreement with experiment is excellent, in particular, for the relative position of the peaks as well as the fine structure. Comparing BSE to supercell core-hole spectra, we find that the latter often qualitatively reproduces the experimental spectrum, however, it sometimes lacks important details. This is shown for the $K$ edges of diamond and nitrogen in hexagonal boron nitride, where we can resolve within the BSE experimental features that are lacking in the supercell core-hole method. Additionally, we show that in certain systems the supercell core-hole method performs better if the excited electron is added to the background charge rather than to the lowest conduction band. We attribute this improved performance to a reduced self-interaction.

Fano resonance is a well-celebrated weak-coupling phenomenon whereby a small interaction between a discrete and a continuum of excited states leads to an interference between the respective two transition amplitudes, resulting in an asymmetric line shape in the excitation spectra. We present the strong-coupling phase of the Fano's model where the interaction is localized in a region $\xi$ that is smaller than the collective coupling between the discrete state and the continuum. We explore the behavior of the excitation probability $\sigma$ as a function of $\xi$, the ratio of the bare transition amplitudes, and the energy of the discrete state. In the strong-coupling phase, the spectral function of the discrete state splits up and produces up to three complete destructive interferences or zeros of $\sigma$ along with ultrasharp features. We also find that the discrete state does not have to lie within the continuum to produce the well-known weak-coupling profiles of $\sigma$ as long as the collective coupling is strong enough. In addition, we consider a broadened state instead of a continuum and present a method that exactly treats such cases by explicitly including the source of the broadening—a weak coupling to the continuum of environment states—in the model.

Structural and electronic properties of neutral native defects in a two-dimensional ${\mathrm{SiO}}_2$ bilayer (2D-${\mathrm{SiO}}_2$) are examined using the SIESTA ab initio approach. We identify scissor and rotation modes of oxygen atoms, in the middle of Si-O-Si chains, as low-energy structural excitations responsible for the response of the 2D-${\mathrm{SiO}}_2$ lattice to the formation of all native defects in the present study. Furthermore, we find that oxygen native defects (single vacancies and interstitials) should be the most abundant defect species in thermal-equilibrated samples. Regarding native-defect electronic states, single oxygen vacancies and interstitials are found to be amphoteric trapping centers in 2D-${\mathrm{SiO}}_2$. Silicon vacancies and interstitials introduce several strongly localized states spanning a large fraction of the gap. Generally, we identify a marked tendency for the appearance of strongly spatially localized defect states, be their energies shallow or deep within the band gap, as well as the emergence of strongly localized resonances in the valence and/or conduction bands. Strong spatial localization of defect states, a hallmark of systems displaying trapping and polaronic effects, is a result of quantum confinement and enhanced Coulombic effects in this 2D system. Carrier trapping and polaron formation are thus expected to be a common feature of defect states and added carriers in 2D-${\mathrm{SiO}}_2$.

Particle entanglement provides information on quantum correlations in systems of indistinguishable particles. Here, we study the one-particle entanglement entropy for an integrable model of spinless, interacting fermions both at equilibrium and after an interaction quantum quench. Using both large-scale exact diagonalization and time-dependent density matrix renormalization group calculations, we numerically compute the one-body reduced density matrix for the $J-V$ model, as well as its postquench dynamics. We include an analysis of the fermionic momentum distribution, showcasing its time evolution after a quantum quench. Our numerical results, extrapolated to the thermodynamic limit, can be compared with field theoretic bosonization in the Tomonaga-Luttinger liquid regime. Excellent agreement is obtained using an interaction cutoff that can be determined uniquely in the ground state.

In this work, the structural, optical, and electronic properties of rare-earth perovskites of the general formula $R{\mathrm{CrO}}_3$, where $R$ represents the rare-earth Gd, Tb, Dy, Ho, Er, and Tm, have been studied in detail. These compounds were synthesized through a facile citrate route. X-ray diffraction, Raman spectroscopy, and UV-Visible spectroscopy were utilized to reveal the structural evolutions in $R{\mathrm{CrO}}_3$. The lattice parameters, ${\mathrm{Cr}}^{3+}–{\mathrm{O}}^2–{\mathrm{Cr}}^{3+}$ bond angle, and ${\mathrm{CrO}}_6$ octahedral distortions were found to strongly depend on the ionic radii of $R$. First-principles calculations based on density-functional theory within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) and strongly constrained and appropriately normed (SCAN) meta-GGA were also employed to calculate the structural and electronic properties of $R{\mathrm{CrO}}_3$. The ground-state energy, lattice constants, electronic structures, and density of states of $R{\mathrm{CrO}}_3$ were calculated. These provide some insights into the electronic characteristics of the $R{\mathrm{CrO}}_3$ compounds. The calculated values of lattice parameters and band gaps with Hubbard $U$ correction ($\mathrm{SCAN}+U$) agree well with values measured experimentally and show more accuracy in predicting the ground-state crystal structure and band structure compared to $\mathrm{PBE}+U$ approximation. The band gap of $R{\mathrm{CrO}}_3$ is found to be independent of the ionic radii of $R$ from both experiments and calculations.

We investigate the role of polarization-photon coupling (specifically, the polarization-oscillation-induced radiation electric field) in the excitation of ferroelectric thin films by an ultrafast terahertz electric field pulse. Analytical formulas are developed to predict how the frequencies and relaxation time of three-dimensional soft mode phonons (intrinsic polarization oscillation) are modulated by the radiation electric field and epitaxial strain. Ultrafast terahertz-pulse-driven excitation of harmonic polarization oscillation in strained single-domain ferroelectric thin film is then simulated using a dynamical phase-field model that considers the coupled strain-polarization-photon dynamics. The frequencies and relaxation time extracted from such numerical simulations agree well with analytical predictions. In relatively thin films, it is predicted that the radiation electric field slightly reduces the frequencies but significantly shortens the relaxation time. These results demonstrate the necessity of considering polarization-photon coupling in understanding and predicting the response of ferroelectric materials to ultrafast pulses of terahertz and higher frequencies.

By investigating the vacancy and substitutional defects in monolayer ${\mathrm{WS}}_2$ with hybrid functionals, we find that there is no unique amount of Fock exchange that concurrently satisfies the generalized Koopmans' condition and reproduces the band-gap and band-edge positions. Fixing the mixing parameter of Fock exchange based on the band gap can lead to qualitatively incorrect defect physics in two-dimensional materials. Instead, excellent agreement is achieved with both experiment and many-body perturbation theory within the $GW$ approximation once the mixing parameters are tuned individually for the defect species and the band edges. We show the departure from a unique optimized mixing parameter is inherent to two-dimensional systems as the band edges experience a reduced screening while the localized defects are subject to bulklike screening.

The Hofstadter model exemplifies a large class of physical systems characterized by particles hopping on a lattice immersed in a gauge field. Recent advancements on various synthetic platforms have enabled highly controllable simulations of such systems with tailored gauge fields featuring complex spatial textures. These synthetic gauge fields could introduce synthetic symmetries that do not appear in electronic materials. Here, in an $\mathrm{SU}\left(2\right)$ non-Abelian Hofstadter model, we theoretically show the emergence of multiple nonsymmorphic chiral symmetries, which combine an internal unitary antisymmetry with fractional spatial translation. Depending on the values of the gauge fields, the nonsymmorphic chiral symmetries can exhibit non-Abelian algebra and protect Kramers quartet states in the bulk band structure, creating general fourfold degeneracy at all momenta. These nonsymmorphic chiral symmetries protect double Dirac semimetals at zero energy, which become gapped into quantum confined insulating phases upon introducing a boundary. Moreover, the parity of the system size can determine whether the resulting insulating phase is trivial or topological. Our work indicates a pathway for creating topology via synthetic symmetries emergent from synthetic gauge fields.

We study the emergence of Berezinskii-Kosterlitz-Thouless (BKT) phases in ${\left({\mathrm{PbTiO}}_3\right)}_3/{\left({\mathrm{SrTiO}}_3\right)}_3$ superlattices by means of second-principles simulations. Between a tensile epitaxial strain of $\epsilon =0.25–1\%$, the local dipole moments within the superlattices are confined to the film-plane, and thus the polarization can be effectively considered as two-dimensional. The analysis of the decay of the dipole-dipole correlation with the distance, together with the study of the density of defects and its distribution as a function of temperature, supports the existence of a BKT phase in a range of temperature mediating the ordered ferroelectric (stable at low $T$), and the disordered paraelectric phase that appears beyond a critical temperature $T_{\mathrm{BKT}}$. This BKT phase is characterized by quasi-long-range order (whose signature is a power-law decay of the correlations with the distance), and the emergence of tightly bound vortex-antivortex pairs whose density is determined by a thermal activation process. The proposed ${\mathrm{PbTiO}}_3/{\mathrm{SrTiO}}_3$ superlattice model and the imposed mechanical boundary conditions are both experimentally feasible, making it susceptible for an experimental observation of these new topological phases in ferroelectric materials.

The study of defect centers in silicon has been recently reinvigorated by their potential applications in optical quantum information processing. A number of silicon defect centers emit single photons in the telecommunication $O$-band, making them promising building blocks for quantum networks between computing nodes. The two-carbon G-center, self-interstitial W-center, and spin-$1/2$ T-center are the most intensively studied silicon defect centers, yet despite this, there is no consensus on the precise configurations of defect atoms in these centers, and their electronic structures remain ambiguous. Here we employ ab initio density functional theory to characterize these defect centers, providing insight into the relaxed structures, band structures, and photoluminescence spectra, which are compared to experimental results. Motivation is provided for how these properties are intimately related to the localization of electronic states in the defect centers. In particular, we present the calculation of the zero-field splitting for the excited triplet state of the G-center defect as the structure is linearly interpolated from the A-configuration to the B-configuration, showing a sudden increase in the magnitude of the $D_{zz}$ component of the zero-field-splitting tensor. By performing projections onto the local orbital states of the defect, we analyze this transition in terms of the symmetry and bonding character of the G-center defect, which sheds light on its potential application as a spin-photon interface.

Motivated by the remarkable experimental observation of all-optical femtosecond-scale magnetization reversal at relatively high temperatures, a stochastic inertial Landau-Lifshitz-Gilbert-Bloch (iLLGB) equation is written to describe nonequilibrium magnetization dynamics in ferromagnets at elevated temperatures and at short enough timescales that an inertial effect manifests. The effect of thermal agitations is described by a Fokker-Planck equation derived from the iLLGB equation including the longitudinal relaxation effect, which is solved with perturbation theory valid at elevated temperatures. Considering a uniaxially symmetric ferromagnet with uniaxial anisotropy, a thermal diffusion-driven exponential mode and alternating field-driven nutation mode of stable magnetization reversal are identified. Our theory proposes a magnetization reversal mechanism based entirely on transfer of angular momentum to the local magnetization, but which takes into account thermal fluctuations and inertial effect at the same time. The theory explains several key observations in all-optical magnetization reversal experiments; the absence of the need for a static field, the subpicosecond switching timescale, the relative roles of thermal and field effects, and the relevance of circularly polarized light. Our results have direct implications for magnetic recording devices operating close to room temperatures and in the ultrafast regime.

Quasiperiodically driven fermionic systems can support topological phases not realized in equilibrium. The fermions are localized in the bulk, but support quantized energy currents at the edge. These phases were discovered through an abstract classification, and few microscopic models exist. We develop a coupled layer construction for tight-binding models of these phases in $d\in \{1,2\}$ spatial dimensions, with any number of incommensurate drive frequencies $D$. The models exhibit quantized responses associated with synthetic two- and four-dimensional quantum Hall effects in the steady state. A numerical study of the phase diagram for $(d+D)=(1+2)$ shows (i) robust topological and trivial phases separated by a sharp phase transition, (ii) charge diffusion and a half-integer energy current between the drives at the transition, and (iii) a long-lived topological energy current which remains present when weak interactions are introduced.

Laves $R{\mathrm{Fe}}_2$ compounds, where $R$ is a rare earth, exhibit technologically relevant properties associated with the interplay between their lattice geometry and magnetism. We apply ab initio calculations to explore how magnetic properties of Fe in $R{\mathrm{Fe}}_2$ systems vary with temperature. We found that the ratio between the orbital magnetic moment ${\mu }_{\mathrm{orb}}$ and the spin magnetic moment ${\mu }_{\mathrm{spin}}$ increases with increasing temperature for $\mathrm{Y}{\mathrm{Fe}}_2, \mathrm{Gd}{\mathrm{Fe}}_2, \mathrm{Tb}{\mathrm{Fe}}_2, \mathrm{Dy}{\mathrm{Fe}}_2$, and $\mathrm{Ho}{\mathrm{Fe}}_2$. This increase is significant and it should be experimentally observable by means of x-ray magnetic circular dichroism. We conjecture that the predicted increase of the ${\mu }_{\mathrm{orb}}/{\mu }_{\mathrm{spin}}$ ratio with temperature is linked to the reduction of hybridization between same-spin-channel states of atoms with fluctuating magnetic moments and to the associated increase of their atomic-like character.

The coupling between phonon and magnon is ubiquitous in magnetic materials and plays a crucial role in many aspects of magnetic properties, most notably in spintronics. Yet, this academically and technologically interesting problem still poses a severe challenge to a general understanding of the issue in certain materials. We report that ${\mathrm{Ni}}_3\mathrm{Te}{\mathrm{O}}_6$ exhibits clear evidence of significant magnon-phonon coupling in both longitudinal thermal conductivity (${\kappa }_{xx}$) and thermal Hall coefficient (${\kappa }_{xy}$). The Debye-Callaway model, a phenomenological description for phonon heat conduction, can explain the measured magnetic field dependence of ${\kappa }_{xx}\left(H\right)$: phonon scattering from spin fluctuation in the paramagnetic phase and additional scattering due to magnon-phonon coupling in the collinear antiferromagnetic phase. We further suggest that a similar approach could be applied to understand the finite ${\kappa }_{xy}$ values in ${\mathrm{Ni}}_3\mathrm{Te}{\mathrm{O}}_6$.

Methods inspired from machine learning have recently attracted great interest in the computational study of quantum many-particle systems. So far, however, it has proven challenging to deal with microscopic models in which the total number of particles is not conserved. To address this issue, we propose a variant of neural network states, which we term neural coherent states. Taking the Fröhlich impurity model as a case study, we show that neural coherent states can learn the ground state of nonadditive systems very well. In particular, we recover exact diagonalization in all regimes tested and observe substantial improvement over the standard coherent state estimates in the most challenging intermediate-coupling regime. Our approach is generic and does not assume specific details of the system, suggesting wide applications.

Thermal transport in condensed matter systems is traditionally formulated as a response to a background gravitational field. In this work, we seek a twisted-boundary-condition formalism for thermal transport in analogy to the U(1) twisted boundary condition for electrical transport. Specifically, using the transfer matrix formalism, we introduce what we call the energy-twisted boundary condition, and study the response of the system to the boundary condition. As specific examples, we obtain the thermal Meissner stiffness of (1+1)-dimensional CFT, the Ising model, and disordered fermion models. We also identify the boost deformation of integrable systems as a bulk counterpart of the energy-twisted boundary condition. We show that the boost deformation of the free fermion chain can be solved explicitly by solving the inviscid Burgers equation. We also discuss the boost deformation of the XXZ model, and its nonlinear thermal Drude weights, by studying the boost-deformed Bethe ansatz equations.