While excitonic instabilities in multiorbital systems recently have come under scrutiny in a variety of transition-metal compounds, understanding the emergence of these instabilities from strong electronic interactions has remained a challenge. Here, we present a sign-problem-free determinant quantum Monte Carlo study of excitonic density orders in a half-filled two-orbital Hubbard-Kanamori model with broken orbital degeneracy, which accounts for the role of Hund's coupling in transition-metal compounds. For strong inverted (negative) Hund's exchange, we find numerical evidence for the emergence of excitonic density order, with competition between anti-ferro-orbital order and $\mathbf{Q}=(\pi ,\pi )$ excitonic density order as a function of orbital splitting and Hund's coupling. While inverted Hund's coupling stabilizes a spin-singlet excitonic density phase for weak orbital splitting, positive Hund's coupling favors a spin-triplet excitonic density phase.

We introduce a generalized Gross-Neveu (GN) model to describe the excitonic instabilities in two different systems: a small overlap semimetal (SM) and a small gap semiconductor (SMC), both in two (2d) and three-dimensions (3d). We identify the excitonic order parameter (EOP) and obtain the effective potential within the large $N$ limit approach where the GN model can be exactly solved. We obtain the excitonic insulator (EI) phase diagrams as a function of temperature, chemical potential, overlap between bands, and gaps of the system. We show that the EI may undergo first- or second-order thermal transitions depending on the regime whereupon this phase is approached. We also investigate the expected thermodynamic signatures for the specific heat above the fine-tuned excitonic quantum critical point (EQCP), in both 2d and 3d, in the SMC regime. We show that the EQCP is a different kind of critical point since although the EOP vanishes at the EQCP, there is always a finite gap in the SMC regime. We find that for high temperatures, the specific heat might exhibit a scaling behavior in the form $C_V/T\propto T^{(d-z)/z}$, where $d$ is the dimension of the system and $z$ is the dynamical critical exponent. The very low-temperature behavior has a dominant exponential thermally activated term due to the presence of a gap that does not vanish at the excitonic transition.

We demonstrate that in photonic gap antennas composed of an epsilon-near-zero (ENZ) layer embedded within a high-index dielectric, hybrid modes emerge from the strong coupling between the ENZ thin film and the photonic modes of the dielectric antenna. These hybrid modes show giant electric field enhancements, large enhancements of the far-field spontaneous emission rate, and a unidirectional radiation response. We analyze both parent and hybrid modes using quasinormal mode theory and find that the hybridization can be well understood using a coupled oscillator model. Under plane-wave illumination, hybrid ENZ antennas can concentrate light and achieve local electric field amplitudes more than two orders of magnitude greater than that of the incident wave. This places them on par with the best plasmonic antennas. In addition, the far-field spontaneous emission rate of a dipole embedded at the antenna hotspot can be increased by more than three orders of magnitude as compared to that in free space, with nearly perfect unidirectional emission.

${\mathrm{CrI}}_3$ is a two-dimensional ferromagnetic (FM) van der Waals material with a charge gap of 1.1–1.2 eV. In this paper, the electronic structure and magnetism of ${\mathrm{CrI}}_3$ are investigated by using density functional theory and dynamical mean-field theory. Our calculations successfully reproduce a charge gap of $\sim 1.1\mathrm{eV}$ in the paramagnetic (PM) state when a Hund's coupling $J_{\mathrm{H}}=0.7\mathrm{eV}$ is included with an onsite Hubbard $U=5\mathrm{eV}$. In contrast, with a large $U$ value of 8 eV and negligible Hund's coupling $J_{\mathrm{H}}, {\mathrm{CrI}}_3$ is predicted to be a moderately correlated metal in the PM state. We conclude that ${\mathrm{CrI}}_3$ is a Mott-Hund's insulator due to the half-filled configuration of the Cr $3d t_{2g}$ orbitals. The Cr $3d e_g$ orbitals are occupied by $\sim 1$ electron, which leads to strong valence fluctuations so that the Cr $3d$ orbitals cannot be described by a single state. Moreover, at finite temperature, the calculated ordered static magnetic moment in the FM state is significantly larger in the $R\overline{3}$ phase than in the $C2/m$ phase. This observation indicates that the structural phase transition from the $C2/m$ phase to the $R\overline{3}$ phase with decreasing temperature is driven by FM spin fluctuations.

The time dependent density functional theory (TDDFT) is used to study the femtosecond laser induced excitation dynamics of electrons in bulk $\alpha$-quartz, and evaluate the laser energy deposition into this material. In order to properly distinguish the contributions of ionization (electron transitions from the valence band to the conduction band) and laser heating (electron transitions in the conduction band), two 10-femtosecond laser pulses exhibiting different wavelengths are used. Short wavelengths are expected to enhance the ionization rate, whereas longer wavelengths should be more suitable for excitation in the conduction band, thus providing a possible control of the whole electron dynamics. The influence of the pulse-to-pulse delay and intensities is studied. A significant enhancement of the interaction efficiency, in terms of excited electron density and their energy density, is observed for zero pulse-to-pulse delay. It is attributed to the opening of new ionization pathways involving various combinations of both photon energies ensuring the energy conservation, i.e., the sum of photon energies bridges the bandgap. This analysis is supported by a semi-analytical quantum model in the multiphoton absorption regime. The role and strength of direct interband transitions for the electron dynamics in the conduction band are highlighted. The associated laser energy deposition into the material is shown to be as efficient as collisional processes.

We present an exact quantum Monte Carlo method for spin systems coupled to dissipative bosonic baths which makes use of nonlocal wormhole updates to simulate the retarded spin-flip interactions originating from an off-diagonal spin-boson coupling. The method is closely related to the stochastic series expansion and extends the scope of the global directed-loop updates to nonlocal moves through a world-line configuration. We test our method for the U(1)-symmetric two-bath spin-boson model, where the off-diagonal components of a spin-1/2 particle are coupled to identical independent baths with power-law spectra, and get a precise estimate of the critical coupling between the critical and the localized phase. Our method applies to impurity systems and lattice models in any spatial dimension coupled to bosonic modes with arbitrary spectral distributions.

We apply the band-unfolding approach to explore the dispersions of different types of photonic quasiperiodic structures, including photonic quasicrystals that have high-rotational symmetry but lack translational symmetry, and moiré superlattices, which are a twist of two primary lattices. The band-unfolding approach provides a simple picture to get the band structure of such quasiperiodic structures, which can be directly compared with experimental results. We envision this approach will be a basic method for introducing the rich concepts of photonics in condensed-matter physics to photonic quasiperiodic systems.

Strong near-infrared absorption of light in semiconducting transition metal dichalcogenides (TMDs) is essential for improving the photocarrier extraction efficiency in optoelectronic devices. Here, we numerically demonstrate that an original TMD Huygens metasurface is specifically designed to overcome the 50% absorptance limit of a subwavelength thin film. The unique metasurface comprising a TMD nanodisk array shows evidence of characteristic Mie resonances including electric and magnetic dipoles. By carefully optimizing the aspect ratio of nanodisks, the extraordinary spectral overlapping of orthogonal electric and magnetic dipole resonances is successfully realized and, thus, enables the super-high absorptance up to 87% in a subwavelength-thin ($\approx 0.19{\lambda }_0$) semiconductor structure by degenerate critical coupling at the wavelength of ${\lambda }_0=903\mathrm{nm}$. All numerical results are confirmed by the temporal coupled-mode theory and again via the multipole decomposition method. Importantly, the near-unity absorptance in this TMD Huygens metasurface by degenerate critical coupling not only is tailored throughout the entire near-infrared region but also is both polarization independent and angle insensitive. The absorptance maintaining >80% is observed with the incident angle up to ±10 ° at least. Our findings are not only of fundamental interest but could also offer a platform for TMD-based high-speed photodetecting, energy harvesting, and thermal emission.

The nonlinear system is a frontier of topological physics. As a prototype, we propose the dimerized sine-Gordon model by combining the sine-Gordon model and the Su-Schrieffer-Heeger model. It has rich varieties of phases depending on the nonlinearity and the dimerization. This system is realized by a simple system of coupled pendulums, which is an ideal playground to explore associated intriguing phenomena. They are observable by the quench dynamics starting from the left-end pendulum. The topological and trivial phases are well defined for the weak nonlinear regime, where a pendulum motion is well approximated by a harmonic oscillator. The emergence of the topological edge state is detected by the standing waves whose amplitude exponentially decays with stationary even-numbered pendulums. In addition, we find nonlinearity-induced phases, i.e., the trap phase with a strictly localized standing wave and the dimer phase with a few coupled standing waves trapped to the left end. We also study a nonlinear dimerized Duffing model, which is obtained by the third-order Taylor expansion of the dimerized sine-Gordon model.

We demonstrate direct transport between two opposing sets of Yu-Shiba-Rusinov (YSR) subgap states realized in a double quantum dot. This bound-state-to-bound-state transport relies on intrinsic quasiparticle relaxation, and the tunable gating of this quantum dot device allows us to explore also an additional relaxation mechanism based on charge transferring Andreev reflections. The transition between these two relaxation regimes is identified in the experiment as a marked gate-induced stepwise change in conductance. We present a transport calculation, including YSR bound states and multiple Andreev reflections alongside quasiparticle relaxation, due to a weak tunnel coupling to a nearby normal metal, and obtain excellent agreement with the data.

The study of quartz and other silica systems under pressure is one of the most prolific domains of research over the past 50 years because of their applications in material science and fundamental relevance to planetary interiors. The characterization of the amorphous state is essential for the comprehension of pressure-induced amorphization of minerals, the metamorphism observed in shocked materials, and the study of melt structures under pressure. Here, we measured in situ, under static compression the density, sound velocities, and electronic structure of quartz as it passes through its pressure-induced amorphization transition. The transition pressure could be derived from the abrupt increase in density and sound velocity at 24 GPa, and from strong changes in the silicon $L_{2,3}$ edge and oxygen $K$ edge between 22 and 27 GPa observed in x-ray Raman scattering data, confirming previous results from x-ray diffraction. Above this pressure, our data show an anomalous behavior in density, sound velocity, and electronic fine structure compared to the cold compressed glass and other silica polymorphs. The pressure-induced amorphous quartz has a lower density relative to that of the compressed glass, consistent with the lower average coordination inferred from a different signature in the Si $L_{2,3}$ and O $K$ electronic absorption edges measured by x-ray Raman scattering spectroscopy. This behavior sheds light on the pressure limit of tetrahedral units in ${\mathrm{SiO}}_2$ components and the existence of polyamorphism in network-forming materials, and highlights the possibility to discriminate between different amorphous states with x-ray Raman scattering spectroscopy.

Lowest-order quantum perturbation theory (Fermi's golden rule) for phonon-disorder scattering has been used to predict thermal conductivities in several semiconducting alloys with surprising success given its underlying hypothesis of weak and dilute disorder. In this paper, we explain how this is possible by focusing on the case of maximally mass-disordered ${\mathrm{Mg}}_2{\mathrm{Si}}_{1-x}{\mathrm{Sn}}_x$. We use a Chebyshev polynomials Green's function method, which allows a full treatment of disorder on very large systems (tens of millions of atoms) to probe individual phonon linewidths and frequency-resolved thermal transport. We demonstrate that the success of perturbation theory originates from the specific form of mass disorder terms in the phonon Green's function and from the interplay between anharmonic and disorder scattering.

We investigate the element-specific spin and orbital states and their roles in magnetic anisotropy in Co-ferrite [${\mathrm{Co}}_x{\mathrm{Fe}}_{3-x}{\mathrm{O}}_{4+\delta }$ (001)] thin films which exhibit perpendicular magnetic anisotropy (PMA). The origin of PMA in the low-$x$ region ($x<$ 1) can be mainly explained by the large perpendicular orbital magnetic moments in the ${\mathrm{Co}}^{2+}$ ($3d^7$) states detected by x-ray magnetic circular dichroism and x-ray magnetic linear dichroism (XMLD). The XMLD for a PMA film ($x=0.2$) with a square hysteresis curve shows the oblate charge distribution in the ${\mathrm{Co}}^{2+}$ site, which is consistent with the change in local nearest-neighbor distance in Co detected by extended x-ray absorption fine structure analysis. Our finding reveals that the microscopic origin of PMA in Co ferrite comes from the enhanced orbital magnetic moments along the out-of-plane [001] direction through in-plane charge distribution by tensile strain, which adds the material functionalities in spinel ferrite thin films from the viewpoint of strain and orbital magnetic moments.

Coupling a conventional $s$-wave superconductor to a ferromagnet allows us, via the proximity effect, to generate superconducting triplet correlations. This feature can be employed to achieve a superconducting triplet spin-valve effect in superconductor-ferromagnet (SF) hybrid structures, for example by switching the magnetizations of the ferromagnets between parallel and antiparallel configurations in ${\text{F}}_1{\text{SF}}_2$ and ${\mathrm{SF}}_1{\mathrm{F}}_2$ trilayers, or in SF bilayers with both Rashba and Dresselhaus spin-orbit coupling (SOC). It was recently reported that geometric curvature can control the generation of long-ranged triplets. We use this property to show that the superconducting critical temperature of an SF hybrid nanowire can be tuned by varying the curvature of the ferromagnetic side alone, with no need of another ferromagnet or SOC. We show that the variation of the critical temperature as a function of the curvature can be exploited to obtain a robust, curvature-controlled, superconducting triplet spin-valve effect. Furthermore, we perform an analysis with the inclusion of spin-orbit coupling and explain how it modifies the spin-valve effect both quantitatively and qualitatively.

The oxide ferromagnet ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) possesses low saturation magnetization $\left(M_s\right)$, a low magnetic damping constant $\left(\alpha \right)$, and high carrier spin polarization, which promise superior functionalities in spintronics devices. For applications, these devices are to be integrated into electronic circuits, which require the control of the magnetization of LSMO by electrical means. However, a reliable electrical switching method remains largely unexplored. Here, we study the current-induced spin-orbit torque (SOT) in an all-oxide $\mathrm{SrIr}{\mathrm{O}}_3/\mathrm{LSMO}$ bilayer. We found the magnetization of a 15 nm LSMO can be switched by electrical current, as confirmed from both the electrical transport measurement and the magnetic optical measurement. By taking advantage of the strain-mediated magnetic anisotropy, the magnetic easy axis of LSMO was controlled to be either perpendicular or parallel to the charge current direction such that a type-$y$ or a type-$x$ SOT switching was achieved, respectively. The dampinglike SOT efficiency is characterized to be 0.15 at room temperature according to the harmonic Hall voltage analysis. We also studied the temperature dependences of the SOT effective field and current-induced switching. Our demonstration of the electrical switching of LSMO magnetization at room temperature may stimulate SOT studies in a wide variety of all-oxide systems.

We theoretically investigate laser-induced spin transport in metallic magnetic heterostructures using an effective spin-transport description that treats itinerant electrons and thermal magnons on an equal footing. Electron-magnon scattering is included and taken as the driving force for ultrafast demagnetization. We assume that in the low-fluence limit, the magnon system remains in a quasiequilibrium, allowing a transient nonzero magnon chemical potential. In combination with the diffusive transport equations for the itinerant electrons, the description is used to chart the full spin dynamics within the heterostructure. In agreement with recent experiments, we find that in the case the spin-current-receiving material includes an efficient spin dissipation channel, the interfacial spin current becomes directly proportional to the temporal derivative of the magnetization. Based on an analytical calculation, we discuss that other relations between the spin current and magnetization may arise in the case the spin-current-receiving material displays inefficient spin-flip scattering. Finally, we discuss the role of (interfacial) magnon transport and show that, a priori, it cannot be neglected. However, its significance strongly depends on the system parameters.

The discovery of van der Waals (vdW) ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ provides a platform for investigating the quantum anomalous Hall effect (QAHE), which is a candidate for quantum computation without dissipation. However, because of the topological requirement, the High Chern number QAHE with extended dissipation-free channels is hard to achieve in atomically thin samples, which can take full advantage of two-dimensional (2D) materials. In this work, by first-principles calculations, the variable stacking order (inherent to vdW materials) was proposed as a means of regulating the Chern number of the QAHE in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ (MBT). The interlayer stacking order in the 2D limit dominates the orbital hybridization, resulting in a tunable topological property. Furthermore, the High Chern number QAHE with $C=N$ was achieved in ultrathin MBT ($2N$ septuple layers) with alternate stacking orders, which have different topological states. This scenario was extended to the vdW heterostructure of MBT-family materials for achieving a stable, zero-field, High Chern number QAHE. Our work reveals stacking tunable topological properties in vdW magnetic topological insulators and provides an effective means of regulating the QAHE.

Starting with general semiclassical equations of motion for electrons in the presence of electric and magnetic fields, we extend the Chambers formula to include, in addition to a magnetic field, time-dependent electric fields, and bands with Berry curvature. We thereby compute the conductivity tensor ${\sigma }_{\alpha \beta }(B,\omega )$ in the presence of magnetic field for bands in two (2D) and three (3D) dimensions with Berry curvature. We focus then on several applications to magnetotransport for metals with Fermi surface topological transitions in 2D. In particular, we consider a rectangular lattice and a model related to overdoped graphene, to investigate the signatures of different types of Fermi surface topological transitions in metals in the Hall coefficient, Hall conductivity ${\sigma }_{xy}$, and longitudinal conductivity ${\sigma }_{xx}$. The behavior of those quantities as a function of frequency, when the electric field is time dependent, is also investigated. As an example of nonzero Berry curvature, we study the magnetotransport of the Haldane model within this context. In addition, we provide the linear and nonlinear electric current formula to order $E^2$.

We examine how dissipation and gate timing errors affect the fidelity of a sequence of swap gates on a chain of interacting qubits in comparison to noise in the interqubit interaction. Although interqubit interaction noise and gate timing errors are always present in any qubit platform, dissipation is a special case that can arise in multivalley semiconductor spin qubit systems, such as Si-based qubits, where dissipation may be used as a general model for valley leakage. In our Hamiltonian, each qubit is coupled via Heisenberg exchange to every other qubit in the chain, with the strength of the exchange interaction decreasing exponentially with distance between the qubits. Dissipation is modeled through the term $-i\gamma \mathbf{1}$ in the Hamiltonian, and $\gamma$ is chosen so as to be consistent with the experimentally observed intervalley tunneling in Si. We show that randomness in the dissipation parameter should have little to no effect on the swap gate fidelity in the currently fabricated Si circuits. We introduce quasistatic noise in the interqubit interaction and random gate timing error and average the fidelities over 10 000 realizations for each set of parameters. The fidelities are then plotted against $J_{\mathrm{SWAP}}$, the strength of the exchange coupling corresponding to the swap gate. We find that dissipation decreases the fidelity of the swap operation—though the effect is small compared to that of the known noise in the interqubit interaction—and that gate timing error creates an effective optimal value of $J_{\mathrm{SWAP}}$, beyond which infidelity begins to increase.