We study the binding energies and optical properties of direct and indirect excitons in monolayers and double-layer heterostructures of Xenes: silicene, germanene, and stanene. It is demonstrated that an external electric field can be used to tune the eigenenergies and optical properties of excitons by changing the effective mass of charge carriers. Our calculations show that the existence of the excitonic insulator phase is plausible in freestanding Xenes at small electric fields. The Schrödinger equation with field-dependent exciton reduced mass is solved by using the Rytova-Keldysh (RK) potential for direct excitons, while both the RK and Coulomb potentials are used for indirect excitons. It is shown that for indirect excitons, the choice of interaction potential can cause huge differences in the eigenenergies at large electric fields and significant differences even at small electric fields. Furthermore, our calculations show that the choice of material parameters has a significant effect on the binding energies and optical properties of direct and indirect excitons. These calculations contribute to the rapidly growing body of research regarding the excitonic and optical properties of this new class of two-dimensional semiconductors.

The phase diagram of solid oxygen at terapascal pressures and several thousand Kelvin has been studied with ab initio density functional theory within the quasiharmonic approximation for the vibrational free energy. Our work extends previous theoretical studies done at zero temperature and shows that temperature has a dramatic effect on the sequence of phases. At low temperature, the transition from onefold (molecular) to fourfold coordination takes place through two intermediate phases with twofold coordination and space groups $I{4}_1/acd$ and $Cmcm$. Above 8000 K we find that these two intermediate phases are no longer stable, and oxygen transforms directly from a molecular phase to a fourfold coordinated phase of space group $Fmmm$. We also find that the transition between $Cmcm$ and $Fmmm$ can be ascribed as a second-order transition driven by an electronic instability.

The Dirac electrons occupying the surface states (SSs) of topological insulators (TIs) have been predicted to exhibit many exciting magnetotransport phenomena. Here we report the experimental observation of an unconventional planar Hall effect (PHE) and a gate-tunable hysteretic planar magnetoresistance in EuS/TI heterostructures, in which EuS is a ferromagnetic insulator (FMI) with an in-plane magnetization. In such exchange-coupled FMI/TI heterostructures, we find a significant (suppressed) PHE when the in-plane magnetic field is parallel (perpendicular) to the electric current. This behavior differs from previous observations of the PHE in ferromagnets and semiconductors. Furthermore, as the thickness of the 3D TI films is reduced into the 2D limit, in which the Dirac SSs develop a hybridization gap, we find a suppression of the PHE around the charge-neutral point indicating the vital role of Dirac SSs in this phenomenon. To explain our findings, we outline a symmetry argument that excludes linear Hall mechanisms and suggest two possible nonlinear Hall mechanisms that can account for all the essential qualitative features in our observations.

We investigate magnetic excitations in the frustrated zigzag spin-1/2 chain compound $\beta \text{−}{\mathrm{TeVO}}_4$ by inelastic neutron scattering. In the magnetically ordered ground state, the excitation spectrum exhibits coexisting magnon dispersion, characteristic of long-range magnetic order, and a spinon-like continuum that prevails above $\sim 2\mathrm{meV}$, indicating the dominance of intrachain interactions. Combining linear-spin-wave-theory and precalculated spinon-continuum results, we reproduce the experimental spectrum. Our analysis offers a minimal exchange-network model, which determines dominant intrachain interactions, their anisotropies, and weak interchain interactions. The obtained parameters explain the magnetic ordering vector and spin excitations in the magnetic ground state.

Magnetic behavior of yavapaiite-type ${\mathrm{BaMoP}}_2{\mathrm{O}}_8$ with the spatially anisotropic triangular arrangement of the $S=1{\mathrm{Mo}}^{4+}$ ions is explored using thermodynamic measurements, neutron diffraction, and density-functional band-structure calculations. A broad maximum in the magnetic susceptibility around 46 K is followed by the stripe antiferromagnetic order with the propagation vector $\mathbf{k}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ formed below $T_N\simeq 21$ K. This stripe phase is triggered by a pronounced one-dimensionality of the spin lattice, where one of the in-plane couplings, $J_2\simeq 4.6$ meV, is much stronger than its $J_1\simeq 0.4$ meV counterpart, and stabilized by the weak easy-axis anisotropy. The ordered moment of 1.42(9)${\mu }_B$ at 1.5 K is significantly lower than the spin-only moment of $2{\mu }_B$ due to a combined effect of quantum fluctuations and spin-orbit coupling.

We investigate the effect of equilibrium topology on the statistics of nonequilibrium work performed during the subsequent unitary evolution, following a sudden quench of the Semenoff mass of the Haldane model. We show that the resulting work distribution function for quenches performed on the Haldane Hamiltonian with broken time reversal symmetry (TRS) exhibits richer universal characteristics as compared to those performed on the time-reversal symmetric massive graphene limit whose work distribution function we have also evaluated for comparison. Importantly, our results show that the work distribution function exhibits different universal behaviors following the nonequilibrium dynamics of the system for small $\varphi$ (argument of complex next nearest neighbor hopping) and large $\varphi$ limits, although the two limits belong to the same equilibrium universality class.

A general perturbation theory for the description of weak damping of linear spin-wave modes in magnetic nanostructures is developed. This perturbative approach allows one to account for the usual uniform Gilbert damping, as well as for the spatially nonuniform (coordinate-dependent) and nonlocal (magnetization-texture-dependent) Gilbert-like dissipation mechanisms. Using the derived general expression, it is possible to calculate the damping rate of a particular spin-wave mode if the frequency and the spatial profile of this mode, along with the relevant parameters of a magnetic material, are known. The examples demonstrating the applications of the developed general formalism include (i) generalization of the damping rate of a spin-wave mode propagating in a magnetic sample for the case of a nonuniform static magnetization or/and bias magnetic field, (ii) calculation of a damping rate of a gyrotropic mode in a vortex-state magnetic nanodot, (iii) evaluation of the spin diffusion influence on the damping rate of spin-wave modes in a conducting ferromagnet, and (iv) calculation of damping rates of spin-wave modes in a ferromagnetic film in the presence of a spin pumping into an adjacent nonmagnetic metal layer. The developed formalism is especially useful in micromagnetic simulations, as it allows one to calculate damping rates of spin-wave modes based on the numerical solution of a conservative eigenmode problem.

Distortion of transition-metal ion-oxygen octahedra in perovskite structure has been a long standing subject for exploring the physics of charge-orbital–spin-lattice couplings. Recent findings in nickelates or $\mathrm{BaBi}{\mathrm{O}}_3$ have suggested that a charge disproportionation in the transition-metal ions can evoke certain cooperative breathing distortions (CBD), constituted by the symmetric expansion and shrinkage of the octahedra in contrast to the Jahn-Teller-type asymmetric distortion. A double perovskite ferrimagnet, $\mathrm{L}{\mathrm{a}}_2\mathrm{CoIr}{\mathrm{O}}_6$ bears inherently the charge disproportionation in the cationic sites with $\mathrm{C}{\mathrm{o}}^{2+}$ and $\mathrm{I}{\mathrm{r}}^{4+}$ sublattices. In light of seeking the clues for the CBD and its coupling to the magnetic order, the crystal structure and magnetic structure of $\mathrm{L}{\mathrm{a}}_2\mathrm{CoIr}{\mathrm{O}}_6$ were examined at various temperatures by using neutron diffraction. Structure refinements and mode decomposition analyses revealed that, indeed, a CBD mode exists substantially in $\mathrm{L}{\mathrm{a}}_2\mathrm{CoIr}{\mathrm{O}}_6$ while preserving the symmetry of the lattices ($P{2}_1/n$) and the magnetism $\left[{\mathrm{\Gamma }}_1\left(A_g\right)\right]$. And more importantly, the magnitude of the symmetric distortion is correlated strongly with the size of the magnetic moment at temperatures below the Curie temperature (∼95 K). This strongly suggests a magnetoelastic coupling of the CBD mode. The order parameters for CBD, e.g., the movement of O1 toward Ir ions at low temperatures, are found to be in a linear-quadratic relationship with the value of the magnetic moment ($\sim 0.002\AA /{{\mu }_{\mathrm{B}}}^2{M_{\mathrm{Co}}}^2$), manifesting the magnetoelastic CBD in double perovskite.

We study the response of a Kitaev spin liquid (KSL) to a local magnetic field perpendicular to the Kitaev honeycomb lattice. The local magnetic field induces a dynamical excitation of a flux pair in the spin liquid and the system can be described by a generally particle-hole asymmetric interacting resonant level model. The dynamical excitation of the flux pair closes the flux gap in the spectrum of the spin correlation function locally for the gapless KSL even from the perturbative response to a weak magnetic field. Beyond the perturbative regime, the p-h asymmetry competes with the magnetic field and results in a rich phase diagram. Moreover, the magnetic field breaks the gauge equivalence of the ferromagnetic and antiferromagnetic Kitaev couplings of the ground state and leads to very different behaviors for the two cases. The antiferromagnetic case experiences a first-order phase transition to the polarized state during magnetization, whereas the ferromagnetic case does not. This study can be generalized to the Kitaev model in a uniform magnetic field and may help understand issues in recent experiments on KSL candidates.

We numerically investigate a lattice-regularized version of quantum electrodynamics in one spatial dimension (Schwinger model). We work at a density where lattice commensuration effects are important and preclude analytic solution of the problem by bosonization. We therefore numerically investigate the interplay of confinement, lattice commensuration, and disorder in the form of a random chemical potential. We begin by pointing out that the ground state at commensurate filling spontaneously breaks the translational symmetry of the lattice. This feature is absent in the conventional lattice regularization, which breaks the relevant symmetry explicitly, but is present in an alternative (symmetric) regularization that we introduce. Remarkably, the spontaneous symmetry breaking survives the addition of a random chemical potential (which explicitly breaks the relevant symmetry), in apparent contradiction of the Imry-Ma theorem, which forbids symmetry breaking in one dimension with this kind of disorder. We identify the long-range interaction as the key ingredient enabling the system to evade Imry-Ma constraints. We examine spatially resolved energy level statistics for the disordered system, which suggest that the low-energy Hilbert space exhibits ergodicity breaking, with level statistics that fail to follow random matrix theory. A careful examination of the structure of the first excited state reveals that disorder-induced localization is responsible for the deviations from random matrix theory and further reveals that the elementary excitations are charge neutral and therefore not long range interacting.

Electrons condensed in a collective mode and behaving as free charge carriers are two important facets induced by the complex intertwining of charge, spin, orbital, and lattice degrees of freedom. Existence of these two electronic ground states in the same structural/chemical phase is as much desired as it is elusive in complex oxides. Using some unique attributes of terahertz (THz) spectroscopy, we show existence of two fundamentally different electronic phenomena in $\mathrm{P}{\mathrm{r}}_{0.5}\mathrm{E}{\mathrm{u}}_{0.5}\mathrm{Ni}{\mathrm{O}}_3$ nickelate in the same structural phase but along two orthogonal in-plane axes $\left[001\right]$ and $\left[1\overline{1}0\right]$ of thin films. While a collective response of charge via charge-density-wave type excitation manifests along the $\left[001\right]$ direction, an entirely different Drude-Smith type free carrier response manifests along the other in-plane orthogonal axis. This anisotropy, on one hand, unveils the strain engineered crystallographic preferences of the underlying charge-ordering phenomenon; on the other hand, the different conduction channels open up possibilities of application of nickelates such as in THz transmission modulators.

The combination of high optical transparency and low electrical resistivity has made transparent conductive oxides (TCOs) a key technology in many optoelectronic applications. Furthermore, the study of TCOs yields insight into many fundamental parameters of semiconductors. For example, the high charge carrier concentration results in an apparent shift in the band gap, the so-called Burstein-Moss shift, in addition to plasmonic resonances in the near infrared regime. While both effects are related to the carrier concentration and band structure, their lateral distribution and interaction with boundary conditions such as interfaces and surfaces are difficult to assess, and a direct observation of the local distribution has remained elusive. Here we employ electron energy-loss spectroscopy in scanning transmission electron microscopy (STEM-EELS) for direct observation of spatially resolved plasmonic resonances and Burstein-Moss shift in gallium doped zinc oxide (GZO), one of the most widely used TCOs. A 25 nm thick GZO film with a carrier concentration of $7\times {10}^{20}{\mathrm{cm}}^{-3}$ has been grown epitaxially on a nominally undoped ZnO film. The GZO film shows a renormalized Burstein-Moss shift of $\sim 0.5$ eV, in accordance with that expected from Hall effect measurements. The plasma resonance of the conduction band electrons is located at 0.82 eV in the bulk GZO, and with a substantial increase in intensity towards the sample surface, where a surface plasmon energy of 0.54 eV is observed. Hence we have directly measured differences in optical properties between the two films, the local variation across the GZO film has been studied, and we are also directly observing the difference between bulk and surface properties of GZO.

We develop a rigorous quantum-mechanical theory of the nonlinear optical process of difference frequency generation of surface plasmon-polaritons in Landau-quantized graphene. Although forbidden in the electric-dipole approximation, the second-order susceptibility is surprisingly high, equivalent to the bulk magnitude above ${10}^{-3}$ m/V. We consider the graphene monolayer as a nonlinear optical component of a monolithic photonic chip with integrated pump fields. The nonlinear power conversion efficiency of the order of tens of $\mu \mathrm{W}/{\mathrm{W}}^2$ is predicted from structures of $10–100\mu \mathrm{m}$ size. We investigate a variety of waveguide configurations to identify the optimal geometry for maximum efficiency.

Two-dimensional (2D) Xenes of a single type of element can offer fascinating electronic properties, such as massless Dirac fermions for extremely high charge-carrier mobility and topological insulators for dissipationless electron transport. However, the realization of either the massless Dirac fermions or the topological insulator in a same element system via a simple physical method has rarely been reported, which is of great importance for the development of next-generation electronic devices. Here, by using first-principles calculations, we identify that a 2D square tellurium system can be effectively tuned to realize either the massless Dirac fermions or the topological insulator phase. The 2D square tellurium system shows three structural phases via strain effect, i.e., buckled square, buckled rectangular, and planar square phases, which exhibit extraordinary topological properties. There are four anisotropic Dirac points in the buckled square phase, in which the Fermi velocity can be as high as $9.44\times {10}^5\mathrm{m}/\mathrm{s}$. The buckled rectangular phase can behave as a quantum spin Hall insulator with a band gap of 0.24 eV, pointing towards promising applications for room-temperature devices. There also exist nodal lines in buckled square/planar square structures in the non-spin-orbit-coupling case. These findings extend the knowledge on single-layer materials and promote future applications of the 2D tellurium systems.

We investigate the magnetic response of a spin interface constituted by MnPc molecules adsorbed on graphene/Co and its robustness against thermal fluctuations by x-ray magnetic circular dichroism. Element-selective hysteresis loops reveal a remarkable antiferromagnetic coupling between MnPc and Co that is strong both in perpendicular and in-plane magnetic orientations, thanks to the magnetic anisotropy properties and electronic configuration of MnPc. The magnetic interaction between MnPc and Co is mediated by the molecular states and the graphene $\pi$ orbitals in a superexchange mechanism that allows a strong exchange coupling while the molecular orbitals symmetries are preserved by the graphene decoupling layer. Our results show that the strength and stability of the magnetic coupling between MnPc molecules and Co layer(s), intercalated at the graphene/Ir(111) interface, is further optimized by the open $3d$ shell of the central Mn ion. The magnetic properties are compared with analogous molecular spin interfaces with high thermal stability, paradigmatic examples to exploit in surface-supported molecular spin electronics.

Bilayer hexagonal borophene, which is bound together through pillars, is a novel topological semimetal. Using density functional theory, we investigate its electronic band structure and show that it is a Dirac material which exhibits a nodal line. A tight-binding model was constructed based on the Slater-Koster approach, which accurately models the electronic spectrum. We constructed an effective four-band model Hamiltonian to describe the spectrum near the nodal line. This Hamiltonian can be used as a new platform to study the new properties of nodal line semimetals. We found that the nodal line is created by edge states and is very robust against perturbations and impurities. Breaking symmetries can split the nodal line, but cannot open a gap.

Using combined scanning tunneling microscopy (STM) measurements and first-principles electronic structure calculations, we extensively studied the atomic and electronic properties of a $\sqrt{7}$-InBi overlayer on Si(111). We propose and demonstrate an effective experimental process to successfully form a large well-ordered $\sqrt{7}$ surface by depositing Bi atoms on the In-Si(111)-$4\times 1$ substrate. The STM images exhibit a honeycomb pattern. After performing an exhaustive computational search, we identified the atomic structures of the surface at In and Bi coverages of 6/7 and 3/7 monolayers, respectively. We discovered a trimer model with a lower energy than the previously proposed model. The simulated STM images of trimer models confirm the presence of the honeycomb pattern in accord with our experimental STM images. Most importantly, we found that the surface is robust, preserving the topologically nontrivial phase. Our edge state calculations verify that the InBi overlayer on Si(111) is indeed a two-dimensional (2D) topological insulator (TI). Moreover, hybrid functional calculations result in band gaps up to 70 meV, which is high enough for room-temperature experiments. Our findings lay the foundation for the materials realization of 2D TIs by growing an InBi overlayer on a Si(111) substrate.

Recent experimental measurements on the layered triangular-lattice antiferromagnet ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ have revealed an interesting interplay among magnetism, charge transport, and optical properties. To explore the nature of the interaction among these degrees of freedom, the electronic structure, magnetic properties, electrical transport, and optical properties of ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ are investigated using density functional theory and Boltzmann transport theory under various approximations such as the generalized gradient approximation (GGA), GGA+$U$, and GGA+$U$+spin orbit (SO). A semimetallic electronic structure with compensating electron and hole pockets is observed. Among the various magnetic states studied, A-type antiferromagnetic (A-AFM) and ferromagnetic (FM) states emerge as having competing total energies even though A-AFM is found to be energetically favored. Further, our GGA+$U$+SO calculations reveal the presence of a magnetic anisotropy which drives the spin moments to align along the crystallographic $b$ direction. We observe from our transport calculations that in the case of A-AFM, the anisotropy between out-of-plane and in-plane resistivities $({\rho }_{zz}/{\rho }_{xx})$ is much higher than that in the FM case, and it increases as temperature goes down, exactly what has been observed experimentally for this system. The huge difference between the resistivities of A-AFM and FM states at lower temperatures is indicative of the presence of negative magnetoresistance in the system as seen in the experimental measurements. In-plane optical reflectivity reveals that intraband contributions play a major role in reproducing the experimental features in the low-frequency regime.

In ultrapure conductors, the collective motion of charge carriers at relatively high temperatures may become hydrodynamic such that electronic transport may be described similarly to a viscous flow. In confined geometries (e.g., in ultrahigh quality nanostructures), the resulting flow is Poiseuille-like. When subjected to a strong external magnetic field, the electric current in semimetals is pushed out of the bulk of the sample towards the edges. Moreover, we show that the interplay between viscosity and fast recombination leads to the appearance of counterflows. The edge currents possess a nontrivial spatial profile and consist of two stripelike regions: the outer stripe carrying most of the current in the direction of the external electric field and the inner stripe with the counterflow.