We investigate theoretically the giant nonlinear optical susceptibilities based on intersubband transitions in the $n$-type ${\mathrm{BaSnO}}_3$ quantum well heterostructures. Validity of the effective mass theory, used to compute susceptibility is demonstrated using first-principles calculations. We predict ${\chi }^{\left(2\right)}$ values up to 200 nm/V and ${\chi }^{\left(3\right)}$ values up to $8000\mathrm{n}{\mathrm{m}}^2/{\mathrm{V}}^2$, both over a much broader spectral range. ${\chi }^{\left(2\right)}$ is estimated from the THz region to the boundary with visible light, a range four times broader than current semiconductor quantum wells. This opens a possibility to make tunable nonlinear optical devices operating in the important range from THz to visible light.

Considering a noncentrosymmetric, nonmagnetic double Weyl semimetal (WSM) $\mathrm{Sr}{\mathrm{Si}}_2$, we investigate the electron and hole pockets in bulk Fermi surface behavior that enables us to characterize the material as a type-I WSM. We study the structural handedness of the material and correlate it with the distinct surface Fermi surface at two opposite surfaces following an energy evolution. The Fermi arc singlet becomes doublet with the onset of spin orbit coupling that is in accordance with the topological charge of the Weyl nodes (WNs). A finite energy separation between WNs of opposite chirality in $\mathrm{Sr}{\mathrm{Si}}_2$ allows us to compute circular photogalvanic effect (CPGE). Followed by the three band formula, we show that CPGE is only quantized for Fermi level chosen in the vicinity of WN residing at a higher value of energy. Surprisingly, for the other WN of opposite chirality in the lower value of energy, CPGE is not found to be quantized. Such a behavior of CPGE is in complete contrast to the time reversal breaking WSM where CPGE is quantized to two opposite plateau depending on the topological charge of the activated WN. We further analyze our finding by examining the momentum resolved CPGE. Finally we show that two band formula for CPGE is not able to capture the quantization that is apprehended by the three band formula.

We present a rigorous analysis of plane-wave-illuminated sinusoidally modulated metasurfaces (MSs) with normal susceptibility components, without limiting the modulation-index values. In contrast to tangentially polarizable scenarios, which are treated within the conventional continued-fraction framework of Meixner and Schäfke, the unconventional structure of the generalized sheet transition conditions with regard to normal components manifests exotic stability conditions, which were not addressed by this framework. By introducing small losses into the constituents of such MSs (inevitable in practice), we resolve these stability issues and establish a valid solution for the scattered fields. Such solutions reveal that for a certain range of modulations these surfaces feature a resonant absorptive notch at an angle related to the period, associated with strong coupling to the weakly evanescent first-order Floquet-Bloch harmonic. Based on our observations, we identify this phenomenon as Wood's anomaly for MSs with normal susceptibilities. We verify these observations numerically and experimentally using suitable, systematically designed, printed-circuit-board prototypes. This work highlights the fundamental intricacies involving theoretical analyses of inhomogeneous normally polarizable MSs and constitutes a firm ground for further exploration and utilization of nonuniform configurations with these mostly ignored degrees of freedom.

The thermophysical properties near the magnetic phase transition point is of great importance in the study of critical phenomenon. Low-dimensional materials are suggested to hold different thermophysical properties comparing to their bulk counterpart due to the dimension induced quantum confinement and anisotropy. In this work, we measured the thermal conductivity of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanowires along the [110] direction (growing direction) with temperature from 100 to 150 K and found a dip of thermal conductivity near the Morin temperature. We found that the thermal conductivity near Morin temperature varies with the angle between magnetic field and the [110] direction of nanowire. More specifically, an angular-dependent thermal conductivity is observed, due to the magnetic field induced movement of the magnetic domain wall. The angle corresponding to the maximum of thermal conductivity varies near the Morin transition temperature, due to the different magnetic easy axis as suggested by our calculation based on magnetic anisotropy energy. This angular dependence of thermal conductivity indicates that the easy axis of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanowires is different from bulk $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ due to the geometric anisotropy.

We propose a projection approach to perform quantum Monte Carlo (QMC) simulation on the infinite-$U$ Hubbard model at some integer fillings where either it is sign problem free or surprisingly has an algebraic sign structure—a power law dependence of average sign on system size. We demonstrate our scheme on the infinite-$USU\left(2N\right)$ fermionic Hubbard model on both a square and honeycomb lattice at half filling, where it is sign problem free, and suggest possible correlated ground states. The method can be generalized to study certain extended Hubbard models applying to cluster Mott insulators or two-dimensional Moiré systems; among one of them at certain non-half-integer filling, the sign has an algebraic behavior such that it can be numerically solved within a polynomial time. Further, our projection scheme can also be generalized to implement the Gutzwiller projection to spin basis such that $SU\left(2N\right)$ quantum spin models and Kondo lattice models may be studied in the framework of fermionic QMC simulations.

Recently a class of quantum systems exhibiting weak ergodicity breaking has attracted much attention. These systems feature in the energy spectrum a special band of eigenstates called quantum many-body scar states. One prototype model that is known to host scar states is the so-called PXP model. In this work we study the fate of quantum many-body scar states in a 2D PXP model against random disorders. We show that in both the square lattice and the honeycomb lattice the scar states can persist up to a finite disorder strength, before eventually being erased by the disorder. We further study the localization properties of the system in the presence of even stronger disorders and show that whether a full localization transition occurs depends on the type of disorder we introduce. Our study thus reveals the fascinating interplay between disorder and quantum many-body scarring in a 2D system.

Cavity optomechanical systems have become a popular playground for studies of controllable nonlinear interactions between light and motion. Owing to the large speed of light, realizing cavity optomechanics in the microwave frequency range requires cavities up to several mm in size, hence making it hard to embed several of them on the same chip. An alternative scheme with much smaller footprint is provided by magnomechanics, where the electromagnetic cavity is replaced by a magnet undergoing ferromagnetic resonance, and the optomechanical coupling originates from magnetic shape anisotropy. Here, we consider the magnomechanical interaction occurring in a suspended magnetic beam, a scheme in which both magnetic and mechanical modes physically overlap and can also be driven individually. We show that a sizable interaction can be produced if the beam has some initial static deformation, as is often the case due to unequal strains in the constituent materials. We also show how the magnetism affects the magnetomotive detection of the vibrations, and how the magnomechanics interaction can be used in microwave signal amplification. Finally, we discuss experimental progress towards realizing the scheme.

Three types of (111)-oriented Pt/Co/Pt trilayers with single-crystalline structures on MgO substrates were fabricated to investigate the mechanisms of the anomalous Hall effect. By introducing 2-monolayer (ML) Cr insertion between a MgO (001) substrate and a Pt underlayer in Pt/Co/Pt epitaxial structures, the Hall resistivity is significantly enhanced compared with that of single-crystalline Pt/Co/Pt deposited on a MgO (111) surface. Furthermore, the detailed crystalline structures were characterized by in situ reflection high-energy electron diffraction, low-energy electron microscopy, and low-energy electron diffraction. It suggests that a twinning structure of the 7-ML ferromagnetic Co layer, which is composed of two twinned traces differing by 90°, can be achieved by crystal engineering with Cr insertion. By fitting with proper scaling of anomalous Hall effect, the dominant contribution to the Hall resistivity can be confirmed as skew scattering, and the remarkable enhancement of ${\rho }_{xy}$ is attributed to the high phonon skew scattering parameter originating from the specific twinning structure.

We present a theory and a simulation of diffuse scattering due to the excitation of magnons in scanning transmission electron microscopy. The calculations indicate that magnons can present atomic contrast when detected by electron energy-loss spectroscopy using atomic-size electron beams. The results presented here indicate that the intensity of the magnon diffuse scattering in bcc iron at 300 K is 4 orders of magnitude weaker than the intensity of thermal diffuse scattering arising from atomic vibrations.

We performed Cu $K$-edge x-ray absorption fine-structure measurements on $T^{\prime }$-type ${\mathrm{La}}_{1.8}{\mathrm{Eu}}_{0.2}{\mathrm{CuO}}_4$ (LECO) and ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$ (NCO) to investigate the variation in the electronic state associated with the emergence of superconductivity due to annealing. The x-ray absorption near-edge structure spectra of as-sintered (AS) LECO are quite similar to those of AS NCO, indicating that the ground state of AS LECO is a Mott insulator. We clarified a significant variation in the electronic state at the Cu sites in LECO due to annealing. The electron density after annealing $n_{\mathrm{AN}}$ was evaluated for both superconducting LECO and nonsuperconducting NCO and was found to be 0.40 and 0.05 electron per Cu atom, respectively. In LECO but not in NCO, extended x-ray absorption fine-structure analysis revealed a reduction in the strength of the Cu-O bond in the ${\mathrm{CuO}}_2$ plane due to annealing, which is consistent with the screening effect on phonons in the metallic state. Since the amounts of oxygen loss due to annealing $\delta$ for LECO and NCO are comparable, these results suggest distinct electron-doping processes in the two compounds. The electron doping in NCO approximately follows the relation $n_{\mathrm{AN}}=2\delta$; this can be understood if electrons are doped through oxygen deficiency, but the anneal-induced metallic nature and large $n_{\mathrm{AN}}$ in LECO suggest that a variation of the electronic band structure causes the self-doping of carriers. The origin of the difference in doping processes due to annealing is discussed in relation to the size of the charge-transfer gap.

We compute the thermal conductivity of water within linear response theory from equilibrium molecular dynamics simulations, by adopting two different approaches. In one, the potential energy surface (PES) is derived on the fly from the electronic ground state of density functional theory (DFT) and the corresponding analytical expression is used for the energy flux. In the other, the PES is represented by a deep neural network (DNN) trained on DFT data, whereby the PES has an explicit local decomposition and the energy flux takes a particularly simple expression. By virtue of a gauge invariance principle, established by Marcolongo, Umari, and Baroni, the two approaches should be equivalent if the PES were reproduced accurately by the DNN model. We test this hypothesis by calculating the thermal conductivity, at the GGA (PBE) level of theory, using the direct formulation and its DNN proxy, finding that both approaches yield the same conductivity, in excess of the experimental value by approximately 60%. Besides being numerically much more efficient than its direct DFT counterpart, the DNN scheme has the advantage of being easily applicable to more sophisticated DFT approximations, such as meta-GGA and hybrid functionals, for which it would be hard to derive analytically the expression of the energy flux. We find in this way that a DNN model, trained on meta-GGA (SCAN) data, reduces the deviation from experiment of the predicted thermal conductivity by about 50%, leaving the question open as to whether the residual error is due to deficiencies of the functional, to a neglect of nuclear quantum effects in the atomic dynamics, or, likely, to a combination of the two.

A technique for spin accumulation quantification, specifically in the ferromagnetic layer of spin-orbit torque inducing heterostructures has been elusive. Here, we demonstrate an easy-to-implement technique to achieve this quantification by applying an additional DC bias during the harmonic Hall measurement in Si/${\mathrm{SiO}}_2$/Ta/Co/Pt. The spin accumulation arising from the DC bias generates an amplitude offset detectable in the first harmonic Hall magnetoresistance. By performing the first harmonic Hall magnetoresistance measurement under a fixed DC bias for two oppositely magnetized states, spin accumulation polarity set by the DC bias enhances the magnetoresistance if it aligns with the magnetization, and vice versa. Thus, the difference in the magnetoresistance amplitude provides the quantitative magnitude of spin accumulation relative to the ferromagnet's saturation magnetization, measured to be up to 0.29% in Si/${\mathrm{SiO}}_2$/Ta/Co/Pt. The strength of both spin accumulation and dampinglike efficiency increased with Ta thickness, further verifying our experimental technique.

Frustrated one-dimensional quantum spin systems are known to exhibit a variety of quantum ground states due to the effects of quantum fluctuations and frustrations. In a spin-$\frac{1}{2}$ kagome-strip chain, which is one of the frustrated one-dimensional spin systems, many quantum phases have been found. However, the magnetic phase diagrams of the kagome-strip chain under magnetic field have not been fully understood. We construct magnetic phase diagrams at 0, 1/5, 3/10, 1/3, 2/5, 7/15, 3/5, and 4/5 magnetization ratio in the kagome-strip chain and investigate magnetic properties in each phase using the density matrix renormalization group method. We find 15 magnetization-plateau phases, one of which is equivalent to the spin-1 Haldane phase.

The new development in sublattice-phase-resolved imaging of an electronic structure now allows for the visualization of the nematic-order state characteristic energy of cuprate superconductors in a wide doping regime. However, it is still unclear how this characteristic energy of the nematic-order state is correlated with the enhancement of superconductivity. Here the doping dependence of the nematic-order state characteristic energy in cuprate superconductors and of its possible connection to the enhancement of superconductivity is investigated within the framework of the kinetic-energy-driven superconductivity. It is shown that the characteristic energy of the nematic-order state is found to be particularly large in the underdoped regime, then it smoothly decreases upon the increase of doping, in full agreement with the corresponding experimental observations. Moreover, the characteristic energy of the nematic-order state as a function of the nematic-order state strength in the underdoped regime presents a similar behavior of the superconducting transition temperature. This suggests a possible connection between the nematic-order state characteristic energy and the enhancement of the superconductivity.

The discovery of the first high critical temperature $\left(T_c\right)$ transition metal diboride superconductor ${\mathrm{MgB}}_2$ structure $\alpha \text{−}{\mathrm{MoB}}_2$ under pressure with $T_c$ up to 32 K at 100 GPa provides new input into some unexplained aspects of electron-phonon coupling in intermetallic compounds. We establish that ${\mathrm{MoB}}_2$ is a phonon-mediated superconductor but has little in common with ${\mathrm{MgB}}_2$ ($T_c=40\mathrm{K}$ at zero pressure). ${\mathrm{MoB}}_2$ is a strongly metallic, three-dimensional, multi-Fermi-surface material, becoming of additional interest because it displays a frequency separation of Mo and B vibrations that mirrors that of metal superhydrides with $T_c$ approaching room temperature. This separation, which is unusual in intermetallic compounds, allows an analysis separately for Mo and B providing, among the other parameters essential for understanding phonon coupling, the matrix elements for scattering by the individual atoms. Strong coupling $({\lambda }_{\mathrm{Mo}}=1.48)$ is provided by Mo (total $\lambda =1.67$). A factor of 15 weaker coupling to each B atom is compensated by that coupling being to mean high-frequency modes around 85–90 meV (maximum of 140 meV), versus 18–20 meV for Mo. As a result, B enhances $T_c$ by 40% over the Mo-only value, to 33 K, corresponding to the experimental value. These results provide a guideline for designing higher-$T_c$ materials from a cooperation of strong coupling from heavy atoms with weakly coupled light atoms. The new high-$T_c$ paradigm discovered here highlights the need for studying and engineering larger ionic scattering matrix elements.

Polarons, that is, charge carriers correlated with lattice deformations, are ubiquitous quasiparticles in semiconductors, and play an important role in electrical conductivity. To date most theoretical studies of so-called large polarons, in which the lattice can be considered as a continuum, have focused on the original Fröhlich model: a simple (nondegenerate) parabolic isotropic electronic band coupled to one dispersionless longitudinal optical phonon branch. The Fröhlich model allows one to understand characteristics such as polaron formation energy, radius, effective mass, and mobility. Real cubic materials, instead, have electronic band extrema that are often degenerate (e.g., threefold degeneracy of the valence band), or anisotropic (e.g., conduction bands at $X$ or $L$), and present several phonon modes. In the present paper, we address such issues. We keep the continuum hypothesis inherent to the large polaron Fröhlich model, but waive the isotropic and nondegeneracy hypotheses, and also include multiple phonon branches. For polaron effective masses, working at the lowest order of perturbation theory, we provide analytical results for the case of anisotropic electronic energy dispersion, with two distinct effective masses (uniaxial) and numerical simulations for the degenerate three-band case, typical of III-V and II-VI semiconductor valence bands. We also deal with the strong-coupling limit, using a variational treatment: we propose trial wave functions for the above-mentioned cases, providing polaron radii and energies. Then, we evaluate the polaron formation energies, effective masses, and localization lengths using parameters representative of a dozen II-VI, III-V, and oxide semiconductors, for both electron and hole polarons. We show that for some cases perturbation theory (the weak-coupling approach) breaks down. In some other cases, the strong-coupling approach reveals that the large polaron hypothesis is not valid, which is another distinct breakdown. In the nondegenerate case, we compare the perturbative approach with the Feynman path integral approach in characterizing polarons in the weak-coupling limit. Thus, based on theoretical results for cubic materials, the present paper characterizes the validity of the continuum hypothesis for a large set of 20 materials.

We study the equilibrium dynamics of magnetic moments in the Mott insulating phase of the Hubbard model on the square and triangular lattice. We rewrite the Hubbard interaction in terms of an auxiliary vector field and use a recently developed Langevin scheme to study its dynamics. A thermal noise, derivable approximately from the Keldysh formalism, allows us to study the effect of finite temperature. At strong coupling, $U\gg t$, where $U$ is the local repulsion and $t$ the nearest-neighbor hopping, our results reproduce the well known dynamics of the nearest-neighbor Heisenberg model with exchange $J\sim \mathcal{O}(t^2/U)$. These include crossover from weakly damped dispersive modes at temperature $T\ll J$ to strong damping at $T\sim \mathcal{O}\left(J\right)$, and diffusive dynamics at $T\gg J$. The crossover temperatures are naturally proportional to $J$. To highlight the progressive deviation from Heisenberg physics as $U/t$ reduces we compute an effective exchange scale $J_{\mathrm{eff}}\left(U\right)$ from the low-temperature spin-wave velocity. We discover two features in the dynamical behavior with decreasing $U/t$: (i) the low-temperature dispersion deviates from the Heisenberg result, as expected, due to longer range and multispin interactions, and (ii) the crossovers between weak damping, strong damping, and diffusion take place at noticeably lower values of $T/J_{\mathrm{eff}}$. We relate this to enhanced mode coupling, in particular to thermal amplitude fluctuations, at weaker $U/t$. A comparison of the square and triangular lattice reveals the additional effect of geometric frustration on damping.

Adsorbed atoms (adatoms) coupled to the matrix of solid state host materials as impurities can significantly modify their properties. Especially in low-dimensional materials, such as one-dimensional organic polymer chains or quasi-one-dimensional graphene nanoribbons, intriguing manipulation of the optical properties, such as the absorption cross section, is possible. The most widely used approach to couple quantum emitters to optical antennas is based on the Purcell effect. This formalism, however, does not comprise charge transfer from the emitter to the antenna, but only spontaneous emission of the quantum emitter into the tailored photonic environment, that is evoked by the antenna. To capture such effects, we present a tight-binding formalism to couple an adatom to a finite Su-Schrieffer-Heeger chain, where the former is treated as a two-level system and the latter acts as an optical antenna. We systematically analyze how the coupling strength and the position of the adatom influence the optical properties of the molecular chains in the model. We take into account charge transfer from the adatom to the chain and vice versa via an intersystem hopping parameter, and also include Coulomb interaction within the chain as well as between the adatom and the chain. We show that coupling the adatom to one of the bulk atoms of the linear chain results in a substantial change in optical properties already for comparatively small coupling strengths. We also find that the position of the adatom crucially determines if and how the optical properties of the chains are altered. Therefore, we identify this adatom-chain hybrid system as a tunable platform for light-matter interaction at the nanoscale.

Black phosphorene (BlackP) is a promising two-dimensional material for next-generation nanoelectronics with a thickness-dependent bandgap and high carrier mobility. Yet to date, epitaxial growth of BlackP on a proper substrate remains a daunting challenge, which in turn prevents its mass production for device applications. Here we propose that BlackP-like group IV-VI substrates can be ideal candidates to promote epitaxial growth of BlackP. We first show, using first-principles approaches, that BlackP is energetically and thermodynamically stable on SnSe(001) as a representative substrate, which can be attributed to the isovalency nature and inherent structural similarity between the two. In contrast, as a close isomer of BlackP, blue phosphorene is thermodynamically unstable on SnSe(001). Next, we show that two phosphorus adatoms not only can readily form a dimer via P-P attraction, but such dimers also can diffuse isotropically and much faster than P monomers, both aspects highly desirable for initial growth and mass production of BlackP. Furthermore, BlackP grown on such semiconducting substrates can be directly exploited as a platform for novel optoelectronic devices, as demonstrated using the BlackP/SnSe heterobilayer that possesses a type-II band alignment and favors the formation of indirect excitons with desirable functionalities.