The solid-state harmonic generation (SSHG) derives from photocurrent coherence. The crystal symmetry, including point-group symmetry and time-reversal symmetry, constrains the amplitude and phase of the photocurrent, thus manipulates the coherent processes in SSHG. We revisit the expression of photocurrent under the electric dipole approximation and give an unambiguous picture of nonequilibrium dynamics of photocarriers on laser-dressed effective bands. In addition to the dynamical phase, we reveal the indispensable roles of the phase difference of transition dipole moments and the phase induced by shift vector in the photocurrent coherence. The microscopic mechanism of the selection rule, orientation dependence, polarization characteristics, time-frequency feature, and ellipticity dependence of harmonics governed by symmetries is uniformly clarified in our theoretical framework. Finally, we propose a nonlinear optical method for detecting the time-reversal symmetry breaking of crystals by using elliptical dichroism of SSHG. This work integrates nonequilibrium electronic dynamics of condensed matter in strong laser fields, and paves a way to explore more nonlinear optical phenomena governed by crystal symmetry.

Graphene corrugations affect hybridization of $\pi$ and $\sigma$ orbitals of carbon atoms in graphene based systems. It can as well break differently the symmetry of the electron transfer integrals for different strip boundaries. Using these facts, we found that the momentum distribution of electrons in ballistically propagating beam can be selective without external electric and/or magnetic fields in the graphene strip under experimentally feasible periodic potential. Such a potential is created by means of the superlattice that consists of periodically repeated graphene elements (flat and rippled junction) with different hybridization of carbon orbits, produced by variation of the graphene surface curvature. As a result it gives rise to the valley dependent focusing effects that can be controlled by alteration of the number of superlattice elements.

The strong chiral light-matter interaction is crucial for various important fields such as chiral optics, quantum optics, and biomedical optics, driving a quest for the extreme intrinsic chirality assisted by ultra-high-quality ($Q$-) factor resonances. In this quest we propose a straightforward method to achieve extreme intrinsic chirality in lossless planar structures by manipulating the quasi-BIC through in-plane perturbation. The temporal coupled-mode theory is employed to derive the conditions necessary for achieving maximal intrinsic chirality. The quasi-BIC should be excited within the transparent spectral range of the structure and couple with $x$- and $y$-polarized waves with the same intensity but a phase difference of $\pi /2$. For an illustration, a planar chiral dielectric dimeric waveguide grating is designed that strongly interacts with left circularly polarized light while decoupling from right circularly polarized light through in-plane symmetry engineering. Furthermore, by adjusting the magnitude of the in-plane asymmetry, we can independently manipulate the $Q$ factors of the chiral quasi-BIC while maintaining near unity circular dichroism. Our results provide a simple yet powerful paradigm for achieving extreme intrinsic chirality on an easily manufacturable platform, which may have potential applications in chiral emission, chiral sensing, and enantiomer separation.

Interfaces between transition-metal oxides can host two-dimensional electron gases (2DEGs) that exhibit intriguing quantum phenomena. The interfacial 2DEG system has attracted significant attention due to its versatile properties and tunability. In this study, we present the discovery of a high-mobility 2DEG characterized by well-developed Shubnikov-de Haas oscillations, alongside superconductivity at temperatures below 230 mK, in a heterostructure comprising bulk-metallic ${\mathrm{SrNbO}}_3$ on ${\mathrm{SrTiO}}_3$. The 2DEG is featured with extremely low carrier density for the quantum oscillations, and the superconductivity with two-step behaviors in various transport properties. Both the 2DEG and the superconductivity exhibit a common dependence on the thickness of the ${\mathrm{SrNbO}}_3$ layer. The dependence underscores the crucial role of epitaxial strain within the metallic layer on these two phenomena. Our findings introduce a novel platform for investigating interfacial high-mobility 2DEGs coexisting with superconductivity, with the bulk-metallic components of ${\mathrm{SrNbO}}_3$ playing a pivotal role in these heterostructures.

Epsilon-near-zero (ENZ) materials offer promising candidates for the study of strong light-matter interactions due to their exotic optical properties. Quasibound states in the continuum (QBICs) supported by all-dielectric metasurfaces possess the characteristics of high quality factors with flexible tunability, allowing for significant field enhancement and more degrees of freedom for manipulating light-matter interaction at the nanoscale. Here, we propose a two-dimensional all-dielectric metasurface combined with a thin polar dielectric film, which supports the ENZ mode and the inherent transverse optical (TO) phonon mode. We demonstrate a strong QBIC-ENZ coupling with giant and highly tunable coupling strengths. The proposed metasurface can not only support QBIC modes, but can also excite the ENZ mode in polar dielectric film. With the help of the strong field localization and flexible modulation properties of QBIC, we further realize a selective QBIC-ENZ-TO three-mode strong coupling in the hybrid system, demonstrating a large Rabi splitting over 22.1 meV (178 ${\mathrm{cm}}^{-1}$) at room temperature, which is four times higher than that of the QBIC-ENZ two-mode coupling. We also demonstrate that the three-mode coupling strength can be easily tuned by varying the thickness of the ENZ thin film, which provides an effective approach for the manipulation of the strong light-matter interaction. The proposed all-dielectric platform exhibits strong coupling strength and flexible design tunability, which is expected to provide a different route for the design of ultracompact optical devices at mid-infrared frequencies.

We study the magnetism of a layered, spin-$\frac{1}{2}$ organic-inorganic copper sulfate, which is a close realization of the star lattice antiferromagnet, one of the playgrounds of geometric frustration and resonating valence bond physics in two spatial dimensions. Our thermodynamic measurements show no ordering down to 0.1 K and a characteristic field-induced entropic shift, revealing the presence of an infinite number of competing states down to very-low-energy scales. The response to external magnetic fields shows, in addition, a peculiar anisotropy, reflected in the formation of a 1/3 magnetization plateau (stable up to full saturation around 105 T) and a paramagnetic, Curie-like susceptibility for one direction of the field $(\mathbf{H}\parallel \mathbf{c})$, and a completely different response in other field directions. Our first-principles density functional theory calculations and exact diagonalizations show that these experimental puzzles are distinctive signatures of a strong interplay between geometric frustration and sizable Dzyaloshinskii-Moriya interactions, and the emergence of a continuous U(1) symmetry at low-energy scales.

Magnets are interesting materials for classical and quantum information technologies. However, the short decoherence and dephasing times that determine the scale and speed of information networks severely limit the appeal of employing the ferromagnetic resonance. Here we show that the lifetime and coherence of the uniform Kittel mode can be enhanced by three-magnon interaction-induced mixing with the long-lived magnons at the minima of the dispersion relation. Analytical and numerical calculations based on this model explain recent experimental results and predict experimental signatures of quantum coherence.

The metal-insulator transition of ${\mathrm{VO}}_2$, which in equilibrium is associated with a structural phase transition, has been intensively studied for decades. It is challenging to disentangle the role of Mott physics from dimerization effects in the insulating phase. Femtosecond time-resolved experiments showed that optical excitations can induce a transient metallic state in the dimerized phase, which is distinct from the known equilibrium phases. In this letter, we combine nonequilibrium cluster dynamical mean-field theory with realistic first-principles modeling to clarify the nature of this laser-induced metallic state. We show that the doublon-holon production by laser pulses with polarization along the V-V dimers and the subsequent interorbital reshuffling of the photocarriers leads to a population of orbital-mixed states and the filling of the gap. The photoinduced metal state is qualitatively like a hot electronic state in the dimerized structure and does not involve a collapse of the Mott gap.

We investigate disorder-driven transitions between trivial and topological insulator (TI) phases in two-dimensional (2D) systems. Our study primarily focuses on the Bernevig-Hughes-Zhang (BHZ) model with Anderson disorder, while other standard 2D TI models exhibit equivalent features. The analysis is based on the local Chern marker (LCM), a local quantity that allows for the characterization of topological transitions in finite and disordered systems. Our simulations indicate that disorder-driven trivial to topological insulator transitions are nicely characterized by ${\mathcal{C}}_0$, the disorder-averaged LCM near the central cell of the system. We show that ${\mathcal{C}}_0$ is characterized by a single-parameter scaling, namely, ${\mathcal{C}}_0(M,W,L)\equiv {\mathcal{C}}_0\left(z\right)$, with $z=[W^{\mu }-W_c^{\mu }\left(M\right)]L$, where $M$ is the Dirac mass, $W$ is the disorder strength, and $L$ is the system size, while $W_c\left(M\right)\propto \sqrt{M}$ and $\mu \approx 2$ stand for the critical disorder strength and the critical exponent, respectively. Our numerical results are in agreement with a theoretical prediction based on a first-order Born approximation analysis. These observations lead us to speculate that the universal scaling function we have found is rather general for amorphous and disorder-driven topological phase transitions.

${\mathrm{LiYbSe}}_2$ was recently discovered and assumed to be a new member of the quantum spin liquid candidates. In the paper, polarization-resolved Raman scattering and temperature-dependent Raman scattering experiments were utilized to study the elementary excitations in ${\mathrm{LiYbSe}}_2$. Four phonon modes with $E_g$, $T_{2g}^1$, $T_{2g}^2$, and $A_{1g}$ symmetry, respectively, and three crystal electric field (CEF) excitations, including CEF1, CEF2, and CEF3 at 116.2, 200.9, and $232.1\mathrm{c}{\mathrm{m}}^{-1}$, respectively, were conclusively identified. With 633 and 532 nm excitation, resonant conditions were satisfied and caused the breakdown of selection rules for $T_{2g}$ and also prompted the observation of higher-order modes. Helicity-resolved CEF modes were studied based on circularly polarized light and the conservation law of angular momentum. Moreover, CEF modes showed abnormal redshift with decreasing temperature which can be attributed to the reduction of electronegativity of Se atoms due to the lattice contraction. Contributing to the close energy between $E_g$ and CEF1, weak CEF-phonon coupling was induced in this system and caused abnormal softening of $E_g$ at low temperature. The CEF excitations and vibration properties studied here provide information about the electronic elementary excitations, which help to further study or control the frustrated magnetism and novel quantum spin liquid state in ${\mathrm{LiYbSe}}_2$.

Aquodiium (${\mathrm{H}}_4{\mathrm{O}}^{2+}$), an isoelectronic analog of the ammonium ion (${\mathrm{NH}}_4^{+}$), can theoretically be formed by combining a molecule of water (${\mathrm{H}}_2\mathrm{O}$) with two protons. However, stable aquodiium has never been reported because of the high energy cost during the second protonation after hydronium (${\mathrm{H}}_3{\mathrm{O}}^{+}$). Here, by performing ab initio evolutionary structure searches combined with first-principles calculations, stable ionic phases, ${\mathrm{H}}_4\mathrm{O}{\mathrm{F}}_2$ and ${\mathrm{H}}_4\mathrm{O}{\mathrm{F}}_2·\mathrm{HF}$, were predicted to be thermodynamically stable at high pressure. Analysis of bond lengths and electron density supports the formation of aquodiium under pressure in these two phases. Moreover, ab initio molecular dynamics simulations reveal that these ionic phases will enter the superionic states at lower temperatures compared to water ice. For ${\mathrm{H}}_4\mathrm{O}{\mathrm{F}}_2·\mathrm{HF}$, there is a plastic phase region where aquodiium ions exhibit free rotation. All aquodiium ions are fully preserved below 1000 K in these ionic phases, while after entering the diffusion state, only the ${\mathrm{H}}_4\mathrm{O}{\mathrm{F}}_2$ phase keeps ${\mathrm{H}}_4{\mathrm{O}}^{2+}$ ions. Our results suggest that pressure stabilizes the ${\mathrm{H}}_4{\mathrm{O}}^{2+}$ ion, presenting an important addition to traditional physical and chemical theories such as the valence shell electron pair repulsion model, proton transfer, and acid-base theory.

Time-varying external magnetic fields can be used to manipulate the dynamics of magnetic nanoparticles. When suspended in viscous media, external fields not only modify the internal Néel relaxation dynamics within the magnetic nanoparticles but also the Brownian particle rotation. For the case of oscillating magnetic fields, Brownian and Néel processes lead to magnetic losses that are dissipated as heat to the neighborhood of the nanoparticle. The mean value of heat dissipated has been studied intensively in recent years, not least because of promising biomedical applications such as magnetic fluid hyperthermia. Here, we use the framework of stochastic thermodynamics to study fluctuations in the dissipated heat. We find that the dynamics of magnetic nanoparticles as modelled by a mesoscopic model obeys the detailed fluctuation theorem in terms of the total entropy production within numerical accuracy. In addition, we observe that the total entropy production is dominated by the dissipated heat and that fluctuations of dissipated heat are rather strong with the standard deviation being of the same order as the mean value. We also find that the probability of observing negative values of dissipated heat is rather large for typical field strengths used in magnetic fluid hyperthermia applications.

Half-Heusler compounds are promising candidates for thermoelectrics. The exploration of multiphonon interaction, including four-phonon interaction, in half-Heusler compounds contributes to the deep understanding of thermal transport, which is helpful to optimize thermoelectric properties. In this work, LuNiBi is taken as the typical half-Heusler compound to investigate multiphonon interaction and thermal conductivity. By employing inelastic x-ray scattering and first-principles calculations, we confirm that the low thermal conductivity ĸ of LuNiBi is closely related to its small group velocity compared with many other half-Heusler compounds. The noteworthy four-phonon scattering between parallel flat acoustic phonon bands is validated through a combination of experiment and calculation, which may be related to its rattlinglike characteristic. Additionally, the calculation confirms the four-phonon scattering further effectively decreases the thermal conductivity at high temperature. The significant four-phonon scattering processes are associated with weakened three-phonon scattering processes due to critical selection rules; this occurs both around parallel flat acoustic phonon bands and in the 13–15 meV energy region. Our work highlights the significance of multiphonon scattering processes, including four-phonon interaction, which plays an important role in the thermal properties of half-Heusler materials.

Charge-density wave phases in quantum materials stem from the complex interplay of electronic and lattice degrees of freedom. Nowadays, various time-resolved spectroscopy techniques allow to actively manipulate such phases and monitor their dynamics in real time. Modeling such nonequilibrium dynamics theoretically is a great challenge and exact methods can usually only treat a small number of atoms and finitely many phonons. We approach the melting of charge-density waves in a Holstein model after a sudden switch-on of the electronic hopping from two perspectives. We prove that in the noninteracting and in the strong-coupling limit, the CDW order parameter on high-dimensional hypercubic lattices obeys a factorization relation for long times, such that its dynamics can be reduced to the one-dimensional case. Secondly, we present numerical results from semiclassical techniques based on the truncated Wigner approximation for two spatial dimensions. A comparison with exact data obtained for a Holstein chain shows that a semiclassical treatment of both the electrons and phonons is required in order to correctly describe the phononic dynamics. This is confirmed, in addition, for a quench in the electron-phonon coupling strength.

Multiphoton emission of electrons has been observed from sharp tips of heavily $p$-doped GaAs caused by laser pulses with, nominally, 800-nm wavelength, 1-nJ/pulse energy, and 90-fs duration. The emission is mostly due to four-photon processes, with some contribution from three-photon absorption as well. When the electron emission current due to two pulses separated by delay $200\mathrm{fs}\le \tau \le 1\mathrm{ns}$ is integrated over all electron energies, it is less than that observed for the sum of the emission from the two individual pulses. This subadditive behavior is consistent with a fast electron emission process, i.e., one in which the electron emission occurs over a time comparable to the laser pulse width. The subadditivity results from Pauli blocking of electron emission by the second pulse due to a population increase of the GaAs conduction band caused by the first pulse. Such subadditive photoemission is a sensitive probe of excited-carrier dynamics. We employ the use of an excited-level population model to characterize the photon absorption process and give us a clearer understanding of the electron dynamics in GaAs associated with multiphoton electron emission. Possible applications of this subadditivity effect to control photoemitted electron spin are discussed.

Unexpected pressure-induced s-d electron transition had been observed in various $s$-block metals. However, it is noteworthy that no such transition has been reported for hydrogen, a unique element within the $s$ block of the periodic table. Here, we propose using a zero-dimensional electride state, similar to a diamond anvil cell, to pressurize hydrogen under high pressure. The s-d electron transition of hydrogen was confirmed in our predicted electride ${\left({\mathrm{Na}}_2\mathrm{He}\right)}_4\mathrm{H}$ at 200 GPa. The electride-state interstitial quasiatoms arranged themselves in a regular octahedral configuration centered at the hydrogen atom. This arrangement established a highly symmetrical Coulomb repulsion field, further compressing and triggering the central hydrogen atom to undergo an s-d transition. Additionally, our results indicate that the hydrogen doping results in the metallization and superconductivity of electride ${\left({\mathrm{Na}}_2\mathrm{He}\right)}_4\mathrm{H}$. The electride-state interstitial quasiatoms around hydrogen atoms dominate the superconductivity of ${\left({\mathrm{Na}}_2\mathrm{He}\right)}_4\mathrm{H}$. The presence of ${\left({\mathrm{Na}}_2\mathrm{He}\right)}_4\mathrm{H}$ provides an example to confirm the promotion effect of the electride state on the superconductivity of electrides.

The response to strains is a crucial issue when applying two-dimensional (2D) materials in flexible electronic devices. Utilizing first-principles calculations based on density functional theory, we systematically studied the structural phase transition in niobium oxide diiodide $\left({\mathrm{NbOI}}_2\right)$ monolayer and the consequent changes in electronic and magnetic properties. The calculated Young's modulus of the monolayer ${\mathrm{NbOI}}_2$ is approximately $80{\mathrm{Nm}}^{\text{−}1}$, and the in-plane Poisson's ratio is less than 0.1. Under uniaxial strain, dissociation of Nb atom dimers occurs, leading to a magnetic phase transition from nonmagnetic to antiferromagnetic states. Moreover, the coupling pattern of magnetic moments exhibits significant anisotropy along different directions with temperature variations. The exchange couplings in two orthogonal crystal directions were analyzed and the Néel temperature was predicted using Monte Carlo simulations. Additionally, the strain also modulates the ferroelectric polarization and switching barriers. A clear physical picture was provided to explain the magnetic and electronic mechanisms in the 2D ${\mathrm{NbOI}}_2$. This study shows the potential applications of such materials in future flexible microelectronic devices.

The impact of weak disorder and its spatial correlation on the topology of a Floquet system is not well understood so far. In this study, we investigate a model closely related to a two-dimensional Floquet system that has been realized in experiments. In the absence of disorder, we determine the phase diagram and identify an exotic phase characterized by edge states with alternating chirality in adjacent gaps. When weak disorder is introduced, we examine the disorder-averaged Bott index and analyze why the anomalous Floquet topological insulator is favored by both uncorrelated and correlated disorder, with the latter having a stronger effect. For a system with a ring-shaped gap in the energy spectrum, the Born approximation fails to explain the topological phase transition, unlike for a system with a pointlike gap.

We analyze the amplitude fluctuations in a diluted 3D classical XY model near the magnetic phase transition, motivated by the unusual localization properties of the amplitude (Higgs) mode recently found at the disordered superfluid-Mott glass quantum phase transition. We calculate the amplitude correlation function and the corresponding scalar susceptibility by means of Monte Carlo simulations. In contrast to the quantum case, in which the scalar susceptibility was found to violate naive scaling, we find that the scalar susceptibility of the classical system fulfills naive scaling (employing the clean critical exponents, as expected from the Harris criterion) as the temperature is varied across the phase transition for several dilutions. We discuss possible reasons for this discrepancy as well as the generality of our findings.

A real Chern insulator (RCI) featuring a real Chern number and a second-order boundary mode appears in a two-dimensional (2D) system with the space-time inversion symmetry $\left(\mathcal{PT}\right)$. Here, we propose a kind of RCI: the mirror real Chern insulator (MRCI), which emerges from the system having additional horizontal mirror symmetry ${\mathcal{M}}_z$. The MRCI generally is characterized by two independent real Chern numbers, respectively defined in the two mirror subsystems of the system. Hence, the MRCI may host the second-order boundary modes different from the conventional RCI. We show that for spinless systems, the definition of the MRCI is straightforward, as $\mathcal{PT}$ keeps each mirror subsystem invariant. For the spinful systems with both $\mathcal{PT}$ and ${\mathcal{M}}_z$, the real Chern number for the total system remain well defined, as ${\mathcal{M}}_z\mathcal{PT}=C_{2z}\mathcal{T}$, and ${\left(C_{2z}\mathcal{T}\right)}^2=1$. However, since $C_{2z}\mathcal{T}$ exchanges the two mirror subsystems, the definition of the MRCI in spinful systems requires the help of projective symmetry algebra. We also discuss the MRCIs in 3D systems, where the MRCI is defined on certain mirror-invariant 2D planes. Compared with its 2D counterpart, the 3D MRCI can exhibit more abundant physics when the systems have additional nonsymmorphic operators. Several concrete MRCI models, including 2D and 3D, spinless and spinful models are constructed to further demonstrate our ideas.