In this paper, we study the dynamic formation of transients when plane waves impinge on a dispersive slab that abruptly changes its electrical properties in time. The time-varying slab alternates between air and metal-like states, whose frequency dispersion is described by the Drude model. It is shown how the physics of this complex system can be well described with the joint combination of two terms: one associated with temporal refractions and the other associated with spatial refractions. To test the validity of the approach, some analytical results are compared with a self-implemented finite-difference time-domain method. Results show how the transients that occur after the abrupt temporal changes can shape the overall steady-state response of the space-time system. In fact, far from always being detrimental, these transient states can be conveniently used to perform frequency conversion or to amplify or attenuate the electromagnetic fields.

The Bethe-Salpeter equation (BSE) is a powerful theoretical approach that is capable to accurately treat electron-hole interactions in materials in an excited state. We developed an ab initio framework based on the BSE to describe a pump-probe experiment, in which an x-ray pulse probes solid-state valence excitons by means of x-ray absorption spectroscopy. Our theoretical framework is of relevance for an accurate modeling of pump-probe experiments of photoexcited materials that utilize novel capabilities offered by x-ray science.

Moiré lattices have served as the ideal quantum simulation platform for exploring novel physics due to the flat electronic bands resulting from the long-wavelength moiré potentials. However, the large sizes of this type of system challenge the first-principles methods for full calculations of their electronic structures, thus bringing difficulties in understanding the nature and evolution of the flat bands. In this study, we investigate the electronic structures of moiré patterns of ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ by combining ab initio and machine learning methods. We find that a flat band with a bandwidth of about 5 meV emerges below the valence band edge at the K point for the H stacking at a twist angle of ${3.89}^{\circ }$ without spin-orbit coupling effect. Then, it shifts dramatically as the twist angle decreases and becomes about 20 meV higher than the valence band maximum for the twist angle of ${3.15}^{\circ }$. Multiple ultraflat bands emerge as the twist angle is reduced to ${1.7}^{\circ }$. The spin-orbit coupling leads to a giant spin splitting comparable to that observed in the untwisted system (about 0.45 eV) and is nearly independent of twisting and stacking. As a result, the K-valley flat band remains the valence band maximum with the inclusion of spin-orbit coupling. Band unfolding reveals that the ultraflat bands formed by the $\mathrm{\Gamma }$ and K valleys show distinct behaviors. The $\mathrm{\Gamma }$-valley flat bands are sensitive to the interlayer coupling, thus experiencing dramatic changes as the twist angle decreases. In contrast, the K-valley flat band, which shows a weak dependence on the interlayer coupling, is mainly modulated by structural reconstruction. Therefore, a relatively small angle $\left({2.13}^{\circ }\right)$ is required to generate the K-valley flat band, which experiences a transition from the honeycomb to the triangular lattice as the twist angle decreases.

The spin of a charged semiconductor quantum dot (QD) coupled to an optical cavity is a promising candidate for high-fidelity spin-photon interfaces; the cavity selectively modifies the decay rates of optical transitions such that spin initialization, manipulation, and readout are all possible in a single magnetic field geometry. By performing cavity QED calculations, we show that a cavity with a single, linearly polarized mode can simultaneously support both high-fidelity optical spin initialization and readout in a single, in-plane (Voigt geometry) magnetic field. Furthermore, we demonstrate that single-mode cavities always outperform bimodal cavities in experimentally favorable driving regimes. Our analysis, when combined with established methods of control in a Voigt geometry field, provides optimal parameter regimes for high-fidelity initialization and readout, and coherent control in both cavity configurations, providing insights for the design and development of QD spin-photon interfaces as the basis of quantum network nodes and for the generation of photonic graph states.

Utilizing the exact diagonalization method, we investigate the one-dimensional Peierls-Hubbard model at quarter filling, where it manifests as an antiferromagnetic Mott insulator in units of dimers. By increasing the on-site Coulomb repulsion $U$, we observe a significant suppression of the Drude peak, based on a nonequilibrium linear-response theory capable of capturing the zero-frequency (Drude) weight of the optical conductivity under periodic boundary conditions. However, after the ultrafast photoirradiation of this model with large $U$, we detect a distinct enhancement of the Drude peak, signifying the onset of a photoinduced insulator-metal transition. Comparing these dynamics with the half-filled Hubbard model and a noninteracting spinless half-filled Su-Schrieffer-Heeger model (corresponding to the quarter-filled Peierls-Hubbard model with infinite $U$), we propose a mechanism for the photoinduced metallic state: The empty-occupied and double-occupied dimers serve as the photoinduced charge carriers, akin to the holons and doublons in the Hubbard model.

Dual-state molecular junctions, with two coupled electronic states in charge transport, showcase a wider range of electrical functionalities in comparison to single-state junctions, attributed to the extra electronic state and the interstate coupling. We carried out a theoretical modeling investigation of charge transport and functionalities of the dual-state junction under the Landauer formula and Marcus theory frameworks. We uncovered a negative differential resistance (NDR) unique to dual-state molecular junctions, which can be fine-tuned by adjusting the polarization and coupling strength between the two electronic states in model molecules with different linking lengths and positions of the electroactive groups. Moreover, using ab initio computation via the density functional theory plus nonequilibrium Green's function method, we demonstrate controlled NDR switching with a photoresponsive chemical bridge to control interstate coupling. Combining the electronic structure analysis based on charge decomposition and the interaction diagram, our study provides valuable insights into the fundamental structure-function relationship of dual-state molecular junctions and elucidates the principles for rational molecular design of controllable NDR functionalities.

We formulate a quantum version of the hard-disk problem on lattices, which exhibits a natural realization in systems of Rydberg atoms. We find that quantum hard disks exhibit unique dynamical quantum features. In 1D, the crystal melting process displays ballistic behavior as opposed to classical subdiffusion. For 2D, crystal structures remain intact against most defects, whereas classically they are washed out completely. We link this peculiar quantum behavior to quantum many-body scars. Our study highlights the potential of constrained 2D quantum matter to display unique dynamical behaviors.

We present results from density functional theory (DFT) calculations of magnetization, induced by an external magnetic field, for δ-phase plutonium. The fully relativistic electronic structure accounts for Zeeman splitting effects through a Hamiltonian that couples the magnetic field to both spin and orbital magnetic moments. The electronic-structure model is further improved by an extension to DFT in terms of an orbital-orbital coupling via the conventional orbital-polarization method, as has routinely been done for plutonium. The response to the applied magnetic field is shown to be weak in the DFT model, with induced magnetic moments of the order of ${10}^{-3}{\mu }_{\mathrm{B}}$ in magnetic fields up to 30 T. These results are in accord with recent assessments from x-ray magnetic circular dichroism measurements in magnetic field on δ-plutonium. Somewhat surprisingly, a model assuming δ-plutonium to be absent magnetic moments shows stronger response to an external static magnetic field than models allowing for antiferromagnetism or magnetic disorder.

We propose an optical cavity with deep subwavelength confinement of light in a region that simultaneously works as a quantum dot. The design is based on a dielectric membrane with a buried quantum well and restricts the electron and hole wave functions to the area of the optical hotspot to overcome the challenge of colocating an optical cavity with a quantum emitter. Combined with proper surface passivation, this geometry points towards the deterministic fabrication of functional quantum dots in optical cavities by lithographic means.

Tuning thermal energy transport via solid-state structure manipulation is a challenge and of vital technological importance in energy-related devices. Herein, we provide microscopic insights into the correlations among atomic motions, anharmonic lattice dynamics, and thermal switch across the ultrafast amorphous-to-crystalline transition in a promising subnanosecond phase-change material (PCM) ${\mathrm{Sc}}_{\text{0.2}}{\mathrm{Sb}}_{\text{2}}{\mathrm{Te}}_{\text{3}}$. We show a reversible octahedron-heptagon reconfiguration related to the Sc-centered atomic motifs with a slight distortion during the phase transition, which is responsible for the exceptional recrystallization kinetics. Our combined density fluctuations analysis with classical hydrodynamic theory demonstrates a jump of thermal conductivity $\left(\kappa \right)$ with the highest predicted switch ratio of 2.4 accompanied by the order-disorder transition, corresponding to the octahedral alignment of Te atoms with an increased long-range translational symmetry. Our unified lattice dynamical approach rationalizes the $\kappa$ evolution by linking Te-dominated anharmonic lattice dynamics and thermal switching, which is further benchmarked by the heat-flux-independent, nonequilibrium simulations. These results provide physical insights into PCM's complex atomic dynamics and thermal switch mechanism.

Higher-order nodal line semimetals represent a recently proposed topological semi-metal class that harbors bulk nodal lines and features gapless hinge Fermi arc excitations, governed by the bulk-hinge correspondence. In this study, we investigate the disorder effect on a higher-order nodal line semimetal and the consequent phase transitions. Within the pristine higher-order nodal line semi-metal model, we unveil three distinct phases: higher-order nodal line semimetal; conventional nodal line semimetal; and normal insulator. The higher-order nodal line semimetal is characterized by one-dimensional hinge Fermi arc states connecting a pair of nodal rings, contrasting with conventional nodal line semimetals that exhibit two-dimensional drumhead surface states. We demonstrate that disorder can trigger multiple phase transitions within this system. Significantly, intermediate disorder can induce higher-order topology in an initial conventional nodal line semi-metal or even an initial normal insulator. Further increase in disorder drives the system through a diffusive metallic phase before ultimately reaching the Anderson insulator regime. Employing a combination of finite-size scaling analysis and an effective medium theory, we construct a comprehensive phase diagram, elucidating the intricate interplay between disorder and topology.

The magnetic field ($H$)-induced second magnetic ordering and exotic multiferroic behavior in the L-type ferrimagnet (L-FiM) ${\mathrm{Fe}}_2{\left(\mathrm{Mo}{\mathrm{O}}_4\right)}_3$, as reported elsewhere [Tiwari et al., Phys. Rev. Mater. 6, 094412 (2022)], requires further exploration. In this study, we investigate the possible origin of multiferroic behavior in the ${\mathrm{Fe}}_2{\left(\mathrm{Mo}{\mathrm{O}}_4\right)}_3$ system by partially substituting W for Mo crystallographic sites [i.e., ${\mathrm{Fe}}_2{\left({\mathrm{Mo}}_{1-x}{\mathrm{W}}_x{\mathrm{O}}_4\right)}_3$]. This substitution induces isotropic expansion of the crystal unit cell and significantly alters the ground-state magnetic properties, transitioning from L-FiM ($x=0$) to antiferromagnetic ($x=0.6$) ordering. A systematic study of magnetization χ ($T, H$), dielectric constant ${\varepsilon }^{\prime }$ ($T, H$) and ferroelectric polarization $P$ ($T, H$) as a function of W substitution ($x$) shows that both $T_{\mathrm{N}2}$ and $H$-induced polarization ($P$) decrease progressively with increasing x, becoming completely suppressed at $x=0.6$, thereby establishing the $x-T$ phase diagram. O $K$-edge x-ray absorption spectroscopy suggests that $p\text{−}d$ orbital hybridization plays a finite role in modulating the magnetic and multiferroic properties at low $T$ upon W substitution. Density functional theory further supports this finding, highlighting variations in the ionic character of the Mo- and W-O bonds. These findings reveal a complex interplay between crystal structure, magnetic, and electric couplings in ${\mathrm{Fe}}_2{\left(\mathrm{Mo}{\mathrm{O}}_4\right)}_3$, providing insight into the possible origin of $H$-induced $T_{\mathrm{N}2}$ and the accompanying ferroelectric polarization.

We present a comprehensive study of the magnetoelastic properties of the Ising pyrochlore oxide ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, known as spin ice, by means of high-field magnetostriction measurements and numerical calculations. When a magnetic field is applied along the crystallographic $\langle 111\rangle$ axis, the longitudinal magnetostriction exhibits a broad maximum in the low-field regime around 30 T, followed by a dramatic lattice contraction due to crystal-field (CF) level crossing at $B_{\mathrm{cf}}\sim 65$ T. The transverse magnetostriction exhibits a contrasting behavior, highlighting the anisotropic nature of the CF striction. By applying a magnetic field at varying sweep rates, we identify distinct timescales of spin dynamics that are relevant to monopole formation and annihilation, as well as CF-phonon dynamics. Our mean-field calculations, based on a point-charge model, successfully reproduce the overall magnetostriction behavior, revealing the competition between the exchange striction and CF striction. A signature of the CF level crossing is also observed through adiabatic magnetocaloric-effect measurements, consistent with our magnetostriction data.

Undoubtedly, machine learning (ML) techniques are being increasingly applied to a wide range of situations in the field of condensed matter. Amongst these techniques, unsupervised techniques are especially attractive, since they imply the possibility of extracting information from the data without previous labeling. In this paper, we resort to the technique known as “anomaly detection” to explore potential exotic phases in skyrmion phase diagrams, using two different algorithms: principal component analysis (PCA) and a convolutional autoencoder (CAE). First, we train these algorithms with an artificial dataset of skyrmion lattices constructed from an analytical parametrization, for different magnetizations, skyrmion lattice orientations, and skyrmion radii. We apply the trained algorithms to a set of snapshots obtained from Monte Carlo simulations for three ferromagnetic skyrmion models: two including in-plane Dzyaloshinskii-Moriya interaction (DMI) in the triangular and kagome lattices, and one with an additional out-of-plane DMI in the kagome lattice. Then, we compare the root mean square error (RMSE) and the binary cross entropy (BCE) between the input and output snapshots as a function of the external magnetic field and temperature. We find that the RMSE error and its variance in the CAE case may be useful to not only detect exotic low-temperature phases, but also to differentiate between the characteristic low-temperature orderings of a skyrmion phase diagram (helical, skyrmions and ferromagnetic order). Moreover, we apply the skyrmion trained CAE to two antiferromagnetic models in the triangular lattice, one that gives rise to antiferromagnetic skyrmions, and the pure exchange antiferromagnetic case, finding that the behavior of the RMSE is still an indicator of different ordering. Finally, we explore the portability of the technique with the same CAE applying it to a ferromagnetic skyrmion model in the square lattice.

We present a comprehensive microscopic insight into the spin configurations within the bond-frustrated cubic spinel compound ${\mathrm{MnCr}}_2{\mathrm{S}}_4$ directly unveiled through extensive single-crystal neutron diffraction studies carried out in zero magnetic field and in fields up to 35 T. While our zero field results confirm the ferrimagnetic structure with an antiparallel arrangement of the magnetic ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Mn}}^{2+}$ sublattices below $T_{\mathrm{FiM}}\approx 65$ K, as well as the presence of the exotic Yafet-Kittel phase below $T_{\mathrm{YK}}\approx 5$ K, our data measured in fields enable us to precisely determine the field-induced magnetic structures and their evolution across the phase transitions at ${\mu }_0{H}_1\approx 11\mathrm{T}$ and ${\mu }_0{H}_2\approx 25\mathrm{T}$ and beyond that towards ${\mu }_0{H}_3$ $(\approx 50\mathrm{T})$. Additionally, combining our experimental findings with mean-field-theory calculations reveals a complex field dependence of the Mn-Mn and Mn-Cr exchange interactions across the different phases, highlighting the significant influence of spin-lattice coupling in this material.

Antiferromagnets without parity-time ($\mathcal{PT}$) symmetry offer novel perspectives in the field of functional magnetic materials. Among them, those with ferromagneticlike responses are promising candidates for future applications such as antiferromagnetic (AFM) memory; however, examples showing large effects are extremely limited. In this paper, we show that the orthorhombic system TaMnP exhibits a large anomalous Hall conductivity (AHC) $\sim 360–370{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$ in spite of the small net magnetization $\sim {10}^{-2}{\mu }_B/\mathrm{Mn}$. Our neutron scattering experiment and the observation of the AH effect indicated that a magnetic structure of TaMnP was dominated by an AFM component represented by $B_{3g}$ with the propagation vector $q=0$. Furthermore, we confirmed that the obtained AHC is among the largest observed in AFM materials at zero fields. Additionally, our first-principles calculations revealed that the spin-orbit interaction originating in the nonmagnetic Ta $5d$ electrons significantly contributes to enhancing Berry curvatures in the momentum space. We found that the magnetic fields along all the crystal axes triggered the AFM domain switching, indicating the possibility of controlling the AFM domain using a small net magnetization, which is symmetrically different.

Rhombohedral graphite (RG), composed of ABC-stacked graphene layers, is a typical topological semimetal that hosts intricate configurations of Dirac lines (nodal lines), predominantly influenced by interlayer hopping. However, the fabrication of stable RG with strong interlayer coupling remains challenging, hindering the exploration of its intriguing properties. Here, we investigate the evolution of Dirac lines from spiral nodal lines to hexagram nodal nets and, finally, to distorted nodal rings in a cavity-tube acoustic RG with controllable interlayer hopping. When projected onto the boundary, these bulk nodal lines give rise to topological drumhead surface states protected by inversion symmetry. Furthermore, the diverse and tunable surface states characterized by nontrivial winding numbers are also demonstrated. Our results advance the understanding of Dirac-line semimetal phases and may provide insights into the study of other stacking configurations of graphene layers.

We study the ultralow-temperature thermodynamics and thermal conductivity ($\kappa$) of the single-crystal rare-earth chalcogenides ${\mathrm{NaYbS}}_2$ and ${\mathrm{NaYbSe}}_2$, which have an ideal triangular lattice of the ${\mathrm{Yb}}^{3+}$ ions and have been proposed to be quantum spin liquid candidates. The magnetic specific heat divided by temperature $C_{\mathrm{mag}}/T$ is nearly constant at $T<200$ mK, which is indeed the indication of the gapless magnetic excitations with a constant density of states. However, we observe a vanishingly small residual term ${\kappa }_0/T$, which points to the absence of mobile fermionic excitations in these materials. Both the weak temperature dependence of $\kappa$ and the strong magnetic-field dependence of $\kappa$ suggest the significant scattering between the spinons and phonons, which actually supports the existence of a gapless or tiny-gapped quantum spin liquid. Moreover, the $\kappa \left(B\right)/\kappa \left(0\right)$ isotherms show a series of field-induced magnetic transitions for $B\parallel a$, confirming the easy-plane anisotropy, which is consistent with the results of ac magnetic susceptibility. We expect our results to inspire further interests in the understanding of the spinon-phonon coupling in the spin liquid systems.