We employ partially collinear two-dimensional electronic spectroscopy to disentangle precisely the ultrafast dynamics of excitons and free carriers in gallium selenide. Femtosecond temporal and meV energy resolution in the visible spectral region allows us to observe ultrafast bleaching at the exciton resonance with a relaxation time of 112 fs that corresponds to the thermal dissociation of excitons at room temperature, and is orders of magnitude faster than carrier relaxation in the bulk crystal. Our method is applicable to other functional materials, two-dimensional systems, and nanostructures.

Recently, few-layer suspended stoichiometric ${\mathrm{BiFeO}}_3$ flakes were synthesized and found to display large tetragonality and giant dipole moments per unit surface. Little is known, however, about the ground state properties of this compound in the two-dimensional (2D) limit. By performing first-principles electronic structure calculations, we determine the ground state structural, magnetic, and electronic properties of suspended stoichiometric ${\mathrm{BiFeO}}_3$ flakes with number of layers ($n$) ranging from 1 to 4. We show that, even if the orthorhombic flakes with G-type antiferromagnetic order are the most stable ones for $n\le 4$, the metastable cubic ${\mathrm{BiFeO}}_3$ multilayers have physical properties in excellent agreement with experimental data, including enhanced tetragonality and large dipole moments per unit surface in the 2D limit. In these cubic multilayers, the broken inversion symmetry, determined by the different top and bottom terminations, results in a strong Fe offset along the $z$ direction coexisting with metallicity. Our work shows that the cubic phase, nonmagnetic and stable only above $927\mathrm{K}$ in bulk and in thin films, is a “polar” magnetic metal and a type I “multiferroic” in the 2D limit at room temperature.

Boron-based clathrate materials, typically with three-dimensional networks of B atoms, have tunable properties through the substitution of guest atoms, but the tuning of B cages themselves has not yet been developed. By combining a crystal structural search with the laser-heated diamond anvil cell technique, we successfully synthesized a new B-based clathrate boride, ${\mathrm{LaB}}_8$, at $\sim 108\mathrm{GPa}$ and $\sim 2100\mathrm{K}$. The novel structure has a B-richest cage, with 26 B atoms encapsulating a single La atom. ${\mathrm{LaB}}_8$ demonstrates phonon-mediated superconductivity with an estimated transition temperature of 14 K at ambient pressure, mainly originating from the electron-phonon coupling of B cage. The replacement of La with alkaline earth metals can remarkably elevate the transition temperature. This work creates a prototype platform for subsequent investigation on tunable electronic properties through the choice of captured atoms.

We theoretically investigate the effect of superconductivity on the ultrafast electron dynamics in graphene interacting with an ultrashort linearly polarized optical pulse. The optical pulse, with frequency greater than 1 THz, quadratically couples to the Higgs (amplitude) mode and therefore can excite the Higgs-mode oscillations in superconductors, leading to quenching superconducting excitation energy, even in $s$-wave pairing symmetry. Since the duration of the used pulse is less than the electron scattering time in graphene (10–100 fs), the electron dynamics driven by the electric field of pulse remains coherent and is described within the tight-binding model of graphene. We show that the electron-electron and hole-hole transitions from valence to conduction bands are substantially irreversible, with a large residual population of the conduction band, which implies quantum electron dynamics being highly nonadiabatic. Such a feature of the system allows us to clearly project the effect of Higgs oscillations on the electric dipole moment created by the electric field of the pulse between the conduction and valence bands. In particular, we study the impact of the superconducting pair potential on the conduction band population, where the strong pulse causes electronlike and holelike quasiparticles transitions between the conduction and valence bands. The conduction band electron redistribution in honeycomb Dirac points results in almost asymmetric hot spots in two different $K$ and $K^{\prime }$ valleys after the pulse ends, which may be interpreted as the valley polarization effect due to the presence of superconductivity. The redistribution of hot spots around Dirac points is in good agreement with the Higgs-mode oscillations (corresponding to the electric dipole moment pattern), and the number of spots weakly depends on the superconducting gap, whereas it has a strong dependence on the ultrashort-pulse intensity.

Motivated by the recent discoveries of magnets harboring short-pitch skyrmion lattices, we investigate the skyrmion-size dependence of the topological Hall effect. By means of large-scale real-space calculations, we find that the Hall conductivity takes its extreme value in the crossover region where both the real-space and momentum-space Berry curvature play a crucial role. We also investigate how the optimum skyrmion size (${\lambda }_{\mathrm{sk}}^{*}$), which separates the above two regions in the adiabatic region, depends on the lifetime of itinerant electrons ($\tau$) and coupling constant between electrons and localized spins ($J$). For the former we show that ${\lambda }_{\mathrm{sk}}^{*}$ is proportional to $\sqrt{\tau }$, which indicates that ${\lambda }_{\mathrm{sk}}^{*}$ is much less sensitive to $\tau$ than the conventional expectation that ${\lambda }_{\mathrm{sk}}^{*}$ is proportional to the mean-free path $\propto \tau$. For the latter we show that the nonadiabaticity considerably suppresses the topological Hall effect when the timescale determined by the skyrmion size and Fermi velocity is shorter than $1/J$. However, its effect on ${\lambda }_{\mathrm{sk}}^{*}$ is not so significant and ${\lambda }_{\mathrm{sk}}^{*}$ is about ten times the lattice constant in a wide range of $J$ and $\tau$.

We consider magnetic field screening and spontaneous magnetic fields in $s+is$ and $s+id$ superconductors both analytically and numerically. We show that in general, the linearized model couples the moduli of order parameters to the magnetic modes. This causes magnetic field screening that does not follow the standard exponential law and hence cannot be characterized by a single length scale: the London penetration length. We also demonstrate that the resulting linear mixed modes, correctly predict spontaneous fields and their orientation. We show that these mixed modes cause external fields to decay nonmonotonically in the bulk. This is observed as the magnetic field twisting direction, up to an angle of $\pi /2$, as it decays in the nonlinear model. Finally, we demonstrate that there are two nondegenerate domain wall solutions for any given parameter set. These are distinguished by either clockwise or anticlockwise interpolation of the intercomponent phase difference, each producing a different solution for the other fields. However, only domain wall solutions in $s+id$ systems exhibit magnetic field twisting.

Signatures of superconductivity at elevated temperatures above $T_c$ in high-temperature superconductors have been observed near 1/8 hole doping for photoexcitation with infrared or optical light polarized either in the $\mathrm{Cu}{\mathrm{O}}_2$ plane or along the $c$ axis. While the use of in-plane polarization has been effective for incident energies aligned to specific phonons, $c$-axis laser excitation in a broad range between 5 $\mu \mathrm{m}$ and 400 nm was found to affect the superconducting dynamics in striped ${\mathrm{La}}_{1.885}{\mathrm{Ba}}_{0.115}\mathrm{Cu}{\mathrm{O}}_4$, with a maximum enhancement in the $1/\omega$ dependence to the conductivity observed at 800 nm. This broad energy range, specifically 800 nm, is not resonant with any phonon modes, yet induced electronic excitations appear to be connected to superconductivity at energy scales well above the typical gap energies in the cuprates. A critical question is, What can be responsible for such an effect at 800 nm? Using time-dependent exact diagonalization, we demonstrate that the holes in the $\mathrm{Cu}{\mathrm{O}}_2$ plane can be photoexcited into the charge reservoir layers at resonant wavelengths within a multiband Hubbard model. This orbitally selective photoinduced charge transfer effectively changes the in-plane doping level, which can lead to an enhancement of $T_c$ near the 1/8 anomaly.

From the phase diagram proposed by Lei et al. [J. Appl. Phys. 101, 084105 (2007)], ${\mathrm{BaSn}}_{0.15}{\mathrm{Ti}}_{0.85}{\mathrm{O}}_3$ is chosen. It also exhibits a diffuse phase transition between cubic and rhombohedral ($C\text{−}R$) near room temperature. Dielectric analysis confirms a phase transition near room temperature ($T_{\mathrm{C}}\approx 290$ K). In addition, frequency dispersion in the dielectric constant, concomitantly, a loss peak is observed at low temperatures ($T<T_{\mathrm{C}}$). Furthermore, in the similar temperature range, a decrease in remanent polarization is seen upon lowering the temperature. All of these findings point to a reentering phenomenon in the system. First-order reversal curves $\mathit{P}({\mathit{E}}_r,\mathit{E})$ are measured at different temperatures and analyzed to better understand the system's interactions and polarization evolution. It was found that the relaxor nature present in the system is responsible for the reentrant behavior. Local probe techniques such as x-ray absorption near-edge spectroscopy, Raman spectroscopy, and Mössbauer spectroscopy are employed to investigate the local environment changes that are associated with the reentrant behavior. A simple ferroelectric exchange model explaining the low-temperature reentrant behavior is presented from these results.

The local structure of ${\mathrm{V}}_2{\mathrm{O}}_3$, an archetypal strongly correlated electron system that displays a metal-insulator transition around 160 K, has been investigated via pair distribution function (PDF) analysis of neutron and x-ray total scattering data. The rhombohedral-to-monoclinic structural phase transition manifests as an abrupt change on all length scales in the observed PDF. No monoclinic distortions of the local structure are found above the transition, although coexisting regions of phase-separated rhombohedral and monoclinic symmetry are observed between 150 and 160 K. This lack of structural fluctuations above the transition contrasts with the known presence of magnetic fluctuations in the high-temperature state, suggesting that the lattice degree of freedom plays a secondary role behind the spin degree of freedom in the transition mechanism.

We compute the dynamical spin structure factor $S(k,\omega )$ of the SU(3) Heisenberg chain variationally using a truncated Hilbert space spanned by the Gutzwiller projected particle-hole excitations of the Fermi sea, introduced by Dalla Piazza et al. [Nat. Phys. 11, 62 (2015)], with a modified importance sampling. We check the reliability of the method by comparing the $S(k,\omega )$ to exact diagonalization results for 18 sites and to the two-soliton continuum of the Bethe ansatz for 72 sites. We get an excellent agreement in both cases. Detailed analysis of the finite-size effects shows that the method captures the critical Wess-Zumino-Witten SU(3)${}_1$ behavior and reproduces the correct exponent, with the exception of the size dependence of the weight of the bottom of the conformal tower. We also calculate the single-mode approximation for the $\mathrm{SU}\left(N\right)$ Heisenberg model and determine the velocity of excitations. Finally, we apply the method to the SU(3) Haldane-Shastry model and find that the variational method gives the exact wave function for the lowest excitation at $k=\pm 2\pi /3$.

We study the $J_1\text{−}{J}_2\text{−}{J}_3$ classical Heisenberg model with ferromagnetic $J_1$ on the triangular lattice using the nematic bond theory. For parameters where the momentum space coupling function $J_{\vec{q}}$ shows a discrete set of minima, we find that the system in general exhibits a single first-order phase transition between the high-temperature ring liquid and the low-temperature single-$\vec{q}$ planar spiral state. Close to where $J_{\vec{q}}$ shows a continuous minimum, we on the other hand find several phase transitions upon lowering the temperature. Most interestingly, we find an intermediate temperature “arc” regime, where the structure factor breaks rotational symmetry and shows a broad arc-shaped maximum. We map out the parameter region over which this arc regime exists and characterize details of its static structure factor over the same region.

Edge states reveal the nontrivial topology of energy band in the bulk. As localized states at boundaries, many-body edge states may obey a special symmetry that is broken in the bulk. When local particle-particle interaction is induced, they may support a particular property. We consider an extended two-dimensional Su-Schrieffer-Heeger Hubbard model and examine the appearance of $\eta$-pairing states, which are excited eigenstates related to superconductivity. In the absence of Hubbard interaction, the energy band is characterized by topologically invariant polarization in association with edge states. In the presence of on-site Hubbard interaction, $\eta$-pairing edge states appear in the topologically nontrivial phase, resulting in the condensation of pairs at the boundary. In addition, as Hamiltonian eigenstates, the edge states contain paired fermions and unpaired fermions. Neither affects the other; they act as two-fluid states. From numerical simulations of many-body scattering processes, a clear manifestation and experimental detection scheme of topologically protected two-fluid edge states are provided.

We report the results of dc magnetization, thermal expansion, and magnetostriction measurements of the Kondo semiconductor ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$. These results provide a magnetic-field–temperature phase diagram for the three principal axes of the cubic crystal [100], [110], and [111] and reveal the magnetic anisotropy of the three ordered phases A, B, and C. The magnetic anisotropy of the transition temperature $T_{\mathrm{C}}$ from the paramagnetic phase to the C phase is $T_{\mathrm{C}}^{\left[111\right]}>T_{\mathrm{C}}^{\left[110\right]}>T_{\mathrm{C}}^{\left[100\right]}$. At low temperatures, the magnetization has a distinct magnetic anisotropy in high magnetic fields: $M^{\left[100\right]}>M^{\left[110\right]}>M^{\left[111\right]}$. This ratio of magnetization is in good agreement with that of the magnetically anisotropic ${\mathrm{\Gamma }}_{67}$ quartet. The observation indicates that the crystalline electric field ground state is a ${\mathrm{\Gamma }}_{67}$ quartet and that the $c\text{−}f$ hybridization effect is nearly isotropic. In addition, comparison of the present experimental results with numerical calculations of a two-sublattice model using the mean-field approximation suggests that the C phase is a ${\mathrm{\Gamma }}_5$-type antiferroquadrupolar ordered state despite the presence of strong $c\text{−}f$ hybridization effects. In this case, the increase in $T_{\mathrm{C}}$ due to the external field is mainly caused by the ${\mathrm{\Gamma }}_2$-type antiferro-octupolar interaction.

Electrides are a hot research topic due to their special electronic distribution. Recently, the coupling effects among electrides, magnetism, and topological electronic states have attracted considerable attention in condensed-matter physics. Here, based on first-principles calculations, we report an electride phase, namely, ${\mathrm{Ba}}_4{\mathrm{Al}}_5·e^{-}$, with combing antiferromagnetic magnetism and multiple types of nontrivial band crossings locating near the Fermi level. The electride feature in ${\mathrm{Ba}}_4{\mathrm{Al}}_5·e^{-}$ has been evidenced by electron contribution calculations and symmetry analysis. The material is found to show an antiferromagnetic ground state. Especially, the magnetism in the system is provided by the excess electrons in the crystal cavity, which is fundamentally different from the orbital-composed one in traditional magnetic materials. In addition, near the Fermi level ($-0.3$ to 0.3 eV) the material shows several regions of band crossings, which are mainly contributed by excess electrons. These band crossings form different topological states, ranging from the zero-dimensional nodal point to the one-dimensional nodal line and two-dimensional nodal surface. The protection mechanism, the effective model, the surface states, and the impacts of spin-orbital coupling on the topological states are systematically discussed. In addition, ${\mathrm{Ba}}_4{\mathrm{Al}}_5·e^{-}$ has a very low work function and good catalysis on the [001] surface. Our work provides an excellent candidate for studying the potential entanglement of electride states, magnetism, and nontrivial band topology.

The reliable ab initio description of strongly correlated materials is a long-sought capability in condensed matter physics. The combination of $GW$ and extended dynamical mean-field theory (EDMFT) method is a promising scheme which provides a self-consistent description of correlations and screening, and does not require user-provided parameters. In order to test the reliability of this $GW$+EDMFT approach we apply it to the experimentally well characterized perovskite compound ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$, in which a temperature-dependent structural deformation drives a paramagnetic metal-insulator transition. Our results demonstrate that the nonlocal polarization and self-energy components introduced by $GW$ are essential for setting the correct balance between interactions and bandwidths, and that the $GW+\mathrm{EDMFT}$ scheme produces remarkably accurate predictions of the electronic properties of this strongly correlated material.

It has recently been reported that a nonlinear anomalous Nernst current (NANC), induced by Berry curvature near the Fermi surface, can be generated as a second-order response to a longitudinal temperature gradient in a wide variety of time-reversal invariant and noncentrosymmetric materials. So far, NANC in two-dimensional Dirac systems has been reported to be finite only in materials with substantial spin-orbit coupling and titled Dirac cones formed from low-energy Dirac quasiparticles. Here, we prove that NANC can also emerge in two-dimensional Dirac materials even in the complete absence of titled Dirac cones and spin-orbit coupling. It's found that the NANC has a quantum origin from the trigonal warping of the Fermi surface. NANC in both trigonal-warping monolayer and bilayer graphene in the presence of uniaxial strain is theoretically investigated. The magnitude of NANC in trigonal-warping bilayer graphene is comparable to those reported which originated from tilted mechanisms in strained ${\mathrm{MoS}}_2$ and bilayer ${\mathrm{WTe}}_2$.

The ability to engineer nonlinear optical processes in all-dielectric nanostructures is both of fundamental interest and highly desirable for high-performance, robust, and miniaturized nonlinear optical devices. Herein, we propose a paradigm for the efficient tuning of a second-harmonic generation (SHG) process in dielectric nanoantennas by integrating with chalcogenide phase-change material. In a design with ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST) film sandwiched between the AlGaAs nanoantennas and ${\mathrm{AlO}}_x$ substrate, the nonlinear SHG signal from the AlGaAs nanoantennas can be boosted via the resonantly localized field induced by the optically induced Mie-type resonances, and further modulated by exploiting the GST amorphous-to-crystalline phase change in a nonvolatile, multilevel manner. The tuning strategy originates from the modulation of resonant conditions by changes in the refractive index of GST. With a thorough examination of tuning performances for different nanoantenna radii, a maximum modulation depth as high as $540\%$ is numerically demonstrated. This work not only reveals the potential of GST in optical nonlinearity control, but also provides a promising strategy in smart designing of tunable and reconfigurable nonlinear optical devices, e.g., light emitters, modulators, and sensors.

Motivated by the recent experimental realization of twisted trilayer graphene and the observed superconductivity that is associated with its flat bands at specific angles, we study trilayer graphene under the influence of different forms of light in the noninteracting limit. Specifically, we study four different types of stacking configurations with a single twisted layer. In all four cases, we study the impact of circularly polarized light and longitudinal light coming from a waveguide. We derive effective time-independent Floquet Hamiltonians and review light-induced changes to the band structure. For circularly polarized light, we find band flattening effects as well as band gap openings. We emphasize that there is a rich band topology, which we summarize in Chern number maps that are different for all four studied lattice configurations. The case of a so-called ABC stacking with top layer twist is especially rich and shows a different phase diagram depending on the handedness of the circularly polarized light. Consequently, we propose an experiment where this difference in typologies could be captured via optical conductivity measurements. In contrast for the case of longitudinal light that is coming from a waveguide, we find that the band structure is very closely related to the equilibrium one but the magic angles can be tuned in situ by varying the intensity of the incident beam of light.