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.

To understand the physical properties of the series of organic conductors $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]X$ with $X=\mathrm{Cl}$, Br, and I, not only electronic correlations, but also the effect of disorder has to be taken into account. While for Cl- and Br-containing salts the influence of both parameters were investigated and a universal phase diagram was proposed, the position of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{I}$ is still not settled. Here we conducted transport, infrared, and dielectric measurements on single crystals of the title compound to clarify its electronic state at low temperatures. The correlation strength was determined as $U/W\approx 2.2$; thus this salt is placed deeper in an insulating state compared to the two sister compounds. We found that inherent disorder leads to a Coulomb localized insulating state similar to the moderately x-ray-irradiated $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Cu}\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$.

A well-established result in condensed matter physics states that materials crystallizing in symmetry groups containing glide reflection symmetries possess nodal lines on the energy bands. These nodal lines are topologically protected and appear on the fixed planes of the reflection in reciprocal space. In the presence of inversion symmetry, the energy bands are degenerate and the band crossings on the fixed plane may be one-dimensional, or may intersect in points, including the case of empty intersection. In the latter case, the crossing is partially or totally avoided, thus producing lines on reciprocal space where the energy gap may be small, and in the former, the nodal lines will endure, thus producing Dirac or double nodal lines. In addition, if the material crystallizes in a ferromagnetic phase where the glide reflection symmetry is broken, the nodal lines hybridize, thus defining lines in reciprocal space where the energy gap may be small. In this work, we concentrate our efforts on the study of nodal lines that hybridize due to magnetization; we have coined the term of quasinodal lines for those lines in reciprocal space where the energy gap is small (less than what can be detected experimentally $\sim 25$ meV). We study magnetic trifluorides and trioxides which crystallize in magnetic space groups 167.107 and 161.71 and we show the existence of quasinodal lines on these materials. We furthermore show that whenever the quasinodal lines are located around the Fermi level then interesting charge and spin transport effects are induced and can be used to detect experimentally these lines. Of particular interest are the half-metallic ferromagnetic phases of ${\mathrm{PdF}}_3$ and ${\mathrm{LiCuF}}_3$ where the large signal of the anomalous Hall conductance is due to the presence of the quasinodal lines on the Fermi level.

We investigate the electronic structure of the new family of kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{K},\mathrm{Rb},\mathrm{Cs})$ using first-principles calculations. We analyze systematically the evolution of the van Hove singularities (vHs's) across the entire family upon applied pressure and hole doping, specifically focusing on the two vHs's closer to the Fermi energy. With pressure, these two saddle points shift away from the Fermi level. At the same time, the Fermi surface undergoes a large reconstruction with respect to the Sb bands while the V bands remain largely unchanged, pointing to the relevant role of the Sb atoms in the electronic structure of these materials. Upon hole doping, we find the opposite trend, where the saddle points move closer to the Fermi level for increasing dopings. All in all, we show how pressure and doping are indeed two mechanisms that can be used to tune the location of the two vHs's closer to the Fermi level, and that they can be exploited to tune different Fermi surface instabilities and associated orders.

Bilayer graphene exhibits outstanding characteristics that can be modified by adjusting the twist angle between two layers. An exact ${30}^{\circ }$-twisted bilayer graphene forms a material comprising two-dimensional quasicrystals accompanied by a relativistic Dirac fermion. In this study, the atomic arrangements of quasicrystal bilayer graphene on a SiC(0001) substrate are identified using positron diffraction. The interlayer distance in quasicrystal bilayer graphene is determined to be 3.46 Å, revealing an expansion of 0.17 Å as compared with that of Bernal-stacked bilayer graphene. This result provides important insights for elucidating the origin of the magnitude of coupling between graphene layers.

Discrete time crystals (DTCs) are nonequilibrium phases of matter with exotic observable dynamics. Among their remarkable features is their response to a periodic drive at a fraction of its frequency. Current successful experiments are, however, only limited to realizing DTCs with period-doubling and period-tripling observable dynamics, forming only a very small subset of DTC phases. Creating larger periodic DTCs in the laboratory remains a longstanding challenge, yet it is necessary for developing the technological applications of DTCs, e.g., as a quantum memory for highly entangled qubits, or exploring interesting features beyond subharmonic dynamics, e.g., condensed matter phenomena in the time domain. By highlighting the connection between DTCs and quantum error correction, we devise a general and realistic scheme for building DTCs exhibiting any large-period observable dynamics, which are observable even at sufficiently small system sizes. Our proposal uses an array of spin-1/2 chains to simulate a repetition code at the hardware level, which has essential properties to realize robust observable dynamics. It is readily implemented with existing superconducting or trapped-ion quantum processors, making new families of DTCs experimentally accessible in the immediate future.

We develop a theory for manipulating the effective band structure of interacting helical edge states realized on the boundary of two-dimensional time-reversal symmetric topological insulators. For a sufficiently strong interaction, an interacting edge band gap develops, spontaneously breaking time-reversal symmetry on the edge. The resulting spin texture, as well as the energy of the time-reversal breaking gaps, can be tuned by an external moiré potential (i.e., a superlattice potential). Remarkably, we establish that by tuning the strength and period of the potential, the interacting gaps can be fully suppressed and interacting Dirac points reemerge. In addition, nearly flat bands can be created by the moiré potential with a sufficiently long period. Our theory provides an unprecedented way to enhance the coherence length of interacting helical edges by suppressing the interacting gap. The implications of this finding for ongoing experiments on helical edge states is discussed.

We report the magnetic-field dependence of the THz absorption and nonreciprocal directional dichroism spectra of $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ measured on the three principal crystal cuts for fields applied along the three principal directions of each cut. From the systematic study of the light polarization dependence, we deduced the optical selection rules of the spin-wave excitations. Our THz data, combined with small-angle neutron scattering results showed that (i) an in-plane magnetic field rotates the $\mathbf{q}$ vectors of the cycloids perpendicular to the magnetic field and (ii) the selection rules are mostly determined by the orientation of the $\mathbf{q}$ vector with respect to the electromagnetic fields. We observed a magnetic field history-dependent change in the strength and the frequency of the spin-wave modes, which we attributed to the change of the orientation and the length of the cycloidal $\mathbf{q}$ vector, respectively. Finally, we compared our experimental data with the results of linear spin-wave theory that reproduces the magnetic-field dependence of the spin-wave frequencies and most of the selection rules, from which we identified the spin-polarization coupling terms relevant for the optical magnetoelectric effect.

The spin-noise spectroscopy (SNS) method implies high efficiency of conversion of the spin-system magnetization to the Faraday rotation angle. Generally, this efficiency cannot be estimated using the characteristics of the regular magnetooptical activity of a paramagnet. However, it may be drastically enhanced in systems with strong inhomogeneous broadening of the optical transitions. This enhancement leads to the giant spin-noise gain effect and previously allowed one to apply the SNS to rare-earth activated crystals. We show that the nonlinear resonant Faraday effect can be used to measure the homogeneous width of the inhomogeneously broadened transition and, thus, to estimate the applicability of the SNS to this type of paramagnet. We present the theoretical description of the effect and perform measurements on intraconfigurational ($4f\text{−}4f$) transitions of the trivalent rare-earth ions of neodymium and ytterbium in fluorite-based crystals. The proposed experimental approach establishes new links between the effects of nonlinear optics and spin-noise characteristics of crystals with paramagnetic impurities and offers new methods of research in the physics of impurity crystals.

Recent experiments have discovered multipolar orders in a variety of $d$-orbital Mott insulators. Motivated by uncovering the exchange interactions which underlie octupolar order proposed in osmate double perovskites, we study a two-site model using exact diagonalization on a five-orbital Hamiltonian, incorporating spin-orbit coupling and interactions, and including both intraorbital and interorbital hopping. Using an exact Schrieffer-Wolff transformation, we then extract an effective pseudospin Hamiltonian for the non-Kramers doublets, uncovering dominant ferrooctupolar coupling driven by the interplay of two distinct intraorbital hopping terms. Using classical Monte Carlo simulations on the face-centered cubic lattice, we obtain a ferrooctupolar transition temperature which is in good agreement with experiments on osmate double perovskites. We also explore the impact of uniaxial strain and dimensional tuning via ultrathin films, which are shown to induce a transverse field on the Ising octupolar order. This suppresses $T_c$ and potentially allows one to access octupolar Ising quantum critical points. We discuss possible implications of our results for a broader class of materials which may host such non-Kramers doublet ions.

We show how measuring real space properties such as the charge density in a quasiperiodic system can be used to gain insight into their topological properties. In particular, for the Fibonacci chain, we show that the total on-site charge oscillates when plotted in the appropriate coordinates, and the number of oscillations is given by the topological label of the gap in which the Fermi level lies. We show that these oscillations have two distinct interpretations, obtained by extrapolating results from the two extreme limits of the Fibonacci chain—the valence bond picture in the strong modulation limit, and perturbation around the periodic chain in the weak modulation limit. This effect is found to remain robust at moderate interactions, as well as in the presence of disorder. We conclude that experimental measurement of the real space charge distribution can yield information on topological properties in a straightforward way.

We present an electronic circuit whose dynamical properties emulate those of a resistively and capacitively shunted Josephson junction. We show how it reproduces the switching properties of a shunted junction and its dependence on the quality factor. A thermal noise source is then used to characterize the temperature dependence of the phase dynamics. In the presence of an AC drive, phase locking is observed at integer and rational multiples of the drive frequency, and it competes with chaotic behavior when the quality factor of the junction exceeds unity. We characterize the stability of phase-locked and chaotic states in the presence of thermal noise.

We report on the experimental study of phonon properties and electron-phonon scattering in thin superconducting NbTiN films, which are intensively exploited in various applications. Studied NbTiN films with sub-10-nm thicknesses are disordered with respect to electron transport, the Ioffe-Regel parameter of $k_F{l}_e=2.5–3.0$ ($k_F$ is the Fermi wave vector, and $l_e$ is the electron mean free path), the inelastic electron-phonon interaction, and the product $q_T{l}_e\ll 1$ ($q_T$ is the wave vector of a thermal phonon). By means of magnetoconductance and photoresponse techniques, we derive the inelastic electron-phonon scattering rate $1/{\tau }_{e\text{-ph}}$ and determine sound velocities and phonon heat capacities. In the temperature range from 12 to 20 K, the scattering rate varies with temperature as $1/{\tau }_{e\text{-ph}}\propto T^{3.45\pm 0.05}$; its value extrapolated to 10 K amounts to approximately 1/16 ps. Making a comparative analysis of our films and other films used in superconducting devices, such as polycrystalline granular NbN and amorphous WSi, we find a systematic reduction of the sound velocity in all these films by about 50% compared to the corresponding bulk crystalline materials. A corresponding increase in the phonon heat capacities in all these films is, however, less than the Debye model predicts. We attribute these findings to reduced film dimensionality and film morphology.

We report a high-frequency/high-magnetic field electron spin resonance (HF-ESR) spectroscopy study in the sub-THz frequency domain of the two representatives of the family of magnetic topological insulators $\left({\mathrm{MnBi}}_2{\mathrm{Te}}_4\right){\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_n$ with $n=0$ and 1. The HF-ESR measurements in the magnetically ordered state at a low temperature of $T=4\mathrm{K}$ combined with the calculations of the resonance modes showed that the spin dynamics in ${\mathrm{MnBi}}_{\text{4}}{\mathrm{Te}}_{\text{7}}$ is typical for an anisotropic easy-axis type ferromagnet whereas ${\mathrm{MnBi}}_{\text{2}}{\mathrm{Te}}_{\text{4}}$ demonstrates excitations of an anisotropic easy-axis type antiferromagnet. However, by applying the field stronger than a threshold value $\sim 6\mathrm{T}$, we observed in ${\mathrm{MnBi}}_{\text{2}}{\mathrm{Te}}_{\text{4}}$ a crossover from the antiferromagnetic (AFM) resonance modes to the ferromagnetic (FM) modes, whose properties are very similar to the FM response of ${\mathrm{MnBi}}_{\text{4}}{\mathrm{Te}}_{\text{7}}$. We attribute this remarkably unusual effect unexpected for a canonical easy-axis antiferromagnet, which, additionally, can be accurately reproduced by numerical calculations of the excitation modes, to the closeness of the strength of the AFM exchange and magnetic anisotropy energies which appears to be a very specific feature of this compound. Our experimental data evidences that the spin dynamics of the magnetic building blocks of these compounds, the Mn-based septuple layers (SLs), is inherently ferromagnetic, featuring persisting short-range FM correlations far above the magnetic ordering temperature as soon as the SLs get decoupled either by introducing a nonmagnetic quintuple interlayer, as in ${\mathrm{MnBi}}_{\text{4}}{\mathrm{Te}}_{\text{7}}$, or by applying a moderate magnetic field, as in ${\mathrm{MnBi}}_{\text{2}}{\mathrm{Te}}_{\text{4}}$, which may have an effect on the surface topological band structure of these compounds.

We report basic physical properties of the quasi-one-dimensional heavy-fermion compound $\mathrm{Yb}{\mathrm{Fe}}_5{\mathrm{P}}_3$. Its magnetic susceptibility follows a modified Curie-Weiss behavior at high temperatures with a Weiss temperature between $-5$ and $-21$ K depending on the direction of the applied magnetic field. Our single crystals of $\mathrm{Yb}{\mathrm{Fe}}_5{\mathrm{P}}_3$ are good metals with a room-temperature resistivity of $\sim 140$ $\mu \mathrm{\Omega }$ cm and a residual resistivity of less than 2 $\mu \mathrm{\Omega }$ cm. Below $\sim 2$ K the resistivity reflects the presence of strong quantum fluctuations, which is supported by specific heat measurements that show a similar strong enhancement in $C/T$ at low temperatures reaching 1.5 J/mol ${\mathrm{K}}^2$ below $T=0.5$ K. No magnetic ordering is observed down to 80 mK. Magnetic fields up to 8 T rapidly suppress the magnetic fluctuations, while resistivity measurements under pressures up to 2.54 GPa indicate the enhancement of quantum fluctuations, although still no order is observed down to 0.3 K. Density functional theory calculations suggest the presence of electronically quasi-one-dimensional (1D) Fermi surface sheets together with three-dimensional electronic pockets. We suggest that the quasi-1D nature of this compound leads to an extended regime of large quantum fluctuations within the paramagnetic state prior to reaching a magnetic instability.

We use density functional theory methods to study the electronic structures of a series of $s\text{−}p$ cubic perovskites, $AB{X}_3$: the experimentally available $\mathrm{Sr}\mathrm{Bi}{\mathrm{O}}_3, \mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3, \mathrm{Ba}\mathrm{Sb}{\mathrm{O}}_3, \mathrm{Cs}\mathrm{Tl}{\mathrm{F}}_3$, and $\mathrm{Cs}\mathrm{Tl}{\mathrm{Cl}}_3$, as well as the hypothetical $\mathrm{Mg}{\mathrm{PO}}_3, \mathrm{Ca}\mathrm{As}{\mathrm{O}}_3, \mathrm{Sr}\mathrm{Sb}{\mathrm{O}}_3$, and $\mathrm{Ra}\mathrm{Mc}{\mathrm{O}}_3$. We use tight-binding modeling to calculate the interatomic hopping integrals $t_{sp\sigma }$ between the $Bs$ and $Xp$ atomic orbitals and charge-transfer energies $\mathrm{\Delta }$, which are the two most important parameters that determine the low-energy electron and hole states of these systems. Our calculations elucidate several trends in $t_{sp\sigma }$ and $\mathrm{\Delta }$ as one moves across the periodic table, such as the relativistic energy lowering of the $Bs$ orbital in heavy $B$ cations, leading to strongly negative $\mathrm{\Delta }$ values. Our results are discussed in connection with the general phase diagram for $s\text{−}p$ cubic perovskites proposed by Khazraie et al. [Phys. Rev. B 98, 205104 (2018)], who find the parent superconductors $\mathrm{Sr}\mathrm{Bi}{\mathrm{O}}_3$ and $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_3$ to be in the regime of negative $\mathrm{\Delta }$ and large $t_{sp\sigma }$. Here, we explore this further and search for different materials with similar parameters, which could lead to the discovery of new superconductors. Also, some considerations are offered regarding a possible relation between the physical properties of a given $s\text{−}p$ compound (such as its tendency to bond disproportionate and the maximal achievable superconducting transition temperature) and its electronic structure.

The effective electron-electron interaction in electron gas depends on both the density and spin local field factors. Variational diagrammatic quantum Monte Carlo calculations of the spin local field factor are reported. These are used together with the charge local field factor from previous diffusion quantum Monte Carlo calculations to quantitatively present the full effective spin-dependent electron-electron interaction in three-dimensional electron gas. Very simple quadratic formulas are presented for the local field factors that quantitatively produce all of the response functions of the electron gas at metallic densities. Exchange and correlation become increasingly important at low densities. At the compressibility divergence at $r_s=5.25$, both the direct (screened Coulomb) term and the charge-dependent exchange term in the electron-electron interaction at $q=0$ are separately divergent. However, due to large cancellations, their difference is finite, well behaved, and much smaller than either term separately. As a result, the spin contribution to the electron-electron interaction becomes an important factor. The static electron-electron interaction is repulsive as a function of density but is less repulsive for electrons with parallel spins. The effect of allowing a deformable, rather than rigid, positive background is shown to be as quantitatively important as exchange and correlation. As a simple concrete example, the electron-electron interaction is calculated using the measured bulk modulus of the alkali metals with a linear phonon dispersion. The net electron-electron interaction in lithium is attractive for wave vectors $0-2k_F$, which suggests superconductivity, and is mostly repulsive for the other alkali metals.