Probing the radial collective oscillation of a trapped quantum system is an accurate experimental tool to investigate interactions and dimensionality effects. We consider a fully polarized quasi-one-dimensional dipolar quantum gas of bosonic dysprosium atoms in a parabolic trap at zero temperature. We model the dipolar gas with an effective quasi-one-dimensional Hamiltonian in the single-mode approximation and derive the equation of state using a variational approximation based on the Lieb-Liniger gas Bethe ansatz wave function or perturbation theory. We calculate the breathing mode frequencies while varying polarization angles by a sum-rule approach and find they are in good agreement with recent experimental findings.

A number of moiré graphene systems have nearly flat topological bands where electron motion is strongly correlated. Though microscopically these systems are only quasiperiodic, they can typically be treated as translation invariant to an excellent approximation. Here we reconsider this question for magic angle twisted bilayer graphene that is nearly aligned with a hexagonal boron nitride (hBN) substrate. We carefully study the effect of the periodic potential induced by hBN on the low energy physics. The combination of this potential and the moiré lattice produced by the twisted graphene generates a quasiperiodic term that depends on the alignment angle between hBN and the moiré graphene. We find that the alignment angle has a significant impact on both the band gap near charge neutrality and the behavior of electrical transport. We also introduce and study toy models to illustrate how a quasiperiodic potential can give rise to localization and change in transport properties of topological bands.

Indium tin oxide is often used as an active material in photonics. Strong correlation between its optical response and a state of conduction electrons, as well as an intermediate density of charges typical for semiconductors, makes it highly tunable using an external electric field. This property is extensively used in various types of electro-optical modulators. Usually for infrared switching the charge concentration is manipulated in order to reach the epsilon-near-zero regime, where the real part of permittivity is small and losses are high. This allows one to switch between an optically transparent state and a highly absorbing state. In this paper we investigate the possibility of a different switching mechanism, namely, between the state where the propagation of surface plasmons is possible and another state where surface waves are forbidden. To obtain modes with high quality factors, which is challenging in indium tin oxide, we suggest using odd pairs of surface plasmons supported by thin films. The accumulation layer, formed at the boundary between indium tin oxide and the insulator under the influence of an external voltage, is considered as a tunable plasmonic waveguide. Both analytical and numerical analysis is used, first, to localize the regime where such modes are possible and, second, to compute the characteristics of corresponding plasmons in realistic conditions. Drift-diffusion equations and Maxwell equations are solved numerically in a self-consistent way to handle electronic and optical aspects of the problem. High tunability of plasmonic modes is demonstrated in infrared. The obtained results can be implemented in the new generation of plasmonic modulators and electro-optical circuits.

Artificial nanostructures enable fine control of electromagnetic fields at the nanoscale, a possibility that has recently been extended to the interaction between polarised light and chiral matter. The theoretical description of such interactions, and its application to the design of optimised structures for chiroptical spectroscopies, brings new challenges to the common set of tools used in nano-optics. In particular, chiroptical effects often depend crucially on the relative orientation of the scatterer and the incident light, but many experiments are performed with randomly oriented scatterers, dispersed in a solution. We derive new expressions for the orientation-averaged local degree of optical chirality of the electromagnetic field in the presence of a nanoparticle aggregate. This is achieved using the superposition $T$-matrix framework, ideally suited for the derivation of efficient orientation-averaging formulas in light scattering problems. Our results are applied to a few model examples and illustrate several nonintuitive aspects in the distribution of orientation-averaged degree of chirality around nanostructures. The results will be of significant interest for the study of nanoparticle assemblies designed to enhance chiroptical spectroscopies, and where the numerically efficient computation of the averaged degree of optical chirality enables a more comprehensive exploration of the many possible nanostructures.

We report on the emergence of bulk, valley-polarized currents in graphene-based devices, driven by spatially varying regions of broken sublattice symmetry, and revealed by nonlocal resistance ($R_{\mathrm{NL}}$) fingerprints. By using a combination of quantum transport formalisms, giving access to bulk properties as well as multiterminal device responses, the presence of a nonuniform local band gap is shown to give rise to valley-dependent scattering and a finite Fermi-surface contribution to the valley Hall conductivity, related to characteristics of $R_{\mathrm{NL}}$. These features are robust against disorder and provide a plausible interpretation of controversial experiments in graphene/hexagonal boron nitride superlattices. Our findings suggest both an alternative mechanism for the generation of valley Hall effect in graphene and a route towards valley-dependent electron optics, by materials and device engineering.

A two-dimensional electron gas at the interfaces of perovskite oxides has unfolded various emergent phenomena. In this work, we fabricate the conducting interface between $\mathrm{La}{\mathrm{VO}}_3$ (LVO) and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ (STO) with two different orientations, (001) and (111), employing a pulsed laser deposition system. A signature of weak antilocalization is observed in the (111)-oriented LVO/STO heterostructure which was not present in the (001) heterostructure. We report the observation of a planar Hall effect (PHE) and anisotropic magnetoresistance (AMR) for both heterointerfaces. The AMR and PHE are measured by driving a current $I$ in the plane of the interface and applying an external magnetic field $B$ in the same plane. The angular dependence (angle $\varphi$ between $B$ and $I$) of AMR and PHE in both cases is observed to be sensitive to applied magnetic field and temperature. The (001)-oriented heterointerface shows larger AMR ($\sim 60\%$) than (111) (<10%), which is the highest among previously reported oxide heterostructures. The PHE shows twofold symmetry as a function of $\varphi$ for both interfaces, and the symmetry remains for all the magnetic field values. In contrast, the AMRs for (001) and (111) have different symmetries. At the same time, they have strong dependence on $B$. A detailed analysis of our results and density functional theory calculations suggests that the origin of these oscillations (AMR and PHE) observed in (001)- and (111)-oriented LVO/STO interfaces can be linked to their unique Fermi surface reconstruction due to its orbital occupancy and polarization/hybridization.

A very large negative magnetoresistance (LNMR) is observed in the insulating regime of the antiferromagnet ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$ when a magnetic field is applied perpendicular to the direction of the sublattice magnetization. A high perpendicular magnetic field eventually suppresses the insulating behavior and allows ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$ to reenter a metallic state. This effect is seemingly unrelated to any field-induced magnetic phase transition, as measurements of magnetic susceptibility and specific heat did not find any anomaly as a function of magnetic fields at temperatures above $2\mathrm{K}$. The LNMR appears in both current-in-plane and current-out-of-plane settings, and Hall effects suggest that its origin lies in an extreme sensitivity of conduction processes of holelike carriers to the infinitesimal field-induced canting of the sublattice magnetization. The LNMR-induced metallic state may thus be associated with the breaking of the antiferromagnetic parity-time symmetry by perpendicular magnetic fields and/or the intricate multiorbital electronic structure of ${\mathrm{BaMn}}_2{\mathrm{Bi}}_2$.

We report de Haas–van Alphen (dHvA) quantum oscillation studies on a layered compound $\mathrm{Lu}{\mathrm{Sn}}_2$. Through the analyses of the dHvA oscillation data, we find this material has several 3D bands and one quasi-2D band hosting relativistic fermions. Compared to previously reported dHvA oscitations in $\mathrm{Y}{\mathrm{Sn}}_{2,}$, the oscillation component arising from the quasi-2D band is significantly enhanced. From structural analyses using single-crystal x-ray diffraction, we find the distorted Sn-square net layer is less corrugated than $\mathrm{Y}{\mathrm{Sn}}_{2,}$, which accounts for the enhancement of two dimensionality of the relativistic fermions created in this layer. This result suggests that the dimensionality of relativistic band in $\mathrm{RE}{\mathrm{Sn}}_2$ ($\mathrm{RE}=\mathrm{rare}$ earth) can be tuned by electronegativity of RE. Moreover, we also find the 3D relativistic bands are pushed closer to the Fermi level with respect to $\mathrm{Y}{\mathrm{Sn}}_{2,}$ due to enhanced spin-orbital coupling. These findings imply $\mathrm{RE}{\mathrm{Sn}}_2$ can be an interesting platform for seeking new topological states via the tuning of electronegativity, spin-orbital coupling, and magnetism.

Theoretical studies have shown that Majorana bound states can be induced at the ends of a one-dimensional wire, a phenomenon possible due to the interplay between $s$-wave superconductivity, spin-orbit coupling, and an external magnetic field. These states have been observed in superconductor-semiconductor hybrid nanostructures in the presence of a Zeeman field, and in the limit of a low density of particles. In this paper, we demonstrate and discuss the possibility of the emergence of Majorana bound states in a superconducting Rashba nanowire deposited on an antiferromagnetically ordered surface. We calculate the relevant topological invariant in several complementary ways. Studying the topological phase diagram reveals two branches of the nontrivial topological phase—the main branch, which is typical for Rashba nanowires, and an additional branch emerging due to the antiferromagnetic order. In the case of the additional topological branch, Majorana bound states can also exist close to half-filling, obviating the need for either doping or gating the nanowire to reach the low-density regime. Moreover, we show the emergence of the Majorana bound states in the absence of the external magnetic field, which is possible due to the antiferromagnetic order. We also discuss the properties of the bound states in the context of real-space localization and the spectral function of the system. This allows one to perceive the band inversion within the spin and sublattice subspaces in the additional branch, contrary to the main branch, where the only band inversion reported in previous studies exists in the spin subspace. Finally, we demonstrate how these topological phases can be confirmed experimentally in transport measurements.

Single-atom-thick two-dimensional material containing $3d$ transition metal was rarely reported in the past. Based on the first principles method, we predict that the cobalt carbonitrides ${\mathrm{CoN}}_4{\mathrm{C}}_{10}$, ${\mathrm{Co}}_2{\mathrm{N}}_8{\mathrm{C}}_6$, and ${\mathrm{Co}}_2{\mathrm{N}}_6{\mathrm{C}}_6$ are a new class of two-dimensional (2D) materials, which are made up of two components: the ${\mathrm{CoN}}_4$ unit and the graphene fragment unit, arranged in a planar pattern. The stability of the ${\mathrm{CoN}}_4{\mathrm{C}}_{10}$, ${\mathrm{Co}}_2{\mathrm{N}}_8{\mathrm{C}}_6$, and ${\mathrm{Co}}_2{\mathrm{N}}_6{\mathrm{C}}_6$ monolayers is demonstrated by the formation energy and phonon spectra calculations, as well as the molecular dynamics simulations. The spin-polarized calculations reveal that all three compounds are magnetic metals, and the electronic states near Fermi energy are dominated by Co $3d$ electronic states. Moreover, there exist a few sharp peaks of density of state in the vicinity of Fermi energy, deriving from the $d_{z^2}$ orbitals of Co atom. Thus, the graphenelike transition-metal carbonitrides are not only a new class of 2D materials with single-atom-thickness but also have rich electronic properties due to the incorporation of transition metal.

We report the observation of the resonant excitation of edge photocurrents in bilayer graphene subjected to terahertz radiation and a magnetic field. The resonantly excited edge photocurrent is observed for both interband (at low carrier densities) and intraband (at high densities) transitions between Landau levels (LL). While the intraband LL transitions can be traced to the classical cyclotron resonance (CR) and produce strong resonant features, the interband-LL resonances have quantum nature and lead to the weaker features in the measured photocurrent spectra. The magnitude and polarization properties of the observed features agree with the semiclassical theory of the intraband edge photogalvanic effect, including its Shubnikov-de-Haas oscillations at low temperatures.

Generating nonclassical states in macroscopic systems is a long-standing challenge. A promising platform in the context of this quest are novel hybrid systems based on magnetic dielectrics, where photons can couple strongly and coherently to magnetic excitations, although a nonclassical state therein is yet to be observed. We propose a scheme to generate a magnetization cat state, i.e., a quantum superposition of two distinct magnetization directions, using a conventional setup of a macroscopic ferromagnet in a microwave cavity. Our scheme uses the ground state of an ellipsoid shaped magnet, which displays anisotropic quantum fluctuations akin to a squeezed vacuum. The magnetization collapses to a cat state by either a single photon or a parity measurement of the microwave cavity state. We find that a cat state with two components separated by $\sim 5\hslash$ is feasible and briefly discuss potential experimental setups that can achieve it.

Electron-phonon interaction plays an important role in metals and can lead to superconductivity and other instabilities. Previous theoretical studies on superconductivity are largely based on the Migdal-Eliashberg theory, which neglects all the vertex corrections to electron-phonon coupling and breaks down in many unconventional superconductors. Here, we go beyond the Migdal-Eliashberg approximation and develop a nonperturbative Dyson-Schwinger equation approach to deal with the superconducting transition. Remarkably, we take into account all the vertex corrections by solving two coupled Ward-Takahashi identities derived from two global U(1) symmetries and rigorously prove that the fully renormalized electron propagator satisfies a self-closed integral equation that is directly amenable to numerical computations. Our approach works equally well in the weak and strong coupling regimes and provides an efficient method to determine superconducting $T_c$ and other quantities. As an application, our approach is used to investigate the high-$T_c$ superconductivity in one-unit-cell FeSe/${\mathrm{SrTiO}}_3$.

Critical properties and anisotropic magnetocaloric effect of hexagonal ferromagnet $\mathrm{Pr}\mathrm{Cr}{\mathrm{Ge}}_3$ single crystals were investigated. The critical exponents $\beta =0.352$ with a critical temperature $T_C=90.5\mathrm{K}$ and $\gamma =1.026$ with $T_C=90.6\mathrm{K}$ are obtained by the modified Arrott plot, whereas $\delta =3.98$ is deduced by a critical isotherm analysis. The critical exponents suggest a long-range magnetic coupling with the exchange distance decaying as $J\left(r\right)\sim r^{-4.6}$. Furthermore, the magnetic entropy change $-\mathrm{\Delta }{S}_M$ reaches a maximum value around $T_C$, i.e., $-\mathrm{\Delta }{S}_M\sim 3.2\left(1.8\right)\mathrm{J}{\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ with in-plane (out-of-plane) field change of 5 T, verifying large magnetic anisotropy. The rescaled $-\mathrm{\Delta }{S}_M(T,H)$ plots collapse onto a universal curve independent of temperature and field, confirming a second-order magnetic phase transition and reliability of the obtained critical exponents.

We report results of magnetization and ${}^{31}\mathrm{P}$ NMR measurements under high pressure up to 6.4 GPa on ${\mathrm{RbMoOPO}}_4\mathrm{Cl}$, which is a frustrated square-lattice antiferromagnet with competing nearest-neighbor and next-nearest-neighbor interactions. Anomalies in the pressure dependencies of the NMR shift and the transferred hyperfine coupling constants indicate a structural phase transition at 2.6 GPa, which is likely to break mirror symmetry and triggers significant change of the exchange interactions. In fact, the NMR spectra in magnetically ordered states reveal a change from the columnar antiferromagnetic (CAF) order below 3.3 GPa to the Néel antiferromagnetic (NAF) order above 3.9 GPa. The spin lattice relaxation rate $1/T_1$ also indicates a change of dominant magnetic fluctuations from CAF-type to NAF-type with pressure. Although the NMR spectra in the intermediate pressure region between 3.3 and 3.9 GPa show coexistence of the CAF and NAF phases, a certain component of $1/T_1$ shows paramagnetic behavior with persistent spin fluctuations, leaving a possibility for a quantum disordered phase. The easy-plane anisotropy of spin fluctuations with unusual nonmonotonic temperature dependence at ambient pressure gets reversed to the Ising anisotropy at high pressures. This unexpected anisotropic behavior for a spin 1/2 system may be ascribed to the strong spin-orbit coupling of Mo-$4d$ electrons.

We performed high-magnetic-field ultrasonic experiments on ${\mathrm{YbB}}_{12}$ up to 59 T to investigate the valence fluctuations in Yb ions. In zero field, the longitudinal elastic constant $C_{11}$, the transverse elastic constants $C_{44}$ and $\left(C_{11}-C_{12}\right)/2$, and the bulk modulus $C_{\mathrm{B}}$ show a hardening with a change of curvature at around 35 K indicating a small contribution of valence fluctuations to the elastic constants. When high magnetic fields are applied at low temperatures, $C_{\mathrm{B}}$ exhibits a softening above a field-induced insulator-metal transition signaling field-induced valence fluctuations. Furthermore, at elevated temperatures, the field-induced softening of $C_{\mathrm{B}}$ takes place at even lower fields and $C_{\mathrm{B}}$ decreases continuously with field. Our analysis using the multipole susceptibility based on a two-band model reveals that the softening of $C_{\mathrm{B}}$ originates from the enhancement of multipole-strain interaction in addition to the decrease of the insulator energy gap. This analysis indicates that field-induced valence fluctuations of Yb cause the instability of the bulk modulus $C_{\mathrm{B}}$.

We investigated the mixed-stack charge-transfer crystal, N,N'-dimethylphenazine-TCNQ (${\mathrm{M}}_2\mathrm{P}$-TCNQ), which is polar at room temperature and just at the neutral-to-ionic interface (ionicity $\rho \approx 0.5$). We detect the typical dielectric signature of a relaxor ferroelectric and an asymmetric positive-up-negative-down behavior. While relaxor ferroelectricity is usually ascribed to disorder in the crystal, we find no evidence for structural disorder in the investigated crystals. To elucidate the origin of ${\mathrm{M}}_2\mathrm{P}$-TCNQ's dielectric properties we perform parallel structural and spectroscopic measurements, associated with theoretical modeling and quantum-mechanical calculations. Our combined effort points to a highly polarizable electronic system that is strongly coupled to lattice vibrations. The found indications for polarization reversal imply flipping of the bent conformation of the ${\mathrm{M}}_2\mathrm{P}$ molecule with an associated energy barrier of a few tens of an eV, broadly consistent with an Arrhenius fit of the dielectric relaxation times. While the polarization is mostly of electronic origin, its possible reversal implies slow collective motions that are affected by solid-state intermolecular interactions.

We provide a combined theoretical and experimental study of the electronic structure and the optical absorption edge of the orthorhombic perovskite $\mathrm{La}\mathrm{In}{\mathrm{O}}_3$, employing density functional theory and many-body perturbation theory. We find the lowest-energy excitation at 0.2 eV below the fundamental gap (5 eV), reflecting a sizable electron-hole attraction. Since the transition from the valence band maximum ($\mathrm{\Gamma }$ point) is, however, dipole forbidden, the onset is characterized by weak excitations from transitions around it. The first intense excitation appears about 0.32 eV above. Interestingly, this value coincides with an experimental value obtained by ellipsometry (4.80 eV) which is higher than the onset from optical absorption spectroscopy (4.35 eV). The latter discrepancy is attributed to the fact that the weak transitions that define the optical gap are not well enough resolved by the ellipsometry measurement. Through temperature-dependent measurements of the optical gap, we assess renormalization effects by electron-phonon coupling, enhancing the quantitative comparison between theoretical and experimental results.