We present a dynamical mean-field study of two-particle dynamical response functions in a two-band Hubbard model across several phase transitions. We observe that the transition between the excitonic condensate and spin-state ordered state is continuous with a narrow strip of supersolid phase separating the two. Approaching transition from the excitonic condensate is announced by softening of the excitonic mode at the $M$ point of the Brillouin zone. Inside the spin-state ordered phase there is a magnetically ordered state with $2\times 2$ periodicity, which has no precursor in the normal phase.

Using continuum model calculations within the framework of the $\mathit{k}·\mathit{p}$ approach and effective mass approximation, we theoretically study the physical properties of the hybrid exciton state in type-II heterostructures composed of two-dimensional monolayer transition metal dichalcogenides (2D TMDs) and zero-dimensional (0D) CdSe-ZnS core-shell quantum dots (QDs). Interactions between an electron located in the lowest unoccupied molecule orbital (LUMO) state of the QD and a hole in the valence band of the 2D TMD along with their effects on charge distribution and exciton binding energy are self-consistently calculated via solving the Schrödinger and Poisson equations. We show that (i) the hole wave function of the 2D TMD is tightly bounded within several unit cells by an electron located in the LUMO state of the QD while the distribution of the electron wave function is marginally changed by the Coulomb attraction from the hole in the 2D TMD; (ii) the binding energies of the hybrid exciton decrease from 110 to 25 meV with the radius of the CdSe core increasing from 2 to 8 nm, which is several times lower than that of the exciton of 2D TMDs; and (iii) the exciton binding energies of heterostructures composed by different 2D TMD materials are similar. We explain the size dependence of exciton binding energy as the result of the delocalization of the electron wave function with QD size and attribute the similar binding energy obtained in different 2D/0D heterostructures to the weak dielectric screening effect of different 2D TMDs on electron-hole interaction. These results indicate that the physical properties of a hybrid exciton in 2D/0D heterostructures is dominated by the properties of the QD instead of the 2D TMDs. We further compare our calculated exciton binding energy with the corresponding experimental result; good agreement between experiment and theory provides evidence for the validity of our theoretical approach.

The Klein paradox, introduced in the study of chiral tunneling in single-layer graphene, is shown to also manifest in graphene bound states with a 1D square-well potential. Such bound states occur both below and above the potential well. We derive bound-state wave functions, in the absence and in the presence of an external transverse magnetic field, and calculate the corresponding electronic currents and dipole transition rates, which can be experimentally probed. The role of parity and time-reversal symmetries is discussed. Our results are also relevant for the physics of bound states of light in periodic optical waveguide structures.

We consider the dynamics of a quantum particle held in a lattice potential and subjected to a time-dependent spin-orbit coupling. Tilting the lattice causes the particle to perform Bloch oscillations, and by suitably changing the Rashba interaction during its motion, the spin of the particle can be gradually rotated. Even if the Rashba coupling can only be altered by a small amount, large spin rotations can be obtained by accumulating the rotation from successive oscillations. We show how the time dependence of the spin-orbit coupling can be chosen to maximize the rotation per cycle, and thus how this method can be used to produce a precise and controllable spin rotator, which we term the Bloch-Rashba rotator, without requiring an applied magnetic field.

Unlike classical fluids, a quantum Fermi liquid can support a long-lived and propagating shear sound wave at arbitrarily small wave vectors and frequencies, reminiscent of the transverse sound in crystals, despite lacking any form of long-range crystalline order. This mode is expected to be present in moderately interacting metals where the quasiparticle mass is renormalized to be more than twice the bare mass in two dimensions (2D), but it has remained undetected because it is hard to excite since it does not involve charge density fluctuations, in contrast to the conventional plasma mode. In this work we propose a strategy to excite and detect this unconventional mode in clean metallic channels. We show that the shear sound is responsible for the appearance of sharp dips in the ac conductance of narrow channels at resonant frequencies matching its dispersion. The liquid resonates while minimizing its dissipation in an analogous fashion to a sliding crystal. Ultraclean 2D materials that can be tuned toward the Wigner crystallization transition such as silicon metal-oxide-semiconductor field-effect transistors, MgZnO/ZnO, p-GaAs, and AlAs quantum wells are promising platforms to experimentally discover the shear sound.

The Brillouin scattering spectra of $\alpha \text{−}{\mathrm{V}}_2{\mathrm{O}}_5$ and Sn-intercalated ${\mathrm{V}}_2{\mathrm{O}}_5$ (${\mathrm{Sn}}_{0.07}{\mathrm{V}}_2{\mathrm{O}}_5$) were measured for sound propagation directions at and near the [001] direction, which is normal to the van der Waals gaps in this layered material. Anisotropic sound velocities and stiffness tensor elements $c_{ij}$ were determined from the Brillouin frequency shifts. Measurements for propagation in the [001] direction in pure ${\mathrm{V}}_2{\mathrm{O}}_5$ yielded $c_{33}=42.20\pm 0.38\mathrm{GPa}$, $c_{44}=23.55\pm 0.27\mathrm{GPa}$, and $c_{55}=25.65\pm 0.27\mathrm{GPa}$ showing substantial disagreement with some published theoretical calculations. High longitudinal stiffness values $\sim 230\mathrm{GPa}$ in the [100] and [010] directions correspond to remarkably high longitudinal sound velocities of 8290 m/s, demonstrating strong anisotropy associated with the layered structure. ${\mathrm{Sn}}_{0.07}{\mathrm{V}}_2{\mathrm{O}}_5$ was found to have stiffnesses of $c_{33}=43.72\pm 0.23\mathrm{GPa}$, $c_{44}=22.91\pm 0.20\mathrm{GPa}$, and $c_{55}=26.54\pm 0.22\mathrm{GPa}$, indicating that some deformation modes are slightly stiffened while others may be unaffected or even softened by the intercalant.

In this work, we investigate the dynamics of interacting particle systems subjected to repulsive forces, such as lattices of magnetized particles. To this end, we first develop a general model capable of capturing the complete dynamical behavior of interacting particle systems governed by arbitrary potentials. The model elucidates the important role played by the static repulsive forces exchanged between particles in the initial equilibrium configuration, which is distilled and mathematically captured by a dedicated component of the stiffness matrix. The implications of the model are then examined through the simple illustrative example of a magnetic particle oscillator, by which we show that the effect associated with the initial static forces is germane to two- or higher-dimensional particle systems and vanishes for one-dimensional (1D) chains. In the context of wave propagation, we show that this type of effect manifests as modal-selective corrections of the dispersion relation of 2D repulsive lattices. To corroborate these findings, we perform laser vibrometry experiments on a lattice prototype consisting of a triangular grid of magnets supported by an elastic foundation of thin pillars. The tests unequivocally confirm the emergence of distinctive dispersive regimes in quantitative accordance to the model.

We consider electron transport in ferromagnets or antiferromagnets sandwiched between metals. When spins in the magnetic materials precess, they emit currents into the surrounding conductors. Generally, adiabatic pumping in mesoscopic systems also enhances current fluctuations. We generalize the description of current fluctuations driven by spin dynamics in three ways using scattering theory. First, our theory describes a general junction with any given electron scattering properties. Second, we consider antiferromagnets as well as ferromagnets. Third, we treat multiterminal devices. Using shot noise-induced current fluctuations to reveal antiferromagnetic resonance appears to be easier than using them to reveal ferromagnetic resonance. The origin of this result is that the associated energies are much higher as compared to the thermal energy. The thermal energy governs the Johnson-Nyquist noise that is independent of the spin dynamics. We give results for various junctions, such as ballistic and disordered contacts. Finally, we discuss experimental consequences.

We present studies on the low-energy magnetic excitations in the Lieb-Mattis (LM) ferrimagnetic phase. We investigate the low-energy magnetic excitations in the mixed spin-($\frac{1}{2},\frac{5}{2}$) chain $(4-\mathrm{Br}-o-\mathrm{Me}\mathrm{Py}-\mathrm{V})\mathrm{Fe}{\mathrm{Cl}}_4$ based on low-temperature specific heat and multifrequency electron spin resonance (ESR) measurements. A clear $\sqrt{T}$ term is observed in the specific heat, which indicates a contribution of the low-energy magnetic excitation with a ferromagnetic quadratic dispersion expected in the LM ferrimagnetic state. Through the ESR mode analysis, we examine the effects of magnetic anisotropy on the excited states in the LM plateau phase.

We present a theory of magnetic response in finite-size two-dimensional superconductors with Rashba spin-orbit coupling. The interplay between the latter and an in-plane Zeeman field leads on the one hand to an out-of-plane spin polarization which accumulates at the edges of the sample over the superconducting coherence length, and on the other hand, to circulating supercurrents decaying away from the edge over a macroscopic scale. In a long finite stripe of width $W$, both the spin polarization and the currents contribute to the total magnetic moment induced at the stripe ends. These two contributions scale with powers of $W$ such that for sufficiently large samples it can be detected by current magnetometry techniques.

The planar topological Hall effect (PTHE) is known to arise from in-plane skyrmion tubes with an external magnetic field applied in the plane of the charge current. Here, we propose that the PTHE can robustly appear also from an unexpected source, the conical spin spiral, realizable in a variety of materials. We show that for both in-plane skyrmion tubes and conical spin spirals, the PTHE is twofold symmetric with respect to the magnetic-field angle, while it is fourfold symmetric for ${90}^{\circ }$-rotated domains of conical spin spirals. We predict that the symmetry and magnetic-field response of the PTHE can distinguish between the two scenarios, unambiguously probing the nature of the antisymmetric spin-exchange interaction and the resulting magnetic texture.

We report on a comprehensive characterization of the newly synthesized ${\mathrm{Cu}}^{2+}$-based molecular magnet $\left[\mathrm{Cu}{\left(\mathrm{pz}\right)}_2{\left(2\text{−}\mathrm{HOpy}\right)}_2\right]{\left({\mathrm{PF}}_6\right)}_2$ (CuPOF), where $\mathrm{pz}={\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2$ and $2\text{−}\mathrm{HOpy}={\mathrm{C}}_5{\mathrm{H}}_4\mathrm{NHO}$. From a comparison of theoretical modeling to results of bulk magnetometry, specific heat, ${\mu }^{+}\mathrm{SR}$, ESR, and NMR spectroscopy, this material is determined as an excellent realization of the two dimensional square-lattice $S=\frac{1}{2}$ antiferromagnetic Heisenberg model with a moderate intraplane nearest-neighbor exchange coupling of $J/k_{\mathrm{B}}=6.80\left(5\right)$ K, and an extremely small interlayer interaction of about 1 mK. At zero field, the bulk magnetometry reveals a temperature-driven crossover of spin correlations from isotropic to $XY$ type, caused by the presence of a weak intrinsic easy-plane anisotropy. A transition to long-range order, driven by the low-temperature $XY$ anisotropy under the influence of the interlayer coupling, occurs at $T_{\mathrm{N}}=1.38\left(2\right)$ K, as revealed by ${\mu }^{+}\mathrm{SR}$. In applied magnetic fields, our ${}^1\mathrm{H}$-NMR data reveal a strong increase of the magnetic anisotropy, manifested by a pronounced enhancement of the transition temperature to commensurate long-range order at $T_{\mathrm{N}}=2.8$ K and 7 T.

It has been well established that for symmetry-protected topological systems, the nontrivial topology prevents a real space representation using exponentially localized Wannier wave functions (WFs) in all directions, unless the protecting symmetry is sacrificed as an on-site transformation. This makes it challenging to determine the symmetry of various physical observables represented using such WFs. In this work, we propose a practical method for overcoming such challenges using the Kane-Mele model as a concrete example. We present a systematic procedure for diagnosing the symmetry of any observables, as well as a method for constructing symmetric operators up to arbitrary truncation accuracy.

A method to analyze optical transitions is developed by combining the Kubo-Greenwood formula with the unfolding method to construct an unfolded electronic band structure with optical transition weights, which allows us to investigate how optical transitions are perturbed by imperfections such as impurity, vacancy, and structural distortions. Based on the Kubo-Greenwood formula, we first calculate frequency-dependent optical conductivity based on the first-principles electronic structure calculations using the linear combinations of atomic orbitals. Benefiting from the atomic orbital basis sets, the frequency-dependent optical conductivity can be traced back to their individual components before summations over all of $k$ points and bands. As a result, optical transition weights of the material can be put on the unfolded electronic band structure to show contributions at different $k$ points and bands. This method is especially useful to study the effects of broken symmetry in the optical transitions due to presence of impurities in the materials. As a demonstration, decomposed optical transition weights of a monolayer Si-doped graphene are shown in the electronic band structure.

Omega bianisotropic metasurfaces (OBMS) provide wave-control capabilities not previously available with Huygens’ metasurfaces (HMS). These enhanced capabilities derive from the additional degree of freedom provided by OBMS, and are based on analyses of zero-thickness surfaces with abstract surface impedance properties. However, the design of practical metasurfaces has proven tedious. Herein we propose an easily designed structure to realize OBMSs. Extending our previous work, we show that an asymmetrical cascade of two judiciously engineered Fabry-Perot (FP) etalons could form an OBMS meta-atom to provide desired wave control capabilities. Implementing this FP-OBMS for anomalous refraction, we show that bianisotropy effectively produces a virtual antireflective coating over a HMS, leading to the OBMS efficiency enhancement. This intriguing observation, backed by an approximate closed-form solution, provides an original physical interpretation of the mechanism underlying perfect anomalous refraction, and is used to explain differences in the angular response of OBMS in comparison to HMS. Implementing the FP-OBMS for the more intricate functionality of beam splitting, we show that the FP-OBMS are capable of nonlocal excitation of surface waves required for this functionality, despite being electrically thick. These results, verified via full-wave computations, demonstrate the ability of the proposed physical structure to meticulously reproduce the scattering properties of ideal (abstract) zero-thickness OBMS, thereby paving the path to practical realization of wave transmission with exotic functionalities, some of which have never before been associated with a physical structure.

Over recent decades, a growing number of systems, many of them quantum critical, have been shown to exhibit non-Fermi-liquid behavior, but a full analytic understanding of such systems out of equilibrium is still lacking. In this paper, we provide a distinct example with broad applications in correlated mesoscopic systems to address this issue—a two-channel Kondo-Luttinger model where a Kondo impurity couples to two voltage-biased interacting electron leads, experimentally realizable in a dissipative quantum dot. Therein, an exotic quantum phase transition has been known to exist since the 1990s from the one-channel to two-channel Kondo ground states by enhancing electron interactions in the leads, but a controlled analytic approach to this quantum critical point has not yet been established due to the breakdown of weak-coupling perturbation theory near this strong-coupling critical point. We present a controlled method to this long-standing problem by mapping the system in the strong-coupling regime to an effective spin-boson-fermion Hamiltonian. Another type of non-Fermi-liquid quantum critical point is discovered with a distinct logarithmic-in-temperature and -voltage dependence in transport. We further obtain an analytical form for the universal differential conductance out of equilibrium near the transition. Our approach can be further generalized to study nonequilibrium physics of other strong-coupling low-dimensional non-Fermi-liquid fixed points. The relevance of our results for recent experiments is discussed.

We present a Raman-scattering study of the interlayer and intralayer $E_{2g}$ Raman-active modes of hexagonal boron nitride $(h-\mathrm{BN})$ under hydrostatic pressure for pressures up to the transition to the wurtzite phase (10.5 GPa). Pressure coefficients and Grüneisen parameters are determined for both modes, and are compared to ab initio calculations based on density functional perturbation theory. The pressure coefficient of the low-energy interlayer mode is higher than that of the high-energy intralayer mode owing to the large compressibility of the $h-\mathrm{BN}$ crystal along the $c$ direction. Both modes exhibit a sublinear phonon frequency increase with pressure, which is more marked in the case of the low-energy mode. The intensity of the low-energy mode increases with pressure, suggesting an enhancement of the mode polarizability as the honeycomb layers become closer. The Raman spectrum of the metastable wurtzite phase is observed at ambient pressure in the transited sample after decompression.

Molecular materials enable a vast variety of functionalities for novel electronic and spintronic devices. The unique possibility to alter organic molecules or metallic substrates offers the opportunity to optimize interfacial properties for almost any desired field of application. For this reason, we extend the successful approach to control metal-organic interfaces by surface alloying. We present a comprehensive characterization of the structural and electronic properties of the interface formed between the prototypical molecule PTCDA and a Sn-Ag surface alloy grown on an Ag(111) single crystal surface. We monitor the changes of adsorption height of the surface alloy atoms and electronic valence band structure upon adsorption of one layer of PTCDA using the normal incidence x-ray standing wave technique in combination with momentum-resolved photoelectron spectroscopy. We find that the vertical buckling and the surface band structure of the $\mathrm{Sn}{\mathrm{Ag}}_2$ surface alloy is not altered by the adsorption of one layer of PTCDA, in contrast to our recent study of PTCDA on a $\mathrm{Pb}{\mathrm{Ag}}_2$ surface alloy [B. Stadtmüller et al., Phys. Rev. Lett. 117, 096805 (2016)]. In addition, the vertical adsorption geometry of PTCDA and the interfacial energy level alignment indicate the absence of any chemical interaction between the molecule and the surface alloy. We attribute the different interactions at these PTCDA/surface alloy interfaces to the presence or absence of local $\sigma$-bonds between the PTCDA oxygen atoms and the surface atoms. Combining our findings with results from literature, we are able to propose an empiric rule for engineering the surface band structure of alloys by adsorption of organic molecules.

We present a study of Hall transport in semi-Dirac critical phases. The construction is based on a covariant formulation of relativistic systems with spatial anisotropy. Geometric data together with external electromagnetic fields is used to devise an expansion procedure that leads to a low-energy effective action consistent with the discrete $\mathcal{P}\mathcal{T}$ symmetry that we impose. We use the action to discuss terms contributing to the Hall transport and extract the coefficients. We also discuss the associated scaling symmetry.

As magnetized media by their nature have broken time reciprocity, the spatial symmetry of a material is also crucial and it imposes some restrictions on observed optical phenomena. Thus, for the normal incidence of light the spatial inversion symmetry makes transmitted and reflected light insensitive to the direction of the in-plane magnetization of the sample. To avoid this limitation, we propose here an approach based on a magnetoplasmonic structure with broken spatial symmetry. Combination of the specially designed spatial asymmetry with magnetism in the presence of optical losses provides a different effect, the transverse magnetophotonic transmittance effect, notable magnetooptical modulation of the optical transmittance at normal incidence enhanced by surface plasmon excitation. As the phenomenon is sensitive to asymmetry, it can serve as a powerful tool to study spin waves and currents in magnonic and optospintronic devices. The approach to marry the concepts of magnetoplasmonics and a lack of spatial symmetry is promising for the design of nanophotonics devices with outstanding properties.