Excitons play a key role for the optoelectronic properties of hybrid systems. We apply steady-state and time-resolved near-field scanning optical microscopy with a 100-nm spatial resolution to study the photoluminescence (PL) of surface excitons (SX) in a 20-nm-thick ZnO film capped with a monolayer of stearic acid molecules. The PL spectra exhibit emission from SX, donor-bound (DX), and—at sample temperatures $T>20\mathrm{K}$—free (FX) excitons. The 4 meV broad smooth envelope of SX emission at $T<10\mathrm{K}$ points to an inhomogeneous distribution of SX transition energies and spectral diffusion caused by diffusive SX transport on a 50-nm scale with a diffusion coefficient of $D_{\mathrm{SX}}(T<10K)=0.30\mathrm{cm}{}^2/\mathrm{s}$.

In many strongly correlated electron metals the thermoelectric power has a nonmonotonic temperature dependence and values that are orders of magnitude larger than for elemental metals. Inspired by Kelvin, Peterson and Shastry derived a particularly simple expression for the thermopower in terms of the temperature dependence of the chemical potential, now known as the Kelvin formula. We consider a Hubbard model on an anisotropic triangular lattice at half filling, a minimal effective Hamiltonian for several classes of organic charge transfer salts. The finite temperature Lanczos method is used to calculate the temperature dependence of the thermopower using the Kelvin formula. We find that electronic correlations significantly enhance the magnitude of the thermopower and lead to a nonmonotonic temperature dependence. The latter reflects a crossover with increasing temperature from a Fermi liquid to a bad metal. Although, the Kelvin formula gives a semiquantitative description of some experimental results it cannot describe the directional dependence of the sign of the thermopower in some materials.

We investigate the interplay between geometrical lattice resonances and surface plasmons mediating the emission of Smith-Purcell visible light via angle-resolved cathodoluminescence spectroscopy. We observe strong modulations in the dispersion curves of Smith-Purcell radiation (SPR) when they intersect the surface plasmons of silver gratings using a 200-kV transmission electron microscope. The decay of the plasmons away from the grating is directly probed by controlling the electron-beam position relative to the sample surface with nanometer precision. Our measurements are in excellent agreement with numerical simulations, clearly revealing the presence of characteristic Fano profiles resulting from the interference of the light continuum and the discrete plasmon states for each direction of emission. The intensity anomaly in the SPR emission pattern can be well explained from the geometrical consideration of the intersections between the dispersion planes of the SPR and surface plasmon polariton (SPP). A strong and directional SPR beam can be realized under the condition that the SPR dispersion plane comes in contact with the band edge of the SPP dispersion plane.

The solid immersion lens is a powerful optical tool that allows light entering material from air or a vacuum to focus to a spot much smaller than the free-space wavelength. Conventionally, however, the lenses rely on semispherical topographies and are nonplanar and bulky, which limits their integration in many applications. Recently, there has been considerable interest in using planar structures, referred to as metasurfaces, to construct flat optical components for manipulating light in unusual ways. Here, we propose and demonstrate the concept of a planar immersion lens based on metasurfaces. The resulting planar device, when placed near an interface between air and dielectric material, can focus electromagnetic radiation incident from air to a spot in the material smaller than the free-space wavelength. As an experimental demonstration, we fabricate an ultrathin and flexible microwave lens and further show that it achieves wireless energy transfer in material mimicking biological tissue.

We report an experimental study of the direct conversion of light into electrical signals in GaAs/AlGaAs Hall-bar microdevices. Our approach, based on different modulation frequencies of the intensity and polarization of the laser beam, allows us to disentangle the charge- and spin-dependent parts of the induced electrical signal and to link them to the incident light intensity and polarization, respectively. We demonstrate that the efficiency of the light to spin conversion in our electrical polarimeter is strongly enhanced by adding a drift component to the transport of the spin-polarized photocarriers, as compared to a purely diffusive transport regime of the device. For a micron-size focused laser beam, the experiments demonstrate that the light to charge and spin conversion efficiency depends on the precise position of the light spot, reflecting the spatially dependent response function of the Hall cross.

We have carried out a systematic study of the tilted magnetic field induced anisotropy at the Landau level filling factor $\nu =5/2$ in a series of high quality GaAs quantum wells, where the setback distance ($d$) between the modulation doping layer and the GaAs quantum well is varied from 33 to 164 nm. We have observed that in the sample of the smallest $d$, electronic transport is anisotropic when the in-plane magnetic field ($B_{\mathrm{ip}}$) is parallel to the $[1–10]$ crystallographic direction, but remains more or less isotropic when $B_{\mathrm{ip}}\parallel \left[110\right]$. In contrast, in the sample of largest $d$, electronic transport is anisotropic in both crystallographic directions. Our results clearly show that the modulation doping layer plays an important role in the tilted field induced $\nu =5/2$ anisotropy.

Indirect excitons in coupled quantum wells are long-living quasiparticles, explored in the studies of collective quantum states. We demonstrate that, despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons, and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons but also one of free electrons by two orders of magnitude.

Topological semimetals in three-dimensions (e.g., a Weyl semimetal) can be built by stacking two-dimensional topological phases. The interesting aspect of such a construction is that even though the topological building blocks in the low dimension may be gapped, the higher dimensional semimetallic phase emerges as a gapless critical point of a topological phase transition between two distinct insulating phases. In this work, we extend this idea by constructing three-dimensional topological semimetallic phases akin to Weyl systems by stacking one-dimensional Aubry-Andre-Harper (AAH) lattice tight-binding models with nontrivial topology. The generalized AAH model is a family of one-dimensional tight-binding models with cosine modulations in both hopping and on-site energy terms. In this paper, we present a two-parameter generalization of the AAH model that can access topological phases in three dimensions within a unified framework. We show that the $\pi$-flux state of this two-parameter AAH model manifests three-dimensional topological semimetallic phases where the topological features are embedded in one dimension. The topological nature of the band touching points of the semimetallic phase in 3D is explicitly established both analytically and numerically from the 1D perspective. This dimensional reduction provides a simple protocol to experimentally construct the three-dimensional Brillouin zone of the topological semimetallic phases using “legos” of simple 1D double well optical lattices. We also propose Zak phase imaging of optical lattices as a tool to capture the topological nature of the band touching points. Our work provides a theoretical connection between the commensurate AAH model in 1D and Weyl semimetals in 3D, and points toward practical methods for the laboratory realization of such three-dimensional topological systems in atomic optical lattices.

Inspired by recent experimental realizations of monolayer Fe membranes in graphene perforations, we perform ab initio calculations of Fe monolayers and membranes embedded in graphene in order to assess their structural stability and magnetization. We demonstrate that monolayer Fe has a larger spin magnetization per atom than bulk Fe and that Fe membranes embedded in graphene exhibit spin magnetization comparable to monolayer Fe. We find that free-standing monolayer Fe is structurally more stable in a triangular lattice compared to both square and honeycomb lattices. This is contradictory to the experimental observation that the embedded Fe membranes form a square lattice. However, we find that embedded Fe membranes in graphene perforations can be more stable in the square lattice configuration compared to the triangular. In addition, we find that the square lattice has a lower edge formation energy, which means that the square Fe lattice may be favored during formation of the membrane.

The ultrafast dynamics of photoexcited carriers closely depends on the excitation processes pertaining to the energy band of the materials and the relevant relaxation pathway relies on the interactions between hot carriers and lattice phonons. By using ultrafast optical-pump terahertz (THz)-probe spectroscopy with an ion-gel gate to tune the Fermi energy level $E_F$ in graphene, we are able to reveal the relaxation dynamics of hot carriers at different $E_F$. It is found that the relaxation time increases while the pump-induced differential transmission decreases as $E_F$ approaches the Dirac point. Through self-consistent model calculations, we quantitatively interpret that a temperature-dependent scattering rate is responsible for a negative photoinduced conductivity, and the relaxation transient directly manifests the Dirac spectrum dependence of the optical phonon emission and the carrier scattering rate. More interestingly, the scattering rate of hot carriers also exhibits a strong $E_F$ dependence, which is the most likely to originate from charged impurities inevitably present in graphene. The diminution of photoresponse efficiency across the Dirac point implies that the graphene-based optoelectronic devices may be operable only in the highly doped regime.

Intrinsic diamagnetism of graphene is studied both theoretically and experimentally, to unravel the magnetic response of chiral massless fermions. Comprehensive formulas predicting the variation of graphene magnetization with magnetic field and temperature are developed. Graphene magnetization $M$ at low temperatures is particularly large and $M\propto -\sqrt{B}$, intrinsically different from normal materials. The quantum Berry phase of $\pi$ and linear energy dispersion are responsible for this intriguing macroscopic behavior. The temperature dependence of magnetization is successfully formulated by a Langevin-like function. The de Haas–van Alphen oscillations are predicted in the case of doping. Correspondingly, experiments at different temperatures are conducted on highly pure, mass-produced graphene flakes derived from SiC single crystals, which exhibit very strong diamagnetism. The measured results agree well with the theoretical ones in both magnitude and trend.

Low-energy magnetic states and finite-temperature properties of Cr nanoclusters in bulk bcc Fe and Fe nanoclusters in bulk Cr are investigated using density functional theory (DFT) and the Heisenberg-Landau Hamiltonian based magnetic cluster expansion (MCE). We show, by means of noncollinear magnetic DFT calculations, that magnetic frustration caused by competing ferromagnetic and antiferromagnetic interactions either strongly reduces local magnetic moments while keeping collinearity or generates noncollinear magnetic structures. Small Cr clusters generally exhibit collinear ground states. Noncollinear magnetic configurations form in the vicinity of small Fe clusters if antiferromagnetic Fe-Cr coupling dominates over ferromagnetic Fe-Fe interactions. MCE predictions broadly agree with DFT data on the low-energy magnetic structures, and extend the DFT analysis to larger systems. Nonvanishing cluster magnetization caused by the dominance of Fe-Cr over Cr-Cr antiferromagnetic coupling is found in Cr nanoclusters using both DFT and MCE. Temperature dependence of magnetic properties of Cr clusters is strongly influenced by the surrounding iron atoms. A Cr nanocluster remains magnetic until fairly high temperatures, close to the Curie temperature of pure Fe in the large cluster size limit. Cr-Cr magnetic moment correlations are retained at high temperatures due to the coupling of interfacial Cr atoms with the Fe environment. Variation of magnetization of Fe-Cr alloys as a function of temperature and Cr clusters size predicted by MCE is assessed against the available experimental data.

Spin ice is a frustrated magnetic system that at low temperatures exhibits a Coulomb phase, a classical spin liquid with topological order and deconfined excitations. This work establishes the presence of a Coulomb phase with coexisting ferromagnetic order in a microscopic model of classical spin ice subject to uniaxial lattice distortion. General theoretical arguments are presented for the presence of such a phase, and its existence is confirmed using Monte Carlo results. This example is used to illustrate generic properties of spin liquids with magnetic order, including deconfinement of monopoles, signatures in the neutron-scattering structure factor, and critical behavior at phase transitions. An analogous phase, a superfluid with spontaneously broken particle-hole symmetry, is demonstrated in a model of hard-core lattice bosons, related to spin ice through the quantum-classical correspondence.

We report specific heat, magnetic, and muon spin relaxation measurements performed on a polycrystalline sample of the normal spinel ${\mathrm{CdHo}}_2{\mathrm{S}}_4$. The rare-earth ions sit on a lattice of corner-sharing regular tetrahedra as in pyrochlore compounds. Magnetic ordering is detected at $T_{\mathrm{c}}\simeq 0.87$ K. From spin-lattice relaxation rate measurements on both sides of $T_{\mathrm{c}}$ we uncover similar magnetic excitation modes driving the so-called persistent spin dynamics at $T<T_{\mathrm{c}}$. Unidimensional excitations are argued to be at its origin. Often observed spin loop structures are suggested to support these excitations. The possibility of a generic mechanism for their existence is discussed.

The manipulation of polarization states in $4\times 4$ vortex crystals using sinusoidal magnetic field bursts is investigated by means of a broadband ferromagnetic-resonance setup. Magnetic field excitation with the proper amplitude and frequency allows tuning different polarization states, which are observed in the measured absorption spectra. The variation of the sinusoidal burst width consecutively identifies the time scale of the underlying process. A memorylike polarization state writing process is demonstrated on the submicrosecond time scale.

We use inelastic neutron scattering (INS) to study the spin excitations in partially detwinned ${\mathrm{NaFe}}_{0.985}{\mathrm{Co}}_{0.015}\mathrm{As}$ which has coexisting static antiferromagnetic (AF) order and superconductivity ($T_c=15$ K, $T_N=30$ K). In previous INS work on a twinned sample, spin excitations formed a dispersive sharp resonance near $E_{r1}=3.25$ meV and a broad dispersionless mode at $E_{r1}=6$ meV at the AF ordering wave vector ${\mathbf{Q}}_{\mathrm{AF}}={\mathbf{Q}}_1=(1,0)$ and its twinned domain ${\mathbf{Q}}_2=(0,1)$. For partially detwinned ${\mathrm{NaFe}}_{0.985}{\mathrm{Co}}_{0.015}\mathrm{As}$ with the static AF order mostly occurring at ${\mathbf{Q}}_{\mathrm{AF}}=(1,0)$, we still find a double resonance at both wave vectors with similar intensity. Since ${\mathbf{Q}}_1=(1,0)$ characterizes the explicit breaking of the spin rotational symmetry associated with the AF order, these results indicate that the double resonance cannot be due to the static and fluctuating AF orders but originate from the superconducting gap anisotropy.

Domain walls can be formed in superconductors with a discrete degeneracy of the ground state, for instance, due to the breaking of time reversal symmetry. We study all cases where the formation of domain walls is possible in a tetragonal superconductor with the point group $D_{4h}$. We discuss both triplet and mixed singlet order parameters. It is found that in all cases the domain walls support subgap Andreev bound states, whose energies strongly depend on the direction of semiclassical propagation. The bound state contribution to the density of quasiparticle states exhibits peculiar features, which can be observed in tunneling experiments.

Nuclear spin diffusion in double quantum wells (QWs) is examined by using dynamic nuclear polarization (DNP) at a Landau level filling factor $\nu =2/3$ spin phase transition (SPT). The longitudinal resistance increases during the DNP of one of the two QW (the “polarization QW”) by means of a large applied current and starts to decrease just after the termination of the DNP. On the other hand, the longitudinal resistance of the other QW (the “detection QW”) continuously increases for approximately 2 h after the termination of the DNP of the polarization QW. It is therefore concluded that the nuclear spins diffuse from the polarization QW to the detection QW. The time evolution of the longitudinal resistance of the polarization QW is explained mainly by the nuclear spin diffusion in the in-plane direction. In contrast, that of the detection QW manifests much slower nuclear diffusion in the perpendicular direction through the AlGaAs barrier.

We study the electronic and optical properties of single- and bilayer black phosphorus with short- and long-range defects by using the tight-binding propagation method. Both types of defect states are localized and induce a strong scattering of conduction states, reducing significantly the charge carrier mobility. In contrast to properties of pristine samples, the anisotropy of defect-induced optical excitations is suppressed due to the isotropic nature of the defects. We also investigate the Landau level spectrum and magneto-optical conductivity and find that the discrete Landau levels are sublinearly dependent on the magnetic field and energy level index, even at low defect concentrations.