In this work, we have demonstrated that self-complementary metasurfaces can provide linear-to-circular polarization conversion. For illustration of this general approach, we studied several particular structures based on periodic self-complementary patterns etched on metal sheets of negligible thickness. The advantage of using self-complementary metasurfaces over previous proposals of thin polarizers is the rigorous constancy of the phase difference between transmission (or reflection) coefficients corresponding to orthogonal linear polarizations, which is just ${90}^{\circ }$ at any frequency. Also, this unusual phenomenon is stable under oblique incidence. The general proposed theory of self-complementary metasurfaces was first validated through numerical simulations for structures made of ideal lossless materials. Through simulations with realistic materials and experimental measurements, we have also demonstrated that this conversion can be reached in reality with high precision.

The Dirac fermions at the surface of a topological insulator can be gapped by introducing magnetic dopants. Alternatively, in an ultrathin slab with thickness on the order of the extent of the surface states, both the top and bottom surface states acquire a common gap value $\left(\mathrm{\Delta }\right)$ but with opposite sign. In a topological insulator, the dominant piece of the Hamiltonian $\left(\widehat{H}\right)$ is of a relativistic nature. A subdominant nonrelativistic piece is also present and, in an external magnetic field $\left(B\right)$ applied perpendicular to the surface, the $N=0$ Landau level is no longer at zero energy but is shifted to positive energy by the Schrödinger magnetic energy. When a gap is present, it further shifts this level down by $-\mathrm{\Delta }$ for positive $\mathrm{\Delta }$ and up by $\left|\mathrm{\Delta }\right|$ for a negative gap. This has important consequences for the magneto-optical properties of such systems. In particular, at charge neutrality, the lowest energy transition displays anomalous nonmonotonic behavior as a function of $B$ in both its position in energy and its optical spectral weight. The gap can also have a profound impact on the spectral weight of the interband lines and on corresponding structures in the real part of the dynamical Hall conductivity. Conversely, the interband background in zero field remains unchanged by the nonrelativistic term in $\widehat{H}$ (although its onset frequency is modified).

It is common knowledge that even a trace amount of a chemisorbed species can strongly perturb the surface electronic structure, in particular the surface states, to the point of their complete eradication. We have confirmed this behavior by adsorbing Li on the Ir(111), but surprisingly, we have discovered that in the presence of graphene Li does not suppress the Ir surface state. By combining the results of the low-energy electron diffraction and angle-resolved photoemission spectroscopy with the density functional theory for modeling of the studied systems we can provide a detailed explanation for the observed phenomena. The quenching of the surface state by the electronic states of disordered Li layer on a bare Ir surface is efficiently deactivated by the presence of graphene which shifts the Li states to lower energies thereby leading to the unexpected reappearance of the surface state. Such protection of the surface state coherence from disorder upon intercalation could be used as a benchmark in the toolbox of surface science.

We have investigated ABA-stacked trilayer graphene under a perpendicular electric field by using the density functional theory (DFT) calculations, which may contribute to the resolution of the discrepancies between experimental and theoretical results on the electric-field-induced band gap and topological phase transition. We found that the electric field opens a band gap at a low field and closes the gap at a high field, supporting one of the experimental results. While the seven electric-field-induced Dirac cones with mass gaps predicted in recent tight-binding (TB) models are confirmed, our DFT calculations demonstrate a phase transition from a quantum valley Hall insulator to a semimetal, contrasting to the TB model prediction of a topological phase transition between topologically nontrivial insulators at a high electric field.

The results of a comprehensive and systematic ab initio based ground-state search for the structural arrangement of oxygen vacancies in rutile phase ${\mathrm{TiO}}_2$ provide new insights into their memristive properties. We find that O vacancies tend to form planar arrangements which relax into structures exhibiting metallic behavior. These metastable arrangements are structurally akin to, yet distinguishable from, the Magnéli phase. They exhibit a more pronounced metallic nature but are energetically less favorable. Our results confirm a clear structure-property relationship between segregated oxygen vacancy arrangement and metallic behavior in reduced oxides.

Nanoscale devices fabricated out of magnetic heterostructures are central to the emerging field of spintronics, so understanding of magnetization dynamics at interfaces between dissimilar materials is essential. Here we report local imaging of magnetization dynamics at the interface formed by a sharp discontinuity in the magnetic properties of a ferromagnetic thin film using localized mode ferromagnetic resonance force microscopy (FMRFM). The behavior of the localized modes near the interface evolves with increasing magnitude of the FMRFM probe field due to its competition with the steplike internal demagnetizing field. We use micromagnetic modeling to visualize the evolution of the localized mode as the magnetic probe is scanned across the interface. Our results demonstrate the ability to image sharp changes in internal magnetic properties in nanoscale devices and provide insights into the mechanisms underlying the generation and manipulation of localized modes near the interface, thus providing a new tool for microscopic studies of spin transport across magnetic interfaces and spin dynamics in their vicinity.

Using the Ong construction for a two-dimensional metal, we show that the sign change in the Hall coefficient $R_{\mathrm{H}}$ of underdoped hole-doped cuprates at low temperature is consistent with the emergence of biaxial charge order recently proposed to explain the observation of low-frequency quantum oscillations. The sharp evolution of $R_{\mathrm{H}}$ with temperature, however, can only be reconciled by incorporating a highly anisotropic quasiparticle scattering rate. The magnitude and form of the scattering rate extracted from the fitting imply that those quasiparticles at the vertices of the reconstructed pocket(s) approach the boundary of incoherence at the onset of charge order.

Single crystals of $\mathrm{YAl}{\mathrm{O}}_3$ doped with Ce have been studied by electron paramagnetic resonance (EPR) at the 9.4 and 34 GHz microwave bands. Besides the single-ion $\mathrm{C}{\mathrm{e}}^{3+}$ spectrum, measurements have revealed many satellite lines which belong to the $\mathrm{C}{\mathrm{e}}^{3+}\text{−}\mathrm{C}{\mathrm{e}}^{3+}$ pair centers. Their spectra have been fitted by a general effective spin Hamiltonian describing two interacting particles with the spin $S=1/2$. Corresponding $g$ factors and spin-spin coupling constants have been determined. The spin-spin coupling constants are in the range from 0.1 up to $0.65\mathrm{c}{\mathrm{m}}^{-1}$ for the nearest and next-nearest neighbors depending on the distance between Ce ions and their position. The exchange interaction between next-nearest neighbors (NNNs) is comparable to or even bigger than that between nearest neighbors (NNs), being in the range $0.4-0.6\mathrm{c}{\mathrm{m}}^{-1}$. For a single $\mathrm{C}{\mathrm{e}}^{3+}$ ion, crystal field parameters, energy sublevels of the ${}^2{F}_{5/2}$ and ${}^2{F}_{7/2}$ multiplets and principal $g$ tensor components were obtained from the density functional theory calculation. They are in satisfactory agreement with those determined experimentally. The principal $g$ tensor components of $\mathrm{C}{\mathrm{e}}^{3+}$ pair centers are also calculated. Nevertheless, it was impossible to assign each of the satellite lines to actual positions of the six NN and 12 NNN Ce pairs in the lattice due to lack of valid information on the sign of the exchange interactions. The influence of $\mathrm{C}{\mathrm{e}}^{3+}$ pairs on the luminescence efficiency is discussed as well.

The electronic structure of ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ is studied by using the relativistic Korringa-Kohn-Rostoker Green function method in the framework of density functional theory. For all concentrations $x, {\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ is a direct semiconductor with a narrow band gap. In contrast to pure lead telluride, tin telluride shows an inverted band characteristic close to the Fermi energy. It will be shown that this particular property can be tuned, first, by alloying PbTe and SnTe and, second, by applying hydrostatic pressure or uniaxial strain. Furthermore, the magnitude of strain needed to switch between the regular and inverted band gap can be tuned by the alloy composition. Thus there is a range of potential usage of ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ for spintronic applications.

Spatially indirect electron-hole exciton condensates stabilized by interlayer Fock exchange interactions have been predicted in systems containing a pair of two-dimensional semiconductor or semimetal layers separated by a thin tunnel dielectric. The layer degree of freedom in these systems can be described as a pseudospin. Condensation is then analogous to ferromagnetism, and the interplay between collective and quasiparticle contributions to transport is analogous to phenomena that are heavily studied in spintronics. These phenomena are the basis for pseudospintronic device proposals based on possible low-voltage switching between high (nearly shorted) and low interlayer conductance states and on near-perfect Coulomb drag-counterflow current along the layers. In this work, a quantum transport simulator incorporating a nonlocal Fock exchange interaction is presented, and used to model the essential transport physics in, for specificity, a graphene-dielectric-graphene system. Finite-size effects, Coulomb drag-counterflow current, critical interlayer currents beyond which interlayer dc conductance collapses at subthermal voltages, nonlocal coupling between interlayer critical currents in multiple lead devices, and an Andreev-like reflection process are illustrated.

In previous studies, an inhomogeneous strain field was used as a trapping mechanism for interwell excitons in coupled GaAs/${\mathrm{Al}}_{0.45}{\mathrm{Ga}}_{0.55}\mathrm{As}$ quantum wells. Photoluminescence measurements in this system demonstrated the presence of a dark population of excitons at trap center in the low-temperature, high-stress, high-density regime. The dramatic appearance of this effect at low temperature and high density initially suggested that it may be indicative of a Bose-Einstein condensation phase transition. Further experiments revealed that this effect appears more readily in wider quantum wells and occurs at strain values near the heavy-hole/light-hole crossover point. In this paper, it will be shown that this effect occurs in a regime where the heavy-hole valence band maximum shifts away from $k_{\left|\right|}=0$, which is expected to strongly suppress the recombination rate at low temperatures. Simulations based on this assumption will be shown to match the experimentally observed behavior, without requiring the appearance of a Bose-Einstein condensate.

We present a method for simulating the time evolution of one-dimensional correlated electron-phonon systems which combines the time-evolving block decimation algorithm with a dynamical optimization of the local basis. This approach can reduce the computational cost by orders of magnitude when boson fluctuations are large. The method is demonstrated on the nonequilibrium Holstein polaron by comparison with exact simulations in a limited functional space and on the scattering of an electronic wave packet by local phonon modes. Our study of the scattering problem reveals a rich physics including transient self-trapping and dissipation.

Claims that dynamic force microscopy has the capability to resolve intermolecular bonds in real space continue to be vigorously debated. To date, studies have been restricted to planar molecular assemblies with small separations between neighboring molecules. Here we report the observation of intermolecular artifacts over much larger distances in 2D assemblies of ${\mathrm{C}}_{60}$ molecules, with compelling evidence that in our case the tip apex is terminated by a ${\mathrm{C}}_{60}$ molecule (rather than the CO termination typically exploited in ultrahigh resolution force microscopy). The complete absence of directional interactions such as hydrogen or halogen bonding, the nonplanar structure of ${\mathrm{C}}_{60}$, and the fullerene termination of the tip apex in our case highlight that intermolecular artifacts are ubiquitous in dynamic force microscopy.

Monolayers of semiconducting van der Waals solids, such as transition metal dichalcogenides, acquire significant electric polarization normal to the layers when placed on a substrate or in a heterogeneous stack. This causes a linear coupling of electrons to electric fields normal to the layers. Irradiation at oblique incidence at frequencies above the gap causes interband transitions due to coupling to both normal and in-plane ac electric fields. The interference between the two processes leads to sizable in-plane photocurrents and valley currents. The direction and magnitude of the currents are controlled by light polarization and are determined by their helical or nonhelical components. The helicity-dependent ballistic current arises due to asymmetric photogeneration. The nonhelical current has a ballistic contribution (dominant in sufficiently clean samples) caused by asymmetric scattering of photoexcited carriers, and a side-jump contribution. A magneto-induced photocurrent is due to the Lorentz force or due to an intrinsic magnetic moment related to Berry curvature.

We analyze the magnetic form factor of ${\mathrm{Cu}}^{2+}$ in low-dimensional quantum magnets by taking the metal-ligand hybridization into account explicitly. In this analysis, we use the form of magnetic Wannier orbitals, derived from first-principles calculations, and identify contributions of different atomic sites. We demonstrate that covalency of metal-ligand interactions has a strong effect on the magnetic form factor and must be taken into account in the evaluation of magnetic neutron scattering. The use of covalent form factors facilitates quantitative description of inelastic neutron-scattering data for ${\mathrm{BaCuSi}}_2{\mathrm{O}}_6$ that would not be possible when ionic form factors are used. We also propose easy-to-use approximations of the partial orbital contributions to the magnetic form factor in order to give microscopic explanation for the results obtained in previous first-principles studies.

We report the quantum phase diagram of a one-dimensional Coulomb wire obtained using the path-integral Monte Carlo method. The exact knowledge of the nodal points of this system permits us to find the energy in an exact way, solving the sign problem which spoils fermionic calculations in higher dimensions. The results obtained allow for the determination of the stability domain, in terms of density and temperature, of the one-dimensional Wigner crystal. At low temperatures, the quantum wire reaches the quantum-degenerate regime, which is also described by the diffusion Monte Carlo method. Increasing the temperature, the system transforms to a classical Boltzmann gas, which we simulate using classical Monte Carlo. At large enough density, we identify a one-dimensional ideal Fermi gas which remains quantum up to higher temperatures than in two- and three-dimensional electron gases. The obtained phase diagram and the energetic and structural properties of this system are relevant to experiments with electrons in quantum wires and to Coulomb ions in one-dimensional confinement.

Harnessing the power of low-dimensional materials in thermal applications calls for a solid understanding of the anomalous thermal properties of such systems. We analyze thermal conduction in one-dimensional systems by determining the frequency-dependent phonon mean free paths (MFPs) for an anharmonic chain, delivering insight into the diverging thermal conductivity observed in computer simulations. In our approach, the MFPs are extracted from the length dependence of the spectral heat current obtained from nonequilibrium molecular dynamics simulations. At low frequencies, the results reveal a power-law dependence of the MFPs on frequency, in agreement with the diverging conductivity and the recently determined equilibrium MFPs. At higher frequencies, however, the nonequilibrium MFPs consistently exceed the equilibrium MFPs, highlighting the differences between the two quantities. Exerting pressure on the chain is shown to suppress the mean free paths and to generate a weaker divergence of MFPs at low frequencies. The results deliver important insight into the anomalous thermal conduction in low-dimensional systems and also reveal differences between the MFPs obtained from equilibrium and nonequilibrium simulations.

We investigate charge conductance and spin and valley polarization along with the tunneling magnetoresistance (TMR) in silicene junctions composed of normal silicene and ferromagnetic silicene. We show distinct features of the conductances for parallel and antiparallel spin configurations and the TMR, as the ferromagnetic-normal-ferromagnetic junction is tuned by an external electric field. We analyze the behavior of the charge conductance and valley and spin polarizations in terms of the independent conductances of the different spins at the two valleys and the band structure of ferromagnetic silicene and show how the conductances are affected by the vanishing of the propagating states at one or the other valley. In particular, unlike in graphene, the band structures at the two valleys are independently affected by the spin in the ferromagnetic regions and lead to nonzero and, in certain parameter regimes, pure valley and spin polarizations, which can be tuned by the external electric field. We also investigate the oscillatory behavior of the TMR with respect to the strength of the barrier potential (both spin-independent and spin-dependent barriers) in the normal silicene region and note that in some parameter regimes, the TMR can even go from positive to negative values, as a function of the external electric field.