We have investigated the conduction-band electronic structure of 1$T$-TaS${}_2$ in the incommensurate (IC) and commensurate (C) charge-density-wave (CDW) phases by means of angle-resolved inverse-photoemission spectroscopy (ARIPES) along the $\Gamma \left(A\right)–M\left(L\right)$ direction. In the ARIPES spectra for the ICCDW phase, a continuous and dispersive Ta 5$d$ band is clearly observed near the Fermi level ($E_{\mathrm{F}}$). For the CCDW phase, on the other hand, the Ta 5$d$ band splits up and down into two manifolds around the $M\left(L\right)$ point, reflecting formation of the $\sqrt{13}\times \sqrt{13}$ superstructure of the Ta plane. In addition, the CCDW-ARIPES spectrum around the $M\left(L\right)$ points exhibits a shoulder structure at 0.7 eV above $E_{\mathrm{F}}$, ascribed to the upper Hubbard band folded from the $\Gamma \left(A\right)$ point. We have also observed an energy gap of $\sim$100 meV in the unoccupied density of states in the CCDW phase.

We investigate the process of condensation of exciton-polaritons in a one-dimensional nanowire, predicting spontaneous formation of domains of uncondensed excitons and condensed polaritons. We find that the system does not follow the standard Kibble-Żurek scenario of defect formation. Nevertheless, the universal scaling laws are still present if the critical exponents are replaced by their “nonequilibrium” counterparts related to the imaginary part of the time-dependent Bogoliubov spectrum.

We show that the direction of coherent electron transport across a cyclic system of quantum dots or a cyclic molecule can be modulated by an external magnetic field if the cycle has an odd number of hopping sites, but the transport becomes completely symmetric if the number is even. These contrasting behaviors, which remain in the case of interacting electrons, are a consequence of the absence or presence of alternance symmetry in the system. These findings are relevant for the design of nanocircuits based on coupled quantum dots or molecular junctions.

We demonstrate that valley polarization can be induced and controlled in semiconducting single-layer transition-metal dichalcogenides by magnetic doping, which is important for spintronics, valleytronics, and photonics devices. As an example, we investigate Mn-doped ${\mathrm{MoS}}_2$ by first-principles calculations. We study how the valley polarization depends on the strength of the spin orbit coupling and the exchange interaction and discuss how it can be controlled by magnetic doping. Valley polarization by magnetic doping is also expected for other honeycomb materials with strong spin orbit coupling and the absence of inversion symmetry.

Using transconductance spectroscopy we study the tunneling dynamics of electrons from a two-dimensional electron gas (2DEG) into excited and ground states of a layer of self-assembled InAs quantum dots (QDs). From an initially selected nonequilibrium condition, we observe the charging dynamics of the QD states and their spectral evolution for one- and two-electron configurations. Furthermore, we measure the electron emission from the QD states into the 2DEG for the corresponding evolution of the QD-hydrogen and QD-helium spectra. The comparison with theoretically predicted energies, as well as the evaluation of the dynamics in charging and emission, allows us to separate and identify ground and excited electron configurations in the spectral evolution and discuss in detail the observed maxima in the different spectra.

A one-dimensional topological superconductor features a single fermionic zero mode that is delocalized over two Majorana bound states located at the ends of the system. We study a pair of spatially separated nanomechanical oscillators tunnel coupled to these Majorana modes. Most interestingly, we demonstrate that the combination of electron-phonon coupling and a finite charging energy on the mesoscopic topological superconductor can lead to an effective superexchange between the oscillators via the nonlocal fermionic zero mode. We further show that this electron teleportation mechanism leads to entanglement of the two oscillators over distances that can significantly exceed the coherence length of the superconductor.

We consider the spin current flowing in a two-dimensional electron gas with Rashba and linear Dresselhaus spin-orbit interaction as a linear response to a temperature gradient, taking into account the contribution due to the thermoelectric effect. We derive a relation connecting the electrically and thermally driven spin and charge conductivities, for the Hamiltonians corresponding to samples grown in the main crystallographic directions. Based on this connection, it is shown that the transverse and longitudinal spin currents generated by the temperature gradient vanish exactly for each spin-orbit Hamiltonian case. This result is in contrast to the recently predicted finite thermospin Hall effect for such class of systems.

Recently, rhenium disulfide (${\mathrm{ReS}}_2$) monolayers were experimentally extracted by conventional mechanical exfoliation technique from as-grown ${\mathrm{ReS}}_2$ crystals. Unlike the well-known members of transition metal dichalcogenides (TMDs), ${\mathrm{ReS}}_2$ crystallizes in a stable distorted-$1T$ structure and lacks an indirect to direct gap crossover. Here we present an experimental and theoretical study of the formation, energetics, and stability of the most prominent lattice defects in monolayer ${\mathrm{ReS}}_2$. Experimentally, irradiation with 3-MeV He${}^{+2}$ ions was used to break the strong covalent bonds in ${\mathrm{ReS}}_2$ flakes. Photoluminescence measurements showed that the luminescence from monolayers is mostly unchanged after highly energetic $\alpha$ particle irradiation. In order to understand the energetics of possible vacancies in ${\mathrm{ReS}}_2$ we performed systematic first-principles calculations. Our calculations revealed that the formation of a single sulfur vacancy has the lowest formation energy in both Re and S rich conditions and a random distribution of such defects are energetically more preferable. Sulfur point defects do not result in any spin polarization whereas the creation of Re-containing point defects induce magnetization with a net magnetic moment of 1–3${\mu }_{\mathrm{B}}$. Experimentally observed easy formation of sulfur vacancies is in good agreement with first-principles calculations.

We solve the disordered Holstein model via the density-matrix renormalization group method to investigate the combined roles of electron-phonon coupling and disorder on the localization of a single charge or exciton. The parameter regimes chosen, namely the adiabatic regime, $\hslash \omega /4t_0={\omega }^{\prime }<1$, and the large polaron regime, $\lambda <1$, are applicable to most conjugated polymers. We show that as a consequence of the polaron effective mass diverging in the adiabatic limit (defined as ${\omega }^{\prime }\to 0$ subject to fixed $\lambda$) self-localized, symmetry-breaking solutions are predicted by the quantum Holstein model for infinitesimal disorder, in complete agreement with the predictions of the Born-Oppenheimer Holstein model. For other parts of the (${\omega }^{\prime }$, $\lambda$) parameter space, however, self-localized Born-Oppenheimer solutions are not expected. If ${\omega }^{\prime }$ is not small enough and $\lambda$ is not large enough, then the polaron is predominately localized by Anderson disorder, albeit more than for a free particle, because of the enhanced effective mass. Alternatively, for very small electron-phonon coupling ($\lambda \ll 1$) the disorder-induced localization length is always smaller than the classical polaron size, $2/\lambda$, so that disorder always dominates. We comment on the implication of our results on the electronic properties of conjugated polymers.

Strong light-matter coupling has recently been demonstrated in subwavelength volumes by coupling engineered optical transitions in semiconductor heterostructures (e.g., quantum wells) to metasurface resonances via near fields. It has also been shown that different resonator shapes may lead to different Rabi splittings, though this has not yet been well explained. In this paper, our aim is to understand the correlation between resonator shape and Rabi splitting, in particular to determine and quantify the physical parameters that affect strong coupling by developing an equivalent circuit network model whose elements describe energy and dissipation. Because of the subwavelength dimension of each metasurface element, we resort to the quasistatic (electrostatic) description of the near field and hence define an equivalent capacitance associated to each dipolar element of a flat metasurface. We show that this is also able to accurately model the phenomenology involved in strong coupling between the metasurface and the intersubband transitions in quantum wells. We show that the spectral properties and stored energy of a metasurface/quantum-well system obtained using our model are in good agreement with both full-wave simulation and experimental results. We then analyze metasurfaces made of three resonator geometries and observe that the magnitude of the Rabi splitting increases with the resonator capacitance in agreement with our theory, providing a phenomenological explanation for the resonator shape dependence of the strong coupling process.

We obtain the phase diagram of the half-filled honeycomb Hubbard model with density matrix embedding theory, to address recent controversy at intermediate couplings. We use clusters from 2–12 sites and lattices at the thermodynamic limit. We identify a paramagnetic insulating state, with possible hexagonal cluster order, competitive with the antiferromagnetic phase at intermediate coupling. However, its stability is strongly cluster and lattice size dependent, explaining controversies in earlier work. Our results support the paramagnetic insulator as being a metastable, rather than a true, intermediate phase, in the thermodynamic limit.

We study the effects of short-ranged interactions on the $Z_2$ topological insulator phase, also known as the quantum spin Hall phase, in the Kane-Mele model at half-filling with staggered potentials, which explicitly breaks the discrete particle-hole symmetry. Within Hartree-Fock mean-field analysis, we conclude that the on-site repulsive interactions help stabilize the topological phase (quantum spin Hall) against the staggered potentials by enlarging the regime of the topological phase along the axis of the ratio of the staggered potential strength and the spin-orbit coupling. In sharp contrast, the on-site attractive interactions destabilize the topological phase. We also examine the attractive interaction case by means of the unbiased determinant projector quantum Monte Carlo and the results are qualitatively consistent with the Hartree-Fock picture.

The electronic structures of two significant atomically well-defined (100)-surface reconstructions of gallium phosphide were investigated with two-photon-photoemission spectroscopy (2PPE). We allocated a series of occupied and unoccupied surface states and deduced the influence of each particular reconstruction on the electronic structure of the surface. Photoemission signals arising from bulk optical transitions were distinguished from surface-state related signals by studying the influence of oxygen exposure to the surfaces and by comparing our results to reflectance anisotropy spectroscopy (RAS) measurements. This revealed that the features in the RAS signal correlate with the transition energies between unoccupied and occupied surface-related states that were observed in the 2PPE measurements. An anisotropy around the $E_0^{\prime }$ critical point, previously known from RA spectra, was also investigated with 2PPE and is ascribed to modifications of the bulk electronic structure in the vicinity of the surface.

We have investigated a large set of individual wurtzite/zinc-blende GaAs heterojunction nanowires using transmission electron microscopy (TEM), photoluminescence (PL), and cathodoluminescence (CL). In several cases we have combined PL and TEM on the same wire. We find a fairly deep emission that shows a strong blueshift with increasing excitation power density. By using a variety of experiments we show that this emission is due to recombination across a heterojunction with a type II band alignment. The data give a valence band offset of about 100 meV, in excellent agreement with theoretical predictions. Spatially resolved CL data on heterojunction nanowires and PL data on pure wurtzite nanowires show that the band gap of wurtzite GaAs is very close to the band gap of zinc-blende GaAs.

We present ab initio density functional theory (DFT) calculation results for electronic and spin structures of both the Te- and Sn-terminated SnTe(111) polar surfaces. Rocksalt narrow-gap semiconductor SnTe belongs to the recently discovered class of topological crystalline insulators in which the topological nature of surface electronic states arises from the crystal symmetry combined with band inversion at the $L$ point. We demonstrate that in contrast to earlier model calculations only trivial spin-split states propagating over the entire two-dimensional Brillouin zone emerge at the SnTe(111) surfaces owing to the surface potential effect which destroys weakly protected topological states. We show that the surface passivation eradicates the trivial surface states and recovers the even number of the helical spin-polarized topological Dirac cones centered at the $\overline{\Gamma }$ and $\overline{\mathrm{M}}$ points prescribed for the topological crystalline insulator by the crystal symmetry.

We study the dynamics of a one-dimensional lattice model of hard core bosons which is initially in a superfluid phase with a current being induced by applying a twist at the boundary. Subsequently, the twist is removed, and the system is subjected to periodic $\delta$-function kicks in the staggered on-site potential. We present analytical expressions for the current and work done in the limit of an infinite number of kicks. Using these, we show that the current (work done) exhibits a number of dips (peaks) as a function of the driving frequency and eventually saturates to zero (a finite value) in the limit of large frequency. The vanishing of the current (and the saturation of the work done) can be attributed to a dynamic localization of the hard core bosons occurring as a consequence of the periodic driving. Remarkably, we show that for some specific values of the driving amplitude, the localization occurs for any value of the driving frequency. Moreover, starting from a half-filled lattice of hard core bosons with the particles localized in the central region, we show that the spreading of the particles occurs in a light-cone-like region with a group velocity that vanishes when the system is dynamically localized.

The one-dimensional diffusion of individual Pb atoms on the Si(553)-Au surface has been investigated by a combination of scanning tunneling microscopy (STM), spectroscopy (STS), and first-principles density functional theory. The obtained results unambiguously prove that the diffusion channels are limited to a narrow region between Au chains and step edges of the surface. Much wider channels observed in STM and STS data have electronic origin and result from an interaction of Pb with surface atoms. The length of the channels is determined by a distance between defects at step edges of the Si(553)-Au surface. The defects can act as potential barriers or potential wells for Pb atoms, depending on their origin.

We study the complex plasmonic band structure of nanoscopically thin metallodielectric layers. It is shown that the lifetime and propagation lengths can be obtained with excellent accuracy using a perturbation technique, thus establishing a quantitative connection between mode dissipation and its electromagnetic fields. Ultimately, the perturbation approach is shown to elucidate the physical origin of the orders-of-magnitude variation of lifetime and propagation lengths across the band structure. The symmetrical Brillouin zone center mode is dissipated the least because its electric field is efficiently screened from the metal layers. For a typical silver–titanium dioxide system at visible and infrared frequencies, maximum lifetimes have been found in the subpicosecond and picosecond range, while the longest propagation lengths are direction dependent and in the 10 to 100-micron range. We have also shown that the ratio of the propagation length for a given direction and the lifetime is equal to the corresponding group velocity component.

The superconducting state formed due to direct intraorbital pairing in the tetragonal multiband superconductor ${\text{Sr}}_2$${\text{RuO}}_4$ has different properties than the state formed due to intraband pairing. In particular, the theory operating with direct intraorbital pairing successfully explains the Kerr rotation of reflected light polarization observed several years ago. Here we apply the intraorbital approach to the problem of the Ginzburg-Landau description of the basal plane upper critical field in this material. It is shown that typically for the two-component superconducting state, additional phase transitions in the vortex state at $H<H_{c2}$ for all four crystallographic directions of magnetic field in the basal plane and the basal plane upper critical field anisotropy still are inevitable properties, even in case of direct intraorbital pairing.

The nucleation of metastable germanium polymorphs on decompression is studied using in situ synchrotron x-ray diffraction. We show that the transition pathway is critically dependent on the hydrostaticity. Quasihydrostatic conditions result in the nucleation of the rhombohedral r8 phase, followed by the cubic bc8 and hexagonal diamond phases. In contrast, the presence of shear yields the tetragonal st12 phase. Thus, targeted nucleation of a metastable polymorph is now possible. This observation has implications for the technological exploitation of Ge, but also for other tetrahedral systems.