Bogoliubov–de Gennes theory is used to investigate the effect of the size of a superconducting square on the vortex states in the quantum confinement regime. When the superconducting coherence length is comparable to the Fermi wavelength, the shape resonances of the superconducting order parameter have strong influence on the vortex configuration. Several unconventional vortex states, including asymmetric ones, giant-multivortex combinations, and states comprising giant antivortices, were found as ground states and their stability was found to be very sensitive on the value of $k_F{\xi }_0$, the size of the sample $W$, and the magnetic flux $\Phi$. By increasing the temperature and/or enlarging the size of the sample, quantum confinement is suppressed and the conventional mesoscopic vortex states as predicted by the Ginzburg-Laudau (GL) theory are recovered. However, contrary to the GL results we found that the states containing symmetry-induced vortex-antivortex pairs are stable over the whole temperature range. It turns out that the inhomogeneous order parameter induced by quantum confinement favors vortex-antivortex molecules, as well as giant vortices with a rich structure in the vortex core—unattainable in the GL domain.

In thin-film ferroelectric (FE) capacitors the chemical and electronic structure of the electrode/FE interface can play a crucial role in determining the kinetics of polarization switching. We investigate the electronic structure of a Pt/BaTiO${}_3$/SrTiO${}_3$:Nb capacitor using time-resolved photoemission spectroscopy. The chemical, electronic, and depth sensitivity of core-level photoemission are used to probe the transient response of different parts of the upper electrode/ferroelectric interface to voltage-pulse-induced polarization reversal. The linear response of the electronic structure agrees quantitatively with a simple $\mathit{RC}$ circuit model. The nonlinear response due to the polarization switch is demonstrated by the time-resolved response of the characteristic core levels of the electrode and the ferroelectric. Adjustment of the $\mathit{RC}$ circuit model allows an estimation of the Pt/BaTiO${}_3$ (BTO) interface capacitance. The experiment shows that the interface capacitance is at least 100 times higher than the bulk capacitance of the BTO film, in qualitative agreement with theoretical predictions from the literature.

We present results for the equation of state of the two-dimensional Hubbard model on an isotropic square lattice as obtained from a controlled and numerically exact large-cluster dynamical mean field simulation. Our results are obtained for large but finite systems and are extrapolated to infinite system size using a known finite-size scaling relation, and are supplemented by reliable error bars accounting for all sources of errors. We establish the importance of examining the decay of spatial spin correlations to determine whether a sufficiently large cluster has been used and with this in mind we present the energy, entropy, double occupancy, and nearest-neighbor spin correlations extrapolated to the thermodynamic limit. We discuss the implications of these calculations on pseudogap physics of the 2D Hubbard model away from half filling, where we find a strong behavioral shift in energy below a temperature $T^{*}$ which becomes more pronounced for larger clusters. Finally, we provide reference calculations and tables for the equation of state for values of doping away from half filling which are of interest to cold-atom experiments.

We compute current correlators of the $CP$${}^{N-1}$ field theory in 2+1 dimensions, both at the critical point and in the phase with spontaneously broken $\mathrm{SU}\left(N\right)$ symmetry. Universal constants are obtained to next-to-leading order in the $1/N$ expansion. Implications are noted for quantum critical points of antiferromagnets and their vicinity.

The electronic properties of macromolecular semiconductor thin films depend profoundly on their solid-state microstructure, which in turn is governed, among other things, by the processing conditions selected and the polymer's chemical nature and molecular weight. Specifically, low-molecular-weight materials form crystalline domains of cofacially π-stacked molecules, while the usually entangled nature of higher-molecular-weight polymers leads to microstructures comprised of molecularly ordered crystallites interconnected by amorphous regions. Here, we examine the interplay between extended exciton states delocalized along the polymer backbones and across polymer chains within the π stack, depending on the structural development with molecular weight. Such two-dimensional excitations can be considered as Frenkel excitons in the limit of weak intersite coupling. We combine optical spectroscopies, thermal probes, and theoretical modeling, focusing on neat poly(3-hexylthiophene) (P3HT)—one of the most extensively studied polymeric semiconductors—of weight-average molecular weight ($M_{\mathrm{w}}$) of 3–450 kg/mol. In thin-film structures of high-molecular-weight materials ($M_{\mathrm{w}}$ > 50 kg/mol), a balance of intramolecular and intermolecular excitonic coupling results in high exciton coherence lengths along chains (∼4.5 thiophene units), with interchain coherence limited to ∼2 chains. In contrast, for structures of low-$M_{\mathrm{w}}$ P3HT (<50 kg/mol), the interchain exciton coherence is dominant (∼30$\%$ higher than in architectures formed by high-molecular-weight materials). In addition, the spatial coherence within the chain is significantly reduced (by nearly 25$\%$). These observations give valuable structural information; they suggest that the macromolecules in aggregated regions of high-molecular-weight P3HT adopt a more planar conformation compared to low-molecular-weight materials. This results in the observed increase in intrachain exciton coherence. In contrast, shorter chains seem to lead to torsionally more disordered architectures. A rigorous, fundamental description of primary photoexcitations in π-conjugated polymers is hence developed: two-dimensional excitons are defined by the chain-length dependent molecular arrangement and interconnectivity of the conjugated macromolecules, leading to interplay between intramolecular and intermolecular spatial coherence.

The strain effects on the spin-orbit-induced splitting of the valence-band maximum and conduction-band minimum in monolayer MoS${}_2$ and the gap in graphene are calculated using first-principles calculations. The dependence of these splittings on the various symmetry types of the strain is described by means of an effective Hamiltonian based on the method of invariants, and the parameters in the model are extracted by fitting to the theory. These splittings are related to acoustic-phonon deformation potentials or electron-phonon-coupling matrix elements which enter the spin-dependent scattering theory of conduction in these materials.

We use a symmetry approach to construct a systematic derivative expansion of the low-energy effective Hamiltonian modifying the continuum Dirac description of graphene in the presence of nonuniform elastic deformations. We extract all experimentally relevant terms and describe their physical significance. Among them there is a new gap-opening term that describes the Zeeman coupling of the elastic pseudomagnetic field and the pseudospin. We determine the value of the couplings using a generalized tight-binding model.

It is shown that universal transport statistics in the ensembles of scattering matrices with the symmetry index $\beta \in \{1,2,4\}$ can be found from a simple microscopic model. A stochastic Riccati equation for the reflection matrix in this model is mapped exactly to the Dorokhov-Mello-Pereyra-Kumar equation for all $\beta$s. The map is used to obtain transport statistics in a quasi-one-dimensional disordered conductor in a wide range of its parameters from ballistic to localizations regime. The conductance statistics, shot noise suppression, and transmission channel density are calculated and compared with available analytical results. Deviation from the universal statistics is also discussed.

We study nonlinear electronic transport symmetries in Aharonov-Bohm interferometers subjected to inelastic-scattering effects and show that odd (even) conductance terms are even (odd) in the magnetic field when the junction is (left-right) spatially symmetric. This observation does not hold when spatial inversion symmetry is broken, as we show numerically. Under elastic dephasing effects, the Onsager-Casimir symmetry is maintained beyond linear response, irrespective of spatial asymmetries.

In the expanding field of plasmonics, accurate control of the degree of plasmon localization is of crucial importance for tailoring optical properties at the nanoscale. In this paper, the degree of plasmon localization is directly probed by recording the momentum transfer dependence (i.e. the dispersion) of plasmon resonance energies using electron energy loss spectroscopy in the aberration-corrected scanning transmission electron microscope. Limited by the uncertainty principle, resolution in momentum space can easily be tuned by the beam convergence, and it is shown that localization is clearly identifiable, even at low-momentum resolution. In this proof-of-principle study, this technique was applied to multilayer graphene cones containing a varying number of topological defects at their apex. It is shown that a high degree of confinement of the $\pi$ and $\pi +\sigma$ volume plasmons is reached for five pentagonal defects at the cone apex. This effect was attributed to the presence of the topological defects themselves. Furthermore, slight negative refraction was observed for the five-pentagon cone, predominantly affecting the collective excitation of the $\pi$ electrons.

A quantum two-level system coupled to a harmonic oscillator represents a ubiquitous physical system. New experiments in circuit QED and nanoelectromechanical systems (NEMS) achieve unprecedented coupling strength at large detuning between qubit and oscillator, thus requiring a theoretical treatment beyond the Jaynes-Cummings model. Here we present a new method for describing the qubit dynamics in this regime, based on an oscillator correlation function expansion of a non-Markovian master equation in the polaron frame. Our technique yields a new numerical method as well as a succinct approximate expression for the qubit dynamics. These expressions are valid in the experimentally interesting regime of strong coupling at low temperature. We obtain a new expression for the ac Stark shift and show that this enables practical and precise qubit thermometry of an oscillator.

A recent article [A. Raeliarijaona et al., Phys. Rev. Lett. 110, 137205 (2013)] demonstrated the existence of a physical energy resulting from the coupling between the angular momentum density and magnetic moment. Here, we show that the force associated with such energy naturally explains, in a straightforward and simple manner, the occurrence of the intrinsic anomalous Hall effect (AHE). Connections between our model and previous theories of AHE are also drawn. Our model further leads to the prediction of another effect, namely, the formation of an electric field along the direction of the magnetic field. This electric field is directly proportional to the magnetic field and to the scalar product between the components of the electric field and magnetization that are transverse to the magnetic field.

Recent tunneling experiments on InSb hybrid superconductor-semiconductor devices have provided hope for a stabilization of Majorana edge modes in a spin-orbit quantum wire subject to a magnetic field and superconducting proximity effect. Connecting the experimental scenario with a microscopic description poses challenges of a different kind, such as accounting for the effect of interactions on the tunneling properties of the wire. We develop a density matrix renormalization group (DMRG) analysis of the tunneling spectra of interacting Majorana chains, which we explicate for the Kitaev chain model. Our DMRG approach allows us to calculate the spectral function down to zero frequency, where we analyze how the Majorana zero-bias peak is affected by interactions. For topological phase transitions between the topological and trivial superconducting phase in the Majorana wire, the bulk gap closure generically affects the proximity peaks and the Majorana peak.

Using atomistic tight-binding theory with a configuration interaction description of Coulomb and exchange effects, we describe excitons in symmetric quantum dots in a vertical electric field to explain how field-induced manipulation of exciton orientation and phase can eliminate the anisotropic exchange, leading to a drastic reduction of the fine-structure splitting and a 90 degree rotation of polarization, similar to experiment. This reorientation and rephasing of the exciton is done plane-by-plane inside the dot without significant squeezing of the exciton.

We study quenches of the interaction and electron-phonon coupling parameter in the Hubbard-Holstein model, using nonequilibrium dynamical mean field theory. The calculations are based on a generalized Lang-Firsov scheme for time-dependent interactions or externally driven phonons, and an approximate strong-coupling impurity solver. The interaction quench calculations reveal the phonon-assisted decay of excess doublons, while the quenches of the electron-phonon coupling lead to persistent oscillations of the phonons and to a phonon-enhanced doublon production.

We present a detailed analysis of the nonanalytic structure of the free energy for the itinerant ferromagnet near the quantum critical point in two and three dimensions. We analyze a model of electrons with an isotropic dispersion interacting through a contact repulsion. A fermionic version of the quantum order-by-disorder mechanism allows us to calculate the free energy as a functional of the dispersion in the presence of homogeneous and spiraling magnetic order. We resum the leading divergent contributions to derive an algebraic expression for the nonanalytic contribution to free energy from quantum fluctuations. Using a recursion which relates subleading divergences to the leading term, we calculate the full $T=0$ contribution in $d=3$. We propose an interpolating functional form, which allows us to track phase transition lines at temperatures far below the tricritical point and down to $T=0$. In $d=2$, quantum fluctuations are stronger, and nonanalyticities are more severe. Using a similar resummation approach, we find that despite the different nonanalytic structures, the phase diagrams in two and three dimensions are remarkably similar, exhibiting an incommensurate spiral phase near the avoided quantum critical point.

We construct a phase diagram of the parent compound Fe${}_{1+x}$Te as a function of interstitial iron $x$ in terms of the electronic, structural, and magnetic properties. For a concentration of $x<10\%$, Fe${}_{1+x}$Te undergoes a “semimetal” to metal transition at approximately 70 K that is also first-order and coincident with a structural transition from a tetragonal to a monoclinic unit cell. For $x\approx 14\%$, Fe${}_{1+x}$Te undergoes a second-order phase transition at approximately 58 K corresponding to a semimetal to semimetal transition along with a structural orthorhombic distortion. At a critical concentration of $x\approx 11\%$, Fe${}_{1+x}$Te undergoes two transitions: the higher-temperature one is a second-order transition to an orthorhombic phase with incommensurate magnetic ordering and temperature-dependent propagation vector, while the lower-temperature one corresponds to nucleation of a monoclinic phase with a nearly commensurate magnetic wave vector. While both structural and magnetic transitions display similar critical behavior for $x<10\%$ and near the critical concentration of $x\approx 11\%$, samples with large interstitial iron concentrations show a marked deviation between the critical response indicating a decoupling of the order parameters. Analysis of temperature dependent inelastic neutron data reveals incommensurate magnetic fluctuations throughout the Fe${}_{1+x}$Te phase diagram are directly connected to the “semiconductor”-like resistivity above $T_N$ and implicates scattering from spin fluctuations as the primary reason for the semiconducting or poor metallic properties. The results suggest that doping driven Fermi surface nesting maybe the origin of the gapless and incommensurate spin response at large interstitial concentrations.

We present a comprehensive study of two of the most experimentally relevant extensions of Kitaev's spinless model of a one-dimensional $p$-wave superconductor: those involving (i) longer-range hopping and superconductivity and (ii) inhomogeneous potentials. We commence with a pedagogical review of the spinless model and, as a means of characterizing topological phases exhibited by the systems studied here, we introduce bulk topological invariants as well as those derived from an explicit consideration of boundary modes. In time-reversal symmetric systems, we find that the longer range hopping leads to topological phases characterized by multiple Majorana modes. In particular, we investigate a spin model that respects a duality and maps to a fermionic model with multiple Majorana modes; we highlight the connection between these topological phases and the broken symmetry phases in the original spin model. In the presence of time-reversal symmetry breaking terms, we show that the topological phase diagram is characterized by an extended gapless regime. For the case of inhomogeneous potentials, we explore phase diagrams of periodic, quasiperiodic, and disordered systems. We present a detailed mapping between normal state localization properties of such systems and the topological phases of the corresponding superconducting systems. This powerful tool allows us to leverage the analyses of Hofstadter's butterfly and the vast literature on Anderson localization to the question of Majorana modes in superconducting quasiperiodic and disordered systems, respectively. We briefly touch upon the synergistic effects that can be expected in cases where long-range hopping and disorder are both present.

We present a computational scheme for extracting the energy-level alignment of a metal/molecule interface, based on constrained density functional theory and local exchange and correlation functionals. The method, applied here to benzene on Li(100), allows us to evaluate charge-transfer energies, as well as the spatial distribution of the image charge induced on the metal surface. We systematically study the energies for charge transfer from the molecule to the substrate as function of the molecule-substrate distance, and investigate the effects arising from image-charge confinement and local charge neutrality violation. For benzene on Li(100) we find that the image-charge plane is located at about 1.8 Å above the Li surface, and that our calculated charge-transfer energies compare perfectly with those obtained with a classical electrostatic model having the image plane located at the same position. The methodology outlined here can be applied to study any metal/organic interface in the weak coupling limit at the computational cost of a total energy calculation. Most importantly, as the scheme is based on total energies and not on correcting the Kohn-Sham quasiparticle spectrum, accurate results can be obtained with local/semilocal exchange and correlation functionals. This enables a systematic approach to convergence.