We analyze the interaction at the surface of a three-dimensional (3D) topological insulator among 2D electron states belonging to the Dirac cone close to the point of the Brillouin zone and the Rayleigh surface phonon mode. The model deals with an elastic continuum in the long-wavelength limit, in Random Phase Approximation (RPA). Screening of the electronic polarization is quite effective at small wave vectors. On the other hand, the absence of backscattering for the Dirac electrons at the Fermi surface is partly responsible for the reduced influence of the electron-phonon interaction in renormalizing the phonon dispersion at finite wave vectors. We infer that softening of the Rayleigh mode appears as unlikely, at least for the case of a clean and defect-free surface to which our approximate treatment applies. The dielectric response to virtual excitation of the Rayleigh phonon could drive the electron-electron interaction attractive at low frequencies, but the average weak coupling pairing interaction is found to be too small to induce a surface superconducting instability.

In this joint experimental and theoretical work, we investigate collective electronic excitations (plasmons) in free-standing, single-layer graphene. The energy- and momentum-dependent electron energy-loss function was measured up to $50\mathrm{eV}$ along two independent in-plane symmetry directions ($\Gamma M$ and $\Gamma K$) over the first Brillouin zone by momentum-resolved electron energy-loss spectroscopy in a transmission electron microscope. We compare our experimental results with corresponding time-dependent density-functional theory calculations. For finite momentum transfers, good agreement with experiments is found if crystal local-field effects are taken into account. In the limit of small and vanishing momentum transfers, we discuss differences between calculations and the experimentally obtained electron energy-loss functions of graphene due to a finite momentum resolution and out-of-plane excitations.

We study the heat transport along an edge state of a two-dimensional electron gas in the quantum Hall regime, in contact to two reservoirs at different temperatures. We consider two exactly solvable models for the edge state coupled to the reservoirs. The first one corresponds to filling $\nu =1$ and tunneling coupling to the reservoirs. The second one corresponds to integer or fractional filling of the sequence $\nu =1/m$ (with $m$ odd), and capacitive coupling to the reservoirs. In both cases, we solve the problem by means of nonequilibrium Green function formalism. We show that heat propagates chirally along the edge in the two setups. We identify two temperature regimes, defined by $\Delta$, the mean level spacing of the edge. At low temperatures, $T<\Delta$, finite size effects play an important role in heat transport, for both types of contacts. The nature of the contacts manifests itself in different power laws for the thermal conductance as a function of the temperature. For capacitive couplings, a highly nonuniversal behavior takes place, through a prefactor that depends on the length of the edge as well as on the coupling strengths and the filling fraction. For larger temperatures, $T>\Delta$, finite-size effects become irrelevant, but the heat transport strongly depends on the strength of the edge-reservoir interactions, in both cases. The thermal conductance for tunneling coupling grows linearly with $T$, whereas for the capacitive case, it saturates to a value that depends on the coupling strengths and the filling factors of the edge and the contacts.

We calculate from first principles the electronic structure and optical properties of a number of transition metal dichalcogenide (TMD) bilayer heterostructures consisting of MoS${}_2$ layers sandwiched with WS${}_2$, MoSe${}_2$, MoTe${}_2$, BN, or graphene sheets. Contrary to previous works, the systems are constructed in such a way that the unstrained lattice constants of the constituent incommensurate monolayers are retained. We find strong interaction between the $\Gamma$-point states in all TMD/TMD heterostructures, which can lead to an indirect gap. On the other hand, states near the $K$ point remain as in the monolayers. When TMDs are paired with BN or graphene layers, the interaction around the $\Gamma$-point is negligible, and the electronic structure resembles that of two independent monolayers. Calculations of optical properties of the MoS${}_2$/WS${}_2$ system show that, even when the valence- and conduction-band edges are located in different layers, the mixing of optical transitions is minimal, and the optical characteristics of the monolayers are largely retained in these heterostructures. The intensity of interlayer transitions is found to be negligibly small, a discouraging result for engineering the optical gap of TMDs by heterostructuring.

We introduce an atomistic approach to the dissipative quantum dynamics of charged or neutral excitations propagating through macromolecular systems. Using the Feynman-Vernon path integral formalism, we analytically trace out from the density matrix the atomic coordinates and the heat bath degrees of freedom. This way we obtain an effective field theory which describes the real-time evolution of the quantum excitation and is fully consistent with the fluctuation-dissipation relation. The main advantage of the field-theoretic approach is that it allows us to avoid using the Keldysh contour formulation. This simplification makes it straightforward to derive Feynman diagrams to analytically compute the effects of the interaction of the propagating quantum excitation with the heat bath and with the molecular atomic vibrations. For illustration purposes, we apply this formalism to investigate the loss of quantum coherence of holes propagating through a poly(3-alkylthiophene) polymer.

The zero and finite temperature spin-Peierls transitions in a quasi-one-dimensional spin-$\frac{1}{2}$ Heisenberg model coupled to adiabatic bond phonons is investigated using the stochastic series expansion (SSE) quantum Monte Carlo (QMC) method. The quantum phase transition from a gapless Néel state to a spin-gapped Peierls state is studied in the parameter space spanned by spatial anisotropy, interchain coupling strength, and spin-lattice coupling strength. It is found that for any finite interchain coupling, the transition to a dimerized Peierls ground state only occurs when the spin-lattice coupling exceeds a finite, nonzero critical value. This is in contrast to the pure 1D model (zero interchain coupling), where adiabatic/classical phonons lead to a dimerized ground state for any nonzero spin-phonon interaction. The phase diagram in the parameter space shows that for a strong interchain coupling, the relation between the interchain coupling and the critical value of the spin-phonon interaction is linear whereas for weak interchain coupling, this behavior is found to have a natural logarithmlike relation. No region was found to have a long range magnetic order and dimerization occurring simultaneously. Instead, the Néel state order vanishes simultaneously with the setting in of the spin-Peierls state. For the thermal phase transition, a continuous heat capacity with a peak at the critical temperature $T_c$ shows a second order phase transition. The variation of the equilibrium bond length distortion ${\delta }_{eq}$ with temperature showed a power law relation which decayed to zero as the temperature was increased to $T_c$, indicating a continuous transition from the dimerized phase to a paramagnetic phase with uniform bond length and zero antiferromagnetic susceptibility.

The Na atomic ordering and the local Co magnetism, in a metallic antiferromagnet made of Na${}_{0.825}$CoO${}_2$, are investigated by nuclear magnetic-resonance (NMR) techniques. Multiple well-resolved NMR peaks of ${}^{23}$Na and ${}^{59}$Co are observed, which suggests a superlattice structure forming in this Na-vacant cobaltate. Examining clues from published x-ray data, we refine the lattice superstructure so as to obtain the best description for the peaks seen in our NMR data. Magnetic Co ions are found to form a striped structure, which produces nontrivial magnetism. Analyzing the NMR frequency shift for each peak, we observe a frequency-shift anomaly, i.e., the breakdown of linearity between the NMR shift (that is, the local susceptibility) and the bulk susceptibility. We find that the shift anomaly can be described by an additional susceptibility component emerging at $\sim$60 K. Slow spin dynamics are observed near a metamagnetic transition where the spins flip between the antiferromagnetic and ferromagnetic orders. Unlike in ordinary spin glasses, this glassy behavior is induced by a magnetic field, which causes competition between the in-plane ferromagnetic and out-of-plane antiferromagnetic interactions, in a magnetic-frustrated triangular lattice.

Ba${}_3$Si${}_6$O${}_{12}$N${}_2$:Eu${}^{2+}$ and Ba${}_3$Si${}_6$O${}_9$N${}_4$:Eu${}^{2+}$ have strikingly similar atomistic structures, but the former is an efficient green phosphor at working temperature while the latter is a bluish-green phosphor whose luminescence decreases quite fast with temperature. Aiming at the understanding of such different behavior, we compute the quasiparticle electronic band structure of the two hosts, Ba${}_3$Si${}_6$O${}_{12}$N${}_2$ and Ba${}_3$Si${}_6$O${}_9$N${}_4$, thanks to many-body perturbation theory in the $G_0{W}_0$ approximation. The gap differs by about 0.43 eV. We analyze the eigenfunctions at the top of the valence band, at the bottom of the conduction band, and also the chemical shifts for the Ba site in the two hosts. The valence bands, directly impacted by the different stoichiometric ratio, are not thought to play a large role in the luminescence. Deceivingly, the dispersive bottom of the conduction band, directly related to luminescent properties, is similar in both compounds. The spatial topology of the probability density of the bottom of the conduction bands differs, as well as the location of the 5d peak, with a much higher energy than the bottom of the conduction band in Ba${}_3$Si${}_6$O${}_{12}$N${}_2$ than in Ba${}_3$Si${}_6$O${}_9$N${}_4$. Electromagnetic absorption spectra are also computed for both compounds.

Heavily potassium-substituted barium iridates ${\mathrm{Ba}}_{2-x}{\mathrm{K}}_x{\mathrm{IrO}}_4$ ($x$≤0.5) and lanthanum-substituted ${\mathrm{Ba}}_{2-y}{\mathrm{La}}_y{\mathrm{IrO}}_4$ ($y$≤0.05) were prepared for the study of the carrier doping effects on the transport properties of the spin-orbit Mott state. The carrier type is holelike for the nondoped ($x$ $=$ $y$ $=$ 0) and K-doped ($x$>0) phases, while it is electronlike for the La-doped ($y$>0) phases. It was found that electron doping is more effective in decreasing the electrical resistivity. This suggests an asymmetry of the density of states between the upper and lower energies of the Fermi level. A semimetallic state emerges for the K-doped phases with $x$≥0.3 at ambient pressure. More conducting metallic states ($\rho \sim {10}^{-2}\Omega \mathrm{cm}$, $d$ρ/$dT$>0) were achieved under high pressure for both the K- and La-doped phases. Notwithstanding, no superconductivity was observed in the metallic states down to 4.2 K. The experimental results are discussed with respect to the electronic phase diagram calculated by Watanabe et al. [Phys. Rev. Lett. 110, 027002 (2013)].

Linear conductance below $2e^2/h$ shows resonance peaks in highly asymmetric quantum point contacts (QPCs). As the channel length increases, the number of peaks also increases. At the same time, differential conductance exhibits zero-bias anomalies (ZBAs) in correspondence with every other peak in the linear conductance. This even-odd effect, observable in the longer channels, is consistent with the formation of correlation-induced quasilocalized states within the QPC. In rare cases, triple peaks are observed, indicating the formation of a spin-1 Kondo effect when the electron filling number is even. Changing the gate voltage tunes this spin triplet to a singlet which exhibits no ZBA. The triple peak provides evidence suggestive of a spin singlet-triplet transition in a QPC, and the presence of a ferromagnetic spin interaction between electrons.

Using time-resolved photoluminescence spectroscopy, we explore the transient behavior of bound and free excitons in GaN nanowire ensembles. We investigate samples with distinct diameter distributions and show that the pronounced biexponential decay of the donor-bound exciton observed in each case is not caused by the nanowire surface. At long times, the individual exciton transitions decay with a common lifetime, which suggests a strong coupling between the corresponding exciton states. A system of nonlinear rate equations taking into account this coupling directly reproduces the experimentally observed biexponential decay.

We investigate strained In${}_x$Ga${}_{1-x}$As/GaAs and strain-free GaAs/Al${}_{0.3}$Ga${}_{0.7}$As quantum dots (QDs) grown on (110)-oriented substrates by means of atomic empirical pseudopotentials and configuration interaction. We find that there is a significant piezoelectric effect on the exciton fine structure splitting (FSS) in strained QDs due to the in-plane character of the electric field. The ground-state bright excitons in these QDs are polarized along the [001] and [$\overline{1}$10] crystal directions, and have large FSSs. The linear degree of polarization is large and decreases linearly with an increase in Ga content, while it is nearly zero and independent of Ga content in In${}_x$Ga${}_{1-x}$As/GaAs QDs with similar shape and size grown on (001) substrates.

In order to increase the coherence time of topological quantum memories in systems with Majorana zero modes, it has recently been proposed to encode the logical qubit states in nonlocal Majorana operators which are immune to localized excitations involving the unpaired Majorana modes. In this encoding, a logical error only happens when the quasiparticles, subsequent to their excitation, travel a distance of the order of the spacing between the Majorana modes. Here, we study the decay time of a quantum memory encoded in a clean topological nanowire interacting with an environment with a particular emphasis on the propagation of the quasiparticles above the gap. We show that the nonlocal encoding does not provide a significantly longer coherence time than the local encoding. In particular, the characteristic speed of propagation is of the order of the Fermi velocity of the nanowire and not given by the much slower group velocity of quasiparticles which are excited just above the gap.

Magnetism in single-side hydrogenated (C${}_2$H) and fluorinated (C${}_2$F) graphene is analyzed in terms of the Heisenberg model with parameters determined from first principles. We predict a frustrated ground state for both systems, which means the instability of collinear spin structures and sheds light on the absence of a conventional magnetic ordering in defective graphene demonstrated in recent experiments. Moreover, our findings suggest a highly correlated magnetic behavior at low temperatures offering the possibility of a spin-liquid state.

We have studied the spin-polarized three-dimensional homogeneous electron gas using the diffusion quantum Monte Carlo method, with trial wave functions including backflow and three-body correlations in the Jastrow factor, and we have used twist averaging to reduce finite-size effects. Calculations of the pair-correlation function, including the on-top pair density, as well as the structure factor and the total energy, are reported for systems of 118 electrons in the density range $r_{\mathrm{s}}=0.5$–20 a.u., and for spin polarizations of 0, 0.34, 0.66, and 1. We consider the spin resolution of the pair-correlation function and structure factor, and the energy of spin polarization. We show that a control variate method can reduce the variance when twist averaging, and we have achieved higher accuracy and lower noise than earlier quantum Monte Carlo studies.

We present the model of a quantum dot (QD) consisting of a spherical core-bulk heterostructure made of three-dimensional (3D) topological insulator (TI) materials, such as PbTe/Pb${}_{0.31}$Sn${}_{0.69}$Te, with bound massless and helical Weyl states existing at the interface and being confined in all three dimensions. The number of bound states can be controlled by tuning the size of the QD and the magnitude of the core and bulk energy gaps, which determine the confining potential. We demonstrate that such bound Weyl states can be realized for QD sizes of few nanometers. We identify the spin locking and the Kramers pairs, both hallmarks of 3D TIs. In contrast to topologically trivial semiconductor QDs, the confined massless Weyl states in 3D TI QDs are localized at the interface of the QD and exhibit a mirror symmetry in the energy spectrum. We find strict optical selection rules satisfied by both interband and intraband transitions that depend on the polarization of electron-hole pairs and therefore give rise to the Faraday effect due to the Pauli exclusion principle. We show that the semiclassical Faraday effect can be used to read out spin quantum memory. When a 3D TI QD is embedded inside a cavity, the single-photon Faraday rotation provides the possibility to implement optically mediated quantum teleportation and quantum information processing with 3D TI QDs, where the qubit is defined by either an electron-hole pair, a single electron spin, or a single hole spin in a 3D TI QD. Remarkably, the combination of interband and intraband transition gives rise to a large dipole moment of up to 450 Debye. Therefore, the strong-coupling regime can be reached for a cavity quality factor of $Q\approx {10}^4$ in the infrared wavelength regime of around $10\mu$m.

Two recent experiments [Radu et al., Science 320, 899 (2008); Lin et al., Phys. Rev. B 85, 165321 (2012)] measured the temperature and voltage dependence of the quasiparticle tunneling through a quantum point contact in the $\nu =5/2$ quantum Hall liquid. The results led to conflicting conclusions about the nature of the quantum Hall state. In this paper, we show that the conflict can be resolved by recognizing different geometries of the devices in the experiments. We argue that in some of those geometries there is significant unscreened electrostatic interaction between the segments of the quantum Hall edge on the opposite sides of the point contact. Coulomb interaction affects the tunneling current. We compare experimental results with theoretical predictions for the Pfaffian, $\mathrm{SU}{\left(2\right)}_2$, 331, and $K=8$ states and their particle-hole conjugates. After Coulomb corrections are taken into account, measurements in all geometries agree with the spin-polarized and spin-unpolarized Halperin 331 states.

We apply the density functional theory for superconductors based on the local density approximation (LDA) to alkali-doped fullerides $A_3$C${}_{60}$ with a face-centered-cubic structure. We evaluate the superconducting transition temperature ($T_{\mathrm{c}}$) from first principles considering the energy dependence of electron-phonon coupling, the mass renormalization, and the retardation effect. The calculated $T_{\mathrm{c}}=7.5$, 9.0, and 15.7 K for $A=\text{K}$, Rb, Cs are approximately 60% smaller than the experimentally observed values. Our results strongly suggest the necessity to go beyond the framework of the Migdal-Eliashberg theory based on the LDA.

We investigate a single vortex reconnection event in superfluid helium at finite temperatures using the vortex filament model. The reconnection induces Kelvin waves which strongly increase energy dissipation. We evaluate the mutual friction dissipation from the reconnection and show that the dissipation power has universal form, which is seen by scaling both time (measured from the reconnection event) and power by the mutual friction parameter $\alpha$. This observation allows us to conclude that the Kelvin wave cascade is not important in the energy dissipation process within the range $\alpha \gtrsim {10}^{-3}$. Rather, the energy is directly transferred from Kelvin waves to the normal component. Moreover, while the excited Kelvin waves greatly enhance energy dissipation, no similar change is seen in angular momentum from the reconnection event. This result has similarities with recent ${}^3$He-B measurements. Our results also confirm another earlier observation that the minimum distance between vortices scales approximately as $d\propto \sqrt{|t-t_{\mathrm{rec}}|}$, both before and after the reconnection event.

How magnetoelectric coupling actually occurs on a microscopic level in multiferroic BiFeO${}_3$ is not well known. By using high-resolution single crystal neutron diffraction techniques, we have determined the electric polarization of each individual element of BiFeO${}_3$, and concluded that magnetostrictive coupling suppresses the electric polarization at the Fe site below $T_N$. This negative magnetoelectric coupling appears to outweigh the spin current contributions arising from the cycloid spin structure, which should produce positive magnetoelectric coupling.