We calculate the tunneling current through a Fabry-Pérot interferometer in the fractional quantum Hall regime. Within linear response theory (weak tunneling but arbitrary source-drain voltage), we find a general expression for the current due to tunneling of quasiparticles in terms of Carlson's $R$ function. Our result is valid for fractional quantum Hall states with an edge theory consisting of a charged channel and any number of neutral channels, with possibly different edge velocities and different chiralities. We analyze the case with a single neutral channel in detail, which applies for instance to the edge of the Moore-Read state. In addition, we consider an asymmetric interferometer with different edge lengths between the point contacts on opposite edges, and we study the behavior of the current as a function of varying edge length. Recent experiments attempted to measure the Aharanov-Bohm effect by changing the area inside the interferometer using a plunger gate. Theoretical analyses of these experiments have so far not taken into account the accompanying change in the edge lengths. We show that the tunneling current exhibits multiple oscillations as a function of this edge length, with frequencies proportional to the injected edge current and inversely proportional to the edge velocities. In particular, the edge velocities can be measured by looking at the Fourier spectrum of the edge current. We provide a numerical scheme to calculate and plot the $R$ function, and include sample plots for a variety of edge states with parameter values, which are experimentally relevant.

In this paper, we report a systematic study on the effect of different van der Waals dispersion correction methods in conjunction with Density Functional Theory on the adsorption characteristics of a monolayer of metal-free phthalocyanine on Au(111). The chosen dispersion corrections were DFT-D2, the Tkatchenko-Scheffler method with and without self-consistent screening, and four sparse-matter density functionals. A comparison among different dispersion corrections was performed and the results are related to available experimental scanning tunnel microscopy and x-ray standing-wave measurements for similar molecules on Au(111). We found that the Tkatchenko-Scheffler method as well as a sparse-matter density functional which employs the exchange potential of optB86b and the nonlocal correlation of Dion describe the adsorbed system, e.g., electronic and geometric structure with an adsorption distance of $3.3$ Å, reasonably well within moderate computational costs.

Magnetotransport experiments on epitaxial GaAs/InAs core/shell nanowires are performed in which the InAs shell forms a tube-like conductive channel around the highly resistive GaAs core. The core/shell nanowires are grown by molecular beam epitaxy. It is found that the nanowire conductance oscillates with the magnetic field oriented parallel to its axis, with a period of the magnetic flux quantum ${\varphi }_0=h/e$. Related to that, it is shown that the electronic transport is mediated by closed loop quantum states encircling the wire axis rather than by electron interference of partial waves. By means of a gate voltage the conductance at zero magnetic field can be changed between an oscillation minimum and maximum. The experimental findings are supported by numerical calculations.

Rare-earth materials epitaxially codeposited with III-V semiconductors form small, spherical rare-earth-monopnictide nanoparticles embedded within the III-V host. The small size of these particles (approximately 1.5 nm diameter) suggests that interesting electronic properties might emerge as a result of both confinement and surface states. However, ErAs nanoparticles do not exhibit any signs of quantum confinement or an emergent band gap, and these experimental observations are understood theoretically. We use ultrafast pump-probe spectroscopy to investigate the electronic structure of TbAs nanoparticles embedded in a GaAs host, which were expected to be similar to ErAs. We study the dynamics of carrier relaxation into the TbAs states, which essentially act as traps, using optical-pump terahertz-probe transient absorption spectroscopy. By analyzing how the carrier relaxation rates depend on pump fluence and sample temperature, we conclude that the TbAs states are saturable. Saturable traps suggest the existence of a band gap for TbAs nanoparticles, in sharp contrast with the results for ErAs.

We present a microscopic study of optical properties of Bernal-stacked bilayer graphene. Our focus lies on the absorption spectrum explicitly including the impact of fully momentum-dependent optical and Coulomb matrix elements. Our approach is based on the density matrix formalism combined with tight-binding wave functions. The energy dispersion of bilayer graphene exhibits four parabolic bands resulting in interesting optical features: We find a pronounced low-energy peak, which can be clearly ascribed to cross transitions at the Dirac point. Furthermore, the bilayer spectrum shows two energetically close absorption peaks in the ultraviolet region resulting from interband transitions at the saddle point of the Brillouin zone. We discuss the impact of the carrier-light coupling element containing the optical selection rules and determining the oscillator strength of all possible optical transitions. Furthermore, we show the influence of the Coulomb interaction accounting for a considerable overall energy renormalization and the formation of electron-hole pairs at the saddle point.

Two-particle scattering in graphene is a multichannel problem, where the energies of the identical or opposite-helicity channels lie in disjoint energy segments. Due to the absence of Galilean invariance, these segments depend on the total momentum $Q$. The dispersion relations for the two opposite-helicity scattering channels are analogous to those of two one-dimensional tight-binding lattices with opposite dispersion relations, which are known to easily bind states at their edges. When an $s$-wave separable interaction potential is assumed, those bound states reveal themselves as three Feshbach resonances in the identical-helicity channel. In the limit $Q\to 0$, one of the resonances survives and the opposite-helicity scattering amplitudes vanish.

The thermal conductivity of a model for solid argon is investigated using nonequilibrium molecular dynamics methods, as well as the traditional Boltzmann transport equation approach with input from molecular dynamics calculations, both with classical and quantum thermostats. A surprising result is that, at low temperatures, only the classical molecular dynamics technique is in agreement with the experimental data. We argue that this agreement is due to a compensation of errors and raise the issue of an appropriate method for calculating thermal conductivities at low (below Debye) temperatures.

The nesting of the Fermi surfaces of an electron and a hole pocket separated by a nesting vector $\mathbf{Q}$ and the interaction between electrons give rise to itinerant antiferromagnetism. The order can gradually be suppressed by mismatching the nesting and a quantum critical point is obtained as the Néel temperature tends to zero. If the vector $\mathbf{Q}$ is commensurate with the lattice (umklapp with $\mathbf{Q}=\mathbf{G}/2$), pairs of electrons can be transferred between the pockets. This process may lead to superconductivity and a superconducting dome above the quantum critical point. We investigate the conditions under which such a dome arises. The specific heat is studied as a function of temperature and nesting mismatch, and the crossover from Fermi liquid to non-Fermi liquid is obtained.

Local and average structures of ceramic ${\mathrm{Ba}}_{1-x}$Sr${}_x$${\text{TiO}}_3$ solid solutions have been reconciled using Reverse Monte Carlo (RMC) refinements of atomic positions that employ simultaneous fitting of neutron total scattering data, x-ray absorption fine structure, and patterns of diffuse scattering in electron diffraction. These refinements enable explicit reconstruction of three-dimensional atomic configurations without any effective parameters. The results reveal cube- and parallelepiped-shaped probability density distributions for Ti atoms in the cubic and tetragonal phases of BaTiO${}_3$, respectively. These distributions are consistent with the split Ti sites (eight for the cubic and four for the tetragonal polymorphs) separated by ≈0.2 Å. The characters of Ti distributions are retained in the solid solutions, but the magnitude of Ti off-centering decreases as $x$ increases. The Ti displacements remain correlated along the octahedral chains, at least up to $x$ = 0.5, as manifested in the sheets of diffuse scattering in electron diffraction patterns; the correlation parameters and lengths have been quantified using RMC analyses. The ion-size difference between Ba and Sr is accommodated through the approximately isotropic relaxation of the oxygen atoms, which shift toward Sr; a similar relaxation is observed for the Ti atoms. Local Ti off-centering diminishes as the number of Sr atoms in the coordination groups [TiO${}_x$Ba${}_{8-n}$Sr${}_n$] increases.

We derive the equations of motionof a domain wall in a thin-film magnet coupled to the surface states of a topological insulator, in the presence of both an electric field along the domain wall and a magnetic field perpendicular to the junction. We show how the electric field acts as a chirality stabilizer holding off the appearance of Walker breakdown and enhancing the terminal velocity of the wall. We also propose a mechanism to reverse the domain wall chirality in a controllable manner, by tuning the chiral current flowing through the wall. An input from a weak perpendicular magnetic field is required in order to break the reflection symmetry that protects the degeneracy of the chirality vacuum.

When an Anderson impurity is immersed in the bulk of a topological insulator that has an “inverted-Mexican-hat” band dispersion, a Kondo resonant peak appears simultaneously with an in-gap bound state. The latter generates another spin state thereby screening the Kondo effect. Using a weak-coupling RG scheme, it is shown that the system exhibits complex crossover behavior between different symmetry configurations and may evolve into a self-screened Kondo or an $\mathrm{SO}\left(n\right)(n=3,4)$ low-energy fixed point. Experimental consequences of these scenarios are pointed out.

Using a recently developed renormalization group method for fermionic superfluids, we determine conditions for $d$-wave superconductivity in the ground state of the two-dimensional Hubbard model at moderate interaction strength, and we compute the pairing gap in the superconducting regime. A pairing instability signaled by a divergent flow in the Cooper channel leads to a superconducting state in all studied cases. The next-to-nearest-neighbor hopping $t^{\prime }$ plays a crucial role in the competition between antiferromagnetism and superconductivity. A sizable $t^{\prime }$ is necessary to obtain a sizable pairing gap.

We report dc magnetic measurements on YbCu${}_2$Si${}_2$ at pressures above 10 GPa using a miniature ceramic anvil cell. YbCu${}_2$Si${}_2$ shows a pressure-induced transition from the nonmagnetic to a magnetic phase at 8 GPa. We find a spontaneous dc magnetization in the pressure-induced phase above 9.4 GPa. The pressure dependence of the ferromagnetic transition temperature $T_{\mathrm{C}}$ and the spontaneous magnetic moment ${\mu }_0$ at 2.0 K have been determined. The value of ${\mu }_0$ in the present macroscopic measurement is less than half of that determined via Mössbauer experiments. The difference may be attributed to a spatial phase separation between the ferromagnetic and paramagnetic phases. This separation suggests that the pressure-induced phase boundary between the paramagnetic and ferromagnetic states is of first order. Further, we have studied the magnetic anisotropy in the pressure-induced ferromagnetic state. The effect of pressure on the magnetization with magnetic field along the magnetic easy $c$ axis is much larger than for field along the hard $a$ axis in the tetragonal structure. The pressure-induced phase has strong Ising-type uniaxial anisotropy, consistent with the two crystal electric field models proposed for YbCu${}_2$Si${}_2$.

We investigate the Majorana physics and its effect on the electron transport in the nontopological superconductor (NS)–topological superconductor (TS) double junctions of a loop geometry. We find that, depending on the ratio between the lengths of two topologically different regions and the localization lengths of the Majorana fermions formed between them, two completely different transport mechanisms are working: perfect crossed Andreev reflection (CAR) for the short NS segment and perfect normal Andreev reflection for the short TS segment. The difference is explained in terms of the topologically distinct properties of subgap states in two regions, which have not been revealed so far. The exotic dependence of the CAR process on the magnetic flux threading the loop is uncovered and can be used to detect the Majorana fermions.

We theoretically analyze the surface density of states of heavy fermion materials such as CeCoIn${}_5$. Recent experimental progress has made it possible to locally probe the formation of heavy quasiparticles in these systems via scanning tunneling microscopy, in which strongly temperature-dependent resonances at the Fermi energy have been observed. The shape of these resonances varies depending on the surface layer, i.e., if cerium or cobalt terminates the sample. We clarify the microscopic origin of this difference by taking into account the layered structure of the material. Our simple model explains all the characteristic properties observed experimentally, such as a layer-dependent shape of the resonance at the Fermi energy, displaying a hybridization gap for the cerium layer and a peak or dip structure for the other layers. Our proposal resolves the seemingly unphysical assumptions in the preceding analysis based on the two-channel cotunneling model.

We report low temperature magnetotransport measurements on a high mobility ($\mu =325000{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$) two-dimensional electron system on a H-terminated Si(111) surface. We observe the integral quantum Hall effect at all filling factors $\nu \le 6$ and find that $\nu =2$ develops in an unusually narrow temperature range. An extended, exclusively even numerator, fractional quantum Hall hierarchy occurs surrounding $\nu =3/2$, consistent with twofold valley-degenerate composite fermions (CFs). We determine activation energies and estimate the CF mass.

We demonstrate a possibility for exciton Bose-Einstein condensation in individual small-diameter ($\sim$$1\text{--}2$ nm) semiconducting carbon nanotubes. The effect occurs under the exciton-interband-plasmon coupling controlled by an external electrostatic field applied perpendicular to the nanotube axis. It requires fields $\sim$1 V/nm and temperatures below 100 K that are experimentally accessible. The effect offers a testing ground for fundamentals of condensed matter physics in one dimension and opens up perspectives to develop tunable coherent polarized light source with carbon nanotubes.

The nonlinear charge mobility and current-induced spin polarization in the nonlinear surface transport of a topological insulator with a hexagonal deforming band are investigated in the presence of a strong dc electric field by means of a balance-equation approach for a single energy band. The nonlinear mobility exhibits a broad peak with respect to increasing electric field in warping systems. The in-plane perpendicular spin polarization is also sensitive to the electric field, warping constant, surface density, and temperature. An out-of-plane spin polarization, the magnitude of which is strongly carrier-density dependent, emerges in systems with warping bands at high electric fields, and drops quickly with increasing temperature.

The mechanical behavior of crystals is dominated by dislocation networks, their structure, and their interactions with impurities or thermal phonons. However, in classical crystals, networks are usually random with impurities often forming nonequilibrium clusters when their motion freezes at low temperature. Helium provides unique advantages for the study of dislocations: Crystals are free of all but isotopic impurities, the concentration of these can be reduced to the parts per ${10}^9$ (ppb) level, and the impurities are mobile at all temperatures and therefore remain in equilibrium with the dislocations. We have achieved a comprehensive study of the mechanical response of ${}^4$He crystals to a driving strain as a function of temperature, frequency, and strain amplitude. The quality of our fits to the complete set of data strongly supports our assumption of stringlike vibrating dislocations. It leads to a precise determination of the distribution of dislocation network lengths and to detailed information about the interaction between dislocations and both thermal phonons and ${}^3$He impurities. The width of the dissipation peak associated with impurity binding is larger than predicted by a simple Debye model, and much of this broadening is due to the distribution of network lengths.

We present the evolution of the initial (up to $\sim 10$ kbar) hydrostatic pressure dependencies of $T_c$ and of the ambient pressure, and the jump in the heat capacity associated with the superconducting transition as a function of Na doping in the Ba${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$ family of iron-based superconductors. For Na concentrations $0.15\le x\le 0.9$, the jump in specific heat at $T_c$, $\Delta C_p{|}_{T_c}$, follows the $\Delta C_p\propto T^3$ (the so-called BNC scaling) found for most BaFe${}_2$As${}_2$-based superconductors. This finding suggests that, unlike the related Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ series, there is no significant modification of the superconducting state (e.g., change in superconducting gap symmetry) in the Ba${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$ series over the whole studied Na concentration range. Pressure dependencies are nonmonotonic for $x=0.2$ and $0.24$. For other Na concentrations, $T_c$ decreases under pressure in an almost linear fashion. The anomalous behavior of the $x=0.2$ and $0.24$ samples under pressure is possibly due to the crossing of the phase boundaries of the narrow antiferromagnetic tetragonal phase, unique for the Ba${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$ series, with the application of pressure. The negative sign of the pressure derivatives of $T_c$ across the whole superconducting dome (except for $x=0.2$) is a clear indication of the nonequivalence of substitution and pressure for the Ba${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$ series.