Magnetically doped topological insulators are studied intensively in the search for exotic phenomena such as the quantum anomalous Hall effect. The interplay of electronic and impurity degrees of freedom leads to the Kondo effect, which is an increase in the resistivity at temperatures $T<T_K$ (the Kondo temperature). We study this effect in chiral surface state transport at $T\ge T_K$ in the metallic regime, starting from the quantum Liouville equation and including Kondo scattering to all orders, as well as phonon and nonmagnetic impurity scattering. Unlike spin-orbit coupled metals and semiconductors, $T_K$ is suppressed by spin-momentum locking, which prevents the formation of a Kondo screening cloud. We expect a resistivity ${\rho }_{xx}\propto T^4$ primarily due to phonons.

We study symmetry-protected features in the quasiparticle interference (QPI) pattern of two-dimensional (2D) systems with mirror symmetries and time-reversal symmetry, around a single static point impurity. We show that, in the Fourier-transformed local density of states (FT-LDOS) $\rho (\mathbf{q},\omega )$, while the position of high-intensity peaks generically depends on the geometric features of the iso-energy contour at energy $\omega$, the absence of certain peaks is guaranteed by the opposite mirror eigenvalues of the two Bloch states that are (i) on the mirror-symmetric lines in the Brillouin zone (BZ) and (ii) separated by scattering vector $\mathbf{q}$. We apply the general result to the QPI on the $\langle 001\rangle$ surface of the topological crystalline insulator Pb${}_{1-x}$Sn${}_x$Te and predict all vanishing peaks in $\rho (\mathbf{q},\omega )$. The model-independent analysis is supported by numerical calculations using an effective four-band model derived from symmetry analysis.

We investigate the scaling of the entanglement spectrum and of the Rényi block entropies and determine its universal aspects in the ground state of critical and noncritical one-dimensional quantum spin models. In all cases, the scaling exhibits an oscillatory behavior that terminates at the factorization point and whose frequency is universal. Parity effects in the scaling of the Rényi entropies for gapless models at zero field are thus shown to be a particular case of such universal behavior. Likewise, the absence of oscillations for the Ising chain in transverse field is due to the vanishing value of the factorizing field for this particular model. In general, the transition occurring at the factorizing field between two different scaling regimes of the entanglement spectrum corresponds to a quantum transition to the formation of finite-range, ordered structures of quasidimers, quasitrimers, and quasipolymers. This entanglement-driven transition is superimposed to and independent of the long-range magnetic order in the broken symmetry phase. Therefore, it conforms to recent generalizations that identify and classify the quantum phases of matter according to the structure of ground-state entanglement patterns. We characterize this form of quantum order by a global order parameter of entanglement defined as the integral, over blocks of all lengths, of the Rényi entropy of infinite order. Equivalently, it can be defined as the integral of the bipartite single-copy or geometric entanglement. The global entanglement order parameter remains always finite at fields below the factorization point and vanishes identically above it.

Spin-orbit interaction in quantum wires leads to a spin resonance at low temperatures, even in the absence of an external dc magnetic field. We study the effect of electron-electron interaction on the resonance. This interaction is strong in quantum wires. We show that the electron-electron interaction changes the shape of the resonance curve and produces an additional cusp at the plasmon frequency. However, except for very strong electron-electron interaction these changes are weak since this interaction by itself does not break the spin-rotation symmetry that is violated weakly by the spin-orbit interaction and external magnetic field.

We show that rf irradiation drastically increases coherence of the charge-density wave (CDW) depinning and sliding in the electric field for NbS${}_3$ (phase II), TaS${}_3$, and (TaSe${}_4$)${}_2$I. The effect is observed in the range of the CDW currents at which the fundamental frequencies are well below the frequency of the irradiation. The result is evident from the shape of $I$-$V$ curves, Shapiro steps, and noise spectra. We treat the effect in terms of the rf-assisted relaxation of metastable states forming at CDW sliding. The rf irradiation can be helpful in experiments, where utmost coherence of the CDW transport is needed, e.g., in studies of the Shapiro steps, expanding the ranges of their observation.

In this paper we use a three-dimensional Pauli master equation to investigate the charge carrier mobility of a two-phase system which can mimic donor-acceptor and amorphous-crystalline bulk heterojunctions. By taking the energetic disorder of each phase, their energy offset, and domain morphology into consideration, we show that the carrier mobility can have a completely different behavior when compared to a one-phase system. When the energy offset is equal to zero, the mobility is controlled by the more disordered phase. When the energy offset is nonzero, we show that the mobility electric field dependence switches from negative to positive at a threshold field proportional to the energy offset. Additionally, the influence of morphology, through the domain size and volume ratio parameters, on the transport is investigated and an approximate analytical expression for the zero field mobility is provided.

We have realized a hybrid solid-state quantum device in which a single-electron semiconductor double quantum dot is dipole coupled to a superconducting microwave frequency transmission line resonator. The dipolar interaction between the two entities manifests itself via dispersive and dissipative effects observed as frequency shifts and linewidth broadenings of the photonic mode respectively. A Jaynes-Cummings Hamiltonian master equation calculation is used to model the combined system response and allows for determining both the coherence properties of the double quantum dot and its interdot tunnel coupling with high accuracy. The value and uncertainty of the tunnel coupling extracted from the microwave read-out technique are compared to a standard quantum point contact charge detection analysis. The two techniques are found to be consistent with a superior precision for the microwave experiment when tunneling rates approach the resonator eigenfrequency. Decoherence properties of the double dot are further investigated as a function of the number of electrons inside the dots. They are found to be similar in the single-electron and many-electron regimes suggesting that the density of the confinement energy spectrum plays a minor role in the decoherence rate of the system under investigation.

We demonstrate a severe Fermi contour anisotropy induced by the application of a parallel magnetic field to high-mobility electrons confined to a 30-nm-wide (001) GaAs quantum well. We study commensurability oscillations, namely, geometrical resonances of the electron orbits with a unidirectional, surface-strain-induced, periodic potential modulation, to directly probe the size of the Fermi contours along and perpendicular to the parallel field. Their areas are obtained from the Shubnikov-de Haas oscillations. Our experimental data agree semiquantitatively with the results of parameter-free calculations of the Fermi contours, but there are significant discrepancies.

The unique versatility of the electronic structures of organic molecules can be potentially utilized to engineer single-molecular electronic devices with specific functionalities. Here, we report on how the electronic structures and the transport properties of molecular junctions containing a $\pi$-conjugated terephthalic acid molecule in a scanning tunneling microscopy (STM) configuration can be tuned by modifying their chemical composition at a single-atom level. More specifically, this strategy implies (i) to change the molecular core through a chemical functionalization process and (ii) to modify the chemical nature of the STM-tip-apex atom. In this respect, our first-principles calculations of the electronic structures and the corresponding electron transport reveal that by the insertion and increase of the number of N atoms in the six-membered benzenelike aromatic ring, the electron transmission at the Fermi level increases. However, the calculated electron transmission at the Fermi level does not depend significantly on the specific position of the N atom in the aromatic ring. Nevertheless, when the tip-apex atom is changed from Cu to W, the electron transmission of the molecular junction significantly changes in an energy range above the Fermi level.

We report an unusual temperature dependence of exciton lifetimes in arrays of GaN nanostructures grown on semipolar (11-22) oriented Al${}_{0.5}$Ga${}_{0.5}$N alloy by molecular beam epitaxy. Atomic force microscopy measurements revealed: (i) a one-dimensional ordering tendency along the [1-100] crystallographic direction together with (ii) an in-plane anisotropy of the nanostructure lateral shape with respect to [1-100] and [11-23] crystallographic axes. As a consequence, a morphological transition from dot-shaped islands forming an array of nanochains to wire-shaped objects elongated along the [1-100] direction was evidenced with the increase of the GaN deposited amount. Nanostructures of different dimensionality were fabricated including quantum dots (QDs), quantum wires (QWRs), and quantum wells (QWs), and the excitonic behavior was investigated as a function of the nanostructure shape. The measured temperature dependencies of the exciton radiative decay revealed its direct correlation with a spatial confinement, resulting in a temperature-independent exciton lifetime in the case of QDs, a square root dependence in the case of QWRs, and a linear dependence for QWs. These results, as well as absolute values of measured lifetimes, are in agreement with theoretical predictions.

We investigate the behavior of low-energy electrons in two-dimensional molybdenum disulfide when submitted to an external magnetic field. Highly degenerate Landau levels form in the material, between which light-induced excitations are possible. The dependence of excitations on light polarization and energy is explicitly determined, and it is shown that it is possible to induce valley and spin polarization, i.e., to excite electrons of selected valley and spin. Whereas the effective low-energy model in terms of massive Dirac fermions yields dipole-type selection rules, higher-order band corrections allow for the observation of additional transitions. Furthermore, inter-Landau-level transitions involving the $n=0$ levels provide a reliable method for an experimental measurement of the gap and the spin-orbit gap of molybdenum disulfide.

The phase transformations of ceramic PbZr${}_{1-x}$Ti${}_x$O${}_3$ are studied at the composition $x=0.05$ close to the morphotropic phase boundary between antiferroelectric orthorhombic $Pbam$ and ferroelectric rhombohedral phases by elastic, dielectric, and x-ray diffraction measurements. Temperature scans at different rates show that the kinetics of the transition from $R3m$ to $Pbam$ is very sluggish, and is actually split into a slower polar component, appearing in the dielectric susceptibility and a nonpolar component, visible as a sharp stiffening of the Young's modulus. The latter is assigned to the establishment of the oxygen octahedral tilt pattern of the $Pbam$ phase, and occurs at once at a temperature that decreases with increasing cooling rate. On heating, the transition occurs at a temperature 65 K higher than on quasistatic cooling, so that both components occur with a faster rate almost together. The Young's modulus presents a variety of apparently different types of anomalies, depending on temperature rate and history, which, however, can be well reproduced in terms of steplike anomalies corresponding to the two components of the $Pbam\leftrightarrow R3m$ transition and the tilt $R3c\leftrightarrow R3m$ transition in the region of coexistence of the FE and AFE phases.

We report the results of various measurements, namely magnetization, complex dielectric permittivity, and electric polarization ($P$), on Dy${}_2$BaNiO${}_5$ as a function of temperature ($T$) and magnetic field ($H$), apart from heat capacity ($C$), with the primary motivation of exploring the existence of magnetoelectric (ME) coupling among Haldane spin-chain systems. The $M$($T$) and $C$($T$) data establish long-range magnetic ordering at 58 K. The most noteworthy observations are: (i) Distinct anomalies are observed in the dielectric constant (ε′) vs $T$ and loss (tanδ) vs $T$ at different temperatures (i.e., 12.5, 30, 50, and 58 K); at low temperatures, three magnetic-field-induced transitions are observed in ε′ vs $H$ at 6, 40, and 60 kOe. These transition temperatures and critical magnetic fields track those obtained from magnetization data, establishing the existence of strong magnetoelectric coupling in this compound. (ii) Correspondingly, electric polarization could be observed as a function of $T$ and $H$ in the magnetically ordered state, thereby indicating magnetism-induced ferroelectricity in this compound; this result suggests that this compound is a possible multiferroic material among spin $=$1 (nickel-containing) compounds, with successive magnetic transitions and strong magnetoelectric coupling.

We point out that essentially all multiband superconductors have an odd-frequency pairing component, as follows from a general symmetry analysis of even- and odd-frequency pairing states. We show that odd-frequency superconducting pairing requires only a finite band hybridization, or scattering, and nonidentical intraband order parameters, of which only one band needs to be superconducting. Under these conditions odd-frequency odd-interband pairing is always present. From a symmetry analysis we establish a complete reciprocity between parity in band index and frequency.

We show that a wide class of unconventional quantum criticality emerges when orbital currents cause quantum phase transitions from zero-gap semiconductors such as Dirac fermions to a topological insulator or a Chern insulator. Changes in Fermi-surface topology concomitant with [SU(2) or time-reversal] symmetry breakings generate quantum critical lines (QCLs) even beyond the quantum critical point. This QCL running at temperature $T=0$ separates two distinct topological phases. This is in contrast to the simple termination of the finite-temperature critical line at the quantum critical point without any extension of it at $T=0$. Topology change causes the unconventionality beyond the concept of simple spontaneous symmetry breaking assumed in the conventional Landau-Ginzburg-Wilson scenario. The unconventional universality implied by mean-field critical exponents $\beta >1/2$ and $\delta <3$ is protected by the existence of the quantum critical line. It emerges for several specific lattice models including the honeycomb, kagome, diamond, and pyrochlore lattices. We also clarify phase diagrams of the topological phases in these lattices at finite temperatures.

We revisit the transport of spin-degrees of freedom across an electrically and thermally biased tunnel junction between two ferromagnets with noncollinear magnetizations. Besides the well-known charge current and spin current we show that a nonzero spin-quadrupole current flows between the ferromagnets. This tensor-valued current describes the nonequilibrium transport of spin anisotropy relating to both local and nonlocal multiparticle spin correlations of the circuit. This quadratic spin anisotropy, quantified in terms of the spin-quadrupole moment, is fundamentally a two-electron quantity. In spin valves with an embedded quantum dot such currents have been shown to result in a quadrupole accumulation that affects the measurable quantum dot spin and charge dynamics. The spin-valve model studied here allows fundamental questions about spin-quadrupole storage and transport to be worked out in detail, while ignoring the detection by a quantum dot. The physical understanding of this particular device is of importance for more complex devices where spin-quadrupole transport can be detected. We demonstrate that, as far as storage and transport are concerned, the spin anisotropy is only partly determined by the spin polarization. In fact, for a thermally biased spin valve the charge current and spin current may vanish, while a pure exchange spin-quadrupole current remains, which appears as a fundamental consequence of Pauli's principle. We extend the real-time diagrammatic approach to efficiently calculate the average of multiparticle spin observables, in particular the spin-quadrupole current. Although the paper addresses only leading-order and spin-conserving tunneling, we formulate the technique for arbitrary order in an arbitrary, spin-dependent tunnel coupling in a way that lends itself to extension to quantum-dot spin-valve structures.

The topological physics of quantum Hall states is efficiently encoded in purely topological quantum field theories of the Chern-Simons type. The reliable inclusion of low-energy dynamical properties in a continuum description, however, typically requires proximity to a quantum critical point. We construct a field theory that describes the quantum transition from an isotropic to a nematic Laughlin liquid. The soft mode associated with this transition approached from the isotropic side is identified as the familiar intra-Landau level Girvin-MacDonald-Platzman mode. We obtain $z=2$ dynamic scaling at the critical point and a description of Goldstone and defect physics on the nematic side. Despite the very different physical motivation, our field theory is essentially identical to a recent “geometric” field theory for a Laughlin liquid proposed by Haldane.

The transition between the long-wavelength and the short-wavelength regimes of light propagation in two-dimensional graded photonic crystal is investigated using a hyperspectral near-field scanning microscope. The experiments show an invariant quantity of only 1.78 times the lattice period as the criterion for the possible application of homogenization theories. These results are discussed in light of Fourier decomposition of the electromagnetic Bloch waves, and a physical interpretation of the observed transition between the two light propagation regimes is proposed. These results indicate the robustness of the homogenization approaches and suggest that the sharp transition between the two light propagation regimes could be profitably combined in graded optical artificial materials.

We study the renormalization group (RG) equations of a pair of spin-boson systems coupled in the $z$ direction with each other. Each spin is coupled to a different bath of harmonic oscillators. We introduce a systematic adiabatic RG, which generalizes the first-order adiabatic renormalization previously used for the single spin-boson model, and we obtain the flow equations for the tunneling constant, the dissipation strength, and the interspin coupling up to third order in the tunneling. If one of the two spins is treated as a constant magnetization, the other spin is described by a biased spin-boson Hamiltonian. In this case the RG equations we find coincide with the ones obtained via a mapping to a long-range Ising chain. If the whole Ohmic two-spin system is considered the Kosterlitz-Thouless phase transition is replaced by a second-order phase transition. In the case of a sub-Ohmic bath our approach predicts that the two-spin system is always localized.

Using scanning tunneling microscopy supplemented with first-principles calculations, we examine all the thermally activated atomistic processes of Ag${}_{\mathrm{n}}$ ($n$ $=$ 1–26) constructed atom-by-atom on a Si(111)-(7×7) substrate, and we exploit such cluster dynamical information to further determine the energetic stability (or magicity) of the clusters. By generalizing the traditional concept of cluster magicity solely based on cluster association/dissociation to also include various complex collective cluster motions, we identify the existence of two classes of magic clusters. The most stable class, Ag${}_{10}$ and Ag${}_{25}$, is defined by geometrical shell closure; the less stable class of Ag${}_n$ ($n$ $=$ 3, 5, 13, 16, 19) is associated with lower kinetic barriers against internal restructuring of, or atom detachment from, their respective clusters of neighboring sizes. Our detailed analysis also reveals that the substrate effect, rather than the number of bonds within the clusters, dominates the cluster stabilities. The conceptual advances gained in the present study are broadly applicable to many related cluster systems in contact with external media, and they are expected to be instrumental in tuning the dynamical behaviors of clusters in surface catalysis, nanoplasmonics, and other technological areas.