We analyze transport through a quantum point contact in fractional quantum Hall states with counter-propagating neutral edge modes. We show that both the noise (as expected and previously calculated by other authors) and (perhaps surprisingly) the average transmitted current are affected by downstream perturbations within the standard edge state model. We consider two scenarios for downstream perturbations. We argue that the change in transmitted current should be observable in experiments that have shown increased noise.

An antiferromagnetically coupled FM/NM/FM (FM = ferromagnet, NM = normal metal) trilayer structure responds to an external magnetic field by the formation of a magnetic-moment spring within the FM layers. We show that the exchange stiffness $\left(A_{\mathrm{ex}}\right)$ of an FM layer can be determined by fitting the field-dependent magnetization, $M\left(H\right)$, of the FM/NM/FM trilayer to a micromagnetic model. Using this method, we have measured the exchange stiffness of thin-film Co alloyed with Cr, Fe, Ni, Pd, Pt, and Ru. The results show that the rate at which a substituent element reduces the exchange stiffness is not directly related to its effect on the magnetization of the alloy. The observed trends have been understood by material-specific modeling based on density functional theory within the local density approximation. The stiffness measurements are in agreement with Brillouin light scattering carried out on thicker Co films.

The transport properties of a large-spin molecule strongly coupled to ferromagnetic leads in the presence of transverse magnetic anisotropy are studied theoretically. The relevant spectral functions, linear-response conductance, and the tunnel magnetoresistance are calculated by means of the numerical renormalization group method. We study the dependence of transport characteristics on orbital level position, uniaxial and transverse anisotropies, external magnetic field, and temperature. It is shown that while uniaxial magnetic anisotropy leads to the suppression of the Kondo effect, finite transverse anisotropy can restore the Kondo resonance. The effect of Kondo peak restoration strongly depends on the magnetic configuration of the device and leads to nontrivial behavior of the tunnel magnetoresistance. We show that the temperature dependence of the conductance at points where the restoration of the Kondo effect occurs is universal and shows a scaling typical for usual spin-one-half Kondo effect.

An analytical study is presented of the double resonant Raman scattering process in graphene, responsible for the D and ${\mathrm{D}}^{\prime }$ features in the Raman spectra. This work yields analytical expressions for the D and ${\mathrm{D}}^{\prime }$ integrated Raman intensities that explicitly show the dependencies on laser energy, defect concentration, and electronic lifetime. Good agreement is obtained between the analytical results and experimental measurements on samples with increasing defect concentrations and at various laser excitation energies. The use of Raman spectroscopy to identify the nature of defects is discussed. Comparison between the models for the edge-induced and the disorder-induced D-band intensity suggests that edges or grain boundaries can be distinguished from disorder by the different dependence of their Raman intensity on laser excitation energy. Similarly, the type of disorder can potentially be identified not only by the intensity ratio $I_{\mathrm{D}}/I_{{\mathrm{D}}^{\prime }}$, but also by its laser energy dependence. Also discussed is a quantitative analysis of quantum interference effects of the graphene wave functions, which determine the most important phonon wave vectors and scattering processes responsible for the D and ${\mathrm{D}}^{\prime }$ bands.

The linear conductance of a two-terminal Aharonov-Bohm interferometer is an even function of the applied magnetic flux, as dictated by the Onsager-Casimir symmetry. Away from linear response this symmetry may be broken when many-body interactions are in effect. Using a numerically exact simulation tool, we study the dynamics and the steady-state behavior of the out-of-equilibrium double-dot Aharonov-Bohm interferometer, while considering different types of interactions: Model I includes a closed interferometer with an interdot electron-electron repulsion energy. In model II the interferometer is interacting with a dissipative environment, possibly driven away from equilibrium. In both cases we show that depending on the (horizontal, vertical) mirror symmetries of the setup, nonlinear transport coefficients obey certain magnetosymmetries. We compare numerically exact simulations to phenomenological approaches and special limits: The behavior of model I is compared to self-consistent mean-field calculations and master equation results in the Coulomb blockade regime. Model II, allowing heat dissipation to a thermal bath, is mimicked by an Aharonov-Bohm junction with a voltage probe. In both cases we find that phenomenological treatments capture the relevant transport symmetries, yet significant deviations in magnitude may show up.

We report on low-temperature scanning tunneling spectroscopy measurements on epitaxial graphene flakes on Au(111). We show that using quasiparticle interference (QPI) mapping, we can discriminate between the electronic systems of graphene and Au(111). Beyond the scattering vectors, which can be ascribed to the elastic scattering within each of the systems, we observe QPI features related to the scattering process between graphene states and the Au(111) surface state. This additional interband scattering process at the graphene/Au(111) interface allows the direct quantitative determination of the Rashba-splitting of the Au(111) surface state, which cannot be evaluated from QPI measurements on pure Au(111). This experiment demonstrates a unique local spectroscopic approach to investigate the Rashba-split bands at weakly interacting epitaxial graphene/substrate interfaces.

We present a systematic topological classification of fermionic and bosonic topological phases protected by time-reversal, particle-hole, parity, and combination of these symmetries. We use two complementary approaches: one in terms of $K$-theory classification of gapped quadratic fermion theories with symmetries, and the other in terms of the $K$-matrix theory description of the edge theory of (2+1)-dimensional bulk theories. The first approach is specific to free fermion theories in general spatial dimensions while the second approach is limited to two spatial dimensions but incorporates effects of interactions. We also clarify the role of the CPT theorem in the classification of symmetry-protected topological phases, and show, in particular, that the topological superconductors discussed before are related by the CPT theorem.

We present a multiscale modeling of the infrared optical properties of the rubrene crystal. The results are in very good agreement with the experimental data that point to nonmonotonic features in the optical conductivity spectrum and small optical effective masses. We find that, in the static-disorder approximation, the nonlocal electron-phonon interactions stemming from low-frequency lattice vibrations can decrease the optical effective masses and lead to lighter quasiparticles. On the other hand, the charge-transport and infrared optical properties of the rubrene crystal at room temperature are demonstrated to be governed by localized carriers driven by inherent thermal disorders. Our findings underline that the presence of apparently light carriers in high-mobility organic semiconductors does not necessarily imply bandlike transport.

We present results of tight-binding spin-dynamics simulations of individual and pairs of substitutional Mn impurities in GaAs. Our approach is based on the mixed quantum-classical scheme for spin dynamics, with coupled equations of motions for the quantum subsystem, representing the host, and the localized spins of magnetic dopants, which are treated classically. In the case of a single Mn impurity, we calculate explicitly the time evolution of the Mn spin and the spins of nearest-neighbor As atoms, where the acceptor (hole) state introduced by the Mn dopant resides. We relate the characteristic frequencies in the dynamical spectra to the two dominant energy scales of the system, namely, the spin-orbit interaction strength and the value of the $p-d$ exchange coupling between the impurity spin and the host carriers. For a pair of Mn impurities, we find signatures of the indirect (carrier-mediated) exchange interaction in the time evolution of the impurity spins. Finally, we examine temporal correlations between the two Mn spins and their dependence on the exchange coupling and spin-orbit interaction strength, as well as on the initial spin-configuration and separation between the impurities. Our results provide insight into the dynamic interaction between localized magnetic impurities in a nanoscaled magnetic-semiconductor sample, in the extremely-dilute (solotronics) regime.

We studied electrical transport through a Pb monolayer on Ge(111), which has a metallic surface state with a large Rashba spin splitting. The Pb monolayer was found to be metallic down to 9 K. The macroscopic sheet conductivity was $10.2\pm 1.0\text{mS}/\square$ at 9 K, which is much higher than those of other metallic monolayers. We found that the microscopic step conductivity is two orders of magnitude larger than those in other metallic monolayers, being the main cause of the high macroscopic conductivity. The transport electron-phonon $(e$-ph) coupling constant obtained by a temperature-dependent conductance measurement is $0.16\pm 0.03$, indicating weak $e$-ph coupling compared to bulk Pb (1.1–1.5). This result is supported by temperature-dependent angle-resolved photoelectron spectroscopy.

Bottom-up synthesized armchair graphene nanoribbons with a width of seven C = C bonds (7-aGNRs) have gained widespread interest due to their large band gap of about 2.8 eV and their atomically precise edge structures. Here, we report on the lifetime of excited states of spatially aligned 7-aGNRs grown on a vicinal Au(788) surface. Time-resolved three-photon photoemission spectroscopy at $h\nu =3.15$ eV was carried out to measure the lifetimes of states located at $E_1-E_F=3.6$ eV and $E_2-E_F=3.8$ eV via a resonant excitation from an unoccupied state at $E-E_F=0.6$ eV. Lifetimes of ${\tau }_1=(110\pm 13)$ fs and ${\tau }_2=(75\pm 10)$ fs are observed, respectively.

We investigate the effects of homogeneous and inhomogeneous deformations and edge disorder on the conductance of gated graphene nanoribbons. Under increasing homogeneous strain the conductance of such devices initially decreases before it acquires a resonance structure and, finally, becomes completely suppressed at higher strain. Edge disorder induces mode mixing in the contact regions, which can restore the conductance to its ballistic value. The valley-antisymmetric pseudomagnetic field induced by inhomogeneous deformations leads to the formation of additional resonance states, which originate either from the coupling into Fabry-Pérot states that extend through the system or from the formation of states that are localized near the contacts, where the pseudomagnetic field is largest. In particular, the $n=0$ pseudo-Landau level manifests itself via two groups of conductance resonances close to the charge neutrality point.

The behavior of ${\mathrm{PbTiO}}_3$ under uniaxial strains and stresses is investigated from first-principles calculations within density functional theory. We show that irrespective of the uniaxial mechanical constraint applied, the system keeps a purely ferroelectric ground state, with the polarization aligned either along the constraint direction $({\mathrm{FE}}_z$ phase) or along one of the pseudocubic axes perpendicular to it $({\mathrm{FE}}_x$ phase). This contrasts with the cases of isotropic and biaxial mechanical constraints for which novel phases combining ferroelectric and antiferrodistortive motions have been previously reported. Under uniaxial strain, ${\mathrm{PbTiO}}_3$ switched from an ${\mathrm{FE}}_x$ ground state under compressive strain to an ${\mathrm{FE}}_z$ ground state under tensile strain beyond a critical strain ${\eta }_{zz}^c\approx +1$%. Under uniaxial stress, ${\mathrm{PbTiO}}_3$ exhibits either an ${\mathrm{FE}}_x$ ground state under compression $\left({\sigma }_{zz}<0\right)$ or an ${\mathrm{FE}}_z$ ground state under tension $\left({\sigma }_{zz}>0\right)$. Here, however, an abrupt jump of the structural parameters is also predicted under both compressive and tensile stresses at critical values ${\sigma }_{zz}\approx$ $+2$ and $-8$ GPa. This behavior appears to be similar to that predicted under negative isotropic pressure and might turn out to be practically useful for enhancing the piezoelectric response in nanodevices.

To clarify the electronic, lattice, and magnetic contribution to the inverse magnetocaloric effect (IMCE) in the metamagnetic shape-memory alloy ${\mathrm{Ni}}_{45}{\mathrm{Co}}_5{\mathrm{Mn}}_{50-x}{\mathrm{In}}_x$, magnetization, magnetocaloric effect, and specific-heat measurements were carried out in a wide range of fields and temperatures. The IMCEs of ${\mathrm{Ni}}_{45}{\mathrm{Co}}_5{\mathrm{Mn}}_{36.7}{\mathrm{In}}_{13.3}$ were directly measured as adiabatic temperature changes in pulsed fields of up to 15 T. A maximum temperature decrease of $-12.8$ K was observed. The low-temperature specific heats in both the austenitic and the martensitic phases of ${\mathrm{Ni}}_{45}{\mathrm{Co}}_5{\mathrm{Mn}}_{36.5}{\mathrm{In}}_{13.5}$ were measured by using steady fields. Through analyses of the data, the entropy changes in charge (1.2 J/kg K), spin $(-29$ J/kg K), and lattice (51 J/kg K) sectors were individually evaluated. The result demonstrates the dominant role of the lattice sector in inverse MCEs in this material.

We report a new and improved approach that uses low-temperature resistivity recovery measurements to study the defect kinetics in metallic binary alloys. This method is able to decouple the effect related to the irradiation defect contribution to the resistivity from that of the short-range order, which is enhanced by the free migration of defects. This approach can provide reliable experimental data which are more suitable for comparisons with current computational models. Furthermore, the difference in this method with respect to the classical one is that our method gives information concerning the role of vacancies and interstitials on short-range order. The method is applied to a model alloy Fe-5%Cr, of interest for fusion applications, where short-range order effects have been previously found to play a role.

Nanoscale stripe structures of the parent iron-11 superconductor ${\mathrm{Fe}}_{1.033}\text{Te}$ were investigated using low-temperature scanning tunnel microscopy-scanning tunnel spectroscopy (STM-STS). STM topographies and $dI/dV$ maps show clear stripe structures with the bias-dependent multiple periods $2\times a_0$ and $a_0$, where $a_0$ is the lattice constant $\sim 0.38$ nm. The form of the stripe structures seen on $dI/dV$ maps strongly depends on the bias voltage. Varying stripe structures are apparently driven by magnetic order appearing below the transition temperature $T_s\sim 72$ K, that is defined by the noticeable drop in the temperature dependence of resistivity, and are strongly influenced by the underlying excess Fe.

In a $D=2+1$ quantum critical system, the entanglement entropy across a boundary with a corner contains a subleading logarithmic scaling term with a universal coefficient. It has been conjectured that this coefficient is, to leading order, proportional to the number of field components $N$ in the associated $\text{O}\left(N\right)$ continuum ${\varphi }^4$ field theory. Using density matrix renormalization group calculations combined with the powerful numerical linked cluster expansion technique, we confirm this scenario for the $\text{O}\left(2\right)$ Wilson-Fisher fixed point in a striking way, through direct calculation at the quantum critical points of two very different microscopic models. The value of this corner coefficient is, to within our numerical precision, twice the coefficient of the Ising fixed point. Our results add to the growing body of evidence that this universal term in the Rényi entanglement entropy reflects the number of low-energy degrees of freedom in a system, even for strongly interacting theories.

Samarium hexaboride $\left({\mathrm{SmB}}_6\right)$, a representative Kondo insulator, has been characterized recently as a likely topological insulator. It is also a material with strong electron correlations, evident by the temperature dependence of its band gap and the existence of a nearly flat collective mode whose energy lies within the band gap. Similar strong correlations can affect or even destabilize the two-dimensional metallic state of topological origin at the crystal boundary. Here we construct the minimal lattice model of the correlated boundary of topological Kondo insulators, and make phenomenological predictions for its possible ground states. Depending on the microscopic properties of the interface between the topological Kondo material and a conventional insulator, the boundary metal can exhibit a varied degree of hybridization between the $d$ and $f$ orbitals of the rare-earth element, yielding a rich two-dimensional heavy-fermion phenomenology. A pronounced participation of the $f$ orbitals is expected to create a heavy-fermion Dirac metal, possibly unstable to a spin density wave, electron localization, or even superconductivity. The opposite limit of “localized magnetic moments” helped by the partial Kondo screening on the crystal boundary can bring about a non-Fermi liquid of $d$ electrons that exhibits two-dimensional quantum electrodynamics, or other unconventional states. In addition, ultrathin films made from topological Kondo insulators could open the possibility of creating exotic incompressible quantum liquids with non-Abelian fractional excitations, whose dynamics shaped by the strong Rashba spin-orbit coupling resembles that of fractional quantum Hall systems.