Experiments on the fractional quantized Hall effect in the zeroth Landau level of graphene have revealed some striking differences between filling factors in the ranges $0<\left|\nu \right|<1$ and $1<\left|\nu \right|<2$. We argue that these differences can be largely understood as a consequence of the effects of terms in the Hamiltonian which break SU(2) valley symmetry, which we find to be important for $\left|\nu \right|<1$ but negligible for $\left|\nu \right|>1$. The effective absence of valley anisotropy for $\left|\nu \right|>1$ means that states with an odd numerator, such as $\left|\nu \right|=5/3$ or 7/5, can accommodate charged excitations in the form of large-radius valley skyrmions, which should have a low energy cost and may be easily induced by coupling to impurities. The absence of observed quantum Hall states at these fractions is likely due to the effects of valley skyrmions. For $\left|\nu \right|<1$, the anisotropy terms favor phases in which electrons occupy states with opposite spins, similar to the antiferromagnetic state previously hypothesized to be the ground state at $\nu =0$. The anisotropy and Zeeman energies suppress large-area skyrmions, so that quantized Hall states can be observable at a set of fractions similar to those in GaAs two-dimensional electron systems. In a perpendicular magnetic field $B$, the competition between the Coulomb energy, which varies as $B^{1/2}$, and the Zeeman energy, which varies as $B$, can explain the observation of apparent phase transitions as a function of $B$ for fixed $\nu$, as transitions between states with different degrees of spin polarization. In addition to an analysis of various fractional states from this point of view and an examination of the effects of disorder on valley skyrmions, we present new experimental data for the energy gaps at integer fillings $\nu =0$ and $\nu =-1$, as a function of magnetic field, and we examine the possibility that valley skyrmions may account for the smaller energy gaps observed at $\nu =-1$.

We investigate the effect of Rashba and intrinsic spin-orbit couplings on the electronic properties and spin configurations of Dirac fermions confined in: (i) a flat graphene sheet, (ii) a graphene wire with $p$-$n$-$p$ structure, and (iii) a superlattice of graphene wires. The interplay between the spin-orbit interaction mechanisms breaks the electron-hole symmetry and the spin configuration induced by Rashba spin-orbit coupling lacks inversion symmetry in $k$ space. We show that the Rashba spin-orbit interaction doubles the Fabry-Pérot resonant modes in the transmission spectrum of a graphene wire and opens new channels for the electron transmission. Moreover, it leads to the appearance of spin split extra Dirac cones in the energy spectrum of a graphene superlattice. It is shown that the spin of the electrons and holes confined in a flat graphene sheet is always perpendicular to their motion while this is not the case for the other nanostructures.

We study the electron transmission through the domain boundary on bilayer graphene separating $\mathit{AB}$ and $\mathit{BA}$ stacking regions. Using the effective continuum model, we calculate the electron transmission probability as a function of the electron energy and the incident angle, for several specific boundary structures. The transmission strongly depends on the crystallographic direction of the boundary and also on the atomic configuration inside. At low energy, the boundary is either insulating or highly transparent depending on the structure. In insulating cases, the transmission sharply rises when the Fermi energy is increased to a certain level, suggesting that the electric current through the boundary can be controlled by the field effect. The boundary parallel to the zigzag direction generally has different transmission properties between the two different valleys, and this enables one to generate the valley polarized current in a certain configuration. We show that those characteristic features can be qualitatively explained by the transverse momentum conservation in the position-dependent band structure in the intermediate region.

We explore the dynamical properties of a one-dimensional Bose-Hubbard model, where two bosonic species interact via Feshbach resonance. We focus on the region in the phase diagram which is described by an effective, low-energy ferromagnetic Ising model in both transverse and longitudinal fields. In this regime, we numerically calculate the dynamical structure factor of the Bose-Hubbard model using the time-evolving block decimation method. In the ferromagnetic phase, we observe both the continuum of excitations and the bound states in the presence of a longitudinal field. Near the Ising critical point, we observe the celebrated $E_8$ mass spectrum in the excited states. We also point out possible measurements which could be used to detect these excitations in an optical lattice experiment.

Here we report on a mathematical description for the neutron dark-field image (DFI) contrast based on the influence of the thickness-dependent beam broadening caused by scattering interactions and multiple refraction in the sample. We conduct radiography experiments to verify that the DFI signal exponentially decays as a function of thickness for both magnetic and nonmagnetic materials. Here we introduce a material-dependent parameter, the so-called linear diffusion coefficient $\Omega$. This allows us to perform a quantitative DFI-computed tomography. Additionally, we conduct correlative small-angle neutron-scattering experiments and validate the mathematical assumption that the angular broadening of the direct beam is proportional to the square root of the number of discrete layers.

Weyl semimetal is a phase of matter that provides a solid state realization of chiral Weyl fermions. Most of its unique physics is a consequence of chiral anomaly, namely nonconservation of the number of particles of a given chirality. Mathematically, this is expressed in the appearance of the so called $\theta$ term in the action of the electromagnetic field, when the Weyl fermions are integrated out. Recently, however, it has been suggested that the analogy between the chiral fermions of quantum field theory with unbounded linear dispersion, and their solid state realization with a dispersion naturally bounded by the bandwidth and crystal momentum defined only within the first Brillouin zone, holds only in a restricted sense, with parts of the $\theta$ term absent. Here we demonstrate that this is not the case. We explicitly derive the $\theta$ term for a microscopic model of a Weyl semimetal by integrating out fermions coupled to electromagnetic field, and show that the result has exactly the same form as in the case of relativistic chiral fermions.

We developed an analytical model to describe hopping transport in organic semiconductors including both energetic disorder and polaronic contributions due to geometric relaxation. The model is based on a Marcus jump rate in terms of the small-polaron concept with a Gaussian energetic disorder, and it is premised upon a generalized effective medium approach yet avoids shortcomings involved in the effective transport energy or percolation concepts. It is superior to our previous treatment [Phys. Rev. B 76, 045210 (2007)] since it is applicable at arbitrary polaron activation energy $E_a$ with respect to the energy disorder parameter $\sigma$. It can be adapted to describe both charge-carrier mobility and triplet exciton diffusion. The model is compared with results from Monte Carlo simulations. We show (i) that the activation energy of the thermally activated hopping transport can be decoupled into disorder and polaron contributions whose relative weight depend nonlinearly on the $\sigma /E_a$ ratio, and (ii) that the choice of the density of occupied and empty states considered in configurational averaging has a profound effect on the results of calculations of the Marcus hopping transport. The $\sigma /E_a$ ratio governs also the carrier-concentration dependence of the charge-carrier mobility in the large-carrier-concentration transport regime as realized in organic field-effect transistors. The carrier-concentration dependence becomes considerably weaker when the polaron energy increases relative to the disorder energy, indicating the absence of universality. This model bridges a gap between disorder and polaron hopping concepts.

Large negative magnetoconductance (MC) of ∼12$\%$ is observed in electrochemically doped polymer light-emitting diodes at sub-band-gap bias voltages ($V_{\mathrm{bias}}$). Simultaneously, a positive magnetoefficiency (Mη) of 9$\%$ is observed at $V_{\mathrm{bias}}$ $=$ 2 V. At higher bias voltages, both the MC and Mη diminish while a negative magnetoelectroluminescence (MEL) appears. The negative MEL effect is rationalized by triplet-triplet annihilation that leads to delayed fluorescence, whereas the positive Mη effect is related to competition between spin mixing and exciton formation leading to an enhanced singlet:triplet ratio at nonzero magnetic field. The resultant reduction in triplet exciton density is argued to reduce detrapping of polarons in the recombination zone at low-bias voltages, explaining the observed negative MC. Regarding organic magnetoresistance, this study provides experimental data to verify existing models describing magnetic field effects in organic semiconductors, which contribute to better understanding hereof. Furthermore, we present indications of strong magnetic field effects related to interactions between trapped carriers and excitons, which specifically can be studied in electrochemically doped organic light-emitting diodes (OLEDs). Regarding light-emitting electrochemical cells (LECs), this work shows that delayed fluorescence from triplet-triplet annihilation substantially contributes to the electroluminescence and the device efficiency.

The time-resolved photoluminescence (PL) characteristics of single CdSe/ZnS nanoparticles, embedded in a poly(methyl methacrylate) layer, is studied at room temperature. We observe a strong spectral jitter of up to 55 meV, which is correlated with a change in the observed linewidth. We evaluate this correlation effect using a simple model, based on the quantum confined Stark effect induced by a diffusing charge in the vicinity of the nanoparticle. This allows us to derive a mean distance between the center of the particle and the diffusing charge of approximately 5 nm on average, as well as a mean charge carrier displacement within the integration time. These results are reproducible, even for particles which exhibit strong blueing, with shifts of up to 150 meV. Both the statistics and its independence of core-shell alterations lead us to conclude that the charge causing the spectral jitter is situated in the ligands or on its surface.

We study low-frequency noise in a non-Ohmic region of metallic single walled and multiwalled carbon nanotubes. The generalized relative noise appears to be independent of applied bias in the power-law regime of the tubes and in agreement with theoretical predictions. Beyond the power-law regime the suppression of conductance due to scattering with optical phonons is accompanied by a reduction of relative noise by an order of magnitude. Mobility fluctuations in the tubes due to optical phonon scattering cause the unexpected reduction in the relative noise magnitude which is modeled using a modified mobility fluctuation picture. The findings have important implications for metallic nanotubes being used as interconnects in nanoelectronic devices.

We present the study of the quantum tunneling through a vertical two-barrier structure sandwiching a graphene nanoribbon quantum object. Scanning tunneling microscopy measurements of the graphene nanoribbon show staircase I-U characteristics and oscillating dI/dU spectra. To identify the physical origin of the observed effect, we varied the tunneling resistance of the tip-ribbon junction and found a tip-to-ribbon distance dependent oscillating period change. Together with the numerical analysis, we confirm that the resonances in the spectroscopy arise from the Coulomb blockade effect. The study of the Coulomb blockade effect in graphene nanoribbons may be of potential usages for the fabrication of superthin quantum dot devices.

We study electronic transport in monolayer and bilayer graphene with single and many short-range defects focusing on the role of edge termination (zigzag versus armchair). Within the tight-binding approximation, we derive analytical expressions for the transmission amplitude in monolayer graphene nanoribbons with a single short-range defect. The analytical calculations are complemented by exact numerical transport calculations for monolayer and bilayer graphene nanoribbons with a single and many short-range defects and edge disorder. We find that for the case of the zigzag edge termination, both monolayer and bilayer nanoribbons in a single- and few-mode regime remain practically insensitive to defects situated close to the edges. In contrast, the transmission of both armchair monolayer and bilayer nanoribbons is strongly affected by even a small edge defect concentration. This behavior is related to the effective boundary condition at the edges, which, respectively, does not and does couple valleys for zigzag and armchair nanoribbons. In the many-mode regime and for sufficiently high defect concentration, the difference of the transmission between armchair and zigzag nanoribbons diminishes. We also study resonant features (Fano resonances) in monolayer and bilayer nanoribbons in a single-mode regime with a short-range defect. We discuss in detail how an interplay between the defect's position at different sublattices in the ribbons, the defect's distance to the edge, and the structure of the extended states in ribbons with different edge termination influence the width and the energy of Fano resonances.

We propose an approach for computing the Gibbs free energy difference between phases of a material. The method is based on the determination of the average force acting on interfaces that separate the two phases of interest. This force, which depends on the Gibbs free energy difference between the phases, is computed by applying an external harmonic field that couples to a parameter which specifies the two phases. Validated first for the Lennard-Jones model, we demonstrate the flexibility, efficiency, and practical applicability of this approach by computing the melting temperatures of sodium, magnesium, aluminum, and silicon at ambient pressure using density functional theory. Excellent agreement with experiment is found for all four elements, except for silicon, for which the melting temperature is, in agreement with previous simulations, seriously underestimated.

We report on a combination of experiments to determine the elastic moduli of a soft poly (dimethylsiloxane) rubber that was utilized in a smart experiment on resonant phononic modes [Liu et al., Science 289, 1734 (2000)] and whose reported moduli became widely used as a model system in theoretical calculations of phononic materials. We found that the most peculiar hallmark of these values, an extremely low longitudinal sound velocity, is not supported by our experiments. Anyhow, performing theoretical band structure calculations, we can reproduce the surprising experimental findings of Liu et al. even utilizing the correct mechanical parameters. Thus, the physical conclusions derived in the theoretical works do not require the use of an extremely low longitudinal velocity, but can be reproduced assuming only a low value of the shear modulus, in agreement with our experiments.

The Brillouin-zone-center phonons and two-magnon excitations in CuB${}_2$O${}_4$ are studied experimentally by polarized Raman spectroscopy. Most of the expected modes are clearly pronounced and their symmetry unambiguously identified from the polarization selection rules. The experimentally observed transverse optical phonon frequencies are in good agreement with those obtained by means of density functional theory. The two-magnon scattering band is centered at 82 cm${}^{-1}$ and is clearly identified at temperatures below the Néel temperature $T_{\text{N}}$. The spectral shape of the two-magnon band confirms the existing theoretical models of magnon dispersion in the commensurate phase of CuB${}_2$O${}_4$ and suggests an exchange integral of $J=33$ cm${}^{-1}$ (48 K) between the nearest-neighbor Cu(A) ions. The quantitative line-shape analysis of the two-magnon band evidences for additional magnon self-energy contributions below the temperature $T^{*}$ of the commensurate-to-incommensurate phase transition.

The paper presents an investigation in the field of orbital physics of strongly correlated oxides. The theoretical study of vibronic mechanism of orbital and magnetic structures forming in BiMnO${}_3$ crystal is carried out. An effect of orbital structure upon superexchange interaction is described. Nonlinear and second-neighbor terms in vibronic interaction on manganese ions play an important role in magnetic ordering of frustrated BiMnO${}_3$. It is shown that the linear vibronic interaction is insufficient to describe the experimentally detected ferromagnetic structure of bismuth manganite. The new approach to orbital structure formation, presented in the paper, could be used not only in manganite physics but also in other Jahn-Teller compounds.

The magnetic susceptibility of the gapped quantum spin liquid compound (C${}_4$H${}_{12}$N${}_2$)Cu${}_2$Cl${}_6$ and its chemically disordered derivatives (C${}_4$H${}_{12}$N${}_2$)Cu${}_2$(Cl${}_{1-x}$Br${}_x$)${}_6$ are systematically studied in magnetic fields of up to 45 T, as a function of Br concentration. The corresponding field-temperature and field-concentration phase diagrams are determined. Measurements on the disorder-free parent compound are not fully consistent with previously published results by other authors. The effect of Br/Cl substitution on the magnetic properties is superficially similar to that of finite temperature. However, important differences are identified and discussed with reference to the previously studied magnetic excitation spectra.

Bad metallic behavior or incoherent charge dynamics is characteristic of the normal state of high-transition-temperature copper-oxide superconductors. A parent compound of iron-based superconductors, BaFe${}_2$As${}_2$, is also a bad metal in the high-temperature phase, whereas its phosphorous counterpart, BaFe${}_2$P${}_2$, is a good metal. Combined measurements of in-plane resistivity and optical conductivity for BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ make it possible to disentangle the coherent channel from the incoherent one in the charge transport. The isovalent P substitution promotes coherent motion of carriers without changing the balance of the electron and hole Fermi-surface volume and hence transforms a bad metal into a good metal.

We discuss the coexistence of antiferromagnetism and $d$-wave superconductivity within the so-called statistically consistent Gutzwiller approximation (SGA) applied to the $t$–$J$–$U$ model. In this approach, the averages calculated in a self-consistent manner coincide with those determined variationally. Such consistency is not guaranteed within the standard renormalized mean-field theory. With the help of SGA, we show that for the typical value $J/\left|t\right|=\frac{1}{3}$, coexistence of antiferromagnetism (AF) and superconductivity (SC) appears only for $U/\left|t\right|>10.6$ and in a very narrow range of doping $(\delta \lesssim 0.006)$ in the vicinity of the Mott insulating state, in contrast to some previous reports. In the coexistent AF+SC phase, a staggered spin-triplet component of the superconducting gap appears also naturally; its value is very small.

We present temperature dependent switching measurements of the Co/Ni multilayered free element of 75-nm-diameter spin-valve nanopillars. Angular dependent hysteresis measurements as well as switching field measurements taken at low temperature are in agreement with a model of thermal activation over a perpendicular anisotropy barrier. However, the statistics of switching (i.e. the mean switching field and the variance of the switching field distribution) from 20 up to 400 K are in disagreement with a Néel-Brown model that assumes a temperature independent barrier height and anisotropy field. We introduce a modified Néel-Brown model that fits the experimental data in which we attribute a $T^{3/2}$ dependence to the barrier height and the anisotropy field due to the temperature dependent magnetization and anisotropy energy.