We present a numerical study on the uniformity of the pseudomagnetic field in graphene as a function of the relative orientation between the graphene lattice and straining directions. We calculated the deformation of a regular micron-sized graphene hexagon by symmetrically displacing three of its edges. We found that the pseudomagnetic field is strongest if the strain is applied perpendicular to the armchair direction of graphene. For a hexagon with a side length of $1\mu \text{m}$, the pseudomagnetic field has a maximum of 1.2 T for an applied strain of 3.5% and it is uniform (variance $<1$%) within a circle with a diameter of $\sim 520$ nm. This diameter is on the order of the typical diameter of the laser spot in a state-of-the-art confocal Raman spectroscopy setup, which suggests that observing the pseudomagnetic field in measurements of shifted magnetophonon resonance is feasible.

Using molecular simulations we investigated the dependence of friction and wear on grain size in nanocrystalline copper. We found that effects of grain size are coupled to the effects of contact size, resulting in a transition from grain size–sensitive regime to grain size–insensitive regime in friction. This transition occurs because for small tips, friction-induced easy-shear planes can be entirely accommodated in a single grain, rendering grain boundaries less relevant to sliding resistance. Trends in friction do not follow trends in hardness, which is sensitive to grain diameter in the entire grain size regime considered in this study. We have also discovered that coupling of the effects of grain diameter and contact size leads to an optimum grain size that minimizes formation of wear chips on the surface.

Magnetic and charge noise have a common microscopic origin in solid-state devices, as described by a universal electron trap model. In spite of this common origin, magnetic (spin) and charge noise spectral densities display remarkably different behaviors when many-particle correlations are taken into account, leading to the emergence of the Kondo effect. We derive exact frequency sum rules for trap noise and perform numerical renormalization-group calculations to show that while spin noise is a universal function of the Kondo temperature, charge noise remains well described by single-particle theory even when the trap is deep in the Kondo regime. We obtain simple analytical expressions for charge and spin noise that account for Kondo screening in all frequency and temperature regimes, enabling the study of the impact of disorder and the emergence of magnetic $1/f$ noise from Kondo traps. We conclude that the difference between charge and spin noise survives even in the presence of disorder, showing that noise can be more manageable in devices that are sensitive to magnetic (rather than charge) fluctuations and that the signature of the Kondo effect can be observed in spin noise spectroscopy experiments.

In a recent work [A. Altland, Y. Gefen, and B. Rosenow, Phys. Rev. Lett. 108, 136401 (2012)], we have addressed the problem of a Luttinger liquid with a scatterer that allows for both coherent and incoherent scattering channels. We have found that the physics associated with this model is qualitatively different from the elastic impurity setup analyzed by Kane and Fisher, and from the inelastic scattering scenario studied by Furusaki and Matveev, thus proposing a paradigmatic picture of Luttinger liquid with an impurity. Here we present an extensive study of the renormalization group flows for this problem, the fixed point landscape, and scaling near those fixed points. Our analysis is nonperturbative in the elastic tunneling amplitudes, employing an instanton calculation in one or two of the available elastic tunneling channels. Our analysis accounts for nontrivial Klein factors, which represent anyonic or fermionic statistics. These Klein factors need to be taken into account due to the fact that higher-order tunneling processes take place. In particular, we find a stable fixed point, where an incoming current is split $\frac{1}{2}-\frac{1}{2}$ between a forward and a backward scattered beams. This intermediate fixed point, between complete backscattering and full forward scattering, is stable for the Luttinger parameter $g<1$.

We propose a class of temperature-tunable semiconductor metamaterials for terahertz applications. These metamaterials are based on doped semiconductor superlattices with ultrathin barriers of about 1 nm thickness. Due to the tunnel transparency of the barriers, layers of the superlattice cannot be considered as isolated and, therefore, the classical homogenization approach is inapplicable. We develop a theory of quantum homogenization which is based on the Kubo formula for conductivity. The proposed approach takes into account the wave functions of the carriers, their distribution function, and energy spectrum. We show that the components of the dielectric tensor of the semiconductor metamaterial can be efficiently manipulated by external temperature and a topological transition from the dielectric to hyperbolic regime of metamaterial can be observed at room temperature. Using a GaAs/${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ superlattice slab as an example, we provide a numerical simulation of an experiment which shows that the topological transition can be observed in the reflectance spectrum from the slab.

Phase competition and the subsequent phase selection are important characteristics of alloy systems exhibiting numerous states of distinct symmetry but comparable energy. The stoichiometric ${\mathrm{Co}}_2\mathrm{NiGa}$ Heusler alloy exhibits a martensitic transformation with concomitant reduction in symmetry from an austenitic $L{2}_1$ phase (cubic) to a martensitic $L{1}_0$ phase (tetragonal). A structural search was carried out for this alloy and it showed the existence of a number of structures with monoclinic and orthorhombic symmetry with ground state energies comparable to and even less than that of the $L{1}_0$ structure, usually reported as the ground state at low temperatures. We describe these structures and focus in particular on the structural transition path from the $L{2}_1$ to tetragonal and orthorhombic structures for this material. Calculations were carried out to study the Bain ($L{2}_1-L{1}_0$) and Burgers ($L{2}_1-\text{hcp}$) transformations. The barrierless Burgers path yielded a stable martensitic phase with orthorhombic symmetry ($O$) with energy much lower—beyond the expected uncertainty of the calculation methods—than the known tetragonal $L{1}_0$ martensitic structure. This low-energy structure ($O$) has yet to be observed experimentally and it is thus of scientific interest to discern the cause for the apparent discrepancy between experiments and calculations. It is postulated that the ${\mathrm{Co}}_2\mathrm{NiGa}$ Heusler system exhibits a classic case of the phase selection problem: although the unexpected $O$ phase may be relatively more stable than the $L{1}_0$ phase, the energy barrier for the ($L{2}_1-O$) transformation may be much higher than the barrier to the ($L{2}_1-L{1}_0$) transformation. To validate this hypothesis, the stability of this structure was investigated by considering the contributions of elastic and vibrational effects, configurational disorder, magnetic disorder, and atomic disorder. The calculations simulating the effect of magnetic disorder/high temperature as well as the atomic disorder simulations showed that the transformation from $L{2}_1$ to $L{1}_0$ is favored over the Burgers path at high temperatures (large magnetic disorder). These conditions are prevalent upon cooling the material from high temperatures (the usual synthesis route), and this provides a plausible explanation of why $L{1}_0$ and not the $O$ phase is observed. Other ground states (not observed in experiments but predicted through calculations) are ruled out in terms of symmetry relations as well as through considerations of elastic barriers to their nucleation.

We carry out a systematic theoretical investigation of magnetocrystalline anisotropy (MCA) of $L{1}_0$ FePt clusters with alternating Fe and Pt planes along the (001) direction. The clusters studied contain 30–484 atoms. We calculate the structural relaxation and magnetic moment of each cluster by using ab initio spin-polarized density functional theory, and the MCA with both the self-consistent direct method and the torque method. We find the two methods give equivalent results for all the structures examined. We find that bipyramidal clusters whose central layer is Pt have higher MCA than their same-sized counterparts whose central layer is Fe. This results from the fact that the Pt atoms in such configurations are coordinated with more Fe atoms than in the latter. By thus participating in more instances of hybridization, they contribute higher orbital moments to the overall MCA of the unit. Our findings suggest that by properly tailoring the structure one can avoid encapsulating the FePt $L{1}_0$ nanoparticles, as has been proposed earlier, to protect a high and stable magnetic anisotropy. Additionally, using a simple model to capture the thermal behavior, we predict that a five-layered nanoparticle with approximately 700 atoms can be expected to be useful in magnetic recording applications at room temperature.

We investigated the crystal and magnetic structures of the $\text{spin}-\frac{1}{2}$ frustrated antiferromagnet ${\mathrm{Cu}}_3{\mathrm{Mo}}_2{\mathrm{O}}_9$ in which the spin system consists of antiferromagnetic chains and dimers. The space group at room temperature has been reported to be orthorhombic $Pnma$ (No. 62). We infer that the space group above $T_{\mathrm{N}}=7.9\mathrm{K}$ is monoclinic $P{2}_1/m$ (No. 11) from the observation of reflections forbidden in $Pnma$ in x-ray powder diffraction experiments at room temperature. We determined the magnetic structure of ${\mathrm{Cu}}_3{\mathrm{Mo}}_2{\mathrm{O}}_9$ in neutron powder diffraction experiments. Magnetic moments on dimer sites lie in the $ac$ planes. The magnitudes are $(0.50\sim 0.74){\mu }_{\mathrm{B}}$. Moments on chain sites may exist but the magnitudes are very small. The magnetic structure indicates that a partially disordered state is realized. We consider the origin of the magnetic structure, weak ferromagnetism, and electric polarization.

We performed a polarized Raman scattering study of the quasi-one-dimensional superconductor ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. We detect two $A_1^{\prime }$ phonons and three $E^{\prime }$ phonons. One of the $A_1^{\prime }$ modes exhibits a nearly temperature-independent Fano line shape. Based on our first-principles calculations, we ascribe this mode to the in-phase vibrations of Cr atoms within one layer. This observation suggests that the magnetic fluctuations in ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ are coupled to the electronic structure via the lattice.

The phase diagram of solid oxygen in the magnetic-field-temperature ($B-T$) plane is revealed by magnetization and magnetotransmission measurements. The high-field phase of solid oxygen, which we term the $\theta$ phase, is induced at the temperature below 42 K. The transition fields at the $\alpha -\theta$ and $\beta -\theta$ transitions are almost temperature independent, with the phase boundary being very steep. This phase boundary indicates that the entropy change is very small at the $\alpha -\theta$ and $\beta -\theta$ transitions, in contrast to the entropy-driven $\beta -\gamma$ transition at zero field. We argue that the $\theta$ phase differs from the high-temperature $\gamma$ phase in terms of entropy; the molecular rotation is inhibited in the $\theta$ phase, whereas permitted in the $\gamma$ phase. This finding consolidates the novelty of the $\theta$ phase.

Coexistence of antiferromagnetic order with superconductivity in many families of newly discovered iron-based superconductors has renewed interest to this old problem. Due to competition between the two types of order, one can expect appearance of the antiferromagnetism inside the cores of the vortices generated by the external magnetic field. The structure of a vortex in type II superconductors holds significant importance from the theoretical and the application points of view. Here we consider the internal vortex structure in a two-band $s_{\pm }$ superconductor near a spin-density-wave instability. We treat the problem in a completely self-consistent manner within the quasiclassical Eilenberger formalism. We study the structure of the $s_{\pm }$ superconducting order and magnetic field-induced spin-density-wave order near an isolated vortex. We examine the effect of this spin-density-wave state inside the vortex cores on the local density of states.

Second-harmonic generation is described by the second-order nonlinear susceptibility ${\chi }^{\left(2\right)}$ which, in the electric-dipole approximation, requires a noncentrosymmetric medium. It is very challenging and of high technological interest to search whether it is possible to find a way to break inversion symmetry in centrosymmetric crystals in order to induce second-order nonlinearities. A new intriguing way to observe second-order nonlinear phenomena is strain. Here, we present a detailed analysis of the correlation between the strain and the ${\chi }^{\left(2\right)}$ in both centrosymmetric and noncentrosymmetric materials. We considered Si and SiC as test materials and we studied different types of strain (tensile/compressive), in different directions (uniaxial/biaxial) and for different light-polarization directions. We found which is the type of strain necessary in order to induce, tune, and enhance second-harmonic generation in different energy regions for centrosymmetric and noncentrosymmetric materials.

We study the conductivity of graphene with a smooth but particle-hole-asymmetric disorder potential. Using perturbation theory for the weak-disorder regime and numerical calculations, we investigate how the particle-hole asymmetry shifts the position of the minimal conductivity away from the Dirac point $\varepsilon =0$. We find that the conductivity minimum is shifted in opposite directions for weak and strong disorder. For large disorder strengths, the conductivity minimum appears close to the doping level for which electron and hole doped regions (“puddles”) are equal in size.

The extreme case of an Fe monolayer deposited onto a manganese arsenide (MnAs) substrate is analyzed using density functional theory. We find that an FeAs quasi-two-dimensional antiferromagnetic surface nanostructure emerges. This nanostructure, which is magnetically nearly decoupled from the substrate, is due to bonding effects arising from the arsenic atoms bridging the Fe magnetic interactions. These interactions are studied and modeled using a Heisenberg-type Hamiltonian. They display an angular dependence which is characteristic of superexchange-like interactions, which are of the same order of magnitude as those appearing in Fe-based pnictides.

The nonequilibrium dynamics beyond the linear response of Weyl semimetals is studied after a sudden switching on of a dc electric field. The resulting current is a nonmonotonic function of time with an initial quick increase in polarization current followed by a power-law decay. Particle-hole creation à la Schwinger dominates for long times when the conduction current takes over the leading role with the total current increasing again. The conductivity estimated from a dynamical calculation within a generalized Drude picture agrees with the one obtained from Kubo's formula. The full distribution function of electron-hole pairs changes from Poissonian for short perturbations to a Gaussian in the long perturbation (Landau-Zener) regime. The vacuum persistence probability of high-energy physics manifests itself in a finite probability of no pair creation and no induced current at all times.

Transient grating spectroscopy has emerged as a useful technique to study thermal phonon transport because of its ability to perform thermal measurements over length scales comparable to phonon mean free path (MFPs). Although several prior works have performed theoretical studies of quasiballistic heat conduction in transient grating, the analysis methods are either restricted to one spatial dimension or require phenomenological fitting parameters. Here, we analyze quasiballistic transport in a two-dimensional transient grating experiment in which heat conduction can occur both in and cross plane using an analytic Green's function of the Boltzmann equation we recently reported that is free of fitting parameters. We demonstrate a method by which phonon MFPs can be extracted from these measurements, thereby extending the MFP spectroscopy technique using transient grating to opaque bulk materials.

We theoretically study the Floquet exciton condensation, which denotes the exciton condensation under an off-resonant circularly polarized light, in graphene bilayers. The polarized light greatly affects the gap function of the condensate as well as the related physical quantities, e.g., the condensate fraction and superfluid density both having maximal values as the photoillumination parameter varies. We find that the critical temperature rises fast and then slowly approaches a constant upon increasing the photoillumination parameter. The variation of the drag conductivity is obvious in a small range just below the critical temperature; furthermore, there exists competition between the photoillumination parameter and temperature in the gap function and drag conductivity. It is believed that these results can facilitate the experimental investigations of the Floquet exciton condensation in graphene bilayers.

We present a magnetospectroscopic study of a doubly negatively charged exciton ${\mathrm{X}}^{2-}$ in a CdTe quantum dot doped with a single $\mathrm{Mn}^{2+}$ ion. The ${\mathrm{X}}^{2-}$ emission leading to the singlet final state of an excited electron pair is demonstrated to consist of six distinct lines corresponding to different projections of the $\mathrm{Mn}^{2+}$ spin, similarly as for the neutral exciton $\mathrm{X}$. We show that the fine structure of ${\mathrm{X}}^{2-}$ energy levels, as well as the effects of the longitudinal magnetic field, are well reproduced by a simple spin Hamiltonian model featuring both carrier-ion and intershell electron-hole exchange interactions. We also point out two important effects distinguishing the ${\mathrm{X}}^{2-}$ from the $\mathrm{X}$, which result from different symmetries of the electron wave function: the field-induced decrease of the anisotropic part of intershell electron-hole exchange, and the negligible value of the $\mathrm{Mn}^{2+}$ exchange integral with the $p$-shell electron.

We study a system of Dirac electrons with finite density of charge carriers coupled to an external electromagnetic field in two spatial dimensions, with a domain wall (DW) mass term. The interface between a thin-film ferromagnet and a three-dimensional topological insulator provides a condensed-matter realization of this model, when an out-of-plane domain wall magnetization is coupled to the topological insulator surface states. We show how, for films with very weak intrinsic in-plane anisotropies, the torque generated by the edge electronic current flowing along the DW competes with an effective in-plane anisotropy energy, induced by quantum fluctuations of the chiral electrons bound to the wall, in a mission to drive the internal angle of the DW from a Bloch configuration towards a Néel configuration. Both the edge current and the induced anisotropy contribute to stabilize the internal angle, so that for weak intrinsic in-plane anisotropies DW motion is still possible without suffering from an extremely early Walker breakdown.

Götze and Wölfle (GW) [Phys. Rev. B 6, 1226 (1972)] wrote the conductivity in terms of a memory function $M\left(\omega \right)$ as $\sigma (\omega +i\eta )=(in{e}^2/m){[\omega +M(\omega +i\eta )]}^{-1}$, where $M(\omega +i\eta )=i/\tau$ in the Drude limit. The analytic properties of $-M(\omega +i\eta )$ are the same as those of the self-energy $\mathrm{\Sigma }$ of a retarded Green's function. In the approximate treatment of GW, $-M$ closely resembles a self-energy with differences, e.g., the imaginary part is twice too large. The correct relation between $-M$ and $\mathrm{\Sigma }$ is known for the electron-phonon case and is conjectured to be similar for other perturbations. When vertex corrections are ignored there is a known relation. A derivation using Matsubara temperature Green's functions is given.