A mechanism of spin structure-driven anomalous Hall effect (AHE) in tilted ferromagnetic metals is proposed by taking account of the $d$-orbital degree of freedom. We find that a conduction electron acquires a Berry phase due to the complex $d$-orbital wave function, in the presence of noncollinear spin structure and the spin-orbit interaction. The AHE driven by this orbital-derived Berry phase is much larger than the AHE induced by spin chirality, and it naturally explains the salient features of spin structure-driven AHE in pyrochlore ${\text{Nd}}_2{\text{Mo}}_2{\text{O}}_7$. Since the proposed AHE can occur even for coplanar spin orders $\left(M_z=0\right)$, it is expected to emerge in other interesting geometrically frustrated systems.

We studied the effect of nonmagnetic and magnetic impurities on superconductivity in ${\text{Lu}}_2{\text{Fe}}_3{\text{Si}}_5$ by small-amount substitution of the Lu site and investigated structural, magnetic, and electrical properties of nonmagnetic ${\left({\text{Lu}}_{1-x}{\text{Sc}}_x\right)}_2{\text{Fe}}_3{\text{Si}}_5$, ${\left({\text{Lu}}_{1-x}{\text{Y}}_x\right)}_2{\text{Fe}}_3{\text{Si}}_5$, and magnetic ${\left({\text{Lu}}_{1-x}{\text{Dy}}_x\right)}_2{\text{Fe}}_3{\text{Si}}_5$. The rapid depression of $T_c$ by nonmagnetic impurities in accordance with the increase in the residual resistivity reveals the strong pair breaking dominated by disorder.

We present a method for constructing parent Hamiltonians for the chiral spin liquid. We find two distinct Hamiltonians for which the chiral spin liquid on a square lattice is an exact zero-energy ground state. We diagonalize both Hamiltonians numerically for 16-site lattices, and find that the chiral spin liquid, modulo its twofold topological degeneracy, is indeed the unique ground state for one Hamiltonian, while it is not unique for the other.

We study the structure of a model surface, i.e., an Fe monolayer on W(110) by nuclear resonant scattering of x rays. We conclude that at room temperature the structure is nearly perfect, whereas when increasing the temperature up to 770 K an increasing fraction of the Fe atoms feels the appearance of defects manifested by a beat structure due to an electric field gradient. The field gradient is caused by an asymmetry of the neighborhood of part of the atoms and attributable to an increasing fraction of vacancies in the monolayer. With increasing temperature the beat structure gets smeared due to the onset of dynamics in the surface, i.e., the motion of the defects and consequently of the atoms. From the temperature dependence of the amplitude of the beat we deduce the vacancy concentration, thus the vacancy formation energy [0.17(5) eV] and from the broadening with less accuracy we deduce the vacancy migration energy [0.16(6) eV] resulting in 0.33(11) eV for the activation energy of diffusion.

The expression for hardness related to crystal orientation is proposed. Because all quantities in the equation are inherently coupled to the atomistic structure of matter, the anisotropy of hardness can be determined by first-principles methods. The calculations show the highest hardness is in ${\text{ReB}}_2$ and ${\text{OsB}}_2$ single crystals along the $c$ axis, 50.3 and 45.5 GPa, respectively, and are in agreement with measurements. The presented model implies that hardness predominantly reflects the strength of bonds transversely oriented to the direction of indentation.

Anisotropic strain relaxation in (Ga,Mn)As nanostructures was studied combining time-resolved Kerr microscopy and ferromagnetic resonance techniques. Local resonance measurements on individual narrow stripes patterned along various crystallographic directions reveal that the easy axis of the magnetization can be forced perpendicular to the strain relaxation direction. Spatially resolved measurements on disk-shaped and rectangular (Ga,Mn)As structures allow us to directly visualize these local changes in the magnetic anisotropy. We show that the strain-induced edge anisotropy allows for an effective control of the coercive field in stripe structures.

Lattice polarization at the ${\text{SrTiO}}_3/{\text{LaAlO}}_3$ interface was investigated by optical second harmonic generation. Superlattices with varying periodicity were employed to study the evolution of interface polarization, while separating substrate contributions. We observed large perpendicular optical nonlinearity, which abruptly increases when the sublattice thickness goes above 3 unit cells. The polarization is primarily in ${\text{SrTiO}}_3$ and develops up to 8 unit cells from the interface.

Electrons in isolated graphene layers are a two-dimensional gas of massless Dirac Fermions. In realistic devices, however, the electronic properties are modified by elastic deformations, interlayer coupling and substrate interaction. Here, we unravel the electronic structure of noninteracting, doped graphene layers by revisiting the stage one graphite intercalation compound ${\text{KC}}_8$. To this end we apply angle-resolved photoemission spectroscopy and ab initio calculations. The full experimental dispersion is in excellent agreement with calculations of doped graphene once electron correlations are included at the $GW$ level (Greens function $G$ of the Coulomb interaction $W$). This highlights that ${\text{KC}}_8$ has negligible interlayer coupling allowing us to access the full experimental Dirac cone.

We report measurements of the thermopower $S$ and Nernst signal $S_{yx}$ in graphene in a magnetic field $H$. Both quantities show strong quantum oscillations vs the gate voltage $V_g$. Our measurements for Landau levels of index $n\ne 0$ are in quantitative agreement with the edge-current model of Girvin and Jonson (GJ). The inferred off-diagonal thermoelectric conductivity ${\alpha }_{xy}$ is consistent in magnitude with GJ’s prediction. At the Dirac point $\left(n=0\right)$, however, the width of the peak in ${\alpha }_{xy}$ is very narrow. We discuss the physical meaning of the peak width and the puzzle raised by ${\alpha }_{xy}$ for states at the Dirac point.

We calculate the dynamical spin-spin correlation functions of a Kondo dot coupled to two noninteracting leads held at different chemical potentials. To this end we generalize a recently developed real-time renormalization group (RG) method in frequency space [H. Schoeller, Eur. Phys. J. Spec. Top. 168, 179 (2009)] to allow the calculation of dynamical correlation functions of arbitrary dot operators in systems describing spin and/or orbital fluctuations. The resulting two-loop RG equations are analytically solved in the weak-coupling regime. This implies that the method can be applied provided either the voltage $V$ through the dot or the external magnetic field $h_0$ are sufficiently large, $\max \{V,h_0\}\gg T_K$, where the Kondo temperature $T_K$ is the scale where the system enters the strong-coupling regime. Explicitly, we calculate the longitudinal and transverse spin-spin correlation and response functions as well as the resulting fluctuation-dissipation ratios. The correlation functions in real-frequency space can be calculated in Matsubara space without the need of any analytical continuation. We obtain analytic results for the line shape, the small- and large-frequency limits, and several other features like the height and width of the peak in the transverse susceptibility at $\Omega \approx \tilde{h}$, where $\tilde{h}$ denotes the renormalized magnetic field. Furthermore, we discuss how the developed method can be generalized to calculate dynamical correlation functions of other operators involving reservoir degrees of freedom as well.

We analyze nonequilibrium states in a tunnel superconductor-normal-metal (N/S/N) structure in the presence of a tunnel current $I$. We use an approximation of an effective temperature $T$ and calculate the current-voltage $I\text{−}V$ characteristics. It is shown that the $I\text{−}V$ dependence may have an S-shaped form. We determine nonuniform current $I\left(x\right)$ and temperature $T\left(x\right)$ distributions that arise as a result of instability of the uniform state with negative differential conductance $\left(dI/dV<0\right)$. We discuss an analogy with equilibrium superconductors with an exchange field in which nonuniform states predicted by Larkin/Ovchinnikov and Fulde/Ferrell are possible.

We calculate the phonon-drag contribution to the transverse (Nernst) thermoelectric power $S_{yx}$ in a bismuth single crystal subjected to a quantizing magnetic field. The calculated heights of the Nernst peaks originating from the hole Landau levels and their temperature dependence reproduce the right order of magnitude for those of the pronounced magneto-oscillations recently reported by Behnia et al. [Phys. Rev. Lett. 98, 166602 (2007)]. A striking experimental finding that $S_{yx}$ is much larger than the longitudinal (Seebeck) thermoelectric power $S_{xx}$ can be naturally explained as the effect of the phonon drag, combined with the well-known relation between the longitudinal and the Hall resistivity ${\rho }_{xx}⪢\left|{\rho }_{yx}\right|$ in a semimetal bismuth. The calculation that includes the contribution of both holes and electrons suggests that some of the hitherto unexplained minor peaks located roughly at the fractional filling of the hole Landau levels are attributable to the electron Landau levels.

The hole states in the valence band of a large class of semiconductors are degenerate in the projections of angular momentum. Here we show that the switching of a hole between the states can efficiently be realized by acoustic solitons. The microscopic mechanism of such a state conversion is related to the valence-band splitting by local elastic strain. The conversion is studied here for heavy holes localized at shallow and deep acceptors in silicon quantum wells.

We present a quantitative analysis of the low-temperature, high parallel-field pairing resonance in ultrathin superconducting Al films with dimensionless conductance $g⪢1$. In this regime we derive an analytical expression for the tunneling density-of-states spectrum from which a variety of normal-state spin parameters can be extracted. We show that by fitting tunneling data at several supercritical parallel magnetic fields we can determine all of the relevant parameters that have traditionally been obtained via fits to tunneling data in the superconducting phase. These include the spin-orbit scattering rate, the antisymmetric Landau parameter $G^0$, and the orbital pair-breaking parameter.

As shown in recent experiments [Auslaender et al., Nat. Phys. 5, 35 (2009)] magnetic force microscopy permits one not only to image but also to manipulate an individual vortex in type-II superconductors, and this manipulation provides a new powerful tool to study vortex dynamics and pinning. We derive equations that describe the deformation of an individual vortex in an anisotropic biaxial type-II superconductor under the action of the microscope’s magnetic tip. These equations take into account the driving force generated by the tip, the elastic force caused by the vortex deformation, and the pinning force exerted by point defects. Using these equations, we reproduce the main features of the experimental data obtained by Auslaender et al.

Within the Dirac theory of the electronic properties of graphene, smoothly varying lattice strain affects the Dirac carriers through a synthetic gauge field. For static lattice strain, the gauge field induces a synthetic magnetic field which is known to suppress weak localization corrections by a dynamical breaking of time-reversal symmetry. When the lattice strain is time dependent, as in connection with phononic excitations, the gauge field becomes time dependent and the synthetic vector potential is also associated with an electric field. In this paper, we show that this synthetic electric field has observable consequences. We find that the Joule heating associated with the currents driven by the synthetic electric field dominates the intrinsic damping, caused by the electron-phonon interaction, of many acoustic phonon modes of graphene and metallic carbon nanotubes when including the effects of disorder and Coulomb interactions. Several important consequences follow from the observation that by time-reversal symmetry, the synthetic electric field associated with the vector potential has opposite signs for the two valleys. First, this implies that the synthetic electric field drives charge-neutral valley currents and is therefore unaffected by screening. This frequently makes the effects of the synthetic vector potential more relevant than a competing effect of the scalar deformation potential which has a much larger bare coupling constant. Second, valley currents decay by electron-electron scattering (valley Coulomb drag) which causes interesting temperature dependence of the damping rates. While our theory pertains first and foremost to metallic systems such as doped graphene and metallic carbon nanotubes, the underlying mechanisms should also be relevant for semiconducting carbon nanotubes when they are doped.

Epitaxial self-assembled quantum dots (QDs) are commonly obtained by the Stranski-Krastanow (SK) growth mode, in which QDs form on top of a thin two-dimensional (2D) wetting layer (WL). In SK QDs, the properties of the WL such as thickness and composition are hard to control independently of those of the overlying QDs. We investigate here strain-free GaAs/AlGaAs QDs located under a GaAs quantum well (QW), analogous to the WL in SK QDs. The thickness of such a QW can be arbitrarily controlled, allowing the optical properties of the QDs to be tuned without modifying the QD morphology and/or composition. By means of single-QD photoluminescence spectroscopy, we observe well-resolved excited-state shell structures with intershell spacing increasing monotonically with decreasing QW thickness. This behavior is well reproduced by eight-band $k\cdot p$ calculations combined with the configuration-interaction model taking the realistic QD morphology as input. Furthermore, for the thinnest GaAs layer investigated here, no QW emission is detected, indicating that it is possible to suppress the two-dimensional layer usually connecting QDs. Finally, we find that all recombination involving an electron-hole pair in the ground state, including the positive trion, occurs at the low-energy side of the neutral exciton emission. This behavior, previously observed for GaAs/AlGaAs QWs, is a consequence of the large lateral extent of the QDs, and hence of pronounced self-consistency and correlation effects.

We study nonlinear dc transport in graphene using a hydrodynamic approach and conclude that in clean samples the drift velocity saturates at a weakly density-dependent value $v_{sat}\sim {10}^7\text{cm}/\text{s}$. We show that saturation results from the interactions between graphene’s Dirac quasiparticles and both acoustic and optical phonons. Saturation is accompanied by substantial electron heating and is not reached at realistic driving fields in moderately or strongly disordered samples. We find that it is essential to account for interactions among graphene’s Dirac quasiparticles, which increase the linear-response resistivity at high temperatures or low densities.

We theoretically and experimentally study a hitherto neglected effect in dynamic force spectroscopy. The time-averaged deflection of the cantilever and the frequency shift of its second flexural mode were measured for 11 oscillation amplitudes between 12.8 and 0.51 nm above a maximum protrusion of the atomically resolved topography on KBr(001). A small but measurable time-averaged deflection was observed and the magnitude increased with decreasing amplitude. Interaction force curves essentially coincident over the whole attractive range were obtained from the measured frequency shifts with a rms noise linearly decreasing with amplitude. The correction of the tip-sample distance with the measured time-averaged deflection changes the strength of the interaction force. The deflection calculated from the extracted interaction force agrees with the direct measurement and is approximately proportional to the frequency shift except at the smallest amplitudes.

We have studied experimentally the spatial properties and the dynamics of magnon Bose-Einstein condensates created in ferromagnetic films by a parametric pumping with different spatial configurations. Using the specific character of dynamic fields produced by pumping resonators of different shapes, we were able to realize the regime, in which two spatially separated condensates of magnons are formed. Our experiments show that while the separation between the condensates is determined by the size of the resonator, their spatial width can be changed by varying the power of the pumping signal.