Using calculations from first principles, we demonstrate that intrinsic carrier-phonon scattering in bilayer graphene is dominated by low-energy acoustic (and acousticlike) phonon modes in a framework that bears more resemblance to bulk graphite than to monolayer graphene. The total scattering rate at low to moderate electron energies can be described by a simple two-phonon model in the deformation potential approximation with effective constants $D_{\mathit{ac}}\approx 15$ eV and $D_{\mathit{op}}\approx 2.8\times {10}^8$ eV/cm for acoustic and optical phonons, respectively. With much enhanced acoustic phonon scattering, the mobility of intrinsic bilayer graphene is estimated to be significantly smaller than that of the monolayer.

The spin-wave excitations of the multiferroic ${\mathrm{MnWO}}_4$ have been measured in its low-temperature collinear commensurate phase using high-resolution inelastic neutron scattering. These excitations can be well described by a Heisenberg model with competing long-range exchange interactions and a single-ion anisotropy term. The magnetic interactions are strongly frustrated within the zigzag spin chain along the $c$ axis and between chains along the $a$ axis, while the coupling between spin along the $b$ axis is much weaker. The balance of these interactions results in the noncollinear incommensurate spin structure associated with the magnetoelectric effect, and the perturbation of the magnetic interactions leads to the observed rich phase diagrams of the chemically doped materials. This delicate balance can also be tuned by the application of external electric or magnetic fields to achieve magnetoelectric control of this type of materials.

The high-temperature ferromagnetism in Sr${}_3$YCo${}_4$O${}_{10+\delta }$ perovskite, whose origin has been the subject of considerable debate, has been studied by neutron powder diffraction and synchrotron x-ray diffraction measurements. Oxygen vacancy ordering creates a complex pattern of zigzag stripes in the oxygen-deficient CoO${}_{4+\delta }$ layers, where the Co ions are found in three distinct coordinations. The symmetry of this unprecedented structural modulation, in conjunction with the existence of different Co spin states, provides a straightforward explanation for the appearance of ferrimagnetism. A model for the magnetic structure compatible with these structural features is proposed, based on the refinement of powder neutron data. The macroscopic moment as a function of temperature that can be calculated from the values of the ordered spins extracted from refinements is in excellent agreement with bulk magnetization. Unlike previous models, a collinear G-type magnetic structure with uncompensated moments due to distinct spin states of Co imposed by different coordination is found.

Pressure-induced superconductivity and its relation to the corresponding Hall coefficient ($R_H$) have been investigated for Bi${}_2$Te${}_3$, a known topological insulator, through in situ measurements of magnetoresistance and ac susceptibility with diamond anvil cells. A full phase diagram is presented which shows a complex dependence of the superconducting transition temperature as a function of pressure over an extensive range. High-pressure $R_H$ measurements reveal a close relation to these complex behaviors; in particular, an abrupt change of ${\mathit{dR}}_H/d$$P$ is observed in crossing from the nonsuperconducting to the superconducting ambient-pressure phase.

Using a structure consisting of two, independently contacted graphene single layers separated by an ultrathin dielectric, we experimentally measure the Coulomb drag of massless fermions in graphene. At temperatures higher than 50 K, the Coulomb drag follows a temperature and carrier density dependence consistent with the Fermi liquid regime. As the temperature is reduced, the Coulomb drag exhibits giant fluctuations with an increasing amplitude, thanks to the interplay between coherent transport in the graphene layer and interaction between the two layers.

An efficient order-$N$ real-space Kubo approach is developed for the calculation of the thermal conductivity of complex disordered materials. The method, which is based on the Chebyshev polynomial expansion of the time evolution operator and the Lanczos tridiagonalization scheme, efficiently treats the propagation of phonon wave packets in real space and the phonon diffusion coefficients. The mean free paths and the thermal conductance can be determined from the diffusion coefficients. These quantities can be extracted simultaneously for all frequencies, which is another advantage in comparison with approaches based on the Green’s function. Additionally, multiple scattering phenomena can be followed through the time dependence of the diffusion coefficient deep into the diffusive regime, and the onset of weak or strong phonon localization could possibly be revealed at low temperatures for thermal insulators. The accuracy of our computational scheme is demonstrated by comparing the calculated phonon mean free paths in isotope-disordered carbon nanotubes with Landauer simulations and analytical results. Then the upscalability of the method is illustrated by exploring the phonon mean free paths and the thermal conductance features of edge-disordered graphene nanoribbons having widths of $~$20 nm and lengths as long as a micrometer, which are beyond the reach of other numerical techniques. It is shown that the phonon mean free paths of armchair nanoribbons are smaller than those of zigzag nanoribbons for the frequency range which dominates the thermal conductance at low temperatures. This computational strategy is applicable to higher-dimensional systems as well as to a wide range of materials.

We introduce a Hubbard model as the simple quantum generalization of the classical capacitance circuit model to study semiconductor quantum-dot spin qubits. We prove theoretically that our model is equivalent to the usual capacitance circuit model in the absence of quantum fluctuations. However, our model naturally includes quantum effects such as hopping and spin exchange. The parameters of the generalized Hubbard model can either be directly read off from the experimental plot of the stability diagram or be calculated from the microscopic theory, establishing a quantitative connection between the two. We show that, while the main topology of the charge stability diagram is determined by the ratio between intersite and on-site Coulomb repulsion, fine details of the stability diagram reveal information about quantum effects. Extracting quantum information from experiments using our Hubbard model approach is simple, but would require the measurement resolution to increase by an order of magnitude.

We present a study of the evolution of magnetism from the quantum critical system YbRh${}_2$Si${}_2$ to the stable trivalent Yb system YbCo${}_2$Si${}_2$. Single crystals of Yb(Rh${}_{1-x}$Co${}_x$)${}_2$Si${}_2$ were grown for $0⩽x⩽1$ and studied by means of magnetic susceptibility, electrical resistivity, and specific heat measurements, as well as photoemission spectroscopy. The results evidence a complex magnetic phase diagram, with a nonmonotonic evolution of $T_N$ and two successive transitions for some compositions resulting in two tricritical points. The strong similarity with the phase diagram of YbRh${}_2$Si${}_2$ under pressure indicates that Co substitution basically corresponds to the application of positive chemical pressure. Analysis of the data proves a strong reduction of the Kondo temperature $T_K$ with increasing Co content, $T_K$ becoming smaller than $T_N$ for $x\approx 0.5$, implying a strong localization of the $4f$ electrons. Furthermore, low-temperature susceptibility data confirm a competition between ferromagnetic and antiferromagnetic exchange. The series Yb(Rh${}_{1-x}$Co${}_x$)${}_2$Si${}_2$ provides an excellent experimental opportunity to gain a deeper understanding of the magnetism at the quantum critical point in the vicinity of YbRh${}_2$Si${}_2$ where the antiferromagnetic phase disappears ($T_N\to 0$).

Using scanning tunneling microscopy (STM), we have imaged local charge density fluctuations in monolayer graphene. By placing a small gold nanoparticle on the end of the STM tip, a charge sensor is created. By raster scanning the tip over the surface and using Coulomb blockade spectroscopy, we map the local potential on the graphene. We observe a series of electron and hole doped puddles with a characteristic length scale of $~$20 nm. Theoretical calculations for the correlation length of the puddles based on the number of impurities are in agreement with our measurements.

Under pressure the spin tetrahedron system Cu${}_2$Te${}_2$O${}_5$Br${}_2$ shows a softening of the collective magnetic mode whereas the energy of local spin-exchange processes increases. This apparent contradiction is due to the system’s proximity to a quantum critical point induced by competing interactions that effectively decouple the tetrahedra. The high-pressure state is therefore nonmagnetic and dominated by valence bond excitations. Anomalies in the frequency and the intensity of magnetic Raman scattering are evidence for a weakly discontinuous evolution of the quantum critical state. In contrast, structural parameters as well as optical phonon modes evolve continuously with pressure without any sign for discontinuous criticality. No new low-energy excitations are observed. In contrast, the expected fingerprints of the spin-liquid phase are broadened due to Dzyaloshinskii-Moriya interactions.

We demonstrate the coexistence of superconductivity and ferromagnetism in Bi${}_3$Ni nanostructures that have been prepared by making use of novel chemical-reaction paths. We have characterized their magnetic and superconducting properties by means of magnetometry and electrical-transport measurements. Other than in bulk geometry, submicrometer-sized particles and quasi-one-dimensional nanoscaled strains of single-phase Bi${}_3$Ni undergo ferromagnetic order. Superconductivity in confined Bi${}_3$Ni emerges in the ferromagnetically ordered phase and is stable up to remarkably high magnetic fields. Uniquely, ferromagnetic hysteresis at zero resistance is observed in nanostructured Bi${}_3$Ni.

There is currently fundamental and technological interest in measuring and manipulating nanoscale magnets, particularly in the quantum coherent regime. To observe the dynamics of such systems one requires a magnetometer with not only exceptional sensitivity but also high gain, wide bandwidth, and low backaction. We demonstrate a dispersive magnetometer consisting of a two-junction superconducting quantum interference device (SQUID) in parallel with an integrated, lumped-element capacitor. Input flux signals are encoded as a phase modulation of the microwave drive tone applied to the magnetometer, resulting in a single quadrature voltage signal. For strong drive power, the nonlinearity of the resonator results in quantum limited, phase sensitive parametric amplification of this signal, which improves flux sensitivity at the expense of bandwidth. Depending on the drive parameters, the device performance ranges from an effective flux noise of 0.29 $\mu {\Phi }_0$Hz${}^{-1/2}$ and 20 MHz of signal bandwidth to a noise of 0.14 $\mu {\Phi }_0$Hz${}^{-1/2}$ and a bandwidth of 0.6 MHz. These results are in excellent agreement with our theoretical model.

A surface-adsorbed molecule is contacted with the tip of a scanning tunneling microscope (STM) at a predefined atom. On tip retraction, the molecule is peeled off the surface. During this experiment, a two-dimensional differential conductance map is measured on the plane spanned by the bias voltage and the tip-surface distance. The conductance map demonstrates that tip retraction leads to mechanical gating of the molecular wire in the STM junction. The experiments are compared with a detailed ab initio simulation. We find that density functional theory (DFT) in the local density approximation (LDA) describes the tip-molecule contact formation and the geometry of the molecular junction throughout the peeling process with predictive power. However, a DFT-LDA-based transport simulation following the nonequilibrium Green's function (NEGF) formalism fails to describe the behavior of the differential conductance as found in experiment. Further analysis reveals that this failure is due to the mean-field description of electron correlation in the local density approximation. The results presented here are expected to be of general validity and show that, for a wide range of common wire configurations, simulations which go beyond the mean-field level are required to accurately describe current conduction through molecules. Finally, the results of the present study illustrate that well-controlled experiments and concurrent ab initio transport simulations that systematically sample a large configuration space of molecule-electrode couplings allow the unambiguous identification of correlation signatures in experiment.

Angle-resolved photoemission spectroscopy measurements on the recently discovered superconductors in the KFe${}_2$Se${}_2$ family with critical temperatures up to $~33$ K suggest that no Fermi pockets of hole character centered on the $\Gamma$ point of the Brillouin zone are present, in contrast to all other known ferropnictide and ferrochalcogenide superconductors. Using a fluctuation exchange approximation and a five-orbital tight-binding description of the band structure, we calculate the effective pairing interaction. We find that the pairing state in this system is most likely to have $d$-wave symmetry due to pair scattering between the remaining electron Fermi pockets at wave vector $\mathbf{q}~(\pi ,\pi )$, but without any symmetry-imposed nodes for the given Fermi surface. We propose experimental tests of this result, including the form of the resonance spectrum probed by inelastic neutron scattering.

The magnetic ordering of superconducting single crystals of ${\mathrm{K}}_{0.80}{\mathrm{Fe}}_{1.76}{\mathrm{Se}}_{2.00}$ has been studied between 10 and 550 K using ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy. Despite being superconducting below $T$${}_{\mathrm{sc}}~30$ K, the iron sublattice in ${\mathrm{K}}_{0.80}{\mathrm{Fe}}_{1.76}{\mathrm{Se}}_{2.00}$ clearly exhibits magnetic order from well below $T$${}_{\mathrm{sc}}$ to its Néel temperature of $T$${}_N=532\pm 2$ K. The iron moments are ordered almost parallel to the crystal $c$ axis. The order collapses rapidly above 500 K and the accompanying growth of a paramagnetic component suggests that the magnetic transition may be first order, which may explain the unusual temperature dependence reported in recent neutron diffraction studies.

The anisotropic upper critical field, $H_{c2}(T)$, curves for ${\mathrm{K}}_{0.8}$Fe${}_{1.76}$Se${}_2$ are determined over a wide range of temperatures down to 1.5 K and magnetic fields up to 60 T. Anisotropic initial slopes of $H_{c2}$ $~$ $-1.4$ T/K and $-4.6$ T/K for magnetic field applied along $\mathit{c}$ axis and $\mathit{ab}$ plane, respectively, were observed. Whereas the $\mathit{c}$ axis $H_{c2}^c$ ($T$) increases quasilinearly with decreasing temperature, the $\mathit{ab}$ plane $H_{c2}^{\mathit{ab}}$($T$) shows a flattening, starting near 25 K above 30 T. This leads to a nonmonotonic temperature dependence of the anisotropy parameter ${\gamma }_H\equiv$ $H_{c2}^{\mathit{ab}}/H_{c2}^c$. The anisotropy parameter is $~$2 near $T_c~32$ K and rises to a maximum ${\gamma }_H~$ 3.6 around 27 K. For lower temperatures, ${\gamma }_H$ decreases with $T$ in a linear fashion, dropping to ${\gamma }_H~2.5$ by $T~18$ K. Despite the apparent differences between the ${\mathrm{K}}_{0.8}$Fe${}_{1.76}$Se${}_2$ and (Ba${}_{0.55}$K${}_{0.45}$)Fe${}_2$As${}_2$ or Ba(Fe${}_{0.926}$Co${}_{0.074}$)${}_2$As${}_2$, in terms of the magnetic state and proximity to an insulating state, the $H_{c2}(T)$ curves are remarkably similar.

We employ angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic structures of two rotational variants of epitaxial, single-layer graphene on Ir(111). As grown, the more-abundant $R$0 variant is nearly charge neutral, with strong hybridization between graphene and Ir bands near the Fermi level. The graphene Fermi surface and its replicas exactly coincide with Van Hove singularities in the Ir Fermi surface. Sublattice symmetry breaking introduces a small gap-inducing potential at the Dirac crossing, which is revealed by $n$ doping the graphene using K atoms. The energy gaps between main and replica bands (originating from the moiré interference pattern between graphene and Ir lattices) is shown to be nonuniform along the minizone boundary owing to hybridization with Ir bands. An electronically mediated interaction is proposed to account for the stability of the $R$0 variant. The variant rotated 30° in plane, $R$30, is $p$ doped as grown, and K doping reveals no band gap at the Dirac crossing. No replica bands are found in ARPES measurements. Raman spectra from the $R$30 variant exhibit the characteristic phonon modes of graphene, while $R$0 spectra are featureless. These results show that the film and substrate interaction changes from chemisorption ($R$0) to physisorption ($R$30) with in-plane orientation. Finally, graphene-covered Ir has a work function lower than the clean substrate but higher than graphite.

In this paper, we describe a mechanism by which the destruction of the Kondo coherence at the same time gives rise to antiferromagnetic ordering. This picture is in contrast to the Doniach picture of the competition of Kondo coherence and antiferromagentic ordering. Our study is done in the honeycomb lattice at half-filling, where Kondo coherence gives rise to a Kondo insulator. We go beyond mean-field (large $N$) formulation of Kondo coherence in Kondo lattices and consider excitations we call Kondo vortices. A Kondo vortex is a configuration where at its core the Kondo amplitude vanishes while far away from the core it retains the uniform Kondo amplitude. A Kondo vortex in our model brings four zero modes to the chemical potential. The zero modes play a crucial role as they allow us to construct spin-1 operators. We further study the transformation of these spin-1 Kondo vortex operators under various symmetry transformations of the Kondo Hamiltonian and find a class of operators that transform like an antiferromagnetic order parameter. This gives a novel picture of how one can create antiferromagnetic ordering by proliferating Kondo vortices inside a Kondo coherent phase. We finish by studying the universality class of this Kondo vortex mediated antiferromagnetic transition and conclude that it is in the $O(3)$ universality class.

We present a compact electrostatic levitator as a sample environment for high quality neutron scattering experiments on melts. By this containerless approach we are able to investigate chemically highly reactive melts in a broad temperature range from high temperatures down to very low temperatures in the metastable liquid. The sample volume typical for former electrostatic levitation facilities was increased by one order of magnitude to a sample diameter of 6 mm. A minimized amount of scattering material in the vicinity of the neutron beam results in a low background of the device and thus in a significantly improved signal-to-noise ratio. Even in these large samples the temperature gradient induced by laser heating is less than 20 K. As a first result the Ni self-diffusion coefficient in Zr${}_{64}$Ni${}_{36}$ was measured by quasielastic neutron scattering at the time-of-flight spectrometer TOFTOF at the FRM II as a function of temperature. At an undercooling of 167 K below the melting point deviations from an Arrhenius-type temperature dependence as observed in bulk metallic glass forming alloys become evident. Neutron diffraction experiments were performed at the high flux diffractometer D20 at the ILL. With a neutron wavelength of 0.94 Å a high quality total structure factor of Zr${}_{64}$Ni${}_{36}$ was measured in a broad temperature range at wave numbers between 0.6 and 12 Å${}^{-1}$.

We demonstrate the injection of pure valley-orbit currents in multivalley semiconductors and present the phenomenological theory of this effect. We studied photoinduced transport in (111)-oriented silicon metal-oxide-semiconductor field effect transistors at room temperature. By shining circularly polarized light on exact oriented structures with six equivalent valleys, nonzero electron fluxes within each valley are generated, which compensate each other and do not yield a net electric current. By disturbing the balance between the valley fluxes, we demonstrate that the pure valley-orbit currents can be converted into a measurable electric current.