Using an ultrafast optical technique we measure coherent phonon-pulse reflection from—and heat flow across—a mechanical contact of nanoscale thickness between a thin metal film and a spherical dielectric indenter. Picosecond phonon wave packets at $\sim 50\text{GHz}$ returning from this interface probe the pressure distribution, the contact area, and the indentation profile to subnanometer resolution, revealing the film deformation in situ. These measurements and simultaneous thermal-wave imaging at $\gtrsim 1\text{MHz}$ are consistent with significant enhancement of phonon transport across the near-contact nanogap.

In the framework of the modern theory of polarization, we rigorously establish the microscopic nature of the electric displacement field $\mathbf{D}$. In particular, we show that the longitudinal component of $\mathbf{D}$ is preserved at a coherent and insulating interface. To motivate and elucidate our derivation, we use the example of LAO/STO interfaces and superlattices, where the validity of the above conservation law is not immediately obvious. Our results generalize the “locality principle” of constrained-$\mathbf{D}$ density-functional theory to the first-principles modeling of charge-mismatched systems.

In situ low-energy electron microscopy (LEEM) of graphene growth combined with measurements of the graphene structure and electronic band structure has been used to study graphene on Pt(111). Growth by carbon segregation produces macroscopic monolayer graphene domains extending continuously across Pt(111) substrate steps and bounded by strongly faceted edges. LEEM during cooling from the growth temperature shows the propagation of wrinkles in the graphene sheet, driven by thermal stress. The lattice mismatch between graphene and Pt(111) is accommodated by moiré structures with a large number of different rotational variants, without a clear preference for a particular interface geometry. Fast and slow growing graphene domains exhibit moiré structures with small [e.g., ${\left(3\times 3\right)}_{\text{G}}$, $\left(\sqrt{6}\times \sqrt{6}\right)\text{R}{2}_{\text{G}}$, and $\left(2\times 2\right)\text{R}{4}_{\text{G}}$] and large unit cells [e.g., $\left(\sqrt{44}\times \sqrt{44}\right)\text{R}{15}_{\text{G}}$, $\left(\sqrt{52}\times \sqrt{52}\right)\text{R}{14}_{\text{G}}$, and ${\left(8\times 8\right)}_{\text{G}}$], respectively. A weak substrate coupling, suggested by the growth and structural properties of monolayer graphene on Pt(111), is confirmed by maps of the band structure, which is close to that of isolated graphene aside from minimal hole doping due to charge transfer from the metal. Finally, the decoupled graphene monolayer on Pt(111) appears impenetrable to carbon diffusion, which self-limits the graphene growth at monolayer thickness. Thicker graphene domains, which can form at boundaries between monolayer domains, have been used to characterize the properties of few-layer graphene on Pt(111).

Heterogeneous gasless reactive systems, including high-energy density metal-nonmetal compositions, have seen increasing study due to their various applications. However, owing to their high reaction temperature, short reaction time, and small scale of heterogeneity, investigation of their reaction mechanisms and kinetics is very difficult. In this study, microstructural changes and the kinetics of product layer growth in the W-Si system was investigated using a high-speed x-ray phase-contrast imaging technique. Using the Advanced Photon Source of Argonne National Laboratory, this method allowed direct imaging of irreversible reactions in the W-Si reactive system at frame rates up to $36000$ frames per second with $4\mu \text{s}$ exposure and spatial resolution of $10\mu \text{m}$. Details of the Si melt and reactions between W and Si, that are unable to be viewed with visible-light imaging, were revealed. These include processes such as the initiation of nucleated melting and other physical phenomena that provide insight into the mixing of reactants and subsequent reaction. Through the use of this imaging technique and future optimization in the imaging process, a model for accurately identifying kinetics of chemical reactions, both spatially and temporally, is also proposed.

In this paper we systematically study a simple class of translation-symmetry protected topological orders in quantum spin systems using slave-particle approach. The lattice spin systems are translation invariant, but may break any other symmetries. We consider topologically ordered ground states that do not spontaneously break any symmetry. Those states can be described by $Z2A$ or $Z2B$ projective symmetry group. We find that the $Z2A$ translation symmetric topological orders can still be divided into 16 subclasses corresponding to 16 translation-symmetry protected topological orders. We introduced four $Z_2$ topological indices ${\zeta }_{\mathbf{k}}=0,1$ at $\mathbf{k}=\left(0,0\right)$, $\left(0,\pi \right)$, $\left(\pi ,0\right)$, and $\left(\pi ,\pi \right)$ to characterize those 16 topological orders on square lattice. We calculated the topological degeneracies and crystal momenta for those 16 topological phases on even-by-even, even-by-odd, odd-by-even, and odd-by-odd lattices, which allows us to physically measure such topological orders. We predict the appearance of gapless fermionic excitations at the quantum phase transitions between those symmetry protected topological orders. Our result can be generalized to any dimensions. We find 256 translation-symmetry protected $Z2A$ topological orders for a system on three-dimensional cubic lattice.

Via a direct coupling between the order parameter and the singlet Josephson supercurrent we detect spin-wave resonances and their dispersion in ferromagnetic Josephson junctions in which the usual insulating or metallic barrier is replaced with a weak ferromagnet. The coupling arises within the Fraunhofer interferential description of the Josephson effect because the magnetic layer acts as a time-dependent phase plate. A spin-wave resonance at a frequency ${\omega }_s$ implies a dissipation that is reflected as a depression in the current-voltage curve of the Josephson junction when $\hslash {\omega }_s=2eV$. We have thereby performed a resonance experiment on only ${10}^7$ Ni atoms.

A non-Abelian geometric method is proposed for rotating of heavy hole spins in a singly positive charged quantum dot in Voigt geometry. The key ingredient is the delay-dependent non-Abelian geometric phase, which is produced by the nonadiabatic transition between the two degenerate dark states. We demonstrate, by controlling the pump, the Stokes and the driving fields, that the rotations about $y$ and $z$ axes with arbitrary angles can be realized with high fidelity. Fast initialization and heavy hole-spin-state readout are also possible.

The spin-orbit interaction strength $g_{\text{so}}$ in helical magnets determines both the pitch wave number $q$ and the critical field $H_{\text{c}1}$ where the helix aligns with an external magnetic field. Within a standard Landau-Ginzburg-Wilson (LGW) theory, a determination of $g_{\text{so}}$ in MnSi and FeGe from these two observables yields values that differ by a factor of 20. This discrepancy is remedied by considering the fermionic theory underlying the LGW theory and in particular the effects of screening on the effective electron-electron interaction that results from an exchange of helical fluctuations.

The carrier density and temperature dependence of the Hall mobility in monolayer, bilayer, and trilayer graphene has been systematically studied. We found that as the carrier density increases, the mobility decreases for monolayer graphene, while it increases for bilayer/trilayer graphene. This can be explained by the different density of states in monolayer and bilayer/trilayer graphenes. In monolayer, the mobility also decreases with increasing temperature primarily due to substrate surface polar phonon scattering. In bilayer/trilayer graphene, on the other hand, the mobility increases with temperature because the electric field of the substrate surface polar phonons is effectively screened by the additional graphene layer(s) and the mobility is dominated by Coulomb scattering. We also find that the temperature dependence of the Hall coefficient in monolayer, bilayer, and trilayer graphene can be explained by the formation of electron and hole puddles in graphene. This model also explains the temperature dependence of the minimum conductance of monolayer, bilayer, and trilayer graphene. The electrostatic potential variations across the different graphene samples are extracted.

We have investigated atomic structure and electrical properties of the $\text{Au}/\text{Si}\left(557\right)\text{−}1\times 2$ surface by using scanning tunneling microscopy. We observe the doubled periodicity $\left(\times 2\right)$ for the step-edge atoms even far away from defects at room temperature (RT), indicating no Peierls-type transition reported earlier. We further identify the Au atoms well resolved from Si atoms in the Au-Si-Au chain at RT, in good accord with the prevailing structural model. Our scanning tunneling spectroscopy data taken along the step-edge atoms unambiguously reveal that these step-edge Si atoms are metallic, and are buckled apparently with a charge transferred from down to up Si atoms. We find no significant thermal fluctuation of the buckled step edges at RT.

We demonstrate that origin of the long-standing contact issue in silicon carbide devices can be understood and technologically manipulated at the atomic level. Using advanced transmission electron microscopy, we attribute qualitatively the formation of ohmic contacts to silicon carbide to an epitaxial, coherent, and atomically ordered interface. Quantitatively, first-principles calculations predict that this interface can trap an atomic layer of carbon and hence enable lowered Schottky barrier and enhanced quantum electron transport. The combined experimental and theoretical studies performed provide insight into the complex electronic and electric effects of the buried contact interface, which are fundamental for improving the contact in future electronics based on wide-band-gap semiconductors such as silicon carbide and diamond.

We present details of carrier properties in high quality $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ single crystals obtained from electronic Raman scattering. The experiments indicate a strong band and momentum anisotropy of the electron dynamics above and below the superconducting transition, highlighting the importance of complex band-dependent interactions. The presence of low-energy spectral weight deep in the superconducting state suggests a gap with accidental nodes, which may be lifted by doping and/or impurity scattering. When combined with other measurements, our observation of band- and momentum-dependent carrier dynamics indicate that the iron arsenides may have several competing superconducting ground states.

A thorough investigation of the surface electronic structure of ferromagnetic Fe(100) films, epitaxially grown on single-crystal W(100), has been conducted using spin- and angle-resolved photoemission combined with state-of-the-art density-functional theory slab computations. The dispersion of the surface emission close to the Fermi level has been assessed quantitatively. The experimental results are in a good agreement with the calculations and, in particular, the presence of a minority surface state with $d_{xz+yz}$ character along the $\overline{\Gamma X}$ high-symmetry direction is unambiguously established. Additionally, the calculations predict the existence of a different unoccupied surface state localized at $\overline{\Gamma }$. The existence of the related minority interface resonance near the Fermi edge and outside of the surface-Brillouin-zone center $\overline{\Gamma }$ is believed to control the tunneling magnetoresistance in Fe/MgO/Fe(001) for very thin MgO spacers thus our results serve as indirect confirmation to these predictions.

In this paper I discuss analytic and numerical calculations of the magnetic-field and sheet-current distributions in superconducting strips of width $2a$ and arbitrary thickness $2b$ at the center when the cross-section is an ellipse, a rectangle, and a shape intermediate between these limits. Using critical-state theory, I use several methods to determine the functional dependence of the ac transport-current losses upon $F=I/I_c$, where $I$ is the peak alternating current and $I_c$ is the critical current, and I discuss how this dependence can be affected by the cross-sectional shape, aspect ratio, and a flux-density-dependent critical current density $J_c\left(B\right)$.

We present a simple phenomenological scaling theory for the pairing instability of a quantum critical metal. It can be viewed as a minimal generalization of the classical Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity (SC) for normal Fermi-liquid metals. We assume that attractive interactions are induced in the fermion system by an external “bosonic glue” that is strongly retarded. Resting on the small Migdal parameter, all the required information from the fermion system needed to address the superconductivity enters through the pairing susceptibility. Asserting that the normal state is a strongly interacting quantum critical state of fermions, the form of this susceptibility is governed by conformal invariance and one only has the scaling dimension of the pair operator as free parameter. Within this scaling framework, conventional BCS theory appears as the “marginal” case but it is now easily generalized to the (ir)relevant scaling regimes. In the relevant regime an algebraic singularity takes over from the BCS logarithm with the obvious effect that the pairing instability becomes stronger. However, it is more surprising that this effect is strongest for small couplings and small Migdal parameters, highlighting an unanticipated important role of retardation. Using exact forms for the finite-temperature pair susceptibility from $1+1\text{D}$ conformal field theory as models, we study the transition temperatures, finding that the gap to transition temperature ratios is generically large compared to the BCS case, showing, however, an opposite trend as a function of the coupling strength compared to the conventional Migdal-Eliashberg theory. We show that our scaling theory naturally produces the superconducting “domes” surrounding the quantum critical points (QCPs), even when the coupling to the glue itself is not changing at all. We argue that hidden relations will exist between the location of the crossover lines to the Fermi liquids away from the quantum critical points and the detailed form of the dome when the glue strength is independent of the zero-temperature control parameter. Finally, we discuss the behavior of the orbital-limited upper critical magnetic field as a function of the zero-temperature coupling constant. Compared to the variation in the transition temperature, the critical field might show a much stronger variation pending the value of the dynamical critical exponent.

Studies have shown that interfaces play a crucial role for efficient spin injection and transport. Here, we address the complex role of interface in spin and charge injection into organic materials by various interface-sensitive characterization tools. Inelastic tunneling spectroscopy and polarized neutron reflectometry were mainly adopted to explore the interfaces of high mobility organic semiconductor rubrene sandwiched by two ferromagnetic electrodes. The dramatic difference in the reported magnetotransport properties and charge injection characteristics in trilayer magnetic junctions has been attributed to the different growth morphology of rubrene molecules at the interface dictated by the presence or absence of a 0.6 nm alumina seed layer. The magnetic contribution of the top ferromagnetic electrode is also influenced by the morphology of the rubrene layer underneath, directly affecting the spin injection efficiency. This work highlights the importance of interface engineering in the development of organic-based spintronics devices.

We report the spin injection and transport in ferromagnet/organic semiconductor/ferromagnet (FM/OSC/FM) heterojunctions using rubrene $\left({\text{C}}_{42}{\text{H}}_{28}\right)$ as an organic semiconductor spacer. For completeness of our study, both tunneling magnetoresistance (TMR) and giant magnetoresistance (GMR) were studied by varying the thickness of the rubrene layer (5–30 nm). A thorough study of the device characteristics reveals spin-polarized carrier injection into and subsequent transport through the OSC layer. When the thickness of the rubrene layers are beyond the tunneling limit, the device currents are limited by carrier injection and bulk transport. The carrier injection is well described with phonon-assisted field emission. The behavior of GMR in response to bias field and temperature shows significant differences from that of TMR.

We present numerical studies of conduction in graphene nanoribbons with different types of disorder. We find that even when defect scattering depresses the conductance to values two orders of magnitude lower than $2e^2/h$, equally spaced conductance plateaus occur at moderately low temperatures due to enhanced electron backscattering near subband edge energies if bulk vacancies are present in the ribbon. This work accounts quantitatively for the surprising conductance quantization observed by Lin et al. [Phys. Rev. B 78, 161409(R) (2008)] in ribbons with such low conductances.

The physics of heat conduction in layered, anisotropic crystals is probed by measurements of the cross-plane elastic constant $C_{33}$ and thermal conductivity $\Lambda$ of muscovite mica as a function of hydrostatic pressure. Picosecond interferometry and time-domain thermoreflectance provide high-precision measurements of $C_{33}$ and $\Lambda$, respectively, of micron-sized samples within a diamond-anvil cell; $\Lambda$ changes from the anomalously low value of $0.46\text{W}{\text{m}}^{-1}{\text{K}}^{-1}$ at ambient pressure to a value more typical of oxides crystals with large unit cells, $6.6\text{W}{\text{m}}^{-1}{\text{K}}^{-1}$, at $P=24\text{GPa}$. Most of the pressure dependence of $\Lambda$ can be accounted for by the pressure dependence of the cross-plane sound velocities.

We study the Shannon entropy of the probability distribution resulting from the ground-state wave function of a one-dimensional quantum model. This entropy is related to the entanglement entropy of a Rokhsar-Kivelson-type wave function built from the corresponding two-dimensional classical model. In both critical and massive cases, we observe that it is composed of an extensive part proportional to the length of the system and a subleading universal constant $S_0$. In $c=1$ critical systems (Tomonaga-Luttinger liquids), we find that $S_0$ is a simple function of the boson compactification radius. This finding is based on a field-theoretical analysis of the Dyson-Gaudin gas related to dimer and Calogero-Sutherland models. We also performed numerical demonstrations in the dimer models and the spin-1/2 $XXZ$ chain. In a massive (crystal) phase, $S_0$ is related to the ground-state degeneracy. We also examine this entropy in the Ising chain in a transverse field as an example showing a $c=1/2$ critical point.