Absolute x-ray absorption cross sections of a regular bacterial surface-layer protein deposited on a naturally oxidized silicon substrate were determined experimentally. Upon separation of the partial cross sections of the three relevant $1s$ absorption edges, the oscillator strengths of the $1s\to {\pi }^{\ast }$ excitations within the peptide-backbone unit were extracted. Comparison with results of first-principles calculations revealed their close correlation to the topology of ${\pi }_{\mathit{\text{peptide}}}^{\ast }$ orbitals of the peptide backbone.

Hopping transport in a one-dimensional system is studied numerically. A fast algorithm is devised to find the lowest-resistance path at arbitrary electric field. Probability distribution functions of individual resistances on the path and the net resistance are calculated and fitted to compact analytic formulas. Qualitative differences between statistics of resistance fluctuations in Ohmic and non-Ohmic regimes are elucidated. The results are compared with prior theoretical and experimental work on the subject.

We present a method to prepare monolayer and multilayer suspended sheets of hexagonal boron nitride (h-BN), using a combination of mechanical exfoliation and reactive ion etching. Ultrahigh-resolution transmission electron microscope imaging is employed to resolve the atoms, and intensity profiles for reconstructed phase images are used to identify the chemical nature (boron or nitrogen) of every atom throughout the sample. Reconstructed phase images are distinctly different for h-BN multilayers of even or odd number. Unusual triangular defects and zigzag and armchair edge reconstructions are uniquely identified and characterized.

As a paradigmatic example of multiscale quantum criticality, we consider the Pomeranchuk instability of an isotropic Fermi liquid in two spatial dimensions, $d=2$. The corresponding Ginzburg-Landau theory for the quadrupolar fluctuations of the Fermi surface consists of two coupled modes, critical at the same point, and characterized by different dynamical exponents: one being ballistic with dynamical exponent $z=2$ and the other one is Landau damped with $z=3$, thus giving rise to multiple dynamical scales. We find that at temperature $T=0$, the ballistic mode governs the low-energy structure of the theory as it possesses the smaller effective dimension $d+z$. Its self-interaction leads to logarithmic singularities, which we treat with the help of the renormalization group. At finite temperature, the coexistence of two different dynamical scales gives rise to a modified quantum-to-classical crossover. It extends over a parametrically large regime with intricate interactions of quantum and classical fluctuations leading to a universal $T$ dependence of the correlation length independent of the interaction amplitude. The multiple scales are also reflected in the phase diagram and in the critical thermodynamics. In particular, we find that the latter cannot be interpreted in terms of only a single dynamical exponent whereas, e.g., the critical specific heat is determined by the $z=3$ mode, the critical compressibility is found to be dominated by the $z=2$ fluctuations.

The elastic properties of hcp ${}^4\text{H}\text{e}$ samples have been investigated using low-frequency (20 Hz to 20 kHz) high sensitivity sound transducers. In agreement with the findings of other workers, most samples studied grew very significantly stiffer at low temperature; Poisson’s ratio $\nu$ was observed to increase from 0.28 below 20 mK to $\sim 0.35$ at 0.7 K. The span of the variation in $\nu$ varies from sample to sample according to their thermal and mechanical history. Crystals carefully grown at the melting curve show a different behavior, the change in $\nu$ taking place at lower $T$ and being more abrupt.

A systematic analysis is presented of the nanocrystalline structures generated due to the intershell C-C bonding between adjacent concentric graphene walls of multiwalled carbon nanotubes (MWCNTs). The analysis combines a comprehensive exploration of the entire parameter space determined by the geometrical characteristics of the individual graphene walls comprising the MWCNT with first-principles density-functional theory calculations of intershell C-C bonding and structural relaxation by molecular-dynamics simulation of the resulting nanocrystalline structures. We find that these structures can provide seeds for the nucleation of the cubic-diamond and hexagonal-diamond phase in the form of nanocrystals embedded in the MWCNTs. The resulting lattice structure is determined by the chirality and relative alignment of adjacent graphene walls in the MWCNT. These crystalline phases are formed over the broadest range of nanotube diameters and for any possible combination of zigzag, armchair, or chiral configurations of graphene walls. The key parameter that determines the size of the generated nanocrystals is the chiral-angle difference between adjacent graphene walls in the MWCNT.

We present a compelling evidence for the opening of a bandgap in exfoliated bottom-gated bilayer graphene by fitting the gate-voltage-modulated infrared reflectivity spectra in a large range of doping levels with a tight-binding model and the Kubo formula. A close quantitative agreement between the experimental and calculated spectra is achieved, allowing us to determine self-consistently the full set of Slonczewski-Weiss-McClure tight-binding parameters together with the gate-voltage-dependent bandgap. The doping dependence of the bandgap shows a good agreement with the existing calculations that take the effects of self-screening into account. We also identify certain mismatches between the tight-binding model and the data, which can be related to electron-electron and electron-phonon interactions.

We present the phase diagram of the frustrated ferromagnetic $S=\frac{1}{2}$ Heisenberg $J_1-J_2$ chain in a magnetic field, obtained by large scale exact diagonalizations and density matrix renormalization group simulations. A vector chirally ordered state, metamagnetic behavior and a sequence of spin-multipolar Luttinger liquid phases up to hexadecupolar kind are found. We provide numerical evidence for a locking mechanism, which can drive spiral states toward spin-multipolar phases, such as quadrupolar or octupolar phases. Our results also shed light on previously discovered spin-multipolar phases in two-dimensional $S=\frac{1}{2}$ quantum magnets in a magnetic field.

We investigate the coupling between the magnetic and superconducting order parameters in an 8-m-long meander line (“wire”) made of a ${\text{La}}_{1.94}{\text{Sr}}_{0.06}{\text{CuO}}_4$ film with a cross section of $0.5\times 100\mu {\text{m}}^2$. The magnetic order parameter is determined using the low-energy muon spin relaxation technique. The superconducting order parameter is characterized by transport measurements and modified by high current density. We find that when the superconducting order parameter is suppressed by the current, the magnetic transition temperature, $T_m$, increases. The extracted sign and magnitude of the Ginzburg-Landau coupling constant indicate that the two orders are repulsive, and that our system is located close to the border between first- and second-order phase transition.

A recently developed high-field technique of atomic chains preparation has made it possible to attain the ultrahigh resolution of field-emission electron microscopy (FEEM), which can be used to direct imaging the intra-atomic electronic structure. By applying cryogenic FEEM, we are able to resolve the spatial configuration of atomic orbitals, which correspond to quantized states of the end atom in free-standing carbon atomic chains. Knowledge of the intra-atomic structure will make it possible to visualize generic aspects of quantum mechanics and also lead to approaches for a wide range of nanotechnological applications.

Recent optical conductivity experiments of doped graphene in the infrared regime reveal a strong background in the energy region between the intra and interband transitions difficult to explain within conventional pictures. We propose a phenomenological model taking into account the marginal Fermi liquid nature of the quasiparticles in graphene near the neutrality point that can explain qualitatively the observed features. We also study the electronic Raman signal and suggest that it will also be anomalous.

We consider the tunneling current through a double point-contact Fabry-Pérot interferometer such as used in recent experimental studies of the fractional quantum Hall plateau at filling fraction $\nu =5/2$. We compare the predictions of several different models of the state of the electrons at this plateau: the Moore-Read, anti-Pfaffian, $\text{SU}{\left(2\right)}_2$ NAF, $K=8$ strong pairing, and (3,3,1) states. All of these predict the existence of charge $e/4$ quasiparticles, but the first three are non-Abelian while the last two are Abelian. We give explicit formulas for the scaling of charge $e/2$ and charge $e/4$ quasiparticle contributions to the current as a function of temperature, gate voltage, and distance between the two point contacts for all three models. Based on these, we analyze several possible explanations of two phenomena reported for recent experiments by Willett et al., namely, halving of the period of the observed resistance oscillations with rising temperature and alternation between the same two observed periods at low temperatures as the area of the interference loop is varied with a side gate. We conclude that the most likely explanation is that the observed alternation is due to switching between even and odd numbers of charge $e/4$ quasiparticles enclosed within the loop as a function of side-gate voltage, which is a clear signature of the presence of non-Abelian anyons. However, there are important features of the data which do not have a simple explanation within this picture. We suggest further experiments which could help rule out some possible scenarios. We make the corresponding predictions for future tunneling and interference experiments at the other observed second Landau level fractional quantum Hall states.

We compute the backscattered current in a double point-contact geometry of a quantum-Hall system at filling fraction as a function of bias voltage in the weak backscattering regime. We assume that the system is in the universality class of either the Pfaffian or anti-Pfaffian state. When the number of charge quasiparticles in the interferometer is odd, there is no interference pattern. However, the coupling between a charge quasiparticle and the edge causes it to be absorbed by the edge at low energies. Consequently, an interference pattern appears at low bias voltages and temperatures, as if there were an even number of quasiparticles in the interferometer. We relate this problem to that of a semi-infinite Ising model with a boundary magnetic field. Using the methods of perturbed boundary conformal field theory, we give an exact expression for this crossover of the interferometer as a function of bias voltage. Finally, we comment on the possible relevance of our results to recent interference experiments.

It has been predicted that the phase sensitive part of the current through a non-Abelian $\nu =5/2$ quantum Hall Fabry-Perot interferometer will depend on the number of localized charged $e/4$ quasiparticles (QPs) inside the interferometer cell. In the limit where all QPs are far from the edge, the leading contribution to the interference current is predicted to be absent if the number of enclosed QPs is odd and present otherwise, as a consequence of the non-Abelian QP statistics. The situation is more complicated, however, if a localized QP is close enough to the boundary so that it can exchange a Majorana fermion with the edge via a tunneling process. Here, we derive an exact solution for the dependence of the interference current on the coupling strength for this tunneling process, and confirm a previous prediction that for sufficiently strong coupling, the localized QP is effectively incorporated in the edge and no longer affects the interference pattern. We confirm that the dimensionless coupling strength can be tuned by the source-drain voltage, and we find that not only does the magnitude of the even-odd effect change with the strength of bulk-edge coupling, but in addition, there is a universal shift in the interference phase as a function of coupling strength. Some implications for experiments are discussed at the end.

We study interactions between localized scatterers on metallic carbon nanotubes by a mapping onto a one-dimensional Casimir problem. Backscattering of electrons between localized scattering potentials mediates long-range forces between them. We model spatially localized scatterers by local and nonlocal potentials and treat simultaneously the effects of intravalley and intervalley backscattering. We find that the long-range forces between scatterers exhibit the universal power-law decay of the Casimir force in one dimension, with prefactors that control the sign and strength of the interaction. These prefactors are nonuniversal and depend on the symmetry and degree of localization of the scattering potentials. We find that local potentials inevitably lead to a coupled valley scattering problem, though by contrast nonlocal potentials lead to two decoupled single-valley problems in a physically realized regime. The Casimir effect due to two-valley scattering potentials is characterized by the appearance of spatially periodic modulations of the force.

We propose a structural model for the $\text{Si}\left(111\right)\text{−}\left(5\times 2\right)\text{-Au}$ reconstruction. The model incorporates a revised experimental value of 0.6 monolayer for the coverage of gold atoms, equivalent to six gold atoms per $5\times 2$ cell. Five main theoretical results, obtained from first-principles total-energy calculations, support the model. (1) In the presence of silicon adatoms the periodicity of the gold rows spontaneously doubles, in agreement with experiment. (2) The dependence of the surface energy on the adatom coverage indicates that a uniformly covered phase is unstable and will phase separate into empty and covered regions, as observed experimentally. (3) Theoretical scanning tunneling microscopy images are in excellent agreement with experiment. (4) The calculated band structure is consistent with angle-resolved photoemission spectra; analysis of their correspondence allows the straightforward assignment of observed surface states to specific atoms. (5) The calculated activation barrier for diffusion of silicon adatoms along the row direction is in excellent agreement with the experimentally measured barrier.

We report that the 122-type ${\text{Sr}}_{1-x}{\text{Ca}}_x{\text{Co}}_2{\text{P}}_2$ solid solution undergoes an anomalous structural transition from the uncollapsed to the collapsed ${\text{ThCr}}_2{\text{Si}}_2$ structure at a distinct onset composition near $x=0.5$. Correlated with the structural changes, the electronic system evolves from a nearly ferromagnetic Fermi liquid to an antiferromagnetic metal, through a complex crossover regime. The structural collapse, driven by P-P bonding across the (Sr,Ca) layers, is much more pronounced in this system than it is in the analogous Fe-based system, indicating a strong sensitivity of structure to total electron count in the transition-metal pnictide 122 family.

We present measurements of silicon (Si) metal-oxide-semiconductor (MOS) nanostructures that are fabricated using a process that facilitates essentially arbitrary gate geometries. Stable Coulomb-blockade behavior showing single-period conductance oscillations that are consistent with a lithographically defined quantum dot is exhibited in several MOS quantum dots with an open-lateral quantum-dot geometry. Decreases in mobility and increases in charge defect densities (i.e., interface traps and fixed-oxide charge) are measured for critical process steps, and we correlate low disorder behavior with a quantitative defect density. This work provides quantitative guidance that has not been previously established about defect densities and their role in gated Si quantum dots. These devices make use of a double-layer gate stack in which many regions, including the critical gate oxide, were fabricated in a fully qualified complementary metal-oxide semiconductor facility.

We investigate the effect of a spin-density wave (SDW) on $s_{\pm }$ superconductivity in Fe-based superconductors. We show that, contrary to the common wisdom, no nodes open at the new, reconnected Fermi surfaces when the hole and electron pockets fold down in the SDW state, despite the fact that the $s_{\pm }$ gap changes sign between the two pockets. Instead, the order parameter preserves its sign along the newly formed Fermi surfaces. The familiar experimental signatures of an $s_{\pm }$ symmetry are still preserved, although they appear in a mathematically different way. For a regular $s$ case $\left(s_{++}\right)$ the nodes do appear in the SDW state. This distinction suggests a specific way to experimentally separate an $s_{\pm }$ state from a regular $s$ in the pnictides. We argue that recently published thermal-conductivity data in the coexisting state are consistent with the $s_{\pm }$, but not the $s_{++}$ state.

Coulomb blockade is observed in a graphene nanoribbon device with a top gate. When two $pn$ junctions are formed via the back gate and the local top gate, electrons are confined between the $pn$ junctions which act as the barriers. When no $pn$ junctions are induced by the gate voltages, electrons are still confined, as a result of strong disorder, but in a larger area. Measurements on five other devices with different dimensions yield consistent results.