Grain boundaries are topological defects that often have a disordered character. Disorder implies that understanding general trends is more important than accurate investigations of individual grain boundaries. Here we present trends in the grain boundaries of graphene. We use density-functional tight-binding method to calculate trends in energy, atomic structure (polygon composition), chemical reactivity (dangling bond density), corrugation heights (inflection angles), and dynamical properties (vibrations), as a function of lattice orientation mismatch. The observed trends and their mutual interrelations are plausibly explained by structure, and supported by past experiments.

We propose and theoretically investigate superconducting spin qubits. Superconducting spin qubit consists of a single spin confined in a Josephson junction. We show that owing to spin-orbit interaction, superconducting phase difference across the junction can polarize this spin. We demonstrate that this enables single-qubit operations and more complicated quantum gates, where spins of different qubits interact via a mutual inductance of the superconducting loop where the junctions are embedded. Recent experimental realizations of Josephson junctions made of semiconductor quantum dots in contact with superconducting leads have shown that the number of electrons in the quantum dot can be tuned by a gate voltage. Superconducting spin qubit is realized when the number of electrons is odd. We discuss the qubit properties at phenomenological level. We present a microscopic theory that enables us to make accurate estimations of the qubit parameters by evaluating the spin-dependent Josephson energy in the framework of fourth-order perturbation theory.

We introduce a method to determine the lateral diffusion constants of planar organic molecules adsorbed on surfaces from the temporal width $\tau$ of pulses in the tunneling current measured for a stationary scanning tunneling microscope tip. The method uses the molecular footprint as a scale bar and evaluates $\tau$ as the first-passage time. Temperature-dependent diffusion constants are reported for the two model molecules copper phthalocyanine and 3,4,9,10-perylene-tetracarboxylic-dianhydride adsorbed on the Ag(100) surface.

Compressed calcium exhibits successive structural transitions: face-centered cubic $\left(\text{Ca-I}\right)\to \text{body}\text{−}\text{centered}$ cubic $\left(\text{Ca-II}\right)\to \text{simple}$ cubic $\left(\text{Ca-III}\right)\to P{4}_1{2}_{1}2$ $\left(\text{Ca-IV}\right)\to Cmca$ (Ca-V). We performed x-ray diffraction measurements of calcium at pressure up to 172 GPa at room temperature and noted a high-pressure phase “Ca-VI” above 158 GPa. The structure was determined to be an orthorhombic $Pnma$ by a Rietveld analysis and confirmed the structure with a density-functional theory calculation. The obtained structure matched to the ones predicted by Yao et al. [Phys. Rev. B 78, 054506 (2008)] well. An orthorhombic model for Ca-III is briefly reported.

We report scanning probe energy loss spectroscopy (SPELS) measurements of a graphite surface taken with microfabricated coaxial tips. The SPELS spectra of graphite obtained with a grounded coaxial tip show the $\pi$ and $\sigma$ plasmon peaks and intense secondary electron emission (SEE) peaks. In comparison, spectra taken with a simple Au cathode tip also showed the $\pi$ and $\sigma$ plasmon peaks but much weaker SEE peaks. The enhanced collection of secondary electrons enables the unoccupied band structure of the surface (i.e., above $E_{\text{vac}}$) to be explored.

Recent precision measurements of mesoscopic persistent currents in normal-metal rings rely on the interaction between the magnetic moment generated by the current and a large applied magnetic field. Motivated by this technique, we extend the theory of mesoscopic persistent currents to include the effect of the finite thickness of the ring and the resulting penetration of the large magnetic field. We discuss both the sample-specific typical current and the ensemble-averaged current which is dominated by the effects of electron-electron interactions. We find that the magnetic field strongly suppresses the interaction-induced persistent current and so provides direct access to the independent-electron contribution. Moreover, the technique allows for measurements of the entire distribution function of the persistent current. We also discuss the consequences of the Zeeman splitting and spin-orbit scattering, and include a detailed and quantitative comparison of our theoretical results to experimental data.

Azurite, ${\text{Cu}}_3{\left({\text{CO}}_3\right)}_2{\left(\text{OH}\right)}_2$, has been considered an ideal example of a one-dimensional diamond chain antiferromagnet. Early studies of this material imply the presence of an ordered antiferromagnetic phase below $T_N\sim 1.9\text{K}$ while magnetization measurements have revealed a 1/3 magnetization plateau. To the best of our knowledge, no corroborating neutron-scattering results have been published to confirm the ordered magnetic moment structure. We present recent neutron-diffraction results which reveal the presence of commensurate magnetic order in azurite which coexists with significant magnetoelastic strain. The latter of these effects may indicate the presence of spin frustration in zero applied magnetic field. Muon spin rotation reveals an onset of short-range order below 3 K and confirms long-range order below $T_N$.

We report on the discovery of superconductivity in the noncentrosymmetric antiferromagnet ${\text{CeIrGe}}_3$ below 1.5 K at a pressure of 20 GPa. We performed electrical resistivity measurements in single crystals of ${\text{CeIrGe}}_3$ in the temperature range 0.04–300 K at pressures up to 24 GPa. From these measurements we deduced the $P\text{−}T$ phase diagram. The data imply a crossover from localized to itinerant $4f$ electrons of the type caused by a Kondo-lattice phenomenon as pressure increases. The antiferromagnetic phase weakens with pressure and eventually vanishes above 22 GPa, indicative of the occurrence of a quantum critical point. Superconductivity appears inside the antiferromagnetic phase near this critical point. After the disappearance of the magnetic ordering a non-Fermi-liquid behavior is observed. Resistivity measurements taken at 24 GPa in magnetic fields up to 8 T strongly suggest that ${\text{CeIrGe}}_3$ has a very large upper critical field for $H\parallel \left[001\right]$.

A general finite-temperature exact-exchange (EXX) formalism derived within the framework of finite-temperature density functional theory for grand canonical ensembles is presented. Based on this formalism a finite-temperature EXX method for solids using plane-wave basis sets is presented. The method is generally applicable, i.e., applicable to insulators, semiconductors, or metals and enables the investigation of temperature effects. More important, however, is that the finite-temperature EXX method enables an EXX treatment of metals by introducing a physically motivated Fermi broadening technique. We tested the method by applying it to sodium, magnesium, and aluminum and compare EXX and LDA (local density approximation) band structures as well as the density of states for the three metals. Differences between LDA and EXX band structures are negligible up to the Fermi level. Above the Fermi level, however, differences between LDA and EXX band structures of magnitudes of 1–2 eV start to build up for energetically higher bands. The magnitude of these differences is of the same order as that of the increases in EXX band gaps compared to LDA band gaps as they are reported for semiconductors and insulators.

The layered structure of the iron-based superconductors gives rise to a more or less pronounced two dimensionality of their electronic structure, most pronounced in LaOFeAs. Consequences are distinct surface states to be expected to influence any surface sensitive experimental probe. In this work a detailed density-functional analysis of the cleavage behavior and the surface electronic structure of LaOFeAs is presented. The surface states are obtained to form two-dimensional bands with their own Fermi surfaces markedly different from the bulk electronic structure.

We use magnetic quantum oscillations in the underdoped high $T_{\text{c}}$ superconductor ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+x}$ $\left(x\approx 0.56\right)$ measured over a broad range of temperatures $100\text{mK}<T<18\text{K}$ to extract the form of the distribution function describing the low-lying quasiparticle excitations in high magnetic fields. Despite the proximity of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6.56}$ to a Mott insulating state, various broken symmetry ground states and/or states with different quasiparticle statistics, we find that our experimental results can be understood in terms of quasiparticle excitations obeying Fermi-Dirac statistics as in the Landau-Fermi liquid theory.

We argue that salient experimental features of the magnetic excitations in the spin-density-wave phase of iron-based superconductors can be understood within an itinerant model. We identify a minimal model and use a multiband random-phase approximation treatment of the dynamical spin susceptibility. Weakly damped spin waves are found near the ordering momentum and it is shown how they dissolve into the particle-hole continuum. We show that ellipticity of the electron bands accounts for the anisotropy of the spin waves along different crystallographic directions and the spectral gap at the momentum conjugated to the ordering one. We argue that our theory agrees well with the existing neutron scattering data.

We show that it is possible to reach one of the ultimate goals of organic electronics: producing organic field-effect transistors with trap densities as low as in the bulk of single crystals. We studied the spectral density of localized states in the band gap [trap density of states (trap DOS)] of small-molecule organic semiconductors as derived from electrical characteristics of organic field-effect transistors or from space-charge-limited current measurements. This was done by comparing data from a large number of samples including thin-film transistors (TFT’s), single crystal field-effect transistors (SC-FET’s) and bulk samples. The compilation of all data strongly suggests that structural defects associated with grain boundaries are the main cause of “fast” hole traps in TFT’s made with vacuum-evaporated pentacene. For high-performance transistors made with small-molecule semiconductors such as rubrene it is essential to reduce the dipolar disorder caused by water adsorbed on the gate dielectric surface. In samples with very low trap densities, we sometimes observe a steep increase in the trap DOS very close $\left(<0.15\text{eV}\right)$ to the mobility edge with a characteristic slope of 10–20 meV. It is discussed to what degree band broadening due to the thermal fluctuation of the intermolecular transfer integral is reflected in this steep increase in the trap DOS. Moreover, we show that the trap DOS in TFT’s with small-molecule semiconductors is very similar to the trap DOS in hydrogenated amorphous silicon even though polycrystalline films of small-molecules with van der Waals-type interaction on the one hand are compared with covalently bound amorphous silicon on the other hand.

Viscosity of liquid boron was measured over the temperature range from 2325 to 2556 K using an electrostatic levitation method combined with an oscillation drop technique. The results obtained revealed that the viscosity increases slowly with decreasing temperature from $2.2\text{mPa}\text{s}$ at 2550 K to $2.6\text{mPa}\text{s}$ at 2370 K, and substantially increases with further decrease in temperature below the melting temperature $\left(T_{\text{m}}=2360\text{K}\right)$, becoming as large as $6.4\text{mPa}\text{s}$ at 2325 K. The increase in the viscosity suggests that clusters with extension may appear in supercooled liquid of boron.

Single-shot measurement of the charge arrangement and spin state of a double quantum dot are reported with measurement times down to 100 ns. Sensing uses radio-frequency reflectometry of a proximal quantum dot in the Coulomb blockade regime. The sensor quantum dot is up to 30 times more sensitive than a comparable quantum point-contact sensor and yields three times greater signal to noise in rf single-shot measurements. Numerical modeling is qualitatively consistent with experiment and shows that the improved sensitivity of the sensor quantum dot results from reduced lifetime broadening and screening.

Landau theory and symmetry considerations lead us to propose an explanation for several seemingly paradoxical behaviors of charge ordering (CO) and spin ordering (SO) in the mixed valence compound ${\text{LuFe}}_2{\text{O}}_4$. Both SO and CO are highly frustrated. We analyze a lattice gas model of CO within mean-field theory and determine the magnitude of several of the phenomenological interactions. We show that the assumption of a continuous phase transitions at which CO or SO develops implies that both CO and SO are incommensurate. To explain how ferroelectric fluctuations in the charge-disordered phase can be consistent with an antiferroelectric-ordered phase, we invoke an electron-phonon interaction in which a low-energy (20 meV) zone-center transverse phonon plays a key role. The energies of all the zone center phonons are calculated from first principles. We give a Landau analysis which explains SO and we discuss a model of interactions which stabilizes the SO state, if it is assumed commensurate. However, we suggest a high-resolution experimental determination to see whether this phase is really commensurate, as believed up to now. The applicability of representation analysis is discussed. A tentative explanation for the sensitivity of the CO state to an applied magnetic field in field-cooled experiments is given.

The oxide pyrochlore ${\text{Bi}}_2{\text{Ti}}_2{\text{O}}_6{\text{O}}^{\prime }$ is known to be associated with large displacements of Bi and ${\text{O}}^{\prime }$ atoms from their ideal crystallographic positions. Neutron total scattering, analyzed in both reciprocal and real space, is employed here to understand the nature of these displacements. Rietveld analysis and maximum entropy methods are used to produce an average picture of the structural nonideality. Local structure is modeled via large-box reverse Monte Carlo simulations constrained simultaneously by the Bragg profile and real-space pair distribution function. Direct visualization and statistical analyses of these models show the precise nature of the static Bi and ${\text{O}}^{\prime }$ displacements. Correlations between neighboring Bi displacements are analyzed using coordinates from the large-box simulations. The framework of continuous symmetry measures has been applied to distributions of ${\text{O}}^{\prime }{\text{Bi}}_4$ tetrahedra to examine deviations from ideality. Bi displacements from ideal positions appear correlated over local length scales. The results are consistent with the idea that these nonmagnetic lone-pair containing pyrochlore compounds can be regarded as highly structurally frustrated systems.

We have studied the growth and electronic/transport properties of ultrathin ${\text{Bi}}_{1-x}{\text{Sb}}_x$ alloy films, which the bulk is reported to be a topological insulator for $0.07<x<0.22$. We found that single crystal epitaxial films as thin as $\sim 30\text{Å}$ could be grown on silicon for $0\le x<0.32$. The $Z_2$ topological number of the films was shown to be nontrivial from the surface Fermi surface. Moreover a crossover from an insulating to a metallic behavior was observed upon reducing the film thickness, revealing the clear detection of the topological-metal conductivity. Our results settle the controversial issues concerning the metallicity of ${\text{Bi}}_{1-x}{\text{Sb}}_x$ at low temperature and verify that the surface/volume ratio must be extensively enhanced to properly understand the nature of these surface states.

Spin-dependent coherent tunneling has been experimentally observed in high-quality sputtered-deposited ${\text{Co}}_2\text{FeAl}/\text{MgO}/\text{CoFe}$ epitaxial magnetic tunneling junctions (MTJs). Consequently, the microfabricated MTJs manifest a very large tunnel magnetoresistance (TMR) at room temperature and an unexpectedly TMR oscillation as a function of MgO barrier thickness. First-principles electronic band calculations confirm the pronounced coherent tunneling effect and are in good agreement with the experimental data. The present work demonstrates the importance of coherent tunneling for large TMR with Heusler alloys

An antiferromagnetic (AF) low-moment solution, $0.35{\mu }_{\text{B}}/\text{Fe}$, is found in the case of LaOFeAs for an intermediately strong Coulomb interaction $U$ of 2.75 eV. This solution is stabilized over a large moment solution due to the gain in exchange energy in the formation of large multipoles of the spin magnetization density. The multipoles are of rank four and can be understood as a type of spin-orbital ordering. Parallels can be drawn to the stabilization of the AF order in, e.g., ${\text{CaCuO}}_2$.