The inter- and intramolecular ordering of the trimetallic nitride endohedral fullerene ${\text{Dy}}_3\text{N}@{\text{C}}_{80}$ with icosahedral cage symmetry $I_h$ on Cu(111) has been studied by scanning tunneling microscopy and synchrotron-based x-ray photoelectron diffraction (XPD). ${\text{Dy}}_3\text{N}@{\text{C}}_{80}$ $\left(I_h\right)$ is found to form ordered islands consisting of domains of equally oriented molecules. As for ${\text{C}}_{60}$ on the same substrate, the cage is facing with a hexagon toward the surface, which is however slightly tilted for ${\text{C}}_{80}$. The endohedral nitrogen atom remains at a position close to the geometrical center of the cage. Resonant XPD on the ${\text{M}}_{\text{V}}$ edge shows that the encaged ${\text{Dy}}_3\text{N}$ unit takes well-defined orientations with respect to the ${\text{C}}_{80}$ cage and the Cu(111) substrate.

The best known method for growing epitaxial oxides on silicon requires the deposition of 1/2 monolayer of an alkaline earth metal, usually Sr, as the first step; however, the details of this process are not well understood. Using first-principles density functional theory calculations, we compute the total energies and vibrational free energies of several classes of Sr on Si(001) structures. These structures, in particular those with submonolayer Si reconstructions, allow us to explain the observed phase diagram of submonolayer Sr on Si(001). We analyze the electronic states of key structures using maximally localized Wannier functions and present simulated scanning tunneling microscopy images of key structures and find that they compare well with recent experiments. Our results clarify the role of temperature in epitaxial oxide growth on silicon, predict an experimentally verified low temperature path to epitaxy, and may be important in finding pathways to epitaxy between silicon and other oxides.

By investigations of phase relations in the alloy system Ba-Pt-Si at $900°\text{C}$ we observe the formation of the compound ${\text{BaPtSi}}_3$, which crystallizes in the noncentrosymmetric ${\text{BaNiSn}}_3$ structure type. Its space group is $I4mm$ with the tetragonal lattice parameters $a=0.44094\left(2\right)\text{nm}$ and $c=1.0013\left(2\right)\text{nm}$ for the arc-melted compound annealed at $900°\text{C}$. The characterization of the physical properties of ${\text{BaPtSi}}_3$ reveals a superconducting transition at 2.25 K with an upper critical field at $T=0\text{K}$ of $\approx 0.05\text{T}$. For analyzing the electronic structure, density-functional theory calculations are performed yielding very good agreement between theory and experiment for the structural properties. From relativistic electronic-structure calculations, Fermi surface nesting features are found for two characteristic double sets of bands. The spin-orbit splitting of the relativistic electronic bands is in general rather small at Fermi energy and, therefore, superconductivity adheres to an almost undisturbed BCS state.

The gain recovery time of a bound-to-continuum terahertz quantum cascade laser is measured using a stimulated emission photocurrent technique. At low temperatures a value $\sim 50\text{ps}$ is found. This tends to decrease with rising temperature, and suggests that class B laser dynamics should be expected from these devices. Both the value and the trend in the gain recovery time are found to be consistent with hopping transport through the miniband. Power-dependent measurements of the photocurrent clearly show the effects of inhomogeneous broadening of the gain transition.

To discuss the gapless spin-liquid-like behavior of the second-layer solid-phase ${}^3\text{H}\text{e}$ adsorbed on graphite, we study a spin-1/2 Heisenberg model on a triangular lattice with two kinds of superexchange couplings due to the corrugation effects from the first-layer by using finite temperature Lanczos method and high-temperature expansions. In some parameter region, it is found that this model can be expressed by an effective Hamiltonian with two different energy scales consisting of kagomé Heisenberg model and triangular Heisenberg model. We find that this effective Hamiltonian well reproduces the experimental behaviors such as the double-peak structure and the low-temperature linear-$T$ behavior of the specific heat as well as the excess enhancement of the spin susceptibility at low temperatures.

We analyze the frequency-dependent noise and the heat production rate for a dynamical quantum capacitor in the regime in which it emits single particles, electrons, and holes. At low temperature and slow driving the relaxation resistance quantum, $R_q=h/\left(2e^2\right)$, defines the heat production rate in both the linear and nonlinear response regimes. If a double-cavity capacitor emits particles in pairs, the noise is enhanced. In contrast the energy dissipated is suppressed or enhanced depending on whether an electron-hole pair or an electron-electron (a hole-hole) pair is emitted.

We have studied transport and magnetic properties of ${\text{Bi}}_{1-x}{\text{Sb}}_x$, which is believed to be a topological insulator—a new state of matter where an insulating bulk supports an intrinsically metallic surface. In nominally insulating ${\text{Bi}}_{0.91}{\text{Sb}}_{0.09}$ crystals, we observed strong quantum oscillations of the magnetization and the resistivity originating from a Fermi surface which has a clear two-dimensional character. In addition, a three-dimensional Fermi surface is found to coexist, which is possibly due to an unusual coupling of the bulk to the surface. This finding demonstrates that quantum oscillations can be a powerful tool to directly probe the novel electronic states in topological insulators.

We used low-energy electron microscopy (LEEM) and scanning tunneling microscopy (STM) to investigate domain structures of epitaxial few-layer graphene grown on SiC(0001). Dark-field (DF) LEEM images formed using (10) and (01) beams clearly indicate that bilayer graphene consists of two types of domains, which have threefold symmetry and are rotated by $180°$ with respect to each other. The DF LEEM images show clear domain contrasts at energies where (10)- and (01)-beam intensities calculated for bulk graphite are largely different. This means that the two types of domains are different in stacking: $AB$ and $AC$ stackings. The stacking domains are also supported by the STM images of bilayer graphene showing both hexagonal and honeycomb patterns.

Thermal conductivity $\left(\kappa \right)$ and Seebeck coefficient $\left(\alpha \right)$ of electron-doped light-metal oxide $12\text{CaO}\cdot 7{\text{Al}}_2{\text{O}}_3$ (C12A7 electride) with a subnanometer-sized cage structure are reported on single crystals with various electron concentrations $\left(N_{\text{e}}\right)$. The semiconducting C12A7 electride exhibits $n$-type conduction with the highest $\alpha$ value of $-100\mu \text{V}{\text{K}}^{-1}$ at 300 K. The $\kappa$ exhibits an amorphouslike $T^2$ dependence at low temperatures and varies between 2.3 and $4.5{\text{Wm}}^{-1}{\text{K}}^{-1}$ at 300 K. This is an order-of-magnitude lower than that of the constituents, CaO $\left(15{\text{Wm}}^{-1}{\text{K}}^{-1}\right)$ and ${\text{Al}}_2{\text{O}}_3$ $\left(30{\text{Wm}}^{-1}{\text{K}}^{-1}\right)$. These properties are attributed to the cage structure, suggesting that the semiconducting electride should be regarded as a phonon glass and electron crystal material. The thermoelectric performance of electrides evaluated by a dimensionless figure of merit $\left(ZT\right)$ shows an optimized value of $2\times {10}^{-3}$ at 300 K for the semiconducting electride with $N_{\text{e}}$ of $5\times {10}^{20}{\text{cm}}^{-3}$.

We revisit the problem of wave-function statistics at the Anderson metal-insulator transition (MIT) of noninteracting electrons in $d>2$ spatial dimensions. At the transition, the complex spatial structure of the critical wave functions is reflected in the nonlinear behavior of the multifractal spectrum of generalized inverse participation ratios (IPRs). Beyond the crossover from narrow to broad IPR statistics, which always occurs for sufficiently large moments of the wave-function amplitude, the spectrum obtained from a typical wave function associated with a particular disorder realization differs markedly from that obtained from the disorder-averaged IPRs. This phenomenon is known as the termination of the multifractal spectrum. We provide a field theoretical derivation for the termination of the typical multifractal spectrum by combining the nonlinear sigma model framework, conventionally used to access the MIT in $d=2+ϵ$ dimensions, with a functional renormalization-group (FRG) technique. The FRG method deployed here was originally pioneered to study the properties of the two-dimensional (2D) random-phase $XY$ model [D. Carpentier and P. Le Doussal, Nucl. Phys. B 588, 565 (2000)]. The same method was used to demonstrate the termination of the multifractal spectrum in the very special problem of 2D Dirac fermions subject to a random Abelian vector potential. Our result shows that the typical multifractal wave-function spectrum and its termination can be obtained at a generic Anderson localization transition in $d>2$, within the standard field theoretical framework of the nonlinear sigma model, when combined with the FRG.

We present an algorithm for generating all derivative superstructures of a nonprimitive parent lattice. The algorithm has immediate application in important materials design problems such as modeling hexagonal-close-packed (hcp) alloys. Extending the work of Hart and Forcade [Phys. Rev. B 77, 224115 (2008)] (which applies only to Bravais lattices), this approach applies to arbitrary multilattices. The algorithm enumerates superlattices and atomic configurations using permutation groups rather than direct geometric comparisons. The key concept is to use the quotient group associated with each superlattice to determine all unique atomic configurations. The algorithm is very efficient; the run time scales linearly with the number of unique structures found. We demonstrate the algorithm in the important case of hcp-derived superstructures. In the list of enumerated hexagonal-close-packed derivative superstructures, we predict several as-yet-unobserved structures as likely candidates for new intermetallic prototypes.

The exact solution of a quantum Bethe lattice model in the thermodynamic limit amounts to solve a functional self-consistent equation. In this paper we obtain this equation for the Bose-Hubbard model on the Bethe lattice, under two equivalent forms. The first one, based on a coherent-state path integral, leads in the large connectivity limit to the mean-field treatment of Fisher et al. [Phys. Rev. B 40, 546 (1989)] at the leading order, and to the bosonic dynamical mean field theory as a first correction, as recently derived by Byczuk and Vollhardt [Phys. Rev. B 77, 235106 (2008)]. We obtain an alternative form of the equation using the occupation number representation, which can be easily solved with an arbitrary numerical precision, for any finite connectivity. We thus compute the transition line between the superfluid and Mott insulator phases of the model, along with thermodynamic observables and the space and imaginary-time dependence of correlation functions. The finite connectivity of the Bethe lattice induces a richer physical content with respect to its infinitely connected counterpart: a notion of distance between sites of the lattice is preserved, and the bosons are still weakly mobile in the Mott insulator phase. The Bethe lattice construction can be viewed as an approximation to the finite-dimensional version of the model. We show indeed a quantitatively reasonable agreement between our predictions and the results of Quantum Monte Carlo simulations in two and three dimensions.

We present a muon-spin relaxation investigation of the Ising chain magnet ${\text{Ca}}_3{\text{Co}}_{2-x}{\text{Mn}}_x{\text{O}}_6$ $\left(x\approx 0.95\right)$. We find dynamic spin fluctuations persisting down to the lowest measured temperature of 1.6 K. The previously observed transition at around $18\text{K}$ is interpreted as a subtle change in dynamics for a minority of the spins coupling to the muon that we interpret as spins locking into clusters. The dynamics of this spin fraction freeze below a temperature $T_{\text{SF}}\approx 8\text{K}$, while a majority of spins continue to fluctuate. An explanation of the low-temperature behavior is suggested in terms of the predictions of the anisotropic next-nearest-neighbor Ising model.

We have studied acoustic phonon-assisted tunneling in a weakly coupled GaAs/AlAs superlattice (SL) in a magnetic field. At zero magnetic field, the phonon-assisted tunnel current, $\delta I$, exhibits maximum at a particular value of the Stark splitting, ${\Delta }_{\text{max}}$, which depends on the spectral distribution of the nonequilibrium phonons. Applying the field $\left(B\le 7\text{T}\right)$ perpendicular to the SL growth direction, $z$, had no significant effect on the phonon-assisted tunneling current compared to $B=0$. However, in a magnetic field parallel to $z$, ${\Delta }_{\text{max}}$ is proportional to $B$ and weakly dependent on the phonon spectrum. This behavior, which we explain in terms of the momentum selection rules for phonon-assisted transitions in a magnetically quantized electron system, suggests that phonon amplification in the SL can be tuned by the magnetic field.

The $d$-Al-Ni-Co quasicrystal exhibits two crystallographically inequivalent twofold symmetric planes: the {10000} and {12110} surface planes. In particular the twofold symmetric surfaces are very interesting since they are spanned by perpendicular periodic and aperiodic axes. We investigated the (12110) surface of two different compositions $d$-${\text{Al}}_{70}{\text{Ni}}_{15}{\text{Co}}_{15}$ and $d$-${\text{Al}}_{72.9}{\text{Ni}}_{10.4}{\text{Co}}_{16.7}$ and the (10000) $d$-${\text{Al}}_{72.9}{\text{Ni}}_{10.4}{\text{Co}}_{16.7}$ surface by means of low-temperature (5 K) scanning tunneling microscopy (STM) in ultrahigh vacuum. All three surfaces are characterized with atomic resolution and display large and flat terraces with a columnar structure along the periodic axis. Both compositions of the (12110) $d$-Al-Ni-Co STM investigations revealed that the surface is faceted into {12110} and {10000} facets with a 1:1 area ratio. The (12110) facets represent the bulk periodicity of 0.4 nm within the prominent columnar structure which is attributed to a stacking of Al dimers. The sequence of the columnar structure along the aperiodic axis as well as the step heights between the (12110) surfaces could be attributed to the pentagonal tiling edge length of the bulk model. Furthermore, the (12110) surface is identified as one of the densest planes in the bulk model possessing a slightly lower Al concentration as the nominal bulk composition. However, a difference in the fine structure within the columns is observed between the two investigated compositions. The (10000) facet presents an identical surface structure as the unfaceted (10000) surface of the quasicrystal. At the (10000) surface two terraces with different surface structures are identified and compared to the bulk model. One of them shows a bias voltage depending structure. In contrast to the bulk model the minimal observed periodicity on the (10000) surface is doubled to 0.8 nm. On the other hand the aperiodic step height sequence and the order of the columnar motifs along the aperiodic axis are in agreement with the bulk model. The height distribution analysis of the (10000) surfaces exhibits that the surface topmost layer consists of planes which are less dense than the (12110) surface planes.

We show that the equilibrium noise can be dramatically suppressed in the case of a tunnel junction with modulated (time-dependent) transparency. We demonstrate how such a contact could be used either as an optimal electron entangler or as a single-electron source with suppressed equilibrium noise at low temperatures.

Strain-dictated epitaxial orientation in MnAs layers on the A and B surfaces of GaAs(113) is investigated by comparing the growths using solid-phase epitaxy and molecular-beam epitaxy. Drastically different crystalline orientation and surface morphology are found for the layer on GaAs(113)A grown by means of solid-phase epitaxy while smooth $\left(1\overline{1}00\right)$- or $\left(1\overline{1}01\right)$-oriented layers are otherwise obtained. We show that $\left(21\overline{3}1\right)$ orientation is realized in the unconventional layer due to the minimization of the strain energy. The layer consists of structural domains associated with in-plane inclinations of the $c$ axis of MnAs by about $\pm 42°$ from the $\left[33\overline{2}\right]$ direction of the substrate, giving rise to a polycrystalline surface morphology. The combination of coexisting crystal orientations is found to lack some reflection symmetries as a consequence of an asymmetry in the bonding energy.

We report the formation and direct observation of self-interstitials in surface proximity of an elemental semiconductor by exploiting subthreshold effects in a new generation of aberration-corrected transmission electron microscopes. We find that the germanium interstitial atoms reside close to hexagonal, tetragonal, and $S$-interstitial sites. Using phase-contrast microscopy, we demonstrate that the three-dimensional position of interstitial atoms can be determined from contrast analysis, with subnanometer precision along the electron-beam direction. Comparison with a first-principles study suggests a strong influence of the surface proximity or a positively charged interstitial. More generally, our investigation demonstrates that imaging of single atom can now be utilized to directly visualize single-defect formation and migration. These high-resolution electron microscopy studies are applicable to a wide range of materials since the reported noise level of the images even allows the detection of single-light atoms.

The recently discovered ${\text{CaFe}}_4{\text{As}}_3$ system displays low-temperature Fermi-liquid behavior, with enhanced electron-electron correlations. At high temperatures, the magnetic susceptibility shows Curie-Weiss behavior, with a large temperature-independent contribution. Antiferromagnetic ordering is observed below $T_N=\left(88.0\pm 1.0\right)\text{K}$, possibly via a spin density wave transition. A remarkably sharp drop in resistivity occurs below $T_2=\left(26.4\pm 1.0\right)\text{K}$, correlated with a similarly abrupt increase in the susceptibility but no visible feature in the specific heat. The electronic specific-heat coefficient $\gamma$ at low temperatures is close to $0.02\text{J}{\text{mol}}_{\text{Fe}}^{-1}{\text{K}}^{-2}$, but a higher value for $\gamma \sim 0.08\text{J}{\text{mol}}_{\text{Fe}}^{-1}{\text{K}}^{-2}$ can be inferred from a linear $Cp/T$ vs $T^2$ just above $T_2$. The Kadowaki-Woods ratio $A/{\gamma }^2=55\ast {10}^{-5}\mu \Omega \text{cm}{\text{mol}}^2{\text{K}}^2{\text{mJ}}^{-2}$ is nearly two orders of magnitude larger than that of heavy fermions.

We utilize a single-atom substitution technique with spectroscopic imaging in a scanning tunneling microscope to visualize the anisotropic spatial structure of magnetic and nonmagnetic transition metal acceptor states in the GaAs (110) surface. The character of the defect states play a critical role in the properties of the semiconductor, the localization of the states influencing such things as the onset of the metal-insulator transition and in dilute magnetic semiconductors the mechanism and strength of magnetic interactions that lead to the emergence of ferromagnetism. We study these states in the GaAs surface finding remarkable similarities between the shape of the acceptor-state wave function for Mn, Fe, Co, and Zn dopants, which is determined by the GaAs host and is generally reproduced by tight-binding calculations of Mn in bulk GaAs [J.-M. Tang and M. E. Flatté, Phys. Rev. Lett. 92, 047201 (2004)]. The similarities originate from the antibonding nature of the acceptor states that arise from the hybridization of the impurity $d$ levels with the host. A second deeper in-gap state is also observed for Fe and Co that can be explained by the symmetry breaking of the surface.