By use of an experimental design that provides for superfluid helium in contact with bulk hcp ${}^4\text{H}\text{e}$ off the melting curve, we have observed the dc transport of mass through a cell filled with solid ${}^4\text{H}\text{e}$ in the hcp region of the phase diagram. Flow, which shows characteristics of a superflow, is seen to be independent of the method used to grow the solid but depends on pressure and temperature. The temperature dependence suggests the possibility of hysteresis.

The exciton dynamics of microcrystalline pentacene films is investigated by femtosecond pump-probe experiments. Measurements are performed with ultrashort laser pulses applied at normal incidence and at an angle of incidence of $65°$ to disentangle singlet and triplet contributions by exploiting the different orientations of the molecular transition dipoles. The results indicate that the initial 70 fs fast relaxation step transforms the optically excited excitons in a reasonable mobile species with strongly reduced radiative transition strength. Fission into triplet excitons takes place on the picosecond time scale as a secondary, thermally activated process and with a small total yield of approximately 2%. Evidence is provided that the dominant species are singlet excitons with excimer character. To the subsequent dynamics contribute diffusion driven exciton-exciton annihilation and trapping of singlet and triplet excitons. Values for diffusion constants and trap densities are extracted by modeling the measurements with rate equations which include singlet and triplet dynamics.

Years of intensive work on perovskite manganites has led to a detailed understanding of the phenomena that emerge from competition between the electronic and lattice degrees of freedom in these correlated electron systems. It is well understood that the related cobaltites provide an additional spin-state degree of freedom. Here, we use the magnetic properties of a particular cobaltite, ${\text{Pr}}_{1-x}{\text{Sr}}_x{\text{CoO}}_3$, to demonstrate the vital role played by a further ingredient often negligible in manganites; magnetocrystalline anisotropy. ${\text{Pr}}_{1-x}{\text{Sr}}_x{\text{CoO}}_3$ exhibits an anomalous “double magnetic transition” that cannot be ascribed to a spin-state transition or the usual charge/orbital/antiferromagnetic ordering and has thus far evaded explanation. We show that this is actually due to a coupled structural/magnetocrystalline anisotropy transition driven, in this case, by Pr-O hybridization. The results point to the existence of a distinct class of phenomena in the cobaltites due to the unique interplay between structure and magnetic anisotropy.

Using the parton construction, we build a three-dimensional (3D) multilayer fractional quantum Hall state with average filling $\nu =1/3$ per layer that is qualitatively distinct from a stacking of weakly coupled Laughlin states. The state supports gapped charge $e/3$ fermionic quasiparticles that can propagate both within and between the layers, in contrast to the quasiparticles in a multilayer Laughlin state which are confined within each layer. Moreover, the state has gapless neutral collective modes, a manifestation of an emergent “photon,” which is minimally coupled to the fermionic quasiparticles. The surface sheath of the multilayer state resembles a chiral analog of the Halperin-Lee-Read state, which is protected against gap-forming instabilities by the topological character of the bulk 3D phase. We propose that this state might be present in multilayer systems in the “intermediate tunneling regime,” where the interlayer tunneling strength is on the same order as the Coulomb energy scale. We also find that the parton construction leads to a candidate state for a bilayer $\nu =1/3$ system in the intermediate tunneling regime. The candidate state is distinct from both a bilayer of $\nu =1/3$ Laughlin states and the single layer $\nu =2/3$ state but is, nonetheless, a fully gapped fractional quantum Hall state with charge $e/3$ anyonic quasiparticles.

Both amorphous and crystalline materials frequently exhibit low-temperature specific heats in excess of what is predicted using the Debye model. The signature of this excess specific heat is a peak observed in $C/T^3$ versus $T$. To understand the curious absence of long-range ordering of local distortions in the crystal structure of pyrochlore ${\text{Bi}}_2{\text{Ti}}_2{\text{O}}_7$, we have measured the specific heat of crystalline ${\text{Bi}}_2{\text{Ti}}_2{\text{O}}_7$ and related compounds. We find that the peak in $C/T^3$ versus $T$ in ${\text{Bi}}_2{\text{Ti}}_2{\text{O}}_7$ falls at a substantially lower temperature than other similar compounds consistent with the presence of disorder. This thermodynamic evidence for disorder in crystalline ${\text{Bi}}_2{\text{Ti}}_2{\text{O}}_7$ is consistent with quenched configurational disorder among Bi lone pairs produced by the geometrical frustration, which could represent a possible realization of “charge ice.”

Motivated by a recent experiment on ${\text{Rb}}_2{\text{Cu}}_3{\text{SnF}}_{12}$, where spin-1/2 ${\text{Cu}}^{2+}$ moments reside on the layers of kagome-like lattices, we investigate quantum ground states of the antiferromagnetic Heisenberg model on a series of deformed kagome lattices. The deformation is characterized by a weaker exchange coupling $\alpha J$ on certain lattice links appropriate for ${\text{Rb}}_2{\text{Cu}}_3{\text{SnF}}_{12}$ with $\alpha =1$ corresponding to the ideal kagome lattice. In particular, we study possible valence bond solid phases using the perturbation theory around isolated dimer limits, dimer series expansion, and self-consistent bond operator mean-field theory. It is shown that the valence bond solid phase with a 36-site unit cell of the ideal kagome lattice is quite sensitive to a small lattice distortion as the kind discovered in ${\text{Rb}}_2{\text{Cu}}_3{\text{SnF}}_{12}$. As a result, we find that a more likely quantum ground state in ${\text{Rb}}_2{\text{Cu}}_3{\text{SnF}}_{12}$ is the valence bond solid phase with a 12-site unit cell, where six dimers form a pinwheel structure, leading to strong modification of the elementary triplet and singlet excitation spectra in the deformed kagome lattices.

We consider the motion of an impurity particle in a general one-dimensional quantum fluid at zero temperature. The dispersion relation $\Omega \left(P\right)$ of the impurity is strongly affected by interactions with the fluid as the momentum approaches $\pm \pi \hslash n,\pm 3\pi \hslash n,\dots$, where $n$ is the density. This behavior is caused by singular $\pm 2\pi \hslash n$ scattering processes and can be understood by analogy to the Kondo effect, both at strong and weak couplings, with the possibility of a quantum phase transition where ${\Omega }^{\prime }\left(\pm \pi n\right)$ jumps to zero with increasing coupling. The low energy singularities in the impurity spectral function can be understood on the same footing.

The diffusion barriers of Ga and N adatoms on the nonpolar $a$- and $m$-plane surfaces have been studied employing density-functional theory calculations. Our calculations reveal a strong in-plane anisotropy of the diffusion barriers for both surfaces: for the $a$-plane surface larger diffusion lengths parallel to the $c$ axis are observed. For the $m$-plane surface the in-plane anisotropy is reversed, and significantly smaller barriers are found for diffusion normal to the [0001] direction. These results are consistent with the experimentally observed in plane growth anisotropy of the nonpolar GaN surfaces. Moreover, they indicate that the large aspect ratio achieved for $c$-axial GaN nanowires is not a consequence of a strong anisotropy in the adatom mobilities but is rather related to surface thermodynamics and nucleation.

We study the effect of the lattice structure on the spin-fluctuation-mediated superconductivity in the iron pnictides adopting the five-band models of several virtual lattice structures of LaFeAsO, as well as actual materials such as NdFeAsO and LaFePO obtained from the maximally localized Wannier orbitals. Random phase approximation is applied to the models to solve the Eliashberg equation. This reveals that the gap function and the strength of the superconducting instability are determined by the cooperation or competition among multiple spin-fluctuation modes arising from several nestings among disconnected pieces of the Fermi surface, which is affected by the lattice structure. Specifically, the appearance of the Fermi surface $\gamma$ around $\left(\pi ,\pi \right)$ in the unfolded Brillouin zone is sensitive to the pnictogen height $h_{\text{Pn}}$ measured from the Fe plane, where $h_{\text{Pn}}$ is shown to act as a switch between high-$T_c$ nodeless and low-$T_c$ nodal pairings. We also find that reduction in the lattice constants generally suppresses superconductivity. We can then combine these to obtain a generic superconducting phase diagram against the pnictogen height and lattice constant. This suggests that NdFeAsO is expected to exhibit a fully gapped, sign-reversing $s$-wave superconductivity with a higher $T_c$ than in LaFeAsO, while a nodal pairing with a low $T_c$ is expected for LaFePO, which is consistent with experiments.

We report a comprehensive study of photoluminescence (PL) quenching of tris-(8-hydroxyquinoline) aluminum $\left({\text{Alq}}_3\right)$ at interfaces with thin films of tin oxide $\left({\text{SnO}}_2\right)$ using both steady-state and time-resolved measurements. Quenching of excitons generated in the ${\text{Alq}}_3$ layer increased with increased conductivity of the ${\text{SnO}}_2$ films, which we relate with the presence of nonradiative energy transfer from excitons in ${\text{Alq}}_3$ to transitions in ${\text{SnO}}_2$. In addition, due to the semitransparency of ${\text{SnO}}_2$, the effects of optical interference on the steady-state PL quenching of ${\text{Alq}}_3$ are determined. We demonstrate that without accounting for the interference effects in the excitation, the extracted exciton diffusion length $\left(L_d\right)$ in ${\text{Alq}}_3$ is in the range of 10–20 nm. However, when using a numerical model to account for the optical interference effects, we find that $L_d$ is in the range of 5–9 nm, which agrees with $L_d$ extracted from time-resolved measurements (4–6 nm). We conclude that time-resolved measurements are least affected by optical interference, yielding the most accurate measurement of $L_d$.

Among the various alkali oxides the sesquioxides ${\text{Rb}}_4{\text{O}}_6$ and ${\text{Cs}}_4{\text{O}}_6$ are of special interest. Electronic-structure calculations using the local spin-density approximation predicted that ${\text{Rb}}_4{\text{O}}_6$ should be a half-metallic ferromagnet, which was later contradicted when an experimental investigation of the temperature-dependent magnetization of ${\text{Rb}}_4{\text{O}}_6$ showed a low-temperature magnetic transition and differences between zero-field-cooled and field-cooled measurements. Such behavior is known from spin glasses and frustrated systems. ${\text{Rb}}_4{\text{O}}_6$ and ${\text{Cs}}_4{\text{O}}_6$ comprise of two different types of dioxygen anions, the hyperoxide and the peroxide anions. The nonmagnetic peroxide anions do not contain unpaired electrons while the hyperoxide anions contain unpaired electrons in antibonding ${\pi }^{\ast }$ orbitals. High electron localization (narrow bands) suggests that electronic correlations are of major importance in these open-shell $p$-electron systems. Correlations and charge ordering due to the mixed valency render $p$-electron-based anionogenic magnetic order possible in the sesquioxides. In this work we present an experimental comparison of ${\text{Rb}}_4{\text{O}}_6$ and the related ${\text{Cs}}_4{\text{O}}_6$. The crystal structures are verified using powder x-ray diffraction. The mixed valency of both compounds is confirmed using Raman spectroscopy, and time-dependent magnetization experiments indicate that both compounds show magnetic frustration, a feature only previously known from $d$- and $f$-electron systems.

We study the few-electron eigenspectrum of a nanotube quantum dot with spin-orbit coupling. The two-electron phase diagram as a function of the length of the dot and the applied parallel magnetic field shows clear signatures of both spin-orbit coupling and electron-electron interaction. Below a certain critical length, ground-state transitions are correctly predicted by a single-particle picture and are mainly independent of the length of the dot despite the presence of strong correlations. However, for longer quantum dots the critical magnetic field strongly decreases with increasing length, which is a pure interaction effect. In fact, the new ground state is spin and valley polarized, which implies a strong occupation of higher longitudinal modes.

The temperature evolution of the nanoscale structure of ${\text{PbSc}}_{0.5}{\text{Nb}}_{0.5}{\text{O}}_3$ (PSN) and $\left(\text{Pb},A^{″}\right){\text{Sc}}_{0.5}{\text{Nb}}_{0.5}{\text{O}}_3$, $\left(A^{″}=\text{Ba},\text{Bi}\right)$ is analyzed by applying synchrotron single-crystal and high-resolution powder x-ray diffraction as well as polarized Raman spectroscopy. The study compares the effect of incorporation of two-valent cations with isotropic electron structure $\left({\text{Ba}}^{2+}\right)$ and three-valent cations with stereochemically active lone pairs $\left({\text{Bi}}^{3+}\right)$ on the structure of Pb-based perovskite-type relaxor ferroelectrics. The results reveal that the violation of the host system of cations with lone-pair electrons $\left({\text{Pb}}^{2+}\right)$, i.e., the reduction in ferroic species with coherent off-centered Pb atoms, is the major factor for the suppression of long-range ferroelectric ordering at low temperatures. The local charge imbalance associated with $A$ site chemical disorder has negligible impact on the development of ferroelectric order if the second type of $A$-positioned cations also forms lone-pair electrons. The substitution of ${\text{Bi}}^{3+}$ for ${\text{Pb}}^{2+}$ even enhances the cubic-to-ferroelectric transformation processes in PSN and results in a structural state consisting of abundant ferroelectric domains, which are large enough to be identified by polarized Raman spectroscopy as crystalline, but with an average size close to the intrinsic detection limit of synchrotron single-crystal x-ray diffraction.

Results are presented for the nonequilibrium infinite-$U$ Anderson model using a large $N$ approach, where $N$ is the degeneracy of the impurity level, and where nonequilibrium is established by coupling the level to two leads at two different chemical potentials so that there is current flow. A slave-boson representation combined with Keldysh functional integral methods is employed. Expressions for the static spin susceptibility ${\chi }_S$ and the conductance $G$ are presented to $\mathcal{O}\left(\frac{1}{N}\right)$ and for an applied voltage difference $V$ less than the Kondo temperature. The correlation function for the slave boson is found to be significantly modified from its equilibrium form in that it acquires a rapid decay in time with a rate that equals the current-induced decoherence rate. Physical observables are found to have a rather complex dependence on the coupling strength to the two leads which can lead to asymmetric behavior ${\chi }_S\left(V\right)\ne {\chi }_S\left(-V\right)$, $G\left(V\right)\ne G\left(-V\right)$ both in the mixed valence and in the Kondo regime.

The electronic structure of electron-doped iron-arsenide superconductors $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ has been measured with angle-resolved photoemission spectroscopy. The data reveal a marked photon energy dependence of points in momentum space where the bands cross the Fermi energy, a distinctive and direct signature of three dimensionality in the Fermi-surface topology. By providing a unique example of high-temperature superconductivity hosted in layered compounds with three-dimensional electronic structure, these findings suggest that the iron arsenides are unique materials, quite different from the cuprates high-temperature superconductors.

The low-energy effective quantum-field theory of the edge excitations of a fully gapped bulk topological phase corresponding to a local Hamiltonian must be local and unitary. Here it is shown that whenever all the edge excitations propagate in the same direction with the same velocity, it is a conformal field theory. In particular, this is the case in the quantum Hall effect for model “special Hamiltonians” for which the ground-state, quasihole, and edge excitations can be found exactly as zero-energy eigenstates, provided the spectrum in the interior of the system is fully gapped. In addition, other conserved quantities in the bulk, such as particle number and spin, lead to affine Lie algebra symmetries in the edge theory. Applying the arguments to some trial wave functions related to nonunitary conformal field theories, it is argued that the Gaffnian state and an infinite number of others cannot describe a gapped topological phase because the numbers of edge excitations do not match any unitary conformal field theory.

We present a scheme to solve the Kohn-Sham equations of density functional theory using orthonormal wave functions and an independent pseudo-Hamiltonian matrix. Our ansatz is based on a direct minimization of the electronic free energy with conjugate-gradient techniques. In contrast to previous approaches, continuous changes in the occupation numbers and subspace rotations are naturally included and allow therefore for exponential convergence. The algorithm is demonstrated for Mo bulk and surfaces.

We argue that Raman study of Fe-pnictides is a way to unambiguously distinguish between various superconducting gaps proposed for these materials. We show that $A_{1g}$ Raman intensity develops a true resonance peak below $2\Delta$ if the pairing gap has $A_{1g}$ symmetry in the folded Brillouin zone $\left(\Delta \left(k=0\right)=\Delta ,\Delta \left(\pi ,\pi \right)=-\Delta \right)$. No such peak develops for a pure $s$-wave gap, a $d$-wave gap, and an extended $s$-wave gap with $\Delta \left(\mathbf{k}\right)=\Delta \text{cos}\frac{k_x}{2}\text{cos}\frac{k_y}{2}$. We show that the peak remains quite strong for the values of interpocket impurity scattering used to fit NMR data.

The inhomogeneous Hubbard model is investigated in the framework of lattice density-functional theory (LDFT) by considering the single-particle density matrix ${\gamma }_{ij}$ with respect to the lattice sites as the basic variable of the many-body problem. The domain of representability of ${\gamma }_{ij}$ is determined for charge-density wave states on finite bipartite lattices. Levy’s constrained search of the interaction-energy functional $W\left[{\gamma }_{ij}\right]$ is numerically solved by applying the Lanczos method to an effective Hubbard-type model. The exact functional dependence of $W\left[{\gamma }_{ij}\right]$ is analyzed by varying systematically the charge transfer $\Delta n={\gamma }_{22}-{\gamma }_{11}$, the degree of electron delocalization $g_{12}$ between the sublattices, the number of sites $N_a$, and the band filling $n=\left({\gamma }_{11}+{\gamma }_{22}\right)/2=N_e/N_a$. For each $\Delta n$ the properties of $W$ are discussed in the limits of weak $\left({\gamma }_{12}\simeq {\gamma }_{12}^0\right)$ and strong $\left({\gamma }_{12}\simeq {\gamma }_{12}^{\infty }\right)$ electronic correlations, as well as in the crossover region $\left({\gamma }_{12}^{\infty }\le {\gamma }_{12}\le {\gamma }_{12}^0\right)$. It is shown that $W$ follows quite closely a simple scaling behavior as a function of $\Delta n$ and $g_{12}=\left({\gamma }_{12}-{\gamma }_{12}^{\infty }\right)/\left({\gamma }_{12}^0-{\gamma }_{12}^{\infty }\right)$. The very good transferability of $W\left(\Delta n,g_{12}\right)$ for different $N_a$, $n$ and lattice structure opens new possibilities of applying LDFT to inhomogeneous many-body models.

Analytical expressions for the noise affecting the different signals of interest in frequency-modulation dynamic force microscopy are derived. Two noise sources are considered, the thermomechanical noise of the cantilever and the noise of the deflection sensor. It is shown that distinguishing between amplitude and phase noise is crucial for a good understanding of the instrument. When the tip is far from the sample surface, in the absence of any tip-substrate interaction, the system reduces to two independent feedback loops, the first one maintaining the amplitude at a set point value and the other one keeping a constant phase lag between the tip oscillation and the cantilever excitation force. Closer to the surface, in the presence of tip-substrate interaction, these two loops become coupled to an extent that is determined by the nonlinear character of the cantilever oscillation induced by this interaction. The approximations introduced in deriving these analytical expressions are validated by numerical simulations and a comparison with experimental measurements is proposed. This modeling of the noise in frequency-modulation dynamic force microscopy allows us to derive most of the previously published results in a clear and unified framework. In addition, it demonstrates that the oscillator nonlinearities play an essential role in determining the noise level of the instrument, an effect that was not considered before.