We report angle-resolved photoemission spectroscopy on two families of high-temperature superconductors (${\mathrm{Ca}}_x{\mathrm{La}}_{1-x}){\mathrm{(Ba}}_{1.75-x}$ La ${}_{0.25+x}{\mathrm{)Cu}}_3{\mathrm{O}}_y$ with $x=0.1$ ($T_c^{\max }=56$ K) and $x=0.4$ ($T_c^{\max }=82$ K). The Fermi surface (FS) is found to be independent of $x$ or $y$, and its size indicates extreme sample-surface overdoping. This universal FS allows the comparison of dynamical properties between superconductors of similar structure and identical doping, but different $T_c^{\max }$. We find that the high-energy ($\left|E\right|>50$ meV) nodal velocity in the $x=0.4$ family is higher than in the $x=0.1$ family. The implied correlation between $T_c^{\max }$ and the hopping rate $t$ supports the notion of kinetic energy driven superconductivity in the cuprates. We also find that the antinodal gap is higher for the $x=0.4$ family.

We have achieved high-quality Fabry-Pérot interference in a suspended graphene device both in conductance and in shot noise. A Fourier analysis of these reveals two sets of overlapping, coexisting interference patterns, with the ratios of the resonance intervals being equal to the width to length ratio of the device. We show that these sets originate from the unique coexistence of longitudinal and transverse resonances, with the longitudinal resonances occurring due to bunching of modes with low transverse momentum. Finally, the high quality of our samples allows us to probe the interaction renormalization of the Fermi velocity as well as the coexistence of Fabry-Pérot oscillations with universal conductance fluctuations.

We study a family of globally smooth, spatially varying, two-dimensional stacking textures in bilayer graphene. We find that the strain-minimizing stacking patterns connecting inequivalent ground states with local $AB$ and $BA$ interlayer registries are two-dimensional twisted textures of an interlayer displacement field. We construct and display these topological stacking textures for bilayer graphene, examine their interactions, and develop the composition rules that allow us to represent other more complex stacking textures, including globally twisted graphenes and extended one-dimensional domain walls.

Absorption and scattering are inherently related, as it is not possible to absorb power without creating a far-field shadow. We show, however, that properly overlapped resonant modes in a suitably designed system may in principle lead to arbitrarily large absorption levels, while at the same time minimizing the total scattering. We discuss the fundamental limits on scattering and absorption of an arbitrary receiving system and envision a composite nanoparticle that demonstrates the concept of a minimum-scattering superabsorber, with potential applications in energy harvesting, sensing, and imaging.

We report the effect of $\delta$ doping at the LaAlO${}_3$/SrTiO${}_3$ interface with LaMnO${}_3$ monolayers on the photoconducting (PC) state. The PC is realized by exposing the samples to broadband optical radiation of a quartz lamp and 325 and 441 nm lines of a He-Cd laser. Along with the significant modification in electrical transport which drives the pure LaAlO${}_3$/SrTiO${}_3$ interface from metal-to-insulator with increasing LaMnO${}_3$ sub-monolayer thickness, we also observe an enhancement in the photoresponse and relaxation time constant. A possible scenario for the PC based on defect clusters, random potential fluctuations, and large lattice relaxation models, along with the role of structural phase transition in SrTiO${}_3$, is discussed. For pure LaAlO${}_3$/SrTiO${}_3$, the photoconductivity appears to originate from interband transitions between Ti-derived $3d$ bands which are $e_g$ in character and O 2$p$–Ti $t_{2g}$ hybridized bands. The band structure changes significantly when fractional layers of LaMnO${}_3$ are introduced. Here the Mn $e_g$ bands ($\approx$1.5 eV above the Fermi energy) within the photoconducting gap lead to a reduction in the photoexcitation energy and a gain in overall photoconductivity.

The dephasing and lifetime of excitons in InSe layered crystals are carefully measured using three-pulse, four-wave mixing and two-dimensional Fourier transform (2DFT) spectroscopy. We obtain a detailed picture of the mechanism of excitonic dephasing in this layered material. The 2DFT spectra reveal contributions from the coherent excitation of continuum states, whereas the temperature dependence provides a detailed description of the phonon-exciton interactions and the low temperature limit of the homogeneous linewidth. The excitation density dependence reveals excitation-induced dephasing due to exciton-exciton scattering. The temperature-dependent homogeneous linewidth is dominated by the interaction of excitons with the $\sim$113 cm${}^{-1}$ out-of-plane phonon mode.

We consider symmetry-projected Hartree-Fock trial wave functions in constrained-path Monte Carlo (CPMC) calculations. Previous CPMC calculations have mostly employed Hartree-Fock (HF) trial wave functions, restricted or unrestricted. The symmetry-projected HF approach results in a hierarchy of wave functions with increasing quality: the more symmetries that are broken and restored in a self-consistent manner, the higher the quality of the trial wave function. This hierarchy is approximately maintained in CPMC calculations: the accuracy in the energy increases and the statistical variance decreases when further symmetries are broken and restored. Significant improvement is achieved in CPMC with the best symmetry-projected trial wave functions over those from simple HF. We analyze and quantify the behavior using the two-dimensional repulsive Hubbard model as an example. In the sign-problem-free region, where CPMC can be made exact but a constraint is deliberately imposed here, spin-projected wave functions remove the constraint bias. Away from half filling, spatial symmetry restoration in addition to that of the spin leads to highly accurate results from CPMC. Since the computational cost of symmetry-projected HF trial wave functions in CPMC can be made to scale algebraically with system size, this provides a potentially general approach for accurate calculations in many-fermion systems.

We report electronic Raman scattering measurements of Sr(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ single crystals in their magnetic–spin density wave (SDW) phase. The spectra display multiple, polarization-resolved SDW gaps as expected in a band-folding itinerant picture for a multiband system. The temperature dependence of the SDW gaps reveals an unusual evolution of the reconstructed electronic structure with at least one gap being activated only well below the magnetic SDW transition $T_N$. A comparison with temperature dependent Hall measurements allows us to assign this activated behavior to a change in the Fermi surface topology deep in the SDW phase, which we attribute to the disappearance of a holelike Fermi pocket. Our results highlight the strong sensitivity of the low energy electronic structure to temperature in iron-arsenide superconductors.

When an applied magnetic field has an arbitrary direction with respect to the lattice axes of a periodically microstructured or nanostructured metamaterial, the effective permittivity tensor of the metamaterial sample becomes anisotropic and all its components can be nonzero. This is true even if the microstructure has a high symmetry, e.g., cubic or triangular. It is found that the strong magneto-induced anisotropy which appears in the macroscopic response leads to an unusually strong anisotropic behavior of the Voigt effect and other magneto-optical effects. That is, these phenomena become strongly dependent on the direction of the applied static magnetic field, as well as on the direction of the time-dependent electromagnetic field, with respect to the symmetry axes of the periodic microstructure.

Time-resolved terahertz quenching studies of the magnetoexcitonic photoluminescence from GaAs/AlGaAs quantum wells are performed. A microscopic theory is developed to analyze the experiments. Detailed experiment-theory comparisons reveal a remarkable magnetic-field controllability of the Coulomb and terahertz interactions in the excitonic system.

Calculations of the properties of monolayer xenon/graphite for temperatures up to its triple point at 100 K are reported. The average lattice constant and orientational epitaxy angle for the monolayer solid are evaluated along its (two-dimensional) sublimation curve. The incommensurate rotated lattice approaches the incommensurate aligned configuration as the melting temperature is approached, as in experiments. The calculated temperature, latent heat of melting, and solid-liquid density difference at the triple point agree with experiment. The methods include molecular dynamics simulations for large submonolayer patches of xenon and both self-consistent-phonon and perturbation-variation approximations. An overall quantitative agreement between the simulations, calculations, and experimental data is achieved with an interaction model that includes the spatially periodic xenon-graphite corrugation energy.

We discuss an electronic interferometer recently measured by Yamamoto et al. This “flying quantum bit” experiment showed quantum oscillations between electronic trajectories of two tunnel-coupled wires connected via an Aharanov-Bohm ring. We present a simple scattering model as well as a numerical microscopic model to describe this experiment. In addition, we present experimental data to which we confront our numerical results. While our analytical model provides basic concepts for designing the flying qubit device, we find that our numerical simulations allow us to reproduce detailed features of the transport measurements such as in-phase and antiphase oscillations of the two output currents as well as a smooth phase shift when sweeping a side gate. Furthermore, we find remarkable resemblance for the magnetoconductance oscillations in both conductance and visibility between simulations and experiments within a specific parameter range.

The electronic and magnetic properties of individual Fe atoms adsorbed on the surface of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$(111) are investigated. Scanning tunneling microscopy and spectroscopy prove the existence of two distinct types of Fe species, while our first-principles calculations assign them to Fe adatoms in the hcp and fcc hollow sites. The combination of x-ray magnetic circular dichroism measurements and angular dependent magnetization curves reveals out-of-plane anisotropies for both species with anisotropy constants of $K_{\text{fcc}}=(10\pm 4)$ meV/atom and $K_{\text{hcp}}=(8\pm 4)$ meV/atom. These values are well in line with the results of calculations.

We report on the superconducting properties of the K${}_x{\mathrm{WO}}_3$ tetragonal tungsten bronze. The highest superconducting transition temperature ($T_c=2.1$ K) was obtained for K${}_{0.38}{\mathrm{WO}}_3$. $T_c$ decreases linearly with increasing K content. Using the measured values for the upper critical field $H_{c2}$ and the specific heat $C$, we estimate the orbital critical field $H_{c2}$(0), coherence length $\xi$(0), Debye temperature ${\Theta }_D$, and coupling constant ${\lambda }_{e\text{−}p}$. The magnitude of the specific-heat jump at $T_c$ suggests that the K${}_x{\mathrm{WO}}_3$ tetragonal tungsten bronze is a weakly coupled superconductor. The superconducting phase diagram of the doped tungsten bronze family is presented.

We present measurements of how a single vortex line in superfluid helium interacts with a macroscopic bump on the chamber wall. Rather than observing a unique pin location, we find that a given applied velocity field can support pinning at multiple sites along the bump, both near its apex and near its edge. We also find that a vortex can pass near or even traverse the bump itself with or without pinning, depending on its path of approach to the bump. We discuss our results in light of past computational work on vortex pinning by a hemispherical bump, as well as our own simulations that incorporate additional aspects of the experimental setup.

Phonon density of states (PDOS) measurements have been performed on polycrystalline ${\mathrm{UO}}_2$ at 295 and 1200 K using time-of-flight inelastic neutron scattering to investigate the impact of anharmonicity on the vibrational spectra and to benchmark ab initio PDOS simulations performed on this strongly correlated Mott insulator. Time-of-flight PDOS measurements include anharmonic linewidth broadening, inherently, and the factor of $\sim$7 enhancement of the oxygen spectrum relative to the uranium component by the increased neutron sensitivity to the oxygen-dominated optical phonon modes. The first-principles simulations of quasiharmonic PDOS spectra were neutron weighted and anharmonicity was introduced in an approximate way by convolution with wave-vector-weighted averages over our previously measured phonon linewidths for ${\mathrm{UO}}_2$, which are provided in numerical form. Comparisons between the PDOS measurements and the simulations show reasonable agreement overall, but they also reveal important areas of disagreement for both high and low temperatures. The discrepancies stem largely from a $\sim$10 meV compression in the overall bandwidth (energy range) of the oxygen-dominated optical phonons in the simulations. A similar linewidth-convoluted comparison performed with the PDOS spectrum of Dolling et al. obtained by shell-model fitting to their historical phonon dispersion measurements shows excellent agreement with the time-of-flight PDOS measurements reported here. In contrast, we show by comparisons of spectra in linewidth-convoluted form that recent first-principles simulations for ${\mathrm{UO}}_2$ fail to account for the PDOS spectrum determined from the measurements of Dolling et al. These results demonstrate PDOS measurements to be stringent tests for ab inito simulations of phonon physics in ${\mathrm{UO}}_2$ and they indicate further the need for advances in theory to address the lattice dynamics of ${\mathrm{UO}}_2$.

Electron paramagnetic resonance (EPR) is used to investigate the triplet ($S$ = 1) ground state of the neutral oxygen vacancy in bulk rutile ${\mathrm{TiO}}_2$ crystals. This shallow donor consists of an oxygen vacancy with two nearest-neighbor, exchange-coupled Ti${}^{3+}$ ions located along the [001] direction and equidistant from the vacancy. The spins of the two trapped electrons, one at each Ti${}^{3+}$ ion, align parallel to give the $S$ = 1 state. These neutral oxygen vacancies are formed near 25 K in as-grown oxidized ${\mathrm{TiO}}_2$ crystals by illuminating with sub-band-gap 442 nm laser light. The angular dependence of the EPR spectra provides the principal values and axes for the $g$ and $D$ matrices. Observations of the Ti and Ti hyperfine lines when the magnetic field is along high-symmetry directions show that the two Ti${}^{3+}$ ions are equivalent; i.e., they have equal hyperfine $A$ matrices. The $A$ matrix for each Ti${}^{3+}$ ion in the neutral $S$ = 1 oxygen vacancy is approximately half of the $A$ matrix reported earlier for the one Ti${}^{3+}$ ion in the singly ionized $S$ = 1/2 oxygen vacancy [Brant et al., J. Appl. Phys. 114, 113702 (2013)]. The neutral oxygen vacancies are thermally unstable above 25 K. They release an electron to the conduction band with an activation energy near 63 meV and convert to singly ionized $S$ = 1/2 oxygen vacancies. When undoped ${\mathrm{TiO}}_2$ is sufficiently oxygen deficient (i.e., reduced), this combination of conduction band electrons and singly ionized oxygen vacancies may result in carrier-mediated ferromagnetism at room temperature.

The work function of a material is commonly used as an intrinsic reference for band alignment; however, it is notoriously susceptible to extrinsic conditions. Following the classification of Bardeen we calculate values for the bulk binding energy of electrons in rutile-structured ${\mathrm{SnO}}_2$ and the effect of the surface on the work function, thus highlighting the role of the surface in determining the energy levels of a material. Furthermore we demonstrate how, through the use of ultrathin heteroepitaxial oxide layers (${\mathrm{SiO}}_2$, ${\mathrm{TiO}}_2$, ${\mathrm{PbO}}_2$) at the surface, the work function can be tuned to achieve energy levels commensurate with important technological materials. This approach can be extended from transparent conducting oxides to other semiconducting materials.

An exotic feature of the fractional quantum Hall effect is the emergence of anyons, which are quasiparticle excitations with fractional statistics. In the presence of a symmetry, such as $U\left(1\right)$ charge conservation, it is well known that anyons can carry fractional symmetry quantum numbers. In this paper we reveal a different class of symmetry realizations, i.e., anyons can “breed” in multiples under symmetry operation. We focus on the global Ising (${\mathbb{Z}}_2$) symmetry and show examples of these unconventional symmetry realizations in Laughlin-type fractional quantum Hall states. One remarkable consequence of such an Ising symmetry is the emergence of anyons on the Ising symmetry domain walls. We also provide a mathematical framework which generalizes this phenomenon to any Abelian topological orders.