We perform an exact diagonalization study of the topological order in topological flat band models through calculating entanglement entropy and spectra of low-energy states. We identify multiple independent minimal entangled states, which form a set of orthogonal basis states for the ground-state manifold. We extract the modular transformation matrices $\mathcal{S}$ ($\mathcal{U}$) which contain the information of mutual (self) statistics, quantum dimensions, and the fusion rule of quasiparticles. Moreover, we demonstrate that these matrices are robust and universal in the whole topological phase against different perturbations until the quantum phase transition takes place.

The free surface of water and the interface between water and a hydrophobic surface both have positive interface energies. The water density near a free surface drops below the bulk density, and thus it is expected that water near a hydrophobic surface will also show a density depletion. However, efforts by multiple groups to detect and characterize the predicted gap at water-hydrophobic interfaces have produced contradictory results. We have studied the interface between water and fluoroalkylsilane self-assembled monolayers using specular x-ray reflectivity and analyzed the parameter-space landscapes of the merit functions being minimized by data fitting. This analysis yields a better understanding of confidence intervals than the customary process of reporting a unique best fit. We conclude that there are unambiguous gaps at water-hydrophobic interfaces when the hydrophobic monolayer is more densely packed.

When time-reversal symmetry is broken, quantum coherent systems with and without spin rotational symmetry exhibit the same universal behavior in their electric transport properties. We show that spin transport discriminates between these two cases. In systems with large charge conductance, spin transport is essentially insensitive to the breaking of time-reversal symmetry. However, in the opposite limit of a single exit channel, spin currents vanish identically in the presence of time-reversal symmetry, but are turned on by breaking it with an orbital magnetic field.

The surface structure of the prototypical topological insulator Bi${}_2$Se${}_3$ is determined by low-energy electron diffraction and surface x-ray diffraction at room temperature. Both approaches show that the crystal is terminated by an intact quintuple layer. Specifically, an alternative termination by a bismuth bilayer is ruled out. Surface relaxations obtained by both techniques are in good agreement with each other and found to be small. This includes the relaxation of the van der Waals gap between the first two quintuple layers.

We investigate the electronic properties of the ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$ interface using density functional theory. In contrast to previous studies, which relied on (semi-)local functionals and the $\text{GGA}+U$ method, we here use a recently developed hybrid functional to determine the electronic structure. This approach offers the distinct advantage of accessing both the metallic and insulating multilayers on a parameter-free equal footing. As compared to calculations based on semilocal GGA functionals, our hybrid functional calculations lead to a considerably increased band gap for the insulating systems. The details of the electronic structure show substantial deviations from those obtained by GGA calculations. This casts severe doubts on all previous results based on semilocal functionals. In particular, corrections using rigid band shifts (“scissors operator”) cannot lead to valid results.

Zintl phases are a class of intermetallic materials that have simultaneously ionic and covalent bonding resulting from charge transfer between two different atomic species. We present a combined first principles and experimental study of Zintl-phase SrAl${}_4$, which is grown in thin film form on the perovskite oxide LaAlO${}_3$ using molecular beam epitaxy. The structural properties are investigated using reflection-high-energy electron diffraction, x-ray diffraction, and cross-section transmission electron microscopy, which reveal relaxed epitaxial island growth. Photoelectron spectroscopy measurements verify the Zintl-Klemm nature of the bonding in the material and are utilized to determine the band offset and the work function of SrAl${}_4$, while transport measurements confirm its metallic behavior. The experimentally observed properties are confirmed using density functional calculations.

Currently little is known about the atomic and electronic structure of Cu${}_2$ZnSnS${}_4$ (CZTS) surfaces, although the efficiency of kesterite-based solar cells has been increased to over 11$\%$. Through the first-principles calculations, we studied the possible surface structures of the frequently observed cation-terminated (112) and anion-terminated ($\overline{1}\overline{1}\overline{2}$) surfaces, and found that the polar surfaces are stabilized by the charge-compensating defects, such as vacancies (V${}_{\mathrm{Cu}}$, V${}_{\mathrm{Zn}}$), antisites (Zn${}_{\mathrm{Cu}}$, Zn${}_{\mathrm{Sn}}$, Sn${}_{\mathrm{Zn}}$), and defect clusters (Cu${}_{\mathrm{Zn}}$+Cu${}_{\mathrm{Sn}}$, 2Zn${}_{\mathrm{Cu}}$+V${}_{\mathrm{Sn}}$). In stoichiometric single-phase CZTS samples, Cu-enriched defects are favored on (112) surfaces and Cu-depleted defects are favored on ($\overline{1}\overline{1}\overline{2}$) surfaces, while in non-stoichiometric samples grown under Cu poor and Zn rich conditions both surfaces favor the Cu-depleted defects, which explains the observed Cu deficiency on the surfaces of the synthesized CZTS thin films. The electronic structure analysis shows that Cu-enriched surfaces produce detrimental states in the band gap, while Cu-depleted surfaces produce no gap states and are thus benign to the solar cell performance. The calculated surface properties are consistent with experimental observation that Cu-poor and Zn-rich CZTS solar cells have higher efficiency.

The formation of an interface between Bi${}_2$O${}_3$, Fe${}_2$O${}_3$, BiFeO${}_3$, Bi${}_{0.5}$Na${}_{0.5}$TiO${}_3$, and the high work function metallic RuO${}_2$ is studied using photoelectron spectroscopy with in situ RuO${}_2$ deposition. Schottky barrier heights are derived and the valence band maximum energies of the studied materials are aligned with respect to each other as well as to other functional oxides like SrTiO${}_3$ and PbTiO${}_3$. The energy band alignment follows systematic trends compared to a large number of oxides, and can be understood in terms of the contribution of Fe $3d$ and Bi $6s/6p$ (lone pair) orbitals to electronic states near the valence band maximum. The results indicate that the valence band maxima are largely determined by the local environment of the cations, which allows to estimate valence band maximum energies of oxides with multiple cations from those of their parent binary compounds. The high valence band maximum of BiFeO${}_3$ is consistent with reported $p$-type conduction of acceptor doped material, while the high conduction band minimum makes $n$-type conduction unlikely.

Topological insulators are characterized by specially protected conduction on their outer boundaries. We show that the protected edge conduction exhibited by 2D topological insulators (and also Chern insulators) is independent of nonmagnetic boundary disorder. In particular, the edge states residing inside the bulk gap remain conducting even when edge-state inhomogeneities destroy the characteristic linear Dirac relation between energy and momentum. The main effects of boundary disorder on the in-gap states are to decrease the Fermi velocity, increase the density of states, pull the states into the disordered region if spin is conserved, and at very large disorder shift the states to the boundary between the disordered edge and the clean bulk. These effects, which may be useful for device engineering, are controlled by a resonance between the disordered edge and the bulk bands. The resonance's energy is set by the bulk band width; protection of the in-gap edge states' plane-wave character is controlled by the bulk band width, not the bulk band gap.

We present a first-principles theoretical approach for evaluating the lattice thermal conductivity based on the exact solution of the Boltzmann transport equation. We use the variational principle and the conjugate gradient scheme, which provide us with an algorithm faster than the one previously used in literature and able to always converge to the exact solution [Omini and Sparavigna, Physica B: Condens. Matter 212, 101 (1995)]. Three-phonon normal and umklapp collisions, isotope scattering, and border effects are rigorously treated in the calculation. Good agreement with experimental data for diamond is found. Moreover we show that by growing more enriched diamond samples it is possible to achieve values of thermal conductivity up to three times larger than those commonly observed in isotopically enriched diamond samples with $99.93\%$ C${}^{12}$ and $0.07$ C${}^{13}$.

The magnetic ordering and its effect on the physical stability of $X_2$Mn${}_{1+x}$Sn${}_{1-x}$ ($0\le x\le 0.5$, and $X=\text{Ru}$, Os, Co, Rh, Ni, Pd, Cu, and Ag) Heusler alloys are investigated systematically by the use of first-principles method. It is found that the ferromagnetic (FM) coupling between Mn on Mn sublattice (Mn${}_1$) and Mn on Sn sublattice (Mn${}_2$) is favorable over the antiferromagnetic (AFM) coupling for $X$ with the number of valence electrons [$N_{\text{v}}\left(X\right)$] of 8 and 9, and vice versa for $X$ with $N_{\text{v}}\left(X\right)=10$ and 11, originated from the competition of the exchange interactions between $X$-Mn${}_2$ and Mn${}_1$-Mn${}_2$. In comparison with the FM Mn${}_1$-Mn${}_2$ coupling, the AFM coupling decreases significantly the shear elastic constant $C^{\prime }$ but increases slightly $C_{44}$, which results in increasing elastic anisotropy ($A=C_{44}/C^{\prime }$) and consequently may facilitate the tetragonal shear lattice deformation. The hybridization of the minority electronic states between $X$ $d$ and Sn $p$ plays a dominant role on the orientation of the magnetic coupling. The smaller change of the density of states in the Fermi level, induced by the lattice distortion for $C^{\prime }$, corresponds to the softer $C^{\prime }$ as well as the larger $A$ in the AFM state than the FM one.

Doping chalcogenide phase change materials was shown to improve the stability of the amorphous phase at high temperature and to strongly increase the crystallization temperature. In this work, we use $ab$ initio molecular dynamics together with Fourier transform infrared spectroscopy to address the stabilization of GeTe doped with nitrogen and carbon in the amorphous phase. The comparison between the simulation and experimental results allows in-depth understanding of the mechanisms. The inclusion of C and N leads to an increase in high frequency vibrational modes and to a lowering of the boson peak intensity. The reduction of the density of floppy vibrational modes and the computed increase of the mechanical rigidity are responsible for the higher activation energy for crystallization. The mechanism described here could apply more generally to stabilize other Ge-Sb-Te phase change materials and ionocovalent glasses at high temperature.

We present results for the ordering process of a two-dimensional Ising model with anisotropic frustrating next-nearest-neighbor interactions. We concentrate on a specific wide temperature and parameter region to confirm the existence of two particular phases of the model. The first phase is an incommensurate algebraically ordered (floating) phase emerging at the transition from the paramagnetic high-temperature phase. Then the model undergoes a transition to an antiferromagnetically ordered second phase with diagonal ferromagnetic stripes [ordering wave vector $\mathbf{q}=(\pi /2,\pi /2)$]. We analyze the unconventional features appearing in several observables, e.g., energy, structure factors, and correlation functions, by means of extensive Monte Carlo simulations and examine carefully the influence of the lattice sizes. For the analytical study of the intermediate phase the Villain-Bak theory is adapted for the present model. Combining both the numerical and analytical work we present the quantitative phase diagram of the model, and, in particular, argue in favor of an intermediate topological floating phase.

The temperature and time dependence of exchange bias for as-deposited Ni${}_{81}$Fe${}_{19}$-NiO films is studied. Using various cooling and measurement protocols, different contributions to the exchange anisotropy are separated. Depending on the cooling procedure, a strong increase or a controlled reversal of sign of exchange bias field is achieved at lower temperatures far away from the antiferromagnetic blocking temperature. The behavior is explained in terms of antiferromagnetic grain instability and the influence of the interfacial spin structure, the latter freezing at very low temperatures.

The first-order versus second-order nature of the phase transition in AuZn has been examined by first-principles calculations. The calculated elastic constants of the high-temperature $B$2 phase have a large anisotropy, which suggests a possible instability in this phase. The first-principles calculations were extended to finite temperature by including vibrational and electronic contributions to the free energy. A small free-energy barrier was found between the high- ($B2$) and low-temperature ($R$) phases, which indicates that this is a weak first-order phase transition. Finally, we find that the calculated theoretical transformation temperature and entropy change (small latent heat) are in excellent agreement with the experimental observations for a first-order transition. Based on the entropy calculations for both phases, the high-temperature phase is found to be stabilized by the contribution of low-energy phonon modes to the lattice entropy.

We describe and demonstrate an architecture for active acoustic metamaterials whose effective material parameters can be tuned independently over a wide range of values, including negative material parameters. The approach is demonstrated experimentally through the design and measurement of two types of unit cells that generate metamaterials in which either the effective density or bulk modulus can be tuned. We show that these unit cells achieve negative refraction, negative effective mass density, significantly nonunity bulk modulus, and are suitable for the design of negative refraction media and media with tunable gain and absorption. In this architecture, a transducer senses the pressure wave incident on the metamaterial and an electronic circuit manipulates the electric signal produced and drives a second transducer that creates the acoustic response consistent with the desired effective material parameters. The electronics between the two transducers allows easy dynamic tuning of these parameters. We show that this approach can be applied to both broadband and narrow band designs.

Competing ferromagnetic metallic (FMM) and insulating phases in the manganite (La${}_{1-y}$Pr${}_y$)${}_{1-x}$Ca${}_x$MnO${}_3$ lead to a phase separated state in which micrometer scale FMM regions behave in a fluidlike manner over a narrow temperature range. Here we show that an electric field can realign the fluidlike FMM phases embedded in an insulating matrix, resulting in anisotropic in-plane resistance in microstructures of (La${}_{0.4}$Pr${}_{0.6}$)${}_{0.67}$Ca${}_{0.33}$MnO${}_3$ thin films. Time and voltage dependent resistance and magnetization measurements show that the dynamic percolation of the FMM regions leads to an insulator to metal transition due to electric-field induced realignment of the FMM regions, which is analogous to the dielectrophoresis of metallic particles suspended in fluid media. In-plane strain anisotropy plays an important role in determining the speed of dynamic percolation of the FMM regions by modifying the local electric fields in the phase separated state.

We analytically derive a rigorous expression for the relative impedance ratio between two photonic structures based on their electromagnetic interaction. Our approach generalizes the physical meaning of the impedance to a measure for the reciprocity-based overlap of eigenmodes. The consistency with known cases in the radio-frequency and optical domain is shown. The analysis reveals where the applicability of simple circuit parameters ends and how the impedance can be interpreted beyond this point. We illustrate our approach by successfully describing a Bragg reflector that terminates an insulator-metal-insulator plasmonic waveguide in the near infrared by our impedance concept.

We introduce a toric code model on the dice lattice which is exactly solvable and displays topological order at zero temperature. In the presence of a magnetic field, the flux dynamics is mapped to the highly frustrated transverse field Ising model on the kagome lattice. This correspondence suggests an intriguing disorder by disorder phenomenon in a topologically ordered system implying that the topological order is extremely robust due to the geometric frustration. Furthermore, a connection between fully frustrated transverse field Ising models and topologically ordered systems is demonstrated which opens an exciting physical playground due to the interplay of topological quantum order and geometric frustration.