Molecular beam epitaxy is used to grow ${\mathrm{TiSe}}_2$ ultrathin films on a graphitized SiC(0001) substrate. ${\mathrm{TiSe}}_2$ films proceed via a nearly layer-by-layer growth mode and exhibit two dominant types of defects, identified as Se vacancy and interstitial, respectively. By means of scanning tunneling microscopy, we demonstrate that the well-established charge density waves can survive in a single unit-cell (one triple-layer) regime, and find a gradual reduction in their correlation length as the density of surface defects in ${\mathrm{TiSe}}_2$ ultrathin films increases. Our findings offer important insights into the nature of charge density waves in ${\mathrm{TiSe}}_2$, and also pave a material foundation for potential applications based on the collective electronic states.

We propose a microscopic description of the bond-disproportionated insulating state in the bismuth perovskites $X{\mathrm{BiO}}_3$ ($X=\text{Ba},\text{Sr}$) that recognizes the bismuth-oxygen hybridization as a dominant energy scale. It is demonstrated by using electronic structure methods that the breathing distortion is accompanied by spatial condensation of hole pairs into local, molecularlike orbitals of the $A_{1g}$ symmetry composed of O-$2p_{\sigma }$ and Bi-$6s$ atomic orbitals of collapsed ${\mathrm{BiO}}_6$ octahedra. The primary importance of oxygen $p$ states is thus revealed, in contrast to a popular picture of a purely ionic ${\mathrm{Bi}}^{3+}/{\mathrm{Bi}}^{5+}$ charge disproportionation. Octahedra tilting is shown to enhance the breathing instability by means of a nonuniform band narrowing. We argue that the formation of localized states upon breathing distortion is, to a large extent, a property of the oxygen sublattice, and we expect similar hybridization effects in other perovskites involving formally high oxidation state cations.

We show that the two recently proposed methods to compute Renyi entanglement entropies in the realm of determinant quantum Monte Carlo methods for fermions are in principle equivalent, but differ in sampling strategies. The analogy allows us to formulate a numerically stable calculation of the entanglement spectrum at strong coupling. We demonstrate the approach by studying static and dynamical properties of the entanglement Hamiltonian across the interaction driven quantum phase transition between a topological insulator and quantum antiferromagnet in the Kane-Mele Hubbard model. The formulation is not limited to fermion systems and can readily be adapted to world-line-based simulations of bosonic systems.

Motivated by the Pati-Salam grand unified theory [J. C. Pati and A. Salam, Phys. Rev. D 10, 275 (1974)], we study $(4+1)d$ topological insulators with $\mathrm{SU}\left(4\right)\times \mathrm{SU}{\left(2\right)}_1\times \mathrm{SU}{\left(2\right)}_2$ symmetry, whose $(3+1)d$ boundary has 16 flavors of left-chiral fermions, which form representations $(\mathbf{4},\mathbf{2},\mathbf{1})$ and $(\overline{\mathbf{4}},\mathbf{1},\mathbf{2})$. The key result we obtain is that, without any interaction, this topological insulator has a $\mathbb{Z}$ classification, namely, any quadratic fermion mass operator at the $(3+1)d$ boundary is prohibited by the symmetries listed above; while under interaction, this system becomes trivial, namely, its $(3+1)d$ boundary can be gapped out by a properly designed short-range interaction without generating nonzero vacuum expectation value of any fermion bilinear mass, or in other words, its $(3+1)d$ boundary can be driven into a “strongly-coupled symmetric gapped (SCSG) phase.” Based on this observation, we propose that after coupling the system to a dynamical $\mathrm{SU}\left(4\right)\times \mathrm{SU}{\left(2\right)}_1\times \mathrm{SU}{\left(2\right)}_2$ lattice gauge field, the Pati-Salam GUT can be fully regularized as the boundary states of a $(4+1)d$ topological insulator with a thin fourth spatial dimension, the thin fourth dimension makes the entire system generically a $(3+1)d$ system. The mirror sector on the opposite boundary will not interfere with the desired GUT, because the mirror sector is driven to the SCSG phase by a carefully designed interaction and is hence decoupled from the GUT.

Utilizing the local-density approximation (LDA) + Gutzwiller method, we have studied the $\alpha \text{−}\gamma$ transition in cerium. Our results indicate that the volume collapse transition between $\alpha$ and $\gamma$ phases is present at zero temperature with negative pressure. By further providing a newly finite temperature generalization of the LDA + Gutzwiller method (using the mean-field potential approach), the entropy contributed by both electronic quasiparticles and lattice vibration included, we obtain the Gibbs free energy at a given volume and temperature, from which we get the $\alpha \text{−}\gamma$ transition at finite temperature and pressure. Our results indicate that the electronic entropy and lattice vibrational entropy both play important roles in the $\alpha \text{−}\gamma$ transition. We also calculated the equation of state and phase diagram of cerium, finding good agreement with the experiments.

The string-net approach by Levin and Wen, and the local unitary transformation approach by Chen, Gu, and Wen, provide ways to classify topological orders with gappable edge in two-dimensional (2D) bosonic systems. The two approaches reveal that the mathematical framework for ($2+1)$-dimensional $\left(2+1\right)\mathrm{D}$ bosonic topological order with gappable edge is closely related to unitary fusion category theory. In this paper, we generalize these systematic descriptions of topological orders to 2D fermion systems. We find a classification of $(2+1)\mathrm{D}$ fermionic topological orders with gappable edge in terms of the following set of data $(N_k^{ij},F_k^{ij},F_{jkn,\chi \delta }^{ijm,\alpha \beta },d_i)$, which satisfy a set of nonlinear algebraic equations. The exactly soluble Hamiltonians can be constructed from the above data on any lattices to realize the corresponding topological orders. When $F_k^{ij}=0$, our result recovers the previous classification of $2+1\mathrm{D}$ bosonic topological orders with gappable edge.

Using molecular dynamics simulation, we study the impact of 3 keV Xe ions at glancing incidence on a $\beta$-SiC (111) surface covered by graphene. On top of a full graphene layer covering the substrate, we add a graphene half-layer; the step forming where the half-layer terminates allows the entrance of glancing-incidence ions into a subsurface channel between graphene layers. We find a high channeling probability which leads to only little sputtering and damage formation. Typically, vacancy defects are formed at periodic intervals when the ion hits the uppermost graphene layer from below. Extended damage occurs when the ion hits the step edge itself. There we find several kinds of defects varying from adatoms over the formation of $s{p}^1$-bonded chains to hillocks.

Scanning tunneling microscopy experiments have been performed to measure the local electron injection in nitrogen-doped graphene on $\text{SiC}\left(000\overline{1}\right)$ and were successfully compared to ab initio calculations. In graphene, a gaplike feature is measured around the Fermi level due to a phonon-mediated tunneling channel. At nitrogen sites, this feature vanishes due to an increase of the elastic channel that is allowed because of symmetry breaking induced by the nitrogen atoms. A large conductance enhancement by a factor of up to 500 was measured at the Fermi level by comparing local spectroscopy at nitrogen sites and at carbon sites. Nitrogen doping can therefore be proposed as a way to improve tunnel-electron injection in graphene.

We demonstrate the photogeneration of loaded dipole plasmonic antennas resonating at THz frequencies. This is achieved by the patterned optical illumination of a semiconductor surface using a spatial light modulator. Our experimental results indicate the existence of capacitive and inductive coupling of localized surface plasmon polaritons. By varying the load in the antenna gap we are able to switch between both coupling regimes. Furthermore, we determine experimentally the effective impedance of the antenna load and verify that this load can be effectively expressed as a LC resonance formed by a THz inductor and capacitor connected in a parallel circuit configuration. These findings are theoretically supported by full electrodynamic calculations and by simple concepts of lumped circuit theory. Our results open new possibilities for the design of active THz circuits for optoelectronic devices.

We present systematic theoretical investigations to explore the microscopic mechanisms leading to the formation of antiferromagnetism in $\mathrm{R}{\mathrm{u}}_2\mathrm{Mn}Z(Z=\mathrm{Sn},\mathrm{Sb},\mathrm{Ge},\mathrm{Si})$ full Heusler alloys. Our study is based on first-principles calculations of interatomic Mn-Mn exchange interactions to set up a suitable Heisenberg spin model and on subsequent Monte Carlo simulations of the magnetic properties at finite temperature. The exchange interactions are derived from the paramagnetic state, while a realistic account of long-range chemical disorder is made in the framework of the coherent potential approximation. We find that in the case of the highly ordered alloys ($Z=\mathrm{Sn}$ and Sb), the exchange interactions derived from the perfectly ordered $L{2}_1$ structure lead to Néel temperatures in excellent agreement with the experiments, whereas, in particular in the case of Si, the consideration of chemical disorder is essential to reproduce the experimental Néel temperatures. Our numerical results suggest that by improving a heat treatment of the samples to suppress the intermixing between the Mn and Si atoms, the Néel temperature of the Si-based alloys can potentially be increased by more than 30%. Based on calculated biquadratic exchange couplings, we evidence a lifting of degeneracy of the antiferromagnetic ground states on a frustrated face-centered-cubic lattice in the fully ordered compounds. Furthermore, we show that in strongly disordered $\mathrm{R}{\mathrm{u}}_2\mathrm{MnSi}$ alloys, a distinct change in the antiferromagnetic ordering occurs.

The initial stages of interface formation for a real-world ferromagnet/antiferromagnet bi-layer system (iron/manganese nitride) are investigated down to the atomic scale using a combination of molecular beam epitaxy, in situ scanning tunneling microscopy, and first-principles theoretical calculations. Submonolayer deposition of iron onto manganese nitride nanopyramid surfaces results in an unexpected yet well-ordered structural and magnetic arrangement. It is shown that although the island structures seen in scanning tunneling microscopy images are of single monolayer height, their chemical composition, based on Auger electron spectroscopy, conductance map imaging, and theoretical models, does not consist of iron. It is found theoretically that models that consider iron on the surface of manganese nitride are highly unfavorable. Instead, models with iron atoms incorporated into specific subsurface layers are most stable, in excellent agreement with Auger spectroscopy measurements. Calculations also reveal the magnetic alignment of iron with the manganese nitride layers.

Magnetic and dielectric properties of the hexagonal triangular lattice antiferromagnet $2H\text{−}{\mathrm{AgFeO}}_2$ have been studied by neutron diffraction, magnetic susceptibility, specific heat, pyroelectric current, and dielectric constant measurements. The ferroelectric polarization, $P\simeq 5\mu \mathrm{C}/{\mathrm{m}}^2$, has been found to appear below 11 K due to a polar nature of the magnetic ground state of the system. In the temperature range of $11 \mathrm{K}\le T\le 18 \mathrm{K}$, an incommensurate spin density wave (ICM1) with the nonpolar magnetic point group $mmm{1}^{\prime }$ and the ${\mathit{k}}_1=(0,q_b^1,0;q_b^1=0.390\text{–}0.405)$ propagation vector takes place. Below 14 K, a proper screw ordering (ICM2) and ${\mathit{k}}_2=(0,q_b^2,0;q_b^2=0.385\text{–}0.396)$ appears as a minor phase which coexists with ICM1 and the ground state down to the lowest measured temperature 5.5 K. No ferroelectric polarization associated with the ICM2 phase was observed in agreement with its nonpolar point group ${2221}^{\prime }$. Finally, a spiral order with cycloid and proper screw components (ICM3), and ${\mathit{k}}_3=(q_a^3,q_b^3,0;q_a^3=0.0467,q_b^3=0.349)$ emerges below 11 K as the ground state of the system. Based on the deduced magnetic point group ${21}^{\prime }$, we conclude that the ferroelectric polarization in ICM3 is parallel to the $c$ axis and is caused by the inverse Dzyloshinskii-Moriya effect with ${\mathit{p}}_1\propto {\mathit{r}}_{ij}\times ({\mathit{S}}_i\times {\mathit{S}}_j)$. Unlike the rhombohedral $3R\text{−}{\mathrm{AgFeO}}_2$ polytype, the additional contribution to the macroscopic polarization ${\mathit{p}}_2\propto {\mathit{S}}_i\times {\mathit{S}}_j$ is not allowed in the present case due to the symmetry constraints imposed by the hexagonal lattice of $2H\text{−}{\mathrm{AgFeO}}_2$.

Based on the Bardeen-Cooper-Schrieffer theory of superconductivity, the coherent splitting of Cooper pairs from a superconductor to two spatially separated quantum dots has been predicted to generate nonlocal pairs of entangled electrons. In order to test this hypothesis, we propose a scheme to transfer the spin state of a split Cooper pair onto the polarization state of a pair of optical photons. We show that the photon pairs produced can be used to violate a Bell inequality, unambiguously demonstrating the entanglement of the split Cooper pairs.

We discuss different ways of generating entanglement in the original picture of circuit QED (XcQED) and several restrictions that arise in the context of a large-scale quantum architecture. To alleviate some of the issues posed by the presence of the nonlinearities inherent to these systems, we introduce a layout for circuit QED, wherein an artificial atom is coupled to a quantized radiation field via its longitudinal degree of freedom (ZcQED). This system is akin to ion traps used in atomic physics, but it relies on fixed coupling between the atom and the resonator. We describe a scalable architecture for processing quantum information with superconducting qubits, which is free from any type of residual interaction between the atomic and photonic degrees of freedom. Tunable interactions can be realized based on sideband transitions, and the system can be operated out of the Lamb-Dicke regime, allowing it to benefit from the possibility of achieving large coupling strengths between atoms and resonators. We also discuss a readout scheme that does not require any extra circuits and allows a qubit-specific measurement of the state of the quantum register inspired by the electron shelving technique. This scheme is quantum nondemolition (QND)-like, and allows for single-shot determination of the qubit states.

We use quasiclassical methods of superconductivity to study the superconducting proximity effect from a topological $p$-wave superconductor into a disordered quasi-one-dimensional metallic wire. We demonstrate that the corresponding Eilenberger equations with disorder reduce to a closed nonlinear equation for the superconducting component of the matrix Green's function. Remarkably, this equation is formally equivalent to a classical mechanical system (i.e., Newton's equations), with the Green function corresponding to a coordinate of a fictitious particle and the coordinate along the wire corresponding to time. This mapping allows us to obtain exact solutions in the disordered nanowire in terms of elliptic functions. A surprising result that comes out of this solution is that the $p$-wave superconductivity proximity induced into the disordered metal remains long range, decaying as slowly as the conventional $s$-wave superconductivity. It is also shown that impurity scattering leads to the appearance of a zero-energy peak.