We introduce a many-body method that combines two powerful many-body techniques, viz., quantum Monte Carlo and coupled cluster theory. Coupled cluster wave functions are introduced as importance functions in a Monte Carlo method designed for the configuration interaction framework to provide rigorous upper bounds to the ground-state energy. We benchmark our method on the homogeneous electron gas in momentum space. The importance function used is the coupled cluster doubles wave function. We show that the computational resources required in our method scale polynomially with system size. Our energy upper bounds are in very good agreement with previous calculations of similar accuracy, and they can be systematically improved by including higher order excitations in the coupled cluster wave function.

Recently, the Grassmann-tensor-entanglement renormalization group (GTERG) algorithm has been proposed as a generic variational approach to study strongly correlated boson/fermion systems [Gu et al., arXiv:1004.2563]. However, the weakness of such a simple variational approach is that generic Grassmann tensor product states (GTPS) with large inner dimension $D$ will contain a large number of variational parameters which are hard to be determined through usual minimization procedures. In this paper, we first introduce a standard form of GTPS which significantly simplifies the representations. Then we describe a simple imaginary-time-evolution algorithm to efficiently update the GTPS based on the fermion coherent state representation and show that all the algorithms developed for usual tensor product states (TPS) can be implemented for GTPS in a similar way. Finally, we study the environment effect for the GTERG approach and propose a simple method to further improve its accuracy. We demonstrate our algorithms by studying some simple two-dimensional free and interacting fermion systems on honeycomb lattice, including both off-critical and critical cases.

The atomic structure of GaP(111)/Si(111), GaP(110)/Si(110), and GaP(113)/Si(113) heterointerfaces was studied by ab initio calculations employing the density functional theory (DFT). Relative formation energies were computed for the interface layers allowing for atomic intermixing. The application of the electron-counting model, a construction principle used for surface reconstructions, to the case of the GaP(111)/Si(111) interfaces leads to electronic compensation at the heterovalent interfaces and to a reduction of the interface formation energy. The specific equilibrium (111) interface reconstruction can be tuned by changing the chemical potential. In particular, the GaP(111)A/Si(111) interface was found to be abrupt and uncompensated under P-rich conditions, whereas it is compensated under Ga-rich conditions. The GaP(111)B/Si(111) interface was found to be compensated. Contrary to the (111) interfaces, stoichiometric abrupt interfaces were found to be the most energetically favorable for the GaP(110)/Si(110) and the GaP(113)/Si(113) interfaces. These interfaces do not reconstruct. Although both interfaces are compensated, the GaP(113)/Si(113) superlattice exhibits a polarization field, in contrast to the (110) superlattice.

We report on the identification and nanometer scale characterization over a large energy range of random, disorder-driven, surface plasmons in silver semicontinuous films embedded in silicon nitride. By performing spatially resolved electron energy loss spectroscopy experiments, we experimentally demonstrate that these plasmons eigenmodes arise when the films become fractal, leading to the emergence of strong electrical fields (“hot spots”) localized over few nanometers. We show that disorder-driven surface plasmons strongly depart from those usually found in nanoparticles, being strongly confined and randomly and densely distributed in space and energy. Beyond that, we show that they have no obvious relation with the local morphology of the films, in stark contrast with surface plasmon eigenmodes of nanoparticles.

The electronic properties of a triaxially strained hexagonal graphene flake with either armchair or zigzag edges are investigated using molecular dynamics simulations and tight-binding calculations. We found that (i) the pseudomagnetic field in strained graphene flakes is not uniform neither in the center nor at the edge of zigzag terminated flakes, (ii) the pseudomagnetic field is almost zero in the center of armchair terminated flakes but increases dramatically near the edges, (iii) the pseudomagnetic field increases linearly with strain, for strains lower than 15$\%$ but increases nonlinearly beyond it, (iv) the local density of states in the center of the zigzag hexagon exhibits pseudo-Landau levels with broken sublattice symmetry in the zeroth pseudo-Landau level, and in addition there is a shift in the Dirac cone due to strain induced scalar potentials, and (v) there is size effect in pseudomagnetic field. This study provides a realistic model of the electronic properties of inhomogeneously strained graphene where the relaxation of the atomic positions is correctly included together with strain induced modifications of the hopping terms up to next-nearest neighbors.

We have performed a systematic representation-theory-based search for the simplest structures allowing isotropic magnetoelectric coupling. We find 30 such structures, all sharing a common pattern of atomic displacements in the direction of atomic magnetic moments. We focus on one of these 30 canonical structures and find that it is generically realized in a class of fractionally substituted pyrochlore compounds with an all-in-all-out magnetic order. Furthermore, we find that these substituted pyrochlore compounds have a substantial Chern-Simons orbital magnetoelectric component ($\theta =0.1$–$0.2$). While this component is also formally present in strong $Z_2$ topological insulators ($\theta =\pi$), its effects are observable there only if time-reversal symmetry is broken at the surface.

A twisted graphene bilayer exhibits a triangular Moiré pattern in the local stacking, that smoothly alternates between the three basic types $A{A}^{\prime }$, $A{B}^{\prime }$, and $B{A}^{\prime }$. Under an interlayer bias $U$, the latter two types develop a spectral gap, characterizsed by opposite valley Chern numbers. We show that for large enough Moiré periods and bias, these regions become depleted electronically, and topologically protected helical modes appear at their boundaries. This gives rise to a delocalized topological network of the Chalker-Coddington type, composed of valley current vortices. This network can be tailored by controlled deposition of valley-mixing adsorbates, which block transmission in selected links, thus opening the possibility of custom topological nanoelectronics.

We have found evidence for a strong magnetoelectric (ME) coupling at room temperature between polycrystalline Co layers ($5-40$ nm) and single-crystalline (001)-oriented BaTiO${}_3$ layers ($15-17$ nm). We took advantage of the quasi-single polarization orientation, perpendicular to the film plane, of the ferroelectric BaTiO${}_3$ domains in the tetragonal phase. Using ferromagnetic resonance spectroscopy with the Co magnetization aligned either parallel or antiparallel to the BaTiO${}_3$ polarization, we assessed a strong anisotropy of about 0.14 T in the Co resonance field positions, indicating a coupling constant of 0.27 s/F. When sweeping the temperature through the phase transitions of BaTiO${}_3$, the two resonance positions are shifted in opposite directions. The ME coupling induces a notable magnetic anisotropy resulting in high values of the out-of-plane remanent magnetization. Our results are promising for future multiferroic devices.

We show that the surface plasmon polariton (SPP) radiation patterns of point-dipole emitters in the vicinity of a metal-dielectric interface are generally asymmetric with respect to the location of the emitter. In particular rotating dipoles, which emit elliptically polarized light, produce highly asymmetric SPP radiation fields that include unidirectional emission. Asymmetric SPP radiation patterns also result when a dipole oscillates tilted with respect to the plane of the interface and optical losses or gains are present in the materials. These effects can be used to directionally control SPP emission and absorption, as well as to study emission and scattering processes close to metal-dielectric interfaces. Possible implementations of asymmetrically emitting SPP sources are discussed.

Reflection and refraction of electromagnetic waves by artificial periodic composites (metamaterials) can be accurately modeled by an effective medium theory only if the boundary of the medium is explicitly taken into account and the two effective parameters of the medium (the index of refraction and the impedance) are correctly determined. Theories that consider infinite periodic composites do not satisfy the above condition. As a result, they can not model reflection and transmission by finite samples with the desired accuracy and are not useful for design of metamaterial-based devices. As an instructive case in point, we consider the “current-driven” homogenization theory, which has recently gained popularity. We apply this theory to the case of one-dimensional periodic medium wherein both exact and homogenization results can be obtained analytically in closed form. We show that, beyond the well-understood zero-cell limit, the current-driven homogenization result is inconsistent with the exact reflection and transmission characteristics of the slab.

We show how symmetry properties can be used to greatly increase the accuracy and efficiency in auxiliary-field quantum Monte Carlo (AFQMC) calculations of electronic systems. With the Hubbard model as an example, we study symmetry preservation in two aspects of ground-state AFQMC calculations, the Hubbard-Stratonovich transformation and the form of the trial wave function. It is shown that significant improvement over state-of-the-art calculations can be achieved. In unconstrained calculations, the implementation of symmetry often leads to shorter convergence time and much smaller statistical errors and thereby a substantial reduction of the sign problem. Moreover, certain excited states become possible to calculate which are otherwise beyond reach. In calculations with constraints, the use of symmetry can reduce the systematic error from the constraint. It also allows release-constraint calculations, leading to essentially exact results in many cases. Detailed comparisons are made with exact diagonalization results. Accurate ground-state energies are then presented for larger system sizes in the two-dimensional repulsive Hubbard model.

We report on the occurrence and properties of photonic surface states in periodic structures of magnetized plasma nanospheres by means of rigorous calculations using the full-electrodynamic layer-multiple-scattering method, properly extended to treat gyrotropic spheres with arbitrarily oriented gyration vector. More specifically, dispersion diagrams of Tamm states at the (001) surface of a semi-infinite fcc crystal of plasma nanospheres and of guided modes of a square array of such spheres supported by a quartz substrate, without and under the action of an in-plane static uniform magnetic field, are analyzed and nonreciprocal optical response, which emerges as a result of the simultaneous lack of space-inversion and time-reversal symmetries, is demonstrated in the Voigt geometry.

Motivated by a recent experiment, we develop a theory of organic magnetoresistance (OMAR) in the presence of a resonant ac drive. To this end, we perform a thorough analysis of the dynamics of ac-driven electron-hole polaron pair in magnetic field, which is a sum of external and random hyperfine fields. Resonant ac drive affects the OMAR by modifying the singlet content of the eigenmodes. This, in turn, leads to the change of recombination rate, and ultimately, to the change of the spin-blocking that controls the current. Our analysis demonstrates that, upon increasing the drive amplitude, the blocking eigenmodes of the triplet type acquire a singlet admixture and become unblocking. Most surprisingly, the opposite process goes in parallel: new blocking modes emerge from nonblocking precursors as the drive increases. These emergent blocking modes are similar to subradiant modes in the Dicke effect. A nontrivial evolution of eigenmodes translates into a nontrivial behavior of OMAR with the amplitude of the ac drive: it is initially linear, then passes through a maximum, drops, and finally saturates.

Based on the first-principles calculations, we recover the silent topological nature of Cd${}_3$As${}_2$, a well known semiconductor with high carrier mobility. We find that it is a symmetry-protected topological semimetal with a single pair of three-dimensional (3D) Dirac points in the bulk and nontrivial Fermi arcs on the surfaces. It can be driven into a topological insulator and a Weyl semimetal state by symmetry breaking, or into a quantum spin Hall insulator with a gap more than 100 meV by reducing dimensionality. We propose that the 3D Dirac cones in the bulk of Cd${}_3$As${}_2$ can support sizable linear quantum magnetoresistance even up to room temperature.

For several years the electronic structure properties of the two-dimensional system silicene have been studied extensively. Electron transport across metal-silicene junctions, however, remains relatively unexplored. To address this issue, we developed and implemented a theoretical framework that utilizes the tight-binding Fisher-Lee relation to span nonequilibrium Green's function (NEGF) techniques, the scattering method, and semiclassical Boltzmann transport theory. Within this hybrid quantum-classical, two-scale framework, we calculated transmission and reflection coefficients of monolayer and bilayer Ag-silicene-Ag junctions using the NEGF method in conjunction with density functional theory; derived and calculated the group velocities; and computed resistance using the semiclassical Boltzmann equation. We found that resistances of these junctions are $\sim 0.08\mathrm{f}\Omega {\mathrm{m}}^2$ for monolayer silicene junctions and $\sim 0.3\mathrm{f}\Omega {\mathrm{m}}^2$ for bilayer ones; factors of $\sim$8 and $\sim$2, respectively, smaller than Sharvin resistances estimated via the Landauer formalism.

We consider the spin-1/2 antiferromagnetic Heisenberg model on three frustrated lattices (the diamond chain, the dimer-plaquette chain, and the two-dimensional square-kagome lattice) with almost dispersionless lowest magnon band. Eliminating high-energy degrees of freedom at high magnetic fields, we construct low-energy effective Hamiltonians, which are much simpler than the initial ones. These effective Hamiltonians allow a more extended analytical and numerical analysis. In addition to the standard strong-coupling perturbation theory, we also use a localized-magnon-based approach leading to a substantial improvement of the strong-coupling approximation. We perform extensive exact diagonalization calculations to check the quality of different effective Hamiltonians by comparison with the initial models. Based on the effective-model description, we examine the low-temperature properties of the considered frustrated quantum Heisenberg antiferromagnets in the high-field regime. We also apply our approach to explore thermodynamic properties for a generalized diamond spin chain model suitable to describe azurite at high magnetic fields. Interesting features of these highly frustrated spin models consist in a steep increase of the entropy at very small temperatures and a characteristic extra low-temperature peak in the specific heat. The most prominent effect is the existence of a magnetic-field-driven Berezinskii-Kosterlitz-Thouless phase transition occurring in the two-dimensional model.

We report the results of a systematic investigation of the phase diagram of the iron-based superconductor system, Ba${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$, from $x=0.1$ to $x=1.0$ using high-resolution neutron and x-ray diffraction and magnetization measurements. We find that the coincident structural and magnetic phase transition to an orthorhombic structure with space group $Fmmm$ and a striped antiferromagnet with space group $F_{C}m{m}^{\prime }{m}^{\prime }$ in Ba${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$ is of first order. A complete suppression of the magnetic phase is observed by $x$ $=$ 30$\%$, and bulk superconductivity occurs at a critical concentration near 15$\%$. We compare our findings to the previously reported results of the hole-doped Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ solid solution in order to resolve the differing effects of band filling and $A$-site cation size on the properties of the magnetic and superconducting ground states. The substantial size difference between Na and K causes various changes in the lattice trends, yet the overarching property phase diagram from the Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ phase diagram carries over to the Ba${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$ solid solution. We note that the composition dependence of the $c$ axis turns over from positive to negative around $x$ $=$ 0.35, unlike the K-substituted materials. We show that this can be understood by invoking steric effects; primarily the Fe${}_2$As${}_2$ layer shape is dictated mostly by the electronic filling, which secondarily induces an interlayer spacing adjusted to compensate for the given cation volume. This exemplifies the primacy of even subtle features in the Fe${}_2$As${}_2$ layer in controlling both the structure and properties in the uncollapsed 122 phases.

Spin transfer torques (STTs) occur when electric currents travel through inhomogeneously magnetized systems and are important for the motion of magnetic textures such as domain walls. Since superconductors are easy-plane ferromagnets in particle-hole (charge) space, it is natural to ask whether any charge duals of STT phenomena exist therein. We find that the superconducting analog of the adiabatic STT vanishes in a bulk superconductor with a momentum-independent order parameter, while the superconducting counterpart of the nonadiabatic STT does not vanish. This nonvanishing superconducting torque is induced by heat (rather than charge) currents and acts on the charge (rather than spin) degree of freedom. It can become significant in the vicinity of the superconducting transition temperature, where it generates a net quasiparticle charge and alters the dispersion and linewidth of low-frequency collective modes.

We present a tradeoff between anharmonicity (related to leakage) and pure dephasing errors for the fluxonium circuit. We show that in the insulating regime, i.e., when the persistent current flowing across the circuit is suppressed, the pure dephasing rate induced by a Markovian environment decreases exponentially as the impedance of the circuit is increased. In contrast to this exponential decrease, the qubit remains sufficiently anharmonic so that gate times can still be short, allowing for significant reduction in the computational error rates. A transition from the insulating to the superconducting phases establishes an upper bound on the Josephson energy below which this tradeoff exists.