We present a multireference configuration mixing scheme for describing ground and excited states, with well-defined spin and space-group symmetry quantum numbers, of the one-dimensional Hubbard model with nearest-neighbor hopping and periodic boundary conditions. Within this scheme, each state is expanded in terms of nonorthogonal and variationally determined symmetry-projected configurations. The results for lattices up to 30 and 50 sites compare well with the exact Lieb-Wu solutions as well as with results from other state-of-the-art approximations. In addition to spin-spin correlation functions in real space and magnetic structure factors, we present results for spectral functions and density of states computed with an Ansatz whose quality can be well controlled by the number of symmetry-projected configurations used to approximate the systems with $N_e$ and $N_e\pm 1$ electrons. The intrinsic symmetry-broken determinants resulting from the variational calculations have rich structures in terms of defects that can be regarded as basic units of quantum fluctuations. Given the quality of the results here reported, as well as the parallelization properties of the considered scheme, we believe that symmetry-projection techniques, which have found ample applications in nuclear structure physics, deserve further attention in the study of low-dimensional correlated many-electron systems.

A key insight from recent studies is that noise, such as dephasing, can improve the efficiency of quantum transport by suppressing coherent single-particle interference effects. However, it is not yet clear whether dephasing can enhance transport in an interacting many-body system. Here, we address this question by analyzing the transport properties of a boundary driven spinless fermion chain with nearest-neighbor interactions subject to bulk dephasing. The many-body nonequilibrium stationary state is determined using large-scale matrix product simulations of the corresponding quantum master equation. We find dephasing enhanced transport only in the strongly interacting regime, where it is shown to induce incoherent transitions bridging the gap between bound dark states and bands of mobile eigenstates. The generic nature of the transport enhancement is illustrated by a simple toy model, which contains the basic elements required for its emergence. Surprisingly, the effect is significant even in the linear response regime of the full system, and it is predicted to exist for any large and finite chain. The response of the system to dephasing also establishes a signature of an underlying nonequilibrium phase transition between regimes of transport degradation and enhancement. The existence of this transition is shown not to depend on the integrability of the model considered. As a result, dephasing enhanced transport is expected to persist in more realistic nonequilibrium strongly correlated systems.

Three dimensional (3D) topological insulators display an array of unique properties such as single Dirac-cone surface states and a strong magnetoelectric effect. Here we show how a 3D topological spectrum can be induced in a trivial insulator by a periodic drive and, in particular, using electromagnetic radiation. In contrast to the two-dimensional analog, we show that a two-photon resonance is required to transform an initially unremarkable band structure into a topological Floquet spectrum. We provide an intuitive, geometrical picture, alongside a numerical solution of a driven lattice model featuring a single surface Dirac mode. Also, we show that the polarization and frequency of the driving electromagnetic field control the details of the surface modes and particularly the Dirac mass. Specific experimental realizations of the 3D Floquet topological insulator are proposed.

We present a plane-wave implementation of the G${}_0$W${}_0$ approximation within the projector augmented wave method code gpaw. The computed band gaps of ten bulk semiconductors and insulators deviate on average by $0.2\mathrm{eV}$ ($\sim 5\%$) from the experimental values, the only exception being ZnO where the calculated band gap is around $1\mathrm{eV}$ too low. Similar relative deviations are found for the ionization potentials of a test set of 32 small molecules. The importance of substrate screening for a correct description of quasiparticle energies and Fermi velocities in supported two-dimensional (2D) materials is illustrated by the case of graphene/$h$-BN interfaces. Due to the long-range Coulomb interaction between periodically repeated images, the use of a truncated interaction is found to be essential for obtaining converged results for 2D materials. For all systems studied, a plasmon-pole approximation is found to reproduce the full frequency results to within $0.2\mathrm{eV}$ with a significant gain in computational speed. Throughout, we compare the G${}_0$W${}_0$ results with different exact exchange-based approximations. For completeness, we provide a mathematically rigorous and physically transparent introduction to the notion of quasiparticle states.

We have performed numerical studies of the Hubbard-Holstein model in two dimensions using determinant quantum Monte Carlo (DQMC). Here, we present details of the method, emphasizing the treatment of the lattice degrees of freedom, and then study the filling and behavior of the fermion sign as a function of model parameters. We find a region of parameter space with large Holstein coupling where the fermion sign recovers despite large values of the Hubbard interaction. This indicates that studies of correlated polarons at finite carrier concentrations are likely accessible to DQMC simulations. We then restrict ourselves to the half-filled model and examine the evolution of the antiferromagnetic structure factor, other metrics for antiferromagnetic and charge-density-wave order, and energetics of the electronic and lattice degrees of freedom as a function of electron-phonon coupling. From this we find further evidence for a competition between charge-density-wave and antiferromagnetic orders at half-filling.

We present first-principles calculations of formation energies of various charge states of V${}_{\mathrm{C}}$, V${}_{\mathrm{Si}}$, and V${}_{\mathrm{C}}$+C${}_{\mathrm{Si}}$ defects in $3C$ and $6H$ silicon carbide to predict if they can be detected by positron annihilation spectroscopy (PAS). We use the two-component density functional theory to compute positron lifetimes in both polytypes, taking into account atomic relaxation due to both the defect and the positron. Three different calculation schemes are used. We find longer lifetimes than the ones obtained in previous theoretical studies in which relaxation was not taken into account or was only estimated. For the neutral carbon vacancy, we find lifetimes from 173 ps in $6H$-SiC to 195 ps in $3C$-SiC, while for the silicon vacancy, we obtain lifetimes from 222 to 227 ps in both polytypes, depending on the charge state. Based on these results, we propose that some experimental positron lifetimes assigned to V${}_{\mathrm{Si}}$ could in fact be related to defects containing V${}_{\mathrm{C}}$. In consequence, the experimental data interpretation and defect identification should be probably reconsidered.

In recent years there have been several theoretical and experimental investigations concerned with the detailed structure of the acceptor states in Mg-doped GaN. Thus, the shallow effective-mass-like acceptor has been attributed to simple substitutional Mg at Ga sites in unstrained regions of the material, while earlier suggestions that the Mg is associated with H appear to have been discounted by more recent studies. Deeper acceptor states have also been attributed to simple substitutional Mg, but in strained regions. The present paper makes use of the extensive data available from electron spin resonance and optically detected magnetic resonance to confirm these assignments and to highlight further the crucial role played by strain in influencing the detailed nature of the acceptor states. In particular, the neutral deep acceptor states are found to be formed by localization of the holes in $p$-like orbits on N atoms that lie in the basal plane, rather than along the $c$ axis, relative to the Mg dopant.

The intrinsic anisotropic optical properties of wurtzite AlN are investigated in absorption and emission. Full access to the anisotropy of the optical response of the hexagonal material is obtained by investigating the $\left(1\overline{1}00\right)$ plane of a high-quality bulk crystal allowing electric field $\mathbf{E}$ polarization perpendicular ($\mathbf{E}\perp \mathbf{c}$) and parallel ($\mathbf{E}\parallel \mathbf{c}$) to the optical axis $\mathbf{c}$. Spectroscopic ellipsometry yields the ordinary (${\varepsilon }_{\perp }$) and extraordinary (${\varepsilon }_{\parallel }$) dielectric functions (DFs) from $0.58$ up to 20 eV. The comparison of the experimental data with recently calculated DFs demonstrates that Coulomb interaction has a strong impact not only on the spectral dependence around the fundamental absorption edge but also on the high-energy features usually discussed in terms of van Hove singularities. The fits of the second-order derivatives of ${\varepsilon }_{\parallel }$ and ${\varepsilon }_{\perp }$ provide the transition energies for the main features in this range. The DFs close to the fundamental absorption edge, dominated by free excitons, exciton-phonon complexes, and the exciton continuum, are independently confirmed by reflectivity and synchrotron-based photoluminescence excitation studies. Values for the band gaps, the crystal field (${\Delta }_{\mathrm{cf}}=-221\pm 2$ meV), and spin-orbit splittings (${\Delta }_{\mathrm{so}}=13\pm 2$ meV) are obtained. Furthermore, we obtain accurate values for the static dielectric constants of ${\varepsilon }_{\mathrm{S}}\perp =7.65$ and ${\varepsilon }_{\mathrm{S}}\parallel =9.21$, entering, e.g., the calculations of exciton binding energies. Photoluminescence is used to investigate the emission properties of the same sample.

When gold is deposited on Si(553), the surface self-assembles to form a periodic array of steps with nearly perfect structural order. In scanning tunneling microscopy these steps resemble quasi-one-dimensional atomic chains. At temperatures below $\sim$50 K the chains develop a tripled periodicity. We recently predicted, on the basis of density-functional theory calculations at $T=0$, that this tripled periodicity arises from the complete polarization of the electron spin on every third silicon atom along the step; in the ground state these linear chains of silicon spins are antiferromagnetically ordered. Here we explore, using ab initio molecular dynamics and kinetic Monte Carlo simulations, the behavior of silicon spin chains on Si(553)-Au at finite temperature. Thermodynamic phase transitions at $T>0$ in one-dimensional systems are prohibited by the Mermin-Wagner theorem. Nevertheless we find that a surprisingly sharp onset occurs upon cooling—at about 30 K for perfect surfaces and at higher temperature for surfaces with defects—to a well-ordered phase with tripled periodicity, in good agreement with experiment.

In quantum wires, the hyperfine coupling between conduction electrons and nuclear spins can lead to a (partial) ordering of both of them at low temperatures. By an interaction-enhanced mechanism, the nuclear spin order, caused by Ruderman-Kittel-Kasuya-Yosida exchange, acts back onto the electrons and gaps out part of their spectrum. In wires with two subbands characterized by distinct Fermi momenta $k_{F1}$ and $k_{F2}$, the nuclear spins form a superposition of two helices with pitches $\pi /k_{F1}$ and $\pi /k_{F2}$, thus exhibiting a beating pattern. This order results in a reduction of the electronic conductance in two steps upon lowering the temperature.

In this article a route to analyze the full optical response of plasmonic structures is developed. First, the simple case of an anisotropic thin plasmonic layer supported on a transparent substrate is analyzed by introducing a quantity named anisotropic surface excess function (ASEF). The spectral features are analyzed in terms of effective dielectric function, demonstrating a more direct relation with the plasmonic response of the layer. The formalism is then generalized using a transfer matrix method. The formalism developed is supported by experimental evidence obtained by measuring the response of anisotropic nanoparticle arrays grown at a glancing angle. The agreement between theory and experiment is clear, suggesting that SE can be conveniently employed to measure the spectroscopic response of plasmonic structures. It is also demonstrated that the figure of merit of the plasmonic resonance for refractive index sensing can be greatly improved, with optimized measurement configurations, using polarized spectroscopy.

The recently synthesized MgFeGe compound is isostructural and isoelectronic with superconducting LiFeAs. Both materials are paramagnetic metals at room temperature. Inspection of their electronic structures without spin polarization reveals hardly any difference between the two. This fact was interpreted as evidence against popular theories relating superconductivity in Fe-based materials with spin fluctuations. We show that in the magnetic domain the two compounds are dramatically different, and the fact that MgFeGe does not superconduct, is, on the contrary, a strong argument in favor of theories based on spin fluctuations.

We investigate the bilayer Ruddlesden-Popper iridate Sr${}_3$Ir${}_2$O${}_7$ by temperature-dependent angle-resolved photoemission. At low temperatures, we find a fully gapped correlated insulator, characterized by a small charge gap and narrow bandwidths. The low-energy spectral features show a pronounced temperature-dependent broadening and non-quasiparticle-like Gaussian line shapes. Together, these spectral features provide experimental evidence for a polaronic ground state. We observe similar behavior for the single-layer cousin Sr${}_2$IrO${}_4$, indicating that strong electron-boson coupling dominates the low-energy excitations of this exotic family of $5d$ compounds.

We have detected a clear magnetic anisotropy in the temperature and field dependent Mn $L_{2,3}$ magnetic circular dichroism (MCD) spectra of 28 Å wide paramagnetic manganese oxide nanostripes, which we relate to the presence of a largely unquenched orbital moment revealed by MCD sum rule analysis. Polarization, angle, and temperature dependent linear dichroism spectra indicate that the observed anisotropy is related to a depletion of 3$d$ states with $m_l=\pm 2$ orbital quantum number in the ground state of the paramagnetic system. Strongly anisotropic electrostatic coupling provides an internal source of spatial symmetry breaking which might generally yield paramagnetic anisotropy phenomena in $3d$ oxide nanostructures.

The electron-phonon interaction in electron-doped single-layer MoS${}_2$ is investigated using first-principles calculations. The electron-phonon coupling is found to be very weak at low doping levels. It then grows rapidly to a maximum of $\lambda \approx 1.7$, after which it begins to decrease with additional doping. The superconducting transition temperature is expected to follow the same trends. This behavior is attributed to the growth and shrinkage of Fermi sheets with different orbital character. These results, which are similar to the experimentally observed superconducting dome in gate-tuned MoS${}_2$ thin flakes, reveal that having the right combination of electronic states at the Fermi level is crucial for optimizing the electron-phonon interaction in this material. By appropriately tuning the band structure, it may be possible to further enhance the superconducting properties of this material.

We study origin of Rashba spin-orbit interaction at SrTiO${}_3$ surfaces and LaAlO${}_3$/SrTiO${}_3$ interfaces by considering the interplay between atomic spin-orbit coupling and inversion asymmetry at the surface or interface. We show that in a simple tight-binding model involving $3d$ $t_{2g}$ bands of Ti ions, the induced spin-orbit coupling in the $d_{xz}$ and $d_{yz}$ bands is cubic in momentum whereas the spin-orbit interaction in the $d_{xy}$ band has linear momentum dependence. We also find that the spin-orbit interaction in one-dimensional channels at LaAlO${}_3$/SrTiO${}_3$ interfaces is linear in momentum for all bands. We discuss implications of our results for transport experiments on SrTiO${}_3$ surfaces and LaAlO${}_3$/SrTiO${}_3$ interfaces. In particular, we analyze the effect of a given spin-orbit interaction term on magnetotransport of LaAlO${}_3$/SrTiO${}_3$ by calculating weak antilocalization corrections to the conductance and to universal conductance fluctuations.

Extraordinary new materials named quasicrystals and characterized by noncrystallographic rotational symmetry and quasiperiodic translational properties have attracted scrutiny. Study of quasicrystals may shed light on the most basic notions related to the quantum critical state observed in heavy-fermion metals. We show that the electronic system of some quasicrystals is located at the fermion condensation quantum phase transition without tuning. In that case the quasicrystals possess the quantum critical state with the non-Fermi-liquid behavior which in magnetic fields transforms into the Landau Fermi-liquid one. Remarkably, the quantum critical state is robust despite the strong disorder experienced by the electrons. We also demonstrate for the first time that quasicrystals exhibit the typical scaling behavior of their thermodynamic properties such as the magnetic susceptibility, and belong to the famous family of heavy-fermion metals. Our calculated thermodynamic properties are in good agreement with recent experimental observations.

We investigate the feasibility of many candidate quantum Hall states for two-component bosons in the lowest Landau level. We identify interactions for which spin-singlet incompressible states occur at filling factors $\nu =2/3$, $4/5$, and $4/3$, and partially spin-polarized states at filling factors $3/4$ and $3/2$, where “spin” serves as a generic label for the two components. We study ground states, excitations, edge states, and entanglement spectrum for systems with up to 16 bosons and construct explicit trial wave functions to clarify the underlying physics. The composite fermion theory very accurately describes the ground states as well as excitations at $\nu =2/3$, $4/5$, and $3/4$, although it is less satisfactory for the $\nu =3/2$ state. For $\nu =4/3$ a “non-Abelian spin-singlet” state, which is the exact ground state of a three-body contact interaction, has been proposed to occur even for a two-body contact interaction; our trial wave functions are very accurate for the excitations of the three-body interaction, but they do not describe the excitations of the two-body interaction very well. Instead, we find that the $\nu =4/3$ state is more likely to be a spin-singlet state of reverse-flux-attached composite fermions at filling ${\nu }^{*}=4$. We also consider incompressible states at integral filling factors $\nu =1$ and 2. The incompressible state at $\nu =1$ is shown to be well described by the parton-based Jain spin-singlet wave function, and the incompressible state at $\nu =2$ as the spin-singlet state of reverse-flux-attached composite fermions at ${\nu }^{*}=2$, which provides an example of the bosonic integer topological phase.

In the charge-ordered phase of strongly doped manganites La${}_{1-x}$Ca${}_x$MnO${}_3$ ($x\ge 0.5$) absorption lines appear in the terahertz spectral range for commensurate $x$ values right below the charge-ordering temperature. They are connected to acoustic phonons that become optically active by folding of the Brillouin zone. At lower temperatures a strongly asymmetric extra absorption band develops at frequencies corresponding to the position of the lowest-energy van Hove singularity in the reduced Brillouin zone. The band is assigned to the boson peak, i.e., to the excess of lattice vibrational states over the standard Debye contribution. The folded phonons and the boson peak do not show up for incommensurate calcium contents when no distinct Brillouin zone folding exists.

The Read-Rezayi wave function is one of the candidates for the fractional quantum Hall effect at filling fraction $\nu =2+⅗$, and thereby also its hole conjugate at $2+⅖$. We study a general class of tripartite composite fermion wave functions, which reduce to the Rezayi-Read ground state and quasiholes for appropriate quantum numbers, but also allow a construction of wave functions for quasiparticles and neutral excitations by analogy to the standard composite fermion theory. We present numerical evidence in finite systems that these trial wave functions capture well the low energy physics of a four-body model interaction. We also compare the tripartite composite fermion wave functions with the exact Coulomb eigenstates at $2+⅗$, and find reasonably good agreement. The ground state as well as several excited states of the four-body interaction are seen to evolve adiabatically into the corresponding Coulomb states for $N=15$ particles. These results support the plausibility of the Read-Rezayi proposal for the $2+⅖$ and $2+⅗$ fractional quantum Hall effect. However, certain other proposals also remain viable, and further study of excitations and edge states will be necessary for a decisive establishment of the physical mechanism of these fractional quantum Hall states.