Quantum wells (QWs) are nanostructures consisting of alternating layers of a low and high band-gap semiconductor. The band gap of QWs can be tuned by changing the thickness of the low band-gap layer, due to quantum confinement effects. Although this principle is well established for crystalline materials, there is still controversy for QWs fabricated from amorphous materials: How strong are the confinement effects in amorphous QWs, where, because of the disorder, the carriers are localized to start with? We prepare an atomistic model of QWs based on a-Si:H to gain insight into this problem. The electronic structure of our atomistic QWs model is described with first-principles density functional theory, allowing us to study the confinement effects directly. We find that the quantum confinement effect is rather weak, compared to experimental results on a similar system.

We investigate the dependence of the structural phase transitions in an infinite quasi-one-dimensional system of repulsively interacting particles on the profile of the confining channel. Three different functional expressions for the confinement potential related to real experimental systems are used that can be tuned continuously from a parabolic to a hard-wall potential in order to find a thorough understanding of the ordering of the chainlike structure transitions. We resolve the long-standing issue why the most theories predicted a 1-2-4-3-4 sequence of chain configurations with increasing density, while some experiments found the 1-2-3-4 sequence.

We develop the theory of magnetic domain-wall motion in coupled double-layer systems where electrons can hop between the layers, giving rise to an antiferromagnetic coupling. We demonstrate that the force from the interlayer coupling drives the walls and the effect of the extrinsic pinning is greatly reduced if the domain walls are initially separated. The threshold current density for metastable spin-aligned configurations is also much lower. We conclude that the interlayer coupling has a significant effect on domain-wall mobility in double-layer systems.

The effect of structural disorder is investigated for an $XY$ pyrochlore antiferromagnet with continuous degeneracy of classical ground states. Two types of disorder—vacancies and weakly fluctuating exchange bonds—lift degeneracy, selecting the same subset of classical ground states. Analytic and numerical results demonstrate that such an “order by structural disorder” mechanism competes with the effect of thermal and quantum fluctuations. Our theory predicts that a small amount of nonmagnetic impurities in ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ will stabilize the coplanar ${\psi }_3$ $\left(m_{x^2-y^2}\right)$ magnetic structure as opposed to the ${\psi }_2$ $\left(m_{3z^2-r^2}\right)$ state found in pure material.

The effects of hydrostatic pressure and partial Na substitution on the normal-state properties and the superconducting transition temperature $\left(T_c\right)$ of ${\mathrm{K}}_{1\text{−}x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ single crystals were investigated. It was found that a partial Na substitution leads to a deviation from the standard $T^2$ Fermi-liquid behavior in the temperature dependence of the normal-state resistivity. It was demonstrated that non-Fermi-liquid like behavior of the resistivity for ${\mathrm{K}}_{1\text{−}x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ and some ${\mathrm{KFe}}_2{\mathrm{As}}_2$ samples can be explained by a disorder effect in the multiband system with rather different quasiparticle effective masses. Concerning the superconducting state our data support the presence of a shallow minimum around 2 GPa in the pressure dependence of $T_c$ for stoichiometric ${\mathrm{KFe}}_2{\mathrm{As}}_2$. The analysis of $T_c$ in ${\mathrm{K}}_{1\text{−}x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ at pressures below 1.5 GPa showed that the reduction of $T_c$ with Na substitution follows the Abrikosov-Gor'kov law with the critical temperature $T_{c0}$ of the clean system (without pair breaking), which linearly depends on the pressure. Our observations also suggest that $T_c$ of ${\mathrm{K}}_{1\text{−}x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ is nearly independent of the lattice compression produced by the Na substitution. Further, we theoretically analyzed the behavior of the band structure under pressure within the generalized gradient approximation (GGA). A qualitative agreement between the calculated and the recently measured—in de Haas–van Alphen experiments [T. Terashima et al., Phys. Rev. B 89, 134520 (2014)]—pressure dependencies of the Fermi-surface cross sections has been found. These calculations also indicate that the observed minimum around 2 GPa in the pressure dependence of $T_c$ may occur without a change of the pairing symmetry.

We report inelastic light scattering measurements of lattice dynamics related to the incommensurate orbital order in ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$. Below the ordering temperature $T_{\mathrm{o}}\approx 250\mathrm{K}$, we observe extra phonon peaks as a result of Brillouin zone folding, as well as a soft vibrational mode with a power-law $T$-dependent energy, $\Omega ={\Omega }_0{(1-T/T_{\mathrm{o}})}^{1/2}$. This temperature dependence demonstrates the second-order nature of the transition at $T_{\mathrm{o}}$, and it indicates that the soft mode can be regarded as the amplitude excitation of the composite order parameter. Our result strongly suggests that the lattice degrees of freedom are actively involved in the orbital-ordering mechanism.

We present an investigation of charge-compensated antiferromagnetic $\left({\mathrm{Ca}}_x{\mathrm{La}}_{1-x}\right){(\mathrm{Ba}}_{1.75-x}{\mathrm{La}}_{0.25+x}{)\mathrm{Cu}}_3{\mathrm{O}}_y$ single crystals using Raman scattering as well as muon spin rotation. In this system the parameter $x$ controls the Cu-O-Cu superexchange interaction via bond distances and buckling angles. The oxygen content $y$ controls the charge doping. In the absence of doping the two-magnon peak position is directly proportional to the superexchange strength $J$. We find that both $x$ and $y$ affect the peak position considerably. The Néel temperature determined from muon spin rotation on the same samples independently confirms the strong dependence of the magnetic interaction on $x$ and $y$. We find a considerable increase in the maximum superconducting transition temperature $T_c^{\max }$ with $J$. This is strong evidence of the importance of orbital overlap to superconductivity in this family of cuprates.

We introduce a two-channel tunneling model to generalize the widely used BTK theory of point-contact conductance between a normal metal contact and superconductor. Tunneling of electrons can occur via localized surface states or directly, resulting in a Fano resonance in the differential conductance $G=dI/dV$. We present an analysis of $G$ within the two-channel model when applied to soft point contacts between normal metallic silver particles and prototypical heavy-fermion superconductors ${\mathrm{CeCoIn}}_5$ and ${\mathrm{CeRhIn}}_5$ at high pressures. In the normal state the Fano line shape of the measured $G$ is well described by a model with two tunneling channels and a large temperature-independent background conductance. In the superconducting state a strongly suppressed Andreev reflection signal is explained by the presence of the background conductance. We report Andreev signal in ${\mathrm{CeCoIn}}_5$ consistent with standard $d_{x^2-y^2}$-wave pairing, assuming an equal mixture of tunneling into [100] and [110] crystallographic interfaces, whereas in ${\mathrm{CeRhIn}}_5$ at 1.8 and 2.0 GPa the signal is described by a $d_{x^2-y^2}$-wave gap with reduced nodal region, i.e., increased slope of the gap opening on the Fermi surface. A possibility is that the shape of the high-pressure Andreev signal is affected by the proximity of a line of quantum critical points that extends from 1.75 to 2.3 GPa, which is not accounted for in our description of the heavy-fermion superconductor.

The nature of the kagome Heisenberg antiferromagnet (HAFM) is under ongoing debate. While recent evidence points towards a ${\mathbb{Z}}_2$ topological spin liquid, the exact nature of the topological phase is still unclear. In this paper, we introduce semionic resonating valence-bond (RVB) states, this is, resonating valence-bond states which are in the ${\mathbb{Z}}_2$-ordered double-semion phase, and study them using projected entangled pair states. We investigate their physics and study their suitability as an ansatz for the HAFM, as compared to a conventional RVB state which is in the toric-code ${\mathbb{Z}}_2$ topological phase. In particular, we find that a suitably optimized “semionic simplex RVB” outperforms the equally optimized conventional “simplex RVB” state, and that the entanglement spectrum (ES) of the semionic RVB behaves very differently from the ES of the conventional RVB, which suggests using the ES to discriminate the two phases. Finally, we also discuss the possible relevance of space-group symmetry breaking in valence-bond wave functions with double-semion topological order.

We present theoretical calculations of quasiparticle energies in closed-shell molecules using the $GW$ method. We compare three different approaches: a full-frequency $G_0{W}_0$ ($\mathrm{FF}\text{−}{G}_0{W}_0$) method with density functional theory (DFT-PBE) used as a starting mean field; a full-frequency $G{W}_0$ ($\mathrm{FF}\text{−}G{W}_0$) method where the interacting Green's function is approximated by replacing the DFT energies with self-consistent quasiparticle energies or Hartree-Fock energies; and a $G_0{W}_0$ method with a Hybertsen-Louie generalized plasmon-pole model (HL $\mathrm{GPP}\text{−}{G}_0{W}_0$). While the latter two methods lead to good agreement with experimental ionization potentials and electron affinities for methane, ozone, and beryllium oxide molecules, $\mathrm{FF}\text{−}{G}_0{W}_0$ results can differ by more than one electron volt from experiment. We trace this failure of the $\mathrm{FF}\text{−}{G}_0{W}_0$ method to the occurrence of incorrect self-energy poles describing shake-up processes in the vicinity of the quasiparticle energies.

The Weyl semimetal surface is modeled by applying the Bogolyubov boundary conditions, in which the quasiparticles have an infinite Dirac mass outside the semimetal. For a Weyl semimetal shaped as a slab of finite thickness, we derive an exact spectral equation for the quasiparticle states and obtain the spectrum of the bulk as well as surface Fermi arc modes. We also show that, in the presence of the magnetic field, the separation between Weyl nodes in momentum space and the length of the Fermi arcs in the reciprocal space are affected by the interactions. As a result, we find that the period of oscillations of the density of states related to closed magnetic orbits involving Fermi arcs has a nontrivial dependence on the orientation of the magnetic field projection in the plane of the semimetal surface. We conclude that the momentum-space separation between Weyl nodes and its modification due the interaction effects in the magnetic field can be measured in the experimental studies of quantum oscillations.

The attractive Hubbard model is investigated in the framework of lattice density-functional theory (LDFT). The ground-state energy $E=T+W$ is regarded as a functional of the single-particle density matrix ${\gamma }_{ij}$ with respect to the lattice sites, where $T\left[\gamma \right]$ represents the kinetic and crystal-field energies and $W\left[\gamma \right]$ the interaction energy. Aside from the exactly known functional $T\left[\gamma \right]$, we propose a simple scaling approximation to $W\left[\gamma \right]$, which is based on exact analytic results for the attractive Hubbard dimer and on a scaling hypothesis within the domain of representability of $\gamma$. As applications, we consider one-, two-, and three-dimensional finite and extended bipartite lattices having homogeneous or alternating onsite energy levels. In addition, the Bethe lattice is investigated as a function of coordination number. Results are given for the kinetic, Coulomb, and total energies, as well as for the density distribution ${\gamma }_{ii}$, nearest-neighbor bond order ${\gamma }_{ij}$, and pairing energy $\Delta E_p$, as a function of the interaction strength $\left|U\right|/t$, onsite potential $\varepsilon /t$, and band filling $n=N_e/N_a$. Remarkable even-odd and super-even oscillations of $\Delta E_p$ are observed in finite rings as a function of band filling. Comparison with exact Lanczos diagonalizations and density-matrix renormalization-group calculations shows that LDFT yields a very good quantitative description of the properties of the model in the complete parameter range, thus providing a significant improvement over the mean-field approaches. Goals and limitations of the method are discussed.

Electron correlation effects are particularly strong in high-temperature superconducting materials. Devising an accurate description of these materials has long been a challenge, with these strong correlation effects historically being considered impossible or impractical to simulate computationally. Using quantum Monte Carlo techniques, we have explicitly simulated electron correlations in several cuprate materials from first principles. These simulations accurately reproduce many important physical quantities of these materials, including the interaction-induced gap and superexchange coupling between copper spins, with no additional parameters beyond fundamental constants. We further investigate the dimensionless spin-lattice coupling parameter in the parent materials, showing that it varies dramatically, between 0.1 and 1.0, depending on the interlayer. This result indicates that the lattice and magnetic degrees of freedom are not independent in these systems, which may have ramifications for the origin of superconductivity.

Ferromagnetic insulating behavior has recently been demonstrated in cobalt-substituted ${\mathrm{SrTiO}}_3$ at room temperature [Posadas et al., Phys. Rev. B 87, 144422 (2013)]. Experimentally it was found that a well-defined hysteresis loop only occurs at high Co concentrations of 30%–40%. X-ray photoelectron spectroscopy also indicated that Co substitutes for Ti with Co being in high-spin +2 oxidation state. In this work we employ density-functional theory to explain the experimentally observed properties of cobalt-substituted ${\mathrm{SrTiO}}_3$. We examine in detail the role of oxygen vacancy (OV) defects and their formation of defect complexes with the Co ions as the origin of the ferromagnetic insulating behavior. Our first-principles thermodynamic calculations indicate that OV defects are much more likely to occur next to Co atoms where their formation energies could be reduced by as much as 1.28 eV compared to that in bulk ${\mathrm{SrTiO}}_3$. We also find that Co in these Co-OV complexes occurs in the high-spin state in agreement with core level spectroscopy, and identify a linear arrangement of the Co-OV defect complexes to be the most energetically favorable structure. These defect complexes are also shown to interact ferromagnetically and that their magnetic interaction is found to be short ranged, consistent with the relatively high Co concentrations needed experimentally for ferromagnetism to be observed in cobalt-substituted ${\mathrm{SrTiO}}_3$.

Disorder scattering and spin-orbit coupling are together responsible for the diffusion and relaxation of spin density in time-reversal invariant systems. We study spin relaxation and diffusion in a two-dimensional electron gas with Rashba spin-orbit coupling and spin-independent disorder, focusing on the role of Rashba spin-orbit coupling in transport. Spin-orbit coupling contributes to spin relaxation, transforming the quantum interference contribution to conductivity from a negative weak localization (WL) correction to a positive weak antilocalization (WAL) correction. The importance of spin channel mixing in transport is largest in the regime where the Bloch state energy uncertainty $\hslash /\tau$ and the Rashba spin-orbit splitting ${\Delta }_{\mathrm{SO}}$ are comparable. We find that as a consequence of this spin channel mixing, the WL-WAL crossover is nonmonotonic in this intermediate regime, which can be related to recent experimental studies of transport at two-dimensional oxide interfaces.

We investigate here how the current flows over a bilayer graphene in the presence of an external electric field perpendicularly applied (biased bilayer). Charge density polarization between layers in these systems is known to create a layer pseudospin, which can be manipulated by the electric field. Our results show that current does not necessarily flow over regions of the system with higher charge density. Charge can be predominantly concentrated over one layer, while current flows over the other layer. We find that this phenomenon occurs when the charge density becomes highly concentrated over only one of the sublattices, as the electric field breaks layer and sublattice symmetries for a Bernal-stacked bilayer. For bilayer nanoribbons, the situation is even more complex, with a competition between edge and bulk effects for the definition of the current flow. We show that, in spite of not flowing trough the layer where charge is polarized to, the current in these systems also defines a controllable layer pseudospin.

Room-temperature light emission from single chemisorbed perylene based molecules adsorbed on silicon carbide (SiC) is probed by scanning tunneling microscopy (STM). A new approach to STM-induced luminescence of a single molecule is explored using a wide-band-gap semiconductor to decouple electronically the molecule from the surface. After molecular adsorption, the lowest unoccupied molecular orbital and the highest occupied molecular orbital (HOMO) both lie within the bulk band gap and below the Fermi level of the substrate. The maximum photon energy of the light emission from the molecule shows a fixed shift of 1.5 eV relative to the maximum energy of the tunnel electrons. This is consistent with the photons being generated by inelastic electron tunneling between the HOMO and the unoccupied electronic states of the STM tip.

An experimental method for detection of bound states around an antidot formed by a hole in a graphene sheet is proposed via measuring the ballistic two-terminal conductance. In particular, we consider the effect of bound states formed by a magnetic field on the two-terminal conductance and show that one can observe Breit-Wigner-like resonances in the conductance as a function of the Fermi level close to the energies of the bound states. In addition, we develop a numerical method utilizing a reduced computational effort compared to the existing numerical recursive Green's function methods.

For many decades, it has been known that rhenium imparts a tremendous resistance to creep to the nickel-based high-temperature alloys colloquially known as superalloys. This effect is so pronounced that is has been dubbed “the rhenium effect.” Its origins are ill-understood, even though it is so critical to the performance of these high-temperature alloys. In this paper we show that the currently known phase diagram is inaccurate, and neglects a stoichiometric compound at 20 at.% Re $\left(\text{Ni}{}_{\mathbf{4}}\text{Re}\right)$. The presence of this precipitate at low temperatures and the short-range ordering of Re in fcc-Ni observed at higher temperatures have important ramifications for the Ni-based superalloys. The $\text{Ni}{}_{\mathbf{4}}\text{Re}$ compound is shown to be stable by quantum mechanical high-throughput calculations at 0 K. Monte Carlo simulations show that it is thermally persistent up to $\approx 930$ K when considering configurational entropy. The existence of this compound is investigated using extended x-ray absorption fine spectroscopy on a $\text{Ni}{}_{96.62}\text{Re}{}_{3.38}$ alloy.