In addition to the eight conventional high-symmetry configurations for self-interstitials in the hexagonal close packed (hcp) structure, we show that four other configurations, obtained by breaking the symmetry of some of the original ones, may be low-energy local minima or saddle points. The first two, BC${}^{\prime }$ and C${}^{\prime }$, consist of the basal crowdion and the crowdion buckled perpendicular to their axes in the pyramidal plane, respectively. The two others, PS and P2S, are obtained by rotating the $c$-axis split dumbbell in the prismatic plane of first and second type, respectively. Using first-principles density functional theory calculations we show that BC${}^{\prime }$, C${}^{\prime }$, and PS are within 0.4 eV of the lowest-energy conventional structure, BO, in Ti, Zr, and Hf. BC${}^{\prime }$ could even be the lowest-energy configuration in hcp-Zr and its symmetry and possible reorientation mechanisms are compatible with internal friction measurements at variance with the conventional structures. The PS and C${}^{\prime }$ configurations exhibit a helicoidal easy glide motion of the dumbbell-crowdion type in the $c$-axis direction. These configurations therefore constitute an important element to take into account when predicting the microstructural evolution of zirconium-based materials under irradiation.

An extensive assessment of the physicochemical factors that control the behavior of dopant-related defects in BaTiO${}_3$ has been performed using high-throughput first-principles computations. Dopants spanning the Periodic Table—44 in total—including K-As, Rb-Sb, and Cs-Bi were considered, and have allowed us to reveal previously unknown correlations, chemical trends, and the interplay between stability, chemistry, and electrical activity. We quantitatively show that the most important factor that determines dopant stability in BaTiO${}_3$ is the dopant ionic size (followed by its oxidation state). Moreover, we are also able to identify definitively dopants that are O vacancy formers and suppressors, and those that would lead to $p$-type versus $n$-type conduction. Our results are in agreement with available experimental data (with no violations thus far), and point to an attractive computational route to dopant selection in BaTiO${}_3$ as well as in other materials.

While the coherent potential approximation (CPA) is the prevalent method for the study of disordered electronic systems, it fails to capture nonlocal correlations and Anderson localization. To incorporate such effects, we extend the dual fermion approach to disordered systems using the replica method. The developed method utilizes the exact mapping to the dual fermion variables, and includes intersite scattering via diagrammatic perturbation theory in the dual variables. The CPA is recovered as a zeroth-order approximation. Results for single- and two-particle quantities show good agreement with a cluster extension of the CPA; moreover, weak localization is captured. As a natural extension of the CPA, our method presents an alternative to existing nonlocal cluster theories for disordered systems, and has potential applications in the study of disordered systems with electronic interactions.

We report local probe investigations of the magnetic interaction between BiFeO${}_3$ films and a ferromagnetic Co${}_{0.9}$Fe${}_{0.1}$ layer. Within the constraints of intralayer exchange coupling in the Co${}_{0.9}$Fe${}_{0.1}$, the multiferroic imprint in the ferromagnet results in a collinear arrangement of the local magnetization and the in-plane BiFeO${}_3$ ferroelectric polarization. The magnetic anisotropy is uniaxial, and an in-plane effective coupling field of order 10 mT is derived. Measurements as a function of multiferroic layer thickness show that the influence of the multiferroic layer on the magnetic layer becomes negligible for 3 nm thick BiFeO${}_3$ films. We ascribe this breakdown in the exchange coupling to a weakening of the antiferromagnetic order in the ultrathin BiFeO${}_3$ film based on our x-ray linear dichroism measurements. These observations are consistent with an interfacial exchange coupling between the CoFe moments and a canted antiferromagnetic moment in the BiFeO${}_3$.

We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa${}_2$Cu${}_3$O${}_{7-x}$/LaMnO${}_{3+\delta }$/YBa${}_2$Cu${}_3$O${}_{7-x}$) tunnel junctions. At a large bias voltage $V\sim 1$ V we observe a steplike onset of excess current that occurs below the superconducting transition temperature $T<T_c$ and is easily suppressed by a magnetic field. The phenomenon is attributed to a different type of the superconducting proximity effect of nonequilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunneling process. The occurrence of a strongly nonequilibrium population is confirmed by the detection of photon emission at large bias voltage. Since the conduction band in our ferromagnetic insulator is strongly spin polarized, the long range (20 nm) of the observed proximity effect provides evidence for an unconventional spin-triplet superconducting state.

Inelastic neutron scattering measurements demonstrate that the magnetic interactions in antiferromagnetic LaFeAsO are two dimensional. Spin-wave velocities within the Fe layer and the magnitude of the spin gap are similar to the $A$Fe${}_2$As${}_2$ based materials. However, the ratio of interlayer and intralayer exchange is found to be less than $\sim {10}^{-4}$ in LaFeAsO, very similar to the cuprates, and $\sim$100 times smaller than that found in $A$Fe${}_2$As${}_2$ compounds. The results suggest that the effective dimensionality of the magnetic system is highly variable in the parent compounds of the iron arsenides and weak three-dimensional interactions may limit the maximum attainable superconducting $T_c$.

The nature of structural relaxation in disordered systems such as amorphous silicon ($a$-Si) remains a fundamental issue in our attempts at understanding these materials. While a number of experiments suggest that mechanisms similar to those observed in crystals, such as vacancies, could dominate the relaxation, theoretical arguments point rather to the possibility of more diverse pathways. Using the kinetic activation-relaxation technique, an off-lattice kinetic Monte Carlo method with on-the-fly catalog construction, we resolve this question by following 1000 independent vacancies in a well-relaxed $a$-Si model at 300 K over a timescale of up to one second. Less than one percent of these survive over this period of time and none diffuse more than once, showing that relaxation and diffusion mechanisms in disordered systems are fundamentally different from those in the crystal.

We argue that many aspects of improper ferroelectric (FE) activity in orthorhombic manganites can be rationalized by considering the limit of infinite intra-atomic splitting between majority- and minority-spin states (or the double-exchange limit), which reduces the problem to the analysis of a spinless double-exchange (DE) Hamiltonian. We apply this strategy to the low-energy model, derived from the first-principles electronic-structure calculations, and combine it with the Berry-phase theory of electric polarization. We start with the analysis of the simplest two-orbital model, describing the behavior of the $e_g$ bands, and apply it to the $E$-type antiferromagnetic (AFM) phase, which in the DE limit effectively breaks up into one-dimensional zigzag chains. We derive an analytical expression for the electronic polarization (${\mathbf{P}}^{\mathrm{el}}$) and explain how it depends on the orbital ordering and the energy splitting $\Delta$ between $e_g$ states. Then, we evaluate parameters of this model for the series of manganites. For these purposes, we start from a more general five-orbital model for all Mn $3d$ bands and construct a new downfolded model for the $e_g$ bands. From the analysis of these parameters, we conclude that the behavior of ${\mathbf{P}}^{\mathrm{el}}$ in realistic manganites always corresponds to the limit of large $\Delta$. This property holds for all considered compounds even in the local-density approximation, which typically underestimates $\Delta$. We further utilize this property in order to derive an analytical expression for ${\mathbf{P}}^{\mathrm{el}}$ in a general twofold periodic magnetic texture, based on the five-orbital model and the perturbation-theory expansion for the Wannier functions in the first order of $1/\Delta$. This expression explains the functional dependence of ${\mathbf{P}}^{\mathrm{el}}$ on the relative directions of spins. Furthermore, it suggests that ${\mathbf{P}}^{\mathrm{el}}$ is related to the asymmetry of transfer integrals, which should have simultaneously symmetric and antisymmetric components in the crystal-field representation. The main contribution to this asymmetry comes from the antiferro-orbital ordering in the $\mathit{ab}$ plane. Finally, we explain how the FE polarization can be switched between orthorhombic $\mathit{a}$ and $\mathit{c}$ directions by inverting the zigzag AFM texture in every second $\mathit{ab}$ plane. We argue that this property is generic and can be realized even in the twofold periodic texture.

Isolated spin tetrahedral systems with weak intertetrahedral couplings, as have been reported for Cu${}_2$Te${}_2$O${}_5$$X_2$ ($X$ $=$ Cl, Br) and the related compound Cu${}_4$Te${}_5$O${}_{12}$Cl${}_4$, have received much attention recently because they represent an interesting class of magnets that consist of weakly coupled magnetic clusters and, in particular, they can directly demonstrate the interplay of intertetrahedral couplings with built-in tetrahedral frustration. However, there is much debate about the structural low dimensionality of the Cu-Te-O-Cl(Br) compounds and its effect on the magnetism of the material. Here, we present a model spin tetrahedral system K${}_4$Cu${}_4$OCl${}_{10}$, with almost isotropic magnetic coupling within the tetrahedron and three-dimensional connection of the tetrahedra. The system enters a spin-singlet state with a susceptibility maximum at $T_{\max }$ $=$ 11 K, and then enters an antiferromagnetic order at $T_{\mathrm{N}}$ $=$ 4.4 K. The ratio $T_{\mathrm{N}}$/$T_{\max }$ $=$ 0.40 is close to the $T_{\mathrm{N}}$/$T_{\max }$ $=$ 0.38 for Cu${}_2$Te${}_2$O${}_5$Br${}_2$, which is viewed as an indicator of closeness to quantum criticality. Evidence in muon-spin rotation or relaxation suggests an incommensurate ordering. This work shows that the previously revealed ground state in anisotropically structured Cu${}_2$Te${}_2$O${}_5$$X_2$ compounds also exists in an isotropic spin tetrahedral system.

Brillouin light scattering was applied to measure the evolution of spin waves in exchange-coupled [Co/Pd]-NiFe films with out-of-plane/in-plane magnetic anisotropy. Tunable tilting of the magnetization has important effects on the spin wave frequency gap: a substantial increase is observed with decreasing the soft NiFe thickness, while seemingly insignificant tilting angles cause a strong reduction with respect to a film with only the hard [Co/Pd] component. The spin wave frequency is reproduced also in samples, with increased thickness of the upper Pd layer, where a sudden modification in the tilting at the hard/soft interface is caused by the weakening of interlayer exchange.

We report a comprehensive study of dc susceptibility, specific-heat, neutron-diffraction, and inelastic neutron-scattering measurements on polycrystalline Ba${}_3$(Cr${}_{1-x}$V${}_x$)${}_2$O${}_8$ samples, where $x=0$, 0.06, 0.15, and 0.53. A Jahn-Teller structure transition occurs for $x=0$, 0.06, and 0.15 samples, and the transition temperature is reduced upon vanadium substitution from 70(2) K at $x=0$ to 60(2) K at $x=0.06$ and 0.15. The structure becomes less distorted as $x$ increases, and such transition disappears at $x=0.53$. The observed magnetic excitation spectrum indicates that the singlet ground state remains unaltered and spin-gap energy $\Delta =1.3\left(1\right)$ meV is identical within the instrument resolution for all $x$. In addition, the dispersion bandwidth $W$ decreases with increase of $x$. At $x=0.53$, $W$ is reduced to 1.4(1) meV from 2.0(1) meV at $x=0$.

Equivalent circuit models are presented for the propagation of noise in one-dimensional negative-index metamaterials based on split-ring resonators (SRRs) and rods. The SRRs are modeled as lossy lumped-element $L$-$C$ resonators, whose reactive components provide the effect of a negative relative permeability over a restricted frequency range but whose resistive elements act as sources of propagation loss and Johnson noise. Similarly, the rod loading is modeled as lumped-element inductors, whose reactive components provide the effect of a negative relative permittivity but whose resistive elements introduce further loss and noise. Coupling between the magnetic resonators allows the propagation of magnetoinductive lattice waves. The effect on the effective medium properties is to shift the apparent magnetic resonance up in frequency by an amount depending on the coupling coefficient and to allow the propagation of noise waves within the resonator array over the entire magnetoinductive band. The model predicts all the details of the electromagnetic and magnetoinductive noise inside the array. However, it is shown that the observable effect outside the array matches the prediction of a simplified model based only on the modified effective medium properties.

Magnetic and electronic properties of a Kondo lattice model with Ising localized spins are studied on an isotropic triangular lattice. By using Monte Carlo simulation, we present that the model shows a rich phase diagram with four dominant states: two-sublattice stripe, three-sublattice ferrimganetic, partially disordered, and Kosterlitz-Thouless-like quasi-long-range ordered states. Among them, the partially disordered state and Kosterlitz-Thouless-like state are intermediate phases induced by thermal fluctuations in the phase competing regime; they are present only at finite temperatures and eventually taken over by another phases as the temperature is further lowered. Although the Kosterlitz-Thouless-like state was found also in triangular Ising antiferromagnets with further-neighbor interactions, the partially disordered state has not been reported in the localized spin-only models in two dimensions. The partially disordered phase is also peculiar in the charge degree of freedom of itinerant electrons; it is insulating and accompanied by charge disproportionation. From a combined analysis of a mean-field calculation of the band structure and Monte Carlo simulation, we conclude that the partial disorder in the present model is stabilized by the Slater mechanism.

We present an approach to calculation of point-defect optical and thermal ionization energies based on the highly accurate quantum Monte Carlo methods. The use of an inherently many-body theory that directly treats electron correlation offers many improvements over the typically employed density functional theory Kohn-Sham description. In particular, the use of quantum Monte Carlo methods can help overcome the band-gap problem and obviate the need for ad hoc corrections. We demonstrate our approach to the calculation of the optical and thermal ionization energies of the $F$-center defect in magnesium oxide, and obtain excellent agreement with experimental and/or other high-accuracy computational results.

We demonstrate high-resolution photocurrent spectroscopy of the positive trion state (X${}^{+}$) in a single InAs/GaAs self-assembled quantum dot (QD). Exploiting a Coulomb-mediated renormalization of the X${}^{+}$ transition energy, our approach allows for a sub-μeV resolution, which is more than four orders of magnitude higher than that in previous approaches. Furthermore, such high-resolution spectroscopy of X${}^{+}$ permits us to obtain precise values for the X${}^{+}$ permanent dipole moment and polarizability, by fitting a theoretical model to the measured transition energy as a function of electric field. Since optical initialization, manipulation, and readout of a QD spin rely on the trion state as an intermediary state, such high-resolution measurements of the X${}^{+}$ spectrum and its dipole moment are essential for performing high-fidelity quantum computing operations on QD spin qubits.

The adsorption of phosphorus molecules evaporated from InP onto the Si(100) surface has been studied by using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Five distinct structures of phosphorus adsorbates (four structures for phosphorus dimers and one for a phosphorus tetramer) were observed on the Si(100) surface by STM. Comparisons between bias-dependent STM images and simulated images based on the DFT calculation determined the adsorption sites and structures of these adsorbates. We also discuss their electronic structures and stabilities. Additionally, STM-induced structural conversions of the phosphorus adsorbates to the state with the minimum energy are shown.

The development of integrated circuits urgently demands improved understanding of interfacial thermal transport at the nanoscale. We report a systematical nonequilibrium Green's function study on thermal transport in a class of experimentally available carbon-based nanostructures, partially unzipped carbon nanotubes (PUCNTs), where unusual interfacial thermal transport behaviors are revealed. We find that thermal conductance of PUCNTs shows a perfect linear dependence on the width of the unzipped part and an exponential decay (to a finite value) with the length of the unzipped part. The exponential length dependence observed at interfaces is a pure quantum phenomenon without classical counterpart. The improved understanding of nanoscale thermal transport can find applications in designing high-performance thermal management and/or thermoelectric nanodevices.

We present a microscopic theory of transport through quantum dot setups coupled to superconducting leads. We derive a master equation for the reduced density matrix to lowest order in the tunneling Hamiltonian and focus on quasiparticle tunneling. For high enough temperatures transport occurs in the subgap region due to thermally excited quasiparticles, which can be used to observe excited states of the system at low bias voltages. On the example of a double quantum dot we show how subgap transport spectroscopy can be done. Moreover, we use the single level quantum dot coupled to a normal and a superconducting lead to give a possible explanation for the subgap features observed in the experiments of Dirks, Chen, Birge, and Mason [Appl. Phys. Lett. 95, 192103 (2009)].

We show that the spin moment induced by $s{p}^3$-type defects created by different covalent functionalizations on a few-layer graphene structure can be controlled by an external electric field. Based on ab initio density functional calculations, including van der Waals interactions, we find that this effect has a dependence on the number of stacked layers and concentration of point defects, but the interplay of both with the electric field drives the system to a half-metallic state. The calculated magnetoelectric coefficient $\alpha$ has a value comparable to those found for ferromagnetic thin films (e.g., Fe, Co, Ni) and magnetoelectric surfaces (e.g., CrO${}_2$). The value of $\alpha$ also agrees with the universal value predicted for ferromagnetic half-metals and also points to a novel route to induce half-metallicity in graphene using surface decoration.

We investigate chiral graphene nanoribbons using projective quantum Monte Carlo simulations within the local Hubbard-model description and study the effects of electron-electron interactions on the electronic and magnetic properties at the ribbons’ edges. Static and dynamical properties are analyzed for nanoribbons of varying width and edge chirality and compared to a self-consistent Hartee-Fock mean-field approximation. Our results show that for chiral ribbons of sufficient width, the spin correlations exhibit exceedingly long correlation lengths, even between zigzag segments that are well separated by periodic armchair regions. Characteristic enhancements in the magnetic correlations for distinct ribbon widths and chiralities are associated with energy gaps in the tight-binding limit of such ribbons. We identify specific signatures in the local density of states and low-energy modes in the local spectral function which directly relate to enhanced electronic correlations along graphene nanoribbons. These signatures in the local density of states might be accessed by scanning tunneling spectroscopy on graphene nanoribbons.