We develop a theoretical model, relying on the well established sp3 bond-orbital theory, to describe the strain-induced ${\chi }^{\left(2\right)}$ in tetrahedrally coordinated centrosymmetric covalent crystals, like silicon. With this approach we are able to describe every component of the ${\chi }^{\left(2\right)}$ tensor in terms of a linear combination of strain gradients and only two parameters $\alpha$ and $\beta$ which can be theoretically estimated. The resulting formula can be applied to the simulation of the strain distribution of a practical strained silicon device, providing an extraordinary tool for optimization of its optical nonlinear effects. The application of the first order theory to the photoelastic effect in C, Si, and Ge showed very good phenomenological and numerical agreement, up to 3% in Si. The model was then used to the second-order nonlinear susceptibility, and we were able not only to confirm the main valid claims known about ${\chi }^{\left(2\right)}$ in strained silicon, but also estimate the order of magnitude of the ${\chi }^{\left(2\right)}$ generated in that device.

Oxygen vacancies in ${\mathrm{LaAlO}}_3$ (LAO) play an important role in the formation of the two-dimensional electron gas observed at the ${\mathrm{LaAlO}}_3\text{/}{\mathrm{SrTiO}}_3$ interface and affect the performance of MOSFETs using LAO as a gate dielectric. However, their spectroscopic properties are still poorly understood, which hampers their experimental identification. Here we predict the absorption spectra and ESR parameters of oxygen vacancies in LAO using periodic and embedded cluster methods and density functional theory (DFT). The structure, charge distribution, and spectroscopic properties of the neutral ($V_{\mathrm{O}}^0$) and charged ($V_{\mathrm{O}}^{+}$ and $V_{\mathrm{O}}^{2+}$) oxygen vacancies in cubic and rhombohedral ${\mathrm{LaAlO}}_3$ are investigated. The highest intensity optical transitions [calculated using time-dependent DFT (TDDFT)], from the oxygen vacancy states to the conduction-band states have onsets at 3.5 and 4.2 eV for $V_{\mathrm{O}}^0$ and 3.6 eV for $V_{\mathrm{O}}^{+}$ in rhombohedral LAO and 3.3 and 4.0 eV for $V_{\mathrm{O}}^0$ and 3.4 eV for $V_{\mathrm{O}}^{+}$ in cubic LAO, respectively. Also reported are the isotropic $g$ value (2.004026) and hyperfine coupling constants of $V_{\mathrm{O}}^{+}$, which are compared to the experimental data obtained using electron spin resonance (ESR) spectroscopy, and accurately predict both the position and the width (3 mT) of its ESR signature. These results may further facilitate the experimental identification of oxygen vacancies in LAO and help to establish their role at the LAO/STO interfaces and in nanodevices using LAO.

A formalism for the vacancy formation energies in random alloys within the single-site mean-filed approximation, where vacancy-vacancy interaction is neglected, is outlined. It is shown that the alloy configurational entropy can substantially reduce the concentration of vacancies at high temperatures. The energetics of vacancies in random ${\mathrm{Cu}}_{0.5}{\mathrm{Ni}}_{0.5}$ alloy is considered as a numerical example illustrating the developed formalism. It is shown that the effective formation energy increases with temperature, however, in this particular system it is still below the mean value of the vacancy formation energy, which would correspond to the vacancy formation energy in a homogeneous model of a random alloy, such as given by the coherent potential approximation.

The singlet ground-state magnetism in pyrochlore osmates ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ and ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ is studied by dc and ac susceptibility, specific heat, and neutron powder diffraction measurements. Despite the expected nonmagnetic singlet in the strong spin-orbit coupling (SOC) limit for ${\mathrm{Os}}^{4+}$ ($5d^4$), ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ exhibits a spin-glass ground state below 4 K with weak magnetism, suggesting possible proximity to a quantum phase transition between the nonmagnetic state in the strong SOC limit and a magnetic state in the strong superexchange limit. ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ has the same structural distortion as in ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$; however, the Os sublattice in ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ shows long-range magnetic ordering below 36 K. The sharp difference of the magnetic ground state between ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ and ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ signals that the singlet ground-state magnetism in $R_2{\mathrm{Os}}_2{\mathrm{O}}_7$ is fragile and can be disturbed by the weak $4f-5d$ interactions.

This work provides a detailed investigation of the measured in-plane field-swept homodyne-detected ferromagnetic resonance (FMR) spectra of an extended Co/Cu/NiFe pseudo-spin-valve stack using a nanocontact (NC) geometry. The magnetodynamics are generated by a pulse-modulated microwave current, and the resulting rectified dc mixing voltage, which appears across the NC at resonance, is detected using a lock-in amplifier. Most notably, we find that the measured spectra of the NiFe layer are composite in nature and highly asymmetric, consistent with the broadband excitation of multiple modes. Additionally, the data must be fit with two Lorentzian functions in order to extract a reasonable value for the Gilbert damping of the NiFe. Aided by micromagnetic simulations, we conclude that (i) for in-plane fields the rf Oersted field in the vicinity of the NC plays the dominant role in generating the observed spectra, (ii) in addition to the FMR mode, exchange-dominated spin waves are also generated, and (iii) the NC diameter sets the mean wave vector of the exchange-dominated spin wave, in good agreement with the dispersion relation.

We have used scanning electron microscopy with polarization analysis and photoemission electron microscopy to image the two-dimensional magnetization of permalloy films patterned into Penrose P2 tilings (P2T). The interplay of exchange interactions in asymmetrically coordinated vertices and short-range dipole interactions among connected film segments stabilize magnetically ordered, spatially distinct sublattices that coexist with frustrated sublattices at room temperature. Numerical simulations that include long-range dipole interactions between sublattices agree with images of as-grown P2T samples and predict a magnetically ordered ground state for a two-dimensional quasicrystal lattice of classical Ising spins.

We consider a short Josephson junction with a phase discontinuity $\kappa$ created, e.g., by a pair of tiny current injectors, at some point $x_0$ along the width of the junction. We derive the effective current-phase relation (CPR) for the system as a whole, i.e., reduce it to an effective pointlike junction. From the effective CPR we obtain the ground state of the system and predict the dependence of its critical current on $\kappa$. We show that in a large range of $\kappa$ values the effective junction behaves as a ${\phi }_0$ Josephson junction, i.e., has a unique ground state phase ${\phi }_0$ within each $2\pi$ interval. For $\kappa \approx \pi$ and $x_0$ near the middle of the junction one obtains a ${\phi }_0\pm \phi$ junction, i.e., a Josephson junction with degenerate ground state phase ${\phi }_0\pm \phi$ within each $2\pi$ interval. Further, in view of possible escape experiments especially in the quantum domain, we investigate the scaling of the energy barrier and eigenfrequency close to the critical currents and predict the behavior of the escape histogram width $\sigma \left(\kappa \right)$ in the regime of the macroscopic quantum tunneling.

The current density-voltage ($J-V$) characteristics of hole-only diodes based on poly(2-methoxy, 5-(2′ ethyl-hexyloxy)-$p$-phenylene vinylene) (MEH-PPV) were measured at a wide temperature and field range. At high electric fields the temperature dependence of the transport vanishes, and all $J-V$ sweeps converge to a power law. Nuclear tunneling theory predicts a power law at high fields that scales with the Kondo parameter. To model the $J-V$ characteristics we have performed master-equation calculations to determine the dependence of charge carrier mobility on electric field, charge carrier density, temperature, and Kondo parameter, using nuclear tunneling transfer rates. We demonstrate that nuclear tunneling, unlike other semiclassical models, provides a consistent description of the charge transport for a large bias, temperature, and carrier density range.

We present the results of transverse-field muon-spin rotation measurements on an epitaxially grown 40-nm-thick film of MnSi on Si(111) in the region of the field-temperature phase diagram where a skyrmion phase has been observed in the bulk. We identify changes in the quasistatic magnetic field distribution sampled by the muon, along with evidence for magnetic transitions around $T\approx 40$ and 30 K. Our results suggest that the cone phase is not the only magnetic texture realized in film samples for out-of-plane fields.

The physical properties and structural evolution of the $M{X}_2$-type solid solution ${\mathrm{Au}}_{1-x}{\mathrm{Pd}}_x{\mathrm{Te}}_2$ are reported. The end member ${\mathrm{AuTe}}_2$ is a normal metal with a monoclinic distorted ${\mathrm{CdI}}_2$-type structure with preformed Te-Te dimers. A monoclinic-trigonal structural phase transition at a finite temperature occurs upon Pd substitution and is suppressed to zero temperature near $x=0.55$, and a superconducting phase with a maximum $T_{\mathrm{c}}=4.65$ K emerges. A clear indication of strong-coupling superconductivity is observed near the composition of the structural instability. The competitive relationship between Te-Te dimers and superconductivity is proposed.

There are three distinct classes of perovskite structured metal oxides, defined by the charge states of the cations: $A^{\text{I}}{B}^{\text{V}}{\mathrm{O}}_3,A^{\text{II}}{B}^{\text{IV}}{\mathrm{O}}_3$, and $A^{\text{III}}{B}^{\text{III}}{\mathrm{O}}_3$. We investigated the stability of cubic quaternary solid solutions $AB{\mathrm{O}}_3\text{−}{A}^{\prime }{B}^{\prime }{\mathrm{O}}_3$ using a model of point-charge lattices. The mixing enthalpies were calculated and compared for the three possible types of combinations of the compounds, both for the random alloys and the ground-state-ordered configurations. The mixing enthalpy of the (I,V)${\mathrm{O}}_3$-(III,III)${\mathrm{O}}_3$ alloy is always larger than the other alloys. We found that, different from homovalent alloys, for these heterovalent alloys a lattice constant mismatch between the constituent compounds could contribute to stabilize the alloy. At low temperatures, the alloys present a tendency to spontaneous ordering, forming superlattices consisting of alternated layers of ${AB\text{O}}_3$ and $A^{\prime }{B}^{\prime }{\mathrm{O}}_3$ along the $\left[110\right]$ direction.

We study the breakdown of an interaction-induced insulator under an imposed bias voltage. A rich voltage-temperature phase diagram is found that contains phases with a spatially patterned charge gap. Nonequilibrium conditions are shown to be able to change the antiferromagnetic nature of the equilibrium correlations. Above a threshold voltage, smaller than the charge gap, the formation of patterns occurs together with the emergence of midgap states yielding a finite conductance. We discuss the experimental implications of this proposed scenario for the breakdown of the insulating state.

Resonant photon modes of a 5-mm-diameter yttrium iron garnet (YIG) sphere loaded in a cylindrical cavity in the 10–30-GHz frequency range are characterized as a function of applied dc magnetic field at millikelvin temperatures. The photon modes are confined mainly to the sphere and exhibited large mode filling factors in comparison to previous experiments, allowing ultrastrong coupling with the magnon spin-wave resonances. The largest observed coupling between photons and magnons is $2g/2\pi =7.11$ GHz for a 15.5-GHz mode, corresponding to a cooperativity of $C=1.51\pm 0.47\times {10}^7$. Complex modifications, beyond a simple multioscillator model, of the photon mode frequencies were observed between 0 and 0.1 T. Between 0.4 and 1 T, degenerate resonant photon modes were observed to interact with magnon spin-wave resonances with different coupling strengths, indicating time-reversal symmetry breaking due to the gyrotropic permeability of YIG. Bare dielectric resonator mode frequencies were determined by detuning magnon modes to significantly higher frequencies with strong magnetic fields. By comparing measured mode frequencies at 7 T with finite element modeling, a bare dielectric permittivity of $15.96\pm 0.02$ of the YIG crystal has been determined at about 20 mK.

By means of one-step model calculations the strong in-plane anisotropy seen in angle-resolved photoemission of the well-known iron pnictide prototype compounds ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and ${\mathrm{Ba}(\mathrm{Fe}}_{1-\mathit{x}}{\mathrm{Co}}_{\mathit{x}}{)}_2{\mathrm{As}}_2$ in their low-temperature antiferromagnetic phases is investigated. The fully relativistic calculations are based on the Korringa-Kohn-Rostoker–Green function approach combined with the coherent potential approximation alloy theory to account for the disorder induced by Co substitution on Fe sites in a reliable way. The results of the calculations can be compared directly to experimental spectra of detwinned single crystals. One finds very good agreement with experiment and can reveal all features of the electronic structure contributing to the in-plane anisotropy. In particular the local density approximation can capture most of the correlation effects for the investigated system without the need for more advanced techniques. In addition, the evolution of the anisotropy for increasing Co concentration $x$ in ${\mathrm{Ba}(\mathrm{Fe}}_{1-\mathit{x}}{\mathrm{Co}}_{\mathit{x}}{)}_2{\mathrm{As}}_2$ can be tracked almost continuously. The results are also used to discuss surface effects and it is possible to identify clear signatures to make conclusions about different types of surface termination.

We develop a scheme to make exactly solvable gauge theories whose electric flux lines host (1+1)-dimensional topological phases. We use this exact “decorated-string-net” framework to construct several classes of interesting models. In particular, we construct an exactly solvable model of a quantum spin liquid whose (gapped) elementary excitations form doublets under an internal symmetry, and hence may be regarded as spin-carrying spinons. The model may be formulated, and is solvable, in any number of dimensions on any bipartite graph. Another example, in any dimension, has ${\mathbb{Z}}_2$ topological order and anyons which are Kramers' doublets of time-reversal symmetry. Further, we make exactly solvable models of three-dimensional topological paramagnets.

The discovery of the charge-density-wave formation in the high-$T_c$ cuprate superconductors has activated intensive theoretical studies for the pseudogap states. However, the microscopic origin of the charge-density-wave state has been unknown so far since the many-body effects beyond the mean-field-level approximations, called the vertex corrections, are essential. Toward solving this problem, we employ the recently developed functional renormalization group method, by which we can calculate the higher-order vertex corrections in a systematic and unbiased way with high numerical accuracy. We discover the critical development of the $p$-orbital-density-wave ($p$-ODW) instability in the strong-spin-fluctuation region. The obtained $p$-ODW state possesses the key characteristics of the charge-ordering pattern in Bi- and Y-based superconductors, such as the wave vector parallel to the nearest Cu-Cu direction, and the $d$-symmetry form factor with the antiphase correlation between $p_x$ and $p_y$ orbitals in the same unit cell. In addition, from the observation of the beautiful scaling relation between the spin susceptibility and the $p$-ODW susceptibility, we conclude that the main driving force of the density wave is the Aslamazov-Larkin vertex correction that becomes very singular near the magnetic quantum-critical point.

We investigate superconducting order in the extended Hubbard model on the two-dimensional graphene lattice using the variational cluster approximation with an exact diagonalization solver at zero temperature. Building on the results of Faye et al. [Phys. Rev. B 92, 085121 (2015)], which identified triplet $p$- and $\left(p+ip\right)$-wave superconductivity as the most favored pairing symmetries in that model, we place uniform superconducting solutions in competition with a nonuniform Kekulé $(p+ip$-K) superconducting pattern, similar to those proposed by Roy and Herbut [Phys. Rev. B 82, 035429 (2010)]. We find that the $p+ip$-K solution is in fact the most favored pairing in most of the phase diagrams. In addition, we show that antiferromagnetism can coexist with the $p+ip$-K state and that both orders are enhanced by their coexistence.

We have investigated the underlying magnetism in newly discovered single crystal Kondo lattices $\mathrm{Ce}X{\mathrm{Al}}_4{\mathrm{Si}}_2$, where $X$ = Rh, Ir. We show that the compound undergoes an incommensurate-to-commensurate magnetic transition at $T_c=9.19$ K (10.75 K in Ir). The spin correlation in the incommensurate phase is described by a spin density wave configuration of Ce ions, which locks in to the long-range antiferromagnetic order at $T=T_c$. The analysis of the experimental data, combined with the calculation of the electronic properties, suggests the role of the Fermi surface nesting as the primary mechanism behind this phenomenon.