We report systematic diffuse x-ray scattering measurements of the elastic strain fields generated by oxygen vacancies in strained bulk ${\mathrm{SrTiO}}_3$ crystals. Diffuse x-ray scattering has been used for decades to measure the elastic strain fields generated by point defects based on the assumptions that the defects are randomly distributed and uncorrelated. We find for ${\mathrm{SrTiO}}_3$ that (i) there are correlations between oxygen vacancies with different orientations, (ii) we can manipulate the relative concentrations of the three orientations via applied stress, and (iii) the oxygen vacancies cluster around the edge dislocations, creating a two-dimensional distribution.

Thermal conductivity reduction is one of the potential routes to improve the performance of thermoelectric materials. However, detailed understanding of the thermal transport of many promising materials is still missing. In this paper, we employ electronic-structure calculations at the level of density functional theory to elucidate thermal transport properties of the $\mathrm{M}{\mathrm{g}}_{2}X$ ($X=\mathrm{C}$, Si, Ge, Sn, and Pb) family of compounds, which includes $\mathrm{M}{\mathrm{g}}_2\mathrm{Si}$, a material already identified as a potential thermoelectric. All these materials crystallize into the same antifluorite structure. Systematic trends in the anharmonic properties of these materials are presented and examined. Our calculations indicate that the reduction in the group velocity is the main driver of the thermal conductivity trend in these materials, as the phonon lifetimes in these compounds are very similar. We also examine the limits of the applicability of perturbation theory to study the effect of point defects on thermal transport and find that it is in good agreement with experiment in a wide range of scattering parameter values. The thermal conductivity of the recently synthesized $\mathrm{M}{\mathrm{g}}_2\mathrm{C}$ is computed and predicted to be 34 W/mK at 300 °C.

A two-dimensional honeycomb lattice of graphene, if heavily doped with electrons or holes, has been predicted to possess a wealth of fascinating properties including high-temperature superconductivity. Although such a material is possible with high concentration of N or B substitution, its experimental realization has been hindered due to its dynamic instability. Using density functional theory combined with a global structural search and phonon dispersion calculations, we show that an ordered $50\%$ N- or B-doped graphene can be made energetically and dynamically stable by simultaneous doping carriers and applying biaxial tensile strain; carrier doping moves the system toward aromaticity while tensile strain reduces adverse effects associated with electrostatic interaction. Electron-phonon coupling calculations show that the N- or B-doped graphene is superconducting with critical temperature reaching above the melting point of nitrogen in the case of $50\%$ N-doped graphene. In addition, the ideal strength of N-doped graphene is even higher than that of pure graphene.

There is increasing evidence supporting the strong potential of twin walls in ferroic materials as distinct, spatially tunable, functional elements in future electronic devices. Here, we report an increase of about one order of magnitude in conductivity and more robust magnetic interactions at (100)-type twin walls in $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ thin films. The nature and microscopic origin of such distinctive behavior is investigated by combining conductive, magnetic, and force modulation scanning force microscopies with transmission electron microscopy techniques. Our analyses indicate that the observed behavior is due to a severe compressive strained state within an $\sim 1\mathrm{nm}$ slab of material centered at the twin walls, promoting stronger Mn $3d-\mathrm{O}2p$ orbital overlapping leading to a broader bandwidth and enhanced magnetic interactions.

Motivated by the recent discovery of high-temperature antiferromagnet ${\mathrm{SrRu}}_2{\mathrm{O}}_6$ [Hiley et al., Angew. Chem. Int. Ed. 53, 4423 (2014); Tian et al., arXiv:1504.03642] and its potential to be the parent of a new superconductor upon doping, we construct a minimal $t_{2g}$-orbital model on a honeycomb lattice to simulate its low-energy band structure. Local Coulomb interaction is taken into account through both random phase approximation and mean-field theory. Experimentally observed antiferromagnetic order is obtained in both approximations. In addition, our theory predicts that the magnetic moments on three $t_{2g}$-orbitals are noncollinear as a result of the strong spin-orbit coupling of Ru atoms.

Recently, a general formalism was presented for gyrotropic, ferroelectric, and multipolar order in spin-orbit coupled metals induced by spin-spin interactions. Here, I point out that the resulting order parameters are equivalent to expectation values of operators that determine natural circular dichroic signals in optical and x-ray absorption. Some general properties of these operator equivalents and the resulting dichroisms are mentioned, and I list several material examples in this connection, including Weyl semimetals. The particular case of the tensor order in the pyrochlore superconductor ${\mathrm{Cd}}_2{\mathrm{Re}}_2{\mathrm{O}}_7$ is treated in more detail, including calculations of the x-ray absorption and circular dichroism at the O $K$ edge.

We show that excitons in $K$ and $K^{\prime }$ valleys of transition metal dichalcogenide monolayers can be entangled with respect to their valley degree of freedom by absorbing linearly polarized single photons. This effect does not require any interaction between $K$ and $K^{\prime }$ excitons in contrast to conventional mechanisms of entanglement that are mediated by coupling between quantum systems (e.g., entanglement of photons in nonlinear optical interactions). The valley entanglement of excitons and free carriers can be verified by measuring the polarization of their photoluminescence or fluctuations of the photocurrent under an applied in-plane dc bias.

The observation of extensive fractional quantum Hall states in graphene brings out the possibility of more accurate quantitative comparisons between theory and experiment than previously possible, because of the negligibility of finite width corrections. We obtain an accurate phase diagram for differently spin-polarized fractional quantum Hall states, and also estimate the effect of Landau level mixing using the modified interaction pseudopotentials given in the literature. We find that the observed phase diagram is in good quantitative agreement with theory that neglects Landau level mixing, but the agreement becomes significantly worse when Landau level mixing is incorporated assuming that the corrections to the energies are linear in the Landau level mixing parameter $\lambda$. This implies that a first order perturbation theory in $\lambda$ is inadequate for the current experimental systems, for which $\lambda$ is typically on the order of or greater than one. We also test the accuracy of the composite-fermion theory and find that all lowest Landau level projection methods used in the literature are very accurate for the states of the form $n/(2n+1)$ but for the states at $n/(2n-1)$ the results are more sensitive to the projection method. An earlier prediction of an absence of spin transitions for the $n/(4n+1)$ states is confirmed by more rigorous calculations, and new predictions are made regarding spin physics for the $n/(4n-1)$ states.

Motivated by the recently developed renormalized second-order perturbation theory for ground-state energy calculations, we propose a second-order screened exchange correction (SOSEX) to the $GW$ self-energy. This correction follows the spirit of the SOSEX correction to the random-phase approximation for the electron correlation energy and can be clearly represented in terms of Feynman diagrams. We benchmark the performance of the perturbative $G_{\text{0}}{W}_{\text{0}}$+SOSEX scheme for a set of molecular systems, including the G2 test set from quantum chemistry as well as benzene and tetracyanoethylene. We find that $G_{\text{0}}{W}_{\text{0}}$+SOSEX improves over $G_{\text{0}}{W}_{\text{0}}$ for the energy levels of the highest occupied and lowest unoccupied molecular orbitals. In addition, it can resolve some of the difficulties encountered by the $GW$ method for relative energy positions as exemplified by benzene where the energy spacing between certain valence orbitals is severely underestimated.

Polarized cross-correlation spectroscopy on a quantum dot charged with a single hole shows the sequential emission of photons with common circular polarization. This effect is visible without magnetic field, but becomes more pronounced as the field along the quantization axis is increased. We interpret the data in terms of electron dephasing in the $X^{+}$ state caused by the Overhauser field of nuclei in the dot. We predict the correlation time scale can be increased by accelerating the emission rate with cavity QED.

We study interaction effects on the topological crystalline insulators protected by time-reversal $\left(T\right)$ and reflection symmetry $\left(R\right)$ in two and three spatial dimensions. From the stability analysis of the edge states with bosonization, we find that the classification of the two-dimensional symmetry-protected topological (SPT) phases protected by $Z_2\times [\text{U(1)}⋊T]$ symmetry is reduced from $\mathbb{Z}$ to ${\mathbb{Z}}_4$ by interactions where the $Z_2$ symmetry denotes the reflection whose mirror plane is the two-dimensional plane itself. By extending the approach recently proposed by Isobe and Fu, we show that the classification of the three-dimensional SPT phases (i.e., topological crystalline insulators) protected by $R\times [\text{U(1)}⋊T]$ symmetry is reduced from $\mathbb{Z}$ to ${\mathbb{Z}}_8$ by interactions.

We examine the electronic properties of two-dimensional electron gas in black phosphorus multilayers in the presence of a perpendicular magnetic field, highlighting the role of in-plane anisotropy on various experimental quantities such as ac magnetoconductivity, screening, and magnetoplasmons. We find that resonant structures in the ac conductivity exhibits a redshift with increasing doping due to interband coupling $\gamma$. This arises from an extra correction term in the Landau energy spectrum proportional to $n^2{\gamma }^2$ $(n$ is Landau index), up to second order in $\gamma$. We found also that Coulomb interaction leads to highly anisotropic magnetoexcitons.

The prototypical exciton model of two interacting Dirac particles in graphene was analyzed in J. Sabio et al., Phys. Rev. B 81, 045428 (2010) and it was found that in one of the electron-hole scattering channels the total kinetic energy vanishes, resulting in a singular behavior. We show that this singularity can be removed by extending the quasiparticle dispersion, thus breaking the symmetry between upper and lower Dirac cones. The dynamics of an electron-electron pair are then mapped onto that of a single particle with negative mass and anisotropic dispersion. We show that the interplay between dispersion and repulsive interaction can result in the formation of bound, Cooper-pair-like, metastable states in double-layered hybrid structures.

Aging is a common feature of the glassy state. In the case of phase-change chalcogenide alloys the aging of the amorphous state is responsible for an increase of the electrical resistance with time. This phenomenon called drift is detrimental in the application of these materials in phase-change nonvolatile memories, which are emerging as promising candidates for storage class memories. By means of combined molecular dynamics and electronic structure calculations based on density functional theory, we have unraveled the atomistic origin of the resistance drift in the prototypical phase-change compound GeTe. The drift results from a widening of the band gap and a reduction of Urbach tails due to structural relaxations leading to the removal of chains of Ge-Ge homopolar bonds. The same structural features are actually responsible for the high mobility above the glass transition which boosts the crystallization speed exploited in the device.

We report the propagation of high-intensity sound beams in a sonic crystal, under self-collimation or reduced-divergence conditions. The medium is a fluid with elastic quadratic nonlinearity, where the dominating nonlinear effect is harmonic generation. The conditions for the efficient generation of narrow, nondiverging beam of second harmonic are discussed. Numerical simulations are in agreement with the analytical predictions made, based on the linear dispersion characteristics in modulated media and the nonlinear interaction in a quadratic medium under phase matching conditions.

By definition, the physics of the $d$-dimensional (dim) boundary of a $(d+1)$-dim symmetry protected topological (SPT) state cannot be realized as itself on a $d$-dim lattice. If the symmetry of the system is unitary, then a formal way to determine whether a $d$-dim theory must be a boundary or not, is to couple this theory to a gauge field (or to “gauge” its symmetry), and check if there is a gauge anomaly. In this paper we discuss the following question: Can the boundary of a SPT state be driven into a fully gapped topological order which preserves all the symmetries? We argue (conjecture) that if the gauge anomaly of the boundary is “perturbative,” then the boundary must remain gapless; while if the boundary only has global gauge anomaly but no perturbative anomaly, then it is possible to gap out the boundary by driving it into a topological state, when $d\ge 2$. We will demonstrate this conjecture with two examples: (1) the $3d$ spin-1/2 chiral fermion with the well-known Witten's global anomaly [Phys. Lett. 117, 324 (1982)], which can be realized on the boundary of a $4d$ topological superconductor with SU(2) or $U\left(1\right)⋊Z_2$ symmetry; and (2) the $4d$ boundary of a $5d$ topological superconductor with the same symmetry. We show that these boundary systems can be driven into a fully gapped ${\mathbb{Z}}_{2N}$ topological order with topological degeneracy, but this ${\mathbb{Z}}_{2N}$ topological order cannot be future driven into a trivial confined phase that preserves all the symmetries due to some special properties of its topological defects. Our study also leads to exotic states of matter in pure $3d$ space.

We present a study of the scaling behavior of the Rényi entanglement entropy (REE) in $\mathrm{SU}\left(N\right)$ spin chain Hamiltonians, in which all of the spins transform under the fundamental representation. These $\mathrm{SU}\left(N\right)$ spin chains are known to be quantum critical and described by a well known Wess-Zumino-Witten (WZW) nonlinear sigma model in the continuum limit. Numerical results from our lattice Hamiltonian are obtained using stochastic series expansion quantum Monte Carlo for both closed and open boundary conditions. As expected for this 1D critical system, the REE shows a logarithmic dependence on the subsystem size with a prefactor given by the central charge of the $\mathrm{SU}\left(N\right)$ WZW model. We study in detail the subleading oscillatory terms in the REE under both periodic and open boundaries. Each oscillatory term is associated with a WZW field and decays as a power law with an exponent proportional to the scaling dimension of the corresponding field. We find that the use of periodic boundaries (where oscillations are less prominent) allows for a better estimate of the central charge, while using open boundaries allows for a better estimate of the scaling dimensions. We also present numerical data on the thermal Rényi entropy which equally allows for extraction of the central charge.

A hierarchical multiscale approach to model the magnetization dynamics of ferromagnetic random alloys is presented. First-principles calculations of the Heisenberg exchange integrals are linked to atomistic spin models based upon the stochastic Landau-Lifshitz-Gilbert (LLG) equation to calculate temperature-dependent parameters (e.g., effective exchange interactions, damping parameters). These parameters are subsequently used in the Landau-Lifshitz-Bloch (LLB) model for multisublattice magnets to calculate numerically and analytically the ultrafast demagnetization times. The developed multiscale method is applied here to FeNi (permalloy) as well as to copper-doped FeNi alloys. We find that after an ultrafast heat pulse the Ni sublattice demagnetizes faster than the Fe sublattice for the here-studied FeNi-based alloys.