We present a method for efficiently recomputing rates in a kinetic Monte Carlo simulation when the existing rate catalog is modified by the presence of a strain field. We use the concept of the dipole tensor to estimate the changes in the kinetic barriers that comprise the catalog, thereby obviating the need for recomputing them from scratch. The underlying assumptions in the method are that linear elasticity is valid, and that the topology of the underlying potential energy surface (and consequently, the fundamental structure of the rate catalog) is not changed by the strain field. As a simple test case, we apply the method to a single vacancy in zirconium diffusing in the strain field of a dislocation, and discuss the consequences of the assumptions on simulating more complex materials.

Room-temperature ferromagnetic insulating behavior is demonstrated in cobalt-substituted SrTiO${}_3$ grown on silicon. Ferromagnetism is exhibited by films with a composition of 30–40$\%$ cobalt. Measurements of stoichiometry by x-ray photoelectron spectroscopy indicate Co replacing Ti with the concomitant creation of an approximately equal number of oxygen vacancies as the cobalt ions. First-principles modeling of this system shows that the observed local magnetic moment originates from a cobalt-oxygen vacancy complex with Co in the $+$2 valence state. The calculations also confirm the insulating nature of the material. The ability to deposit a room-temperature ferromagnetic insulator onto silicon may be useful for certain spintronics applications such as spin filters for spin-injection contacts.

We study a class of $\text{SU}\left(N\right)$ Heisenberg models, describing Mott insulators of fermionic ultracold alkaline earth atoms on the three-dimensional simple cubic lattice. Based on an earlier semiclassical analysis, magnetic order is unlikely, and we focus instead on a solvable large-$N$ limit designed to address the competition among nonmagnetic ground states. We find a rich phase diagram as a function of the filling parameter $k$, composed of a variety of ground states spontaneously breaking lattice symmetries, and in some cases also time-reversal symmetry. One particularly striking example is a state spontaneously breaking lattice rotation symmetry, where the cubic lattice breaks up into bilayers, each of which forms a two-dimensional chiral spin liquid state.

We propose an experimentally realizable hybrid quantum circuit for achieving a strong coupling between a spin ensemble and a transmission-line resonator via a superconducting flux qubit used as a data bus. The resulting coupling can be used to transfer quantum information between the spin ensemble and the resonator. In particular, in contrast to the direct coupling without a data bus, our approach requires far less spins to achieve a strong coupling between the spin ensemble and the resonator (e.g., three to four orders of magnitude less). This proposed hybrid quantum circuit could enable a long-time quantum memory when storing information in the spin ensemble, and allows the possibility to explore nonlinear effects in the ultrastrong-coupling regime.

We perform in-plane optical spectroscopy measurement on (00l) FeSe thin film grown on SrTiO${}_3$ substrate by the pulsed laser deposition method. The study indicates that the low-frequency conductivity consists of two Drude components: a broad one which takes up most of the spectral weight and a very narrow one roughly below 100–150 cm${}^{-1}$. The narrow Drude component locates at so low frequencies that no such behavior was observed in iron pnictides. The overall plasma frequency is smaller than that of the FeAs-based compounds, suggesting a stronger correlation effect. Similar to iron pnictides, a temperature-induced spectral weight transfer is observed for FeSe. However, the relevant energy scale is smaller. Additionally, different from a recent angle-resolved photoemission spectroscopy measurement which revealed a spin density wave (SDW) order at low temperature for FeSe thin films grown on SrTiO${}_3$ substrate, no signature of the band structure reconstruction arising from the formation of the SDW order is seen by optical measurement in the thick FeSe films.

In charge-coupled circuit QED systems, transition amplitudes and dispersive shifts are governed by the matrix elements of the charge operator. For the fluxonium circuit, these matrix elements are not limited to nearest-neighbor energy levels and are conveniently tunable by magnetic flux. Previously, their values were largely obtained numerically. Here, we present analytical expressions for the fluxonium charge matrix elements. We show that new selection rules emerge in the asymptotic limit of large Josephson energy and small inductive energy. We illustrate the usefulness of our expressions for the qualitative understanding of charge matrix elements in the parameter regime probed by previous experiments.

We study the charge response of conformal field theories (CFTs) at nonzero temperature in 2+1 dimensions using the anti-de Sitter (AdS)/CFT correspondence. A central role is played by the quasinormal modes (QNMs), specifically, the poles and zeros of the current correlators. We generalize our recent study of the QNMs of the ac charge conductivity to include momentum dependence. This sheds light on the various excitations in the CFT. We begin by discussing the $R$-current correlators of the $\mathcal{N}=8$ SU$\left(N_c\right)$ super-Yang-Mills theory at its conformal fixed point using holography. For instance, transitions in the QNM spectrum as a function of momentum clearly identify “hydrodynamic-to-relativistic” crossovers. We then extend our study to include four-derivative terms in the gravitational description allowing us to study more generic charge response as well as the role of the S duality, which plays a central role in understanding the correlators. The presence of special purely damped QNMs can lead to new behavior, distinct from what occurs in the aforementioned gauge theory. We also extend previous conductivity sum rules to finite momentum and discuss their interpretation in the gravity picture. A comparison is finally made with the conformal fixed point of the vector O($N$) model in the large-$N$ limit.

Loop condensed phases are scale-invariant quantum liquid phases of matter. These phases include topologically ordered liquid phases such as the toric code as well as critical liquids such as the Rokhsar-Kivelson point of the quantum dimer model on the square lattice. To investigate the extent to which nonlocal geometric observables capture a signature of the nonlocal quantum order present in these phases, we compute geometric properties of such loop condensed states using directed loop Monte Carlo calculations. In particular, we investigate the loop condensed nature of ground state of the square lattice quantum dimer model at the Rokhsar-Kivelson point and compare with other loop condensed states on the square lattice, including those of the toric code and fully packed loop model. The common features of such liquids are a scale invariant distribution of loops and a fractal dimensionality of spanning loops. We find that the fractal dimension of the loop condensate of the square lattice quantum dimer model at the Rokhsar-Kivelson point is $3/2$, which provides quantitative confirmation of the effective height model that is commonly used to describe this critical dimer liquid.

As a long-lived theme in solid-state physics, the Kondo effect reflects the many-body physics involving the short-range Coulomb interactions between itinerant electrons and localized spins in metallic materials. Here we show that the Kondo effect is present in ZnO, a prototypical wide-band-gap oxide, doped with a rare-earth element (Gd). The localized 4$f$ electrons of Gd ions do not produce remanent magnetism, but interact strongly with the host electrons, giving rise to a saturating resistance upturn and negative magnetoresistance at low temperatures. Furthermore, the Kondo temperature and resistance can be electrostatically modulated using electric-double-layer gating with liquid ionic electrolyte. Our experiments provide the experimental evidence of tunable Kondo effect in ZnO, underscoring the magnetic interactions between localized and itinerant electrons and the emergent transport behaviors in such doped wide-band-gap oxides.

We investigate the electronic reconstruction at the interface between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in superlattices using a two-orbital double-exchange model including superexchange interactions, Jahn-Teller lattice distortions, and long-range Coulomb interactions. The magnetic and transport properties critically depend on the thickness of the AFI layers. We focus on superlattices where the constituent parent manganites have the same electron density, $n=0.6$. The induced ferromagnetic moment in the AFI layers decreases monotonically with increasing layer width, and the electron-density profile and the magnetic structure in the center of the AFI layer gradually return to the bulk limit. The width of the AFI layers and the charge-transfer profile at the interfaces control the magnitude of the magnetoresistance and the metal-insulator transition of the FM/AFI superlattices.

Temperature dependence of the x-ray absorption near-edge structure (XANES) spectra at the Pr $L_3$ and Tb $L_3$ edges was measured for the (Pr${}_{1-y}$Tb${}_y{)}_{0.7}$Ca${}_{0.3}$CoO${}_3$ system, in which a sharp spin-state (SS) transition took place at a critical temperature $T_{\mathrm{SS}}$. A small increase in the valence of the terbium ion was found below $T_{\mathrm{SS}}$, besides the enhancement of the praseodymium valence; the trivalent states, which are stable at room temperature, change to a 3+/4$+$ ionic mixture at low temperatures. In particular for the $y=0.2$ sample, the average valence determined at 8 K amounts to 3.25$+$ and 3.03$+$ for the Pr and Tb ion, respectively. In analogous (Pr${}_{1-y}{\mathit{RE}}_y{)}_{0.7}$Ca${}_{0.3}$CoO${}_3$ samples ($\mathit{RE}$ = Sm and Eu), in which the SS transition also took place, no valence shift of the $\mathit{RE}$ ion was detected in the XANES spectra at the $\mathit{RE}$ ion $L_3$ edge. The role of the substituted $\mathit{RE}$ ion for the Pr site on the SS transition is discussed.

We show a technique that images the intensity distribution and local state of polarization of the optical field of high-quality factor optical modes confined in dielectric planar photonic crystal nanocavities. Based on energy-loss spectroscopy of swift electrons, the technique gives a spatial resolution improved by a factor of 30 compared to the optical diffraction limit. Moreover, because the energy loss is induced by coupling of the moving charges with the local density of states of the dielectric cavity, it is sensitive to the entire volume of the confined electric field, not just its evanescent contributions. This three-dimensional sensitivity paves the way for a highly resolved tomography of confined modes in dielectric photonic nanostructures.

The doping of graphene to tune its electronic properties is essential for its further use in carbon-based electronics. Adapting strategies from classical silicon-based semiconductor technology, we use the incorporation of heteroatoms in the 2D graphene network as a straightforward way to achieve this goal. Here, we report on the synthesis of boron-doped graphene on Ni(111) in a chemical vapor deposition process of triethylborane on the one hand and by segregation of boron from the bulk of the substrate crystal on the other hand. The chemical environment of boron was determined by x-ray photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy was used to analyze the impact on the band structure. Doping with boron leads to a shift of the graphene bands to lower binding energies. The shift depends on the doping concentration and for a doping level of 0.3 ML a shift of up to 1.2 eV is observed. The experimental results are in agreement with density-functional calculations. Furthermore, our calculations suggest that doping with boron leads to graphene preferentially adsorbed in the top-fcc geometry, since the boron atoms in the graphene lattice are then adsorbed at substrate fcc-hollow sites. The smaller distance of boron atoms incorporated into graphene compared to graphene carbon atoms leads to a bending of the doped graphene sheet in the vicinity of the boron atoms. By comparing calculations of doped and undoped graphene on Ni(111), as well as the respective freestanding cases, we are able to distinguish between the effects that doping and adsorption have on the band structure of graphene. Both doping and bonding to the surface result in opposing shifts on the graphene bands.

A strong-coupling series expansion for the Green's function and the extremely correlated Fermi liquid (ECFL) theory are used to calculate the moments of the electronic spectral functions of the infinite-$U$ Hubbard model. Results from these two complementary methods agree very well at both low densities, where the ECFL solution is the most accurate, and at high to intermediate temperatures, where the series converge. We find that a modified first moment, which underestimates the contributions from the occupied states and is accessible in the series through the time-dependent Green's function, best describes the peak location of the spectral function in the strongly correlated regime. This is examined by the ECFL results at low temperatures, where it is shown that the spectral function is largely skewed towards the occupied states.

Motivated by the rich interplay among electronic correlation, spin-orbit coupling (SOC), crystal-field splitting, and geometric frustrations in the honeycomblike lattice, we systematically investigated the electronic and magnetic properties of Li${}_2$RhO${}_3$. The material is semiconducting with a narrow band gap of $\Delta \sim 78$ meV, and its temperature dependence of resistivity conforms to a three-dimensional variable range hopping mechanism. No long-range magnetic ordering was found down to 0.5 K, due to the geometric frustrations. Instead, single atomic spin-glass behavior below the spin-freezing temperature ($\sim$6 K) was observed and its spin dynamics obeys the universal critical slowing down scaling law. A first-principles calculation suggested it to be a relativistic Mott insulator mediated by both electronic correlation and SOC. With moderate strength of electronic correlation and SOC, our results shed light on the research of the Heisenberg-Kitaev model in realistic materials.

We propose a variational method to calculate the two-hole propagators relevant for Auger spectroscopy in transition-metal oxides. This method can be thought of as an intermediary step between the full solution (which is difficult to generalize to systems with partially filled bands) and the impurity approximation. Like the former, our solution has full translational invariance, and like the latter, it can be generalized to certain types of systems with partially filled bands. Here we compare both our variational approximation and the impurity approximation against the exact solution for a simple one-dimensional model with filled bands. We show that when the energies of the eigenstates residing primarily on the transition-metal ions do not overlap with those of the eigenstates residing primarily on oxygen ions, both approximations are valid but the variational approach is superior.

Mn${}_5$Ge${}_3$(001) thin films grown on Ge(111)-$c(2\times 8)$ reconstructed surfaces were studied by angle-resolved photoemission using synchrotron radiation in the 14–94 eV photon energy range. The results obtained in the $\Gamma$ALM plane and in the $\Gamma$AHK plane are in agreement with simulations starting with band structure calculations based on the density functional theory. This provides a unique validation of band structure calculations for a proper description of the electronic properties of Mn${}_5$Ge${}_3$. Only the spectral feature very close to the Fermi level cannot be well explained by the simulation. This departure is discussed in terms of the three-dimensional nature of the sample and of correlation effects.

Ultra-high-quality factor photonic crystal nanocavities are attracting much attention because they can confine light into an ultra-small region and enable strong interaction between light and matter. One important step toward the realization of quantum information processing with cavity quantum electrodynamics approaches is a robust method that transfers photons among such cavities. We propose and theoretically investigate a scheme to transfer photons between strongly coupled, distant photonic crystal nanocavities by an adiabatic change in the cavity characteristics in the time domain. Adding one intermediate nanocavity for control allows efficient light transfer between objective nanocavities. Because our scheme utilizes adiabatic transfer, its operation is robust for both the control parameters and the control timing.

Motivated by the recently synthesized cubic phase BaRuO${}_3$ under high pressure and high temperature, a thorough study has been conducted on its temperature-dependent electronic properties by using the state-of-the-art ab initio computing framework of density-functional theory combined with dynamical mean-field theory. At ambient condition the cubic phase BaRuO${}_3$ should be a correlated Hund's metal with frozen spin magnetic moment. The spin-spin correlation function and local magnetic susceptibility can be well described by the Curie-Weiss law over a wide temperature range. The calculated low-frequency self-energy functions of Ru$$$4d$ states apparently deviate from the behaviors predicted by Landau Fermi-liquid theory. Beyond that, the low-frequency optical conductivity can be fitted to a power law $\Re \sigma \left(\omega \right)\sim {\omega }^{-0.98}$, which further confirms the non-Fermi-liquid metallic state.