We present a comprehensive investigation of optical parametric oscillation in resonantly excited one-dimensional semiconductor microcavities with embedded quantum wells. Such solid-state structures feature a fine control over light-matter coupling and produce a photonic/polaritonic mode fan that is exploited for the efficient emission of parametric beams. We implement an energy-degenerate optical parametric oscillator with balanced signal and idler intensities via a polarization-inverting mechanism. In this paper, we (i) precisely review the multimode photonic/polaritonic structure of individual emitters, (ii) provide a thorough comparison between experiment and theory, focusing on the power and the threshold dependence on the exciton-photon detuning, (iii) discuss the influence of inhomogeneous broadening of the excitonic transition and finite size, and (iv) find that a large exciton-photon detuning is a key parameter to reach a high output power and a high conversion efficiency. Our study highlights the predictive character of the polariton interaction theory and the flexibility of one-dimensional semiconductor microcavities as a platform to study parametric phenomena.

We examine the role of N environment on persistent photoconductivity (PPC) in GaAs${}_{1-x}$N${}_x$ films. For $x$ > 0.006, significant PPC is observed at cryogenic temperatures, with the PPC magnitude increasing with increasing $x$ due to an increase in the density of N-induced levels. Interestingly, rapid thermal annealing suppresses the PPC magnitude and reduces the N interstitial fraction; thus, the N-induced level is likely associated with N interstitials. PPC is attributed to the photogeneration of carriers from N-induced levels to the conduction-band edge, leading to a modified N molecular bond configuration. With the addition of thermal energy, the ground state configuration is restored; the N-induced level is then able to accept carriers and the conductivity decays to its preillumination value.

The morphology of cobalt, palladium, and platinum nanoclusters grown on a gold surface is analyzed from both thermodynamic and kinetic viewpoints. Although the thermodynamic equilibrium shape as a function of cluster size is similar for all three elements and shows a morphology transition from monolayer to bilayer, only Co clusters meet their stable state and undergo a transition. Atomistic simulations on a picosecond to nanosecond time scale evidence kinetic limitations for Pt and Pd, and allow us to understand the experimentally observed morphology for the different species. It is shown that stress relaxation, by strongly influencing the energy activation for atom hopping from first to second cluster layer and the magnitude of vibration of the atoms, is the determinant parameter for the existence or absence of the cluster morphology transition.

Quantum phase transitions (QPTs) in qubit systems are known to produce singularities in the entanglement, which could in turn be used to probe the QPT. Recent proposals have suggested that the QPT in a spin chain could be probed via the entanglement between external qubits coupled to the spin chain. Such experiments may be technically challenging, because the probe qubits are nonlocal. Here we show that a double quantum dot coupled locally to a spin chain provides an alternative and efficient probe of a QPT. To demonstrate this method in a simple geometry, we propose an experiment to observe a QPT in a triple quantum dot, based on the well-known singlet projection technique.

We discuss the relation between particle number conservation and topological phases. In four spatial dimensions, we find that systems belonging to different topological phases in the presence of a $\text{U}(1)$ charge conservation can be connected adiabatically, i.e., without closing the gap, upon intermediately breaking this local symmetry by a superconducting term. The time reversal preserving topological insulator states in two and three dimensions which can be obtained from the four-dimensional parent state by dimensional reduction inherit this protection by charge conservation. Hence, all topological insulators can be adiabatically connected to a trivial insulating state without breaking time reversal symmetry, provided an intermediate superconducting term is allowed during the adiabatic deformation. Conversely, in one spatial dimension, non-symmetry-protected topological phases occur only in systems that break $\text{U}(1)$ charge conservation. These results can intuitively be understood by considering a natural embedding of the classifying spaces of charge conserving Hamiltonians into the corresponding Bogoliubov–de Gennes classes.

We present optically tunable magnetic three-dimensional (3D) metamaterials at terahertz (THz) frequencies which exhibit a tuning range of $\sim$30$\%$ of the resonance frequency. This is accomplished by fabricating 3D array structures consisting of double-split-ring resonators (DSRRs) on silicon on sapphire, fabricated using multilayer electroplating. Photoexcitation of free carriers in the silicon within the capacitive region of the DSRR results in a redshift of the resonant frequency from 1.74 to 1.16 THz. The observed frequency shift leads to a transition from a magnetic-to-bianisotropic response as verified through electromagnetic simulations and parameter retrieval. Our approach extends dynamic metamaterial tuning to magnetic control, and may find applications in switching and modulation, polarization control, or tunable perfect absorbers.

We present experimental data and first-principles calculations of the x-ray resonant magnetic scattering (XRMS) line shapes of Er and Tm metals. Band structure calculations are presented that illustrate the existence of 4$f$/5$d$ hybridization of unoccupied electron levels. This leads to spin and orbital features in the XRMS spectra that are not representative of the 5$d$ occupied moments. General trends through the heavy rare earth series are discussed with reference to previous work.

Ferropericlase (Mg,Fe)O is a major lower mantle mineral, and studying its properties is a fundamental step toward understanding the Earth's interior. Here, we performed a first-principles investigation on the properties of iron as an isolated impurity in magnesium oxide, which is the condition of ferropericlase under which iron-iron interactions could be neglected. The calculations were carried using the all-electron full-potential linearized augmented plane wave method within the density functional theory in the generalized gradient approximation plus the on-site Hubbard correction. We present the electronic and magnetic properties, electric and magnetic hyperfine splitting of this impurity in high and low spin states for several charge states at zero pressure, which were then extended to high pressures. For the impurity in the neutral charge state, our results indicated that there is a metastable intermediate spin state ($S=1$), in addition to the high ($S=2$) and low ($S=0$) spin states. Those results were discussed in the context of an intermediate spin state, experimentally identified in ferrosilicate perovskite.

Based on the semiclassical model Hamiltonian of the surface plasmon polariton and the nonequilibrium Green-function approach, we present a microscopic kinetic theory to study the influence of the electron scattering on the dynamics of the surface plasmon polariton in semiconductor nanowires. The damping of the surface plasmon polariton originates from the resonant absorption by the electrons (Landau damping), and the corresponding damping exhibits size-dependent oscillations and distinct temperature dependence without any scattering. The scattering influences the damping by introducing a broadening and a shifting to the resonance. To demonstrate this, we investigate the damping of the surface plasmon polariton in InAs nanowires in the presence of the electron-impurity, electron-phonon, and electron-electron Coulomb scatterings. The main effect of the electron-impurity and electron-phonon scatterings is to introduce a broadening, whereas the electron-electron Coulomb scattering can not only cause a broadening, but also introduce a shifting to the resonance. For InAs nanowires under investigation, the broadening due to the electron-phonon scattering dominates. As a result, the scattering has a pronounced influence on the damping of the surface plasmon polariton: the size-dependent oscillations are smeared out and the temperature dependence is also suppressed in the presence of the scattering. These results demonstrate the important role of the scattering on the surface plasmon polariton damping in semiconductor nanowires.

Based on microscopic calculations, we predict a transient femtosecond population inversion in graphene suggesting graphene as a new active gain material covering a broad frequency range. In this paper, we microscopically shed light on the underlying elementary many-particle processes: Transient gain and population inversion occur due to an interplay of strong optical pumping and carrier cooling that fills states close to the Dirac point giving rise to a relaxation bottleneck. The subsequent femtosecond decay of the optical gain is mainly driven by Coulomb-induced Auger recombination. Our findings are in excellent agreement with recent experimental data.

We study the effect of a finite proximity superconducting (SC) coherence length in SN and SNS junctions consisting of a semiconducting topological insulating wire whose ends are connected to either one or two $s$-wave superconductors. We find that such systems behave exactly as SN and SNS junctions made from a single wire for which some regions are sitting on top of superconductors, the size of the topological SC region being determined by the SC coherence length. Moreover, we study the effects of continuous phase gradients in both an open and closed (ring) SNS junction. We find that such phase gradients play an important role in the spatial localization of the Majorana fermions.

Single-layer materials open up tremendous opportunities for nanoelectronic devices. Using a first-principles design approach we identify a previously unrecognized family of single-layer III-V materials. We determine their energetic and dynamical stability, identify a surprising reconstruction, and calculate their electronic properties using a hybrid density functional and the $G_0{W}_0$ method. Finally, we find that metal substrates stabilize these as-yet hypothetical materials. Our results provide guidance for experimental synthesis efforts and future searches of single-layer materials suitable for device applications.

Gauge symmetries have been identified in graphene and associated with specific physical properties. For instance, the $U\left(1\right)$ gauge group is related to electrodynamics in $(1+2)$-dimensional [$(1+2)$D] space-time and non-Abelian gauge groups can describe curvature and torsion. Here we demonstrate that the Dirac Lagrangian for massless electrons near the Dirac points is also invariant under the group $SU\left(2\right)$ related to local spin rotations, leading to the correct spin-orbit interactions and a rigorous definition for the spin-current density. Furthermore, we computed the charge and spin conductivity within the framework of Kubo linear response theory, using the algebra of relativistic Dirac spinors in $(1+2)$D space-time. The minimal value of electrical conductivity is predicted to be $\pi q^2/h$, in agreement with typical experimental findings.

Co nanotubes (NTs) were electrodeposited inside nanoporous alumina templates by a potentiostatic method and then left in air for the natural oxidation of their inner walls. Temperature-dependent magnetic measurements evidenced the existence of exchange-bias (EB) coupling between the Co ferromagnetic (FM) outer wall and the CoO antiferromagnetic (AFM) inner wall of the NTs. The values of the EB field measured at 6 K were found in the same order of magnitude as those already reported for Co/CoO bilayer thin films and core-shell nanoparticles. The effect of temperature and cooling field on the EB effect between the Co/CoO walls of the NTs is here reported. The training effect of the EB in the Co/CoO NT arrays, measured at 6 K, suggests the existence of a stronger coupling between the AFM and the FM layers that are shaped into a core-shell nanotubular structure, when compared with flat interfaces. A bimodal distribution of blocking temperatures is also found when applying the magnetic field parallel and perpendicular to the NTs’ axis.

Here we discuss how to account for short-range spin correlations on the measured muon spin relaxation spectra. The shape of the zero-field relaxation function is found to be highly sensitive to short-range spin correlations, as clearly inferred from the analysis of the published longitudinal relaxation function for the pyrochlore quantum spin-liquid system Yb${}_2$Ti${}_2$O${}_7$ at low temperature. The field distribution at the muon site in this compound is found to be strongly asymmetric, i.e., to deviate substantially from the usually expected Gaussian function. This is a clear signature of short-range correlations of the spins. Our analysis yields the characteristic function of the component field distribution at the muon site of Yb${}_2$Ti${}_2$O${}_7$, a well-defined statistical physics quantity accessible to theory once an interaction Hamiltonian is available and the muon site is determined. As a by-product of our study, we have found that the validity of the so-called golden formula due to Kubo is limited.

In this work we show that a tunable coupling between microwave resonators can be engineered by means of simple Josephson junctions circuits, such as dc and rf superconducting quantum interference devices. We show that by controlling the time dependence of the coupling it is possible to switch on and off and modulate the cross-talk and boost the interaction towards the ultrastrong regime, as well as to engineer red and blue sideband couplings, nonlinear photon hopping, and classical gauge fields. We discuss how these dynamically tunable superconducting circuits enable key applications in the fields of all-optical quantum computing, continuous-variable quantum information, and quantum simulation—all within the reach of the state of the art in circuit-QED experiments.

Slow noise processes, with characteristic timescales $\sim$1 s, have been studied in planar superconducting resonators. A frequency-locked loop is employed to track deviations of the resonator center frequency with high precision and bandwidth. Comparative measurements are made in varying microwave drive and temperature, and between bare resonators and those with an additional dielectric layer. All resonators are found to exhibit flicker frequency noise which increases with decreasing microwave drive. We also show that an increase in temperature results in a saturation of flicker noise in resonators with an additional dielectric layer, while bare resonators stop exhibiting flicker noise, instead showing a random frequency walk process.

Quasiparticle recombination in a superconductor with an $s$-wave gap is typically dominated by a phonon bottleneck effect. We have studied how a magnetic field changes this recombination process in metallic thin-film superconductors, finding that the quasiparticle recombination process is significantly slowed as the field increases. The magnetic field disrupts the time-reversal symmetry of the pairs, giving them a finite lifetime and decreasing the energy gap. The field could also polarize the quasiparticle spins, producing different populations of spin-up and spin-down quasiparticles. Both processes favor slower recombination; in our materials we conclude that strong spin-orbit scattering reduces the spin polarization, leaving the field-induced gap reduction as the dominant effect and accounting quantitatively for the observed recombination rate reduction.

We study the low-temperature thermal conductivity ($\kappa$) of Dy${}_2$Ti${}_2$O${}_7$ along and perpendicular to the (111) plane and under the magnetic field along the [111] direction. Besides the steplike decreases of $\kappa$ at the field-induced transitions from the spin-ice state to the kagomé-ice state and then to the polarized state, an abnormal phenomenon is that the $\kappa \left(H\right)$ isotherms show a clear irreversibility at very low temperatures upon sweeping the magnetic field up and down. This phenomenon surprisingly has no correspondence with the well-known magnetization hysteresis. Possible origins for this irreversibility are discussed; in particular, a pinning effect of magnetic monopoles in a spin-ice compound by the weak disorders is proposed.