The magnetophotoluminescence in modulation doped core-multishell nanowires is predicted as a function of photoexcitation intensity in nonperturbative transverse magnetic fields. We use a self-consistent field approach within the effective mass approximation to determine the photoexcited electron and hole populations, including the complex composition and anisotropic geometry of the nanomaterial. The evolution of the photoluminescence is analyzed as a function of (i) photoexcitation power, (ii) magnetic field intensity, (iii) type of doping, and (iv) anisotropy with respect to field orientation.

We transform the two-dimensional Dirac-Weyl equation, which governs the charge carriers in graphene, into a nonlinear first-order differential equation for scattering phase shift, using the so-called variable-phase method. This allows us to utilize the Levinson theorem, relating scattering phase shifts of a slow particle to its bound states, to find zero-energy bound states created electrostatically in realistic structures. These confined states are formed at critical potential strengths, which leads us to posit the use of “optimal traps” to combat the chiral tunneling found in graphene: this could be explored experimentally with an artificial network of point charges held above the graphene layer. We also discuss scattering on these states and find that the $s$ states create a dominant peak in the scattering cross section as the energy tends towards the Dirac point energy, suggesting a dominant contribution to the resistivity.

The modification of the graphene spin structure is of interest for novel possibilities of application of graphene in spintronics. The most exciting of them demand not only high value of spin-orbit splitting of the graphene states, but non-Rashba behavior of the splitting and spatial modulation of the spin-orbit interaction. In this work we study the spin and electronic structure of graphene on Ir(111) with intercalated Pt monolayer. Pt interlayer does not change the $9.3\times 9.3$ superlattice of graphene, while the spin structure of the Dirac cone becomes modified. It is shown that the Rashba splitting of the $\pi$ state is reduced, while hybridization of the graphene and substrate states leads to a spin-dependent avoided-crossing effect near the Fermi level. Such a variation of spin-orbit interaction combined with the superlattice effects can induce a topological phase in graphene.

We study the high magnetic field regime of the antiferromagnetic insulator ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ by expressing the spin-1/2 operators in the relevant Heisenberg model in terms of hard-core bosons and implementing the hard-core constraint via an infinite on-site interaction. We focus on the case where the external magnetic field exceeds the saturation field $B_c\approx 8.5\mathrm{T}$ and is oriented along the crystallographic $a$ axis perpendicular to the lattice plane. Because in this case the excited states are separated by an energy gap from the ground state, we may use the self-consistent ladder approximation to take the strong correlations due to the hard-core constraint into account. In ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$, there are additional interactions besides the hard-core interaction which we treat in self-consistent Hartree-Fock approximation. We calculate the spectral function of the hard-core bosons from which we obtain the in-plane components of the dynamic structure factor, the magnetic susceptibility, and the specific heat. Our results for the specific heat are in good agreement with the available experimental data. We conclude that the self-consistent ladder approximation in combination with a self-consistent Hartree-Fock decoupling of the non-hard-core interactions gives an accurate description of the physical properties of gapped hard-core bosons in two dimensions at finite temperatures.

We investigate how hybridization (single-quasiparticle scattering) between two superconducting bands induces odd-frequency superconductivity in a multiband superconductor. An explicit derivation of the odd-frequency pairing correlation and its full frequency dependence is given. We also find that the density of states is modified, at higher energies, from the sum of the two BCS spectra to also include additional hybridization gaps with strong coherence peaks when odd-frequency pairing is present. These gaps constitute clear experimentally measurable signatures of odd-frequency pairing in multiband superconductors.

We predict a thermodynamic magnon recoil effect for domain wall motions in the presence of temperature gradients. All current thermodynamic theories assert that a magnetic domain wall must move toward the hotter side, based on equilibrium thermodynamic arguments. Microscopic calculations, on the other hand, show that a domain wall can move either along or against the direction of heat currents, depending on how strong the magnonic heat currents are reflected by the domain wall. We have resolved the inconsistency between these two approaches by augmenting the theory in the presence of thermal gradients by incorporating in the free energy of domain walls a heat current term present in nonequilibrium steady states. The condition to observe a domain wall propagation toward the colder regime is derived analytically and can be tested by future experiments.

We studied the structural modifications of a Pt/Co/Pt trilayer epitaxial film under ${\mathrm{Ga}}^{+}$ 30-keV ion irradiation by means of classical molecular dynamics and Monte Carlo simulations. The semiclassical tight-binding second-moment approximation potential was adjusted to reproduce the enthalpies of formation, the lattice constants, and the order-disorder transition temperatures for Co-Pt alloys. We found that during irradiation, the sandwich-type Pt(fcc)/Co(hcp)/Pt(fcc) film structure underwent a transition to the new solid solution $\alpha -\mathrm{Co}/\mathrm{Pt}\left(\mathrm{fcc}\right)$ phase. Our analysis of the short-range order indicates the formation, within a nanosecond time scale, of a homogeneous chemically disordered solution. The longer time-scale simulations employing a Monte Carlo algorithm demonstrated that the transition from the disordered phase to the ordered $L{1}_0$ and $L{1}_2$ phases was also possible but not significant for the changes in perpendicular magnetic anisotropy (PMA) observed experimentally. The strain analysis showed that the Co layer was under tensile strain in the lateral direction at the fluences of $1.5\times {10}^{14}–3.5\times {10}^{14}{\mathrm{ions}\mathrm{cm}}^{-2}$; this range of fluences corresponds to the appearance of PMA. This strain was induced in the initially relaxed hcp Co layer due to its partial transformation to the fcc phase and to the influence of atomic layers with larger lattice constants at upper/lower interfaces.

Propagation of elastic waves in acoustic metamaterials based on locally resonant viscoelastic phononic crystals is discussed. A variational formulation of the complex band structure for in-plane polarized waves is proposed and used to formulate a finite element model. Two different types of locally resonant band gaps are found for quasilongitudinal and quasishear waves, with distinct features in terms of complex bands and transmission bandwidth. The influence of viscosity on the complex band structure, transmission properties, and effective dynamic mass density of two-dimensional locally resonant metamaterials is further investigated. It is found that bands that were degenerate in the elastic case are separated when viscosity is introduced, and that sharp corners at high symmetry points become rounded. Transmission is generally worsened in passing bands, while it is enhanced inside locally resonant band gaps, contrary to what was observed previously for Bragg band gaps. All changes in the complex band structure and transmission spectra are solely due to the dispersive and dissipative effects of viscosity. It is also found that the negative mass density property may also disappear when viscosity is introduced. These results are relevant to practical applications of elastic and viscoelastic metamaterials.

We report equilibrium and nonequilibrium optical measurements on the recently synthesized “harmonic” honeycomb iridate $\gamma -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ (LIO), as well as the layered honeycomb iridate ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ (NIO). Using Fourier transform infrared microscopy we performed reflectance measurements on LIO, from which we obtained the optical conductivity below 2 eV. In addition, we measured the photoinduced changed in reflectance $\mathrm{\Delta }R$, as a function of time $t$, temperature $T$, and probe field polarization in both LIO and NIO. In LIO, $\mathrm{\Delta }R(t,T)$ is anisotropic and comprises three $T$-dependent components. Two of these components are related to the onset of magnetic order and the third is related to a photoinduced population of metastable electronic excited states. In NIO, $\mathrm{\Delta }R(t,T)$ has a single $T$-dependent component that is strikingly similar to the electronic excitation component of $\mathrm{\Delta }R$ in LIO. Through analysis and comparison of $\mathrm{\Delta }R(t,T)$ for two compounds, we extract information on the onset of magnetic correlations at and above the transition temperature in LIO, the bare spin-flip scattering rate in equilibrium, the lifetime of low-lying quasiparticle excitations, and the polarization dependence of optical transitions that are sensitive to magnetic order.

We show that the entanglement spectrum associated with a certain class of strongly correlated many-body states—the wave functions proposed by Laughlin and Jain to describe the fractional quantum Hall effect—can be very well described in terms of a simple model of noninteracting (or weakly interacting) composite fermions.

Fourier transform spectroscopy is used to determine/control the degree of spectral diffusion in high-quality site-controlled GaN nanowire quantum dots. Detailed analysis, including the development of a statistical model and Monte Carlo simulations, provides evidence that the broadening is caused by photoinduced excitation of defects. Furthermore, performing the experiment under weak excitation allows for an estimate of the homogeneous linewidth to be made $\left(135\mu \mathrm{eV}\right)$. The origins of this linewidth are discussed, and the existence of an alternate dephasing mechanism is inferred. A limit on the emission linewidth at zero excitation is also discussed.

Efficient operation sequences to couple and interchange quantum information between quantum dot spin qubits of different kinds are derived using exchange interactions. In the qubit encoding of a single-spin qubit, a singlet-triplet qubit, and an exchange-only (triple-dot) qubit, some of the single-qubit interactions remain on during the entangling operation; this greatly simplifies the operation sequences that construct entangling operations. In the ideal setup, the gate operations use the intraqubit exchange interactions only once, and entangling operations with gate times similar to typical single-qubit operations are constructed. The limitations of the entangling sequences are discussed, and it is shown how quantum information can be converted between different kinds of quantum dot spin qubits. These gate sequences are useful for large-scale quantum computation because they show that different kinds of coded spin qubits can be combined easily, permitting the favorable physical properties of each to be employed.

Electron beams have the ability of exciting vibrational modes (phonons) in molecules and nanoclusters, which can be currently probed with atomic spatial resolution through electron energy-loss spectroscopy (EELS). This scenario is similar to the excitation of plasmons with light, except that electrons allow much tighter spatial focusing. Additionally, phonons possess larger quality factors and lifetimes. In an effort to exploit these appealing properties, we theoretically investigate the interaction of focused electron beams with the vibrational modes of carbon nanographenes. The EELS probability is found to mimic the density of vibrational states, both of which evolve smoothly with cluster size towards the limit of extended graphene. Our results support the use of phonons as long-lived, tightly confined substitutes of plasmons, combined with electron-beam-mediated mode-selective excitation.

We investigate theoretically the photoacoustic generation by a gold nanosphere in water in the thermoelastic regime. Specifically, we consider the long-pulse illumination regime, in which the time for electron-phonon thermalization can be neglected and photoacoustic wave generation arises solely from the thermoelastic stress caused by the temperature increase of the nanosphere or its liquid environment. Photoacoustic signals are predicted based on the successive resolution of a thermal diffusion problem and a thermoelastic problem, taking into account the finite size of the gold nanosphere, thermoelastic and elastic properties of both water and gold, and the temperature dependence of the thermal expansion coefficient of water. For sufficiently high illumination fluences, this temperature dependence yields a nonlinear relationship between the photoacoustic amplitude and the fluence. For nanosecond pulses in the linear regime, we show that more than $90\%$ of the emitted photoacoustic energy is generated in water, and the thickness of the generating layer around the particle scales close to the square root of the pulse duration. The amplitude of the photoacoustic wave in the linear regime is accurately predicted by the point-absorber model introduced by Calasso et al. [Phys. Rev. Lett. 86, 3550 (2001)], but our results demonstrate that this model significantly overestimates the amplitude of photoacoustic waves in the nonlinear regime. We therefore provide quantitative estimates of a critical energy, defined as the absorbed energy required such that the nonlinear contribution is equal to that of the linear contribution. Our results suggest that the critical energy scales as the volume of water over which heat diffuses during the illumination pulse. Moreover, thermal nonlinearity is shown to be expected only for sufficiently high ultrasound frequency. Finally, we show that the relationship between the photoacoustic amplitude and the equilibrium temperature at sufficiently high fluence reflects the thermal diffusion at the nanoscale around the gold nanosphere.

A spin-spin coupling mechanism that occurs during the transport of spin-polarized minority electrons in semiconductors is described. Unlike the Coulomb spin drag, this coupling arises from the ambipolar electric field, which is created by the differential movement of the photoelectrons and the photoholes. Like the Coulomb spin drag, it is a pure spin coupling that does not affect charge diffusion. Experimentally, the coupling is studied in $p^{+}$-GaAs using polarized microluminescence. The coupling manifests itself as an excitation power-dependent reduction in the spin polarization at the excitation spot without any change of the spatially averaged spin polarization.

Here, we formally develop theoretical methods to topologically classify a wide class of bianisotropic continuous media. It is shown that for continuous media, the underlying wave vector space may be regarded as the Riemann sphere. We derive sufficient conditions that ensure that the pseudo-Hamiltonian that describes the electrodynamics of the continuous material is well behaved so that the Chern numbers are integers. Our theory brings the powerful ideas of topological photonics to a wide range of electromagnetic waveguides and platforms with no intrinsic periodicity and sheds light over the emergence of edge states at the interfaces between topologically inequivalent continuous media.

We report on a systematic ${}^{31}\mathrm{P}$-NMR study on an iron- (Fe-) based superconductor ${\mathrm{SrFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ (Sr122AsP) in which a superconducting (SC) transition temperature $T_c$ at $x=0.35$ increases from $T_c=26\mathrm{K}$ up to 33 K by annealing an as-grown monocrystalline sample. The present NMR study has unraveled that $T_c$ reaches the highest value of 33 K at $x=0.35$ around a quantum critical point at which antiferromagnetic (AFM) order disappears. When noting that the SC transition disappears at $x=0.6$ where the AFM spin fluctuations (SFs) are no longer present, we remark that the onset and increase in $T_c$ are apparently associated with the emergence and enhancement, respectively, of the AFM-SFs. In the SC state, the residual density of state (RDOS) at the Fermi energy $E_{\mathrm{F}}$ in the SC state becomes much smaller for the annealed sample than for the as-grown one, suggesting that some inhomogeneity and/or imperfection for the latter increases the RDOS as expected for the unconventional SC state with a nodal gap. These findings in Sr122AsP are consistent with the unconventional $s_{\pm }$-wave Cooper pairing state that is mediated by the AFM-SFs. We also discuss other key ingredients besides the AFM-SFs to increase $T_c$ further.

Transitional processes accompanying switches between steady states in multistable cavity polariton systems are studied experimentally in a low-symmetry high-$Q$ microcavity under resonant optical excitation at normal incidence. We show that the establishment of a high-energy polariton state is influenced by an optical parametric oscillation. Therefore, the emission spectrum reveals the energy-separated signal-idler pairs in both spin components. Accordingly, the time dependencies of the polariton emission exhibit oscillations whose periods correspond to the spectral positions of the scattered states. Thus, the sharp transitions between the steady states of a no-equilibrium polariton condensate enable one to visualize the near-condensate eigenmodes and explore their spectral and spin properties depending on the condensate amplitude and average spin.

We report results of the atomic structure, alloying behavior, and magnetism in $\mathrm{F}{\mathrm{e}}_m\mathrm{P}{\mathrm{t}}_n(m+n=2–10)$ clusters using projector augmented wave (PAW) pseudopotential method and spin-polarized generalized gradient approximation (GGA) for the exchange-correlation energy. These results are compared with those obtained by using HCTH exchange-correlation functional and LANL2DZ basis set in the Gaussian program and the overall trends are found to be similar. As in bulk Fe-Pt alloys, clusters with equal composition of Fe and Pt have the largest binding energy and the largest heat of nanoalloy formation for a given number of atoms in the cluster. There are some deviations due to the different symmetries in clusters and in cases where the total number of atoms is odd. The lowest energy isomers tend to maximize bonds between unlike atoms with Fe (Pt) atoms occupying high (low) coordination sites in the core (surface) of the cluster. The binding energy, heat of formation, and the second order difference of the total energy show $\mathrm{F}{\mathrm{e}}_2\mathrm{P}{\mathrm{t}}_2, \mathrm{F}{\mathrm{e}}_4\mathrm{P}{\mathrm{t}}_4$, and $\mathrm{F}{\mathrm{e}}_4\mathrm{P}{\mathrm{t}}_6$ clusters to be the most stable ones among the different clusters we have studied. The magnetic moments on Fe atoms are high in Pt-rich clusters as well as in small Fe-rich clusters and decrease as the aggregation of Fe atoms and the cluster size increases. The maximum value of the magnetic moments on Fe atoms is $\sim 3.8{\mu }_B$, whereas for Pt atoms it is $1 {\mu }_B$. These are quite high compared with the values for bulk Fe as well as bulk FePt and $\mathrm{F}{\mathrm{e}}_3\mathrm{Pt}$ phases while bulk Pt is nonmagnetic. There is significant charge transfer from those Fe atoms that interact directly with Pt atoms. We discuss the hybridization between the electronic states of Pt and Fe atoms as well as the variation in the magnetic moments on Fe and Pt atoms. Our results provide insight into the understanding of the nanoalloy behavior of Fe-Pt and we hope that this would help to design Fe based nanoalloys and their assemblies with high magnetic moments for strong magnets without rare earths as well as Pt alloy catalysts.