Tunable high harmonic pulses from nanorings swirled by optical vortices.

Irradiating intercalated nanorings by optical vortices ignites a charge flow that emits coherent trains of high harmonic bursts with frequencies and time structures that are controllable by the topological charge of the driving vortex beam. Similar to synchrotron radiation, the polarization of emitted harmonics is also selectable by tuning to the appropriate emission angle with respect to the ring plane. The nonequilibrium orbital magnetic moment triggered in a ring tunnels quantum mechanically to smaller and larger rings leading respectively to high and low-frequency harmonic generation. The frequencies of the emitted harmonics are tunable by simply changing the waist and/or the winding number of the optical vortex, without the need to increase the pulse intensity which can lead to material damage. These findings follow from full-fledged quantum dynamic simulations for realistic material and laser parameters. The proposed setup is non-destructive as only short vortex pulses of moderate intensities are needed, and it offers a versatile tool for nanoscale optical and spectroscopic applications such as local, single beam pump-probe experiments.

Electrically driven magnetic antenna based on multiferroic composites.

We suggest and demonstrate via large scale numerical simulations an electrically operated spin-wave inducer based on composite multiferroic junctions. Specifically, we consider an interfacially coupled ferromagnetic/ferroelectric structure that emits controllably spin waves in the ferromagnets if the ferroelectric polarization is poled by an external electric field. The roles of geometry and material properties are discussed.

Positive-Negative Birefringence in Multiferroic Layered Metasurfaces.

We uncover and identify the regime for a magnetically and ferroelectrically controllable negative refraction of a light-traversing multiferroic, oxide-based metastructure consisting of alternating nanoscopic ferroelectric (SrTiO3) and ferromagnetic (Y3Fe2(FeO4)3, YIG) layers. We perform analytical and numerical simulations based on discretized, coupled equations for the self-consistent Maxwell/ferroelectric/ferromagnetic dynamics and obtain a biquadratic relation for the refractive index. Various scenarios of ordinary and negative refraction in different frequency ranges are analyzed and quantified by simple analytical formula that are confirmed by full-fledge numerical simulations. Electromagnetic waves injected at the edges of the sample are propagated exactly numerically. We discovered that, for particular GHz frequencies, waves with different polarizations are characterized by different signs of the refractive index, giving rise to novel types of phenomena such as a positive-negative birefringence effect and magnetically controlled light trapping and accelerations.

Superadiabatic quantum heat engine with a multiferroic working medium.

A quantum thermodynamic cycle with a chiral multiferroic working substance such as LiCu_{2}O_{2} is presented. Shortcuts to adiabaticity are employed to achieve an efficient, finite-time quantum thermodynamic cycle, which is found to depend on the spin ordering. The emergent electric polarization associated with the chiral spin order, i.e., the magnetoelectric coupling, renders possible steering of the spin order by an external electric field and hence renders possible an electric-field control of the cycle. Due to the intrinsic coupling between the spin and the electric polarization, the cycle performs an electromagnetic work. We determine this work's mean-square fluctuations, the irreversible work, and the output power of the cycle. We observe that the work mean-square fluctuations are increased with the duration of the adiabatic strokes, while the irreversible work and the output power of the cycle show a nonmonotonic behavior. In particular, the irreversible work vanishes at the end of the quantum adiabatic strokes. This fact confirms that the cycle is reversible. Our theoretical findings evidence the existence of a system inherent maximal output power. By implementing a Lindblad master equation we quantify the role of thermal relaxations on the cycle efficiency. We also discuss the role of entanglement encoded in the noncollinear spin order as a resource to affect the quantum thermodynamic cycle.

Centrifugal photovoltaic and photogalvanic effects driven by structured light.

Much efforts are devoted to material structuring in a quest to enhance the photovoltaic effect. We show that structuring light in a way it transfers orbital angular momentum to semiconductor-based rings results in a steady charge accumulation at the outer boundaries that can be utilized for the generation of an open circuit voltage or a photogalvanic (bulk photovoltaic) type current. This effect which stems both from structuring light and matter confinement potentials, can be magnified even at fixed moderate intensities, by increasing the orbital angular momentum of light which strengthens the effective centrifugal potential that repels the charge outwards. Based on a full numerical time propagation of the carriers wave functions in the presence of light pulses we demonstrate how the charge buildup leads to a useable voltage or directed photocurrent whose amplitudes and directions are controllable by the light pulse parameters.

Dynamics of localized modes in a composite multiferroic chain.

In a coupled ferroelectric-ferromagnetic system, i.e., a composite multiferroic, the propagation of magnetic or ferroelectric excitations across the whole structure is a key issue for applications. Of special interest is the dynamics of localized magnetic or ferroelectric modes (LM) across the ferroelectric-ferromagnetic interface, particularly when the LM's carrier frequency is in the band of the ferroelectric and in the band gap of the ferromagnet. For a proper choice of the system's parameters, we find that there is a threshold amplitude above which the interface becomes transparent and an in-band ferroelectric LM penetrates the ferromagnetic array. Below that threshold, the LM is fully reflected. Slightly below this transmission threshold, the addition of noise may lead to energy transmission, provided that the noise level is neither too low nor too high, an effect that resembles stochastic resonance. These findings represent an important step towards the application of ferroelectric and/or ferromagnetic LM-based logic.

Photovoltaic effect of light carrying orbital angular momentum on a semiconducting stripe.

We investigate the influence of a light beam carrying an orbital angular momentum on the current density of an electron wave packet in a semiconductor stripe. It is shown that due to the photo-induced torque the electron density can be deflected to one of the stripe sides. The direction of the deflection is controlled by the direction of the light orbital momentum. In addition the net current density can be enhanced. This is a photovoltaic effect that can be registered by measuring the generated voltage drop across the stripe and/or the current increase.

Nonlinear anomalous Hall effect and negative magnetoresistance in a system with random Rashba field.

We predict two spin-dependent transport phenomena in two-dimensional electron systems, which are induced by a spatially fluctuating Rashba spin-orbit interaction. When the electron gas is magnetized, the random Rashba interaction leads to the anomalous Hall effect. An example of such a system is a narrow-gap magnetic semiconductor-based symmetric quantum well. We show that the anomalous Hall conductivity reveals a strongly nonlinear dependence on the magnetization, decreasing exponentially at large spin density. We also show that electron scattering from a fluctuating Rashba field in a two-dimensional nonmagnetic electron system leads to a negative magnetoresistance arising solely due to spin-dependent effects.

Spin-orbital phase synchronization in the magnetic field-driven electron dynamics in a double-well potential.

We study the dynamics of an electron confined in a one-dimensional double-well potential in the presence of driving external magnetic fields. The orbital motion of the electron is coupled to the spin dynamics by spin-orbit interaction of the Dresselhaus type. We derive an effective time-dependent model Hamiltonian for the orbital motion of the electron and obtain a condition for synchronization of the orbital and the spin dynamics. We find an analytical expression for the Arnold 'tongue' and propose an experimental scheme for realizing the proposed synchronization.

Communication: Superatom molecular orbitals: new types of long-lived electronic states.

We present ab initio calculations of the quasiparticle decay times in a Buckminsterfullerene based on the many-body perturbation theory. A particularly lucid representation arises when the broadening of the quasiparticle states is plotted in the angular momentum (l) and energy (ε) coordinates. In this representation the main spectroscopic features of the fullerene consist of two occupied nearly parabolic bands, and delocalized plane-wave-like unoccupied states with a few long-lived electronic states (the superatom molecular orbitals, SAMOs) embedded in the continuum of Fermi-liquid states. SAMOs have been recently uncovered experimentally by Feng et al. [Science 320, 359 (2008)] using scanning tunneling spectroscopy. The present calculations offer an explanation of their unusual stability and unveil their long-lived nature making them good candidates for applications in the molecular electronics. From the fundamental point of view these states illustrate a concept of the Fock-space localization [B. L. Altshuler, Y. Gefen, A. Kamenev, and L. S. Levitov, Phys. Rev. Lett. 78, 2803 (1997)] with properties drastically different from the Fermi-liquid excitations.

Orbital and spin dynamics of intraband electrons in quantum rings driven by twisted light.

We theoretically investigate the effect that twisted light has on the orbital and spin dynamics of electrons in quantum rings possessing sizable Rashba spin-orbit interaction. The system Hamiltonian for such a strongly inhomogeneous light field exhibits terms which induce both spin-conserving and spin-flip processes. We analyze the dynamics in terms of the perturbation introduced by a weak light field on the Rasha electronic states, and describe the effects that the orbital angular momentum as well as the inhomogeneous character of the beam have on the orbital and the spin dynamics.

Dynamic switching of the magnetization in a driven molecular nanomagnet.

We study the magnetization dynamics of a single molecular nanomagnet driven by static and variable magnetic fields within a classical treatment. The underlying analysis is valid for a regime where the energy is definitely lower than the anisotropy barrier, but still a substantial number of states are excited. We find the phase space to contain a separatrix line. Solutions far from it are oscillatory whereas the separatrix solution is of a soliton type. States near the separatrix are extremely sensitive to small perturbations, a fact that we utilize in obtaining dynamically induced magnetization switching. A new type of magnetization switching is proposed.

Electric currents induced by twisted light in Quantum Rings.

We theoretically investigate the generation of electric currents in quantum rings resulting from the optical excitation with twisted light. Our model describes the kinetics of electrons in a two-band model of a semiconductor-based mesoscopic quantum ring coupled to light having orbital angular momentum (twisted light). We find the analytical solution, which exhibits a "circular" photon-drag effect and an induced magnetization, suggesting that this system is the circular analog of that of a bulk semiconductor excited by plane waves. For realistic values of the electric field and material parameters, the computed electric current can be as large as microA; from an applied perspective, this opens new possibilities to the optical control of the magnetization in semiconductors.

Local control of ultrafast dynamics of magnetic nanoparticles.

Using the local control theory we derive analytical expressions for magnetic field pulses that steer the magnetization of a monodomain magnetic nanoparticle to a predefined state. Finite-temperature full numerical simulations confirm the analytical results and show that a magnetization switching or freezing is achievable within few precessional periods and that the scheme is exploitable for fast thermal switching.

Stochastic dynamics and control of a driven nonlinear spin chain: the role of Arnold diffusion.

We study a chain of nonlinear interacting spins driven by a static and a time-dependent magnetic field. The aim is to identify the conditions for the locally and temporally controlled spin switching. Analytical and full numerical calculations show the possibility of stochastic control if the underlying semiclassical dynamics is chaotic. This is achievable by tuning the external field parameters according to the method described in this paper. We show analytically for a finite spin chain that Arnold diffusion is the underlying mechanism for the present stochastic control. Quantum mechanically we consider the regime where the classical dynamics is regular or chaotic. For the latter we utilize the random matrix theory. The efficiency and the stability of the non-equilibrium quantum spin states are quantified by the time dependence of the Bargmann angle related to the geometric phases of the states.

Decay of hybridized electronic states of a Na cluster on Cu(001).

The electronic properties of a Na9+ cluster are markedly changed when deposited on a Cu(001) surface. Particularly, the lifetime of the (hybridized) single-particle electronic states are drastically enhanced indicating a change in electronic correlations upon absorption. To capture this effect, we developed a Green's function approach based on the configuration interaction technique. The calculated lifetimes (16.5 to 33 fs) of excited electronic states are in line with experimental observations. Our new method demonstrates the feasibility of accounting accurately for electronic correlation in large, nonperiodic systems.

Pathways of polaron and bipolaron transport in DNA double strands.

We investigate the pathways of polaron and bipolaron transports in DNA double strands with an extended Su-Schrieffer-Heeger model involving the effects of solvent polarization. We find that the long-range transport of polaron/bipolaron under high electric field at low temperature is the field-facilitated sequential tunneling through spatial-disordered potential barriers via multiple intrastrand and interstrand pathways. Although the interstrand pathways may be very active and effective in some DNA sequences, the intrastrand ones always dominate the charge transfer when the excess charge moves close to the final acceptor.

Controlling the spin polarization of nanostructures on magnetic substrates.

It is shown that, by utilizing spin-selective quantum interference, the spin polarization of nanostructures deposited on a magnetic substrate with a surface state can be strongly modulated locally and energetically by an appropriate structural design. This finding is deduced from state-of-the-art ab initio calculations and interpreted within an analytical model. We present results for hexagonal Cu corrals and mesoscopic triangular Co islands on Co-covered Cu(111). These systems are experimentally feasible, and the effect should be detectable with current technology.

Tunable conductance of magnetic nanowires with structured domain walls.

We show that in a magnetic nanowire with double magnetic domain walls, quantum interference results in spin-split quasistationary states localized mainly between the domain walls. Spin-flip-assisted transmission through the domain structure increases strongly when these size-quantized states are tuned on resonance with the Fermi energy, e.g., upon varying the distance between the domain walls which results in resonance-type peaks of the wire conductance. This novel phenomenon is shown to be utilizable to manipulate the spin density in the domain vicinity. The domain wall parameters are readily controllable, and the predicted effect is hence exploitable in spintronic devices.

Mapping out electron-electron interactions at surfaces.

Using a high resolution coincidence technique, we measured for the first time the angular and energy correlation of an electron pair emitted from the valence band of a single crystal upon the impact of an electron with a specified momentum. We observe a hole in the measured two-particle correlation function when the two excited electrons have comparable momentum vectors, a fact traced back to exchange and repulsion among the electrons. We find the hole is not isotropic, has a finite extension, and is strongly suppressed when decoherence is operating.