Electronic and magnetic properties of the interface between metal-quinoline molecules and cobalt

It was recently established that spin injection from a ferromagnetic metal into an organic semiconductor depends largely on the formation of hybrid interface states. Here we investigate whether the magnetic properties of the interface between cobalt and tris( 8-hydroxyquinolinato)-Al( III) ( Alq3), the most prominent molecular candidate for organicspin-valve devices, can be modified by substituting the aluminum atom with either gallium or indium. The electronic structure of Alq3, Gaq3, and Inq3 and the properties of their interfaces with ferromagnetic cobalt are probed experimentally, by using different photoemission spectroscopy methods, and theoretically, through density functional theory calculations. For all cases, the results highlight the presence of spin-polarized interface states. However no striking difference between the properties of the various molecules and interfaces is observed. This is a consequence of the fact that the molecules frontier orbitals are mainly localized on the ligands and they show only a negligible contribution coming from the metal ion

Spin- and time-resolved photoemission studies of thin Co2FeSi Heusler alloy films

We have studied the possibly half metallic Co2FeSi full Heusler alloy by means of spin- and time-resolved photoemission spectroscopy. For excitation, the second and fourth harmonic of femtosecond Ti:sapphire lasers were used, with photon energies of 3.1 eV and 5.9 eV, respectively. We compare the dependence of the measured surface spin polarization on the particular photoemission mechanism, i.e. 1-photon-photoemission (1PPE) or 2-photon photoemission (2PPE). The observed differences in the spin polarization can be explained by a spin-dependent lifetime effect occurring in the 2-photon absorption process. The difference in escape depth of the two methods in this context suggests that the observed reduction of spin polarization (compared to the bulk) cannot be attributed just to the outermost surface layer but takes place at least 4-6 nm away from the surface.Comment: 7 pages, 3 figures; submitted to Journal of Magnetism and Magnetic Material

Photoemission Electron Microscopy as a tool for the investigation of optical near fields

Photoemission electron microscopy was used to image the electrons photoemitted from specially tailored Ag nanoparticles deposited on a Si substrate (with its native oxide SiO$_{x}$). Photoemission was induced by illumination with a Hg UV-lamp (photon energy cutoff $\hbar\omega_{UV}=5.0$ eV, wavelength $\lambda_{UV}=250$ nm) and with a Ti:Sapphire femtosecond laser ($\hbar\omega_{l}=3.1$ eV, $\lambda_{l}=400$ nm, pulse width below 200 fs), respectively. While homogeneous photoelectron emission from the metal is observed upon illumination at energies above the silver plasmon frequency, at lower photon energies the emission is localized at tips of the structure. This is interpreted as a signature of the local electrical field therefore providing a tool to map the optical near field with the resolution of emission electron microscopy.Comment: 10 pages, 4 figures; submitted to Physical Review Letter

Topological States on the Gold Surface

Gold surfaces host special electronic states that have been understood as a prototype of Shockley surface states (SSs). These SSs are commonly employed to benchmark the capability of angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy. We find that these Shockley SSs can be reinterpreted as topologically derived surface states (TDSSs) of a topological insulator (TI), a recently discovered quantum state. Based on band structure calculations, the Z2 topological invariant can be well defined to characterize the nontrivial features of gold that we detect by ARPES. The same TDSSs are also recognized on surfaces of other well-known noble metals (e.g., silver, copper, platinum, and palladium). Besides providing a new understanding of noble metal SSs, finding topological states on late transition metals provokes interesting questions on the role of topological effects in surface-related processes, such as adsorption and catalysis.Comment: 21 pages, 3 figure

Light-induced magnetization reversal of high-anisotropy TbCo alloy films

Magnetization reversal using circularly polarized light provides a new way to control magnetization without any external magnetic field and has the potential to revolutionize magnetic data storage. However, in order to reach ultra-high density data storage, high anisotropy media providing thermal stability are needed. Here, we evidence all-optical magnetization switching for different TbxCo1-x ferrimagnetic alloy composition and demonstrate all-optical switching for films with anisotropy fields reaching 6 T corresponding to anisotropy constants of 3x106 ergs/cm3. Optical magnetization switching is observed only for alloys which compensation temperature can be reached through sample heating

Phase covariant quantum cloning

We consider an N -> M quantum cloning transformation acting on pure two-level states lying on the equator of the Bloch sphere. An upper bound for its fidelity is presented, by establishing a connection between optimal phase covariant cloning and phase estimation. We give the explicit form of a cloning transformation that achieves the bound for the case N=1, M=2, and find a link between this case and optimal eavesdropping in the quantum cryptographic scheme BB84.Comment: 9 pages, 1 figur