Molecular switches at surfaces

1\. Auflag

Microscopic insights to spin transport–driven ultrafast magnetization dynamics in a Gd/Fe bilayer

Laser-induced spin transport is a key ingredient in ultrafast spin dynamics. However, it remains debated to what extent ultrafast magnetization dynamics generates spin currents and vice versa. We use time- and spin-resolved photoemission spectroscopy to study an antiferromagnetically coupled Gd/Fe bilayer, a prototype system for all-optical switching. Spin transport leads to an ultrafast drop of the spin polarization at the Gd surface, demonstrating angular-momentum transfer over several nanometers. Thereby, Fe acts as spin filter, absorbing spin majority but reflecting spin minority electrons. Spin transport from Gd to Fe was corroborated by an ultrafast increase of the Fe spin polarization in a reversed Fe/Gd bilayer. In contrast, for a pure Gd film, spin transport into the tungsten substrate can be neglected, as spin polarization stays constant. Our results suggest that ultrafast spin transport drives the magnetization dynamics in Gd/Fe and reveal microscopic insights into ultrafast spin dynamics

Azobenzene versus 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) at Au(111): Characterizing the role of spacer groups

We present large-scale density-functional theory (DFT) calculations and temperature programmed desorption measurements to characterize the structural, energetic and vibrational properties of the functionalized molecular switch 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) adsorbed at Au(111). Particular emphasis is placed on exploring the accuracy of the semi-empirical dispersion correction approach to semi-local DFT (DFT-D) in accounting for the substantial van der Waals component in the surface chemical bond. In line with previous findings for benzene and pure azobenzene at coinage metal surfaces, DFT-D significantly overbinds the molecule, but seems to yield an accurate adsorption geometry as far as can be judged from the experimental data. Comparing the trans adsorption geometry of TBA and azobenzene at Au(111) reveals a remarkable insensitivity of the structural and vibrational properties of the -N=N- moiety. This questions the established view of the role of the bulky tert-butyl-spacer groups for the switching of TBA in terms of a mere geometric decoupling of the photochemically active diazo-bridge from the gold substrate.Comment: 9 pages including 6 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Spin-dependent lifetime and exchange splitting of surface states on Ni(1 1 1)

We report on a spin-resolved two-photon photoemission study of the Ni(1 1 1) surface states. Nickel thin films were grown by molecular beam epitaxy on a W(1 1 0) substrate. The first image-potential state is used as a sensor to map the spin polarization of the occupied surface states. This allows us to identify the majority spin component of the Shockley surface state as well as a majority and minority d-derived surface resonance. The n = 1 image-potential state is found to be exchange split by 14 ± 3 meV. In spite of the fact that the band structure at the Fermi level exhibits a strongly discerned density of states in both spin channels, we observe low spin asymmetries in the decay and dephasing rates of the photoexcited electrons. Varying the sample preparation reveals that the Shockley surface state contributes about 40% to the spin- dependent decay rate

The role of spin-lattice coupling for ultrafast changes of the magnetic order in rare earth metals

By comparing femtosecond laser-pulse-induced spin dynamics in the surface state of the rare earth metals Gd and Tb, we show that the spin polarization of valence states in both materials decays with significantly different time constants of 15 ps and 400 fs, respectively. The distinct spin polarization dynamics in Gd and Tb are opposed by similar exchange splitting dynamics in the two materials. The different time scales observed in our experiment can be attributed to weak and strong 4f spin to lattice coupling in Gd and Tb, suggesting an intimate coupling of spin polarization and 4f magnetic moment. While in Gd the lattice mainly acts as a heat sink, it contributes significantly to ultrafast demagnetization of Tb. This helps explain why all optical switching is observed in FeGd—but rarely in FeTb-based compounds

Transient band structures in the ultrafast demagnetization of ferromagnetic gadolinium and terbium

We compare the laser-driven demagnetization dynamics of the rare earths gadolinium and terbium by mapping their transient valance band structures with time- and angle-resolved photoelectron spectroscopy. In both metals, the minority and majority spin valence bands evolve independently with different time constants after optical excitation. The ultrafast shift of the partially unoccupied minority spin bulk band to higher binding energy and of the majority spin surface state to lower binding energy suggests spin transport between surface and bulk. The slower response of the fully occupied majority spin band follows the lattice temperature and is attributed to Elliott-Yafet type spin-flip scattering. Terbium shows a stronger and faster decay of the exchange splitting, pointing to ultrafast magnon emission via 4f spin-to- lattice coupling

Fluence-dependent dynamics of the 5d6s exchange splitting in Gd metal after femtosecond laser excitation

We investigate the fluence-dependent dynamics of the exchange-split 5d6s valence bands of Gd metal after femtosecond, near-infrared (IR) laser excitation. Time- and angle-resolved photoelectron spectroscopy (tr-ARPES) with extreme ultraviolet (XUV) probe pulses is used to simultaneously map the transient binding energies of the minority and majority spin valence bands. The decay constant of the exchange splitting increases with fluence. This reflects the slower response of the occupied majority-spin component, which we attribute to Elliot–Yafet spin-flip scattering in accordance with the microscopic three-temperature model (M3TM). In contrast, the time constant of the partly unoccupied minority-spin band stays unaffected by a change in pump fluence. Here, we introduce as an alternative to superdiffusive spin transport exchange scattering, which is an ultrafast electronic mechanism explaining the observed dynamics. Exchange scattering can reduce the spin polarization in the partially unoccupied minority-spin band and thus its energetic position without effective demagnetization

Charge-transfer dynamics in azobenzene alkanethiolate self-assembled monolayers on gold

We have studied the charge transfer dynamics in azobenzene-functionalized alkanethiolate self-assembled monolayers. We compare the core-hole clock technique, i.e., resonant vs. non-resonant contributions in the azobenzene autoionization of the C1s-pi∗ core exciton, with the lifetime of a molecular resonance determined by two-photon photoemission spectroscopy using femtosecond laser pulses. Both techniques yield comparable charge-transfer times of 80+-20 fs for a linker consisting of three CH2 groups and one oxygen unit. Thus the quenching of the excitation is about one order of magnitude faster than the time required for the trans to cis isomerization of the azobenzene photoswitch in solution

The Valence Band Structure of Gadolinium Studied with Time-Resolved Photoemission

We have studied the response of the exchange split valence bands of ferromagnetic gadolinium tofemtosecond laser excitation. We observe a drop of the exchange splitting with a time constant of 0.9 ps but different response times of minority and majority spin bands. Furthermore, even above the Curie temperature there is a finite exchange splitting, which also decreases with laser excitation