⇒ complete CFEL Seminar List
⇒ Upcoming Seminars at HASYLAB (MPSD and CFEL seminars included)
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MPSD Seminar

May 11^th 2012, 11:00h

Tuning Magnetic Interactions in Semiconductors by STM

Location:  DESY Building 49, seminar room 108

Dilute magnetic semiconductors have the electrical properties of semiconductors while allowing for magnetic alignment of spins. In particular, manganese-doped gallium arsenide is a ferromagnetic semiconductor with a Curie temperature of ~200K. These manganese dopants act as electron acceptors while preserving a magnetic moment. Low-temperature (5K) scanning tunneling microscopy (STM) allows us to study these dopants individually. By moving point charges with atomic precision, we adjust the binding energy of single acceptors embedded into the surface, and tune the interaction between multiple acceptors. Because of the anisotropy of the zincblende crystal lattice of GaAs, the magnetic coupling between Mn dopants can be ferromagnetic or antiferromagnetic depending on the orientation of the acceptors. Control of this magnetic interaction will lead to deeper understanding of these dilute magnetic semiconductors, hopefully leading to designs for spintronic materials that can function at room temperature.

MPSD Seminar

May 25^th 2012, 11:00h

Kondo effect in the presence of a spin-polarized current

Location:  DESY Building 49, seminar room 108

The Kondo effect occurs when a localized magnetic moment is screened by the spins of the host metal electrons. Below a typical temperature known as Kondo temperature, this many-body interaction results in the emergence of a resonance in the density of states located near the Fermi level. For more than a decade, such a resonance has been investigated by transport measurements in single Kondo impurities, consisting of magnetic atoms or artificial quantum dots (QD). Of particular interest for the emerging field of spintronics is the interaction of single Kondo impurities with ferromagnetic leads, where a splitting of the Kondo resonance is predicted. Several experimental and theoretical studies have been published on QDs, but are still lacking for single magnetic atoms. Here, we present the first low-temperature STM measurements showing a Kondo splitting of a single atom in the presence of a spin-polarized current. A cobalt atom on the Cu(100) surface presents a Kondo resonance, which we are able to split by approaching a magnetic tip covered by copper. The original aspect of this study is to use copper as a spacer between the magnetic tip and the Co atom to minimize the magnetic direct couplings. With the additional support of equation of motion (EOM) calculations, we show that the splitting is produced mainly by the spinpolarized current flowing across the junction. We also evidence that the Kondo splitting is weakened when a direct ferromagnetic coupling is present. This study demonstrates the impact of magnetic interactions and of the spin-polarized current in the Kondo effect.

CFEL-Colloquium:

June 11^th 2012, 14:30h

Picosecond photocurrents and terahertz generation in nanoscale circuits

Location:  DESY Building 49, seminar room 108

The time-resolved dynamics of photogenerated charge carriers in nanoscale systems are typically detected by optical methods such as the transient absorption technique and the time-resolved photoluminescence spectroscopy. Many questions remain concerning the separation and transport of photogenerated charge carriers to source and drain leads, when the nanosystems shall be functional modules in (opto-)electronic circuits. Typical propagation times of ballistic photogenerated charge carriers in nanoscale circuits are in the ps-regime. Conventional electronic measurements cannot resolve such ultrafast dynamics because available electronic equipment cannot produce trigger signals and detect transients faster than tens of picoseconds. Furthermore, nanosystems typically exhibit a high impedance of several kilo-ohms, and ultrafast charge-carrier dynamics are therefore obscured by the response time of the high-frequency circuits. We recently introduced an experimental on-chip pump/probe scheme to measure the photocurrent dynamics of electrically contacted nanosystems with a picosecond time-resolution [1]. In the talk, I highlight the picosecond photocurrent dynamics in semiconducting nanowires and graphene [2,3]. In particular, our ultrafast experiments clarify the optoelectronic mechanisms contributing to the photocurrent generation at graphene-metal interfaces. We verify that both built-in electric fields, similar to those in semiconductor-metal interfaces, and a photo-thermoelectric effect give rise to the photocurrent at graphene-metal interfaces at different time scales. Our results open the possibility to design and fabricate graphene-based and nanowire-based ultrafast photodetectors, photoswitches, photovoltaic cells, and THz-sources. [1] L. Prechtel, L. Song, S. Manus, D. Schuh, W. Wegscheider, A.W. Holleitner, Nano Lett. 11, 269 (2011). [2] L. Prechtel, M. Padilla, N. Erhard, H. Karl, G. Abstreiter, A. Fontcuberta i Moral, A.W. Holleitner, Nano Lett. accepted 10.1021/nl300262j (2012). [3] L. Prechtel, L. Song, P. Ajayan, D. Schuh, W. Wegscheider, A.W. Holleitner, Nature Comm. 3, 646 (2012).

Fig.: Graphene is incorporated into a coplanar stripline circuit. A pump laser pulse focused onto the graphene-sheet generates a photocurrent jphoto. The time-resolved photocurrent is measured with a probe pulse focused onto an ultrafast Auston-switch as a function of the time delay.

CFEL-Colloquium:

June 22^nd 2012, 13:15h

Solids in Ultrafast and Strong Optical Fields

Location:  FLASH Hall Seminar Room, DESY-Building 28c

This talk will consider phenomena in insulator nanofilms and bulk crystals subjected to strong and ultrafast optical fields with carrier frequency much below the bandgap. Such fields cause adiabatic phenomena such as the Wannier-Stark localization, formation of quantum bouncers at the surfaces, and anticrossings of adiabatic levels. In the dielectric nanofilms subjected to sufficiently slow fields, the anticrossings of the quantum-bouncer levels of the valence and conduction bands is predicted to lead to adiabatic metallization of the solid¹. In the ultrafast optical fields, a combination of the adiabatic and nonadiabatic effects leads to the increased polarizability of the system making it similar to semiconductors or plasmonic metals². We will also discuss response of a dielectric solid to near-single cycle strong optical fields, where new theoretical and experimental results have been recently obtained in collaboration with MPQ at Garching.

¹ M. Durach, A. Rusina, M. F. Kling, and M. I. Stockman, Metallization of Nanofilms in Strong Adiabatic Electric Fields, Phys. Rev. Lett. 105, 086803-1-4 (2010)

² M. Durach, A. Rusina, M. F. Kling, and M. I. Stockman, Predicted Ultrafast Dynamic Metallization of Dielectric Nanofilms by Strong Single-Cycle Optical Fields, Phys. Rev. Lett. 107, 0866021-5 (2011)