The Winners of the SPS Awards 2011

As every year, it was not an easy task for the SPS Award committee, presided by Prof. Louis Schlapbach, to select the three awardees from all the submitted, high quality canditatures.
The winners had the opportunity to present their outstanding work in the course of the joint annual meeting. The laudationes (written by L. Schlapbach) and summaries (written by the respective authors) are printed below.

Leander Schulz (IBM Award), Amaury Triaud (ABB Award)
Dirk van der Marel (representative of the award committee), Maria Amanti (Oerlikon Award), Christophe Rossel (SPS President)
SPS Award for General Physics, sponsored by ABB

A team of scientists of the "Wide Area Search for Planets" (WASP) project of a consortium of UK universities in collaboration with the Geneva Observatory has found a new exoplanet (planet outside our solar system) of unusually large size and very low density, named WASP-17. The extraordinary characteristics of WASP-17 - which sets it apart from the earlier discovered other 16 exoplanets - is, that WASP-17 orbits the "wrong way" around its host star, 1000 light years away from the solar system. Since planets form out of the same swirling gas cloud that creates a star, they are expected to orbit in the same direction that the star spins.

Amaury Triaud (Geneva Observatory) and David Anderson (Keele University), both PhD students, made the main observations resulting in the discovery of the first planet known to have a "retrograde" orbit. Their likely explanation is that WASP-17 was involved in a near collision with another planet early in its history. The discovery casts new light on how planetary systems form and evolve, and confirms that newly formed solar systems can be violent places. The WASP-South camera array that led to the discovery of WASP-17 is hosted by the South African Astronomical Observatory. The results were published under the title "Spin-orbit angle measurements for six southern transiting planets: New insights into the dynamical origins of hot Jupiters" in Astronomy & Astrophysics 524, A 25 (2010) with Amaury Triaud as first author.

The Swiss Physical Society honors Amaury Triaud with the ABB prize 2011.

Spin-orbit angle measurements for six southern transiting planets: New insights into the dynamical origins of hot Jupiters

Several competing scenarios for planetary-system formation and evolution seek to explain how hot Jupiters came to be so close to their parent stars. Most planetary parameters evolve with time, making it hard to distinguish between models. The obliquity of an orbit with respect to the stellar rotation axis is thought to be more stable than other parameters such as eccentricity. Most planets, to date, appear aligned with the stellar rotation axis; the few misaligned planets so far detected are massive ( > 2 MJ ).
Our goal is to measure the degree of alignment between planetary orbits and stellar spin axes, to search for potential correlations with eccentricity or other planetary parameters and to measure long term radial velocity variability indicating the presence of other bodies in the system.
For transiting planets, the Rossiter-McLaughlin effect allows the measurement of the sky-projected angle β between the stellar rotation axis and a planet’s orbital axis. Using the HARPS spectrograph, we observed the Rossiter-McLaughlin effect for six transiting hot Jupiters found by the WASP consortium. We combine these with long term radial velocity measurements obtained with CORALIE. We used a combined analysis of photometry and radial velocities, fitting model parameters with the Markov Chain Monte Carlo method. After obtaining β we attempt to statistically determine the distribution of the real spin-orbit angle ψ.
We found that three of our targets have β above 90°, the other three have angles compatible with 0°. We find no dependence between the misaligned angle and planet mass nor with any other planetary parameter. All six orbits are close to circular, with only one firm detection of eccentricity. No long-term radial acceleration was detected for any of the targets. Combining all previous 20 measurements of β and our six and transforming them into a distribution of ψ we find that between about 45 and 85% of hot Jupiters have ψ > 30°.
Most hot Jupiters are misaligned, with a large variety of spin-orbit angles. We find observations and predictions using the Kozai mechanism match well. If these observational facts are confirmed in the future, we may then conclude that most hot Jupiters are formed from a dynamical and tidal origin without the necessity to use type I or II migration. At present, standard disc migration cannot explain the observations without invoking at least another additional process.

Triaud, A. H. M. J., Collier Cameron A., Queloz, D. et al. 2010, Astronomy & Astrophysics 524, 25
Triaud, A. H. M. J., Queloz, D., Hellier, C., et al. 2011, Astronomy & Astrophysics 531, 24

 

 

SPS Award for Condensed Matter Physics, sponsored by IBM

Semiconductor physics and its so successful application started with the availability of highest purity semiconductor crystals and their accurate doping with impurities which deliver negative and positive charge carriers. Spintronics as a future technology will also make use of the spin of the charge carriers. Leander Schulz together with an European team of scientists made a substantial step in the development of spintronics. They succeded in the control and the direct detection of the spin polarization of charge carriers extracted from an intelligently engineered polar interface layer of an organic-inorganic semiconductor device. Low energy muon spin rotation was the experimental tool of the work. The results were published under the title "Engineering spin propagation across a hybrid organic/inorganic interface using a polar layer" in Nature Materials 10, 39 (2011). Leander Schulz is first author of that publication, he was responsable for the experimental measurements and for the analysis and interpretation of the results, together with colleagues.

The Swiss Physical Society honors Leander Schultz with the IBM prize 2011.

Engineering spin propagation across a hybrid organic/inorganic interface using a polar layer

State-of-the-art organic spintronics devices are typically based on ferromagnetic metallic layers, which are separated by at least one semiconducting organic thin film. One of the main aims of the current research in this field is the controlled manipulation of the spin degree of freedom of the charge carriers, since this is a main prerequisite of future spintronics devices, like the spin organic light-emitting diode (spin-OLED) or the spin organic field-effect transistor (spin-OFET). A classical approach to manipulate the spin-related properties in these devices is the modification or the functionalisation of the interfaces, a research field which was recently named “spinterface science” [1].
In this work, it is shown that the sign of the spin polarisation of the charge carriers, which are injected from the ferromagnetic metal electrode into the semiconducting organic layer, is reversed by inserting an additional polar layer between the metal and the organic layer. This is demonstrated by comparing a sample consisting of permalloy (NiFe), aluminium quinolate (Alq3) and iron-cobalt (FeCo) with a second sample of nominally identical composition apart from an additional polar layer (lithium fluoride, LiF) between the NiFe and the Alq3 layer. The insertion of the polar layer gives rise to a shift of the vacuum level in the Alq3 layer relative to the vacuum level of the NiFe layer due to the electric field generated by the polar LiF layer. The highest occupied molecular orbital level of the Alq3, in which in this case the spin-polarised charge carriers (holes) move, is therefore also shifted and is now aligned with a different energy level of the NiFe, where the density of states in the NiFe is now higher for the other spin species as compared to the sample without the vacuum level shift (without LiF). Since only one interface is modified, the magnetoresistive properties are thus reversed, i.e. the magnetoresistance changes from negative to positive values. Furthermore, the obtained results imply that the spin polarisation of the injected charge carriers in such a device can in the future be controlled by choosing the sign and the magnitude of the polarity of such an interface layer.
The measurements were performed with the low-energy muon spin rotation technique, which allows for the measurement of the spin polarisation of charge carriers at buried interfaces at a local, i.e. microscopic, level [2]. It was found that the measured spin polarisation at the NiFe/Alq3 interface was reversed by introducing the LiF layer. The obtained results were confirmed by macroscopic magneto-transport measurements, where an inversion of the magnetoresistance was observed.

[1] S. Sanvito, “The rise of spinterface science”, Nature Materials 6, 562 (2010).
[2] A. J. Drew et al., “Direct measurement of the electronic spin diffusion length in a fully functional organic spin valve by low-energy muon spin rotation”, Nature Materials 8, 109 (2009).

 

 

SPS Award for Applied Physics, sponsored by OC Oerlikon

Terahertz waves excite the vibrational/rotational frequencies of large molecules and can penetrate disordered, e.g. polymeric materials. There is a great interest for terahertz sources in security, fabrication and environmental control as well as for medical applications. Quantum cascade lasers, semiconductor laser sources based on intersubband transitions in quantum wells, are the only solid-state fundamental oscillators covering the terahertz region. Maria I. Amanti has developed as her PhD work a novel approach for cavities of terahertz quantum cascade lasers to overcome processing and conceptual difficulties: By using a dry etching technique, she achieved the fabrication of high aspect ratio ridge structures with a lateral distributed feedback operating as a third order grating which provides at the same time the feedback for the laser mode and the outcoupling to the free space. In this way, a single mode operation was achieved at a frequency defined by the grating periodicity. The teeth of the grating do operate as a phased array and collimate the beam into a narrow, symmetric spot. She combined excellent experimental and theoretical skills to achieve these results. They were published in four articles with Maria Amanti as the first author in high impact journals, among which a publication in Nature Photonics.

The Swiss Physical Society honors Maria I. Amanti with the Oerlikon prize 2011.

Photonics for terahertz quantum cascade lasers

This work deals with novel strategies for the quantum design of the active material for THz quantum cascade lasers as well as with innovative resonator concepts for these sources.
The first part of the work led to realization of a new active region design for high temperature operation. This design, based on a four wells scheme, exhibits a very nice combination of good high temperature performance, relative insensitivity to slight inaccuracies in the epitaxial growth and good high power operation [1].
The second part of this work was devoted to the engineering of the laser resonators. The most innovative result was the development of third-order distributed feedback lasers. The idea, borrowed from the use of such gratings in antenna structures, is to exploit the whole device surface for radiation emission but to collimate the latter in the plane of the device, as in a conventional ridge laser. In this way the impedance matching problem at exit of the metal-metal waveguide is solved in a configuration that is perfectly suited to the TM polarization rule of intersubband transitions. The lasers realized neatly demonstrate the usefulness of the approach, with record-high output slope-efficiencies and almost ideal single-lobe radiation patterns [2]. Furthermore, the concept has been extended to ridges of complete sub-wavelength dimensions, showing that also in this case a nice collimated beam can be produced, defying in some sense the expected diffraction limit.

[1] M. I Amanti, G. Scalari, R. Terazzi, M. Fischer, M. Beck, J. Faist, A. Rudra, P. Gallo and E. Kapon, "Bound-to-continuum terahertz quantum cascade laser with a single quantum well phonon extraction/injection stage", New. J. Phys. 11 125022b (2009)
[2] M. I. Amanti, M. Fischer, G. Scalari, M. Beck and J. Faist, "Low-divergence single-mode terahertz quantum cascade laser", Nature Photonics 3, 586-590 (2009)