Die Gewinner der SPG-Preise 2008

Thorsten Lisker obtained his diploma in physics in 2003 at the University of Erlangen-Nürnberg. The topic of his diploma thesis was already in his preferred field, namely astrophysics. He then realized that the small neighboring country in the south might offer him further interesting insight into the world of stars and galaxies. After one year at ETHZ he went to the University of Basel for his PhD work that he completed a year ago with the highests honors – summa cum laude. His thesis asks a question: "Early-type Dwarf Galaxies in the Virgo Cluster: Nature or Nurture ?"

Galaxy formation and evolution is a central problem of today’s astrophysics. Analysing data obtained by an impressive photometric and spectroscopic sky survey project, Thorsten Lisker performed the first systematic classification of the dwarf galaxy population of the Virgo cluster. He identified subpopulations with statistically significant different age, chemical composition, structure and dynamics within the galaxy cluster. He even succeeded – in a highly competitive process – in obtaining observing time for follow-up studies with the Very Large Telescope of the European southern Observatory in Chile. He has already 13 papers in his publication list and his PhD work has significantly advanced the understanding of galaxy evolution.

Dr. Lisker is now back in his home country as a post-doctoral researcher at the "Zentrum für Astronomie" of the University of Heidelberg where he is organizing his research group showing convincing leadership abilities – as we can read in one of the recommendation letters. He has already obtained a prize in 2007: the Camille & Henry Dreyfus Award, given by the University of Basel. So – in case he continues to have a prize every year – the Nobel prize may be his in due time….

Early-type Dwarf Galaxies in the Virgo Cluster: Nature or Nurture ?

The light of galaxy clusters, the largest gravitationally bound structures in the Universe, is dominated by a few dozen very bright and massive galaxies. However, a closer look reveals a huge number of faint and small "dwarf" galaxies, of which more than a thousand reside in the nearby Virgo galaxy cluster, 50 million light years away. Most of them belong to the class of dwarf elliptical galaxies, which appear to be of rather simple and homogeneous structure and consist mostly of old stars. Surprisingly, though, our detailed study [1] of these galaxies with the multicolour imaging data of the Sloan Digital Sky Survey revealed a pronounced complexity with regard to their internal composition, their location in areas of different mass density, and most importantly, their origins.

For many galaxies, a careful image analysis uncovered weak spiral arms and other disk features, with a structure similar to that of giant spiral galaxies and unlike that of faint star-forming galaxies that are commonly discussed as possible progenitors. But there are also other subpopulations of dwarf ellipticals that do not show spiral features, yet still violate the common picture of a spheroidal object located in the galaxy cluster's central region: instead, they exhibit a flat shape, similar to a thick disk, and are preferentially found in the outskirts of the cluster. In some of them, we are even witnessing the very last stages of star formation, in which the final few percent of stellar mass will be created before all gas is consumed.

Only those dwarf ellipticals hosting a compact, bright star cluster in their center follow the classical conception of this galaxy class: they are spheroidal, prefer high-density regions, and consist of stars that are older and more enriched in heavy elements than those of the other subclasses. Even when taking into account possible transitions between the subclasses, this diversity in the galaxy properties cannot be explained with just a single formation scenario responsible for all dwarf ellipticals [2]. Instead, there must be at least two different mechanisms creating these galaxies. One might be the combined physical processes acting in galaxy clusters, continuously producing new dwarf ellipticals through gas stripping and structural transformation of infalling galaxies. The other one might simply be cold dark matter structure formation, which should have yielded a large number of small dark matter "halos" that today are found deep within the gravitational potential of the galaxy cluster, each one being illuminated by its own little dwarf galaxy.

[1] T. Lisker, E. K. Grebel, B. Binggeli, and K. Glatt, "Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. III. Subpopulations: Distributions, Shapes, Origins", The Astrophysical Journal 660, 1186 (2007)
[2] T. Lisker, E. K. Grebel, and B. Binggeli, "Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. IV. The Color-Magnitude Relation", The Astronomical Journal 135, 380 (2008)

Lorenz Meier has been what we call a "mobile student": he studied physics at ETH Zürich and at the University of Lund in Sweden. His PhD work for which he gets his prize was done in the framework of a joint research project between the Laboratory of Solid State Physics at ETH in Zürich and the IBM Research Lab in Rüschlikon (I can garantee you that the IBM member in the prize committee took a very neutral point of view when we evaluated the different candidates…).

I can imagine that electrons in quantum wells are really afraid of Lorenz Meier since they have to dance according to his will ! He has developed methods allowing to manipulate the spin of these electrons, either by magnetic or – what is less obvious – by electric fields. The latter case is more promising for practical applications, particularly in computing and information processing. He is using basic physics that tells us that – in the reference frame of a moving electron – electric fields transform into magnetic fields which then influences the electronic spin degree of freedom.

His work is already internationally recognized and a Nature Physics paper presents his achievements in electric-dipole-induced spin resonance. However, his CV shows that his interests go far beyond physics: he has been a useful employee of his home canton of St. Gallen for a couple of years by programming software for tax calculation and staff management, and our SBB company has profited from his implementation and testing of the braking parameters for a new high speed train line running with the European Train Control System (ETCS). This is perhaps not so surprising, since, as a good student, Lorenz knows what European Credit Transfer System (ECTS) means and then easily switched to ETCS…

Manipulation of electron spins in quantum wells with magnetic and electric fields

We report on the manipulation of electron spins confined in a GaAs/InGaAs quantum well with magnetic stray fields from ferromagnetic structures and with electric fields via the spin-orbit interaction.

The coherent precession of electron spins in the magnetic stray field below an array of Fe stripes is measured. Comparing with reference stripes made of non-magnetic Au, we find an enhancement of the spin precession frequency proportional to the Fe magnetization, which we can attribute to the effect of the magnetic stray field emanating from the magnetized Fe bars. By applying a gate voltage to an interdigitated grating of Fe stripes, the electrons are displaced within the inhomogeneous magnetic stray field and we achieve electric control of the spin precession frequency.

Electric fields can manipulate the electrons spin via the spin-orbit interaction. We present a method that allows the separate determination of both Rashba and Dresselhaus contributions to the effective spin-orbit magnetic field. We use an external a.c. electric field to bring the electrons into an in-plane oscillatory motion. Depending on the orientation of this motion with respect to the crystal lattice, the electrons are subject to a varying spin-orbit magnetic field. By investigating the electron spin precession frequency as a function of their movement direction, the strength of the Rashba and Dresselhaus spin-orbit magnetic fields, and their coupling constants, can be extracted. In another experiment, we use these spin-orbit fields to trigger electron spin resonance with solely electric fields, in this context referred to as "electric–dipole–induced spin resonance".

References:
L. Meier, G. Salis, N. Moll, C. Ellenberger, I. Shorubalko, U. Wahlen, K. Ensslin, E. Gini, "Optimized stray-field-induced enhancement of the electron spin precession by buried Fe gates", Appl. Phys. Lett 91, 162507 (2007).
Lorenz Meier, Gian Salis, Ivan Shorubalko, Emilio Gini, Silke Schön, Klaus Ensslin, "Measurement of Rashba and Dresselhaus spin–orbit magnetic fields", Nature Physics 3, 650– 654 (2007).

Andrea Guarino, of Swiss and Italian nationality, obtained his university degree in nuclear engineering – with mathematical-physical option – at the Politecnico di Milano. With his well equilibrated background in theoretical and applied aspects of science he performed his PhD work in the Nonlinear Optics Group at the ETH in Zürich, dealing with "Electro-optic microring resonators in inorganic crystals for photonic applications". Dr. Guarino is now investing his skills and knowledge into applied projects: he has a "High Power Laser Engineer" position in the R&D Department of the Bookham company in Zürich.

The optical microresonators studied by Dr. Guarino, with the shape of a ring with dimensions of 100 μ, have a light transmission spectrum that can be tuned by the size and the optical properties of the material. Using new technology he has succeeded, for the first time, to produce such resonators made of lithium niobate. This is already a well known and appreciated material for different kinds of optical applications, but Dr. Guarino’s innovation consisted in using the "ion slicing" technique in order to put submicrometer thin films of the material onto all kinds of substrates. This allows to produce a large difference in refractive index between the wave guide and the cladding material. This achievement opens fascinating new prospects for integrated nonlinear photonic devices.

Andrea Guarino says in his CV: "I love physics, optics and technology in general". No doubt he will continue with great enthusiasm to build a bridge between basic quantum physics and useful technological applications.

Electro-optic microring resonators in inorganic crystals for photonic applications.

Optical microring resonators are one of the essential tools for the development of new highly integrated photonic applications. The resonator consists of a waveguide bent in a ring-shape of micrometric size, coupled to an external straight waveguide which carries the light signal to and from the cavity. These resonators can be used as ultrafast compact filters and switches for telecommunication applications, as sensors, or as miniaturized wavelength generators. Very attractive features can be obtained by embedding electrooptical and nonlinear optical properties in the cavity. This work investigates the fabrication of microring resonators in ferroelectric crystals, which have outstanding electro-optical and nonlinear optical coefficients. In particular, the first realization of an electro-optic microring resonator in lithium niobate is presented. The ring is structured on a submicrometric crystalline film of lithium niobate obtained by an improved crystal ion slicing and bonding technique. The slicing of a virgin crystal is obtained by a proper combination of light ion implantation and heat treatment of lithium niobate, whereas the bonding of the thin film on a lowrefractive index substrate is ensured by an adhesive polymer. The method allows for high refractive index contrast thin films, the ultimate requirement for ultra-small bent resonators. The preservation of the electro-optical properties of the thin film is shown by tuning the resonance frequency of the resonator by applying an external electrical field.

Reference: A. Guarino et al, Electro-optically tunable microring resonators in lithium niobate, Nature Photonics, 1, p. 407 (2007).