The Winners of the SPS Awards 2006

The SPS award committee, presided by Prof. Piero Martinoli (Uni Neuchâtel), has again been suc­cessful in nominating three young physicists for their outstanding work, presented below.

SPS Award for General Physics, sponsored by ABB

Christian Rüegg  (AG) studied at the ETH Zürich, where he obtained his Diploma in Physics with distinction in 2001 and the PhD in 2005 with a thesis at the Laboratory for Neutron Scattering of the ETH Zürich and the Paul Scherrer Institute under the supervision of Prof. Albert Furrer. At present he is a post-doc at the London Centre for Nanotechnology of the University and Imperial College in London. In his work he used inelastic neutron scatterring techniques to investigate the rich low-temperature phase diagram of quantum magnetic systems by tuning their spin-gap energy with a magnetic field, hydrostatic pressure or composition, thereby discovering a wealth of novel physical phenomena, in particular an ordered phase which is best described by a Bose-Einstein condensa­tion of magnetic quasi-particles.

Investigation and Characterization of the Excitation Spectrum and the Field-, Pressure- and Doping-Induced Quantum Phase Transitions in Quantum Spin Systems

Quantum magnetic systems have in recent years offered diverse opportunities for the study of a broad range of novel physical phenomena including BEC of magnons, Bose-glasses, resonating valence-bond phases and Luttinger-liquid regimes [1]. A possible starting point is hereby a spin Hamiltonian with a quantum-disorder ground-state and gapped singlet-triplet excitations, e.g. Hal­dane, ladder or dimer spin systems. Tuning the spin-gap energy by a magnetic field, hydrostatic pressure or composition results in rich low-temperature phase diagrams, which can be investigated by inelastic neutron scattering (INS) [2]. A magnetic field tunes TlCuCl3 through a quantum critical point (QCP) to an ordered phase, which consists of a mixture of triplet states into the sea of non-magnetic singlets and is best described by a BEC of magnetic quasi-particles. Related to the nature of this novel ground-state for solid-state systems, TlCuCl3 above the field-induced transition shows a very characteristic excitation spectrum, which could be observe for the first time. In contrast, the closely related compound NH4CuCl3 surprisingly features distinct plateaus at fractional values of the magnetic saturation. The microscopic origin of such unconventional behaviour is revealed also by INS in high magnetic fields. The fundamental difference between the two classes of magnetic insulators, which include many other systems currently under investigation, can be understood in analogy with phase transitions observed in ultra-cold atomic gases and electronic conductors [1]. A pressure-induced QCP could additionally be achieved in TlCuCl3 and was investigated in detail by measuring the elementary excitations across the transition. In consequence of these recent studies, the concept of magnetic order as known since a long time needs to be extended by a new class of magnetically ordered phases occurring beyond a quantum phase transition at the lowest temperatures - an exciting topic of current research in solid state physics.

[1] T. M. Rice, Science 298, 760 (2002).
[2] Ch. Rüegg et al., Nature 423, 62 (2003); Phys. Rev. Lett. 93, 037207 (2004); Phys. Rev. Lett. 93, 257201 (2004); Phys. Rev. Lett. 95, 267201 (2005)

SPS Award for Condensed Matter Physics, sponsored by IBM

Patrycja Paruch is Polish. She obtained her Bachelor of Arts "magna cum laude" at the Physics Harvard College in 2000 and the PhD in 2004 with a thesis at the Département de Physique de la Matière Condensée of the University of Geneva under the supervision of Prof. Jean-Marc Triscone. At present she is a post-doc at the Laboratory of Atomic and Solid State Physics of the Cornell University. In her work she used the nanoscale resolution provided by atomic force microscopy to investigate the dynamics of ferroelectric domains and of the thin elestic domain walls separating these domains in epitaxial perovskite thin films, thereby providing a deeper understanding of the mechanisms controlling the pinning and propagation of elastic objects in disordered media. This is of great importance for the electromechanical and information storage applications of perovskite materials.

Ferroelectric domain walls as elastic objects in disordered media

Understanding the mechanisms controlling the pinning and propagation of elastic objects in dis­ordered media is important for a wide range of physical systems. We have used the existing theo­retical framework, combined with the nanoscale resolution provided by atomic force microscopy, to investigate the behavior of ferroelectric domains, or regions with opposite polarization, and of the thin elastic “walls” separating these regions from each other in epitaxial perovskite thin films. In these materials, particularly interesting from the point of view of micro-electromechanical applicati­ons and information storage, we have written ultra-high density domain arrays, with feature size as small as 15-20 nm, which remain stable throughout the period of observation (up to 5 months) [1].

In two series of independent experiments carried out on the same samples, we measured the cha­racteristic exponents  ~ 0.25 and µ ~ 0.6 governing static domain wall roughness [2], and the non-linear response (creep) of domain walls when subjected to a small applied force, respectively [3]. These results give rise to a clear physical picture of domain walls in ferroelectrics as elastic sheets in the presence of “random-bond” disorder, and where dipolar interactions play an important role, effectively increasing the dimensionality of the system, in agreement with theoretical predictions.

[1] P. Paruch, T. Tybell and J.-M. Triscone, APL 79, 530 (2001).
[2] P. Paruch, T. Giamarchi and J.-M. Triscone, PRL 94, 197601 (2004)
[3] T. Tybell, P. Paruch, T. Giamarchi and J.-M. Triscone, PRL 89, 097601 (2002)

SPS Award for Applied Physics, sponsored by Unaxis

Giacomo Scalari is Italian. He obtained is laurea in fisica with "marks 108/110" at the University of Pisa in 1999 and the PhD in 2005 with a thesis at the Institut de Physique of the University of Neuchâtel under the supervision of Prof. Jérôme Faist. Berfore attending the mesoscopic physics group in Neuchâtel, Dr. Scalari worked on the development of innovative micro-instrumentation for surgery at the Scuola Superiore S.Anna in Pisa. At present, he is pursuing his research work as a post-doc in Faist’s group. The work of Giacomo Scalari led to the first demonstration of a quantum cascade laser based on a bound-to-continuum transition operating at Terahertz frequencies and above the technologically important temperature of liquid nitrogen. By investigating the confine­ment effects in QCLs induced by a magnetic field, in a second line of research he has been able to extend the operation of these devices up to wavelength of 220 µm, the longest demonstrated to-date.

Magneto-spectroscopy and development of terahertz quantum cascade lasers

The first research line of this work has led to the demonstration of a THz quantum cascade laser based on a bound-to-continuum transition [1]. The device was the first one to operate above the technologically important temperature of liquid nitrogen. A further development of the bound-to-continuum design has led to laser action with high power (50 mW) at different wavelengths (87-130 µm). In the framework of a collaboration, such devices have been employed by Agilent Technolo­gies to demonstrate THz imaging at different wavelengths.

The second research line has been focused on the magneto-spectroscopy study of the THz quan­tum cascade lasers and on the development of devices specially designed for operation in a mag­netic field. With this approach it has been possible to extend the frequency operation of the quan­tum cascade laser down to 1.39 THz (220 µm wavelength), the lowest demonstrated to-date. The confinement induced by the magnetic field radically modifies the physics of the system allowing laser action with extremely reduced threshold current densities (less than 1 A/cm2) [2] and leading to the demonstration of a two color THz quantum cascade laser.

[1] G. Scalari, L. Ajili, J. Faist, H. Beere, E. Linfield, D. Ritchie, and G. Davies, “Far-infrared (λ= 87 µm) bound-to-continuum quantum-cascade lasers operating up to 90 K”, Appl. Phys. Lett., vol. 82, no. 19, pp. 3165-3167 (2003).
[2] G. Scalari, S. Blaser, J. Faist, H. Beere, E. Linfield, D. Ritchie, and G. Davies, “Terahertz emission from quantum cascade lasers in the quantum Hall regime: evidence for many body resonances and localization effects”, Phys. Rev. Lett.,vol. 93, pp. 237403-1,237403-4