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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)}

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 ζ ~ 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)}

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}