Luciano Reatto retired from his Chair in Physics at the University of Milan at the end of 2010. On this occasion, the international scientific communities in the fields of Classical and Quantum Fluids, Statistical Physics and Soft Matter gathered to celebrate one of their most distinguished members by dedicating a Special Issue of Molecular Physics to Luciano. 35 contributions were received from all over the world, spanning a broad range of topics, reflecting the strongly interdisciplinary character of Luciano's activity, as well as the esteem and affection of a large and diverse community. Molecular Physics is particularly well suited for such a compendium, being a reference journal for such a broad range of subjects.
Luciano ‘formally’ retired one year ago, but those who work at the Physics Department in Milano are likely to run into him talking to students and collaborators, attending seminars or lecturing, happy to be exempt from committee meetings and bureaucratic nuisances. There is no doubt that he is still very active and definitely intrigued by the latest advances in Condensed Matter Physics, and prepared to contribute to the cross-fertilization of many fields, which has always been one of the most distinctive marks of his research style.
Luciano was born in Northern Italy at the foot of the Dolomites. He obtained the Laurea in Physics at the University of Milan in 1963, where he spent two more years as a Research Associate. Then he moved to the Laboratory of Atomic and Solid State Physics of Cornell University until 1967 before returning to Italy, where he lectured at the University of Parma and at the Polytechnic University of Milan. In 1976 he was appointed full professor, thereafter pollinating several institutions: the Universities of Catania, Ferrara, Parma and finally Milano. A small but enthusiastic group of young scientists active in Theoretical Condensed Matter Physics grew in all the Physics Departments encountered by Luciano, who played a key role in spreading across the Italian community the fascination of the subtle and deep conceptual issues pervading the intriguing world of Statistical Physics.
Since his return to Italy in 1971 Luciano was invited as visiting professor by several foreign universities where he spent sabbatical leaves, including Helsinki, Cornell, New York University, Toronto, Barcelona, Paris VI and Paris VII. He also served as a member or chairman of several scientific boards, such as the Liquids Section of the European Physical Society (1997–1999), the Scientific Board of the Italian Institute of the Physics of Matter (2001–2003) or the Scientific Board of the International Centre for Theoretical Physics in Trieste (2000–2004).
It is impossible for us to summarize in a few lines almost 50 years of intense scientific activity in Theoretical Condensed Matter Physics, which gave rise to more than 250 papers in international journals. Within the variety of subjects of Luciano's contributions, ranging from quantum magnetism to superfluidity, from critical phenomena to pattern formation in colloids, one can always detect a leitmotiv: a unifying view of the mechanisms leading to phase transitions and to the onset of order in complex systems, whose understanding requires universal concepts and methods, despite the apparent diversity of the theoretical frameworks.
In the mid 1960s, at Cornell, Luciano engaged a long lasting collaboration with Geoffrey Chester, leading to the development of variational Jastrow-type wave-functions for the description of both the ground state and the low energy spectrum of superfluid Helium. The interest in the superfluid properties of correlated quantum fluids has been a central theme in the scientific activity of Luciano over almost 50 years. Already in these early seminal papers the ‘contamination’ between the Quantum Many Body formalism and some concepts taken from Classical Statistical Physics clearly emerges. Methods borrowed from Liquid State Theory allowed him to unveil the relationship between the structure of the variational wave-function and the occurrence of off-diagonal long-range order in the system. Later, at the University of Parma, Luciano, in collaboration with E. Rastelli and A. Tassi, examined the possibility of exotic ordering in frustrated quantum magnets. The investigation of the liquid state of matter, which occupies a central part of Luciano's activity, includes among the most significant results, the development of integral equations for dense fluids, in collaboration with Neil Aschroft, and an ingenious solution of the inverse problem via a combination of integral equations and Monte Carlo simulations. The latter, aimed at extracting the interaction potential from structural data, was obtained in collaboration with two other key figures of the research in liquid state theory, namely Dominique Levesque and Jean-Jaques Weis, to whom this journal dedicated earlier Special Issues. Around that period, one of us (AP) had the lucky opportunity to become Luciano's first PhD student. The main outcome of a long and ongoing collaboration was the Hierarchical Reference Theory of Fluids (HRT) designed to reconcile the field theoretical treatment of critical phenomena, embodied in the Renormalization Group approach developed some ten years earlier, with the microscopic point of view at the basis of Liquid State Theory. HRT was later successfully generalized to lattice models, fluid mixtures and quantum systems with the help of several students and collaborators, among whom Alberto Meroni, Davide Pini and Maria Tau. Switching from simple to complex fluids, Luciano investigated soft-core particle and pattern formation in systems with competing interactions, in collaboration with Federica Lo Verso and Alessandra Imperio.
But Luciano never forgot his ‘first love’, namely Quantum Fluids. During a discussion with Mal Kalos at New York University the Shadow Wavefunction was conceived. This very successful variational representation of superfluid Helium allows one to accurately describe the freezing transition, to obtain unprecedented accuracy in the excitation spectrum, to understand the properties of superfluidity in confined geometry and naturally led to the investigation of supersolidity. These studies were carried out by Luciano's group at the University of Milano, including Silvio Vitiello and Davide Galli. Finally, the experimental discovery of Bose–Einstein condensation in cold atoms in the mid 1990s triggered a new line of research. Thanks to the driving force of Luca Salasnich, a simple but remarkably accurate non-linear Schrödinger equation for the study of the dynamics of a Bose condensate in anisotropic traps was derived.
Although on first approach Luciano appears somewhat formal and reserved, after spending some time (typically a few decades) with him will unveil unexpected sides of his personality, like a definite talent for cooking, and a pronounced appreciation of haute cuisine. In the best Italian tradition, Luciano is a very civilized and highly cultivated person who will engage in lively conversations and debates on a broad range of subjects, once the initial ‘potential barrier’ has been overcome.
Both those who had a chance to work with him and those who just met him through his papers or lectures are well aware of the depth and breadth of Luciano's contribution to the Theory of Classical and Quantum Fluids, which remain a landmark for all his former students as well as for an ever growing community in Statistical Physics. Wishing a happy and always productive ‘retirement’, the Classical and Quantum Fluid community is proud to pay tribute to Luciano, a discreet but highly respected player in the scientific arena.