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## Bio

Oakland University

Department of Physics

Associate Proessor

Ph.D. Instituto Balseiro, Bariloche, Argentina

Research Interests

• Electron transport at low temperatures

• Quantum fluctuations

Many electron properties in two layer systems. In 1992, together with G. D. Mahan, Dr. Rojo discovered the effect of non-dissipative drag (NDD) on superconductors and mesoscopic systems. He plans to continue this line of research, exploring various applications of this fascinating effect. Dr. Rojo's work in this area has stimulated significant experimental and theoretical activity. NDD results from the coupling of the zero point charge fluctuations between two systems with no tunneling from one to the other. Dr. Rojo has discussed and summarized its current status and its relation with the dissipative current drag in his recent review article. In collaboration with his graduate student Joe Baker he has studied both analytically and by two different numerical methods the effect of disorder on NDD in order to make contact with experiments. A related effect that has bearing on the coupling between non-tunneling systems is the eddy current coupling between a superconductor and a normal, highly conducting system. He is involved in an ongoing collaboration with the experimental group of C. Thomsen and A. Goñi at the Technische Universität in Berlin, where the effect was observed for the first time in the InSb/GaAs system. The experimental results are in quantitative agreement with Dr. Rojo's theoretical predictions. He is seeking external funding to strengthen the collaboration in which further ramifications of this very interesting and significant effect will be explored.

Squeezing and control of quantum noise. Another project that has been particularly successful since Dr. Rojo's arrival at Michigan was his work on phonon squeezing, a field that falls within his interest in zero point fluctuations. In preliminary calculations he had identified the mechanism of pulses acting on harmonic systems as a means of producing squeezing. For the case of phonons the effect corresponds to a time modulation of the amplitude of the zero point fluctuations in the atomic positions within the solid. Dr. Rojo started collaboration with R. Merlin’s group, who measured the effect using ultra fast optical pulses. The experiment constituted the first observation of the squeezing effect in condensed matter, and could have exciting future applications in device physics and in several areas where, in general, a “stroboscopic” control over the quantum noise might be necessary. A very important question to be addressed in the future is: what other excitations can be squeezed in condensed matter, and what are the possible applications? Part of Dr. Rojo's future research effort will be devoted to answering these questions.

Role of confinement in high temperature superconductivity. Before arriving in Michigan Dr. Rojo did some important work on high temperature superconductors. Since his arrival he has continued working on some problems within this field. With his former graduate student Mathew Reilly, Dr. Rojo solved the two-magnon Raman scattering problem, showing that some recent experiments can be understood using a spin-phonon model without disorder in the non-adiabatic approximation. The study of High Tc superconductors has motivated an intense study of spin systems and Heisenberg, e.g. spin ladders, where the issue of a gapless versus gapped spectrum of excitations is the subject of experimental and theoretical study. He contributed to that subfield by providing a proof, extending the Lieb-Mattis theorem, that spin ladders with an odd number of legs are gapless. His recent work on confinement on c-axis transport addresses the fundamental issue of whether correlations can give rise to a “confined” phase in which transport is coherent in two spatial directions, and incoherent in the third. This is an unresolved many-body problem, the detailed study of which originates in P. W. Anderson’s conjecture that the ideas and paradigms of one-dimensional non-Fermi liquids can be extended to two and three-dimensional systems. In collaboration with C. Balseiro from Bariloche (Argentina) Dr. Rojo considered the strongly correlated anisotropic system, proposed and solved a model using a new slave-fermion scheme, and showed that a confinement transition emerges naturally from the solution. This collaboration is funded by the National Science Foundation through its international program, and has proven very fruitful. The researchers have also approached two other significant problems within High Tc superconductivity: the effect of disorder on d-wave pairing, and the problem of resistance at the melting point of a vortex lattice. Dr. Rojo plans to continue studying the issue of confinement. This will be the subject of the Ph.D. thesis of a graduate student in Bariloche who is studying finite anisotropic systems using the Lanczos method.

Bose-Einstein condensation. The field of Bose-Einstein condensation is one of the most exciting problems in physics. Due to its observation in supercooled atomic systems, the problem combines knowledge from condensed matter and atomic physics. For example, a condensate can be produced of Rb atoms in two internal states, which invites analogies with anisotropic magnetic systems. Dr. Rojo has proven an interesting theorem that establishes the regimes of phase separation for these kind of condensates. Also, in collaboration with P. Berman (Atomic Physics) at the University of Michigan, he studied the so-called Talbot oscillations, already well known for independent atoms, and their modification in the presence of a Bose-Einstein condensate. The goal was to understand the effects that atom-atom interactions will have on the Talbot oscillations. Since the atom-atom interaction makes the problem an unsolved many-body problem, one has to resort to approximations. To approach this problem I have proposed a simplified version that can be solved exactly. The simplification consists of treating the problem in one dimension, and mapping strongly interacting (hard-core) bosons to free fermions. This trick, originally introduced by M. Girardeaux, can be proven to work in this case and we describe the interplay of collision and quantum coherence in an exact framework. Dr. Rojo's work has already attracted some attention and has motivated interesting extensions.