Critical Behavior of the Supersolid transition in Bose-Hubbard Models

We study the phase transitions of interacting bosons at zero temperature between superfluid (SF) and supersolid (SS) states. The latter are characterized by simultaneous off-diagonal long-range order and broken translational symmetry. The critical phenomena is described by a long-wavelength effective action, derived on symmetry grounds and verified by explicit calculation. We consider two types of supersolid ordering: checkerboard (X) and collinear (C), which are the simplest cases arising in two dimensions on a square lattice. We find that the SF--CSS transition is in the three-dimensional XY universality class. The SF--XSS transition exhibits non-trivial new critical behavior, and appears, within a $d=3-\epsilon$ expansion to be driven generically first order by fluctuations. However, within a one--loop calculation directly in $d=2$ a strong coupling fixed point with striking ``non-Bose liquid'' behavior is found. At special isolated multi-critical points of particle-hole symmetry, the system falls into the 3d Ising universality class.Comment: RevTeX, 24 pages, 16 figures. Also available at http://www.cip.physik.tu-muenchen.de/tumphy/d/T34/Mitarbeiter/frey.htm

Quantum and Classical Ballistic Transport in Constricted Two-Dimensional Electron Gases

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Stepwise unfolding of collapsed polymers

Motivated by recent experimental data on DNA stretching in presence of polyvalent counterions, we study the force-induced unfolding of a homopolymer on and off lattice. In the fixed force ensemble the globule unravels via a series of steps due to surface effects which play an important role for finite-size chains. This holds both for flexible and stiff polymers. We discuss in a qualitative way how this result may impact on the interpretation of DNA stretching experiments showing peaks in the characteristic curves, by extracting from the raw data the corresponding elongation-versus-force characteristic curves. Furthermore, approximate analytical and numerical calculations, valid in a quasi-equilibrium fixed stretch ensemble, and if the initial low-temperature state is ordered in a spool, show that the average force versus elongation displays peaks related to the geometry of the initial configuration. We finally argue how the proposed mechanisms identified for the arising of peaks may couple in the experiments, and comment on the role of dynamic effects