Fractional quantization of the magnetic flux in cylindrical unconventional superconductors.

The magnetic flux threading a conventional superconducting ring is typically quantized in units of Φ0=hc/2e. The factor of 2 in the denominator of Φ0 originates from the existence of two different types of pairing states with minima of the free energy at even and odd multiples of Φ0. Here we show that spatially modulated pairing states exist with energy minima at fractional flux values, in particular, at multiples of Φ0/2. In such states, condensates with different center-of-mass momenta of the Cooper pairs coexist. The proposed mechanism for fractional flux quantization is discussed in the context of cuprate superconductors, where hc/4e flux periodicities were observed.

Quasiparticle anisotropy and pseudogap formation from the weak-coupling renormalization group point of view.

Using the one-loop functional renormalization group technique, we evaluate the self-energy in the weak-coupling regime of the 2D t-t(') Hubbard model. At van Hove (vH) band fillings and at low temperatures, the quasiparticle weight along the Fermi surface (FS) continuously vanishes on approaching the (pi,0) point where the quasiparticle concept is invalid. Away from vH band fillings the quasiparticle peak is formed inside an anisotropic pseudogap and the self-energy has the conventional Fermi-liquid characteristics near the Fermi level. The spectral weight of the quasiparticle features is reduced on parts of the FS between the near vicinity of hot spots and the FS points closest to (pi,0) and (0,pi).

Spin-charge separation in Aharonov-Bohm rings of interacting electrons.

We investigate the properties of strongly correlated electronic models on a flux-threaded ring connected to semi-infinite free-electron leads. The interference pattern of such an Aharonov-Bohm ring shows sharp dips at certain flux values, determined by the filling, which are a consequence of spin-charge separation in a nanoscopic system.

Detection of topological transitions by transport through molecules and nanodevices.

We analyze the phase transitions of an interacting electronic system weakly coupled to free-electron leads by considering its zero-bias conductance. This is expressed in terms of two effective impurity models for the cases with and without spin degeneracy. Using the half-filled ionic Hubbard ring, we demonstrate that the weight of the first conductance peak as a function of external flux or of the difference in gate voltages between even and odd sites allows one to identify the topological charge transition between a correlated insulator and a band insulator.