262 research outputs found
Numerical Renormalization Group Analysis of Interacting Quantum Dots
Wilson's Numerical Renormalization Group (NRG) is so far the only
nonperturbative technique that can reliably access low-energy properties of
quantum impurity systems. We present a recent extension of the method, the
DM-NRG, which yields highly accurate results for dynamical quantities at
arbitrary frequencies and temperatures. As an application, we determine the
spectrum of a quantum dot in an external magnetic field. Furthermore, we
discuss magnetic impurities with orbital degeneracy, which have been inferred
in recent experiments on quantum dots in an Aharonov-Bohm geometry. It is
demonstrated that for spinless electrons, interference between neighbouring
levels sets the low-energy scale of the system. Switching on an external field
leads to a remarkable crossover into a regime dominated by orbital Kondo
screening. We predict that the broadening-induced level splitting should be
clearly visible in measurements of the optical absorption power. A more general
model including the electron spin is studied within an extended two-band NRG
procedure. We observe competition between interference and Kondo screening,
similar to the situation in two-impurity models (RKKY).Comment: Invited talk at the DPG spring meeting 2001, to appear in Advances in
Solid State Physics 4
Flavor Degeneracy and Effects of Disorder in Ultracold Atom Systems
Cold atoms in optical lattices offer an exciting new laboratory where quantum
many-body phenomena can be realized in a highly controlled way. They can even
serve as quantum simulators for notoriously difficult problems like
high-temperature superconductivity. This review is focussed on recent
developments and new results in multi-component systems. Fermionic atoms with
SU(N) symmetry have exotic superfluid and flavor-ordered ground states. We
discuss symmetry breaking, collective modes and detection issues. Bosonic
multi-flavor ensembles allow for engineering of spin Hamiltonians which are
interesting from a quantum computation point of view. Finally, we will address
the competition of disorder and interaction in optical lattices. We present a
complete phase diagram obtained within dynamical mean-field theory and discuss
experimental observability of the Mott and Anderson phases.Comment: 13 pages, 9 eps figures included, Adv. Solid State Phys. (in press
Supersolid Phase of Cold Fermionic Polar Molecules in 2D Optical Lattices
We study a system of ultra-cold fermionic polar molecules in a
two-dimensional square lattice interacting via both the long-ranged
dipole-dipole interaction and a short-ranged on-site attractive interaction.
Singlet superfluid, charge density wave, and supersolid phases are found to
exist in the system. We map out the zero temperature phase diagram and find
that the supersolid phase is considerably stabilized by the dipole-dipole
interaction and thus can exist over a large region of filling factors. We study
the melting of the supersolid phase with increasing temperature, map out a
finite temperature phase diagram of the system at fixed filling, and determine
the parameter region where the supersolid phase can possibly be observed in
experiments.Comment: 8 pages, 5 figure
Ultracold fermions and the SU(N) Hubbard model
We investigate the fermionic SU(N) Hubbard model on the two-dimensional
square lattice for weak to moderate interaction strengths using one-loop
renormalization group and mean-field methods. For the repulsive case U>0 at
half filling and small N the dominant tendency is towards breaking of the SU(N)
symmetry. For N>6 staggered flux order takes over as the dominant instability,
in agreement with the large-N limit. Away from half filling for N=3 the system
rearranges the particle densities such that two flavors remain half filled by
cannibalizing the third flavor. In the attractive case and odd N a full Fermi
surface coexists with a superconductor in the ground state. These results may
be relevant to future experiments with cold fermionic atoms in optical
lattices.Comment: 4 pages, 3 figure
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