11,453 research outputs found
Correlated spinless fermions on the honeycomb lattice revisited
We investigate the quantum many-body instabilities of the extended Hubbard
model for spinless fermions on the honeycomb lattice with repulsive
nearest-neighbor and 2nd nearest-neighbor density-density interactions. Recent
exact diagonalization and infinite density matrix renormalization group results
suggest that a putative topological Mott insulator phase driven by the 2nd
nearest-neighbor repulsion is suppressed, while other numerically exact
approaches support the topological Mott insulator scenario. In the present
work, we employ the functional renormalization group (fRG) for correlated
fermionic systems. Our fRG results hint at a strong suppression of the
scattering processes stabilizing the topological Mott insulator. From analyzing
the effects of fermionic fluctuations, we obtain a phase diagram which is the
result of the competition of various charge ordering instabilities.Comment: 9 pages, 8 figure
Bond-ordered states and -wave pairing of spinless fermions on the honeycomb lattice
Spinless fermions on the honeycomb lattice with repulsive nearest-neighbor
interactions are known to harbour a quantum critical point at half-filling,
with critical behaviour in the Gross-Neveu (chiral Ising) universality class.
The critical interaction strength separates a weak-coupling semimetallic regime
from a commensurate charge-density-wave phase. The phase diagram of this basic
model of correlated fermions on the honeycomb lattice beyond half-filling is,
however, less well established. Here, we perform an analysis of its many-body
instabilities using the functional renormalization group method with a basic
Fermi surface patching scheme, which allows us to treat instabilities in
competing channels on equal footing also away from half-filling. Between
half-filling and the van-Hove filling, the free Fermi surface is hole-like and
we again find a charge-density wave instability to be dominant at large
interactions. Moreover, its characteristics are those of the half-filled case.
Directly at the van-Hove filling the nesting property of the free Fermi surface
stabilizes a dimerized bond-order phase. At lower filling the free Fermi
surface becomes electron-like and a superconducting instability with -wave
symmetry is found to emerge from the interplay of intra-unitcell repulsion and
collective fluctuations in the proximity to the charge-density wave
instability. We estimate the extent of the various phases and extract the
corresponding order parameters from the effective low-energy Hamiltonians.Comment: 11 pages, 11 figure
Generalized Polarizabilities in a Constituent Quark Model
We discuss low-energy virtual Compton scattering off the proton within the
framework of a nonrelativistic constituent quark model. A simple interpretation
of the spin-averaged generalized polarizabilities is given in terms of the
induced electric polarization (and magnetization). Explicit predictions for the
generalized polarizabilities obtained from a multipole expansion are presented
for the Isgur-Karl model and are compared with results of various models.Comment: 10 pages, Latex, 2 figures, uses ws-p8-50x6-00.cls, talk given at
NSTAR2001, Workshop on "The Physics of Excited Nucleons," Mainz, Germany,
March 7-10, 200
Pion polarizabilities: No conflict between dispersion theory and ChPT
Recent attempts to determine the pion polarizability by dispersion relations
yield values that disagree with the predictions of chiral perturbation theory.
These dispersion relations are based on specific forms for the absorptive part
of the Compton amplitudes. The analytic properties of these forms are examined,
and the strong enhancement of intermediate-meson contributions is shown to be
connected to non-analytic structuresComment: 9 pages, 4 figures; Proceedings of 6th International Workshop on
Chiral Dynamics, 6-10 July 2009, Bern, Switzerlan
Path integral quantization for massive vector bosons
A parity-conserving and Lorentz-invariant effective field theory of
self-interacting massive vector fields is considered. For the interaction terms
with dimensionless coupling constants the canonical quantization is performed.
It is shown that the self-consistency condition of this system with the
second-class constraints in combination with the perturbative renormalizability
leads to an SU(2) Yang-Mills theory with an additional mass term.Comment: 16 pages, 2 figures, REVTeX
Spin-Orbit Coupling and Magnetic Anisotropy in Iron-Based Superconductors
We determine theoretically the effect of spin-orbit coupling on the magnetic
excitation spectrum of itinerant multi-orbital systems, with specific
application to iron-based superconductors. Our microscopic model includes a
realistic ten-band kinetic Hamiltonian, atomic spin-orbit coupling, and
multi-orbital Hubbard interactions. Our results highlight the remarkable
variability of the resulting magnetic anisotropy despite constant spin-orbit
coupling. At the same time, the magnetic anisotropy exhibits robust universal
behavior upon changes in the bandstructure corresponding to different materials
of iron-based superconductors. A natural explanation of the observed
universality emerges when considering optimal nesting as a resonance
phenomenon. Our theory is also of relevance to other itinerant system with
spin-orbit coupling and nesting tendencies in the bandstructure.Comment: 15 pages, 9 figure
Unconventional pairing and electronic dimerization instabilities in the doped Kitaev-Heisenberg model
We study the quantum many-body instabilities of the Kitaev-Heisenberg Hamiltonian on the honeycomb lattice as a
minimal model for a doped spin-orbit Mott insulator. This spin- model is
believed to describe the magnetic properties of the layered transition-metal
oxide NaIrO. We determine the ground-state of the system with finite
charge-carrier density from the functional renormalization group (fRG) for
correlated fermionic systems. To this end, we derive fRG flow-equations adapted
to the lack of full spin-rotational invariance in the fermionic interactions,
here represented by the highly frustrated and anisotropic Kitaev exchange term.
Additionally employing a set of Ward identities for the Kitaev-Heisenberg
model, the numerical solution of the flow equations suggests a rich phase
diagram emerging upon doping charge carriers into the ground-state manifold
( quantum spin liquids and magnetically ordered phases). We
corroborate superconducting triplet -wave instabilities driven by
ferromagnetic exchange and various singlet pairing phases. For filling , the -wave pairing gives rise to a topological state with protected
Majorana edge-modes. For antiferromagnetic Kitaev and ferromagnetic Heisenberg
exchange we obtain bond-order instabilities at van Hove filling supported by
nesting and density-of-states enhancement, yielding dimerization patterns of
the electronic degrees of freedom on the honeycomb lattice. Further, our flow
equations are applicable to a wider class of model Hamiltonians.Comment: 24 pages, 18 figures, corresponds to journal versio
Imaging of Thermal Domains in ultrathin NbN films for Hot Electron Bolometers
We present low-temperature scanning electron microscopy (LTSEM)
investigations of superconducting microbridges made from ultrathin NbN films as
used for hot electron bolometers. LTSEM probes the thermal structure within the
microbridges under various dc current bias conditions, either via
electron-beam-induced generation of an unstable hotspot, or via the
beam-induced growth of a stable hotspot. Such measurements reveal
inhomogeneities on a micron scale, which may be due to spatial variations in
the NbN film or film-interface properties. Comparison with model calculations
for the stable hotspot regime confirm the basic features of common hot spot
models.Comment: 3 pages, 3 figure
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