6,006 research outputs found
A BCS-BEC crossover in the extended Falicov-Kimball model: Variational cluster approach
We study the spontaneous symmetry breaking of the excitonic insulator state
induced by the Coulomb interaction in the two-dimensional extended
Falicov-Kimball model. Using the variational cluster approximation (VCA) and
Hartree-Fock approximation (HFA), we evaluate the order parameter,
single-particle excitation gap, momentum distribution functions, coherence
length of excitons, and single-particle and anomalous excitation spectra, as a
function of at zero temperature. We find that in the weak-to-intermediate
coupling regime, the Fermi surface plays an essential role and calculated
results can be understood in close correspondence with the BCS theory, whereas
in the strong-coupling regime, the Fermi surface plays no role and results are
consistent with the picture of BEC. Moreover, we find that HFA works well both
in the weak- and strong-coupling regime, and that the difference between the
results of VCA and HFA mostly appears in the intermediate-coupling regime. The
reason for this is discussed from a viewpoint of the self-energy. We thereby
clarify the excitonic insulator state that typifies either a BCS condensate of
electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate of
preformed excitons (strong-coupling regime).Comment: 11 pages, 9 figure
Magnetic digital flop of ferroelectric domain with fixed spin chirality in a triangular lattice helimagnet
Ferroelectric properties in magnetic fields of varying magnitude and
direction have been investigated for a triangular-lattice helimagnet
CuFe1-xGaxO2 (x=0.035). The magnetoelectric phase diagrams were deduced for
magnetic fields along [001], [110], and [1-10] direction, and the in-plane
magnetic field was found to induce the rearrangement of six possible
multiferroic domains. Upon every 60-degree rotation of in-plane magnetic field
around the c-axis, unique 120-degree flop of electric polarization occurs as a
result of the switch of helical magnetic q-vector. The chirality of spin helix
is always conserved upon the q-flop. The possible origin is discussed in the
light of the stable structure of multiferroic domain wall.Comment: 5 pages, 4 figures. Accepted in Phys. Rev. Let
Impurity-doping induced ferroelectricity in frustrated antiferromagnet CuFeO2
Dielectric responses have been investigated on the triangular-lattice
antiferromagnet CuFeO2 and its site-diluted analogs CuFe1-xAlxO2 (x=0.01 and
0.02) with and without application of magnetic field. We have found a
ferroelectric behavior at zero magnetic field for x=0.02. At any doping level,
the onset field of the ferroelectricity always coincides with that of the
noncollinear magnetic structure while the transition field dramatically
decreases to zero field with Al doping. The results imply the further
possibility of producing the ferroelectricity by modifying the frustrated spin
structure in terms of site-doping and external magnetic field.Comment: 4 pages, 4 figure
Theory of the waterfall phenomenon in cuprate superconductors
Based on exact diagonalization and variational cluster approximation
calculations we study the relationship between charge transfer models and the
corresponding single band Hubbard models. We present an explanation for the
waterfall phenomenon observed in angle resolved photoemission spectroscopy
(ARPES) on cuprate superconductors. The phenomenon is due to the destructive
interference between the phases of the O2p orbitals belonging to a given
Zhang-Rice singlet and the Bloch phases of the photohole which occurs in
certain regions of k-space. It therefore may be viewed as a direct experimental
visualisation of the Zhang-Rice construction of an effective single band model
for the CuO2 plane.Comment: 11 pages, 9 Postscript figure
Mott transition and ferrimagnetism in the Hubbard model on the anisotropic kagom\'e lattice
Mott transition and ferrimagnetism are studied in the Hubbard model on the
anisotropic kagom\'e lattice using the variational cluster approximation and
the phase diagram at zero temperature and half-filling is analyzed. The
ferrimagnetic phase rapidly grows as the geometric frustration is relaxed, and
the Mott insulator phase disappears in moderately frustrated region, showing
that the ferrimagnetic fluctuations stemming from the relaxation of the
geometric frustration is enhanced by the electron correlations. In metallic
phase, heavy fermion behavior is observed and mass enhancement factor is
computed. Enhancement of effective spatial anisotropy by the electron
correlations is also confirmed in moderately frustrated region, and its effect
on heavy fermion behavior is examined.Comment: 5 pages, 6 figure
Variation of Entanglement Entropy in Scattering Process
In a scattering process, the final state is determined by an initial state
and an S-matrix. We focus on two-particle scattering processes and consider the
entanglement between these particles. For two types initial states; i.e., an
unentangled state and an entangled one, we calculate perturbatively the change
of entanglement entropy from the initial state to the final one. Then we show a
few examples in a field theory and in quantum mechanics.Comment: 13 pages; v2: refs. adde
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