19,559 research outputs found
SU(2) gauge theory of the Hubbard model: Emergence of an anomalous metallic phase near the Mott critical point
We propose one possible mechanism for an anomalous metallic phase appearing
frequently in two spatial dimensions, that is, local pairing fluctuations.
Introducing a pair-rotor representation to decompose bare electrons into
collective pairing excitations and renormalized electrons, we derive an SU(2)
gauge theory of the Hubbard model as an extended version of its U(1) gauge
theory\cite{Florens,LeeLee} to allow only local density fluctuations. Since our
effective SU(2) gauge theory admits two kinds of collective bosons
corresponding to pair excitations and density fluctuations respectively, an
intermediate phase appears naturally between the spin liquid Mott insulator and
Fermi liquid metal of the U(1) gauge theory,\cite{Florens,LeeLee} characterized
by softening of density-fluctuation modes as the Fermi liquid, but gapping of
pair-excitation modes. We show that this intermediate phase is identified with
an anomalous metallic phase because there are no electron-like quasiparticles
although it is metallic
NuTeV Structure Function Measurement
The NuTeV experiment obtained high statistics samples of neutrino and
anti-neutrino charged current events during the 1996-1997 Fermilab fixed target
run. The experiment combines sign-selected neutrino and anti-neutrino beams and
the upgraded CCFR iron-scintillator neutrino detector. A precision continuous
calibration beam was used to determine the muon and hadron energy scales to a
precision of 0.7% and 0.43% respectively. The structure functions F_2(x,Q^2)
and xF_3(x,Q^2) obtained by fitting the y-dependence of the sum and the
difference of the neutrino and anti-neutrino differential cross sections are
presented.Comment: Proceedings of the XIII international workshop on Deep Inelastic
Scattering DIS 2005, 4 pages, 4 figure
Role of disorder in the Mott-Hubbard transition
We investigate the role of disorder in the Mott-Hubbard transition based on
the slave-rotor representation of the Hubbard model, where an electron is
decomposed into a fermionic spinon for a spin degree of freedom and a bosonic
rotor (chargon) for a charge degree of freedom. In the absence of disorder the
Mott-Hubbard insulator is assumed to be the spin liquid Mott insulator in terms
of gapless spinons near the Fermi surface and gapped chargons interacting via
U(1) gauge fields. We found that the Mott-Hubbard critical point becomes
unstable as soon as disorder is turned on. As a result, a disorder critical
point appears to be identified with the spin liquid glass insulator to the
Fermi liquid metal transition, where the spin liquid glass consists of the U(1)
spin liquid and the chargon glass. We expect that glassy behaviors of charge
fluctuations can be measured by the optical spectra in the insulating phase of
an organic material . Furthermore, since
the Mott-Anderson critical point depends on the spinon conductivity,
universality in the critical exponents may not be found
Effect of nonmagnetic disorder on criticality in the "dirty" U(1) spin liquid
We investigate the effect of nonmagnetic disorder on the stability of the
algebraic spin liquid () by deriving an effective field theory, nonlinear
model (). We find that the anomalous critical exponent
characterizing the criticality of the causes an anomalous gradient in the
. We show that the sign of the anomalous gradient exponent or the
critical exponent of the determines the stability of the "dirty" . A
positive exponent results in an unstable fixed point separating delocalized and
localized phases, which is consistent with our previous study [Phys. Rev. B
{\bf 70}, 140405 (2004)]. We find power law suppression for the density of
spinon states in contrast to the logarithmic correction in the free Dirac
theory. On the other hand, a negative exponent destabilizes the , causing
the Anderson localization. We discuss the implication of our study in the
pseudogap phase of high cuprates
Bandwidth-control vs. doping-control Mott transition in the Hubbard model
We reinvestigate the bandwidth-control and doping-control Mott transitions
(BCMT and DCMT) from a spin liquid Mott insulator to a Fermi liquid metal based
on the slave-rotor representation of the Hubbard model,\cite{Florens} where the
Mott transitions are described by softening of bosonic collective excitations.
We find that the nature of the insulating phase away from half filling is
different from that of half filling in the respect that a charge density wave
coexists with a topological order (spin liquid) away from half filling because
the condensation of vortices generically breaks translational symmetry in the
presence of "dual magnetic fields" resulting from hole doping while the
topological order remains stable owing to gapless excitations near the Fermi
surface. Performing a renormalization group analysis, we discuss the role of
dissipative gauge fluctuations due to the Fermi surface in both the BCMT and
the DCMT
Critical field theory of the Kondo lattice model in two dimensions
In the context of the U(1) slave boson theory we derive a critical field
theory near the quantum critical point of the Kondo lattice model in two
spatial dimensions. First we argue that strong gauge fluctuations in the U(1)
slave boson theory give rise to confinement between spinons and holons, thus
causing "neutralized" spinons in association with the slave boson U(1) gauge
field. Second we show that critical fluctuations of Kondo singlets near the
quantum critical point result in a new U(1) gauge field. This emergent gauge
field has nothing to do with the slave boson U(1) gauge field. Third we find
that the slave boson U(1) gauge field can be exactly integrated out in the low
energy limit. As a result we find a critical field theory in terms of
renormalized conduction electrons and neutralized spinons interacting via the
new emergent U(1) gauge field. Based on this critical field theory we obtain
the temperature dependence of specific heat and the imaginary part of the
self-energy of the renormalized electrons. These quantities display non-Fermi
liquid behavior near the quantum critical point
Competition between superconductivity and charge density waves
We derive an effective field theory for the competition between
superconductivity (SC) and charge density waves (CDWs) by employing the SO(3)
pseudospin representation of the SC and CDW order parameters. One important
feature in the effective nonlinear model is the emergence of Berry
phase even at half filling, originating from the competition between SC and
CDWs, i.e., the pseudospin symmetry. A well known conflict between the previous
studies of Oshikawa\cite{Oshikawa} and D. H. Lee et al.\cite{DHLee} is resolved
by the appearance of Berry phase. The Berry phase contribution allows a
deconfined quantum critical point of fractionalized charge excitations with
instead of in the SC-CDW quantum transition at half filling. Furthermore,
we investigate the stability of the deconfined quantum criticality against
quenched randomness by performing a renormalization group analysis of an
effective vortex action. We argue that although randomness results in a weak
disorder fixed point differing from the original deconfined quantum critical
point, deconfinement of the fractionalized charge excitations still survives at
the disorder fixed point owing to a nonzero fixed point value of a vortex
charge.Comment: adding a renormalization group analysis with a random fugacity term
as an effect of randomness on a deconfined quantum critical poin
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