1,854 research outputs found
Nernst Effect and Superconducting Fluctuations in Zn-doped YBaCuO
We report the measurements of in-plane resistivity, Hall effect, and Nernst
effect in Zn doped YBaCuO epitaxial thin films grown
by pulsed laser deposition technique. The pseudogap temperature, ,
determined from the temperature dependence of resistivity, does not change
significantly with Zn doping. Meanwhile the onset temperature () of
anomalous Nernst signal above , which is interpreted as evidence for
vortex-like excitations, decreases sharply as the superconducting transition
temperature does. A significant decrease in the maximum of vortex
Nernst signal in mixed state is also observed, which is consistent with the
scenario that Zn impurities cause a decrease in the superfluid density and
therefore suppress the superconductivity. The phase diagram of ,
, and versus Zn content is presented and discussed.Comment: 6 pages, 5 figures, Latex; v2: to be published in PR
Frustration and the Kondo effect in heavy fermion materials
The observation of a separation between the antiferromagnetic phase boundary
and the small-large Fermi surface transition in recent experiments has led to
the proposal that frustration is an important additional tuning parameter in
the Kondo lattice model of heavy fermion materials. The introduction of a Kondo
(K) and a frustration (Q) axis into the phase diagram permits us to discuss the
physics of heavy fermion materials in a broader perspective. The current
experimental situation is analysed in the context of this combined "QK" phase
diagram. We discuss various theoretical models for the frustrated Kondo
lattice, using general arguments to characterize the nature of the -electron
localization transition that occurs between the spin liquid and heavy Fermi
liquid ground-states. We concentrate in particular on the Shastry--Sutherland
Kondo lattice model, for which we establish the qualitative phase diagram using
strong coupling arguments and the large- expansion. The paper closes with
some brief remarks on promising future theoretical directions.Comment: To appear in a special issue of JLT
Investigation of the Spin Density Wave in NaxCoO2
Magnetic susceptibility, transport and heat capacity measurements of single
crystal NaxCoO2 (x=0.71) are reported. A transition to a spin density wave
(SDW) state at Tmag = 22 K is observable in all measurements, except chi(ac)
data in which a cusp is observed at 4 K and attributed to a low temperature
glassy phase. M(H) loops are hysteretic below 15 K. Both the SDW transition and
low temperature hysteresis are only visible along the c-axis. The system also
exhibits a substantial (~40%) positive magnetoresistance below this
temperature. Calculations of the electronic heat capacity gamma above and below
Tmag and the size of the jump in C indicate that the onset of the SDW brings
about the opening of gap and the removal of part of the Fermi surface. Reduced
in-plane electron-electron scattering counteracts the loss of carriers below
the transition and as a result we see a net reduction in resistivity below
Tmag. Sodium ordering transitions at higher temperatures are observable as
peaks in the heat capacity with a corresponding increase in resistivity.Comment: 14 pages, 6 figure
Large corrections to asymptotic and in the light-cone perturbative QCD
The large- behavior of - and -
transition form factors, and
are analyzed in the framework of light-cone perturbative QCD with the heavy
quark ( and ) mass effect, the parton's transverse momentum dependence
and the higher helicity components in the light-cone wave function are
respected. It is pointed out that the quark mass effect brings significant
modifications to the asymptotic predictions of the transition form factors in a
rather broad energy region, and this modification is much severer for
than that for due to the
-quark being heavier than the -quark. The parton's transverse momentum
and the higher helicity components are another two factors which decrease the
perturbative predictions. For the transition form factor
, they bring sizable corrections in the present
experimentally accessible energy region (). For the
transition form factor , the corrections coming from
these two factors are negligible since the -quark mass is much larger than
the parton's average transverse momentum. The coming collider (LEP2)
will provide the opportunity to examine these theoretical predictions.Comment: 8 pages, RevTex, 5 PostScript figure
Global Phase Diagram of the Kondo Lattice: From Heavy Fermion Metals to Kondo Insulators
We discuss the general theoretical arguments advanced earlier for the T=0
global phase diagram of antiferromagnetic Kondo lattice systems, distinguishing
between the established and the conjectured. In addition to the well-known
phase of a paramagnetic metal with a "large" Fermi surface (P_L), there is also
an antiferromagnetic phase with a "small" Fermi surface (AF_S). We provide the
details of the derivation of a quantum non-linear sigma-model (QNLsM)
representation of the Kondo lattice Hamiltonian, which leads to an effective
field theory containing both low-energy fermions in the vicinity of a Fermi
surface and low-energy bosons near zero momentum. An asymptotically exact
analysis of this effective field theory is made possible through the
development of a renormalization group procedure for mixed fermion-boson
systems. Considerations on how to connect the AF_S and P_L phases lead to a
global phase diagram, which not only puts into perspective the theory of local
quantum criticality for antiferromagnetic heavy fermion metals, but also
provides the basis to understand the surprising recent experiments in
chemically-doped as well as pressurized YbRh2Si2. We point out that the AF_S
phase still occurs for the case of an equal number of spin-1/2 local moments
and conduction electrons. This observation raises the prospect for a global
phase diagram of heavy fermion systems in the Kondo-insulator regime. Finally,
we discuss the connection between the Kondo breakdown physics discussed here
for the Kondo lattice systems and the non-Fermi liquid behavior recently
studied from a holographic perspective.Comment: (v3) leftover typos corrected. (v2) Published version. 32 pages, 4
figures. Section 7, on the connection between the Kondo lattice systems and
the holographic models of non-Fermi liquid, is expanded. (v1) special issue
of JLTP on quantum criticalit
Long-term monitoring of the TeV emission from Mrk 421 with the ARGO-YBJ experiment
ARGO-YBJ is an air shower detector array with a fully covered layer of
resistive plate chambers. It is operated with a high duty cycle and a large
field of view. It continuously monitors the northern sky at energies above 0.3
TeV. In this paper, we report a long-term monitoring of Mrk 421 over the period
from 2007 November to 2010 February. This source was observed by the
satellite-borne experiments Rossi X-ray Timing Explorer and Swift in the X-ray
band. Mrk 421 was especially active in the first half of 2008. Many flares are
observed in both X-ray and gamma-ray bands simultaneously. The gamma-ray flux
observed by ARGO-YBJ has a clear correlation with the X-ray flux. No lag
between the X-ray and gamma-ray photons longer than 1 day is found. The
evolution of the spectral energy distribution is investigated by measuring
spectral indices at four different flux levels. Hardening of the spectra is
observed in both X-ray and gamma-ray bands. The gamma-ray flux increases
quadratically with the simultaneously measured X-ray flux. All these
observational results strongly favor the synchrotron self-Compton process as
the underlying radiative mechanism.Comment: 30 pages, 8 figure
Effect of Na content and hydration on the excitation spectrum of the cobaltite Na_xCoO_2 yH_2O
We report on a Raman scattering study on the superconducting cobaltite
as function of Na content and hydration (x1/3,
3/4 and y0, 2/3, 4/3). The observed phonon scattering and scattering
continua are analyzed in terms of lattice strain due to the structural misfit
and disorder. Hydration, due to the intercalation of one or two layers,
releases a part of this strain. Our Raman data suggest a connection between
disorder on the partly occupied Na sites, the split off of the level
from the other states of and superconductivity.Comment: 10 pages, 4 figures, for further information see
http://www.peter-lemmens.d
Anomalous Transport Phenomena in Fermi Liquids with Strong Magnetic Fluctuations
In many strongly correlated electron systems, remarkable violation of the
relaxation time approximation (RTA) is observed. The most famous example would
be high-Tc superconductors (HTSCs), and similar anomalous transport phenomena
have been observed in metals near their antiferromagnetic (AF) quantum critical
point (QCP). Here, we develop a transport theory involving resistivity and Hall
coefficient on the basis of the microscopic Fermi liquid theory, by considering
the current vertex correction (CVC). In nearly AF Fermi liquids, the CVC
accounts for the significant enhancements in the Hall coefficient,
magnetoresistance, thermoelectric power, and Nernst coefficient in nearly AF
metals. According to the numerical study, aspects of anomalous transport
phenomena in HTSC are explained in a unified way by considering the CVC,
without introducing any fitting parameters; this strongly supports the idea
that HTSCs are Fermi liquids with strong AF fluctuations. In addition, the
striking \omega-dependence of the AC Hall coefficient and the remarkable
effects of impurities on the transport coefficients in HTSCs appear to fit
naturally into the present theory. The present theory also explains very
similar anomalous transport phenomena occurring in CeCoIn5 and CeRhIn5, which
is a heavy-fermion system near the AF QCP, and in the organic superconductor
\kappa-(BEDT-TTF).Comment: 100 pages, Rep. Prog. Phys. 71, 026501 (2008
Electromagnetic Transition Form Factor of Pseudoscalar Meson and Mixing
The electromagnetic transition form factors of and are
calculated in the light-cone perturbation theory. We show that it is unreliable
to determine the \eta-\etap'c\bar{c}c\bar{c}c\bar{c}$ component.Comment: 13 pages, RevTeX, 4 ps figures, to appear in Phys. Rev.
Gamma-ray observations of Tycho's SNR with VERITAS and Fermi
High-energy gamma-ray emission from supernova remnants (SNRs) has provided a
unique perspective for studies of Galactic cosmic-ray acceleration. Tycho's SNR
is a particularly good target because it is a young, type Ia SNR that is
well-studied over a wide range of energies and located in a relatively clean
environment. Since the detection of gamma-ray emission from Tycho's SNR by
VERITAS and Fermi-LAT, there have been several theoretical models proposed to
explain its broadband emission and high-energy morphology. We report on an
update to the gamma-ray measurements of Tycho's SNR with 147 hours of VERITAS
and 84 months of Fermi-LAT observations, which represents about a factor of two
increase in exposure over previously published data. About half of the VERITAS
data benefited from a camera upgrade, which has made it possible to extend the
TeV measurements toward lower energies. The TeV spectral index measured by
VERITAS is consistent with previous results, but the expanded energy range
softens a straight power-law fit. At energies higher than 400 GeV, the
power-law index is . It
is also softer than the spectral index in the GeV energy range, , measured by this study using
Fermi--LAT data. The centroid position of the gamma-ray emission is coincident
with the center of the remnant, as well as with the centroid measurement of
Fermi--LAT above 1 GeV. The results are consistent with an SNR shell origin of
the emission, as many models assume. The updated spectrum points to a lower
maximum particle energy than has been suggested previously.Comment: Accepted for publication in The Astrophysical Journa
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