158 research outputs found
Doping dependence of superconducting gap in YBa_2Cu_3O_y from universal heat transport
Thermal transport in the T -> 0 limit was measured as a function of doping in
high-quality single crystals of the cuprate superconductor YBa_2Cu_3O_y. The
residual linear term kappa_0/T is found to decrease as one moves from the
overdoped regime towards the Mott insulator region of the phase diagram. The
doping dependence of the low-energy quasiparticle gap extracted from kappa_0/T
is seen to scale closely with that of the pseudogap, arguing against a
non-superconducting origin for the pseudogap. The presence of a linear term for
all dopings is evidence against the existence of a quantum phase transition to
an order parameter with a complex (ix) component.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
Field-induced thermal metal-to-insulator transition in underdoped LSCO
The transport of heat and charge in cuprates was measured in undoped and
heavily-underdoped single crystal La_{2-x}Sr_xCuO_{4+delta} (LSCO). In
underdoped LSCO, the thermal conductivity is found to decrease with increasing
magnetic field in the T --> 0 limit, in striking contrast to the increase
observed in all superconductors, including cuprates at higher doping. The
suppression of superconductivity with magnetic field shows that a novel thermal
metal-to-insulator transition occurs upon going from the superconducting state
to the field-induced normal state.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
Multiband superconductivity in NbSe_2 from heat transport
The thermal conductivity of the layered s-wave superconductor NbSe_2 was
measured down to T_c/100 throughout the vortex state. With increasing field, we
identify two regimes: one with localized states at fields very near H_c1 and
one with highly delocalized quasiparticle excitations at higher fields. The two
associated length scales are most naturally explained as multi-band
superconductivity, with distinct small and large superconducting gaps on
different sheets of the Fermi surface.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
Field-induced quantum critical point in CeCoIn_5
The resistivity of CeCoIn_5 was measured down to 20 mK in magnetic fields of
up to 16 T. With increasing field, we observe a suppression of the non-Fermi
liquid behavior, rho~T, and the development of a Fermi liquid state, with its
characteristic rho=rho_0+AT^2 dependence. The field dependence of the T^2
coefficient shows critical behavior with an exponent of ~4/3. This is evidence
for a new field-induced quantum critical point, occuring in this case at a
critical field which coincides with the superconducting upper critical field
H_c2.Comment: 2 pages, 2 figures, submitted to M2S-Rio 2003 Proceeding
Effect of Hund coupling in the one-dimensional SU(4) Hubbard model
The one-dimensional SU(4) Hubbard model perturbed by Hund coupling is
studied, away from half-filling, by means of renormalization group and
bosonization methods. A spectral gap is always present in the spin-orbital
sector irrespective of the magnitude of the Coulomb repulsion. We further
distinguish between two qualitatively different regimes. At small Hund
coupling, we find that the symmetry of the system is dynamically enlarged to
SU(4) at low energy with the result of {\it coherent} spin-orbital excitations.
When the charge sector is not gapped, a superconducting instability is shown to
exist. At large Hund coupling, the symmetry is no longer enlarged to SU(4) and
the excitations in the spin sector become {\it incoherent}. Furthermore, the
superconductivity can be suppressed in favor of the conventional charge density
wave state.Comment: 10 pages, 1 figur
Universality class of S=1/2 quantum spin ladder system with the four spin exchange
We study s=1/2 Heisenberg spin ladder with the four spin exchange. Combining
numerical results with the conformal field theory(CFT), we find a phase
transition with central charge c=3/2. Since this system has an SU(2) symmetry,
we can conclude that this critical theory is described by k=2 SU(2)
Wess-Zumino-Witten model with Z symmetry breaking
The 3D Structure of N132D in the LMC: A Late-Stage Young Supernova Remnant
We have used the Wide Field Spectrograph (WiFeS) on the 2.3m telescope at
Siding Spring Observatory to map the [O III] 5007{\AA} dynamics of the young
oxygen-rich supernova remnant N132D in the Large Magellanic Cloud. From the
resultant data cube, we have been able to reconstruct the full 3D structure of
the system of [O III] filaments. The majority of the ejecta form a ring of
~12pc in diameter inclined at an angle of 25 degrees to the line of sight. We
conclude that SNR N132D is approaching the end of the reverse shock phase
before entering the fully thermalized Sedov phase of evolution. We speculate
that the ring of oxygen-rich material comes from ejecta in the equatorial plane
of a bipolar explosion, and that the overall shape of the SNR is strongly
influenced by the pre-supernova mass loss from the progenitor star. We find
tantalizing evidence of a polar jet associated with a very fast oxygen-rich
knot, and clear evidence that the central star has interacted with one or more
dense clouds in the surrounding ISM.Comment: Accepted for Publication in Astrophysics & Space Science, 18pp, 8
figure
From nodal liquid to nodal Mottness in a frustrated Hubbard model
We investigate the physics of frustrated 3-leg Hubbard ladders in the band
limit, when hopping across the ladder's rungs (t) is of the same
order as hopping along them (t) much greater than the onsite Coulomb repulsion
(U). We show that this model exhibits a striking electron-hole asymmetry close
to half-filling: the hole-doped system at low temperatures develops a
Resonating Valence Bond (RVB)-like d-wave gap (pseudogap close to (,0))
coinciding with gapless nodal excitations (nodal liquid); in contrast, the
electron-doped system is seen to develop a Mott gap at the nodes, whilst
retaining a metallic character of its majority Fermi surface. At lower
temperatures in the electron-doped case, d-wave superconducting correlations --
here, coexisting with gapped nodal excitations -- are already seen to arise.
Upon further doping the hole-doped case, the RVB-like state yields to d-wave
superconductivity. Such physics is reminiscent of that exhibited by the high
temperature cuprate superconductors--notably electron-hole asymmetry as noted
by Angle Resolved PhotoEmission Spectroscopy (ARPES) and the resistivity
exponents observed. This toy model also reinforces the importance of a more
thorough experimental investigation of the known 3-leg ladder cuprate systems,
and may have some bearing on low dimensional organic superconductors.Comment: 26 pages, 16 figure
at Two Loops In Chiral Perturbation Theory
We calculate the decay at next-to-next-to-leading order or
order in Chiral Perturbation Theory. The corrections are somewhat larger
than was indicated by dispersive estimates. We present numerical results for
the Dalitz plot parameters, the ratio of the neutral to charged decay and
the total decay rate. In addition we derive an inequality between the slope
parameters of the charged and neutral decay. The experimental charged decay
rate leads to central values for the isospin breaking quantities and
.Comment: 35p, some misprints corrected and a wrong figure replace. Some
sentences reformulated and Sect. 2 shortene
Electromagnetic corrections in eta --> 3 pi decays
We re-evaluate the electromagnetic corrections to eta --> 3 pi decays at
next-to-leading order in the chiral expansion, arguing that effects of order
e^2(m_u-m_d) disregarded so far are not negligible compared to other
contributions of order e^2 times a light quark mass. Despite the appearance of
the Coulomb pole in eta --> pi+ pi- pi0 and cusps in eta --> 3 pi0, the overall
corrections remain small.Comment: 21 pages, 11 figures; references updated, version published in EPJ
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