469 research outputs found
High-frequency gate manipulation of a bilayer graphene quantum dot
We report transport data obtained for a double-gated bilayer graphene quantum
dot. In Coulomb blockade measurements, the gate dielectric Cytop(TM) is found
to provide remarkable electronic stability even at cryogenic temperatures.
Moreover, we demonstrate gate manipulation with square shaped voltage pulses at
frequencies up to 100 MHz and show that the signal amplitude is not affected by
the presence of the capacitively coupled back gate
Anomalous Fermi Liquid Behavior of Overdoped High-Tc Superconductors
According to a generic temperature vs. carrier-doping (T-p) phase diagram of
high-temperature superconductors it has been proposed that as doping increases
to the overdoped region they approach gradually a conventional (canonical)
Fermi Liquid. However, Hall effect measurements in several systems reported by
different authors show a still strong \emph{T}-dependence in overdoped samples.
We report here electrical transport measurements of
Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7-delta} thin films presenting a temperature
dependence of the Hall constant, R_H, which does not present a gradual
transition towards the T-independent behavior of a canonical Fermi Liquid.
Instead, the T-dependence passes by a minimum near optimal doping and then
increases again in the overdoped region. We discuss the theoretical predictions
from two representative Fermi Liquid models and show that they can not give a
satisfactory explanation to our data. We conclude that this region of the phase
diagram in YBCO, as in most HTSC, is not a canonical Fermi Liquid, therefore we
call it Anomalous Fermi Liquid.Comment: 9 pages, 12 figures, to be published in Phys. Rev.
MgB2 single crystals substituted with Li and with Li-C: Structural and superconducting properties
The effect of Li substitution for Mg and of Li-C co-substitution on the
superconducting properties and crystal structure of MgB2 single crystals has
been investigated. It has been found that hole doping with Li decreases the
superconducting transition temperature Tc, but at a slower rate than electron
doping with C or Al. Tc of MgB2 crystals with simultaneously substituted Li for
Mg and C for B decreases more than in the case where C is substituted alone.
This means that holes introduced by Li cannot counterbalance the effect of
decrease of Tc caused by introduction of electrons coming from C. The possible
reason of it can be that holes coming from Li occupy the pi band while
electrons coming from C fill the sigma band. The temperature dependences of the
upper critical field Hc2 for Al and Li substituted crystals with the same Tc
show a similar dHc2/dT slope at Tc and a similar Hc2(T) behavior, despite of
much different substitution level. This indicates that the mechanism
controlling Hc2 and Tc is similar in both hole and electron doped crystals.
Electrical transport measurements show an increase of resistivity both in Li
substituted crystals and in Li and C co-substituted crystals. This indicates
enhanced scattering due to defects introduced by substitutions including
distortion of the lattice. The observed behavior can be explained as a result
of two effects, influencing both Tc and Hc2. The first one is doping related to
the changes in the carrier concentration, which may lead to the decrease or to
the increase of Tc. The second one is related to the introduction of new
scattering centers leading to the modification of the interband and/or
intraband scattering and therefore, to changes in the superconducting gaps and
to the reduction of Tc.Comment: 22 pages, 17 figures, submitted to PR
Lattice Study of Anisotropic QED-3
We present results from a Monte Carlo simulation of non-compact lattice QED
in 3 dimensions on a lattice in which an explicit anisotropy between
and hopping terms has been introduced into the action. This formulation is
inspired by recent formulations of anisotropic QED as an effective theory
of the non-superconducting portion of the cuprate phase diagram, with
relativistic fermion degrees of freedom defined near the nodes of the gap
function on the Fermi surface, and massless photon degrees of freedom
reproducing the dynamics of the phase disorder of the superconducting order
parameter. Using a parameter set corresponding to broken chiral symmetry in the
isotropic limit, our results show that the renormalised anisotropy, defined in
terms of the ratio of correlation lengths of gauge invariant bound states in
the and directions, exceeds the explicit anisotropy introduced
in the lattice action, implying in contrast to recent analytic results that
anisotropy is a relevant deformation of QED. There also appears to be a
chiral symmetry restoring phase transition at , implying
that the pseudogap phase persists down to T=0 in the cuprate phase diagram.Comment: 24 pages, 9 figures, 3 tables. This (the published version) has the
following alterations: i) An expanded discussion of the empirical aspects of
HT superconductivity, ii) An updated version of Figure 4, iii) The removal of
the consistency check in section 3.3.1 for reasons of brevit
Evidence of Water-related Discrete Trap State Formation in Pentacene Single Crystal Field-Effect Transistors
We report on the generation of a discrete trap state during negative gate
bias stress in pentacene single crystal "flip-crystal" field-effect transistors
with a SiO2 gate dielectric. Trap densities of up to 2*10^12 cm^-2 were created
in the experiments. Trap formation and trap relaxation are distinctly different
above and below ~280 K. In devices in which a self-assembled monolayer on top
of the SiO2 provides a hydrophobic insulator surface we do not observe trap
formation. These results indicate the microscopic cause of the trap state to be
water adsorbed on the SiO2 surface.Comment: 13 pages, 4 figures, submitted to Applied Physics Letter
Interaction-induced singular Fermi surface in a high-temperature oxypnictide superconductor
In the family of iron-based superconductors, LaFeAsO-type materials possess
the simplest electronic structure due to their pronounced two-dimensionality.
And yet they host superconductivity with the highest transition temperature
Tc=55K. Early theoretical predictions of their electronic structure revealed
multiple large circular portions of the Fermi surface with a very good
geometrical overlap (nesting), believed to enhance the pairing interaction and
thus superconductivity. The prevalence of such large circular features in the
Fermi surface has since been associated with many other iron-based compounds
and has grown to be generally accepted in the field. In this work we show that
a prototypical compound of the 1111-type, SmFe0.92Co0.08AsO, is at odds with
this description and possesses a distinctly different Fermi surface, which
consists of two singular constructs formed by the edges of several bands,
pulled to the Fermi level from the depths of the theoretically predicted band
structure by strong electronic interactions. Such singularities dramatically
affect the low-energy electronic properties of the material, including
superconductivity. We further argue that occurrence of these singularities
correlates with the maximum superconducting transition temperature attainable
in each material class over the entire family of iron-based superconductors.Comment: Open access article available online at
http://www.nature.com/srep/2015/150521/srep10392/full/srep10392.htm
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