207 research outputs found
Origin of the T enhancement in heterostructure cuprate superconductors
Recent experiments on heterostructures composed of two or more films of
cuprate superconductors of different oxygen doping levels\cite{Yuli,Gozar} have
shown a remarkable T enhancement (up to 50%) relative to single compound
films. We provide here a simple explanation of the enhancement which arises
naturally from a collection of experimental works. We show that the enhancement
could be caused by a structural change in the lattice, namely an increase in
the distance of the apical oxygen from the copper-oxygen plane. This increase
modifies the effective off-site interaction in the plane which in turn enhances
the d-wave superconductivity order parameter. To illustrate this point we study
the extended Hubbard model using the fluctuation exchange approximation
Magnetoconductance signatures of chiral domain-wall bound states in magnetic topological insulators
Recent magnetoconductance measurements performed on magnetic topological
insulator candidates have revealed butterfly-shaped hysteresis. This hysteresis
has been attributed to the formation of gapless chiral domain-wall bound states
during a magnetic field sweep. We treat this phenomenon theoretically,
providing a link between microscopic magnetization dynamics and butterfly
hysteresis in magnetoconductance. Further, we illustrate how a spatially
resolved conductance measurement can probe the most striking feature of the
domain-wall bound states: their chirality. This work establishes a regime where
a definitive link between butterfly hysteresis in longitudinal
magneto-conductance and domain-wall bound states can be made. This analysis
provides an important tool for the identification of magnetic topological
insulators.Comment: v2: Final published version; 7 pages, 3 figure
Tunnelling theory of Weyl semimetals in proximity to a metallic band
We study the effects of tunnelling on the band structure and Fermi arc of a
time-reversal broken Weyl semimetal (WSM). When coupled to a non-magnetic
parabolic band, the WSM's chiral arc state lowers in energy and forms, together
with a previously extended state, a noticeable spin-dependent asymmetry in the
interface spectrum in the vicinity of the Weyl nodes. We study these effects
with a lattice model which we solve numerically on a finite sample and
analytically through using an ansatz on an infinite sample. Our ansatz agrees
very well with the numerical simulation as it accurately describes the
behaviour of the chiral state, from its energy asymmetry to the spin canting at
the interface. We find that the tunnelling effectively increases the Fermi arc
length, allowing for the presence of interface states beyond the bare Weyl
nodes. These additional states may carry current along the interface and their
contribution can be detected in the conductance. Across the interface, the
spin-independent conductance reproduces the results of an electron tunnelling
experiment to reveal the WSM's density of states. Besides conductivity, the
effect of tunnelling between the WSM and the metallic band can be seen in
quantum oscillations experiments which we briefly comment about.Comment: 15 pages, 8 figures; typos corrected, reference adde
The Origin of T_c Enhancement in Heterostructure Cuprate Superconductors
Recent experiments on heterostructures composed of two or more films of cuprate superconductors of different oxygen doping levels have shown a remarkable T_c enhancement (up to 50%) relative to single compound films. We provide a simple explanation of the enhancement which arises naturally from a collection of experimental works. We show that the enhancement could be caused by a structural change in the lattice, namely an increase in the distance of the apical oxygen from the copper-oxygen plane. This increase modifies the effective off-site interaction in the plane which in turn enhances the d-wave superconductivity order parameter. To illustrate this point we study the extended Hubbard model using the fluctuation exchange approximation
Quasiparticle interference patterns as a test for the nature of the pseudogap phase in the cuprate superconductors
Electrons, when scattered by static random disorder, form standing waves that
can be imaged using scanning tunneling microscopy. Such interference patterns,
observable by the recently developed technique of Fourier transform scanning
tunneling spectroscopy (FT-STS), are shown to carry unique fingerprints
characteristic of the electronic order present in a material. We exploit this
feature of the FT-STS technique to propose a test for the nature of the
enigmatic pseudogap phase in the high- cuprate superconductors. Through
their sensitivity to the quasiparticle spectra and coherence factors, the
FT-STS patterns in principle carry enough information to unambiguously
determine the nature of the condensate responsible for the pseudogap
phenomenon. We argue that the next generation of FT-STS experiments, currently
underway, should be able to distinguish between the pseudogap dominated by the
remnants of superconducting order from the pseudogap dominated by some
competing order in the particle-hole channel. Using general arguments and
detailed numerical calculations, we point to certain fundamental differences
between the two scenarios and discuss the prospects for future experiments.Comment: 15 pages REVTeX + 9 ps figures. For related work and info visit
http://www.physics.ubc.ca/~franz; version 2 to appear in IJMP
Duality and the vibrational modes of a Cooper-pair Wigner crystal
When quantum fluctuations in the phase of the superconducting order parameter
destroy the off-diagonal long range order, duality arguments predict the
formation of a Cooper pair crystal. This effect is thought to be responsible
for the static checkerboard patterns observed recently in various underdoped
cuprate superconductors by means of scanning tunneling spectroscopy. Breaking
of the translational symmetry in such a Cooper pair Wigner crystal may, under
certain conditions, lead to the emergence of low lying transverse vibrational
modes which could then contribute to thermodynamic and transport properties at
low temperatures. We investigate these vibrational modes using a continuum
version of the standard vortex-boson duality, calculate the speed of sound in
the Cooper pair Wigner crystal and deduce the associated specific heat and
thermal conductivity. We then suggest that these modes could be responsible for
the mysterious bosonic contribution to the thermal conductivity recently
observed in strongly underdoped ultraclean single crystals of YBCO tuned across
the superconductor-insulator transition.Comment: 14 pages; 3 figures; corrected the sample size value; version 3 to
appear in PR
Transport through a disordered topological-metal strip
Features of a topological phase, and edge states in particular, may be obscured by overlapping in energy with a trivial conduction band. The topological nature of such a conductor, however, is revealed in its transport properties, especially in the presence of disorder. In this work, we explore the conductance behavior of such a system with disorder present, and contrast it with the quantized conductance in an ideal two-dimensional topological insulator. Our analysis relies on numerics on a lattice system and analytics on a simple toy model. Interestingly, we find that as disorder is increased from zero, the edge conductivity initially falls from its quantized value; yet, as disorder continues to increase, the conductivity recovers, and saturates at a value slightly below the quantized value of the clean system. We discuss how this effect can be understood from the tendency of the bulk states to localize, while the edge states remain delocalized
Physicochemical and Sensory Evaluation Data to Drive the Development of a Green Chili Pepper Hot Sauce from Unexploited Raw Materials
The present study shows the set of analyses conducted during the development of a hot chili pepper sauce to valorize green peppers usually discarded in the Espelette region (France). A traditional production process was used as the inspiration for product development, and two different fermentation processes were assessed and characterized by measuring pH, sugar content, instrumental color, volatile composition, and conducting sensory (discriminant test) and microbiological analyses (total plate count). Significant differences were observed among pepper mash samples with respect to their physicochemical characteristics, but the products were considered similar from a sensory standpoint. Both sensory and physicochemical tests suggested that the ingredients added to make the sauces were determinant and had a higher impact on the organoleptic profile of the final product than the fermentation process. Finally, a Napping® test was conducted to determine the attributes that could differentiate the product from the hot sauces found in the current market. The results of the present research allowed the optimization of the elaboration process of the new product, saving time and ingredient costs. The procedures shown in the study could be used as an example of a new product development process in which physicochemical and sensory data are collected and used for decision making.The present study shows the set of analyses conducted during the development of a hot chili pepper sauce to valorize green peppers usually discarded in the Espelette region (France). A traditional production process was used as the inspiration for product development, and two different fermentation processes were assessed and characterized by measuring pH, sugar content, instrumental color, volatile composition, and conducting sensory (discriminant test) and microbiological analyses (total plate count). Significant differences were observed among pepper mash samples with respect to their physicochemical characteristics, but the products were considered similar from a sensory standpoint. Both sensory and physicochemical tests suggested that the ingredients added to make the sauces were determinant and had a higher impact on the organoleptic profile of the final product than the fermentation process. Finally, a Napping® test was conducted to determine the attributes that could differentiate the product from the hot sauces found in the current market. The results of the present research allowed the optimization of the elaboration process of the new product, saving time and ingredient costs. The procedures shown in the study could be used as an example of a new product development process in which physicochemical and sensory data are collected and used for decision making
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