954 research outputs found
Quantum Noise Limits for Nonlinear, Phase-Invariant Amplifiers
Any quantum device that amplifies coherent states of a field while preserving
their phase generates noise. A nonlinear, phase-invariant amplifier may
generate less noise, over a range of input field strengths, than any linear
amplifier with the same amplification. We present explicit examples of such
nonlinear amplifiers, and derive lower bounds on the noise generated by a
nonlinear, phase-invariant quantum amplifier.Comment: RevTeX, 6 pages + 4 figures (included in file; hard copy sent on
request
Josephson scanning tunneling microscopy
We propose a set of scanning tunneling microscopy experiments in which the
surface of superconductor is scanned by a superconducting tip. Potential
capabilities of such experimental setup are discussed. Most important
anticipated results of such an experiment include the position-resolved
measurement of the superconducting order parameter and the possibility to
determine the nature of the secondary component of the order parameter at the
surface. The theoretical description based on the tunneling Hamiltonian
formalism is presented.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Asymmetric magnetic interference patterns in 0-pi Josephson junctions
We examine the magnetic interference patterns of Josephson junctions with a
region of 0- and of pi-phase shift. Such junctions have recently been realized
as c-axis YBCO-Pb junctions with a single twin boundary in YBCO. We show that
in general the junction generates self-fields which introduces an asymmetry in
the critical current under reversal of the magnetic field. Numerical
calculations of these asymmetries indicate they account well for the
unexplained features observed in single twin boundary junctions.Comment: 4 pages, 3 figure
Phase transition from a to superconductor
We study the phase transition from a to
superconductor using the tight-binding model of two-dimensional cuprates. As
the temperature is lowered past the critical temperature , first a superconducting phase is created. With further reduction of
temperature, the phase is created at temperature
. We study the temperature dependencies of the order parameter,
specific heat and spin susceptibility in these mixed-angular-momentum states on
square lattice and on a lattice with orthorhombic distortion. The
above-mentioned phase transitions are identified by two jumps in specific heat
at and .Comment: Latex file, 5 pages, 6 postscript figures, Accepted in Physical
Review
ac Josephson effect in superconducting d-wave junctions
We study theoretically the ac Josephson effect in superconducting planar
d-wave junctions. The insulating barrier assumed to be present between the two
superconductors may have arbitrary strength. Many properties of this system
depend on the orientation of the d-wave superconductor: we calculate the ac
components of the Josephson current. In some arrangements there is substantial
negative differential conductance due to the presence of mid-gap states. We
study how robust these features are to finite temperature and also comment on
how the calculated current-voltage curves compare with experiments. For some
other configurations (for small barrier strength) we find zero-bias conductance
peaks due to multiple Andreev reflections through midgap states. Moreover, the
odd ac components are strongly suppressed and even absent in some arrangements.
This absence will lead to a doubling of the Josephson frequency. All these
features are due to the d-wave order parameter changing sign when rotated
. Recently, there have been several theoretical reports on parallel
current in the d-wave case for both the stationary Josephson junction and for
the normal metal-superconductor junction. Also in our case there may appear
current density parallel to the junction, and we present a few examples when
this takes place. Finally, we give a fairly complete account of the method used
and also discuss how numerical calculations should be performed in order to
produce current-voltage curves
Single-Particle Properties of a Two-Dimensional Fermi Liquid at finite Frequencies and Temperatures
We review the leading momentum, frequency and temperature dependences of the
single particle self-energy and the corresponding term in the entropy of a two
dimensional Fermi liquid (FL) with a free particle spectrum. We calculate the
corrections to these leading dependences for the paramagnon model and the
electron gas and find that the leading dependences are limited to regions of
energy and temperature which decrease with decreasing number density of
fermions. This can make it difficult to identify the frequency and temperature
dependent characteristics of a FL ground state in experimental quantities in
low density systems even when complications of band structure and other degrees
of freedom are absent. This is an important consideration when the normal state
properties of the undoped cuprate superconductors are analyzed.Comment: Revtex, 15 pages with 13 figures. minor corrections. Accepted for
publication in Phy. Rev.
Composición y capacidad antioxidante de especies aromáticas y medicinales con alto contenido de timol y carvacrol
Se determinó la composición química de extractos de seis especies vegetales (tomillo, oréganos común, cimarrón, de castilla, rastrero y mejorana) y cuatro aceites esenciales obtenidos por destilación-extracción simultánea con solvente (SDE) y hidrodestilación asistida por la radiación de microondas (MWHD), respectivamente. La separación, detección e identificación de los compuestos se realizó por GC-MS y por comparación de sus espectros de masas con los de patrones y de las bases de datos e índices de retención de Kovàts. Se evaluó la capacidad de atrapamiento del catión-radical ABTS+. ácido 2,2´-azino-bis-(3- etilbenzo-tiazolina-6-sulfónico) por los aceites esenciales
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