6,022 research outputs found
Phonon Squeezing in a Superconducting Molecular Transistor
Josephson transport through a single molecule or carbon nanotube is
considered in the presence of a local vibrational mode coupled to the
electronic charge. The ground-state solution is obtained exactly in the limit
of a large superconducting gap, and is extended to the general case by
variational analysis. Coherent charge fluctuations are entangled with
non-classical phonon states. The Josephson current induces squeezing of the
phonon mode, which is controlled by the superconducting phase difference and by
the junction asymmetry. Optical probes of non-classical phonon states are
briefly discussed
Spin-orbit induced chirality of Andreev states in Josephson junctions
We study Josephson junctions (JJs) in which the region between the two
superconductors is a multichannel system with Rashba spin-orbit coupling (SOC)
where a barrier or a quantum point contact (QPC) is present. These systems
might present unconventional Josephson effects such as Josephson currents for
zero phase difference or critical currents that \textit{depend on} the current
direction. Here, we discuss how the spin polarizing properties of the system in
the normal state affect the spin characteristic of the Andreev bound states
inside the junction. This results in a strong correlation between the spin of
the Andreev states and the direction in which they transport Cooper pairs.
While the current-phase relation for the JJ at zero magnetic field is
qualitatively unchanged by SOC, in the presence of a weak magnetic field a
strongly anisotropic behavior and the mentioned anomalous Josephson effects
follow. We show that the situation is not restricted to barriers based on
constrictions such as QPCs and should generically arise if in the normal system
the direction of the carrier's spin is linked to its direction of motion.Comment: 19 pages, 9 figures. To appear in PR
Anomalous Josephson Current in Junctions with Spin-Polarizing Quantum Point Contacts
We consider a ballistic Josephson junction with a quantum point contact in a
two-dimensional electron gas with Rashba spin-orbit coupling. The point contact
acts as a spin filter when embedded in a circuit with normal electrodes. We
show that with an in-plane external magnetic field an anomalous supercurrent
appears even for zero phase difference between the superconducting electrodes.
In addition, the external field induces large critical current asymmetries
between the two flow directions, leading to supercurrent rectifying effects.Comment: 4 pages, 4 figures, to appear in PR
Emergence of a negative charging energy in a metallic dot capacitively coupled to a superconducting island
We consider the hybrid setup formed by a metallic dot, capacitively coupled
to a superconducting island S connected to a bulk superconductor by a Josephson
junction. Charge fluctuations in S act as a dynamical gate and overscreen the
electronic repulsion in the metallic dot, producing an attractive interaction
between two additional electrons. As the offset charge of the metallic dot is
increased, the dot charging curve shows positive steps () followed by
negative ones () signaling the occurrence of a negative differential
capacitance. A proposal for experimental detection is given, and potential
applications in nanoelectronics are mentioned.Comment: Revised version, 4 pages, 4 figure
Edge channel mixing induced by potential steps in an integer quantum Hall system
We investigate the coherent mixing of co-propagating edge channels in a
quantum Hall bar produced by step potentials. In the case of two edge channels
it is found that, although a single step induces only a few percent mixing, a
series of steps could yield 50% mixing. In addition, a strong mixing is found
when the potential height of a single step allows a different number of edge
channels on the two sides of the step. Charge density probability has been also
calculated even for the case where the step is smoothened.Comment: final version: 7 pages, 6 figure
What is the value of the superconducting gap of a F/S/F trilayer ?
Based on the model of F/S/F trilayer with atomic thickness [A. Buzdin and M.
Daumens, cond-mat/0305320] we discuss the relative roles of pair-breaking and
proximity effects, as a function of the exchange field, of disorder and of a
finite thickness in the superconducting layer. The exchange field can be small
(weak ferromagnets) or large (strong ferromagnets) compared to the
superconducting gap. With weak ferromagnets we show the existence of a
reentrant superconducting gap for the F/S/F trilayer with atomic thickness in
the parallel alignment (equivalent to the F/S bilayer). Qualitatively small
disorder is equivalent to reducing the value of the hopping parameters. In the
presence of a finite thickness in the superconducting layer the superconducting
gap in the antiparallel alignment is larger than in the parallel alignment,
meaning that pair breaking dominates over the proximity effect.Comment: 7 pages, 3 figure
The Redox Couple of the Cytochrome \u3cem\u3ec\u3c/em\u3e Cyanide Complex: The Contribution of Heme Iron Ligation to the Structural Stability, Chemical Reactivity, and Physiological Behavior of Horse Cytochrome \u3cem\u3ec\u3c/em\u3e
Contrary to most heme proteins, ferrous cytochrome c does not bind ligands such as cyanide and CO. In order to quantify this observation, the redox potential of the ferric/ferrous cytochrome c–cyanide redox couple was determined for the first time by cyclic voltammetry. Its E0′ was −240 mV versus SHE, equivalent to −23.2 kJ/mol. The entropy of reaction for the reduction of the cyanide complex was also determined. From a thermodynamic cycle that included this new value for the cyt c cyanide complex E0′, the binding constant of cyanide to the reduced protein was estimated to be 4.7 × 10−3 LM−1 or 13.4 kJ/mol (3.2 kcal/mol), which is 48.1 kJ/mol (11.5 kcal/mol) less favorable than the binding of cyanide to ferricytochrome c. For coordination of cyanide to ferrocytochrome c, the entropy change was earlier experimentally evaluated as 92.4 Jmol−1K−1 (22.1 e.u.) at 25 K, and the enthalpy change for the same net reaction was calculated to be 41.0 kJ/mol (9.8 kcal/mol). By taking these results into account, it was discovered that the major obstacle to cyanide coordination to ferrocytochrome c is enthalpic, due to the greater compactness of the reduced molecule or, alternatively, to a lower rate of conformational fluctuation caused by solvation, electrostatic, and structural factors. The biophysical consequences of the large difference in the stabilities of the closed crevice structures are discussed
Addendum to Finite-size effects on multibody neutrino exchange
The interaction energy of the neutrons due to massless neutrino exchange in a
neutron star has recently been proved, using an effective theory, to be
extremely small and infrared-safe. Our comment here is of conceptual order: two
approaches to compute the total interaction energy density have recently been
proposed. Here, we study the connection between these two approaches. From CP
invariance, we argue that the resulting interaction energy has to be even in
the parameter , which expresses the static neutrino
potential created by a neutron medium of density .Comment: Latex file (Revtex), 9 pages, 1 figure, one reference change
Spin noise and Bell inequalities in a realistic superconductor-quantum dot entangler
Charge and spin current correlations are analyzed in a source of
spin-entangled electrons built from a superconductor and two quantum dots in
parallel. In addition to the ideal (crossed Andreev) channel, parasitic
channels (direct Andreev and cotunneling) and spin flip processes are fully
described in a density matrix framework. The way they reduce both the
efficiency and the fidelity of the entangler is quantitatively described by
analyzing the zero-frequency noise correlations of charge current as well as
spin current in the two output branches. Spin current noise is characterized by
a spin Fano factor, equal to 0 (total current noise) and -1 (crossed
correlations) for an ideal entangler. The violation of the Bell inequalities,
as a test of non-locality (entanglement) of split pairs, is formulated in terms
of the correlations of electron charge and spin numbers counted in a specific
time window . The efficiency of the test is analyzed, comparing to
the various time scales in the entangler operation.Comment: 8 pages, 5 figures, references added, to appear in Phys. Rev.
Dynamical Generation of Extended Objects in a Dimensional Chiral Field Theory: Non-Perturbative Dirac Operator Resolvent Analysis
We analyze the dimensional Nambu-Jona-Lasinio model non-perturbatively.
In addition to its simple ground state saddle points, the effective action of
this model has a rich collection of non-trivial saddle points in which the
composite fields \sigx=\lag\bar\psi\psi\rag and \pix=\lag\bar\psi
i\gam_5\psi\rag form static space dependent configurations because of
non-trivial dynamics. These configurations may be viewed as one dimensional
chiral bags that trap the original fermions (``quarks") into stable extended
entities (``hadrons"). We provide explicit expressions for the profiles of
these objects and calculate their masses. Our analysis of these saddle points
is based on an explicit representation we find for the diagonal resolvent of
the Dirac operator in a \{\sigx, \pix\} background which produces a
prescribed number of bound states. We analyse in detail the cases of a single
as well as two bound states. We find that bags that trap fermions are the
most stable ones, because they release all the fermion rest mass as binding
energy and become massless. Our explicit construction of the diagonal resolvent
is based on elementary Sturm-Liouville theory and simple dimensional analysis
and does not depend on the large approximation. These facts make it, in our
view, simpler and more direct than the calculations previously done by Shei,
using the inverse scattering method following Dashen, Hasslacher, and Neveu.
Our method of finding such non-trivial static configurations may be applied to
other dimensional field theories
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