24 research outputs found
Influence of randomly distributed magnetic nanoparticles on surface superconductivity in Nb films
We report on combined resistance and magnetic measurements in a hybrid
structure (HS) of randomly distributed anisotropic CoPt magnetic nanoparticles
(MN) embedded in a 160 nm Nb thick film. Our resistance measurements exhibited
a sharp increase at the magnetically determined bulk upper-critical fields
Hc2(T). Above these points the resistance curves are rounded, attaining the
normal state value at much higher fields identified as the surface
superconductivity fields Hc3(T). When plotted in reduced temperature units, the
characteristic field lines Hc3(T) of the HS and of a pure Nb film, prepared at
exactly the same conditions, coincide for H10 kOe
they strongly segregate. Interestingly, the characteristic value H=10 kOe is
equal to the saturation field of the MN. The behavior mentioned above is
observed only for the case where the field is normal to the HS, while is absent
when the field is parallel to the film. Our experimental results suggest that
the observed enhancement of surface superconductivity field Hc3(T) is possibly
due to the not uniform local reduction of the external magnetic field by the
dipolar fields of the MN.Comment: to be published in Phys. Rev.
Holographic Josephson Junctions and Berry holonomy from D-branes
We construct a holographic model for Josephson junctions with a defect system
of a Dp brane intersecting a D(p+2) brane. In addition to providing a
geometrical picture for the holographic dual, this leads us very naturally to
suggest the possibility of non-Abelian Josephson junctions characterized in
terms of the topological properties of the branes. The difference between the
locations of the endpoints of the Dp brane on either side of the defect
translates into the phase difference of the condensate in the Josephson
junction. We also add a magnetic flux on the D(p+2) brane and allow it evolve
adiabatically along a closed curve in the space of the magnetic flux, while
generating a non-trivial Berry holonomy.Comment: 20 pages, 2 figure
Fermion correlators in non-abelian holographic superconductors
We consider fermion correlators in non-abelian holographic superconductors.
The spectral function of the fermions exhibits several interesting features
such as support in displaced Dirac cones and an asymmetric distribution of
normal modes. These features are compared to similar ones observed in angle
resolved photoemission experiments on high T_c superconductors. Along the way
we elucidate some properties of p-wave superconductors in AdS_4 and discuss the
construction of SO(4) superconductors.Comment: 49 pages, 11 figure
Quantum States and Phases in Driven Open Quantum Systems with Cold Atoms
An open quantum system, whose time evolution is governed by a master
equation, can be driven into a given pure quantum state by an appropriate
design of the system-reservoir coupling. This points out a route towards
preparing many body states and non-equilibrium quantum phases by quantum
reservoir engineering. Here we discuss in detail the example of a \emph{driven
dissipative Bose Einstein Condensate} of bosons and of paired fermions, where
atoms in an optical lattice are coupled to a bath of Bogoliubov excitations via
the atomic current representing \emph{local dissipation}. In the absence of
interactions the lattice gas is driven into a pure state with long range order.
Weak interactions lead to a weakly mixed state, which in 3D can be understood
as a depletion of the condensate, and in 1D and 2D exhibits properties
reminiscent of a Luttinger liquid or a Kosterlitz-Thouless critical phase at
finite temperature, with the role of the ``finite temperature'' played by the
interactions.Comment: 9 pages, 2 figure
Antiferromagnetic Order Induced by an Applied Magnetic Field in a High-Temperature Superconductor
One view of the cuprate high-transition temperature (high-Tc) superconductors
is that they are conventional superconductors where the pairing occurs between
weakly interacting quasiparticles, which stand in one-to-one correspondence
with the electrons in ordinary metals - although the theory has to be pushed to
its limit. An alternative view is that the electrons organize into collective
textures (e.g. charge and spin stripes) which cannot be mapped onto the
electrons in ordinary metals. The phase diagram, a complex function of various
parameters (temperature, doping and magnetic field), should then be approached
using quantum field theories of objects such as textures and strings, rather
than point-like electrons. In an external magnetic field, magnetic flux
penetrates type-II superconductors via vortices, each carrying one flux
quantum. The vortices form lattices of resistive material embedded in the
non-resistive superconductor and can reveal the nature of the ground state -
e.g. a conventional metal or an ordered, striped phase - which would have
appeared had superconductivity not intervened. Knowledge of this ground state
clearly provides the most appropriate starting point for a pairing theory. Here
we report that for one high-Tc superconductor, the applied field which imposes
the vortex lattice, also induces antiferromagnetic order. Ordinary
quasiparticle pictures cannot account for the nearly field-independent
antiferromagnetic transition temperature revealed by our measurements
A single-photon transistor using nano-scale surface plasmons
It is well known that light quanta (photons) can interact with each other in
nonlinear media, much like massive particles do, but in practice these
interactions are usually very weak. Here we describe a novel approach to
realize strong nonlinear interactions at the single-photon level. Our method
makes use of recently demonstrated efficient coupling between individual
optical emitters and tightly confined, propagating surface plasmon excitations
on conducting nanowires. We show that this system can act as a nonlinear
two-photon switch for incident photons propagating along the nanowire, which
can be coherently controlled using quantum optical techniques. As a novel
application, we discuss how the interaction can be tailored to create a
single-photon transistor, where the presence or absence of a single incident
photon in a ``gate'' field is sufficient to completely control the propagation
of subsequent ``signal'' photons.Comment: 20 pages, 4 figure
Electron-Spin Excitation Coupling in an Electron Doped Copper Oxide Superconductor
High-temperature (high-Tc) superconductivity in the copper oxides arises from
electron or hole doping of their antiferromagnetic (AF) insulating parent
compounds. The evolution of the AF phase with doping and its spatial
coexistence with superconductivity are governed by the nature of charge and
spin correlations and provide clues to the mechanism of high-Tc
superconductivity. Here we use a combined neutron scattering and scanning
tunneling spectroscopy (STS) to study the Tc evolution of electron-doped
superconducting Pr0.88LaCe0.12CuO4-delta obtained through the oxygen annealing
process. We find that spin excitations detected by neutron scattering have two
distinct modes that evolve with Tc in a remarkably similar fashion to the
electron tunneling modes in STS. These results demonstrate that
antiferromagnetism and superconductivity compete locally and coexist spatially
on nanometer length scales, and the dominant electron-boson coupling at low
energies originates from the electron-spin excitations.Comment: 30 pages, 12 figures, supplementary information include
The pseudogap: friend or foe of high Tc?
Although nineteen years have passed since the discovery of high temperature
superconductivity, there is still no consensus on its physical origin. This is
in large part because of a lack of understanding of the state of matter out of
which the superconductivity arises. In optimally and underdoped materials, this
state exhibits a pseudogap at temperatures large compared to the
superconducting transition temperature. Although discovered only three years
after the pioneering work of Bednorz and Muller, the physical origin of this
pseudogap behavior and whether it constitutes a distinct phase of matter is
still shrouded in mystery. In the summer of 2004, a band of physicists gathered
for five weeks at the Aspen Center for Physics to discuss the pseudogap. In
this perspective, we would like to summarize some of the results presented
there and discuss its importance in the context of strongly correlated electron
systems.Comment: expanded version, 20 pages, 11 figures, to be published, Advances in
Physic