128 research outputs found
Enhancement of flux-line pinning in all-oxide superconductor/ferromagnet heterostructures
We have studied the local critical current density, jc, in the superconductor
thin film of bilayer structures consisting of YBa2Cu3O7 and the ferromagnets
La2/3Ca1/3MnO3 and SrRuO3, respectively, by means of quantitative
magneto-optics. A pronounced hysteresis of jc was observed which is ascribed to
the magnetization state of the ferromagnetic layer. The results are discussed
within the frame of magnetic vortex - wall interactions.Comment: 9 page
Is There Still Room for Warm/Hot Gas? Simulating the X-ray Background Spectrum
At low redshifts, a census of the baryons in all known reservoirs falls a
factor of two to four below the total baryon density predicted from Big Bang
nucleosynthesis arguments and observed light element ratios. Recent
cosmological hydrodynamic simulations suggest that a significant fraction of
these missing baryons could be in the form of warm/hot gas in the filaments and
halos within which most field galaxies are embedded. With the release of source
count results from Chandra and recent detections of this gas in O VI quasar
absorption lines, it becomes interesting to examine the predictions and limits
placed on this component of the X-ray background (XRB). We have used new
hydrodynamical simulations to predict the total X-ray spectrum from the gas in
the 100 eV to 10 keV range. We find that, when uncertainties in the
normalization of the observed XRB and the value of Omega_b are taken into
account, our results are consistent with current observational limits placed on
the contribution of emission from gas to the XRB. In the 0.5-2 keV range, we
expect the contribution from this component to be 0.63 10^{-12} erg s^-1 cm^-2
deg^-2 or between 6% and 18% of the extragalactic surface brightness. The peak
fraction occurs in the 0.5-1 keV range where the predicted line emission
mirrors a spectral bump seen in the latest ASCA/ROSAT XRB data.Comment: 5 pages with 1 figure; submitted to ApJ Letter
Quantum phase transition to unconventional multi-orbital superfluidity in optical lattices
Orbital physics plays a significant role for a vast number of important
phenomena in complex condensed matter systems such as high-T
superconductivity and unconventional magnetism. In contrast, phenomena in
superfluids -- especially in ultracold quantum gases -- are commonly well
described by the lowest orbital and a real order parameter. Here, we report on
the observation of a novel multi-orbital superfluid phase with a {\it complex}
order parameter in binary spin mixtures. In this unconventional superfluid, the
local phase angle of the complex order parameter is continuously twisted
between neighboring lattice sites. The nature of this twisted superfluid
quantum phase is an interaction-induced admixture of the p-orbital favored by
the graphene-like band structure of the hexagonal optical lattice used in the
experiment. We observe a second-order quantum phase transition between the
normal superfluid (NSF) and the twisted superfluid phase (TSF) which is
accompanied by a symmetry breaking in momentum space. The experimental results
are consistent with calculated phase diagrams and reveal fundamentally new
aspects of orbital superfluidity in quantum gas mixtures. Our studies might
bridge the gap between conventional superfluidity and complex phenomena of
orbital physics.Comment: 5 pages, 4 figure
Suppression of Superconductivity in YBCO/LCMO Superlattices
The competition of superconductivity and magnetism in superlattices composed
of alternating YBaCuO and LaCaMnO thin
films is investigated using low-energy optical spectroscopy. The thickness of
the superconducting YBCO layers is varied from 30 nm to 20 nm while the
thickness of the magnetic LCMO layers is kept constant at 20 nm. We clearly
observe that the superconducting condensate density in the superconducting
state of superlattice is drastically reduced by the magnetic subsystem which
may be connected with proximity effects that distort the gap symmetry and thus
suppress superconductivity.Comment: 4 pages, 4 figure
Creating, moving and merging Dirac points with a Fermi gas in a tunable honeycomb lattice
Dirac points lie at the heart of many fascinating phenomena in condensed
matter physics, from massless electrons in graphene to the emergence of
conducting edge states in topological insulators [1, 2]. At a Dirac point, two
energy bands intersect linearly and the particles behave as relativistic Dirac
fermions. In solids, the rigid structure of the material sets the mass and
velocity of the particles, as well as their interactions. A different, highly
flexible approach is to create model systems using fermionic atoms trapped in
the periodic potential of interfering laser beams, a method which so far has
only been applied to explore simple lattice structures [3, 4]. Here we report
on the creation of Dirac points with adjustable properties in a tunable
honeycomb optical lattice. Using momentum-resolved interband transitions, we
observe a minimum band gap inside the Brillouin zone at the position of the
Dirac points. We exploit the unique tunability of our lattice potential to
adjust the effective mass of the Dirac fermions by breaking inversion symmetry.
Moreover, changing the lattice anisotropy allows us to move the position of the
Dirac points inside the Brillouin zone. When increasing the anisotropy beyond a
critical limit, the two Dirac points merge and annihilate each other - a
situation which has recently attracted considerable theoretical interest [5-9],
but seems extremely challenging to observe in solids [10]. We map out this
topological transition in lattice parameter space and find excellent agreement
with ab initio calculations. Our results not only pave the way to model
materials where the topology of the band structure plays a crucial role, but
also provide an avenue to explore many-body phases resulting from the interplay
of complex lattice geometries with interactions [11, 12]
Long-range transfer of electron-phonon coupling in oxide superlattices
The electron-phonon interaction is of central importance for the electrical
and thermal properties of solids, and its influence on superconductivity,
colossal magnetoresistance, and other many-body phenomena in
correlated-electron materials is currently the subject of intense research.
However, the non-local nature of the interactions between valence electrons and
lattice ions, often compounded by a plethora of vibrational modes, present
formidable challenges for attempts to experimentally control and theoretically
describe the physical properties of complex materials. Here we report a Raman
scattering study of the lattice dynamics in superlattices of the
high-temperature superconductor and the
colossal-magnetoresistance compound that suggests
a new approach to this problem. We find that a rotational mode of the MnO
octahedra in experiences pronounced
superconductivity-induced lineshape anomalies, which scale linearly with the
thickness of the layers over a remarkably long range of
several tens of nanometers. The transfer of the electron-phonon coupling
between superlattice layers can be understood as a consequence of long-range
Coulomb forces in conjunction with an orbital reconstruction at the interface.
The superlattice geometry thus provides new opportunities for controlled
modification of the electron-phonon interaction in complex materials.Comment: 13 pages, 4 figures. Revised version to be published in Nature
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