464 research outputs found
Vortex core shrinkage in a two gap superconductor: application to MgB2
As a model for the vortex core in MgB2 we study a two band model with a clean
sigma band and a dirty pi band. We present calculations of the vortex core size
in both bands as a function of temperature and show that there exists a
Kramer-Pesch effect in both bands even though only one of the bands is in the
clean limit. We present calculations for different pi band diffusivities and
coherence lengths.Comment: Submitted to M2S-HTSC-VIII conference proceeding
Fermi surface topology and vortex state in MgB2
Based on a detailed modeling of the Fermi surface topology of MgB2 we
calculated the anisotropy of the upper critical field Bc2 within the two gap
model. The sigma-band is modeled as a distorted cylinder and the pi-band as a
half-torus, with parameters determined from bandstructure calculations. Our
results show that the unusual strong temperature dependence of the Bc2
anisotropy, that has been observed recently, can be understood due to the small
c-axis dispersion of the cylindrical Fermi surface sheets and the small
interband pairing interaction as obtained from bandstructure calculations. We
calculate the magnetic field dependence of the density of states within the
vortex state for field in c-axis direction and compare with recent measurements
of the specific heat on MgB2 single crystals.Comment: 2 pages, 2 figure
Interdependence of magnetism and superconductivity in the borocarbide TmNi2B2C
We have discovered a new antiferromagnetic phase in TmNi2B2C by neutron
diffraction. The ordering vector is Q_A = (0.48,0,0) and the phase appears
above a critical in-plane magnetic field of 0.9 T. The field was applied in
order to test the assumption that the zero-field magnetic structure at Q_F =
(0.094,0.094,0) would change into a c-axis ferromagnet if superconductivity
were destroyed. We present theoretical calculations which show that two effects
are important: A suppression of the ferromagnetic component of the RKKY
exchange interaction in the superconducting phase, and a reduction of the
superconducting condensation energy due to the periodic modulation of the
moments at the wave vector Q_A
Exploring the fragile antiferromagnetic superconducting phase in CeCoIn5
CeCoIn5 is a heavy fermion Type-II superconductor which exhibits clear
indications of Pauli-limited superconductivity. A variety of measurements give
evidence for a transition at high magnetic fields inside the superconducting
state, when the field is applied either parallel to or perpendicular to the c
axis. When the field is perpendicular to the c axis, antiferromagnetic order is
observed on the high-field side of the transition, with a magnetic wavevector
of (q q 0.5), where q = 0.44 reciprocal lattice units. We show that this order
remains as the magnetic field is rotated out of the basal plane, but the
associated moment eventually disappears above 17 degrees, indicating that the
anomalies seen with the field parallel to the c axis are not related to this
magnetic order. We discuss the implications of this finding.Comment: Accepted Physical Review Letters, September 2010. 4 pages, 4 figure
Magnetic field induced orientation of superconducting MgB crystallites determined by X-ray diffraction
X-ray diffraction studies of fine polycrystalline samples of MgB in the
superconducting state reveal that crystals orient with their \emph{c}-axis in a
plane normal to the direction of the applied magnetic field. The MgB
samples were thoroughly ground to obtain average grain size 5 - 10 m in
order to increase the population of free single crystal grains in the powder.
By monitoring Bragg reflections in a plane normal to an applied magnetic field
we find that the powder is textured with significantly stronger (\emph{0,0,l})
reflections in comparison to (\emph{h,k,0}), which remain essentially
unchanged. The orientation of the crystals with the \emph{ab}-plane parallel to
the magnetic field at all temperatures below demonstrates that the sign
of the torque under magnetic field does not alter, in disagreement with current
theoretical predictions
Scanning Tunneling Spectroscopy on Single Crystal MgB2
We report on the results of scanning tunneling spectroscopy measurements on
single crystals of Mg2. Tunneling was performed both parallel and perpendicular
to the crystalline c-axis. In the first case, a single superconducting gap
(Delta_pi = 2.2 meV) associated with the pi-band is observed. Tunneling
parallel to the ab-plane reveals an additional, larger gap (Delta_sigma ~ 7
meV) originating in the highly two-dimensional sigma-band. Vortex imaging in
the pi-band was performed with the field and tunnel current parallel to the
c-axis. The vortices have a large core size compared to estimates based on Hc2,
and show an absence of localized states in the core. Furthermore,
superconductivity between the vortices is rapidly suppressed by an applied
field. A comparison to specific heat measurements is performed.Comment: 12 pages, 7 figs. Submitted to Physica
Fluxon dynamics by microwave surface resistance measurements in MgB2
Field-induced variations of the microwave surface resistance, Rs(H), have
been investigated in high-density ceramic MgB2. At low temperatures, several
peculiarities of the Rs(H) curves cannot be justified in the framework of
models reported in the literature. We suggest that they are ascribable to the
unconventional vortex structure in MgB2, related to the presence of two gaps.
On the contrary, the results near Tc can be accounted for by the Coffey and
Clem model, with fluxons moving in the flux-flow regime, provided that the
anisotropy of the upper critical field is taken into due account.Comment: 6 pages, 4 figure
Structural Transition Kinetics and Activated Behavior in the Superconducting Vortex Lattice
Using small-angle neutron scattering, we investigated the behavior of a
metastable vortex lattice state in MgB2 as it is driven towards equilibrium by
an AC magnetic field. This shows an activated behavior, where the AC field
amplitude and cycle count are equivalent to, respectively, an effective
"temperature" and "time". The activation barrier increases as the metastable
state is suppressed, corresponding to an aging of the vortex lattice.
Furthermore, we find a cross-over from a partial to a complete suppression of
metastable domains depending on the AC field amplitude, which may empirically
be described by a single free parameter. This represents a novel kind of
collective vortex behavior, most likely governed by the nucleation and growth
of equilibrium vortex lattice domains.Comment: 5 pages plus 3 pages of supplemental materia
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