63 research outputs found
Shape- and orientation-dependence of surface barriers in single crystalline d-wave Bi_2Sr_2CaCu_2O_8+delta
7 pages, submitted to Phys. Rev. BMagneto-optical imaging and Hall-probe array magnetometry are used to measure the field of first flux entry, H_p, into the same Bi_2Sr_2CaCu_2O_8+delta single crystal cut to different crystal thickness-to-width ratios (d/w), and for two angles alpha between the edges and the principal in-plane crystalline (a,b) axes. At all temperatures, the variation with aspect ratio of H_p is qualitatively well described by calculations for the so-called geometric barrier [E.H. Brandt, Phys. Rev. B 60, 11939 (1999)]. However, the magnitude of H_p is strongly enhanced due to the square shape of the crystal. In the intermediate temperature regime (T < ~ 50 K) in which the Bean-Livingston barrier limits vortex entry, there is some evidence for a tiny crystal-orientation dependent enhancement when the sample edges are at an angle of 45° with respect to the crystalline axes, rather than parallel to them
In-plane field-induced vortex liquid correlations in underdoped Bi_2Sr_2CaCu_2O_8+\delta
The effect of a magnetic field component parallel to the superconducting
layers on longitudinal Josephson plasma oscillations in the layered high
temperature superconductor BiSrCaCuO is shown to
depend on the thermodynamic state of the underlying vortex lattice. Whereas the
parallel magnetic field component depresses the Josephson Plasma Resonance
(JPR) frequency in the vortex solid phase, it may enhance it in the vortex
liquid. There is a close correlation between the behavior of microwave
absorption near the JPR frequency and the effectiveness of pancake vortex
pinning, with the enhancement of the plasma resonance frequency occurring in
the absence of pinning, at high temperature close to the vortex melting line.
An interpretation is proposed in terms of the attraction between pancake
vortices and Josephson vortices, apparently also present in the vortex liquid
state.Comment: 8 pages, 7 Figures, submitted to Phys. Rev.
Thermodynamics of the vortex liquid in heavy ion-irradiated superconductors
It is shown that the large effect of heavy ion-irradiation on the
thermodynamical properties of the anisotropic superconductor
YBaCuO extends well into the superconducting
fluctuation regime. The presence of the induced amorphous columnar defects
shifts the specific heat maximum at the normal-to-superconducting transition.
This effect is similar to that recently put into evidence in cubic
KBaBiO (). In both compounds, vortex pinning
manifests itself as a sharp angular dependence of the \em equilibrium \rm
torque. In YBaCuO, pinning by the defects appears at
the temperature of the specific heat maximum, well above the
magnetic irreversibility line . In isotropic
KBaBiO, the onset of the pinning-related torque anomaly
tracks the onset of the specific heat anomaly and the irreversibility line. In
YBaCuO, fluctuations of the amplitude of the order
parameter (and not vortex line wandering) are ultimately responsible for the
vanishing of pinning. In KBaBiO, vortex pinning disappears
only at the superconducting-to-normal transition. The results indicate that in
both compounds, the pinning energy at the ``Bose glass'' transition is large
with respect to the total free energy gain in the superconducting state. By
implication, the mechanism of this latter transition should be reconsidered.Comment: 9 pages, 9 figures, resubmitted to PRB 23-09-200
Singular robust room-temperature spin response from topological Dirac fermions
Topological insulators are a class of solids in which the nontrivial inverted
bulk band structure gives rise to metallic surface states that are robust
against impurity scattering. In three-dimensional (3D) topological insulators,
however, the surface Dirac fermions intermix with the conducting bulk, thereby
complicating access to the low energy (Dirac point) charge transport or
magnetic response. Here we use differential magnetometry to probe spin rotation
in the 3D topological material family (BiSe, BiTe, and
SbTe). We report a paramagnetic singularity in the magnetic
susceptibility at low magnetic fields which persists up to room temperature,
and which we demonstrate to arise from the surfaces of the samples. The
singularity is universal to the entire family, largely independent of the bulk
carrier density, and consistent with the existence of electronic states near
the spin-degenerate Dirac point of the 2D helical metal. The exceptional
thermal stability of the signal points to an intrinsic surface cooling process,
likely of thermoelectric origin, and establishes a sustainable platform for the
singular field-tunable Dirac spin response.Comment: 20 pages, 14 figure
Ultrafast photocurrents at the surface of the three-dimensional topological insulator
Topological insulators constitute a new and fascinating class of matter with
insulating bulk yet metallic surfaces that host highly mobile charge carriers
with spin-momentum locking. Remarkably, the direction and magnitude of surface
currents can be controlled with tailored light beams, but the underlying
mechanisms are not yet well understood. To directly resolve the "birth" of such
photocurrents we need to boost the time resolution to the scale of elementary
scattering events ( 10 fs). Here, we excite and measure photocurrents in
the three-dimensional model topological insulator
with a time resolution as short as 20 fs by sampling the concomitantly emitted
broadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafast
surface current response is dominated by a charge transfer along the Se-Bi
bonds. In contrast, photon-helicity-dependent photocurrents are found to have
orders of magnitude smaller magnitude than expected from generation scenarios
based on asymmetric depopulation of the Dirac cone. Our findings are also of
direct relevance for optoelectronic devices based on topological-insulator
surface currents
Detection of discretized single-shell penetration in mesoscopic vortex matter
We investigated configurational changes in mesoscopic vortex matter with less
than thousand vortices during flux penetration in freestanding 50 m
diameter disks of BiSrCaCuO. High-resolution AC
and DC local magnetometry data reveal oscillations in the transmittivity echoed
in peaks in the third-harmonics magnetic signal fainting on increasing vortex
density. By means of extra experimental evidence and a simple geometrical
analysis we show that these features fingerprint the discretized entrance of
single-shells of vortices having a shape that mimics the sample edge
Vortex creep and critical current densities in superconducting (Ba,K)FeAs single crystals
The surprisingly rapid relaxation of the sustainable current density in the
critical state of single crystalline BaKFeAs is
investigated for magnetic fields oriented parallel to the c-axis and to the
--plane respectively. Due to the inadequacy of standard analysis procedures
developed for flux creep in the high temperature superconducting cuprates, we
develop a simple, straightforward data treatment technique that reveals the
creep mechanism and the creep exponent . At low magnetic fields, below the
second magnetization peak, varies only slightly as function of
temperature and magnetic flux density . From the data, we determine the
temperature- and field dependence of the effective activation barrier for
creep. At low temperatures, the measured current density approaches the
zero--temperature critical current density (in the absence of creep) to within
a factor 2, thus lending credence to earlier conclusions drawn with respect to
the pinning mechanism. The comparable values of the experimental screening
current density and the zero-temperature critical current density reveals the
limited usefulness of the widely used "interpolation formula".Comment: Physical Review B (2012) Accepte
Disorder, critical currents, and vortex pinning energies in isovalently substituted BaFe(AsP)
We present a comprehensive overview of vortex pinning in single crystals of
the isovalently substituted iron-based superconductor
BaFe(AsP), a material that qualifies as an
archetypical clean superconductor, containing only sparse strong point-like
pins [in the sense of C.J. van der Beek {\em et al.}, Phys. Rev. B {\bf 66},
024523 (2002)]. Widely varying critical current values for nominally similar
compositions show that flux pinning is of extrinsic origin. Vortex
configurations, imaged using the Bitter decoration method, show less density
fluctuations than those previously observed in charge-doped
Ba(FeCo)As single crystals. Analysis reveals that the
pinning force and -energy distributions depend on the P-content . However,
they are always much narrower than in Ba(FeCo)As, a
result that is attributed to the weaker temperature dependence of the
superfluid density on approaching in
BaFe(AsP). Critical current density measurements and
pinning force distributions independently yield a mean distance between
effective pinning centers nm, increasing with
increasing P-content . This evolution can be understood as being the
consequence of the P-dependence of the London penetration depth. Further
salient features are a wide vortex free "Meissner belt", observed at the edge
of overdoped crystals, and characteristic chain-like vortex arrangements,
observed at all levels of P-substitution.Comment: 11 page
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