138 research outputs found
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Tunneling spectroscopy of the superconducting state of URu2Si2
We present measurements of the superconducting gap of URuSi made with
scanning tunneling microscopy (STM) using a superconducting tip of Al. We find
tunneling conductance curves with a finite value at the Fermi level. The
density of states is V shaped at low energies, and the quasiparticle peaks are
located at values close to the expected superconducting gap from weak coupling
BCS theory. Our results point to rather opened gap structures and gap nodes on
the Fermi surface
Reply to Comment by Borisenko et al. on article `A de Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAs'
Recently, Borisenko et al have posted a Comment (arXiv:1108.1159) where they
suggest an alternative interpretation of our de Haas-van Alphen (dHvA)
measurements on the superconductor LiFeAs. In our original paper
(arXiv:1107.4375) we concluded that our measurements of the bulk Fermi surface
were not consistent with the surface bands observed thus far by ARPES.
Borisenko et al dispute this and suggest the two measurements are consistent if
some of the orbits we observe are due to magnetic breakdown. We argue here that
this scenario is inconsistent with the experimental data and therefore that our
original conclusion stands.Comment: 4 pages with figure
Direct observation of melting in a 2-D superconducting vortex lattice
Topological defects such as dislocations and disclinations are predicted to
determine the twodimensional (2-D) melting transition. In 2-D superconducting
vortex lattices, macroscopic measurements evidence melting close to the
transition to the normal state. However, the direct observation at the scale of
individual vortices of the melting sequence has never been performed. Here we
provide step by step imaging through scanning tunneling spectroscopy of a 2-D
system of vortices up to the melting transition in a focused-ion-beam
nanodeposited W-based superconducting thin film. We show directly the
transition into an isotropic liquid below the superconducting critical
temperature. Before that, we find a hexatic phase, characterized by the
appearance of free dislocations, and a smectic-like phase, possibly originated
through partial disclination unbinding. These results represent a significant
step in the understanding of melting of 2-D systems, with impact across several
research fields, such as liquid crystal molecules, or lipids in membranes.Comment: Submitted to Nature Physic
Intrinsic atomic scale modulations of the superconducting gap of 2H-NbSe2
We present scanning tunneling microscopy and spectroscopy measurements at
100mK in the superconducting material 2H-NbSe2 that show well defined features
in the superconducting density of states changing in a pattern closely
following atomic periodicity. Our experiment demonstrates that the intrinsic
superconducting density of states can show atomic size modulations, which
reflect the reciprocal space structure of the superconducting gap. In
particular we obtain that the superconducting gap of 2H-NbSe2 has six fold
modulated components at 0.75 mV and 1.2 mV.Moreover, we also find related
atomic size modulations inside vortices, demonstrating that the much discussed
star shape vortex structure produced by localized states inside the vortex
cores, has a, hitherto undetected, superposed atomic size modulation. The tip
substrate interaction in an anisotropic superconductor has been calculated,
giving position dependent changes related to the observed gap anisotropy.Comment: 9 pages, 5 figures. Discussion extended, references added, one more
figur
Scanning microscopies of superconductors at very low temperatures
We discuss basics of scanning tunneling microscopy and spectroscopy (STM/S)
of the superconducting state with normal and superconducting tips. We present a
new method to measure the local variations in the Andreev reflection amplitude
between a superconducting tip and the sample. This method is termed Scanning
Andreev Reflection Spectroscopy (SAS). We also briefly discuss vortex imaging
with STM/S under an applied current through the sample, and show the vortex
lattice as a function of the angle between the magnetic field and sample's
surface
Superconducting density of states and vortex cores of 2H-NbS2
Scanning tunneling microscopy and spectroscopy (STM/S) measurements in the
superconducting dichalcogenide 2H-NbS2 show a peculiar superconducting density
of states with two well defined features at 0.97 meV and 0.53 meV, located
respectively above and below the value for the superconducting gap expected
from single band s-wave BCS model (D=1.76kBTc=0.9 meV). Both features have a
continuous temperature evolution and disappear at Tc = 5.7 K. Moreover, we
observe the hexagonal vortex lattice with radially symmetric vortices and a
well developed localized state at the vortex cores. The sixfold star shape
characteristic of the vortex lattice of the compound 2H-NbSe2 is, together with
the charge density wave order (CDW), absent in 2H-NbS2.Comment: 5 pages, 4 figure
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