127 research outputs found
Cooper pair islanding model of insulating nanohoneycomb films
We first review evidence for the Cooper pair insulator (CPI) phase in
amorphous nanohoneycomb (NHC) films. We then extend our analysis of
superconducting islands induced by film thickness variations in NHC films to
examine the evolution of island sizes through the magnetic field-driven SIT.
Finally, using the islanding picture, we present a plausible model for the
appearance and behavior of the CPI phase in amorphous NHC films.Comment: 7 pages, 3 figure
Collapse of the Cooper pair phase coherence length at a superconductor to insulator transition
We present investigations of the superconductor to insulator transition (SIT)
of uniform a-Bi films using a technique sensitive to Cooper pair phase
coherence. The films are perforated with a nanohoneycomb array of holes to form
a multiply connected geometry and subjected to a perpendicular magnetic field.
Film magnetoresistances on the superconducting side of the SIT oscillate with a
period dictated by the superconducting flux quantum and the areal hole density.
The oscillations disappear close to the SIT critical point to leave a
monotonically rising magnetoresistance that persists in the insulating phase.
These observations indicate that the Cooper pair phase coherence length, which
is infinite in the superconducting phase, collapses to a value less than the
interhole spacing at this SIT. This behavior is inconsistent with the gradual
reduction of the phase coherence length expected for a bosonic, phase
fluctuation driven SIT. This result starkly contrasts with previous
observations of oscillations persisting in the insulating phase of other films
implying that there must be at least two distinct classes of disorder tuned
SITs
Modification of electronic surface states by graphene islands on Cu(111)
We present a study of graphene/substrate interactions on UHV-grown graphene
islands with minimal surface contamination using \emph{in situ} low-temperature
scanning tunneling microscopy (STM). We compare the physical and electronic
structure of the sample surface with atomic spatial resolution on graphene
islands versus regions of bare Cu(111) substrate. We find that the Rydberg-like
series of image potential states is shifted toward lower energy over the
graphene islands relative to Cu(111), indicating a decrease in the local work
function, and the resonances have a much smaller linewidth, indicating reduced
coupling to the bulk. In addition, we show the dispersion of the occupied
Cu(111) Shockley surface state is influenced by the graphene layer, and both
the band edge and effective mass are shifted relative to bare Cu(111).Comment: 12 pages, 3 figure
Cooper pair insulator in amorphous films induced by nanometer-scale thickness variations
Unusual transport properties of superconducting (SC) materials, such as the
under doped cuprates, low dimensional superconductors in strong magnetic
fields, and insulating films near the Insulator Superconductor Transition
(IST), have been attributed to the formation of inhomogeneous phases.
Difficulty correlating the behaviors with observations of the inhomogeneities
make these connections uncertain. Of primary interest here are proposals that
insulating films near the IST, which show an activated resistance and giant
positive magnetoresistance, contain islands of Cooper Pairs (CPs). Here we
present evidence that these types of inhomogeneities are essential to such an
insulating phase in amorphous Bi (a-Bi) films deposited on substrates patterned
with nanometer-sized holes. The patterning induces film thickness variations,
and corresponding coupling constant variations, that transform the composition
of the insulator from localized electrons to CPs. Analyses near the
thickness-tuned ISTs of films on nine different substrates show that weak links
between SC islands dominate the transport. In particular, the ISTs all occur
when the link resistance approaches the resistance quantum for pairs. These
observations lead to a detailed picture of CPs localized by spatial variations
of the superconducting coupling constant.Comment: 4 pages, 3 figures, 1 supplemental page with 1 supplemental figur
Thermal Expansion and Magnetostriction of the Ising Antiferromagnet TbNi2Ge2
We have measured the linear thermal expansion and magnetostriction of the
Ising antiferromagnet TbNi2Ge2 along its c-axis from room temperature to 2 K
and in magnetic fields to 14 T. We find a magnetic phase diagram that agrees
with earlier work and estimate aspects of its uniaxial pressure dependence. We
also find a new high field feature near 10 T which may signal the onset of an
additional field-induced phase.Comment: 2 pages, to apear in proceedings of 24th International Conference on
Low Temperature Physic
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