107 research outputs found
Probing the Phase Diagram of Bi2Sr2CaCu2O8+d with Tunneling Spectroscopy
Tunneling measurements are performed on Ca-rich single crystals of
Bi2Sr2CaCu2O8+d (Bi2212), with various oxygen doping levels, using a novel
point contact method. At 4.2 K, SIN and SIS tunnel junctions are obtained with
well-defined quasiparticle peaks, robust dip and hump features and in some
cases Josephson currents. The doping dependence of tunneling conductances of
Ca-rich Bi2212 are analyzed and compared to stoichiometric Bi2212. A similar
profile of energy gap vs. doping concentration is found although the Ca-rich
samples have a slighly smaller optimum Tc and therefore smaller gap values for
any doping level. The evolution of tunneling conductance peak height to
background ratios with hole concentration are compared. For a given doping
level, the Ca-rich spectra showed more broadened features compared to the
stoichiometric counterparts, most likely due to increased disorder from the
excess Ca. Comparison of the dip and hump features has provided some potential
insights into their origins.Comment: 4 pages, 4 figures; presented at the Applied Superconductivity
Conference (August 4-9, 2002) in Houston, TX; to be published in IEEE Trans.
Appl. Supercon
Single Junction and Intrinsic Josephson Junction Tunneling Spectroscopies of Bi2Sr2CaCu2O8+d
Tunneling spectroscopy measurements are reported on optimally-doped and
overdoped BiSrCaCuO single crystals. A novel
point contact method is used to obtain superconductor-insulator-normal metal
(SIN) and SIS break junctions as well as intrinsic Josephson junctions (IJJ)
from nanoscale crystals. Three junction types are obtained on the same crystal
to compare the quasiparticle peaks and higher bias dip/hump structures which
have also been found in other surface probes such as scanning tunneling
spectroscopy and angle-resolved photoemission spectroscopy. However, our IJJ
quasiparticle spectra consistently reveal very sharp conductance peaks and no
higher bias dip structures. The IJJ conductance peak voltage divided by the
number of junctions in the stack consistently leads to a significant
underestimate of when compared to the single junction values. The
comparison of the three methods suggests that the markedly different
characteristics of IJJ are a consequence of nonequilibrium effects and are not
intrinsic quasiparticle features.Comment: 4 pages, 4 figures; presented at the Applied Superconductivity
Conference (October 3-8, 2004) in Jacksonville, FL; to be published in IEEE
Trans. Appl. Supercon
Rebuttal to "Comment by V.M. Krasnov on 'Counterintuitive consequence of heating in strongly-driven intrinsic junctions of Bi2Sr2CaCu2O8+d Mesas' "
In our article [1], we found that with increasing dissipation there is a
clear, systematic shift and sharpening of the conductance peak along with the
disappearance of the higher-bias dip/hump features (DHF), for a stack of
intrinsic Josephson junctions (IJJs) of intercalated Bi2Sr2CaCu2O8+{\delta}
(Bi2212). Our work agrees with Zhu et al [2] on unintercalated, pristine
Bi2212, as both studies show the same systematic changes with dissipation. The
broader peaks found with reduced dissipation [1,2] are consistent with broad
peaks in the density-of-states (DOS) found among scanning tunneling
spectroscopy [3] (STS), mechanical contact tunneling [4] (MCT) and inferred
from angle (momentum) resolved photoemission spectroscopy [5] (ARPES); results
that could not be ignored. Thus, sharp peaks are extrinsic and cannot
correspond to the superconducting DOS. We suggested that the commonality of the
sharp peaks in our conductance data, which is demonstrably shown to be
heating-dominated, and the peaks of previous intrinsic tunneling spectroscopy
(ITS) data implies that these ITS reports might need reinterpretation.Comment: Rebuttal to Comment of Krasnov arXiv:1007.451
Full Bulk Spin Polarization and Intrinsic Tunnel Barriers at the Surface of Layered Manganites
Transmission of information using the spin of the electron as well as its
charge requires a high degree of spin polarization at surfaces. At surfaces
however this degree of polarization can be quenched by competing interactions.
Using a combination of surface sensitive x-ray and tunneling probes, we show
for the quasi-two-dimensional bilayer manganites that the outermost Mn-O
bilayer, alone, is affected: it is a 1-nm thick insulator that exhibits no
long-range ferromagnetic order while the next bilayer displays the full spin
polarization of the bulk. Such an abrupt localization of the surface effects is
due to the two-dimensional nature of the layered manganite while the loss of
ferromagnetism is attributed to weakened double exchange in the reconstructed
surface bilayer and a resultant antiferromagnetic phase. The creation of a
well-defined surface insulator demonstrates the ability to naturally
self-assemble two of the most demanding components of an ideal magnetic tunnel
junction.Comment: 19 pages, 5 figure
Characteristic Energy of the Coulomb Interactions and the Pileup of States
Tunneling data on crystals confirm
Coulomb interaction effects through the dependence of the
density of states. Importantly, the data and analysis at high energy, E, show a
pileup of states: most of the states removed from near the Fermi level are
found between ~40 and 130 meV, from which we infer the possibility of universal
behavior. The agreement of our tunneling data with recent photoemission results
further confirms our analysis.Comment: 4 pages, 4 figures, submitted to PR
Effect of bilayer coupling on tunneling conductance of double-layer high T_c cuprates
Physical effects of bilayer coupling on the tunneling spectroscopy of high
T cuprates are investigated. The bilayer coupling separates the bonding
and antibonding bands and leads to a splitting of the coherence peaks in the
tunneling differential conductance. However, the coherence peak of the bonding
band is strongly suppressed and broadened by the particle-hole asymmetry in the
density of states and finite quasiparticle life-time, and is difficult to
resolve by experiments. This gives a qualitative account why the bilayer
splitting of the coherence peaks was not clearly observed in tunneling
measurements of double-layer high-T oxides.Comment: 4 pages, 3 figures, to be published in PR
Hypothesis of two-dimensional stripe arrangement and its implications for the superconductivity in high-Tc cuprates
The hypothesis that holes doped into high-Tc cuprate superconductors organize
themselves in two-dimensional (2D) array of diagonal stripes is discussed, and,
on the basis of this hypothesis, a new microscopic model of superconductivity
is proposed and solved. The model describes two kinds of hole states localized
either inside the stripes or in the antiferromagnetic domains between the
stripes. The characteristic energy difference between these two kinds of states
is identified with the pseudogap. The superconducting (SC) order parameter
predicted by the model has two components, whose phases exhibit a complex
dependence on the the center-of-mass coordinate. The model predictions for the
tunneling characteristics and for the dependence of the critical temperature on
the superfluid density show good quantitative agreement with a number of
experiments. The model, in particular, predicts that the SC peaks in the
tunneling spectra are asymmetric, only when the ratio of the SC gap to the
critical temperature is greater than 4. It is also proposed that, at least in
some high-Tc cuprates, there exist two different superconducting states
corresponding to the same doping concentration and the same critical
temperature. Finally, the checkerboard pattern in the local density of states
observed by scanning tunneling microscopy in Bi-2212 is interpreted as coming
from the states localized around the centers of stripe elements forming the 2D
superstructure.Comment: Text close to the published version. This version is 10 per cent
shorter than the previous one. All revisions are mino
Modeling Study of the Dip/Hump Feature in BiSrCaCuO Tunneling Spectroscopy
The tunneling spectra of high temperature superconductors on
BiSrCaCuO (Bi-2212) reproducibly show a high bias
structure in the form of a dip-hump at voltages higher than the gap voltage. Of
central concern is whether this feature originates from the normal state
background or is intrinsic to the superconducting mechanism. We address this
issue by generating a set of model conductance curves - a ''normal state''
conductance that takes into account effects such as the band structure and a
possible pseudogap, and a pure superconducting state conductance. When
combined, the result shows that the dip-hump feature present in the
experimental conductance curves cannot be naively attributed to a normal state
effect. In particular, strong dip features found in
superconductor-insulator-superconductor data on optimally-doped Bi-2212,
including negative dI/dV, cannot be a consequence of an extrinsic pseudogap.
However, such features can easily arise from states-conserving deviations in
the superconducting density of states, e.g., from strong-coupling effects.Comment: 4 pages, 4 figure
On the correct formula for the lifetime broadened superconducting density of states
We argue that the well known Dynes formula [Dynes R C {\it et al.} 1978 {\it
Phys. Rev. Lett.} {\bf 41} 1509] for the superconducting quasiparticle density
of states, which tries to incorporate the lifetime broadening in an approximate
way, cannot be justified microscopically for conventional superconductors.
Instead, we propose a new simple formula in which the energy gap has a finite
imaginary part and the quasiparticle energy is real. We prove that
in the quasiparticle approximation 2 gives the quasiparticle decay
rate at the gap edge for conventional superconductors. This conclusion does not
depend on the nature of interactions that cause the quasiparticle decay. The
new formula is tested on the case of a strong coupling superconductor
PbBi and an excellent agreement with theoretical predictions is
obtained. While both the Dynes formula and the one proposed in this work give
good fits and fit parameters for PbBi, only the latter formula
can be justified microscopically.Comment: 6 pages, 4 figure
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