175 research outputs found
Tunneling Spectroscopy of Tl2Ba2CuO6
New results from tunneling spectroscopies on near optimally-doped single
crystals of Tl_{2}Ba_{2}CuO_{6} (Tl-2201) junctions are presented. The
superconductor-insulator-normal metal (SIN) tunnel junctions are obtained using
the point-contact technique with a Au tip. The tunneling conductances
reproducibly show a sharp cusp-like subgap, prominent quasiparticle peaks with
a consistent asymmetry, and weakly decreasing backgrounds. A rigorous analysis
of the SIN tunneling data is performed using two different models for the
(d-wave) density of states (DOS). Based on these and earlier
results, the tunneling DOS of Tl-2201 have exhibited the most reproducible data
that are consistent with a d-wave gap symmetry. We show that the dip feature at
that is clearly seen in SIN tunneling data of
Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} is also present in Tl-2201, but at a weaker
level. The gap values for crystals with a bulk T_c = 86 K are in the range of
19-25 meV.Comment: 7 pages, 5 figure
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
Two energy gaps in cuprates: pairing and coherence gaps: The interpretation of tunneling and inelastic neutron scattering measurements
Tunneling and inelastic neutron scattering (INS) measurements in cuprates are
discussed. There is a clear discrepancy among energy-gap values for different
90 K cuprates, inferred from tunneling measurements. By using the phase diagram
of hole-doped cuprates we interpret tunneling measurements in 90 K cuprates and
INS data in YBCO.Comment: 2 pages (including 3 figures) Physica C (in press). Proceedings of
M2S-HTSC-VI Conference. Houston, February 20-25, 200
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
Tunneling conductance of SIN junctions with different gap symmetries and non-magnetic impurities by direct solution of real-axis Eliashberg equations
We theoretically investigate the effect of various symmetries of the
superconducting order parameter Delta(omega) on the normalized tunneling
conductance of SIN junctions by directly solving the real-axis Eliashberg
equations (EEs) for a half-filled infinite band, with the simplifying
assumption mu*=0. We analyze six different symmetries of the order parameter:
s, d, s+id, s+d, extended s and anisotropic s, by assuming that the spectral
function alpha^{2}F(Omega) contains an isotropic part alpha^{2}F(Omega)_{is}
and an anisotropic one, alpha^{2}F(Omega)_{an}, such that
alpha^{2}F(Omega)_{an} = g alpha^{2}F(Omega)_{is}, where g is a constant.
We compare the resulting conductance curves at T=2 K to those obtained by
analytical continuation of the imaginary-axis solution of the EEs, and we show
that the agreement is not equally good for all symmetries. Then, we discuss the
effect of non-magnetic impurities on the theoretical tunneling conductance
curves at T=4 K for all the symmetries considered.
Finally, as an example, we apply our calculations to the case of
optimally-doped high-T_{c} superconductors (HTSC). Surprisingly, although the
possibility of explaining the very complex phenomenology of HTSC is probably
beyond the limits of the Eliashberg theory, the comparison of the theoretical
curves calculated at T=4 K with the experimental ones obtained in various
optimally-doped copper-oxides gives fairly good results.Comment: 11 pages, 11 figures, 2 tables. References added, figs. 6,7,10 and 11
changed, text change
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
Evidence of Strong-Coupled Superconductivity in CaC6 from Tunneling Spectroscopy
Point-contact tunneling on CaC crystals reproducibly reveals
superconducting gaps, , of 2.30.2 meV which are ~40% larger
than earlier reports. That puts CaC into the class of very strong-coupled
superconductors since 2/kT~4.6. Thus soft Ca phonons will be
primarily involved in the superconductivity, a conclusion that explains the
large Ca isotope effect found recently for CaC. Consistency among
superconductor-insulator-normal metal (SIN), SIS and Andreev reflection (SN)
junctions reinforces the intrinsic nature of this result.Comment: 2nd version, 4 pages, 4 figures, re-submitted to Physical Review
Letter
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