5,761 research outputs found
Subpicosecond timeâresolved studies of coherent phonon oscillations in thinâfilm YBa2Cu3O6+x (x<0.4)
We report the results of the first timeâresolved observation of impulsively generated coherent optical phonon oscillations in the semiconducting cuprate compound YBa2Cu3O6+x (x<0.4). The oscillations, which were probed through timeâresolved transmissivity modulation, had a period of 237 fs at room temperature, corresponding to a Raman active mode of A1g symmetry at 142 cmâ1. No oscillations were observed in the superconducting form of YâBaâCuâO either above or below Tc. The amplitude, frequency, and linewidth of this mode were measured over a temperature range from âź7 K to room temperature.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70769/2/APPLAB-58-9-980-1.pd
Femtosecond optical absorption studies of nonequilibrium electronic processes in high Tc superconductors
We report the results of femtosecond optical transient absorption experiments performed on the superconducting compounds YBa2Cu3O7âx(x âź 0) and Bi2Sr2Ca2Cu3O10+δ(δ âź 0) and nonsuperconducting YBa2Cu3O6+y(y<0.4) for sample temperatures ranging from âź7 K to room temperature. Nonequilibrium heating was found to occur on a subpicosecond time scale. A distinct, dramatic increase in the relaxation time was observed for the superconducting samples as the sample temperature was lowered below the critical temperatures of the respective films. Accompanying the increase in relaxation time was an increase in the peak fractional transmissivity change. No such changes were observed for the nonsuperconducting YBCO sample. We believe the above described behavior is electronic in origin and intimately related to the superconductivity of the compounds.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69780/2/APPLAB-57-16-1696-1.pd
Comparison of s- and d-wave gap symmetry in nonequilibrium superconductivity
Recent application of ultrafast pump/probe optical techniques to
superconductors has renewed interest in nonequilibrium superconductivity and
the predictions that would be available for novel superconductors, such as the
high-Tc cuprates. We have reexamined two of the classical models which have
been used in the past to interpret nonequilibrium experiments with some
success: the mu* model of Owen and Scalapino and the T* model of Parker.
Predictions depend on pairing symmetry. For instance, the gap suppression due
to excess quasiparticle density n in the mu* model, varies as n^{3/2} in d-wave
as opposed to n for s-wave. Finally, we consider these models in the context of
S-I-N tunneling and optical excitation experiments. While we confirm that
recent pump/probe experiments in YBCO, as presently interpreted, are in
conflict with d-wave pairing, we refute the further claim that they agree with
s-wave.Comment: 14 pages, 11 figure
Spatial Symmetry of Superconducting Gap in YBa2Cu3O7-\delta Obtained from Femtosecond Spectroscopy
The polarized femtosecond spectroscopies obtained from well characterized
(100) and (110) YBa2Cu3O7-\delta thin films are reported. This bulk-sensitive
spectroscopy, combining with the well-textured samples, serves as an effective
probe to quasiparticle relaxation dynamics in different crystalline
orientations. The significant anisotropy in both the magnitude of the
photoinduced transient reflectivity change and the characteristic relaxation
time indicates that the nature of the relaxation channel is intrinsically
different in various axes and planes. By the orientation-dependent analysis,
d-wave symmetry of the bulk-superconducting gap in cuprate superconductors
emerges naturally.Comment: 8 pages, 4 figures. To be published in Physical Review B, Rapid
Communication
Angular position of nodes in the superconducting gap of YBCO
The thermal conductivity of a YBCO single crystal has been studied as a
function of the relative orientation of the crystal axes and a magnetic field
rotating in the Cu-O planes. Measurements were carried out at several
temperatures below T_c and at a fixed field of 30 kOe. A four-fold symmetry
characteristic of a superconducting gap with nodes at odd multiples of 45
degrees in k-space was resolved. Experiments were performed to exclude a
possible macroscopic origin for such a four-fold symmetry such as sample shape
or anisotropic pinning. Our results impose an upper limit of 10% on the weight
of the s-wave component of the essentially d-wave superconducting order
parameter of YBCO.Comment: 10 pages, 4 figure
External electro-optic integrated circuit probing
An external electro-optic measurement system with subpicosecond resolution has been developed. This electro-optic sampling system is designed to operate as a non-contact probe of voltages in electrical devices and circuits with modified wafer-level test equipment and no special circuit preparation. Measurements demonstrate the system's ability to probe continuous and pulsed signals on microwave integrated circuits on arbitrary substrates with single-micron spatial resolution. We also discuss the application of external electro-optic sampling to various aspects of time-domain circuit studies, including the generation of short electrical test pulses using novel photoconductive techniques and the propagation of pulses on interconnects.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28606/1/0000415.pd
Dynamical Properties of Josephson Junctions Coupled by a Transmission Line
A system composed of two Josephson junctions connected by a transmission line has been studied by means of electronic analog simulation. Under external current bias, the resistive component of the coupling induces frequency locking between the two junctions at commensurate ratios. The resonant modes of the transmission line give rise to steps in the I-V characteristics of the system
Measurement of the Helicity Fractions of W Bosons from Top Quark Decays Using Fully Reconstructed top-antitop Events with CDF II
We present a measurement of the fractions F_0 and F_+ of longitudinally
polarized and right-handed W bosons in top quark decays using data collected
with the CDF II detector. The data set used in the analysis corresponds to an
integrated luminosity of approximately 318 pb -1. We select ttbar candidate
events with one lepton, at least four jets, and missing transverse energy. Our
helicity measurement uses the decay angle theta*, which is defined as the angle
between the momentum of the charged lepton in the W boson rest frame and the W
momentum in the top quark rest frame. The cos(theta*) distribution in the data
is determined by full kinematic reconstruction of the ttbar candidates. We find
F_0 = 0.85 +0.15 -0.22 (stat) +- 0.06 (syst) and F_+ = 0.05 +0.11 -0.05 (stat)
+- 0.03 (syst), which is consistent with the standard model prediction. We set
an upper limit on the fraction of right-handed W bosons of F_+ < 0.26 at the
95% confidence level.Comment: 11 pages, 2 figures, submitted to Phys. Rev.
Search for a Higgs Boson Produced in Association with a W Boson in pbar-p Collisions at sqrt{s} = 1.96 TeV
We present a search for a standard model Higgs boson produced in association
with a W boson using 2.7 1/fb of integrated luminosity of pbar-p collision data
taken at sqrt{s} = 1.96 TeV. Limits on the Higgs boson production rate are
obtained for masses between 100 GeV and 150 GeV. Through the use of
multivariate techniques, the analysis achieves an observed (expected) 95%
confidence level upper limit of 5.6 (4.8) times the theoretically expected
production cross section for a standard model Higgs boson with a mass of 115
GeV.Comment: submitted to Phys. Rev. Let
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
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