6,325 research outputs found
Field-induced gapless electron pocket in the superconducting vortex phase of YNi2B2C as probed by magnetoacoustic quantum oscillations
By use of ultrasound studies we resolved magneto-acoustic quantum oscillation
deep into the mixed state of the multiband nonmagnetic superconductor YNi2B2C.
Below the upper critical field, only a very weak additional damping appears
that can be well explained by the field inhomogeneity caused by the flux-line
lattice in the mixed state. This is clear evidence for no or a vanishingly
small gap for one of the bands, namely, the spheroidal alpha band. This
contrasts de Haas--van Alphen data obtained by use of torque magnetometry for
the same sample, with a rapidly vanishing oscillation signal in the mixed
state. This points to a strongly distorted flux-line lattice in the latter case
that, in general, can hamper a reliable extraction of gap parameters by use of
such techniques.Comment: 6 pages, 6 figure
Field and Temperature Dependence of the Superfluid Density in LaO_{1-x}F_xFeAs Superconductors: A Muon Spin Relaxation Study
We present zero field and transverse field \muSR experiments on the recently
discovered electron doped Fe-based superconductor LaO_{1-x}F_xFeAs. The zero
field experiments on underdoped (x=0.075) and optimally doped (x=0.1) samples
rule out any static magnetic order above 1.6 K in these superconducting
samples. From transverse field experiments in the vortex phase we deduce the
temperature and field dependence of the superfluid density. Whereas the
temperature dependence is consistent with a weak coupling BCS s-wave or a dirty
d-wave gap function scenario, the field dependence strongly evidences
unconventional superconductivity. We obtain the in-plane penetration depth of
\lambda_{ab} (0) = 254(2)nm for LaO_{0.9}F_{0.1}FeAs and \lambda_{ab} (0) =
364(8)nm for LaO_{0.925}F_{0.075}FeAs. Further evidence for unconventional
superconductivity is provided by the ratio of T_c versus the superfluid
density, which is close to the Uemura line of hole doped high-T_c cuprates.Comment: 5 pages, 5 figure
Geometry of the Grosse-Wulkenhaar Model
We define a two-dimensional noncommutative space as a limit of finite-matrix
spaces which have space-time dimension three. We show that on such space the
Grosse-Wulkenhaar (renormalizable) action has natural interpretation as the
action for the scalar field coupled to the curvature. We also discuss a natural
generalization to four dimensions.Comment: 16 pages, version accepted in JHE
Electronic properties of LaOFFeAs in the normal state probed by NMR/NQR
We report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear
quadrupole resonance (NQR) measurements on powders of the new LaO1-xFxFeAs
superconductor for x = 0 and x = 0.1 at temperatures up to 480 K, and compare
our measured NQR spectra with local density approximation (LDA) calculations.
For all three nuclei in the x = 0.1 material, it is found that the local Knight
shift increases monotonically with an increase in temperature, and scales with
the macroscopic susceptibility, suggesting a single magnetic degree of freedom.
Surprisingly, the spin lattice relaxation rates for all nuclei also scale with
one another, despite the fact that the form factors for each site sample
different regions of q-space. This result suggests a lack of any q-space
structure in the dynamical spin susceptibility that might be expected in the
presence of antiferromagnetic correlations. Rather, our results are more
compatible with simple quasi-particle scattering. Furthermore, we find that the
increase in the electric field gradient at the As cannot be accounted for by
LDA calculations, suggesting that structural changes, in particular the
position of the As in the unit cell, dominate the NQR response.Comment: 17 pages, 6 figure
Ce-L3-XAS study of the temperature dependence of the 4f occupancy in the Kondo system Ce2Rh3Al9
We have used temperature dependent x-ray absorption at the Ce-L3 edge to
investigate the recently discovered Kondo compound Ce2Rh3Al9. The systematic
changes of the spectral lineshape with decreasing temperature are analyzed and
found to be related to a change in the occupation number, n_f, as the
system undergoes a transition into a Kondo state. The temperature dependence of
indicates a characteristic temperature of 150K, which is clearly related
with the high temperature anomaly observed in the magnetic susceptibility of
the same system. The further anomaly observed in the resistivity of this system
at low temperature (ca. 20K) has no effect on n_f and is thus not of Kondo
origin.Comment: 7 pages, three figures, submitted to PR
Evidence for Pauli-limiting behaviour at high fields and enhanced upper critical fields near T_c in several disordered FeAs based Superconductors
We report resistivity and upper critical field B_c2(T) data for disordered
(As deficient) LaO_0.9F_0.1FeAs_1-delta in a wide temperature and high field
range up to 60 T. These samples exhibit a slightly enhanced superconducting
transition at T_c = 28.5 K and a significantly enlarged slope dB_c2/dT = -5.4
T/K near T_c which contrasts with a flattening of B_c2(T) starting near 23 K
above 30 T. The latter evidences Pauli limiting behaviour (PLB) with B_c2(0)
approximately 63 T. We compare our results with B_c2(T)-data from the
literature for clean and disordered samples. Whereas clean samples show almost
no PLB for fields below 60 to 70 T, the hitherto unexplained pronounced
flattening of B_c2(T) for applied fields H II ab observed for several
disordered closely related systems is interpreted also as a manifestation of
PLB. Consequences are discussed in terms of disorder effects within the frames
of (un)conventional superconductivity, respectively.Comment: 2 pages, 3 figures, submitted to M2S Tokyo 0
Performance of the EUDET-type beam telescopes
Test beam measurements at the test beam facilities of DESY have been
conducted to characterise the performance of the EUDET-type beam telescopes
originally developed within the EUDET project. The beam telescopes are equipped
with six sensor planes using MIMOSA26 monolithic active pixel devices. A
programmable Trigger Logic Unit provides trigger logic and time stamp
information on particle passage. Both data acquisition framework and offline
reconstruction software packages are available. User devices are easily
integrable into the data acquisition framework via predefined interfaces.
The biased residual distribution is studied as a function of the beam energy,
plane spacing and sensor threshold. Its standard deviation at the two centre
pixel planes using all six planes for tracking in a 6\,GeV
electron/positron-beam is measured to be
(2.88\,\pm\,0.08)\,\upmu\meter.Iterative track fits using the formalism of
General Broken Lines are performed to estimate the intrinsic resolution of the
individual pixel planes. The mean intrinsic resolution over the six sensors
used is found to be (3.24\,\pm\,0.09)\,\upmu\meter.With a 5\,GeV
electron/positron beam, the track resolution halfway between the two inner
pixel planes using an equidistant plane spacing of 20\,mm is estimated to
(1.83\,\pm\,0.03)\,\upmu\meter assuming the measured intrinsic resolution.
Towards lower beam energies the track resolution deteriorates due to increasing
multiple scattering. Threshold studies show an optimal working point of the
MIMOSA26 sensors at a sensor threshold of between five and six times their RMS
noise. Measurements at different plane spacings are used to calibrate the
amount of multiple scattering in the material traversed and allow for
corrections to the predicted angular scattering for electron beams
Fermions on spontaneously generated spherical extra dimensions
We include fermions to the model proposed in hep-th/0606021, and obtain a
renormalizable 4-dimensional SU(N) gauge theory which spontaneously generates
fuzzy extra dimensions and behaves like Yang-Mills theory on M^4 \times S^2. We
find a truncated tower of fermionic Kaluza-Klein states transforming under the
low-energy gauge group, which is found to be either SU(n), or SU(n_1) x SU(n_2)
x U(1). The latter case implies a nontrivial U(1) flux on S^2, leading to
would-be zero modes for the bifundamental fermions. In the non-chiral case they
may pair up to acquire a mass, and the emerging picture is that of mirror
fermions. We discuss the possible implementation of a chirality constraint in 6
dimensions, which is nontrivial at the quantum level due to the fuzzy nature of
the extra dimensions.Comment: 34 pages. V2: references added, minor corrections V3: discussion
added, final versio
Dynamical generation of fuzzy extra dimensions, dimensional reduction and symmetry breaking
We present a renormalizable 4-dimensional SU(N) gauge theory with a suitable
multiplet of scalar fields, which dynamically develops extra dimensions in the
form of a fuzzy sphere S^2. We explicitly find the tower of massive
Kaluza-Klein modes consistent with an interpretation as gauge theory on M^4 x
S^2, the scalars being interpreted as gauge fields on S^2. The gauge group is
broken dynamically, and the low-energy content of the model is determined.
Depending on the parameters of the model the low-energy gauge group can be
SU(n), or broken further to SU(n_1) x SU(n_2) x U(1), with mass scale
determined by the size of the extra dimension.Comment: 27 pages. V2: discussion and references added, published versio
Nanoscale Electronic Order in Iron Pnictides
The charge distribution in RFeAs (R=La, Sm) iron pnictides is probed using As NQR. Whereas undoped and optimally-doped/overdoped compounds feature a single charge environment, two charge environments are detected in the underdoped region. Spin- lattice relaxation measurements show their coexistence at the nanoscale. Together with the quantitative variations of the spectra with doping, they point at a local electronic order in the iron layers, where low- and high-doping-like regions would coexist. Implications for the interplay of static magnetism and superconductivity are discussed
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