23,760 research outputs found
Scattering by a contact potential in three and lower dimensions
We consider the scattering of nonrelativistic particles in three dimensions
by a contact potential which is defined
as the limit of . It is
surprising that it gives a nonvanishing cross section when and
. When the contact potential is approached by a spherical square
well potential instead of the above spherical shell one, one obtains basically
the same result except that the parameter that gives a nonvanishing
cross section is different. Similar problems in two and one dimensions are
studied and results of the same nature are obtained.Comment: REVTeX, 9 pages, no figur
Experimental tests of the chiral anomaly magnetoresistance in the Dirac-Weyl semimetals NaBi and GdPtBi
In the Dirac/Weyl semimetal, the chiral anomaly appears as an "axial" current
arising from charge-pumping between the lowest (chiral) Landau levels of the
Weyl nodes, when an electric field is applied parallel to a magnetic field . Evidence for the chiral anomaly was obtained from the longitudinal
magnetoresistance (LMR) in NaBi and GdPtBi. However, current jetting
effects (focussing of the current density ) have raised general concerns
about LMR experiments. Here we implement a litmus test that allows the
intrinsic LMR in NaBi and GdPtBi to be sharply distinguished from pure
current jetting effects (in pure Bi). Current jetting enhances along the
mid-ridge (spine) of the sample while decreasing it at the edge. We measure the
distortion by comparing the local voltage drop at the spine (expressed as the
resistance ) with that at the edge (). In Bi,
sharply increases with but decreases (jetting effects are
dominant). However, in NaBi and GdPtBi, both and
decrease (jetting effects are subdominant). A numerical simulation allows the
jetting distortions to be removed entirely. We find that the intrinsic
longitudinal resistivity in NaBi decreases by a factor of
10.9 between = 0 and 10 T. A second litmus test is obtained from the
parametric plot of the planar angular magnetoresistance. These results
strenghthen considerably the evidence for the intrinsic nature of the
chiral-anomaly induced LMR. We briefly discuss how the squeeze test may be
extended to test ZrTe.Comment: 17 pages, 8 figures, new co-authors added, new Fig. 6a added. In
press, PR
Large magnetic penetration depth and thermal fluctuations in a Ca(PtAs)[(FePt)As] (x=0.097) single crystal
We have measured the temperature dependence of the absolute value of the
magnetic penetration depth in a
Ca(PtAs)[(FePt)As] (x=0.097)
single crystal using a low-temperature magnetic force microscope (MFM). We
obtain (0)1000 nm via extrapolating the data to .
This large and pronounced anisotropy in this system are responsible
for large thermal fluctuations and the presence of a liquid vortex phase in
this low-temperature superconductor with critical temperature of 11 K,
consistent with the interpretation of the electrical transport data. The
superconducting parameters obtained from and coherence length
place this compound in the extreme type \MakeUppercase{\romannumeral 2} regime.
Meissner responses (via MFM) at different locations across the sample are
similar to each other, indicating good homogeneity of the superconducting state
on a sub-micron scale
model and Higgs mass in standard model calculated by Gaussian effective potential approach with a new regularization-renormalization method
Basing on new regularization-renormalization method, the
model used in standard model is studied both perturbatively and
nonperturbatively (by Gaussian effective potential). The invariant property of
two mass scales is stressed and the existence of a (Landau) pole is emphasized.
Then after coupling with the SU(2)U(1) gauge fields, the Higgs mass in
standard model (SM) can be calculated as 138GeV. The critical
temperature () for restoration of symmetry of Higgs field, the critical
energy scale (, the maximum energy scale under which the lower
excitation sector of the GEP is valid) and the maximum energy scale
(, at which the symmetry of the Higgs field is restored) in the
standard model are 476 GeV, GeV
and GeVv respectively.Comment: 12 pages, LaTex, no figur
Building a 3.5 m prototype interferometer for the Q & A vacuum birefringence experiment and high precision ellipsometry
We have built and tested a 3.5 m high-finesse Fabry-Perot prototype
inteferometer with a precision ellipsometer for the QED test and axion search
(Q & A) experiment. We use X-pendulum-double-pendulum suspension designs and
automatic control schemes developed by the gravitational-wave detection
community. Verdet constant and Cotton-Mouton constant of the air are measured
as a test. Double modulation with polarization modulation 100 Hz and
magnetic-field modulation 0.05 Hz gives 10^{-7} rad phase noise for a 44-minute
integration.Comment: This draft has been presented in the 5th Edoardo Amaldi Conference on
Gravitational Wave
Growth to early adulthood following extremely preterm birth: the EPICure study.
OBJECTIVE: To investigate growth trajectories from age 2.5 to 19 years in individuals born before 26 weeks of gestation (extremely preterm; EP) compared with term-born controls. METHODS: Multilevel modelling of growth data from the EPICure study, a prospective 1995 birth cohort of 315 EP participants born in the UK and Ireland and 160 term-born controls recruited at school age. Height, weight, head circumference and body mass index (BMI) z-scores were derived from UK standards at ages 2.5, 6, 11 and 19 years. RESULTS: 129 (42%) EP children were assessed at 19 years. EP individuals were on average 4.0 cm shorter and 6.8 kg lighter with a 1.5 cm smaller head circumference relative to controls at 19 years. Relative to controls, EP participants grew faster in weight by 0.06 SD per year (95% CI 0.05 to 0.07), in head circumference by 0.04 SD (95% CI 0.03 to 0.05), but with no catch-up in height. For the EP group, because of weight catch-up between 6 and 19 years, BMI was significantly elevated at 19 years to +0.32 SD; 23.4% had BMI >25 kg/m2 and 6.3% >30 kg/m2 but these proportions were similar to those in control subjects. EP and control participants showed similar pubertal development in early adolescence, which was not associated with height at 19 years in either study group. Growth through childhood was related to birth characteristics and to neonatal feeding practices. CONCLUSIONS: EP participants remained shorter and lighter and had smaller head circumferences than reference data or controls in adulthood but had elevated BMI
Suspension of the fiber mode-cleaner launcher and measurement of the high extinction-ratio (10^{-9}) ellipsometer for the Q & A experiment
The Q & A experiment, first proposed and started in 1994, provides a feasible
way of exploring the quantum vacuum through the detection of vacuum
birefringence effect generated by QED loop diagram and the detection of the
polarization rotation effect generated by photon-interacting (pseudo-)scalar
particles. Three main parts of the experiment are: (1) Optics System (including
associated Electronic System) based on a suspended 3.5-m high finesse
Fabry-Perot cavity, (2) Ellipsometer using ultra-high extinction-ratio
polarizer and analyzer, and (3) Magnetic Field Modulation System for generating
the birefringence and the polarization rotation effect. In 2002, the Q & A
experiment achieved the Phase I sensitivity goal. During Phase II, we set (i)
to improve the control system of the cavity mirrors for suppressing the
relative motion noise, (ii) to enhance the birefringence signal by setting-up a
60-cm long 2.3 T transverse permanent magnet rotatable to 10 rev/s, (iii) to
reduce geometrical noise by inserting a polarization-maintaining optical fiber
(PM fiber) as a mode cleaner, and (iv) to use ultra-high extinction-ratio
(10^{-9}) polarizer and analyzer for ellipsometry. Here we report on (iii) &
(iv); specifically, we present the properties of the PM-fiber mode-cleaner, the
transfer function of its suspension system, and the result of our measurement
of high extinction-ratio polarizer and analyzer.Comment: 8 pages, 6 figures, presented in the 6th Edoardo Amaldi Conference on
Gravitational Waves, Okinawa, Japan, June 2005, and accepted by "Journal of
Physics: Conference Series". Modifications from version 2 were made based on
the referees' comments on figures. Ref. [31] were update
Controlling the Momentum Current of an Off-resonant Ratchet
We experimentally investigate the phenomenon of a quantum ratchet created by
exposing a Bose-Einstein Condensate to short pulses of a potential which is
periodic in both space and time. Such a ratchet is manifested by a directed
current of particles, even though there is an absence of a net bias force. We
confirm a recent theoretical prediction [M. Sadgrove and S. Wimberger, New J.
Phys. \textbf{11}, 083027 (2009)] that the current direction can be controlled
by experimental parameters which leave the underlying symmetries of the system
unchanged. We demonstrate that this behavior can be understood using a single
variable containing many of the experimental parameters and thus the ratchet
current is describable using a single universal scaling law.Comment: arXiv admin note: substantial text overlap with arXiv:1210.565
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