220 research outputs found
Testing Einstein's time dilation under acceleration using M\"ossbauer spectroscopy
The Einstein time dilation formula was tested in several experiments. Many
trials have been made to measure the transverse second order Doppler shift by
M\"{o}ssbauer spectroscopy using a rotating absorber, to test the validity of
this formula. Such experiments are also able to test if the time dilation
depends only on the velocity of the absorber, as assumed by Einstein's clock
hypothesis, or the present centripetal acceleration contributes to the time
dilation. We show here that the fact that the experiment requires -ray
emission and detection slits of finite size, the absorption line is broadened;
by geometric longitudinal first order Doppler shifts immensely. Moreover, the
absorption line is non-Lorenzian. We obtain an explicit expression for the
absorption line for any angular velocity of the absorber.
The analysis of the experimental results, in all previous experiments which
did not observe the full absorption line itself, were wrong and the conclusions
doubtful. The only proper experiment was done by K\"{u}ndig (Phys. Rev. 129
(1963) 2371), who observed the broadening, but associated it to random
vibrations of the absorber. We establish necessary conditions for the
successful measurement of a transverse second order Doppler shift by
M\"{o}ssbauer spectroscopy. We indicate how the results of such an experiment
can be used to verify the existence of a Doppler shift due to acceleration and
to test the validity of Einstein's clock hypothesis.Comment: 11 pages, 4 figure
Testing the influence of acceleration on time dilation using a rotating M\"Ossbauer absorber
The aim of the experiment series was to test the influence of acceleration on
time dilation by measuring the relative spectral shift between the resonance
spectra of a rotating Mossbauer absorber with acceleration anti-parallel and
parallel to the direction of the incident beam. Based on the experiences and
know-how acquired in our previous experiments, We collected data for rotation
frequencies up to 510Hz in both directions of rotation and also used different
slits. For each run with high rotation, we observed a stable statistically
significant relative shift between the spectra of the two states with opposite
acceleration. This indicates the influence of acceleration on time dilation.
However, we found that this shift also depends on the choice of the slit, and
on the direction of rotation. These new unexpected findings, resulting from the
loss of symmetry in obtaining the resonant lines in the two states, could
overshadow the relative shift due to acceleration. This loss of the symmetry is
caused by the deflection of the radiative decay due to the Nuclear Lighthouse
effect from the rotating Mossbauer absorber. We also found that it is
impossible to keep the alignment (between the optical and the dynamical rotor
systems) with accuracy needed for such experiment, for long runs, which
resulted in the reduction of the accuracy of the observed relative shift. These
prevent us to claim with certainty the influence of acceleration on time
dilation using the currently available technology. An improved KB optics with
focal spot of less than 1 micron to avoid the use of a slit and a more rigid
mounting of the rotor system, are necessary for the success of such experiment.
Hopefully, these findings together with the indispensable plan for a conclusive
experiment presented in the paper, will prove useful to future experimentalists
wishing to pursue such an experiment
The combinatorics of the Baer-Specker group
Denote the integers by Z and the positive integers by N.
The groups Z^k (k a natural number) are discrete, and the classification up
to isomorphism of their (topological) subgroups is trivial. But already for the
countably infinite power Z^N of Z, the situation is different. Here the product
topology is nontrivial, and the subgroups of Z^N make a rich source of examples
of non-isomorphic topological groups. Z^N is the Baer-Specker group.
We study subgroups of the Baer-Specker group which possess group theoretic
properties analogous to properties introduced by Menger (1924), Hurewicz
(1925), Rothberger (1938), and Scheepers (1996). The studied properties were
introduced independently by Ko\v{c}inac and Okunev. We obtain purely
combinatorial characterizations of these properties, and combine them with
other techniques to solve several questions of Babinkostova, Ko\v{c}inac, and
Scheepers.Comment: To appear in IJ
Magneto-transport and magnetic susceptibility of SmFeAsO1-xFx (x = 0.0 and 0.20)
Bulk polycrystalline samples, SmFeAsO and the iso-structural superconducting
SmFeAsO0.80F0.20 are explored through resistivity with temperature under
magnetic field {\rho}(T, H), AC and DC magnetization (M-T), and Specific heat
(Cp) measurements. The Resistivity measurement shows superconductivity for x =
0.20 sample with Tc(onset) ~ 51.7K. The upper critical field, [Hc2(0)] is
estimated ~3770kOe by Ginzburg-Landau (GL) theory. Broadening of
superconducting transition in magneto transport is studied through thermally
activated flux flow in applied field up to 130 kOe. The flux flow activation
energy (U/kB) is estimated ~1215K for 1kOe field. Magnetic measurements
exhibited bulk superconductivity with lower critical field (Hc1) of ~1.2kOe at
2K. In normal state, the paramagnetic nature of compound confirms no trace of
magnetic impurity which orders ferromagnetically. AC susceptibility
measurements have been carried out for SmFeAsO0.80F0.20 sample at various
amplitude and frequencies of applied AC drive field. The inter-granular
critical current density (Jc) is estimated. Specific heat [Cp(T)] measurement
showed an anomaly at around 140K due to the SDW ordering of Fe, followed by
another peak at 5K corresponding to the antiferromagnetic (AFM) ordering of
Sm+3 ions in SmFeAsO compound. Interestingly the change in entropy (marked by
the Cp transition height) at 5K for Sm+3 AFM ordering is heavily reduced in
case of superconducting SmFeAsO0.80F0.20 sample.Comment: 18 pages text + Figs: comments/suggestions welcome
([email protected]
57Fe Mossbauer spectroscopy and magnetic measurements of oxygen deficient LaFeAsO
We report on the magnetic behavior of oxygen deficient LaFeAsO1-x (x-0.10)
compound, prepared by one-step synthesis, which crystallizes in the tetragonal
(S.G. P4/nmm) structure at room temperature. Resistivity measurements show a
strong anomaly near 150 K, which is ascribed to the spin density wave (SDW)
instability. On the other hand, dc magnetization data shows paramagnetic-like
features down to 5 K, with an effective moment of 0.83 mB/Fe. 57Fe Mossbauer
studies (MS) have been performed at 95 and 200 K. The spectra at both
temperatures are composed of two sub-spectra. At 200 K the major one (88%), is
almost a singlet, and corresponds to those Fe nuclei, which have two oxygen
ions in their close vicinity. The minor one, with a large quadrupole splitting,
corresponds to Fe nuclei, which have vacancies in their immediate neighborhood.
The spectrum at 95 K, exhibits a broadened magnetic split major (84%)
sub-spectrum and a very small magnetic splitting in the minor subspectrum. The
relative intensities of the subspectra facilitate in estimating the actual
amount of oxygen vacancies in the compound to be 7.0(5)%, instead of the
nominal LaFeAsO0.90. These results, when compared with reported 57Fe MS of
non-superconducting LaFeAsO and superconducting LaFeAsO0.9F0.1, confirm that
the studied LaFeAsO0.93 is a superconductivity-magnetism crossover compound of
the newly discovered Fe based superconducting family.Comment: 7 pages text + Figs : Comments/suggestions welcome
([email protected]
Thermomagnetic history effects in SmMnGe
The intermetallic compound SmMnGe, displaying multiple magnetic phase
transitions, is being investigated in detail for its magnetization behavior
near the 145 K first order ferromagnetic to antiferromagnetic transition
occuring on cooling, in particular for thermomagnetic history effects in the
magnetization data. The most unusual finding is that the thermomagnetic
irreversibility, [= M(T)-M(T)] at 135 K is higher in
intermediate magnetic field strengths. By studying the response of the sample
(i.e., thermomagnetic irreversibility and thermal hysteresis) to different
histories of application of magnetic field and temperature, we demonstrate how
the supercooling and superheating of the metastable magnetic phases across the
first order transition at 145 K contribute to overall thermomagnetic
irreversibility.Comment: 15 pages, 5 figures, to appear in Physical Review
Theory of the first-order isostructural valence phase transitions in mixed valence compounds YbIn_{x}Ag_{1-x}Cu_{4}
For describing the first-order isostructural valence phase transition in
mixed valence compounds we develop a new approach based on the lattice Anderson
model. We take into account the Coulomb interaction between localized f and
conduction band electrons and two mechanisms of electron-lattice coupling. One
is related to the volume dependence of the hybridization. The other is related
to local deformations produced by f- shell size fluctuations accompanying
valence fluctuations. The large f -state degeneracy allows us to use the 1/N
expansion method. Within the model we develop a mean-field theory for the
first-order valence phase transition in YbInCu_{4}. It is shown that the
Coulomb interaction enhances the exchange interaction between f and conduction
band electron spins and is the driving force of the phase transition. A
comparison between the theoretical calculations and experimental measurements
of the valence change, susceptibility, specific heat, entropy, elastic
constants and volume change in YbInCu_{4} and YbAgCu_{4} are presented, and a
good quantitative agreement is found. On the basis of the model we describe the
evolution from the first-order valence phase transition to the continuous
transition into the heavy-fermion ground state in the series of compounds
YbIn_{1-x}Ag_{x}Cu_{4}. The effect of pressure on physical properties of
YbInCu_{4} is studied and the H-T phase diagram is found.Comment: 17 pages RevTeX, 9 Postscript figures, to be submitted to Phys.Rev.
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