1,329 research outputs found
BaV3O8: A possible Majumdar-Ghosh system with S=1/2
BaV3O8 contains both magnetic V4+(S=1/2) ions and non-magnetic V5+(S=0) ions.
The V4+ ions are arranged in a coupled Majumdar-Ghosh chain like network. Our
magnetic susceptibility chi(T) data fit well with the Curie-Weiss formula in
the temperature range of 80-300K and it yields a Curie constant
C=0.39cm3K/mole-V4+ and an antiferromagnetic Weiss temperature theta=-26K. The
chi(T) curve shows a broad maximum at T~25K indicative of short-range order
(SRO) and an anomaly corresponding to long-range order (LRO) at TN~6K. The
value of the frustration index (f=mod[theta/TN]~5) suggests that the system is
moderately frustrated. Above the LRO temperature the experimental magnetic
susceptibility data match well with the coupled Majumdar-Ghosh chain model with
the ratio of the nnn (next-nearest neighbor) to nn (nearest neighbor) magnetic
coupling alpha=2 and Jnnn/kB=40K. In a mean-field approach when considering the
inter-chain interactions, we obtain the total inter-chain coupling to be about
16K. The LRO anomaly at TN is also observe in the specific heat Cp(T) data and
is not sensitive to an applied magnetic field up to 90kOe. A 51V NMR signal
corresponding to the non-magnetic vanadium was observed. Anomalies at 6K were
observed in the variation with temperature of the 51V NMR linewidth and in the
spin-lattice relaxation rate 1/T1, indicating that they are sensitive to the
LRO onset and fluctuations at the magnetic V sites. The existence of two
components (one short and another long) is observed in the spin-spin relaxation
rate 1/T2 data in the vicinity of TN. The shorter component seems to be
intimately connected with the magnetically ordered state. We suggest that both
magnetically ordered and non-long range ordered (non-LRO) regions coexist in
this compound below the long range ordering temperature.Comment: Accepted in Phys. Rev.
No evidence for gamma-ray halos around active galactic nuclei resulting from intergalactic magnetic fields
We analyze the gamma-ray halo around stacked AGNs reported in Ap.J.Lett.,
2010, 722, L39. First, we show that the angular distribution of gamma-rays
around the stacked AGNs is consistent with the angular distribution of the
gamma-rays around the Crab pulsar, which is a point source for Fermi/LAT. This
makes it unlikely that the halo is caused by an electromagnetic cascade of TeV
photons in the intergalactic space. We then compare the angular distribution of
gamma-rays around the stacked AGNs with the point-spread function (PSF) of
Fermi/LAT and confirm the existence of an excess above the PSF. However, we
demonstrate that the magnitude and the angular size of this effect is different
for photons converted in the front and back parts of the Fermi/LAT instrument,
and thus is an instrumental effect.Comment: accepted to A&
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