34,217 research outputs found
Robust optical emission polarization in MoS2 monolayers through selective valley excitation
We report polarization resolved photoluminescence from monolayer MoS2, a
two-dimensional, non-centrosymmetric crystal with direct energy gaps at two
different valleys in momentum space. The inherent chiral optical selectivity
allows exciting one of these valleys and close to 90% polarized emission at 4K
is observed with 40% polarization remaining at 300K. The high polarization
degree of the emission remains unchanged in transverse magnetic fields up to 9T
indicating robust, selective valley excitation.Comment: 5 pages, 3 figure
Vacancy-tuned paramagnetic/ferromagnetic martensitic transformation in Mn-poor Mn_{1-x}CoGe alloys
It is shown that a temperature window between the Curie temperatures of
martensite and austenite phases around the room temperature can be obtained by
a vacancy-tuning strategy in Mn-poor Mn1-xCoGe alloys (0 <= x <= 0.050). Based
on this, a martensitic transformation from paramagnetic austenite to
ferromagnetic martensite with a large magnetization difference can be realized
in this window. This gives rise to a magnetic-field-induced martensitic
transformation and a large magnetocaloric effect in the Mn1-xCoGe system. The
decrease of the transformation temperature and of the thermal hysteresis of the
transformation, as well as the stable Curie temperatures of martensite and
austenite, are discussed on the basis of the Mn-poor Co-vacancy structure and
the corresponding valence-electron concentration.Comment: Manuscript revised. 18 pages, 6 figures and 1 table. Submitted for
publication
Observation of Y(2175) in
The decays of are analyzed using a sample of events collected with the BESII detector at the Beijing
Electron-Positron Collider (BEPC). A structure at around GeV/ with
about significance is observed in the invariant mass
spectrum. A fit with a Breit-Wigner function gives the peak mass and width of
GeV/ and GeV/, respectively, that are consistent with those
of Y(2175), observed by the BABAR collaboration in the initial-state radiation
(ISR) process . The production branching
ratio is determined to be , assuming that the Y(2175) is a state.Comment: 5 pages, 4 figures, accepted by Phys. Rev. Let
A Unified Approach to the Classical Statistical Analysis of Small Signals
We give a classical confidence belt construction which unifies the treatment
of upper confidence limits for null results and two-sided confidence intervals
for non-null results. The unified treatment solves a problem (apparently not
previously recognized) that the choice of upper limit or two-sided intervals
leads to intervals which are not confidence intervals if the choice is based on
the data. We apply the construction to two related problems which have recently
been a battle-ground between classical and Bayesian statistics: Poisson
processes with background, and Gaussian errors with a bounded physical region.
In contrast with the usual classical construction for upper limits, our
construction avoids unphysical confidence intervals. In contrast with some
popular Bayesian intervals, our intervals eliminate conservatism (frequentist
coverage greater than the stated confidence) in the Gaussian case and reduce it
to a level dictated by discreteness in the Poisson case. We generalize the
method in order to apply it to analysis of experiments searching for neutrino
oscillations. We show that this technique both gives correct coverage and is
powerful, while other classical techniques that have been used by neutrino
oscillation search experiments fail one or both of these criteria.Comment: 40 pages, 15 figures. Changes 15-Dec-99 to agree more closely with
published version. A few small changes, plus the two substantive changes we
made in proof back in 1998: 1) The definition of "sensitivity" in Sec. V(C).
It was inconsistent with our actual definition in Sec. VI. 2) "Note added in
proof" at end of the Conclusio
Partial wave analysis of J/psi to p pbar pi0
Using a sample of 58 million events collected with the BESII
detector at the BEPC, more than 100,000 events are
selected, and a detailed partial wave analysis is performed. The branching
fraction is determined to be . A long-sought `missing' , first observed in , is observed in this decay too, with mass and width of
MeV/c and MeV/c,
respectively. Its spin-parity favors . The masses, widths, and
spin-parities of other states are obtained as well.Comment: Add one author nam
Measurement of \psip Radiative Decays
Using 14 million psi(2S) events accumulated at the BESII detector, we report
first measurements of branching fractions or upper limits for psi(2S) decays
into gamma ppbar, gamma 2(pi^+pi^-), gamma K_s K^-pi^++c.c., gamma K^+ K^-
pi^+pi^-, gamma K^{*0} K^- pi^+ +c.c., gamma K^{*0}\bar K^{*0}, gamma pi^+pi^-
p pbar, gamma 2(K^+K^-), gamma 3(pi^+pi^-), and gamma 2(pi^+pi^-)K^+K^- with
the invariant mass of hadrons below 2.9GeV/c^2. We also report branching
fractions of psi(2S) decays into 2(pi^+pi^-) pi^0, omega pi^+pi^-, omega
f_2(1270), b_1^\pm pi^\mp, and pi^0 2(pi^+pi^-) K^+K^-.Comment: 5 pages, 4 figure
Measurements of and decays into and
Using 58 million and 14 million events collected by the
BESII detector at the BEPC, branching fractions or upper limits for the decays
and and are measured. For the isospin violating decays, the upper
limits are determined to be and at the 90% confidence level. The isospin
conserving process is observed for the
first time, and its branching fraction is measured to be , where the
first error is statistical and the second one is systematic. No signal is observed in decays, and is set at the 90%
confidence level. Branching fractions of decays into and are also reported, and the sum
of these branching fractions is determined to be .Comment: 7 pages, 10 figures. Phys.Rev.D comments considere
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