Spin interactions of interstitial Mn ions in ferromagnetic GaMnAs

The recently reported Rutherford backscattering and particle-induced X-ray emission experiments have revealed that in low-temperature MBE grown GaMnAs a significant part of the incorporated Mn atoms occupies tetrahedral interstitial sites in the lattice. Here we study the magnetic properties of these interstitial ions. We show that they do not participate in the hole-induced ferromagnetism. Moreover, Mn interstitial double donors may form pairs with the nearest substitutional Mn acceptors - our calculations evidence that the spins in such pairs are antiferromagnetically coupled by the superexchange. We also show that for the Mn ion in the other, hexagonal, interstitial position (which seems to be the case in the GaMnBeAs samples) the p-d interactions with the holes, responsible for the ferromagnetism, are very much suppressed.Comment: 4 pages, 3 figures, submitted to PR

Study of the B^0 Semileptonic Decay Spectrum at the Upsilon(4S) Resonance

We have made a first measurement of the lepton momentum spectrum in a sample of events enriched in neutral B's through a partial reconstruction of B0 --> D*- l+ nu. This spectrum, measured with 2.38 fb**-1 of data collected at the Upsilon(4S) resonance by the CLEO II detector, is compared directly to the inclusive lepton spectrum from all Upsilon(4S) events in the same data set. These two spectra are consistent with having the same shape above 1.5 GeV/c. From the two spectra and two other CLEO measurements, we obtain the B0 and B+ semileptonic branching fractions, b0 and b+, their ratio, and the production ratio f+-/f00 of B+ and B0 pairs at the Upsilon(4S). We report b+/b0=0.950 (+0.117-0.080) +- 0.091, b0 = (10.78 +- 0.60 +- 0.69)%, and b+ = (10.25 +- 0.57 +- 0.65)%. b+/b0 is equivalent to the ratio of charged to neutral B lifetimes, tau+/tau0.Comment: 14 page, postscript file also available at http://w4.lns.cornell.edu/public/CLN

Radiative Decay Modes of the D0D^{0} Meson

Using data recorded by the CLEO-II detector at CESR we have searched for four radiative decay modes of the $D^0$ meson: $D^0\to\phi\gamma$, $D^0\to\omega\gamma$, $D^0\to\bar{K}^{*}\gamma$, and $D^0\to\rho^0\gamma$. We obtain 90% CL upper limits on the branching ratios of these modes of $1.9\times 10^{-4}$, $2.4\times 10^{-4}$, $7.6\times 10^{-4}$ and $2.4\times 10^{-4}$ respectively.Comment: 15 page postscript file, postscript file also available through http://w4.lns.cornell.edu/public/CLN

Measurement of the Mass Splittings between the bbˉχb,J(1P)b\bar{b}\chi_{b,J}(1P) States

We present new measurements of photon energies and branching fractions for the radiative transitions: Upsilon(2S)->gamma+chi_b(J=0,1,2). The masses of the chi_b states are determined from the measured radiative photon energies. The ratio of mass splittings between the chi_b substates, r==(M[J=2]-M[J=1])/(M[J=1]-M[J=0]) with M the chi_b mass, provides information on the nature of the bbbar confining potential. We find r(1P)=0.54+/-0.02+/-0.02. This value is in conflict with the previous world average, but more consistent with the theoretical expectation that r(1P)<r(2P); i.e., that this mass splittings ratio is smaller for the chi_b(1P) triplet than for the chi_b(2P) triplet.Comment: 11 page postscript file, postscript file also available through http://w4.lns.cornell.edu/public/CLN

Studies of the Cabbibo-Suppressed Decays D+→π0ℓ+νD^+ \to \pi^0 \ell^+ \nu and D+→ηe+νeD^+ \to \eta e^+ \nu_e

Using 4.8 fb$^{-1}$ of data taken with the CLEO II detector, the branching fraction for the Cabibbo-suppressed decay $D^+\to\pi^0\ell^+\nu$ measured relative to the Cabibbo favored decay $D^+\to\bar{K^0}\ell^+\nu$ is found to be $0.046\pm 0.014\pm 0.017$. Using $V_{cs}$ and $V_{cd}$ from unitarity constraints, we determine $| f_+^{\pi}(0)/f_+^K(0)|^2=0.9\pm 0.3\pm 0.3$ We also present a 90% confidence level upper limit for the branching ratio of the decay $D^+ \to \eta e^+\nu_e$ relative to that for $D^+ \to \pi^0 e^+\nu_e$ of 1.5.Comment: 10 page postscript file, postscript file also available through http://w4.lns.cornell.edu/public/CLN