Theory of band gap bowing of disordered substitutional II-VI and III-V semiconductor alloys

For a wide class of technologically relevant compound III-V and II-VI semiconductor materials AC and BC mixed crystals (alloys) of the type A(x)B(1-x)C can be realized. As the electronic properties like the bulk band gap vary continuously with x, any band gap in between that of the pure AC and BC systems can be obtained by choosing the appropriate concentration x, granted that the respective ratio is miscible and thermodynamically stable. In most cases the band gap does not vary linearly with x, but a pronounced bowing behavior as a function of the concentration is observed. In this paper we show that the electronic properties of such A(x)B(1-x)C semiconductors and, in particular, the band gap bowing can well be described and understood starting from empirical tight binding models for the pure AC and BC systems. The electronic properties of the A(x)B(1-x)C system can be described by choosing the tight-binding parameters of the AC or BC system with probabilities x and 1-x, respectively. We demonstrate this by exact diagonalization of finite but large supercells and by means of calculations within the established coherent potential approximation (CPA). We apply this treatment to the II-VI system Cd(x)Zn(1-x)Se, to the III-V system In(x)Ga(1-x)As and to the III-nitride system Ga(x)Al(1-x)N.Comment: 14 pages, 10 figure

Multiband tight-binding theory of disordered ABC semiconductor quantum dots: Application to the optical properties of alloyed CdZnSe nanocrystals

Zero-dimensional nanocrystals, as obtained by chemical synthesis, offer a broad range of applications, as their spectrum and thus their excitation gap can be tailored by variation of their size. Additionally, nanocrystals of the type ABC can be realized by alloying of two pure compound semiconductor materials AC and BC, which allows for a continuous tuning of their absorption and emission spectrum with the concentration x. We use the single-particle energies and wave functions calculated from a multiband sp^3 empirical tight-binding model in combination with the configuration interaction scheme to calculate the optical properties of CdZnSe nanocrystals with a spherical shape. In contrast to common mean-field approaches like the virtual crystal approximation (VCA), we treat the disorder on a microscopic level by taking into account a finite number of realizations for each size and concentration. We then compare the results for the optical properties with recent experimental data and calculate the optical bowing coefficient for further sizes

Measurement of the branching fraction for Υ(1S)→τ+τ−\Upsilon (1S) \to \tau^+ \tau^-

We have studied the leptonic decay of the $\Upsilon (1S)$ resonance into tau pairs using the CLEO II detector. A clean sample of tau pair events is identified via events containing two charged particles where exactly one of the particles is an identified electron. We find $B(\Upsilon(1S) \to \tau^+ \tau^-) = (2.61~\pm~0.12~{+0.09\atop{-0.13}})$. The result is consistent with expectations from lepton universality.Comment: 9 pages, RevTeX, two Postscript figures available upon request, CLNS 94/1297, CLEO 94-20 (submitted to Physics Letters B

Lifetime Differences, direct CP Violation and Partial Widths in D0 Meson Decays to K+K- and pi+pi-

We describe several measurements using the decays D0->K+K- and pi+pi-. We find the ratio of partial widths, Gamma(D0->K+K-)/Gamma(D0->pi+pi-), to be 2.96+/-0.16+/-0.15, where the first error is statistical and the second is systematic. We observe no evidence for direct CP violation, obtaining A_CP(KK) = (0.0+/-2.2+/-0.8)% and A_CP(pipi = (1.9+/-3.2+/-0.8)%. In the limit of no CP violation we measure the mixing parameter y_CP = -0.012+/-0.025+/-0.014 by measuring the lifetime difference between D0->K+ K- or pi+pi- and the CP neutral state, D0->K-pi+. We see no evidence for mixing.Comment: 14 pages postscript, also available through http://w4.lns.cornell.edu/public/CLNS, submitted to PRD, Rapid Communicatio

Search for the Flavor-Changing Neutral Current Decay D0→μ+μ−D^0 \to \mu^+\mu^- with the HERA-B Detector

We report on a search for the flavor-changing neutral current decay $D^0 \to \mu^+\mu^-$ using $50 \times 10^6$ events recorded with a dimuon trigger in interactions of 920 GeV protons with nuclei by the HERA-B experiment. We find no evidence for such decays and set a 90% confidence level upper limit on the branching fraction $Br(D^0 \to \mu^+\mu^-) <2.0 \times 10^{-6}$.Comment: 17 pages, 4 figures (of which 1 double), paper to be submitted to Physics Letters

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

Measurement of the Decay Asymmetry Parameters in Λc+→Λπ+\Lambda_c^+ \to \Lambda\pi^+ and Λc+→Σ+π0\Lambda_c^+ \to \Sigma^+\pi^0

We have measured the weak decay asymmetry parameters (\aLC ) for two \LC\ decay modes. Our measurements are \aLC = -0.94^{+0.21+0.12}_{-0.06-0.06} for the decay mode $\Lambda_c^+ \to \Lambda\pi^+$ and \aLC = -0.45\pm 0.31 \pm 0.06 for the decay mode $\Lambda_c \to \Sigma^+\pi^0$. By combining these measurements with the previously measured decay rates, we have extracted the parity-violating and parity-conserving amplitudes. These amplitudes are used to test models of nonleptonic charmed baryon decay.Comment: 11 pages including the figures. Uses REVTEX and psfig macros. Figures as uuencoded postscript. Also available as http://w4.lns.cornell.edu/public/CLNS/1995/CLNS95-1319.p

Observation of the Ξc+\Xi_c^+ Charmed Baryon Decays to Σ+K−π+\Sigma^+ K^-\pi^+, Σ+Kˉ∗0\Sigma^+ \bar{K}^{*0}, and ΛK−π+π+\Lambda K^-\pi^+\pi^+

We have observed two new decay modes of the charmed baryon $\Xi_c^+$ into $\Sigma^+ K^-\pi^+$ and $\Sigma^+ \bar{K}^{*0}$ using data collected with the CLEO II detector. We also present the first measurement of the branching fraction for the previously observed decay mode $\Xi_c^+\to\Lambda K^-\pi^+\pi^+$. The branching fractions for these three modes relative to $\Xi_c^+\to\Xi^-\pi^+\pi^+$ are measured to be $1.18 \pm 0.26 \pm 0.17$, $0.92 \pm 0.27 \pm 0.14$, and $0.58 \pm 0.16 \pm 0.07$, respectively.Comment: 12 page uuencoded postscript file, postscript file also available through http://w4.lns.cornell.edu/public/CLN

Measurement of Lepton Momentum Moments in the Decay bar{B} \to X \ell \bar{\nu} and Determination of Heavy Quark Expansion Parameters and |V_cb|

We measure the primary lepton momentum spectrum in B-bar to X l nu decays, for p_l > 1.5 GeV/c in the B rest frame. From this, we calculate various moments of the spectrum. In particular, we find R_0 = [int(E_l>1.7) (dGam/dE_sl)*dE_l] / [int(E_l>1.5) (dGam/dE_sl)*dE_l] = 0.6187 +/- 0.0014_stat +/- 0.0016_sys and R_1 = [int(E_l>1.5) E_l(dGam/dE_sl)*dE_l] / [int(E_l>1.5) (dGam/dE_sl)*dE_l] = (1.7810 +/- 0.0007_stat +/- 0.0009_sys) GeV. We use these moments to determine non-perturbative parameters governing the semileptonic width. In particular, we extract the Heavy Quark Expansion parameters Lambda-bar = (0.39 +/- 0.03_stat +/- 0.06_sys +/- 0.12_th) GeV and lambda_1 = (-0.25 +/- 0.02_stat +/- 0.05_sys +/- 0.14_th) GeV^2. The theoretical constraints used are evaluated through order 1/M_B^3 in the non-perturbative expansion and beta_0*alpha__s^2 in the perturbative expansion. We use these parameters to extract |V_cb| from the world average of the semileptonic width and find |V_cb| = (40.8 +/- 0.5_Gam-sl +/- 0.4_(lambda_1,Lambda-bar)-exp +/- 0.9_th) x 10^-3. In addition, we extract the short range b-quark mass m_b^1S = (4.82 +/- 0.07_exp +/- 0.11_th) GeV/c^2. Finally, we discuss the implications of our measurements for the theoretical understanding of inclusive semileptonic processes.Comment: 21 pages postscript, also available through http://w4.lns.cornell.edu/public/CLNS, submitted to PR

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