The ${\mathrm{}\left[\mathrm{Li}\right]\mathrm{}}^0$ center has been formed in lithium-doped single crystals of MgO by high temperature quenching. In contrast to the ${\mathrm{}\left[\mathrm{Li}\right]\mathrm{}}^0$ center formed by low-temperature $\gamma$ irradiation, the center formed by quenching is stable at room temperature. A heavy electron irradiation also produces stable ${\mathrm{}\left[\mathrm{Li}\right]\mathrm{}}^0$ centers. Investigations using EPR and electron-nuclear double resonance techniques have established that, in all three cases, the ${\mathrm{}\left[\mathrm{Li}\right]\mathrm{}}^0$ center has the same local configuration. The increased stability has enabled us to observe the motionally averaged ${\mathrm{}\left[\mathrm{Li}\right]\mathrm{}}^0$ spectrum for the first time. Optical absorption data indicate that the center is associated with an absorption band at 1.83 eV.

Phonon echoes, the acoustic analog of spin and photon echoes, are observed in fused silica glass at ultralow temperatures. From the phonon echo decay, we infer a homogeneous transverse lifetime of 14×${10}^{-6\mathrm{}}$ sec and a longitudinal relaxation time of 200×${10}^{-6\mathrm{}}$ sec for tunneling states at 0.68 GHz at a temperature of 20 mK. The coupling between the two-level systems and phonons is estimated to be 1.6 eV.

Neutron scattering measurements are reported on the linewidth $\Gamma$ of spin waves in the simple cubic antiferromagnet RbMn${\mathrm{F}}_3$ at relatively low temperatures $T$ and spin-wave energies $E_q$. The results are well fitted by $\Gamma =AE_q^{}{}_{}{}^{\alpha }{T}^{\beta }$, where $\alpha =2.13\mathrm{}\pm 0.18$, consistent with the scaling law prediction $\alpha =2\mathrm{}$ of Halperin and Hohenberg, and $\beta =3.29\mathrm{}\pm 0.39$, in agreement with the prediction $\beta =3\mathrm{}$ of two regimes considered by Harris, Kumar, Halperin, and Hohenberg, although the magnitude $A$ is not in agreement with theory.

We report measurements of spin-polarized tunnel currents from a wide compositional range of alloys of Ni with Fe, Mn, Cr, or Ti. The interpretation of the data indicates that all the alloys have an electron spin polarization $P$ which is positive and whose magnitude follows closely the saturation magnetic moment of the alloy. This simple result greatly constrains possible explanations of spin-polarized tunneling from $3d$ ferromagnetic metals.

We present the first measurements of time-resolved luminescence spectra during the growth of electron-hole liquid in Si excited by 15-nsec optical pulses. The results clearly show that at high excitation intensities, the electron-hole liquid forms directly from a hot, dense plasma rather than from a gas of excitions.

Charge-transfer excitations of halogen impurities in potassium have been detected by optical methods. The data are used to establish experimentally that optical threshold profiles in metals are determined by chemical properties of optical centers.

Variable-range hopping as evidenced by a resistivity proportional to $\exp T^{-\frac{1}{4}}$ and by the two-dimensional behavior of thin films has been observed for the first time in a chalcogenide glass. This was accomplished by sputtering ${\mathrm{As}}_2$ ${\mathrm{Te}}_3$ at 77°K. The localized paramagnetic hopping states have also been established by the existence of an ESR signal.

Measurements of optical absorption and gain in CdS and gain in CdSe show that for high excitation density optically excited carriers form an electron-hole liquid with binding energy of 13 meV for CdS and 2 meV for CdSe. The measurements are compared to a recent theory for electron-hole liquid formation in polar semiconductors.

We calculate the ground-state energy of the electron-hole liquid including the coupling to LO phonons. We find that the electron-phonon interaction increases the stability of the liquid phase relative to excitons. We apply the theory to AgBr and to the direct bandgap semiconductors CdS, CdSe, and ZnS. We find that the electron-hole liquid is strongly bound in CdS and AgBr but only weakly so in CdSe and ZnS.

It is shown that an appropriately defined magnetization has a zero thermal average above a critical temperature and a nonzero average below it for a specific lattice Hamiltonian with three-spin interactions in two or more dimensions. This work implies the existence of a phase transition in Reggeon field theory.

Accepted without review at the request of Daniel Sinclair under policy announced 26 April 1976

Orthopositronium ($o-\mathrm{Ps}$), formed on the cone of a channel electron multiplier by 400-eV positrons, is shown to diffuse quickly into an MgO-coated evacuated chamber with energies up to 0.8 ± 0.2 eV. The decay rate of $o-\mathrm{Ps}$ is measured to be 7.09 ± 0.02 ${\mathrm{\mu }\sec }^{-1\mathrm{}}$, where the error is due principally to systematic effects. This result is interpreted to be the first direct measurement of the vacuum decay rate of the $o-\mathrm{Ps}$. It is in disagreement with the theoretical value of 7.242 ± 0.008 μ ${\sec }^{-1\mathrm{}}$ and previous experiments in gases as well.

We have searched for the charmed and other new particles by measuring the hadron pair mass spectra in proton-nucleus collisions at 400 GeV/c. Results are presented for ${\pi }^{-}{K}^{+}$, ${\pi }^{+}{K}^{-}$, and ${\pi }^{+}{\pi }^{-}$ pairs in the mass region from 1.5 to 4.0 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^2$. No evidence for narrow resonances was found. The sensitivity of the search varied from 2 × ${10}^{-30\mathrm{}}$ ${\mathrm{cm}}^2$ at 2 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^2$ to 5 × ${10}^{-32\mathrm{}}$ ${\mathrm{cm}}^2$ at 4 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^2$. Experimental checks were provided by the observation of a clean $\frac{J}{\psi \mathrm{}\left(3.1\right)\mathrm{}}\to {\mu }^{+}{\mu }^{-}$ signal.

The average charged multiplicity, $〈n_x〉$, has been determined for ${\pi }^{-}+p\to \pi _{\mathrm{fast}}^{}{}_{}{}^{-}+X$ at 147 GeV/c and is found to increase linearly with $\mathrm{}\left|t\right|\mathrm{}$ as well as $\ln M_x^{}{}_{}{}^2$. The coefficient of the $\ln M_x^{}{}_{}{}^2$ term is approximately equal to that for other reactions, but the absolute value of $〈n_x〉$ is significantly smaller even when it is analyzed in terms of the energy available. This result is discussed in the framework of a simple model.

Electrodisintegration of the deuteron has been measured for an incident energy of 300 MeV at 30° and 90° scattering angles. The maximum energy transfer was 116 MeV. Very good agreement with theory was achieved when the pionic-exchange currents were included. Near the threshold and for momentum transfer $\overrightarrow{\mathrm{q}}_{\mathrm{c}.\mathrm{m}.\mathrm{}}^{}{}_{}{}^2=3.9\mathrm{}$ ${\mathrm{fm}}^{-2\mathrm{}}$, the meson-exchange currents contribute about 40% to the total cross section.

Differential cross sections for $\mathrm{Na}{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ have been measured in collisions with Hg at thermal energies. In the angular range between 4° and 30°, the coarse structure of the cross section relative to the well-known ground-state interaction is given. In the region between 40° and 70°, an oscillatory behavior of the cross section is observed. Because of their velocity dependence, these oscillations are attributed to fine-structure transitions.

It is shown from a quasilinear theory that an initially isotropic magnetized plasma will be forced into an anisotropic state in ion-cyclotron-resonance heating. Strong heating of the perpendicular ion temperature and strong cooling of the longitudinal temperature should occur simultaneously. The maximum temperature ratio predicted by the quasilinear theory is in exact agreement with that predicted from the basic thermodynamic arguments by Busnardo-Neto, Dawson, Kamimura, and Lin. Heating by fast hydromagnetic waves is also examined.

The influence of finite plasma $\beta$ on the lower-hybrid-drift instability is examined including the effects of (a) transverse electromagnetic perturbations ($\delta \overrightarrow{B}\ne 0$) and (b) resonant and nonresonant $\nabla B_0$ electron-orbit modifications. For typical post-implosion $\theta$-pinch parameters, it is found that there is a critical value of local $\beta \left({\beta }_{\mathrm{cr}}\right)$ such that the instability is completely stabilized ($\gamma <0\mathrm{}$) for $\beta >{\beta }_{\mathrm{cr}}$. For example, ${\beta }_{\mathrm{cr}}=1.6\mathrm{}$, when $\frac{T_e}{T_i}=1\mathrm{}$, $\frac{\omega _{\mathrm{pe}}^{}{}_{}{}^2}{\omega _{\mathrm{ce}}^{}{}_{}{}^2}\gg 1$, and $\left|{\gamma }_{\mathrm{L}i}\frac{\partial \ln n}{\partial x}\right|=2\mathrm{}$.

An extraordinary wave is excited in a plasma column with a long rf coil. The plasma absorption exhibits resonances for frequencies below the second ion gyroharmonic ($\omega <2\mathrm{}{\Omega }_i$). The resonances are shown to be similar, in the ion frequency range, to the Buchsbaum-Hasegawa resonances since they originate from a conversion at the lower hybrid layer of the electromagnetic wave into an ion Bernstein wave. Because of hot plasma effects, the short-wavelength converted wave is observed to propagate into the plasma core even for $\omega >2\mathrm{}{\Omega }_i$.

We show that very dilute concentrations of dipole impurities distributed in alkali halides (for example, ${\mathrm{OH}}^{-}$ impurities in KCl) undergo a phase transition provided that the average impurity-impurity interaction is greater than the tunnel-splitting energy of a single dipole. This theory predicts a temperature-dependent cusp in the dielectric constant. The polarization in the absence of an applied field is predicted to be zero even below the transition temperature.

The nuclear-magnetic-resonance properties of dilute ${\mathrm{He}}^3$ impurities in solid ${\mathrm{He}}^4$ are analyzed on the basis of "gaslike" motions using a model Hamiltonian whose only parameters are the nearest-neighbor exchange matrix elements $J_{34}$ and $J_{33}$. It is shown that ${\mathrm{He}}_2^3$ "molecular" bound states exist in this model. The value of $J_{34}$ required to fit the experiment is not so "anomalously small" as previously reported.