Physical Review Letters

New Phenomena in Excitoa Condensation in Germanium

Thomas K. Lo, Bernard J. Feldman, and C. D. Jeffries

Phys. Rev. Lett. 31, 224 (1973) - Published 23 July, 1973

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We report a sharp optical pumping threshold for the 709-meV luminescence in pure Ge, the simultaneous onset of $10^{- 11}$ -C charge pulses in Ge junctions, and a pronounced optical hysteresis characteristic of supersaturation. Our results favor the model of exciton condensation into electron-hole droplets over the biexciton model. Our droplet-free exciton coexistence curves yield $ϕ = 1.54 \pm 0.25$ meV for the condensation energy.

Diffuse Mössbauer Scattering Applied to Dynamics of Phase Transformations

B. W. Batterman, G. Maracci, A. Merlini, and S. Pace

Phys. Rev. Lett. 31, 227 (1973) - Published 23 July, 1973

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Bcc Zr-20% Nb has a lattice instability, forming the so-called hexagonal $ω$ phase. We show that it is possible to measure diffusely scattered Mössbauer $γ$ rays associated with this transformation and determine whether this scattering is energy shifted. The results strongly suggest a dynamical aspect to the transformation and indicate that Mössbauer scattering can be a significant complement to inelastic neutron diffraction techniques.

Interband Masses of Higher Interband Critical Points in Ge

D. E. Aspnes

Phys. Rev. Lett. 31, 230 (1973) - Published 23 July, 1973

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Interband reduced masses for the critical points $E_{0}$ , $E_{0} + Δ_{0}$ , $E_{1}$ , $E_{1} + Δ_{1}$ , ${E_{0}}^{'}$ , ${E_{0}}^{'} + {Δ_{0}}^{'}$ , ${E_{0}}^{'} + {Δ_{0}}^{'} + Δ_{0}$ , and $E_{2}$ have been determined accurately for Ge from Franz-Keldysh oscillations. The observation of a well-defined interband reduced mass for the $E_{2}$ structure suggests a relatively well-localized critical-point origin for this spectral feature, in contrast to presently accepted interpretations.

Multipole Waves in Periodic Lattices

U. Fano

Phys. Rev. Lett. 31, 234 (1973) - Published 23 July, 1973

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Coulomb Effects on the Gain Spectrum of Semiconductors

W. F. Brinkman and P. A. Lee

Phys. Rev. Lett. 31, 237 (1973) - Published 23 July, 1973

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We have investigated the effect of Coulomb interactions on the gain spectra of highly excited semiconductors, specifically GaAs. It is shown that at low temperatures there is an enhancement of the direct excitation of approximately a factor of 2 and a sideband due to particle-hole pair excitations that extends into the gap. As the temperature increases, it is argued by analogy to the electron-phonon problem that the sideband changes into an Urbach tail with gain of the order of 10-100 ${cm}^{- 1}$ .

Critical Resistivity near an Order-Disorder Transition

G. A. Thomas, A. B. Giray, and R. D. Parks

Phys. Rev. Lett. 31, 241 (1973) - Published 23 July, 1973

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An anomaly in the electrical resistivity is observed in the vicinity of the order-disorder transition in an alloy of composition near ${Fe}_{3}$ Al, which is of the form predicted by Suezaki and Mori for antiferromagnets and, hence, order-disorder systems. The anomaly is qualitatively different from the "ferromagnetic" anomaly observed in $β$ - brass.

Nuclear Reactions of Silver with 300-GeV Protons

G. English, Y. W. Yu, and N. T. Porile

Phys. Rev. Lett. 31, 244 (1973) - Published 23 July, 1973

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Cross sections of 27 radionuclides formed by the interaction of 300-GeV protons with silver were determined on the assumption that the cross section of the reaction ${}^{27}{Al} (p, 3 p n)$ remains the same as at 10-30 GeV. The results are compared with the corresponding values obtained at 11.5 GeV. The average value of the ratio $\frac{σ_{300}}{σ_{11.5}}$ for all products ranging from ${}^{7}{Be}$ to ${}^{106 m}{Ag}$ is 0.91±0.07 and is independent of mass number.

Subthreshold Fission Induced by Neutrons on ${}^{238}U$

R. C. Block, R. W. Hockenbury, R. E. Slovacek, E. B. Bean, and D. S. Cramer

Phys. Rev. Lett. 31, 247 (1973) - Published 23 July, 1973

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Subthreshold fission is observed in ${}^{238}U (n, f)$ at 0.720, 1.210, 2.5, 7.5, 11, 15, 27, and 35 keV. Fission widths are obtained for the resonances at 720 and 1210 eV. The average fission cross section is 87 ± 26 μb for $10 <~ E_{n} <~ 30$ keV and 40 ± 12 μb for $30 <~ E_{n} <~ 100$ keV. The second potential minimum is deduced to lie ≈ 2.2 MeV above the first minimum.

( ${}^{10}B$ , ${}^{7}{Li}$ ) and ( ${}^{10}B$ , ${}^{7}{Be}$ ) Analog Reactions on ${}^{12}C$

K. Nagatani, D. H. Youngblood, R. Kenefick, and J. Bronson

Phys. Rev. Lett. 31, 250 (1973) - Published 23 July, 1973

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The reactions ${}^{12}C$ ( ${}^{10}B$ , ${}^{7}{Li}$ ) ${}^{15}O$ and ${}^{12}C$ ( ${}^{10}B$ , ${}^{7}{Be}$ ) ${}^{15}N$ were investigated at 100 MeV incident energy. These analog reactions show almost identical features, as expected, because the incident energy is much higher than the Coulomb barriers. Therefore, these reactions represent a powerful reaction-mechanism-independent method of finding corresponding analog states in the residual nuclei. Furthermore, the data are consistent with the assumption that the processes are direct three-nucleon transfers populating highspin states. We identify the states having two-particle, three-hole and three-particle, four-hole configurations coupled to high spins.

Experimental Evidence for the Direct Nature of the Reaction ${}^{12}C ({}^{6}{Li}, α) {}^{14}N$ at $E ({}^{6}{Li}) = 33$ MeV

R. L. White, K. W. Kemper, L. A. Charlton, and W. J. Courtney

Phys. Rev. Lett. 31, 254 (1973) - Published 23 July, 1973

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The reaction ${}^{12}C ({}^{6}{Li}, α) {}^{14}N$ has been studied in the energy range 32.5-35.2 MeV. As estimated from the back-angle cross section to the 3.94-MeV $1^{+}$ state, the compound-nucleus contribution to the other $T = 0$ transitions in ${}^{14}N$ is small. A distorted-wave Born-approximation analysis was performed with the usual zero-range approximation and with a full finite-range treatment. Only the finite-range calculations gave cross sections in phase with the data at forward angles.

$π$ Condensation in Nuclear Matter

A. B. Migdal

Phys. Rev. Lett. 31, 257 (1973) - Published 23 July, 1973

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Anomalous $μ^{+}$ Precession in Silicon

J. H. Brewer, K. M. Crowe, F. N. Gygax, R. F. Johnson, B. D. Patterson, D. G. Fleming, and A. Schenck

Phys. Rev. Lett. 31, 143 (1973) - Published 16 July, 1973

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Time Dependence of Vortex-Ring Creation in He II

J. A. Titus and J. S. Rosenshein

Phys. Rev. Lett. 31, 146 (1973) - Published 16 July, 1973

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Measurements of vortex-ring current created by ions in pulsed electric fields of durations ranging from $10^{- 6}$ to $10^{- 4}$ seconds were made in a temperature range of 0.55 to 0.70 K. The results show that the number of vortex rings created is an exponential function of time as predicted by Donnelly and Roberts. However, the velocity dependence of measured nucleation probabitilities is only in qualitative agreement with the theory.

Ion Cyclotron and Fast Hydromagnetic Wave Generation in the $ST$ Tokamak

J. C. Hosea and W. M. Hooke

Phys. Rev. Lett. 31, 150 (1973) - Published 16 July, 1973

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A test coil operating at 25 MHz is used to generate waves in the vicinity of the ion-cyclotron frequency and its harmonics in the $ST$ tokamak. Coil-loading and magnetic-probe signals show that propagating natural modes are excited efficiently in this plasma torus for both slow (ion cyclotron) and fast hydromagnetic waves. These data permit the formulation of an empirical model for wave propagation in the toroidal plasma.

Transport in Relaxation Semiconductors

H. Kiess and A. Rose

Phys. Rev. Lett. 31, 153 (1973) - Published 16 July, 1973

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Optical Investigation of $π$ Bands in Graphite

G. Guizzetti, L. Nosenzo, E. Reguzzoni, and G. Samoggia

Phys. Rev. Lett. 31, 154 (1973) - Published 16 July, 1973

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Observation of Excitonic Polarons at Cyclotron Resonance in Germanium

E. Otsuka, T. Ohyama, and T. Sanada

Phys. Rev. Lett. 31, 157 (1973) - Published 16 July, 1973

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New Model for Amorphous Magnetism

R. Harris, M. Plischke, and M. J. Zuckermann

Phys. Rev. Lett. 31, 160 (1973) - Published 16 July, 1973

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We propose a new model for magnetism in an amorphous material. The model is particularly appropriate for rare-earth compounds such as Tb $F_{2}$ . It is a Heisenberg model in which each ionic spin is subjected to a local anisotropy field of random orientation. We discuss the magnetic properties of two simple ionic configurations, and show that the model is in qualitative agreement with the experimental data.

Simultaneous Excitation of a Magnon plus a Phonon in Co ${Br}_{2}$ ·2 $H_{2}$ O: A New Type of Bound State?

J. B. Torrance and K. A. Hay

Phys. Rev. Lett. 31, 163 (1973) - Published 16 July, 1973

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The first observation of the excitation of a magnon plus a phonon is reported. The agreement between far-infrared measurements on Co ${Br}_{2}$ ·2 $H_{2}$ O and a simple theory is excellent. In addition, the transition probability is shown to be a consequence of a simple magnon-phonon interaction. We also discuss the possibility and evidence that this excitation corresponds to a bound state of a magnon plus a phonon.

Bound Magnon-Phonon Pairs in Metamagnetic Systems

K. L. Ngai, J. Ruvalds, and E. N. Economou

Phys. Rev. Lett. 31, 166 (1973) - Published 16 July, 1973

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We calculate the magnon-phonon coupling for metamagnetic systems of the type FeCl ·2 $H_{2}$ O. The coupling, which is derived from a microscopic point of view, is strong enough to bind a magnon to a phonon in some metamagnetic materials. An analysis of the coupling is presented for different systems, and the results are related to recent experimental data of Torrance and Hay.

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New Phenomena in Excitoa Condensation in Germanium

Diffuse Mössbauer Scattering Applied to Dynamics of Phase Transformations

Interband Masses of Higher Interband Critical Points in Ge

Multipole Waves in Periodic Lattices

Coulomb Effects on the Gain Spectrum of Semiconductors

Critical Resistivity near an Order-Disorder Transition

Nuclear Reactions of Silver with 300-GeV Protons

Subthreshold Fission Induced by Neutrons on ${}^{238}U$

( ${}^{10}B$ , ${}^{7}{Li}$ ) and ( ${}^{10}B$ , ${}^{7}{Be}$ ) Analog Reactions on ${}^{12}C$

Experimental Evidence for the Direct Nature of the Reaction ${}^{12}C ({}^{6}{Li}, α) {}^{14}N$ at $E ({}^{6}{Li}) = 33$ MeV

$π$ Condensation in Nuclear Matter

Anomalous $μ^{+}$ Precession in Silicon

Time Dependence of Vortex-Ring Creation in He II

Ion Cyclotron and Fast Hydromagnetic Wave Generation in the $ST$ Tokamak

Transport in Relaxation Semiconductors

Optical Investigation of $π$ Bands in Graphite

Observation of Excitonic Polarons at Cyclotron Resonance in Germanium

New Model for Amorphous Magnetism

Simultaneous Excitation of a Magnon plus a Phonon in Co ${Br}_{2}$ ·2 $H_{2}$ O: A New Type of Bound State?

Bound Magnon-Phonon Pairs in Metamagnetic Systems

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