C. D. Jeffries

Electron-Hole Condensation in Semiconductors: Electrons and holes condense into freely moving liquid metallic droplets, a plasma phase with novel properties

In Ge and Si, and also in Ge-Si alloys (74), there is extensive evidence for the stable binding of electrons and holes into a cold plasma of constant density, which undergoes a phase separation. Liquid metallic drops 1 to 300 �m in size are formed, with lifetimes ranging from 0.1 to 600 �sec. For Ge a surprising amount is known: the phase diagram, the surface energy, the work function, the decay kinetics. Much less is known for Si. There is good agreement between theoretical and experimental values of the liquid density, the critical density, the critical temperature, and the binding energy. The stability of the liquid phase is strikingly dependent on band structure. The multivalley structure and mass anisotropy of Si, Ge, and Ge-Si, together with their indirect band gap, are no doubt responsible for the observed stability in these crystals. In the similar semiconductor gallium phosphide, drops have not yet been observed, most likely because the high impurity content traps the excitons. In gallium arsenide the existence of drops is controversial (75). Undoubtedly drops will be found to exist in other semiconductors, perhaps at even higher temperatures. This is an exciting field for the experimentalist; new phenomena are being rapidly discovered, usually before they are predicted. For the theorist, the electron-hole drop is of high intrinsic interest. It represents the first example of a quantum liquid of constant density in a periodic crystal lattice. A number of challenging experimental and theoretical problems remain.

Spin-wave turbulence

Abstract A high resolution study is made of spin-wave dynamics above the Suhl threshold in a sphere of yttrium iron garnet driven by microwave ferromagnetic resonance. In different regions of parameter space observed behavior includes: excitation of a single spatial spin-wave mode at threshold; when two modes are excited, low frequency collective oscillations with a period doubling route to chaos; nonperiodic relaxation oscillations when three modes are excited, quasiperiodic route to chaos; abrupt hysteretic onset of wide-band chaos. These results are accounted for in a unified way by numerical iteration of a model: coupled quantum oscillators representing the photons of the cavity and the magnons, including four-magnon scattering processes.

Chaotic Dynamics of Instabilities in Solids

A review is given of studies of chaotic dynamics in several solid state systems. In each case the physical system is described, relevant equations of motion are given, experimental results are presented and interpreted, more or less, from the relevant equations, including numerical solutions. The systems are: (1) An electron-hole helical plasma density wave in a Ge crystal in parallel electric and magnetic fields; this shows period doubling and quasiperiodic routes to chaos. (2) Standing mode spin wave packets in ferrite spheres, excited by driving ferromagnetic resonance of the uniform mode; this system shows period doubling to chaos and periodic windows. (3) Resonantly driven p-n junctions in Si show extremely nonlinear behavior due to charge stored during injection; one junction shows period doubling to chaos and period adding (frequency locking); coupled junctions show, in addition, quasiperiodicity, entrainment, and behavior generic to coupled nonlinear oscillators. The fractal dimension is measured for these systems.

DIRECT OBSERVATION OF A TANGENT BIFURCATION IN A NONLINEAR OSCILLATOR

Abstract We report the direct observation of a tangent bifurcation at the period-five window of a driven nonlinear semiconductor oscillator, by observing the fifth iterate of the dynamical variable of the system in real time. The tangent bifurcation is accompanied by an intermittency as predicted by Manneville and Pomeau.

Nonlinear dynamics of spin waves (invited)

For a yttrium‐iron‐garnet sphere at room temperature, an experimental study is made of the first‐order Suhl spin‐wave instability using perpendicular pumping at 9.2 GHz with the dc field parallel to the [111] crystal axis. The dynamical behavior of the magnetization is observed with high resolution by varying two control parameters, dc field (580<H0<2100 G) and microwave pump power (1<Pin <200 mW). Within this parameter space quite varied behavior is found: (i) onset of the Suhl instability by excitation of a single spin‐wave mode with very narrow linewidth (<0.5 G); (ii) when two or more modes are excited, interactions lead to collective oscillations (‘‘auto‐oscillations’’) with a systematic dependence of frequency (104–106 Hz) on pump power, these oscillations displaying period‐doubling to chaos; (iii) quasiperiodicity, locking, and chaos occur when three or more modes are excited; (iv) abrupt transition to wide band power spectra (i.e., turbulence), with hysteresis; (v) irregular relaxation oscillation...

Luminescence profiles of large electron-hole drops in stressed germanium

Abstract A large electron-hole drop may be stably formed in a potential well of maximum shear strain in a suitably stressed Ge crystal. We scan a magnified image of this drop across a narrow spectrometer slit and measure the time-resolved spatial distribution of the recombination luminescence following pulsed laser excitation. The drop is approximately spherical with an initial radius ≈0.2 mm, which decays with a time constant of 1500 μs.

Rapid formation kinetics of a long-lived electron-hole drop in Ge under pulsed excitation

Abstract The buildup of a large long-lived electron-hole drop in stressed Ge is studied using its Alfven wave resonances. This microwave method permits rapid time resolution of the formation process after a short intense (100 nsec, 7 W) laser excitation pulse. A delay of about 1 μsec is observed before the drop radius rapidly increases from zero to ∼ 100 μm within the next μsec. This is consistent with the interpretation that the photo-produced electron-hole pairs are quickly accelerated to the strain-induced potential minimum which is well within the crystal.

Observation of luminescence from bound multiexciton complexes in gallium doped germanium

Abstract Spectrally resolved luminescence associated with the decay of bound multiexciton complexes in optically excited Ge:Ga is observed. This is the first reported observation of multiexciton complexes in p-type germanium. The observed spectra are consistent with the shell model for bound multiexciton complexes. No-phonon, TA, LA, and TO phonon assisted luminescence are observed. From these spectra, the energies of the LA, TA, and TO phonons in Ge:Ga are determined.

Proton Spin‐Lattice Relaxation in Paramagnetic Crystals in High Fields and Low Temperatures

For nuclei in diamagnetic atoms a shell‐of‐influence model of the nuclear relaxation rate T1n−1 in dilute paramagnetic crystals at low temperature T and high fields H, predicts that 1/T1n ≈ 3/10(gβ/H)2〈1/r6〉[sech2(gβH/2kT)/T1e], where r is the separation between the nucleus and the paramagnetic ion, and T1e is the relaxation time of the ion. The factor sech2(gβH/2kT) ≈ 4 exp(−gβH/kT) arises from the fact that nuclei are relaxed only by mutual ion‐nuclear spin flips, which are proportional to the ion population in the upper Zeeman level. The over‐all consequence is that the nuclear relaxation time becomes exponentially longer at high fields and low temperatures. The above expression well fits our data for T1n for protons in (Nd0.01, La0.99)2Mg3(NO3)12· 24H2O over the ranges 10≤H≤50 kOe, 0.5≤T≤3°K. At 19.5 kOe, T = 0.5°K, we find T1n = 40 h; dynamically induced proton polarization can be maintained for very long times.