Tension responses due to small and rapid length changes (completed within 40 microseconds) were obtained from skinned single-fiber segments (4- to 7-mm length) of the iliofibularis muscle of the frog incubated in relaxing, rigor, and activating solution. The fibers were skinned by freeze-drying. The first 500 microseconds of the responses for all three conditions could be described with a linear model, in which the fiber is regarded as a rod composed of infinitesimally small identical segments, containing an undamped elastic element, two damped elastic elements and a mass in series. An additional damped elastic element was needed to describe tension responses of activated fibers up to the first 5 ms. Consequently phase 1 and phase 2 of activated fibers can be described with four apparent elastic constants and three time constants. The results indicate that fully activated fibers and fibers in rigor have similar elastic properties within the first 500 microseconds of tension responses. This points either to an equal number of attached cross-bridges in rigor and activated fibers or to a different number of attached cross-bridges in rigor and activated fibers and nonlinear characteristics in rigor cross-bridges. Mass-shift measurements obtained from equatorial x-ray diffraction patterns support the latter possibility.

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A discussion is given of the contribution to the Mossbauer linewidth in quasi-one-dimensional antiferromagnets with Ising anisotropy due to thermal excitation of moving domain walls ("kinks" or "solitons"). Experimental data on Fe 2+ chain compounds are presented that confirm the predicted exponential temperature dependence of the linewidth.PACS numbers: 76.80.+y, 76.60.Es, 75.10.Hk The study of solitary excitations is an exciting new topic that has an impact on a great many branches of physics. 1 In contrast to the rapid development of theoretical understanding of nonlinear dynamics, very few experimental results bearing upon this problem have been obtained up till now. It is therefore of great importance to find new ways in which to probe the presence of these nonlinear excitations and to establish their influence on experimentally measurable quantities.It has recently become apparent that magnetic chains offer quite promising possibilities in this respect. Following a suggestion of Mikeska, 2 an inelastic neutron scattering study on the quasione-dimensional (ID) XY ferromagnet CsNiF 3 was performed by Kjems and Steiner, 3 who found an anomalous increase in intensity of the central peak attributed to scattering of the neutrons by the solitons.It has subsequently been shown, 4 that analogous local excitations exist in a ID XY antif erromagnet. In a series of field-dependent neutron inelastic scattering and NMR studies on (CH 3 ) 4 NMnCl 3 (TMMC), contributions ascribed to solitons have been observed which were very much more pronounced than those found in CsNiF 3 .It is the purpose of this note to point out that Mossbauer spectroscopy can provide an additional tool for studying such nonlinear excitations. Similarly to NMR, the Mossbauer effect measures the hyperfine interaction between electronic and nuclear spins. Spin fluctuations arising from linear as well as from nonlinear excitations may contribute to the Mossbauer linewidths, just as they contribute to the NMR nuclear spin lattice relaxation time T im After a brief outline of the formal analogy between the Mossbauer linewidth F, the 1/T 1 in NMR, and the neutron scattering cross section, we present experimental linewidth data on a number of linear chain Fe 2+ antiferromagnets with different Ising-type spin anisotropy. In this class of soliton-bearing systems solitons have not been studied before experimentally. Because of the Ising type of anisotropy there is no need for a symmetry-breaking applied magnetic field. 5 The contribution to r from the solitons reveals itself as an anomalous line broadening that extends to temperatures far above the 3D magnetic ordering temperature T c (brought about by the weaker interchain couplings), and shows an exponential dependence on temperature.In theoretical treatments of spin fluctuations, one considers the space-time correlation between spins via the pair correlation functions G aa (r,t) = (S a (r,t)S a (0,Q)).By taking the space-time Fourier transform of G aa (r,t), the dynamical structure factor (Van Hove scatt...

Tension responses due to small, rapid length changes (completed within 40 microseconds) were obtained from skinned single frog muscle fiber segments (4-10 mm length) incubated in relaxing and rigor solutions at various ionic strengths. The first 2 ms of these responses can be described with a linear model in which the fiber is regarded as a rod, composed of infinitesimally small, identical segments, containing one undamped elastic element and two or three damped elastic elements and a mass in series. Rigor stiffness changed less than 10% in a limited range, 40-160 mM, of ionic strength conditions. Equatorial x-ray diffraction patterns show a similar finding for the filament spacing and intensity ratio I(11)/I(10). Relaxed fibers became stiffer under low ionic strength conditions. This stiffness increment can be correlated with a decreasing filament spacing and (an increased number of) weakly attached cross-bridges. Under low ionic strength conditions an additional recovery (1 ms time constant) became noticeable which might reflect characteristics of weakly attached cross-bridges.