We present the temperature dependence of elastic modes of Fe3−xZnxO4 with x = 0, x = 0.02 and x = 0.032. The c44 shear modes show a pronounced softening which can be normalized to a common behavior for all these substances. We can explain the softening with a bilinear coupling of the elastic strain to an order parameter linked to charge ordering processes. The other elastic modes (c11, c12 and the bulk modulus) do not show any softening. We present a symmetry analysis for the charge ordering model.
PACS. 71.30.+h Metal-insulator transitions and other electronic transitions -62.65.+k Acoustical properties of solids -75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
The intermediate state of (TMTSF)2C104 is characterized by resistance and magnetoresistance measurements to consist of an inhomogeneous mixture of superconducting and spin-density-wave regions, whereas the quenched state is purely insulating with a phase-transition temperature TsD+=6, 05 K and a zerotemperature gap 25(0) =22 K =3. 52TsDw, close to the mean-field value.In the family of the Bechgaard salts (TMTSF)2X (TMTSF denotes the organic molecule tetramethyltetraselenafulvalenium and Xstands for a variety of inorganic anions) the perchlorate compound (X = C104) is the only member up to now that exhibits superconductivity at ambient pressure. This ground state is in close competition with an insulating one. Experimentally either state may be realized by modification of the cooling procedure below about 40 K. After Takahashi, Jerome, and Bechgaard, ' the slowly cooled ground state (0.1 K/min)the "relaxed state"is completely superconducting below 1.2 K, whereas the rapidly cooled state ( )50 K/min)the "quenched state"is insulating. The latter one was identified by resistivity, NMR, EPR, and antiferromagnetic resonance measurements as a magnetic, namely, a spin-density-wave (SDW) state. ' 3 The origin of this "ground-state degeneracy" is a structural phase transition at 24 K due to an ordering of the noncentrosymmetric perchlorate anions characterized by a wave vector q = (0, 2, 0). 4 In the relaxed state, the anions are ordered. Fast cooling through the phase-transition region freezes the high-temperature disorder, thus creating the quenched state. It is possible to switch reversibly between these two states by simply warming up the sample above 40 K and using either cooling procedure. Intermediate states with a SOW precursor to the superconducting state may be obtained in two different ways, either by using intermediate cooling speeds or by quenching from different temperatures T~i n the critical region around 24 K with a rapid cooling rate.In this Rapid Communication we report on lowtemperature rnagnetoresistance measurements of (TMTSF)2C104 in states with various degrees of anion disorder characterized by the different quenching temperatures Tp (thus ranging from the relaxed states via several intermediate states to the quenched state).The preparation of the (TMTSF)2C104 crystal is discussed in detail in Ref. 5. The resistance measurements were carried out with a standard low-frequency (30 Hz) ac method. The electrical leads (copper wire of 0.017-mm diam) were attached to the single-crystal sample with silver paint. The sample was thermally anchored to the mixing chamber of a dilution refrigerator and its crystal a axis was roughly oriented parallel (to within 15') to the magnetic field of a small superconducting solenoid (maximum field 9 kG). The intention was to compare the properties of the crystal as a function of magnetic field in its various intermediate states rather than to study the magnetic-field dependence on a distinct orientation.Initially the sample was measured in its relaxed state (cooling spee...
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