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Études calorimétriques des comportements multicritiques des phases ondes de densité de spin dans un composé moléculaire

Abstract : My works have been devoted to the experimental study of a low-dimensional molecular compound, (TMTSF)2ClO4. This is a superconductor at temperatures below 1.2 K, however at low temperature a magnetic field induces a cascade of quantised Spin Density Wave phases (FISDW), beyond about 3 teslas. I developed several experimental set-ups and techniques, in particular the simultaneous measurement of specific heat and, either thermal conductivity (in fixed magnetic field), or magnetocaloric effect (in varying field). My calorimetric investigations have led to a series of striking discoveries that initiated number of theoretical works. -1- In the superconducting state, in zero field, a pair-breaking effect is caused by anion disorder, similar to the effect caused by magnetic impurities in a conventional superconductor. -2- At low field (B<8T), the metal-FISDW transition exhibits a weak coupling character. -3- Some transitions are partially reentrant. -4- The specific heat jump at the transition - i.e., the coupling strength! - oscillates as a function of the magnetic field, and its value presents discontinuities at the limits between quantised SDW phases. -5- In higher fields, the system enters a strong coupling limit (even very strong). -6- The behaviour appears to evolve from 3D towards 2D when crossing the border. -7- Thermal conductivity seems to be dominated by lattice vibrations, however the critical behaviour is associated with the electronic transition. -8- Close to the end of the cascade of transitions, the latter become exothermic whatever the sweeping direction through the last transition line, the signature of a highly irreversible behaviour. -9- For a sample cooled slowly enough through the anion ordering transition, these transition lines separate during an iterative process, leading to the observation of an arborescent phases diagram. -10- We have revealed the existence of a tetracritical point, which corresponds to the meeting of four transition lines in a single point. -11- As anion disorder is increased, the tetracritical behaviour evolves towards a bicritical one, the arborescence of the transition lines disappears, and we observe a pair-breaking effect similar to the one observed in the superconducting phase of the same compound, however of a much larger magnitude.
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Submitted on : Friday, April 18, 2003 - 9:47:53 AM
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François Pesty. Études calorimétriques des comportements multicritiques des phases ondes de densité de spin dans un composé moléculaire. Matière Condensée [cond-mat]. Université Paris Sud - Paris XI, 1993. ⟨tel-00002744v2⟩

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