Capacité thermique | EPFL Graph Search

La capacité thermique (anciennement capacité calorifique) d'un corps est une grandeur qui mesure la chaleur qu'il faut lui transférer pour augmenter sa température d'un kelvin. Inversement, elle permet de quantifier la possibilité qu'a ce corps d'absorber ou de restituer de la chaleur au cours d'une transformation pendant laquelle sa température varie. Elle s'exprime en joules par kelvin (). C'est une grandeur extensive : plus la quantité de matière est importante, plus la capacité thermique est grande. Il peut donc être défini une capacité thermique molaire (exprimée en joules par mole kelvin, ) et une capacité thermique massique ou spécifique (exprimée en joules par kilogramme kelvin, , anciennement appelée chaleur spécifique). La capacité thermique dépend des conditions opératoires mises en œuvre lors de sa détermination. Elle dépend notamment du phénomène mis en jeu pour augmenter la température : force de pression, force électromagnétique, etc. Par conséquent, il peut être déf

Order and dynamics in Kagome like compound Cu2OSO4 and other quantum magnets

Virgile Yves Favre

This thesis presents results of studies of novel compounds modeling complex fundamental physics phenomena. Cu2OSO4 is a copper based magnetic Mott Insulator system, where spin half magnetic moments form a new type of lattice. These intrinsically quantum pins are exhibiting atypical magnetic order and spin dynamics. The recent success in the growth of large single crystals of Cu2OSO4 enabled to perform measurements probing its static and fluctuating properties. The peculiarity of this sample is that its atoms are forming layers, with a geometry close to the intensively studied Kagomé lattice, but with a third of its spins replaced by dimers. This quantum magnetism system has been probed in its bulk, by the means of heat capacity and DC-susceptibility measurements, revealing a transition to a magnetically long range ordered state upon cooling, the details of which are revealed by neutron scattering. Single crystal inelastic neutron scattering shed light on the spin-dynamics in the system, with clear spin waves appearing as fluctuations around the peculiar ground state of the system: a 120 degrees spin configuration where the magnetic moment of the spin-dimer causes the sample to be globally ferrimagnetic. The presented results indicate that Cu2OSO4 represents a new type of model lattice with frustrated interactions where interplay between magnetic order, thermal and quantum fluctuations can be explored. The magnetic excitations of the compound can be modeled by a yet-to-be-understood internal effective mean-field that no simple magnetic coupling seems to reproduce. K2Ni2(SO4)3 is another compound that allows for the existence of non-trivial topological phases. This thesis presents results of the study of the unusual magnetic behavior of K2Ni2(SO4)3. No clear sign of well-established magnetic long range order has been observed down to dilution temperatures. Neutron scattering reveals the details of the competition between frustration and fluctuations that prevent order from settling in. Low temperature spin excitations take the form of a continuum at 500 mK, but also of broad, energy independent continua at higher temperatures. Bulk and neutron scattering measurements are put in perspective and linked together with a view to building up a better understanding of how quantum spin liquids can be stabilized in general, and in particular in this model compound. Finally, the last contribution of this thesis to the field of condensed matter physics regards the establishment of a state-of-the-art technique to fit heat capacity and unit cell volume of samples to try and make the extraction of magnetic information from specific heat measurements more robust. This newly-developed technique consists in modeling lattice contributions with better accuracy by using data from multiple experimentally accessible quantities to consolidate the fitting scheme. This method has been cautiously applied to several compounds at the forefront of research in experimental physics.

EPFL2021

Possible spin-orbit driven spin-liquid ground state in the double perovskite phase of Ba

Arash Alahgholipour Omrani, Henrik Moodysson Rønnow

We report the structural transformation of hexagonal Ba3YIr 2O9 to a cubic double perovskite form (stable in ambient conditions) under an applied pressure of 8 GPa at 1273 K. While the ambient pressure synthesized sample undergoes long-range magnetic ordering at ∼4 K, the high-pressure (HP) synthesized sample does not order down to 2 K as evidenced from our susceptibility, heat capacity, and nuclear magnetic resonance (NMR) measurements. Further, for the HP sample, our heat capacity data have the form γT+βT3 in the temperature (T) range of 2-10 K with the Sommerfeld coefficient γ=10 mJ/mol-Ir K2. The 89Y NMR shift has no T dependence in the range of 4-120 K and its spin-lattice relaxation rate varies linearly with T in the range of 8-45 K (above which it is T independent). Resistance measurements of both the samples confirm that they are semiconducting. Our data provide evidence for the formation of a 5d-based, gapless, quantum spin-liquid in the cubic (HP) phase of Ba3YIr 2O9. In this picture, the γT term in the heat capacity and the linear variation of 89Y 1/T1 arises from excitations out of a spinon Fermi surface. Our findings lend credence to the theoretical suggestion that strong spin-orbit coupling can enhance quantum fluctuations and lead to a QSL state in the double perovskite lattice. © 2013 American Physical Society.

2013

Order and dynamics in Kagome like compound Cu2OSO4 and other quantum magnets

Virgile Yves Favre

EPFL2021