Abstract
When a magnetic field confines the carriers of a Fermi sea to their lowest Landau level, electron−electron interactions are expected to play a significant role in determining the electronic ground state. Graphite is known to host a sequence of magnetic field-induced states driven by such interactions. Three decades after their discovery, thermodynamic signatures of these instabilities are still elusive. Here we report the detection of these transitions with sound velocity measurements. The evolution of elastic constant anomalies with temperature and magnetic field allows to draw a detailed phase diagram which shows that the ground state evolves in a sequence of thermodynamic phase transitions. Our analysis indicates that the electron−electron interaction is not the sole driving force of these transitions and that lattice degrees of freedom play an important role.
Highlights
When a magnetic field confines the carriers of a Fermi sea to their lowest Landau level, electron−electron interactions are expected to play a significant role in determining the electronic ground state
A variety of electronic instabilities driven by the electron−electron interactions, which may arise in this context, have been proposed[2,3,4]
In the case of graphite it is generally assumed that the density wave (DW) forms as a result of a nesting process along the magnetic field direction driven by electron−electron interactions[8]
Summary
When a magnetic field confines the carriers of a Fermi sea to their lowest Landau level, electron−electron interactions are expected to play a significant role in determining the electronic ground state. Graphite is known to host a sequence of magnetic field-induced states driven by such interactions. Three decades after their discovery, thermodynamic signatures of these instabilities are still elusive. The evolution of elastic constant anomalies with temperature and magnetic field allows to draw a detailed phase diagram which shows that the ground state evolves in a sequence of thermodynamic phase transitions. Due to the variety of degrees of freedom competing for the ground state (orbital, spin, and valley), several instabilities have been proposed over the years: charge[8, 9, 10] and spin[11], density wave (respectively labeled CDW and SDW) and more recently excitonic phases[12, 13]. The interaction of electrons with the lattice may favor a DW phase with an in-plane component modulation reminiscent of the CDW state[17, 18] and excitonic state[19] proposed in the case of graphene
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