Quantum Tricritical Points
Dr Sven Friedemann (Physics, Bristol)
Frank Lecture Theatre H H Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL
Phase transitions are a very old concept in thermodynamics. The thermal fluctuations at a second-order phase transition give rise to critical behaviour. In the limit of zero temperature, however, thermal fluctuations are absent and second-order phase transitions are dominated by quantum fluctuations due to Heisenberg’s uncertainty principle. Such a case is called a quantum critical point. Here, I present our latest progress on ferromagnetism that is suppressed towards a quantum critical point. The suppression of the transition temperature in antiferrromagnets has been studied intensively and lead to the discovery of emergent behaviour like unconventional superconductivity. In ferromagnets, however, the quantum critical point appears to be avoided: Ferromagnetism is pre-empted by other types of order and the transition turns first order. Thus, no significant fluctuations emerge. In our recent work, however, we have studied NbFe2 – a prototypical material for the suppression of ferromagnetism – using a Landau order parameter expansion. We show very consistently that the ferromagnetic quantum critical point is indeed avoided. In addition, we discover quantum tricritical points at which both the uniform (ferromagnetic) susceptibility and the finite wavevector (antiferromagnetic) susceptibility diverge. Because of the general nature of the model used, the results are expected to apply to a whole class of ferromagnets Three dimensional phase diagram of NbFe2 with the newly discovered quantum tricritical points (QCTP) S. Friedemann et al., Nature Physics AOP, DOI: 10.1038/nphys4242
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