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Quantum oscillations as a probe of hidden order in heavy fermion systems

15/01/2013, Alix McCollam, HFML, Radboud University, Nijmegen, The Netherlands

Quantum oscillations are well known as a highly sensitive probe of electronic structure, and have a long history of providing valuable information about the Fermi surface and quasiparticle properties.

In the first part of this talk, I will discuss measurements of the de Haas-van Alphen effect in PrOs4Sb12, which show that quantum oscillations may also be useful as a probe of hidden order in strongly correlated electron systems. PrOs4Sb12 has an unusual antiferroquadrupolar (AFQ) ordered phase that forms at low temperature and high magnetic field due to a field-induced level-crossing of triplet and singlet crystal field states. I will show that AFQ order has a subtle, but clearly measurable, influence on quantum oscillations, which allows the order parameter to be tracked down to very low temperatures, and provides information on the coupling between conduction electrons and localised f-electrons that has not been accessible by other means. Moreover, the low temperature sensitivity of the de Haas-van Alphen technique reveals a strong and unexpected influence of the hyperfine coupling on the AFQ order parameter below 300 mK.
I will then describe a new type of magnetometer [1], based on a piezoresistive microcantilever and a small, moveable ferromagnet. Depending on the position of the ferromagnet, a strong but highly local field gradient of up to 7000 T/m can be applied at the sample or removed completely during a single measurement, allowing both the magnetic force and torque on the sample to be independently determined. Sensitivity of 10−14 J/T is achievable for the isotropic and anisotropic magnetisation, so that even weak magnetic effects can be measured in very small samples, at millikelvin temperatures, and in the highest available magnetic fields.
[1] A. McCollam, P. G. van Rhee et al., Rev. Sci. Instrumen. 82, 053909 (2010).