Study of magnetic fluctuations and ordering in uranium compounds by heat capacity and neutron scattering measurements
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Date
29/11/2018Author
Entwisle, Oliver John
Metadata
Abstract
URhGe is the first ferromagnet discovered that shows superconductivity at
ambient pressure. It shows a rich temperature-magnetic field phase diagram with
a re-emergence of superconductivity at high magnetic field where the moments
rotate. This suggests that the quantum fluctuations associated with the moment
rotation may provide the pairing interaction for superconductivity. The objective
of this thesis was to study these critical fluctuations with inelastic neutron
scattering and heat capacity measurements, using the latter to test the bulk
nature of the superconductivity and determine the types of gap nodes to help
test this hypothesis.
To perform the heat capacity measurements, it was necessary to develop an
apparatus that measures milligram samples in the temperature range 50-1000
mK, and magnetic field range 0-12 T. The field exerts a mechanical force upon
the sample, which causes it to rotate, perturbing the system destructively. The
apparatus developed in this thesis overcomes this diffculty by holding the sample
with tensioned kevlar wires. Testing was done by making measurements on UPt3,
a well characterised superconductor. It was then used to measure URhGe in zero
magnetic field. The extension to measurements in high magnetic field were not
performed however, due to the structural integrity of the apparatus being weak -
this was in an attempt to reduce the thermodynamic signature of the background.
After many iterations of apparatus design and build, the device was proved not
appropriate for high fields. A discussion of the zero-field data, as well as the
design and build process, is given.
The Curie temperature of URhGe is suppressed with magnetic field (applied
along the b-axis), reaching zero temperature at the moment rotation transition
referred to above. Small angle neutron scattering (SANS) was measured
at both zero and finite fields to detect the evolution and relaxation of the
critical fluctuations. The scattering is inelastic and the SANS measurement
integrates over energy. Nevertheless it was possible to compare models with
different dynamical dependences for the magnetic relaxation. In field, however,
the magnitude of the fluctuations was strongly reduced, falling below the
detection limit at half the critical field. Comparing Landau damping to various
forms of non-Landau damping, a result was found that agrees with that for
the ferromagnetic superconductors UGe2 and UCoGe, but the lack of critical
scattering at field is found to be in contradiction with NMR measurements, which
is discussed.
UAu2 is a new material on the heavy fermion landscape. The crystal structure
found suggests some frustrated magnetism, culminating in a Neél temperature
of 43 K and a further transition at 400 mK; this suggests some new quantum
criticality not seen before, and so heat capacity measurements were performed
with the already-tested apparatus to see if, as the resistivity measurements
suggest, a Fermi-liquid state is found. Results revealed differences between
annealed and non-annealed samples in their thermodynamic signature, and the
behaviour expected for antiferromagnetic spin-fluctuations is found to continue
to temperatures below 150 mK, suggesting the existence of a quantum critical
point. The validity of these results along with implications are discussed.