Hall resistivity and torque magnetometry studies of the ferromagnetic superconductors UGe2 and URhGe
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Date
29/11/2016Author
Lithgow, Calum Thomas
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Abstract
Ferromagnetism (FM) and superconductivity (SC) are traditionally thought of as
competing states of matter, since the opposite-spin electron pairing mechanism
required for conventional SC is rendered impossible by FM spin alignment.
However, recently discovered heavy-fermion compounds UGe2 and URhGe are
examples where SC and FM are cooperative, and rather than antagonistic the
presence of FM is actually necessary for the occurrence of the SC phase.
A cooperative state of FM and SC is a topic of interest because it presents a
possible solution to one of the two main problems with present superconductors:
technology inhibiting limits on the highest temperature and highest magnetic field
to which the SC phase can exist. Although both UGe2 and URhGe cease to be
superconductors before even reaching 1 K, unlike the various `high temperature'
superconductors currently known that easily surpass 100 K, it is their magnetic
properties that are interesting, the inherent FM ordering allowing them to exceed
conventional limits on the maximum magnetic field that SC can withstand. For
example, URhGe remains superconducting above 35 T and the upper limit is so
high that it is still experimentally undetermined.
How exactly the FM SC phase arises in these compounds is as yet unknown. The
necessary opposite-spin pairing mechanism is theoretically provided by magnetic
fluctuations in an easily polarizable system right on the edge of a magnetic phase
transition, and indeed SC emerges in UGe2 and URhGe around a first-order
quantum critical point (QCP) where the temperature of the transition to an FM
phase is reduced to absolute zero, by application of pressure in the case of UGe2
and by application of a magnetic field for URhGe.
The aim of the research detailed in this thesis is to probe the FM phase
transition and the associated QCP related to the emergence of SC in these
compounds, to gather more information about the precise nature of the phases
either side of the transition and exactly what changes occur in the system
crossing the QCP. Specifically, the main objective is to characterise the magnetic
fluctuations at the phase boundary and determine whether, by current FM
SC theory, these fluctuations could be responsible for SC or if instead other,
modified, unconventional theories are required to explain the unconventional
electron pairing.
The probes of choice for this PhD were Hall effect and magnetoresistance
measurements of UGe2, and capacitive torque magnetometry and simultaneous
magnetoresistance measurements of URhGe.
The main result of the UGe2 project is an observed order-of-magnitude change in
the Hall coefficient crossing the FM transition as a function of temperature and a
dramatic change, similar in magnitude but also accompanied by a sign reversal,
crossing the QCP as a function of pressure. Furthermore, the sign reversal at the
critical pressure persists up to roughly 12 K, far beyond the 7 K critical end point
of the phase transition, suggesting that in fact three different phases converge at
the QCP where fluctuations between them presumably lead to the emergence of
SC. Further investigation of the Fermi surface, either by deeper analysis of the
Hall effect results or by other experimental methods, will be required to complete
the main objective and determine exactly what the differences are between these
newly identified phases.
The main result of the URhGe project is actually the successful development of
the capacitive torque magnetometry technique itself and the proof of operation
for simultaneous measurement of all the individual components of both the
magnetization and differential susceptibility tensors in a high magnetic field,
which is currently not possible by any other technique. Completing the
main objective was hampered by the extremely high susceptibility components
encountered in the vicinity of the QCP, which in itself could be considered
evidence for the theoretical relationship between strong FM fluctuations and the
emergence of SC in URhGe.
A number of results incidental to the main aim of the URhGe project are also
summarised in this thesis, including the characterisation of quantum oscillations
frequencies not previously reported in scientific literature and a variety of
subtle features in resistivity measurements, which could, in conjunction with
evidence from the susceptibility measurements, suggest the presence of another
superconducting state such as surface or domain wall SC.
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