Itinerant to localized views on f-electron systems: A multiprobe study

In this thesis, various f-electron systems, ranging in their f-electron character
from itinerant to localized, have been investigated by means of bulk and
microscopic methods.
The alloying series CePt3B1-xSix is of interest as it might shed more light
onto CePt3Si, the first heavy fermion superconductor without inversion symmetry.
The phase diagram of the alloying series CePt3B1-xSix has been determined
by means of bulk measurements. Furthermore, susceptibility and
µSR measurements of CePt3B have shown, that the antiferromagnetic order at
low temperatures is a bulk property with a magnetic moment of the order of
1 µB. Surprisingly, no magnetic signal has been observed in neutron scattering
experiments, an observation that will require further investigation.
The influence of structural disorder on the physical properties of the antiferromagnetic
heavy fermion system UPd2Sb has been investigated by means of
neutron scattering. The unusual semiconductor like behavior of the resistivity
could be associated to crystallographic disorder, present in UPd2Sb. The type
of structural disorder has been determined as a kind of phase segregation, with
small crystallites embedded in quasi amorphous regions. Furthermore, the antiferromagnetic
structure has been determined to consist of ferromagnetically
ordered planes, which are antiferromagnetically coupled along the third cubic
axis. We have shown that the size of the crystallites affects on the correlation
length of the magnetically order phase.
A detailed analysis of structural disorder on the Fermi liquid properties, as
observed in the resistivity of UPd2-xSn, revealed that there is no disorder induced
non-Fermi liquid behavior and the Fermi liquid temperature T * is not
significantly altered by structural disorder. In context of the prediction of disorder
induced non-Fermi liquid behavior in the vicinity of a QCP by Rosch
[23], these observations bring up the question, how to measure the distance to
a QCP, viz., what is the meaning of "vicinity to a QCP". Furthermore, the results
of the study on the influence of disorder on the coherent state in the Hall
effect contradict the predictions of the theory of Fert and Levy [49]. Therefore,
new theoretical studies on the anomalous Hall effect in heavy fermion systems
are desirable.
The reinvestigation of the resistivity and magnetoresistivity, in combination
with new high magnetic field measurements, of UPt2Si2 revealed that this
compound has to be discussed as an itinerant f-electron system. New field
induced phases have been found, bearing resemblance to the the hidden order/
heavy fermion superconductor URu2Si2. It suggests that UPt2Si2 might be
a key to understand the exotic properties of the hidden order/heavy fermion
superconductor URu2Si2. To achieve this, further investigations, and in particular
additional high field measurements ought to be carried out on UPt2Si2 in
the future.
UPd3 is one of the rare examples of metallic, quadrupolar ordered systems. In
this study, for the first time a phase diagram of U(Pd1-xPtx)3 has been determined
by means of specific heat, susceptibility, and resistivity. A small amount
of ~ 1% Pt doping destroys long range quadrupolar ordering in UPd3, implying
that the quadrupolar phases in UPd3 are even more sensitive to Pt doping
than the superconducting phase in UPt3 to Pd doping. Because of substantial
broadening of the quadrupolar transitions with Pt alloying, most likely no
quadrupolar QCP exists in the phase diagram.
Furthermore, the magnetic phase diagram of U(Pd1-xPtx)3, x = 0.005, has
been derived. The principle behavior of U(Pd1-xPtx)3, x = 0.005 in a magnetic
field is the same as observed for UPd3, aside from a shift of the transitions
to lower temperatures and a reduction of the entropy Smag associated to the
quadrupolar phase transition, and which possibly might indicate a reduction of
the quadrupolar moment. A splitting of the susceptibility between FC and ZFC
mode at low temperatures has been observed for the first time in U(Pd1-xPtx)3,
x = 0.005. We speculate that the splitting provides an easy avenue to determine
the associated transition temperature T2. In order to fully resolve the
open questions of the quadrupolar properties of U(Pd1-xPtx)3, resonant x-ray
experiments would be desirable.
The resonant x-ray experiments on the rare earth hexaboride PrB6 revealed a
splitting of the structural peaks in the antiferromagentic commensurate phase.
Furthermore, a charge peak in the commensurate phase has been observed,
which splits in the incommensurate phase and vanishes in the paramagnetic
phase. A detailed investigation of the magnetic peaks could verify the earlier
proposed magnetic structure, while no unambiguous evidence for quadrupolar
ordering has been found.
In conclusion, the investigations presented in this thesis reveal that the heavy
fermion related systems investigated in this study have a pronounced itinerant
character. Both, UPd2Sn and UPd2Sb are well understood within an itinerant
approach, and even UPt2Si2, which was considered a prime example of a local
moment U-based antiferromagnet, is more adequately described within an
itinerant Fermi surface picture. Furthermore, in UPd3, again considered to be
an archetypical localized system, Fermi surface effects seems to play an important
role. This is indicated by the suppression of the entropy with 0.5% of
Pt doping and a complete elimination of the quadrupolar ordering with only
~ 1.5% Pt content. Only PrB6 is a localized system with the physical properties
being well described in a crystal electric field scheme. In conclusion, with
this study, it appears that Fermi surface effects ought to be emphasized if a
detailed understanding of heavy fermion related materials should be attained.