Table Of ContentSpringer Theses
Recognizing Outstanding Ph.D. Research
Jennifer Fowlie
Electronic and
Structural Properties
of LaNiO -Based
3
Heterostructures
Springer Theses
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Jennifer Fowlie
Electronic and Structural
Properties of LaNiO -Based
3
Heterostructures
Doctoral Thesis accepted by
the University of Geneva, Geneva, Switzerland
123
Author Supervisor
Dr. Jennifer Fowlie Prof. Dr. Jean-MarcTriscone
Department ofQuantum Matter Physics Department ofQuantum Matter Physics
University of Geneva University of Geneva
Geneva, Switzerland Geneva, Switzerland
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-030-15237-6 ISBN978-3-030-15238-3 (eBook)
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’
Supervisor s Foreword
During the last 20 years, a large effort has been made to realize and study oxide
epitaxialthinfilmsandnoveloxideheterostructures.Itisrecognizedtodaythatthe
quality of the oxide structures that can be synthesized using advanced growth
techniques such as oxide molecular beam epitaxy, pulsed laser deposition, or
sputtering is at the level of III-V semiconductor heterostructures.
Complex transition metal oxides display a variety of electronic properties, and
superconducting metallic, ferromagnetic, dielectric, ferroelectric, multiferroic, and
Mott insulating compounds are found within this family of materials. Oxides also
often display complex phase diagrams with competing phases. This competition
makes these materials very sensitive to external parameters, and it is thus possible
using pressure, magnetic, or electric field to dramatically change their electronic
properties.
In oxide thin films and heterostructures, epitaxial strain, quantum confinement,
and interfacial effects can further lead to interesting new phases and exciting
physics.
Among thematerialsofinterest today, thefamily ofnickelate compounds,with
formula RENiO , is attracting a lot of attention. These materials display, for most
3
ofthem,ametal–insulatortransition(MIT)whosetemperaturedependsonthechosen
rare earth. Additionally, there have been exciting theoretical predictions regarding
artificial structures containing nickelates—for instance, possible high-temperature
superconductivitywaspredictedinsuperlatticesinwhichoneofthebuildingblocks
isaone-unitcellthicklayerofLaNiO .
3
In her Ph.D. work, Jennifer Fowlie has been exploring the physics of some
of these fascinating compounds—in particular LaNiO and a solid solution of
3
LaNiO and NdNiO in thin film form.
3 3
Jennifer starts her manuscript with chapters describing the physics of transition
metal oxides at the center of her work. She introduces the characteristics of
d-orbitals,theroleofthecrystalfield,andtheimportantconceptsofMott–Hubbard,
charge transfer, and negative charge transfer insulators. She then describes the
nickelate family by introducing the phase diagram that relates the metal–insulator
and paramagnetic to antiferromagnetic (AFM) phase transition temperatures to the
vii
viii Supervisor’sForeword
rare earth size or Ni-O-Ni bond angle. She explains the interest in LaNiO —the
3
only member of the family not displaying an MIT and the goals of her work.
Jennifer then develops a more technical chapter in which she is describing the
growth and characterization techniques used during her Ph.D. illustrated by
examples of materials grown during this work.
In Chap. 6, Jennifer describes how she used X-ray diffraction and half-order
peak analyses to determine the film’s structure (a distorted perovskite) and the
octahedralrotationangles.FollowingGlazer,shefirstdeterminedtheabsenceorthe
presence of specific half-order peaks that allow the distortion pattern of a system
(a-b-c+—for instance) to be determined. To go further, it is necessary to carefully
measure the intensities of the half-order peaks and to compare those to the ones
obtained by calculating the diffraction pattern of a specific “distorted” structure.
Amodelsystem—LaAlO on(001)SrTiO —wasused,andtheresultsobtainedare
3 3
inlinewithwhatonewouldexpectfromgeometricalconsiderations.Then,LaNiO
3
films grown on (001) SrTiO and (001) LaAlO were studied versus the LaNiO
3 3 3
thickness and the average rotation angles could be obtained. This approach is of
great interest for many systems since it allows the determination of the rotation
pattern and angles in thin perovskite films.
Chapter 7 focuses on the properties of ultrathin LaNiO films grown by sput-
3
tering.Thistechniqueallowedverymanyfilmstobegrownondifferentsubstrates,
althoughthechapterismostlyfocusingonfilmsgrownon(001)LaAlO .Transport
3
measurementsrevealedanunexpectedmaximuminconductivityforfilms6–11unit
cells thick. This maximum is robust and observed at room temperature and at low
T. TEM analyses and ab initio calculations suggest that there are three different
regions in “thick” films; an interfacial one with distortions “matching” the ones
of the substrate, an intermediate “bulk-like” region, and a surface region, very
distorted.Asthethicknessisreduced,themiddle“bulk-like”regionfirstdisappears
followedbytheinterfacialone.Inafewunitcellsofthickfilm,thesurfaceregionis
the only one left with strong distortions. This study on ultrathin LaNiO films has
3
allowed the substrate-film and film-surface interactions to be better understood.
In the last chapter of her Ph.D., Jennifer studies a solid solution of LaNiO and
3
NdNiO3—Nd1-xLaxNiO3.Thereareseveralmotivationsforthisstudy.Asafunction
of x and at T = 0 K, the system should go from an insulator to a metal for a given
compositionx .Forthisparticularcomposition,theMITshouldthusbeatT=0K;
c
the system should also display a magnetic transition from an AFM state to a para-
magneticstateaswellasatransitionfromamonoclinicstructuretoarhombohedral
structure. The x composition is of interest, maybe surprisingly, for superconduc-
c
tivity. Jennifer makes ananalogy with valence skippingelements suchas bismuth.
For these elements, some valences are not energetically favorable and thus the
system charge disproportionates—a phenomenon sometimes called negative U
physics—the system preferring to have “2” electrons on one site and zero on the
neighboringone.Suchaneffectcanprovidealocalpairingand,withsometuningof
parameters,mayleadtosuperconductivity.Intheinsulatingphaseofthenickelates,
achargedensitydisproportionationtakesplaceallowingsomeanalogywithvalence
skipping elements. For x , the targeted composition for searching for
c
Supervisor’sForeword ix
superconductivity, the MIT is at 0 K and the system “hesitates” to bond dispro-
portionate. The first tests with Nd1-xcLaxcNiO3 did not reveal a superconducting
state, but the studies are still ongoing. Also of great interest is the study of the
dependence of the MIT on x that reveals an unexpected sharp drop at a particular
x—a behaviorstill unexplained.
The thesis of Jennifer Fowlie treats several aspects of the physics of nickelates
whose metal–insulator transition and unusual magnetic order are only some the
hallmarks of this fascinating transition metal oxide system. As will be obvious
going through this Ph.D. work, the presented studies open several research direc-
tions that will certainly be explored in the future.
Geneva, Switzerland Prof. Dr. Jean-Marc Triscone
December 2018
Acknowledgements
As is always the case, a thesis is produced not only by the person named on the
front but also by the help, support, and contribution of an almost uncountable
number of people.
Naturally, the first acknowledgment must go to Prof. Jean-Marc Triscone who
directedthisprojectandwasinstrumentalateachstep.FromhimIhavelearned,of
course,atremendousamountaboutphysicsbutalso,andperhapsmoreimportantly,
the value of scientific integrity. I believe that I could not have found a better
environment in which to carry out my thesis work than that of Jean-Marc’s group.
I was greatly honored that my work was evaluated by such distinguished sci-
entistssoIwishtothankProf.CharlesAhn,Prof.AntoineGeorges,andProf.Phil
Willmott for having agreed toserve asmythesisjury. I was lucky enough tohave
had the opportunity to discuss or collaborate with all three of them during my
studies.
Over the last few years, I have found myself working alongside many talented
scientists and wonderful people.
IwasfirstintroducedtothefascinatingfieldofoxidesbyNickPlumbandMilan
Radovic at the Paul Scherrer Institute and by Cliff Hicks and Andy MacKenzie in
St Andrews so I owe a big thank you to them for helping me to discover this
subject.
Morerecently,ithasbeenwonderfultoworkaspartofthe“NickelatesTeam”in
Geneva, that is, Sara Catalano, Claribel Dominguez, and Marta Gibert. Marta, in
particular, deserves one of the biggest acknowledgments of all for going through
my manuscript with a fine-tooth comb.
Anyone who has worked in the Triscone group knows that it is a special
experience so thank you to all the group members, past and present, for the
excursions and coffee breaks as well as the day-to-day work in the laboratories:
MargheritaBoselli,JoerideBruickere,RitsukoEguchi,StéphanieFernandez,Elías
Ferreiro, Alexandre Fête, Stefano Gariglio, Dirk Groenendijk, Jorrit Hortensius,
Denver Li, Céline Lichtensteiger, Wei Liu, Hugo Meley, Gernot Scheerer, Giulio
xi
