Nonequilibrium Dynamics of Collective Excitations in Quantum Materials

360 Pages·2018·13.197 MB·English

by  Edoardo Baldini

Springer Theses
Recognizing Outstanding Ph.D. Research
Edoardo Baldini

Nonequilibrium
Dynamics
of Collective
Excitations in
Quantum Materials
Springer Theses
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Edoardo Baldini
Nonequilibrium Dynamics
of Collective Excitations
in Quantum Materials
Doctoral Thesis accepted by
É é é
the cole Polytechnique F d rale de Lausanne, Switzerland
123
Author Supervisors
Dr. Edoardo Baldini Prof. Majed Chergui
Department ofPhysics ÉcolePolytechnique Fédérale
Massachusetts Institute of Technology deLausanne (EPFL)
Cambridge, MA Lausanne
USA Switzerland
Prof. Fabrizio Carbone
ÉcolePolytechnique Fédérale
deLausanne (EPFL)
Lausanne
Switzerland
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-77497-8 ISBN978-3-319-77498-5 (eBook)
https://doi.org/10.1007/978-3-319-77498-5
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Nothing in life is to be feared, it is only to be
understood. Now is the time to understand
more, so that we may fear less.
—Marie Skłodowska Curie
To my parents, Luisella and Primo
In memory of Teresa, Francesco and Silvio
’
Supervisors Foreword
One of the historical goals of condensed matter physics is to explain the diversity
that characterizes different quantum states of matter in the condensed phase.
Understanding its origin would unravel new quantumphenomena, witha potential
for advanced functionalities in devices. At the core of this diversity lies a subtle
balance between different forces on a microscopic scale. Such forces govern the
couplingamong thefundamental particles that characterize thematerial properties.
As a result, the interplay between the constituent particles makes the study of
condensed matter an intrinsic many-body problem.
Inorder toaddressthedetailsofthemany-bodyproblem, intheearly twentieth
century, band theory provided a first intuitive classification of the quantum
mechanicalbehaviorofelectronsinsolids,leadingtothedistinctionbetweenmetals
and band insulators. However, most complex solids nowadays escape this simple
classification, requiring more advanced and refined models for treating the inter-
actionsamongseveralelementaryexcitations.Thesesolidsarecurrentlyreferredto
as “Quantum materials”, as their fundamental properties arise from a complex
interplay between electron–boson interactions and electron–electron correlations.
Phenomena such as high-temperature superconductivity; spin, charge, and orbital
ordering; metal–insulator transitions, colossal magnetoresistance, and many others
are believed to be direct manifestations of this phenomenology.
The most promising strategy to reveal the details of complex many-body sys-
tems is understanding the spectrum of their elementary excitations, since the latter
reflect the balance among the forces within the solid. Some of these elementary
excitations can still be described as fermionic particles with renormalized proper-
ties, i.e., dressed of interactions. Other elementary excitations, mostly of bosonic
nature, involve a cooperative, wave-like motion of many particles in the system
simultaneously. These are dubbed “collective excitations”, which represent the
maintopicofthisbook.Thespectrumoftheelementaryexcitations,similarlytothe
underlying interactions, depends on a number of external parameters that can be
tunedatwill.Forexample,itiswellknownthattheenergy,linewidth,andintensity
ix
x Supervisors’Foreword
of acollective excitation canvary asa functionof temperature, pressure, magnetic
or electric field, and chemical doping depending on the specific material that is
investigated. This in turn reflects how the combination of several microscopic
forces inside the solid is affected by the same subset of external parameters. More
recently, another external parameter has become available, which can be tuned to
investigateandmodifythepropertiesofquantummaterials:Light.Indeed,photons
in the form of short, tailored light pulses interact with matter and can prepare a
system in a highly nonequilibrium state that depends on the nature of the pho-
toexcitation process itself. As a result, the dynamical response of a system can be
followed with a temporal resolution comparable to the timescale of the relevant
microscopic processes in the material.
In this book, Dr. Baldini focuses on the role of collective excitations in gov-
erning the properties of quantum materials beyond conventional band theory,
namely strongly interacting metals and insulators, and strongly correlated super-
conductors and multiferroics. In so doing, he develops a novel spectroscopic
technique that relies on ultrafast broadband optical probes in the visible and
deep-ultravioletrange.Thiscutting-edgeapproachhasamainadvantageoverother
conventionalspectroscopicprobes,asitallowstosimultaneouslytrackhigh-energy
collective modes (such as plasmons and excitons) in the frequency domain and
low-energycollectivemodes(suchasphonons,magnons,andexoticcharge-density
excitations) in the time domain. The dynamical behavior of these collective exci-
tations in turn unveils the real-time evolution of complex many-body phenomena
occurring in the solid at low energies. As such, the methodology implemented by
Dr. Baldini hits deep inside the paradigm of correlated electron systems and
allowed him to obtain a number of important results. To name a few, the first
observation of the phonon-mediated mechanism connecting distinct electronic
subsystems in the multiband superconductor MgB , the discovery of strongly
2
bound excitons in anatase TiO , and the use of the latter excitons to detect the
2
interfacial electron transfer in dye-sensitized systems.
All the results presented in this book are cast and described in a broad per-
spective, providing an extensive and simplified introduction to concepts. At the
same time, deep physical insights are offered by the combination of novel exper-
imental data and state-of-the-art theoretical calculations. The book reads like an
advanced treatise in condensed matter physics that, by a few examples, explains
topics ofhugeinterest inthefield,discussed withacritical eyeandaveryoriginal
vision.Italsocontainsanexhaustiveandrichlistofreferences.Thehighqualityof
Dr. Baldini’s work has also been recognized by the members of the jury that
evaluated his thesis, comprising distinguished names in the field such as Prof.
Anthony James Leggett (Nobel Prize in Physics 2003—University of Illinois at
Urbana-Champaign, USA), Prof. Thomas Elsaesser (Head of the Max Born
Institute, Germany), Prof. Joël Mesot (Head of the Paul Scherrer Institute,
Switzerland), and Prof. Vincenzo Savona (Director of the Doctoral program in
Physics at EPFL, Switzerland). The thesis has been also awarded the prestigious
Supervisors’Foreword xi
DimitrisN.ChorafasPrize2017,whichisassignedforoutstandingworkinselected
fields in the engineering sciences, medicine, and the natural sciences. We are
convinced that this work provides a highly valuable reference textbook for any
scientist starting a project in this field of research and in particular young ones.
Lausanne, Switzerland Prof. Majed Chergui
December 2017 Prof. Fabrizio Carbone