Emergent Transport Properties of Magnetic Topological Insulator Heterostructures

109 Pages·2020·6.253 MB·English

by  Kenji Yasuda

Springer Theses
Recognizing Outstanding Ph.D. Research
Kenji Yasuda
Emergent Transport
Properties of Magnetic
Topological Insulator
Heterostructures
Springer Theses
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Kenji Yasuda
Emergent Transport
Properties of Magnetic
Topological Insulator
Heterostructures
Doctoral Thesis accepted by The
University of Tokyo, Tokyo, Japan
123
Author Supervisor
Dr. KenjiYasuda Prof. Yoshinori Tokura
Department ofPhysics RIKEN Centerfor Emergent
Massachusetts Institute of Technology Matter Science(CEMS)
Cambridge, MA, USA Wako, Japan
TheUniversity of Tokyo
Tokyo,Japan
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-981-15-7182-4 ISBN978-981-15-7183-1 (eBook)
https://doi.org/10.1007/978-981-15-7183-1
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’
Supervisor s Foreword
The application of quantum mechanics to condense matter has made a significant
impact in understanding and controlling solid-state materials over the last century.
Despite the huge success of semiconductor devices in modern electronics, they do
notharnessthefullpotentialofquantummechanicalwavefunctions.Namely,they
utilize only the energy-momentum dispersion of the band structure but not
explicitly the phase information of the wave function. Over the past few decades,
people have started to appreciate the importance of the geometrical phases and
topology in momentum space motivated primarily by quantum Hall effect and
anomalous Hall effect. The concept of topology now forms one of the central
schemes in condensed matter physics with the discovery of novel quantum mate-
rials, such as topological insulators. The exotic band topology of these quantum
materialsisexpectedtoserveasanovelplatformforemergentphysicalproperties.
This thesis by Dr. Kenji Yasuda presents such possibilities by studying the
electrical transport properties of topological insulator heterostructures. Employing
state-of-the-art thin film growth andfabrication techniques, he hasfound emergent
transport phenomena; dissipationless chiral edge conduction along the magnetic
domain wall, skyrmion formation, current-induced magnetization switching, and
nonreciprocal nonlinear resistance, i.e., diode-like effect, in magnetic and super-
conductingtopologicalinsulators.Theseresponsesallresultfromhighlyentangled
electron spin and momentum enabled by the topological nature of the band struc-
ture. The unique spin-momentum locking of the topological surface state, coupled
with magnetism, enabled the electrical control of magnetism and vice versa. The
presentstudyprovidessuperiorinsightintothetransportphenomenaoftopological
materials, offering a guideline for utilizing topological band structure to produce
novel physical functionalities.
Wako/Tokyo, Japan Prof. Yoshinori Tokura
August 2020
v
Preface
One ofthe central themesofcondensed matter physicsisto understand andutilize
the characteristics of phases of matter. Roughly speaking, there are two types of
phasesincondensedmatter:oneisaspontaneoussymmetrybrokenorderedphase,
andtheotherisatopologicalphaseofmatter.Thenotionofspontaneoussymmetry
breaking leads to the appearance of a variety of orders such as ferroelectricity,
ferromagnetism and superconductivity with fixed order parameters. These effects
wereexperimentallydiscoveredandtheoreticallyformulatedbythe1950sandhave
longbeenthecentralthemeofcondensedmatterphysicseversince.Theconceptof
topological phase, on the other hand, is relatively new. The most prominent
exampleofthetopologicalphaseisthequantumHalleffectdiscoveredinthe1980s
in two-dimensional electronic systems under the magnetic field. Thanks to this
discovery, the importance ofband topology in theBrillouin zone isrecognizedfor
thefirsttime.Fromthismoment,theideaoftopologyoriginallydevelopedinpure
mathematics began to prevail in condensed matter physics. Followed by the for-
mulation of anomalous Hall effect and spin Hall effect, the extension of quantum
Hall effect has lead to the discovery of various topological materials such as
topological insulator, Dirac semimetal and Weyl semimetal. These materials are
characterizedbyatopologicalintegersuchaswindingnumber,Chernnumber,and
Z invariant. Since these values are integers, the continuous deformation cannot
2
changethetopologicalvalue.Asaresult,thesematerialsyieldcharacteristicsurface
state at the interface between the vacuum or trivial materials, associated with the
discrete change of the topological integer. Such surface states gained interests of
physicists because of their robustness against perturbations as well as the charac-
teristic band structures that cannot be realized in conventional bulk materials. The
surface state possesses a spin-nondegenerate massless Dirac dispersion, where its
spin directionisuniquelylocked toitsmomentum called spin-momentumlocking.
Following the theoretical prediction, the experimental study of topological insula-
tors has developed rapidly over the past 10 years. Together with material devel-
opments, the detailed characteristics of the surface state are now well understood.
vii
viii Preface
Now that the respective characteristics of the two phases, symmetry broken
orderedphaseandtopologicalphasearewellunderstood,itisnaturaltoexpectthat
the combination of these two will result in more exotic quantum states. A naive
consideration can readily predict the appearance of exotic phenomena: When
magnetization is combined with the spin-momentum locked topological surface
state, the magnetization dramatically modifies the band structure giving rise to
various magnetoelectric responses. In fact, quantum anomalous Hall effect, or the
magnetically induced quantum Hall effect, and various spintronic phenomena are
experimentally found in a magnetically coupled topological insulator. Meanwhile,
when superconductivity is combined with spin-momentum locking, the exotic
pairingisexpectedbecauseofthelimitedpairingsymmetrybythespinmomentum
locking. This results in the emergence of topological superconductivity and asso-
ciated formation of Majorana fermion.
Although there have been a lot of seminal works in this direction from around
2010,thisfieldisstillimmature.Thegoalofthethesisistoinvestigatetheemergent
transport properties and functionalities of topological insulators under broken
symmetry. To study the interaction between magnetization and surface state, we
study the transport properties of Crx(Bi1(cid:1)ySby)2(cid:1)xTe3 and (Bi1(cid:1)ySby)2Te3/Crx
(Bi1(cid:1)ySby)2(cid:1)xTe3 the research has found the high-temperature realization of the
quantum Hall effect and the chiral edge state at the domain wall in quantum
anomalous Hall state. In addition, we found various spintronic functionalities such
as unidirectional magnetoresistance, current-nonlinear Hall effect, current-induced
magnetization switching, and skyrmionformation. Finally,the interaction between
surface state and superconductivity is investigated in the heterostructure of Bi Te
2 3
and FeTe. The largely enhanced nonreciprocal transport is found associated with
the appearance of superconductivity.
Cambridge, USA Dr. Kenji Yasuda
List of publications
Parts of this thesis have been published in the following journal articles:
1. Quantum Hall states stabilized in semi-magnetic bilayers of topological
insulators
R.Yoshimi*,K.Yasuda*,A.Tsukazaki,K.S.Takahashi,N.Nagaosa,M.Kawasaki
andY.Tokura
Nature Communications 6, 8530 (2015) (* equal contribution).
2. Magnetic modulation doping in topological insulators toward
higher-temperature quantum anomalous Hall effect
M. Mogi, R. Yoshimi, A. Tsukazaki, K. Yasuda, Y. Kozuka, K. S. Takahashi,
M. Kawasaki and Y. Tokura
Applied Physics Letters 107, 182401 (2015).
3. Geometric Hall effects of topological insulator heterostructures
K.Yasuda,R.Wakatsuki,T.Morimoto,R.Yoshimi,A.Tsukazaki,K.S.Takahashi,
M.Ezawa,M.Kawasaki,N.NagaosaandY.Tokura
Nature Physics 12, 555–559 (2016).
4. Zero-bias photocurrent in ferromagnetic topological insulator
N.Ogawa,R.Yoshimi,K.Yasuda,A.Tsukazaki,M.KawasakiandY.Tokura
Nature Communications 7, 12246 (2016).
5. Large Unidirectional Magnetoresistance in a Magnetic Topological
Insulator
K.Yasuda,A.Tsukazaki,R.Yoshimi,K.S.Takahashi,M.KawasakiandY.Tokura
Physical Review Letters 117, 127202 (2016).
6. Current-Nonlinear Hall Effect and Spin-Orbit Torque Magnetization
Switching in a Magnetic Topological Insulator
K. Yasuda, A. Tsukazaki, R. Yoshimi, K. Kondou, K. S. Takahashi, Y. Otani,
M. Kawasaki and Y. Tokura
Physical Review Letters 119, 137204 (2017).
7. Quantized chiral edge conduction on domain walls of a magnetic topolog-
ical insulator
K.Yasuda,M.Mogi,R.Yoshimi,A.Tsukazaki,K.S.Takahashi,M.Kawasaki,
F. Kagawa and Y. Tokura
Science 358, 1311–1314 (2017).
8. Nonreciprocal charge transport at topological insulator/superconductor
interface
K.Yasuda,H.Yasuda,R.Yoshimi,A.Tsukazaki,K.S.Takahashi,N.Nagaosa,
M. Kawasaki, Y. Tokura
Nature Communications 10, 2734 (2019).
9. Magnetic topological insulators
Y Tokura, K. Yasuda, A Tsukazaki
Nature Reviews Physics 1, 126–143 (2019).
ix
x Listofpublications
Some related works, which not presented here, are published in the following
articles:
(cid:129) Interface-driven topological Hall effect in SrRuO -SrIrO bilayer
3 3
J. Matsuno, N. Ogawa, K. Yasuda, F. Kagawa, W. Koshibae, N. Nagaosa,
Y. Tokura, M. Kawasaki
Science Advances 2, e1600304 (2016).
(cid:129) Current-driven magnetization switching in ferromagnetic bulk Rashba
semiconductor (Ge,Mn)Te
R. Yoshimi, K. Yasuda, A. Tsukazaki, K. S. Takahashi, M. Kawasaki,
Y. Tokura
Science Advances 4, eaat9989 (2018).