Phase Diagram and Magnetic Excitations of BaFe2-xNixAs2: A Neutron Scattering Study

127 Pages·2017·7.414 MB·English

by  Xingye Lu (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Xingye Lu

Phase Diagram and
Magnetic Excitations
of BaFe Ni As : A
2–x x 2
Neutron Scattering
Study
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Xingye Lu
Phase Diagram and
Magnetic Excitations
of BaFe Ni As :
−x x
2 2
A Neutron Scattering Study
Doctoral Thesis accepted by
University of Chinese Academy of Sciences, Beijing, China
123
Author Supervisor
Dr. Xingye Lu Prof. Pengcheng Dai
Centerfor AdvancedQuantum Studies Department ofPhysics andAstronomy
andDepartmentof Physics RiceUniversity
Beijing NormalUniversity Houston
Beijing USA
China
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-981-10-4997-2 ISBN978-981-10-4998-9 (eBook)
DOI 10.1007/978-981-10-4998-9
LibraryofCongressControlNumber:2017944213
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’
Supervisor s Foreword
The discovery of superconductivity in Fe-based materials in 2008 boosted the
researchesinhigh-Tc superconductivity.Althoughthehigh-Tc superconductivityin
copper oxides (cuprates) was discovered in 1986, no consensus has been achieved
on the microscopic mechanism in spite of considerable efforts. The discovery of
Fe-based superconductors (FeSC) provided a new route to high-Tc superconduc-
tivity. Since superconductivity in both cuprates and FeSC arises from doped
antiferromagnets, unveiling the interplay between superconductivity and magnetic
order, spin dynamics, and related exotic phases are essential for understanding the
microscopic origin of high-Tc superconductivity.
The aim of this thesis is to reveal this interplay between superconductivity and
other orders and their dynamics in a specific electron-doped iron-pnictide system
BaFe2−xNixAs2. Using high-resolution X-ray diffraction, neutron diffraction, and
inelastic neutron scattering (INS), Xingye Lu studied the phase diagram and the
spin excitations of BaFe2−xNixAs2. In his first work (Chap. 4), he used
high-resolution X-ray and neutron diffraction to establish a detailed structural,
magnetic, and electronic phase diagram of BaFe2−xNixAs2. The magnetic and
orthorhombic structural order parameters reveal a strong magnetoelastic coupling,
which was expected to lead to a quantum bicritical point near optimal doping.
However, the emergence of an incommensurate antiferromagnetic order close to
optimal superconductivity strongly modifies the electronic properties and pre-
empted the purported quantum critical point. This work clarifies the quantum
criticality and its connection to superconductivity in electron-doped BaFe As
2 2
systems. To further investigate the nature of the short-range incommensurate
magneticorder,heexploitedneutronresonancespin-echotechnique,cold,thermal,
and polarized neutron triple-axis spectrometers to study the slow dynamics of the
incommensurate magnetic order, and determined its cluster spin glass nature
(Chap. 5). These two works have settled the doping evolution of the magnetic
ground states and their effects on the electronic properties of this system.
After determining the phase diagram of BaFe2−xNixAs2, Xingye used INS to
investigate the newly discovered electronic nematic order located between Ts and
TN (Ts[TN)inthephasediagram(Chap. 6).Thisstudyisdifficultbecauseitwill
v
vi Supervisor’sForeword
need to be performed on large detwinned crystals (*0.2 g). After putting great
efforts,hedevelopedamethodtopreparesampleanddesignedadetwinningdevice
thatcandetwinlargesinglecrystal.HisINSstudiesonBaFe2−xNixAs2findthatthe
low-energyspinexcitationsevolvefromC symmetry(withequalintensityforthe
4
excitations at (1, 0, 1) and (0, 1, 1)) to C symmetry (with intensity difference
2
between the excitations at (1,0,1) and (0, 1, 1)). The onset temperature of this
spin-excitation anisotropy (T(cid:1)) is well above Ts and coincident with that for the
resistivity anisotropy of uniaxial-stress detwinned BaFe2−xNixAs2. They also show
similardopingdependenceandthereforeindicatethattheyareintimatelycorrelated
or share the same origin. This is the first study of this type and inspired many
follow-up works.
In the third part of this thesis, he unveiled the direct correlation between
low-energy spin excitations (including spin resonance) and superconductivity, and
the persistence of the diffusive high-energy spin excitations (paramagnon)
throughout the phase diagram, using the state-of-art time-of-flight neutron scatter-
ing spectrometers in ISIS (Chap. 7). This work provided solid evidence that the
low-energy spin excitations are correlated with the formation of cooper pairs. The
systematic work was based on large amount of single crystals (*100 g of single
crystals in total for three dopings) and careful INS experiments. This impressive
samplegrowthworkwasdonewithinseveralmonthsbyXingyeandhiscolleagues.
Besides the important discoveries discussed above, this thesis contains a brief
introduction to FeSC and a relative detailed introduction to the principles and
techniques associated with neutron scattering. Both parts are easy to follow by
non-experts. Overall, the work of Xingye has broadened considerably our under-
standing of the interplay between magnetism and superconductivity in FeSC.
Through his dedication, patience, and hard work, he was able to accomplish a
tremendousamountofworkwithinarelativelyshortperiod.Ibelievehisthesissets
a new standard of excellence in condensed matter physics and will be appreciated
for many years to come.
Houston, USA Prof. Pengcheng Dai
April 2017
Publications Associated with This Thesis
Xingye Lu, Jitae Park, Rui Zhang, Huiqian Luo, A.H. Nevidomskyy, Qimiao Si
and Pengcheng Dai. Nematic spin correlations in the tetragonal state of uniaxial
strained BaFe2−xNixAs2. Science 345, 657–660 (2014).
Xingye Lu, David W. Tam, Chenglin Zhang, Huiqian Luo, Meng Wang, Rui
Zhang, Leland W. Harriger, T. Keller, B. Keimer, L.-P. Regnault, Thomas
A. Maier, and Pengcheng Dai. Short-range cluster spin glass near optimal super-
conductivity in BaFe2−xNixAs2. Phys. Rev. B 90(2), 024509 (2014).
MengWang*,ChenglinZhang*,XingyeLu*,GuotaiTan,HuiqianLuo,YuSong,
Miaoyin Wang, Xiaotian Zhang, E.A. Goremychkin, T.G. Perring, T.A. Maier,
Zhiping Yin, Kristjan Haule, Gabriel Kotliar, and Pengcheng Dai. Doping depen-
dence of spin excitations and its correlations with high-temperature superconduc-
tivity in iron pnictides. Nature Communications 4, 2874 (2013).
XingyeLu,H.Gretarsson,RuiZhang,XuerongLiu,HuiqianLuo,WeiTian,Mark
Laver,Z.Yamani,Young-JuneKim,A.H.Nevidomskyy,QimiaoSi,andPengcheng
Dai. Avoided quantum criticality and magnetoelastic coupling in BaFe2−xNixAs2.
PhysicalReviewLetters110(25),257001(2013).
Huiqian Luo*, Xingye Lu*, Rui Zhang, Meng Wang, E.A. Goremychkin,
D.T. Adroja, Sergey Danilkin, Guochu Deng, Zahra Yamani, and Pengcheng Dai.
Electron doping evolution of the magnetic excitations in BaFe2−xNixAs2. Physical
Review B 88(14), 144516(2013).
vii
Acknowledgements
I wish to express my sincere thanks and appreciation to my supervisor
Prof.PengchengDai.Ihavebeendeeplyinfluencedbyhispassionanddiligencein
doing science. The atmosphere of freedom for discussions and the chances for
doingneutronscatteringexperimentsprovidedbyPengcheng,whichIenjoyedvery
much, are the most important factors leading to my dissertation and publications.
I would like tothank Dr. Huiqian Luo and Dr. Shiliang Lifor bringingme into
the SC08 group and teaching me sample growth and various neutron scattering
techniques.Ithankmycolleaguesinourgroup,Dr.XiaoyanMa,Dr.MengWang,
RuiZhang,DingHu,YanchaoChen,WenliangZhang,ZhaoyuLiu,andDongliang
Gong, who give me help in preparing samples or doing neutron scattering exper-
iments.IalsothankDr.LelandHarriger,Dr.MiaoyinWang,Dr.MengshuLiu,and
YuSongfortheirhelpduringmyvisittotheUniversityofTennesseeatKnoxville.
I also owe special thanks to Dr. Jitae Park, Prof. Toby Perring, Dr. Russell
Ewing,Dr.DevashibhaiAdroja,Dr.WeiTian,Dr.ZahraYamani,Dr.MarkLaver,
Dr. Thomas Keller, and Mr. Kuo-Feng Tseng, for their help during my neutron
scatteringexperimentsattheMLZ,ISIS,HFIR,C5,andSINQneutronsources.The
collaborations with Jitae, Kuo-Feng, and Thomas are very fruitful, which
I appreciate very much.
Last but not least, I would like to thank my wife and my parents for their
unfailing support during my Ph.D. years, without which all the works I have done
could not be possible.
ix
Contents
1 Motivation and Overview.. ..... .... .... .... .... .... ..... .. 1
1.1 Superconductivity. .... ..... .... .... .... .... .... ..... .. 1
1.2 Scope of This Thesis.. ..... .... .... .... .... .... ..... .. 3
References .. .... .... .... ..... .... .... .... .... .... ..... .. 5
2 Iron-Based Superconductors .... .... .... .... .... .... ..... .. 9
2.1 Basic Properties.. .... ..... .... .... .... .... .... ..... .. 10
2.1.1 Structure and Magnetic Structure .... .... .... ..... .. 10
2.1.2 Electronic Structure .. .... .... .... .... .... ..... .. 14
2.2 Phase Diagrams.. .... ..... .... .... .... .... .... ..... .. 15
2.2.1 Spin Density Wave... .... .... .... .... .... ..... .. 17
2.2.2 Superconductivity and Fermi Surface Nesting .. ..... .. 19
2.2.3 Coexisting and Competing Orders ... .... .... ..... .. 21
References .. .... .... .... ..... .... .... .... .... .... ..... .. 23
3 Neutron Scattering ... .... ..... .... .... .... .... .... ..... .. 29
3.1 Historical Overview... ..... .... .... .... .... .... ..... .. 29
3.2 Principles... .... .... ..... .... .... .... .... .... ..... .. 30
3.2.1 Cross Section.. ..... .... .... .... .... .... ..... .. 31
3.2.2 Nuclear Scattering ... .... .... .... .... .... ..... .. 33
3.2.3 Magnetic Interactions and Elastic Magnetic Scattering .... 34
3.2.4 Coherent Inelastic Magnetic Scattering.... .... ..... .. 36
3.2.5 Dynamic Structure Factor SðQ;x) and Generalized
Susceptibility v00(Q, x).... .... .... .... .... ..... .. 37
3.3 Neutron Scattering Spectrometry .. .... .... .... .... ..... .. 38
3.3.1 Triple-Axis Spectrometry .. .... .... .... .... ..... .. 38
3.3.2 Time-of-Flight Spectrometry.... .... .... .... ..... .. 42
References .. .... .... .... ..... .... .... .... .... .... ..... .. 49
xi