Table Of ContentSpringer Theses
Recognizing Outstanding Ph.D. Research
Tatsuya Kobayashi
Study of Electronic
Properties of 122
Iron Pnictide Through
Structural, Carrier-
Doping, and Impurity-
Scattering Effects
Springer Theses
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Tatsuya Kobayashi
Study of Electronic
Properties of 122 Iron
Pnictide Through Structural,
Carrier-Doping,
and Impurity-Scattering
Effects
Doctoral Thesis accepted by
Osaka University, Toyonaka, Japan
123
Author Supervisor
Dr. TatsuyaKobayashi Prof. SetsukoTajima
Department ofPhysics Osaka University
Osaka University Toyonaka
Toyonaka,Osaka Japan
Japan
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-981-10-4474-8 ISBN978-981-10-4475-5 (eBook)
DOI 10.1007/978-981-10-4475-5
LibraryofCongressControlNumber:2017939068
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’
Supervisor s Foreword
Since the discovery of high-temperature superconductivity in iron-based super-
conductors (FeSC), a large number of studies have been reported to clarify its
electronicstateandthemechanismofsuperconductivity. Thereareseveralreasons
why so much attention has been paid to this material. First, its superconducting
transition temperature is very high (the second highest at ambient pressure).
Second, the conductionelectrons originate from iron atom, which isferromagnetic
and has been assumed to be harmful for superconductivity. We have observed
quantum phenomena in 3d electrons. Fortunately, ab-initio band calculation seems
toworkwellforFeSC,incontrasttothecaseofcupratesuperconductors,inwhich
theelectronicstateisfarfromthebandpictureowingtostrongelectroncorrelation.
Nevertheless, there remain numerous unresolved problems such as the unusual
in-plane electronic anisotropy, gap symmetry, and pairing mechanism.
Inthisthesis,theauthorattemptedtoelucidatetheseproblemsthroughthestudy
of a typical FeSC, Sr(Ba)Fe As , by controlling physical parameters via various
2 2
element substitutions and/or by controlling disorders via the annealing process.
Thethesisconsistsofthreemainparts.Thefirstisadiscussionofgapsymmetry
basedontheeffectofdisorderonspecificheat.Thiswastriggeredbythesuccessful
growthofahigh-qualitycrystalofSrFe2(As1(cid:1)xPx)2(Sr122).Beforethissuccessful
growth,itwasbelievedthattheT ofSr122islowerthanthatofBa122becausethe
c
Fermisurfaceismorethree-dimensionalintheformerowingtothesmallerionsize
of Sr. However, the author found that the T of Sr122 can be higher than that of
c
Ba122 with appropriate annealing. Surprisingly, it was also found that the
remarkable effect of annealing is caused by structural change. Owing to the
high-qualitycrystals,theauthorhasestablishedacompletephasediagramofSr122.
Furthermore,thedisordereffectofspecificheatwasexamined,andthechangefrom
a clean superconductor to a dirty one with an s(cid:3)-wave full gap was suggested.
The subject of the second part is the unusual in-plane anisotropy of resistivity
observed in this material. By preparing a series of crystals of Ba(Fe1(cid:1)xTMx)2As2
(TM = Co, Cr and Mn), the author found that, with hole doping, the anisotropy
becomesinvertedfromq [q toq \q .Thisfindingcannotbeexplainedbythe
b a b a
anisotropic spin scattering mechanism that was proposed before, but it strongly
v
vi Supervisor’sForeword
suggeststheimportanceoftheFermisurfacetopology(sizeandshape)thatchanges
with carrier doping.
The third part concerns the impurity effects on the charge dynamics, which has
revealed the unusual electronic state of this system. The author measured the
temperature dependence of optical reflectivity spectra for all the crystals of Ba
(Fe1(cid:1)xTMx)2As2 overa wide frequency range. Two remarkable effects were found
for the magnetic impurity doping (Cr and Mn substitution). One is a strong sup-
pression of the coherent Drude component, which never occurs in the case of
non-magnetic Co substitution. This indicates that the magnetic impurities act as
strong carrier-scattering centers. The loss of coherent carriers prevents the
appearance of superconductivity. The other effect is the appearance of an unusual
localized state, the energy of which increases with impurity doping. The author
attributed it to the cooperative effect between conduction electrons and local
magnetic moment around impurities.
Alltheseresultssupporttheelectronicpictureinwhichspinfluctuationplaysan
importantrolebothinthenormalandthesuperconductingstateofthismaterial.The
presentstudymakesasignificantcontributiontounderstandtheelectronicstateand
the superconductivity mechanism of FeSC through work including high-quality
crystal growth; careful measurement of resistivity under uniaxial pressure; precise
measurements of x-ray diffraction, specific heat, and optical spectra; as well as
detailed analyses of the data.
Osaka, Japan Setsuko Tajima
December 2016
Parts of this thesis have been published in the following journal articles
1. T. Kobayashi, M. Nakajima, S. Miyasaka, and S. Tajima, Phys. Rev. B 94,
224516 (2016)
2. T.Adachi,Y.Nakamatsu,T.Kobayashi,S.Miyasaka,S.Tajima,M.Ichimiya,
M.Ashida,H.Sagayama,H.Nakao,R.Kumai,andY.Murakami,J.Phys.Soc.
Jpn. 85, 063705 (2016)
3. J. Jandke, P. Wild, M. Schackert, S. Suga, T. Kobayashi, S. Miyasaka,
S. Tajima, and W. Wulfhekel, Phys. Rev. B 93, 104528 (2016)
4. E. Uykur, T. Kobayashi, W. Hirata, S. Miyasaka, S. Tajima, and
C. A. Kuntscher, Phys. Rev. B 92, 245133 (2015)
5. N.Murai,T.Fukuda,T.Kobayashi,M.Nakajima,H.Uchiyama,D.Ishikawa,
S. Tsutsui, H. Nakamura, M. Machida, S. Miyasaka, S. Tajima, and A. Q. R.
Baron, Phys. Rev. B 93, 020301(R) (2016) (Editors’ Suggestion)
6. M. Miyamoto, H. Mukuda, T. Kobayashi, M. Yashima, Y. Kitaoka,
S. Miyasaka, and S. Tajima, Phys. Rev. B 92, 125154 (2015)
7. T. Kobayashi, K. Tanaka, S. Miyasaka, and S. Tajima, J. Phys. Soc. Jpn. 84,
094707 (2015)
8. C.P.Strehlow,M.Kończykowski,J.A.Murphy,S.Teknowijoyo,K.Cho,M.
A.Tanatar,T.Kobayashi,S.Miyasaka,S.Tajima,andR.Prozorov,Phys.Rev.
B 90, 020508(R) (2014)
9. H. Suzuki, T. Kobayashi, S. Miyasaka, T. Yoshida, K. Okazaki,
L. C. C. Ambolode, II, S. Ideta, M. Yi, M. Hashimoto, D. H. Lu, Z.-X. Shen,
K.Ono,H.Kumigashira,S.Tajima,andA.Fujimori,Phys.Rev.B89,184513
(2014)
10. T. Kobayashi, S. Miyasaka, S. Tajima, and N. Chikumoto, J. Phys. Soc. Jpn.
83, 104702 (2014)
11. S. Miyasaka, A. Takemori, T. Kobayashi, S. Suzuki, S. Saijo, S. Tajima,
J. Phys. Soc. Jpn. 82, 124706 (2013)
12. T.Kobayashi,S.Miyasaka,S.Tajima,T.Nakano,Y.Nozue,N.Chikumoto,H.
Nakao, R. Kumai, and Y. Murakami, Phys. Rev. B 87, 174520 (2013)
13. J. Murphy, C. P. Strehlow, K. Cho, M. A. Tanatar, N. Salovich,
R.W.Giannetta,T.Kobayashi,S.Miyasaka,S.Tajima,andR.Prozorov,Phys.
Rev. B 87, 140505(R) (2013)
14. M.Ikeda,M.Hagiwara,T.Kobayashi,W.Hirata,S.Miyasaka,andS.Tajima,
J. Korean Phys. Soc. 62, 2007 (2013)
15. T. Kida, T. Kobayashi, S. Miyasaka, S. Tajima, and M. Hagiwara, J. Low
Temp. Phys. 170, 346 (2013)
16. S.Yeninas,M.A.Tanatar,C.Strehlow,J.Murphy,O.E.Ayala-Valenzucla,R.
D.McDonald,U.Welp,W.K.Kwok,T.Kobayashi,S.Miyasaka,S.Tajima,and
R.Prozorov,Phys.Rev.B,87,094503(2013)
vii
viii Partsofthisthesishavebeenpublishedinthefollowingjournalarticles
17. T.Dulguun,H.Mukuda,T.Kobayashi,F.Engetsu,H.Kinouchi,M.Yashima,
Y. Kitaoka, S. Miyasaka, and S. Tajima, Phys. Rev. B 85, 144515 (2012)
18. T. Kobayashi, S. Miyasaka, and S. Tajima, J. Phys. Soc. Jpn. Suppl. B 81,
SB045 (2012)
Acknowledgements
Firstandforemost,IwouldliketoexpressmysincerestgratitudetoProf.S.Tajima,
for her guidance, discussions, suggestions, support, and patience throughout this
research.
I thank Prof. S. Miyasaka for his guidance, discussions, suggestions, and
encouragement in the course of this work.
I also wish to thank Prof. T. Masui, Prof. K. Tanaka, and Prof. M. Nakajima
for their experimental support and discussions.
Additionally, I would like to appreciate the following individuals: Professor
N.Chikumoto(EPMAmeasurement);Prof.H.MukudaandProf.Y.Kitaoka(NMR
measurement); Prof. T. Nakano and Prof. Y. Nozue (specific-heat measurement);
Prof. T. Kida and Prof. M. Hagiwara (high-magnetic-field measurement); Prof.
A. Fujimori (photoemission measurement); Prof. Y. Murakami, Prof. R. Kumai,
Prof. H. Nakao, and Prof. Y. Wakabayashi (x-ray diffraction measurement);
Dr. A.Q.R. Baron and Dr. T. Fukuda (inelastic x-ray measurement); Prof. S. Suga
(STM measurement); Prof. M. Ashida and Prof. M. Ichimiya (SEM-EDX mea-
surement); andProf. R. Prozorov (penetration-depthmeasurement).
I thank all the members of Tajima group for miscellaneous help. Particularly,
I am grateful to W. Hirata, who taught me everything from the basics.
Finally, I would like to express special gratitude to my dear family for fully
trusting my decisions and providing support over the years.
ix
