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Yukio Watanabe Last modified date:2021.11.01

Professor / Condensed Matter Physics
Department of Physics
Faculty of Sciences


Graduate School
Undergraduate School


Homepage
https://kyushu-u.pure.elsevier.com/en/persons/yukio-watanabe
 Reseacher Profiling Tool Kyushu University Pure
http://www.sci.kyushu-u.ac.jp/e/departments/phys/labo/quantum.html#quantum-a
Quantum Nanophysics A .
Academic Degree
Ph.D University of Tokyo (1991)
Country of degree conferring institution (Overseas)
No
Field of Specialization
Solid state physics
Total Priod of education and research career in the foreign country
02years08months
Outline Activities
Major achievements

1. Industrialized in Mitsubishi Chemical: magnetooptical disk (once 70% of world share) as a principal researcher in basic and designing phase.
(The first papers for this products from Mitsubishi Chemical: Y. Watanabe, J. Sasaki, Y. Kobayashi and T. Yoshitomi, IEEE Trans. Mag-23, 2623(1987)). Y. Watanabe et al, J. Magn. Soc. Jpn. 11, Suppl., 313-316 (1987)
U.S. Patent No. 4743502 (Also EU, Germany, Canada,France, Britain)


2. Industrialized in Mitsubishi Chemical : coal-tar based carbon fiber (As proposer of the principle strength( Contrary to the general belief, I proposed to suppress the crystallization of graphite plane in order to distribute stress. For this, I also proposed to let coal-tar go through narrow holes during spinning.)


3. First ferroelectric field effect conduction modulation with memory retention.
In US, Ahn et al (Science,1995) is often regarded as the first of this kind. But, it was submitted after our 2 papers were published.
US patent was awarded to this invention and was applied to almost all perovskite oxide systems including Ahn's and Mathews's (Science,1997).

Y. Watanabe, Field effect transistor with perovskite oxide channel United State Patent 5418389 (Priority date:1992 11 9, Grant:1995, May) citation 158 times
Y. Watanabe, Epitaxial all-perovskite ferroelectric field effect transistor with a memory retention, Appl. Phys. Lett.66, 1770(1995,Apr) cited 171 (Google Scholar, 2020)

Y. Watanabe et al., Ferroelectrics/(La,Sr)2CuO4 epitaxial hetero-structure with high thermal stability, Appl. Phys. Lett.66, 299(1995, Jan) cited 21 (Google Scholar, 2020)

Y. Watanabe et al., Memory retention and switching speed of ferroelectric field effect in (Pb,La)(Ti,Zr)O3/La2CuO4:Sr heterostructure, Jpn. J. Appl. Phys. 35(2B), 1564-1568 (1996) cited 41 (Google Scholar, 2020)


4. First nondestructive writing & reading in perovskite oxides with appropriate mechanism
The first report is by Blom et al(Phys. Rev. Lett.) but with an inappropriate mechanism from a present viewpoint.
Our paper appeared one month behind but proposed mechanisms including injection and oxygen ordering, which are the bases of present R-RAM.

Y. Watanabe et al., Ferroelectric/(La,Sr)2CuO4 epitaxial hetero-structure and hysteretic diode property,Physica C235-240, 739(1994,Nov) cited 22 (Google Scholar)

Y. Watanabe, A reproducible memory effect in the leakage current of epitaxial ferroelectrics/conductive perovskite hetero-structures Appl. Phys. Lett.66, 28(1995, Jan) cited 84 (Google Scholar, 2020)

Y. Watanabe et al., Recurrent local resistance breakdown of an epitaxial BaTiO3 /SrTiO3 heterostructure, Appl. Phys. Lett.72 (19), 2415-2417(1998) cited 32 (Google Scholar, 2020)

Okano, Watanabe, Nonvolatile programmable two-terminal diodes using ferroelectric semiconductor,  Appl. Phys. Lett.76 (2), 233-235 (2000). cited 26 (Google Scholar, 2020)

Y. Watanabe, Review of Resistance Switching of Ferroelectrics and Oxides in Quest for Unconventional Electronic Mechanisms, Ferroelectrics 349,190–209 (2007)    cited 44 (Google Scholar, 2020)

Y. Watanabe et al., Current-driven insulator–conductor transition and nonvolatile memory in chromium-doped SrTiO3 single crystals, Appl. Phys. Lett. 78, 3738 (2001) cited 657 (Google Scholar, 2020)


5. First discovery of surface conduction at the polar discontinuity of ferrroelectric (e.g., surface)
The first discovery of the conduction at the interface of insulators due to polar discontinuity is usually regarded as SrTiO3/LaAlO3 by Ohtomo and Hwang (Nature, 2004). However, 3 years before, we presented the similar conduction based on our theory below.

Y. Watanabe et al., Surface conduction on insulating BaTiO3 crystal suggesting an intrinsic surface electron layer, Phys. Rev. Lett. 86, 332(2001) cited 124 (Google Scholar, 2020)

In Fig.1 of this paper, we showed also the conduction at charged domain boundaries (domain boundary conduction), which is recently published in many Nature-related journals (2008-2017).


6. A new principle for depolarization based on excitation of electron and holed by this field. This predicted a significant reduction of constraints posed by depolarization field and the formation of electron and hole layer, which is recently reported in many papers as mentioned above.

Thesis: Y. Watanabe, Theoretical stability of the polarization in a thin semiconducting ferroelectric, Phys. Rev. B57, 789(1998) cited 145 (Google Scholar 2020)

Y. Watanabe, D. Sawamura, Thermodynamic Stability of the Spontaneous Polarization and the Space Charge layer in Ferroelectric/Semiconductor Heterostructures, Jpn. J. Appl. Phys. 36(9B) 6162-6166 (1997) (cited 19 Google Scholar, 2020)

Y. Watanabe, A. Masuda, Theoretical Stability of Polarization in Metal/Ferroelectric Insulator /Semiconductor and Related Structure, Jpn. J. Appl. Phys. 40(9B) 5610-5614(2001) (cited 33, Google Scholar, 2020)

Antithesis showing the necessity of the thesis: Y. Watanabe, Theoretical stability of the polarization in insulating-ferroelectric/semiconductor structures, J. Appl. Phys. 83, 2179(1998);Erratum: J. Appl. Phys. 84, 3428 (1998) cited 52 (Google Scholar,2020)

In this antithesis, we showed an abrupt nanodomain formation by slight increase of insulating layer,
which is used by Bratkovsky and Levanyuk (2001) and is believed until now(2017). However, these thin nanodomain had free energy higher than its non-ferroelectric phase and we concluded that this state was unphysical and suggested the necessity of a new theory (the thesis).


7. Theoretical prediction of bulk rectification and space-charge limited conduction with J-Vn (1
Y. Watanabe, Unidirectional bulk conduction and the anomalous temperature dependence of drift current under a trap-density gradient, Phys. Rev. B 81,195210-1-14 (2010).
Y. Watanabe, Unidirectional current flow due to nonlinear bulk conduction under trap density gradient, J. Phys. Soc. Jpn.78, 104712-1-10 (2009)


8. First discoveries of pn junction in transit metal oxides, tunneling pn junction in transit metal oxides, and photovoltaic effectin transit metal oxides.

Y. Watanabe, Electrical transport through Pb(Zr,Ti)O3 pn and pp heterostructures modulated by bound charges at a ferroelectric surface: Ferroelectric pn diode, Phys. Rev. B 59, 11257(1999)         cited 179 (Google Scholar, 2020)

Y. Watanabe, Tunneling current through a possible all-perovskite oxide pn junction, Phys. Rev. B57, R5563 (1998) cited 101 (Google Scholar,2020)

Y. Watanabe and M. Okano, Photodiode properties of epitaxial Pb(Ti,Zr)O3/SrTiO3 ferroelectric heterostructures, Appl. Phys. Lett.78, 1906(2001). cited 34 (Google Scholar, 2020)



1980 Univ Tokyo Faculty of Science BS
1982 Univ Tokyo Faculty of Science MS Institute of Space and Astronautical Physics (at present JAXA)
1982-1995 Mitsubishi Chemical researcher- principal researcher
1983-1984 Kyoto Univ. Dept. of electrical engineering visiting scientist
1987-1989 Princeton Univ. D.C. Tsui (1998 nobel prize) visiting staff
1991 Univ. Tokyo visiting scientist
1995-2001 Kyushu Institute of Technology associate professor
1998 Bell lab(Murray Hill) Physics department consultant
2000 IBM Zurich Physics department visiting professor (host:Dr. J. G. Bednorz 1987 nobel prize)
2001-      Kyushu Univ. Physics department  professor
2004.4-2007.3 The Japan Society for the Promotion of Science  program officer
2007.4-2008.3  Toyotariken external researcher
Research
Research Interests
  • nanoscopic quantum physics, Physics of ferroelectrics and insulators
    keyword : nanoscopic, quantum physics,ferroelectrics, insulators, surface
    1995.08Solid state physics.
Academic Activities
Books
1. Yukio Watanabe, Ferroelectric Thin Films - Basic and Device Physics in FerroelectricThin Films for Memory Application (Topics in Applied Physics 98), Springer, Physics of ferroelectric interface: Attempt at nanoferroelectric physics, 2005.06, (citation 5 2014.1).
Reports
1. Yukio Watanabe, Review of Resistance Switching of Ferroelectrics and Oxides in Quest for Unconventional Electronic Mechanisms    (被引用44), Ferroelectrics 349,190–209 (2007), Google Scholar による被引用回数は47回(2021年), 2007.04, Resistance switching effects in perovskite oxides, which acquire recently intense interests for the application to the resistance-random-access memories, are reviewed with emphasis on paraelectric and ferroelectric oxides. We examine whether the switching mechanism is the electric field effect or the current injection effect as well as the in nanometer scale writing/reading/erasing. Based on these examinations, we search for the traces of unconventional electronic switching, as opposed to thermal and defect mechanisms that are considered responsible for the most results in the past..
2. Y. Watanabe, Study of an epitaxial ferroelectrics /semiconductor /insulator structure using perovskite oxides for application to the field effect transistor, Extended Abstracts of Solid State Device and Materials conf.94 SSDM'94, (Yokohama, 1994,Aug), pp784-786, 1994.08.
Papers
1. Y Watanabe, Examination of Permittivity for Depolarization Field of Ferroelectric by Ab initio Calculation, Suggesting Hidden Mechanisms, Scientific Reports, 10.1038/s41598-021-81237-0, 1, 2155, https://rdcu.be/ceaeW, 2021.01, [URL], Electrostatics of depolarization field Ed in relation to the polarization is studied. In particular, the value of permittivity for Ed (εd) in prototypical situations of ferroelectrics, including Mehta formula, is examined by ab initio calculations. By using spontaneous polarization PS corresponding to accurate experiment ones, we show εd = 1, which suggests that the results of εd ≫ 1 indicate hidden mechanisms; εd = 1 suggests that the effect of Ed is significant to induce intriguing important phenomena overlooked by εd ≫ 1. A bridge between εd = 1 and εd ≫ 1, i.e. the consistency of εd = 1 with conventional results is presented. The exact electrostatic equality of head-to-head–tail-to-tail domains to free-standing ferroelectrics is deduced. Hence, most stoichiometric clean freestanding monodomain ferroelectrics and head-to-head–tail-to-tail domains are shown unstable regardless of size, unless partially metallic. This verifies the previous results in a transparent manner. This conclusion is shown consistent with a recent hyperferroelectric LiBeSb and “freestanding” monolayer ferroelectrics, of which origin is suggested to be adsorbates. In addition, this restriction is suggested to break in externally strained ultrathin ferroelectrics. The macroscopic formulas of Ed are found valid down to a several unit-cells, when electronic and atomic-scale surface effects are unimportant and accurate PS is used..
2. Y Watanabe, Breakdown of ion-polarization-correspondence and born effective charges: Algebraic formulas of accurate polarization under field, Physical Review Materials, 10.1103/PhysRevMaterials.4.104405, 4, 104405, 2020.10, [URL], Polarization, especially of ferroelectrics FEs, is conventionally described by ion positions, e.g., by Born
effective charges, where the complete entanglement of electron polarization with that of ions is implicitly assumed. We find that such descriptions or Born effective charge polarization-type approaches break down partially in the presence of high field, owing to the partial disentanglement of electrons with ions. To overcome this, we propose a correction (non-Born effective charge polarization) that calculates both macroscopic and unit-cell-by-unit-cell total polarization accurately. The accuracy of this method is demonstrated in prototypical situations of depolarization field Ed that exists in finite-size or inhomogeneous insulating FEs: paraelectric/FE, FE capacitors, and FE/vacuum. Here, FE/vacuum are shown to be electrically identical to encountering domains. This method provides simple algebraic formulas to calculate total polarization PS and Ed using conventionally estimated polarizations that are obtained from local ion positions. Therefore, it can be easily used in experimental estimations of PS and Ed , including 3D cases. For example, this method reveals that PS varies across ferroelectric/insulator far less than the conventional estimate, which explains substantially reduced Ed and the absence of metallicity. In addition, vortexlike domains are discussed in view of Ed . The partial disentanglement of ion and electron polarization would imply limitation of Ginzburg-Landau framework of ferroelectrics under high field..
3. Y Watanabe, Ferroelectricity of stress-free and strained pure SrTiO3 revealed by ab initio calculations with hybrid and density functionals, Physical Review B, DOI: 10.1103/PhysRevB.99.064107, 99, 064107-1-064107-14, 2019.02, [URL], The properties of stress-free and biaxially strained stoichiometric SrTiO3 in the absence and presence ofantiferrodistortive (AFD) distortion were calculated ab initio. To obtain reliable results, multiple exchangecorrelation (XC) functionals, including the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, were used. HSEwas the primary XC functional, while another functional provided a good lower bound of ferroelectricity (FE).The reliability of the calculations was further reinforced by the calculations of the strain and AFD dependence.In contrast with previous works, we show that the ferroelectric phase (C14v, C104v ) is more stable in the absenceof quantum and thermal fluctuations than the paraelectric phase (Oh1, D184h ), even for the stress-free case, andclarify the properties of these FE phases. The energy gain of stress-free FE, in comparison with the thermaland quantum fluctuation energy, indicates that a paraelectric phase emerges at room temperature by thermalfluctuations, but is near 0 K marginally close to the FE phase, which aligns with the experimental incipientFE. This implies that the paraelectricity of stress-free SrTiO3 in experiments contains incoherent atomic-scaleFE regions. These results are consistent with the FE microregions (FMR), signatures of polar disorders, andthe emergence of FE in SrTiO3 due to impurities and defects. The value of spontaneous polarization PS couldreach 10 μC/cm2 in the absence of fluctuations, even for the stress-free case. In view of the earlier theoryof the carrier layer at polar discontinuities, the present results may explain the conduction at the interfacesof the LaAlO3/SrTiO3. In addition, an “enhancement of FE due to symmetry constriction” is proposed as anadditional mechanism to the strain-enhanced FE in the epitaxial effect. For large compressive strain, e.g., 2%,the Perdew-Burke-Ernzerhof density functional for solids (PBEsol) yielded properties with PS > 20 μC/cm2,agreeing with HSE, and therefore is usable as a practical substitute of HSE for SrTiO3..
4. Y. Watanabe, Calculation of strained BaTiO3 with different exchange correlation functionals examined with criterion by Ginzburg-Landau theory, uncovering expressions by crystallographic parameters, Journal of Chemical Physics , DOI: 10.1063/1.5022319, 148, 194702-1-194702-15, 2018.03, [URL], In the calculations of tetragonal BaTiO3, some exchange-correlation (XC) energy functionals such as local density approximation (LDA) have shown good agreement with experiments at room temperature (RT), e.g., spontaneous polarization (PS), and superiority compared with other XC functionals.
This is due to the error compensation of the RT effect and, hence, will be ineffective in the heavily strained case such as domain boundaries. Here, ferroelectrics under large strain at RT are approximated as those at 0 K because the strain effect surpasses the RT effects. To find effective XC energy functionals for strained BaTiO3, we propose a new comparison, i.e., a criterion. This criterion is the properties at 0 K given by the Ginzburg-Landau (GL) theory because GL theory is a thermodynamic description of experiments working under the same symmetry-constraints as ab initio calculations.
With this criterion, we examine LDA, generalized gradient approximations (GGA), meta-GGA, meta-GGA + local correlation potential (U), and hybrid functionals, which reveals the high accuracy of some XC functionals superior to XC functionals that have been regarded as accurate. This result is examined directly by the calculations of homogenously strained tetragonal BaTiO3, confirming the validity of the new criterion. In addition, the data points of theoretical PS vs. certain crystallographic parameters calculated with different XC functionals are found to lie on a single curve, despite their wide variations. Regarding these theoretical data points as corresponding to the experimental results, analytical expressions of the local PS using crystallographic parameters are uncovered. These expressions show the primary origin of BaTiO3 ferroelectricity as oxygen displacements. Elastic compliance and electrostrictive coefficients are estimated. For the comparison of strained results, we show that the effective critical temperature TC under strain <0.01 is >1000 K from an approximate method combining ab initio results with GL theory. In addition, in a definite manner, the present results show much more enhanced ferroelectricity at large strain than the previous reports..
5. Y. Watanabe, Unidirectional bulk conduction and the anomalous temperature dependence of drift current under a trap-density gradient, Phys. Rev. B, 81, 195210, 2010.05,

Nonlinear drift conduction under a trap-density gradient is mathematically formulated. Semianalytical and numerical solutions demonstrate bulk-induced unidirectional current flow, i.e., rectification. The present theory is in excellent agreement with various experimental J-V characteristics J: current density and V: applied voltage. At low V, the J-V characteristics are ohmic and bidirectional. As the injection increases, the J-V characteristics become nonlinear and exhibit unidirectionality under proper conditions. The major requirements for a large unidirectionality are the trap-density gradient G1, an intermediate V, and not too large trap-filling factor , which requires the presence of acceptorlike traps. The unidirectional J-V characteristics due to the difference in trap-filled-to-trap-free-limit voltage VTFL for forward and reverse bias markedly resemble the standard rectification. In addition, the trap-density gradient yields a positive T dependence of resistance for a proper set of parameters, evident JV1.5 characteristics, and a photovoltaic effect. The present results suggest that bulk conduction under trap-density gradient explains fractions of resistance switching and rectification phenomena. The semianalytical solutions are verified by numerical solutions and comparison with experiments. In particular, semianalytical solutions for shallow-trap case excellently fit the experimental data by three parameters in practice: two scaling factors and G.
.
6. Y. Watanabe, Review of Resistance Switching of Ferroelectrics and Oxides in Quest for Unconventional Electronic Mechanisms, Ferroelectrics 349,190-209 (2007) (被引用47, Google Scholar (2021)), 2007.04.
7. M. Okano, Y. Watanabe, S. W. Cheong , Nonlinear positive temperature coefficient of resistance of Schottky contact on strained epitaxial BaTiO3 film,  Appl. Phys. Lett. 82(12), 1923-1925 (2003) (被引用 36, Google Scholar, 2020), 10.1063/1.1563061, 82, 12, 1923-1925, 2003.03.
8. Y. Watanabe, M. Okano, Photodiode properties of epitaxial Pb(Ti,Zr)O3/SrTiO3 ferroelectric heterostructures,  Appl. Phys. Lett.78(13), 1906-1908 (2001) (被引用 34 Google Scholar, 2020), 2001.03.
9. Y. Watanabe, M. Okano, A. Masuda, Surface conduction on insulating BaTiO3 crystal suggesting an intrinsic surface electron layer,  Phys. Rev. Lett. 86 (2), 332-335 (2001). (被引用 127 Google Scholar, 2021), https://doi.org/10.1103/PhysRevLett.86.332, 2001.01, [URL].
10. M. Okano(修士2年) and Y. Watanabe , Nonvolatile programmable two-terminal diodes using ferroelectric semiconductor,  Appl. Phys. Lett.76 (2), 233-235 (2000). (被引用 26 Google Scholar, 2020), 2000.01.
11. Y. Watanabe, Electrical transport through Pb(Zr,Ti)O3 pn and pp heterostructures modulated by bound charges at a ferroelectric surface: Ferroelectric pn diode,  Phys. Rev. B 59(17)11257-11266(1999). (被引用 184 Google Scholar, 2021), 1999.12.
12. Y. Watanabe, D. Sawamura(修士卒), and M. Okano(修士1年) , Recurrent local resistance breakdown of an epitaxial BaTiO3 /SrTiO3 heterostructure,  Appl. Phys. Lett.72 (19), 2415-2417(1998) (被引用 32 Google Scholar, 2020), 1998.05.
13. Y. Watanabe, Tunneling current through a possible all-perovskite oxide pn junction,  Phys. Rev. B57(10) (Rapid Commun.) R5563-R5566 (1998) (被引用 109 Google Scholar, 2021), 1998.03.
14. Y. Watanabe, Theoretical stability of the polarization in insulating-ferroelectric/semiconductor structures,  J. Appl. Phys. 83 (4), 2179-2193(1998) 被引用 58 (Google Scholar, 2021), Erratum: J. Appl. Phys. 84(6), 3428 (1998) , 1998.02.
15. Y. Watanabe, Theoretical stability of the polarization in a thin semiconducting ferroelectric,  Phys. Rev. B57(2), 789-804 (1998) (被引用 155 Google Scholar, 2021), https://doi.org/10.1103/PhysRevB.57.789, 1998.01, [URL].
16. Y. Watanabe, Epitaxial all-perovskite ferroelectric field effect transistor with a memory retention,  Appl. Phys. Lett.66(14), 1770-1772 (1995) (被引用 178 Google Scholar, 2021), 1995.04.
17. Y. Watanabe, Y. Matsumoto, H. Asami, M.Tanamura, Ferroelectrics / (La,Sr)2CuO4 epitaxial hetero-structure with high thermal stability,  Appl. Phys. Lett.66(3), 299-301 (1995) 被引用 24 (Google Scholar, 2021), 1995.01.
18. Y. Watanabe, A reproducible memory effect in the leakage current of epitaxial
ferroelectrics/conductive perovskite hetero-structures,  Appl. Phys. Lett.66(1), 28-30 (1995) (被引用 86 Google Scholar, 2021), 1995.01.
19. Y. Watanabe, Ferroelectric/ (La,Sr)2CuO4 epitaxial hetero-structure and hysteretic diode property, Physica C235-240, 739-740 (1994) (被引用 24 Google Scholar, 2021), 1994.12, [URL].
20. Y. Watanabe, Y. Matsumoto, H. Kunitomo, M. Tanamura, E. Nishimoto, Crystallographic and electronic properties of epitaxial BaTiO3 film grown on conductive and insulating perovskite oxides,  Jpn J. Appl. Phys. 33(9B), 5182-5186 (1994) (被引用 81 Google Scholar, 2021), 1994.09, [URL].
21. Y. Watanabe, D.C. Tsui, J.T. Birmingham, N.P. Ong, J.M. Tarascon, Infrared reflectivity of single-crystal Bi2Mm+1ComOy (M= Ca, Sr, Ba; m=1, 2), Bi2Sr3Fe2O9.2 and Bi2Sr2MnO6.25 isomorphic to Bi-Cu-based high-Tc oxides,  Phys. Rev. B 43(4), 3026-3033 (1991). (被引用 37 Google Scholar, 2021), 1991.11.
22. Y. Watanabe, Z.Z. Wang, S.A. Lyon, S.A. Lyon, D.C. Tsui, N.P. Ong, J.M. Tarascon, P.Barboux, Mid-Infrared reflectivity and ellipsometry measurements on single crystal YBa2Cu3O7 and Bi2Sr2CuO6+y,  Phys. Rev. B 40(10), 6884-6889 (1989). (被引用 36 Google Scholar, 2020), 1989.10.
23. Y. Watanabe, J. Sasaki, Y. Kobayashi and T. Yoshitomi, Magneto-optical disc with alumina based oxide layer,  IEEE Trans. Mag-23(5), 2623-2625, 1987.11.
24. Y. Watanabe, T. Terasawa, On the excitation mechanism of the low-frequency upstream waves, J. Geophys. Res. 89(A8), 6623- 6630 (1984). (被引用 33 Google Scholar, 2021), 1984.08.
25. Y. Watanabe, A. Nishida, Field line distortion and Joule heating by the Corotation Enforcement Currents in the Jovian Magneto-Ionosphere, J. Geophys. Res. 87(A10), 8111-8117 (1982)., 1982.10.
Presentations
1. Y. Watanabe, Renovation of Electrostatics of Ferroelectrics Liberating Ferroelectricity from Established Restrictions (招待講演), 14th European Meeting on Ferroelectricity(EMF) (2019 IEEE ISAF-ICE-EMF-IWPM-PFM Joint Conference内), 2019.07, Ultimate smallest sizeand domainconfigurationsof ferroelectricshave beenconsideredto be severely restricted by the electric field from the spontaneous polarization (PS).This has been a conventional principlefor nano-ferroelectrics, insulator/ferroelectric interfacesin thin film applications, ferroelectric surface, and some kinds of domains (Fig.1), imposing fundamental limitations.

Decades ago, we showed for the first time1
1) spontaneous formation of a free electron/hole (e-/h+) layer generally at thediscontinuity of PS(Fig.1),
2) its thermodynamic meta-stability, and
3)its drastic impact on the reduction of the aforementioned restrictionsand limitations.

Thepredictions 1)2)1were proved by the firstexperimental discovery of this free e-/h+layer2and are supported by numerous recent works reporting possiblee-/h+layer at domain boundariespresumably due to PS

The extreme resistanceagainst the size limitationsgiven by the e-/h+ layer1mentioned in 3)also begins to be confirmed by ab initio theories3,4in specialferroelectrics named as “hyper-ferroelectrics”.3,4

We will show that prototypical ferroelectrics BaTiO3and PbTiO3exhibit hyper-ferroelectricity by ab initiocalculation for the first time, based on an extensive study of functionals.5,6

Furthermore, we will present an easy-to-use-by-everybody approximate material-independent algebraic formula to predict the ultimatesize, which will be examined by ab initiocalculations and experiments. This shall show that hyper-ferroelectricityis a propertyof generalferroelectrics.

Using these results, we will clarify the origins of instabilities of encounteringdomains and their e-/h+layer,considering also charge injection and defect formation as observed in resistance-switching in general metal oxides.

In addition, the renowned electron layer at LaAlO3/SrTiO3will be shown to be significantly contributed by the present mechanism.7

References
[1]Y. Watanabe, Phys. Rev.B57, 789 (1998); Y. Watanabe and A. Masuda, Integrated Ferroelectrics27, 51 (1999).
[2]Y. Watanabe et al., Phys. Rev. Lett.86, 332 (2001).
[3] K. Garrity et al.,Phys. Rev. Lett. 112, 127601 (2014).
[4]S. Liu andR. E. Cohen, J. Phys. 29, 244003 (2017).
[5] Y. Watanabe, J. Chem. Phys.148, 194702 (2018),
[6]Y. Watanabe, Comp. Mat.Sci.158, 315 (2019),
[7] Y. Watanabe, Phys. Rev.B 99, 064107 (2019)..
2. Conduction at domain-boundaries, ferroelectric/insulator like SrTiO3/LaAlO3 (STO/LAO), and free surface is a hot topic in physics of ferroics. Although, as predicted*, these conductions are often attributed to polarization discontinuity of spontaneous polarization Ps, the importance of the extrinsic origins as defects, impurities, and field-induced defects have been established, and emphasized by Szot and Roleder.**

Indeed, these mechanisms exist in most of conduction phenomena. In STO/LAO the defects and intermixtures are unavoidable because of the growth at high temperature in vacuum. In the low-mobility and artificially-formed domain-boundaries, the pinning by defects and high-field-induced defects should exist, respectively; the domain boundaries are formed along the path of the high field. Moreover, most of the clean surfaces of metal oxides are prepared through the annealing in vacuum that forms defects such as oxygen vacancy.

On the other hand, the Ps–induced conduction at domain boundaries and ferroelectric/insulator boundary was earlier predicted* to show its important influence on size effect and domains. Therefore, the rigorous examinations whether Ps is the dominant origin of these conductions are of primary importance. The intrinsicness of Ps–induced conduction in the view of sample preparation, measurement methods and conduction characteristics will be discussed together with their implications.
* Watanabe,PRB57,789(1998);PRL 86,332(2001),**IMF Abst. 245 (2017)..
3. Y. Watanabe, D. Matsumoto, Y. Urakami and M. Okano, Polarization-Discontinuity Conductions at Domain Boundaries, Interfaces & Surfaces: Unification and Implications for Domain, Size Effects & Hyper-ferroelectricity   (招待講演)        , 2018 ISAF-FMA-AMF-AMEC-PFM Joint Conference IFAAP2018, 2018.05,
Conductions at charged domain boundaries and the interface of ferroelectrics are much interested [1-4]. The conduction at the interface of an incipient ferroelectric SrTiO3(STO) and LaAlO3(LAO), are likely due to adjacent chemical doping [5], and defects [5,6] assisted by ferroelectric polarization [7-9]. Here, the metallization of STO by La doping [10,11] or oxygen vacancy [6] and modulation doping by adjacent layer [12] were established long ago, and intediffusion and oxygen vacancy are unavoidable when formed in vacuum at high temperature [5].
Therefore, both conductions at domain boundaries and STO/LAO are considered as polarization-discontinuity conductions or polarization-discontinuity-assisted conductions predicted theoretically and experimentally (Fig. 1) [13, 14]. The application of these conductions to electronics is exceptionally challenging owing to the competitions with other candidates like resistance switching [15] and Si that can have higher electron/hole mobility than STO/LAO at 4 K. However, if these conductions are intrinsic, i.e., due substantially to the polarization-discontinuity, they imply the existence of intrinsic screening in ferroelectric metal oxides that changes domains and size effects [14]. The necessity of this theory [14] was also shown through a conventional framework that showed the nano-domain formation in electrodeless ultrathin ferroelectric and its instability [16], which was used later by Bratkovsky-Levanyuk for nanodomain by fatigue [17].
To date, ohmic bipolar conduction at low applied voltage in initially high resistive ferroelectrics has been absent in conductions at domain boundaries and STO/LAO, indicating the extrinsicness such as defects and current injection by too large applied field [15]. Therefore, our presentation focuses on the properties of nominally stoichiometric clean surfaces of a high-purity BaTiO3 single crystals in ultrahigh vacuum as an idealized model of these conductions, especially, of a ferroelectric/insulator interface. The choice of BaTiO3 is because of a low coercive field to avoid high fields and a far lower defect concertation than Pb- and Bi-based ferroelectrics such as PbTiO3 and BiFeO3, which is essential for assuring intrinsicness. These surfaces exhibit ohmic bipolar conduction at low applied voltage with initially high resistivity and domain properties different from conventional views. Their implications will be discussed..
4. Yukio Watanabe, H. Nakahara(学部卒), S. Kaku(博士卒), D. Matsumoto(博士卒), S.-W Cheong, Natural domains of BaTiO3 in ultra-high vacuum, air & acid: properties & invariant domain-size proving intrinsic screening & a Review of polarization induced conduction  (招待講演), International Meeting on Ferroelectricity (IMF14), 2017.09,
Conduction at SrTiO3/LaAlO3[1] is intensively studied. Before this, conduction at clean surface of fully oxygenated BaTiO3 in UHV (ultrahigh vacuum) was reported.[2] This verified also conduction at charged domain boundaries and dielectric/ferroelectric interfaces.[2] These conductances are too small for electronics but are fundamental for domains, because they screens depolarization field.[3]

Using nm-scale and macroscopic probes, we’ll clarify static and dynamic properties of natural domains of unpolished atomically flat surface of fully oxygenated stoichiometric BaTiO3 single crystal in UHV, air, and acid, which will be explained by DFT/GL calculations.

We show experimentally that the electrostatic domain patterns agree excellently with piezoelectric force response images (PFM) nearly complete screening of electric field from domains of atomically cleaned surface in UHV without extraneous screening.

Our experiments find conduction of both +PS surface and -PS surface. Our calculations show the location of each conduction is titanium of one unitcell deeper than top surface and the outermost oxygen, respectively.

The domain patterns in UHV, air and acid are found mutually very similar. In particular, we find a perfect agreement of domain patterns obtained by etching and PFM for the first time and invariance of domains of air in acid.

A critical finding is that the domain size/width in UHV, air, and acid was the same, despite 100-times difference of permittivity (screening efficiency) between UHV and acid.

These results verify intrinsic screening of depolarization field is inside ferroelectric, consistently with the evidence by conduction.[2] This screening explains the absence of natural vortex domains on free surface as well as properties of nano-ferroelectrics such as compulsory nano-domain.

1. Ohtomo-Hwang, Nature427(2004), 2. Watanabe et al, PRL86(2001), 3. Watanabe, PRB57(1998)..
5. Yukio Watanabe, Y. Urakami(修士卒), D. Matsumoto(博士卒), S. Kaku(博士卒), S.-W Cheong, G.A. Thomas, S. Miyauchi(修士卒), Novel Electrical Conduction of Oxide Insulators under examination of Defects & Injection & Relationship to theories of Ferroelectric Domain  (招待講演)                , Materials Research Society (MRS) 2014 Spring Meeting, 2014.04,

As ferroelectric is miniaturized, it faces extreme conditions such as enormous depolarization field Ed and surface effects, of which intrinsic properties are unclarified. PS-field-effect (field effect by spontaneous polarization PS) may exist in such condition, relaxing the restrictions by Ed. However, it is usually regarded as extrinsic and is neglected in theories of nano-ferroelectrics and domains.
This is reasonable, because reported PS-field-effect may be due to (1) defects/impurities, especially, oxygen vacancies, and (2) electron/hole injection, which are examined in this talk. For example, resistance switching in Schottky [1-3], pn and tunnel [4] diodes using ferroelectrics including nano-scale ones [5] is often attributed to (1)(2) or PS. Similar switching in ordinary insulators (R-RAM), attributed to (1)(2), indicates that (1)(2) are unavoidable in 2-terminal structures. Indeed, the mechanism, PS [1,2] or injection/oxygen,[3] has been controversial until now. Here, rigorous treatment of current-voltage characteristics is crucial, and those assigned to PS [1] are precisely explained by injection.[6]
These difficulties (1)(2) exist also in PS-field-effect on the conductance perpendicular to PS, i.e., transconductance in 3-terminal. It is noted that the mechanisms (1)(2) exists in MOSFET, the most well-controlled clean system. Indeed, the mechanisms (1)(2) have been unavoidable since the proposals of PS-field-effect transconductance in 1957,[7] and transconductance modulation with retention mainly by PS were achieved after introduction of perovskite-oxide semiconductors.[8-10] For these reasons, this talk examines extrinsic effects (1)(2) including PS-induced modulation doping.
Nonetheless, it reports convincing evidences that self-PS-field-effect [11-14] exists in ferroelectric chemically same as that in air. Moreover, although imperfections cannot be reduced be below thermodynamic limit in O2 (samples contain impurities and defects of a few ppm), the analyses show that this effect is intrinsic and exists in ideal ferroelectrics without imperfections. These conclusions are drawn from transconductance in combination with nanoscopic measurements in UHV. To this end, the choice and preparation of samples are the most essential: BaTiO3 single crystals are used.
[1] T. Choi et al., Science324, 63(2009). [2] P. Blom et al., PRL73, 2107(1994). [3] Y.W. et al.,APL66, 28 (1995); Physica C235, 739(1994). [4] Y.W., PRB57, R5563(1998); JAP94, 7187(2003). [5] Y.W., APL72, 2415(1998). [6] Y.W., PRB81, 195210(2010). [7] C. Seager et al., Integrated Ferroelectr.6, 47 (1995). [8] Y.W.,  U.S. Patent 5418389(May, 1995). [9] Y.W. APL66, 1770(April,1995); Ext. Abst. SSDM’94, 784 (1994). [10] C. Ahn et al., Science269, 373 (July,1995). [11] M. Krômar and C. Fu, PRB 68, 115404(2003). [12] R. Cohen, Ferroelectr. 194, 323(1997). [13] Y.W. et al, Jpn. J. Appl. Phys. 36, 6162(1997); PRB 789 (1998). [14] Y.W. et al., PRL 86, 332(2001)..
6. Y. Watanabe, S. Kaku(博士卒), S. Miyauchi(修士卒), S.-W Cheong(Rutgers Univ), D. Matsumoto(博士卒) and Y. Urakami(修士卒), Statics and Dynamics of cc and ac Domains of Virtually Intrinsic Ferroelectric in Clean Limit (招待講演), International Meeting on Ferroelectricity (IMF13), 2013.09.
7. Xian .
8. Lithuania.
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Mechanics, Electrodynamics and thermodynamics, Solid State Physics III, Electrodynamics II, Quantum Physics of atoms and molecules, Advanced Semiconductor Physics, Elementary Excitations, Mathematics for Physics I
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