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Jeong, J.,Oh, D.,Song, D.,Eisaki, H.,Kim, C.,Park, S.R. The Korea Institute of Applied Superconductivity a 2019 한국초전도저온공학회논문지 Vol.21 No.4
We performed Raman spectroscopy on two different over-doped Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+x</sub> (BSCCO), of which superconducting transition temperatures are 89 K and 77 K. Line-shape analysis of the Raman-spectrum was done, focused on B<sub>1g</sub> bond buckling mode which have drawn a lot of attention, since photoemission studies showed an evidence for strong coupling between the mode and electron. The line-shapes show asymmetry and are well fitted by the Fano line-shape formula. Remarkably, we found that the peak line-widths from B<sub>1g</sub> bond buckling mode in BSCCO show much broader than those in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-x</sub>. The broad line width can be attributed to the superstructure modulation of BSCCO. Our results imply that B<sub>1g</sub> bond buckling mode may have close relation to the origin of superconductivity or to boosting the superconducting transition temperature in BSCCO.
Hamidian, M. H.,Edkins, S. D.,Joo, Sang Hyun,Kostin, A.,Eisaki, H.,Uchida, S.,Lawler, M. J.,Kim, E.-A.,Mackenzie, A. P.,Fujita, K.,Lee, Jinho,Davis, J. C. Sé,amus Nature Publishing Group, a division of Macmillan P 2016 Nature Vol.532 No.7599
<P>The quantum condensate of Cooper pairs forming a superconductor was originally conceived as being translationally invariant. In theory, however, pairs can exist with finite momentum Q, thus generating a state with a spatially modulated Cooper-pair density(1,2). Such a state has been created in ultracold Li-6 gas(3) but never observed directly in any superconductor. It is now widely hypothesized that the pseudogap phase(4) of the copper oxide superconductors contains such a 'pair density wave' state(5-21). Here we report the use of nanometre-resolution scanned Josephson tunnelling microscopy(22-24) to image Cooper pair tunnelling from a d-wave superconducting microscope tip to the condensate of the superconductor Bi2Sr2CaCu2O8+x. We demonstrate condensate visualization capabilities directly by using the Cooper-pair density variations surrounding zinc impurity atoms(25) and at the Bi2Sr2CaCu2O8+x crystal supermodulation(26). Then, by using Fourier analysis of scanned Josephson tunnelling images, we discover the direct signature of a Cooper-pair density modulation at wavevectors Q(P) approximate to (0.25, 0)2 pi/a(0) and (0, 0.25)2 pi/a(0) in Bi2Sr2CaCu2O8+x. The amplitude of these modulations is about five per cent of the background condensate density and their form factor exhibits primarily s or s' symmetry. This phenomenology is consistent with Ginzburg-Landau theory(5,13,14) when a charge density wave(5,27) with d-symmetry form factor(28-30) and wavevector Q(C) = Q(P) coexists with a d-symmetry superconductor; it is also predicted by several contemporary microscopic theories for the pseudogap phase(18-21).</P>
Line-shape analysis of the Raman-spectrum from B1g bond buckling phonon in Bi2Sr2CaCu2O8+x
S. R. Park,J. Jeong,D. Oh,D. Song,H. Eisaki,C. Kim 한국초전도.저온공학회 2019 한국초전도저온공학회논문지 Vol.21 No.4
We performed Raman spectroscopy on two different over-doped Bi2Sr2CaCu2O8+x (BSCCO), of which superconducting transition temperatures are 89 K and 77 K. Line-shape analysis of the Raman-spectrum was done, focused on B1g bond buckling mode which have drawn a lot of attention, since photoemission studies showed an evidence for strong coupling between the mode and electron. The line-shapes show asymmetry and are well fitted by the Fano line-shape formula. Remarkably, we found that the peak line-widths from B1g bond buckling mode in BSCCO show much broader than those in YBa2Cu3O7−x. The broad line width can be attributed to the superstructure modulation of BSCCO. Our results imply that B1g bond buckling mode may have close relation to the origin of superconductivity or to boosting the superconducting transition temperature in BSCCO.
Lee Seokbae,Seo Yu-Seong,Roh Seulki,Song Dongjoon,Eisaki Hirosh,Hwang Jungseek 한국물리학회 2022 Current Applied Physics Vol.39 No.-
We introduce an analysis model, an extended Drude–Lorentz model, and apply it to Fe-pnictide systems to extract their electron–boson spectral density functions (or correlation spectra). The extended Drude–Lorentz model consists of an extended Drude mode for describing correlated charge carriers and Lorentz modes for interband transitions. The extended Drude mode can be obtained by a reverse process starting from the electron–boson spectral density function and extending to the optical self-energy and, eventually, to the optical conductivity. Using the extended Drude–Lorentz model, we obtained the electron–boson spectral density functions of K-doped BaFe2As2 (Ba-122) at four different doping levels. We discuss the doping-dependent properties of the electron– boson spectral density function of K-doped Ba-122. We also can include pseudogap effects in the model using this approach. Therefore, this approach is very helpful for understanding and analyzing measured optical spectra of strongly correlated electron systems, including high-temperature superconductors (cuprates and Fe-pnictides).
Kyung, W. S.,Huh, S. S.,Koh, Y. Y.,Choi, K.-Y.,Nakajima, M.,Eisaki, H.,Denlinger, J. D.,Mo, S.-K.,Kim, C.,Kim, Y. K. Nature Publishing Group 2016 NATURE MATERIALS Vol.15 No.12
<P>The superconducting transition temperature (T-c) in a FeSe monolayer on SrTiO3 is enhanced up to 100 K (refs 1-4). High T-c is also found in bulk iron chalcogenides with similar electronic structure(5-7) to that of monolayer FeSe, which suggests that higher T-c may be achieved through electron doping, pushing the Fermi surface (FS) topology towards leaving only electron pockets. Such an observation, however, has been limited to chalcogenides, and is in contrast to the iron pnictides, for which the maximum T-c is achieved with both hole and electron pockets forming considerable FS nesting instability(8-11). Here, we report angle-resolved photoemission characterization revealing a monotonic increase of T-c from 24 to 41.5 K upon surface doping on optimally doped Ba(Fe1-xCox)(2)As-2. The doping changes the overall FS topology towards that of chalcogenides through a rigid downward band shift. Our findings suggest that higher electron doping and concomitant changes in FS topology are favourable conditions for the superconductivity, not only for iron chalcogenides, but also for iron pnictides.</P>
각분해 광전자분석 실험을 이용한 $Sm_{1.82}Ce_{0.18}CuO_4$ 물질의 온도에 따른 가짜 갭 연구
송동준,최환영,김철,박승룡,김창영,Song, D.J.,Choi, H.Y.,Kim, Chul,Park, S.R.,Kim, C.,Eisaki, H. 한국초전도학회 2010 Progress in superconductivity Vol.11 No.2
There are theoretical and experimental evidences for the pseudo-gap in electron doped cuprates being due to interaction between electrons and anti-ferromagnetism(AFM). A remaining issue is on how AFM correlates with pseudo-gap, and eventually with superconductivity. To elucidate the issue, we have performed temperature dependent angle-resolved photoemission studies of an e-doped cuprate superconductor $Sm_{2-x}Ce_xCuO_4$(SCCO) x=0.18 at 20K and 150K. In the case of $Nd_{2-x}Ce_xCuO_4$, the most well known e-doped cuprate, pseudo-gap disappears at around 100 K for x=0.17. Our experimental result reveals that the pseudo-gap of SCCO exists even at 150K for x=0.18. This result implies that the AFM of SCCO survives even in x=0.18, which agrees with previously reported phase diagram of SCCO. Yet, the superconductivity disappears around x=0.18 for both NCCO and SCCO in spite of the difference in the magnetic order. This result sheds a light on the disappearance of superconductivity on the over-doped side.
Simultaneous Transitions in Cuprate Momentum-Space Topology and Electronic Symmetry Breaking
Fujita, K.,Kim, Chung Koo,Lee, Inhee,Lee, Jinho,Hamidian, M. H.,Firmo, I. A.,Mukhopadhyay, S.,Eisaki, H.,Uchida, S.,Lawler, M. J.,Kim, E.-A.,Davis, J. C. American Association for the Advancement of Scienc 2014 Science Vol.344 No.6184
<P><B>Under the Dome</B></P><P>The superconducting transition temperature <I>T<SUB>c</SUB></I> of copper oxides has a dome-shaped dependence on chemical doping. Whether there is a quantum critical point (QCP) beneath the dome, and whether it is related to the enigmatic pseudogap, has been heavily debated. Two papers address this question in two different families of Bi-based cuprates. In (Bi,Pb)<SUB>2</SUB>(Sr,La)<SUB>2</SUB>CuO<SUB>6+δ</SUB>, <B>He <I>et al.</I></B> (p. 608) found that the Fermi surface (FS) undergoes a topological change as doping is increased, which points to the existence of a QCP at a doping close to the maximum in <I>T<SUB>c</SUB></I>, seemingly uncorrelated with the pseudogap. <B>Fujita <I>et al.</I></B> (p. 612) studied a range of dopings in Bi<SUB>2</SUB>Sr<SUB>2</SUB>CaCu<SUB>2</SUB>O<SUB>8+δ</SUB> to find an FS reconstruction simultaneous with the disappearance of both rotational and translational symmetry breaking, the latter of which has been associated with the pseudogap. These findings point to a concealed QCP.</P>
Oxygen-content-dependent electronic structures of electron-doped cuprates
Song, Dongjoon,Park, Seung Ryong,Kim, Chul,Kim, Yeongkwan,Leem, Choonshik,Choi, Sungkyun,Jung, Wonsig,Koh, Yoonyoung,Han, Garam,Yoshida, Yoshiyuki,Eisaki, Hiroshi,Lu, D. H.,Shen, Z.-X.,Kim, Changyoung American Physical Society 2012 Physical review. B, Condensed matter and materials Vol.86 No.14