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ABSTRACT: We report on an ^{75}As-NMR study on the Fe-pnictide high-T_{c} superconductor Y_{0.95}La_{0.05}FeAsO_{1-y} (Y_{0.95}La_{0.05}1111) with T_{c}=50 K that includes no magnetic rare-earth elements. The measurement of the nuclear-spin lattice-relaxation rate ^{75}(1/T_{1}) has revealed that the nodeless bulk superconductivity takes place at T_{c}=50 K while antiferromagnetic spin fluctuations develop moderately in the normal state. These features are consistently described by the multiple fully gapped s_{±}-wave model based on the Fermi-surface nesting. Incorporating the theory based on band calculations, we propose that the reason that T_{c}=50 K in Y_{0.95}La_{0.05}1111 is larger than T_{c}=28 K in La1111 is that the Fermi-surface multiplicity is maximized, and hence the Fermi-surface nesting condition is better than that in La1111.
Physical Review Letters 10/2012; 109(15):157001. · 7.37 Impact Factor
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M. Yashima,
N. Tagami,
S. Taniguchi,
T. Unemori,
K. Uematsu, H. Mukuda,
Y. Kitaoka,
Y. Ota,
F. Honda,
R. Settai,
Y. Onuki
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ABSTRACT: We report on a pressure-induced evolution of exotic superconductivity and
spin correlations in CeIr(In$_{1-x}$Cd$_{x}$)$_5$ by means of
In-Nuclear-Quadrupole-Resonance (NQR) studies. Measurements of an NQR spectrum
and nuclear-spin-lattice-relaxation rate $1/T_1$ have revealed that
antiferromagnetism induced by the Cd-doping emerges locally around Cd dopants,
but superconductivity is suddenly induced at $T_c$ = 0.7 and 0.9 K at 2.34 and
2.75 GPa, respectively. The unique superconducting characteristics with a large
fraction of the residual density of state at the Fermi level that increases
with $T_c$ differ from those for anisotropic superconductivity mediated by
antiferromagnetic correlations. By incorporating the pressure dependence of the
NQR frequency pointing to the valence change of Ce, we suggest that
unconventional superconductivity in the CeIr(In$_{1-x}$Cd$_{x}$)$_5$ system may
be mediated by valence fluctuations.
08/2012;
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ABSTRACT: We report $^{31}$P- and $^{75}$As-NMR studies on
(Ca$_4$Al$_2$O$_{6}$)Fe$_2$(As$_{1-x}$P$_x$)$_2$ with an isovalent substitution
of P for As. We present the novel evolution of emergent phases that the
nodeless superconductivity (SC) in 0$\le x \le$0.4 and the nodal one around
$x$=1 are intimately separated by the onset of a commensurate stripe-type
antiferromagnetic (AFM) order in 0.5$\le x \le$ 0.95, as an isovalent
substitution of P for As decreases a pnictogen height $h_{Pn}$ measured from
the Fe plane. It is demonstrated that the AFM order takes place under a
condition of 1.32\AA$\le h_{Pn} \le$1.42\AA, which is also the case for other
Fe-pnictides with the Fe$^{2+}$ state in (Fe$Pn$)$^{-}$ layers. This novel
phase evolution with the variation in $h_{Pn}$ points to the importance of
electron correlation for the emergence of SC as well as AFM order.
07/2012;
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ABSTRACT: We report 75As-NMR study on the Fe-pnictide high-Tc superconductor
Y0.95La0.05FeAsO_{1-y} (Y0.95La0.051111) with Tc=50 K that includes no magnetic
rare-earth elements. The measurement of the nuclear-spin lattice-relaxation
rate 75(1/T1) has revealed that the nodeless bulk superconductivity takes place
at Tc=50 K while antiferromagnetic spin fluctuations (AFSFs) develop moderately
in the normal state. These features are consistently described by the multiple
fully-gapped s_\pm-wave model based on the Fermi-surface (FS) nesting.
Incorporating the theory based on band calculations, we propose that the reason
that Tc=50 K in Y0.95La0.051111 is larger than Tc=28 K in La1111 is that the FS
multiplicity is maximized, and hence the FS nesting condition is better than
that in La1111.
07/2012;
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ABSTRACT: High-temperature superconductivity (HTSC) in copper oxides emerges on a
layered CuO2 plane when an antiferromagnetic Mott insulator is doped with
mobile hole carriers. We review extensive studies of multilayered copper oxides
by site-selective nuclear magnetic resonance (NMR), which have uncovered the
intrinsic phase diagram of antiferromagnetism (AFM) and HTSC for a
disorder-free CuO2 plane with hole carriers. We present our experimental
findings such as the existence of the AFM metallic state in doped Mott
insulators, the uniformly mixed phase of AFM and HTSC, and the emergence of
d-wave SC with a maximum Tc just outside a critical carrier density, at which
the AFM moment on a CuO2 plane disappears. These results can be accounted for
by the Mott physics based on the t-J model. The superexchange interaction J_in
among spins plays a vital role as a glue for Cooper pairs or mobile
spin-singlet pairs, in contrast to the phonon-mediated attractive interaction
among electrons established in the Bardeen-Cooper-Schrieffer (BCS) theory. We
remark that the attractive interaction for raising the $T_c$ of HTSC up to
temperatures as high as 160 K is the large J_in (~0.12 eV), which binds
electrons of opposite spins to be on neighboring sites, and that there are no
bosonic glues. It is the Coulomb repulsive interaction U(> 6 eV) among Cu-3d
electrons that plays a central role in the physics behind high-Tc phenomena. A
new paradigm of the SC mechanism opens to strongly correlated electron matter.
01/2012;
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ABSTRACT: We report 75As nuclear quadrupole resonance studies on (Ca4Al2O(6-y))(Fe2As2) with T(c) = 27 K. Measurement of nuclear-spin-relaxation rate 1/T1 has revealed a significant development of two-dimensional antiferromagnetic spin fluctuations down to T(c) in association with the smallest As-Fe-As bond angle. Below T(c), the temperature dependence of 1/T1 without any trace of the coherence peak is well accounted for by a nodeless s(±)-wave multiple-gaps model. From the fact that its T(c) is comparable to T(c) = 28 K in the optimally doped LaFeAsO(1-y) in which antiferromagnetic spin fluctuations are not dominant, we remark that antiferromagnetic spin fluctuations are not a unique factor for enhancing T(c) among Fe-based superconductors, but a condition for optimizing superconductivity should be addressed from the lattice structure point of view.
Physical Review Letters 07/2011; 107(4):047002. · 7.37 Impact Factor
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ABSTRACT: We report 75As-nuclear quadrupole resonance (NQR) studies on
(Ca_4Al_2O_{6-y})(Fe_2As_2) with Tc=27K, which unravel unique normal-state
properties and point to unconventional nodeless superconductivity (SC).
Measurement of nuclear-spin-relaxation rate 1/T_1 has revealed a significant
development of two dimensional (2D) antiferromagnetic (AFM) spin fluctuations
down to Tc, in association with the fact that FeAs layers with the smallest
As-Fe-As bond angle are well separated by thick perovskite-type blocking layer.
Below Tc, the temperature dependence of 1/T_1 without any trace of the
coherence peak is well accounted for by an s(+-)-wave multiple gaps model. From
the fact that Tc=27K in this compound is comparable to Tc=28K in the
optimally-doped LaFeAsO_{1-y} in which AFM spin fluctuations are not dominant,
we remark that AFM spin fluctuations are not a unique factor for enhancing Tc
among existing Fe-based superconductors, but a condition for optimizing SC
should be addressed from the lattice structure point of view.
06/2011;
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ABSTRACT: We report on the observation of high-T_c superconductivity (SC) emerging with
the background of an antiferromagnetic (AFM) order in the five-layered cuprate
Ba_2Ca_4Cu_5O_10(F,O)_2 through 19F-NMR and zero-field Cu-NMR studies. The
measurements of spectrum and nuclear spin-lattice relaxation rates 19(1/T_1) of
19F-NMR give convincing evidence for the AFM order taking place below T_N = 175
K and for the onset of SC below T_c = 52 K, hence both coexisting. The
zero-field Cu-NMR study has revealed that AFM moments at Cu sites are 0.14 mu_B
at outer CuO_2 layers and 0.20 mu_B at inner ones. We remark that an intimate
coupling exists between the AFM state and the SC order parameter below T_c = 52
K; the spin alignment in the AFM state is presumably changed in the SC-AFM
mixed state.
02/2011;
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ABSTRACT: We report an 75As-nuclear quadrupole resonance (NQR) study on heavily
electron-doped LaFeAsO_1-xF_x(La1111) with Tc=5 K. Nuclear spin relaxation rate
(1/T1) measurement has revealed that a Hebel-Slichter (HS) peak partially
recovers in heavily electron-overdoped regimes where the nesting condition of
hole and electron Fermi surfaces(FSs) becomes significantly worse. This is in
contrast to previous results reported in optimally doped La1111 with Tc=28 K
where a lack of the HS peak was reported. It indicates that the interband
scattering between the hole and electron FSs is strongly suppressed by an
almost vanishing hole FS through the heavily electron-overdoping. Our findings
strongly suggest that the sign reversal of the gap functions on the different
FSs, that is, s(+-)-wave state is realized in La1111 compounds. We remark that
interband scattering on well-nested FSs is essential for stabilizing the
s(+-)-wave state and enhancing the Tc up to 28 K in LaFeAsO-based
superconductors.
09/2010;
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ABSTRACT: We report our extensive 57Fe-, 75As-, 139La-, and 1H-NMR studies of
La_{0.8}Y_{0.2}FeAsO_{1-y} (La_{0.8}Y_{0.2}1111) and
LaFeAsO_{1-y}H_{x}(La1111H), where doping yttrium (Y) and hydrogen (H) into
optimally doped LaFeAsO_{1-y} (La1111(OPT)) increases T_c=28 K to 34 and 32 K,
respectively. In the superconducting (SC) state, the measurements of
nuclear-spin lattice-relaxation rate 1/T_1 have revealed in terms of a multiple
fully gapped s_\pm-wave model that the SC gap and T_c in La_{0.8}Y_{0.2}1111
become larger than those in La1111(OPT) without any change in doping level. In
La1111H, the SC gap and T_c also increase slightly even though a decrease in
carrier density and some disorders are significantly introduced. As a
consequence, we suggest that the optimization of both the structural parameters
and the carrier doping level to fill up the bands is crucial for increasing T_c
among these La1111-based compounds through the optimization of the Fermi
surface topology.
07/2010;
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K. Miyazawa,
S. Ishida,
K. Kihou,
P. M. Shirage,
M. Nakajima,
C. H. Lee,
H. Kito,
Y. Tomioka,
T. Ito,
H. Eisaki,
H. Yamashita, H. Mukuda,
K. Tokiwa,
S. Uchida,
A. Iyo
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ABSTRACT: We report that the incorporation of hydroxide ions ( OH )<sup>-</sup> significantly enhances the superconducting transition temperature (T<sub>c</sub>) in the Ln FeAsO -based superconductors ( Ln1111 : Ln= La , Ce, and Pr). For La1111, T<sub>c</sub> of the ( OH )<sup>-</sup> incorporated sample synthesized using high-pressure technique becomes 35 K, which is higher by 7 K than the typical optimally doped La1111 superconductors. Similar enhancement in T<sub>c</sub> is also observed for Ce1111 and Pr1111. <sup>1</sup> H -NMR measurement have confirmed the existence of hydrogen atoms in the samples. Accompanying the ( OH )<sup>-</sup> incorporation, the lattice parameters are largely contracted, down to the values which have never been attained by any other dopings/substitutions.
Applied Physics Letters 03/2010; · 3.84 Impact Factor
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ABSTRACT: We report the superconducting characters in CeIrIns studied by In-NQR measurements under pressure (P). In CeCoIn5 and CeRhIn5, the occurrence of superconductivity (SC) is related with the antiferrmagnetic spin fluctuations (AFM-SFs) originating from the antiferromagntic quantum-critical point (AFM-QCP). The high-Tc SC (Tcmax > 2 K) is realized in both compounds. However, in CeIrIn5 which is apart from the AFM-QCP, SC occurs even without AFM-SFs and the quite small value of Tcmax (~ 1 K) is observed around P = 3 GPa. The mechanism of SC in CeIrIn5 may be different from that in CeCoIn5 and CeRhIn5.
Journal of Physics Conference Series 02/2010; 200(1):012087.
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H Mukuda,
N Terasaki,
M Yashima,
Y Kitaoka,
K Miyazawa,
P M Shirage,
S Suzuki,
S Miyasaka,
S Tajima,
H Kito,
H Eisaki,
A Iyo
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ABSTRACT: We report systematic 57Fe-NMR study on an underdoped sample (Tc = 20 K), an optimally doped sample (Tc = 28 K), and an overdoped sample (Tc = 22 K) of oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO1−y. The 57Fe-1/T1T in the normal state decreases significantly upon cooling regardless of an electron-doping level, revealing that 1/T1T is not always enhanced by antiferromagnetic fluctuations close to an antiferromagnetic phase in the underdoped superconducting sample. This contrasts with the behavior in hole-doped Ba0.6K0.4Fe2As2(Tc = 38 K), which exhibits a significant increase in 1/T1T upon cooling. We suggest that the crucial difference between the normal-state properties of LaFeAsO1−yand Ba0.6K0.4Fe2As2 originates from the fact that the relevant Fermi surface topologies are differently modified depending on whether electrons or holes are doped into the FeAs layers.
Journal of Physics Conference Series 02/2010; 200(1):012132.
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ABSTRACT: We report on superconducting and magnetic characteristics in four-layered high-Tc superconductors Ba2Ca3Cu4O8(FyO1−y)2 with apical fluorine through 63Cu-NMR measurements. The temperature dependence of the Knight shift shows that the substitution of oxygen for fluorine at the apical site increases carrier density (Nh) and Tc from 55 up to 102 K. The 63Cu-NMR spectra suggest that antiferromagnetic (AFM) order takes place in underdoped CuO2 layers; especially, the AFM order coexists with superconductivity in a single CuO2 plane at y=1.0.
Journal of Physics Conference Series 02/2010; 200(1):012186.
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ABSTRACT: We present a phase diagram for the antiferromagnetism and superconductivity in Ce2RhIn8 probed by In-NQR studies under pressure (P). The quasi-2D character of antiferromagnetic spin fluctuations in the paramagnetic state at P = 0 evolves into a 3D character because of the suppression of antiferromagnetic order for P > PQCP ~ 1.36 GPa (QCP: antiferromagnetic quantum critical point). Nuclear-spin-lattice-relaxation rate 1/T1 measurements revealed that the superconducting order occurs in the P range 1.36 − 1.84 GPa, with maximum Tc ~ 0.9 K (PQCP ~ 1.36 GPa).
Journal of Physics Conference Series 02/2010; 200(1):012238.
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ABSTRACT: We report 29Si NMR study on a single crystal of the heavy-fermion superconductor CeIrSi3 without an inversion symmetry along the c axis. The 29Si Knight-shift measurements under pressure have revealed that the spin susceptibility for the ab plane decreases slightly below T(c), whereas along the c axis it does not change at all. The result can be accounted for by the spin susceptibility in the superconducting state being dominated by the strong antisymmetric (Rashba-type) spin-orbit interaction that originates from the absence of an inversion center along the c axis and it being much larger than superconducting condensation energy. This is the first observation which exhibits an anisotropy of the spin susceptibility below T(c) in the noncentrosymmetric superconductor dominated by strong Rashba-type spin-orbit interaction.
Physical Review Letters 01/2010; 104(1):017002. · 7.37 Impact Factor
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ABSTRACT: We report 29Si-NMR study on a single crystal of the heavy-fermion
superconductor CeIrSi3 without an inversion symmetry along the c-axis. The
29Si-Knight shift measurements under pressure have revealed that the spin
susceptibility for the ab-plane decreases slightly below Tc, whereas along the
c-axis it does not change at all. The result can be accounted for by the spin
susceptibility in the superconducting state being dominated by the strong
antisymmetric (Rashba-type) spin-orbit interaction that originates from the
absence of an inversion center along the c-axis and it being much larger than
superconducting condensation energy. This is the first observation which
exhibits an anisotropy of the spin susceptibility below Tc in the
noncentrosymmetric superconductor dominated by strong Rashba-type spin-orbit
interaction.
12/2009;
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ABSTRACT: We present a phase diagram for the antiferromagnetism and superconductivity in Ce2RhIn8 probed by In-NQR studies under pressure (P). The quasi-two-dimensional character of antiferromagnetic spin fluctuations in the paramagnetic state at P=0 evolves into a three-dimensional character because of the suppression of antiferromagnetic order for P>PQCP∼1.36 GPa (QCP: quantum critical point). Nuclear-spin-lattice relaxation rate 1/T1 measurements revealed that the superconducting order occurs in the P range 1.36–1.84 GPa, with maximum Tc∼0.9 K around PQCP∼1.36 GPa.
Phys. Rev. B. 11/2009; 80(18).
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ABSTRACT: We present a novel phase diagram for the antiferromagnetism and superconductivity in Ce$_2$RhIn$_8$ probed by In-NQR studies under pressure ($P$). The quasi-2D character of antiferromagnetic spin fluctuations in the paramagnetic state at $P$ = 0 evolves into a 3D character because of the suppression of antiferromagnetic order for $P > P_{\rm QCP}\sim$ 1.36 GPa (QCP: antiferromagnetic quantum critical point). Nuclear-spin-lattice-relaxation rate $1/T_1$ measurements revealed that the superconducting order occurs in the $P$ range 1.36 -- 1.84 GPa, with maximum $T_c\sim$ 0.9 K around $P_{\rm QCP}\sim$ 1.36 GPa. Comment: 5 pages, 5 figures, submitted to PRB
10/2009;
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ABSTRACT: We report on a novel pressure ($P$)-induced evolution of magnetism and superconductivity (SC) in a helical magnet CeRhIn$_5$ with an incommensurate wave vector $Q_i=({1/2},{1/2},0.297)$ through the $^{115}$In nuclear quadrupole resonance (NQR) measurements under $P$. Systematic measurements of the $^{115}$In-NQR spectrum reveal that the commensurate antiferromagnetism (AFM) with $Q_c=({1/2},{1/2},{1/2})$ is realized above $P_m \sim$ 1.7 GPa. An important finding is that the size of SC gap and $T_c$ increase as the magnitude of the AFM moment decreases in the $P$ region, where SC uniformly coexists with the commensurate AFM. This result provides evidence of strong coupling between the commensurate AFM order parameter (OP) and SC OP. Comment: 5 pages, 5 figures
05/2009;
