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Y. Itoh

Publications and source records attributed to Y. Itoh.

At least 55 records · Page 3Linked to original sources
arXiv2008

Emergence of Antiferromagnetic Correlation in LiTi2-xVxO4 via 7Li NMR

We report 7Li NMR studies of V-substitution effects on spinel oxide superconductor LiTi2O4 (Tc = 13.4 K). In LiTi2-xVxO4 (x = 0-0.4), the V substitution for the Ti site suppressed the relative volume fraction of superconductivity faster than Tc. From the observation of fairly homogeneous enhancement in a 7Li nuclear spin-lattice relaxation rate, we conclude that the V substitution changes electron correlation effects through electron carrier doping from quarter electron filling 3d0.5 to 3d1.5 and then the antiferromagnetic correlation emerges.

cond-mat.supr-con↗
arXiv2008

Hg-Based Superconducting Cuprates: High Tc and Pseudo Spin-Gap

A brief review of microscopic studies of high-Tc superconductors HgBa2CuO4+d and HgBa2CaCu2O6+d is presented. The topics concerned are the pseudo spin-gap, the antiferromagnetic spin fluctuations near the quantum critical point, and the mechanism of the high Tc via NMR. The crystal structure with the flat CuO2 plane, two-dimensional electrical resistivity, Cu NQR, NMR experimental results are discussed.

cond-mat.supr-con↗
arXiv2008

59Co, 23Na, and 1H NMR Studies of Double-Layer Hydrated Superconductors NaxCoO2-yH2O

We present our NMR studies of double-layer hydrated cobalt oxides NaxCoO2-yH2O (x ~ 0.35, y ~ 1.3) with various Tc = 0 - 4.8 K and magnetic transition temperatures. High-resolution $^{1}$H NMR spectrum served as an evidence for the existence of H$_{3}$O$^{+}$ oxonium ions. $^{23}$Na nuclear spin-lattice relaxation rates served to detect local field fluctuations sensitive to Tc. $^{59}$Co nuclear quadrupole resonance (NQR) spectra served to classify the various Tc samples. From the classification by $^{59}$Co NQR frequency, the double-layer hydrated compounds were found to have two superconducting phases closely located to a magnetic phase. In the normal state and at a magnetic field in the $ab$-plane, two $^{59}$Co NMR signals with different Knight shifts and different $^{59}$Co nuclear spin-lattice relaxation times $^{59}T_{1}$ were observed. The two $^{59}$Co NMR signals suggest magnetic disproportionation of two Co sites or in-plane (XY) anisotropy of a single Co site. Non Korringa behavior and power law behavior in zero-field NQR 1/$^{59}T_{1}$ above and below Tc suggest non-Fermi liquid and unconventional superconductivity.

cond-mat.supr-con↗
arXiv2007

Recent advances of NMR and NQR studies of YBa2Cu3O7-d and YBa2Cu4O8

Some of recent advances in the NMR and NQR studies of high-Tc superconductors YBa2Cu3O7-d and YBa2Cu4O8 are presented. The topics concerned are the understanding of magnetic phase diagram, impurity substitution effects and vortex core states in the mixed states.

cond-mat.supr-con↗
arXiv2007

Site-selective $^{63}$Cu NMR study of the vortex cores of Tl$_{2}$Ba$_{2}$CuO$_{6+δ}$

We report site-selective $^{63}$Cu NMR studies of the vortex core states of an overdoped Tl$_{2}$Ba$_{2}$CuO$_{6+δ}$ with $T_{c}$ = 85 K. We observed a relatively high density of low-energy quasi-particle excitations at the vortex cores in a magnetic field of 7.4847 T along the c axis, in contrast to YBa$_{2}$Cu$_{3}$O$_{7-δ}$.

cond-mat.supr-con↗
arXiv2006

23Na NMR study of non-superconducting double-layer hydrate NaxCoO2.yH2O

We report 23Na NMR studies of the polycrystalline samples of double-layer hydrated cobalt oxides NaxCoO2.yH2O (x ~ 0.35 and y ~ 1.3) with the superconducting transition temperatures Tc < 1.8K and ~4.5K, and the dehydrated NaxCoO2 (x ~ 0.35). The hyperfine field and the electric field gradient at the Na sites in the non-hydrated Na0.7CoO2 and the dehydrated Na0.35CoO2 are found to be significantly reduced by the hydration, which indicates a strong shielding effect of the intercalated water molecules on the Na sites. The temperature dependence of 23 Na nuclear spin-lattice relaxation rate 1/23T1 of the non-superconducting double-layer hydrate NaxCoO2.yH2O is found to be similar to that of the non-hydrated Na0.7CoO2, whose spin dynamics is understood by A-type (intra-layer ferromagnetic and inter-layer antiferromagnetic) spin fluctuations. The superconducting phase is located close to the quantum critical point with the A-type magnetic instability.

cond-mat.supr-con↗
arXiv2006

In-plane Anisotropy of the Magnetic Fluctuations in NaxCoO2-yH2O

We report the $^{59}$Co NMR studies of the in-plane anisotropy of bilayer hydrated Na$_{x}\mathrm{CoO_{2}} \cdot$$y\mathrm{H_{2}O}$ using a oriented powder sample by a magnetic field in Fluorinert FC70. We found for the first time the $ab$-plane anisotropy of the $^{59}$Co NMR Knight shift $K$, the nuclear spin-lattice relaxation rate 1/$T_{1}$ and the nuclear spin-spin relaxation rate 1/$T_{2}$ at a magnetic field $H \sim$ 7.5 T up to 200K. Below 75 K, the anisotropy of $K$ is large compared with that at high temperatures. The hyperfine coupling constants seem to change around the temperature 150 K, in which the bulk susceptibility $χ$ shows broad minimum, suggesting a change of the electronic state of CoO$_{2}$ plane. 1/$T_{1}$ also shows a significant anisotropy, which cannot be explained only by the anisotropy of the hyperfine coupling constants nor the anisotropic uniform spin susceptibility. The difference in the in-plane anisotropy of $T_{1}$ from that of $K$ indicates that the magnetic fluctuation at a finite wave vector $\vec{q} \neq 0$ is also anisotropic and the anisotropy is different from that at $\vec{q} = 0$.

cond-mat.supr-con↗
arXiv2006

Hole Doping Effects on Spin-gapped Na2Cu2TeO6 via Topochemical Na Deficiency

We report the magnetic susceptibility and NMR studies of a spin-gapped layered compound Na2Cu2TeO6 (the spin gap $Δ\sim$ 250 K), the hole doping effect on the Cu2TeO6 plane via a topochemical Na deficiency by soft chemical treatment, and the static spin vacancy effect by nonmagnetic impurity Zn substitution for Cu. A finite Knight shift at the $^{125}$Te site was observed for pure Na2Cu2TeO6. The negative hyperfine coupling constant $^{125}A_{tr}$ is an evidence for the existence of a superexchange pathway of the Cu-O-Te-O-Cu bond. It turned out that both the Na deficiency and Zn impurities induce a Curie-type magnetism in the uniform spin susceptibility in an external magnetic field of 1 T, but only the Zn impurities enhance the low-temperature $^{23}$Na nuclear spin-lattice relaxation rate whereas the Na deficiency suppresses it. A spin glass behavior was observed for the Na-deficient samples but not for the Zn-substituted samples. The dynamics of the unpaired moments of the doped holes are different from that of the spin vacancy in the spin-gapped Cu2TeO6 planes.

cond-mat.str-el↗
arXiv2006

59Co Nuclear Quadrupole Resonance Studies of Superconducting and Non-superconducting Bilayer Water Intercalated Sodium Cobalt Oxides NaxCoO2.yH2O

We report 59Co nuclear quadrupole resonance (NQR) studies of bilayer water intercalated sodium cobalt oxides NaxCoO2.yH2O (BLH) with the superconducting transition temperatures, 2 K < T_c <= 4.6 K, as well as a magnetic BLH sample without superconductivity. We obtained a magnetic phase diagram of T_c and the magnetic ordering temperature T_M against the peak frequency nu_3 59Co NQR transition I_z = +- 5/2 <=> +-7/2 and found a dome shape superconducting phase. The 59Co NQR spectrum of the non-superconducting BLH shows a broadening below T_M without the critical divergence of 1/T_1 and 1/T_2, suggesting an unconventional magnetic ordering. The degree of the enhancement of 1/T_1T at low temperatures increases with the increase of nu_3 though the optimal nu_3~12.30 MHz. In the NaxCoO2.yH2O system, the optimal-T_c superconductivity emerges close to the magnetic instability. T_c is suppressed near the phase boundary at nu_3~12.50 MHz, which is not a conventional magnetic quantum critical point.

cond-mat.supr-con↗
arXiv2005

Anisotropic Behavior of Knight Shift in Superconducting State of Na_xCoO_2yH_2O

The Co Knight shift was measured in an aligned powder sample of Na_xCoO_2yH_2O, which shows superconductivity at T_c \sim 4.6 K. The Knight-shift components parallel (K_c) and perpendicular to the c-axis (along the ab plane K_{ab}) were measured in both the normal and superconducting (SC) states. The temperature dependences of K_{ab} and K_c are scaled with the bulk susceptibility, which shows that the microscopic susceptibility deduced from the Knight shift is related to Co-3d spins. In the SC state, the Knight shift shows an anisotropic temperature dependence: K_{ab} decreases below 5 K, whereas K_c does not decrease within experimental accuracy. This result raises the possibility that spin-triplet superconductivity with the spin component of the pairs directed along the c-axis is realized in Na_xCoO_2yH_2O.

cond-mat.supr-con↗
arXiv2005

Slow transverse relaxation of the Zn-neighbor Cu nuclear spins in YBa2(Cu{1-x}Znx)4O8

An explanation is given for the recent observation in the plane-site Cu nuclear quadrupole resonance (NQR) study of a heavily Zn-substituted YBa$_2$(Cu$_{1-x}$Zn$_x$)$_4$O$_{8}$ that the Cu nuclear spin-spin relaxation time $T_{2G}$ near the impurity Zn is longer than that away from Zn. The long $T_{2G}$ of the Zn-neighbor Cu nuclear spins results from a distant coupling between only a few nuclear spins near Zn, which does not indicate the absence of local enhancement of the staggered spin susceptibility around Zn.

cond-mat.supr-con↗
arXiv2005

23Na NMR Studies of Non-superconducting and Superconducting Na0.35CoO2-1.3H2O (Tc < 1.8 and ~ 4.5 K)

We report 23Na NMR studies of the polycrystalline samples of bilayer hydrated cobalt oxides Na0.35CoO2-1.3H2O with the superconducting transition temperatures Tc < 1.8 and ~ 4.5 K. The magnitude of the 23Na nuclear spin-lattice relaxation rate 23(1/T1) significantly decreased from non-hydrated Na0.7CoO2 to the bilayer hydrates. We found that the temperature dependence of 23(1/T1) of the non-superconducting bilayer hydrate is similar to that of the non-hydrated Na0.7CoO2, the spin dynamics of which is understood by A-type (intra-layer ferromagnetic and inter-layer antiferromagnetic) spin fluctuations. The superconducting phase is located close to the A-type magnetic instability.

cond-mat.supr-con↗
arXiv2005

Zeeman Perturbed $^{63}$Cu Nuclear Quadrupole Resonance Study of the Vortex State of YBa$_2$Cu$_3$O$_{7-δ}$

We report a $^{63}$Cu nuclear quadrupole resonance (NQR) study of the vortex state for an aligned polycrystalline sample of a slightly overdoped high-$T_c$ superconductor YBa$_2$Cu$_3$O$_{7-δ}$ ($T_{c}\sim$92 K) at a low magnetic field of 96 mT along the c axis, near a lower critical field $H_{c1}$. We observed the frequency distribution of the nuclear spin-lattice relaxation time $^{63}T_1$ in the Zeeman-perturbed $^{63}$Cu NQR spectrum below $T_c$. The characteristic behavior of 1/$^{63}T_1$, taking the minimum values with respect to temperature and frequency, indicates the significant role of antiferromagnetic spin fluctuations in the Doppler-shifted quasiparticle energy spectrum inside and outside vortex cores.

cond-mat.supr-con↗
arXiv2005

$^{23}$Na NMR Study of Layered Superconductor $Na_xCoO_2- yH_2O$

We measured $^{23}$Na-NMR spectra and nuclear spin-lattice relaxation rates $^{23}(1/T_1)$ of superconducting and non-superconducting bilayer hydrate $Na_xCoO_2-yH_2O$ (x ~ 0.3, y ~ 1.3). The central resonance frequency shows a small but various shift due to the difference in the shielding effect by intercalated $H_2O$ molecules. The different shielding effect also gives a large difference in the magnitude of $^{23}(1/T_1)$.

cond-mat.supr-con↗
arXiv2005

Possible Spin-triplet Superconductivity in NaxCoO2yH2O - 59Co NMR Study

We report 59Co NMR studies on the magnetically oriented powder samples of Co-oxide superconductors NaxCoO2yH2O with Tc ~ 4.7 K. From two-dimensional powder pattern in the NMR spectrum, the ab-plane Knight shift in the normal state was estimated by the magnetic field dependence of second-order quadrupole shifts at various temperatures. Below 50 K, the Knight shift shows a Curie-Weiss-like temperature dependence, similarly to the bulk magnetic susceptibility chi. From the analysis of so-called K-chi plot, the spin and the orbital components of K and the positive hyperfine coupling constant were estimated. The onset temperature of superconducting transition in the Knight shift does not change so much in an applied magnetic field up to 7 T, which is consistent with the reported high upper critical field Hc2. The Knight shift at 7 T shows an invariant behavior below Tc. No coherence peak just below Tc was observed in the temperature dependence of the nuclear spin-lattice relaxation rate 1/T1 in both cases of NMR and NQR. We conclude that the invariant behavior of the Knight shift below Tc and unconventional behaviors of 1/T possibly indicate the spin-triplet superconductivity with p- or f-wave symmetry.

cond-mat.supr-con↗
arXiv2005

Vortex State of Tl$_2$Ba$_2$CuO$_{6+δ}$ via $^{205}$Tl NMR at 2 Tesla

We report a $^{205}$Tl NMR study of vortex state for an aligned polycrystalline sample of an overdoped high-$T_c$ superconductor Tl$_2$Ba$_2$CuO$_{6+δ}$ ($T_{c}\sim$85 K) with magnetic field 2 T along the c axis. We observed an imperfect vortex lattice, so-called Bragg glass at $T$=5 K, coexistence of vortex solid with liquid between 10 and 60 K, and vortex melting between 65 and 85 K. No evidence for local antiferromagnetic ordering at vortex cores was found for our sample.

cond-mat.supr-con↗
arXiv2004

Effects of carbon incorporation on doping state of YBa2Cu3Oy

Effects of carbon incorporation on the doping state of YBa2Cu3Oy (Y-123) were investigated. Quantitative carbon analysis revealed that carbon could be introduced into Y-123 from both the precursor and the sintering gas. Nearly carbon-free (< 200 ppm) samples were prepared from a vacuum-treated precursor by sintered at 900 ˚C and cooling with 20 ˚C /min in flowing oxygen gas. The lower Tc (= 88 K) and higher oxygen content (y = 6.98) strongly suggested the overdoping state, which was supported by the temperature dependence of resisitivity and thermoelectric power. The nuclear quadrapole resonance spectra and the Raman scattering spectra indicated that there was almost no oxygen defect in the Cu-O chain in these samples. On the other hand, in the same cooling condition, the samples sintered in air stayed at optimal doping level with Tc = 93 K, and the intentionally carbon-doped sample was in the underdoping state. It is revealed that about 60% of incorporated carbon was substituted for Cu at the chain site in the form of CO32+, and the rest remains at the grain boundary as carbonate impurities. Such incorporation affected the oxygen absorption process in Y-123. It turned out that the oxygen content in Y-123 cannot be controlled only by the annealing temperature and the oxygen partial pressure but also by the incorporated carbon concentration.

cond-mat.supr-con↗