arXiv subjects

K. Takada

Publications and source records attributed to K. Takada.

At least 19 recordsLinked to original sources

arXiv2009

Magnetic anomalies of hydrous cobaltate compound Na_x(H_3O)_zCoO_2 \cdot yH_2O detected by NMR and NQR measurements

In order to investigate the relationship between superconductivity and magnetism in bilayer-hydrate cobaltate Na_x(H_3O)_zCoO_2 \cdot yH_2O, Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements were performed on three different samples, which demonstrate various ground states at low temperatures. The appearance of small internal fields is observed in the NQR spectra below approximately 6 K on one of the samples that possesses the largest c-axis length and the highest NQR frequency. The other two samples exhibit superconducting transition in zero magnetic field, while these two samples show different ground states in the magnetic fields greater than 5 T. The comparison of the NMR spectra of these two samples obtained in high magnetic fields reveals the appearance of static internal magnetic fields at the Co site below 4 K in the sample that possesses the intermediate c-axis length and the NQR frequency.

cond-mat.supr-con↗

arXiv2006

Unconventional Superconductivity Induced by Quantum Critical Fluctuations in Hydrate Cobaltate Na$_{x}$(H$_3$O)$_{z}$CoO$_{x}\cdot$ $y$H$_{2}$O -- Relationship between Magnetic Fluctuations and the Superconductivity Revealed by a Co Nuclear Quadrupole Resonance --

Co nuclear-quadrupole-resonance (NQR) measurements were performed on various bilayered hydrate cobaltate Na_x(H_3O)_zCoO_2\cdot yH_2O with different values of the superconducting and magnetic-ordering temperatures, T_c and T_M, respectively. From measurements of the temperature and sample dependence of the NQR frequency, it was revealed that the NQR frequency is changed by the change of the electric field gradient (EFG) along the c axis ν_{zz} rather than the asymmetry of EFG within the ab-plane. In addition, it is considered that the change of ν_{zz} is gaverned mainly by the trigonal distortion of the CoO_2 block layers along the c axis, from the relationships between ν_{zz} and the various physical parameters. We found the tendency that samples with ν_{zz} larger than 4.2 MHz show magnetic ordering, whereas samples with lower ν_{zz} show superconductivity. We measured the nuclear spin-lattice relaxation rate 1/T_1 in these samples, and found that magnetic fluctuations depend on samples. The higher-ν_{zz} sample has stronger magnetic fluctuations at T_c. From the relationship between ν_{zz} and T_c or T_M, we suggest that the NQR frequency can be regarded as a tuning parameter to determine the ground state of the system, and develop the phase diagram using ν_{zz}. This phase diagram shows that the highest-T_c sample is located at the point where T_M is considered to be zero, which suggests that the superconductivity is induced by quantum critical fluctuations. We strongly advocate that the hydrate cobaltate superconductor presents an example of the magnetic-fluctuation-mediated superconductivity argued in the heavy-fermion compounds. The coexistence of superconductivity and magnetism observed in the sample with the highest ν_{zz} is also discussed on the basis of the results of our experiments.

cond-mat.supr-con↗

arXiv2005

Superconducting phase diagram of NaxCoO2.yH2O

We synthesized Nax(H3O)zCoO2yH2O samples with various Na/H3O ratios but with the constant Co valence of s = +3.40, and measured their magnetic properties to draw phase diagrams of the system. The superconductivity is very sensitive to the Na/H3O ratio. With varying x under fixed s of +3.40, magnetically ordered phase appears in the intermediate range of x sandwiched by two separated superconducting phases, suggesting that the superconductivity is induced by moderately strong magnetic interactions. In the vicinity of the magnetic phase, transition from the superconducting state to the magnetically ordered state was induced by applying high magnetic field. This transition is of the second order, at least, above 1.8 K. The upper-critical field is expected to be much higher than the Pauli limit for a phase located far away from the magnetic phase regarding the Na/H3O ratio.

cond-mat.supr-con↗

arXiv2005

Valence and Na content dependences of superconductivity in NaxCoO2.yH2O

Various samples of sodium cobalt oxyhydrate with relatively large amounts of Na$^{+}$ ions were synthesized by a modified soft-chemical process in which a NaOH aqueous solution was added in the final step of the procedure. From these samples, a superconducting phase diagram was determined for a section of a cobalt valence of $\sim$+3.48, which was compared with a previously obtained one of $\sim$+3.40. The superconductivity was significantly affected by the isovalent exchanger of Na$^{+}$ and H$_{3}$O$^{+}$, rather than by variation of Co valence, suggesting the presence of multiple kinds of Fermi surface. Furthermore, the high-field magnetic susceptibility measurements for one sample up to 30 T indicated an upper critical field much higher than the Pauli limit supporting the validity of the spin-triplet pairing mechanism.

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

$^{23}$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

Evidence of nodal superconductivity in Na_0.35CoO2-1.3H_2O: a specific heat study

Comprehensive low-temperature specific heat data C(T,H) of Na_0.35CoO2-1.3H_2O with temperature T down to 0.6 K and the magnetic field H up to 8 T are presented. For the normal state, the values of gamma_n=13.94 mJ/mol K2, and Debye temperature 362 K are reported. At zero field, a very sharp superconducting anomaly was observed at Tc=4.5 K with DeltaC/gamma_nTc=1.45. The superconducting volume fraction is estimated to be 47.4 % based on the consideration of entropy balance at Tc for the second-order superconducting phase transition. In the superconducting state, the electronic contribution C_es at H=0 can be well described by the model of the line nodal order parameter. In low H, gamma(H) H^1/2 which is also a manifestation of the line nodes. The behaviors of both Tc(H) and gamma(H) suggest the anisotropy of Hc2 or possible crossovers or transitions occurring in the mixed state.

cond-mat.supr-con↗

arXiv2004

Weak Magnetic Order in the Bilayered-hydrate Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O Structure Probed by Co Nuclear Quadrupole Resonance - Proposed Phase Diagram in Superconducting Na$_x$CoO$_{2} \cdot$ $y$H$_2$O

A weak magnetic order was found in a non-superconducting bilayered-hydrate Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O sample by a Co Nuclear Quadrupole Resonance (NQR) measurement. The nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ shows a prominent peak at 5.5 K, below which a Co-NQR peak splits due to an internal field at the Co site. From analyses of the Co NQR spectrum at 1.5 K, the internal field is evaluated to be $\sim$ 300 Oe and is in the $ab$-plane. The magnitude of the internal field suggests that the ordered moment is as small as $\sim 0.015$ $μ_B$ using the hyperfine coupling constant reported previously. It is shown that the NQR frequency $ν_Q$ correlates with magnetic fluctuations from measurements of NQR spectra and $1/T_1T$ in various samples. The higher-$ν_Q$ sample has the stronger magnetic fluctuations. A possible phase diagram in Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O is depicted using $T_c$ and $ν_Q$, in which the crystal distortion along the c-axis of the tilted CoO$_2$ octahedron is considered to be a physical parameter. Superconductivity with the highest $T_c$ is seemingly observed in the vicinity of the magnetic phase, suggesting strongly that the magnetic fluctuations play an important role for the occurrence of the superconductivity.

cond-mat.supr-con↗

arXiv2004

Unconventional superconductivity in NaxCoO2 yH2O

We synthesized powder samples of Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O changing the volume of the water in the hydration process, then investigated their superconducting properties,. It was proved that the volume of water is one of key parameters to obtain a single phase of Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O with good superconducting properties. The transition temperature, $T_{c}$, of the sample changed gradually while it was stored in the atmosphere of 70% humidity. Superconducting behavior under high magnetic field was very sensitive to $T_{c}$. $H_{c2}$ of a high quality sample with high $T_{c}$ seemed very high.

cond-mat.supr-con↗

arXiv2004

Correlation between Tc and Lattice Parameters of Novel Superconducting NaxCoO2 yH2O

We synthesized the five batches of the samples of the novel P3 type superconductor, Na$_{x}$(H$_{3}$O)$_{y'}$CoO$_{2}\cdot y''$H$_{2}$O, by the soft chemical process starting from $α$-NaCoO$_{2}$. The chemical and structural properties varied rather widely from batch to batch, with a result that $T_{c}$ varied from 4.6 K to 3.2 K. The magnetic susceptibility above $T_{c}$ shows upturn at low temperature as in the case of the P2 phase. The $T_{c}$ seems to be well correlated to the lattice parameters.

cond-mat.supr-con↗

arXiv2004

Correlation between Superconducting Transition Temperature $T_c$ and Increase of Nuclear Spin-Lattice Relaxation Rate Devided by Temperature $1/T_1T$ at $T_c$ in the Hydrate Cobaltate Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O

We have performed Co-nuclear quadrupole resonance (NQR) studies on Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O compounds with different Na ($x$) and hydrate ($y$) contents. Two samples with different Na contents but nearly the same $T_c$ values ($x$ = 0.348, $T_c$ = 4.7 K ; $x$ = 0.339, $T_c$ = 4.6 K) were investigated. The spin-lattice relaxation rate $1/T_1$ in the superconducting (SC) and normal states is almost the same for the two samples except just above $T_c$. NQR measurements were also performed on different-hydrate-content samples with different $T_c$ values, which were prepared from the same Na-content ($x$ = 0.348) sample. From measurements of $1/T_1$ using the different-hydrate-content samples, it was found that a low-$T_c$ sample with $T_c = 3.9$ K has a larger residual density of states (DOS) in the SC state and a smaller increase of $1/T_1T$ just above $T_c$ than a high-$T_c$ sample with $T_c$ = 4.7 K. The former behavior is consistent with that observed in unconventional superconductors, and the latter suggests the relationship between $T_c$ and the increase in DOS just above $T_c$. This increase, which is seemingly associated with the two-dimensionality of the CoO$_2$ plane, is considered to be one of the most important factors for the occurrence of superconductivity.

cond-mat.supr-con↗

arXiv2004

Laser-excited ultrahigh-resolution photoemission spectroscopy of NaxCoO2.yH2O:Evidence for pseudogap formation

We have studied the temperature-dependent electronic structure near the Fermi level (EF) of the layered cobaltate superconductor, Na0.35CoO2.1.3H2O, and related materials, using laser-excited ultrahigh-resolution photoemission spectroscopy. We observe the formation of a pseudogap with an energy scale of ~ 20 meV in Na0.35CoO2.1.3H2O and Na0.35CoO2.0.7H2O, which is clearly absent in Na0.7CoO2. The energy scale of the pseudogap is larger than the expected value for the superconducting gap, suggesting an additional competing order parameter at low temperatures. We discuss implications of the pseudogap in relation to available transport and magnetic susceptibility results.

cond-mat.str-el↗

arXiv2004

Specific heat of unconventional superconductivity in NaxCoO2-yH2O

Comprehensive studies of the low-temperature specific heat C(T,H) in NaxCoO2-yH2O are presented. At H=0, a very sharp anomaly was observed at T=4.7 K indicating the existence of bulk superconductivity. There exists the T^2 term in C(T,H=0) in the superconducting state manifesting the line nodal superconducting order parameter. The superconducting volume fraction is estimated to be 26.6 % based on the consideration of entropy conservation at Tc for the second-order superconducting phase transition. An abrupt change of the slope in Tc(H) was observed. Possible scenarios such as the multiple phase transitions in the mixed state are discussed.

cond-mat.supr-con↗

arXiv2004

Determination of The Spin Contributed Knight Shift in $Na_{0.35} Co O_2 \cdot 1.3 H_2 O$

The spin contributed Knight shift $^{59}K_{s}$ of $^{59}\mathrm{Co}$ in the triangular lattice superconductor, $\mathrm{Na_{0.35}CoO_{2} \cdot 1.3H_{2}O}$, has been estimated by NMR experiments in an oriented 2-dimensional powder sample. The Knight shift in the paramagnetic state was estimated by $δν/(γ\mathrm{_{N}}H_{i})$ vs. $(γ\mathrm{_{N}}H_{i})^{-2}$ plot at various temperatures. A Knight shift vs. magnetic susceptibility diagram revealed that at least 0.3 % for $K_{x}$ and 0.1 % for $K_{y}$ are attributed to the spin contribution. Below $T\mathrm{_{c}}$, $δν/(γ\mathrm{_{N}}H_{i})$ vs. $(γ\mathrm{_{N}}H_{i})^{-2}$ plot does not give unique Knight shift because of the diamagnetic effect and/or the other ones. We concluded that the invariant behavior at about 7 T (between $H\mathrm{_{c1}}$ and $H\mathrm{_{c2}}$) below $T\mathrm{_{c}}$ possibly indicates the nature of the spin-triplet superconductivity.

cond-mat.supr-con↗

arXiv2004

Bulk Electronic structure of Na$_{0.35}$CoO$_{2}$.1.3H$_{2}$O

High-energy (h$ν$ = 5.95 keV) synchrotron Photoemission spectroscopy (PES) is used to study bulk electronic structure of Na$_{0.35}$CoO$_{2}$.1.3H$_{2}$O, the layered superconductor. In contrast to 3-dimensional doped Co oxides, Co $\it{2p}$ core level spectra show well-separated Co$^{3+}$ and Co$^{4+}$ ions. Cluster calculations suggest low spin Co$^{3+}$ and Co$^{4+}$ character, and a moderate on-site Coulomb correlation energy U$_{dd}\sim$3-5.5 eV. Photon dependent valence band PES identifies Co $\it{3d}$ and O $\it{2p}$ derived states, in near agreement with band structure calculations.

cond-mat.mtrl-sci↗

arXiv2003

The role of the water molecules in novel superconductor, Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O

In order to investigate the role of the water molecules in Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O, we synthesized superconducting Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O and nonsuperconducting Na$_{0.35}$CoO$_{2}\cdot$0.7H$_{2}$O, and measured their normal-state magnetic susceptibilities. The susceptibility of Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O has an enhancement below ~150 K probably caused by ferromagnetic fluctuation, whereas no such enhancement was observed in Na$_{0.35}$CoO$_{2}\cdot$0.7H$_{2}$O. The water molecules in Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O may work to shield random coulomb potential of the Na ions with smoother potential at the CoO$_{2}$ layer. This effect may account for the appearance of superconductivity in Na$_{0.35}$CoO$_{2}\cdot$1.3H$_{2}$O.

cond-mat.supr-con↗

arXiv2003

Effect of Na content and hydration on the excitation spectrum of the cobaltite Na_xCoO_2 yH_2O

We report on a Raman scattering study on the superconducting cobaltite $Na_xCoO_2\cdot yH_2O$ as function of Na content and hydration (x$\approx$1/3, 3/4 and y$\approx$0, 2/3, 4/3). The observed phonon scattering and scattering continua are analyzed in terms of lattice strain due to the structural misfit and disorder. Hydration, due to the intercalation of one or two $H_2O$ layers, releases a part of this strain. Our Raman data suggest a connection between disorder on the partly occupied Na sites, the split off of the $a_{1g}$ level from the other $t_{2g}$ states of $Co^{4+}$ and superconductivity.

cond-mat.str-el↗