arXiv subjects

C. Michioka

Publications and source records attributed to C. Michioka.

At least 19 recordsLinked to original sources

arXiv2014

Magnetic phase diagram of Sr$_{1-x}$Ca$_x$Co$_2$P$_2$

In order to study the phase diagram from a microscopic viewpoint, we have measured wTF- and ZF-$μ^+$SR spectra for the Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ powder samples with $x=0$, 0.2, 0.4, 0.5, 0.6, 0.8, and 1. Due to a characteristic time window and spatial resolution of $μ^+$SR, the obtained phase diagram was found to be rather different from that determined by magnetization measurements. That is, as $x$ increases from 0, a Pauli-paramagnetic phase is observed even at the lowest $T$ measured (1.8~K) until $x=0.4$, then, a spin-glass like phase appears at $0.5\leq x\leq0.6$, and then, a phase with wide field distribution probably due to incommensurate AF order is detected for $x=0.8$, and finally, a commensurate $A$-type AF ordered phase (for $x=1$) is stabilized below $T_{\rm N}\sim80~$K. Such change is most likely reasonable and connected to the shrink of the $c$-axis length with $x$, which naturally enhances the magnetic interaction between the two adjacent Co planes.

cond-mat.str-el↗

arXiv2010

Magnetic Phase Diagram of Alternating Chain Compound Pb2V3O9

Pb2V3O9 was found to have antiferromagnetic alternating chains of S=1/2 and show the antiferromagnetic long range ordered (AFLRO) state under the magnetic field between 4 and 38 T. We succeeded in growing single crystals of Pb2V3O9 by floating zone method and determined the asymmetric H-T phase diagram of Pb2V3O9 from measurements of the magnetization and the heat capacity. Previous reports focused on the AFLRO state in the low magnetic field region and that in high magnetic field region was not discussed so much. In this paper, we examined the AFLRO state in terms of the mean field theory and the picture of Bose-Einstein condensation of magnons to discuss the whole phase diagram. Consequently, the asymmetry of the phase boundary in Pb2V3O9 is explained by the mixing contribution of excited states and by interchain interactions.

cond-mat.str-el↗

arXiv2008

Critical Slowing Down of Triangular Lattice Spin-3/2 Heisenberg Antiferromagnet Li7RuO6 via 7Li NMR

We report 7Li NMR studies of single crystals of triangular-lattice Heisenberg antiferromagnet Li7RuO6. Slow critical divergence with a wide critical region of |T/TN - 1|< 7 was observed in 7Li nuclear spin-lattice relaxation rate. The slowing down of staggered spin fluctuations was analyzed in a renormalized classical region of a two-dimensional triangular-lattice non-linear sigma model. A spin stiffness constant was found to reduce to about 20 % from the value in a spin-wave approximation. The effect of spin frustration, e.g., Z2 vortex excitations on the critical phenomena is suggested.

cond-mat.str-el↗

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

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

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↗

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

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

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

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

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

Magnetic Fluctuations in the Metallic State of Na$_{0.7}$CoO$_2$ Revealed by $^{23}$Na NMR

Nuclear-magnetic-resonance (NMR) studies were performed in Na$_{0.7}$CoO$_2$ to investigate its magnetic properties from the microscopic point of view. Na$_{0.7}$CoO$_2$ is a starting compound of the recently discovered superconductor Na$_{0.35}$CoO$_2$$\cdot 1.3$H$_2$O, in which Co atoms form a triangular structure. Although $^{23}$Na-NMR results show no magnetic anomaly down to 1.5 K, Knight shift ($^{23}K$) and $^{23}(1/T_1T)$ of $^{23}$Na continue to increase, suggestive of the vicinity of the magnetic phase. From the relation between $^{23}K$ and $^{23}(1/T_1T)$, it is shown that ferromagnetic fluctuations are dominant below 100 K. In addition to the ferromagnetic correlations, antiferromagnetic fluctuations are also developed in a low temperature region below 4 K. Magnetic fluctuations suggested by our experiment are discussed.

cond-mat.str-el↗