arXiv subjects

Masatoshi Sato

Publications and source records attributed to Masatoshi Sato.

At least 181 records · Page 10Linked to original sources

arXiv2006

Magnetic properties of superconducting cobalt oxides NaxCoO2.yH2O

Studies of the NMR Knight shift K of Na0.3CoO2.yH2O have been carried out in detail. The suppression of K by the occurrence of the superconductivity reported previously by the present authors in both magnetic field directions perpendicular and parallel to the c axis has been confirmed, indicating that the Cooper pairs are in the singlet state. The anisotropy of the suppression amplitudes is consistent with the anisotropy of the hyperfine coupling constant Aspin estimated by the K-kai plot. It has also been found that even samples, which do not exhibit a significant amount of the Curie-Weiss-like increase of the uniform magnetic susceptibility kai with decreasing temperature T, exhibit the superconducting transition, which indicates that the superconducting Na0.3CoO2.1.3H2O is not necessarily be in the proximity region of the ferromagnetic phase. It has also been confirmed that the superconducting transition temperature Tc of the samples prepared by mixing Na0.7CoO2, H2O (or D2O) and bromine for 4 h, and separating the resultant powder from the solution by filtration, depends on the elapsed time t after filtration. For these samples, the Curie constant of kai estimated at low temperatures increases with t, supporting the idea that the number of the lattice imperfection possibly due to the oxygen-vacancy-formation increases with t. Even for such samples, the superconductivity appears, which seems to exclude the possibility of the anisotropic superconducting order parameter. These results are favorable for the full-gapped superconductivity.

cond-mat.supr-con↗

arXiv2006

Search for Magnetic Excitation Spectra of NaxCoO2.yD2O -Neutron Scattering-

Neutron inelastic scattering studies on aligned crystals of NaxCoO2.yD2O have been carried out. Significant scattering has been observed for phonons originating from the vibrations of the short-range ordered D2O molecules. No firm evidence for the magnetic fluctuations has been found for the NaxCoO2.yD2O, while the very weak intensity modulation observed in the j scans along (1/2, 1/2, j) suggests the existence of the antiferromagnetic fluctuations in the non-deuterated parts of the sample, the volume fraction of which is estimated to be about 26 %. The present results indicate that magnetic fluctuations, especially ferromagnetic ones are rather weak, in the deuterated system, which is consistent with the results of the NMR Knight shift reported by the present authors' group. We discuss the short-range ordered state of D2O, too by analyzing the observed diffuse scattering.

cond-mat.supr-con↗

arXiv2006

Magnetic Structure of YBaCo4O7 with Kagome- and Triangular-Lattices

Neutron diffraction studies have been carried out in the temperature(T) range between 10 K and 130 K on a single crystal of YBaCo4O7, which has the stacking of kagome and triangular lattices of CoO4 tetrahedra along the c-axis. Structural transitions have been found at two temperatures Tc1~70 K and Tc2~105 K. With decreasing T, magnetic order appears along with the transition at Tc2, but it does not grow to an ideal long-range order even at the lowest T studied (~10 K). Two groups of magnetic diffuse reflections, which originate from the Co-moments of the kagome and triangular lattices, are observed separately in the reciprocal space. With further decreasing T, the growth rates of the intensities of these two are enhanced by another transition at Tc1. These magnetic behaviors seem to be related to the release of the geometrical frustration. At 10 K, the patterns of the magnetic short-range order have been determined for both sites of the kagome and triangular lattices.

cond-mat.str-el↗

arXiv2005

Magnetic Correlation of NaxCoO2 and Successive Phase Transitions of Na0.5CoO2 -NMR and Neutron Diffraction Studies-

Data taken by various kinds of methods including NMR/NQR and neutron scattering are presented for NaxCoO2, in particular, for Na0.5CoO2. Attentions have also been paid to the x dependence of the electronic nature. The pseudo-gap-like behavior observed in the region of x<xc~0.6 is emphasized. For samples with x<0.6, signals from two kinds of Co sites with different quadrupole frequencies nQ, ~4.1 MHz and ~2.8 MHz were observed, and the former value of nQ, which is nearly equal to that of the superconducting system Na0.3CoO2.yH2O, becomes dominant with decreasing x to 0.3. For Na0.5CoO2, the Co sites with the larger nQ value have the larger magnetic moments. They align antiferromagnetically at Tc1~87 K with their direction within the plane, while the Co sites with the smaller nQ value have the smaller moments. They align along the direction parallel to the c axis. The ordered moments at the two distinct sites exhibit the same T dependences, indicating that the existence of these sites is not due to a macroscopic phase separation. Although we have observed the splitting of the zero-field NMR signals from the Co sites with the smaller nQ value at the second transition temperature Tc2~53 K, any other anomaly of the ordered magnetization has not been observed there in the T dependences of the neutron and the NMR data. As x decreases, the two-dimensional nature of the electrons is enhanced and the antiferromagnetic correlation increases. Based on these results, the superconducting pair state in Na0.3CoO2.yH2O is discussed.

cond-mat.str-el↗

arXiv2005

59Co-NMR Knight Shift of Aligned Crystals and Polycrystalline Samples of Superconducting Na0.3CoO2.1.3H2O

Temperature (T) dependence of 59Co-NMR Knight shifts K of Na0.3CoO2.1.3H2O has been studied, where samples of randomly oriented powder and aligned crystals have been used for the applied magnetic fields H // ab plane and H //c-axis, respectively. For both directions of H, the shift K decreases below the superconducting transition temperature Tc(H) with decreasing T, indicating that the superconducting electron pairs are in the singlet state. The upper critical fields Hc2(T) determined from the K(H)-T curves are found to be consistent with the values reported by the resistivity measurements for both directions of H.

cond-mat.supr-con↗

arXiv2005

On the Mechanism of the Spin State Transition of (Pr1-ySmy)1-xCaxCoO3

Transport, thermal and magnetic measurements have been carried out on (Pr1-ySmy)1-xCaxCoO3. The system exhibits a structural phase transition accompanied by the spin state change from the intermediate spin (IS) state to the low spin (LS) state with decreasing temperature T. We have constructed a T-y phase diagram for x=0.3 and T-x ones for y=0.2 and 0.3. By analyzing their magnetic susceptibilities, the number of Co ions excited to IS state (or the electron number in the eg orbitals), nIS, are roughly estimated. With increasing y or with decreasing x, nIS decreases, and the phase transition changes gradually to the (IS-LS) crossover-like one. We discuss on the possible role of the Pr atoms in realizing the transition.

cond-mat.str-el↗

arXiv2005

On Disagreement about Nonperturbative Corrections in Triple-well Potential

We examine in detail nonperturbative corrections for low lying energies of a symmetric triple-well potential with non-equivalent vacua, for which there have been disagreement about asymptotic formulas and controversy over the validity of the dilute gas approximation. We carry out investigations from various points of view, including not only a numerical comparison of the nonperturbative corrections with the exact values but also the prediction of the large order behavior of the perturbation series, consistency with the perturbative corrections, and comparison with the WKB approximation. We show that all the results support our formula previously obtained from the valley method calculation beyond the dilute gas approximation.

quant-ph↗

arXiv2004

Pump process of the rotatory molecular motor and its energy efficiency

The pump process of the ratchet model inspired by the $F_o$ rotatory motor of ATP synthase is investigated. In this model there are two kinds of characteristic time. One is dynamical, the relaxation time of the system. Others are chemical, the chemical reaction rates at which a proton binds to or dissociates from the motor protein. The inequalities between them affect the behavior of the physical quantities, such as the rotation velocity and the proton pumping rates across the membrane. The energy transduction efficiency is calculated and the condition under which the efficiency can become higher is discussed. The proton pumping rate and the efficiency have a peak where a certain set of inequalities between the chemical reaction rates and the reciprocal of the relaxation time holds. The efficiency also has a peak for a certain value of the load. The best efficiency condition for the pump process is consistent with that for the motor process.

physics.bio-ph↗

arXiv2004

On the Magnetic Excitation Spectra of High Tc Cu Oxides up to the Energies far above the Resonance Energy

Magnetic excitation spectra c"(q,w) of YBa2Cu3Oy and La214 systems have been studied. For La1.88Sr0.12CuO4, c"(q,w) have been measured up to ~30 meV and existing data have been analyzed up to the energy w~150 meV by using the phenomenological expression of the generalized magnetic susceptibility c(q,w)=c0(q,w)/{1+J(q)c0(q,w)}, where c0(q,w) is the susceptibility of the electrons without the exchange coupling J(q) among them. In the relatively low energy region up to slightly above the resonance energy Er, it has been reported by the authors' group that the expression can explain characteristics of the q- and w-dependence of the spectra of YBa2Cu3Oy (YBCO or YBCOy). Here, it is also pointed out that the expression can reproduce the rotation of four incommensurate peaks of c"(q,w) within the a*-b* plane about (p/a, p/a) {or so-called (p, p)} point by 45 degree, which occurs as w goes to the energy region far above Er from E below Er. For La2-xSrxCuO4 and La2-xBaxCuO4, agreements between the observed results and the calculations are less satisfactory than for YBCO, indicating that we have to take account of the existence of the "stripes" to consistently explain the observed c"(q,w) of La214 system especially near x=1/8.

cond-mat.supr-con↗

arXiv2004

Transport and Magnetic Studies on the Spin State Transition of Pr1-xCaxCoO3 up to High Pressure

Transport and magnetic measurements and structural and NMR studies have been carried out on (Pr1-yR'y)1-xAxCoO3 {R'=(rare earth elements and Y); A=(Ca, Ba and Sr)} at ambient pressure or under high pressure. The system exhibits a phase transition from a nearly metallic to an insulating state with decreasing temperature T, where the low spin (LS) state of Co3+ is suddenly stabilized. For y=0, we have constructed a T-x phase diagram at various values of the external pressure p. It shows that the (T, x) region of the low temperature phase, which is confined to a very narrow region around x=0.5 at ambient pressure, expands as p increases, suggesting that the transition is not due to an order-disorder type one. For the occurrence of the transition, both the Pr and Ca atoms seem to be necessary. The intimate relationship between the local structure around the Co ions and the electronic (or spin) state of Co3+ ions is discussed: For the smaller unit cell volume or the smaller volume of the CoO6 octahedra and for the larger tilting angle of the octahedra, the temperature of the transition becomes higher. The role of the carriers introduced by the doping of the A atoms, is also discussed. By analyzing the data of 59Co-NMR spectra and magnetic susceptibilities of Pr1-xCaxCoO3 the energy separations among the different spin states of Co3+ and Co4+ are roughly estimated.

cond-mat.str-el↗

arXiv2003

Ru-NMR Studies and Specific Heat Measurements of Bi3Ru3O11 and La4Ru6O19

Specific heats measurements and Ru-NMR studies have been carried out for Bi3Ru3O11 and La4Ru6O19, which commonly have three-dimensional linkages of edge-sharing pairs of RuO6 octahedra. The Knight shifts, the nuclear spin-lattice relaxation rates 1/T1 and the electronic specific heats Cel of these systems exhibit anomalous temperature (T) dependence at low temperatures, as was pointed out by Khalifah et al. [Nature 411 (2001) 660.] for the latter system based on their experimental data of the resistivity, magnetic susceptibility and electronic specific heat. Ratios of 1/T1T to the square of the spin component of the isotropic Knight shift, Kspin estimated for these systems at low temperatures suggest that they have antiferromagnetic (AF) spin fluctuations. It is confirmed by the fact that the T-dependences of 1/T1T and Cel/T of the present systems can be explained by the self-consistent renormalization theory for three dimensional itinerant electron systems with AF spin fluctuations. All these results suggest that the AF fluctuations are the primary origin of the characteristics of their low temperature physical behavior.

cond-mat.str-el↗

arXiv2003

Studies on Magnetic Excitation Spectra of High-Tc Superconductors

Magnetic excitation spectra c"(q,w) of YBa2Cu3Oy, La2-xSrxCuO4, La2-y-xNdySrxCuO4 and Nd2-xCexCuO4 are compared with those of model calculations, where q and w are the wave vector and the energy of the excitations. In the model, the dynamical spin susceptibility is calculated by the expression c(q,w)= c0(q,w)/{1+J(q)c0(q,w)}, where c0(q,w) is obtained by adopting the effective energy dispersion of the quasi particles which can reproduce the shapes of the Fermi surface, and J(q) is the exchange coupling between the Cu-Cu electrons. It is important to consider the experimentally observed quasi particle broadening (G) to reproduce the observed spectra of YBa2Cu3Oy, while to reproduce the data of La2-xSrxCuO4 and La2-y-xNdySrxCuO4, very small G values have to be used, even in the phases without the static "stripe" order. This may be caused by the existence of the "stripe" correlations or "stripe" fluctuations.

cond-mat.supr-con↗

arXiv2003

Magnetic Structures and Spin States of NdBaCo2O5

Neutron diffraction studies have been carried out on a single crystal of oxygen-deficient perovskite NdBaCo2O5 in the temperature range of 7-370 K. There have been observed, with decreasing temperature T, two magnetic transitions accompanied with structural changes, to the antiferromagnetic phase at ~360 K and to the charge ordered antiferromagnetic one at ~250 K. We have studied the magnetic structures at 300 K and 7 K. At these temperatures the Co-moments have the so-called G-type antiferromagnetic structure, and Co ions are considered to be in the high spin state over the whole temperature range below TN 360 K irrespective of whether the charge ordering is present or not.

cond-mat.str-el↗

arXiv2003

Non-Abelian Statistics of Axion Strings

We examine an axion string coupled to a Majorana fermion. It is found that there exist a Majorana-Weyl zero mode on the string. Due to the zero mode, the axion strings obey non-abelian statistics.

hep-th↗

arXiv2003

Motion of rotatory molecular motor and chemical reaction rate

We examine the dependence of the physical quantities of the rotatory molecular motor, such as the rotation velocity and the proton translocation rate, on the chemical reaction rate using the model based only on diffusion process. A peculiar behavior of proton translocation is found and the energy transduction efficiency of the motor protein is enhanced by this behavior. We give a natural explanation that this behavior is universal when certain inequalities between chemical reaction rates hold. That may give a clue to examine whether the motion of the molecular motor is dominated by diffusion process or not.

physics.bio-ph↗

arXiv2003

59Co-NQR study on superconducting NaxCoO2.yH2O

Layered Co oxide NaxCoO2.yH2O with a superconducting transition temperature Tc =4.5 K has been studied by 59Co NQR. The nuclear spin relaxation rate 1/59T1 is nearly proportional to temperature T in the normal state. In the superconducting state, it exhibits the coherence peak and decreases with decreasing T below ~0.8Tc. Detailed comparison of the 1/T1T values and the magnetic susceptibilities between NaxCoO2.yH2O and NaxCoO2 implies that the metallic state of the former system is closer to a ferromagnetic phase than that of the latter. These experimental results impose a restriction on the mechanism of the superconductivity.

cond-mat.supr-con↗

arXiv2003

59Co-NMR Knight Shift of the Superconducting NaxCoO2.yH2O

Layered Co oxide NaxCoO2.yH2O with the superconducting transition temperature Tc =4.5 K has been studied by 59Co-NMR. The Knight shift K estimated from the observed spectra for powder sample exhibits almost temperature(T)-independent behavior above Tc and decreases with decreasing T below Tc. This result and the existence of the coherence peak in the spin-lattice-relaxation-rate versus T curve reported by the present authors indicate, naively speaking, that the singlet order parameter with s-wave symmetry is realized in NaxCoO2.yH2O. Differences of the observed behaviors between the spectra of the non-aligned sample and the one aligned in epoxy adhesive by applying the external magnetic field are discussed.

cond-mat.supr-con↗

arXiv2003

Magnetic Structures of High Temperature Phases of TbBaCo2O5.5

Neutron diffraction studies have been carried out on a single crystal of oxygen-deficient perovskite TbBaCo2O5.5 in the temperature range of 7-370 K. There have been observed several magnetic or structural transitions. Among these, the existence of the transitions to the insulating phase from the metallic one at ~340 K, to the one with the ferromagnetic moment at ~280 K and possibly to the antiferromagnetic one at ~260 K, with decreasing temperature T correspond to those reported in former works. We have studied the magnetic structures at 270 K and 250 K and found that all Co3+ ions of the CoO6 octahedra are in the low spin state and those of the CoO5 pyramids carry spins which are possibly in the intermediate spin state. Non-collinear magnetic structures are proposed at these temperatures. Two other transitions have also been observed at the temperatures, ~100 K and ~250 K.

cond-mat.str-el↗