arXiv subjects

Takayuki Kawamata

Publications and source records attributed to Takayuki Kawamata.

34 records · Page 2Linked to original sources

arXiv2016

Superconductivity and Intercalation Statein the Lithium-Hexamethylenediamine-Intercalated Superconductor Li$_x$(C$_6$H$_{16}$N$_2$)$_y$Fe$_{2-z}$Se$_2$: Dependence on the Intercalation Temperature and Lithium Content

The superconductivity and intercalation statein the lithium-and hexamethylenediamine (HMDA)-intercalated superconductor Li$_x$(C$_6$H$_{16}$N$_2$)$_y$Fe$_{2-z}$Se$_2$ have been investigated from powder x-ray diffraction, thermogravimetric and magnetic susceptibility measurements, changing the intercalation temperature, $T_i$, and the Li content, $x$. Both Li and HMDA have been co-intercalated stably up to $x$ = 2 roughly in the molar ratio of $x : y = 2 : 1$. In the case of $T_i$ = 45$^\circ$C, it has been found that both Li and HMDA are co-intercalated locally at the edge of FeSe crystals, indicating that both Li and HMDA are hard to diffuse into the inside of FeSe crystals at 45$^\circ$C. In the case of $T_i$ = 100$^\circ$C, on the other hand, it has been found that both Li and HMDA diffuse into the inside of FeSe crystals, so that $T_c$ tends to increase with increasing $x$ from ~30 K at $x$ = 1 up to 38 K at $x$ = 2 owing to the increase of electron carriers doped from Li into the FeSe layers.

cond-mat.supr-con↗

arXiv2016

New Superconducting Phase of Li$_x$(C$_6$H$_{16}$N$_2$)$_y$Fe$_{2-z}$Se$_2$ with $T_\textrm{c}$ = 41 K Obtained through the Post-Annealing

Post-annealing effects on the crystal structure and superconductivity of the lithium- and hexamethylenediamine (HMDA)-intercalated superconductor Lix(C6H16N2)yFe2-zSe2 have been investigated. Through the post-annealing, a two-step reduction of the interlayer spacing between neighboring Fe layers, d, has been observed. It has been found that a new phase of Lix(C6H16N2)yFe2-zSe2 with d= 10.30(2) Å and Tc = 41 K different from the as-intercalated phase is stabilized owing to the possible stable inclination of HMDA intercalated between FeSe layers. This result supports the domic relation between Tc and d in the FeSe-based intercalation superconductors. The reason why Tc increases with a decrease in d through the post-annealing is discussed.

cond-mat.supr-con↗

arXiv2015

Thermal Conductivity and Spin State of the Spin Diamond-Chain System Azurite Cu3(CO3)2(OH)2

In order to investigate the spin state of azurite, Cu3(CO3)2(OH)2, we have measured the thermal conductivity along the c-axis, $κ$c, perpendicular to the spin diamond-chains. It has been found that the temperature dependence of $κ$c shows a broad peak at ~ 100 K, which is explained as being due to the strong phonon-scattering by the strong spin-fluctuation owing to the spin frustration at low temperatures below ~ 100 K. Furthermore, it has been found that the temperature dependence of $κ$c shows another peak at low temperatures below 20 K and that $κ$c is suppressed by the application of magnetic field along the c-axis at low temperatures below ~ 35 K. In high magnetic fields above ~ 8 T at low temperatures below ~ 6 K, it has been found that $κ$c increases with increasing field. The present results have indicated that both spin-singlet dimers with a spin gap of ~ 35 K and antiferromagnetically correlated spin-chains with the antiferromagnetic exchange interaction of ~ 5.4 K are formed at low temperatures, which is consistent with the recent conclusion by Jeschke et al. [Phys. Rev. Lett. 106, 217201 (2011)] that the ground state of spins in azurite in zero field is a spin-fluid one. In addition, a new quantum critical line in magnetic fields at temperatures above 3 K has been proposed to exist.

cond-mat.str-el↗

arXiv2015

Magnetic State of the Geometrically Frustrated Quasi-One-Dimensional Spin System Cu$_3$Mo$_2$O$_9$ Studied by Thermal Conductivity

We have measured the thermal conductivity of the geometrically frustrated quasi-onedimensional spin system Cu$_3$Mo$_2$O$_9$ in magnetic fields. A contribution of the thermal conductivity due to spins has been observed in the thermal conductivity along the spin chains. The thermal conductivity due to phonons, $κ_{\rm phonon}$, has been found to decrease by the application of a magnetic field, which has been explained as being due to the reduction in the spin gap originating from the spin-singlet dimers. Moreover, it has been found that $κ_{\rm phonon}$ increases with increasing field in high fields above ~7 T at low temperatures. This suggests the existence of a novel field-induced spin state and is discussed in terms of the possible spin-chirality ordering in a frustrated Mott insulator.

cond-mat.str-el↗

arXiv2014

Synthesis and post-annealing effects of alkaline-metal-ethylenediamine-intercalated superconductors Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) with Tc = 45 K

New iron-based intercalation superconductors Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) with Tc = 45 K have successfully been synthesized via intercalation of dissolved alkaline metal in ethylenediamine. The c-axis lengths of Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) are 20.74(7) Å and 21.9(1) Å, respectively, and are about 50 % larger than that of KxFe2Se2, indicating that not only alkaline metal but also ethylenediamine is intercalated between the Se-Se layers of FeSe. It seems that the high-Tc of Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) is caused by the possible two-dimensional electronic structure due to the large c-axis length. Through the post-annealing in an evacuated glass tube, it has been found that Tc decreases with increasing post-annealing temperature and that deintercalation of EDA from the as-intercalated sample takes place at low temperatures below 250 °C.

cond-mat.supr-con↗

arXiv2014

Thermal Conductivity due to Spinons in the One-Dimensional Quantum Spin System Sr2V3O9

We have measured the thermal conductivity along different directions of the S = 1/2 one-dimensional (1D) spin system Sr2V3O9 in magnetic fields up to 14 T. It has been found that the thermal conductivity along the [10-1] direction, \k{appa}[10-1], is large and markedly suppressed by the application of magnetic field, indicating that there is a large contribution of spinons to \k{appa}[10-1] and that the spin chains run along the [10-1] direction. The maximum value of the thermal conductivity due to spinons is ~14 W/Km along the [10-1] direction, supporting the empirical law that the magnitude of the thermal conductivity due to spinons is roughly proportional to the antiferromagnetic interaction between the nearest neighboring spins.

cond-mat.str-el↗

arXiv2014

Observation of the Thermal Conductivity due to Spins in the One-Dimensional Antiferromagnetic Ising-Like Spin System ACoX3 (A = Rb, Cs; X = Cl, Br)

Thermal conductivity measurements have been carried out for single crystals of the onedimensional (1D) antiferromagnetic (AF) Ising-like spin system ACoX3 (A = Rb, Cs; X = Cl, Br). A shoulder originating from the thermal conductivity due to spin excitations, \k{appa}?spin, has been observed around 60 K in the temperature dependence of the thermal conductivity along the c-axis parallel to spin chains for every ACoX3. To our knowledge, this is the first observation of ?\k{appa}spin in 1D AF Ising-like spin systems. It has been found that the magnitude of ?\k{appa}spin in ACoX3 is a little smaller than those in 1D AF Heisenberg spin systems. The reason is discussed in terms of the Heisenberg-like character in ACoX3.

cond-mat.str-el↗

arXiv2014

Development of spatial inhomogeneity of internal magnetic field above $T_{\rm c}$ in Bi$_2$Sr$_2$Ca$_{1-x}$Y$_x$Cu$_2$O$_{8+δ}$ observed by longitudinal-field muon-spin-relaxation

Longitudinal-field muon-spin-relaxation measurements have revealed inhomogeneous distribution of the internal magnetic field at temperatures above the bulk superconducting (SC) transition temperature, $T_{\rm c}$, in slightly overdoped Bi$_2$Sr$_2$Ca$_{1-x}$Y$_x$Cu$_2$O$_{8+δ}$. The distribution width of the internal magnetic field, $Δ$, evolves continuously with decreasing temperature toward $T_{\rm c}$. The origin of the increase in $Δ$ is discussed in terms of the creation of SC domains in a sample.

cond-mat.supr-con↗

arXiv2013

New Li-Ethylenediamine-Intercalated Superconductor Li$_x$(C$_2$H$_8$N$_2$)$_y$Fe$_{2-z}$Se$_2$ with $T_c$ = 45 K

A new iron-based superconductor Li$_x$(C$_2$H$_8$N$_2$)$_y$Fe$_{2-z}$Se$_2$ with $T_c$ = 45 K has successfully been synthesized via intercalation of dissolved lithium metal in ethylenediamine. The distance between neighboring Fe layers is 10.37 Å and much larger than those of FeSe with $T_c$ = 8 K and K$_x$Fe$_2$Se$_2$ with $T_c$ ~ 30 K. It seems that the high-$T_c$ of Li$_x$(C$_2$H$_8$N$_2$)$_y$Fe$_{2-z}$Se$_2$ is caused by the possible two-dimensional electronic structure due to the large $c$-axis length.

cond-mat.supr-con↗

arXiv2013

Superconducting Symmetry Studied from Impurity Effects in Single-Crystal Fe1-yMySe0.3Te0.7 (M = Co, Ni, Zn)

We have investigated the suppression of the superconducting transition temperature, Tc, with an increase of the residual resistivity, ρ_0, through the substitution of M (M = Co, Ni, Zn) for Fe in Fe1-yMySe0.3Te0.7 single crystals, in order to clarify the symmetry of the superconducting gap in FeSe1-xTex. Small, large and very small suppression of Tc have been observed through the Co, Ni and Zn substitution, respectively. The magnitude of the suppression rate is hardly explained in terms of the pair-breaking effect due to potential scattering calculated based on the Abrikosov-Gor'kov theory, even if errors in the estimation of the carrier concentration, effective mass and ρ_0 were taken into account. Accordingly, these results suggest that the superconducting symmetry is the s++-wave in FeSe1-xTex.

cond-mat.supr-con↗

arXiv2013

Thermal conductivity and annealing effects in the iron-based superconductor FeSe$_{0.3}$Te$_{0.7}$

Thermal conductivity measurements in magnetic fields have been carried out for FeSe$_{0.3}$Te$_{0.7}$ single crystals as-grown and annealed at 400 {\degree}C for 100 h in vacuum ($\sim$ 10$^{-4}$ Pa). It has been found that the thermal conductivity in the {\it ab}-plane, {\kab}, of the annealed crystal shows an enhancement at low temperatures just below the superconducting transition temperature, {\tc}, owing to the thermal conductivity due to quasiparticles, while {\kab} of the as-grown crystal does not. This suggests that FeSe$_{1-x}$Te$_{x}$ is a strongly correlated electron system. It has also been found that both the degree of the enhancement of {\kab} just below {\tc} and the behavior of the suppression of {\kab} by the application of magnetic field for the annealed crystal depend on the thickness of the crystal. These results indicate that bulk superconductivity is absent in the as-grown crystal, appears only in the surface area in the annealed-thick crystal and appears in the almost whole region in the annealed-thin crystal. It has been concluded that the excess Fe included in the as-grown crystal is removed from the surface through the vacuum-annealing.

cond-mat.supr-con↗

arXiv2011

Study of Ni-doping Effect of Specific Heat and Transport Properties for LaFe1-yNiyAsO0.89F0.11

Specific heats and transport quantities of the LaFe1-yNiyAsO0.89F0.11 system have been measured, and the results are discussed together with those reported previously by our group mainly for LaFe1-yCoyAsO0.89F0.11 and LaFeAsO0.89-xF0.11+x systems. The y dependence of the electronic specific heat coefficient gamma can basically be understood by using the rigid-band picture, where Ni ions provide 2 electrons to the host conduction bands and behave as nonmagnetic impurities. The superconducting transition temperature Tc of LaFe1-yNiyAsO0.89F0.11 becomes zero, as the carrier density p (=2y+0.11) doped to LaFeAsO reaches its critical value p_c_ ~0.2. This p_c_ value of ~0.2 is commonly observed for LaFe1-yCoyAsO0.89F0.11 and LaFeAsO0.89-xF0.11+x systems, in which the relations p = x+0.11 and p = y+0.11 hold, respectively. As we pointed out previously, the critical value corresponds to the disappearance of the hole-Fermi surface. These results indicate that the carrier number solely determines the Tc value. We have not observed appreciable effects of pair breaking, which originates from the nonmagnetic impurity scattering of conduction electrons and strongly suppresses T_c_ values of systems with sign-reversing of the order parameter over the Fermi surface(s). On the basis of the results, the so-called s_+-_ symmetry of the order parameter with the sign-reversing is excluded.

cond-mat.supr-con↗

arXiv2010

Thermal Conductivity in the Bose-Einstein Condensed State of Triplons in the Bond-Alternating Spin-Chain System Pb2V3O9

In order to clarify the origin of the enhancement of the thermal conductivity in the Bose-Einstein Condensed (BEC) state of field-induced triplons, we have measured the thermal conductivity along the [101] direction parallel to spin-chains, $kappa_{\|[101]}$, and perpendicular to spin-chains, $kappa_{\perp[101]}$, of the S=1/2 bond-alternating spin-chain system Pb2V3O9 in magnetic fields up to 14 T. With increasing field at 3 K, it has been found that both $kappa_{\|[101]}$ and $kappa_{\perp[101]}$ are suppressed in the gapped normal state in low fields. In the BEC state of field-induced triplons in high fields, on the other hand, $kappa_{\|[101]}$ is enhanced with increasing field, while $kappa_{\perp[101]}$ is suppressed. That is, the thermal conductivity along the direction, where the magnetic interaction is strong, is markedly enhanced in the BEC state. Accordingly, our results suggest that the enhancement of $kappa_{\|[101]}$ in the BEC state is caused by the enhancement of the thermal conductivity due to triplons on the basis of the two-fluid model, as in the case of the superfluid state of liquid 4He.

cond-mat.str-el↗

arXiv2010

Change of the Ground State upon Hole Doping Unveiled by Ni Impurity in High-$T_{\rm c}$ Cuprates

The electronic ground state in high-$T_{\rm c}$ cuprates where the superconducting state is suppressed by Ni substitution has been investigated in La$_{2-x}$Sr$_x$Cu$_{1-y}$Ni$_y$O$_4$ from the specific heat and muon spin relaxation measurements. It has been found that the ground state changes from a magnetically ordered state with the strong hole-trapping by Ni to a metallic state with the Kondo effect of Ni with increasing hole-concentration. Moreover, the analysis of the results has revealed that a phase separation into the magnetically ordered phase and the metallic phase occurs around the boundary of two phases.

cond-mat.supr-con↗

arXiv2010

Pressure-induced new magnetic phase in Tl(Cu$_{0.985}$Mg$_{0.015}$)Cl$_3$ probed by muon spin rotation

We carried out zero-field muon-spin-rotation (ZF-$μ$SR) measurements in hydrostatic pressures in impurity-doped quantum spin gap system Tl(Cu$_{0.985}$Mg$_{0.015}$)Cl$_3$ to investigate microscopic magnetic properties of the pressure-induced phase. The spontaneous muon spin precession, which indicates the existence of a long-range coherent order, is observed in pressures. With decreasing temperature in 3.1 kbar, the internal static magnetic field at the muon sites {\it H}$_{\rm int}$ tends to saturate to 280 Oe around 4 K, however, decreases to 240 Oe at 2.3 K. These results suggest a rearrangement of ordered spins, and we speculate that the oblique antiferromagnetic phase, which is observed in the pressure of 14 kbar on the pure TlCuCl$_3$, appears in the Mg-doped system in lower pressures.

cond-mat.str-el↗

arXiv2008

Evidence for Ballistic Thermal Conduction in the One-Dimensional S=1/2 Heisenberg Antiferromagnetic Spin System Sr2CuO3

We have measured the thermal conductivity of the one-dimensional (1D) S=1/2 Heisenberg antiferromagnetic spin system of Sr2Cu1-xPdxO3 single crystals including nonmagnetic impurities of Pd2+. It has been found that the mean free path of spinons along the 1D spin chain at low temperatures is very close to the average length of finite spin chains between spin defects estimated from the magnetic susceptibility measurements. This proves that the thermal conduction due to spinons at low temperatures in Sr2CuO3 is ballistic as theoretically expected [Zotos et al.: Phys. Rev. Lett. 55 (1997) 11029].

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