arXiv subjects

Yoji Koike

Publications and source records attributed to Yoji Koike.

At least 37 records · Page 2Linked to original sources

arXiv2016

Pairing Symmetry Studied from Impurity Effects in the Undoped Superconductor T'-La$_{1.8}$Eu$_{0.2}$CuO$_4$

We have investigated the effects of magnetic Ni and nonmagnetic Zn impurities on the superconductivity in undoped T'-La$_{1.8}$Eu$_{0.2}$CuO$_4$ (T'-LECO) with the Nd$_2$CuO$_4$-type structure, using the polycrystalline bulk samples, to clarify the pairing symmetry. It has been found that both suppression rates of the superconducting transition temperature $T_\mathrm{c}$ by Ni and Zn impurities are nearly the same and are very similar to those in the optimally doped and overdoped regimes of hole-doped T-La$_{2-x}$Sr$_{x}$CuO$_4$ with the K$_2$NiF$_4$-type structure. These results strongly suggest that the superconductivity in undoped T'-LECO is of the $d$-wave symmetry and is mediated by the spin fluctuation.

cond-mat.supr-con↗

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↗

arXiv2015

Influence of Optically Quenched Superconductivity on Quasiparticle Relaxation Rates in Bi2Sr2CaCu2O8+delta

We use time- and angle-resolved photoemission to measure quasiparticle relaxation dynamics across a laser-induced superconducting phase transition in Bi2Sr2CaCu2O8+delta. Whereas low-fluence measurements reveal picosecond dynamics, sharp femtosecond dynamics emerge at higher fluence. Analyses of data as a function of energy, momentum, and doping indicate that the closure of the near-nodal gap and disruption of macroscopic coherence are primary mechanisms driving this onset. The results demonstrate the important influence of transient electronic structure on relaxation dynamics, which is relevant for developing an understanding of nonequilibrium phase transitions.

cond-mat.str-el↗

arXiv2015

Frequency Extension to THz Range in High Pressure ESR System and Its Application to Shastry-Sutherland Model Compound SrCu$_{2}$(BO$_{3}$)$_{2}$

We have made a survey of ceramics for the inner parts of the transmission-type pressure cell to achieve the high pressure and the high transmission in THz range. By using the optimal combination of ZrO$_{2}$-based ceramic and Al$_{2}$O$_{3}$ ceramic, we have succeeded in obtaining the pressure up to 1.5 GPa and the frequency region up to 700 GHz simultaneously. We show the high-pressure ESR results of Shastry-Sutherland compound SrCu$_{2}$(BO$_{3}$)$_{2}$ as an application. We observed the direct ESR transition modes between the singlet ground state and the triplet excited states up to the pressure of 1.51 GPa successfully, and obtained the precise pressure dependence of the gap energy. The gap energy is directly proved to be suppressed by the pressure. Moreover, we found that the system approaches the quantum critical point with pressure by comparing the obtained data with the theory. This result also shows the usefulness of high-pressure ESR measurement in THz region to study quantum spin systems.

cond-mat.mtrl-sci↗

arXiv2015

Successive magnetic transitions relating to itinerant spins and localized Cu spins in La2-xSrxCu1-yFeyO4: Possible existence of stripe correlations in the overdoped regime

In order to investigate the relationship between the so-called stripe correlations and the high-Tc superconductivity, we have carried out magnetic susceptibility and zero-field muon-spin-relaxation measurements in partially Fe-substituted La2-xSrxCu1-yFeyO4 (LSCFO). It has been found that the Fe substitution induces successive magnetic transitions in the overdoped regime, namely, a spin-glass transition of Fe3+ spins due to the RKKY interaction based on itinerant spins at higher temperatures and a stripe-order transition of localized Cu2+ spins at lower temperatures. The stripe-order transition disappears at the hole concentration of ~0.28 per Cu which coincides with the endpoint of the superconductivity in pristine La2-xSrxCuO4, suggesting that the stripe correlations are closely related to the high-Tc superconductivity in the overdoped regime as well as in the underdoped regime. As for the spin-glass state, it has been found that the contribution of polarized itinerant spins to the muon-spin depolarization is not negligible. Taking into account neutron-scattering results by R.-H. He et al. [Phys. Rev. Lett. 107, 127002 (2011)] that a spin-density-wave (SDW) order has been observed in overdoped LSCFO, the present results demonstrate the presence of a novel coexistent state of a SDW order and a spin-glass state.

cond-mat.supr-con↗

arXiv2015

Resolving unoccupied electronic states with laser ARPES in bismuth-based cuprate superconductors

Angle-resolved photoemission spectroscopy (ARPES) is typically used to study only the occupied electronic band structure of a material. Here we use laser-based ARPES to observe a feature in bismuth-based superconductors that, in contrast, is related to the unoccupied states. Specifically, we observe a dispersive suppression of intensity cutting across the valence band, which, when compared with relativistic one-step calculations, can be traced to two final-state gaps in the bands 6 eV above the Fermi level. This finding opens up possibilities to bring the ultra-high momentum resolution of existing laser-ARPES instruments to the unoccupied electron states. For cases where the final-state gap is not the object of study, we find that its effects can be made to vanish under certain experimental conditions.

cond-mat.supr-con↗

arXiv2014

Unscaling Superconducting Parameters with $T_c$ for Bi-2212 and Bi-2223: A Magnetotransport Study in the Superconductive Fluctuation Regime

To investigate the origin of the enhanced $T_c$ ($\approx$ 110 K) of the trilayer cuprate superconductor Bi$_{2}$Sr$_{2}$Ca$_{2}$Cu$_{3}$O$_{10+δ}$ (Bi-2223), we have performed systematic magnetoresistance (MR) measurements on this superconductor, as well as on the bilayer superconductor, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ (Bi-2212). The in-plane coherence length, $ξ_{ab}$, and the specific-heat jump, $ΔC$, have been estimated using the theory of renormalized superconductive fluctuations, and the doping dependence of these parameters has been qualitatively explained using the Fermi arc approach. A detailed comparison of the superconducting parameters with $T_c$ for these compounds suggests that an additional superconducting condensation energy exists, due to an increase in the number of stacking CuO$_{2}$ planes in a unit cell.

cond-mat.supr-con↗

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

Doping Dependencies of Onset Temperatures for the Pseudogap and Superconductive Fluctuation in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$, Studied from both In-Plane and Out-of-Plane Magnetoresistance Measurements

To investigate the relationship between the pseudogap and superconductivity, we measured both the in-plane ($ρ_{ab}$) and out-of-plane ($ρ_c$) resistivity for oxygen-controlled Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ single crystals subject to magnetic fields (parallel to the $c$ axis) of up to 17.5 T. The onset temperature for the superconductive fluctuation, $T_{scf}$, is determined by the large positive in-plane magnetoresistance (MR) and negative out-of-plane MR observed near $T_c$, whereas the pseudogap opening temperature $T^*$ is determined by the semiconductive upturn of the zero-field $ρ_c$. $T_{scf}$ was found to scale roughly as $T_c$, with a decreasing temperature interval between them upon doping. On the other hand, $T^*$ starts out much higher than $T_{scf}$ but decreases monotonically upon doping; finally, at a heavily overdoped state, it is not observed above $T_{scf}$. These results imply that the pseudogap is not a simple precursor of superconductivity, but that further study is needed to determine whether or not $T^*$ exists below $T_{scf}$ in the heavily overdoped state.

cond-mat.supr-con↗

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↗