arXiv subjects

S. Takeshita

Publications and source records attributed to S. Takeshita.

25 records · Page 2Linked to original sources

arXiv2009

Insular superconductivity in Co-doped iron pnictide CaFe$_{1-x}$Co$_x$AsF

The presence of macroscopic phase separation between the superconducting and magnetic phases in \cfcaf is demonstrated by muon spin rotation (muSR) measurements conducted across their phase boundaries (x=0.05-0.15). The magnetic phase tends to retain the high transition temperature (T_m > T_c), while Co-doping induces strong randomness. The volumetric fraction of superconducting phase is nearly proportional to the Co content $x$ with constant superfluid density. These observations suggest the formation of superconducting "islands" (or domains) associated with Co ions in the Fe$_2$As$_2$ layers, indicating a very short coherence length.

cond-mat.supr-con↗

arXiv2009

Competition/Coexisitence of Magnetism and Superconductivity in Iron Pnictides Probed by Muon Spin Rotation

The presence of macroscopic phase separation into superconducting and magnetic phases in LaFeAsO$_{1-x}$F$_x$ (LFAOF) and CaFe$_{1-x}$Co$_x$AsF (CFCAF) is demonstrated by muon spin rotation (muSR) measurement across their phase boundaries ($x=0.06$ for LFAOF and $x=0.075$--0.15 for CFCAF). In LFAOF, both magnetism and superconductivity develop simultaneously below a common critical temperature, $T_m =~ T_c =~ 18$ K, where the magnetism is characterized by strong randomness. A similar, but more distinct segregation of these two phases is observed in CFCAF, where the magnetic phase retains $T_m$ as close to that of the parent compound ($T_c << T_m =~ 80$--120 K) and the superconducting volume fraction is proportional to the Co content $x$. The close relationship between magnetism and superconductivity is discussed based on these experimental observations. Concerning superconducting phase, an assessment is made on the anisotropy of order parameter in the superconducting state of LFAOF, CFCAF, and Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (BKFA, $x=0.4$) based on the temperature dependence of superfluid density [$n_s(T)$] measured by muSR. The gap parameter, $2Δ/k_BT_c$, determined from $n_s(T)$ exhibits a tendency that hole-doped pnictides (BKFA) is much greater than those in electron-doped ones (LFAOF, CFCAF), suggesting difference in the coupling to bosons mediating the Cooper pairs between relevant $d$ electron bands.

cond-mat.supr-con↗

arXiv2008

Full Gap Superconductivity in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Probed by Muon Spin Rotation

Superfluid density ($n_s$) in the mixed state of an iron pnictide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ is determined by muon spin rotation for a sample with optimal doping ($x=0.4$). The temperature dependence of $n_s$ is perfectly reproduced by the conventional BCS model for s-wave paring, where the order parameter can be either a single-gap with $Δ=8.35(6)$ meV [$2Δ/k_BT_c=5.09(4)$], or double-gap structure with $Δ_1=12$ meV (fixed) [$2Δ_1/k_BT_c=7.3$] and $Δ_2=6.8(3)$ meV [$2Δ_2/k_BT_c=4.1(2)$]. The latter is consistent with the recent result of angle-resolved photo-emssion spectroscopy. The large gap parameters ($2Δ/k_BT_c$) indicate extremely strong coupling of carriers to bosons that mediate the Cooper pairing.

cond-mat.supr-con↗

arXiv2008

Magnetic response of noncentrosymmetric superconductor La2C3: Effect of double-gap and spin-orbit interaction

The presence of spin-orbit (SO) interaction in a noncentrosymmetric superconductor, La2C3 (T_c~11 K) is demonstrated by muon spin rotation (muSR) in its normal state, where muSR spectra exhibit field-induced weak depolarization due to van Vleck-like local susceptibility. In the mixed state, muon spin relaxation due to inhomogeneity of internal field (sigma_v) exhibits a field dependence that is characterized by a kink, where sigma_v (and hence the superfluid density) is more strongly reduced at lower fields. This is perfectly in line with the presence of a secondary energy gap previously inferred from the temperature dependence of sigma_v, and also consistent with the possible influence of asymmetric deformation of the Fermi surface due to the SO interaction.

cond-mat.supr-con↗

arXiv2008

Effect of Zn substitution for Cu on Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ near the hole concentration of 1/8 per Cu

A weakening of superconductivity upon substitution of Cu by Zn (0.5~1 %) is observed in a high-T_c cuprate, Ca_{2-x}Na_xCuO2Cl2, near the hole concentration of 1/8 per Cu. The superconducting transition temperature and its volume fraction, estimated by magnetic susceptibility, exhibit a sizable anomaly for x=0.12~0.14, where the slowing down of Cu spin fluctuations below 5 K is demonstrated by muon spin relaxation experiments. These observations are in close resemblance to other typical cuprates including YBa2Cu3O_{7-d}, and Bi2Sr2Ca_{1-x}Y_xCu2O_{8+d}, providing further evidence that Zn-induced "stripe" correlation is a universal feature of high-T_c cuprate superconductors common to that of La_{2-x}A_{x}CuO4 (A=Ba, Sr).

cond-mat.supr-con↗

arXiv2008

Coexistence of Supercondcutivity and Magnetism in LaFeAs(O0.94F0.06) Probed by Muon Spin Relaxation

Recent discovery of oxypnictide superconductor LaFeAs(O,F) (LFAO-F) with the critical temperature (Tc) of 26 K and succeeding revelation of much increased Tc upon substitution of La for other rare earth elements (such as Sm, leading to ~43 K) and application of pressure for LFAO-F (~ 43 K) has triggered broad interest in the mechanism yielding relatively high Tc in this new class of compounds. While they share a feature with high-Tc cuprates that superconductivity occurs upon carrier doping to pristine compound which exhibits magnetism, they also resemble the heavy-fermion compounds in the sense that superconductivity appears in the vicinity of magnetic phase. Investigation of electronic states near the boundary between these two phases might provide some useful information on the mechanism of superconductivity, as it has been proved to be the case in many exotic superconductors. Here we show by muon experiment in the LFAO-F compound that a macroscopic phase separation into superconducting and spin glass-like magnetic phases occurs at x=0.06 that is near the phase boundary, where both the magnetism and superconductivity develop simultaneously below a common Tc ~ 18 K. This accordance strongly suggests intimate relationship between magnetism and superconductivity typically found in heavy-fermion systems near the quantum critical point.

cond-mat.mtrl-sci↗

arXiv2008

Multigap superconductivity in sesquicarbides La$_2$C$_3$ and Y$_2$C$_3$

A complex structure of the superconducting order parameter in $Ln_2$C$_3$ ($Ln$ = La, Y) is demonstrated by muon spin relaxation ($μ$SR) measurements in their mixed state. The muon depolarization rate [$σ_{\rm v}(T)$] exhibits a characteristic temperature dependence that can be perfectly described by a phenomenological double-gap model for nodeless superconductivity. While the magnitude of two gaps is similar between La$_2$C$_3$ and Y$_2$C$_3$, a significant difference in the interband coupling between those two cases is clearly observed in the behavior of $σ_{\rm v}(T)$.

cond-mat.supr-con↗