In order to investigate details of the superconducting (SC) gap in the iron-chalcogenide superconductors, the specific heat, C, of FeSe_1-x_Te_x_ with x=0.6-1 has been measured in magnetic fields. Using the two-gap model, it has been found that the s
maller SC gap is significantly depressed by the application of magnetic field, resulting in the increase of the slope of the C/T vs T^2^ plot at low temperatures. From the specific-heat measurements at very low temperatures down to 0.4 K, it has been found that the enhancement of the residual electronic-specific-heat-coefficient in the ground state, gamma_0_, by the application of magnetic field is much smaller than that expected for superconductors with the typical s-wave or d-wave SC paring symmetry, which is in sharp contrast to the significant enhancement of gamma_0 observed in the iron-pnictide superconductors. These results are discussed in relation to the multi-band effect in the iron-based superconductors.
The evolution of the electronic state through the reduction annealing has been investigated in electron-doped Pr_1.3-x_La_0.7_Ce_x_CuO_4+delta_ (x=0.10) single crystals with the so-called T structure. From the ab plane and c axis electrical resistivi
ty measurements in magnetic fields, it has been found that, through the reduction annealing, the strongly localized state of carriers accompanied by the antiferromagnetic (AF) pseudogap in the as-grown crystal changes to a metallic state bringing about the Kondo effect without AF pseudogap and to a superconducting state. These results are able to be understood in terms of a model based on the strong electron correlation. The complete removal of excess oxygen in the T-cuprates is expected to result in the appearance of superconductivity in a wide range of the Ce concentration including the parent compound of x=0.
The electronic specific heat of as-grown and annealed single-crystals of FeSe1-xTex (0.6<=x<=1) has been investigated. It has been found that annealed single-crystals with x=0.6-0.9 exhibit bulk superconductivity with a clear specific-heat jump at th
e superconducting (SC) transition temperature, Tc. Both 2Delta_0/kBTc [Delta_0: the SC gap at 0 K estimated using the single-band BCS s-wave model] and Delta C/(gamma_n-gamma_0)Tc [Delta C$: the specific-heat jump at Tc, gamma_n: the electronic specific-heat coefficient in the normal state, gamma_0: the residual electronic specific-heat coefficient at 0 K in the SC state] are largest in the well-annealed single-crystal with x=0.7, i.e., 4.29 and 2.76, respectively, indicating that the superconductivity is of the strong coupling. The thermodynamic critical field has also been estimated. gamma_n has been found to be one order of magnitude larger than those estimated from the band calculations and increases with increasing x at x=0.6-0.9, which is surmised to be due to the increase in the electronic effective mass, namely, the enhancement of the electron correlation. It has been found that there remains a finite value of gamma_0 in the SC state even in the well-annealed single-crystals with x=0.8-0.9, suggesting an inhomogeneous electronic state in real space and/or momentum space.
We have measured the thermal conductivity along the c-axis parallel to the spin-chains, kappa_c, of the one-dimensional antiferromagnetic spin system SrCuO_2, using as-grown and O_2-annealed single-crystals grown from raw materials with 99.9% (3N) an
d 99.99% (4N) purity. The value of kappa_c around 50K, where large contribution of the thermal conductivity due to spinons, kappa_spinon, is observed, is markedly enhanced by both the increase of the purity of raw materials and the O_2-annealing. Therefore, the increase of kappa_c implies that kappa_spinon is enhanced due to the decrease of spin defects caused by impurities in raw materials and by oxygen defects. The mean free path of spinons is as large as about 24000 angstrom at low temperatures in the O_2-annealed single-crystal grown from raw materials with 4N purity.
We have measured the thermal conductivity along the [101] direction, kappa_[101]_, along the [10-1] direction, kappa_[10-1]_, and along the b-axis, kappa_b_, of the quasi one-dimensional S=1/2 spin system Sr_2_V_3_O_9_ in magnetic fields up to 14 T,
in order to find the thermal conductivity due to spinons and to clarify whether the spin-chains run along the [101] or [10-1] direction. It has been found that both kappa_[101]_, kappa_[10-1]_ and kappa_b_ show one peak around 10 K in zero field and that the magnitude of kappa_[10-1]_ is larger than those of kappa_[101]_ and kappa_b_. By the application of magnetic field along the heat current, the peak of kappa_[10-1]_ is markedly suppressed, while the peaks of kappa_[101]_ and kappa_b_ little change. These results indicate that there is a large contribution of spinons to kappa_[10-1]_ and suggest that the spin-chains run along the [10-1] direction.
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.
Muon-spin-relaxation (muSR) measurements have been performed for the partially Zn-substituted electron-doped high-T_c_ superconductor Pr_0.86_LaCe_0.14_Cu_1-y_Zn_y_O_4+alpha-delta_ with y=0-0.05 and the reduced oxygen content delta=0-0.09, in order t
o investigate nonmagnetic Zn-impurity effects on the Cu-spin dynamics. For all the measured samples with delta=0.01-0.09, it has been found that a fast depolarization of muon spins is observed below 100 K due to the effect of Pr^3+^ moments and that the muSR time spectrum in the long-time region above 5 mu-sec increases with decreasing temperature at low temperatures below 30 K possibly due to slowing down of the Cu-spin fluctuations assisted by Pr^3+^ moments. No Zn-induced slowing down of the Cu-spin fluctuations has been observed for moderately oxygen-reduced samples with delta=0.04-0.09, which is very different from the muSR results of La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_. The possible reason may be that there are no dynamical stripe correlations of spins and electrons in the electron-doped high-T_c_ cuprates or that the effect of Pr^3+^ moments on the muSR spectra is stronger than that of a small amount of Zn impurities.
Temperature dependence of the Hall coefficient, R_H_, has been investigated in charge-spin stripe-ordered La-214 high-T_c_ superconductors. Using the simplest stripe-ordered system of La_2-x_Ba_x_CuO_4_, it has been clarified that both the behavior o
f R_H_ and its sign exhibit significant dependences on the hole concentration. That is, R_H_ is zero in the ground state of the charge-spin stripe order at x=1/8, while it is negative in the less-stabilized state of the charge stripe for x<1/8. These are interpreted as owing to the delicate balance of the contributions of the hole-like Fermi surface and the possible electron pocket arising from the formation of the charge-spin stripe order.
We have investigated effects of Zn and Ni on the Cu-spin dynamics and superconductivity from the zero-field muon-spin-relaxation (ZF-muSR) and magnetic-susceptibility, chi, measurements for La_2-x_Sr_x_Cu_1-y_(Zn,Ni)_y_O_4_ with x=0.15-0.20, changing
y up to 0.10 in fine step. In the optimally doped x=0.15, it has been concluded that the formation of a magnetic order requires a larger amount of Ni than that of Zn, which is similar to our previous results of x=0.13. From the estimation of volume fractions of superconducting (SC) and magnetic regions, it has been found for x=0.15 that the SC region is in rough correspondence to the region where Cu spins fluctuate fast beyond the muSR frequency window for both Zn- and Ni-substituted samples. According to the stripe model, it follows that, even for x=0.15, the dynamical stripe correlations of spins and holes are pinned and localized around Zn and Ni, leading to the formation of the static stripe order and the suppression of superconductivity. These may indicate an importance of the dynamical stripe in the appearance of the high-T_c_ superconductivity in the hole-doped cuprates. In the overdoped regime of x=0.18 and 0.20, on the other hand, the SC region seems to be in rough correspondence to the region where Cu spins fluctuate fast beyond the muSR frequency window, though it appears that the Cu-spin dynamics and superconductivity are affected by the phase separation into SC and normal-state regions.
Muon-spin-relaxation measurements have been performed for the partially Zn-substituted La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ with y=0-0.10 in the overdoped regime up to x=0.30. In the 3 % Zn-substituted samples up to x=0.27, exponential-like depolarization of
muon spins has been observed at low temperatures, indicating Zn-induced slowing-down of the Cu-spin fluctuations. The depolarization rate decreases with increasing x and almost no fast depolarization of muon spins has been observed for x=0.30 where superconductivity disappears. The present results suggest that the dynamical stripe correlations exist in the whole superconducting regime of La_2-x_Sr_x_CuO_4_ and that there is no quantum critical point at x~0.19.
