摘要 :
We present detailed measurements of the longitudinal resistivity ρ_(xx)(T,H) and the upper critical field H_(c2) of NdFeAsO_(0.7)F_(0.3) single crystals in strong dc and pulsed magnetic fields up to 45 and 60 T, respectively. We ...
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We present detailed measurements of the longitudinal resistivity ρ_(xx)(T,H) and the upper critical field H_(c2) of NdFeAsO_(0.7)F_(0.3) single crystals in strong dc and pulsed magnetic fields up to 45 and 60 T, respectively. We found that the field scale of H_(c2) is comparable to H_(c2) ～ 100 T of high-T_c cuprates. H_(c2)(T) parallel to the c axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrys-talline LaFeAs(O,F), NdFeAs(O,F), and SmFeAs(O,F) samples. Thus, this behavior of H_(c2)~⊥,(T) is indeed an intrinsic feature of oxypnictides rather than manifestation of vortex lattice melting or granularity. The orien-tational dependence of H_(c2)(θ) as a function of the angle θ between H and the c axis shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy parameter γ(T) = (m_c/m_(ab))~(1/2)=H_(c2)~‖/H_(c2)~⊥ obtained from these measurements decreases as temperature decreases from γ approx= 9.2 at 44 K to γ approx= 5 at 34 K, where ‖ and ⊥ correspond to H parallel and perpendicular to the ab planes, respectively. Spin-dependent magnetore-sistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent of that in diluted magnetic semiconductors. The Ohmic resistivity ρ_(xx)(T,H) measured below T_c but above the irreversibility field exhibits a clear Arrhenius thermally-activated behavior ρ=ρ_0 exp[-E_a(T,H)/T] over 4-5 decades of ρ_(xx). The activation energy E_a(T,H) has very different field dependencies for H‖ab and H⊥ab varying from 4× 10~3 K at H=0.2 T to ～200 K at H = 35 T. We discuss to what extent different pairing scenarios suggested in the literature can manifest themselves in the observed behavior of H_(c2), using the two-band model of superconductivity in oxypnictides. The results indicate the importance of paramagnetic effects on H_(c2)(T) in oxypnictides, which may significantly reduce H_(c2)(0) as compared to H_(c2)(0)～ 200-300 T based on extrapolations of H_(c2)(T) near T_c down to low temperatures.
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摘要 :
We present a thermodynamic study of the phase diagram of single-crystal Ba!_(1-x)K_xFe_2As_2 using specilic-heat measurements. In zero-magnetic field a clear step in the heat capacity of △C/T_c=0.1 J/mol K~2 is observed at T_c ≈...
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We present a thermodynamic study of the phase diagram of single-crystal Ba!_(1-x)K_xFe_2As_2 using specilic-heat measurements. In zero-magnetic field a clear step in the heat capacity of △C/T_c=0.1 J/mol K~2 is observed at T_c ≈ 34.6 K for a sample with x=0.4. This material is characterized by extraordinarily high slopes of the upper critical field of μ_0 partial deriv H_(c2)~c/partial derivT=-6.5 T/K and μ_0 partial deriv H_(c2)~(ab)/ partial derivT=-17.4 T/K and a surprisingly low anisotropy of Γ ～2.6 near T_c. A consequence of the large field scale is the effective suppression of superconducting fluctua-tions. Using thermodynamic relations we determine Ginzburg-Landau parameters of κ_c ～ 100 and κ_(ab) ～ 260 identifying Ba_(1-x)K_xFe_2As_2 as extreme type II. The large value of the normalized discontinuity of the slopes of the specific heat at T_c, (T_c/△C)△(dC/dT)_(T_c) ～6, indicates strong-coupling effects in Ba_(1-x)K_xFe_2As_2.
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摘要 :
We report the study of temperature- and magnetic-field dependent in-plane tunneling conductance on the electron-doped Nd_(1. 85)Ce_(0.15)CuO_(4-y). and Pr_(1-x)LaCe_xCuO(4-y) single crystals. The previously reported normal state g...
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We report the study of temperature- and magnetic-field dependent in-plane tunneling conductance on the electron-doped Nd_(1. 85)Ce_(0.15)CuO_(4-y). and Pr_(1-x)LaCe_xCuO(4-y) single crystals. The previously reported normal state gap (NSG) Δ_(ps) and superconducting gap Δ_(sc) were observed simultaneously in superconducting state. Combined with an investigation on the evolution of low-energy spectral weight with temperature, our data present evidence for electron-doped cuprates that Δ_(ps)and Δ_(sc) are most possibly two different energy scales with very different field dependencies, and that superconductivity is condensed from the residual quasiparticle density of states (DOS) of the NSG state.
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We report an optical study of transition metal dichalcogenide 2H-TaS_2 and the Na intercalated superconductor Na_xTaS_2 over a broad frequency range at various temperatures. A clear gap feature was observed for 2H-TaS_2 when it un...
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We report an optical study of transition metal dichalcogenide 2H-TaS_2 and the Na intercalated superconductor Na_xTaS_2 over a broad frequency range at various temperatures. A clear gap feature was observed for 2H-TaS_2 when it undergoes the charge-density wave (CDW) transition. The existence of a Drude component in σ_1(ω) below T_(CDW) indicates that the Fermi surface of 2H-TaS_2 is only partially gapped in the CDW state. The spectral evolution of two different Na_xTaS_2 crystals further confirms that the partial gap structure observed in 2H-TaS_2 has a CDW origin. The CDW mechanism for 2H-TaS_2 and the competition between CDW and superconductivity in the Na_xTaS_2 system are discussed.
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We have studied the current-carrying capability of high quality e'pitaxial MgB_2 films synthesized using the hybrid physical-chemical vapor deposition technique by both transport measurement in nanobridge constrictions and magneti...
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We have studied the current-carrying capability of high quality e'pitaxial MgB_2 films synthesized using the hybrid physical-chemical vapor deposition technique by both transport measurement in nanobridge constrictions and magnetization measurement. An extremely high self-field critical current density J_c(0) > 10~8 A/cm~2, approaching the theoretical depairing current of MgB_2, was observed on a 150 nm bridge, indicating an excellent current-carrying capability in these films. The magnetization measurement also showed very high J_c.
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摘要 :
An unusual anisotropy of thermopower and magnetothermopower has been observed in the powerful thermoelectric Ca_3Co_4O_(9+δ) single crystal. The in-plane thermopower is about twice as big as the out-of-plane thermopower. Combinin...
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An unusual anisotropy of thermopower and magnetothermopower has been observed in the powerful thermoelectric Ca_3Co_4O_(9+δ) single crystal. The in-plane thermopower is about twice as big as the out-of-plane thermopower. Combining ab initio band structure calculation with semiclassical model analysis, we understand this anisotropy with band structure effects and especially with anisotropic Fermi surface. We find that a strong anisotropy in the topology of Fermi surface leads to the anisotropy of (magneto)thermopower. This study may also shed light on anisotropic properties of other layered cobalt oxides.
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