摘要 :
A large nonlinear response of a single crystal of ZrB_(12) to an ac field (frequency 40-2500 Hz) for H_0 > H_(c2) has been observed. Direct measurements of the ac wave form and the exact numerical solution of the Ginzburg-Landau e...
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A large nonlinear response of a single crystal of ZrB_(12) to an ac field (frequency 40-2500 Hz) for H_0 > H_(c2) has been observed. Direct measurements of the ac wave form and the exact numerical solution of the Ginzburg-Landau equations, as well as the phenomenological relaxation equation, permit the study of the surface superconducting state dynamics. It is shown that the low-frequency response is defined by transitions between the metastable superconducting states under the action of an ac field. The relaxation rate that determines such transition dynamics is found.
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In several iron-arsenide superconductors there is strong evidence for a fully gapped superconducting state consistent with either a conventional s-wave symmetry or an unusual s± state where the gap changes sign between the electr...
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In several iron-arsenide superconductors there is strong evidence for a fully gapped superconducting state consistent with either a conventional s-wave symmetry or an unusual s± state where the gap changes sign between the electron and hole Fermi-surface sheets. Here we report measurements of the penetration depth λ(T) in very clean samples of the related iron-phosphide superconductor, LaFePO, at temperatures down to ~100 mK. We find that λ(T) varies approximately linearly with T strongly suggesting the presence of gap nodes in this compound. Taken together with other data, this suggests the gap function is not universal for all pnictide superconductors.
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We report ~(63)Cu-NMR/NQR and ~(19)F-NMR studies on the multilayered high-T_c copper oxides Ba_2Ca_(n-1)Cu_nO_(2n)F_2 with n = 2,3,4, where n is the number of CuO_2 planes. It is revealed that bilayered Ba_2CaCu_2O_4F_2 is an unde...
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We report ~(63)Cu-NMR/NQR and ~(19)F-NMR studies on the multilayered high-T_c copper oxides Ba_2Ca_(n-1)Cu_nO_(2n)F_2 with n = 2,3,4, where n is the number of CuO_2 planes. It is revealed that bilayered Ba_2CaCu_2O_4F_2 is an underdoped superconductor with hole carriers, which are introduced into CuO_2 planes by an unexpected deviation from the nominal content of apical fluorines. In a previous paper, we proposed a self-doping mechanism as the origin of carrier doping in n = 3 and n = 4 in the mechanism, electrons are transferred from the inner CuO_2 plane (IP) to the outer one (OP). However, since it has been found that the bilayered compound is hole doped, we have re-examined the superconducting and magnetic properties in n =3 and n=4 by ~(63)Cu-NMR/NQR and ~(19)F-NMR. The extensive NMR studies have confirmed that the apical-fluorine compounds are not self-doped but hole doped and that antiferromagnetism (AFM) and superconductivity (SC) coexist in a single CuO_2 plane. In n=4, the AFM ordering occurs at T_N=80 K well above T_c =55 K, where the respective AFM moments are M_(AFM)=0.1 1μ_B and 0.18μ_B at the OP and the IP. In n=3, on the other hand, the underdoped single IP exhibits a spontaneous moment M_(AFM)=0.12 μ_B at low temperatures and a peak in the nuclear-spin-lattice relaxation rate 1/T_1 of ~(19)F at T_N=23 K much lower than T_c=76 K. We note that the increase in the number of IPs from one to two results in the strengthening of the interlayer coupling; T_N increases as the interlayer coupling becomes stronger,-although the doping levels for both compounds are comparable. Consequently, we conclude that the uniform mixing of AFM and SC is a general property inherent to a single CuO_2 plane in an underdoped regime for hole doping. This conclusion incorporates the angle-resolved photoemission spectroscopy results on the n=4 compound [Chen et al., Phys. Rev. Lett. 97, 236401 (2006)]; it was found that the two Fermi sheets of the IP and OP are observed and that the SC gap opens at the IP and OP below T_c=55 K.
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The expulsion of flux in five type-Ⅰ materials in a slow, continuously decreasing, perpendicular magnetic field provides evidence for the possible existence of a barrier in the superconductive transition. The variation of the obs...
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The expulsion of flux in five type-Ⅰ materials in a slow, continuously decreasing, perpendicular magnetic field provides evidence for the possible existence of a barrier in the superconductive transition. The variation of the observed critical fields with temperature yields Ginzburg-Landau parameter determinations for the materials which suggests their behavior to be more strongly type Ⅰ than generally considered.
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We have measured the nonlinear response to the ac magnetic field in the superconducting weak ferromagnet Ru-1222, at different regimes of sample cooling which provides unambiguous evidence of the interplay of the domain structure ...
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We have measured the nonlinear response to the ac magnetic field in the superconducting weak ferromagnet Ru-1222, at different regimes of sample cooling which provides unambiguous evidence of the interplay of the domain structure and the flux distribution in the superconducting state. This is a proof of coexistence of ferromagnetic and superconductive order parameters in high-T_c ruthenocuprates.
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We report a careful ~(59)Co nuclear quadrupolar resonance measurement on the recently discovered cobalt oxyhydrate Na_(0.35)CoO_2 · yH_2O superconductor from T = 40 K down to 0.2 K. We find that in the normal state the spin-latti...
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We report a careful ~(59)Co nuclear quadrupolar resonance measurement on the recently discovered cobalt oxyhydrate Na_(0.35)CoO_2 · yH_2O superconductor from T = 40 K down to 0.2 K. We find that in the normal state the spin-lattice relaxation rate 1/T_1 follows a Curie-Weiss type temperature (T) variation, 1/T_1T = C/(T ― θ), with θ = ―42 K, suggesting two-dimensional antiferromagnetic spin correlations. Below T_c = 3.9 K, 1/T_1 decreases with no coherence peak and follows a T~n dependence with n approx= 2.2 down to ～2.0 K but crosses over to a 1/T_1 ∝ T variation below T = 1.4 K, which suggests non-s-wave superconductivity. The data in the superconducting state are most consistent with the existence of line nodes in the gap function.
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摘要 :
We analyzed nonexponential ~(63)Cu nuclear spin-lattice relaxation curves for ~(63)Cu-enriched high-T_c superconductors: La_(2-X)Sr_xCuO_4 with x = 0.13 (slightly underdoped) and 0.18 (slightly overdoped), and studied the applicab...
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We analyzed nonexponential ~(63)Cu nuclear spin-lattice relaxation curves for ~(63)Cu-enriched high-T_c superconductors: La_(2-X)Sr_xCuO_4 with x = 0.13 (slightly underdoped) and 0.18 (slightly overdoped), and studied the applicability of an impurity-induced nuclear spin-lattice relaxation theory. We found a remnant of pseudo-spin-gap effect on the host ~(63)Cu nuclear spin-lattice relaxation time and slow inhomogeneous spin fluctuation via the impurity-induced relaxation time. The effect of slow spin dynamics was also observed in ~(139)La nuclear spin-lattice relaxation. The inhomogeneous electron-spin fluctuation, which is associated with randomly distributed staggered moments on the CuO_2 plane, smears the pseudo-spin-gap. The fact that the optimal T_c ～38 K is smaller than T_c ～96 K of HgBa_2CuO_(4+δ) can be attributed to the depairing effect due to the slow spin fluctuation.
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We report simultaneous NMR and electrical transport experiments in the pressure range near the boundary of the antiferromagnetic spin density wave (SDW) insulator and the metallic/superconducting (SC) phase in (TMTSF)_2PF_6. Measu...
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We report simultaneous NMR and electrical transport experiments in the pressure range near the boundary of the antiferromagnetic spin density wave (SDW) insulator and the metallic/superconducting (SC) phase in (TMTSF)_2PF_6. Measurements indicate a tricritical point separating a line of second-order SDW/metal transitions from a line of first-order SDW/metal(SC) transitions with coexistence of macroscopic regions of SDW and metal(SC) order, with little mutual interaction but strong hysteretic effects. NMR results quantify the fraction of each phase.
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Long-standing discrepancies within determinations of the Ginzburg-Landau parameter k from supercritical field measurements on superconducting microspheres are reexamined. The discrepancy in tin is shown to result from differing me...
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Long-standing discrepancies within determinations of the Ginzburg-Landau parameter k from supercritical field measurements on superconducting microspheres are reexamined. The discrepancy in tin is shown to result from differing methods of analyses, whereas the discrepancy in indium is a consequence of significantly differing experimental results. The reanalyses, however, confirm the lower k determinations to within experimental uncertainties.
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Extensive measurements of the microwave conductivity of highly pure and oxygen-ordered YBa_2Cu_3O_(6+y) single crystals have been performed as a means of exploring the intrinsic charge dynamics of a d-wave superconductor. Broadban...
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Extensive measurements of the microwave conductivity of highly pure and oxygen-ordered YBa_2Cu_3O_(6+y) single crystals have been performed as a means of exploring the intrinsic charge dynamics of a d-wave superconductor. Broadband and fixed-frequency microwave apparatus together provide a very clear picture of the electrodynamics of the superconducting condensate and its thermally excited nodal quasiparticles. The measurements reveal the existence of very long-lived excitations deep in the superconducting state, as evidenced by sharp cusplike conductivity spectra with widths that fall well within our experimental bandwidth. We present a phenomenological model of the microwave conductivity that captures the physics of energy-dependent quasiparticle dynamics in a d-wave superconductor which, in turn, allows us to examine the scattering rate and oscillator strength of the thermally excited quasiparticles as functions of temperature. Our results are in close agreement with the Ferrell-Glover-Tinkham sum rule, giving confidence in both our experiments and the phenomenological model. Separate experiments for currents along the a and b directions of detwinned crystals allow us to isolate the role of the CuO chain layers in YBa_2Cu_3O_(6+y), and a model is presented that incorporates both one-dimensional conduction from the chain electrons and two-dimensional transport associated with the CuO_2 plane layers.
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