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Register a new userSpin gap and magnetic coherence in a clean high-T_c superconductor
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B. Lake
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A notable aspect of high-temperature superconductivity in the copper oxides is the unconventional nature of the underlying paired-electron state. A direct manifestation of the unconventional state is a pairing energy - that is, the energy required to remove one electron from the superconductor - that varies (between zero and a maximum value) as a function of momentum or wavevector: the pairing energy for conventional superconductors is wavevector-independent. The wavefunction describing the superconducting state will include not only the pairing of charges, but also of the spins of the paired charges. Each pair is usually in the form of a spin singlet, so there will also be a pairing energy associated with transforming the spin singlet into the higher energy spin triplet form without necessarily unbinding the charges. Here we use inelastic neutron scattering to determine the wavevector-dependence of spin pairing in La_{2-x}Sr_xCuO_4, the simplest high-temperature superconductor. We find that the spin pairing energy (or spin gap) is wavevector independent, even though superconductivity significantly alters the wavevector dependence of the spin fluctuations at higher energies.
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Polarized and unpolarized neutron scattering was used to measure the wave vector- and frequency-dependent magnetic fluctuations in the normal state (from the superconducting transition temperature, T_c=35, up to 350 K) of single crystals of La_{1.86}Sr_{0.14}CuO_4. The peaks which dominate the fluctuations have amplitudes that decrease as T^{-2} and widths that increase in proportion to the thermal energy, k_B T (where k_B is Boltzmanns constant), and energy transfer added in quadrature. The nearly singular fluctuations are consistent with a nearby quantum critical point.
In the stripe-ordered state of a strongly-correlated two-dimensional electronic system, under a set of special circumstances, the superconducting condensate, like the magnetic order, can occur at a non-zero wave-vector corresponding to a spatial period double that of the charge order. In this case, the Josephson coupling between near neighbor planes, especially in a crystal with the special structure of La_{2-x}Ba_xCuO_4, vanishes identically. We propose that this is the underlying cause of the dynamical decoupling of the layers recently observed in transport measurements at x=1/8.
Relationship between the superconducting gap and the pseudogap has been the subject of controversies. In order to clarify this issue, we have studied the superconducting gap and pseudogap of the high-Tc superconductor La2-xSrxCuO4 (x=0.10, 0.14) by angle-resolved photoemission spectroscopy (ARPES). Through the analysis of the ARPES spectra above and below Tc, we have identified a superconducting coherence peak even in the anti-nodal region on top of the pseudogap of a larger energy scale. The superconducting peak energy nearly follows the pure d-wave form. The d-wave order parameter Delta_0 [defined by Delta(k)=Delta_0(cos(kxa)-cos(kya)) ] for x=0.10 and 0.14 are nearly the same, Delta_0 ~ 12-14 meV, leading to strong coupling 2Delta_0/kB Tc ~ 10. The present result indicates that the pseudogap and the superconducting gap are distinct phenomena and can be described by the two-gap scenario.
Extensive Cu-NMR studies on multilayered high-Tc cuprates have deduced the following results;(1) Antiferromagnetic (AFM) moment M_{AFM} is decreased with doping, regardless of the number of CuO_2 layers n, and collapses around a carrier density N_h = 0.17. (2) The AFM ordering temperature is enhanced as the out-of-plane coupling J_{out} increases with increasing n. (3) The in-plane superexchange J_{in} is invariant with doping, but is even increased. (4) The dome shape of T_c from the underdoped to the overdoped regime with a maximum T_c at N_h = 0.22 does not depend on n, but its maximum value of T_c seems to depend on n moderately. The present results strongly suggest that the AFM interaction plays the vital role as the glue for the Cooper pairs, which will lead us to a genuine understanding of why the T_c of cuprate superconductors is so high.
We report the magnetic susceptibility and the magnetization under pressures up to 1.7GPa above the critical pressure, Pc ~ 1.5GPa, for H // a, b, c-axes in the novel spin triplet superconductor UTe2. The anisotropic magnetic susceptibility at low pressure with the easy magnetization a-axis changes to the quasi-isotropic behavior at high pressure, revealing a rapid suppression of the susceptibility for a-axis, and a gradual increase of the susceptibility for the b-axis. At 1.7GPa above Pc, magnetic anomalies are detected at T_MO ~ 3K and T_WMO ~ 10K. The former anomaly corresponds to long-range magnetic order, most likely antiferromagnetism, while the latter shows a broad anomaly, which is probably due to the development of short range order. The unusual decrease and increase of the susceptibility below T_WMO for H // a and b-axes, respectively, indicate the complex magnetic properties at low temperatures above Pc. This is related to the interplay between multiple fluctuations dominated by antiferromagnetism, ferroamgnetism, valence and Fermi surface instabilities.
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