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21.
A phenomenological phase diagram which has been derived for high-temperature superconductors from NMR Knight-shift measurements of the pseudogap is compared to the phase diagram that is obtained for organic superconductors and spin-ladder superconductors, both low-dimensional systems. This is contrasted to the phase diagram of some Heavy Fermion superconductors, i.e. superconductors not constrained to a low dimensionality. 相似文献
22.
R.S. Markiewicz Susmita Basak A. Bansil 《Journal of Electron Spectroscopy and Related Phenomena》2010,181(1):23-27
We demonstrate that many features ascribed to strong correlation effects in various spectroscopies of the cuprates are captured by a calculation of the self-energy incorporating effects of spin and charge fluctuations. The self-energy is calculated over the full doping range from half-filling to the overdoped system. In the normal state, the spectral function reveals four subbands: two widely split incoherent bands representing the remnant of the two Hubbard bands, and two additional coherent, spin- and charge-dressed in-gap bands split by a spin-density wave, which collapses in the overdoped regime. The resulting coherent subbands closely resemble our earlier mean-field results. Here we present an overview of the combined results of our mean-field calculations and the newer extensions into the intermediate coupling regime. 相似文献
23.
K. Levin Qijin ChenIoan Kosztin Boldizsár JankóYing-Jer Kao Andrew Iyengar 《Journal of Physics and Chemistry of Solids》2002,63(12):2233-2236
In the last few years evidence has been accumulating that there are a multiplicity of energy scales which characterize superconductivity in the underdoped cuprates. In contrast to the situation in BCS superconductors, the phase coherence temperature Tc is different from the energy gap onset temperature T∗. In addition, thermodynamic and tunneling spectroscopies have led to the inference that the order parameter Δsc is to be distinguished from the excitation gap Δ; in this way, pseudogap effects persist below Tc. It has been argued by many in the community that the presence of these distinct energy scales demonstrates that the pseudogap is unrelated to superconductivity. In this paper, we show that this inference is incorrect. We demonstrate that the difference between the order parameter and excitation gap and the contrasting dependences of T∗ and Tc on hole concentration x and magnetic field H follow from a natural generalization of BCS theory. This simple generalized form is based on a BCS-like ground state, but with self-consistently determined chemical potential in the presence of arbitrary attractive coupling g. We have applied this mean field theory with some success to tunneling, transport, thermodynamics, and magnetic field effects. We contrast the present approach with the phase fluctuation scenario and discuss key features which might distinguish our precursor superconductivity picture from that involving a competing order parameter. 相似文献
24.
M. Erbudak A. Hensch J. Keller B. Rssner A. R. Kortan 《Journal of Electron Spectroscopy and Related Phenomena》2001,120(1-3):47-53
Using the line-shape analysis of the Mn 2p3/23d3d Auger transitions and by observing the Mn 3p→3d transitions in electron energy-loss spectroscopy, we have studied the local density of electronic states of the crystalline cubic and the quasicrystalline icosahedral phases of Al70Pd20Mn10. We find that, in the cubic phase the Mn 3d states coincide with the Fermi level EF, but in the quasicrystalline state the Mn 3d band is fully occupied and shifted to lower binding energies causing a significant reduction in the density of states at EF. 相似文献
25.
T. Suzuki E. Negishi H. Uozaki H. Matsui N. Toyota 《Physica C: Superconductivity and its Applications》2006,440(1-2):17-24
We have investigated the microwave response at 45 GHz in an organic superconductor λ-(BEDT-TSF)2GaCl4 with Tc = 4.8 K. We determine the μ0Hc2–T phase diagram from microwave loss and find that the superconducting state is in the pure limit (l/ξGL 10). Although the real part of the complex conductivity (=σ1 + iσ2) does not show a coherence peak just below Tc, the London penetration depth completely saturates at low temperatures down to T/Tc = 0.2, which may provide an evidence for a conventional s-wave pairing. In the metallic state below about 50 K, (parallel to the c-axis) deviates downward from , while σ2, which should be zero in a conventional metal, increases exponentially toward Tc. In spite of the fact that the Hagen–Rubens limit is well satisfied as far as the dc conductivity is concerned, a Drude model is unable to explain the large positive σ2. In order to explain such anomalies in the metallic state, we propose a possible existence of so-called a pseudogap near a Fermi level. The anomalous increase of the positive σ2 may be attributed to an appearance of pre-formed electron pairs in the pseudogap state. This appearance can be regarded as a precursor to the superconducting transition. Such a precursory phenomenon has been observed also in the isostructural FeCl4 salt with the anomalous metallic states, which shows a negative σ2 in contrast to the GaCl4 salt. Just the opposite of ground states in between the GaCl4 and FeCl4 salts may result in the contrasting anomalous metallic states with different precursory phenomena with opposite signs of σ2. 相似文献
26.
The incommensurate antiferromagnetism (AF) in metallic underdoped cuprates is interpreted with several complementary approaches, unified by a consistent disambiguation of the copper and oxygen degrees of freedom. Collinear (with respect to the copper lattice) peaks in the neutron response at low frequency are due to the oxygen-dominated arcs, while the diagonal peaks at high frequency or low doping come from the copper-dominated vH points. The latter switch from collinear to diagonal as the frequency increases. The direct O–O overlap induces an AF which is weaker and thermodynamically preferred to the AF of strongly localized copper sites, and which coexists with Fermi arcs. A theoretical understanding of the strongly k-dependent AF gap observed in ARPES and STM measurements is proposed. 相似文献