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تسجيل مستخدم جديدObservation of a d-wave nodal liquid in highly underdoped Bi_2Sr_2CaCu_2O_{8+delta}
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We use angle resolved photoemission spectroscopy to probe the electronic excitations of the non-superconducting state that exists between the antiferromagnetic Mott insulator at zero doping and the superconducting state at larger dopings in Bi_2Sr_2CaCu_2O_{8+delta}. We find that this state is a nodal liquid whose excitation gap becomes zero only at points in momentum space. Despite exhibiting a resistivity characteristic of an insulator and the absence of coherent quasiparticle peaks, this material has the same gap structure as the d-wave superconductor. We observe a smooth evolution of the spectrum across the insulator-to-superconductor transition, which suggests that high temperature superconductivity emerges when quantum phase coherence is established in a non-superconducting nodal liquid.
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We have used a scanning tunneling microscope to demonstrate that a single CuO_2 plane can form a stable and atomically ordered layer at the surface of Bi_2Sr_2CaCu_2O_{8+delta}. In contrast to previous studies on high-T_c surfaces, the CuO_2-terminat
ed surface exhibits a strongly suppressed tunneling conductance at low voltages. We consider a number of different explanations for this phenomena and propose that it may be caused by how the orbital symmetry of the CuO_2 planes electronic states affects the tunneling process.
The temperature and magnetic-field (H) dependences of thermal conductivity (kappa) of Bi_2Sr_2CaCu_2O_{8+delta} (Bi2212) are systematically measured for a broad doping range by using both pure Bi2212 single crystals with tuned oxygen contents and Bi_
2Sr_2Ca_{1-x}Dy_xCu_2O_{8+delta} (Dy-Bi2212) single crystals with different Dy contents x. In the underdoped samples, the quasiparticle (QP) peak below T_c is strongly suppressed, indicating strong QP scattering by impurities or oxygen defects, whereas the phonon conductivity is enhanced in moderately Dy-doped samples and a phonon peak at 10 K is observed for the first time in Bi2212 system, which means Dy^{3+} ions not only introduce the impurities or point defects but also stabilize the crystal lattice. The subkelvin data show that the QP heat conductivity gradually decreases upon lowering the hole doping level. The magnetic-field dependence of kappa at temperature above 5 K is mainly due to the QP scattering off vortices. While the underdoped pure Bi2212 show very weak field dependence of kappa, the Dy-doped samples present an additional dip-like term of kappa(H) at low field, which is discussed to be related to the phonon scattering by free spins of Dy^{3+} ions. For non-superconducting Dy-Bi2212 samples with x simeq 0.50, an interesting plateau feature shows up in the low-T kappa(H) isotherms with characteristic field at 1 -- 2 T, for which we discuss the possible revlevance of magnon excitations.
We report a c-axis-polarized electronic Raman scattering study of Bi_2Sr_2CaCu_2O_{8+delta} single crystals. In the normal state, a resonant electronic continuum extends to 1.5 eV and gains significant intensity as the incoming photon energy increase
s. In the superconducting state, a coherence 2Delta peak appears around 50 meV, with a suppression of the scattering intensity at frequencies below the peak position. The peak energy, which is higher than that seen with in-plane polarizations, signifies distinctly different dynamics of quasiparticle excitations created with out-of-plane polarization.
We study sharp low-energy resonance peaks in the local density of states (LDOS) induced by Zn impurities or possible Cu vacancies in superconducting Bi_2Sr_2CaCu_2O_{8+delta}. The measured structure of these near-zero-bias resonances is quantitativel
y reproduced by an extended impurity potential without invoking internal impurity states or sophisticated tunneling models. The Zn potential extends at least to the nearest-neighbor Cu sites, and the range of order parameter suppression extends at least 8 AA away from the Zn site. We further show that the local spin susceptibilities near Zn impurities increase rather than decrease with decreasing temperature in the superconducting state due to the sharp increase of LDOS near the Fermi level.
We present a detailed study on the behaviour of vortex cores in Bi_2Sr_2CaCu_2O_{8+delta} using scanning tunneling spectroscopy. The very irregular distribution and shape of the vortex cores imply a strong pinning of the vortices by defects and inhom
ogeneities. The observed vortex cores seem to consist of two or more randomly distributed smaller elements. Even more striking is the observation of vortex motion where the vortex cores are divided between two positions before totally moving from one position to the other. Both effects can be explained by quantum tunneling of vortices between different pinning centers.
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