Imaging the granular structure of high-Tc superconductivity in underdoped Bi2Sr2CaCu2O8+δ

Abstract

Granular superconductivity occurs when microscopic superconducting grains are separated by non-superconducting regions through which they communicate by Josephson tunneling to establish the macroscopic superconducting state 1 . Although crystals of the cuprate high-T c superconductors are not granular in a structural sense, theory indicates that at low hole densities the holes can become concentrated at some locations resulting in hole-rich superconducting domains 2-5 . Granular superconductivity due to Josephson tunneling through 'undoped' regions between such domains would represent a new paradigm for the underdoped cuprates. Here we report studies of the spatial interrelationships between STM tunneling spectra in underdoped Bi 2 Sr 2 CaCu 2 O 8+d . They reveal an apparent spatial segregation of the electronic structure into ~3nm diameter domains (with superconducting characteristics and local energy gap ∆<50 meV) in an electronically distinct background. To explore whether this represents nanoscale segregation of two distinct electronic phases, we employ scattering-resonances at Ni impurity atoms 6 as 'markers' for the local existence of superconductivity [7][8][9] . No Ni-resonances are detected in any regions where ∆>50±2.5 meV. These observations suggest that underdoped Bi 2 Sr 2 CaCu 2 O 8+d is a mixture of two different short-range electronic orders with the long-range characteristics of a granular superconductor.

Condensation, excitation, pairing, and superfluid density in high-Tc superconductors: the magnetic resonance mode as a roton analogue and a possible spin-mediated pairing

“…Recently, spontaneous formation of spatially heterogeneous areas, with similar length scales, was discovered by a series of STM measurements [10,11] in under to optimally doped Bi2212 systems without impurity doping. As shown in Fig.…”

“…The results demonstrate the trade off between superconducting and magnetic volume fractions, expected in the "magnetic island formation" illustrated at the top. (d) The "gap-map" obtained by STM in moderately underdoped Bi2212 [10,11]. The red-yellow regions represent the area where the sharp superconducting gap is observed, while blue and black region has a response similar to those found in the "pseudogap" region [68] or the nonsuperconducting very underdoped region [69].…”

“…They include: 1) scanning tunnelling microscope results which confirmed the "swiss-cheese" like situation in Zn doped cuprates [9], and further established spontaneous formation of spatial heterogeneity in the superconducting state in a wide range of doping levels even without impurity substitution [10,11]; 2) highfrequency conductivity measurements of n s /m * , which showed a frequency-dependent "dynamic" response remaining above T c and provided the first direct evidence for a wide range of superconducting phase fluctuations in the normal state [12]; 3) the Nernst effect measurements which obtained further evidence for the dynamic superconducting response above T c , and mapped out the region of this response as a function of external field H, temperature T , and the doping concentration x [13,14]; 4) neutron scattering studies which established that the "41 meV magnetic resonance mode", first found in YBCO [15], is generic to many of the cuprate systems, and that the mode energy accurately scales with T c [16][17][18]; 5) Raman scattering studies which obtained the response in the A1 g mode at T → 0 whose energy precisely follows that of the neutron resonance peak [19][20][21][22]; and 6) a sharp "coherence peak" which appears in the antinodal response of ARPES studies, with the intensity proportional to the superfluid density [23].…”

On the nature of the superconducting gap in the cuprates

“…More generally, a pseudogap may be a generic feature of systems that are intrinsically inhomogeneous due to the competition between ordered states separated by a first-order phase transition [26,27]. Such picture becomes especially appealing in the light of recent scanning tunnelling microscope studies of BSCCO [28,29]. It could be investigated in our model by evaluating the density fluctuations induced by the attractive interaction, in addition to the pairing fluctuations considered in the present work.…”

Pebbles in the nodal pond

“…The raw-silk texture disappears, and is replaced by a very different pattern (Fig. 1c) 1,8 . I call these patterns 'quantum salad-dressing' 9 : there seem to be two very different states of electron matter present that, like oil and vinegar, do not want to mix.…”