Pairing fluctuations in the pseudogap state of copper-oxide superconductors probed by the Josephson effect

Abstract

The phase diagram of high-temperature superconductors is still to be understood 1 . In the low-carrier-doping regime, a loss of spectral weight in the electronic excitation spectrum-the so-called pseudogap-is observed above the superconducting temperature T c , and below a characteristic temperature T * (ref. 2). First observed in the spin channel by NMR measurements, the pseudogap has also been observed in the charge channel by scanning probe microscopy and photoemission experiments, for instance 2 . An important issue to address is whether this phenomenon is related to superconductivity or to a competing 'hidden' order. In the superconductivity case, it has been suggested that superconducting pairing fluctuations may be responsible, but this view remains to be tested experimentally. Here, we have designed a Josephson-like experiment to probe directly the fluctuating pairs in the normal state. We show that fluctuations survive only in a restricted range of temperature above T c , well below T * , and therefore cannot explain the opening of the pseudogap at higher temperature.Angle-resolved photoemission spectroscopy 3,4 and scanning tunnelling spectroscopy 5 showed a characteristic energy of the pseudogap that merges with the superconducting gap when the temperature is lowered below T c . This reveals a smooth crossover rather than a sharp transition line between the pseudogap regime and the superconducting state, and has led to the superconducting precursor scenario. As opposed to the conventional Bardeen-Cooper-Schrieffer (BCS) transition, where pairing and condensation occur simultaneously at T c , in underdoped cuprates fluctuating pairs may form at T * , with no long-range coherence, and condense in the superconducting state at T c (refs 6,7). Difficulties in confirming (or invalidating) this scenario arise from the fact that most of the experimental techniques used to investigate the pseudogap are sensitive only to the one-particle excitations, and therefore cannot provide a test of pairing above T c . Owing to its ability to probe the properties of the superconducting wavefunction, the Josephson effect is a natural way to address the fluctuation issue.In a second-order phase transition, the susceptibility is given by the linear response of the order parameter to a suitable external field. In the case of the superconducting phase transition, the role of the external field can be played by the rigid pair field of a second superconductor below its own T c (refs 8,9). In a Josephson junction in which one side of the junction is the fluctuating superconductor of interest above its T c , whereas the other side is a superconductor below its T c , the coupling between the pairing fluctuations and the well-established pair field gives rise to an excess current I ex proportional to the imaginary part of the frequency-and wavenumber-dependent pair susceptibility χ(ω, q). For a conventional superconductor above its T c (ref. 9)where Γ 0 = (16k B /h)(T − T c ) is the relaxation rate of the fluctuations, ξ(T ) is the ...

Amperean Pairing and the Pseudogap Phase of Cuprate Superconductors

“…The shape of this peak will contain information about the degree of the PDW order. This kind of tunneling structure has in fact been fabricated and studied in the absence of a magnetic field [47]. Hence, the proposed experiment should be quite feasible.…”

Ultrafast pump-probe spectroscopic signatures of superconducting and pseudogap phases in YBa2Cu3O7−δ films

“…In contrast, the intense pulses combined with the low repetition rate used in this study are clearly superior for revealing the strong response in the pseudo-gap regime. While the origin of such PG phase in cuprates is still under active debate, [35][36][37] recently the incoherent pair formation has been identified in PG phase. 38 By virtue of the same observation, we conjecture that perhaps similar to the saturated response of the SC phase, the pulse energy in a highflux regime (> 10 0 Φ ) seems sufficient to break the majority of incoherent pairs in the PG phase .…”

Magnetic order in the pseudogap phase of HgBa2CuO4+δstudied by spin-polarized neutron diffraction

“…[2][3][4][5][6][7][8][9][10][11][12][13][14] In particular, time-reversal symmetry is broken below the pseudogap temperature T * in at least four hole-doped cuprate families. [2][3][4][5][6][7][8][9][10] Meanwhile, there is increasing evidence that pre-formed Cooper pairs 15,16 exist only in a relatively limited temperature range above T c , 11,[17][18][19][20][21][22][23] well below T * , and are therefore unlikely to be responsible for the pseudogap phenomenon. These experimental facts strongly suggest that the pseudogap regime of the phase diagram is a genuine new phase of matter in close proximity to superconductivity.…”