24 research outputs found
Cooper pairs without 'glue' in high- superconductors
We address the origin of the Cooper pairs in high- cuprates and the
unique nature of the superconducting (SC) condensate. Itinerant holes in an
antiferromagnetic background form pairs spontaneously, without any `glue',
defining a new quantum object the `pairon'. In the incoherent pseudogap phase,
above or within the vortex core, the pairon binding energies are
distributed statistically, forming a `Cooper-pair glass'. Contrary to
conventional SC, it is the mutual pair-pair interaction that is responsable for
the condensation. We give a natural explanation for the {\it ergodic rigidity}
of the excitation gap, being uniquely determined by the carrier concentration
and . The phase diagram can be understood, without spin fluctuations, in
terms of a single energy scale , the exchange energy at the
metal-insulator transition
Local Symmetries and Order-Disorder Transitions in Small Macroscopic Wigner Islands
The influence of local order on the disordering scenario of small Wigner
islands is discussed. A first disordering step is put in evidence by the time
correlation functions and is linked to individual excitations resulting in
configuration transitions, which are very sensitive to the local symmetries.
This is followed by two other transitions, corresponding to orthoradial and
radial diffusion, for which both individual and collective excitations play a
significant role. Finally, we show that, contrary to large systems, the focus
that is commonly made on collective excitations for such small systems through
the Lindemann criterion has to be made carefully in order to clearly identify
the relative contributions in the whole disordering process.Comment: 14 pages, 10 figure
Sign Reversals of ac Magnetoconductance in Isolated Quantum Dots
We have measured the electromagnetic response of micron-size isolated
mesoscopic GaAs/GaAlAs square dots down to temperature T=16mK, by coupling them
to an electromagnetic micro-resonator. Both dissipative and non dissipative
responses exhibit a large magnetic field dependent quantum correction, with a
characteristic flux scale which corresponds to a flux quantum in a dot. The
real (dissipative) magnetoconductance changes sign as a function of frequency
for low enough density of electrons. The signal observed at frequency below the
mean level spacing corresponds to a negative magnetoconductance, which is
opposite to the weak localization seen in connected systems, and becomes
positive at higher frequency. We propose an interpretation of this phenomenon
in relation to fundamental properties of energy level spacing statistics in the
dots.Comment: 4 pages, 4 eps figure
Statistics of the cuprate pairon states on a square lattice
International audienceIn this paper the fundamental parameters of high- T c superconductivity are shown to be connected to the statistics of pairons (hole pairs in their antiferromagnetic (AF) environment) on a square lattice. In particular, we study the density fluctuations and the distribution of the area surrounding each pairon on the scale of the AF correlation length Ο A F , for the complete range of hole concentration. We show that the key parameters of the phase diagram, the T c dome, and the pseudogap (PG) temperature T â , emerge from the statistical properties of the pairon disordered state. In this approach, the superconducting and the PG states appear as inseparable phenomena. The condensation energy, which fixes the critical temperature, is directly proportional to the correlation energy between pairons and not to the energy gap, contrary to conventional superconductors. When the correlation energy between pairons is suppressed by fluctuations, either thermally, by disorder, or in the vortex core, the PG state of disordered pairons is obtained. We attribute the unique features of cuprate superconductivity to this orderâdisorder transition in real space, which clearly differs from the BardeenâCooperâSchrieffer mechanism. Our predictions are in quantitative agreement with low-temperature tunneling and photoemission spectroscopy experiments
Origin of the Fermi arcs in cuprates: a dual role of quasiparticle and pair excitations
International audienceAngle resolved photoemission spectroscopy (ARPES) mesurements in cuprates have given key information on the temperature and angle dependence of the gap (d-wave order parameter, Fermi arcs and pseudogap). We show that these features can be understood in terms of a Bose condensation of interacting pairons (preformed hole pairs which form in their local antiferromagnetic environment). Starting from the basic properties of the pairon wavefunction, we derive the corresponding k-space spectral function. The latter explains the variation of the ARPES spectra as a function of temperature and angle up to T *, the onset temperature of pairon formation. While Bose excitations dominate at the antinode, the fermion excitations dominate around the nodal direction, giving rise to the Fermi arcs at finite temperature. This dual role is the key feature distinguishing cuprate from conventional superconductivity