BEC to BCS crossover picture for high-Tc cuprate and unconventional superconductors, and pairing due to comparable spin-charge energy scales: message from muons and neutrons

Speaker: Yasutomo J. Uemura from Columbia University

Title:    BEC to BCS crossover picture for high-Tc cuprate and unconventional superconductors, and pairing due to comparable spin-charge energy scales: message from muons and neutrons

Abstract:  

Soon after the discovery of high-Tc superconductors in 1986, we started muon spin relaxation (MuSR) measurements of the magnetic field penetration depth, and demonstrated nearly linear correlations between Tc and the superfluid density n_{s}/m^{*}, which are manifest not only in cuprates but also in many other unconventional superconductors including Alkali doped C60, FeAs and some of the heavy-fermion systems [1,2].  The strong dependence of Tc on carrier density is not expected in the BCS condensation, but is a main characteristic of Bose Einstein Condensation.  Another important scaling of Tc is seen with the energy of  the magnetic resonance mode measured by neutron scattering, which can be viewed as a spin–charge soft mode related to competing antiferromagnetic state, analogous to rotons in superfluid He [3] related to competing solid HCP phase. This roton-like scaling is expected when condensed bosons are excited with phase fluctuations to another bosonic state, without pair breaking.  This again is different from pair-breaking excitations across the energy gap in BCS condensation. We discuss these features and adopting BEC-BCS crossover picture for the cuprates, which I proposed in 1994 [4].  Further analyses of the Tc-superfluid density (charge energy) scaling and Tc-spin wave energy (mediating boson energy) correlations lead to a possible pairing mechanism based on comparable spin and charge energy scales and dynamic synchronized charge motion in “traffic-light resonance” [5].

[1] Y.J. Uemura et al., Universal Correlations between Tc and n_s/m∗ (Carrier Density over Effective

      Mass) in High-Tc Cuprate Superconductors, Phys. Rev. Lett. 62 (1989) 2317-2320.

[2] Y.J. Uemura et al., Basic Similarities among Cuprate, Bismuthate, Organic, Chevrel Phase, and

      Heavy-Fermion Superconductors Shown by Penetration Depth Measurements, Phys. Rev. Lett. 66

      (1991) 2665-2668.

[3] Y.J. Uemura, Superconductivity: Exotic Commonalities in Phase and Mode, Nature Materials 8

      (2009) 253-255.

[4] Y.J. Uemura, Energy Scales of Exotic Superconductors, an invited talk presented at the International

      Workshop on Polarons and Bipolarons in High-Tc Superconductors and Related Materials,

      Cambridge, UK, April, 1994, in Polarons and Bipolarons in High-Tc Superconductors and Related

      Materials, ed. by E. Salje, A.S. Alexandrov and Y. Liang, Cambridge University Press (1995), p.p.

      453-460.

[5] Y.J. Uemura, Energy-scale phenomenology and pairing via resonant spin-charge motion in

      FeAs, CuO, heavy-fermion and other exotic superconductors (Plenary Talk at SCES2008,

      Buzios, Brazil, Aug., 2008), Physica B404 (2009) 3195-3201.