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

Event Dates: 

Wednesday, November 4, 2015 - 4:00pm

CTQM Seminar Type: 

  • Related Seminar Series

Speaker Name(s): 

Tomo Uemura

Speaker Affiliation(s): 

Columbia University

Abstract, Event Details: 

 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 ns/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.