PHYSICS /BK21 Colloquium (08/03/28)

"Spin and charge dynamics in the 2-D systems of strongly correlated electrons: High Tc superconductiv


연사: Sung-Ho Salk[KIAS]

일시: 2008 년 3 월 28 일 오후 3시
장소: 무은재 기념관 307호 (멀티미디어 강의실)

ABSTRACT:

For the last two decades since the advent of high temperature superconductivity, noticeable progress
has been made to understand both normal and superconducting states by paying attention to phase
diagrams[1], electronic structures[2], thermal properties, super°uidity[3] and both spin[4, 5] and
charge dynamics[6]. Despite the progress there exists no satisfactory theory which consistently repr
oduces various physical properties of cuprate oxides in association with the phase diagram. High Tc
superconductivity is involved with low (two) dimensional systems of strongly correlated electrons,
namely the cuprate oxides acting as Mott insulators. To tackle this problem, theoretical efforts have
been made using either the Hubbard Hamiltonian or the t-J Hamiltonian. Electrons have threefaces:
namely, the spin, charge and quantum phase. To meet the three essential faces, earlier we[7] propo
sed both U(1) and SU(2) gauge theoretic slave-boson theories of the t . J Hamiltonian in which coup
ling between the spin pairing and charge paring orders are properly taken care of, contrary to other
slave-boson theories[8, 9]. Thanks to the fact that the Cooper pair is realized as a composite of the
spinon (spin) and holon (charge) pairing orders in our slave-boson treatment, not only the dome-shap
ed phase diagram but other physical properties are consistently reproduced in agreements with meas
urements : superfluid weight, spectral function[10], optical conductivity[11], magnetic susceptibility
and the universal scaling behavior of T ¤=Tc[12]. In this talk we present a study of spin dynamics and
charge dynamics. We explain how the two different dynamics are coupled to cause various exotic
physical phenomena such as the dome-shaped bose condensation temperature, the boomerang
behavior of superfluid weight and peak-dip-hump structures of both spectral function and optical
conductivity. Regarding the charge dynamics we .find that the peak-dip-hump structure in optical
conductivity is attributed to coupling between the spin and charge degrees of freedom, but not to
the spin-charge separation. Further, quantum critical point will found in light of charge dynamics.
Regarding the spin dynamics we unveil physics involved with both temperature and doping depen
dence of magnetic resonance. We find from this study that the onset temperature of magnetic reso
nance which occurs in spin-exciton channel is the pseudogap (spin gap) temperature T . in agree
ment with measurements of Mook and coworkers[4]. The resonance peak energy Eres is shown to
have a linear scaling behavior with the superconducting transition temperature, Tc, in agreements
with observations of Keimer and coworkers[5]. We demonstrate that the spin pairing correlations are
responsible for both the linear scaling of Eres ∼ Tc and the universal scaling behavior of T ¤=Tc[12].
Finally an integrated view of physics of the above novel features will be highlighted in the language of
coupling between the spin and charge degrees of freedom and the spin and charge dynamics.

문의처: 김재삼(054-279-2077, jsk@postech.ac.kr)
"