Transcript Search of Quantum Critical Point in Cuprate

Search of a Quantum Critical Point
in High Tc Superconductors
Yang Xie
(with great help from Prof. Eun-Ah Kim)
Hidden Critical Point
Recall the phase diagram of 1-d quantum Ising model
Non-Fermi liquid: strange power law of resistivity
Hidden Critical Point
Compared with phase diagram of 1-d quantum Ising model
We suspect that there is a quantum critical point in the
superconducting phase
STM Experiment
Electron density of
states in a band picture
STM Experiment
is density of states (DOS)
of quasi-particles, at excitation
energies E, at position
Take E=0 at EF , i.e. take E w.r.t EF
Cuprate Physics
High Tc Superconductors show d-wave gap, different
from s-wave superconductor (metallic superconductor)
d-wave
Peak in DOS
Gap
(BSCCO)
Quasi-particle
density of states
(DOS)
s-wave
(NbSe2)
(w.r.t EF , i.e.
E=0 is the
Fermi level)
Phase Diagram
Non-Fermi Liquid:
strange power law
behavior of resistivity
Non-Fermi Liquid
• Strange resistivity: increase linear vs T for a
narrow range of doping
• Generally,
increase as Tx, x ~ 0.8-1.5
• Fermi liquid: increase as T2
Phase Diagram
To understand Non-Fermi
liquid, we need to study
regions next to it.
(Fermi liquid is wellunderstood)
ARPES
Pseudo Gap
Tc=85 K
Pink line:
•Fermi liquid (Platinum)
•Excitation across the Fermi level
(take EF =0)
Black line:
•High Tc (BSCCO)
•Suppressed excitation below EF in
SC phase Tc<85 K
•‘Partially’ suppressed excitation
above Tc (at T=95K, 120K) –
Pseudo Gap
STM Experiment
During the experiment, the
samples are always in the
SC phase
DOS evolution for
increasing doping
STM Experiment
DOS
Fourier
Transform
STM Experiment
EPG has strong correlation
to pseudogap crossover
During the experiment, the
samples are always in the
SC phase
Quantum Critical Point
During the experiment, the
samples are always in the
SC phase
•Power laws
•Scaling function
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• Prof. Eun-Ah Kim
• Quantum Nematics Theory
• Next Tuesday in class
Acknowledgement
• James
• Eun-Ah & Michael
• Stefan, Jeehye & Hitesh