Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 Advertisement • Prof. Eun-Ah Kim • Quantum Nematics Theory • Next Tuesday in class Acknowledgement • James • Eun-Ah & Michael • Stefan, Jeehye & Hitesh