Physical review letter vol. 92, 086401

1. Quasiparticle Spectra, Charge-Density waves, Superconductivity, and electron –phonon coupling in 2H-NbSe2 Physical review letter vol. 92, 086401

2. Fermi liquid is a system of interacting electrons, whose properties can be mpped onto those of a system of non-interacting electrons by 1:1 transformation Four distinct difference from non-interacting case. Energy renormalization A finite quasi-particle lifetime An incoherent background created by e-h paurs PES weight outside Fermi surface Fermi liquid pheonomena in ARPES Jdluv

3. Self energy of quasi particle • Describes the interaction in Fermi liquid • Conserving total Q • ReΣ(k,w) – the shift in energy – Renormalization ~ E • ImΣ(k,w) – scattering rate or inverse lifetime ~ Lorentzian shape. ~ E2. Jdluv

4. 2H-Dichalcogenides system • 2 : number of layerH : hexagonal symmetry • 2D layered system • CDW and Superconductivity coexist. • NbSe2, NbS2, TaSe2, TaS2 • If TCDW increases, TSC decreases. • NbSe2 TCDW 35K/TSC 7.2 K • TaSe2 TCDW 90K/TSC0.15 K • Two order parameters compete. Jdluv

5. 2H – TaSe2 large kink in spectra • Below TCDW, kink shown near EF. (@34 K) • Interaction can be estimated by direct fitting. • ReΣ ~ shift in energy • ImΣ ~ Lorentzian shape • 70meV signature in both parts • Not only e-ph coupling and electron scattering also drive this change. Jdluv

6. 2H-NbSe2 Superconductivity • Fermi surface sheet-dependent Superconductivity • Below TSC(=7.2K) , small gap(~1meV) open. • Different e-ph coupling from different states. Se 4p Nb 4d Jdluv

7. Experiment • High resolution ARPES performed on U13UB @ NSL combined with SES-200. • Energy resolution 4 meV @ T ~ 15K • Angular resolution < 0.1° (0.0025Å-1 @ 15.2eV) • 2H-NbSe2 was grown by iodine vapor transport method. • Cleaved in situ in UHV • 3 X 10-9 Pa during measurement Jdluv

8. PES intensities near EF crossing • Smaller kinks (10 ~ 35meV) than those in 2H-TaSe2 case. • Similar coupling constant ~0.85 (change in slopes) T = 10 K Jdluv

9. Optical phonon Acoustic phonon Energy range Self energy ReΣ • Different peak positions and magnitude. • 13 < wmax < 35meV • Acoustic phonon w ~ 12 meV • Optical branch 15 < w < 40 meV • Different phonon coupled different states • Except for point 6(1.9), λ ~ 0.85 ± 0.15 • Good agreement with various experimental results, such as specific heat and c-axis optical measurement and so on. Specific heat coefficient γ ~ 18.5mJmol-1K-2 ~ N(0)(1+λ) Jdluv

10. Relation with CDW? • Gap opening of CDW is not noticed in this system. • However, large λ (1.9) coincide with the result from c-axis optical conductivity. • K-H inner sheet • Also, in TaSe2, gap opening observed in K-H inner shell. • Author’s suggestion. • Both SC and CDW originate from inner K-sheet driven by strong e-ph coupling. Contrary to STM result ~ 35meV CDWgap Jdluv

11. Compare to 2H-TaSe2? • Different temperature dependence • Kink and scattering rate are insensitive to the CDW transition in NbSe2. Jdluv

12. SC gap on K sheet • Gap opening is shown within experimental error bar. • No evidence for CDW gap. Jdluv

13. Discussion • NbSe2 : better conduction in the CDW state. • Duality exist. ( In normal state, strongly coupled portion acting as scattering “sinks”) for the weak coupled regions. • Hot spots (ordered state) vs Cold spot (Conductivity) • Similarity with cuparates : competing mechanism bet’n CDW and SC. (doping in HTSC) -but by other mechanism rather than phonon. Jdluv

14. Summary • They detected strong anisotropy of the self-energy in 2H-NbSe2. • Electron-phonon coupling constant – 0.8 ~ 1.9 on Nb derived sheets. • The strongest coupling found on the inner K-H sheet. • Play a central role in both CDW and SC transition in 2H dichalcogenides. • However, anisotropy in coupling strength does not induce the anisotropy in SC gap. Jdluv