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

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Fermi liquid pheonomena in ARPES
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 renormalizationA finite quasi-particle lifetimeAn incoherent background created by e-h paursPES weight outside Fermi surface

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Self energy of quasi particle
Describes the interaction in Fermi liquid

Conserving total Q

Re(k,w) the shift in energy Renormalization ~ E

Im(k,w) scattering rate or inverse lifetime ~ Lorentzian shape. ~ E2.

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2H-Dichalcogenides system
2 : number of layer H : hexagonal symmetry

2D layered system

CDW and Superconductivity coexist.

NbSe2, NbS2, TaSe2, TaS2

If TCDW increases, TSC decreases. NbSe2 TCDW 35K/TSC 7.2 KTaSe2 TCDW 90K/TSC0.15 K Two order parameters compete.

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2H TaSe2 large kink in spectra
Below TCDW, kink shown near EF. (@34 K)

Interaction can be estimated by direct fitting. Re ~ shift in energyIm ~ Lorentzian shape

70meV signature in both parts

Not only e-ph coupling and electron scattering also drive this change.

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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

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Experiment
High resolution ARPES performed on U13UB @ NSL combined with SES-200. Energy resolution 4 meV @ T ~ 15KAngular resolution < 0.1 (0.0025-1 @ 15.2eV)

2H-NbSe2 was grown by iodine vapor transport method.Cleaved in situ in UHV3 X 10-9 Pa during measurement

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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

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Self energy Re
Different peak positions and magnitude.

13 < wmax < 35meV Acoustic phonon w ~ 12 meVOptical branch 15 < w < 40 meVDifferent phonon coupled different states

Except for point 6(1.9), ~ 0.85 0.15Good agreement with various experimental results, such as specific heat and c-axis optical measurement and so on.
Specific heat coefficient ~ 18.5mJmol-1K-2 ~ N(0)(1+)

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Relation with CDW?
Gap opening of CDW is not noticed in this system.

However, large (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.

Authors suggestion.Both SC and CDW originate from inner K-sheet driven by strong e-ph coupling.
Contrary to STM result ~ 35meV CDWgap

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Compare to 2H-TaSe2?
Different temperature dependence

Kink and scattering rate are insensitive to the CDW transition in NbSe2.

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

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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 betn CDW and SC. (doping in HTSC) -but by other mechanism rather than phonon.

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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.