Electron relaxation in La2xSrxCuO4 measured with ultrafast laser spectroscopy

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Electron relaxation in La2-xSrxCuO4 measured with
ultrafast laser spectroscopy

• Primo Kuar
• advisors
• Prof. Dragan Mihailovic, and Dr. Viktor Kabanov

Complex Matter - IJS
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Contents

• Introduction
• Experiment
• Measurements and results
• Conclusion

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Superconductivity

• Discovered in 1911(Onnes),
• No DC resistivity
• Perfect diamagnetism Meissner Effect
• Critical temperature, current, magnetic field
• Phenomenological theory
• London equations
• Meissner effect penetration depth
• Ginzburg-Landau 1950
• Meissner effect penetration depth
• Microscopic theory BCS 1957
• Cooper pairs
• Electron-phonon interaction
• Energy gap at Fermi level 2?

B
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• High temperature superconductivity
• 1986 Ba-La-Cu-O - cuprates
• Tc gt 30K-40K (BCS limit)
• Record Tc gt 130K (HgBa2Ca2Cu3O8x)
• Type I and II superconductors
• Type II no perfect diamagnetism in high magnetic
  fields magnetic vortices

V. L. Ginzburg (phenomenological theory) A. A.
Abrikosov (Vortices) 2/3 of the Nobel prize in
2003
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Cuprates

• Based on Cu O planes

La2-xSrxCuO4 Tc lt 40K
YBa2Cu3O7-? Tc lt 90K
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La2-xSrxCuO4

• La (3e) -gt Sr (2e)
• Doped with holes
• 0.1-0.2 cooper pars per u. cell
• 2? 30meV
• Simple structure
• Doping does not change the structure

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Ultrafast laser spectroscopy

• Laser with very short pulses (lt100fs)
• ModeLocked TiSapphire

ART1 ?R ? -?A
Reflectivity change as a function of a delay as a
consequence of different electron configurations
-gt electron relaxation
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Experimental setup
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Scheme of the electron relaxation in case of the
gap in the density of states
Pump pulse
fast el. el. relaxation
Undisturbed state at low temperatures
Just after the pump pulse
Phonons ?? gt 2? and Excited quasiparticles in
temporary thermal equilibrium
At the end slow relaxation by the phonon decay
takes place
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Thermodynamic description of slow relaxation
Phonons
Pseudoparticles above the gap
Energy conservation
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Measurements
? 800nm Epump pulse 0.1nJ Spot size
100?m Absorbtion depth 100nm Nexcited lt
210-3/u. cell ltlt 0.1/u. cell Small perturbation
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Temperature scan
La1.9Sr0.1CuO4 Tc 31K
Tc
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Analysis
Relaxation rise time
?Reflexivity/Reflexivity
Superconducting gap
pseudogap
La1.9Sr0.1CuO4
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Different dopings
Relaxation rise time
?Reflexivity/Reflexivity
Tc x0.1 31K x0.15 39K x0.2 30K
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Conclusions

• Signal up to the room T for all samples
• Qualitatively similar for all dopings
• Visible relaxation over the superconducting gap
  below TC
• Existence of pseudogap up to room temperatures