Ultrafast electronic and structural dynamics in NbSe43I

1
Ultrafast electronic and structural dynamics in
(NbSe4)3I
Dept. of Complex Matter, Jozef Stefan Institute,
Ljubljana, Slovenia Faculty of Mathematics and
Physics, University of Ljubljana, Slovenia

• D. Mihailovic
• V.V.Kabanov,
• Samples K.Biljakovic (IFS Zagreb)
• Discussions P.Monceau

Skradin 2006
2
Outline

• The femtosecond time-resolved optical
  spectroscopy
• The example of (NbSe4)3I an electronically
  driven structural phase transition
• Compare the dynamics of different collective
  phenomena

3
Femtosecond optical spectroscopy the pump-probe
technique.
h?phonon
QP states
D
t 200 - 3000 fs
sample

• Pump laser typically 400 or 800 nm, 50 fs, Mira
  or REGA

• Probe laser typically 800 nm, but also NIR and
  MIR (OPA and DFG)

4
(NbSe4)3I

• A quasi one-dimensional small-gap semiconductor
• with a pseudo-Jahn-Teller structural instability
  near T 274 K

tetragonal, D4h at 300K
Gressier et al, 1984-85, Izumi 1984-88
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The vibronic interaction in (NbSe4)3I mixes the
B2u and A2g states by a B1u-symmetry phonon.
B1u
B1u Vibration
mixes
Nd dz2 sub-bands
A1u
B2u
B2u and A2g electronic states
EF
A2g
B1g
Sekine Izumi, 1988 Kristoffel et al, 1968
Gressier, Monceau et al, 1984
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The order parameter
Kristoffel Konsin, 1968, Sekine and Izumi, 1988
The order parameter h is related to the
difference in occupancy of the two levels h ?
(n2 - n1)
Pseudo Jahn-Teller effect
population n2
population n1
7
A TA (acoustic) soft-phonon is observed by
neutron scattering. (Tc274 K)
a B1u soft mode
Tc
B1u
Lorenzo et al, 1995
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Femtosecond dynamics of the order parameter
relaxation in (NbSe4)3I
pump
Pump
sample
B2u
A2g
Since ? ? n1-n2 (the difference in populations of
the ground state and first excited states), ? we
can use femtosecond spectroscopy to directly
probe the structural and electronic degrees of
freedom in real time.
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The data Photoinduced reflectivity vs. time

• The response of the system to an external
  electronic perturbation is described in terms
  of
• The relaxation of a zero frequency (overdamped)
  electronic mode (order parameter)
• Oscillatory modes of A symmetry below Tc, coupled
  to the order parameter.

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Model time-relaxation of the order parameter
(Kabanov and Dvorsek, 2005)
Landaus theory with ?Q coupling
1. Landau-Khalatnikov equation
2. Equation of motion for phonon
The solution gives the photoinduced reflectivity
where the renormalized frequency of each mode is
Note both the amplitude of the soft mode
oscillations and exponentially decaying
component behave as
The relaxation time of the order parameter
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T-dependence of the relaxation time t and h for
E//c and E?c
Theory predictions (TltTc)
Theory holds well for E?c. but does not hold for
E//c.
Lifetime 1/t (Tc-T)
Amplitude A h n1 - n2
12
A Fourier transform of DR/R gives the spectrum
FFT
in (NbSe4)3I
Non-oscillatory part can be described by
13
The phonon spectrum
in (NbSe4)3I
14
T-dependence of the soft modes
Soft phonon frequency
(w2- w02 ) n2 - n1
(this is true for all modes of appropriate (A)
symmetry.)
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Conclusions

• The relaxation of the order parameter describing
  the structural relaxation can be observed in
  real-time.
• In spite of the soft mode like behaviour, the 12
  cm-1 mode is actually not the soft mode of the
  phase transition.
• The observed relaxation dynamics of different
  macroscopic states (structural ordering,
  superconductivity and CDW formation) show
  qualitatively similar behaviour.
• A new very anomalous state below 100 K in Nb-Se-I