phd dissertation brescia 20-12-2004 infmd.m.f. dynamics of non-equilibrium states in solids induced...
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PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Dynamics of Non-Equilibrium States in SolidsInduced by Ultrashort Coherent Pulses
INFM and
Università Cattolica del Sacro CuoreDipartimento di Matematica e Fisica, Via Musei 41, Brescia.
Claudio Giannetti
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Investigation of Photoinduced non-equilibrium states in solids
High-Intensity femtosecond coherent pulses →
Introduction
10-100 fs
Photodiode
reflectivity variatione-Spectrometer
Photoemissionsamplepump
probe
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Investigation of Photoinduced non-equilibrium states in solids
High-Intensity femtosecond coherent pulses →
Introduction
•Time-resolved non-linear photoemission on METALS. [W.S. Fann et al., Phys. Rev. Lett. 68, 2834 (1992)][U. Höfer et al., Science 277, 1480 (1997)][G. Ferrini et al., Phys. Rev. Lett. 92, 2668021 (2004)]
•Structural and electronic phase transitions in SOLIDS and MOLECULAR CRYSTALS. [A. Cavalleri et al., Phys. Rev. Lett. 87, 2374011 (2001)][E. Collet et al., Science 300, 612 (2003)]
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
OPTICAL CONTROL OF ELECTRON INTERACTIONS AND PHASE TRANSITIONS
IN TWO SPECIFIC SYSTEMS:
Introduction
•Image Potential States on Ag(100)By selecting the excitation photon energy it is possible to investigate the properties of IPS in different regimes.
•Insulator-Metal phase transition of VO2 By selecting the excitation photon energy it is possible to clarify the physical mechanisms responsible for the photoinduced phase-transition.
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
IPS on Ag(100)
IMAGE-POTENTIAL STATES (IPS)
P.M. Echenique et al., Surf. Sci. Rep. 52, 219 (2004).
IPS: 2-dim electron gas in the forbidden gap of bulk states
Ag(100)
zπε
eEzV vac 4
1
4)(
0
2
Image Potential:
*
2||
2
2 2m
k
n
RyE
Eigenvalues: •Ry: Rydberg-likeConstant•n=1, 2,…•m* : electron effective mass
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
IPS on Ag(100)
MEASUREMENTS on IPS
)',()',(2
'
)2(),(
2*
qkGqW
dqdik
•Relaxation dynamics•IPS effective mass
Important test for many-body theories (GW)
),(
1),(
*0
kkG
k
Electron Green functionScreened
interaction potential
Electronself-energy
damping:Γ = 1/τ = ImΣ*
Effective mass:o
k + ReΣ*≈ħ2k2/2m*Quasi-particleEnergy spectrum
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
ToF
e-
UHV
sample
4th4.2eV
2nd2.1eV
Amplified Ti:Sapphire Oscillator
Pulse width: 150 fsRep. rate: 1kHzAverage power: 1WWavelenght: 790nm (1.57eV)
Source: TOPG Tunability 1150-1500 nm
(0.8-1.1 eV) Pulse width 150 fs Average power 50mW
Travelling Wave Optical Parametric Generator
Energy resolution: 10 meV @ 2eV
IPS on Ag(100)
EXPERIMENTAL SET-UP
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
IPS on Ag(100)
NON-LINEAR PHOTOEMISSION on IPS
ToF
hν = 4.2 eV > Φ
150 fs
Ekin = hν - En
Population of empty states via resonant 2-photon photoemission
τ = ħ / Γ
Phys. Rev. B 67, 235407 (2003)
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
IPS on Ag(100)
ANGLE-RESOLVED PHOTOEMISSION on IPS
Phys. Rev. B 67, 235407 (2003)
*
2//
2
// 2)(
m
kEkE n
sin2||
KinmEk
m*/m=0.970.02in agreement with calculated values
→ 2-dimensionalfree electron gas
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Non-Equilibrium Electron Distribution
NON-LINEAR PHOTOEMISSION on METALS
when hν < Φ a non-equilibrium electron population is excited in the s-p bands of Ag
investigation of the non-equilibrium electron distribution
↓•Excitation mechanisms•Relaxation dynamics
•Photoemission processes
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
*
2||
2
2m
kE
Free-electrondispersion
E
k||Γ
PHOTON ABSORPTION MECHANISMSPROBLEMS:
ΔE
Δk||
The intraband transition between s-s states within the same branch is FORBIDDEN for the conservation of the momentum.
THE ENERGY ABSORPTION IS DUE TO A THREE-BODY PROCESS AND NOT TO A DIPOLE TRANSITION
Recently the excitation mechanism has been attributed to:
•Laser quanta absorption in electron collisions with phonons. [A.V. Lugovskoy and I. Bray, Phys. Rev. B 60, 3279 (1999)]
•Photon absorption in electron-ion collisions. •[B. Rethfeld et al., Phys. Rev. B 65, 2143031 (2002)]
Non-Equilibrium Electron Distribution
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Non-Equilibrium Electron Distribution
2-Photon Photoemissionwith p-polarized light
hν=3.14eV
Log Scale106 sensitivity
Iabs=13 μJ/cm2
Occupied states
Non-equilibrium Distribution
n=1 IPS
hν
NON-LINEAR PHOTOEMISSION on AgThe excitation of a non-equilibrium electron population results
in a high-energy electron tail: E > nhνΦ
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.submitted to Phys. Rev. B
Non-Equilibrium Electron Distribution
We exclude:
• Direct 3-photon photoemission
• Coherent 3-photon photoemission
↓• Scattering-mediated
transition
The high-energy electron tail is a fingerprint of the non-equilibrium electron distribution at k||≠0
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
NON-EQUILIBRIUM ELECTRON DYNAMICSRESULTS:
Time-Resolved Photoemission SpectroscopyPhotemitted charge autocorrelation of different energy regions
The Relaxation Time of the high-energy region is
τ<150 fs
Non-Equilibrium Electron Distribution
submitted to Phys. Rev. B
fs23)()()(
)(
12
1
En
En
EdnEN 2
0)(
F
F
EE
EE Fermi-liquid
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
ENERGY TRANSFER non-equilibrium electrons
↓Equilibrium distribution
Non-Equilibrium Electron Distribution
submitted to Phys. Rev. B
Two-temperature model:
)()(
)()()(
lel
ll
lee
ee
TTGt
TTC
tPTTGt
TTC
The heating of the equilibrium distribution can be neglected
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
IPS as a Probe of Non-Equilibrium Distribution
Phys. Rev. Lett 92, 2568021 (2004)
IPS INTERACTING WITH NON-EQUILIBRIUM ELECTRON DISTRIBUTION
hν = 4.28 eV> En-EF
RESONANCE
Iinc= 30 μJ/cm2
90% d→d
ρe~ 2∙1018 cm-3
hν = 3.14 eV< En-EF
NO DIRECTPOPULATION
Iinc= 300 μJ/cm2
0% d→d
ρe~ 1020 cm-3
when hν = 3.14 eV a high-density non-equilibrium electron distribution cohexists with electrons on IPS
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
IPS as a Probe of Non-Equilibrium Distribution
hν=3.15eV hν=3.54eV
Shifting with photon energy Δhν=0.39eV
n=1
Fermi edge
Dispersion of IPS in k||-space
Ag(100)
Ekin = hν-Ebin
Ebin 0.5 eV
n=1
Ag(100)
K||=0
IMAGE POTENTIAL STATE
Phys. Rev. Lett 92, 2568021 (2004)
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
IPS as a Probe of Non-Equilibrium Distribution
ELECTRIC DIPOLE SELECTION RULESRESULTS:
n
P
S
Ag(100)
Dipole selection rules
Expected dipole selection rules: J=0 in S-polJ≠0 in P-pol
Violated in non-resonant case
Phys. Rev. Lett 92, 2568021 (2004)
EF
Ev
occupied states
emptystates
Φn=1
Indirect population of IPS
Scattering Assisted Population and Photoemission
NO DIPOLETRANSITIONRespected in resonant case
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
IPS as a Probe of Non-Equilibrium Distribution
)',()',(2
'
)2(),(
2*
qkGqW
dqdik
IPS EFFECTIVE MASS
Phys. Rev. Lett 92, 2568021 (2004)
s-polarizationm*/m = 0.88±0.04
p-polarizationm*/m = 0.88±0.01
2-D electron system interacting with 3-D electron system
Role of IPS interaction with the non-equilibrium distribution in W
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Insulator-Metal Phase Transition in VO2Insulator-Metal Phase Transition in VO2
Insulator-to-Metal photoinduced phase transition in
VO2
Solid State properties in highly non-equilibrium regimes
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Temperature-Driven IMPT in VO2
High-TRutile phaseConductor
Low-TMonoclinic phase
Insulator:Egap~0.7 eV
Tc=340K
3d energy levels
[S. Shin et al., Phys. Rev. B 41, 4993 (1990)]
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Origin of the Insulating Band-Gap
Origin of the insulating band-gap:
A comprehensive review: [M. Imada et al.., Rev. Mod. Phys. 70, 1039 (1998)]
electron-electron correlations in the d|| band(Mott-Hubbard insulator)
IMPT Dynamics:the electronic structurestabilizes the distorted
Monoclinic phase
minimization of theground-state lattice energy
(Peierls or band-like insulator)
IMPT Dynamics:a phononic mode drives
the phase transition
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Photo-Induced IMPT in VO2
The Insulator-to-Metal phase transition can be induced by ultrashort coherent
pulses.τ=150 fs hν=1.55 eV I=10 mJ/cm2
[M. Becker et al.., Appl. Phys. Lett. 65, 1507 (1994)]
•It is the same structural and electronic phase transition?
•Which is the mechanism driving the highly non-equilibrium phase transition?
•Structural and electronic transitions are simultaneous?
Questions opened:
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
•It is the same structural and electronic phase transition?Photo-Induced IMPT in VO2
StructuralYES
Electronic?
probe: hν=1.55 eVstructuraldynamicsτ~500 fs
electronicdynamicsτ~500 fs
[A. Cavalleri et al.., Phys. Rev. Lett. 87, 2374011 (2001)]
[M. Becker et al.., Appl. Phys. Lett. 65, 1507 (1994)]
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Optical Properties of VO2
j jj
PjP
iEEE
E
iEE
EE
//)(
20
2
2
2
2
DRUDE Harmonic Oscillator
[H. Verleur et al., Phys. Rev. 172, 788 (1968)]
@ 790 nmΔR/R ~ -20%
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Experimental Set-Up
time-resolved (τ~150 fs)near-IR (0.5-1 eV) reflectivity
PUMP + PROBE
three-layer sample
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Near-IR Reflectivity0.5-1 eV reflectivity:
signature of the band-gap
Multi-film calculation
Ein Eout
L1=20 nm L2=330 nm
L1
L2
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Femtosecond Band-Gap Closing
The Insulator-to-Metal phase transition is induced by 1.57 eV-pulses
and probed by 0.54 eV-pulses (under gap)
Signature of Femtosecond band-gap closing
150 fs
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Photo-Induced IMPT Mechanism
•Which is the mechanism driving the highly non-equilibrium phase transition?
d||
π*
hole - doping
e-
with Ipump>10 mJ/cm2
hole-doping ~ 20-100%
•Removal of the d|| electron-electron correlations→band-gap collapse and lattice stabilization•Coherent excitation of the phonon responsible of the IMPT→lattice transition and electronic rearrangmentIn this experimental scheme it is not possible to discriminate!
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Photo-Induced IMPT Mechanism
d||
π*
hole - doping
e-
Near-IR photoinduction of the phase transition
0.7 eV
in the under-gap region
the hole-doping is highly reduced
we can discriminate between the two mechanisms
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Near-IR Photoinduction of the IMPT
Pump: 0.95 eVProbe: 1.57 eV-pulses (under gap)
ZOOM:IMPT completed in 150 fs:NO thermal effect
Metastable metallic phase
Two dynamics: τ1=200 fsτ2=1000 fs
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Near-IR Photoinduction of the IMPT
The Insulator-to-Metal phase transition can be induced in the under-gap region, through near-IR pulses (0.5-1 eV)
The pump fluence necessary for the IMPT is about constant!
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Conclusions
We have demonstrated that selecting a particular excitation channel:
•It is possible to photoinduce the IMPT of VO2 and clarify the physical mechanisms responsible for the VO2 electronic properties
•It is possible to investigate IPS on Ag interacting with a photoinduced non equilibrium electron distribution
PhD Dissertation Brescia 20-12-2004
INFM D.M.F.
Publications
•G. Ferrini, C. Giannetti, D. Fausti, G. Galimberti, M. Peloi, G.P. Banfi and F. Parmigiani, Phys. Rev. B 67, 235407 (2003).
•G. Ferrini, C. Giannetti, G. Galimberti, S. Pagliara, D. Fausti, F. Banfi and F. Parmigiani, Phys. Rev. Lett. 92, 2568021 (2004).
•C. Giannetti, G. Galimberti, S. Pagliara, G. Ferrini, F. Banfi, D. Fausti and F. Parmigiani, Surf. Sci. 566-568, 502 (2004).
•G. Ferrini, C. Giannetti, S. Pagliara, F. Banfi, G. Galimberti and F. Parmigiani, in press on J. Electr. Spectrosc. Relat. Phenom.
•F. Banfi, C. Giannetti, G. Ferrini, G. Galimberti, S. Pagliara, D. Fausti and F. Parmigiani, accepted for publication on Phys. Rev. Lett.
•C. Giannetti, S. Pagliara, G. Ferrini, G. Galimberti, F. Banfi and F. Parmigiani, submitted to Phys. Rev. B.
•E. Pedersoli, F. Banfi, S. Pagliara, G. Galimberti, G. Ferrini, C. Giannetti and F. Parmigiani, in preparation.