ADRESS
Pump-probe RIXS:
Cu L3 and O K edge vs. Cu K-edge
Thorsten Schmitt
Paul Scherrer Institut
CH-5232 Villigen PSI
Switzerland
People
ADRESS
T. Schmitt, V.N. Strocov, J. Schlappa, K. J. Zhou, F. Vernay, B. Delley,
B. Thielemann, J. Mesot, and L. Patthey
Paul Scherrer Institut, Switzerland
H.M. Rønnow
EPFL Lausanne, Switzerland
V. Ilakovac (Collaboration Sr14Cu24O41)
Pierre and Marie Curie University, Paris, France
Vanishri S., and C. Marin (Sr14Cu24O41 samples)
INAC/SPSMS/DRFMC, CEA-Grenoble, France
J. van den Brink et al.
IFW Dresden
G. Ingold (ack. for discussions on time resolved experiments)
Paul Scherrer Institut, Switzerland
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Strongly correlated electron systems
Understand materials with novel electronic properties
Coupling of degrees
of freedom
• High temerature superconductors
• CMR –materials
• heavy fermions / Kondo systems
• Mott-Hubbard systems
• Spin / charge / orbital order
• unconventional superconductors
Different time scales:
Tvib ~ 100 fs
Telectron-phonon ~ 1 ps
Telectron-electron ~ 10 fs
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Femtosecond spin-lattice relaxation in Ni
Ultrafast quenching of ferromagnetic
order → transfer of spin angular
momentum to lattice (angular
momentum conservation)
Initial x-ray pulse: 100±20 fs
Ni L3 XAS (lin. Pol.)
XAS: 200 fs after laser excitation
XAS without laser excitation
C. Stamm et al., Nature Materials 6, 740 (2007).
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Discriminating between electronic and magnetic structure
linear polarized XAS signal
Temporal evolution of electron-hole
excitations: 120 ± 50 fs
(valence electron localization)
XMCD XAS signal
Time to quench ferromagnetic order:
120 ± 70 fs (ultrafast demagnetization)
C. Stamm et al., Nature Materials 6, 740 (2007).
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Femtosecond time resolved resonant
inelastic X-ray scattering (fs-RIXS)
Probing ultrafast electron dynamics in strongly correlated materials
time resolved pump-probe RIXS
Study of matter in excited states with RIXS at an X-ray Free Electron Laser:
- Dynamics of ultrafast phase transitions in complex systems (e.g. metalinsulator transitions)
- Excite specific phonon or magnetic mode and follow temporal evolution
RIXS
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hωout = hωin - Eexc
Em‘
Em
Typical time scale
Low energy
excitations Eexc
hωin
B
Ef
A
Eexc
Ei
TM
3d*
3d
hv
2p
hv‘
CT excitations
Mott gap
4 eV
1 fs
Orbital excitations
dd-excitations
2 eV
2 fs
100 meV
Magnons
Spin-flips
Optical phonons
40 fs
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Q-dispersion of CT-excitation by Cu K- & Ni K-RIXS
Momentum transfer:
q = k2 – k1
La2CuO4
La2NiO4
k1
q
k2
- k1
E. Collart et al., PRL 96, 157004 (2006).
localized CT-exciton
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SAXES RIXS station
Collaboration:
PSI (T. Schmitt et al.) & Politecnico di Milano (L. Braicovich et al.) & EPFL Lausanne (M. Grioni et al.)
CCD
camera
spherical 3200 lines/mm VLS grating
entrance sample
slit
detector (L-N2 cooled CCD)
on movable frame
rotating platform on air cushions (5 positions)
G. Ghiringhelli et al., Rev. Sci. Instrum. 77,
77 113108 (2006).
SAXES
& Swiss Light Source
Politecnico di Milano
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RIXS: Sr14Cu24O41
Cu-L3 RIXS: Cu2p3/2 → Cu3d → Cu2p3/2
O-K
RIXS: O1s → O2p → O1s
~ 931 eV
~ 525 eV
elastic peak
Charge transfer excitations
Crystal field exc.
Spin excitations:
(local) spin flips,
collective spin excitations
(mutiple spin waves)
J. Schlappa, T. Schmitt et al., Phys. Rev. Lett 103, 047401 (2009)
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RIXS intensity map
Sr14Cu24O41
Two-triplon
continuum
One-triplon
dispersion
(K. P. Schmidt and G. S. Uhrig,
Mod. Phys. Lett. B 19, 1179
(2005))
• Intensity follows
lower boundary for 2triplon continuum
J. Schlappa, T. Schmitt et al., Phys. Rev. Lett 103, 047401 (2009)
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RIXS vs. Theory - Neutrons
RIXS Experiment:
INS from La4Sr10Cu24O41
S. Notbohm et al., PRL 98, 027403 (2007).
Homogeneous cross-section
over the full Brillouin zone !
low sensitivity @ small
q-transfer
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Two-magnon excitations in Cu K-RIXS
1
La2CuO4
0.9
0.8
(π, 0)
−1
Intensity (s )
0.7
0.6
0.5
on-resonance
0.4
0.3
0.2
off-resonance
0.1
0
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Energy Loss (eV)
J. Hill et al., Phys. Rev. Lett. (2008).
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Charge transfer excitations in Cu K-RIXS
M.Z. Hasan et al., Science 288, 1811 (2000).
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Summary / Conclusions
RIXS
• photon-in / photon-out → bulk-sensitive, ideal for insulating samples
• probes low-energy valence excitations, phonons and spin-excitations
• charge neutral: no interference with magnetic and electric fields
• couples directly to charge and indirectly to spin degrees of freedom
• resolution only limited by final state lifetime
• q-transfer (Cu K) >> q-transfer (Cu L, O K)
• Cu K (TM K) RIXS excellent for charge transfer excitations
• Cu K (TM K) much better for in-situ experiments (pressure, liquids etc.)
Time resolved - RIXS
• pump and probe RIXS: dynamics of phase transitions
• will help understanding the coupling between the degrees of freedom:
Pump one DOF and monitor the other with RIXS
• Combine TM K- with O K- and TM L-RIXS
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Femtosecond X-ray absorption spectroscopy
of photoinduced MIT in VO2
A. Cavalleri et al., PRL 95, 067405 (2005).
MIT in VO2
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800 nm laser pump – x-ray absorption probe
Fluence 25 mJ/cm2
collapse of band gap
(nonequilibrium electron distribution)
transient core level shifts
A. Cavalleri et al., PRL 95, 067405 (2005).
hot electron
distribution
thermalizes
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Metal-insulator transition in VO2
VO2: 1st order metal-insulator transition
T> 340K: Metallic phase: Rutile
T< 340K: Insulating phase: Monoclinic
F. J. Morin, PRL (1959)
Still object of controversy!
Concepts for MIT:
Rutile (R) (Metallic)
Monoclinic (M1) (Insulating)
• crystal structure distortion
(Peierls-transition)
cr
• electron-electron correlation
(Mott-Hubbard-transition)
• or both?
V.Eyert et. al., Ann.Phys. (2002)
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RIXS on VO2 (before ADRESS / SAXES)
Raman features around 0-2 eV loss
in RIXS on insulating phase of VO2
V Lβ
V Lα
O 2p – V 3d
hybridized PDOS
V 3d PDOS
L. Braicovich et al., PRB (2007)
T. Schmitt et al., Surf. Rev. Lett.(2002).
T. Schmitt, Ph.D. thesis, Uppsala 2004.
RP ~ 900: mostly fluorescence
MIT in VO2: high resolution
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VO2 Temperature-dependent RIXS @ ADRESS - BL
30o
π d∗
//
∗
σ∗
O K and
V L2, 3 XAS
V L RIXS
K.-J. Zhou et al.,
to be published
initiate MIT with laser pulse
(pump) and probe with RIXS:
direct sensitivity
to transition across gap
Sharp peak at 0.45 eV in V L –RIXS for LV-pol. @ insulating phase:
dd-excitation across band gap OR inter-site charge transfer within a dimer
Sr14Cu24O41
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Temperature dependence of Cu L3 RIXS spectrum in Sr14Cu24O41
Dominant spin
excitations in RIXS:
2-triplon excitation
J┴
J||
J. Schlappa, T. Schmitt et al.,
Phys. Rev. Lett 103, 047401 (2009)
ladder sub-system
spin-singlet ground state
Elementary excitations:
at cost of finite energy (Spin gap)
Triplons
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q-dependent RIXS
Sr14Cu24O41:
T=15K
90°
130°
J. Schlappa, T. Schmitt et al., Phys. Rev. Lett 103, 047401 (2009)