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The puzzling CDW-phase of 1T-TiSe2 from a TR-ARPES point of view
Institut für Experimentelle und Angewandte Physik Christian-Albrechts-Universität zu Kiel
Michael Bauer
http://www.ieap.uni-kiel.de/solid/ag-bauer/
outline
the problem: TiSe2 - excitonic insulator or Peierls insulator?
strategy and experiment: reference systems and time-resolved XUV-ARPES
collapsing gap dynamics: 1T-TiSe2 1T-TaS2 , Rb:1T-TaS2 outlook on technical improvements
1T-TiSe2
CDW at T < 200 K
transition metal dichalcogenide
CDW instability in 1T-TiSe2
Peierls: electron-phonon interaction Excitonic: electron-electron interaction
0 π/a 0 π/a
W. Kohn, in Many Body Physics, Gordon and Breach, New York (1968)
1T-TiSe2 - nature of CDW state?
Se 4p
Ti 3d
• excitonic insulator? • Peierls-type transition? • mixed process?
K. Rossnagel et al., PRB 65, 235101 (2002)
T.E. Kidd, et al., PRL 88, 22402 (2002)
H. Cercellier et al., PRL 99, 146403 (2007)
hierachy of melting times
electronic driven lattice driven
melting time
TiSe2 reference systems
strategy
reference systems
K. Rossnagel et al., PRL 95, 126403 (2005)
1T-TaS2 - Peierls-Mott insulator Rb-intercalated 1T-TaS2
S
S Ta
Rb
S
S Ta
B. Dardel et al., PRB 45, 1462 (1992)
HHG-ARPES: experimental scheme
• time resolution: sub 30 fs (XUV-pulses < 10 fs) • energy resolution: ∆E ≈ 400 meV • photon flux: ≈ 2⋅109 photons/s at 43 eV (sample position)
opticaldelay-line
fs XUV pulse generation
Infrared-pulse
1.4 mJ/pulse, 800 nm,25 fs, 3 kHz
XUV-Multilayeroptics
Aluminumfilter
Argon
energy analyzer with 2D detector
sample
DPSS
DPSS Ti:Saphire Osc.
Ti:Saphire amplifier
UHV system
S. Mathias et al.; Rev. Sci. Instr. 78, 083105 (2007)
1T-TiSe2 – ARPES with fs XUV pulses
T = 115 K hν = 43 eV ∆E ≈ 400 meV
trARPES 1T-TiSe2
-70 fs
IR-pump: 33 fs, 1.5 eV, 5 mJcm-2, p-pol
XUV-probe: < 10 fs, 43 eV, s-pol
0 fs 30 fs
T. Rohwer et al., Nature 471, 490 (2011)
trARPES 1T-TiSe2
300 Jcm-3
trARPES 1T-TiSe2 - fluence dependence
T. Rohwer et al., Nature 471, 490 (2011)
trARPES 1T-TaS2
L. Perfetti et al., PRL 97, 067402 (2006) J. C. Peterson et al., PRL 107, 177402 (2011)
trARPES 1T-TaS2
Mott-gap
CDW-gap
∆Mott
∆CDW 412 fs
300 Jcm-3
trARPES Rb:1T-TaS2
∆CDW
300 Jcm-3
< 20 fs
44 fs
131 fs
232 fs
600 Jcm-3
comparison of melting times
electronic driven
lattice driven Rb:1T-TaS2 - CDW
1T-TaS2 - CDW
120 fs
215 fs
electronic driven
1T-TaS2 - Mott
1T-TiSe2 - CDW
< 20 fs
75 fs
Mott insulator
Excitonic insulator
Peierls insulator
Peierls insulator
300 Jcm-3
S. Hellmann et al., will be published next week
CDW dynamics 1T-TiSe2 – screening
n1∝
v v
L. Bányai et al., PRL 81 , 882 (1998). R. Huber et al. Nature 414, 286 (2001).
mel
ting
time
[fs]
summary
1T-TaS2 1T-TiSe2 1T-TaS2/Rb-1T:TaS2
S. Hellmann et al., will be published next week
R. Carley et al., PRL 109, 057401 (2012)
HHG-based trARPES - energy resolution magnetization dynamics in Gd
J. C. Peterson et al., PRL 107, 177402 (2011)
Mott dynamics in 1T-TaS2
grating-based approaches ∆E ≈ 150 meV
narrow-bandwidth HHG generation
BBO IR
IR + SHG
HHG generation
grating
7th order SHG
IR HHG background IR HHG background
Argon 30 torr
waevlength/nm
hν = 21,7 eV ∆E < 200 meV
SHG 7th harmonic: − small bandwidth − singular − rigid
27nd harmonic IR 7th harmonic SHG
7th harmonic SHG - ARPES
1T-TiSe2
∆E = 150 meV
0 fs
IR fluence: 0.2 mJ/cm2
100 fs -500 fs
E-E F
(eV)
7th harmonic SHG - time-resolved ARPES
signa
l am
plitu
de [a
rb. u
nits
]
time delay [fs]
collaborators
Ch. Sohrt, M. Wiesenmayer, T. Rohwer, A. Stange, S. Hellmann, G. Rohde, K. Hanff, B. Slomski, M. Kalläne
technical aspects of trARPES S. Eich, M. Aeschlimann, S. Mathias, Univ. of Kaiserslautern
X-ray optics/HHG generation L. Miaja Avila - A. Carr - H. Kapteyn - M. Murnane, JILA Colorado Y. Liu, Center for X-Ray Optics, Berkeley
idea and experiment AG K. Rossnagel/L. Kipp, AG M. Bauer, University of Kiel
response - summary 300 Jcm-3 600 Jcm-3
Brillouin zone
1T-TiSe2 – CDW reconstruction
real space reciprocal space
normal phase CDW phase: (2x2x2) - reconstructed
unit cell Brillouin zone
F.J. DiSalvo et al., PRB 14, 4321 (1976)
trARPES 1T-TiSe2
IR-pump: 33 fs, 1.5 eV, 1.5 mJcm-2, p-pol XUV-probe: < 10 fs, 43 eV, p-pol
T. Rohwer et al., Nature 471, 490 (2011)
E. Möhr-Vorobeva et al., PRL 107, 036403 (2011)
1T-TiSe2 : time-resolved reflectivity
photoemission transients
ROI
time-resolved photoemission
NUV probe XUV probe
115 K CDW phase
1T-TaS2 – dynamics
14.09.2012 Melting of CDWs studied by TR-XUV ARPES 31
t =-200 fs
t = 220 fs, 1.1 mJ/cm-2
2.75 mJ/cm-2
Γ
0.8 mJ/cm-2
Similar data by A. Cavalleri et al., PRL 107, 177402 (2011)
Ta
S Γ
M
M
Photoemission transients
Monochromator
12% total reflectivity R40,46eV / R43eV = 5/1000
Multilayer optics: CXRO at LBNL, Berkeley
planar
curved, R = 2m
HHG source
sample
27th 25th 29th
HHG source
HHG mode (43 eV) HHG intensity (43 eV)
Image intensifier
(calibrated X-ray photodiode)
4 x 10 9 photons / s at 43 eV at sample position ~1011 photons / s at 43 eV at HHG source output
Maximum
Standard
time resolution
IR-pump/XUV-probe crosscorrelation
Laser assisted PE*
33.4 ±1.1 fs
34.9 ±3.2 fs
time delay [fs]
Phot
oem
issi
on in
tens
ity [a
rb. u
nits
]
IR-pulse: 32 fs (FROG) XUV-pulse: < 10 fs
*L. Miaja-Avila et al., PRL 97, 113604 (2006)
trARPES 1T-TiSe2: EDCs
IR-pump: 33 fs, 1.5 eV, 5 mJcm-2, p-pol XUV-probe: < 10 fs, 43 eV, s-pol
trARPES 1T-TiSe2
-70 fs
IR-pump: 33 fs, 1.5 eV, 5 mJcm-2, p-pol
XUV-probe: < 10 fs, 43 eV, s-pol
0 fs 30 fs
3000 fs
T. Rohwer et al., Nature 471, 490 (2011)