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)