The “normal” state of layered dichalcogenides

Arghya Taraphder

Indian Institute of Technology Kharagpur

Department of Physics and Centre for Theoretical Studies

Workshop @ Harish Chandra Research Institute, November 12-14, 2010

Salient Features

Transition metal dichalcogenide – TM atoms

separated by two layers of chalcogen atoms

TM atoms form 2D triangular lattice

CDW & Superconductivity (likely to be anisotropic)

Partially filled TM d band or chalcogen p band:[]d1/0

1T and 2H type lattice structure

Both I and C CDW at moderate temperature

Normal to SC transition with pressure/doping

Normal transport unusual (cf. HTSC)

Dichalcogenides: crystal structure

Glossary

Typical Phase diagram

D.B. Mcwhan, et al. PRL 45,269(1980)(2H-TaSe2)

A. F. Kusmartseva, et al. PRL 103, 236401(2009) (1T-TiSe2)

B. Sipos, et al. Nat. mater. 7, 960 (2008) (IT-TaS2)

2H-TaSe2

1T-T

iSe2

1T-TaS2

2H-TaS2Cava et al.

Phase diagram of 1T-TiSe2 : doping and pressure

Quantum critical?

Castro-Neto, loc cit

Cava, PRL (2008)

DC Resistivities

Aebi, loc cit

Resistivity of TMDs: 1T and 2H

Y. Ueda, et al. Journal of Physical Societyof Japan 56 2471-2476, (1987).

P. Aebi, et al. Journal of Electron Spectroscopy and Related Phenomena117–118 (2001)

Vescoli et al, PRL 81, 453 (1998)

2H-TaSe2

Op

tica

l co

nd

uct

ivit

y(0

.04

< E

< 5

eV

ran

ge)

R C

Dyn

es,

et a

l.,

EP

JB 3

3, 1

5 (

200

3)

Features of dc transport and Re σ (ω)

•“Drude-like” peak at ω=0 for both systems along both ab and C-directions, narrowing at low T, indicating freezing of scattering of charge carriers at low energy

•Tccdw does not affect transport at all, in fact thermodynamics is also unaffected

•Broad conductivity upto large energies (~0.5 eV)

Dynes loc cit

Spectral weight distribution

•Spectral weight is non-zero even upto 5 eV and beyond – “recovery” of total n uncertain

•Shifts progressively towards FIR as T is lowered - condensation at lower frequency

• Nothing abrupt happens as T_CDW is crossed

Transport scattering rateab-plane

Tra

nsp

ort

sca

tter

ing

rat

ec-

axis

Scattering rate from transport

• Strongly frequency dependent. Rapid suppression of both Γab and Γc below characteristic freq. ~ 500 /cm Possible “pseudogap” in 20K curve

•High and low T Γab cross each other for TaSe2 at some frequency

•No saturation of Γab upto 0.6 eV

•Both Γab and Γc are above Γ= ω line upto 2000 /cm and nearly linear in ω

“QP” Scattering Rate & SE from ARPES

Valla, PRL 85, 4759 (2000)

Valla, loc. cit..Fit with momentum-indep. SE

Aebi, JES 117, 433 (2001)

Electronic structure

Self-energy from ARPES

•Local - no k-dependence

•Re Σ peaks at 65 meV, Im Σ drops there – characteristic of a photo-hole scattering off a collective ‘mode’ ~ 65 mev (too large for all phonons in TaSe2)

•Im Σ(0) matches excellently with transport Γ(0) in its T-dependence

2H-TaSe2

H.E. Brauer,et al. J. Phys. Cond. Matter 13, 9879 (2001)

Band structure

Aebi, JES 117, 433 (2001)

Tight Binding Description

N V Smith, et al. J. Phys. C:Solid State Phys. 18 (1985) 3175-3189

Tight binding fit near FL for 2H-TaSe2

N V Smith, et al. J. Phys. C:Solid State Phys. 18 (1985) 3175-3189.

Fermi surface map for the TB bands

2H-TaSe2 1T-TaS2

ARPES - 2H-TaSe2

Liu

, P

RL

80

, 57

62

(199

8)

Liu

, P

RL

80

, 57

62

(199

8)

Valla et al, PRL 85, 4759 (2000)

CDW Gap ? Castro_Neto, PRL 86, 4382 (2001)

)Pseudogap in 2H-TaSe2, Borisenko et al, PRL 100, 196402 (2008)

Fermi surface and ARPES - 2H type

N V Smith, et al. J. Phys. C: Solid State Phys. 18 (1985) 3175-3189.

S V Borisenko, et al.Phys. Rev. Lett. 100, 196402 (2008)

Fermi surface and ARPES - 1T type

N V Smith, et al. J. Phys. C:Solid State Phys. 18 (1985) 3175-3189.

F.Clerc, et al. Physica B 351 245-249, (2004)

Fermi surface of 1T-TiSe2

P. Aebi, et al. Phys.Rev.B 61 16213, (2000)

Superlattice & BZ in the CDW phaseof Dichalcogenides

N V Smith, et al. J. Phys. C:Solid State Phys. 18 (1985) 3175-3189.

2H-TaSe2

1T-TaS2

Our Work: LDA - tight binding fit near FL for 2H-TaSe2

N V Smith, et al. J. Phys. C:Solid State Phys. 18 (1985) 3175-3189.

Fermi surface map for the TB bands

2H-TaSe2 1T-TaS2

Spectral Function for 2H-TaSe2

Before DMFT After DMFT

Evolution of Spectral Function and fitting ARPES

Conductivity and resistivity from DMFT

DMFT with inter-orbital hopping for 2H-TaSe2

Opening of gap with increase in temperature

Pressure dependence of Fermi Surface

Change in spectral function with pressure

Temperature dependent Spectral function at different pressure

Change in resistivity at different pressure

Conclusion

• DMFT Spectral function is broadened.

• With application of Inter-orbital coulomb interaction

the system goes to insulator.

• With application of Inter-orbital hopping DMFT orbital

occupation changes from LDA.

• There is a opening of gap with increasing temperature

up-to 140K.

• With decreasing pressure hole pockets in the Fermi

surface disappear.

• With increasing pressure the gap formed at the Fermi

surface decreases.