Evolution of the charge density wave in Sulfur substituted ... · STM/STS x = 0.12 • x = 0.34 T...

Evolution of the charge density wave in Sulfur substituted 1T-TiSe2

A combined ARPES and STM/STS study

M.-L. Mottas, T. Jaouen, B. Hildebrand, E. Razzoli, G. Monney and P. Aebi DFT calculations : D. R. Bowler F. Vanini Growth of Single Crystals : E. Giannini, C. Barreteau

University of Fribourg London Centre for Nanotechnology University of Fribourg University of Geneva

Charge Density Wave

Superconductivity

F. Di Salvo, PRB, vol. 14, 4321 (1976)

Cu doped 1T-TiSe2

E. Morosan, Nat. Phys. 2, 544 (2006)

Under pressure

A. F. Kusmartseva, PRL, vol. 103, 236401 (2009)

TCDW ~ 200 K, new 2x2x2 structure F. Di Salvo, PRB, vol. 14, 4321 (1976)

vdW gap

Se

Se

Ti TiSe2-xSx

x

In Ti self-doped 1T-TiSe2

B. Hildebrand, PRB, vol. 95, 081104 (2017)

0%

2%

Ti self-doped 1T-TiSe2

Anomalous resistivity

Local resilience of the CDW

1T-TiSe2 : an intriguing compound

Motivation

Charge Density Wave

Superconductivity

Cu doped 1T-TiSe2

E. Morosan, Nat. Phys. 2, 544 (2006)

Under pressure

A. F. Kusmartseva, PRL, vol. 103, 236401 (2009)

TCDW ~ 200 K, new 2x2x2 structure F. Di Salvo, PRB, vol. 14, 4321 (1976)

vdW gap

Se

Se

Ti

TiSe2-xSx

Local resilience of the CDW In Ti self-doped 1T-TiSe2


Influence of Sulfur on the transition temperature and CDW behavior?

x≈0%

x≈200%

F. Di Salvo, PRB, vol. 14, 4321 (1976)

Sulfur doped 1T-TiSe2

Anomalous resistivity

1T-TiSe2 : an intriguing compound

Motivation

1x1 2x2

The topmost layer of 1T-TiSe2 is a Se layer.

Vbias=-1 V Vbias=0.15 V

Real space

STM on 1T-TiSe2-xSx

Characterization of samples 1T-TiSe2-xSx

Vbias+0.6 V, It=0.15nA, 15x15 nm2 at 4.5 K

Identification of S atoms


S Sevac S

Z (

pm

)

profile (nm)

S substitution – Se vacancy distinction

S

Sevac

S




Vbias+0.6 V Vbias+0.3 V

Vbias-0.5 V Vbias-1.0 V

Sulfur depletion of ῀ 14 pm

Topographic effect

S2- Se2-

14 pm




Vbias+0.6 V Vbias+0.3 V

Vbias-0.5 V Vbias-1.0 V

Sulfur depletion of ῀ 14 pm

DFT simulation*

Vbias+0.6 V

Measured image

Vbias+0.6 V

Topographic effect

S2- Se2-

14 pm


*D. R. Bowler, London Centre for Nanotechnology



x = 0.12 x = 0.34

Precise determination of Sulfur concentrations




x = 0.12 x = 0.34



Ensure negligible Intercalated-Ti concentration

Characterization of samples 1T-TiSe2-xSx Effect of intercalated-Ti

Vbias-0.05 V, It=0.2nA, 17x17 nm2 at 4.5 K

Vbias0.1 V, It=0.2nA, 17x17 nm2 at 4.5 K

Ensure negligible Intercalated-Ti concentration

Phase-shifted domains : break of the CDW long-range coherence

~ 2.5 % intercalated-Ti (no sulfur)

~ 1.2 % intercalated-Ti (no sulfur)




x = 0.12 x = 0.34


Less than 0.2% of intercalated-Ti effects of Sulfur substitution only


Characterization of samples 1T-TiSe2-xSx Long-range coherence of the CDW

What about ARPES measurements ?

Long-range coherence of the 2x2 in-plane electronic modulation

Inexistence of phase-slip



x = 0.12 x = 0.34

H A

Γ

L

K M

L

A

Γ

M

K

ARPES on 1T-TiSe2

Brillouin zone of the 1x1x1 structure

Top view near-EF band structure

Reciprocal space

H A

Γ

L

K M

L

A

Γ

M

K

ARPES on 1T-TiSe2



T-dependent ARPES study from TROOM to 10 K

UPS with He I, Eh= 21.2 eV (close to A and L points)

Reciprocal space

H A

Γ

L

K M

L

A

Γ

M

K

E

EF

k

Γ

ARPES on 1T-TiSe2





• Se 4p VB at Γ (Cut // Γ − K )

Reciprocal space


H A

Γ

L

K M

L

A

Γ

M

K

k

E

EF

k

Γ M



• Se 4p VB at Γ (Cut // Γ − K )

• Ti 3d CB at M (Cut // Γ − M (long axis))

Reciprocal space

ARPES on 1T-TiSe2


Γ

M

K


H A

Γ

L

K M

L

A k

E

EF

k

Γ M

-0.33 eV below EF

Normal phase (RT)

ARPES on 1T-TiSe2 Pristine at RT

Reduced Brillouin zone of the 2x2x2 structure


Γ

Γ

M

K

k

E

EF

k

Γ

A L

M

CDW phase

ARPES on 1T-TiSe2

M


Γ

Γ

M /Γ *

K

k

E

EF

k

Γ M /Γ *

A/Γ* L/Γ*

M/Γ*

ARPES on 1T-TiSe2


CDW phase


Γ

Γ

M /Γ *

K

k

E

EF

k

Γ M /Γ *

A/Γ* L/Γ*

M/Γ*

ARPES on 1T-TiSe2 Pristine at 10 K

CDW phase (10 K)



Γ

Γ

M /Γ *

K

k

E

EF

k

Γ M /Γ *

A/Γ* L/Γ*

M/Γ*

ARPES on 1T-TiSe2

Semiconductor or semimetal ?


CDW phase (10 K)

Pristine at 10 K

Γ

M

K

Γ M

ARPES on 1T-TiSe2

Semiconductor at TROOM

Ti 3d CB above EF Se 4p VB slightly below EF

Pristine

Normal phase (RT) Normal phase (RT) CDW phase (10 K) CDW phase (10 K)

+10 meV

-84 meV

Γ

M

K

Γ M

ARPES on 1T-TiSe2


Ti 3d CB above EF Se 4p VB slightly below EF

Pristine


+10 meV

-84 meV

Doped samples?

ARPES on 1T-TiSe2-xSx Γ

M

K

Γ M

Ti 3d CB below EF Se 4p VB below EF


Semimetal at TROOM

x = 0.12

-18 meV

-67 meV

ARPES on 1T-TiSe2-xSx

Γ M

Γ

M

K

x = 0.34


Ti 3d CB slightly above EF Se 4p VB below EF


+2 meV

-64 meV


Γ M

Γ

M

K

x = 0.34


Ti 3d CB slightly above EF Se 4p VB below EF


+2 meV

-64 meV

Evolution of the gap size with temperature?

k

EF

k

Γ M /Γ *

Γ

M

K ARPES on 1T-TiSe2-xSx T-dependence Pristine

= ( - )2

k

EF

k

Γ M /Γ *

Indirect band gap squared

Γ

M


= ( - )2

k

EF

k

Γ M /Γ *

Γ

M



= ( - )2

TCDW = 220 K ± 5 K

k

EF

k

Γ M /Γ *

P. Chen et al., Nat. Comm. 6, 8943 (2015)

Δ 2 T − Δ 2 TC ∝ tanh2 ATC

T− 1 )

Semi-empirical BCS-type gap equation

Γ

M

K ARPES on 1T-TiSe2-xSx

What about the Sulfur doped samples?

T-dependence Pristine


= ( - )2

k

EF

k

Γ M /Γ *

Γ

M

K ARPES on 1T-TiSe2-xSx T-dependence S-doped

x = 0.12 x = 0.34


= ( - )2

k

EF

k

Γ M /Γ *

Γ

M

K

TCDW = 185 K ± 5 K TCDW = 195 K ± 5 K

ARPES on 1T-TiSe2-xSx T-dependence S-doped

x = 0.12 x = 0.34


= ( - )2

k

EF

k

Γ M /Γ *

Γ

M

K

TCDW = 185 K ± 5 K TCDW = 195 K ± 5 K


Consistent with temperature-dependent resistivity measurements?

T-dependence S-doped

x = 0.12 x = 0.34


Resistivity of 1T-TiSe2-xSx

dρ

/dT

(Ω*

cm/K

)

Transition temperatures from resistivity in agreement with ARPES

x =0.12 TCDW = 186 K x =0.34 TCDW = 193 K

ρ (

Ω*cm

)

dρ/dT

ρ(T)

Summary

ARPES

ρ (T)

STM/STS x = 0.12 x = 0.34

TCDW = 185 K ± 5 K TCDW = 195 K ± 5 K

E

EF

k

Γ

M E

EF

k

Γ

M

x = 0.12 x = 0.34

Normal phase

f(T)

Outlook

ARPES

ρ (T)

STM/STS x = 0.12 x = 0.34

TCDW = 185 K ± 5 K TCDW = 195 K ± 5 K

E

EF

k

Γ

M E

EF

k

Γ

M

x = 0.12 x = 0.34

Normal phase

• pristine TiSe2 semiconductor

• sulfur lowers TCDW nonlinearly

• slight reentrant behavior for low sulfur concentration

• TiSe2 under pressure becomes semimetallic

• TiS2 is a semiconductor with no CDW

Outlook

x = 0.12 x = 0.34

Normal phase

• pristine TiSe2 semiconductor

• sulfur lowers TCDW nonlinearly

• slight reentrant behavior for low sulfur concentration

• TiSe2 under pressure becomes semimetallic

• TiS2 is a semiconductor with no CDW

Competition between a positive chemical pressure effect and band reconstruction

Y. Miyahara et al., J. Phys.: Condens. Matter 8, 7453 (1996)

structural effect of isovalent S substitution

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Evolution of the charge density wave in Sulfur substituted ... · STM/STS x = 0.12 • x = 0.34 T...

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