Neutron scattering investigation of (TMTTF)2PF6

P. Foury-Leylekiana, S. Petitb, B. Hennionb, A. Moradpoura and J.-P. Pougeta

a. Laboratoire de Physique des Solides, CNRS-UMR 8502, Université Paris-sud, 91405 Orsay, France

b. Laboratoire Léon Brillouin,CEA-CNRS, UMR 12, 91191 Gif-sur-Yvette, France

ECRYSCargèse, Corse (F) August 25-29 2008

Phase diagram of (TMTCF)2X

Loc

SP ground state 4kF charge ordering at TCO=90K

O

PF6(D12)

Spin susceptibility measurements: (TMTTF)2X

TSP

Thermal behavior of a S=1/2 Heisenberg chain

J~410-420K(Dumm et al PRB 62,

6510 (2000))

TCO

no effect at TCO: spin – charge decoupling

Ground states of the 1D Heisenberg chain

=1 (1 electron per site) Mott-Hubbard localization

case of CuGeO3

AF

SP

=1/2 (1 electron per dimer)Case of (TMTTF)2X

2kF SDW

2kF bond « CDW » (BOW)Spin-

Peierls

Spin-Peierls transition

S→0 at T=0: S=0 ground state

(TMTTF)2PF6: superlattice reflexions below TSP

h= ½ a* : 2a periodicity

chain dimerization (pairing of S=1/2 units into

S=0 singlet)

Elastic neutron scattering

(P. Foury-Leylekian et al PRB 7 R180405 (2004))

TSP=13K

Thermal dependence of the (TMTTF)2PF6 (D12) (1/2,1/2,1/2) superlattice peak intensity

max d/dT: TSP=12.9K

Magnetic excitations in the SP ground state: T=0K

q/qSP

Δσ spin-peierls gap

Triplet excitations of the SP dimer: S=1 magnon mode gapped at Δσ

Continuum of excitations gapped

(Uhrig et Schulz PRB 54, R9624 (1996))

at2 Δσ

S=0 S=1

Magnetic excitations in the SP ground state 1 - magnon mode:propagation of the triplet excitations (S=1) of the SP dimer

dispersion: ħωM(q)= E(q) with E(q)=[Δ2(q)+ε2(q)]1/2

Δ(q)= Δcos2πq , ε(q)=Jeff sin2πq

Jeff=J (πJ/2) for the XY(Heisenberg) chain

(Bonner & Blote PRB 25, 6959 (1982))

2 - continuum of double magnon excitations located in between:

ħωl(q)= Δ +E(q) and ħωs(q)= 2E(q/2)

(Uhrig & Schulz PRB 54, R9624 (1996))

Inelastic neutron scattering study

• performed with ~1cm3 of 98% deuterated (TMTTF)2 PF6 powder(A. Moradpour LPS)

• 2T triple axis study at Orphée reactor

(LLB Saclay)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Q

Ene

rgy

(meV

)

0 0.5 1 1.5 2 2.5 3 0

20

40

60

80

100

120

1D magnon collective mode

Min of E(q): Δσ

Max of E(q): Jeff

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

QE

nerg

y (m

eV)

0 0.5 1 1.5 2 2.5 3 0

20

40

60

80

100

120

1D continuum

Simulation of powder average*

Energy scan in QSP = (3/2,-1/2,1/2)= 1.66Å-1

S. Petit

Min of ħωl(q): ≥ 2Δσ

Max of ħωl(q): Δσ+Jeff

* Simulation ignoring factor structure effects

neutron count variation inside the SP phase of (TMTTF)2 PF6 (d12): I(4K)-I(11K)

-100

-50

0

50

100

150

0 5 10 15 20

Cou

nts

Energy (meV)

Difference 4K-11K avec correctionDifference 4K-11K

Fit1(x)

broad excitation ΔU

narrow excitation ΔL

Double gap structure previously observed in CuGeO3 single crystals(Ain et al PRL 78, 1560, 1997)

evidence of two excitation energies

Δσ 2Δσ

Δmax

The continuum of excitations is more peaked in (TMTTF)2PF6 than in CuGeO3

(magnetism more 1D in (TMTTF)2X than in CuGeO3?)

Low T excitation gaps

Powder ΔL~67K ΔU~150K

Single crystalΔL=68K

0

-100

-50

0

50

100

150

0 5 10 15 20

Cou

nts

Energy (meV)

Difference 4K-18K avec correctionDifference 4K-18K

Fit1(x)

ΔL~ ΔU/2

I (4K) – I(18K) reference of intensity 18-20K well above TSP=13K

negative intensity because of the formation of a gap in the excitations of the Heisenberg chain (reference of intensity above TSP)

ΔL ΔU

Spin-Peierls ground state: Singlet-Triplet splitting gapexp-/T

PF6(D12): = 75K = U/2 ~ L

(C. Coulon)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Q

Ene

rgy

(meV

)

0 0.5 1 1.5 2 2.5 3 0

20

40

60

80

100

120

1D magnon collective mode

Min of E(q): Δσ

Max of E(q): Jeff

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

QE

nerg

y (m

eV)

0 0.5 1 1.5 2 2.5 3 0

20

40

60

80

100

120

1D continuum

Simulation of powder average*

High energy scan

S. Petit

Min of ħωl(q): ≥ 2Δσ

Max of ħωl(q): Δσ+Jeff

* Simulation ignoring factor structure effects

High energy scan at Q = 3.4 Å-1

-100

-50

0

50

100

150

40 45 50 55 60 65 70 75

Cou

nts

Energy (meV)

Fit1(x)Difference 4K-18K

response at 55meV ~ Jeff (if Δ neglected)

Jeff= πJ/2 leads to J~400KOK with J=410-420K obtained for the « Bonner et Fisher » fit of χspin(T)

Thermal dependence of the magnetic excitations in the SP phase

scan difference: I(T)-I(18K)

Reference scan

Only the « ΔU » continuum of excitations is

detected at 7K (~TSP/2)!

divergence of the density of state of the continuum of

1D magnetic excitations

gapped excitations

Reasons for the non–observation of the ΔL magnon peak

• Peak merges in the continuum?• Peak intensity vanishes?• Peak broadens?

life time effect: efficient decay mode• Possible explanation: magnon mode decays into two

bounded spinons on approachingTSP

X X

S=1

S=1/2

Binding due to unfavorable (out of phase) interchain coupling

S=1/2

Decay of magnon into two bound spinons

• Possible near TSP when the cost of interchain coupling is not large

• Creation of bound spinons inside the SP phase of (TMTTF)2PF6 is possible because the 3D spin-Peierls distortion (i.e. SP satellite intensity) is very weak(P. Foury-Leylekian et al PRB 70, R180405 (2004))

• By this scenario one passes continuously (through a 2nd order transition) when Δ→0 from the excitations of the SP chain to those (only a continuum of free spinons) of the Heisenberg chain

In the vicinity of TSP the intensity of the ΔU peak drops and a larger gap in the excitations of the

Heisenberg chain is revealed!

A broad max of intensity at « ΔU » and a large gap in the excitations of the Heisenberg chain are still observed

above TSP!I<0 below 20meV: pseudo-gap formation?

difference with thermal correction

Thermal evolution of the upper gap: ΔU

TSP

Δu does not vanishes at TSP! (linear extrapolation to zero at ~35K)

2Δχspin

χRPE du (TMTTF)2PF6 (D12)

C. Coulondrops below ~ 40K

vanishing of U in thepseudo-gap region?

Pouget et al Mol. Cryst.Liq. Cryst. 79, 129 (1982)

Pseudo-gap built by SP 1D structural fluctuations:1D X-ray diffuse scattering observed above TSP in

(TMTTF)2PF6and AsF6

1D structural SP fluctuationsabove TSP

mean-field energy scale

• 1D structural fluctuations detected until:

~60-80K PF6 (H12)- 40K AsF6

If one takes TSPMF~60K for the PF6 (D12), the BCS relationship

gives:

2Δ1DMF~215K=18.5meV

In this energy range inelastic neutron scattering reveals a drop of the magnetic excitation spectrum of the Heisenberg chain

negative intensity in the scan difference I(T) - I(18K)

2ΔMF

2ΔMF

Scan difference:

Summary• This is the first time that magnetic excitations have been measured by

neutron scattering in an organic conductor

• The SP transition of (TMTTF)2PF6 and of CuGeO3 differs:

in (TMTTF)2PF6: - the magnon mode decay inside the SP phase- above TSP: there are pretransitional SP fluctuations and a pseudo gap formation (adiabatic limit)

in CuGeO3 :

- a sharp and intense magnon mode is followed until TSP where Δ vanishes abruptly- no pseudo gap effects are observed above TSP (non adiabatic limit)

Crossover of S(q,ω) from the SP ground state (with magnon excitations) to the uniform Heisenberg chain (with spinon excitations) need to be calculated

Chain fluctuations needed to be included in the treatment of excitations