Temperature dependent locally resolved 13C Knight shifts in the organic conductor TTF[Ni(dmit)2]2

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Submitted on 1 Jan 1990

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Temperature dependent locally resolved 13C Knight

shifts in the organic conductor TTF[Ni(dmit)2]2

A. Vainrub, E. Canadell, D. Jérome, P. Bernier, T. Nunes, M.-F. Bruniquel,

P. Cassoux

To cite this version:

Temperature dependent

locally

resolved

13C

_Knight

shifts

in

the

_organic

conductor

_{TTF[Ni(dmit)2]2}

A. Vainrub

_{(1, *),}

E. Canadell

_(2),

D. Jérome

_(1),

P. Bernier

_(3),

T. Nunes

_(3),

M.-F.

_Bruniquel

₍₄₎

and P. Cassoux

₍₄₎

(1)

Laboratoire de

_Physique

des _Solides, Université de _Paris-Sud, 91405

_Orsay,

France

(2)

Laboratoire de Chimie

_Théorique,

Université de

_Paris-Sud,

91405

_Orsay,

France

(3)

Groupe

de

_Dynamique

des Phases Condensées, Université des Sciences et

_Techniques

du

Languedoc,

34060

_Montpellier,

France

(4)

Laboratoire de Chimie de Coordination du CNRS, Université Paul

Sabatier,

31077 _Toulouse, France

(Received

18

_May

1990,

accepted

in

_{final form}

18

_{July 1990)}

Abstract. 2014

High

resolution 13C NMR results for the

_organic

conductor

_{TTF[Ni(dmit)2]2}

enriched

by

13C

isotope

in

Ni(dmit)2

molecules are

_reported.

Variable temperature

magic angle

spinning experiments

were

_performed

between 160 and 295 K.

_{Paramagnetic Knight}

shifts in the range 41-106.5 ppm at room _temperature are observed for all

_Ni(dmit)2

carbon sites. These shifts are

_slightly

_temperature

_dependent

for inner carbons. However, for outer _carbons, the shifts

strongly

decrease towards low _temperature due to _{precursor effects of the}

_{charge density}

wave

(CDW)

transition. The results confirm the occurrence of both HOMO

_{(highest occupied}

molecular

_orbital)

and LUMO

_{(lowest unoccupied}

molecular

_orbital)

conduction bands and are

compared

with calculated

_partial

contributions of individual carbon atoms to the

_density

of states at the Fermi level. We suggest that CDW effects above 160 K are associated with the LUMO bands.

Classification

Physics

Abstracts 74.70K - 71.45L

1. Introduction.

Organic

conductors based on transition metal

_(M

=

Ni, Pd,

Pt)

complexes

of dmit

(dmit

=

1,3-dithia-2-thione-4,5-dithiolato) [1]

]

have attracted _a

_great

deal of attention since

the

_discovery

of

_{superconductivity}

in

_{TTF[Ni(dmit)2]2 (TTF}

=

tetrathiafulvalene)

_at 1.6 K

under a _pressure of 7 kbar

_[2].

Two other

_{superconducting compounds}

of this

_family,

(CH3)4N[Nl(dmlt)2]2

(Tc = 5 K

at

_{7 kbar) [3]}

and

_{a’-TTF[Pd(dmit)A2 (Tc = 6.5 K}

at

20

kbar)

[4]

were found later.

Moreover,

an

_interesting

feature of

_{TTF[M(dmit)j2 (M}

=

Ni,

Pd)

is the occurrence of

charge density

wave

_(CDW)

instabilities observed in

_X-ray

diffuse

scattering

studies at _{ambient pressure}

_[5].

_Thus,

these materials

_provide

an

_interesting

example

of

_competition

between CDW and

_{superconductivity}

in molecular metals.

In contrast with other

_organic

conductors,

the CDW coexists with metal like

_conductivity

in

TTF[Ni(dmit)2]2.

Proton

_spin-lattice

relaxation studies of the TTF stacks show that their

metallic character is retained down to 1 K with no indication of CDW effects

_[6].

We have

recently reported

13C

NMR

_experiments

on

_samples

enriched

_by

13C

_isotope

in

_Ni(dmit)2

molecules

_only

[7].

These measurements

_give

evidence that the CDW involves the

_Ni(dmit)2

stacks. The

_Knight

shift decreases but not vanishes at low

temperature

showing

that the

Ni(dmit)2 subsystem

is metallic in the entire

_temperature

domain. Two

_{possible explanations}

for

_why

the CDW does not induce a metal-insulator transition on the

_Ni(dmit)2

stacks were

suggested [7].

The first assumed

_{imperfect nesting}

of a

_{wraped planar}

1 D Fermi surface. The

second was based on the

_prediction

of a multiconduction band for the

_Ni(dmit)2

stacks

consisting

of the HOMO and LUMO bands

_[8].

The CDW _{would open} a gap in

only

some of them so that some metallic bands will remain. We were unable to

_distinguish

between these

two

_{possibilities}

from broad band NMR data. As it will be seen, the results of the

present

work

_provide

_support

for the second

_explanation.

We

_report

here

_high

resolution variable

_{temperature 13C}

NMR measurements on

TTF[Ni(dmit)2]2

enriched

_by

13C

_isotope

in

_Ni(dmit)2

_{molecules. The paper} is

_organized

as

follows. In section 2 we

_present

_experimental

results,

_analyze

the

_assignments

of the resonance lines and the local

Knight

shifts for different carbon sites are determined. A brief

account of the band electronic structure of the

_Ni(dmit)2

slabs in

_{TTF[Ni(dmit)2]2}

and the calculated densities of states at the Fermi level are

_presented

in section 3. These results are

used in section 4 to

_roughly

estimate the contributions of the HOMO and LUMO bands to the

Knight

shifts and to

_inquire

about the

_origin

of the CDW.

_Finally,

section 5 contains a

summary of the results and some

_suggestions

for further

_experiments.

2.

_High

resolution NMR

_spectra.

_Assignment

of the lines.

The

_experiments

were carried out on a

_{microcrystalline powder sample (40 mg)}

of

’ITF[Ni(dmit)212 nonselectively

enriched to 50 %

13C

_isotope

content on

_Ni(dmit)2

molecules. The

_{preparation procedure}

has been

_{already reported [7].}

13C

_{magic angle}

spinning (MAS)

spectra

were measured at 50.3 MHz

_(Bruker

CXP-200 or MSL-200

_high

power

spectrometers)

.with or without cross

_{polarization (CP) using}

variable

_temperature

(160-300 K)

MAS

_probehead

with

_cylindrical

rotor

_spinning

_up to 4.6 kHz rotation rate.

Some

_experiments

were

_performed

at 90.5 MHz on a Bruker MSL-360

_{spectrometer.}

All the

NMR shifts were measured with

_{tetramethyl-silane (TMS)}

scale as the reference.

Figure

1 shows

_examples

of

13C

MAS NMR

_spectra

at room

_temperature

and 185 K. These

spectra

were obtained without

_polarization

transfer since some resonances, as we discuss

below,

are _{very weak in} cross

_{polarization experiments.}

Six

_isotropic

lines are observed and

denoted as

A,

_B,

_{C, D,}

E and F. All other

_signals

are well known

_spinning

side bands

_(SSB)

which

_give

for each

_isotropic

line a

_pattern

_{of uniformly separated}

lines with the

_spacing

_equal

to a

_frequency

of the

_sample

rotation. As

_usual,

we have

_{distinguished}

betwèen

_isotropic

lines

and SSB’s

_{by change}

of the rotation rate.

_Figure

1 illustrates the

temperature

behaviour of different lines. The overall

_temperature

_dependence

of the resonance

_positions

is shown in

figure

2. The A and B lines shift

_upfield

whereas the D and E ones shift less downfield with

the

_temperature

decrease. The

_positions

of the C and F lines are almost

_temperature

independent.

The

_spectrum

in the extended _range and the

_assignments

of the SSB’s are shown in

_figure

3.

Several SSB’s are observed within a _{range of about 350 ppm for} each of the

_{C, D,}

E and F

Fig.

1. - 13C MAS NMR _spectra of

_{TTF[Ni(dmit)J2}

measured without cross

_polarization

at 185 K. A, B, C, D, E and F denote

_isotropic

lines. All other

_{signa.ls signals}

are

_spinning

side bands.

Fig.

2. 2013 Positions of 13C

Fig.

3. -

50.3 MHz 13C MAS NMR _spectrum of

TTF[Ni(dmit)A2.

The identification of

_isotropic

lines and their

_spinning

side bands

(SSB)

is shown.

_Spinning

rate is 3 690 Hz. Shown in the insert is the

Ni(dmit)2

formula with the

_numbering

for different carbon sites.

with MAS

_speed

_{v r} = 3 690 Hz = 73.4 ppm. So we estimate 150 ppm as an upper limit for the

anisotropy

of the

_magnetic

shift tensor for the A and B lines and 350 ppm as a lower limit for

that of the

_C,

_D,

E and F lines.

We would like to consider now the

_assignment

of the lines to different carbon sites of

Ni(dmit)2

stacks. The four

_Ni(dmit)2

molecules in the monoclinic unit cell of

TTF[Ni(dmit)2]2

are

_{crystallographically equivalent [1].}

But the six carbon sites in a

_given

Ni(dmit)2

molecule are

_{nonequivalent}

in the

_{crystal (see}

insert in

_Fig.

3 for the

_Ni(dmit)2

formula and

_numbering

of

_carbons).

We summarize NMR data in table 1 where the results of

our

_spin-lattice

relaxation measurements are also included. The relaxation of A and B lines is 3-4 times slower than those for other lines. It is clear from table I that on the basis

_{of similarity}

Table I. 2013 Characteristics

of

the

13C

MAS NMR spectrum

of

TTF(Ni(dmit)2)2.

For each

resonance line are

_{given :}

_possible

_assignments,

the

_shift

at room _temperature

_(3,

in

_ppm)

and

of behaviour all lines can be divided into two groups. The first one consists of A and B lines

characterized

_by

a

_{strong temperature}

_dependence

of the

_position,

small

_anisotropy

of the

magnetic

shift and slow

_spin-lattice

relaxation.

C, D,

E and F lines with weak

_temperature

dependence,

stronger

anisotropy

and faster relaxation form the second _{group. Since} similar

NMR

_properties

of alike sites on the

_Ni(dmit)2

unit can be

_expected,

we _{propose that A} and

B can be associated with the outer sites C2 and C5 and

_C,

_D,

E and F with the inner sites

_Cl,

C3,

C4 and C6. This

_analysis

corroborates the

_{inhomogeneity}

of relaxation rate which was

noticed in the wide-band

_spectra

_[7].

_{The absence of any}

_{Korringa-like relationship}

between values of

_Ti

and

K

_(namely

_1/T1

_{a K)}

in table 1 could

_possibly

be ascribed to the existence of an enhancement factor for the relaxation due to correlations which is not uniform over the

dmit molecule.

The cross

_{polarization experiments provide}

further evidence for these

_assignments.

Figure

4 shows

_spectra

with the same number of accumulations for different

_polarization

transfer contact times at room

_temperature.

_By

_comparison

of these results with those without

CP shown in

_figures

1 and 3 two observations can be made.

_First,

there is a

_strong

enhancement of the A and B lines with

_respect

to the other ones.

Second,

the intensities of

the A and B lines grow faster and saturate earlier with a contact time increase. Thus the cross

polarization

is more effective and faster for the A and B lines. This is in accordance with their

assignment

to outer carbons since the distances from TTF

_protons

to outer carbons

_(the

shortest contact is 4.2

_Â)

are shorter than those to inner carbons

_(about

6.1

Â).

Since our

_assignments

are

_incomplete,

we have tried to

_get

additional information from rotational resonance

_{experiments [10].}

The effect is known for isolated homonuclear

coupled spin pairs.

The

_{normally sharp}

resonance lines broaden and

_split

if the

_frequency

of

magic

angle

spinning

vr is

adjusted

to

satisfy

the condition

Aw i,. = 2 Ir l’, n.

Here,

Oca iso

is the difference in

_{isotropic magnetic}

shifts and n is an

_integer.

The

_{dipolar coupling}

of

two inner

13C

_spins

of the same dmit

_moiety

is

_stronger

than their other homonuclear

_dipolar

interactions.

_{Consequently,}

C 1-C3 and C4-C6 are

fairly

isolated

spin pairs.

The

_spinning

rate of about 1 700 Hz is sufficient to obtain narrow lines for

_{TTF[Ni(dmit)2]2. Using}

50.3 or

90.5 MHz

13C

MAS NMR we were able to

_adjust

resonance rotation

_speeds

for all

_pairs

of lines with

_{separation larger}

_{than 19 ppm. However} we did not succeed to find

_{spin pairs :}

in all

cases, no

_changes

of line

_shapes

were observed. So we believe the differences of the shifts within C1-C3 and C4-C6

_pairs

do not _{exceed 19 ppm. These}

_experiments

have shown that

C-E,

C-F and D-F

_pairs

of inner carbon lines do not

_belong

to

_{spin pairs.}

Therefore the

_only

possibility

which remains is to associate C-D and E-F

_pairs

of lines with

_{spin pairs}

C1-C3 and C4-C6.

_{Unfortunately,}

we could not

_perform

rotational resonance for C-D and E-F line

_pairs

because of the small shift differences. It was

_possible

however to

_study

the most

_separated

A line

_together

with each other line and so its

_assignement

to an outer site was confirmed. The

final results of our

_assignments

are shown in table I.

The total

_magnetic

shift for an

_organic

conductor is a sum of the chemical shift and the

Knight

shift. We have measured the chemical shifts to _{be 205.6 ppm for} outer sites and 137 ppm

(really

a doublet 136.5 and 137.5 ppm is

observed)

for inner carbon sites in

nonconducting

(NBu4)2[Zn(dmit)2]. Using

these values we have determined the

_locally

resolved

Knight

shifts of 41-106.5 ppm as shown in table 1.

3. Electronic band structure calculations.

In order to

_gain

some

_insight

into the

_origin

of the

_temperature

_dependence

of the

13C

MAS

NMR resonances we carried out

_{tight-binding}

band structure calculations on the

_Ni(dmit)2

slabs of

_{TTF[Ni(dmit)2]2.}

An effective one-electron hamiltonian of the extended Hückel

_type

[11]

was

used. All valence electrons were

_explicitely

taken into account in the calculations.

The basis set consisted of Slater

_type

orbitals of

_double-£

_quality

for nickel 3d functions and of

single-C quality

for carbon 2s and

_2p,

sulfur 3s and

_3p

and nickel 4s and

_{4p [12].}

The

exponents,

contraction coefficients and atomic

_parameters

used in the calculations are

summarised in table II. As usual in

_{organometallic compounds,}

the

_off-diagonal

matrix elements of the hamiltonian were calculated

_according

to the modified

_{Wolfsberg-Helmholz}

formula

_[13].

The calculations were

_performed

on the room

_temperature

and ambient

pressure structure

_[1]

which is the

_{only presently}

available.

Table II.

_{- Exponents}

and parameters used in the calculations.

As

_{reported previously [8], although}

the

_Ni(dmit)2

molecules

_play

the role of

_acceptors

in

TTF[Ni(dmit)2]2,

both the

_{highest occupied (HO)}

and lowest

_{unoccupied (LU)}

molecular

orbitals of

_Ni(dmit)2

lead to

_partially

filled bands. This is due to the fact that the

_splitting

between the HOMO and LUMO in the monomer - due to

_metal-ligand

interactions - is overriden

_by

the

_dispersion

_{of energy levels}

_{produced by}

the monomer-monomer interactions

along

the

_Ni(dmit)2

stacks. Since the

_repeat

unit cell of a

_Ni(dmit)2

slab contains four

molecules,

the band structure of the

_Ni(dmit)2

slab in the

_region

near the Fermi level

_(see

Fig. 5)

contains two sets of four bands

_each,

the _upper one

_coming

from the

_LUMO’s

and the lower one

_coming

from the HOMO’s.

It is clear from

_figure

5 that the

_density

of states at the Fermi level

_{(N (£p))}

will contain contributions from both the HOMO

_{(N ( EF)HOMO)}

and LUMO

_(N(EF)LUMO)

bands. In order

to calculate these

_N(EF)

_values,

we used a mesh of 125

k-points

in the irreducible

wedge

of

the

_rectangular

Brillouin zone of the slab. The

_computed

values were smoothed

_using

gaussian

functions with a half-width of 0.04 eV. The

_partial

contributions of the different

atoms were obtained via a Mulliken

_{population analysis}

of the

_crystal

orbitals

_[14].

The actual

_degree

of

_charge

transfer

_(CT)

is not

_presently

known. In _consequence, we

performed

our

_study

for different values of CT in between 0.5 and 1.0 electrons transferred

per TTF molecule. Since the

qualitative

results of our

_study

do not

_depend

on the actual

degree

of

_charge

transfer in between the above mentioned

limits,

we

_only

_report

_{(Tab. III)}

the

_N(EF)TOTAL,

_N(EF)HOMO

and

_N(EF)LUMO

values calculated for the

_{representative}

case of 0.75 electrons transferred _{per TTF molecule. The} values in table III are in units of electrons

per eV and per molecule. As could be

expected

from the nature of the HOMO and LUMO of

Ni(dmit)2 [8],

the

_{N ( EF)}

values for the four inner carbons are

_greater

than those for the two outer ones. It is worth

_pointing

out that

_although

the total values associated with similar

atoms

_(inner

or

_outer)

in the two different moieties

_(left

or

_right)

of the molecule are very

Fig.

5. - Band structure of the

Ni(dmit)2

slabs in

TTF[Ni(dMit)A2

taken from reference

_[8].

F, Y, Z

Table III. - Total

_{and partial}

densities

_{of states}

associated with the six

_different

carbon atoms in the slab

_{of Ni(dmit)2}

assuming

a

_{transfer of}

0.75 electrons per TTF molecule

(a)

(a)

In units of electrons per eV and per molecule.

(b)

For

numbering

see

_figure

3.

(c)

Contributions associated with the

eight

bands of

figure

5.

(d)

Contributions associated with the four lower bands of

_figure

5.

(e)

Contributions associated with the four upper bands of

figure

5.

similar,

the contributions

_originating

from the HOMO and LUMO bands are not. The

possible

relation between these results and the

_temperature

_{dependences reported}

before will be discussed in the next section.

4. Discussion.

Figure

6 shows the

_locally

resolved

13C

_Knight

shifts for

_{Ni(dmit)2 TTF[Ni(dmit)2]2}

as a

function of

_temperature

between 160 and 295 K. Those associated with outer carbons

Fig.

6. - 13 C

_Knight

shifts of outer

_{(A, B)}

and inner

_(C,

D, E,

F)

sites of

_Ni(dmit)2

versus

temperature. The solid curves are fits obtained from

experimental

data for resonance A under the

decrease

_by

a factor of about 1.7 towards low

_temperature.

By

contrast, the variations are

small for inner carbons and do not exceed 7 %. To our

_knowledge,

this is the first observation

of such a different

_temperature

behaviour of the local

_Knight

shifts for the same molecule in an

_organic

conductor.

_{Temperature dependent}

_Knight

shifts have been

_reported

for

(TMTSF)2PF6 [15], (TMTSF)2Re04 [16]

and

_{(FA)2AsF6 [17].}

In these cases,

however,

they

decrease at low

temperatures

for all carbon sites of the cation radical.

In the case of a

_single

conduction band the

Knight

shift can be written as

_[18]

where the

_{index j}

denotes

_{nonequivalent}

crystal sites,

_yp is the Pauli

_{susceptibility}

per one

molecule, aj

is the

_{isotropic hyperfine}

interaction constant in _{radians per inverse second and}

Pj

is the

spin density

normalized to the

unity

over the

_{ion-radical. aj}

and

_pj

are

_{mainly given}

by

the free ion-radical

_parameters

which are

_temperature

independent

and are

_slightly

modified

_{by crystal}

effects. So the

temperature

dependence

of the

_Knight

shift is

_expected

to

be of the

_type

_Kj(T)

_aXp(T),

i.e. the same for the nuclei at different

_crystal

sites. Our

experimental

data

_disagree

with this

_prediction

and

_consequently

_suggest

that we are out of a

single

conduction band situation.

_Therefore,

in the discussion which follows we use the

HOMO and LUMO bands

_{picture [8]}

described in the

_previous

section.

We associate the

_temperature

_dependence

of the

_Knight

shifts in

_{’ITF[Ni(dMit)212}

with the CDW instabilities known from

_X-ray

diffuse

_scattering

studies

_[5].

At room

_temperature

and

below,

1 D structural fluctuations with the wave vector 0.4 b*

_{(the crystal}

b-axis is

_along

the

conducting chains)

have been observed.

They

become 3D ordered at about 40 K. Two additional weak 1D distortions at wave vectors - 0.18 b * and - 0.22 _{b * appear} at - 25 K

[19].

From our

_previous

broad band

13C

NMR studies the first CDW was

_proved

to involve the

Ni(dmit)2

stacks and to decrease the

_Knight

shift

_averaged

over the six carbon atoms

(9)

below 100 K

[7].

The

_present

_{experiments clearly}

show that this decrease starts

_already

at

room

_temperature

but the

_change

J1K ~

7 ppm is too small to be detectable

_by

broad band NMR.

Theoretical studies

_[20]

have shown that the CDW fluctuation

_regime

is characterized

_by

a

pseudogap

in the electronic

_density

of states which transforms into a real energy gap at the Fermi level below the

_temperature

of the CDW 3D

_ordering.

The

_temperature

domain of the fluctuations is

large

for

strongly

1 D

_systems.

In this

_temperature

_region

the

_density

of states at

the Fermi level

_{and, hence,}

the

_Knight

shift decrease towards low

_temperature.

The effect

was observed _up to the room

_temperature

for

13C

_Knight

shifts of TTF

_[21]

_and

_TCNQ

_[22]

in

TTF-TCNQ although

the CDW’s at the

_TCNQ

and TTF stacks become 3D ordered at

sufficiently

low

_{temperatures,}

54 K and 38

_K,

_{respectively.}

Let us now compare the

experimental Knight

shifts with the calculated band structure

parameters

of section 3. We rewrite

_{equation (1),}

as usual for a

_metal,

as

_[23]

where

_{Hhf - -}

_aj/2

_yn is the

_hyperfine

_{field per}

electron,

_Nj(£p)

is the

_partial

contribution of the

_{atom j}

to the

_density

of states at the Fermi level related to the Pauli

_{susceptibility}

Xp

by

Ipj

1 Xp

= 1_ B 2Nj(--F).

Taking

into account both the HOMO and LUMO conduction

where the contributions

_KHOMO

and

_KLUMO

are

given by equation

_{(2). Therefore,}

for the outer

carbons C2 and C5 we

_get

where

N H

and

N L

are the densities of states at the Fermi level for the HOMO and LUMO

bands. It should be noted that the same

hyperfine

fields

Hô

and

H;

(index

« o » states the

outer

_carbons)

are

_supposed

for both C2 and C5 sites because of their

_{equivalence by}

symmetry

in the free molecule. A

_{nonequivalence}

of these atoms in the

_crystal

is taken into

account

_by

their

_partial

densities of states at the Fermi level

_{(Tab. III).}

Because of this reason we also use the same

_hyperfine

fields

HiH

and

HiL

for all the four inner carbons. We solve the

system

of

_{equations (4) using}

the

_{experimental Knight}

shifts

_{(Tab. I)}

and the calculated densities of states

_{(Tab. III)}

to

_get

H0H

and

_HoL.

Then from relation

₍₂₎

we find the

contributions

KHOMO

and

KLUM0.

Since there are two

_{possible assignments}

of the resonance

lines to the C2 and C5

_sites,

we

_get

two solutions as shown in table IV.

In

_analogy

with

_{equations (4)}

we _may write for the inner carbon sites :

Here each

_equation

describes a

_pair

of the inner carbons of the same dmit

_moiety

and not the

individual carbons. This

_approach

has two

_advantages.

_First,

we

_only

have to consider two

possible assignments

of the C-D and E-F

_pairs

of lines to two dmit moieties of

_Ni(dmit)2

instead of

_eight

for individual lines.

_Second,

an

_analysis

reveals that solutions for the

pairs

are

much less sensitive to errors in the calculated densities of states

_{(Tab. III)}

than solutions for individual sites. Thus this

_approach

is believed to be more _{reliable. The calculated average} over a

_pair

of the contributions of the HOMO and LUMO bands to the

_Knight

shifts are

presented

in table IV.

The results in table IV show

_obviously

that

_only

one set of solutions

_(at

the

_right

side of Tab.

_IV)

is in

_agreement

with the

_experimental

_temperature

_dependences

of the

_Knight

shifts. It

_corresponds

to the

_{assignments A (C5), B (C2),}

C-D

_{(CI, C3)}

and

_{E-F (C4, C6).}

The dominant contribution to the

_Knight

shifts of the outer sites C2 and C5 arise from the LUMO bands whereas the contribution of the HOMO bands is the main one for the inner site

pairs

C1-C3 and C4-C6.

_Assuming

that the CDW

_instability

involves

_only

the LUMO

bands,

Table IV. - Contributions

_{of the}

HOMO

_(KHOMO)

and the

LUMO

_(KLUMO)

bands to the total

Knight

shift (K) calculated for different

assignments

of

the resonance lines. For

C,

D and

E,

F

we

_expect

_decreasing

and almost constant

_Knight

shifts for the outer and inner

carbons,

respectively. Using

as the reference the

_position

of the

_{strongest temperature}

_dependent

line A and data of table

_IV,

we can estimate the

_Knight

shifts of the other lines versus

temperature.

Figure

5 shows that a

_satisfactory

fit for line B and almost

_temperature

independent

average

positions

for the C-D and E-F

_pairs

of lines are obtained. The

assignement

of the CDW _gap to the

_Ni(dmit)2

LUMO band _agrees with one of the

suggestions

made after

_comparison

between the

_X-ray

_pattern

and the band calculation

_[8].

5.

_Concluding

remarks.

The main result of the

_present

work is the observation of two different behaviours of the

locally

resolved 13C

_Knight

shifts for the same

_Ni(dmit)2

molecule of the

_{1TF[Ni(dmit)212}

organic

conductor. We observe a

_strong

decrease towards low

_temperature

for the outer

carbons and a weak

_temperature

_dependence

for the inner carbons. We believe these data

provide

evidence for an unusual multiband structure in

_organic

conductors in which both the HOMO and LUMO bands of the

_Ni(dmit)2

_acceptor

stacks cross the Fermi

level,

as it was

predicted by

band structure calculations

_[8].

The

_drop

of the outer carbon

_Knight

shifts is associated with 1 D structural fluctuations

_arising

from the CDW

_instability.

Furthermore,

we have tried to ascertain if the calculated contributions of the different

carbon atoms to the

_density

of states at the Fermi level are consistent with the measured distribution of their

_Knight

shifts. Such

_analysis

allows to test the

_ability

of band structure

calculations

_ignoring

electron correlations to

_predict

not

_{only qualitative}

features

_(such

as an

overlap

of the

_bands)

but also more delicate details of the electronic structure.

_{Incompleteness}

of the

_assignment

of the NMR lines

_prevents

an

_unambiguous

answer.

_However,

there is

_good

agreement

for one of four

_{possible assignments.}

In this case the

_high

_temperature

instability

is

driven

_by

the LUMO bands as it was

_suggested

for

a’-TTF[Pd(dMit)A2 [8].

The dominant

contribution to the

_Knight

shifts of the outer and inner carbons arise from the LUMO and

HOMO

bands,

respectively.

In view of these

_results,

a

13C

MAS NMR

_study

of

(CH3)4Ni(dmit)2)2 [24],

where the conduction band is built from the LUMO

[8],

would be very

interesting.

The same

_type

of

_experiments

on a

_{’-TTF[Pd(dmit)2]2}

would

provide

an

interesting comparison

with the

_present

results.

Acknowledgments.

The authors would like to thank F. Creuzet for his

_{participation}

on the initial

stages

of this

work. We also thank F. Devreux for

_allowing

us to

_perform

measurements on a MSL

360 NMR

_spectrometer

and M. Smith from Bruker

Laboratory

at Karisruhe for his

_help

in

some

_experiments.

The

_stay

of A. V. at

_Orsay

was

_{supported by}

the

European

contract

ESPRIT 3121.

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