The ubiquity of the new organic conductor ditetramethyldithiadiselenafulvalene-hexafluorophosphate (TMDTDSF )2PF6

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The ubiquity of the new organic conductor ditetramethyldithiadiselenafulvalene- hexafluorophosphate (TMDTDSF )2PF6

P. Auban, D. Jérome, K. Lerstrup, I. Johannsen, M. Jorgensen, K. Bechgaard

To cite this version:

P. Auban, D. Jérome, K. Lerstrup, I. Johannsen, M. Jorgensen, et al.. The ubiquity of the new organic

conductor ditetramethyldithiadiselenafulvalene-hexafluorophosphate (TMDTDSF )2PF6. Journal de

Physique, 1989, 50 (18), pp.2727-2739. �10.1051/jphys:0198900500180272700�. �jpa-00211097�

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The ubiquity ^of ^the ^new organic ^conductor ditetramethyl-

dithiadiselenafulvalene-hexafluorophosphate (TMDTDSF )2PF6

P. Auban (1), ^{D. Jérome} (1), ^K. Lerstrup (2), I. Johannsen (2), ^M. Jorgensen (2) ^and

K. Bechgaard (2)

(1) Laboratoire de Physique des Solides (associé

^au

CNRS), Université Paris Sud, ⁹¹⁴⁰⁵ Orsay,

France

(2) H. C. Oersted Institute, Universitetsparken ^5, ^DK ²¹⁰⁰ Copenhagen, ^Denmark

(Reçu le 6 avril 1989, révisé le 12 mai 1989, accepté ^{le 1 er} juin 1989)

Résumé.

²⁰¹⁴

Nous présentons

^une

méthode de préparation ^et les résultats d’une étude des

propriétés ^de transport

^sous

^haute pression ^du ^matériau (TMDTDSF )2PF6 ^{élaboré à} partir ^{de la}

nouvelle molécule di-tétraméthyldithiadisélénafulvalène. ^Le composé ^est isostructural à la série

(TMTTF-TMTSF )2X. Une localisation induite par les interactions coulombiennes ^est observée au-dessous de 180 K

sous

pression atmosphérique. ^Un ^{ordre à} longue portée ^de type spin-Peierls

et

un

état à ondes de densité de spins ^sont ôbservés ^à ²⁰ êt ⁷ ^K respectivement. L’existence d’un état antiferromagnétique êst bien établie par la disparition ^de ^la ^résonance électronique ^{du mode}

Zeeman et l’apparition simultanée d’un

nouveau

mode à

un

champ de résonance (Hr ~ ⁵ kOe) largement supérieur

^au

champ de résonance Zeeman. La dépendence angulaire ^de

^{ce nouveau}

mode est la signature ^d’une ^résonance antiferromagnétique. ^On peut ^considérer (TMDTDSF )2PF6

^comme^un

^matériau prototype pour l’étude des différentes instabilités de basse

température d’un gaz électronique quasi 1-D en fonction de la pression. Les effets de localisation et l’instabilité spin-Peierls ^sont supprimés

^sous

pression. Au-delà de 20 kbar,

^un

^état ^à ^conduction de type métallique ^est ^stable ^à ^basse température. Toutefois l’absence d’état supraconducteur

^au-

dessus de 0,56 ^K pourrait ^{être liée}

^au

désordre structural des empilements cationiques

^ou

^à ^la

faiblesse du couplage interchaines.

Abstract.

²⁰¹⁴

Preparation, magnetic ^and transport properties under pressure of

^an

organic

conductor based

on

the

new

molecule di-tetramethyldithiadiselenafulvalene (TMDTDSF )2PF6

are

reported. ^This compound is isostructural to the (TMTTF-TMTSF )2X series. At ambient pressure, Coulomb induced localization is observed below 180 K. Spin-Peierls ordering

^occurs

^at

20 K and

a

spin density

^wave

^state is stabilized below 7 K. The onset of

an

antiferromagnetic ^state

has been conclusively established by ^the vanishing of the ESR spectrum ^at the Zeeman frequency (X-band) and the concomitant

occurrence

of

a new

resonance line at

a

resonance field

(Hr ~ ⁵ kOe) far above the Zeeman field (g

⁼

2) ^{and whose} angular dependence provides ^the signature ^for antiferromagnetic

resonance.

Both charge localization and spin-Peierls ^states

^are

suppressed under pressure. Above 20 kbar, ⁱⁿ spite of the stabilization of the conducting ^state (metallic-like) ^at ^low temperatures

^no

superconductivity

^can

be observed down to 0.56 K. This feature could be related ^to the existence of structural disorder

or

to the weakness of the interchain

coupling in the title compound. (TMDTDSF)2PF6

^can

^be considered

as a

prototype material for the study of the instabilities occurring ⁱⁿ

^a

quasi ^1-D electron gas ^at low temperatures ^via

^an

adequate adjustment of the pressure.

Classification

Physics ^Abstracts

71.30

^-

72.80L

^-

74.70K

^-

75.30F

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198900500180272700

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Introduction.

Superconductivity ⁱⁿ organic conductors was first discovered in the quasi-one dimensional radical-cation salt (TMTSF)2PF6, ^{below 1 K} ûnder â pressure of 9 kbar [1]. ^Below â ^critical

pressure of about 9 kbar this organic ^conductor undergoes ^a sharp ^conductor ^to semiconductor transition at a temperature ^which depends ^on ^the applied hydrostatic pressure ( TN ^{= 12 K}

under ambient pressure). Furthermore, ^NMR experiments [2] have shown that the transition towards the dielectric state is accompanied by ^the ônset ôf ân înternal magnetic modulation.

Single crystal susceptibility [3] ^NMR [4, 5] ^and antiferromagnetic ^resonance experiments [6]

have firmly established the existence of a static spin modulation, ^a spin density ^wave (SDW)

characterized by â modulation wave vector with components (2 kF, ^0.246b, 0.06c) ând amplitude ^0.08 Blmolecule ât ^{4.2 K.}

(TMTSF )2PF 6 belongs ^to ^a ^broader family ^of conducting materials with different anions X

[7]. ^{One of} ^them, ^X

⁼

C104 displays superconductivity ^at ^{1.2 K} under ambient pressure [8].

Furthermore, ^the (TMTSF)2X series possesses ^an all-sulfur analog series, (TMTTF)2X,

which is isomorphous ^to ^the (TMTSF)2X series. This series has been known for quite ^some

time [9] ^but ^never triggered much interest due to the highly insulating character of the

conductivity below 200 K or so [10].

The investigation ^{of the} magnetic properties ^of (TMTTF )2PF6 ^and (TMTTF)2Br ^has

revealed the existence of a non-magnetic ground ^state (spin-Peierls) [11, 12] ^with ^a ^structural

distorsion [13] ^{and of} ân antiferromagnetic ^SDW ground ^state [14] in the former and latter materials below 19 and 13 K respectively. Furthermore, a cross-over between the SP and the SDW ground ^states is observed in (TMITF)2PF6 under pressure (around ¹⁰ kbar). ^The insulating ^SDW ground ^state ôf (TMTTF)2Br îs suppressed by â pressure of 25 kbar [15]. Â possibility ôf superconductivity ^{below 4 K} ^{has been} reported ⁱⁿ this material under pressure

[15]. ^The ability ^to span ^a broad variety of physical properties ^{in the} (TM’ITF)2X series either

by looking ât different salts with different anions or by ^the ûse ôf high pressure has strongly

stimulated the development ^{of the} theory of these conductors. It has also been realized that

unravelling ^the properties of various (TMTTF)2X compounds might be very helpful ^{for the} understanding ^{of the} pairing mechanism in the superconducting phase ^of (TMTSF )2PF6 ^since

this phase ^shares ^{a common} border with the SDW ground ^state [16].

The freezing ^out ^of charge degrees of freedom at high temperatures together ^{with the}

existence of phase transitions at low temperatures 20 K) involving ^the spin degrees ^of

freedom has led theoreticians to propose ^an interpretation ⁱⁿ ^terms ^{of 1D} physics concepts.

As emphasized by Barisié and Brazovskii [17], the existence of a potential at wave-vector 4 kF ^has important consequences ^on the physics of the 1D electron gas. As noticed by Pouget [18], ^the ^two organic molecules in the unit cell are equivalent by ^inversion symmetry.

However, ^the intermolecular intrastack bonds display ^a dimerization due to the anion 3-D environment. Consequently, ^the holes in the conduction band see a small anion potential ^with periodicity ⁴ kF ^which ^is superimposed ^to ^{the main} organic ^lattice potential ^whose periodicity

is 8 kF. ^This additional anion potential ^creates small gaps à (4 kF ) ^at ^± ² kF in the 1-D bands and in turn makes the conduction band 1/2-filled instead of 3/4-filled as inferred from purely stoechiometry consideration [19]. ^The relative size of the 4 kF potential in different

compounds ^can ^{indeed be} ^estimated from the existence of ^a slight dimerization of the organic

stack which is revealed by ^the X-ray ^structure analysis. ^The dimerization is rather large ^{for the}

all-sulfur materials (> ^{1 %} ^or so) but remains relatively ^small ^{or even} non-existing in the all- selenium series. The loss of the charge degrees ^of freedom below 200 K has been attributed to a 4 kF localization due to the repulsive interactions between 1-D carriers in the stacks [19].

When a band is half-filled a new scattering channel is open within the 1-D theoretical

framework besides backward (gl) ^and ^forward (g2) scattering, this is the Umklapp scattering

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of two carriers from ± kF ^to :i: kF ^{with the} corresponding coupling ^constant

93 - g 1 A (4 A:F)/Ep [17]. The dimerization gap A (4 kF) ^is usually small in the (TMTTF )2X

series. Thus, the effect of g3 «.c gl) ^on the electronic properties of the 1-D conductors are

expected ^to ^be important only in the low temperature regime (say ^below ^room temperature around Tp). In the weak coupling ^limit, Tp ^is proportional ^to g3 [19].

Since the spin susceptibility ôf (TMTTF )2X materials is significantly ênhanced ôver ^the

bare Pauli value and does not show any sign of activation down to the low temperature domain, ^the sign ^of gl ^is positive [17].

If the inequality gl : 2 g2 + g3 îf fulfilled, â gap àp opens up in the charge degrees ôf

freedom at Tp (= Llp /7T) ^driven by ^a 4 kF Wigner localization of the carriers. Below

7p ^the conductivity becomes activated but the spin degrees of freedom remain unaffected by

the charge localization.

Whenever 7p ^is ^large, ^say 7p

>

t 1- /7r where t_L ^is ^a transverse overlap integral, 1-D physics

remains applicable ⁱⁿ ^the temperature ^domain Tp:> T:> t 1- / 7T. Consequently, ^the ^two competing correlations, ^{the 2} KF-CDW (bond) ^{and the 2} KF-SDW diverge according ^to ^the

same power law (- Tp/T). ^{The 2} ^KF-CDW ^may ^couple ^to the lattice through the electron-

phonon interaction and gives ^rise ^to ^a ² kF-lattice modulation observable by X-ray ^diffuse scattering [19].

Thus, ^{if the} electron-phonon interaction is large enough, ^the resulting ² kF ^lattice

modulation can be detected by ^the occurrence of ^a 1-D softening ^{of the 2} kF phonon. ^This

additional dimerization (of the 1-D chain of localized Heisenberg spins) ^is accompanied by ^a

reduction of the spin degrees of freedom as the uniform chain of spins evolves towards a

dimerized chain of spin singlets. ^1-D spin-Peierls fluctuations exist in this temperature domain. These effects (2 kF lattice fluctuations and the concomitant reduction of Xs) ^become

visible below the mean-field SP temperature esp. ^In (TMTTF)2PF6, TsP is about 40 K ^at ambient pressure. Below esp ^the growth ^{of the 2} KF-SDW fluctuations is weakened by ^the

reduction of the spin degrees of freedom and ^a transition towards ^a 3-D SP long range ordered state is achieved at Tsp via the interstack coupling ^{between 2} kF bond CDW and lattice fluctuations. Below Tsp( , 20 K ⁱⁿ (TMITF)2PF6) ^the properties of the material are

characterized by ^a ^{3-D 2} kF lattice modulation and ^a dimerized assembly of electrons in singlet

states.

In case of weak 4 kF ^Coulomb localization (for example ^if Tp is much smaller than room

temperature) ^the growth of the 1-D SP fluctuations may ^not be large enough in the 1-D

regime for the SP instability ^to ^win ^the competition against ^{the SDW} ^state. Thus, ^a ^SDW ^state gets stabilized instead. This situation is encountered in (TMTTF )2Br ^at ambient pressure

(Tp -- ¹⁰⁰ ^{K) [20]} and also in (TMTTF)2PF6 above 10 kbar [11]. ^The crossing-over ^between

SP and SDW phases ^{has been} extensively studied in (TN=F)2PF6 under pressure [21].

The P-T diagram of the all-selenium compound (TMTSF)2PF6 has also shed light ^on ^the

well-known cross-over between SDW and Superconductivity. However, ^so ^far the sequence of SP, ^SDW ^and conducting (superconducting) ^states ^has ^yet ^never been observed in the same

compound by ^a proper adjustement of the pressure. Therefore, ^we have decided to elaborate

a new material with the same structure as the (TMTTF-TMTSF )2X series but based on a new

hybrid molecule, TMDTDSF, hoping ^to get ^a ^material behaving ^{as a} prototype ^{for the} physical properties.

Material preparation.

The TMDTDSF molecule can be viewed as the hybrid between TMTTF and TMTSF molecules since the two rings of the molecule look like TMTSF and TMTTF respectively. ^{It is}

different from another di-selena-dithia molecule reported by Wudl et al. [22] ^as the molecule

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in our work does not contain the cis-trans mixture which is a built-in source of disorder in the

stacking ^{of hetero} ^atoms ^{for the} compound of Wudl et al. [22].

So far, ^the only ^{method of} preparation ^for

^«

^mixed

^»

molecules has involved the mixing ^of

the two halves and carrying ôut ône of the standard coupling ^reactions ôn the mixture. This

procedure ^{will of} ^course ^{result in} ^a mixture of compounds, ^{and the} separation ^{of the} products

is tedious and difficult as best and outright impossible ^at ^worse.

We have developed ^a procedure in which these problems ^{have been} eliminated and in this

manner prepared

^-

among other mixed ^or

^«

unsymmetrical » tetrachalcogenfulvalenes-the tetramethyl-dithia-diselenafulvalene, TMDTDSF, [1] ⁱⁿ figure ^1.

Fig. ^1.

^-

Preparative scheme for TMDTDSF.

The procedure îs loosely ^based ^{on a} method described by Cava et al. [23], ^{but the} shortcomings ôf ^this ^method [24] ^{have been} eliminated as well, ^{and thus} ôur ^method provides exclusively ^the desired, ^mixed product.

We prepared ^the 2-bisethylphosphonates of various 1,3-dithioles by mixing ^the appropriate

dithiolium-iodide with 1.2 equivalent ôf triethylphosphite in acetonitrile at slightly êlevated temperature [25]. Evaporation of the solvent provided ^the phosphonate êster ^{in almost} quantitative yield.

The 4,5-dimethyl-1,3-dithiole-2-bisethylphosphonate [2] ^was treated with 1.2 eq. potas-

sium-tert-butoxide in THF at - 72 °C, whereupon ^one eq. of 4,5-dimethyl-1,3-diselenole-2-

(1-piperidinium) hexafluorophosphate [3] ^was added. The temperature ^was ^allowed ^to ^rise

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slowly ^{to -} ^{20 °C} ^and stirring ^was continued until all solids had disappeared. Diethyl ^ether

was added, and the reaction mixture extracted several times with icewater to remove THF, tert-butanol, and salts. The organic phase, containing the adduct [4] ^was ^dried ôver magnesium sulfate, filtered and added a large êxcess ôf glacial acetic acid and stirred at room

temperature for several hours. The product ^was ^filtered ^off, the filtrate washed with water, dried, the solvent evaporated ^{and the} ^remanence subjected ^to ^column chromatography (silica/toluene) ^to obtain further product. The total yield ^was typically ^around ^{25 %.}

Elemental analysis, ^NMR spectroscopy, and gas chromatography ^{of the} product ^revealed ^no signs of contamination with the symmetrical ^TMTTF ^or ^TMTSF.

Single crystals ôf (TMDTDSF)2PF6 âre ôbtained by electrolysis of solutions (10- 3 M) ôf

the recrystallized ^and gradient ^sublimed material in THF solutions containing ^n- BU4NPF6(0.1 M).

The determination of the crystal ^structure [26] provides ^a ^triclinic ^structure ^P1 ^with ^two

donors per unit cell ^as for the (TMTTF-TMTSF)2X series. Unit cell parameters ^at T

⁼

295 K are a

⁼

7.206 Â, ^b

⁼

^7.632 À, ^c

⁼

^13.374 Â, a

⁼

^83.23 _(deg), /3

⁼

^85.59 (deg),

y

⁼

71.63 (deg), ^with interplanar distances 3.61 and 3.59 Â. These values are nearly midway

between those of all-sulfur and all-selenium compounds. ^At ^first sight ^the ^structure ^is

disordered in terms of the heteroatoms stacking.

In this work we report the results of a preliminary investigation ^{of the} transport ^and magnetic properties ^of (TMDTDSF )2PF 6 ^{between 1} bar and 23 kbar.

Experimental.

All experiments except ^NMR ^were performed ^on single crystals ^with ^a typical ^size

2 x 0.2 x 0.2 mm3. The conductivity ^was ^measured by the usual low frequency ^four probe technique. ^The production ôf high pressures and low temperatures ^was âchieved by ^the regular pressure and temperature equipment âvailable ât Orsay. Single crystal ÊSR spectra

were obtained in a standard microwave (9.4 GHz) equipment ^with ^{Hllc *} ^or ^{in the}

b’ - c * plane. ^NMR experiments ^were performed ^{on a} 40 mg powdered sample ^at

VL

⁼

45 MHz.

We report ^the transport property ^{data in} figures 2 ^{and 3.} ^The conductivity (u (300 K)

⁼

²⁰⁰ ^fl-1 cm-1) increases almost linearly ^under pressure up ^to 23 kbar with an

Fig. 2. - Pressure dependence of the electrical conductivity up ^to 20 kbar at

room

temperature.

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initial pressure coefficient of ^a In o- / 8P

⁼

⁺ ^{33 %} kbar-1, _figure ^{2 . As} ^{shown in} _figure ^{3 the} profile ^{of the} resistivity ^versus temperature is very dependent ^on ^the applied pressure. Several

voltage jumps (not ^{shown in} figure 3) ^are usually observed while cooling ^the sample ^below

200 K at atmospheric pressure. These jumps ^are presumably ^related ^to ^the development ^of

microcracks in the sample ^or (and) ^at ^the ^contacts between the evaporated gold layer ^{and the}

thin gold ^wires. Consequently, ^no ^detailed analysis ^{of the} temperature dependence ^can ^be performed ^on ^the ambient pressure resistivity ^data. However, resistivity ^{data do} ^not ^reveal

any sharp anomaly which could be related to the onset of the metal-insulator transition,

instead a shallow resistivity minimum is observed at T. - ¹⁸⁰ ^K. The shallow resistivity

minimum is shifted down in temperature under pressure and is fully suppressed above 9 kbar.

Thus, â sharp conductor-to-semiconductor transition is observed on the resistivity in the range of 20 to 5 K depending ôn ^the applied pressure. At 23 kbar, ^the resistivity êxhibits â ^metallic-

like temperature dependence ôver the entire temperature domain with a ratio p (300 K)/p (4.2 K ) ., ^{100 and} ^no sign of saturation at low temperatures. ^We have been able to cool the sample ^down ^to ^0.56 ^K ât the pressure of 23 kbar without _any sign ôf superconductivity.

Fig. ^3.

^-

Temperature dependence ^{of the} resistivity ^of (TMDTDSF )2PF6 ^at ambient pressure, 16.5 kbar and 23 kbar.

The zero pressure behaviour in figure ^{3 is} reminiscent of the temperature dependence ^of

p in (TMTTF)2PF6 ^at ambient pressure [20]. ^A similarity is also observed between the ESR data of the mixed sulfur-selenium compound and that of the (TMTTF )2X ^like compounds.

The ESR provides â single Lorentzian line whose characteristics are AH r--- 160 G, g - 2.0087 ^for ^{Hl/c } ând ^dH=175G, g=2.0094 ^for HI/b’, ât ^{300 K.} ^For Hl/c , ^the

linewidth decreases smoothly ^with temperature ^down ^to ^{70 G} ât ^{10 K} (with gc*(10 K ) --. 2.0094), figure ^4. ^{Below 9} ^K, ^we ^have ^noticed â ^clear ^cut change ^{in the} shape ôf

the EPR signal ât the Zeeman field, which is the superposition ôf â relatively

^«

^narrow

^»

signal ( ^=$ ⁶⁵ G) and of another weaker signal with similar g-factor, becoming broader upon

cooling ^{below 9 K} (65 G). ^The origin ^{of the}

^«

^narrow

^»

signal is still unclear. If this signal ^is

due to paramagnetic impurities following ^a ^Curie law, ^its weight in the ESR spectrum ^should

be negligible above 15 K. However, the remarkable feature displayed by ^X-band magnetic

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Fig. ^4.

^-

Temperature dependence of the ESR linewidth àHp.,. (HIlc *).

resonance experiments îs the onset, below 7 K, ôf a new resonance line in the fiel range 4- 5.5 kOe (i.e. ^far ^from the Zeeman resonance field ouf - 3.3 kOe), figure ^5. ^{When the} magnetic field is rotated in the plane perpendicular ^to ^the â (stacking) axis, ^the angular dependence ^{of the} ^resonance field, figure 5, îs reminiscent of the typical

^«

^bubble

^»

pattern which is observed in antiferromagnets with the rotation axis aligned ^close ^to ^the

^«

^hard

^»

magnetic ^axis [27]. ^So ^far, only ^the high field branch of the bubble has been studied, ^at

T

⁼

4.5 K. The axis of symmetry (the easy axis) ^{for the} angular dependence ^is ^located

12.5 deg. away from the c * axis in the b’ - c * plane. ^The angular distance between the two maxima of the resonance field ait = 5.53 kOe, is about 20 deg. Rotating ^the field in the b’ - c * plane decreases the value of the resonance field down to the lowest observable

resonance at 4.1 kOe. No resonance could be detected over the entire field domain when the

angle exceeds - 45 deg. ^or ⁺ ¹⁵ deg. ^with respect ^to ^the ^{c *} axis. The value of the resonance

Fig. ^5.

^-

Angular dependence of the AFMR field

resonance

Hr ^{when the} sample is rotated around the

a-axis (T = 4.5 K ).

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field corresponding ^to the central cusp anomaly (Hr ^{= 5.4} kOe ) provides ^an estimate for the zero-field antiferromagnetic ^resonance frequency [28], f2- - 4.2 kOe which is very close ^to values obtained in other organic ^materials displaying ^a ^SDW ^state [29].

The spin susceptibility, figure 6, has been derived from the area under the ESR spectrum and a calibration with a (TMTSF)2PF6 single crystal ^{of known} susceptibility [30].

The proton spin-lattice measurements provide data very similar ^to those obtained in

(TMTTF)2PF6 ^{in the} ^35-20 ^K temperature ^domain [31]. ^However, below 20 K an exponential vanishing ôf Tîl is observed for the all sulfur material whereas an upturn of the relaxation rate is noticed for (TMDTDSF )2PF6 below = 18 K giving ^rise ^to â Tï1 ^maximum ât ⁷ ^K.

Below about 10 K the recovery of the magnetization after the usual saturation comb becomes

non-exponential. ^{The time} ^constant ^{which is} plotted ⁱⁿ figure ⁷ corresponds ^to ^{the time}

constant which is obtained after a long delay. The insert of figure ^{7 shows} ^{that the}

Ti 1 singularity ^which develops ^at TN

⁼

^{7 K} ^can ^be analyzed up ^to 15 K or so by ^a square ^root temperature divergence. Below 7 K we have not tried to analyse ^the Ti ¹ temperature dependence in details because of the strong non-exponential character of the magnetization

recovery. However, ^we ^have ^also plotted ⁱⁿ figure 7, ^around TN, ^the temperature dependence

of the time constant corresponding ^to ^the growth ^{of the} magnetization after saturation up ^to M

⁼

0.63 Mo. ^In the limit of exponential recovery, this time ^constant is nothing ^but Tl. ^As ^{shown in} figure 7, ^the anomaly of this time constant at TN ^{is far} ^more pronounced ^than

that observed in the T-dependence ^{of the} long time recovery. Non exponential ^recoveries

have been systematically ôbserved ât ^the ônset ôf aniferromagnetic ordering ⁱⁿ organic

conductors [14]. ^So ^far, ^no satisfying interpretation ^{has been} proposed. ^One possibility ^{lies in}

a possible quenching of the nuclear spin diffusion process between neighbouring nuclei due to

large ^local magnetic ^fields gradients in the SDW state or even in the critical region ^near TN. ^Such â decoupling ^{is known} ^to êxist ât ^the ônset ôf â magnetic ground ^state ⁱⁿ (TMTTF )2Br ât ^{19 K} ^since T2 decreases and T2 increases below the phase transition [14].

Consequently, the maximum of T11 ^at TN instead of a divergence ^{and the} apparent ^saturation of In Ti ^near TN, ^{which is} displayed in the insert of figure ^7, may be attributed ^to the above- discussed quenching ^of spin ^diffusion.

Fig. ^6.

^-

Spin susceptibility XS

^versus

temperature deduced from ESR measurements (Holl c *). ^The

low temperature region shows the spin-Peierls transition around 20 K. The whole temperature behaviour is presented in the insert. The absolute value of _XS is derived from

a

calibration with the

(TMTSF)2PF6 compound.

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Fig. ^7.

^-

Temperature dependence ^{of the} long-time ^tail Tl (lH) ^at ambient pressure showing ^the ^onset

of the SDW state at 7 K. Around 7 K, the temperature dependence of the time constant corresponding

to the growth ^{of the} magnetization ^after saturation up ^to M

⁼

0.63 Mo is ^also plotted (*). ^{The insert}

shows

a

fit of the experimental ^{data with}

^a

power law Ti’ - (F- T,)- "2 ( TN

⁼

⁷ K).

Discussion and conclusion.

As already emphasized, ^the transport ^{data in} figure ³ ^are very similar ^to those obtained in the series of all-sulfur compounds. The Coulomb-induced localization below 180 K is rapidly

removed under pressure. Above 10 kbar ^a large regime ⁱⁿ temperatures exhibiting ^metallic-

like conductivity extends down to the sharp phase transition ait = 20 K and even lower temperatures ^at higher pressures. As for (TMTTF )2Br under very high pressure, the

insulating ground ^state ^is fully suppressed ^above 20 kbar. The material retains then its

conducting ^state ^down ^to ^the ^lowest temperatures. În (TMTTF)2PF6 â pressure induced stabilization of the conducting ground ^state ^was âlso expected under pressure but ât â pressure which is beyond ^the possibilities ^{of the} hydrostatic pressure equipment [15].

However, ^the highlight ^{of the} present compound îs provided by ^{the low} temperature behaviour at ambient pressure. The Ti anomaly ât ^{7 K} and the square ^root divergence ôf

Ti ^at higher temperatures both argue in favour of the establishment of ^a SDW state at 7 K.

Similar features have been observed by ^NMR ât ^the ônset ôf â ^SDW ^state ⁱⁿ (TMTTF )2Br [14], (TMTTF )2PF6 under 13 kbar [11] ând (TMTSF)2PF6 [31]. The existence of â SDW state is indeed firmly established by the observation of a new (orientation dependent) ^resonance

mode below 7 K.

The maximum of Ti ^is attributed to the divergence ^of antiferromagnetic fluctuations at the onset of a SDW state and the square ^root temperature dependence ^{is the} signature of critical fluctuations in a 3-D critical regime [32].

As noticed in the previous section, ^the change in the EPR line shape ^{below 9 K} ^{makes the}

discussion of the ESR properties ^more ^delicate ^at ^the ^onset of the SDW state. We believe that the broadening of the small amplitude signal ^can be attributed to the onset of antiferromagnet-

ism. The line broadening îs admittedly smaller than what is observed in other materials with SDW states occurring ât higher temperatures [20]. În the latter compounds, ^the possible

existence of an extrinsic impurity ^resonance line interferes less severely with the intrinsic

resonance provided by itinerant electrons since they exhibit somewhat higher ^{values of}

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TN. Â closely resembling situation of low TN îs encountered in the quenched-state ôf (TMTSF)2CI04, ^{In this} ^case, the existence of a SDW state was suggested by ^NMR ^data [33]

below 7 K and confirmed by ^AFMR experiments ^at ^{1.6 K} [34]. However, ^the approach ^to ^the

SDW above 3.6 K is characterized by ^an only ^moderate broadening of the ESR spectrum [35].

The anomaly ^at ^{20 K} ^{in the} temperature dependence ^{of the} susceptibility, figure 6, ^{which is}

not accompanied by any broadening ôr shift of the ESR line is the signature ôf â SP transition

affecting ^the spin degrees of freedom. However, this reduction remains rather modest when

compared ^to ^the ^drastic drop ^of susceptibility which is observed below 19 K in (TMTTF )2?F6 where X (10 K) amounts to only X (20 K)/20 [11]. The weak reduction of the spin degrees ^of

freedom below 20 K in the S-Se material leaves the 2 kF spin fluctuations active enough ^to

enable the establishment of a SDW state at a lower temperature. Apparently, ^{the BCS}

relation between the magnitude ^{of the} spin-Peirls gap and the ordering temperature ^is ^not followed in the present ^situation (otherwise there would be no spin degrees of freedom left for the establishment of antiferromagnetism ^below Tsp/2). ^We attribute the failure of usual

descriptions ^of phase transitions to account for the development ^{of the} spin-Peirls gap below

Tsp ^to ^{the close} competition between SP and SDW long range orders (vide-infra).

Summarizing transport ^and magnetic ^data ^a phase diagram ^can ^be ^{drawn for} (TMDTDSF)2PF6 under pressure, figure 8. In the low pressure regime ^we ^have ^sketched ^{as a} guide for the eye the pressure dependence of the SP transition according ^to ^{the known}

pressure dependence ⁱⁿ (TMTTF)2PF6 [21].

Fig. ^8.

^-

^Phase diagram Temperature-Pressure deduced from resistivity measurements under pressure

(*, A), ^ESR susceptibility (e) ^{and NMR} determination of Tl (9). ^The diagram shows the presence of

charge localization (CL), spin-Peierls (SP) ^and spin density

^wave

(SDW) ^states. ^{The data} points (o ) ^above 20 kbar indicate

«

symbolically

^»

^{that the} compound ^remains conducting ^down ^to ^{the lowest} temperature reached in this work, i.e. 0.56 K.

As usual in the physics ^of quasi ^1-D conductors, ^the occurrence of transitions towards 3D

long range ordered ^states and the nature of these states are governed by ^the ^transverse coupling between 1-D correlations. When both SP and SDW 1-D channels become equally divergent ^at ^low temperatures (in ^the vicinity ^{of 9} kbar) there exists a competition ^{for the}

establishment of one or the other of the two mutually exclusive orders. In the very vicinity ^of

the phase transitions the two competing instabilities can thus be treated phenomenologically

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by ^a ^Landau expansion of the free energy comprising ^two ^order parameters of different

symmetries ^as outlined in reference [21].

where in equation (1) the coefficient of the quadratic ^term ^first vanishing determines the nature of the long range order, ASDW, Asp::> 0 and the fourth order term is positive.

A prediction of the Landau expansion ^{is the} possibility ^for â phase transition between the two states of low symmetry [20] (via â first order transition). This transition between SP and SDW orderings is indeed observed at 7 K in the present study ând provides â strong support for the description ⁱⁿ ^terms ôf â ^Landau expansion. ^Such â phase transition had never been observed so far in the study ôf organic conductors. However, ^we have failed to detect any first order character associated with the SP - SDW transition. We attribute this failure to the smallness of the jump of the SP order parameter ^{which is} only weakly developed ^{when the}

SDW state becomes stabilized at the 7 K transition. In addition, ^the repulsive interaction between the two order parameters îs responsible ^{for the} depression ^{of the} ônset ^{of SDW} ât

P

⁼

1 bar and for its rise under pressures between 1 bar and 10 kbar.

The absence of superconducting transition down to 0.56 K when the SDW state is

suppresed under pressure (P

⁼

²³ kbar ) ^{could be} ^a consequence of the structural disorder in the packing of sulfur and selenium atoms as inferred from the structure determination [26]

and from monochromatic Laue X-ray diffraction patterns [36].

In conclusion, ^the ^new ^material (TMDTDSF)2PF6 îs â remarkable prototype material for the study ôf quasi 1-D conductors belonging ^to ^the quasi ^1-D (TMTTF-TMTSF )2X family.

Competitions ^between spin-Peierls ^{and SDW} ^on ^the ^one hand and between SDW and

conducting ground ^states ôn the other hand have been clearly observed. The novelty ôf ôur investigation ^{is the} finding ôf â phase transition between SP and SDW long range ordered states. The rapid suppression of the SP state which is observed under pressure ^can be attributed to the pressure induced decrease of the dimerization gap 2l(4 kF ) âs inferred from the temperature 7p ât which Coulomb localization becomes important. ^The depression ôf 7p ^removes ^the opening ^{of the} pseudo-gap ^{in the} density ôf states at the Fermi level at

7’so, (Tsop proportional ^to Tp). ^When esp ^{is small} enough ^to be of the order of magnitude ^{of the} temperature TN at ^which long range SDW alone can establish, ^the stability of the SDW state wins over that of the SP state [21]. ^The ^SDW-SC competition has been attributed to the frustration of the SDW state due to large deviations to the complete nesting of the Fermi surface and to the concomitant rise of the 1-D to 3-D cross-over temperature. ^Both ^features

are induced under pressure ;

The absence of superconductivity ^raises ^an interesting problem since it may be due ^to the disordered nature of the material. Although ^{the 3-D} superconducting ^state ^{of these} ^Q-1-D

conductors is very effectively suppressed by ^a ^weak ^disorder potential, ^the ^amount ^{of Fermi}

surface nesting above 23 kbar may ^not be good enough ^to ^stabilize ^a ^{3-D SDW} ^state ^instead.

Such a behaviour is different from what is observed in (TMTSF)2CI04 ^when varying ^the cooling ^rate below 40 K. In this compound ^the existing anion disorder in rapidly ^cooled samples suppresses superconductivity but instead a SDW state sets in around 5 K. However,

for the moment, ^we ^cannot rule out the possible existence of superconductivity below 0.56 K, since some well ordered S-Se conductors belonging ^to ^the (DMET)2X ^series ^{do exhibit} superconductivity in the range of 0.4-0.6 K [37]. We may also suggest ^another possibility ^for

the low value (or êven absence) ôf T, ^{in the} presently investigated compound. ^Within â tight binding description ^{of the} quasi ^1-D ^band ^structure ^the transverse energy ^can be approximated by £.1 (k) = - 2 tb ^cos kb - 2 tb ^cos 2 kb b which takes into account the overlap ^between ^near

and next near neighbour ^chains along the b-direction with t’ - tl/ta [38]. ^The stability ^{of the}

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SDW state depends ^on ^the nesting of the Fermi surface. As long ^{as a} good nesting ^{of the} quasi

1-D Fermi surface can be preserved, ^the ground ^state îs â ^SDW ^state. This situation prevails when t’ is smaller than â critical value th * which is of the order of the mean field SDW transition temperature in the limit Of tb ^{= 0,} î.e. th * = TsoDw [39]. As similar values of

TSDw = ¹² ^to ^{14 K} ^are found for the (TMTSF)2PF6 ^salt ^at ambient pressure and for the present mixed S-Se compound ^{around 10} kbar, ^we may infer that tb * ^{is the} ^same ^{for both}

series of materials (t’* , ^12-14 K). On the other hand, ^the superconducting ^critical temperature depends ^on the interchain coupling tb ^but ^not ^on ^the nesting properties (t’). Therefore, the situation at P

>

20 kbar may be such that tb ^is actually ^too large ^{for the}

stabilization of the SDW state but yet ^too ^small b t,,, TsDw ) for ^the ônset ôf superconduc- tivity within the attainable temperature ^domain. Very likely ^the amplitude of ta ^for (TMDTDSF )2PF6 lies in between the all-sulfur and all-selenium values according ^to ^the computation ôf overlap integrals ^{for the} (TMTTF-TMTSF )2X ^series [40].

The likeliness of randomly distributed disorder S-Se, ^as ^inferred ^from X-ray analysis [36],

and the role it plays ^on various low temperature instabilities is still an open problem ^{and will}

be the matter of future thorough investigations.

The present material is particularly ^well adapted for future experimental investigations. ^1-

D 2 kF ^lattice softening should be observable below 60 K as it is for (TMTTF )2PF6 by X-ray

diffuse scattering techniques. ^The study of the SP - SDW transition at 7 K would also be very valuable. This compound ^{is also} â ^very good candidate for more thorough ÂFMR experiments ât very low temperatures ând extensive NMR studies since the presence of

77 Se nuclei (I

⁼

1/2 ) in the TMDTDSF molecule is very appropriate ^for spin-lattice ^relaxation

studies. Other TMDTDSF salts with non-centrosymmetric ânions âre presently ûnder investigation.

Acknowledgments.

We wish to thank J. C. Ameline for his skillful help ⁱⁿ high pressure and low temperature techniques. ^We thank J. P. Pouget ^for pointing ^out ^reference [18]. This work has been partly supported by the ESPRIT-Basic Research Action 3121.

Note added in proof : ^The assumption ^of ^a ^SDW ground ^state above 5 kbar has been made by comparison with other TMTTF and TMTSF salts. So far, ^no magnetic data have been obtained for this compound under pressure.

The first order transition SP - SDW is not in contradiction with strong fluctuation effects when the first order character of the transition is weak.

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