93Nb NMR study of NbSe3

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93Nb NMR study of NbSe3

F. Devreux

To cite this version:

F. Devreux. 93Nb NMR study of NbSe3. Journal de Physique, 1982, 43 (10), pp.1489-1495.

�10.1051/jphys:0198200430100148900�. �jpa-00209530�

93Nb NMR study ^of NbSe3

F. Devreux (*)

Département de Recherche Fondamentale, Centre d’Etudes Nucléaires de Grenoble, 85X, 38041 Grenoble Cedex, ^France

(Reçu ^{le 17 mai} 1982, accepté ^le 28 juin 1982)

Résumé. 2014 Nous présentons ^une étude par RMN du métal anisotrope NbSe3. ^La raie de résonance de 93Nb a été

enregistrée ^en utilisant des méthodes de RMN pulsée ^{et en} balayant ^le champ magnétique ^{autour de 65 et de}

140 kG. Des mesures ont été effectuées à température ambiante, ^{77 et 4,2 K sur un} échantillon en poudre ^{et sur}

un échantillon composé ^{de cristaux} alignés ^le long ^{de l’axe b} (axe ^{de la} chaîne). ^A température ambiante, ^les spectres obtenus pour la direction du champ magnétique parallèle ^{à b} permettent ^de distinguer trois sites non équivalents

avec des déplacements paramagnétiques ^{et des} fréquences quadrupolaires différents. A basse température, ^un

des sites n’est pas affecté par l’apparition des ondes de densité de charge ^{à 144 et} 59 K. Pour les deux autres, la structure quadrupolaire disparaît du fait de la modulation du champ électrique local par les ondes de densité de

charge; ^de plus, ^le déplacement paramagnétique ^est modifié par suite de la destruction partielle ^de la ^{surface de}

Fermi. Une interprétation ^{en termes de structure de bande est} proposée : ^{le site} qui n’est pas affecté par les ondes de densité de charge appartiendrait ^{à une} chaîne isolante possédant ^une sous-bande d pleine, tandis que les deux autres sites correspondraient ^{aux chaînes} métalliques qui, ^avec des bandes quart-pleines, participent ^{aux deux}

ondes de densité de charge successives. Une décomposition ^{de la} susceptibilité paramagnétique ^{en une} partie

orbitale et une partie ^{relative au} spin ^{est donnée.}

Abstract.

We present ^{a NMR} study ^{of the} anisotropic ^metal NbSe3. ^{93Nb NMR} lines have been recorded by

conventional pulsed ^NMR techniques ^{and field} sweeping around 65 and ,140 kG. Measurements were made at room temperature, 77 and 4.2 K on a powder sample ^{and on a} sample ^{made of} crystals aligned along ^{the b axis} (chain axis). ^{At room} temperature, spectra ^{with the} magnetic ^field along ^{b allow us to} distinguish ^three inequivalent

sites with different Knight shifts and quadrupolar frequencies. ^{At low} temperature, ^one site remains unaffected by

the occurrence of the charge density ^{waves at 144} and 59 K. For the two others, ^the quadrupolar structures disappear

as a consequence of the modulation of the local electric field by ^the charge density waves, and the Knight ^shifts

are changed because of the partial destruction of the Fermi surface. An interpretation ^is proposed ^{in terms of}

band structure : the site which is not affected by ^the charge density ^{waves would} belong ^{to an} insulating ^{chain with}

filled d subband, ^{while the two other} sites would correspond ^to the metallic chains with nearly one-quarter-filled bands, ^{which are} involved in the two successive charge density ^wave transitions. A decomposition of the para-

magnetic susceptibility in orbital and spin contributions is given.

Classification Physics ^Abstracts

76.60C - 72.15N

1. Introduction.

Niobium triselenide, NbSe3,

is an anisotropic metal which presents ^two very specta- cular resistivity ^anomalies [ 1], that have been asso-

ciated with the occurrence of two apparently ^inde- pendent charge density ^waves (CDW) ^{at 144 and}

59 K. This discovery ^has focused interest on ’ the transport properties ^{of this} compound. Considerable

experimental work has revealed a number of unusual

features, including field and frequency dependence

of the conductivity [2-7] and the existence of periodic

components in the noise spectrum [5-9], ^{so that} NbSe3

has become one of the best candidates for the study

of conducting CDWs. On the other hand, the relation between the CDWs and the crystal ^structure [10, 11]

is not clearly established, ⁱⁿ spite ^of interesting sug-

gestions based upon chemical considerations [12]

and band structure calculations [13-15].

It is the aim of the present ^NMR study ^{to relate}

the physical properties

especially the CDWs - to the microscopic structure. 93Nb NMR is, ⁱⁿ prin- ciple, ^a suitable method for this because it is sensi- tive to both magnetic ^{and electric} properties through

the Knight shift and the quadrupolar interactions, respectively. ^{Such a} study ^was completed ^some

years ago in the parent compound NbSe2 ^{and led} (*) E. R. C.N.R.S. 216.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198200430100148900

1490

actually ^{to a better} understanding of the CDW mechanism in this material [16-19].

The crystal ^structure [10] ^of NbSe3 is made of infinite stacks of trigonal prisms of selenium atoms

parallel ^{to the} monoclinic b axis, the niobium atoms

being ^{located at the centre of the} prisms. ^{Each unit}

cell contains three inequivalent pairs ^{of chains,}

hereafter referenced as I, II and III. A projection ^of

the structure perpendicular ^to the b axis is shown in figure ^{1. The wave vectors} of the CDWs have been obtained by ^electron [11, 20] ^and X-ray [21]

diffraction : _q, ⁼ (0, 0.243, 0) ^and q2 ⁼ (0.5, 0.263, 0.5) for the distortion at 144 and 59 K, respectively.

The puzzling point is that the distortion wave vector

component along ^{the Nb chain seems to} suggest nearly quarter-filled bands for both transitions. Taking

into account the different Se-Se distances within the prisms (see Fig. 1), ^Wilson [12] ^has proposed ^an

electronic distribution, ^which offers the advantage

of explaining this feature. It consists in putting ^two

electrons on each Se atom, except ^on those in chains I and III which are very close to each other and form

a Se2 - ^{bond. From the 30 valence electrons} given by ^{the six} Nb atoms, there thus remain only ^two

electrons _per unit cell, ^{which are} distributed in the d-orbitals of NbI and NbIII, giving ^{rise to four one-} quarter-filled ^{bands. In this} model, ^{chains II are} insulating ^and diamagnetic. Charge density ^waves

at T ⁼ 144 and 59 K appear in chains III and I, respectively.

Fig. ^1.

Projection ^{of the} NbSe3 ^structure perpendicular

to the b axis (from ^Ref. [11]). ^{The three} inequivalent ^Nb

chains are labelled I, II, III ; pairs ^of equivalent ^{chains are}

related by ^{an inversion centre.} Inter- and intra-chain Se-Se and Nb-Se distances are given.

Checking ^{this model was the} starting point ^of

our NMR study. Indeed, it should give ^{rise to} very distinctive features in the 93Nb NMR spectrum, namely the absence of Knight shift for one niobium line and the effects of the CDWs on the quadrupolar

structure of the two others. The experimental ^results,

which are exposed in section 2, ^{lead to a} slightly

different picture. This is discussed in section 3 with reference to the band structure, ^a quantitative ^ana- lysis ^is proposed ^{in section 4} and conclusions are

given in section 5.

2. Experimental.

^{The main} difficulties in this

study ^were the absence of crystals ^of sufficient size for NMR and the need for high magnetic ^{field to} separate the lines of the inequivalent niobium, ^which

are presumably ^{close to} each other. The first problem

was solved by aligning ^and holding ^{in grease a} large

number of ribbon-shaped NbSe3 monocrystals parallel

to the b axis (needle axis). ^The resulting sample (about ⁴⁰ mg) ^can thus be considered as a pseudo- monocrystal ^{when the} magnetic ^{field H is} parallel

to b, while it remains randomly oriented for H 1 b.

The high ^field (140 kG) ^was supplied by ^{a Bitter}

coil at the Service National des Champs Intenses, Grenoble. The field inhomogeneity ^{over the} sample

volume and the short time instability ^{were about}

20 G. Complementary measurements were performed

at 65 kG in a superconducting ^coil (field homoge- neity better than 1 G) ^to distinguish ^between magne- tic shift, first order and second order quadrupolar splitting. ^{Because of the} poor signal ^to noise ratio

requiring long accumulation time, measurements were only performed ^{at room} temperature, ^{77 K and} 4.2 K. Fortunately, ^these temperatures ^{are such as} it is possible ^to study separately the effects of each CDW.

The 93Nb NMR signal ^{was obtained} using ^a

Bruker SXP pulsed spectrometer working ^at

144.15 MHz and at 69.83 and 63.69 MHz for the

high and low field measurements, respectively. ^A

nuclear spin ^{echo was} produced ^{with a} pulse sequence

Pd - T - Pd, - ^{T, where} Pd ^and Pd, ^are pulses ^of

durations d and d’, respectively ^{and i is the refoca-}

lization time. The rather long T2 ^{allowed to choose}

T ⁼ 50 ps without loosing signal amplitude, d ^and

d’ were set to get the maximum echo intensity; they

were typically ^{1 and} 2 ps in low field measurements, and 3 and _{6 ps} in high ^field measurements. The

signal obtained after phase ^{detection was} digitalized using ^{a Bruker BC 104} transient recorder and inte-

grated ^over the echo width (about ²⁰ ps) by ^{a PDP8E}

computer. The difference between the signals ^obtained by switching ^{the first} pulse alternatively ^{on and off}

was accumulated while sweeping ^the field conti-

nuously. ^{After 2 N} accumulations, ^a spectrum point

was obtained and the accumulation for the ^next

point began immediately. This defines the field reso-

lution as AH (2 Nt/T) where AH is the sweeping

range, T the sweeping ^{time and t the} periodicity

of the pulse sequence. Typical ^{values were 5 kG for}

AH and 5 hours for T. N and t depend ⁱⁿ opposite

manner on the temperature : ^{N was about 2 000 at}

room temperature, ^{400 at 77 K and 5 at 4.2} K ;

t was 10 ms at room temperature, ^{50 ms at} 77 K and 3 s at 4.2 K. This gives ^a field resolution of the order of 5 to 10 G _per point. ^{Part of the} spectrum

were recorded with a higher resolution, ^{but this}

did not give any ^more information.

Spectra ^{at room} temperature, ^{77 K and 4.2 K} for high ^{and low} fields with H// b are shown in figure ^2.

The magnetic ^field measurements have been cali- brated by using ^{the resonance of 65Cu and} 63Cu, taking 65y ^{= 1.2119} MHz/kG ^and 63 7 ^{= 1.1312} MHz/kG. ^{For the} measurements in the

superconducting magnet, ^the delay ^{between the field}

and the current has been taken into account to determine the instantaneous field value from the current reading. Position of the diamagnetic ^niobium

resonance shown on figure ² is obtained by using 93y ^{= 1.040 7. The} imprecision in the absolute value of the field is estimated to be about 10 G and 20 G in low field and high ^field measurements, respec-

tively.

The most evident feature in figure ² is that the well-defined but complicated quadrupolar ^structure

which exists at room temperature disappears par-

tially ^{at 77 K and almost} completely ^{at 4.2 K. This is}

a clear sign of the presence of the CDWs ^at low tem-

perature. However, ^{even at room} temperature, ^the lines are broader (typically ⁵⁰ G) ^{than is} expected

for a dipolar ^width (less ^{than 5} G). ^This may be due to various effects : crystal misalignment, field inhomo-

geneity ^and partial overlap of different lines.

In non-cubic symmetry ^the spectrum ^of 93Nb (I = 9/2) ^is composed of nine lines. Thus the three

inequivalent Nb sites in NbSe3, ^should give ^{rise to}

27 different lines. As the crystal symmetry ^{set the b} axis as a principal axis for the Knight ^{shift and}

electric field gradient ^{tensors, the resonance fields}

for H// b are :,

where the second order quadrupolar splitting ^has

been neglected. ^In equation (1) vo is the fixed spectro-

meter frequency, y is the gyromagnetic ^ratio, K I I is the Knight ^shift component in the b direction and vQ II is the parallel quadrupolar frequency, ^which

is related to the nuclear quadrupole ^moment Q ^and

to the parallel electric field gradient ^{VZZ =} eqll II by

The indexation of the lines in figure ² has been done

using equation (1). The results are given ^{in table I}

with the corresponding ^{values in} NbSe2 for reference purpose. At room temperature, equation (1) ^allows unambiguously the indexation of two sets of lines, corresponding ^to sites referenced as sites 1 and 2 in table I. Once these lines have been indexed, ^there

remain only ^two well-defined lines (shown by ^arrows

in Fig. 2). Comparison ^{between the} high and low field spectra shows that they ^are quadrupolar ^satellites

Fig. ^2.

^{93Nb NMR} spectra ^{for H//b in} NbSe3. X-axis is labelled in kG. The position ^{of the resonance of the} diamagnetic

Nb (y ^{= 1.040 7} MHz/kG) is shown. The peaks ^are labelled with symbols, which refer to the different sites : 0 site 1, ^{+ site} 2,

40 site 3.

1492

Table I.

Experimental Knight shifts ^and quadrupolar interactions for ^{H//b in} NbSe3. ^Values for NbSe2 ^are

taken from reference [19].

with sa small quadrupolar interaction. One is led ^to

assume that the central line and the other satellites

overlap with the lines of sites 1 and 2. In table I, ^we

suppose that these two lines correspond ^to the transi-

tions - 2 and § - ), giving ^a quadrupolar interaction e2 Qqlllh ⁼ 3.5 MHz. However it would also be possible ^{to assume that} they correspond ^to

± -1 --+ ⁺ 1, which would give e2Qqll/h ^{= 5 MHz}

and a different Knight shift. At 77 K, ^the noteworthy

features are the disappearance ^of the well-defined structure corresponding ^{to site 1 and the} splitting ^of

the central line into three lines instead of two lines at

room temperature. ^The Knight ^{shift and} quadrupolar frequency ^{of site 2 are} practically ^not affected and the

quadrupolar satellites are only slightly ^smoothed.

There remains one moderately ^smoothed quadrupolar

structure with e2 Qq 11 Ih ^{= 5 MHz, which} corresponds

to the less shifted central peak (site 3). ^{At 4.2} K, ^this quadrupolar ^structure disappears ⁱⁿ turn, while the central lines of sites 1 and 3 have almost merged again

into one line. The Knight ^{shift and} quadrupolar ^inter-

actions of site 2 remain unchanged.

Spectra have been also recorded for the aligned sample ^{with H 1 b and for a} powder sample. ^{In the}

absence of cylindrical symmetry, ^we expect ^{for H 1 b}

a powder-like spectrum, ^which depends ^{on three} parameters (two Knight ^shift components ^{and the} asymmetry parameter ^{of the field} gradient tensor)

for each unequivalent ^{Nb site. The} resulting spectrum is rather complicated ^{and we have not succeeded in} determining ^these transverse components. However, from the measurements on the powder sample, ^{we have}

been able ^to estimate the mean isotropic Knight

shift Kiso by calculating ^{the first moment} of the whole line by integration. ^The resulting ^{values are} given ⁱⁿ

table II. The rather large uncertainty arises from the

difficulty ^of choosing ^an appropriate integration

range.

Finally ^{we have not} attempted ^{to make} precise

determination of the nuclear relaxation time Ti,

which has been recently ^{measured at low} temperature

by Wada et al. [22]. However, ^the repetition ^{time t,}

which has been set to the minimum value that does

Table II.

Temperature ^variation of ^the isotropic Knight shift ^and of ^the (Tt)-l product ⁱⁿ NbSe3.

not reduce the signal amplitude, ^{can be used as a} qualitative indication about the relative variation of

Tl. ^{In this} respect ^the temperature variation of (Tt)-1

given ^{in table II indicates} clearly ^{a decrease of the}

relaxation rate (T T 1 )-1 ^with decreasing temperature.

It _may be noticed that our (Tt)-’ ^{values are in}

agreement ^{with the} (T T 1 )-1 ^measured by ^Wada

et al. [22].

3. Discussion.

Thus, _contrary ^{to our} naive _expec- tation, ^{there is no} diamagnetic ^{chain in} NbSe3.

Moreover, ^the only ^{chain which is not affected} by ^the CDWs, ^is just ^{the one} that has the largest quadrupolar

interaction and the largest Knight ^{shift. In fact this last}

contradiction is only apparent, ^{since the} spin Knight

shift arising from d electrons is mainly ^{due to core} polarization and therefore is expected ^{to be} negative.

On the other hand, the orbital Knight shift, ^which may be important for 4d electrons is positive. Thus it is rather natural to propose the following picture : ^site

2 corresponds ^{to the} insulating ^{chain; it has a small,} possibly vanishing, spin susceptibility ^{and its} tempera-

ture independent Knight ^{shift is} mainly ^{due to the}

orbital susceptibility. ^This implies the existence of filled d subbands in this chain. As far as sites 1 and 3

are concerned, ^the interpretation is less clear. One _may first assume that the transitions at 144 K and 59 K take place primarily ^{in two} different chains corres-

ponding ^to the sites 1 and 3, respectively. ^{At each}

transition temperature, ^the quadrupolar ^{structure of}

the related chain disappears because of the spatial

modulation of the electric field gradient ^{and the} Knight

shift increases due to the decrease of the density

of states at the Fermi level, which reduces the negative

spin Knight shift contribution. An alternative expla-

nation is to suppose that both sites are involved in both transitions and that the two less shifted central

peaks ^{at 77 K and 4.2 K are the} singularities ^{of the} Knight shift distributions corresponding ^{to the incom-}

mensurate CDWs. In this _case, the 5 MHz quadrupo-

lar structure at 77 K would be caused by ^a redistribu- tion of the electronic density between the sites 1 and 3 at the first transition. This structure, partially ^smooth-

ed by ^{the first} CDW, ^{would be} completely destroyed by ^{the second one.}

To connect this picture ^{with the} crystal structure, it is interesting ^to consider the band structures calcu- lated by Hoffman et al. [15]. Figure 3 shows the band

diagram along the b axis for different cases : two isolated chains (Fig. 3a), ^{a two} dimensional lattice

Fig. ^3.

^{Band structure of} NbSe3 along the b axis : a) ^two

isolated chains, b) ^{a two} dimensional lattice, c) ^{the real}

structure of NbSe3 (from ^Ref. [15]).

(Fig. 3b) and the real structure of NbSe3 (Fig. 3c).

Among the bands of the real structure, ^two pairs ^of

bands are reminiscent of the situation for isolated

chains, while four others look like the dz2 bands of the two dimensional structure. According ^{to reference} [15],

this change ^{in the band} shape is related to a change ⁱⁿ

the Nb environment, going from the six-coordinate distorted trigonal prism for the isolated chain to the

approximately eight-coordinate geometry ^{for the two} dimensional lattice. Looking ^{at the} crystal ^structure (Fig. 1), ^it appears that the four dz2 ^bands belong ^to

chains I and III, ^for which the distance between Nb and the extra-chain Se (2.73 A) is about the same as the distance between Nb and the intra-chain Se (2.65 A).

The other bands with mixed dz2 ^and dx2 - y2 ^character belong ^{to chain II,} for which this distance is larger (2.86 ^{and 2.95} A).

Thus it is tempting ^to interpret ^{our NMR results in}

the following way : site 2 is identified with the Nb

atoms of chains II, ^which accommodate four electrons in two filled lowlying d subbands. There remain two electrons for the four dZ2 cosine-like bands of chains I and III, giving ^{rise to the} expected one-quarter-filled

bands. These chains, ^which _carry ^the CDWs, ^corres- pond ^{to the sites 1 and 3 of our NMR} study, ^{without it} being possible ^{at the moment to} distinguish ^between

them. This model is qualitatively supported by ^the larger quadrupolar interaction in site 2. Although ^the

electric field gradient ^results from different contribu- tions which _may compensate ^each other, ^recent empirical studies show that the valence electron contribution of the metallic atom is dominant [23].

Therefore it is quite consistent that chains II with two electrons in Nb-based d-orbitals give ^a markedly larger quadrupolar frequency than chains I and III, which have only ^{half an electron.}

4. Local susceptibilities.

^{In this section, we shall}

attempt ^a quantitative joint analysis ^{of the} Knight ^shift

and susceptibility ^{data. We} begin ^{with the} isotropic paramagnetic susceptibility, ^{which can be} decomposed

into spin ^and orbital contribution :

where the symbol x ^{means the} average susceptibility

over the three inequivalent ^sites. Assuming ^{there are}

no Nb s-electrons at the Fermi level, ^{the mean} isotropic Knight shift is written as :

where f3 is the Bohr magneton, ^{A the} Avogadro ^num- ber, ^{H’P and Horb the core} polarization ^{and the}

orbital hyperfine ^fields, respectively. Taking [24]

H’P ^{= -} 0.21 ^x 106 G and Horb = 0.28 ^x 106 G and the values for the paramagnetic susceptibility_as given by Kulick and Scott [25], ^we get ^for X’ ^and x so

the results which are given ^{in table III.} Actually, ^due

to the uncertainty ^{on the} hyperfine fields and _espe-

cially ^on the static susceptibility these values should be only considered as indicative. For example, taking

the susceptibility given by DiSalvo et al. [26] (Xiso ⁼

14 ^x 10-6 emu/mole ^{at room} temperature), ^one

obtains a negative spin susceptibility. Furthermore it has been assumed that there is no Se contribution to the paramagnetic susceptibility. ^{From the small}

Knight shift and relaxation rate of "Se (K;,. ^0.05 %

and ( Tl 7) - ’ -- ^0.018 (sK) - I ^{at low} temperature [27]),

it _may be inferred that this contribution ^{is of the} order of 10- 5 emu/mole. ^{This is not} negligible ^{but, as this is}

smaller than the uncertainty ^on the measured bulk

susceptibility, it has been consistently neglected ^{in the}

present analysis. Nevertheless, results of table III suggest that less than half the paramagnetic suscep-

tibility ^{is due to the} spin contribution ^which explains

the rather small relative change of the total suscepti-

bility ^at the transitions [25, 26]. ^{The room} temperature

spin susceptibility corresponds ^{to a} density ^{of states}

1494

Table III.

Decomposition of ^{the mean} isotropic para-

magnetic susceptibility X in NbSe3 ; X values are taken

from reference [25].

of 0.31 states per eV, spin ^{and Nb} atom, ^consistent with band calculations [13-15]. The decrease of the spin susceptibility ^{from room} temperature ^{to 4.2 K is} corroborated by ^the decrease of (Tt)-’, ^{which is} expected ^{to be} proportional ^{to the} square of the

density ^of states at the Fermi level. Both indicate a

destruction of about 50 % of the Fermi surface at 4.2 K, ^while transport measurements [3] give ⁸⁰ %.

At the same time, the orbital susceptibility ^{does not} change appreciably, in contradiction with ^some theo- retical predictions [28].

Let us now consider ^the parallel Knight ^{shift. For}

each chain (i ⁼ 1, 2, 3), ^{one has :}

where Hdip is the dipolar hyperfine ^field. Assuming ^a dz2 ^band symmetry, ^{it is} given by ^{Hdip =} 2/7 r- 3 >

where r > ^{is the mean value} of r - ’ in the metallic d orbitals ^at the Fermi level. Taking [24, 29] ; r - 3 > =

2 _a.u., one has : Hdip = 0.04 ^{x 106 G. With the mean} spin susceptibility : xs - 1/3 Y ^{Xis, this gives four}

i

relations with six unknown susceptibilities. ^{To solve}

the system, ^{one should make two} hypotheses. ^The

most natural one is to assume X,,b ⁼ X,,,b , ^{because of}

the similarity of the band structure for these two chains. For the second hypothesis, ^{one can} tentatively

suppose either that the third orbital susceptibility

orb ^{is equal to the two first ones or that the spin}

susceptibility x2 ^{vanishes. In the first case, one gets a} negative X’; in the second _case, assuming ^{that each}

central peak ⁱⁿ figure ² corresponds ^{to one kind of} chains, ^one obtains the set of values which are given

in table IV. In the alternative model of a Knight ^shift

Table IV.

Decomposition of ^the parallel paramagne- tic susceptibility ⁱⁿ NbSe3.

distribution, xl ^and x3 should be considered as the minimum and maximum values of the local spin susceptibility distribution. Although these results should be considered cautiously, they give ^{a rather}

attractive picture ^{with a constant} orbital paramagne- tism and a spin susceptibility ^which drops ^{as the}

CDWs appear. It should also be remarked that the

mean perpendicular ^orbital susceptibility ^{which is}

obtained from x sa ⁼ 1/3( ^{+ 2 Xvv) is markedly}

smaller than the parallel ^{one :}

This is quite consistent with the fact that, ⁱⁿ C2, symmetry, ^{the z} component ^{of the} orbital momentum

1_, has matrix elements between the fundamental x2, y2, ^z2 manifold and the _xy orbital, which has lower energy [15] ^{than the} yz and xz orbitals, the latter being

connected to the fundamental subbands by lx ^and ly, respectively.

5. Conclusion.

Our 93Nb NMR study ^{allowed a} microscopic approach of the CDWs in NbSe3. ^We

have been able to distinguish the role of the inequi-

valent Nb chains. One chain is not affected by ^the

CDWs and is characterized by ^{a smaller,} possibly vanishing density ^of states at the Fermi level. From considerations based _upon the band structure it is

tempting ^to identify ^this less-conducting ^{chain with}

chain II. This is consistent with the larger ^Se-Se

distance in that chain which allows the accommodation of more electrons [12] ^{than in chains I and} III. However

our study shows that these electrons belong ^{to orbitals}

which are built _up with Nb d-orbitals, ^{at least} par-

tially. ^In fact, ^{it seems} reasonable to assume an impor-

tant hybridization between Se p-orbitals ^{and Nb}

d-orbitals. It is difficult, ^{from our} data, ^{to decide}

whether both the other chains are involved in both transitions or whether they ^{are affected} separately.

Moreover, in the latter _case, it does not seem possible

to distinguish which of chains I or III is concerned with the high ^{or low} temperature transition. From the

perpendicular components of the distortion vector it has been suggested [12] that chain III is involved in the high temperature ^{CDW and chain I} in the lower temperature ^{CDW. In such a case, site 1 would corres-} pond ^{to chain III and site 3 to chain I.} Finally, ^{we have} proposed ^a decomposition ^{of the} paramagnetic ^sus- ceptibility in local contributions. We would be more

confident in this decomposition ^{if the} uncertainty ^on

the bulk susceptibility (a factor of four between the two values published in the literature [25, 26]) ^was

reduced. It would be also interesting ^{to measure the} anisotropy ^{of the} susceptibility. ^{As far as the NMR is}

concerned further experiments would include "Se Knight ^{shift and} T 1 measurements and T 1 ^determina-

tion for the different 93Nb sites.

Acknowledgments. ^{- The} samples ^were produced by ^L. Guermas, A. Meerschaut and J. Rouxel at the Laboratoire de Chimie Min6rale in Nantes and _pro- vided by P. Monceau. Further acknowledgments ^are

due to P. Monceau for patiently aligning ^the crystals ^to

make a sample ^of sufficient size. I thank the Service National des Champs Intenses in Grenoble for high

field facilities and M. Picoche for technical assistance.

Finally, ^I have benefited from fruitful discussions on

the physics of NbSe3 ^{with P.} Monceau and M. Renard.

References

[1] CHAUSSY, J., HAEN, P., LASJAUNIAS, ^J. C., MONCEAU, P., WAYSAND, G., WAINTAL, A., MEERSCHAUT, A., MOLINIÉ, ^{P. and} ROUXEL, J., Solid State Commun.

20 (1976) ^759.

[2] MONCEAU, ^P., ONG, ^N. P., PORTIS, ^A. M., ^MEERS-

CHAUT, A. and ROUXEL, J., Phys. Rev. Lett. 37

(1976) 602.

[3] ONG, ^{N. P. and} MONCEAU, P., Phys. ^{Rev. B 16} (1977)

3443.

[4] GRÜNER, G., TIPPIE, ^L. C., SANNY, J., CLARK, ^{W. C.}

and ONG, ^N. P., Phys. ^{Rev. Lett. 45} (1980) ^935.

[5] FLEMING, ^{P. M. and} GRIMES, ^C. C., Phys. ^{Rev. Lett. 42} (1979) ^1923.

[6] GILL, ^J. C., ^J. Phys. ^{F 10} (1980) ^281.

[7] MONCEAU, P., RICHARD, ^{J. and} RENARD, M., Phys. ^Rev.

B25 (1982) 931;

RICHARD, J., MONCEAU, ^{P. and} RENARD, M., Phys.

Rev. B25 (1982) ^948.

[8] MONCEAU, P., RICHARD, ^{J. and} RENARD, M., Phys.

Rev. Lett. 45 (1980) ^43.

[9] WEGER, M., GRÜNER, ^{G. and} CLARK, ^W. C., ^Solid

State Commun. 35 (1980) ^243.

[10] MEERSCHAUT, ^{A. and} ROUXEL, J., ^J. Less-Common Metals 39 (1975) ^197.

[11] HODEAU, ^J. L., MAREZIO, M., ROUCAU, C., AYROLES, R., MEERSCHAUT, A., ROUXEL, ^{J. and} MONCEAU, P., ^J. Phys. ^{C 11} (1978) ^4117.

[12] WILSON, ^J. A., Phys. ^{Rev. B 19} (1979) ^6456.

[13] BULLETT, ^D. W., Solid State Commun. 26 (1978) ^563.

[14] ^{BULLETT, D.} W., ^J. Phys. ^{C 12} (1979) ^277.

[15] HOFFMANN, R., SHAIK, S., SCOTT, ^J. C., WHANGBO,

M. H. and FOSHEE, ^M. J., ^J. Solid State Chem.

34 (1980) ^263.

[16] TORGESON, ^{D. R. and} BORSA, F., Phys. ^{Rev. Lett. 37} (1976) 956.

[17] STILES, J. A. R. and WILLIAMS, ^{D. L.} G., ^J. Phys. ^{C 9} (1976) 3941.

[18] WADA, S., ^J. Phys. ^Soc. Japan ⁴⁰ (1976) ^1263.

[19] BERTHIER, C., JÉROME, ^{D. and} MOLINIÉ, P., ^J. Phys.

C 11 (1978) ^797.

[20] TSUTSUMI, K., TAGAGAKI, T., YAMAMOTO, M., ^SHIO-

ZAKI, Y., IDO, M., SAMBONGI, T., YAMAYA, ^{K. and} ABE, Y., Phys. ^{Rev. Lett. 39} (1977) ^1675.

[21] ^{FLEMING, R.} M., MONCTON, ^{D. E. and} MCWHAN, D. B., Phys. ^{Rev. B 18} (1978) ^5560.

[22] WADA, S., AOKI, ^{R. and} FUJETA, O., Preprint.

[23] RAGHAVAN, P., KAUFMAN, ^E. N., RAGHAVAN, ^R. S., ANSALDO, ^{E. J. and} NAUMANN, ^R. A., Phys. ^Rev.

B 13 (1976) ^2835.

[24] YAFET, ^{Y. and} JACCARINO, V., Phys. ^{Rev. 133} (1964)

1630.

[25] KULICK, J. D. and SCOTT, ^J. C., Solid State Commun.

32 (1979) ^217.

[26] DISALVO, ^F. J., WASZCZAK, J. V. and YAMAYA, K., J. Phys. ^{Chem. Solids 41} (1980) ^1311.

[27] PANISSOD, P., unpublished ^work.

[28] BORIACK, M. L., Phys. ^{Rev. B 21} (1980) ^4478.

[29] FREEMANN, ^{A. J. and} WATSON, ^R. E., in Magnetism,

ed. by Rado G. T. and Suhl H., Volume II A

(Academic Press, ^{New York and} London) 1965,

p. 167.