PHYSICAL PROPERTIES RELATED TO CHARGE DENSITY WAVES IN LAYERED TRANSITION METAL DICHALCOGENIDES

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PHYSICAL PROPERTIES RELATED TO CHARGE

DENSITY WAVES IN LAYERED TRANSITION

METAL DICHALCOGENIDES

C. Berthier, D. Jérome, P. Molinié

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C7, supplkment uu no 12, Tome 38, dkcembre 1977, puge C7-250

PHYSICAL PROPERTIES RELATED

TO

CHARGE DENSITY WAVES

IN LAYERED TRANSITION lMETAL DICHALCOGENIDES

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C. B E R T H I E R

Laboratoire d e Spectrometrie Physique, B P 53, 38041 Grenoble Cedex, France

D. JEROME

Laboratoire d e Physique des Solides, Universite d e Paris-Sud, 91405 Orsay, France a n d P. MOLINIE

Laboratoire d e Chimie Minerale A, B P 1044, Nantes Cedex, France

RkumC. - I1 a et6 observe dans la plupart des polytypes de dichalcogCnures lamellaires de mCtaux de transition TX, (T = V, Nb, Ta, X = S, Se) des distorsions periodiques du reseau (D.P.R.) accompagnkes d'ondes de densite de charge (O.D.C.). L'apparition de ces instabilites structurales est favorisee par la forme cylindrique de la surface de Fermi et (ou) par I'existence dans la structure de bande de singularites de Van Hove proche du niveau de Fermi. Nous donnerons une revue succincte des rksultats expkrimentaux obtenus ces quelques dernieres anntes dans ces mattriaux. Dans les composks a coordination trigonale prismatique (2 H-NbSe,, 2 H-TaSe,, 2 H-TaS,). l'etablissement de I'onde de densite de charge i T = To s'accompagne d'une faible anomalie dans la rCsistivitC et ces systkmes deviennent supraconducteurs a plus basse temperature. Une etude sous haute pression demontre I'existence d'une relation entre I'augmentation de T, sous pression et I'existence d'une instabilitt structurale B To > T,. Nous termincrons par un bref rappel des principaux resultats

obtenus par Resonance Magnttique NuclCaire sur 1'Ctat avec DPR/ODC dans le compose 2 H-NbSe,. Abstract. - Periodic lattice distortions (PLD) accompanied by chat-yc: density waves (CDW) have been observed in most of the polytypes of the lamellar d1 transition metal dicha~co~enides TX; (T = V, Nb, Ta, X = S, Se). These structural instabilities are favoured by the two dimensional character of the Fermi surface of these materials and (or) by the existence of Van Hove singularities at the Fermi level. The experimental work on these phase transitions will be briefly reviewd. In the trigonal prismatic polytypes(2 H-NbSe,, 2 H-TaSe,, 2 H-TaS,) the onset of the CDW is accompanied by a weak resistivity anomaly and these compounds become superconductors at lower temperature. A high pressure investigation shows a connection between the pressure enhancement of T , and the existence of a structural phase transition at To > T,. Endly we shall briefly recall the main results

of a detailed N M R investigation of the PLD-CDW state in 2 H-NbSez.

Periodic lattice distortion (PLD) accompanied by charge density waves (CDW) have been observed [l, 21

in most of the polytypes of the lamellar d1 transition metal dichalcogenides TX, (T = V, Nb, Ta, X = S , Se). These structural instabilities are favoured by the two-dimensionnal character of the Fermi surface of these materials, which allows t o find some parti- cular wavevectors Q, spanning large parallel pieces (nesting condition) and (or) the existence of Van Hove singularities near the Fermi level [3]. Both mechanisms lead t o peaks in the electronic suscepti- bility for special q values, which combined with

(*) This work has been supported at Orsay by the DGRST contract n u 74-70877.

electron-phonon interaction may provide the neces- sary condition [4] for the onset of a PLD accompanied with a C D W o f same wavevector. This P L D / C D W usually is incommensurate, i.e. its wavelength does not correspond t o a translation of the host lattice. Such a transition from the normal metallic state to

a n incommensurate P L D I C D W state at T = To is generally second order, because of the Coulomb interaction between adjacent layers [5]. T h e amplitude of the distortion (or that of CDW), which is the order parameter of the phase transition, grows when the temperature is lowered below To, a n d when it becomes t o o large, t h e incommensurate P L D / C D W turns into a commensurate P L D I C D W in a first order transition a t T = T,. These successive phase transi-

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PHYSICAL PROPERTIES RELATED TO CHARGE DENSITY WAVES C7-251 tions have been described in a phenomenological

Ginzburg-Landau theory by McMillan [5,6] and Bhatt and McMillan [7].

Direct evidence for the existence of PLDICDW on the lamellar d1 transition metal dichalcogenides have been obtained by electron diffraction [ l , 21 and neutron scattering [g, 91 in all the 1 T, 2 H, and 4 Hb polytypes of TaS,, as well as in 2 H-NbSe, and 1 T-VSe, [2]. Besides these diffraction techniques, the PLDICDW state has been studied at microscopic scale by various techniques like Raman scattering [10], ESCA [ll], Mossbauer effect [12], and NMR [l31 which will be considered below in more details.

The onset of these phase transitions are accompanied by various anomalies in the electronic properties of the crystal like conductivity, suscepti- bility [l], Hall effect [14], specific heat [15], etc. In polytypes where the coordination of the transition metal ion is octaedral, the onset of the PLDICDW induces metal-semiconductor transitions (at 325 K in 1 T-TaS,) or metal-semi metal transitions (at 473 K in 1 T-TaSe,). In trigonal prismatic compounds, the transition temperatures To are much lower, and the amplitude of the distortion smaller. Conse- quently, one simply observes at the onset of the PLD/CDW metal-metal transitions with slight resisti- ~ v i t y anomalies, and these polytypes become super-

conductor at lower temperature.

The study of the pressure dependence of To and Tc through resistivity measurements shows a strong connection between CDW and superconductivity in these materials [16]. It has been established that the observed strong positive dTc/dP coefficient [7] was due to a progressive removal of the PLDICDW state by the pressure; this was particularly demons- trated in the case of 2 H-NbSe, [l81 and 4 Hb-TaS, [19], in which above a critical pressure P * (respectively 36 and 8.5 kbar) the PLDICDW is fully suppressed and dTc/dP becomes an order of magnitude smaller. Except the case of 2 H-TaSe, [20] for which the experimental situation is not quite clear as far as dT,/dP is concerned [21] the results obtained in the whole series of the trigonal prismatic compounds are well accounted by the suggestion of Friedel [22] who explains the change in T, in terms of a variation of

the density of states at the Fermi level. The results obtained in 2 H-NbSe, and 4 Hb-TaS, rule out the alternative interpretation based on soft modes asso- ciated to the structural instability; such a model predicts a decrease of Tc above P*, in contradiction with experimental results.

We shall end by recalling the main results obtained from a detailed NMR study on the PLDICDW in 2 H-NbSe, [13, 16, 231. Below the onset temperature, a redistribution of conduction electrons causes a broadening of 93Nb quadrupolar satellites. The corresponding lineshape of these lines is characteristic of the symmetry of the PLDICDW state and of its incommensurate (commensurate) character : one can easily distinguish between a single CDW (one q vector) and the local superposition of three CDW's with different q vectors. So in 2 H-NbSe,, the observed lineshape indicates the local coexistence of three incommensurate CDW, confirming the results obtained from neutron scattering [g], and eliminates the possibility of equivalent domains with single q vectors. This triple CDW remains incommensurate at 21 kbar, with To = 26 K. The amplitude of the broadening gives a measure of the amplitude of the CDW, and the maximum redistribution of conduction electrons within on atomic cell was found equal to 9 % at 4.2 K in 2 H-NbSe,. The amplitude of the PLD/CDW was also found proportional to To, as expected in mean field theory.

Above To, a pretransitional broadening of lines is observed, which has been shown [16, 231 to be due to static fluctuations of CDW induced by impurities [6], although its temperature dependence is the same as predicted by Bhatt and McMillan [7] for dynamical fluctuations. This demonstrates the strong coupling of CDW fluctuations with impurities in low dimensional systcms.

Finally, the difference between the NMR lineshape of nuclei submitted to a commensurate PLDICDW, and those placed in an area where the PLD/CDW is incommensurate should provide an unique tool to investigate the incommensurate-commensurate PLDICDW transition and the possible existence of discommensurations [5].

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