DIELECTRIC PHASE TRANSITION IN SUPERCONDUCTING ZrV2, HfV2 COMPOUNDS

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DIELECTRIC PHASE TRANSITION IN

SUPERCONDUCTING ZrV2, HfV2 COMPOUNDS

V. Pan, I. Bulakh, A. Kasatkin, A. Shevchenko

To cite this version:

V. Pan, I. Bulakh, A. Kasatkin, A. Shevchenko. DIELECTRIC PHASE TRANSITION IN SU-

PERCONDUCTING ZrV2, HfV2 COMPOUNDS. Journal de Physique Colloques, 1978, 39 (C6),

pp.C6-1068-C6-1069. �10.1051/jphyscol:19786473�. �jpa-00217954�

JOURNAL DE PHYSIQUE Colloque C6, supplgment au no 8, Tome 39, aozit 1978, page

C6-1068

DIELECTRIC PHASE TRANSITION IN SUPERCONDUCTING ZrV2, HfV2 COMPOUNDS V.M.Pan, I.E.Bulakh,A.L.Kasatkin and A.D.Shevchenko

Inseitute of Metal Physics, Acad. Sci. UkrSSR, Kiev-142, Vemadsky str. 36.

R6sumG.- L16tude de la rdsistance Glectrique et de la susceptibilitd magn6tique en fonction de la tempdrature pour ZrVZ et HfVZ a permis de mettre en dvidence l'existence d'une transition de pha- se avec didlectrisatlon partlelle du spectre d161ectrons pour les composds considdrds. Aux tempdra- tures infdrieures 1 la tempsrature de transition la conductivitd 6lectrique n'est pas ohmique.

Abstract.- Investigations carried out on the temperature dependence of electrical resistivity and magnefic susceptibility in ZrV2 and HfV suggest the existence of a phase transition with a partial 2

dielectrization of electron spectrum in the compounds under study. At temperatures below the tran- sition temperature significant deviation from the conduction ohmic character is observed.

The obtained results indicate that the electron spectrum ,is essentially changed during the structu- ral phase transition in ZrV2 and HfV compounds this

2

being the cause of marked anomalies in temperature dependences of electrical resistance and magnetic susceptibility. Alloys were melted in the arc fur- nace in the atmosphere of pure argon and then sujec- ted to thermal treatment which ensured that samples consisted of a single phase. Samples had a crystal structure C15 with the lattice parameters~a= 7.430

0

j:0.001 A for ZrV2 and a = 7.380 0.001 A b r HfV2.

All the measurements were performed in the tempera- ture range 4.2

-

Fig.2: Magnetic susceptibility vs temperature;

8.9 0.1 K while for HfV2 it makes 9.0 +_ 0.1 K. solid line refers to theorical results.

Figures 1 and 2 show the results of resistivi- ty measurements, p(T,E) at variofis current densi- ties (E is the electric field in the sample), as well as the measurement of magnetic susceptibility,

X (T), in HfV2 alloys. These dependences are of a similar shape for ZrV2 samples. As may be seen from figures 1,2 at T < T = 130 K an obvious anomaly

P

is observed in the behaviour of the characteristics unGer study which manifests itself in the presence of a region where electrical resistivity has a negative temperature coefficient and the tempera- ture run of magnetic susceptibility is changed. It is found from magneto-resistance measurements in the field H = 40 kG that the part in the p(T) =

p(T,E + 0) dependence which has a negative tempe- Fig.1 : Ekectrical resistivity vs.temperature at rature caafficient is shifted to higher -temperatu- various values of the field E; solid line refers

res and the value of T is accordingly risen by

to the theorical results. P

5 K (the electrical resistance in the magnetic field is not observed to vary at temperature above

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786473 Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786473

T and below TV350 K, that is when the temperature P

dependence of the electrical resistance is of metal- lic character). It must be said that the departure of the alloy composition from a stoichimetric one and the introduction of impurities are responsible tor disappearance of anoinalies in the temperaturede- pendences p(T) and X(T).The.lattice distortion at structur,al transition is small (%10-3 /l/), and the- refore we have little ground to suppose that the observed anomalies are due only to the change in lattice properties. Experimental results given abo- ve indicate, as to our minds, that the nuniber of free carriers varies because of a dielectric gap which is formed on some part of Fermi surface. To illustrate this the model of a semi-metal has been considered where the phase transition with a partial dielectrization of carriers occurs by way of pairing of electrons and holes from different bands whose Fermi-surfaces are similar by their shape 121.

Besides, the presence of a partially filled band of nondielectrizable carriers is presumed. Compari- son with the experimental data reveals good agree- men t

.

The model under consideration describes quali- tatively the experimental dependences p(T) and x(T); anomalies observed there stem from the out- freezing of some free carriers. The most interes- ting result is that the conduction in the tempera- ture range T.<T has an obviously non-ohmic charac- ter. Figure 1 gives the results for electrical con- P ductivity measured at different current densities that is at different values of the electric field E in the sample. The observed conductivity increase in the field does not relate to thermal effects because such increase was found throughout the tem- perature range T <T but was absent at T> T The

P P'

value of T was independent on E. Figure 3 shows, P

in a setqi-log scale, A U (T,E) = a(T,E)- a (T,O) as a function of the field magnitude at various tem- peratures. It may be seen that the value of Ao(T,E) is well described by the law

A (J(T,E) = a(T) exp EE~(T)/~

The same figure gives the temperature dependence of Eo. Similar nonlinear conduction character has re- cently been 'observed in a quasi-one-dimensional me- tal TTF-TCNQ / 3 / and in the NbSe compound

1/41

3

the dielectric phase transition. Small values of E.

as in / 4 / make it difficult to ascribe this fact to single-particle properties of a dielectrical phase'

According to / 3 , 4 / it seems quite possible that the conductivity increase observed in the electric field results from the dynamics of the wave of char- ge density arising at the dielectrical phase tran- sition /5,6/.

Fig.3 : Dependence of Aaon E at different tempe- rature values; the temperature dependence of the

"activation field", Fo(T>

References

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-

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