PRESSURE ENHANCED SUPERCONDUCTIVITY
IN NbSe2

Denis Jerome, A.D. Yoffe, A.J. Grant

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Denis Jerome, A.D. Yoffe, A.J. Grant. PRESSURE ENHANCED SUPERCONDUCTIVITY IN NbSe2. Solid State Communications, 1971. hal-04981373

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Distributed under a Creative Commons Attribution 4.0 International License
PRESSURE ENHANCED SUPERCONDUCTIVITY IN NbSe$_2$

D. Jérome

Laboratoire de Physique du Solide - 91 - Orsay (France)†

A.J. Grant and A.D. Yoffe

Cavendish Laboratory - Cambridge (England)

(Received 15 September 1971 by P.G. de Gennes)

The superconducting transition temperature $T_c$ of the layer crystal niobium diselenide has been found to increase from 6.9°K to 8.5°K on the application of a hydrostatic pressure of nearly 17 kbar. This corresponds to one of the highest known pressure coefficients $dT_c/dP = 9.5 \times 10^{-6}$ deg/bar. Evidence for two dimensional fluctuations are found above $T_c$.

THE DIVERSITY of properties which is observed in the large family of transition metal layer crystals has attracted considerable attention to these materials.¹

Some compounds belonging to group V with trigonal prism coordination NbS$_2$, NbSe$_2$, TaS$_2$ and TaSe$_2$ are superconductors. According to the optical data obtained on layer structure dichalcogenides the superconducting electrons in NbSe$_2$ are expected to be those which reside in a narrow energy band (~ 0.5 eV).

It is generally believed that this conduction band is formed from the $dz^2$ orbitals of the metal ions. In NbSe$_2$ the conduction band is thus half-filled with one electron per metal atom.

The layer crystals are highly anisotropic both electrically¹ and mechanically.

Since a large difference between the compressibility $\parallel$ and $\perp$ to the c-axis has been observed,² it was interesting to investigate the effect of hydrostatic pressure on the superconducting transition.

The superconducting transition temperature of 2H - NbSe$_2$ single crystals has been reported as 7°K³ or 6.9°K⁴ in the literature.

We have used single crystals of 2H - NbSe$_2$ as grown with the surface perpendicular to the c-axis.

The hydrostatic pressure is transmitted to the sample via isopentane contained in a pressure vessel at low temperature. The system is capable of up to 17.5 kbar at liquid helium temperature and is described fully elsewhere.⁵

A four probe potentiometer method was used to measure the longitudinal resistance. The temperature was measured by a copper-constantan thermocouple. The 'warm' junction was mounted inside the pressure cell near the crystal whereas the reference junction was immersed in the liquid helium bath in the pressure-cell cryostat. The temperature stabilisation system held the temperature of the sample constant to within 0.05°K. The absolute temperature is known with an accuracy of ± 0.15°K. Our data are consistent with an undetectable pressure effect on the thermal e.m.f. up to 17 kbar at low temperature.⁶

The resistive transition shows a high temperature tail with a width of ~ 0.1°K, Fig. 1. $T_c$ was taken as the temperature corresponding to the

<footnote>* The research at Orsay was sponsored by the D.R.M.E. Contract No. 69/280</footnote>

<footnote>† Associé au C.N.R.S.</footnote>
<figure><img src="image_3.png" /><figcaption>FIG. 1. The variation of the resistance through the superconducting transition at P = 6.2 kbar.</figcaption></figure>

steep departure from the superconducting state. No sizeable change on the width of the transition has been observed under pressure.

The variation of $T_c$ under pressure is plotted on Fig. 2 and corresponds to the very high positive coefficient.

$$ \frac{dT_c}{dP} = 9.5 \times 10^{-5} \, \text{°K/bar} $$

This value is many times greater than these found for transition metal alloys of niobium,⁷ although comparable positive values have been observed in $V_3Ge^8$ and in lanthanum.⁹

On the application of pressure at room temperature, the resistance of NbSe₂ fell linearly by about 5% in 15 kbar. The variation of resistance with temperature with the crystal under pressure was closely similar to that reported elsewhere for zero pressure,¹⁰ with linear metallic behaviour down to about 80°K, and d ln ρ/dT = 8 × 10⁻¹ deg⁻¹. The resistance falls more rapidly to about 22°K and is then constant between 22°K and $T_c$.

<figure><img src="image_4.png" /><figcaption>FIG. 2. The pressure variation of the superconducting transition temperature in NbSe₂.</figcaption></figure>

which is evidence that the scattering is impurity dominated in this last temperature interval near $T_c$.

Intercalation is a valuable technique in the study of layer crystals. For example, alkali metal intercalation of NbSe₂ introduces extra electrons which fill the half-filled band based on the dz² orbital, to produce a semiconductor similar to MoS₂¹¹. Intercalation of layer crystals with organic molecules such as pyridine does not add electrons to the host system, but increases the interlayer spacing to accept the organic molecules between each layer of host crystal and the next. The effect of this is to depress the superconducting transition temperature from 6°K to 4°K in NbS₂¹² but to raise it from 0.8°K to 4.5°K in TaS₂¹³ using a substituted pyridine.

The density of states at the Fermi level reaches a particularly high value in NbSe₂, N(0) ~ 8.7 eV⁻¹ molecule⁻¹.⁴ According to a simple tight binding calculation this value is expected to decrease under pressure. This latter fact leads to a $T_c$ decreasing with pressure, with the assumption of a constant electron-electron interaction within the B.C.S. approach.

Even more refined theories such as strong coupling theories¹⁴, ¹⁵, ¹⁶ are not likely to fit the situation of layer superconductors since none of them include the highly anisotropic character.¹⁷
It appears from our data that interlayer coupling must be introduced in an appropriate theory since $T_c$ depends critically on the interlayer spacing, Moreover, the tail observed above $T_c$ on Fig. 1 may very well be related to strong fluctuation-induced conductivity within the layers.¹⁸,¹⁹

In conclusion, in contrast to intercalated TaS₂,¹³ this work has shown that the interlayer spacing has a large effect on the well defined transition temperature of 2H-NbSe₂. We have also found evidence for two dimensional fluctuations above $T_c$.

REFERENCES

1. WILSON J.A. and YOFFE A.D., *Adv. Phys.*, **18**, 193 (1969).

2. $β_{‖} = 1/c (dc/dP) 300°K = 11 ± 1 \ 10^{-7}/bar$, $β_{⊥} = 1/a (da/dP) 300°K = 1.5 ± 0.3 \ 10^{-7}/bar$  
Dr. Flack, private communication: see GRANT A.J. and YOFFE A.D. (1971) to be published.

3. REVOLINSKY E., SPIERING G.A. and BEERNSTEN D.J., *J. Phys. Chem. Solids*, **26**, 1029, (1965).

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19. ASLAMAZOV L.G. and LARKIN A.I., *Soviet Physics Solid State*, **10**, 875 (1968).

Nous avons mesuré une augmentation de la température de transition supraconductrice de 6,9°K jusqu'à 8,5°K dans le disélenure de niobium sous pression hydrostatique d'environ 17 kbar. Cette augmentation correspond à l'un des coefficients de variation les plus élevés connu actuellement $dT_c/dP = 9,5 \times 10^{-6}$ deg/bar. Nous avons remarqué des indices de fluctuations à 2-dimensions au dessus de $T_c$.