Resistivity anisotropy and charge density wave in 2H - NbSe2 and 2H - TaSe2

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Abstract

The in-plane and out-of-plane resistivities of both 2H-TaSe2 and 2H-NbSe2 were determined down to 10 K. For both compounds, the resistivity anisotropy shows notably a slope change at temperatures where a CDW transition is expected to occur. On the other hand, for both compounds the resistivity anisotropy at the lowest temperature of measurement is much greater than expected by the Lawrence–Doniach model, which relates the critical magnetic field anisotropy to the normal state resistivity anisotropy for 3D-anisotropic superconductors.

Introduction

The structure of 2H-MSe2 (M: Nb, Ta) consists of three-layer packets, inside which the layers are ordered in the Se–M–Se sequence with covalent binding between them, whereas the packets are bound by van der Waals coupling. Due to this weak coupling, adjacent packets may be oriented in different ways relative one to another and as a consequence the possibility of polytypism. The most widely encountered polytype is the 2H where 2 is the number of packets in the unit cell and H stands for hexagonal. The structure of 2H-NbSe2 was recently reviewed in [1].

These two compounds have been extensively studied for their superconductivity and the formation of a CDW state.

2H-NbSe2 and 2H-TaSe2 show a superconducting transition at 7.2 K [2], [3], [4] and 133 mK [5] respectively, with an upper critical magnetic field anisotropy, induced by the layered structure, of about 3. The CDW state appears in 2H-NbSe2 below TCDW33K[6], [7], [8], though 2H-TaSe2 exhibits two CDW phase transitions, at 88 and 122 K [6], [8], [9], [10], [11].

It is generally believed that superconductivity in the dichalcogenides is of conventional BCS character, mediated by strong electron–phonon coupling [12]. However, consensus on the exact mechanism that drives the system into the CDW state has still not been reached. A CDW gap is expected in these systems only along certain wave-vectors, remaining gapless (and metallic) on other regions of the Fermi Surface. However, the electronic structure of these two systems in the CDW state is still an open question and motivating numerous experimental [13], [14], [15], [16] and theoretical works [17].

We present in this work precise measurements of the in-plane and out-of-plane resistivities of these two compounds down to 10 K. It is shown that, for both compounds, the resistivity anisotropy shows notably a slope change at temperatures where a CDW transition is expected to occur.

On the other hand, at the lowest temperature of measurement, the resistivity anisotropy is much greater than expected by the Lawrence–Doniach model [18], which relates the critical magnetic field anisotropy to the resistivity anisotropy just above the critical temperature.

Section snippets

Experimental

The NbSe2 and TaSe2 powders were prepared starting from constituting elements at stoichiometric ratio, and then the crystals were grown by thermal gradient using the iodine as a transport agent. Single crystals of these compounds are in platelet form with a diameter in the range of 2 mm and a thickness of 50 μm.

In-plane and out-of-plane resistivities are determined as follows: Single crystals are first cut into a parallelepipedic form. Two contacts extended along the sample width are then put

Results and discussion

Resistivities of 2H-TaSe2 are sketched on Fig. 2. This compound shows a drop in both resistivities at about T=88K. Variations resemble that of ρab reported in [11], although, the drop occurs, in our sample, at a lower temperature. There seems to be an anomaly at about 122 K, though it is less clear than the voltage drop at 88 K.

Let β=ρc/ρab the resistivity anisotropy. We reported on Fig. 3 the resistivity anisotropy β for 2H-TaSe2.

Fig. 4(a) shows both resistivities as well as the resistivity

Conclusion

The in-plane and out-of-plane resistivities of both 2H-NbSe2 and 2H-TaSe2 were determined down to 10 K. The resistivity anisotropy for both compounds notably shows a clear slope change at the same temperatures where a CDW transition is expected to occur. The value of the resistivity anisotropy at the lowest temperature of measurement is much greater than expected by the model of Lawrence and Doniach.

Acknowledgement

A. Nader wishes to thank Prof. I. Othmann, Director General of the Atomic Energy Commission of Syria for his continuous support.

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