Critical field of 2H-NbSe2 down to 50mK
© Nader and Monceau; licensee Springer. 2014
Received: 14 June 2013
Accepted: 26 December 2013
Published: 9 January 2014
Critical field of 2H-NbSe2 is determined for the field perpendicular to the conducting planes down to 50 mK, by magnetoresistance measurements. It is the first time that such a measurement is extended below 1 K. Variations are almost linear down to 1 K, with a little upward curvature, and the slope of Hc 2⊥ (T) decreases below 1 K. The reduced critical field extrapolates to 0.9 when the temperature approaches zero, higher than the WHH upper limit of 0.69 conformably with the extension of this model for anisotropic superconductors.
Angular dependence of the critical field is also determined at 5.5 K. Variations are the same as expected for a 3D-anisotrpic superconductor and the anisotropy value confirms previous results.
The structure of 2H-NbSe2 consists of three-layer packets, inside which the layers are ordered in the Se-Nb-Se sequence with covalent binding between them, whereas the packets are bound by van der Waals coupling. Due to this weak coupling in the dichacogenides family, adjacent packets may be oriented in different ways relative one to another and as a consequence the possibility of polytypism. The most widely encountered poly-type for NbSe2 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 reviewed by Meerschaut and Deudon (2001).
This compound has been extensively studied for its superconductivity and the formation of a CDW state. It shows a superconducting transition at 7.2 K (Toyota et al. 1976 Sanchez et al. 1995 Soto et al. 2007) with an upper critical magnetic field anisotropy, induced by the layered structure, of about 3.
The CDW state appears in 2H-NbSe2 below TCDW ~ 33 K (Moncton et al. 1977 Higemoto et al. 2003). It can be seen as an anomaly in the resistivity curve that the amplitude is tightly proportional with the sample purity (Iwaya et al. 2003)
An interesting aspect in the superconducting state of this compound which still attracts lots of attention is the peak effect (Banerjee et al. 1998). This effect appears as a maximum in the critical current curve in function of temperature.
Though, it has been believed for a long time that 2H-NbSe2 has just an anisotropic superconducting gap and some experimental results were explained into this framework (Toyota et al. 1976)
The behavior of the critical magnetic field has been subject to discussion since the seventies since its behavior depends tightly on the Fermi Surface geometry. As a result many theoretical calculations (Werthamer et al. 1966 Arai and Kita 2004) concluded that the critical field perpendicular to the layers vary linearly near Tc and its slope decreases at lower temperatures. Though an upward curvature is expected for the critical field in the parallel direction (Arai and Kita 2004).
The perpendicular critical field saturation at very low temperatures has never been investigagted experimentally.
In this work critical magnetic field of this compound is determined by mean of magneto-resistance measurement down to 50mK for the first time. It shows a behavior as expected by theoretical models mentioned above with almost a linear variations near Tc and a slope decreas at very low temperature. Angular dependence of the critical field is also determined at 5.5 K, it confirms previous results.
The NbSe2 powder is prepared starting from constituting elements at stoechiometric ratio, and then the crystals are 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.
Measurements down to 50 mK are done using a top loading dilution refrigerator and a 8 Tesla superconducting magnet. The anisotropy is measured at 5.5 K using a rotating system with an accuracy of 0.1°.
Results and discussion
Our results are similar to that found in (Toyota et al. 1976) where the measurements were done only down to 1.3 K. Though, no upward curvature near Tc was mentioned in (Toyota et al. 1976), probably because samples were of a much better quality; their RRR ratio was always higher than 30, although, for our sample it is less than 10.
Werthamer et al. (1966) extended the calculation of the upper critical field for a bulk superconductor by taking in account Pauli spin paramagnetism and spin-orbit impurity scattering. According to this model this value should not be greater than 0.69. Though, in (Hohenberg and Werthamer 1967) it was shown that this is possible for anisotropic superconductors when Fermi surface is no more spherical.
Also, the sketch of h (t) seems to be in good agreement with the reduced critical field curve calculated using the Fermi surface obtained by local density approximation (LDA) by Arai and Kita (2004) (see Figure 1 in this reference). According to this calculation h (0)extrapolates to about 0.95.
Though, Lawrence and Doniach simple model based on the effective mass anisotropy was proven not to be sufficient to describe the critical field anisotropy in layered superconductors as shown by Toyota et al. (1976) and that other assumptions should be made such as the gap anisotropy, but it could be used as a first approximation to extract anisotropy from the Hc 2 (θ, T) curve.
In plane and out-of-plane coherence lengths are then ξ|| (0) =74 Å and ξ⊥ (0) = 23 Å respectively. ξ⊥ (0) is much greater than the distance separating the centers of two successive conducting planes, which confirms that 2H-NbSe2 is a 3D-anisotropic superconductor.
Coherence lengths as well as the critical field anisotropy are of the same range as found in (Sanchez et al. 1995) by specific heat measurements.
Critical magnetic field measurement of 2H-NbSe2 is extended down to 50 mK. The slope of H c 2⊥ (T) decreases below 1 K and varies almost linearly near Tc with a little upward curvature, probably induced by a rather large superconducting transition.
Also, the anisotropy is the same as obtained in previous works.
I wish to thank Prof. I. Othmann Director General of Atomic Energy Commission of Syria for his support.
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