Anisotropy of 2H-NbSe2 in the superconducting and charge density wave states

Abstract

Anisotropy is an important feature of layered materials, and a large anisotropy is usually related to the two-dimensional characteristics. We investigated the anisotropy of the layered transition metal dicalcogenide 2H-NbSe2 in the superconducting and charge density wave (CDW) states using magnetotransport measurements. In the superconducting state, the normalized H c 2 | | c / H p is independent of the thickness of 2H-NbSe2, while H c 2 | | a b / H p increases significantly with decreasing thickness, where H p is the Pauli limiting magnetic field and H c 2 | | c and H c 2 | | a b are the upper critical fields in the c and ab directions, respectively. It is found that the superconducting anisotropy parameter γ H c 2 = H c 2 | | a b / H c 2 | | c increases with reduction in the thickness of 2H-NbSe2. In the CDW state, the angular (θ) dependence of magnetoresistance, R(H,θ) scales with H ( cos 2 θ + γ CDW − 2 sin 2 θ ) 1 / 2 , which decreases with increasing temperature and disappears at about 40 K. It is found that the CDW anisotropy parameter γ CDW is much larger than the effective mass anisotropy but does not change a lot for ultrathin and bulk samples. Our results suggest the existence of three-dimensional superconductivity and quasi-two dimensional CDWs in bulk 2H-NbSe2.