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

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 supercon-ducting and charge density wave(CDW)states using magnetotransport measurements.In the superconducting state,the normalized Hc2‖c/Hp is independent of the thickness of 2H-NbSe2,while Hc2‖ab/Hp increases significantly with decreasing thickness,whereHpis the Pauli limiting magnetic field andHc2‖c andHc2‖ab are the upper critical fields in the c and ab di-rections,respectively.It is found that the superconducting anisotropy parameter γHc2=HC2‖ab/Hc2‖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(cos2 θ+γCD-2W sin2 θ)1/2,which decreases with increasing temperature and disappears at about 40 K.It is found that the CDW anisotropy parameter γCDW 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.