Anisotropy of 2H-NbSe ₂ 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-NbSe ₂ in the superconducting and charge density wave (CDW) states using magnetotransport measurements. In the superconducting state, the normalized $H_{\mathrm{c}2}^{\left|\right|c}/H_{\mathrm{p}}$ is independent of the thickness of 2H-NbSe ₂, while $H_{\mathrm{c}2}^{\left|\right|ab}/H_{\mathrm{p}}$ increases significantly with decreasing thickness, where H _p is the Pauli limiting magnetic field and $H_{\mathrm{c}2}^{\left|\right|c}$ and $H_{\mathrm{c}2}^{\left|\right|ab}$ are the upper critical fields in the c and ab directions, respectively. It is found that the superconducting anisotropy parameter ${\gamma }_{H_{\mathrm{c}2}}=H_{\mathrm{c}2}^{\left|\right|ab}/H_{\mathrm{c}2}^{\left|\right|c}$ increases with reduction in the thickness of 2H-NbSe ₂. In the CDW state, the angular ( θ) dependence of magnetoresistance, R( H, θ) scales with $H{({\cos }^2\theta +{\gamma }_{\mathrm{CDW}}^{-2}{\sin }^2\theta )}^{1/2}$ , 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-NbSe ₂.