High Hole Mobility and Nonsaturating Giant Magnetoresistance in the New 2D Metal NaCu₄Se₄ Synthesized by a Unique Pathway

Abstract

The new compound NaCu₄Se₄ forms by the reaction of CuO and Cu in a molten sodium polyselenide flux, with the existence of CuO being unexpectedly critical to its synthesis. It adopts a layered hexagonal structure (space group P6₃/ mmc with cell parameters a = 3.9931(6) Å and c = 25.167(5) Å), consisting of infinite two-dimensional [Cu₄Se₄]^- slabs separated by Na⁺ cations. X-ray photoelectron spectroscopy suggests that NaCu₄Se₄ is mixed-valent with the formula (Na⁺)(Cu⁺)₄(Se^2-)(Se^-)(Se₂)^2-. NaCu₄Se₄ is a p-type metal with a carrier density of ∼10²¹ cm^-3 and a high hole mobility of ∼808 cm² V^-1 s^-1 at 2 K based on electronic transport measurements. First-principles calculations suggest the density of states around the Fermi level are composed of Cu-d and Se-p orbitals. At 2 K, a very large transverse magnetoresistance of ∼1400% was observed, with a nonsaturating, linear dependence on field up to 9 T. Our results indicate that the use of metal oxide chemical precursors can open reaction paths to new low-dimensional compounds.