Strain induced metal-semiconductor transition in two-dimensional topological half metals

Abstract

Spintronic applications of two-dimensional (2D) magnetic half metals and semiconductors are thought to be very promising. Here, we suggest a family of stable 2D materials ${Mn}_2$ $X_7$ (X = Cl, Br, and I). The monolayer ${Mn}_2$ ${Cl}_7$ exhibits an in-plane ferromagnetic (FM) ground state with a Curie temperature of 118 K, which is unveiled to be a 2D Weyl half semimetal with two Weyl points of opposite chirality connected by a remarkable Fermi arc. In addition, it appears that a biaxial tensile strain can lead to a metal-semiconductor phase transition as a result of the increased anomalous Jahn-Teller distortions, which raise the degeneracy of the $e_g$ energy level and cause a significant energy splitting. A 10% biaxial tensile strain also increases the Curie temperature to about 159 K, which originates from the enhanced Mn-Cl-Mn FM superexchange. Moreover, the metal-semiconductor transition can also be induced by a uniaxial strain. Our findings provide an idea to create 2D magnetic semiconductors through metal-semiconductor transition in half metals.

Keywords: Condensed matter physics; Magnetism.

Graphical abstract

Figure 1

The crystal structure and stability (A) Top and side views of the crystal structure of ${\mathrm{M}\mathrm{n}}_2$${\mathrm{X}}_7$. (B) Convex hull of binary system ${\mathrm{M}\mathrm{n}}_{1-x}$Cl_x. (C) First Brillouin zone with high-symmetry points indicated. (D) Phonon spectrum of monolayer ${\mathrm{M}\mathrm{n}}_2$${\mathrm{C}\mathrm{l}}_7$.

Figure 2

Magnetic and electronic properties (A–E) FM and four AFM spin configurations. (F) Temperature-dependent magnetization and specific heat for ${\mathrm{M}\mathrm{n}}_2$${\mathrm{C}\mathrm{l}}_7$. (G) Magnetic anisotropy energy with respect to the ground state. (H and I) (H) Electronic band structures without SOC and with spin orientation along the y axis (in-plane) and (I) its corresponding spectral function on the (010) edge.

Figure 3

Biaxial strain induced metal-semiconductor transition (A and B) (A) Electronic band structures without and with SOC and (B) temperature-dependent magnetization and specific heat for ${\mathrm{M}\mathrm{n}}_2$${\mathrm{C}\mathrm{l}}_7$ under a 5% biaxial tensile strain. (C and D) (C) Electronic band structures without and with SOC and (D) temperature-dependent magnetization and specific heat for ${\mathrm{M}\mathrm{n}}_2$${\mathrm{C}\mathrm{l}}_7$ under a 10% biaxial tensile strain. (E and F) Bandgaps as a function of (E) biaxial tensile strain and (F) uniaxial (x axis) compressive strain.

Figure 4

Micromechanisms of biaxial strain induced metal-semiconductor transition and enhanced Curie temperature (A and B) Comparison between octahedral distortions of monolayer ${\mathrm{M}\mathrm{n}}_2$${\mathrm{C}\mathrm{l}}_7$ (A) without strain and (B) with a 10% biaxial tensile strain. (C) Evolution of $e_g$ states under anomalous asymmetric JT distortions. (D and E) Partial density of states corresponding to monolayer ${\mathrm{M}\mathrm{n}}_2$${\mathrm{C}\mathrm{l}}_7$ (D) without strain and (E) with a 10% biaxial tensile strain.

Figure 5

Uniaxial strain induced metal-semiconductor transition (A and B) (A) Electronic band structures without and with SOC and (B) temperature-dependent magnetization and specific heat for ${\mathrm{M}\mathrm{n}}_2$${\mathrm{C}\mathrm{l}}_7$ under a 5% uniaxial (x axis) compressive strain. (C and D) (C) Electronic band structures without and with SOC and (D) temperature-dependent magnetization and specific heat for ${\mathrm{M}\mathrm{n}}_2$${\mathrm{C}\mathrm{l}}_7$ under a 10% uniaxial (x axis) compressive strain. (E–H) Same as (A–D), but for ${\mathrm{M}\mathrm{n}}_2$${\mathrm{C}\mathrm{l}}_7$ under tensile strains along the y axis.

Figure 6

Similar properties for ${\mathrm{M}\mathrm{n}}_2$${\mathrm{B}\mathrm{r}}_7$ and ${\mathrm{M}\mathrm{n}}_2$${\mathrm{I}}_7$ (A–D) (A) Phonon spectrum, (B) electronic band structures without and with SOC, (C) temperature-dependent magnetization and specific heat for ${\mathrm{M}\mathrm{n}}_2$${\mathrm{B}\mathrm{r}}_7$, and (D) its band structures under a 10% biaxial tensile strain. (E–H) Same as (A–D), but for ${\mathrm{M}\mathrm{n}}_2$${\mathrm{I}}_7$.