Weyl node assisted conductivity switch in interfacial phase-change memory with van der Waals interfaces

The interfacial phase-change memory (iPCM) ${\mathrm{GeTe}/\mathrm{Sb}}_2{\mathrm{Te}}_3$, a promising candidate for the next generation nonvolatile random-access memories, exhibits fascinating topological properties. Depending on the atomic-layer-stacking sequence of the GeTe block, the iPCM can be either in the SET (Ge-Te-Ge-Te) or RESET (Te-Ge-Ge-Te) states, where the former exhibits ferroelectric polarization and electrical conductivity two orders of magnitude larger than that of the RESET state. Yet, its microscopic origin remains elusive. Here, we predict the emergence of a Weyl semimetal phase in the SET state induced by the ferroelectric polarization which breaks the crystal inversion symmetry. We show that the giant conductivity enhancement of the SET phase is due to the appearance of gapless Weyl nodes. The Ge-Te- or Sb-Te-terminated surfaces of Weyl semimetal iPCM exhibit surface states with completely distinctive topology, where the former consists solely of Fermi arcs while the latter consists of both closed Fermi surface and open Fermi arcs. The iPCM with van der Waals interfaces offers an ideal platform for exploiting the exotic Weyl properties as well as for future memory device applications.

Figure 1

Atomic and electronic structures of iPCM in the RESET and SET states. Side view of the hexagonal ${\left(\mathrm{GeTe}\right)}_2{\left({\mathrm{Sb}}_2{\mathrm{Te}}_3\right)}_1$ superlattice in the (a) RESET inverted-Petrov structure (${\mathrm{Sb}}_2{\mathrm{Te}}_3$-Te-Ge-Ge-Te- stacking) and (b) SET ferroelectic-GeTe structure (${\mathrm{Sb}}_2{\mathrm{Te}}_3$-Ge-Te-Ge-Te- stacking). MBJLDA bulk band structure along symmetry lines of (c) inverted-Petrov and (d) ferro-GeTe band structures with the Fermi level set at 0 eV. The bands denoted with red (cyan) derive from anion (cation) states. Inset shows the bulk BZ with the high symmetry points and the projected 2D BZ for the (001) surface.

Figure 2

Weyl nodes and conductivity enhancement in the SET state. (a) Top view and (b) oblique view of locations of Weyl nodes in the BZ of the ferro-GeTe structure, where red (blue) circles denote Weyl nodes with positive (negative) chirality. The hexagon denotes a cross section of BZ at $k_z=\pi /c$. The position of Weyl nodes relative to $A$ point is scaled fivefold bigger for visibility. (c) Location of Weyl nodes along the $k_z$ direction vs the azimuthal angle $\theta$ in the $k_x\text{−}{k}_y$ plane. The yellow curve is a fitted sinusoidal curve for a guide. [(d)–(f)] Calculated bulk band structures of ferro-GeTe structure on the 2D $k_x\text{−}{k}_y$ plane at (d) $k_z=\pi /c+\delta$, (e) $\pi /c$, and (f) $\pi /c-\delta$, respectively, where $\delta =0.0143{\AA }^{-1}$ is the deviation of Weyl nodes from the BZ cross section. Red and blue colors in panel (e) denote the $\widehat{z}$ component of Berry curvature, ${\mathrm{\Omega }}_z$, while those in panels (d) and (f) denote the chirality, ${\sum }_i\mathrm{\Delta }{\mathbf{k}}_i·\mathrm{\Omega }\left(\mathbf{k}\right)/\left(\right|\mathrm{\Delta }{\mathbf{k}}_i\left|\text{exp}\right(|\mathrm{\Delta }{\mathbf{k}}_i|/r_0)$. Here $\mathrm{\Delta }{\mathbf{k}}_i=\mathbf{k}-{\mathbf{k}}_i$, where ${\mathbf{k}}_i$ is each Weyl node position and $r_0$ is an arbitrary cutoff radius. (g) Distribution of Berry curvature on a part of the $k_y\text{−}{k}_z$ plane [yellow highlighted square in panel (b)]. (h) Calculated in- and out-of-plane conductivity ratio of the SET to RESET states as a function of the chemical potential.

Figure 3

(001) surface states of the semi-infinite iPCM structure in the SET state. Surface band structures for (a) GTT surface and (b) STT surface. Fermi surfaces on (c) the GTT and (d) the STT surfaces as a function of the change of vacancy layer thickness from its equilibrium value, $\mathrm{\Delta }t=t-t_{eq}$, where $t$ is shown in Fig. 1. Red dots denote Weyl nodes projected on the surface BZ.

Figure 4

Schematic view of Fermi arc connectivity. Two types of surface-state evolution as the system undergoes a topological phase transition from normal insulator (NI) to Weyl semimetal (WSM) to topological insulator (TI) phases by changing an external parameter $m$. (a) Fermi arc connecting creation pair. (b) Fermi arcs connecting annihilation pair. ${\lambda }_i$ denotes time-reversal-invariant momenta on the surface BZ and $w_i$ are the Weyl nodes.