Prediction of topological insulating behavior in crystalline Ge-Sb-Te

Abstract

We report a discovery, through first-principles calculations, that crystalline Ge-Sb-Te ͑GST͒ phase-change materials exhibit the topological insulating property. Our calculations show that the materials become topological insulator or develop conducting surfacelike interface states depending on the layer stacking sequence. It is shown that the conducting interface states originate from topological insulating Sb 2 Te 3 layers in GSTs and can be crucial to the electronic property of the compounds. These interface states are found to be quite resilient to atomic disorders but sensitive to the uniaxial strains. We presented the mechanisms that destroy the topological insulating order in GSTs and investigated the role of Ge migration that is believed to be responsible for the amorphorization of GSTs.Topological insulator ͑TI͒ has a bulk-phase energy gap but contains conducting surface states that have linear energy-momentum dispersions near time-reversal invariant momenta ͑TRIM͒. 1-4 These surface states are chiral and robust to external perturbations because they are protected by time-reversal symmetry. Finding new TI materials and exploring implications to device applications have been the primary focus of current research on TI. 5-8 Also the change in topological insulating property and detailed emergent behaviors of the surface states when the composition and structure of TI are tailored are still yet to be investigated.Phase-change materials such as Ge-Sb-Te ͑GST͒ compounds are considered the best candidates for nextgeneration nonvolatile memories because of their rapid, and reversible cycles between the crystalline and amorphous structures. 9-12 Detailed atomic and electronic structures associated with the structural transition of GST compounds have been extensively studied 13-15 but the mechanism and factors responsible for the very fast atomic rearrangement are still unknown. Nor is the electronic structure of GST understood sufficiently to explain the conducting properties. Movement of Ge atoms from octahedral to tetrahedral sites has been proposed as the mechanism of structural transitions from metastable rocksalt or stable hexagonal structures to nonconducting amorphous phase. 11,13 Several candidate models have been suggested for ͑meta-͒ stable crystalline phases. Petrov proposed the layer sequence of Te-Sb-Te-Ge-Te-Te-Ge-Te-Sb ͑Ref. 16͒ ͑the Petrov sequence͒. Kooi and De Hosson ͑KH͒ proposed a different layer sequence of Te-Ge-Te-Sb-Te-Te-Sb-Te-Ge ͑Ref. 17͒ ͑the KH sequence͒. In firstprinciples calculations, the Petrov sequence is slightly less stable than the KH sequence. 18 GeTe and Sb 2 Te 3 are the main components of GSTs ͑Refs. 11, 12, and 19͒ and have finite band gaps in the bulk phase. Sb 2 Te 3 is topological insulator that has gapless edge states protected by time-reversal symmetry while maintaining bulk energy gap. 20,21 GeTe does not have such properties. For gapless edge states to exist, strong spin-orbit coupling ͑SOC͒ is needed to produce a parity inversion at TRIM. 4,20 Since the crystallin...

Disorder-induced structural transitions in topological insulating Ge-Sb-Te compounds

“…7 First-principles calculations show that crystalline GST compounds can also have a topological insulating phase depending on the composition and layer stacking sequence. [8][9][10] Experimental observation of weak antilocalization in GeSb 2 Te 4 , angle-resolved photoemission study for Ge 2 Sb 2 Te 5 (GST225), and large magneto-resistance in interfacial PCMs supports these theoretical predictions. [11][12][13] GST225 undergoes a series of structural transitions from amorphous to metastable rock-salt structure (or cubic) at $150 C and then to stable hexagonal structure at >300 C. 14 For the hexagonal structure, three types of layer sequencing have been suggested: Petrov sequence Te-Sb-TeGe-Te-Te-Ge-Te-Sb-, 15 Kooi and De Hosson (KH) sequence Te-Ge-Te-Sb-Te-Te-Sb-Ge-, 16 and intermixed sequence TeGe/Sb-Te-Te-Sb/Ge-.…”

“…The band inversion indicates that cubic GST225 transforms to a topological insulating phase above this exchange ratio as found in the hexagonal structure. 8 Our calculations suggest that cationic disorder by Ge-Sb exchange affects mostly the low energy states near the Fermi level and eventually induces formation of the topological insulating phase in our cubic GST225. 13 This trend demonstrates that the disorder not only affects the electronic band structure but also can control its topological nature.…”

“…4(e)); this change is critical in restoring the topological insulating property. 8 Finally, we discuss a model ( Fig. 5(a)) of transition path from cubic to hexagonal structures based on our calculations.…”

Fe K-edge X-ray resonant magnetic scattering from Ba(Fe1−x Co x )2As2 superconductors

“…The FWHM of the single peak gives an upper limit for the potential incommensurability ε ≈ 4.9 × 10 −4 in the longitudinal direction and ε ≈ 4.7 × 10 −3 in the transverse direction. We also measured the longitudinal and transverse scans at T = 20 K (not shown) and did not find changes of FWHM [38].…”

REXS contribution to electronic ordering investigation in solids

“…Traditionally, Cu based superconductors have been essentially studied by soft RXD (see specific citations in [121]) although with the advent of the pnictide superconductors, hard RXD investigations have been revitalized [169][170][171][172].…”