A theoretical study of an interfacial phase change memory made of a GeTe-SbTe superlattice with W electrodes is presented to identify the high and low resistance states and the switching mechanism. The ferro structure of the GeTe layer block in the Te-Ge-Te-Ge sequence can be in the low resistance state only if the SET/RESET mode consists of a two step dynamical process, corresponding to a vertical flip of the Ge layer with respect to the Te layer, followed by lateral motion driven by thermal relaxation. The importance of spin-orbit coupling at the GeTe/SbTe interface to the "bias polarity-dependent" SET/RESET operation is shown, and an analysis of the two-dimensional states confined at the GeTe/SbTe interface inside the resistive switching layer is presented. Our results allow us to propose a phase diagram for the transition from a topologically nontrivial to a trivial gap state of these two-dimensional compounds.

The chalcogenide alloy Ge-Sb-Te (GST) has not only been used in rewritable digital versatile discs, but also in nonvolatile electrical phase change memory as a key recording material. Although GST has been believed for a long time not to show magnetic properties unless doped with magnetic impurities, it has recently been reported that superlattices (SLs) with the structure [(GeTe) L (Sb 2 Te 3 ) M ] N (where L, M, and N are usually integers) have a large magnetoresistance at room temperature for particular combinations of L and M. Here it is reported that when [(GeTe) L (Sb 2 Te 3 ) M ] N chalcogenide SL films are thermally annealed at 470 K and cooled down to room temperature under an external magnetic field accompanied by current pulse injections, a large magnetoresistance change (>2500 Ω) is induced. This study shows that the phenomenon has a strong correlation with the GeTe thickness and the periodic structure of the SL films, and that it is induced by the structural phase transition between electrically nonpolar and polar phases in the GeTe layers in the SLs. This study proposes that the relationship between the polar (ferroelectric) phase and the Berry curvature in the SLs is responsible for the magnetoresistance change.

We investigated the resistive switching mechanism between the high-resistance state (HRS) and the low-resistance state (LRS) of the GeTe–Sb2Te3 (GST) superlattice.

We extended the conventional Douglas–Kroll (DK) and infinite order two-component (IOTC) methods to a technique applicable to Fock matrices, called extended DK (EDK) and extended IOTC (EIOTC), respectively. First, we defined a strategy to divide the Dirac–Fock operator into zero- and first-order terms. We then demonstrated that the first-order extended DK transformation, which is the Foldy–Wouthuysen transformation for the zero-order term, as well as the second- and third-order EDK and EIOTC, could be well defined. The EDK- and EIOTC-transformed Fock matrix, kinetic energy operator, nuclear attraction operator, and density matrix were derived. These equations were numerically evaluated, and it was found that these methods were accurate. In particular, EIOTC was consistent with the four-component approach. Four-component and extended two-component calculations are more expensive than non-relativistic calculations due to small-component-type two-electron integrals. We developed a new approximation formula, RIS-V, for small-component-type two-electron integrals, including the spin–orbit interaction between electrons. These results suggest that the RIS-V formula effectively accelerates the four-component and extended two-component methods.

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