Switching ferroelectricity in SnSe across diffusionless martensitic phase transition

Abstract

We experimentally investigate transport properties of a hybrid structure, which consists of a thin single crystal SnSe flake on a top of 5 µm spaced Au leads. The structure initially is in highly-conductive state, while it can be switched to low-conductive one at high currents due to the Joule heating of the sample, which should be identified as α-P nmaβ-Cmcm diffusionless martensitic phase transition in SnSe. For highly-conductive state, there is significant hysteresis in dI/dV (V ) curves at low biases, so the sample conductance depends on the sign of the applied bias change. This hysteretic behavior reflects slow relaxation due to additional polarization current in the ferroelectric SnSe phase, which we confirm by direct measurement of time-dependent relaxation curves. In contrast, we observe no noticeable relaxation or low-bias hysteresis for the quenched β-Cmcm low-conductive phase. Thus, ferroelectric behavior can be switched on or off in transport through hybrid SnSe structure by controllable α-P nmaβ-Cmcm phase transition. This result can also be important for nonvolatile memory development, e.g. phase change memory for neuromorphic computations or other applications in artificial intelligence and modern electronics.

Thermoelectric response as a tool to observe electrocaloric effect in a thin conducting ferroelectric SnSe flake

“…The possibility to induce polarization current by source-drain field variation has been shown for WTe2 as a direct consequence of ferroelectricity and metallic conductivity coexistence [28]. We have demonstrated qualitatively similar ferroelectric behavior of dV/dI(I) curves for thin SnSe flakes [23], as it is shortly shown in the present text as small hysteresis for V 1ω xx (I) in the upper inset to Fig. 3.…”

“…1 temperature gradient ∇T is measured between 80 µm spaced contacts C2 and C6, while the distance between heating contacts C1 and C4 is about 40 µm. Since the contact resistance exceeds the bulk SnSe value [23], Joule heating is mostly concentrated in the contact areas. Since two-pont resistance strongly depends on the dc bias in the upper inset to Fig.…”

“…Ferroelectric domains are much smaller than the contact size in our samples [22]: the domains are about 100 nm, the domain wall region is about 20-50 nm. In this case, any variation of the sourcedrain bias I dc affects ferroelectric polarization due to the domain wall shift for varying E ∼ I dc in-plane electric field [23]. It leads to the temperature variation because of the electrocaloric effect [13,14,29], so the sign of the temperature δT variation δT is determined by the direction of the electric field E.…”

“…Ultra-thin [23] SnSe flakes (about 100-200 nm) are obtained by regular mechanical exfoliation from the initial layered ingot. For electrical measurements the exfoliated flake is placed on the top of the pre-defined Au leads pattern, the pattern specifies experimental geometry for charge transport as it is depicted in Fig.…”

“…Even for small absolute values of the effect, corresponding temperature gradients should be detectable by in-situ thermoelectric response measurements [18][19][20]. Among different materials, layered SnSe single crystals can be convenient for these investigations: thin SnSe flakes (300 nm and below) are characterized by the in-plane ferroelectric polarization [22,23] in addition to significant thermoelectric properties [21].…”