Tuning domain wall conductivity in bulk lithium niobate by uniaxial stress

Conductive domain walls (DWs) in insulating ferroelectrics have recently attracted considerable attention due to their unique topological, optical, and electronic properties, and offer potential applications such as in memory devices or rewritable circuitry. The electronic properties of DWs can be tuned by the application of strain, hence controlling the charge carrier density at DWs. In this paper, we study the influence of uniaxial stress on the conductivity of DWs in the bulk single crystal lithium niobate $\left({\mathrm{LiNbO}}_3\right)$. Using conductive atomic force microscopy, we observe a large asymmetry in the conductivity of DWs, where only negatively screened walls, so called head-to-head DWs, are becoming increasingly conductive, while positively screened, tail-to-tails DWs, show a decrease in conductivity. This asymmetry of DW conductivity agrees with our theoretical model based on the piezoelectric effect. In addition, we observed that the current in the DW increases up to an order of magnitude for smaller compressive stresses of 100 MPa. This response of DWs remained intact for multiple stress cycles over two months, opening a path for future applications.

Figure 1

Sketch: (a) Neutral domain walls in a ferroelectric, where red arrows are representing the spontaneous polarization parallel to domain walls. (b) Charged domain walls: Spontaneous polarizations meet head-to-head (h2h) and tail-to-tail (t2t) perpendicular to domain walls. (c) Neutral domain walls but charged due to induced polarization, represented by yellow arrows, when stress is applied to a crystal. Charges shown in the images are screening charge carriers. (d) Piece of single crystal Z-cut ${\mathrm{LiNbO}}_3$ containing a hexagonal domain. (e) Two different ${\mathrm{LiNbO}}_3$ samples cut along different axes from the parent crystal in image (d). The samples are cut such that stress can be applied to the crystallographic $x$ and $y$ axes.

Figure 2

(a) Three-dimensional sketch of the uniaxial stress cell. (b) Zoom-in image of the stress cell: Z-cut 5% MgO-doped ${\mathrm{LiNbO}}_3$ (sample: LNO-01) mounted on the cell with 2850FT epoxy. (c) Sketch of the setup for cAFM measurement with applied stress. Domains and domain walls are electrically connected by a 20-nm-thick chromium electrode at the $-z$ side and grounded by the cantilever at the $+z$ side.

Figure 3

Projection of induced polarization ($P_j$ = $d_{jkl}{X}_{kl}$) at DWs along different axes at zero external electric field when ${\mathrm{LiNbO}}_3$ is stressed along (a) $x$ axis and (b) $y$ axis, where for ${\mathrm{LiNbO}}_3$, $d_{22}=20.8\times {10}^{-12}$ C/N and $d_{31}=-0.863\times {10}^{-12}$ C/N at room temperature [32]. The sign of stress should be negative for compression and positive for tension. Each sketch belongs to the respective column above depicts directions of induced polarization at DWs for compression configuration. The directions should be reversed for tension. Charges shown at DWs are screening charges, which are responsible for the conductivity in DWs (detailed calculations are given in the Supplemental Material [30]).

Figure 4

(a) Stitched PFM phase image of LNO-01 sample ($x$ compression), arrows on top left shows crystallographic axes. (b) Current distribution in DWs under compressive stress according to a model based on direct piezoelectricity. (c) SHG image of LNO sample. (d) Current distribution of DWs under tensile stress according to a model based on direct piezoelectricity. Stitched cAFM image of DWs: (e) at 0 MPa stress, (f) at $-129.0$ MPa compressive stress, (g) at 0 MPa after compression, and (h) at 64.5 MPa tensile stress. The DWs in (f) and (h) show expected response as in sketches above them, (b) and (d), respectively.

Figure 5

Change in current with stress along the line profiles A–C taken from different sections of DWs of sample LNO-01($x$ compression), provided in Figs. 4 and 4.

Figure 6

(a) Stitched PFM phase image of LNO-02 sample ($y$ compression). Arrows on top left show crystallographic axes. (b) Current distribution in DWs under compressive stress according to a model based on direct piezoelectricity predicts opposite response to the sample LNO-01. Stitched cAFM image of DWs: (c) at 0 MPa stress, (d) at $-300.6$ MPa compressive stress.