Enhancing the Domain Wall Conductivity in Lithium Niobate Single Crystals

Abstract

Domain walls (DWs) in ferroelectric/ferroic materials have been a central research focus for the last 50 years; DWs bear a multitude of extraordinary physical parameters within a unit-cell-sized lateral confinement. Especially, one outstanding feature has recently attracted a lot of attention for room-temperature applications, which is the potential to use DWs as two-dimensional (2D) conducting channels that completely penetrate bulk compounds. Domain wall currents in lithium niobate (LNO) so far lie in the lower pA regime. In this work, we report on an easy-to-use and reliable protocol that allows enhancing domain wall conductivity (DWC) in single-crystalline LNO (sc-LNO) by 3 to 4 orders of magnitude. sc-LNO thus has become one of the most prospective candidates to engineer DWC applications, notably for domain wall transport both with and without photoexcitation. DWs were investigated here for several days to weeks, both before and after DWC enhancement. 2D local-scale inspections were carried out using adequate local-probe techniques, i.e., piezoresponse force microscopy and conductive atomic force microscopy, while Cerenkov second-harmonic generation was applied for mapping the DW constitution in three-dimensional space across the full LNO single crystal. The comparison between these nano- and microscale inspections allows us to unambiguously correlate the DW inclination angle α close to the sample surface to the measured domain wall current distribution. Moreover, ohmic or diode-like electronic transport characteristics along such DWs can be readily interpreted when analyzing the DW inclination profile.

Resistor Network Modeling of Conductive Domain Walls in Lithium Niobate

“…Note that only cases (a) and (b) are treated in this paper, because they match to the experiments of Godau et al [17] For these two cases (a) and (b), a linear potential drop across the sample was assumed, with the potential distribution normalized to the arbitary value V c = 10. In order to choose appropriate numerical parameters, the stability of the model was firstly evaluated by checking against several parameters while using the dataset of the DW after tuning.…”

“…[1] Although DWs measure just a few unit cells in width [2][3][4][5] they exhibit completely different physical properties as compared to the material's bulk, [6] such as altered dielectric, [7] optical, [8] magnetic, [9] and also conductive properties. [27] The 2D-RN-DWC-modeling used the 3D distribution of inclination angles α of one single DW as an input, as measured by Godau et al [17] [see Figure 1b]. [27] The 2D-RN-DWC-modeling used the 3D distribution of inclination angles α of one single DW as an input, as measured by Godau et al [17] [see Figure 1b].…”

“…DWs thus are very promising candidates for both the bottom-up assembly of functional nanoelectronic broad variety of different electronic transport scenarios, such as topological Kondo-Insulators, [21] manganite thin films upon Mott phase transitions, [22] as well as the currents flowing in disordered semiconductors, [23] in composite materials, [24] or in low-dimensional nanostructures [25] like graphene [26] or singlestranded DNA molecules. [17,28,29]. Details on how to prepare and analyze such a single, highly conductive DW can be found in refs.…”

“…[30,31] In the present work, these two data sets of DW inclination angles (before and after sample tuning) were used as the input for the RN modeling. [19] The data in Figure 2a,b is displayed in a so-called "developed" or "world-map" view, unrolling the quasi hexagonal [17]). For simplicity all simulations presented in this work were carried out on a square lattice; a hexagonal lattice was tested as well for comparison reasons yielding the same results for sufficiently large N. The result of the N = 200 interpolation is illustrated in Figure 2a,b before and after DWC tuning, respectively.…”

“…This so-called domain wall conductivity (DWC) has been reported first for multiferroic bismuth ferrite [11] and lead-zirconate-titanate [12] thin films, followed by research on (improper) erbium manganite [13] and barium titanate [14] bulk ferroelectric crystals. [15,16] However, recent experiments by Godau et al [17] on LNO single crystals (sc) have shown that reshaping the DW to larger inclination angles by applying a dedicated electrical tuning protocol enhances DWC by 3-4 orders of magnitude, and gives rise to DW currents in the upper µA range at room temperature and in the dark.Theoretical approaches applied to explain the increased DWC in these materials include phenomenological Landau [18] and Landau-Ginzburg-Devonshire theory, [19] and more recently also the combination of quantum mechanics with phenomenological Landau theory. [15,16] However, recent experiments by Godau et al [17] on LNO single crystals (sc) have shown that reshaping the DW to larger inclination angles by applying a dedicated electrical tuning protocol enhances DWC by 3-4 orders of magnitude, and gives rise to DW currents in the upper µA range at room temperature and in the dark.…”