Colossal flexoresistance in dielectrics

ARTICLE https://doi.org/10.1038/s41467-020-16207-7 OPEN Colossal flexoresistance in dielectrics 1234567890():,; Sung Min Park1,2, Bo Wang 3, Tula Paudel 4, Se Young Park1,2,5, Saikat Das1,2, Jeong Rae Kim1,2, Eun Kyo Ko1,2, Han Gyeol Lee1,2, Nahee Park6, Lingling Tao4, Dongseok Suh 6, Evgeny Y. Tsymbal Long-Qing Chen3, Tae Won Noh 1,2 ✉ & Daesu Lee 7,8 ✉ 4, Dielectrics have long been considered as unsuitable for pure electrical switches; under weak electric fields, they show extremely low conductivity, whereas under strong fields, they suffer from irreversible damage. Here, we show that flexoelectricity enables damage-free exposure of dielectrics to strong electric fields, leading to reversible switching between electrical states —insulating and conducting. Applying strain gradients with an atomic force microscope tip polarizes an ultrathin film of an archetypal dielectric SrTiO3 via flexoelectricity, which in turn generates non-destructive, strong electrostatic fields. When the applied strain gradient exceeds a certain value, SrTiO3 suddenly becomes highly conductive, yielding at least around a 108-fold decrease in room-temperature resistivity. We explain this phenomenon, which we call the colossal flexoresistance, based on the abrupt increase in the tunneling conductance of ultrathin SrTiO3 under strain gradients. Our work extends the scope of electrical control in solids, and inspires further exploration of dielectric responses to strong electromechanical fields. 1 Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 08826, Korea. 2 Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea. 3 Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA. 4 Department of Physics and Astronomy & Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588, USA. 5 Department of Physics, Soongsil University, Seoul 07027, Korea. 6 Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea. 7 Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea. 8 Asia Pacific Center for Theoretical Physics, Pohang 37673, Korea. ✉email: ; NATURE COMMUNICATIONS | (2020)11:2586 | https://doi.org/10.1038/s41467-020-16207-7 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-16207-7 Results Concept of flexoelectric control of electrical states in dielectrics. To achieve intrinsic, static control of electrical states in dielectrics, we could utilize a non-destructive electrostatic field developed in ultrathin polar materials (Fig. 1a). When a polar material is sufficiently thin but still maintains polarization P, a depolarization field Edep arises from the unscreened bound charges on its surface7,8: Edep ¼  P  σS ; ε ð1Þ where σS is the screening charge (e.g., by adjacent metal a Δ Δbg P ¼ ε  feff  ð2Þ Colossal flexoresistance in an archetypal dielectric SrTiO3. We choose SrTiO3 (STO) as a model dielectric system, as it shows enhanced flexocoupling strength at the nanoscale19, as well as a reasonably large bandgap of 3.2 eV. Importantly, furthermore, its conductivity responds negligibly to the applied strain itself (Supplementary Fig. 3)23,24, thereby maximizing the contribution from strain gradient-induced flexoelectricity. We prepare 10-unitcell-thick (i.e., 3.9-nm thick) stoichiometric STO films on a (001)oriented STO single crystal substrate, with a conductive SrRuO3 buffer layer (Supplementary Figs. 4 and 5). The stoichiometric homoepitaxial STO should remain paraelectric down to 0 K b Δbg 100 10–2 10–4 10–6 10–8 Valence band ∂u ; ∂x where ∂u/∂x and feff are the strain gradient and effective flexocoupling coefficient, respectively. Applying loading forces through an atomic force microscope (AFM) tip (Fig. 2a) generates strain gradients as large as 107 m−1 in ultrathin dielectrics13,17–19. Such giant strain gradient could then induce flexoelectric polarization, up to a few 0.1 Cm−2 (ref. 19), much larger than the polarization values typically attainable in ultrathin ferroelectrics21,22. P Conduction band 0 electrodes) and ε is the dielectric permittivity of the polar material. In the ultrathin limit, σS tends to zero8 and Edep becomes increasingly saturated at Edep = −P/ε, largely modifying the band structure (Fig. 1a). In particular, when the polarization exceeds a certain threshold, both the conduction band minimum and valence band maximum could cross the Fermi level, as confirmed in our first-principles calculation (Supplementary Fig. 1). In such a case, the tunnel-barrier width of ultrathin dielectrics would abruptly decrease, whereas the tunnel-barrier height remains fixed to the bandgap Δbg (Fig. 1a and Supplementary Fig. 2). This would result in a significant enhancement of tunneling conductance across ultrathin dielectrics, leading to a colossal decrease in electrical resistance, as predicted in our Wentzel–Kramers–Brillouin (WKB) simulation (Fig. 1b). Therefore, it would be of great interest to explore tunnel transport across a highly polarized ultrathin dielectric. To this end, we can induce and stabilize large polarization in an ultrathin dielectric via flexoelectricity9–20. All dielectric materials polarize in response to strain gradients, as follows: R (a.u.) C ontrolling electron dynamics in solids has opened avenues for fascinating physical phenomena1–3 and has formed the basis of electronic applications. In semiconductors with a relatively small but nonzero bandgap, applying moderate electric fields could switch their electrical state, i.e., from insulator to conductor, which makes them a building block for contemporary digital electronics. In dielectrics with a large bandgap, controlling their electrical states is quite complicated, as it usually involves a combination of intrinsic and extrinsic processes. Zener4 predicted that strong electric fields (≥109 V m−1) could intrinsically lead to electrical breakdown in dielectrics through tunneling processes across the valence and conduction bands. As this dielectric breakdown naturally guarantees the largest and fastest electrical response, recent works have aimed to realize it by applying strong femtosecond fields1,2. Under strong static fields, however, the dielectric breakdown has been unavoidably subject to extrinsic effects5,6, such as Joule heating and irreversible damage. This situation complicates our understanding of the intrinsic mechanism of dielectric breakdown and limits device application. Here, we demonstrate that electrical states in dielectrics can be controlled by means of depolarization field induced by flexoelectric polarization. By applying the strain gradients from a conductive scanning probe tip, we simultaneously polarize and measure the current across the film. Above the certain critical strain gradients, the current–voltage (I–V) characteristic chang (...truncated)