Symmetry Breakdown in Franckeite: Spontaneous Strain, Rippling, and Interlayer Moire

Franckeite is a naturally occurring layered mineral with a structure composed of alternating stacks of SnS2-like and PbS-like layers. Although this superlattice is composed of a sequence of isotropic two-dimensional layers, it exhibits a spontaneous rippling that makes the material structurally anisotropic. We demonstrate that this rippling comes hand in hand with an inhomogeneous in-plane strain profile and anisotropic electrical, vibrational, and optical properties. We argue that this symmetry breakdown results from a spatial modulation of the van der Waals interaction between layers due to the SnS2-like and PbS-like lattices incommensurability

Electrical half-wave rectification at ferroelectric domain walls

Ferroelectric domain walls represent multifunctional 2D-elements with great potential for novel device paradigms at the nanoscale. Improper ferroelectrics display particularly promising types of domain walls, which, due to their unique robustness, are the ideal template for imposing specific electronic behavior. Chemical doping, for instance, induces p- or n-type characteristics and electric fields reversibly switch between resistive and conductive domain-wall states. Here, we demonstrate diode-like conversion of alternating-current (AC) into direct-current (DC) output based on neutral 180$^{\circ}$ domain walls in improper ferroelectric ErMnO$_3$. By combining scanning probe and dielectric spectroscopy, we show that the rectification occurs for frequencies at which the domain walls are fixed to their equilibrium position. The practical frequency regime and magnitude of the output is controlled by the bulk conductivity. Using density functional theory we attribute the transport behavior at the neutral walls to an accumulation of oxygen defects. Our study reveals domain walls acting as 2D half-wave rectifiers, extending domain-wall-based nanoelectronic applications into the realm of AC technology

In-plane anisotropic optical and mechanical properties of two-dimensional MoO3

Molybdenum trioxide (MoO3) in-plane anisotropy has increasingly attracted the attention of the scientific community in the last few years. Many of the observed in-plane anisotropic properties stem from the anisotropic refractive index and elastic constants of the material but a comprehensive analysis of these fundamental properties is still lacking. Here we employ Raman and micro-reflectance measurements, using polarized light, to determine the angular dependence of the refractive index of thin MoO3 flakes and we study the directional dependence of the MoO3 Young’s modulus using the buckling metrology method. We found that MoO3 displays one of the largest in-plane anisotropic mechanical properties reported for 2D materials so far

Strongly Anisotropic Strain-Tunability of Excitons in Exfoliated ZrSe3

The effect of uniaxial strain on the band structure of ZrSe3, a semiconducting material with a marked in-plane structural anisotropy, is studied. By using a modified three-point bending test apparatus, thin ZrSe3 flakes are subjected to uniaxial strain along different crystalline orientations monitoring the effect of strain on their optical properties through micro-reflectance spectroscopy. The obtained spectra show excitonic features that blueshift upon uniaxial tension. This shift is strongly dependent on the direction along which the strain is being applied. When the flakes are strained along the b-axis, the exciton peak shifts at ≈60–95 meV %−1, while along the a-axis, the shift only reaches ≈0–15 meV %−1. Ab initio calculations are conducted to study the influence of uniaxial strain, applied along different crystal directions, on the band structure and reflectance spectra of ZrSe3, exhibiting a remarkable agreement with the experimental results

Anisotropic magnetotransport in SrTiO_3 surface electron gases generated by Ar^+ irradiation

Metallic surface layers are fabricated by doping (100) SrTiO_3 (STO) single crystals with oxygen vacancies generated by bombardment with Ar ions from an rf plasma source. The presence of oxygen vacancies is confirmed by cathodoluminescence and x-ray photoemission spectroscopy. This technique produces a surface electron gas with high values of the sheet carrier density (n_(2D) = 2.45x10^17 cm^(-2)). A strong increase (300%) of the low-temperature magnetoresistance is observed when the magnetic field is rotated away from the surface, characteristic of orbital effects of confined electrons. We estimate the width of the confinement region to be in the 200-300 nm range. When a magnetic field is applied in the surface plane and parallel to the current direction, a large negative magnetoresistance is found below the structural transition of the STO, which is discussed in terms of spin-orbit scattering. On further reduction of temperature, there is a change to a positive magnetoresistance regime due to the scattering of charge carriers at the disordered surface region

Paving the way to nanoionics: atomic origin of barriers for ionic transport through interfaces

The blocking of ion transport at interfaces strongly limits the performance of electrochemical nanodevices for energy applications. The barrier is believed to arise from space-charge regions generated by mobile ions by analogy to semiconductor junctions. Here we show that something different is at play by studying ion transport in a bicrystal of yttria (9% mol) stabilized zirconia (YSZ), an emblematic oxide ion conductor. Aberration-corrected scanning transmission electron microscopy (STEM) provides structure and composition at atomic resolution, with the sensitivity to directly reveal the oxygen ion profile. We find that Y segregates to the grain boundary at Zr sites, together with a depletion of oxygen that is confined to a small length scale of around 0.5 nm. Contrary to the main thesis of the space-charge model, there exists no evidence of a long-range O vacancy depletion layer. Combining ion transport measurements across a single grain boundary by nanoscale electrochemical strain microscopy (ESM), broadband dielectric spectroscopy measurements, and density functional calculations, we show that grain-boundary-induced electronic states act as acceptors, resulting in a negatively charged core. Besides the possible effect of the modified chemical bonding, this negative charge gives rise to an additional barrier for ion transport at the grainboundary.Fil: Frechero, Marisa Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto de Química del Sur; Argentina. Universidad Complutense de Madrid; EspañaFil: Rocci, M.. Universidad Complutense de Madrid; EspañaFil: Sanchez Santolino, G.. Universidad Complutense de Madrid; EspañaFil: Kumar, Amit. Oak Ridge National Laboratory. Center for Nanophase Materials Sciences; Estados UnidosFil: Salafranca, Juan. Universidad Complutense de Madrid; EspañaFil: Schmidt, Rainer. Universidad Complutense de Madrid; EspañaFil: Diaz Guillen, M.R.. Universidad Complutense de Madrid; EspañaFil: Durá, O. J.. Universidad Complutense de Madrid; EspañaFil: RiveraCalzada, A.. Universidad Complutense de Madrid; EspañaFil: Mishra, R.. Vanderbilt University; Estados UnidosFil: Jesse, Stephen. Oak Ridge National Laboratory. Center for Nanophase Materials Sciences; Estados UnidosFil: Pantelides, S.T.. Vanderbilt University; Estados UnidosFil: Kalinin, Sergei. Oak Ridge National Laboratory. Center for Nanophase Materials Sciences; Estados UnidosFil: Varela, M.. Universidad Complutense de Madrid; EspañaFil: Pennycook, Steve. University Of Tennessee; Estados UnidosFil: Santamaria, J.. Universidad Complutense de Madrid; EspañaFil: Leon, C.. Universidad Complutense de Madrid; Españ