Recent theoretical predictions of ferroelectricity in two-dimensional (2D) van der Waals materials reveal exciting possibilities for their use in scalable low-power electronic devices with polarization-dependent functionalities. These prospects have been further invigorated by the experimental evidence of the polarization response in some transition metal chalcogenides (TMCs)—a group of narrow-band semiconductors and semimetals with a wealth of application potential. Among the TMCs, molybdenum disulfide (MoS2) is known as one of the most promising and robust 2D electronic materials. However, in spite of theoretical predictions, no ferroelectricity has been experimentally detected in MoS2, while the emergence of this property could enhance its potential for electronics applications. Here, we report the experimental observation of a stable room-temperature out-of-plane polarization ordering in 2D MoS2 layers, where polarization switching is realized by mechanical pressure induced by a tip of a scanning probe microscope. Using this approach, we create the bi-domain polarization states, which exhibit different piezoelectric activity, second harmonic generation, surface potential, and conductivity. Ferroelectric MoS2 belongs to the distorted trigonal structural 1T” phase, where a spontaneous polarization is inferred by its P3m1 space-group symmetry and corroborated by theoretical modeling. Experiments on the flipped flakes reveal that the 1T”-MoS2 samples consist of the monolayers with randomly alternating polarization orientation, which form stable but switchable “antipolar” head-to-head or tail-to-tail dipole configurations. Mechanically written domains are remarkably stable facilitating the application of 1T”-MoS2 in flexible memory and electromechanical devices.

Application of conducting ferroelectric domain walls (DW) as functional elements may facilitate development of conceptually new resistive switching devices. In a conventional approach, several orders of magnitude change in resistance can be achieved by controlling the DWs density using super-coercive voltage. However, a deleterious characteristic of this approach is high-energy cost of polarization reversal due to high leakage current. Here, we demonstrate a new approach based on tuning the conductivity of DWs themselves rather than on domain rearrangement. Using LiNbO3 capacitors with graphene, we show that resistance of a device set to a polydomain state can be continuously tuned by application of sub-coercive voltage. The tuning mechanism is based on the reversible transition between the conducting and insulating states of DWs.The developed approach allows an energy-efficient control of resistance without the need for domain structure modification. The developed memristive devices are promising for multi-level memories and neuromorphic computing applications.

Strain engineering is a powerful strategy to control the physical properties of material-enabling devices with enhanced functionality and improved performance. Here, we investigate a modulation of the transport behavior of the two-dimensional MoS2 junctions under the mechanical stress induced by a tip of an atomic force microscope (AFM). We show that the junction resistance can be reversibly tuned by up to 4 orders of magnitude by altering a tip-induced force. Analysis of the stress-induced evolution of the I–V characteristics indicates a combined effect of the tip-induced strain and strain gradient on the energy barrier height and profile. In addition, we show that the tip-generated flexoelectric effect leads to significant enhancement of the photovoltaic effect in the MoS2 junctions. A combination of the optical and mechanical stimuli facilitates reversible photomechanical tuning of resistance of the narrow-band 2D semiconductors and development of devices with an enhanced photovoltaic response.

Pairing two-dimensional semiconductors with ferroelectric films may allow for the development of hybrid electronic devices that would not only exhibit a combination of the functional properties of both material groups but would also reveal unusual characteristics emerging from coupling between these properties. Here, we report the observation of a considerable (up to 103 at 0.8 V read bias) polarization-mediated tunneling electroresistance (TER) effect in Hf0.5Zr0.5O2 (HZO) ferroelectric tunnel junctions (FTJs) employing MoS2 as one of the electrodes. It was found that for this type of hybrid FTJs, a change in resistance upon polarization reversal could be described by Fowler–Nordheim tunneling. The underlying mechanism for the enhanced TER effect is a polarization-mediated accumulation or depletion of the majority carriers at the MoS2/HZO interface, which results in a change in the effective barrier shape seen by the tunneling electrons. Given the compatibility of HfO2-family ferroelectrics with CMOS technology and a possibility of large scale growth and transfer of MoS2 films, our results provide a pathway for fabrication of high-density nonvolatile memory and data storage systems based on hybrid FTJs.

A strong polaron pairing model of high-temperature cuprate superconductors is presented. The normal and anomalous one-particle Green’s functions are derived from a system with strong electron-phonon coupling. Self-consistent equation for the superconducting order parameter (Δ) is derived using Green’s function technique and following Lang and Firsov transformations. Expressions for specific heat, density of states, free energy, and critical field based on this model have been derived. The theory is applied to explain the experimental results in the systemYBa2Cu3O7-x. There is convincing evidence that the theory is fully compatible with the key experiments.