Controlling many-body states by the electric-field effect in a two-dimensional material

Abstract

To be able to control the properties of a system that has strong electron–electron interactions using only an external electric field would be ideal, but the material must be thin enough to avoid shielding of the electric field in the bulk material; here pure electric-field control of the charge-density wave and superconductivity transition temperatures is achieved by electrolyte gating through an electric-field double layer transistor in the two-dimensional material 1T-TiSe2. The properties of a system with strong electron–electron interactions are ideally studied using only an external electric field, but this is only effective if the material is thin enough to avoid the shielding effect of the bulk material. Various fabrication techniques have been developed in recent years to produce ultrathin, two-dimensional forms of electronic materials. Antonoi Castro-Neto and colleagues use one such method to study the layered transition-metal dichalcogenide 1T-TiSe2 in the form of a flake less than 10 nanometres thick and encapsulated between hexagonal boron nitride. By varying the electric field, magnetic field and temperature, they reveal details about the transition between different electronic phases, such as a correlation between the existence of superconductivity and appearance of spatially modulated electronic states. To understand the complex physics of a system with strong electron–electron interactions, the ideal is to control and monitor its properties while tuning an external electric field applied to the system (the electric-field effect). Indeed, complete electric-field control of many-body states in strongly correlated electron systems is fundamental to the next generation of condensed matter research and devices1,2,3. However, the material must be thin enough to avoid shielding of the electric field in the bulk material. Two-dimensional materials do not experience electrical screening, and their charge-carrier density can be controlled by gating. Octahedral titanium diselenide (1T-TiSe2) is a prototypical two-dimensional material that reveals a charge-density wave (CDW) and superconductivity in its phase diagram4, presenting several similarities with other layered systems such as copper oxides5, iron pnictides6, and crystals of rare-earth elements and actinide atoms7. By studying 1T-TiSe2 single crystals with thicknesses of 10 nanometres or less, encapsulated in two-dimensional layers of hexagonal boron nitride, we achieve unprecedented control over the CDW transition temperature (tuned from 170 kelvin to 40 kelvin), and over the superconductivity transition temperature (tuned from a quantum critical point at 0 kelvin up to 3 kelvin). Electrically driving TiSe2 over different ordered electronic phases allows us to study the details of the phase transitions between many-body states. Observations of periodic oscillations of magnetoresistance induced by the Little–Parks effect show that the appearance of superconductivity is directly correlated with the spatial texturing of the amplitude and phase of the superconductivity order parameter, corresponding to a two-dimensional matrix of superconductivity. We infer that this superconductivity matrix is supported by a matrix of incommensurate CDW states embedded in the commensurate CDW states. Our results show that spatially modulated electronic states are fundamental to the appearance of two-dimensional superconductivity.