Speaker:
Benjamin M. Hunt
Institution:
Carnegie Mellon University
Date:
Wednesday, August 25, 2021
Time:
11:00 am
Location:
Zoom Seminar
Abstract:
Ferroelectricity is the electrostatic counterpart to ferromagnetism; it occurs in materials that have a permanent electric dipole that can be switched by an external electric field. The many applications of ferroelectricity include sensors, actuators, high-permittivity dielectrics, and non-volatile memory devices. Reports of ferroelectricity in two-dimensional (2D) van der Waals materials have been fairly rare because of the set of required crystal symmetries that cause the materials to be polar, though some recent works have used the stacking-order control possible in van der Waals heterostructures to engineer switchable dipole moments. Still rarer are reports of the coexistence of ferroelectricity and metallic behavior, which are generally incompatible due to screening, at least in three dimensions. In my talk, I will discuss our experiments on semimetallic bilayer WTe2, which exhibits a switchable, out-of-plane electric polarization whose magnitude is gate-tunable. We use a layer-sensitive capacitance technique to directly measure the charge distribution on the two layers as the overall carrier density is varied. Our layer-resolved capacitance measurements agree remarkably well with a model of the bilayer WTe2 Hamiltonian, which allows us to reveal the roles of interlayer coupling and spin-orbit coupling in the layer distribution of the wavefunctions that determine the out-of-plane ferroelectric polarization. Both the capacitance-based measurement technique and the theoretical understanding are widely applicable to the emerging field of ferroelectricity in two-dimensional materials.
Ferroelectricity is the electrostatic counterpart to ferromagnetism; it occurs in materials that have a permanent electric dipole that can be switched by an external electric field. The many applications of ferroelectricity include sensors, actuators, high-permittivity dielectrics, and non-volatile memory devices. Reports of ferroelectricity in two-dimensional (2D) van der Waals materials have been fairly rare because of the set of required crystal symmetries that cause the materials to be polar, though some recent works have used the stacking-order control possible in van der Waals heterostructures to engineer switchable dipole moments. Still rarer are reports of the coexistence of ferroelectricity and metallic behavior, which are generally incompatible due to screening, at least in three dimensions. In my talk, I will discuss our experiments on semimetallic bilayer WTe2, which exhibits a switchable, out-of-plane electric polarization whose magnitude is gate-tunable. We use a layer-sensitive capacitance technique to directly measure the charge distribution on the two layers as the overall carrier density is varied. Our layer-resolved capacitance measurements agree remarkably well with a model of the bilayer WTe2 Hamiltonian, which allows us to reveal the roles of interlayer coupling and spin-orbit coupling in the layer distribution of the wavefunctions that determine the out-of-plane ferroelectric polarization. Both the capacitance-based measurement technique and the theoretical understanding are widely applicable to the emerging field of ferroelectricity in two-dimensional materials.
Host:
Javier Sanchez-Yamagishi