First native ferroelectric metal observed

Materials World magazine
,
7 Jul 2019
Ferroelectric domains in a WTe2 single crystal. Image credit: Fleet.

Australian researchers have documented the first observation of a native ferroelectric metal.

A study by the University of New South Wales (UNSW) showed a native metal with bistable and electrically switchable spontaneous polarisation states - the hallmark of ferroelectricity.

A ferromagnetic material is a 'magnet' with a north and south pole. Ferroelectric materials display permanent electric polarisation, which originates from electric dipoles consisting of equal and oppositely charged poles. In ferroelectric materials, these electric dipoles exist at the unit cell level and enable a non-vanishing permanent electric dipole moment.

Such materials are used in nano-electronic computer memory, RFID cards, medical ultrasound transducers, infrared cameras, submarine sonar, vibration and pressure sensors, and precision actuators.

In the study, bulk single-crystalline tungsten ditelluride (WTe2) was probed by spectroscopic electrical transport measurements, conductive-atomic force microscopy, and by piezo-response force microscopy (PFM) to map the polarisation, detecting lattice deformation because of an applied electric field.

Ferroelectric domains — the regions with oppositely oriented direction of polarisation — were seen in cleaved WTe2 crystals.

Spectroscopic-PFM measurements with a top electrode in a capacitor geometry were used to demonstrate the ferroelectric polarisation switching.

First-principles density functional theory (DFT) calculations at the University of Nebraska confirmed the findings.

This particular moment in such materials could be used as a gate for the underlying 2D electron system in an artificial topological insulator.

Compared to conventional semiconductors, the sub-nanometre proximity of a ferroelectric's electron dipole moment to the electron gas in the atomic crystal ensures more effective switching, overcoming limitations of conventional semiconductors where the conducting channel is buried tens of nanometres below the surface.

Study author Dr Pankaj Sharma said, ‘We found coexistence of native metallicity and ferroelectricity in bulk crystalline tungsten ditelluride (WTe2) at room temperature.

‘We demonstrated that the ferroelectric state is switchable under an external electrical bias and explain the mechanism for 'metallic ferroelectricity' in WTe2 through a systematic study of the crystal structure, electronic transport measurements and theoretical considerations.’

Dr Feixiang Xiang said, ‘A van der Waals material that is both metallic and ferroelectric in its bulk crystalline form at room temperature has potential for new nano-electronics applications’.

The study was published in Science Advances in July 2019.