Ferroelectricity modulates polaronic coupling at multiferroic interfaces

Soft X-ray angle-resolved photoelectron spectroscopy is employed here to record the band structure of buried multiferroic interfaces where ferroelectric-induced charge modulation propagates into tunable electron-phonon interaction.
Published in Physics
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Artificial multiferroic interfaces can be prepared by joining materials with different ferroic orders (ferromagnetic, ferroelastic, ferroelectric). Such design create the opportunity to enrich the functionality of the interface by the coupling of the relevant order parameters from the two materials or even by stabilizing completely new order parameters, thus new functionality.

For example, in the case of the interface between a metal (ferromagnetic or not) and a ferroelectric material, the stabilization of the ferroelectric state (i.e, with electric dipoles oriented in a well-defined direction) often induces a charge modulation (accumulation or depletion)  into the joining metal [1,2] or different occupation of electronic orbitals propagating into the metallic electrode [3] with consequences on, for example, their spin polarization [4].

In  Ferroelectricity modulates polaronic coupling at multiferroic interfaces, Commun Phys 5, 209 (2022) we took a step forward to understanding how we can design new interface functionality by exploring the impact of ferroelectric-induced charge modulation on the electronic structure of the joining metal.

We focused on a prototypical oxide, La1-xSrxMnO3 (LSMO) which has mixed electron and hole conduction and is featured by metallic conductivity when ideally doped around x=0.3. LSMO used as a bottom electrode was covered with thin (~3nm) ferroelectric layers (PbZrTiO3 and BaTiO3) and we recorded the band structure of LSMO by probing the interface through the ferroelectric layer. 

For the success of this pioneering experiment, carried out at ADRESS beamline from the Swiss Light Source synchrotron radiation facility in Switzerland, the large probing depth of the soft X-ray angle resolved photoelectron spectroscopy (SX-ARPES) is of critical importance. Unlike conventional ARPES experiments performed with UV radiation which are extremely surface sensitive, the excitation source in the soft X-ray range (hundreds of eV) generates photoelectrons with enough kinetic energy to escape the capping layer and reveal the intrinsic band structure of the interface.

By comparing the band dispersions with the reference ones obtained on bare LSMO film, we were able to separate the contribution of the electrons and holes at the total induced charge modulation.  More exactly, we identified the gradual electron accumulation accompanied by hole depletion as a function of the strength of the ferroelectric polarization which occurs as a requirement to compensate for the depolarizing field in the thin ferroelectric layer. Figure 1 shows how the different ferroelectric polarizations modify the Fermi energy of LSMO, resulting in electron accumulation/(depletion) and hole depletion/(accumulation) in response to the P-/(P+) ferroelectric state of the top layer. 

Separating the distinct signature of the electrons and hole bands, which manifest in different regions of the k-space is particularly challenging for 3D materials such as LSMO. Here, this was possible due to high resolution in the kz direction of the reciprocal space, which is another consequence of the using soft X-rays as excitation source.

graphical description
Ferroelectric-dependent modification of carrier concentration and of the polaronic coupling at a multiferroic interface, resulting in weaker electron-phonon interaction and smaller effective mass

We turned then our attention to what are the consequences of such charge modulation on the electronic properties of LSMO. It is well established that the charge carriers (electrons and holes) in LSMO are heavily coupled with the lattice vibrations in the form of large polarons. These quasiparticles can intuitively be imagined as the charge carrier moving through the crystal, accompanied by the lattice distortion which it generates due to the interaction with the phonons. For the electron or hole, dragging of the phonon cloud while moving across the crystal feels like an increase of the effective mass, m*, fundamentally limiting their mobility.

In the experimental electronic band structure, the signature of the electron-phonon coupling at low and intermediate coupling strengths is seen as deviations from the parabolic dispersions at the phonon frequency in the form of "kinks". By analyzing such deviations, we were able to show that, compared to LSMO surface, during the interface formation with the ferroelectric pointing downwards, the effective mass of both the electrons and holes decreases. Since at the interface the hole density decreases, accompanied by electron accumulation, this means that the latter are involved in better screening of the electron (and hole) - phonon interaction. Thus the consequence is the decrease of the polaronic coupling.

Although this hypothesis still awaits experimental demonstration, we predict that the ferroelectric state can be effectively used to tune the mobility of interfaces, thus enriching the functionality of multiferroic interfaces

References

[1]  D. G. Popescu et al. Impact on Ferroelectricity and Band Alignment of Gradually Grown Au on BaTiO3, Phys. Status. Solidi - RRL 13, 1900077 (2019)  

[2] D. G. Popescu et al. The interplay of work function and polarization state at the Schottky
barriers height for Cu/BaTiO3 interface,
Appl. Surf. Sci. 502, 144101 (2020)

[3] D. Preziosi et al. Electric-Field Control of the Orbital Occupancy and Magnetic Moment
of a Transition-Metal Oxide, Phys. Rev. Lett. 115, 157401 (2015)

[4] C. A. F. Vaz et al. Origin of the Magnetoelectric Coupling Effect in Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 Multiferroic Heterostructures, Phys. Rev. Lett 104, 127202 (2010)

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