A talk by P A Lee on SC and FM coexisting in oxide interface

http://videochannel.ust.hk/Watch.aspx?Section=Channels&Channel=2&SubType=All&View=Icon&Sort=Date&Page=1&Current=3&Mode=Play

Giant Electroresistance

There are many devices that are of fundamental interest. The most latest and famous may be the one invented by Fert et al., which displays giant magnetoresistance. In the past few years, there have appeared a flurry of work exploiting ferroelectrics as the barrier layer. I have mentioned quite a number of them in this blog. Here comes a new one. It is made up by sandwiching a BTO layer with two LSMO at different dopings. All these efforts are clearly directed to manipulate the interweaving properties of spin, charge and orbital degrees.

A giant tunneling electroresistance effect may be achieved in a ferroelectric tunnel junction by exploiting the magnetoelectric effect at the interface between the ferroelectric barrier and a magnetic La1 xSrxMnO3 electrode. Using first-principles density-functional theory we demonstrate that a few magnetic monolayers of La1 xSrxMnO3 near the interface act, in response to ferroelectric polarization
reversal, as an atomic-scale spin valve by filtering spin-dependent current. This produces more than an order of magnitude change in conductance, and thus constitutes a giant resistive switching effect. [PRL 106, 157203 (2011)]

2DEG switchable by electric field ?

Perovskite materials are cool as they frequently exhibit exotic properties and thus offer opportunities to fabricate new electronic components.

Here i talk about a perovskite-based interface structure that traps electrons within a few layers (2DEG). 2DEG has been the focus of extensive investigations for many years, examples concerning cuprate superconductors and transistors.

This structure consists of a NbO2 layer sanwitched by strontium STO on one end and KNO on the other. Electrons shall pool around that NbO2 sheet. As we know, the d orbitals on every Nb atom in bulk KNO are nominally empty. So does the pure NbO2 sheet. As one incorperates this sheet into that structure, due to electronic reconstruction that happens often at interfaces, the d orbitals shall be taken up by electrons, but only partially, which forms the so-called Hubbard layer. For partial filling, these electrons shall conduct electricity, with conductivity proportional to the electron density.

Now that KNO is a ferroelectric (STO is only incipient), one may wonder if the spontaneous polarization appearing in it shall affect the electron density and hence the conductivity. Yes, it is, as recently demonstrated by first-principles computations [1]. The physics is simple: the electric field produced by this polarization shall deplet or enrich electrons (screening effect), depending on the field direction, resembling what takes place to a conventional p-n jucntion in the presence of an ecternal electric field. Hence, by inverting the spontaneous polarization in KNO, one is able to switch the conduction states of the NbO2 layer.

For the moment, it may be interesting to see how this prediction will be confirmed experimentally and to undrstand the switch time required for the polarization reversal. Obviously, this time shall be crucial for applications.

[1]PRL, 103:016804(2009)