Field effects

The electric field effect is a remarkably simple and enormously successful approach that provides new opportunities for basic science and innovative device applications. In this approach, an electrostatic field is used to modulate the charge density and hence the conductivity of a transport medium. The realization that field effects can be employed to modify the electronic properties of materials underpins the ubiquitous role of semiconductors in current information technology; there are ~10^18 field effect transistors (FETs) fabricated each year. An extension of this approach is to employ field effects to control the charge density of correlated oxides, allowing one to modify their electronic and magnetic properties in a reversible fashion [1]. An illustration of the amplitude and range of behavior accessed by field effects is given in Fig. 1(b). Two examples of the application of these concepts are given below.

Fig. 1: (a) 80 nm ferroelectric domains written in a PZT thin film using piezoelectric scanning probe microscopy. (b)  Illustration of the diverse behavior displayed by correlated materials as a function of sheet charge density. From Ref. [1]. (c) Resistivity as a function of temperature for a PZT/ 3.8 nm LSMO heterostructure for the two polarization states of PZT. From Ref. [4].

One striking manifestation of the field effect in correlated oxides involves high temperature superconductors that are subjected to the electric polarization field of a ferroelectric oxide (e.g., PbZrxTi1-xO3). Ferroelectrics can possess spontaneous electric fields that are more than an order of magnitude larger than the breakdown field of SiO2, the most widely used insulating medium for field effect devices. The use of the ferroelectric polarization field thus allows for extremely large modulation of the charge carrier density and electronic properties of transport media, including switching between superconducting and insulating behavior in the high temperature superconductors [2,3]. This approach demonstrates the potential of the field effect to control and manipulate complex behavior in a reversible fashion.

Another example of the application of the field effect involves the modulation of the electric charge density in colossal magnetoresistive (CMR) materials, (e.g., La1-xSrxMnO3 (LSMO)), which are magnetic oxides where the conductivity depends sensitively on externally applied magnetic fields.  In these materials, the competition between interactions, such as strong Coulomb repulsion and electron-phonon coupling, leads to strong correlations between the structural, transport, and magnetic properties that characterize the colossal effects.  The effect of charge density has usually been studied via chemical doping, which has the disadvantage that the effects of chemical disorder cannot be easily disentangled from intrinsic charge correlations. The field effect approach provides a unique tool to study the effect of charge density on the properties of these materials, independent of the influence of structural modifications, changes in disorder, and modification of the band structure that accompany chemical doping. By engineering field effect heterostructures containing CMR materials, we have modulated the magnetic properties of CMR oxides, and in optimized samples, we have achieved reversible switching between metallic and insulating behavior, [Fig. 1(c)] [4]. 

[1] Ahn et al, Nature 424: 1015, 2003
[2] Gariglio et al, Phys. Rev. Lett. 88: 067002, 2002
[3] Ahn et al, Science 284:1152, 1999
[4] Hong et al., Appl. Phys. Lett., 86:142501, 2005
[5] Hong et al., Phys. Rev. B 74: 174406, 2006
[6] Ahn et al., Rev. Mod. Phys. 78: 1185, 2006