Electrolyte Gating and Complex Oxides

The mobile ions in an electrolyte, in the form of room-temperature ionic liquid (IL) or ionic gel composed of IL suspended in a matrix of PMMA-based copolymer, are used to gate (accumulate or deplete) low-dimensional electron systems via the electric field effect. Sheet carrier density swings up to 10^15 cm-2 can be induced, and hence electrolyte gating can be used to study systems where the relevant electron/hole density scales are significant fractions of the Brillouin zone volume (especially transition metal oxide interfaces and thin films, and even normal metal films). By comparison, typical gate dielectrics can sustain electric fields that affect carrier density changes up to 10^13 cm-2.

Electrolyte-Gated SrTiO3

Ionic gels formed by a matrix of PS-PMMA-PS copolymer in ionic liquids (e.g., EMI-TFSA) are used to gate the bare surface of undoped SrTiO3 (STO), a semiconducting perovskite oxide. Accumulating a high concentration of cations over the crystal surface causes electronic reconstruction, forming a two-dimensional and highly-mobile system of electrons direclty beneath the surface, residing on the Ti sites and partially filling the Ti 3d conduction band. Carrier density swings on the order of 10^14 cm-2 electrons can be induced by tuning the gate voltage within the electrochemical window of the electrolyte. The strong electric fields of the mobile ions in the electrolyte bear close resemblance to the "polar catastrophe" caused by the polarity of lanthanum aluminate (LAO) overlayer in LAO/STO heterostructures. Low-temperature, high-field magnetotransport measurements show that the Kondo effect due to the magnetism of +3-coordinated Ti ions is the dominant scattering mechanism at high electron density [1].


Figure taken from Ref. 2, showing the analogy between the LAO/STO interface and electrolyte-gated STO.

Ionic liquid gating of strontium titanate nanostructures

We aim to study the superconducting properties of strontium titanate interfaces at the nanoscale using ionic liquid gating in conjunction with nanostructured metal gates. We are interested in the following questions: What is the phase diagram in small devices, and how does it compare to bulk data? What information about the nature of the transport can we extract by measuring mesoscopic effects?


Left: SEM micrograph of a 1μm long by 300nm wide hall bar of ionic liquid gated STO. Center: Differential resistance of hall bar as a function of DC bias current, showing 7 nA critical current. Right: Universal conductance fluctuations of resistance.

Using nanopatterned gates, we can define a 1μm long by 300nm wide hall bar. This allows us to both study superconductivity at the nanoscale in STO and access the mesoscopic transport regime. Furthermore, these gates can be used to modulate the Fermi level and effective width of the Hall bar by changing the gate voltages. This device shows a clear superconducting feature at 12mK, with a critical current of around 7nA. The critical current feature is strongly suppressed as a function of temperature, disappearing around 200mK. The density of electrons can be tuned between 1-2x1013cm-2, dramatically changing superconducting features. The conductance as a function of magnetic field shows reproducible fluctuations, which can only be UCF. The magnitude of the fluctuations is ~0.05e^2/h, and decreases with temperature as expected. We still see fluctuations at temperatures up to 500mK.

Gold Point Contacts

Using ionic liquid (electrolyte) gating, we can achieve charge density changes greater than 10^15 cm-2 in thin gold films. We can prepare gold atomic contacts by electromigration in-situ, and the conductance of small contacts exhibits step-like changes when a gate voltage is applied.

References

  1. Menyoung Lee, J. R. Williams, Sipei Zhang, C. Daniel Frisbie, and D. Goldhaber-Gordon, "Electrolyte Gate-Controlled Kondo Effect in SrTiO3," Physical Review Letters 107, 256601 (2011) [See Ref. 2, the accompanying Physics Viewpoint]. Supplementary info .
  2. Johann Kroha, "Tuning Correlations in a 2D Electron Liquid," Physics 4, 106 (2011).
Contact Menyoung Lee (menyoung@), Sam Stanwyck (stanwyck@), Trevor Petach (petach@), or Patrick Gallagher (pgallagh@) for more information.