Large capacitance enhancement and negative compressibility of two-dimensional electronic systems at LaAlO3_3/SrTiO3_3 interfaces

Novel electronic systems forming at oxide interfaces comprise a class of new materials with a wide array of potential applications. A high mobility electron system forms at the LaAlO$_3$/SrTiO$_3$ interface and, strikingly, both superconducts and displays indications of hysteretic magnetoresistance. An essential step for device applications is establishing the ability to vary the electronic conductivity of the electron system by means of a gate. We have fabricated metallic top gates above a conductive interface to vary the electron density at the interface. By monitoring capacitance and electric field penetration, we are able to tune the charge carrier density and establish that we can completely deplete the metallic interface with small voltages. Moreover, at low carrier densities, the capacitance is significantly enhanced beyond the geometric capacitance for the structure. In the same low density region, the metallic interface overscreens an external electric field. We attribute these observations to a negative compressibility of the electronic system at the interface. Similar phenomena have been observed previously in semiconducting two-dimensional electronic systems. The observed compressibility result is consistent with the interface containing a system of mobile electrons in two dimensions.Comment: 4 figures in main text; 4 figures in the supplemen

Compressibility of a two-dimensional hole gas in tilted magnetic field

We have measured compressibility of a two-dimensional hole gas in p-GaAs/AlGaAs heterostructure, grown on a (100) surface, in the presence of a tilted magnetic field. It turns out that the parallel component of magnetic field affects neither the spin splitting nor the density of states. We conclude that: (a) g-factor in the parallel magnetic field is nearly zero in this system; and (b) the level of the disorder potential is not sensitive to the parallel component of the magnetic field

The Droplet State and the Compressibility Anomaly in Dilute 2D Electron Systems

We investigate the space distribution of carrier density and the compressibility of two-dimensional (2D) electron systems by using the local density approximation. The strong correlation is simulated by the local exchange and correlation energies. A slowly varied disorder potential is applied to simulate the disorder effect. We show that the compressibility anomaly observed in 2D systems which accompanies the metal-insulator transition can be attributed to the formation of the droplet state due to disorder effect at low carrier densities.Comment: 4 pages, 3 figure

Thermodynamic Signature of a Two-Dimensional Metal-Insulator Transition

We present a study of the compressibility, K, of a two-dimensional hole system which exhibits a metal-insulator phase transition at zero magnetic field. It has been observed that dK/dp changes sign at the critical density for the metal-insulator transition. Measurements also indicate that the insulating phase is incompressible for all values of B. Finally, we show how the phase transition evolves as the magnetic field is varied and construct a phase diagram in the density-magnetic field plane for this system.Comment: 4 pages, 4 figures, submitted to Physical Review Letters; version 1 is identical to version 2 but didn't compile properl

Physics of the Insulating Phase in the Dilute Two-Dimensional Electron Gas

We propose to use the radio-frequency single-electron transistor as an extremely sensitive probe to detect the time-periodic ac signal generated by sliding electron lattice in the insulating state of the dilute two-dimensional electron gas. We also propose to use the optically-pumped NMR technique to probe the electron spin structure of the insulating state. We show that the electron effective mass and spin susceptibility are strongly enhanced by critical fluctuations of electron lattice in the vicinity of the metal-insulator transition, as observed in experiment.Comment: 5 pages, 2 figures, uses jetpl.cls (included). v.4: After publication in JETP Letters, two plots comparing theory and experiment are added, and a minor error is correcte

Universal flow diagram for the magnetoconductance in disordered GaAs layers

The temperature driven flow lines of the diagonal and Hall magnetoconductance data (G_{xx},G_{xy}) are studied in heavily Si-doped, disordered GaAs layers with different thicknesses. The flow lines are quantitatively well described by a recent universal scaling theory developed for the case of duality symmetry. The separatrix G_{xy}=1 (in units e^2/h) separates an insulating state from a spin-degenerate quantum Hall effect (QHE) state. The merging into the insulator or the QHE state at low temperatures happens along a semicircle separatrix G_{xx}^2+(G_{xy}-1)^2=1 which is divided by an unstable fixed point at (G_{xx},G_{xy})=(1,1).Comment: 10 pages, 5 figures, submitted to Phys. Rev. Let

Phase diagram of the integer quantum Hall effect in p-type Germanium

We experimentally study the phase diagram of the integer quantized Hall effect, as a function of density and magnetic field. We used a two dimensional hole system confined in a Ge/SiGe quantum well, where all energy levels are resolved, because the Zeeman splitting is comparable to the cyclotron energy. At low fields and close to the quantum Hall liquid-to-insulator transition, we observe the floating up of the lowest energy level, but NO FLOATING of any higher levels, rather a merging of these levels into the insulating state. For a given filling factor, only direct transitions between the insulating phase and higher quantum Hall liquids are observed as a function of density. Finally, we observe a peak in the critical resistivity around filling factor one.Comment: 4 pages, 4 figures, some changes in the tex