Figure 4
From: Anomalous enhancement of the sheet carrier density beyond the classic limit on a SrTiO3 surface
Interpretation of 
enhancement by a negative κ model.
(a) Top: sheet carrier density n2D (open circles) obtained by the Hall effect measurement for the multi-terminal FET device. Solid line (blue) is a least-square fit of the data to 
, where 
, 
is 0.28 μF/cm2 and 
is 1.88 V. For 
, we used a model shown in the bottom panel (See main text for details). Bottom: the dash-dotted line (purple) represents a case that 
in the subthreshold region with the body factor m of 2.8 changes to 
of the ideal metal. The solid line (blue) becomes negative which explains the enhancement of n2D. Inset shows the capacitance of the HfO2/Parylene-C gate insulator as a function of time measured while continuously applying the voltage. The variation is less than 2% for one hour even for the application of 8 V which is close to the breakdown voltage. (b) Schematic picture of ISD, 
and n2D with respect to VG. In the metal region, 
and comes back from −∞. However, 
changes continuously, which explains the observed n2D. (c) Negative capacitance means the charge compressibility 
is negative, i.e., dμ/dn2D is negative. In the general rigid-band model, dμ/dn2D > 0. If the density of states D(ω) is changed by the carrier doping, dμ/dn2D < 0 can be realised. Closure of the correlation gap such as the Mott transition and a band-splitting such as the Rashba effect are the typical examples.
