Like ion channels, nanochannels are known to exhibit curious non-Ohmic current-voltage (I-V) characteristics with an approximate piece-wise constant differential resistance. Using a nanoslot model and a nonequilibrium ion transport theory, we attribute the nonlinear resistance to overlapping double layers inside and an extended polarized layer of space charge outside the nanochannel. The overlimiting current beyond a critical voltage is shown to develop when the polarized layer is destabilized by a microvortex instability at one entrance. By extending earlier nanochannel and polarized layer models to include this instability, nonideal ion permselectivity and field-focusing effect, quantitative predictions-together with explicit differential resistance expressions-are offered for the nonlinear I-V features of a nanochannel surrounded by microreservoirs from a simple pseudo-one-dimensional model.

A peculiar and undesirable current-voltage characteristic of nanoporous membranes is that it exhibits a voltage window with a near-constant limiting-current density when bulk ions near one surface of the membrane are depleted. We show both theoretically and experimentally that this interval disappears for an isolated circular nanopore (or narrow nanoslot) because radial field focusing at the pore entrance enhances the depletion effect and drives an ejecting hydrodynamic vortex pair that amplifies ion flux into the nanopore. This vortex pair is distinct from the vortex arrays that appear in front of a wide nanoslot or a nanoporous membrane with small inter-nanopore separation. It hence suggests that an optimal pore radius/separation ratio exists for maximum current density across a membrane.

A nanoslot array with a uniform surface charge and height but with asymmetric slot entrances is shown to exhibit strong rectification, gating type current-voltage characteristics and a total current higher than the sum of isolated slots at a large voltage. Unlike previous reports of low-voltage current rectification within nanopores and nanochannels with a nonuniform surface charge and/or height, the asymmetry is due to asymmetric space-charge polarization and interslot communication at only one of the two different entrances.

The effects of the external electric field on 14N nuclear quadrupole resonance (NQR) have been investigated in the ferroelectric phase of sodium nitrite (NaNO,) single crystals with either the single domain or the multidomain structure. NQR measurements of two resonance lines of v + and u -were made at 77 K under the external electric field up to 25 kV/cm. The rates of the Stark shift in the crystal of single domain are du+/dE+ = 6.8 f 3.0, dv'ldE-= -9.8 f 2.2, dv-ldE' = 5.0 f 2.0, and dv-/dE-= -2.5 f 2.4 Hz . cm/kV, respectively. Also the rates of the line broadening in the crystal of multidomain can be expressed by d(Av+)/dE+ = 18 f 3 and d(Av+)/dE-= 12 f 3 Hz . cm/kV, respectively. These results are found to be consistent with the sum of two slopes of the v + line with the single domain. The present work provides valuable information on the state of the ferroelectric domains and be applicable to other ferroelectric materials with the 180" domain.