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The rf impedance of superconducting point contacts has been measured as a function of the quantummechancial phase difFerence $ across the point contact. By representing the point contact as a parallel combination of an inductor Z($) = (h/2e) (&&"/BQ

Crystal structures and low field principal crystal axis magnetic measurements are reported for the antiferromagnetic iron (III) salts A2FeCl5⋅H2O; A=Rb+, Cs+. The two compounds undergo a zero field paramagnetic to antiferromagnetic phase transition at 6.5 K (A=Cs+) and 10.05 K (A=Rb+). The spin flop magnetic phase is observed in high field measurements. The magnetic field-temperature phase diagrams are reported. All magnetic measurements were recorded on a superconducting susceptometer. The bicritical points are 6.25 K, 14.5 kOe (A=Cs+), 9.75 K, 17.8 KOe (A=Rb+). Crystal data for Cs2FeCl5⋅H2O: space group Cmcm, Z=4, a=7.442(3) Å, b=17.307(7) Å, c=8.077(7) Å, V=1040 Å3, R=5.2%, 561 reflections. Crystal data for Rb2FeCl5⋅H2O, space group Pnma, Z=4, a=13.825(4) Å, b=9.918(3) Å, c=7.100(2) Å, V=4.9 Å3, 5.7%, 712 reflections.

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We present a nonoptical shear-force feedback method to regulate tip–sample distance for near-field scanning optical microscopy. In the shear force setup, the dither piezo and the attached fiber tip form an electromechanical system, whose power dissipation on resonance is sensitive to the change in damping force as the tip approaches and interacts with the sample. At the frequencies of interest (∼10–100 kHz), the change in power dissipation is conveniently manifested as a change in the electrical impedance of the dither piezo. We demonstrate that tip–sample distance feedback control can be achieved by measuring this change in dither piezo impedance. The sensitivity is currently ∼0.5 Å. This new technique is compared to other methods currently used for distance control in near-field scanning optical microscopy.