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Transport properties of nanoscale Nb and NbN Josephson junctions fabricated by focused-ion-beam milling
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Image of FIG. 1.
FIG. 1.

Fabrication of SETs by FIB milling. (A1) After photolithography. (A2) Side view of (A1). (B) Carbon deposition. (C) Milling from 0°. (D1) Side view of dotted circle in (D2). (D2) Milling from 85°.

Image of FIG. 2.
FIG. 2.

SET fabricated by FIB milling. (A) FIB secondary-electron image taken from the direction perpendicular to the substrate. (B) FIB secondary-electron image taken from the direction indicated by the arrow in (A). (C) Scanning electron microscope image. The substrate was tilted by 30°.

Image of FIG. 3.
FIG. 3.

Anodization. (A) Schematic diagram. (B) Scanning electron micrograph of an anodized Nb SET.

Image of FIG. 4.
FIG. 4.

Current-voltage characteristics of Nb-SJJ1 (see Table I). The curve can be divided into three parts: (A) supercurrent, (B) superconducting gap, and (C) quasiparticle current.

Image of FIG. 5.
FIG. 5.

(A) Current-voltage characteristics (upper data set) and the differential resistance vs bias voltage (lower data set) of Nb-SET1. (B) Enlarged part of the dotted circle in (A). JQP peaks are in the two encircled fields.

Image of FIG. 6.
FIG. 6.

Diamonds of JQP-peak positions in the superconducting state at . Lower data set: Current vs gate voltage at (dotted line in the upper data set of Fig. 6), where is the bias voltage. Upper data set: JQP peak current positions on the plane.

Image of FIG. 7.
FIG. 7.

(A) Electrostatic energy [ in Eqs. (4) and (5), in units of the single-electron charging energy ] of the charge states of a SET for a given number of extra electrons in the island, plotted as a function of the normalized gate charge . The solid curves are parabolas of the lowest even- states, and the dashed curves are parabolas of the lowest odd- states when in Eq. (5). (B) Theoretical gate modulation of the normalized critical current of the SET in the superconducting state. The solid curve is for the case that the period of dependence is 2 ( periodic), and the dashed curve is for the case that the period is 1 ( periodic). For both (A) and (B), was used for the calculations. (C) Measured critical current of Nb-SET1 at as a function of the gate voltage.

Image of FIG. 8.
FIG. 8.

Current-voltage characteristics of single junctions at . (A) NbN-SJJ1, estimated junction area is . (B) NbN-SJJ2, .


Generic image for table
Table I.

List of samples. SJJ and SET represent SJJ and SET, respectively; is the measurement temperature; is the normal-state resistance; is the measured superconducting gap energy; for SJJ and for SET is the estimated junction area, where is the critical current density and we assumed for Nb junctions and for NbN junctions; is the measured critical current; for SJJ and for SET is the estimated Josephson energy, where is the flux quantum; is the Boltzmann constant; is the single-electron charging energy estimated from the positions of JQP peaks (see Fig. 6); for both SJJ and SET is the theoretical maximum of the critical current; is the ratio of the theoretical maximum to when dependence of has a period of 1 ( periodic), where is the normalized gate charge.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Transport properties of nanoscale Nb and NbN Josephson junctions fabricated by focused-ion-beam milling