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Using ion irradiation to make high- Josephson junctions
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10.1063/1.2796105
/content/aip/journal/jap/102/8/10.1063/1.2796105
http://aip.metastore.ingenta.com/content/aip/journal/jap/102/8/10.1063/1.2796105

Figures

Image of FIG. 1.
FIG. 1.

(Color online) Schematic description of the JnJ fabrication. (a) A thick -axis YBCO film is covered by a thick in situ gold layer. A PMMA resist mask is patterned and an additional of gold later is deposited and lifted off. (b) The in situ gold layer is removed by Ar ion beam etching. (c) The sample is irradiated with a high fluence of oxygen ions such as the unprotected part of the YBCO film become insulating. This allows us to create superconducting channels underneath the gold mask, which are completely encapsulated in the insulating YBCO layer. (d) The gold mask is partially removed such as we obtained a superconducting microbridge ( wide) connected to four gold contacts. (e) Sectional view of the sample taken along the microbridge. A PMMA resist is deposited all over the sample, and a wide slit is opened across the superconducting microbridge. The junction is defined by a second irradiation performed with typical fluences of a few . (f) At the end of the fabrication we obtain a junction embedded in the insulating layer and connected to four terminals.

Image of FIG. 2.
FIG. 2.

(Color online) Resistance as a function of temperature of two microbridges with the same aspect ratio but with different widths (1 and ). The resistance is the same within 10%. Inset: example of TRIM calculation for oxygen ions in a thick YBCO film deposited on a STO substrate. Each dot represents a defect in the crystallographic structure. The projected lateral range is found to be in this case.

Image of FIG. 3.
FIG. 3.

Resistance as a function of temperature of wide JnJ irradiated with the following fluences (from right to left): , , , and . The higher transitions at refer to the transition temperature of the undamaged microbridges (bulk ) and the lower ones to the Josephson coupling . Inset: curves of six junctions on the same chip irradiated with the same fluence of , showing the reproducibility of the process.

Image of FIG. 4.
FIG. 4.

(Color online) (a) Current-voltage characteristics of a wide JnJ irradiated with a fluence of measured at in the Josephson regime . The dissipative branch displays an upward curvature, which fits (solid line) with the quadratic dependence of the RSJ model. (b) Current-voltage characteristics of the same junctions measured at in the flux-flow regime . In this case, the dissipative branch displays a characteristic downward curvature.

Image of FIG. 5.
FIG. 5.

(a) Current-voltage characteristics of a JnJ ( wide) irradiated with a fluence of at different temperatures in the Josephson regime . From top to bottom , 75.2, 76.2, and . (b) Current-voltage characteristics of a JnJ ( wide) irradiated with a fluence of at different temperatures in the Josephson regime . From top to bottom , 43.9, 46.3, and . (c) Modulation of the critical current of a JnJ ( wide, ) as a function of a magnetic field perpendicular to the sample. The dots correspond to data and the solid line to the Fraunhofer pattern . (d) characteristic of a JnJ ( wide, ) under microwave radiation of frequency of . Despite thermal fluctuations at this rather high temperature measurement , the curve displays clear Shapiro steps.

Image of FIG. 6.
FIG. 6.

Critical current as function of temperature for four JnJs ( wide) irradiated with different fluences (, , , and ). The experimental data (dots) are in good agreement with relation (3) (solid lines).

Image of FIG. 7.
FIG. 7.

The left axis displays the computed dpa profiles along the junction for two different fluences of and obtained from TRIM calculation. The right axis displays the corresponding along the junctions according to relation (1). The center of the junction gives the lower value of named in the text. The figure illustrates how the effective length of the Josephson changes as a function of temperature in the case of the fluence. At temperatures T1 and T2, the effective lengths are, respectively, L1 and L2.

Image of FIG. 8.
FIG. 8.

Current height of the Shapiro steps , 1, and 2 normalized by as a function of the microwave voltage , measured at on a JnJ irradiated with a fluence of . The dots correspond to experimental data and the solid line to the Bessel functions of orders of 0, 1, and 2. Inset: Typical curve of the JnJ under microwave radiations showing Shapiro steps , 1, and 2.

Image of FIG. 9.
FIG. 9.

(Color online) Schematic description of the trilayer junction fabrication: (a) A -axis oriented trilayer structure is pulsed laser deposited on a substrate and in situ covered by a thick gold layer. A gold mask is made using UV lithography, gold deposition, and lift-off technique. (b) The in situ gold layer is removed by Ar ion beam etching. Regions A are covered by of gold, the regions B by , and the region C has no gold left. (c) Sectional view. The sample is irradiated with a high dose of oxygen ions. During this process the trilayer in the unprotected regions C becomes entirely insulating and the trilayer in regions A remains undamaged since the ion range is smaller than the gold thickness. The thickness of the gold layer covering the regions B has been chosen such as the oxygen ions stop in the upper part of the bottom layer. This irradiation allows us to define a mesa in the trilayer structure as well as two superconducting paths connected to the bottom layer of the mesa. (d) The gold in regions B is removed using UV lithography and ion bean etching. As a result, we obtain a mesa junction whose dimensions are typically or , completely encapsulated in the insulating structure. The top of the junction is directly connected to a gold contact 1 and the bottom of the junction is connected to two larger gold contacts 2 and 3 through superconducting path. (e) Additional gold pads are then deposited in order to connect the junction. (f) Contact 1 can eventually be divided into two contacts to do four probe measurements.

Image of FIG. 10.
FIG. 10.

Current-voltage characteristics of a junction made with a thick barrier. A , , and . The finite slope of the Josephson current is due to the gold contact resistance 1 in serial with the junction. Inset: Critical current of the junction as a function of temperature.

Image of FIG. 11.
FIG. 11.

(a) Positive part of the characteristics of a junction ( area and barrier thickness) under microwave radiations of frequency . Shapiro steps (, 1, 2, and 3) appear when the radiation power is increased (from top to bottom). Inset: Voltage width of the Shapiro steps as a function of frequency showing the linear dependence expected from the Josephson relationship. (b) Oscillations of the current height of the Shapiro steps (left axis) and (right axis) as a function of the microwave power.

Image of FIG. 12.
FIG. 12.

Conductance of a and thick barrier junction as a function of bias voltage, measured at a temperature just above the of the underdoped YBCO layer. The dots correspond to experimental data and solid line to Glazman and Matveev formula [Eq. (4)] for .

Image of FIG. 13.
FIG. 13.

Conductance of a and thick barrier junction as a function of bias voltage measured at . The arrows indicate peaks in the conductance corresponding to mutliple Andreev reflection. Note that the peak at due to Josephson effect has been cut.

Tables

Generic image for table
Table I.

Typical values of and for different fluences of irradiation.

Generic image for table
Table II.

Typical parameters for trilayer junctions.

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/content/aip/journal/jap/102/8/10.1063/1.2796105
2007-10-17
2014-04-18
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Using ion irradiation to make high-Tc Josephson junctions
http://aip.metastore.ingenta.com/content/aip/journal/jap/102/8/10.1063/1.2796105
10.1063/1.2796105
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