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Josephson Junctions with Tunable Weak Links

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Science  13 Apr 2001:
Vol. 292, Issue 5515, pp. 252-254
DOI: 10.1126/science.1058812

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Abstract

The electrical properties of organic molecular crystals, such as polyacenes or C60, can be tuned from insulating to superconducting by application of an electric field. By structuring the gate electrode of such a field-effect switch, the charge carrier density, and therefore also the superfluid density, can be modulated. Hence, weak links that behave like Josephson junctions can be fabricated between two superconducting regions. The coupling between the superconducting regions can be tuned and controlled over a wide range by the applied gate bias. Such devices might be used in superconducting circuits, and they are a useful scientific tool to study superconducting material parameters, such as the superconducting gap, as a function of carrier concentration or transition temperature.

Superconductivity is a most intriguing macroscopic quantum state, known for almost a century (1), yet still of great intellectual challenge and attraction, even as new classes of materials and new pairing mechanisms and order parameter symmetries are discovered. A particular consequence of the macroscopic quantum state is the occurrence of the Josephson effect when two superconductors are weakly connected. This effect is at the heart of practical devices such as ultrasensitive magnetic field detectors. In these devices, a carefully crafted thin insulating layer typically provides the coupling between superconductors. The coupling strength is exponentially sensitive to the insulator thickness and is fixed once the junction has been fabricated. However, it would be desirable to have an adjustable link, both in single-junction devices or in circuits where many superconductors could be Josephson-coupled and decoupled in an externally controlled way, as might be needed in quantum computing. We describe a method to create Josephson junctions where the coupling strength between two superconductors can be varied over the full possible range simply through the variation of an external voltage. It is based on the idea of controlling the superconducting properties of materials by an applied electric field (2). Such modulation has been demonstrated in a variety of field-effect devices (3–9). The technique of gate-induced superconductivity was used to exploit the capability of creating a controlled spatial modulation of the superfluid density within the same material through a suitable modification of the gate potential profile. Hence, an external voltage controls the carrier density and width of a normal conducting region, resulting in a modulation of the coupling strength between the two adjacent superconductors. We have produced tunable weak links in a series of organic compounds, including polyacenes and fullerenes. Controllable weak links have been demonstrated using superconducting metal electrodes in inorganic semiconductor field-effect structures (10–12). In addition, we determined the superconducting gap Δ from the current-voltage (I-V) characteristics of such devices for transition temperaturesT c ranging from 2 to 52 K. For hole-doped C60, T c has been varied from 7 to 52 K. The ratio of 2Δ/k B T c varies between 3.3 and 3.8, indicating weak to intermediate coupling strength of the organic superconductors.

Molecular crystals of anthracene, tetracene, pentacene, and C60 were grown from the vapor phase in a stream of hydrogen (13). Field-effect transistors were prepared on cleaved crystal surfaces (14) using gold electrodes and an insulating Al2O3 layer (50 to 100 nm thick). Although all single crystal–based field-effect devices that were investigated showed ambipolar activity (15), electron, as well as hole, superconductivity was only observed in C60. The mobilities at low temperatures and at the high carrier concentrations are in the range from 1 to 100 cm2V–1 s–1. The corresponding mean free path is in the range of several nm. The critical temperatures ranged from 2 K in pentacene (7) to 52 K in hole-doped C60 (9). Josephson junctions (16) were prepared by creating a narrow region of a thicker gate dielectric layer. This was achieved by evaporating source- and drain-electrodes (gold or aluminum), and by sputtering of a first uniform layer of the gate dielectric (Al2O3). Then, shallow-angle shadow-mask evaporation was used to deposit the part of the gate electrode, leaving a gap of >50 nm. An additional 25 to 75 nm Al2O3 layer was deposited on top of the gapped electrode, followed by deposition of a second gate layer, which is connected to the first gate layer. As a result, the electric field under the narrow, thicker part of the gate oxide is reduced and the carrier concentration in the channel region exhibits a spatial variation (Fig. 1). Details of the field and carrier profile will have to be investigated in a further study. As the gate voltage increased, the carrier concentration increases, but is suppressed under the ridge of the gate dielectric. At a certain threshold voltage, superconductivity is induced underneath the two thinner parts of the dielectric layer. Upon further increase of the gate voltage V g, the two sides are Josephson coupled and a supercurrent flows at zero bias between source and drain. A priori, it is difficult to predict whether the weak link is of superconductor–normal metal–superconductor (SNS) or of superconductor-insulator-superconductor (SIS) type. Obviously, in the present experiments, a high gate voltage is necessary to induce superconductivity and then an additional variation by a few volts is needed to tune the coupling strength. In such a device, no voltage gain can be achieved.

Figure 1

Schematic of a “weak link” prepared on an organic single crystal. The cross-section of such a weak link shows the structured gate dielectric layer. By adjusting the gate voltage, a spatial variation of the carrier concentration can be achieved, which leads to the formation of a weak link [superconductor–normal conductor–superconductor (SNS) structure], although the details depend sensitively on the gate voltage, as seen in the insets of Fig. 3. Rather than forming a normal conducting region, a nonconducting (insulating) region appears to form.

Such a I-V characteristic for a C60(electron-doped) Josephson junction is shown at 2 K (Fig. 2). The transition temperature of electron-doped C60 is 11 K. Similar characteristics have been observed for the other materials, and we did not observe any systematic differences between polyacenes and C60, despite their different superconducting coherence lengths (30 to 100 Å). As we discuss later, however, the I-V characteristics depend on the tuning voltage. The current at zero voltage represents the critical Josephson current I c, which is given by the maximum amount of current that the tunneling Cooper pairs can carry (Fig. 2). In order to investigate the microscopic nature of the weak link, further experiments will be needed. The tunneling characteristics reveal a nonideal junction with rather high transparency (Figs. 2 and3) (17). The transparency increases with increasing gate voltage, which might be explained by the increased density of charge carriers under the ridge. Hence, the devices first resemble SIS junctions, and at higher voltages, change over to more SNS-type behavior.

Figure 2

Current-voltage characteristic of an electron-doped C60 Josephson junction at 2 K. The inset shows the temperature dependence of the critical Josephson currentI c.

Figure 3

Variation of the critical Josephson currentI c in a C60-based device at 2 K (electron-doping; T c = 11 K, which is constant within 5% in the shown gate voltage range). By adjusting the gate voltage, the thickness of the normal conducting region can be varied, giving rise to a modulation of I c. The critical Josephson current is normalized to the maximumI c obtained for this junction (V g = 180 V). The insets show the current-voltage characteristics for different gate voltages (the current is normalized to the value at 6 mV).

In addition, the preparation of Josephson junctions with gate-induced superconductors allows the adjustment of the strength of the weak link by the applied gate bias. Because the gate bias controls the carrier concentration in the channel, the width of the normal or nonconducting region (Fig. 1), as well as the coherence length, can be adjusted by the applied gate bias. Consequently, the strength of the weak link, which depends on both parameters, the width, and the coherence length (18), can be modified in a controlled way. The critical Josephson current and the properties of the junction can then be controlled precisely over a wide range (Fig. 3). Gate-controlled supercurrents have also been achieved in a variety of metal-inorganic semiconductor (10–12) and high-T c superconductor devices (4). Our approach is different in that the same material acts as superconductor and normal conductor/insulator simply by the variation of the carrier density. By using two (or more) gate electrodes, the Josephson coupling between gate-induced superconductors might be refined.

The strength of the Josephson coupling varies rather rapidly as the gate voltage is ramped up (Fig. 3). The magnitude of the Josephson current I c increases withV g, reaches a maximum, and then drops quickly again. The details, however, depend on several factors, including the gate capacitance profile (given by the gate oxide thickness) and the electronic properties of the superconductor, such as the carrier concentration dependence ofT c, and the superconducting coherence length. In all the various junctions, we find similarI c(V g) dependencies, i.e., a strong variation of I cwithin a few percent of V g. The temperature dependence of I c is shown in the inset of Fig. 2. The rapidly changing character of the junction is also evident from the junction resistance, decreasing from ∼20 kilohms at V g = 177 V to ∼1 kilohms at maximum I c (at 180 V).

The junction geometry used to study Josephson coupling also offers the opportunity to measure the superconducting gap Δ and its dependence on the carrier density, i.e.,T c. Because such investigations can be performed on the same device, the effects of additional disorder or structural changes due to chemical doping of the sample are eliminated. If the applied voltage exceeds the energy 2Δ to break the Cooper pairs, the current through the junction is carried by quasiparticles. In Figs. 2 and 3, we show several current-voltage characteristics, measured at 2 K in electron-doped C60, for an electron concentration that results in a T c of 11 K. Although the current is not zero below the gap voltage, the value of the gap Δ can nevertheless be clearly discerned by inspection of the derivative (dI/dV).

The experiments for different materials followed different protocols. For materials with low T c, we measured the I-V characteristics at 1.7 K. Thus, the experimentally deduced values of Δ are reduced compared to Δ(T→0), especially for polyacene crystals. However, Δ(T→0) can be estimated from the temperature dependence of Δ (19). For C60, we applied gate voltages over a wide range, resulting in a variation of T c up to 11 K for electron-doping and up to 52 K for hole-doping. Then, we measured theI-V characteristics at several fixed gate voltages as a function of temperature in order to determine Δ(T,V g), thereby calibrating the full T c dependence on the gate bias. For other gate voltages, T c is deduced from the previously measuredT c(V g) (9). Figure 4 shows an almost linear dependence of Δ on T c. More insight is gained by plotting 2Δ/k B T c versusT c (Fig. 5). The experimental values for this ratio vary between 3.3 and 3.8, which is within the considerable experimental uncertainties consistent with weak to intermediate strong coupling [2Δ(0) = 3.52·k B T c] (19). Because the subgap current was larger in C60 junctions with the highestT c, Δ might be slightly overestimated.

Figure 4

Dependence of the superconducting gap Δ as a function of the critical temperatureT c, revealing a linear dependence. The slope is slightly higher than predicted in the Bardeen-Cooper-Schrieffer (BCS) weak coupling limit.

Figure 5

Ratio of the superconducting gap Δ and the transition temperature T c as a function of T c for the various investigated organic materials. The open symbols are the experimental data for the gap at the measurement temperature Δ(T) and the solid symbols are the estimation of the zero temperature gap Δ(T→0). The observed values are in reasonable accordance with the weak coupling limit of the BCS theory (2Δ = 3.52·k B T c).

The demonstration that Josephson junctions with tunable weak links can be fabricated for materials exhibiting gate-induced superconductivity opens up possibilities for scientific research as well as superconducting electronic circuits. The control of the superconducting and junction properties by an applied voltage might be useful in the fabrication of circuits for superconducting electronics or Josephson junction arrays. For example, the use of Josephson junction arrays as qubits has been demonstrated, which are suitable for scaling to large numbers of devices (20–22). However, at this point it is difficult to use standard lithographic processes on top of the molecular crystals without modifying their properties. Therefore, the use of a bottomgate field-effect device instead of the top-gate configuration will facilitate the implementation of such tunable weak links and more complex circuitry because the gate structures can be defined by standard lithography. It has recently been demonstrated that large-scale integration of organic electronic devices on prepatterned Si circuitry is possible (23).

  • * To whom correspondence should be addressed. E-mail: hendrik{at}lucent.com

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