Thiolating cuprate superconductors for nanoelectronic devices fabrication
Despite current efforts, single-molecule devices are still away to replace semiconductor technologies during this new decade. Some challenging problems in this matter, such as lack of direct microscopy to confirm the presence of a particular molecule in the junction of the devices, the microscopy variation in configuration from device to device, together with the necessity of great care and many control experiments to draw useful conclusions from any particular device makes silicon to continue dominating nano-electronics technologies for more time ahead. On the other hand the present semiconductor-based microelectronics is not believed to achieve circuit density sufficient for maintaining Moore's law and electronics requires suitably sized functional elements for the development and design of new architectures and devices. Nanocrystals could be one of these functional elements as they exhibit small size and size-tunable electrical behaviour, potentially useful for customizing physical properties to meet the requirements of nanoelectronic devices in the nearest future. Superconducting nano-crystals are an attractive alternative to this purpose. The advantages of superconducting electronics over semiconductor electronics are its high mobility of carriers which allows ultrafast switching speed (for digital applications) and high sensitivity and response to electromagnetic phenomena over a very wide frequency spectrum (for analogue applications). There have been some attempts to fabricate superconductig transistors, but most of them based on thin film superconductors attached to metal electrodes. However, the drawbacks of these devices are the intense interdiffusion between layers obtained during growing and the typical wide area of contact between metal electrodes and superconduting layer (the order of μm2). In that sense, an alternative to overcome this problem is using already prepared superconductor nanocrystals and attach it between two metal electrodes. We have succeeded in functionalizing cuprate superconductor crystals with octane di-thiol and attached them on gold surfaces through self assembly monolayers of alkane di-thiol (Fig 1).
Fig 1. Attaching functionalized cuprate superconductor crystals on Au surfaces.
It is extremely important to check this type of crystals do not loose their superconducting properties by degradation after functionalization. According to the M(T) measurements (Fig 2), after functionalizing the La1113 superconductor with octane-dithiol molecules, the sample does not loose its superconducting properties, the critical temperature is not notoriously affected and it remains 80K.
Fig 2. M(T) measurements of the La1113 superconductor before functionalization (black spheres) and after functionalization (white spheres).
The aim of this project is to fabricate nanoelectronic superconducting devices such as that shown in Fig. 3. At T < TC, the presence or absence of an external magnetic field (H) could be used as a gate (G). When H is higher than the maximum critical magnetic field the crystal superconducts, in contrast, when H is lower than the lower critical magnetic field or suppressed the crystal superconducts. The non-superconducting/superconductor states can be achieved for current-flow to two corresponding off and on states, thus achieving a superconductive magnetic controlled gating nano-device. For instance, taking an applied voltage (at the drain) VD=1 and the critical magnetic field H=1 units, and considering the presence (absence) of the resulting voltage as 1/0; the result is that when H=0 then V=1 and vice versa when H=1 then V=0; thus such device could work as an inverter switch. Logic gates such as AND or NOR could be obtained using two inverters in series or in parallel respectively. Further digital circuits could be logically built on combination of various logic gates. Eventually, when HC1 < H < HC2 is in the mixed state meaning partial conduction of the crystal. Alternatively, the SiO2/Si could be tested as a substrate gate as well. This project is being performed in collaboration with the University of San Marcos (Peru).
Fig 3. Nanocrystal superconducting transistor with external magnetic gate.
Preparation and characterization of high critical temperature cuprate crystals with low critical magnetic field.
Cuprate superconductor nanocrystals which are good candidates for nano-crystal superconducting transistors fabrications (see project Superconducting Nanoelectronic Devices) are those whit both properties at the same time high critical temperature (TC) and low critical fields (HC). Superconductors with TC above 77K require nitrogen instead of helium to make such transistors useful, whereas the low HC avoids high current sources for the magnetic gates operation. LaCaBaCuO7 (La1113) is a good candidate for this purpose. La1113 is a high temperature cuprate superconductor (HTCS) with TC around 80K. The La1113 structure is tetragonal and similar to the non-superconductor YBaCu3O6 (YBCO-6). Fig 1 shows schematically a structural comparison of La1113 with YBCO. YBaCu3O7 (YBCO-7) has an orthorhombic Pmmm structure and becomes superconductor below TC(onset) of around 90K (Fig 1(a)). YBCO-6 has tetragonal P4/mmm structure and is antiferromagnetic insulator (Fig 1(b)). La1113 is superconductor and has tetragonal structure similar to YBCO-6 (Fig 1(c)). The cell parameters of La1113 are a=b=3.88Å and c=11.60Å whereas for YBCO superconductor are a=3.83Å, b=3.89Å and c=11.70Å [15, 16]. In addition, other important differences between both structures are:
* While the rare earth Y in the YBCO structure is sandwiched between two oxygen-deficient Ba-Cu perovskites, the (1/2, 1/2, 1/2) position in the La1113 structure is shared by La and Ca atoms.
* The O(1)(0,1/2,0) of the Cu-O chains is fully occupied in the superconducting YBCO, whereas O(1) is partially occupied (occupancy ≈ 0.7) in the La1113 structure.
Fig 4. Comparison between LaCaBaCu3O7 and YBCO structures: a) Orthorhombic structure of YBCO (superconductor), b) Tetragonal structure of YBCO (insulator) and c) Tetragonal structure of La1113. The positions of Cu(1) and Cu(2) in all the structures are (0,0,0) and (0,0,1/3) respectively.
The magnetic phase diagram of the La1113 is shown in Fig 2. The various parts of the diagram are explained in the following. As long as the external flux density does not exceed HC1, the La1113 is in a diamagnetic Meissner state, where the magnetic flux is completely expelled from the interior by surface currents. In the area between HC1(T) and HC2(T), the magnetic flux penetrates the bulk La1113 in the form of flux lines (mixed state). Magnetic flux motion has to be prevented by flux pinning, i.e the magnitude of FP. As long as the flux lines are pinned, the maximum supercurrent density JC can flow without any loss. The critical current density JC depends on the applied magnetic field and on the temperature. If the current density exceeds JC (Hirr < H < HC2), then the flux line lattice (or parts of it) starts to move. Because of thermal fluctuations, the mobility of the flux line lattice strongly increases with applied magnetic field and temperature. Above the irreversible field Hirr the vortex lattice becomes so strong that currents cannot flow without losses although the superconductor La1113 is not yet in the normal state. This means that possible applications of La1113 are restricted to the field range below the Hirr. According to the plot, Hirr scales with the applied field following a trend:
Hirr=Hirr(0) ( 1 - T/Tc ) ^ n
Where Hirr(0)=48kOe, TC=76K and the exponent n provides an indication for applications of the superconductor (it varies considerably in different HTCS). The shape of this irreversibility field line is quite similar to that obtained for bulk YBCO in the range 55K - 90K . The exponent n in the temperature range 41K - 76K is 2.8, this value is double than that obtained for bulk YBCO (n ≈ 1.4). Therefore, La1113 can support low applied fields and thus seems to be potentially more applicative in the design of magnetic field-gated superconducting transistors than YBCO. Probably the partial Oxygen occupancy in the Cu-O chains in the La1113 structure is responsible for the vortex lattice becoming disordered at temperatures close to the TC and thus Hirr and n are lower than YBCO. Extrapolating HC2 and HC1 in the figure to T=0, values of HC2(0)≈53.5kOe and HC1(0)≈0.5kOe can be estimated. The aim of this project is obtaining La1113 nanocrystals with lower critical fields without affecting the TC by doping the La1113 structure with anions such as PO4, BO3, etc for applied magnetic gated superconducting nano-transistors. This project is being performed in collaboration with the University of San Marcos (Peru).
Fig. 5 Magnetic phase diagram of LaCaBaCu3O7