Switchable superconductive inductor

[0002] The present invention relates to an inductor to be used in microwave integrated circuits, in particular an inductor being formed by a microstrip line.

[0003] In transmission paths in microwave integrated circuits, there is a need for various components such as inductors. In particular, there may be a need for an inductor, the characteristics of which can be varied, such as an inductor which can be switched between two inductance values as controlled by an electrical signal.

[0004] In Japanese patent application JP 2/101801, a microwave band-rejection filter is disclosed having transmission lines designed as linear microstrip, metal elements placed on top of an area of a layer of superconducting material. The superconducting material area has a pattern substantially agreeing with that of the metal conductor, except in some regions where the width of the superconducting area is larger than that of the metal conductor. When the superconducting material is in a non-superconducting state, most of the electric current passes through the common metal material of the metal conductor, whereas, in superconducting state, the electrical current passes only through the superconducting underlying material. The microstrip metal elements thereby obtain a variable filtering effect. However, a disadvantage of this design resides in providing a region having some, though it may be low, electrical conductivity placed under the normal conductor, since this region causes losses in the transmission line. The conductivity of materials, which are superconducting at a low temperature and are suitable for microwave integrated circuits, have in their normal state an electrical conductivity corresponding to some 10⁻³to 10⁻²times that of the electrical conductivity of the material of the always normal metal conductor.

[0005] It is an object of the invention to provide an electrical inductor of the microstrip type for microwaves exhibiting low losses.

[0006] An inductor for primarily microwave frequencies is constructed of a transmission line designed as a linear microstrip element made of a central line comprising normal electrically conducting material, such as a suitable metal. The microstrip element has a width which is varied by making areas at the sides of the central line superconducting. By changing the effective width of the microstrip, the inductance thereof is changed accordingly. The areas at the sides of the microstrip element are electrically connected along at least portions of the sides or of the edges of the central metal conductor. These superconducting areas, in their non-superconducting state, have some electrical conductivity which can be rather low. However, because they contact the central metal conductor only at a very low, thin or narrow edge, instead of contacting it at a large surface, they do not significantly affect the transmission characteristics of the transmission path when the superconducting areas are in the normal, not superconducting state.

[0007] Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the example and non-limiting methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.

[0008] While the novel features of the invention are set forth with particularly in the appended claims, a complete understanding of the invention, both as to organization and content, and of the above and other features thereof may be gained from and the invention will be better appreciated from a consideration of the following detailed description of non-limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which:

[0009] FIG. 1 is a cross-sectional view of a planar, switchable microwave inductor,

[0010] FIG. 2ais a cross-sectional view identical to that of FIG. 1 illustrating electrical current distribution when some regions are in a superconducting state,

[0011] FIG. 2bis a cross-sectional view similar to that of FIG. 2aillustrating electrical current distribution when some regions have changed from a superconducting state to a normal state, and

[0012] FIG. 3 is a diagram of the inductance of an inductor as a function of time illustrating the case where some regions of an inductor change from a superconducting state to a normal state.

[0013] In the cross-sectional view of FIG. 1 an inductor having a variable inductance intended to be connected in e.g., a microwave circuit is illustrated. The inductor is built on a dielectric substrate 1 having an electrically conducting ground plane layer 3, such as a metal layer of e.g. Cu, Ag or Au, on its bottom surface, the ground plane layer covering substantially all of the bottom surface as a contiguous layer. On the top surface there is a patterned electrically conducting layer 5 having a high electrical conductivity, such as a suitable metal, e.g., of the same metal as the bottom layer, i.e. of copper (Cu), silver (Ag) or gold (Au). The patterned layer 5 has a uniform width W_cand forms a transmission or propagation path for microwaves. The strip 5 has electrically conducting areas or regions 7 located directly at the side or sides of the conductor strip 5. These regions 7 are made of a potentially superconducting material, preferably a high temperature superconducting (HTS) material. The regions 7 comprise strips located at both sides of the central metallic strip 5, preferably symmetrically in relation thereto, these strips thus having the same uniform width. The width of the superconducting strips together with the central conductor is denoted by W.

[0014] In the “normal” state, the potentially superconducting regions 7 may have, e.g., an electrical conductivity σ_nof about 5·10⁵S/m. The electrical conductivity σ_cof the central metal conductor 5 may be, e.g., about 10⁸S/M. When the potentially superconducting regions 7 are in the normal state, electrical current flows almost entirely in the central conductor 5. The current distribution (J) for this normal, non-superconducting state appears from the diagram of FIG. 2b. The current distribution (J) is here substantially uniform over the width W_cof the conductor 5.

[0015] When the regions 7 are in a superconducting state, all of the electrical current passes in the lateral superconducting areas 7, and at the outer edges thereof, (see the current distribution (J) in the diagram of FIG. 2a), according to the Meissner effect.

[0016] The inductance (L) of a microstrip line is mainly determined by the total width W of the line, e.g., being approximately inversely proportional to the width, provided that the height h of the dielectric 1 is fixed. Thus, changing the state of the potentially superconducting regions 7 changes the inductance (L) of the microstrip line between a lower and a higher value over time (t) as shown in the diagram of FIG. 3.

[0017] A switching between the superconducting state and the normal state of the potentially superconducting regions 7 can be achieved in any conventional way, such as by varying the temperature, by varying the magnetic field, or by varying a direct current level as to what is required or desired. This switching is symbolized by the control unit 9 shown in FIG. 1. Preferably, the control unit 9 provides an electrical current higher than the critical current of the superconducting material so that it passes through the microstrip line. With a fixed bias current having an intensity slightly lower than that of the critical current passing through the line, and by adding (or not adding) thereto a small control current such as a current pulse, the reversible switching between the superconducting state and the normal state can be made extremely fast. The general appearance of the switching operation appears in the diagram of FIG. 3. When the regions 7 of the microstrip line are in a superconducting state, the microstrip line has a first low inductance L_super. When the state is changed to normal, the inductance is changed to a higher value L_normal. There is a small transition time τ before the change of inductance is actually effected, for instance when the current through the microstrip line is suddenly increased.

[0018] Numerical simulation has indicated that the inductance L of a microstrip line can be easily doubled for a suitable width of the superconducting regions 7, working at microwave frequencies. herein, it is realized that numerous additional advantages, modifications and changes will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within a true spirit and scope of the invention.