U.S. patent number 3,676,718 [Application Number 05/128,445] was granted by the patent office on 1972-07-11 for supercurrent structures utilizing mobil flux vortices.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated. Invention is credited to Philip Warren Anderson, Robert Carr Dynes, Theodore Alan Fulton.
United States Patent |
3,676,718 |
Anderson , et al. |
July 11, 1972 |
SUPERCURRENT STRUCTURES UTILIZING MOBIL FLUX VORTICES
Abstract
A supercurrent logic structure of extended dimensions is capable
of sustaining a plurality of trapped magnetic field vortices each
of which supports one flux quantum. Such a vortex prefers to
position itself in a region such that a local minimum of the sum of
the magnetic energy plus the Josephson coupling energy is
established. A variety of ways to create such preferred regions are
disclosed. A vortex is moved from one such region to another in
shift register fashion by applying a force thereto as, for example,
by applying a local current or magnetic field near to the
vortex.
Inventors: |
Anderson; Philip Warren (New
Vernon, NJ), Dynes; Robert Carr (Berkeley Heights, NJ),
Fulton; Theodore Alan (Berkeley Heights, NJ) |
Assignee: |
Bell Telephone Laboratories,
Incorporated (Murray Hill, NJ)
|
Family
ID: |
22435422 |
Appl.
No.: |
05/128,445 |
Filed: |
March 26, 1971 |
Current U.S.
Class: |
326/4; 257/36;
377/93; 505/865; 327/527; 257/E39.014 |
Current CPC
Class: |
G11C
19/32 (20130101); H01L 39/223 (20130101); G11C
11/44 (20130101); Y10S 505/865 (20130101) |
Current International
Class: |
G11C
19/00 (20060101); G11C 19/32 (20060101); G11C
11/44 (20060101); H01L 39/22 (20060101); G11C
11/21 (20060101); H03k 003/38 (); G11c
019/00 () |
Field of
Search: |
;307/212,221,277,306
;317/234T |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Zazworsky; John
Claims
What is claimed is:
1. Supercurrent apparatus comprising
a weak-link supercurrent structure,
creating means for creating in said structure a plurality of
magnetic field vortices each characterized by a circulating
supercurrent and a flux quantum induced by said supercurrent,
means of establishing in said structure preferred locations about
each of which a vortex distributes itself so that a local minimum
of the sum of the total magnetic energy plus the Josephson coupling
energy is created with respect to such vortex, and
control means for causing selected ones of said vortices to
propagate in said structure from one of said preferred locations to
another.
2. The apparatus of claim 1 wherein said preferred locations are
centered in regions separated by a distance S which to a first
approximation satisfies the relationship LI.sub.c = .PHI..sub.o
where L is the self-inductance of a circulating supercurrent path,
.PHI..sub.o is the flux quantum supported by the circulating
supercurrent and I.sub.c is the net critical current of a region of
length S.
3. The apparatus of claim 1 wherein said control means comprises
means for applying control current in control regions near to each
of said preselected vortices and in a direction opposite to the
flow of supercurrent in said control regions so that said
preselected vortices propagate away from said control regions.
4. The apparatus of claim 1 wherein said control means comprises
means for applying control current in control regions near to each
of said preselected vortices and in the same direction as the flow
of supercurrent in said control regions so that said preselected
vortices propagate toward said control region.
5. The apparatus of claim 1 wherein
said weak-link structure is elongated,
said creating means comprises a current source connected to one end
of said structure, and
said control means comprises means for selectively applying control
current along the elongated dimension of said structure at spaced
points near to said preferred locations so that preselected ones of
said vortices are made to move toward the other end of aid
structure.
6. The apparatus of claim 5 including utilization means connected
to the other end of said structure to detect said vortices as each
reaches said other end.
7. The apparatus of claim 6 wherein the output of said creating
means comprises a pulse code modulated train of current pulses,
each pulse being separated by a blanking interval, and
said control means is turned on during said blanking intervals.
8. The apparatus of claim 7 wherein the current from said control
means is applied sequentially to each of said spaced points
beginning with the point nearest to said other end and ending with
the point nearest said one end.
9. The apparatus of claim 7 wherein the current from said control
means is applied substantially simultaneously to each of said
spaced points.
10. The apparatus of claim 1 wherein each of said preferred
locations is a region in which the critical supercurrent is
zero.
11. The apparatus of claim 10 including
a pair of superconductive layers,
a nonsuperconductive layer contiguous with and separating said
superconductive layers,
said nonsuperconductive layer having a plurality of apertures
extending therethrough between said superconductive layers, said
apertures forming said preferred locations and the remaining
portions of said nonsuperconductive layer in conjunction with said
superconductive layers contiguous therewith forming a plurality of
said weak-link regions electrically connected in parallel.
12. The apparatus of claim 11 wherein said nonsuperconductive layer
comprises a material selected from the group consisting of
insulators and normal metal.
13. The apparatus of claim 10 including
a pair of superconductive layers,
a nonsuperconductive layer contiguous with and separating said
superconductive layers,
said nonsupercondutive layer having a plurality of separated first
regions of thickness effective to prevent supercurrent tunneling
therethrough and a plurality of second regions interleaving said
first regions and of thickness effective to permit supercurrent
tunneling therethrough,
said first regions constituting said preferred locations in which
the critical supercurrent is substantially zero and said second
regions constituting a plurality of weak-link regions electrically
connected in parallel.
14. The apparatus of claim 13 wherein said nonsuperconductive layer
comprises a material selected from the group consisting of
insulators and normal metals.
15. The apparatus of claim 10 including
a first superconductive layer in the form of a strip having a
plurality of first appendages of the same material as said strip
extending therefrom,
plurality of nonsuperconductive layers, one formed on at least a
portion of each of said first appendages,
a second superconductive layer contacting each of said
nonsuperconductive layers, thereby forming a plurality of
superconducting loops each of which includes a pair of weak-link
regions electrically connected in parallel, each of said loops
being adapted so that to a first approximation LI.sub.c =
.PHI..sub.o where L is the self inductance of the loop in which a
supercurrent flows supporting a flux quantum .PHI..sub.o, and
I.sub.c is the critical current of each of said weak-link
regions.
16. The apparatus of claim 15 where said nonsuperconductive layers
comprise a material selected from the group consisting of
insulators and normal metals.
17. The apparatus of claim 16 including a separate resistive shunt
electrically connected in parallel with each of said weak-link
regions.
18. The apparatus of claim 17 wherein
said creating means comprises a current source connected between a
first one of said first appendages and said second layer,
said control means includes means for applying a current between
said second layer and selected ones of said other first appendages,
and including
utilization means connected between a different one of said first
appendages and said second layer.
19. The apparatus of claim 18 including a separate weak-link
magnetometer magnetically coupled to each of said loops.
20. The apparatus of claim 19 wherein
said first appendages are located on one side of said strip and
said magnetometers each comprise
a pair of second appendages extending from the other side of said
strip, opposite one of said loops,
a plurality of nonsuperconductive layers, one formed on a portion
of each of said second appendages,
a third superconductive layer contacting each of said
nonsuperconductive layers thereby forming a pair of weak-link
regions electrically connected in parallel, and
means for detecting changes induced in the critical supercurrent of
each of said latter pairs of weak-link regions.
21. The apparatus of claim 1 including
a pair of elongated superconductive layers,
an elongated nonsuperconductive third layer of substantially
uniform thickness contiguous with and disposed between said
superconductive layers, thereby forming an elongated multilayered
structure,
means for applying along said third layer a localized magnetic
field at a plurality of points equally spaced by a distance of
approximately twice the Josephson penetration depth and in a
direction substantially parallel to said third layer, thereby
defining said preferred locations about which said vortices prefer
to distribute themselves, said magnetic field being applied in
magnetic sense opposite to that of said vortex in said third
layer.
22. The apparatus of claim 21 wherein
said creating means comprises a current source connected across
said superconductive layer at one end of said structure, and
said control means comprises means for selectively applying a
control current near to predetermined ones of said vortices so that
said predetermined vortices propagate toward the other end of said
structure.
23. The apparatus of claim 22 including utilization means connected
across said superconductive layers at the other end of said
structure to detect said vortices as each reaches said other
end.
24. The apparatus of claim 23 wherein said nonsuperconductive layer
comprises a material selected from the group consisting of
insulators and normal metals.
25. The apparatus of claim 1 including
a first portion in which the critical supercurrent is substantially
uniform and nonzero,
contiguous with said first portion, a second portion in which the
critical supercurrent is substantially zero and which is
characterized by a variable self-inductance per unit length as
measured in a direction parallel to the direction of vortex
propagation.
26. The apparatus of claim 25 wherein the width of said second
portion undulates along a face parallel to said direction of vortex
propagation.
27. The apparatus of claim 26 wherein the width of said second
portion undulates periodically.
28. The apparatus of claim 27 wherein
said first portion comprises
a first pair of elongated superconductive layers,
a first elongated nonsuperconductive layer of substantially uniform
thickness effective to permit the flow of supercurrent
therethrough, said first nonsuperconductive layer being contiguous
with and disposed between said first pair of superconductive
layers, and
said second portion comprises
a second pair of superconductive layers,
a second elongated nonsuperconductive layer of substantially
uniform thickness effective to prevent the flow of supercurrent
therethrough, said second nonsuperconductive layer being contiguous
with and disposed between said second pair of superconductive
layers,
one elongated face of said first portion being contiguous with one
elongated face of said second portion.
29. The apparatus of claim 28 wherein the width of said second
portion undulates in the shape of a periodic square wave.
30. The apparatus of claim 28 wherein
said creating means comprises a current source connected across
said first pair of superconductive layers at one end of said
structure, and
said control means comprises means for selectively applying a
control current near to predetermined ones of said vortices so that
said predetermined vortices propagate toward the other end said
structure.
31. The apparatus of claim 30 including utilization means connected
across said first pair of superconductive layers at the other end
of said structure to detect said vortices as each reaches said
other end.
32. The apparatus of claim 31 wherein said nonsuperconductive
layers comprise a material selected from the group consisting of
insulators and normal metals.
33. For use in weak-link supercurrent apparatus, a structure
comprising a plurality of weak-link supercurrent regions
electrically connected in parallel to form a plurality of closed
circuit paths coupled to one another, adjacent ones of said regions
being uniformly separated by a distance S effective to sustain in
selected ones of said paths a circulating supercurrent which
supports a single magnetic flux quantum linking said path.
34. The structure of claim 33 wherein said distance S is selected
so that to a first approximation the following relationship is
satisfied
LI.sub.c = .PHI..sub.o
where L is the self-inductance of each of said paths, .PHI..sub.o
is the flux quantum supported by each circulating supercurrent and
I.sub.c is the net critical current of a region of length S.
35. The structure of claim 34 including
a pair of superconductive layers,
a nonsuperconductive layer separating said superconductive
layers,
said nonsuperconductive layer having a plurality of apertures
extending therethrough between said superconductive layers, thereby
defining between said apertures a plurality of spaced
nonsuperconductive regions which constitute said plurality of
weak-link regions connected in parallel.
36. The structure of clam 34 including
a pair of superconductive layers
a nonsuperconductive layer separating said superconductive
layers,
said nonsuperconductive layer having a plurality of spaced first
regions of thickness effective to prevent supercurrent tunneling
therethrough and interleaving said first regions a plurality of
second regions of thickness effective to permit supercurrent
tunneling therethrough, said second regions constituting said
plurality of weak-link regions connected in parallel.
37. For use in weak-link supercurrent apparatus for guiding the
propagation of magnetic flux vortices in a first direction, a
structure comprising
a first portion in which the critical supercurrent is substantially
uniform and nonzero,
contiguous with said first portion, a second portion in which the
critical supercurrent is substantially zero and which is
characterized by a variable self-inductance per unit length as
measured in a direction parallel to the direction of vortex
propagation.
38. The apparatus of claim 37 where the width of said second
portion undulates along a face parallel to said direction of vortex
propagation.
39. The apparatus of claim 38 wherein the width of said second
portion undulates periodically.
40. The apparatus of claim 39 wherein
said first portion comprises
a first pair of elongated superconductive layers,
a first elongated nonsuperconductive layer of substantially uniform
thickness effective to permit the flow of supercurrent
therethrough, said first nonsuperconductive layer being contiguous
with and disposed between said first pair of superconductive
layers, and
said second portion comprises
a second pair of superconductive layers,
a second elongated nonsuperconductive layer of substantially
uniform thickness effective to prevent the flow of supercurrent
therethrough, said second nonsuperconductive layer being contiguous
with and disposed between said second pair of superconductive
layers,
one elongated face of said first portion being contiguous with one
elongated face of said second portion.
41. The apparatus of claim 40 wherein the width of said second
portion undulates in the shape of a periodic square wave.
Description
BACKGROUND OF THE INVENTION
This invention relates to weak-link supercurrent structures and,
more particularly, to such structures capable of sustaining trapped
magnetic field vortices.
In the early stages of the superconductive art the basic switching
device was the classical cryotron a current controlled device
capable of being switched between a superconducting state and a
normal conducting state. As the art progressed, more sophisticated
switches such as the Josephson junction were developed. The
Josephson junction, as well as the now well known SNS, point
contact and bridge configurations, are characterized by the ability
to sustain a supercurrent at zero voltage up to a certain maximum
critical current J.sub.c, and by a "normal" superconducting state
at a finite voltage. Such devices are now described as "weak-link"
devices. Switching between these states is typically effected by
varying an applied current above and below J.sub.c or by fixing the
applied current and varying a magnetic field which in turn changes
J.sub.c. The basic Josephson junction and its aforementioned
properties are described in U.S. Pat. No. 3,281,609 issued to J. M.
Rowell on Oct. 25, 1966 and assigned to the assignee hereof.
Improved forms of weak-link devices are disclosed in U.S. Pat. No.
3,564,351 issued to D. E. McCumber on Feb. 16, 1971, also assigned
to the assignee hereof. In addition, the superconductive totalizer
or analog-to-digital converter disclosed in U.S. Pat. No. 3,450,735
issued on July 29, 1969 to M. D. Fiske is exemplary of the prior
supercurrent art in which binary information is represented by a
Josephson junction being in either its supercurrent-zero voltage
state or its "normal" superconducting-finite voltage state.
It is an object of our invention, however, to represent logic
information in a plurality of coupled weak-link devices, or in a
plurality of coupled weak-link regions of an extended single
junction device, which during operation remain in a supercurrent
state.
It is another object of our invention to represent such information
by a plurality of trapped magnetic vortices capable of being
controllably created or annihilated at preferred locations in a
logic device.
It is another object of our invention to controllably move such
vortices in shift register fashion from one such preferred location
to another.
SUMMARY OF THE INVENTION
These and other objects are accomplished in an illustrative
embodiment of our invention, a weak-link supercurrent logic
structure which is able to sustain one or more trapped magnetic
field vortices. In an extended Josephson junction device which is
large compared to the Josephson penetration depth (.lambda..sub.J),
such a vortex is induced by a spatial variation of the supercurrent
J(x) in which a positive supercurrent flows through the oxide layer
and into the contiguous superconductor to a depth .lambda..sub.L,
the London penetration depth, then along the superconductor a
distance of about 2.lambda..sub.J, thence through the oxide again
as a negative supercurrent into the opposite superconductor to a
depth .lambda..sub.L and finally back to the point of beginning.
Such a vortex supports a net magnetic flux of precisely .PHI..sub.o
= 2.07 .times. 10.sup..sup.-15 Wb, the well-known flux quantum.
Hereinafter, the term "vortex" shall define an entity which
includes both the circulating supercurrent J(x) and the flux
quantum .PHI..sub.o induced thereby.
Once a vortex is created it prefers to position and distribute
itself in a region so that a local minimum of the sum of the
magnetic energy plus the Josephson coupling energy is established.
Where a plurality of such preferred locations are present in a
single weak-link structure, it is possible to move the vortex from
one such location to another by applying a force thereto as, for
example, by applying a local current or magnetic field to a region
near to the vortex.
A number of ways are hereinafter described to create such preferred
locations including, for example: (1) creating regions in the
structure at which J.sub.c (x) = 0, e.g., by intentionally
fabricating the oxide with gaps in it, thereby forming a structure
having periodic oxide regions; a similar structure may also be
fabricated utilizing discrete isolated junctions connected in
parallel; (2) fabricating the oxide of a variable thickness to
create regions of variable J.sub.c (x) in a fashion similar to (1)
above; (3) applying a local point source of magnetic field at
periodic locations along the oxide layer; (4) applying a local
current at periodic locations along the oxide layer to create a
local magnetic field analogous to (3) above; and (5) fabricating
the structure such that it has a variable self-inductance per unit
length, e.g., in top view the width of the device undulates in a
prescribed manner such that a vortex prefers to position itself
about a region of minimum width.
In a similar fashion vortices and preferred locations can be
created in other types of weak-link structures. As used herein, the
term "weak-link structure" includes, but is not limited to, not
only a structure having a single uniform weak-link region (e.g.,
structures (3), (4) and (5) supra) but also a structure having a
plurality of smaller separated weak-link regions connected in
parallel e.g., structures (1) and (2) supra).
BRIEF DESCRIPTION OF THE DRAWING
These and other objects of the invention, together with its various
features and advantages, can be more easily understood from the
following more detailed description taken in conjunction with the
accompanying drawing, in which:
FIG. 1 is an end view of a typical Josephson junction;
FIG. 2, Parts A - E, indicate the gradual change in supercurrent
spatial distribution as applied current is increased;
FIG. 3A shows schematically a trapped vortex having a supercurrent
J(x) and a magnetic field B;
FIGS. 3B and 3C are graphs of the approximate distribution of
vortex magnetic field in the junction of the devices of FIG.
3A;
FIGS. 4A-4C are end views of a Josephson junction structure in
accordance with an illustrative embodiment of our invention in
which a hole in the oxide creates a region where J.sub.c = 0;
FIG. 5 is an isometric view of a second embodiment of our invention
utilizing discrete Josephson junctions;
FIG. 6 is an end view of a third embodiment of our invention in
which the oxide layer is of variable thickness;
FIG. 7A is an end view of a fourth embodiment of our invention
utilizing point sources of magnetic field to create preferred
vortex locations;
FIG. 7B is an en view of a fifth embodiment of our invention in
which preferred locations are made to propagate;
FIG. 8 is an end view of a sixth embodiment of our invention
utilizing current sources to create preferred vortex locations;
FIG. 9 is an isometric view of a seventh embodiment of our
invention utilizing variable self-inductance to create preferred
vortex locations;
FIG. 10A is a top view of a working example of our invention
utilizing a pair of the devices of FIG. 5 connected in parallel;
and
FIG. 10B is a schematic of FIG. 10A.
DETAILED DESCRIPTION
Before discussing in detail the various embodiments of our
invention, it will be helpful to consider the spatial buildup of
supercurrent in an SIS Josephson junction and the subsequent
creation of a trapped magnetic field vortex. An SIS Josephson
junction is depicted for the purposes of illustration only, it
being understood that the following comments apply equally as well
to other types of weak-link structures.
Turning then to fig. 1, there is shown an end view of an elongated
Josephson junction structure 10 of length l>>.lambda..sub.J,
the Josephson penetration depth. The structure 10 is multilayered
including an oxide layer 12 (e.g., PbO) formed between a pair of
superconductive layers 14 and 16 (e.g., Pb). A current source 18 is
connected across the superconductors at he left-hand edge 19.
As the current from source 18 is increased, the supercurrent J(x)
penetrates farther and farther into the device from left to right
until at some current I = I.sub.1, as shown in FIG. 2, Part A, the
maximum supercurrent is at the left-hand edge (x = 0) and the
distribution J.sub.1 (x) gradually decreases to zero at a distance
.lambda..sub.J from the left-hand edge. Note that in FIG. 1, as
well as in embodiments to be subsequently described, the
supercurrent is shown on the end face for the purpose of clarity
only. In practice, the supercurrent would be interior to the device
and generally located opposite the contacts of source 18. As the
current is increased further, the point of maximum J(x) moves away
from the left-hand edge and to the right, as shown in FIG. 2, Parts
B and C. In fact, at I > I.sub.3 the supercurrent flows across
the junction in both directions as represented by the positive and
negative values of J.sub.4 (x) in FIG. 2, Part D.
At this point it should be noted that the spatial supercurrent
distributions J.sub.1 (x) to J.sub.4 (x) are fixed in space. At a
current I = I.sub.5 > I.sub.4 (FIG. 2, Part E), however, a limit
is reached where the supercurrent distribution cannot adjust itself
to carry additional current. Consequently, an incipient vortex is
formed which propagates to the right at a velocity, and to a
distance, determined by damping processes (e.g., single particle
tunneling). Simultaneously, another vortex begins to form at the
left-hand edge as described with reference to FIG. 2, Parts A - D.
The process is repeated until other factors intervene, e.g., the
applied current is reduced or a propagating vortex stops at a
preferred location somewhere between the left and right-hand edges.
Techniques for creating such preferred locations will be discussed
hereinafter.
A schematic of an isolated vortex is shown in FIG. 3A, where it has
been assumed that the vortex is stationary at some arbitrary
preferred location designated 20. From point 20 where J(x) = 0 the
supercurrent increases (positively and negatively) on either side
thereof to a distance .lambda..sub.J (typically about 100 .mu. in
SIS devices, but typically considerably shorter in SNS and other
weak-link devices) so that the total length of the spatial
distribution of J(x) extends over a length of about
2.lambda..sub.J. This current tunnels from one superconductor to
the other through the oxide layer penetrating to a depth
.lambda..sub.L, the London penetration depth (typically about 0.1
.mu. is SIS devices). As shown, the lateral skin current flows in
each superconductor parallel to the oxide layer 12 but in opposite
directions thus forming a closed supercurrent loop centered at
point 20. This current supports precisely one magnetic flux quantum
.PHI..sub.o = 2.07 .times. 10.sup..sup.-15 Wb which extends through
the oxide layer from one end face 19 to the other (not shown)
closing upon itself through space. The magnetic field B.sub.v
associated with the vortex is related to .PHI..sub.o by the
well-known relation that
where A is the area bounded by J(x). For simplicity only, B.sub.v
is shown in FIG. 3A as consisting of a single closed flux line. In
actuality, in the x-direction along the junction, B.sub.v (x) is
approximately Gaussian in shape as shown in FIG. 3B, whereas in the
z-direction across the junction, B.sub.v (z) is uniform in the
oxide and decays exponentially to a distance of about
.lambda..sub.L as shown in FIG. 3C.
A vortex once created as previously described will prefer to
position and distribute itself in a region such that a local
minimum (not necessarily an absolute minimum) of the sum E of the
magnetic energy E.sub.m plus the Josephson coupling energy E.sub.J
is created, i.e., a minimum of E = E.sub.m + E.sub.J given by
The first term on the right-hand side of Equation (1) is E.sub.m
and the second is E.sub.J, where .mu..sub.o is the permeability of
free space, B is the total magnetic field equal to B.sub.ext +
B.sub.v, where B.sub.ext is the magnetic field other than that of
the vortex and B.sub.v is the magnetic field associated with the
vortex; V is volume, .PHI..sub.o is the flux quantum; J.sub.c (x,y)
is the critical supercurrent density in the junction (x,y) plane;
and .phi.(x,y) is the spatially dependent phase difference between
the wave-functions in the superconductors on either side of the
junction.
In effect, a vortex "seeks out" regions in which this minimization
can be accomplished. Once located in such region, a vortex will
remain there until a force is applied to the vortex as described
hereinafter.
Consequently, it is desirable to construct a device so that
vortices can distribute themselves to minimize E.sub.m and/or to
maximize E.sub.j (since its contribution to E is negative). Of
course, trade-offs may be required since in a particular structure
the position of a vortex in a particular region may, for example,
both decrease E.sub.m and decrease E.sub.J or conversely may both
increase E.sub.J and increase E.sub.m. In the latter cases,
therefore, the relative magnitude of changes should be considered.
Utilizing the above criteria, we have determined that preferred
vortex locations may be created in a number of illustrative ways,
to wit:
A. by fabricating a weak-link structure with one or more regions
where J.sub.c = 0, as by the use of discrete Josephson
junctions;
B. by fabricating an oxide layer of a Josephson junction, for
example, to have a thickness which varies in the z-dimension along
the layer;
C. by utilizing a uniform junction and point sources of magnetic
field placed at periodic points along the junction to establish a
field in opposition to B.sub.v ;
D. by utilizing locally applied currents to establish the field of
(C) above;
E. by fabricating the weak-link structure to have a variable
self-inductance per unit length.
Before describing each of the foregoing illustrative embodiments,
however, the determination of the separation between preferred
locations will be discussed.
PREFERRED LOCATION SPACING
In a uniform extended junction (e.g., FIGS. 3A, 7A, 8) in order to
support flux vortices the appropriate separation of adjacent
preferred locations is about 2.lambda..sub.J. On the other hand, in
a nonuniform junction (e.g., FIGS. 4C, 5, 6) the appropriate
separation S is determined as follows: given an extended structure
having arbitrary spacing S between tentatively selected preferred
locations, but otherwise having a fixed geometry, one can readily
calculate numerically in a well-known fashion the extent and
precise form of a supercurrent distribution J(x,y) which will
support a single quantum of flux .PHI..sub.o ; i.e., the form of a
vortex in such a structure can be determined. This calculation is
performed by utilizing Josephson's equations (see, for example,
Physical Review, 41, 2,047 (1970) by C. S. Owen and D. J.
Scalapino), as they relate the spatial variation of .phi.(x,y) to
magnetic fields, in conjunction with Maxwell's equations relating
the supercurrent flow to the magnetic field distribution.
Having thus chosen an arbitrary S and calculated the vortex shape,
one compares the two. If the vortex dimension in the x-direction is
substantially larger than the spacing S, the calculation is
iterated for a smaller S until a value of S is determined which is
substantially equal to the vortex dimension in the x-direction.
Conversely, if a value S were initially chosen to be too large, it
is possible that more than one vortex could exist within a region
of length S, a possibility which can readily be checked by
numerical calculation. If so, a smaller S should be chosen until,
again, it matches the vortex dimension in the x-direction. Precise
equality is not, however, required as long as S is chosen to
confine a single vortex.
To a first approximation, the preceding criterion is equivalent to
satisfying the relationship
L I.sub.c = .PHI..sub.o (2)
where L is the self-inductance of a typical supercurrent loop which
supports a single flux quantum .PHI..sub.o and I.sub.c is the net
critical current of a region of the device of length S.
EMBODIMENT A: REGIONS WHERE J.sub.c = 0
One way to create a region where J.sub.c = 0 is to form the oxide
layer 12, as shown in FIG. 4A, with a hole 22 therein extending
between the superconductors. Accordingly, the supercurrent J(x)
centers itself on the hole with current flowing in opposite
directions on either side thereof. The flux .PHI..sub.o (not shown)
is directed into the page and is substantially confined in the hole
22.
To appreciate qualitatively the reason that the vortex prefers to
"sit" on the hole it must be recognized that the right hand term
(E.sub.J) of Equation (1) would be a maximum if cos .phi. equaled
unity (.phi. = 0, 2.pi.) everywhere in the junction. However, the
presence of a magnetic field B(y) in the y-direction, for example,
either from an external source or the vortex itself, causes .phi.
to vary spatially in the x-direction because the derivative
d.phi./dx is proportional to B(y). Consequently, the presence of a
vortex alone dictates that .phi. cannot be zero everywhere in the
x-dimension. In fact, with reference again to FIG. 3A, cos .phi. =
1 at points 21 and 23, the extreme edges of J(x) and cos .phi. = -1
at the center 20. At intermediate points cos .phi. takes on values
between +1 and -1. Since .phi. varies with x, it follows that cos
.phi. does also. To increase E.sub.J it would be desirable to
reduce the contribution of the regions corresponding to values of
cos .phi. .ltoreq. 1 (e.g., negative contributions of regions where
cos .phi. .ltoreq. 0). One way of effecting this result is to
locate the center of the vortex at a point where J.sub.c = 0 since
E.sub.J involves the product of J.sub.c (x,y)cos .phi. (x,y).
Consequently, the negative contributions to E.sub.J are eliminated,
E.sub.J is increased and, as desired, E is decreased.
Consider now that the hole is made wider as shown at 24 in FIG. 4B.
Since no supercurrent can flow in the hole, the distribution J(x)
accommodates the hole by increasing its density in the extreme
regions at 26 and 28. Note that the device of FIG. 4B is beginning
to resemble two discrete junctions, one on either side the hole 24.
Ultimately, as shown in FIG. 4C, a structure having a plurality of
such holes 24 between discrete oxide regions 30 will support a
plurality of vortices, one centered on each hole. As discussed
previously, the oxide regions are separated by a distance S
calculated to satisfy equation (2). Therefore, a single flux
quantum is induced by each supercurrent loop 32 flowing between
adjacent oxide regions 30. Note also that supercurrent from
adjacent vortices may flow through a common junction in opposite
directions, thereby producing substantially total cancellation of
the supercurrent therein. Thus, adjacent loops 32.1 and 32.2 flow
through common junction 30.2 producing substantially total
cancellation. The loop, therefore, in effect extends between
junctions 30.1 and 30.3 and supports two vortices, one centered at
point 24.1 and one at point 24.2.
Instead of utilizing holes in an otherwise uniform junction
structure to create preferred vortex locations, it is possible as
shown in FIG. 5 to utilize a discrete junction configuration which
satisfies Equation (2). In this case the "hole" is created by
fabricating a U-shaped superconductor 40 on the end portions of
which are formed discrete oxide layers 42 and 44. Subsequently
superconductor 46 is deposited to join the oxide layers, thereby
forming a pair of Josephson SIS junctions electrically connected in
parallel. Current source 48 causes a supercurrent J(x) to flow in
path shown by the dashed line. This supercurrent supports a
magnetic field B.sub.v which threads the hole 50 formed by U-shaped
superconductor 40 and superconductor 46. The use of this type of
device in a shift register will be described hereinafter with
reference to FIGS. 10A and 10B.
EMBODIMENT B: VARIABLE INSULATOR THICKNESS
Recognizing that thick insulator regions (e.g., about 20 A. thick)
also produce regions where J.sub.c = 0 (since electron pair
tunneling is effectively prevented), it follows that a structure
such as shown in FIG. 6 will support a plurality of trapped
vortices. More specifically, an insulator, such as oxide layer 12,
is fabricated with a plurality of thin oxide regions 62, each
capable of carrying a supercurrent, separated from one another by
thick oxide regions 60 in each of which J.sub.c = 0. As before the
thin oxide regions 62 are spaced from one another by a distance S
calculated to satisfy Equation (2). Accordingly, a single vortex
prefers to center itself on a thick oxide region 60 with the
supercurrent J(x) flowing through adjacent thin oxide regions
(i.e., the operative junctions).
EMBODIMENT C: POINT SOURCES OF MAGNETIC FIELD
In FIG. 7A there is shown a cross-sectional view of a two-stage
Josephson shift register in accordance with an illustrative
embodiment of our invention in which a substantially uniform oxide
layer 59 is sandwiched between a pair of elongated superconductive
layers 63 and 64. Vortices are created by means of current source
61 connected across the superconductors 62 and 64 at the left-hand
edge 57. Illustratively, a pair of preferred vortex locations 66
and 68 are established by directing at each of these locations an
external magnetic field B.sub.ext generated by point sources 70 and
72. The magnetic sense of B.sub.ext is made to be opposite of that
of B.sub.v thereby reducing the total field B = B.sub.ext +
B.sub.v. Since E.sub.m is thereby reduced, vortices distribute
themselves around points 66 and 68 so that local minima of Equation
(1) are created. Since FIG. 7A depicts a uniform junction, joints
66 and 68 are separated by about 2.lambda..sub.J to satisfy
Equation (2).
In order to cause the vortices to propagate to the right, a current
source 74 is selectively connectable by means 52, switches 75 and
77 to points 52 and 54 intermediate each vortex location. With
switch 77 closed a current I.sub.2 flows across the junction in a
region adjacent to the right-hand side of supercurrent loop J.sub.2
(x). The current I.sub.2 flows in the same direction as J.sub.2 (x)
does in region 69. As a result, J.sub.2 (x) will shift to the right
(toward I.sub.2). At the extreme right-hand end the vortex is
detected by utilization device 76, typically a weak-link double
junction magnetometer, (or alternatively a voltmeter) connected
across the right-hand end of superconductors 63 and 64. Note that
both vortices may be moved together by closing switches 75 and 77
simultaneously. In addition, adjacent vortices can be moved
simultaneously if I.sub.1 is sufficiently large (thus J.sub.1
"pushes" J.sub.2 to the right). Care should be exercised, however,
since too much control current I.sub.1 may drive the junctions into
a finite-voltage state. Moreover, whereas control current flows
between preferred vortex locations, as at point ,, it should be
made to flow much nearer to the preferred vortex location it is
designated to control than to the adjacent vortex location, e.g.,
since I.sub.1 is designated to control J.sub.1, point 52 should be
closer to vortex location 66 than location 68.
A similar embodiment is shown in FIG. 7B. The sequential
application of point sources of magnetic field generated by control
sources M1-M3, M1'-M3' and M1"-M3", causes the preferred vortex
locations, and thus the vortices themselves to propagate. For
example, with only M1-M3 ON, the preferred locations are designated
by an x at locations P1-P3, respectively. If now M1'-M3' are also
turned ON (and then M1-M3 turned OFF), the preferred location moves
to the right to intermediate positions P1'-P3'. Consequently,
vortices originally centered at P1-P3 move to P1'-P3',
respectively. Similarly, activation of M1"-M3" causes vortices to
move to the left to P1"-P3", respectively. This form of control can
be applied equally as well to the other embodiments of our
invention.
EMBODIMENT D: CURRENT SOURCES TO GENERATE B.sub.ext
As discussed above with reference to Embodiment C, FIG. 7A, point
sources of magnetic field B.sub.ext applied at periodic locations
along the oxide layer define preferred vortex locations provided
the sense of B.sub.ext is opposite to that of B.sub.v in the oxide.
In FIG. 8, the point sources 70 and 72 of FIG. 7A have been
replaced by local current sources 80 and 82, respectively. The
resultant current flow in oxide layer 59 from sources 80 and 82
generates at each preferred location a local magnetic field
B.sub.ext. Tee operation and structure of this embodiment are
otherwise identical to that of FIG. 7A. In addition, however, an
alternate form of vortex source means is shown. Current source 61,
instead of being connected across superconductors 63 and 64, is
connected across an inductor 65 which is positioned to produce a
magnetic field at point 66 near to the left-hand end and in the
plane of the junction. This field in turn induces a circulating
supercurrent J(x). Either of these source means may be used
interchangeably with each of the embodiments of our invention.
EMBODIMENT E: VARIABLE SELF-INDUCTANCE
In FIG. 9 there is shown an illustrative embodiment of our
invention comprising a first portion 84, where J.sub.c (x,y) is
uniform and nonzero, and a second laterally contiguous portion 86
where J.sub.c (x,y) = 0 and the self-inductance per unit length is
variable. The self-inductance per unit length L(x) of the combined
portions is therefore also variable.
More specifically, portion 84 illustratively comprises a uniform
Josephson junction having an oxide layer 91.1 of uniform thickness
sandwiched between superconductive layers 93.1 and 95.1. Portion 86
is similarly constructed except that oxide layer 91.2 is thicker in
order that J.sub.c be made zero therein. In addition, the width of
portion 86 is made variable as measured in a direction (y-axis)
normal to the direction of vortex propagation (x-axis). To produce
a variable L(x) at least one edge 90 parallel to the direction of
propagation is made to have an undulating, preferably periodic,
shape. As shown in FIG. 9, edge 90 illustratively has a square wave
shape. A current source 92 connected across superconductors 93.1
and 95.1 creates a supercurrent flow across the junction and
generates trapped vortices as previously described. These vortices
prefer to position themselves at points 96 of minimum width (i.e.,
in the notches). As before, current from control source 94 is
applied to regions 89 intermediate preferred vortex locations 96 to
shift the vortices to the right, selectively or simultaneously,
depending on the manner in which switches 87 are closed. Of course,
the device of FIG. 9 may also be symmetrical by fabricating on one
end face 88, a structure which is a substantial mirror image of
portion 86.
In order for this embodiment to operate effectively in trapping
vortices in the notches 90.2, it is important that the notch
separation s be properly chosen. In devices in which the
undulations take on complicated shapes, a proper s would be
calculated by numerical analysis to satisfy Equation (2). However,
the embodiment of FIG. 9 utilizes a simplified undulating shape, a
square wave, in which the maximum width (y-dimension) is 2w, the
minimum width (in a notch 90.2) is w, the width of a notch is s and
the notches are separated from one another by a distance s. In this
structure the self-inductance per unit length L in the notches 90.2
is given by
L = .mu..sub.o (2.lambda..sub.L + d)/w (3)
where .mu..sub.o is the permeability of free space, .lambda..sub.L
is the London penetration depth and d is the oxide thickness in
portion 86. In the wide sections 90.1 the inductance per unit
length is one-half Equation (3).
When s is properly chosen, a vortex will position itself in the
center of a notch 90.2 and extend on either side thereof. The
supercurrent flow I of the vortex is, with a factor of about 2,
equal to J.sub.c s (the approximation arises because .phi.(x,y)
depends on position in the notch so that sin .phi..noteq.1 in the
entire notch).
Moreover, the supercurrents on the average will circulate around a
loop of approximately length s centered in a notch so that the loop
inductance is approximately .mu..sub.o (2.lambda..sub.L + d)s/w.
Since Equation (2) dictates that LI = .PHI..sub.o, s to a first
approximation is given by
That the vortices prefer to sit in the notches, which are points of
high self-inductance per unit length, can be understood by
reference to the E.sub.m term of Equation (1). More specifically,
the magnetic field of the vortex, which is mainly concentrated in a
notch, has a value B.sub.v = .mu..sub.o I/w, where .mu..sub.o is
the permeability of free space, I is the supercurrent associated
with the vortex and w is the width of the structure in a notch.
Since w is smaller in a notch, B.sub.v is larger thereby
disadvantageously increasing the value of B.sup.2 in the E.sub.m
term. However, the spatial extent of the vortex in the x-direction
is smaller in a notch which more than compensates for the larger
B.sup.2 contribution. Another way of viewing this principle is to
recognize that the magnetic energy E.sub.m is approximately equal
to .PHI..sub.o.sup.. I. In the notches a higher self-inductance
prevails so that the current I required to support .PHI..sub.o is
smaller.
EXAMPLE
A two-loop, three-junction shift register 100 as shown in FIG. 10A
has been successfully constructed and operated as follows.
On a rectangular glass substrate 102 there was evaporated a
rectangular Sn film 104 filling the central portion of the
substrate to serve as a superconducting ground plane. Both the Sn
and glass were then covered with an evaporated germanium film (not
shown) to electrically insulate the ground plane. The region of the
ground plane forms the surface on which were evaporated the thin
films which form the actual shift register.
On the ground plane 104 there was next deposited an evaporated Sn
film 106 having an elongated central member 106a, three equally
spaced (by a distance of about 3 mm) appendages 106 (b-d) on one
side thereof, and five appendages 106 (e-i) on the other side
thereof. Thereafter the surface of the Sn film 106 was oxidized in
a glow discharge of oxygen. Subsequently, evaporated Sn strips 107,
108 and 109 were deposited so that strip 108 coupled members 106e
and 106f, strip 109 coupled members 106h and 106i and strip 107
coupled members 106b, 106c and 106d. Josephson junctions were thus
formed at the regions of overlap between film 106 and strips 107,
108 and 109. The configuration of the junctions and the loops 113
and 114 were adapted to satisfy Equation (2). In a final
evaporation, silver shunts 110, 111 and 112 were deposited in
parallel with each of the junctions.
In FIG. 10B the pattern of the films 106 to 109 of FIG. 10A is
shown schematically. The circles E, F and G designate the three
junctions which, together with the two large right-hand loops 113
and 114 comprise a two-stage shift register (using structures of
the type shown in FIG. 5). The other junctions, designated A, D, K
and L comprise two separate double-junction interferometers
(magnetometers) which were used to monitor the magnetic flux
contained in the loops 113 and 114 of the shift register. More
specifically, the junction pair A-D comprises a magnetometer to
monitor loop 113 and junction pair K-L comprises a magnetometer to
monitor loop 114. The two magnetometer loops are designated 115 and
116. Current leads were attached at the various numbered points
number 1 to 10.
In our structure the Sn films were of the order of 1,000 A. thick
and the Ge film was about 10,000 A. thick. The ground plane was
about 1 cm by 1/2 cm and the upper films 106-109 were about 0.2 mm
wide and a few mm long. The dimensions of the various loops were
about 1-3 mm by 1-3 mm.
The entire structure was cooled in liquid He below the
superconducting transition of Sn, and was shielded from the earth's
magnetic field by well-known mu-metal. Sn was chosen because
oxidation thereof is relatively easy. In practice Pb, Nb or Ta
which are superconducting at 4.2.degree. K. may also be used.
The I-V curves of the two magnetometers were displayed on an
oscilloscope by applying current and measuring voltage between
leads 7 and 8 and between leads 9 and 10 for the magnetometers A-D
and K-L, respectively. The critical supercurrents of these
magnetometers depend upon the magnetic field linking the loops 115
and 116. Since any flux supported in, say, the register loop 113
requires a current flowing around that loop, and in particular
through the portion BC, some flux from that loop will also link the
A-D magnetometer loop 115. Thus, the critical supercurrent of
magnetometer A-D is affected by any flux present in the register
loop 113, and one may observe changes in this flux as changes in
the critical supercurrent of the magnetometer.
In the structure built the self-inductance of each register loop
113 and 114 was about 2.times.10.sup..sup.-11 H, and of each
magnetometer loop 115 and 116 about 4.times.10.sup..sup.-11 H. The
critical supercurrents of the magnetometers were about 50 .mu.A,
depending on temperature and other factors. The coupling between
the magnetometer loops and the register loops was about 0.2, i.e.,
one-fifth of the flux trapped in a register loop linked a
magnetometer loop.
The critical current of each register junction E, F and G was
inferred from operational behavior to be in the approximate range
100 -500.mu.A.
In operation, with both magnetometer I-V curves displayed on an
oscilloscope so that their critical currents could be monitored,
current was passed through leads 1-2, starting with zero current
and gradually increasing. At a current, typically of the order of
100-200.mu.A, a sudden change in the critical current of the
magnetometer A-D was noted. No corresponding change was noted in
the critical current of the magnetometer 116. We concluded
therefore that a flux quantum (i.e., vortex) entered the register
loop 113.
A subsequent decrease of the current in leads 1-2 to zero produced
no change -- the flux quantum remained in the register loop 113. A
negative current of sufficient magnitude, again about 100 .mu.A
applied to leads 1-2, however, caused the flux quantum to be
annihilated in the register loop 113. A still larger negative
current introduced a flux quantum of the opposite sign in loop 113.
(Note that if a clockwise current can be made to give a positive
flux quantum, the same size current counterclockwise will produce a
negative flux quantum.)
Similarly current made to flow into lead 5 and out of lead 6
produced analogous results, i.e., a flux quantum entered the
register loop 114 but not the register loop 113.
When current was applied to leads 3-4, a different result was
observed; namely, that a sufficiently large current caused flux to
suddenly appear in both loops 113 and 114, i.e., a positive flux
was placed in the upper loop 113 and negative flux in the loop 114.
If there had been a positive flux quantum in the loop 114
initially, this would have cancelled out the negative flux quantum
resulting from application of the current to leads 3-4.
Consequently, applying current to leads 3-4 caused in effect the
transfer of a flux quantum from the loop 113 to loop 114. Thus, a
two-stage shift register was demonstrated.
In principle, if the critical currents of the register junctions
are all the same and are properly chosen, then the loops can hold
only, one, zero, or minus one flux quantum. Application of larger
currents to leads 1-2 than that required to produce one flux
quantum would have the effect of driving flux into the next loop
(by "passing it along"). Illustratively, a flux quantum is
transferred from one register loop to another in a time
corresponding to the inverse of the Josephson plasma frequency,
i.e., about 10 picoseconds.
The purpose of the silver shunts 110, 111 and 112 will now be
discussed. If current is applied to, say, leads 1-2, at the moment
when the flux enters the register loop 113 a voltage pulse is
developed across the junction E (which is the mechanism for causing
currents to flow around the loop). The existence of the pulse at
the same time as the current is applied to the leads 1-2 means
energy is given to the circuit
which is partially the 1/2 LI.sup.2 magnetic energy due to creating
a current flowing around the loop 113 and partly a charging of the
junction capacitance. This latter charging causes LC oscillations
which can have the effect of allowing a second flux quantum to
enter loop 113 or of transferring the flux down into the adjoining
loop 114. The purpose of the shunts is to damp these oscillations
(by providing low resistance in parallel to the junction
capacitance).
It is to be understood that the above-described arrangements are
merely illustrative of the many possible specific embodiments which
can be devised to represent application of the principles of the
invention. Numerous and varied other arrangements can be devised in
accordance with these principles by those skilled in the art
without departing from the spirit and scope of the invention. More
particularly, while the preceding embodiments relate specifically
to the propagation of vortices in one-dimension, it is possible to
move such vortices in more than one dimension, e.g., in two
dimensions in a plane.
* * * * *