Abstract:
The invention is about cascading high-transition-temperature superconducting quantum interference devices (SQUIDs) for sensing magnetic fields. These SQUIDs in series are connected with coils for picking up detected magnetic signals. Depending on the patterns of pick-up coils, magnetometers or gradiometers, which sense the magnetic field intensity and magnetic field gradient respectively, are achieved. Examples of magnetometers and gradiometers includes cascading high-T c  SQUIDs in series are provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority benefit of U.S. provisional application Ser. No. 60/815,517, filed on Jun. 20, 2006, all disclosures are incorporated therewith. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of Invention  
         [0003]     The present invention relates to sensing structure for sensing magnetic field or magnetic flux. More particularly, the present invention relates to a technology of magnetometer and gradiometer of superconducting quantum interference device (SQUID) to sense magnetic field/flux.  
         [0004]     2. Description of Related Art  
         [0005]     The conventional superconducting quantum interference device (SQUID) with ultra-high sensitivity to the magnetic flux has been proposed. The SQUID is, for example, popularly applied to sense weak magnetic signals, for example biomagnetic signals.  FIG. 1  is a drawing, schematically illustrating a conventional SQUID. A SQUID  100  is usually formed on a substrate. The substrate has a boundary  101 . The boundary is, for example, formed two grain region  102   a  and  102   b  with a grain boundary. Alternatively for example, the two regions  102   a  and  102   b  may have a step height to form a step boundary. The SQUID  100  has the superconducting film as shown in  FIG. 1  by shading. The SQUID  100  includes two Josephson junctions  110  in parallel induced by the boundary  101 . The electrode lead  104   a  is disposed on the substrate at the region  102   a , usually having two lead terminals. One terminal I  106  is for applying a current through the Josephson junctions  110  and the other terminal V  108  is for detecting an induced voltage signal. The electrode lead  104   b  is grounded.  
         [0006]     The basic detecting mechanism of SQUID is following. When a certain current slightly higher than the critical current of Josephson junctions  110  flows through the Josephson junctions  110 , a resistance at the Josephson junction occurs. Then, the resistance induces a voltage level, which can be detected. Due to the property of superconducting material without having magnetic flux, when an external magnetic flux is shone onto a SQUID, a circulating current through these two junctions is induced to compensate the external magnetic field. Thus, a voltage cross the junctions is generated in response to the external magnetic flux.  
         [0007]     However, the conventional SQUID can still only detect the intensity of magnetic field having magnetic flux through a small area. To increase the sensing area for achieving a higher sensitivity, SQUIDs are usually hooked with superconducting coils to form magnetometers or gradiometers. On the other hand, with the discovery of high-T c  superconductors, SQUID magnetometers or gradiometers made of high-T c  superconductors show impact to practical applications because of low system cost and easy cryogenic handling. Thus, various designs of high-T c  SQUID magnetometers and gradiometers are still under developing.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention provides a magnetometer or a gradiometer having a plurality of SQUIDs to more efficiently measuring magnetic flux or intensity gradient of magnetic field. The SQUID can be formed by high-T c  superconductors.  
         [0009]     The invention provides an embodiment of a SQUID magnetometer, suitable for sensing a magnetic field. The magnetometer includes a plurality of SQUID units. A plurality of superconducting connection parts connects the SQUID units to have a cascade connection. A plurality of electrode leads is respectively connected to the separated SQUID units. Different pair of the electrode leads are taken, the different sensitivity is achieved. This depends on the actual need in use. The present invention can indeed effectively improve the sensitivity of the SQUID magnetometer and can have more application in various choices.  
         [0010]     The invention also provides an embodiment of a SQUID magnetometer, including a SQUID set, divided by a boundary into a first part and a second part. The SQUID set has multiple electrode leads respectively at the first part and the second part, and multiple superconducting bars crossing the boundary and connecting the electrode leads in the first part and the second part. A coil-type magnetic-flux sensing part is disposed at the on the same side of the first part with respect to the grain boundary to connect the first part of the SQUID set at the superconducting bars, wherein a material of the coil-type magnetic-flux sensing part is a superconducting material.  
         [0011]     The invention also provides a SQUID gradiometer, including at least one SQUID set. Each SQUID set has multiple SQUID units connected side by side and divided by a boundary into a first part and a second part. Multiple electrode leads are connecting to the SQUID units. Different pair of the electrode leads are taken, the different sensitivity is achieved. This depends on the actual need in use. The present invention can indeed effectively improve the sensitivity of the SQUID gradiometer and can have more application in various choices. A first coil-type magnetic-flux sensing part of superconducting material is disposed at the first part. A second coil-type magnetic-flux sensing part of superconducting material, disposed at the second part. A common connection portion is connecting between the SQUID units and connecting to the first coil-type magnetic-flux sensing part and the second coil-type magnetic-flux sensing part. The first coil-type magnetic-flux sensing part senses a first magnetic flux and the second coil-type magnetic-flux sensing part senses a second magnetic flux, to obtain a magnetic field gradient.  
         [0012]     It will be apparent to those ordinarily skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing descriptions, it is intended that the present invention covers modifications and variations of this invention if they fall within the scope of the following claims and their equivalents. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0014]      FIG. 1  is a drawing, schematically illustrating a conventional SQUID.  
         [0015]      FIG. 2  is drawing, schematically illustrating structure of a bare SQUID, according to an embodiment of the invention.  
         [0016]      FIG. 3  is a drawing, schematically illustrating a SQUID magnetometer, according to an embodiment of the invention.  
         [0017]      FIG. 4  is a drawing, illustrating a performance of the SQUID magnetometer in  FIG. 3 , according to an embodiment of the invention.  
         [0018]      FIG. 5  is a drawing, illustrating a performance of the SQUID magnetometer in  FIG. 3  about the relation of the induced voltage with the magnetic flux, which has been converted into a modulation current, according to embodiment of the invention.  
         [0019]      FIG. 6  is a drawing, schematically illustrating the magnetometer of SQUID, according to another embodiment of the invention.  
         [0020]      FIG. 7  is a drawing, illustrating a performance of the SQUID magnetometer in  FIG. 6  about the variation of induced voltage with the magnetic flux, which has been converted into a modulation current, according to embodiment of the invention.  
         [0021]      FIG. 8  is a drawing, illustrating a performance of the SQUID magnetometer in  FIG. 6  about the frequency dependence of magnetic field sensitivity.  
         [0022]      FIGS. 9-11  are drawings, schematically illustrating another SQUID magnetometer, according to other embodiments of the invention.  
         [0023]      FIG. 12  is a drawing, schematically illustrating a SQUID gradiometer, according to other embodiment of the invention.  
         [0024]      FIG. 13  is a drawing, schematically illustrating a mechanism of gradiometer.  
         [0025]      FIG. 14  is a drawing, illustrating a performance of the SQUID gradiometer in  FIG. 12  about the variation of induced voltage with the gradient magnetic flux, which has been converted into a modulation current, according to embodiment of the invention.  
         [0026]      FIG. 15  is a drawing, illustrating a performance of the SQUID gradiometer in  FIG. 12  about the frequency dependence of magnetic field sensitivity.  
         [0027]      FIG. 16  is a drawing, schematically illustrating another SQUID gradiometer, according to another embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]      FIG. 2  is drawing, schematically illustrating structure of a bare SQUID, according to an embodiment of the invention. In  FIG. 2 , a SQUID unit  120  is similar to the SQUID  100  in  FIG. 1 . However, the electrode leads  104   a  and  104   b  in  FIG. 1  can be modified into the large electrode leads  104   a  and  104   b . No magnetic flux exits in the electrode leads  104   a ′ and  104   b ′, according to the phenomenon of superconducting material. Due to the larger area of the electrode leads  104   a ′ and  104   b ′ in superconducting material, the SQUID unit  120  can pick up more magnetic flux, and induce more compensating current in the SQUID unit  120  and then induce the higher voltage signal for detection.  
         [0029]     In order to further improve the performance in sensing magnetic flux, which is proportional magnetic field intensity, a magnetometer with multiple SQUID units in cascade connection provided as an embodiment.  FIG. 3  is a drawing, schematically illustrating a magnetometer of multiple SQUID units, according to an embodiment of the invention. In  FIG. 3 , for example, 10 SQUID units are connected together in cascade. The sensing part  130  (also called washer) in superconducting material of the SQUID unit can be the large bars, so as to squeeze more magnetic flux to the central region of the SQUID and induce more compensating current. Several superconducting bridging parts  131  connected the SQUID units as the cascade connection. One bare SQUID unit is shown in larger scale. The bare SQUID unit can be, for example, identical to the one shown in  FIG. 2 . Then, several electrode leads, such as the electrode leads  132 ,  134 ,  136 , and  138 , are respectively connected to the separated SQUID units, for example. Any pair of the electrode leads can form a sensing set of SQUID units. The connection of the electrode leads to the SQUID units can have several ways. For example, the electrode lead  132  is connected to the first SQUID unit, counting from right to left. The electrode leads  134  and  136  are connected to the intended connecting parts  131 . The electrode lead  138  is, for example, connected to the last SQUID unit. Each electrode lead, corresponding to voltage signal and applying current, can have two terminal pads for applying current and detect the induced voltage signal.  
         [0030]     In the structure of SQUID as shown in  FIG. 3 , any pair of the electrode leads can include at least one SQUID unit, connected in cascade. For example, if the electrode leads  132  and  134  are taken, then one SQUID unit is in use. If the electrode leads  132  and  136  are taken, then five SQUID units are in use to sense the magnetic flux. Further example, if the electrode leads  132  and  138  are taken, then ten SQUID units are in use to sense the magnetic flux. The more the SQUID unit is in used, the more the sensitivity is achieved.  FIG. 4  is a drawing, illustrating a performance of the SQUID magnetometer in  FIG. 3 , according to an embodiment of the invention. In  FIG. 4 , the horizontal axis is the applying current I. The right vertical axis is the induced voltage level for one SQUID unit for dashed line, and the left vertical axis is the induced voltage level for ten SQUID units for dotted line. As one can see, the induced voltage level with ten SQUID units is about ten times of the induced voltage level with one SQUID unit. As one can see, different pair of the electrode leads are taken, the different sensitivity is achieved. This depends on the actual need in use. The present invention can indeed effectively improve the sensitivity of the SQUID magnetometer and can have more application in various choices.  
         [0031]      FIG. 5  is a drawing, illustrating a performance of the SQUID magnetometer in  FIG. 3  about the relation of the induced voltage with the magnetic flux, which has been converted into a modulation current, according to embodiment of the invention. In  FIG. 5 , the voltage-flux characteristics are shown in V-I mod  curves for a single-SQUID magnetometer and the 10-SQUID array magnetometer at a temperature of 77 K. It is clear that not only the voltage of the single-SQUID magnetometer, but also of the 10-serial-SQUIDs magnetometer vary with the applied magnetic flux. The magnetic flux has been represented by the modulation current I mod . Due to quantum effect, the voltage V varies in period with the magnetic flux. The line curve without symbol is a result from single SQUID unit, in which the induce voltage level is not much. However, the line curve with square symbol is a result from 10 SQUID units connected in cascade, in which the induced voltage level is about ten time larger. In thus situation, the slope is much larger. This indicated that the sensitivity to the magnetic flux is improved.  FIG. 5  reveals the fact that the washer-type magnetometer having SQUIDs in series can be used to sense the magnetic flux via measuring the voltage variation.  
         [0032]     The sensing part  130  in washer-type may also picking up certain noise. Alternatively, in order to at least reduce the noise level, the washer-type film can be, for example, replaced by a coil-type.  FIG. 6  is a drawing, schematically illustrating a performance of the SQUID magnetometer, according to another embodiment of the invention. In  FIG. 6 , for example, a coil-type SQUID magnetometer  140  can include a SQUID set, which for example includes two SQUID units formed across the boundary  101 , dividing each SQUID unit into a first part (upper part) and a second part (lower part). For example, one SQUID unit has electrode leads  142   a  and  142   b , respectively at the first part and the second part. Likewise, the other SQUID unit has the similar electrode leads  143   a  and  143   b . These two SQUID units are cascaded with a superconducting connection between the second parts of the two SQUID units.  
         [0033]     Then, a coil-type magnetic-flux sensing part  144  is disposed at, for example, the second part to connect the SQUID units of the SQUID magnetometer. The material of the coil-type magnetic-flux sensing part  144  is also the same superconducting material. If there are many coils included, the coils are separated by a gap  146 . The central portion is a free space for adapting the electrode leads of the SQUID units. It should be noted that  FIG. 6 , just as an example, shows three coils and the three coils  144  are connected to the same line, so as to connect to each of the SQUID unit. However, the number of the coils can be one or several. The coils can also be separately connected to the sides of the SQUID units. The one in  FIG. 6  can save the occupied space. With the superconducting properties, each coil increases the sensing capability of magnetic flux. If the electrode pair of A 1  and A 2  is taken, then one SQUID unit is in use. If the electrode pair of A 1  and A 3  is taken, then two SQUID units are in use because two sets of Josephson junctions are involved. However, under the basic principle, the cascade connection can be included to use more SQUID units. For example, the connection portion is alternatively changed in two part of the boundary, then the applying current can flow through more number sets of Josephson junctions. It should be noted that the number of SQUID units is not limited to way as shown in  FIG. 6 , too.  
         [0034]      FIG. 7  is a drawing, illustrating a performance of the SQUID magnetometer in  FIG. 6  about the variation of induced voltage with the magnetic flux, which has been converted into a modulation current, according to embodiment of the invention. In  FIG. 7 , the square dotted line is the result from single SQUID unit in use. When two SQUID units are in use, the induced voltage level is shown by open-circle dotted line. Again, the slope of the voltage level is increased. It indicates that the sensitivity is increased by using two SQUID units.  
         [0035]      FIG. 8  is a drawing, illustrating a performance of the SQUID magnetometer in  FIG. 6  about magnetic field sensitivity S B   1/2  as a function of the frequency of the sensed magnetic field. In  FIG. 8 , the curve  1  is the result from the magnetometer with single SQUID unit in use, which shows a filed sensitivity of 42-50 fT/Hz 1/2  at 1 kHz and 120-150 fT/Hz 1/2  at 1 Hz. When two SQUID units are in use, the magnetic field sensitivity is shown by the curve  2 , which shows a field sensitivity of ˜33 fT/Hz 1/2  at 1 kHz and ˜80-100 fT/Hz 1/2  at 1 Hz. The lower value for the magnetic field sensitivity means that the SQUID magnetometer can sense lower magnetic-field intensities. It indicates that the sensitivity is increased by using a magnetometer having more SQUID units.  
         [0036]     Further,  FIG. 9  is a drawing, schematically illustrating another SQUID magnetometer, according to other embodiments of the invention. In  FIG. 9 , based on the same structure in  FIG. 6 , a superconducting flux focuser  150  can be further included, disposing over the coil-type magnetometer  140 . The superconducting flux focuser  150  is, for example, a C-like shape with an open gap  152  and a free space  154 . Since the superconducting flux focuser  150  is also made of superconducting material, in which the magnetic flux cannot exit in side the superconducting material, the superconducting flux focuser  150  can squeeze more magnetic flux into the coil-type magnetometer  140  for sensing. The focusing phenomenon is therefore achieved. With the magnetic focuser  150 , the sensibility can be further improved, as shown by star dotted line in FIG.  7 . Even though the maximum voltage level for two SQUID units is about the same, the period of flux is reduced with an aid of superconducting flux focuser  150 . In this situation, the slope of voltage to the magnetic flux is increased. This phenomenon with focuser also indicates that the sensitivity is improved.  
         [0037]      FIG. 10  is a drawing, schematically illustrating another SQUID magnetometer, according to other embodiments of the invention.  FIG. 11  shows the magnified structure about the region  178  of  FIG. 10 . In further consideration, with the same design principle, several SQUID units can be included and connected side by side. The electrode leads  174  and  176  of the SQUID units can be properly arranged without specific choice. However, for example, the location of the electrode leads  176  can be located at the other far opposite side at the periphery of the free space. The number of electrode leads is not limited to a specific quantity. Basically, since there are several electrode leads, when one SQUID is broken, the other SQUID can be used instead. This also true for all of the examples shown in the invention. The magnetic flux focuser  182  may also be included. The coil  180  may be also included. However, in this example of  FIGS. 10-11 , the gear-like dam structure  179  is presented. The flux dam structure  179  is, for example, connected between the side one of SQUID units and the coil  180 , and for example crossing on the boundary  172 . According to study of the flux dam, the 1/f noise level at the low frequency can be effectively reduced while the flux dam is included. In addition, the flux dam may also further include a floating SQUID unit  184 .  
         [0038]     Based on the similar principle, the magnetometer can be further designed into a superconducting gradiometer, which can measure, for example, the gradient of magnetic field intensity.  FIG. 12  is a drawing, schematically illustrating a SQUID gradiometer, according to other embodiment of the invention. In  FIG. 12 , the gradiometer  210  in left drawing can, for example, include two SQUID sets in SQUID region  200  with the coil-type design being put together. The right drawing in  FIG. 12  is a magnified structure at the SQUID region  200  having two SQUID sets  200   a  and  200   b.    
         [0039]     In general, each of the two SQUID sets  200   a ,  200   b  has multiple SQUID units  200   c  at the SQUID region  200 , connected side by side and divided by a boundary  208  into a first part and a second part. Multiple electrode leads  204   a ,  204   b ,  204   c , and  204   d  are connecting to the SQUID units. In this example, each SQUID set  200   a ,  200   b  has six SQUID units  200   c , for example. Each SQUID unit  200   c  has two electrode leads with, for example, the lead pads for applying current and sensing induced voltage. For a better space distribution, for example, three of the electrode leads go to left direction while the other three electrode leads go to right direction. The lead pads are distributed at the periphery of the free space. It should be noted that the drawing in  FIG. 12  is just a schematic drawing. The actual design may be changed under the same principle. One coil set  202   a , serving as a coil-type magnetic-flux sensing part, is at one part of the boundary  208  while the other coil set  202   b  is at the other part of the boundary  208 .  
         [0040]     A common connection portion  205  is connected between the SQUID units  200   c , and connected to the two coil-type magnetic-flux sensing parts  202   a ,  202   b . Wherein, the coil-type magnetic-flux sensing part  202   a  senses a magnetic flux and another coil-type magnetic-flux sensing part  202   b  senses another magnetic flux, so as to obtain a magnetic field gradient. This measuring mechanism is shown in  FIG. 13 .  FIG. 13  is a drawing, schematically illustrating a mechanism of gradiometer. For one SQUID unit  304  across the grain boundary  305 , the two coils located at different positions  300  and  302  and enclosed the two side of the SQUID unit. With a common connection. For example, when the magnetic flux at the position  300  is entering the drawing paper while the magnetic flux at the position  302  is going out the drawing paper. Due to the different direction of magnetic flux, the induced current, flowing into the SQUID unit  304 , is enhanced. As a result, a non-zero voltage V can be detected. The quantity of V is related to the gradient degree. For the situation with uniform magnetic flux, them the magnetic flux at the position  300  is substantially equal to that at the position  302 . The induced currents cancel to each other, causing a zero induced current to the SQUID unit. Then, the voltage is not induced, either, that is V=0. According to this mechanism, the intensity gradient of magnetic field can be measured. For example, if the electrode leads A 1  and A 2  in  FIG. 12  are taken, one SQUID unit is in use. If the electrode leads A 2  and A 3  are taken, then two SQUID units are in use with better sensitivity. As mentioned above in  FIG. 6 , the choice and the design of the electrode leads can be changed, according to the actual design. More SQUID units can be included in use.  
         [0041]      FIG. 14  is a drawing, illustrating a performance of the SQUID gradiometer in  FIG. 12  about the variation of induced voltage with the gradient magnetic flux, which has been converted into a modulation current, according to embodiment of the invention. In  FIG. 14 , the voltage-gradient-flux characteristics are shown in V-I mod  curves for 1-SQUID gradiometer and 2-SQUIDs gradiometer at 77 K. It is clear that not only the voltage of the 1-SQUID gradiometer, but also of the 2-serial-SQUIDs gradiometer vary with the gradient magnetic flux. The gradient magnetic flux has been represented by the modulation current I mod . Due to quantum effect, the voltage V varies in period with the magnetic flux. The curve  1  is a result from the gradiometer with single SQUID unit, in which the induce voltage level is about 17 μV. However, the curve  2  is a result from 2 SQUID units connected in cascade for the gradiometer, in which the induced voltage level is about twice larger. In thus situation, the slope is much larger. This indicated that the sensitivity to the gradient magnetic flux is improved.  FIG. 14  reveals the fact that the gradiometer having SQUIDs in series can be used to sense the gradient magnetic flux via measuring the voltage variation.  
         [0042]      FIG. 15  is a drawing, illustrating a performance of the SQUID grasiometer in  FIG. 12  about sensitivity S B   1/2  in the gradient magnetic field as a function of the frequency of the sensed gradient magnetic field. In  FIG. 15 , the curve  1  is the result from the gradiometer with single SQUID unit in use, which shows a gradient filed sensitivity of 90˜150 fT/cmHz 1/2  at 1 kHz and of 1-2 pT/cmHz 1/2  at 1 Hz. When two SQUID units are in use, the gradient magnetic field sensitivity is shown by the curve  2 , which shows a field sensitivity of 50 fT/cmHz 1/2  at 1 kHz and 100 fT/cmHz 1/2  at 1 Hz. The lower value for the gradient magnetic field sensitivity means that the SQUID gradiometer can sense lower gradient magnetic-field intensities. It indicates that the sensitivity is increased by using a gradiometer having more SQUID units.  
         [0043]     It should also be noted that the foregoing embodiments can be partially or fully combined, according to the actual design. The magnetometer and the gradiometer are based on the same design principle of the present invention. For example, the flux focuser can be furthered used in gradiometer.  FIG. 16  is a drawing, schematically illustrating another SQUID gradiometer, according to another embodiment of the invention. In  FIG. 16 , the flux focuser  212  is over the gradiometer  210 , so as to pick up more magnetic flux. However, since the gradiometer includes two sensing locations, the flux focuser  212  is formed in accordance with the structure of the gradiometer  210 . For example, the flux focuser  212  is a superconducting film has two E-like structures against to each other with the gaps  214 . However, the middle horizontal lines  216  are connected together. As a result, the free space  218   a  and  218   b  expose the sensing coils of the gradiometer  210 . The size of the flux focuser  212  can be sufficient large to pick up more magnetic flux and squeeze the magnetic flux into the sensing coils.  
         [0044]     The present invention has proposed the magnetometer and the gradiometer based on multiple SQUID units being in cascade connections. As a result, the present invention can indeed effectively improve the sensitivity of the magnetometer and the gradiometer, and can have more application in various choices by taking different pair of the electrode leads of SQUID units. This depends on the actual need in use. Further for example, the coil-type and the washer-type for the SQUID can be chosen in option. The flux focuser can be optionally included, too, for increasing the sensitivity with larger sensing area.  
         [0045]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing descriptions, it is intended that the present invention covers modifications and variations of this invention if they fall within the scope of the following claims and their equivalents.