Abstract:
A quantum junction device having three or more wires connected to a loop surrounding a magnetic flux is used to act as a switch responsive to magnetic flux and therefore useable for mass storage devices or as a flux detector by sensing current direction, conductance tensor, in response to a magnetic field under test.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0001]     This invention was made with Government Support under Contract Numbers DMR-0203159 and DMR-9876208 awarded by the National Science Foundation. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Known devices such as SQUID&#39;s are superconducting devices which operate at extremely low temperatures to achieve superconductivity conditions. These and other devices also operate in the domain of thousands of conductivity channels or electron modes for conductivity. Such large numbers of channels typically indicate operation as a so-called Fermi liquid and provides relative insensitivity to magnetic fields either for detection or for conduction switching above superconducting temperatures.  
         [0003]     These prior devices can be used as magnetic field sensors and data storage read-out devices. The problems of requiring superconductivity, and relatively large size as well as limited response speeds make such prior art devices less competitive with with conventional magnetic field sensors above superconducting temperatures and conventional read-out devices such as magnetic based hard drives read-heads in terms of storage density.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     According to the present invention a quantum junction device is provided which has small size, rapid response times and relatively high sensitivity. This is achieved by providing a conductive loop to which three conductors lead at roughly equally spaced locations around the loop. The structure is dimensioned with a thinness in the nanometer or micrometer scale in order to limit the number of conduction channels, for example, to 100 or less, so as to operate as a Luttinger liquid or as a non-Fermi liquid.  
         [0005]     Using appropriate voltage and current sources and detectors as well as flux sources, the device can be used for data storage read-out because of its potential to switch states or conductance tensors in response to an applied magnetic field. Similarly the device can be used, because of its propensity to switch in the presence of a magnetic field, to detect such magnetic fields either singly or over an area in the case of arrayed junction devices. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0006]     These and other features of the invention are more fully set forth below in the detailed description of the invention and the accompanying drawing of which:  
         [0007]      FIG. 1  shows a quantum junction device according to the invention;  
         [0008]     FIGS.  2 A-C show a quantum junction device of the type illustrated in  FIG. 1  in association with electronics, electric current and voltage and magnetic flux for use as a switch or reading magnetic fields;  
         [0009]      FIG. 3  illustrates the application of the invention for high-density data storage readout; and  
         [0010]      FIG. 4  illustrates a device of the invention for detecting magnetic flux. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]     The present invention contemplates using a conductive loop accessed by three wires dimensioned on micrometer and nanometer scales to permit operation in a non-Fermi liquid regime. Preferably the device is maintained at a condition where electron interactions are characterized by a Luttinger (or Tomonaga/Luttinger) liquid. The device of claim  1  has a conductive loop  10  and three conductors  12 ,  14  and  16  preferably, but not necessarily, spaced equally apart around the loop connected to voltage and/or current sources and detectors as discussed below. A magnetic field illustrated by flux  18  is applied through the center of the loop  10  along an axis perpendicular thereto. Addition of conductor(s)  8  and source/detector(s) may be used as well as to enhance the functions. For example, with four conductors, opposite pairs can operate as source and switched outputs respectively.  
         [0012]     For operation as a quantum junction device, the device of  FIG. 1  is dimensioned, particularly in thickness, so as to provide a limited number of conduction channels, such as 1, 10, 20, 50, 80 but no more than about 100. A thickness, material type and temperature condition determines the existence of a non-Fermi liquid operating regime. Considerations of a theoretical nature are found in the attached Appendix comprising a paper entitled “Junctions of Three Quantum Wires.” 
         [0013]     In particular, in the quantum regime the magnetic flux  18 , for example, can cause switching of the current between connectors  12  and  14  in one flux condition and conductors  12  and  16  in another flux condition. The switching speed can be extremely fast making such device useful as a digital data bit retrieving element in mass storage devices for data storage. The device is not required to operate at superconducting temperatures though switching sensitivity to the magnetic flux is substantially enhanced at such low temperatures. The switching profile stretches out as the temperature rises to such conditions as room temperature.  
         [0014]     From a theoretical standpoint, the electrical response of this device is characterized by the conductance tensor G jk (ø) that relates the currents I j ,  FIG. 1, 2  or  3 , flowing into the junction from wire j ( 12 ,  14 ,  16 ) when voltages V k  are applied to wire k ( 12 ,  14 ,  16 ): 
 
 I   j   =G   jk (ø) V   k   (1) 
 
 From current conservation, ΣG jk (ø)V k =0. The conductance depends explicitly on the magnetic flux that is encircled by the ring because of quantum mechanical phase effects. 
 
         [0015]     The two functions of the device, as a switch and as a detector, follow from the magnetic flux dependence of the conductances G jk (ø).  
         [0016]     (1) To function as a switch: a current is injected through one of the leads, say lead  12 , and the magnetic flux  18  controls through which of the other two leads,  14  or  16 , most of the current flows out. In extreme cases, the current is fully diverted from lead  12  to either lead  14  or  16  depending on the magnetic flux  18  encircled by the loop.  
         [0017]     (2) To function as a detector: a current is injected through one of the leads, say lead  12 , and the presence of a magnetic flux  18  is detected by reading the current that flows through the other leads,  14  or  16 . An asymmetry in the flow of currents to leads  14  and  16 , quantified by I 2  and I 3 , which depends on ø, provides a means to detect small magnetic fields that give rise to a flux that threads the inside of the ring.  
         [0018]     The principle behind the device functioning is quantum interference in the case of both micrometer and nanometer scale wire-junctions. For nanometer scale devices, electron-electron interaction effects are responsible for non-linearities that make the device&#39;s response to an applied flux more pronounced.  
         [0019]     The device differs from the so-called SQUID or superconducting devices by allowing scaling to very small size and limiting the conduction channels to be on the order of 100 or less channels. By limiting the conduction channels, electron behavior no longer is visualized as a fluid of independent electrons but more as a Luttinger liquid of interacting electrons, which can exhibit an attraction between electrons that provides for the switching function but at a warmer than a superconducting temperature such as, for example, room temperature if desired. The electron attraction interaction and the limited channel dimensions are what particularly characterize the device.  
         [0020]     The ring can be formed of any number of materials including, for example, copper, gold, silver, aluminum or of structures as carbon nanotubes or of semiconductor materials. The term channel is used in this invention to represent the number of electron modes in the conduction process. The well known Schroedinger equation and the lateral confinement potential of the wire defines the allowed quantum modes or channels that can propagate along the wire. As a result, the thinner the wire structure of the invention is, the fewer the channels or electron conduction modes. The material used for the junction device of the invention also has an effect on the electron density in the wire and thus the number of channels or electron modes for a given thickness.  
         [0021]      FIG. 2A  illustrates the use of the quantum junction device as a magnetic field detector. As shown there, a source for voltage and/or current  30  applies an input to the lead  12  coupled to the quantum loop  10 . First and second output leads  14  and  16  are connected to respective voltage and/or current detectors  32  and  34 . The loop  10  is positioned near a source  36  for a magnetic field  18  such that the flux passes through the loop  10 . The signal on leads  14  and  16  indicate the presence and/or strength of the field.  
         [0022]      FIG. 2B  illustrates the use of the quantum loop as a switch. In this case, the loop  10 ′ has an input  12 ′ and first and second output conductors  14 ′ and  16 ′. A voltage and/or current signal from a source  30 ′ is applied through the input conductor  12 ′ to the loop  10 ′. A magnetic field source  36 ′ applies a magnetic field  18 ′ or not depending upon whether the signal on input lead  12 ′ is to be directed to output conductor  14 ′ or  16 ′. Voltage and/or current destinations  32 ′ and  34 ′ are provided to receive the majority of the switched signal for subsequent processing.  
         [0023]      FIG. 2C  illustrates one application for the quantum loop device of the present invention as a detector. In this case, the device is used to detect magnetic flux from minute sources within a human brain  42 . A quantum loop  10 ″ is placed on a probe  44  which is inserted into a region of the brain  42  where it is desired to detect the presence of a magnetic field for medical diagnostic purposes.  
         [0024]      FIG. 3  illustrates the use of a quantum loop of the invention as a readout device for data stored magnetically within a structure  50 . In particular, data is stored within minute magnetic elements  52  within an array  54 . Each magnetic element has a quantum loop  56  associated with it with corresponding input conductors  58  and output conductors  60  and  62 . Each of the input connectors is supplied with a voltage and/or current signal from a bus  64 , typically supplied from a multiplexer, while the output leads  60  and  62  provide signals to respective voltage and/or current output buses  66  and  68 , similarly connected through a multiplexer for connection to electronics to provide readout of the state of each of the magnetic elements  52 . Magnetic elements  52  and  54  may, typically be permanent magnets or magnetic regions of a magnetic sheet material on a tape or hard drive. Alternatively, a single loop  56  may be scanned over the array  50 .  
         [0025]     For use in detecting magnetic flux, the device of  FIGS. 1 and 2  are used as illustrated in  FIG. 4 . A quantum junction device  17  of the type illustrated in  FIG. 1  is provided either singly or in an array  72  for detecting the magnetic flux  74  over an area. Electronics  76  is provided, which in the case of multiple devices  70 , is provided as an array of voltage/current source/detector elements or multiplexed over a bus  75  as above. The direction in which current proceeds through the quantum junction device  70  is an indication of the presence and/or level of the magnetic flux  70  at a particular point or over an area where multiple devices are provided.  
         [0026]     These and other embodiments of the invention are to limited in scope only in accordance with the following claims.