3.1. Transient State
3.1.1. Spatial Electron Concentration at Transient State
First, we assume that the voltage of the voltage source and the voltage of the load are equal. Then, the diffusion current of the current source is introduced. Considering the conductivity, introducing an average electric field, and substituting a diffusion constant result in a specific and special electron concentration. The voltage balance is
where
VE,
V,
R, and
I denote the voltage from the voltage source, the voltage of the load, the resistance of the load, and the current from the current source, respectively. In Eq. (1-2),
ρ,
S, and
l denote the resistivity, the area of the load, and the length of the load, respectively. The diffusion current
I is expressed as [
18,
30]
where
q,
D, and
n denote the electron charge, the diffusion constant, and the electron concentration, respectively. Therefore, the Ohmic voltage becomes
where
σ denotes the conductivity of the load. Here the following equations are substituted:
where
μ,
kB,
T, and
Ei denote the average mobility, the Boltzmann constant, the temperature, and the local electric field, respectively. Therefore, the local electric field is
Solving this equation gives
3.1.2. Attractive Potential and Coiled New Electric Field at the Transient State
We employed the Poisson equation to derive the interaction potential between two electrons at the transient state. Equation (6) gives the concentration in terms of the electrons; substituting this concentration into the Poisson equation produces the following attractive interaction potential (
:
where
ξ is variable and relative macroscopic distance between the two electrons. Hereafter, we refer to this potential as the transient state potential. It is very important to note that the Coulomb repulsive interaction is also determined using Poisson’s equation. Accordingly, this repulsive interaction does not appear at a macroscopic scale. Instead, the abovementioned interactive potential
Vα works. Note that, at the steady state, the macroscopic relative distance
ξ becomes zero but microscopic one is not zero. In this sense, at the steady state, we will consider the Schrodinger equation.
In the process of solving the Poisson equation, we derived the following new electric field at the transient state:
When defining the circle integral of eq. (8) (i.e., a voltage), this integral must end up within a material. This implies that, in the material, a coil voltage must be defined. The spatial integral of Eq. (8) for ξ results in a negative voltage. As discussed later, we will see that, immediately prior to the transition from the normal state to the superconducting state, this negative voltage appears in our simulation. This new electric field appears at the transient state and thus an inductor voltage will be essential. That is, as discussed later, the spatial integral of eq. (8) will form a coil.
3.1.3. Combination of Two Electrons and Critical Current Density
The transient state attractive interaction potential Vα results in two electrons that approach each other until their distance is only the lattice diameter due to the lack of the Coulomb repulsive interaction at the macroscopic scale. At this moment, the macroscopic relative distance ξ becomes zero. Note that, however, microscopic one is not zero. Therefore, the transient state up to the combination of the two electrons is a macroscopic phenomenon. However, only when the potential Vα vanishes do we need to consider the microscopic scale. As described later, however, at this moment, when the potential Vα vanishes, the relative kinetic energy also vanishes. Therefore, we do not need to consider the Hamiltonian of the quantum mechanics only before the combination of two electrons. Instead, it is necessary to consider the Hamiltonian for quantum mechanics for the center-of mass motion as described later.
As a result of the transient interaction
Vα, the two electrons take the location of a lattice, and at a lattice (i.e., macroscopic variable
ξ = 0), the total energy
ET of the system is expressed as follows:
Here,
V is the spin magnetic potential [
18] expressed as
where
m and
zm denote the electron mass and the microscopic relative coordinate, respectively. In Eq. (9),
P and
Vp are the kinetic and zero-point energies in terms of the Debye temperature at the lattices, respectively.
Vp is expressed as follows:
where
ωD and
θD indicate the Debye angular frequency and temperature, respectively. The following conditions should be satisfied for the two-electron combination:
For Eq. (12-1) to be satisfied, both electrons must be located at the zero point of a lattice. This condition, i.e., critical current density, will be considered later. To satisfy Eq. (12-2), the microscopic relative distance
zm (i.e., the coherence) should be
If Eq. (12-1) is satisfied, then, Eqs. (12-2) and (13) are also satisfied because the zero-point energy implies the quantum fluctuation energy of a lattice. Therefore, when current density is less than the critical current density, only the spin magnetic potential V remains, and the entire energy takes on a negative value, which implies that the two electrons have a net combination, in which a collision between the two electrons becomes completely inelastic. Consequently, these two electrons combine to form a pair.
Even though the kinetic energy in terms of the relative motions exists in the transient state, as mentioned, in the steady state, only the spin attractive force
V remains:
where
a denotes the constant coherence of an electron pair.
Therefore, this potential is taken to be the superconducting energy gap of our superconductivity. As mentioned, the two-electron pair combines at the distance of a lattice, which is typically estimated to be 1 Å, and, therefore, the magnitude of the energy gap is estimated to be approximately 10-18 J. The typical room temperature energy kBT (where the temperature T = 300 K) is approximately on the order of 10−21 J. Therefore, this superconducting energy gap is much larger than that of room temperature. This implies that the individual electron pair will not be destroyed via normal heating.
Next, let us consider an equation of the critical current density.
At the critical point, i.e., in the case where the inequality of Eq. (12-1) is maximum,
From this equation, the concentration
n0 can be expressed as
In Eq. (14), considering the second equation of Eq. (9), the left-hand side of Eq. (14) implies a kinetic energy
P. Therefore,
where
m and
v denote the mass of the electron and the velocity in terms of the relative motions, respectively. Substituting Eq. (15),
Therefore, the relative velocity is derived to be
Consequently, the critical current density equation is obtained such that
Note that, in the Results section, we will calculate numerical values of the critical current density using Eq. (20).
3.2. Macroscopic Wave Function and the London Equation at the Steady State
Given that a two-electron pair binds strongly, from the discussion in the previous sections, we now precede to a discussion of Bose–Einstein condensation and the macroscopic wave function at the steady state.
As described later, the Hamiltonian equation of an electron pair in terms of the center-of-mass motion is expressed simply as
The potential
U in the above equation indirectly implies the electrostatic potential of the two electrons, i.e., the internal electric field at a lattice:
where
x1 and
x2 denote the positions of the two electrons.
In the steady state, we are now considering the combined electron pair at a lattice, without collisions (i.e., the interactions) with another pair, and thus this temperature-independence allows us to consider that the dimension of the center-of-mass motion whose direction is along
Ei (i.e., the movement depends on eq. (22)) is single. Note that the distribution of internal electric field
Ei at a lattice is also single dimension. The reason of the absence of interactions between pairs and pairs will be discussed soon. However, at the transient state, the existence of the potential
Vα implies that each electron motion depends on temperature
T. Moreover, because there are not net collisions between pairs and pairs at the steady state, it is potential for a phase transition that is related to temperatures not to exist [
25]. The absence of the interactions with another pair guarantees the physical picture of one body.
The local placements at the identical lattice of the two electrons (i.e., the local and strong combination of the two electrons at a lattice) imply that
x1 =
x2. Employing the Hamiltonian equation, a wave function of the pair in terms of the center-of-mass motion can be obtained. Note that, at the moment when the electrons have an identical location at a lattice, the potential
U in the Hamiltonian equation converts to an eigenvalue (i.e., the kinetic energy in terms of the center-of-mass motion) that stems from the local electric field
Ei. Accordingly, the distribution of the local electric field
Ei within the lattice vanishes.
where
μi denotes the mobility of the pair at the lattice. Using the following equations,
The wave function of an electron pair can be derived such that
where
σi, △
Vi, and
J denote the conductivity at the lattice, the variation quantity of the volume with respect to the increase in the index
i, and the current density, respectively.
Equation (24-1) implies the proportion of Joule electric and electrostatic fields at the lattice-scale level. As mentioned, the above wave function of an electron pair is indexed by i, and a wave function at a subsequent lattice does not interact with this wave function because the distance between the two neighboring lattices is much larger than the coherence of the two electrons. Note that, in the previous section, we mentioned that the coherence of a combined pair is estimated to be approximately 1 Å (i.e, the typical diameter of a lattice). This is the reason why each electron pair does not interact with each other.
The entire wave function is represented in the following equation:
Assuming that the index
i is infinite and that the sample volume of a load is kept constant, the lattice volume △
Vi /
i converges to the differential
dV. Accordingly, the countable lattice concept becomes ineffective, and a macroscopic continuous body appears. That is, we obtain a macroscopic wave function that is mandatory when considering the mechanism of the new superconductivity, and this macroscopic wave function is needed to derive a London equation (i.e., a Meissner effect):
In Eq. (27), the wave number
K is
where
M denotes the spin magnetic moment of an electron and
a is the constant coherence in terms of our new superconductivity. The spin magnetic moment in this equation can be correlated to the combination energy (i.e., the spin interaction) from Eq. (10) when the microscopic ordinate
zm is replaced with the constant coherence
a.
where
V denotes the spin interaction given in Eq. (10-2).
In Eq. (28-2), the macroscopic current I is independent of the collision time and is therefore a superconducting current. Further, the wave number K in this equation is uniform and is not related to space, e.g., it is not related to the lattice index i. Therefore, all the quantum states converge to a single state, and Bose–Einstein condensation is derived.
Let us consider the Meissner effect derived from the macroscopic wave function. As mentioned, the macroscopic wave function is
In this equation, the macroscopic current I appears. In general, a current needs to be continuous in space and time. In this case, therefore, the current I is continuous for the output current I from the current source. Accordingly, this superconducting current must generate a self-magnetic field in the load sample and, if a new superconductivity is generated, this self-magnetic field must be canceled out.
Considering the above self-magnetic field
A, the Aharonov–Bohm (AB) effect [
11] can be introduced to the macroscopic wave function, Eq. (27):
Based on the property of phases,
Taking the differentials to both sides for variable
x,
Introducing a small angle parameter φ to the differential at the right side,
Assuming a differential for integrals for the surface is very small, Eq. (33) becomes
Therefore, we derive a London equation:
3.3. Consideration of the Meissner Effect
When considering a sample with a cylindrical shape, it is necessary to introduce an internal toroid whose large radius corresponds to that of the cylindrical sample. Then, we consider the renewable coordinate that is tangential to the large circumference as corresponding to the
z-axis in cylindrical coordinates; see
Figure 3. We refer to this type of coordinates as the “specific cylindrical coordinates.”
From the initial time to the transition time tc, there is a normal self-magnetic field distribution B0 in the sample along the direction of the z-axis of the specific cylindrical coordinates. However, due to the existence of the new electric field eq. (8) at the transient state, internal voltages must be defined within a material. This implies that coils appear at the transient state within the material. In the process of forming coils, negative voltage appears and currents are produced. Owing to the continuous property of the current, this generated current becomes the persistent one of eq. (35) at the steady state. In this process, the generated magnetic field cancels the existing self-magnetic field B0 whose direction is along the z-axis of the specific cylindrical coordinates. Therefore, the internal net magnetic flux density becomes zero despite the existence of the current I. That is, from energy conservation, the magnetic field energy derived from B0 is transformed as a discharged current, which generates a temporal negative voltage, as mentioned previously.
Let us consider the above more concretely. Here, in the right panel of
Figure 3, we consider one element coil. However, as discussed soon, this element coil is divided into many local coils. Assuming that the length
l in the left panel of
Figure 3 is considered to be
Na, where
N and
a denote arbitrary integer and the coherence of a pair respectively, in the conductor, many local divided coils and currents exist. However, considering the analogy of the Stokes’ theorem, these internal local currents of each local divided coil cancel with each other and the net current eventually becomes the most outer current of the blue surface in
Figure 3. These phenomena imply that, as described in the theory section, the cancelation of the local currents indicate the perfect inelastic collisions of two electrons and the two electrons bind by the spin magnetic force eq. (10). After that, these combined electrons move uniformly by the local electric field
Ei and thus they result in eq. (27) (i.e., the Bose-Einstein condensation). However, the superconducting current
I in eq. (27) still generates a self-magnetic field
B0 and thus AB effect leads to the London equation (35) whose current distributes along the most outer line of the blue surface in Figure3. Note that, however, forming of BE condensation and London equation actually occur simultaneously. Thus, the exclusion of the internal magnetic field
B0 at the transient state is taken over to the steady state. Note that, importantly, if the further magnetic field is applied after the steady state, in order to maintain the value of the persistent current of eq. (35), a current is also discharged.