A Stability of persistent currents in superconductors regardless of temperature

[maxpi]

Thanks for clarifying! So the lifetime experiments are still proposals - that makes sense. It's indeed surprising how little work has been done on systematic decay measurements, given how fundamental they are.

Regarding your macroscopic rings (20 mm / 10 mm) - that's interesting because at those scales, you're firmly in the regime where my framework (TCTQ) predicts topology becomes negligible. The crossover scale d_c ~ 50-100 nm means topological effects only matter for nano-scale structures.

But here's where our perspectives might connect: you're asking about *pair permanency* (whether the same electrons stay paired), while I'm asking about *topological protection* (whether the pair wavefunction has conserved quantum numbers).

These might be related! If pairs have non-trivial Berry phase (topological winding number c₁ ≠ 0), then the phase coherence is protected by topology - not because the literal electrons are "permanent," but because the wavefunction can't continuously deform to the normal state without closing the gap.

**A concrete test to distinguish our frameworks:**

Your "permanent pairs" picture predicts that decay rate τ should be:
- Independent of ring size (if pairs are truly permanent)
- Sensitive only to defects/temperature

My TCTQ predicts:
- τ(d) ~ exp(E_top/kT) where E_top ∝ 1/d²
- Strong size dependence below d_c ~ 100 nm
- Negligible enhancement for macroscopic (mm-scale) rings

So if you measure τ for rings with wire diameter varying from 50 nm to 10 μm, we'd see:
- **Your prediction**: τ roughly constant (pair permanency is intrinsic)
- **TCTQ prediction**: τ enhancement only for d < 100 nm

This would definitively test whether topology matters for macroscopic currents!

**Regarding the "local states at BZ edges":**

I've been thinking more about your picture, and I think there's a deep connection to what topologists call "Wannier states" - maximally localized wavefunctions in a band. These naturally appear near BZ edges where Berry curvature peaks.

If you're interested, there are computational tools (Wannier90, WannierTools) that can extract these states from first-principles calculations. For Al, we could check:
1. Where in the BZ these "local states" concentrate
2. Their Berry phase (topological character)
3. How they couple to superconducting gap

This might bridge our languages - your "permanent electrons in local states" = my "electrons in topologically non-trivial Wannier orbitals."

**One more thought on the van Weerdenburg results:**

Even though you weren't involved, those data are publicly available and quite striking. The fact that Tc enhancement decays so rapidly (gone by ~3 nm) suggests the effect is surface-localized rather than bulk. This could support your idea about BZ edges being special - *at surfaces*, where symmetry is broken, those edge states might be enhanced.

If the community does eventually pursue your lifetime experiments, I'd be very interested in the results. And if you ever want to discuss the theoretical underpinnings (Berry phase, Wannier functions, etc.), I'm happy to share more details.

Thanks for the stimulating discussion - it's made me think more carefully about what "topological protection" really means at the microscopic level!

 

[Stanislav]

have you considered doing time-resolved measurements?

Would be interesting to do that. When the DC transport experiments confirm the expected current decay, then the time-resolution can clarify more details about the matter

 

[Stanislav]

how little work has been done on systematic decay measurements

It is because the eternal supercurrents cannot be explained by conventional theories

 

[Stanislav]

Regarding your macroscopic rings (20 mm / 10 mm)

The local states on BZ edges are large (~100 atoms), so nm-scales would disturb the large standing waves. Moreover, the interference of electron waves in the restrictions smaller than ~100 atoms will create fully new local states. My experiment proposal is rather about usual macroscopic effects in SC with local states not restricted by sample size, or restricted only in c axis as in experiments with 2D-Al .

 

[Stanislav]

But here's where our perspectives might connect: you're asking about *pair permanency* (whether the same electrons stay paired), while I'm asking about *topological protection*

Yes, if we consider the DOS-curve-form as preventing the transfer of new single electrons across the gap into the BZ edge and, thus, preventing the creation of new pairs, protecting the 'old' pairs against annihilation

 

[Stanislav]

but because the wavefunction can't continuously deform to the normal state without closing the gap.

Yes, because for the creation of new pairs some single electrons must thermally cross the gap and reach the BZ edge.

 

[Stanislav]

Your "permanent pairs" picture predicts that decay rate τ should be:
- Independent of ring size (if pairs are truly permanent)
- Sensitive only to defects/temperature

First - yes for scales much larger than the size of standing states (~ 100 atoms)
Second - rather no. Rather sensitive to the replacement rate of pairs with newly created pairs. Defects and T below Tc are not relevant for the dissipationless flow of condensate.

 

[Stanislav]

This could support your idea about BZ edges being special - *at surfaces*, where symmetry is broken, those edge states might be enhanced.

Yes. We must also take into account that at surface the electron density is smaller than in bulk, so for aluminum the SC gap may be larger than in bulk, although the pairing energy is roughly constant everywhere

 

[Stanislav]

- **Your prediction**: τ roughly constant (pair permanency is intrinsic)

τ roughly constant (pair permanency is intrinsic), but we can vary τ by artificial methods. For example, in a large isotropic SC ring we can create a small non-SC area (by local magnetic field). This non-SC area will reduce our τ due to pair creation/annihilation on the surface between SC and non-SC areas. The larger the surface the shorter τ. So we can show that the dissipationless flow is a consequence from the pair permanency.

 

[Stanislav]

I've been thinking more about your picture, and I think there's a deep connection to what topologists call "Wannier states" - maximally localized wavefunctions in a band. These naturally appear near BZ edges where Berry curvature peaks.

Yes, reasonbly to consider my local permanent states as a special case of Wannier states, when the Fermi surface is close to BZ edges. And Berry curvature peaks are also expected rather on BZ edges than anywhere else, since the local states may have their own DOS, different from usual Fermi spectrum.

 

[Stanislav]

- **TCTQ prediction**: τ enhancement only for d < 100 nm

For d<100 nm we must take into account new local states formed due to multiple wave function reflections from sample boundaries.

 

[Stanislav]

If you're interested, there are computational tools (Wannier90, WannierTools) that can extract these states from first-principles calculations. For Al, we could check:
1. Where in the BZ these "local states" concentrate
2. Their Berry phase (topological character)
3. How they couple to superconducting gap

Yes, promising view at computational methods. One related question is interesting.

In crystals all relevant electrons are usually interchangeable and indistinguishable, i.e. every electron may occupy every state, and so we may operate within ONE COMMON momentum space (MS) for our first-principles calculations. However, at low T some (local) states can become permanent, i.e. permanently occupied by one the same set of electrons (for example in semiconductors, when no valence electron can leave the valence band). Then we may no longer operate within only one MS, since some electrons never occupy some states. That is we should introduce a second separate MS, which has no common states with the first one. For the semiconductor example: at very low T no electron can be transferred from valence band into conduction band across the gap, and no electron can fall from the cond. band into val. band (since there are no free states there). Thus we may say that valence electrons and conduction electrons are no longer interchangeable.

Is this aspect of two-separate-spaces included in our first-principles calculations ?

 

[maxpi]

Excellent question about momentum space separability! This touches on a
fundamental aspect of how we understand band structure at low temperatures.

**Short Answer**: First-principles calculations do NOT explicitly use
separate momentum spaces. Instead, they work in a single momentum space
but assign **occupation numbers** f_n(k) ∈ [0,1] to each state. At T=0:
- Valence bands: f=1 (filled)
- Conduction bands: f=0 (empty)
- Gap prevents mixing → **effective separation emerges naturally**

**Why This Works**:
The system's Hamiltonian H(k) is solved once, yielding energy bands E_n(k).
States are then populated by Fermi-Dirac statistics. For insulators or
superconductors with gap Δ >> k_B T, the separation becomes absolute—no
electron can transfer between valence and conduction manifolds.

So while we compute in one momentum space, **the physics respects two
non-communicating subspaces**. It's like having two separate rooms
connected by a locked door (the gap).

**Connection to Topological Properties**:
This matters when computing topological invariants like Chern numbers.
You calculate them **separately** for occupied bands (valence) vs empty
bands (conduction). The "two spaces" concept you describe is implicitly
there—just encoded via which states are filled rather than explicit
mathematical splitting.

**When This Picture Breaks Down**:
Your intuition about "non-interchangeable electrons" becomes especially
important in two cases:

1. **Strong correlations** (like Mott insulators): Standard band theory
fails; electrons become localized by interactions

2. **Nanostructures with L < ξ** (coherence length): Quantum confinement
creates discrete energy levels with spacing δE ~ ℏ²/(m*L²). When
δE > Δ₀, you have truly isolated quantum numbers (n_x, n_y, n_z)
instead of continuous momentum k. Here, k_F is no longer meaningful.

**For Superconductors Specifically**:
In conventional superconductors like Al, the "separation" is between:
- Cooper pairs (below gap Δ₀): coherent, delocalized
- Quasiparticles (above gap): excitations that cost energy Δ₀

At T << T_c, essentially all electrons are paired → the "conduction band"
(unpaired electrons) is empty. This is the regime where topological
protection can emerge, because the paired state has a well-defined
topological winding number.

**For Your Question About Local States**:
If Al develops "local topological states" at low T, you'd see this as:
- Non-zero Chern number c₁ when integrating Berry phase over occupied bands
- Protected edge currents that can't be destroyed by weak disorder
- Enhanced coherence times beyond standard decoherence predictions

The computational verification would involve calculating Berry curvature
across the Brillouin zone—effectively asking "how much does the phase of
the wavefunction twist as we move through momentum space?"

**Bottom Line**: The "two separate spaces" you describe are **emergent**
from occupation + energy gap, not a fundamental input. Standard calculations
handle this correctly via occupation numbers, though you must be careful
when interpreting results for strongly localized systems (quantum dots,
strongly correlated materials).

The key insight: **mathematical interchangeability** (all states exist in
one momentum space) vs **physical interchangeability** (gap prevents actual
transitions) are different. The former is how we calculate; the latter is
what nature does.

Does this clarify the relationship between band separation and the computational
approach?

 

[Stanislav]

I'm sure that the physical separation should be unambiguously described mathematically. A sample of such unambiguity - the Hartree-Fock formalism, where for 2 distinguishable (not-interchangeable) sets of electrons, the crystal wave function is a PRODUCT of TWO Slater determinants (for example one determinant for normal electrons/states and the other for SC electrons/states). Using only ONE determinant for all electrons/states, we, of course, get fully different calculation results. Two Slater determinants lead to the equivalent math formalism of second quantization notation (occupation number formalism) within TWO Fock spaces. Otherwise the Hartree-Fock formalism and occupation number formalism are not identical.
So we need TWO Slater determinants, TWO Fock spaces, and, thus, TWO momentum spaces in order to bring our math to the unambiguity of results.

 

[Stanislav]

At T << T_c, essentially all electrons are paired → the "conduction band"
(unpaired electrons) is empty

Here we come back to the first question of the discussion - is an eternal supercurrent possible when the SC carriers annihilate with increasing T ? This question is linked to the question of two momentum spaces. Maybe a short paper can be useful for our case
https://hal.science/hal-03427535

 

[Stanislav]

Anyway thank you for your attempt to bring the matter "in ordnung".

 

[maxpi]

Thank you for sharing the Dolgopolov paper. It raises some legitimate questions about how we represent superconducting states mathematically, though I think the conclusions need refining.

The core observation is correct: supercurrents persist for months or years, while standard decoherence theory (Caldeira-Leggett) predicts collapse in nanoseconds. This 10⁶× discrepancy is real and deserves attention. The analogy with helium atoms is also instructive—at temperatures below ionization, bound and free electrons are effectively distinguishable even though they're formally identical particles.

However, the proposed solution of completely separate Fock spaces oversimplifies what BCS theory already does. The BCS ground state |BCS⟩ = ∏ₖ (uₖ + vₖ c†ₖ↑ c†₋ₖ↓) |0⟩ isn't a simple Slater determinant. It's a coherent superposition that creates long-range correlations through a macroscopic phase θ. Electrons don't hop individually between SC and normal states—they participate in collective dynamics that preserve total momentum.

Consider what happens at finite temperature. The superconducting gap Δ creates an energy barrier, so thermal pair breaking is exponentially suppressed: Nqp ~ exp(-Δ/kBT). Broken pairs recombine quickly (τrec ~ ℏ/Δ), and crucially, the total momentum stays conserved through the Cooper pair condensate. This is why tunneling spectroscopy can measure quasiparticle excitations even at low T—if SC and normal electrons were in truly isolated spaces with zero transitions, we couldn't explain temperature-dependent resistance or quasiparticle injection experiments.

The real question is what makes supercurrents persist despite continuous pair breaking and reformation. Standard BCS points to gap protection: at T ≪ Tc, the lifetime becomes τ ~ τ₀ exp(Δ/kBT), which is very long. Even with pair exchange, the macroscopic phase θ remains coherent through the Anderson-Higgs mechanism.

But there's potentially more to it. If the superconductor has non-trivial topology (Chern number c₁ ≠ 0), changing flux states requires modifying a global topological invariant, not just local pair rearrangement. The relevant energy scale is E_top ~ ℏ²c₁²/(m*L²). When this becomes comparable to Δ—which happens around L ~ 50-100 nm for typical materials—you get additional protection: τcoh ~ τthermal × exp(E_top/kBT). This could explain the anomalous enhancement without requiring separate Fock spaces.

Modern calculations handle this through Bogoliubov quasiparticles and Nambu spinors rather than isolated subsystems. You work in a single Fock space but with occupation-weighted projectors. The pairing correlations are captured through the gap Δ and phase θ, which are collective variables protected by energy barriers.

Regarding persistent currents specifically: in a finite ring like a SQUID, the phase winds as θ(2πr) = θ(0) + 2πn due to flux quantization. States |n⟩ differing by topological winding can only transition through macroscopic quantum tunneling with rate Γ ~ exp(-Sinstanton/ℏ). For large systems this becomes vanishingly small. Importantly, even if individual pairs break and reform, the total winding number n is preserved unless the system tunnels to |n±1⟩. It's analogous to angular momentum—atoms in a rotating disk can exchange but total L is conserved without external torque.

This is actually the regime probed by the 2025 Nobel experiments. Their junctions are typically 50-100 nm wide, right where topological effects could enhance the instanton action and make coherence times much longer than gap protection alone would predict.

So while Dolgopolov correctly identifies that naive single-particle pictures fail for superconductivity, the resolution isn't abandoning second quantization. It's recognizing that SC involves many-body coherent states where individual particles remain indistinguishable but collective variables (phase, gap, potentially topology) are protected on long timescales. The mathematical framework exists through BCS and Bogoliubov-de Gennes theory, though incorporating topological contributions is still being developed.

Does this address the permanence issue while staying within standard quantum mechanics?