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Current-induced re-entrant superconductivity and extreme nonreciprocal superconducting diode effect in valley-polarized systems
by Yu-Chen Zhuang and Qing-Feng Sun
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Submission summary
| Authors (as registered SciPost users): | Yu-Chen Zhuang |
| Submission information | |
|---|---|
| Preprint Link: | scipost_202505_00037v1 (pdf) |
| Date submitted: | May 16, 2025, 2:48 p.m. |
| Submitted by: | Yu-Chen Zhuang |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approach: | Theoretical |
Abstract
The superconducting diode effect (SDE) refers to the nonreciprocity of superconducting critical currents. Generally, the SDE has a positive and a negative critical currents jc± corresponding to two opposite directions with unequal amplitudes. It is demonstrated that an extreme nonreciprocity where two critical currents can become both positive (or negative) has been observed in twisted graphene systems. In this work, we theoretically propose a possible mechanism to realize an extreme nonreciprocal SDE. Based on a simple microscopic model, we demonstrate that depairing currents required to dissolve Cooper pairs can be remodulated under the interplay between valley polarizations and applied currents. Near the disappearance of the superconductivity, the remodulation is shown to induce extreme nonreciprocity and also the current-induced re-entrant superconductivity where the system has two different critical current intervals. Our study may provide new horizons for understanding the coexistence of superconductivity and spontaneous valley polarizations, and pave a way for designing SDE with 100% efficiency.
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Reports on this Submission
Report
How charge current influences VP in normal state has been discussed in several previous works eg [49], and similarly for how VP influences SDE eg [57]. The discussion here attempts to put these two together phenomenologically. It appears previous works do not consider the mechanism on SDE discussed here.
The current-VP coupling is described by Eqs. (4,5), which are argued to hold in normal state, similar to Ref. [49]. It is not shown in the text they are valid with same coefficients in superconducting state. The argument why this would not matter appears not rigorous but important for conclusions (see detailed point 1).
Consequences of this assumption are then explored in fairly straightforward calculations within a simple model with some electron dispersion, with several numerical results in Sec 3.
In principle, the results on supercurrent-VP coupling and their effect on SDE is interesting, even though it is not demonstrated in this work this mechanism is relevant for some specific realistic system. The main results however appear to hinge on an assumption whose validity is not clear and sufficiently explored in the text. If this question can be clarified, the results may be suitable for publication.
Requested changes
1 - On p. 7 line 204: The argumentation for Eqs. (4,5) discusses only nonequilibrium normal (N) state, and its validity is not shown for the superconducting (S) state.
It is asserted in the text that this does not matter, arguing one can approach the transition from the normal side. This appears to still implicitly assume that VP is continuous across the S/N transition. However, at the critical current on the S-side the order parameter Delta has a nonzero finite value, whereas the N-side one is in nonequilibrium state with different Delta, so it's not clear why the assumption would work.
Generally, Eqs. (4,5), the critical current, and the SDE should be found from the same equilibrium free energy. There, if e.g. VP changes discontinuously in current-carrying S/N transition, the transition point cannot be seen from Fig. 1(b) by approaching from the "normal" region and does not need to lie on the \tilde{j}_c line in the figure.
I think the discussion in the manuscript is not rigorous enough on this point. As the qualitative and quantitative main results rely on this, a more complete argument should be presented, eg. a consistent equilibrium model would improve the manuscript.
2 - Does the mean-field model of Eq. (6) possibly combined with (1,2) contain coupling between VP and supercurrent in equilibrium current-carrying state? This would be useful to evaluate and discuss in the text.
3 - Estimates for the values of the \alpha_+- coefficients in Eqs. (4,5) in some more realistic system would be useful. It is currently not easy to see that the values used in calculations are in a regime that is similar to an experimental system. Some values are mentioned in the discussion. Are the figures calculated with parameters that are similar in relative sizes to those required for the magnetization switching in eg. TBG or trilayer?
Recommendation
Ask for major revision
Report #1 by Jin-Xin Hu (Referee 1) on 2025-8-21 (Invited Report)
Strengths
- This manuscript provide a theoretical analysis of superconducting diode effect in graphene system without SOC.
- The interesting point is that the authors clarify a possible mechanism to explain the SDE in twisted graphene systems: the current-induced valley polarization modulation.
Weaknesses
- The main weakness of this work is the too simplified model. To be specific, in twisted graphene system, the trigonal warping of the Fermi surface is important, so the finite momentum pairing should be three-fold degenerate. In this case, applying current may change the populations among the three pockets.
Report
Recommendation
Ask for minor revision
We are very grateful for Dr. Hu of reviewing our work and giving a valuable report.
We have summarized and presented our reply to the referee’s report in an attached PDF profile in File attachment: replySci-T. pdf
Author: Yu-Chen Zhuang on 2025-11-15 [id 6038]
(in reply to Report 2 on 2025-10-10)See reply in the attached file: ReplySci-2T.pdf
Attachment:
ReplySci-2T.pdf