REVIEW 2 major objections 4 minor 62 references
Flat-band formation and chiral superconductivity in driven topological insulators
T0 review · 2 major / 4 minor · reviewed 2026-08-15 · deepseek-v4-flash
Pith's one-line read This paper claims that circularly polarized light can flatten the surface band of a topological insulator and make the electrons pair into a chiral superconductor at about 7 K.
desk verdict Clever Floquet flat-band mechanism with a real quantitative concern: the first-order expansion used for Tc is not controlled at the claimed value. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the first-order Floquet Hamiltonian $H_F=\hbar v_F \mathbf{k}\cdot\boldsymbol{\sigma}+D(k^2+e^2A_0^2/\hbar^2)\sigma_0-\frac{v_F^2e^2A_0^2}{\hbar\omega}\sigma_z$, derived by a Peierls substitution followed by an inverse-frequency expansion. The drive induces a mass term that gaps the Dirac cone while also shifting the band bottom; when the curvature $Dk^2$ cancels the $k^2$ part of the square-root dispersion, the band flattens. The superconductivity mechanism is the RPA-screened Rytova-Keldysh Coulomb interaction, projected onto the lower band, decomposed into angular momentum channels $\tilde{V}_\ell(k,k')$, and solved self-consistently for the gap $\eta_\ell(k)$. Spin-orbit locking freezes the spin on the topological-insulator surface, leaving only odd-parity pairing channels and allowing the form factors of the projected interaction to select a definite chirality.
What would settle it
Recompute the gap equation with the second-order Floquet term quoted in the Supplement, which renormalizes $v_F$ to $v_F(1-e^2A_0^2v_F^2/\hbar^2\omega^2)$; if the $\mathcal{O}(k^4)$ flatness and a finite $T_c$ do not survive at the renormalized flat-band condition, the central quantitative claim fails. On the experimental side, time-resolved photoemission of Bi2Se3 at 50 THz with $E_0$ near $1.4\times10^8$ V/m should reveal the lower band flattening to $\mathcal{O}(k^4)$; observing an essentially unchanged quadratic dispersion would falsify the claim.
Extended reading notes
Core claim
At the level of the effective two-by-two Dirac Hamiltonian $H_0=\hbar v_F \mathbf{k}\cdot\boldsymbol{\sigma}+Dk^2\sigma_0$, circularly polarized light with amplitude $E_0$ and frequency $\omega$ creates a Floquet band with dispersion $\varepsilon_k=D(k^2+e^2A_0^2/\hbar^2)-\sqrt{\hbar^2v_F^2k^2+(v_F^2e^2A_0^2/\hbar\omega)^2}$. When $E_0=E_0^{\mathrm{flat}}=\frac{\hbar\omega}{e}\sqrt{\frac{\hbar\omega}{2|D|}}$, the quadratic term in $k$ cancels and the lower band disperses only at $\mathcal{O}(k^4)$; for slightly smaller $E_0$ the Fermi surface becomes an annulus, giving a Mexican-hat-like band. In that regime the lower band is nearly pseudospin-polarized, so the surviving pairing channels are odd-parity. Using a Rytova-Keldysh-type screened Coulomb interaction, RPA charge susceptibility, and a metallic gate to control screening, the paper finds that the screened interaction becomes attractive at short range, and the angular-momentum-decomposed gap equation yields a chiral $p_x\pm ip_y$ order parameter with $T_c\sim 7$ K at low electron densities. The paper argues this realizes, in a driven system, the same physics proposed for rhombohedral graphene under a displacement field.
Load-bearing premise
The quantitative superconductivity calculation rests on a first-order Floquet expansion whose expansion parameter $eA_0v_F/(\hbar\omega)$ is close to one at the flat-band limit, so the neglected higher-order terms could change the band structure and the predicted $T_c$.
Editorial extensions
If this is right
- At drive amplitude $E_0^{\mathrm{flat}}$, the lower Floquet band of a 3D topological-insulator surface disperses only at $\mathcal{O}(k^4)$, giving a nearly flat band whose curvature can be flipped into a Mexican-hat shape by varying $E_0$.
- With a metallic gate at roughly 10 nm, the screened Coulomb interaction becomes attractive at short range and the self-consistent gap equation admits chiral $p_x\pm ip_y$ pairing with $T_c\sim 7$ K at densities $10^{11}$--$10^{12}$ cm$^{-2}$, in a regime where the Wigner-crystal phase is suppressed.
- The same effective Hamiltonian applies to other Dirac materials, including topological crystalline insulators, spin-orbit-coupled transition-metal dichalcogenides, and graphene with proximity-induced spin-orbit coupling, so the flat-band and pairing mechanism transfers to those settings.
- Because the lower band is nearly pseudospin-polarized, only odd-parity pairing channels survive, and the projection form factors lift the degeneracy between the $\ell=+1$ and $\ell=-1$ channels, favoring one chirality of the order parameter.
Reading between the lines
- A direct next step is to carry the second-order Floquet term through the RPA screening and the gap equation; the Supplement shows the expansion parameter is near one at the flat-band limit, so the precise location of the flat-band condition and the numerical value of $T_c$ could shift.
- If the flat band is as narrow as claimed, an external magnetic field or increased interaction strength might drive fractional Chern insulator states, since the ingredients of band topology and a nearly dispersionless band would be present; the paper does not address this.
- The gate-distance window suggests a device design in which a patterned or split gate tunes different regions of one sample between superconducting and Wigner-crystal behavior, something the uniform-gate calculation does not explore.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes using circularly polarized light on the surface of three-dimensional topological insulators to Floquet-engineer nearly flat or Mexican-hat electronic bands. It derives an effective Floquet Hamiltonian to first order in the inverse frequency, gives an explicit flat-band condition E0_flat, and shows that near this condition the lower band disperses only at O(k^4). Using a Rytova-Keldysh screened Coulomb interaction with a nearby metallic gate, the authors solve a self-consistent mean-field gap equation for odd-parity pairing and report chiral p-wave superconductivity with Tc ~ 7 K for densities in the range 10^11-10^12 cm^-2. They also discuss competition with the Wigner crystal phase as a function of gate distance.
Significance. If the quantitative result holds, the paper would demonstrate a new route to interaction-driven topological superconductivity in a driven system, with a falsifiable prediction of Tc~7 K at specific drive amplitudes and densities. The strengths of the work include an explicit, parameter-specific flat-band condition, a self-consistent mean-field calculation with no parameter fitted to Tc, and a careful treatment of screening and the competition with Wigner crystallization. The analogy to rhombohedral graphene under displacement field is well motivated. However, the central numerical prediction rests on an inverse-frequency expansion whose control parameter is O(1) near the flat-band limit, as the paper's own supplementary material states, so the quantitative claim is not yet secured.
major comments (2)
- [Supplementary Material, Eqs. (17)-(21) and Fig. 3] The quantitative superconducting Tc in the main text (Eqs. (10)-(13) and Fig. 1(b)) is computed with the first-order Floquet Hamiltonian Eq. (17), but the SM explicitly states that the expansion parameter eA0 vF/(hbar omega) is approximately 1 near the flat-band limit for Bi2Se3, so higher-order terms are not negligible. The second-order term (Eq. (20)) renormalizes the Dirac velocity to vF_tilde = vF[1 - (eA0 vF/(hbar omega))^2] (Eq. (21)); at the claimed flat-band amplitude this factor is negative, so the curvature cancellation leading to the O(k^4) dispersion in Eq. (3) of the main text is destroyed. The SM retunes to a revised flat-band amplitude and compares only band structures (Fig. 3), but does not carry the second-order Hamiltonian through the RPA screening and the gap equation. Because the expansion is uncontrolled, the central numerical prediction Tc ~ 7 K is not secured.
- [Main text, Eq. (3) and Fig. 1(b)] The phase diagram in Fig. 1(b) is plotted versus E0/E0_flat with E0_flat defined by the first-order condition. Since the second-order correction changes the flat-band amplitude by a large amount (by roughly 40% or more for the Bi2Se3 parameters), the horizontal axis of Fig. 1(b) does not correspond to the field at which the band is actually flat once second-order terms are retained. The authors should either recompute the Tc and the phase diagram using the second-order Floquet Hamiltonian, or restrict the quantitative claim to parameter regimes where (eA0 vF/(hbar omega))^2 is genuinely small and present the Bi2Se3 result as qualitative only.
minor comments (4)
- [Supplementary Material, text before Fig. 3] The displayed expression for the revised flat-band amplitude E_tilde_flat^0 is missing parentheses and is very difficult to parse; please rewrite the formula in a clearly parenthesized form.
- [Abstract and Fig. 1(b)] The abstract quotes Tc ~ 7 K, but the color bar in Fig. 1(b) has a maximum of 6 K and Fig. 5(b) shows values below 6 K; please clarify the maximum value and indicate at which parameters the 7 K value is obtained.
- [Main text, paragraph after Eq. (13)] The sentence "We get Eq.(30) straightforwardly from equations (22, 23) by using definitions in Eq.(24)" refers to supplementary-material equation numbers from within the main text; this cross-referencing is confusing because the main text itself numbers only up to Eq. (13), and should be rephrased.
- [Main text, Eq. (1) and following paragraph] The statement that "to a good approximation, the pseudospin sigma aligns with the physical spin orientation" is used later to justify spin-polarized pairing, but no quantitative estimate of the deviation is given; a brief justification or reference would help.
Circularity Check
No significant circularity: the flat-band amplitude is analytically derived from the input Hamiltonian, and Tc emerges from a self-consistent gap equation with no parameter fitted to produce it.
full rationale
The derivation chain is self-contained. Starting from the low-energy surface Hamiltonian H0 (Eq. 1) with literature values for Bi2Se3, the paper computes the Floquet Hamiltonian (Eqs. 2, 17), obtains the lower-band dispersion (Eq. 3), and derives the flat-band condition E0_flat = (hbar omega/e) sqrt(hbar omega/(2|D|)) by requiring the k^2 curvature coefficient to vanish. That condition is an analytic output of the model, not a parameter fitted to the later superconductivity result. The superconducting Tc is then obtained by solving the RPA-screened Coulomb interaction (Eqs. 7-8) together with the self-consistent gap equation (Eqs. 10, 13, 25-29); Tc is an emergent output, not an input. No load-bearing step reduces to its own conclusion, and no fitted quantity is renamed as a prediction. The only identified self-citation is Ref. [27] (Farrell, Arsenault, and Pereg-Barnea), used as background for Floquet-gapped Dirac cones and not relied on for the flat-band condition or the Tc calculation. The Supplementary Material's comment that the expansion parameter eA0 vF/(hbar omega) is about 1 near the flat-band limit, and the resulting second-order renormalization of vF, is a genuine truncation-validity concern about the quantitative Tc value, but it is not a circularity: the first-order result is not equivalent to its inputs by construction. Thus the paper is largely self-contained and has no meaningful circular dependence.
Assumptions & free parameters
free parameters (7)
- D (Dirac mass curvature) =
0.2 eV nm^2
- hbar v_F =
0.3 eV nm
- drive frequency omega =
2 pi * 50 THz (hbar omega approx 0.2 eV)
- gate distance d_reg =
10 nm (scanned from 1 to 30 nm)
- Rytova-Keldysh parameter r_K =
1 nm
- effective dielectric constant epsilon =
(20 epsilon0 + epsilon0) / 2 = 10.5 epsilon0
- drive amplitude E0 / E0_flat =
0.8 to 1.2 (swept)
assumptions (6)
- domain assumption The high-frequency inverse-frequency Floquet expansion converges, and the first non-trivial order is sufficient for all quantitative results.
- domain assumption Under driving, the electronic system reaches a steady state described by a static thermal distribution, so a static Coulomb-interacting Hamiltonian applies.
- domain assumption Projection form factors in the interaction can be set to 1 because hbar^2 k_F^2 / (4 m_flat^2 v_F^2) is much less than 1.
- domain assumption RPA gives a quantitatively accurate screened interaction, and mean-field theory gives a quantitatively accurate Tc.
- domain assumption Electron spin is frozen in the lower Floquet band, so only odd-parity, effectively spinless pairing survives.
- domain assumption Top and bottom surface states do not hybridize for slab thickness L satisfying 6 nm < L < d_reg.
Cite this review
Pith. "Pith review of Flat-band formation and chiral superconductivity in driven topological insulators." pith.science (2026). https://pith.science/paper/ZSZWALO7
@misc{pith2026260727355,
author = {Pith},
title = {Pith review of: Flat-band formation and chiral superconductivity in driven topological insulators},
year = {2026},
howpublished = {\url{https://pith.science/paper/ZSZWALO7}},
note = {Machine review of arXiv:2607.27355}
}
abstract
We demonstrate that circularly polarized light can be used to Floquet-engineer nearly flat or Mexican-hat like electronic bands on the surface of three-dimensional topological insulators (3D TIs), which under suitable conditions, can support topological superconductivity via purely repulsive Coulomb interactions. The driving acts not merely by gapping out the Dirac cone on the surface of the 3D TI, but can be used to diminish, and even flip in sign, the intrinsic curvature of the surface state dispersion away from the Dirac point. Using parameters for canonical 3D TIs, we find that the flat band limit is attained for reasonable electric fields and the bands realized by changing the strength of the driving field have a similar energetic and spatial profile to those obtained in rhombohedral graphene under varying displacement field, where the case for superconductivity with purely repulsive interactions has recently been made. We find that, with the aid of appropriately placed screening metallic gate, one can obtain $T_c \sim 7 $K in this setup while avoiding Wigner crystallization for low electron densities in the range of $10^{11}-10^{12}/\text{cm}^2$.
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Works this paper leans on
-
[1]
Leykam, A
D. Leykam, A. Andreanov, and S. Flach, Advances in Physics: X3, 1473052 (2018)
2018
-
[2]
Tasaki, Progress of Theoretical Physics99, 489 (1998)
H. Tasaki, Progress of Theoretical Physics99, 489 (1998)
work page 1998
-
[3]
K. Sun, Z. Gu, H. Katsura, and S. Das Sarma, Phys. Rev. Lett.106, 236803 (2011)
2011
-
[4]
Wang and Y
F. Wang and Y. Ran, Phys. Rev. B84, 241103(R) (2011)
2011
-
[5]
S. A. Parameswaran, R. Roy, and S. L. Sondhi, Comptes Rendus. Physique14, 816 (2013)
2013
-
[6]
Y. Cao, V. Fatemi, A. Demir, S. Fang, S. L. Tomarken, J. Y. Luo, J. D. Sanchez-Yamagishi, K. Watanabe, T. Taniguchi, E. Kaxiras, R. C. Ashoori, and P. Jarillo- Herrero, Nature556, 80 (2018)
2018
- [7]
-
[8]
Yankowitz, S
M. Yankowitz, S. Chen, H. Polshyn, Y. Zhang, K. Watan- abe, T. Taniguchi, D. Graf, A. F. Young, and C. R. Dean, Science363, 1059 (2019)
2019
Show all 62 references
-
[9]
Stepanov, I
P. Stepanov, I. Das, X. Lu, A. Fahimniya, K. Watanabe, T. Taniguchi, F. H. L. Koppens, J. Lischner, L. Levitov, and D. K. Efetov, Nature583, 375 (2020)
2020
-
[10]
Bistritzer and A
R. Bistritzer and A. H. MacDonald, Proceedings of the National Academy of Sciences108, 12233 (2011)
2011
-
[11]
Geier, M
M. Geier, M. Davydova, and L. Fu, Nature Communica- tions17, 232 (2025)
2025
-
[12]
Y. Zeng, Z. Xia, K. Kang, J. Zhu, P. Kn¨ uppel, C. Vaswani, K. Watanabe, T. Taniguchi, K. F. Mak, and J. Shan, Nature622, 69 (2023)
2023
-
[13]
J. Xie, Z. Huo, X. Lu, Z. Feng, Z. Zhang, W. Wang, Q. Yang, K. Watanabe, T. Taniguchi, K. Liu, Z. Song, X. C. Xie, J. Liu, and X. Lu, Nature Materials24, 1042 (2025)
2025
-
[14]
J. H. Shirley, Phys. Rev.138, B979 (1965)
1965
-
[15]
Rahav, I
S. Rahav, I. Gilary, and S. Fishman, Phys. Rev. A68, 013820 (2003)
2003
-
[16]
Goldman and J
N. Goldman and J. Dalibard, Phys. Rev. X4, 031027 (2014)
2014
-
[17]
Goldman, J
N. Goldman, J. Dalibard, M. Aidelsburger, and N. R. Cooper, Phys. Rev. A91, 033632 (2015)
2015
-
[18]
Eckardt and E
A. Eckardt and E. Anisimovas, New Journal of Physics 17, 093039 (2015)
2015
-
[19]
N. H. Lindner, G. Refael, and V. Galitski, Nature Physics 7, 490 (2011)
2011
-
[20]
Oka and S
T. Oka and S. Kitamura, Annual Review of Condensed Matter Physics10, 387 (2019)
2019
-
[21]
M. S. Rudner and N. H. Lindner, Nature Reviews Physics 2, 229 (2020)
2020
-
[22]
C. Bao, P. Tang, D. Sun, and S. Zhou, Nature Reviews Physics4, 33 (2022)
2022
-
[23]
J. W. McIver, B. Schulte, F. U. Stein, T. Matsuyama, G. Jotzu, G. Meier, and A. Cavalleri, Nature Physics16, 38 (2020)
2020
-
[24]
Oka and H
T. Oka and H. Aoki, Phys. Rev. B79, 081406(R) (2009)
2009
-
[25]
Kitagawa, T
T. Kitagawa, T. Oka, A. Brataas, L. Fu, and E. Demler, Phys. Rev. B84, 235108 (2011)
2011
-
[26]
G. Usaj, P. M. Perez-Piskunow, L. E. F. Foa Torres, and C. A. Balseiro, Phys. Rev. B90, 115423 (2014)
2014
-
[27]
Farrell, A
A. Farrell, A. Arsenault, and T. Pereg-Barnea, Phys. Rev. B94, 155304 (2016)
2016
-
[28]
L. Fu, C. L. Kane, and E. J. Mele, Phys. Rev. Lett.98, 106803 (2007)
2007
-
[29]
Fu and C
L. Fu and C. L. Kane, Phys. Rev. B76, 045302 (2007)
2007
-
[30]
Zhang, C.-X
H. Zhang, C.-X. Liu, X.-L. Qi, X. Dai, Z. Fang, and S.-C. Zhang, Nature Physics5, 438 (2009)
2009
-
[31]
Liu, X.-L
C.-X. Liu, X.-L. Qi, H. Zhang, X. Dai, Z. Fang, and S.-C. Zhang, Phys. Rev. B82, 045122 (2010)
2010
-
[32]
Y. Xia, D. Qian, D. Hsieh, L. Wray, A. Pal, H. Lin, A. Bansil, D. Grauer, Y. S. Hor, R. J. Cava, and M. Z. Hasan, Nature Physics5, 398 (2009)
2009
-
[33]
Y. L. Chen, J. G. Analytis, J.-H. Chu, Z. K. Liu, S.-K. Mo, X. L. Qi, H. J. Zhang, D. H. Lu, X. Dai, Z. Fang, S. C. Zhang, I. R. Fisher, Z. Hussain, and Z.-X. Shen, Science325, 178 (2009)
2009
-
[34]
M. Z. Hasan and C. L. Kane, Rev. Mod. Phys.82, 3045 (2010)
2010
-
[35]
K. I. Seetharam, C.-E. Bardyn, N. H. Lindner, M. S. Rudner, and G. Refael, Phys. Rev. X5, 041050 (2015)
2015
-
[36]
K. I. Seetharam, C.-E. Bardyn, N. H. Lindner, M. S. Rudner, and G. Refael, Phys. Rev. B99, 014307 (2019)
2019
-
[37]
F. C. Peiris, E. T. Holmgren, J. W. Lyons, X. Li, X. Liu, M. Dobrowolska, and J. K. Furdyna, Journal of Vacuum Science and Technology B37, 031205 (2019)
2019
-
[38]
Y. H. Wang, H. Steinberg, P. Jarillo-Herrero, and N. Gedik, Science342, 453 (2013)
2013
-
[39]
N. S. Rytova, Screened potential of a point charge in a thin film (2020), arXiv:1806.00976 [cond-mat.mes-hall]
2020 arXiv
-
[40]
L. V. Keldysh, Soviet Journal of Experimental and The- oretical Physics Letters29, 658 (1979)
1979
-
[41]
M. Fang, Z. Wang, H. Gu, M. Tong, B. Song, X. Xie, T. Zhou, X. Chen, H. Jiang, T. Jiang, and S. Liu, Applied Surface Science509, 144822 (2020)
2020
-
[42]
Kohn and J
W. Kohn and J. M. Luttinger, Phys. Rev. Lett.15, 524 (1965)
1965
-
[43]
A. V. Chubukov, Phys. Rev. B48, 1097 (1993)
1993
-
[44]
Maiti and A
S. Maiti and A. V. Chubukov, AIP Conference Proceed- ings1550, 3 (2013)
2013
-
[45]
Alexandrov,Theory of Superconductivity: From Weak to Strong Coupling, Condensed Matter Physics (CRC Press, 2003)
A. Alexandrov,Theory of Superconductivity: From Weak to Strong Coupling, Condensed Matter Physics (CRC Press, 2003)
2003
-
[46]
Mahan,Many-Particle Physics, Physics of Solids and Liquids (Springer US, 1990)
G. Mahan,Many-Particle Physics, Physics of Solids and Liquids (Springer US, 1990)
1990
-
[47]
Giuliani and G
G. Giuliani and G. Vignale,Quantum Theory of the Elec- tron Liquid, Masters Series in Physics and Astronomy (Cambridge University Press, 2005)
2005
-
[48]
Wigner, Phys
E. Wigner, Phys. Rev.46, 1002 (1934)
1934
-
[49]
Ceperley, Phys
D. Ceperley, Phys. Rev. B18, 3126 (1978). 6
1978
-
[50]
Tanatar and D
B. Tanatar and D. M. Ceperley, Phys. Rev. B39, 5005 (1989)
1989
-
[51]
N. D. Drummond and R. J. Needs, Phys. Rev. Lett.102, 126402 (2009)
2009
-
[52]
Zhang, K
Y. Zhang, K. He, C.-Z. Chang, C.-L. Song, L.-L. Wang, X. Chen, J.-F. Jia, Z. Fang, X. Dai, W.-Y. Shan, S.-Q. Shen, Q. Niu, X.-L. Qi, S.-C. Zhang, X.-C. Ma, and Q.-K. Xue, Nature Physics6, 584 (2010)
2010
-
[53]
Sakamoto, T
Y. Sakamoto, T. Hirahara, H. Miyazaki, S.-i. Kimura, and S. Hasegawa, Phys. Rev. B81, 165432 (2010)
2010
-
[54]
Lu, W.-Y
H.-Z. Lu, W.-Y. Shan, W. Yao, Q. Niu, and S.-Q. Shen, Phys. Rev. B81, 115407 (2010)
2010
-
[55]
Fu, Physical Review Letters106, 106802 (2011)
L. Fu, Physical Review Letters106, 106802 (2011)
2011
-
[56]
Ando and L
Y. Ando and L. Fu, Annual Review of Condensed Matter Physics6, 361 (2015)
2015
-
[57]
Xiao, G.-B
D. Xiao, G.-B. Liu, W. Feng, X. Xu, and W. Yao, Phys- ical Review Letters108, 196802 (2012)
2012
-
[58]
X. Xu, W. Yao, D. Xiao, and T. F. Heinz, Nature Physics 10, 343 (2014)
2014
-
[59]
Gmitra and J
M. Gmitra and J. Fabian, Physical Review B92, 155403 (2015)
2015
-
[60]
Zhang, C
J. Zhang, C. Triola, and E. Rossi, Physical Review Let- ters112, 096802 (2014)
2014
-
[61]
Bukov, L
M. Bukov, L. D’Alessio, and A. Polkovnikov, Advances in Physics64, 139 (2015)
2015
-
[62]
Mikami, S
T. Mikami, S. Kitamura, K. Yasuda, N. Tsuji, T. Oka, and H. Aoki, Phys. Rev. B93, 144307 (2016). 7 Supplementary Material for: Flat-band formation and chiral superconductivity in driven topological insulators FLOQUET HAMIL TONIAN AND FLA T-BAND CONDITION In the presence of cir...
2016
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