A well-resolved 0.7 conductance anomaly (a shoulder at roughly 0.7×2e2/h0.7\times 2e^2/h below the first quantized-conductance plateau) was recently reported in bilayer graphene (BLG) proximitized by WSe2_2, where proximity-enhanced spin-orbit coupling splits the fourfold ground state into spin-valley-locked Kramers pairs. The temperature, source-drain-bias, and Kondo-temperature phenomenology closely mirrors GaAs devices, yet the anomaly is essentially unchanged by a magnetic field parallel to the plane, showing that valley degrees of freedom play an essential role — something no conventional-semiconductor mechanism contains. The microscopic mechanism remains open: the relative roles of valley-exchange and spin-exchange interactions, the nature of the Kondo-like correlated state and what fixes its energy scale, how bulk valley- or spin-polarized phases and Lifshitz-transition regimes (a Lifshitz transition being a topological change of the Fermi surface, such as the appearance of new carrier pockets at low density) modify the channel correlations (untestable in the present single-back-gate design), and why weak spin-orbit-coupled samples show only indistinct 0.7 signatures.

Background

A quantum point contact (QPC) is a short, electrostatically defined ballistic constriction in a two-dimensional electron gas whose conductance rises in quantized steps of 2e2/h2e^2/h (ee is the electron charge and hh Planck's constant), each step corresponding to the opening of a one-dimensional transport mode. Since its 1996 report in GaAs, a persistent shoulder at roughly 0.7×2e2/h0.7\times 2e^2/h below the first plateau — the 0.7 anomaly — has been observed in many material systems, and its microscopic origin has been debated for three decades. The leading proposals are spontaneous spin polarization of the one-dimensional channel, a Kondo-like correlated state arising when a quasi-bound electron in the constriction acts as a localized magnetic moment screened by itinerant electrons (the Kondo effect, characterized by a Kondo temperature TKT_K below which screening sets in), and interaction-enhanced local spin moments with spin-incoherent transport. No consensus exists even in GaAs: a statistical study of 571 devices found that neither the spontaneous-polarization nor the Kondo model fits its data, and a 2025 on-chip sensing experiment reports that the 0.7 anomaly and the Kondo spin singlet have distinct physical origins. Bilayer graphene (BLG) adds a valley degree of freedom: low-energy states sit at the two inequivalent valleys K+K^+ and K−K^- of the graphene band structure, so each QPC subband is a fourfold spin-valley multiplet that time-reversal symmetry organizes into two Kramers pairs (pairs of time-reversed states protected against splitting at zero magnetic field). Placing BLG on the transition-metal dichalcogenide WSe2_2 induces proximity spin-orbit coupling that locks spin to valley and separates the two Kramers pairs; in the devices of Gerber et al. the splitting ΔSO=1.4\Delta_{\mathrm{SO}}=1.4 meV allows the individual states to be resolved. Earlier BLG QPC work saw only apparent fourfold degeneracy or contested intermediate plateaus, and one prior report of a 0.7 anomaly in plain BLG modeled it with spontaneous spin splitting while keeping the valley degenerate. Gerber et al. now report a well-resolved 0.7 anomaly in BLG/WSe2_2 QPCs, emerging between the lowest opposite spin-valley Kramers states: its temperature and bias behavior and its Kondo temperatures closely match GaAs, but it is essentially unaffected by an in-plane magnetic field (which would reorient spins in a purely spin-based mechanism) while a perpendicular field converts it into a resolved e2/he^2/h plateau via valley-Zeeman splitting (the lifting of the valley degeneracy by a perpendicular magnetic field) — evidence that valley exchange interactions are essential. Concurrently, Ingla-Aynés et al. observed the 0.7 anomaly in a non-proximitized BLG QPC and interpreted it as Kondo physics crossing over between SU(4) (spin and valley both degenerate; SU(4) Kondo screening involves a moment carrying all four spin-valley flavors) and spin-valley-locked SU(2) regimes. What no work provides is a microscopic account of the anomaly in the spin-valley-locked regime, where the interacting states have opposite valley quantum numbers rather than opposite spin.

Problem Statement

Identify the microscopic mechanism of the 0.7 anomaly in spin-valley-locked bilayer-graphene/WSe2_2 quantum point contacts, as posed by Gerber et al. Specifically: (i) determine the relative roles of valley-exchange and spin-exchange interactions between the opposite spin-valley Kramers-pair states (time-reversal-related pairs of states whose separation is set by the proximity spin-orbit coupling) in producing the conductance shoulder at roughly 0.7×2e2/h0.7\times 2e^2/h; (ii) establish the nature of the Kondo-like correlated state — whether a localized moment forms in the open channel, which spin and valley flavors it involves, and what sets the Kondo temperatures and their gate dependence, which are observed to closely match GaAs values; (iii) determine how the bulk phases of bilayer graphene — valley- or spin-polarized reservoirs and the Lifshitz-transition regimes at low carrier density (a Lifshitz transition is a topological change of the Fermi surface, such as the appearance of new carrier pockets) that the present single-global-back-gate design cannot reach — modify the channel correlation physics, a question the source proposes to attack with dual-gated structures allowing independent control of the displacement field (the perpendicular electric field across the two graphene layers) and the carrier density; and (iv) clarify why samples with weak proximity spin-orbit coupling (ΔSO=0.13\Delta_{\mathrm{SO}}=0.13 meV) show only indistinct 0.7 signatures, in particular whether all four states contribute when the Kramers-pair separation is small. A complete answer is a single microscopic account of this mechanism, validated by reproducing the reported temperature, source-drain-bias, perpendicular- and parallel-magnetic-field phenomenology (including the near-invariance of the anomaly under parallel field and its evolution into a resolved e2/he^2/h plateau under perpendicular field), that addresses all four items (i)-(iv); item (iii) is addressed not by measurement, which the reported single-global-back-gate data cannot provide, but by stating what the proposed mechanism implies for how bulk valley- or spin-polarized phases and Lifshitz-transition regimes would modify the channel correlations.

References

  1. Jonas D. Gerber, Efe Ersoy, Michele Masseroni, Markus Niese, Artem O. Denisov, Christoph Adam, Lara Ostertag, Jessica Richter, Takashi Taniguchi, Kenji Watanabe, Yigal Meir, Thomas Ihn, Klaus Ensslin, 'Spin-valley 0.7 anomaly in bilayer graphene/WSe2_2 quantum point contacts', Physical Review B 114, L111401 (2026), DOI 10.1103/zss4-lmlm, arXiv:2511.06384, https://doi.org/10.1103/zss4-lmlm
  2. Josep Ingla-Aynés, Serhii Volosheniuk, Talieh S. Ghiasi, Angelika Knothe, Kenji Watanabe, Takashi Taniguchi, Vladimir I. Fal'ko, Herre S. J. van der Zant, 'Tunable Kondo effect in a bilayer graphene quantum channel', arXiv:2511.17188 (2025), https://arxiv.org/abs/2511.17188
  3. Vanessa Gall, Rainer Kraft, Igor V. Gornyi, Romain Danneau, 'Spin and valley degrees of freedom in a bilayer graphene quantum point contact: Zeeman splitting and interaction effects', Physical Review Research 4, 023142 (2022), DOI 10.1103/PhysRevResearch.4.023142, arXiv:2108.10098, https://doi.org/10.1103/PhysRevResearch.4.023142
  4. K. J. Thomas, J. T. Nicholls, M. Y. Simmons, M. Pepper, D. R. Mace, D. A. Ritchie, 'Possible spin polarization in a one-dimensional electron gas', Physical Review Letters 77, 135 (1996), DOI 10.1103/PhysRevLett.77.135, https://doi.org/10.1103/PhysRevLett.77.135
  5. Yigal Meir, Kenji Hirose, Ned S. Wingreen, 'Kondo model for the "0.7 anomaly" in transport through a quantum point contact', Physical Review Letters 89, 196802 (2002), DOI 10.1103/PhysRevLett.89.196802, https://doi.org/10.1103/PhysRevLett.89.196802
  6. S. M. Cronenwett, H. J. Lynch, D. Goldhaber-Gordon, L. P. Kouwenhoven, C. M. Marcus, K. Hirose, N. S. Wingreen, V. Umansky, 'Low-temperature fate of the 0.7 structure in a point contact: a Kondo-like correlated state in an open system', Physical Review Letters 88, 226805 (2002), DOI 10.1103/PhysRevLett.88.226805, https://doi.org/10.1103/PhysRevLett.88.226805
  7. Florian Bauer, Jan Heyder, Enrico Schubert, David Borowsky, Daniela Taubert, Benedikt Bruognolo, Dieter Schuh, Werner Wegscheider, Jan von Delft, Stefan Ludwig, 'Microscopic origin of the "0.7 anomaly" in quantum point contacts', Nature 501, 73 (2013), DOI 10.1038/nature12421, https://doi.org/10.1038/nature12421
  8. A. P. Micolich, 'What lurks below the last plateau: experimental studies of the 0.7 × 2e^2/h conductance anomaly in one-dimensional systems', Journal of Physics: Condensed Matter 23, 443201 (2011), DOI 10.1088/0953-8984/23/44/443201, https://doi.org/10.1088/0953-8984/23/44/443201
  9. Pengcheng Ma, Kaveh Delfanazari, Reuben K. Puddy, Jiahui Li, Moda Cao, Teng Yi, Jonathan P. Griffiths, Harvey E. Beere, David A. Ritchie, Michael J. Kelly, Charles G. Smith, 'Statistical evaluation of 571 GaAs quantum point contact transistors showing the 0.7 anomaly in quantized conductance using cryogenic on-chip multiplexing', Chip 3, 100095 (2024), DOI 10.1016/j.chip.2024.100095, arXiv:2404.06784, https://doi.org/10.1016/j.chip.2024.100095
  10. Shun-Tsung Lo, Che-Cheng Wang, Sheng-Chin Ho, Jun-Hao Chang, Ming-Wei Chen, G. L. Creeth, L. W. Smith, Shih-Hsiang Chao, Yu-Chiang Hsieh, Pei-Tzu Wu, Yi-Cheng Wu, Chi-Te Liang, M. Pepper, J. P. Griffiths, I. Farrer, G. A. C. Jones, D. A. Ritchie, Tse-Ming Chen, 'On-chip quantum sensing of Kondo spins in a high-mobility quasi-one-dimensional nanoconstriction', Nano Letters 25, 7740 (2025), DOI 10.1021/acs.nanolett.5c00560, https://doi.org/10.1021/acs.nanolett.5c00560
  11. Hiske Overweg, Angelika Knothe, Thomas Fabian, Lukas Linhart, Peter Rickhaus, Lucien Wernli, Kenji Watanabe, Takashi Taniguchi, David Sánchez, Joachim Burgdörfer, Florian Libisch, Vladimir I. Fal'ko, Klaus Ensslin, Thomas Ihn, 'Topologically nontrivial valley states in bilayer graphene quantum point contacts', Physical Review Letters 121, 257702 (2018), DOI 10.1103/PhysRevLett.121.257702, https://doi.org/10.1103/PhysRevLett.121.257702
  12. L. Banszerus, B. Frohn, T. Fabian, S. Somanchi, A. Epping, M. Müller, D. Neumaier, K. Watanabe, T. Taniguchi, F. Libisch, B. Beschoten, F. Hassler, C. Stampfer, 'Observation of the spin-orbit gap in bilayer graphene by one-dimensional ballistic transport', Physical Review Letters 124, 177701 (2020), DOI 10.1103/PhysRevLett.124.177701, https://doi.org/10.1103/PhysRevLett.124.177701