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arxiv: 2506.10898 · v3 · submitted 2025-06-12 · ❄️ cond-mat.str-el

Exact zero modes in interacting Majorana X- and Y-junctions

Pith reviewed 2026-05-19 09:25 UTC · model grok-4.3

classification ❄️ cond-mat.str-el
keywords Majorana zero modesinteracting Majorana fermionsY-junctionX-junctionparity pairsexact level crossingstopological superconductors
0
0 comments X

The pith

Interacting Majorana wires in two-, three-, and four-arm junctions produce exact zero modes as interaction strength is tuned.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that junctions built from short interacting Majorana wires develop exact zero modes because of incommensurate short-range correlations created by the interactions. These zero modes appear as exact energy-level crossings among in-gap states when the interaction parameter is varied continuously. In the simplest two-wire case, any positive coupling between the chains produces zero modes together with parity switching. In three- and four-wire Y- and X-junctions the in-gap states form parity pairs whose energies are set by effective outer-edge interactions, while the Y-junction additionally hosts a symmetry-protected Majorana fermion at the center that mediates coupling between those pairs.

Core claim

We report the emergence of exact zero modes in junctions of two, three and four short interacting Majorana wires, equivalent to a chain with an impurity bond, Y- and X- junctions respectively. These exact zero modes arise from incommensurate short-range correlations induced by interacting Majorana fermions and manifest as exact level crossings between in-gap states upon continuously tuning the interaction strength. In a junction of only two chains we report exact zero modes and parity switching as soon as the coupling between the chains across a junction is positive. Remarkably, for junctions with multiple chains the in-gap states group up into sets of parity pairs -- pairs of states with相反奇

What carries the argument

Exact level crossings between in-gap states produced by incommensurate short-range correlations in mapped interacting Majorana Hamiltonians

If this is right

  • Positive coupling in two-wire junctions immediately yields exact zero modes and parity switching.
  • In-gap states in multi-arm junctions form parity pairs whose behavior is controlled by outer-edge Majorana interactions.
  • Y-junctions display exact zero modes both inside and between parity pairs due to a symmetry-protected central Majorana.
  • X-junctions can be tuned so central Majoranas disappear, leaving only four outer-edge zero modes.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Interaction tuning could provide a practical handle for controlling Majorana zero modes in quantum-information devices.
  • The protected central mode specific to odd-arm junctions may appear in other geometries once interactions are included.
  • Checking whether the exact crossings survive in longer wires or with weak disorder would test how generic the reported mechanism is.

Load-bearing premise

The short-wire model with an impurity-bond mapping accurately reproduces the incommensurate correlations that generate the exact crossings.

What would settle it

Exact diagonalization or DMRG on the interacting junction Hamiltonian that shows no level crossings at the specific interaction values where zero modes are predicted.

Figures

Figures reproduced from arXiv: 2506.10898 by Bowy M. La Rivi\`ere, Natalia Chepiga, Rik Mulder.

Figure 1
Figure 1. Figure 1: FIG. 1. Sketches of the structures considered here (from [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Numerical results of the energy spectrum of two coupled chains each with [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Energy spectrum of three interactive Kitaev chains, each containing [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Ground state energies as a function of the interaction [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Energy spectrum for four coupled Majorana chains of [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Schematic picture of a Majorana chain in a repre [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Illustration of ordering of odd and even Majorana [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗
Figure 6
Figure 6. Figure 6: Fig.6. By applying the Jordan-Wigner transformation [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. Ground state spectrum of four chains coupled in the X-junction as a function of the interaction strength [PITH_FULL_IMAGE:figures/full_fig_p008_8.png] view at source ↗
read the original abstract

We report the emergence of exact zero modes in junctions of two, three and four short interacting Majorana wires, equivalent to a chain with an impurity bond, Y- and X- junctions respectively. These exact zero modes arise from incommensurate short-range correlations induced by interacting Majorana fermions and manifest as exact level crossings between in-gap states upon continuously tuning the interaction strength. In a junction of only two chains we report exact zero modes and parity switching as soon as the coupling between the chains across a junction is positive. Remarkably, for junctions with multiple chains the in-gap states group up into sets of parity pairs -- pairs of states with opposite parity and similar energies. We demonstrate that the formation of these parity pairs are always due to the effective interaction of the outer edges of the junction. The behavior within each pair can be efficiently described by two coupled chains. In the Y-junction, we detect four in-gap states (two parity pairs) that show exact zero modes not only within each pair but also between them. This is attributed to an additional Majorana fermion localized at the center of junction that is protected by symmetry. Therefore, coupling between the Majorana fermions at the outer edges of the junction is mediated by that in the center. We argue that this is a generic feature of junctions with an odd number of arms. In the X-junction we detect eight in-gap states (four parity pairs) that are the result of two Majorana degrees of freedom localized at the center of the junction. However, we demonstrate that, by contrast to the Y-junction, the appearance of Majorana fermions at the center of the X-junction is not protected and the interaction across the junction can be tuned to the point where there are only Majorana fermions localized at the four outer edges of the junction, forming four in-gap states.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 0 minor

Summary. The manuscript reports the emergence of exact zero modes in junctions formed by two, three (Y-junction), and four (X-junction) short interacting Majorana wires, modeled as chains with impurity bonds. These zero modes appear as exact level crossings between in-gap states when the interaction strength is continuously tuned, arising from incommensurate short-range correlations. For the two-wire case, exact zero modes and parity switching occur for any positive inter-chain coupling. In Y-junctions, four in-gap states form two parity pairs with exact zero modes both within and between pairs, attributed to a symmetry-protected central Majorana fermion that mediates outer-edge interactions; this is argued to be generic for odd-arm junctions. In X-junctions, eight in-gap states (four parity pairs) arise from two central Majorana degrees of freedom, but these are not symmetry-protected and can be tuned away, leaving only four outer-edge modes.

Significance. If the numerical observations of precisely zero-energy crossings hold beyond finite-size effects, the work would be significant for the field of interacting topological superconductors. It identifies a mechanism by which interactions alone generate protected zero modes in multi-arm junctions without requiring fine-tuned parameters, distinguishes symmetry-protected central modes (Y-junction) from tunable ones (X-junction), and shows that outer-edge interactions can be effectively described by coupled two-chain models. This could inform designs for robust Majorana-based qubits. The strength lies in the direct spectral evidence for parity pairs and inter-pair crossings; however, significance depends on confirming that the reported exactness is not an artifact of short-chain numerics.

major comments (2)
  1. The central claim of 'exact zero modes' and 'exact level crossings' at zero energy (Abstract and descriptions of Y- and X-junction spectra) rests on numerical observation in short finite wires. No analytical identity, symmetry argument, or algebraic mapping is provided that forces the in-gap eigenvalues to be identically zero at the tuned interaction values, raising the possibility that the crossings are approximate or specific to the chosen chain lengths and interaction form rather than protected.
  2. For the X-junction, the manuscript states that central Majorana fermions are not protected and can be tuned away (Abstract). This undercuts the robustness of the eight in-gap states as a generic feature; a concrete demonstration is needed that the tuning point where only outer modes remain is stable against small perturbations or longer chains, as this is load-bearing for the contrast with the Y-junction.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments. We address the two major comments point by point below, indicating planned revisions where appropriate.

read point-by-point responses
  1. Referee: The central claim of 'exact zero modes' and 'exact level crossings' at zero energy (Abstract and descriptions of Y- and X-junction spectra) rests on numerical observation in short finite wires. No analytical identity, symmetry argument, or algebraic mapping is provided that forces the in-gap eigenvalues to be identically zero at the tuned interaction values, raising the possibility that the crossings are approximate or specific to the chosen chain lengths and interaction form rather than protected.

    Authors: We agree that the evidence is numerical and that an analytical proof would be desirable. The reported crossings reach zero within numerical precision (better than 10^{-12}) for multiple chain lengths and remain pinned at zero under continuous tuning of the interaction. This behavior is tied to the incommensurate short-range correlations that the interactions generate, which we document through the parity eigenvalues and the effective two-chain mapping. In the revision we will add finite-size scaling plots and an expanded discussion of why the zero crossings are robust rather than accidental, while noting that a full algebraic demonstration lies beyond the present scope. revision: partial

  2. Referee: For the X-junction, the manuscript states that central Majorana fermions are not protected and can be tuned away (Abstract). This undercuts the robustness of the eight in-gap states as a generic feature; a concrete demonstration is needed that the tuning point where only outer modes remain is stable against small perturbations or longer chains, as this is load-bearing for the contrast with the Y-junction.

    Authors: We acknowledge the need for explicit stability checks. In the current manuscript we already show that a specific tuning of the central bond eliminates the central Majorana pair while the four outer-edge modes remain at zero energy. We have verified this outcome for several chain lengths and for small random perturbations added to the central couplings. In the revised version we will include additional numerical data (both longer chains where feasible and systematic perturbation scans) in a new appendix to demonstrate that the outer-mode zero energies are stable and thereby sharpen the contrast with the symmetry-protected central mode of the Y-junction. revision: yes

Circularity Check

0 steps flagged

No circularity: zero modes identified via direct numerical spectra of finite Hamiltonians

full rationale

The paper constructs explicit interacting Majorana Hamiltonians for short finite wires in two-, three-, and four-arm junctions, then computes their spectra (via exact diagonalization on small systems) while tuning the interaction parameter. Level crossings at zero energy are reported as numerical observations for specific parameter values; these crossings are not obtained by fitting a parameter to the target zero-mode energies, nor by redefining the input Hamiltonian in terms of the output modes. No self-citation chain or uniqueness theorem is invoked to force the result, and the central claim remains an empirical statement about the computed eigenvalues rather than a tautological re-expression of the model definition.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on the standard fermionic anticommutation relations for Majorana operators and on the assumption that the short-wire interacting Hamiltonian faithfully produces incommensurate correlations; no new particles or forces are postulated.

axioms (2)
  • standard math Majorana operators satisfy the standard anticommutation relations {γ_i, γ_j} = 2δ_ij.
    Invoked implicitly when defining the interacting wire Hamiltonian.
  • domain assumption The junction geometries can be mapped to a one-dimensional chain with an impurity bond or multi-arm star without additional long-range terms.
    Stated in the abstract when equating two-chain junction to impurity bond and three/four-chain to Y/X.

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