arXiv subjects

Eslam Ahmed

Publications and source records attributed to Eslam Ahmed.

5 recordsLinked to original sources

arXiv2026

Disorder-robust trivial Majorana-like states from smooth confinement in chiral superconducting nanowires

Near-zero-energy states in Majorana nanowires can arise from topologically trivial mechanisms such as smooth spatial inhomogeneity and disorder, making zero-energy pinning alone insufficient evidence of bulk topology. Here we identify a real-space mechanism governing their robustness to symmetry-preserving disorder. For a chiral-symmetric Bogoliubov-de Gennes Hamiltonian, we decompose a low-energy state into two normalized components of opposite chirality and show that disorder-induced splitting is bounded by their spatial overlap. We demonstrate this result in a finite Rashba nanowire with smooth chemical potential and pairing profiles. Below the bulk topological transition, smooth confinement produces partially separated chiral components with exponentially small overlap, yielding globally trivial Majorana-like Andreev bound states that remain near zero energy even under strong scalar, nonmagnetic disorder. The chiral overlap therefore provides a direct diagnostic of the protection of low-energy states against local perturbations, independent of the bulk topological invariant.

cond-mat.mes-hall↗

arXiv2026

Universal Transport Theory for Paired Fractional Quantum Hall States in the Quantum Point Contact Geometry

Even-denominator fractional quantum Hall (FQH) states can be viewed as topological superconductors of composite fermions, supporting a charged chiral mode and $|\mathcal{C}_{cf}|$ neutral Majorana modes set by the Chern number $\mathcal{C}_{cf}$. Despite ongoing efforts, distinguishing the many competing paired phases remains an open problem. In this work, we propose a unified theory of charge transport across a quantum point contact (QPC) for general paired FQH states described by an $so(N)_1 \times u(1)$ conformal field theory. We derive the boundary effective action for an arbitrary number of Majorana fermions $N=|\mathcal{C}_{cf}|$ and develop a non-perturbative instanton approximation to describe tunneling processes. We establish a weak-strong duality relating strong quasiparticle tunneling to weak electron tunneling. We calculate the scaling dimensions of the tunneling operators and demonstrate that while the weak-coupling fixed point is generally unstable, the strong-coupling fixed point is stable for physically relevant filling fractions and number of Majorana fermions. These transport exponents provide a distinct experimental fingerprint to identify the topological phases of even-denominator FQH states.

cond-mat.mes-hall↗

arXiv2025

Anomalous proximity effect under Andreev and Majorana bound states

We theoretically study the anomalous proximity effect in a ballistic normal metal/diffusive normal metal/superconductor junction based on Rashba semiconductor nanowire model. The system hosts two distinct phases: a trivial helical phase with zero-energy Andreev Bound States and a topological phase with Majorana Bound States. We analyze the local density of states and induced pair correlations at the edge of the normal metal region. We investigate their behavior under scalar onsite disorder and changing the Superconductor and diffusive regions lengths in the trivial helical and topological phases. We find that both phases exhibit a zero-energy peak in the local density of states and spin-triplet pair correlations in the clean limit, which we attribute primarily to odd-frequency spin-triplet pairs. Disorder rapidly splits the zero-energy peak in the trivial helical phase regardless of the lengths of the superconductor and diffusive normal regions. The zero-energy peak in the topological phase show similar fragility when the superconductor region is short. However, for long superconductor regions, the zero-energy peak in the topological phase remain robust against disorder. In contrast, spin-singlet correlations are suppressed near zero energy in both phases. Our results highlight that the robustness of the zero-energy peak against scalar disorder, contingent on the Superconductor region length, serves as a key indicator distinguishing trivial Andreev bound states from topological Majorana bound states.

cond-mat.supr-con↗

arXiv2025

Odd-frequency pairing due to Majorana and trivial Andreev bound states

Majorana and trivial Andreev bound states are predicted to appear in superconductor-semiconductor hybrid systems, but their identification is still a challenging task. Here we consider superconducting junctions with Rashba spin-orbit coupling and explore the signatures of Majorana and trivial Andreev bound states in the emergent superconducting correlations when the systems are subjected to an external Zeeman field. We first show that robust zero-energy Andreev bound states naturally appear due to confinement and helicity when the normal sector of the junction becomes helical. These Andreev states can evolve into Majorana states, developing alike oscillations around zero energy as a function of Zeeman field. Unlike Majorana states located at both ends, helical Andreev states are located at the interface. We then demonstrate that the emergent superconducting correlations are locally composed of even-frequency spin-singlet even-parity and odd-frequency spin-triplet even-parity pair amplitudes, which coexist due to the interplay of spin-orbit coupling, Zeeman field, and spatial translation invariance breaking. In the helical regime, trivial Andreev states enhance odd-frequency spin-triplet pairing, which decays in the superconductor and has a homogeneous long-range profile in the normal region. At zero frequency, however, odd-frequency spin-triplet pairing vanishes in the helical regime. In the topological phase, Majorana states enhance odd-frequency spin-triplet pairing, producing a long-range homogeneous leakage into the normal region. Interestingly, we discover that when Majorana states are truly zero-energy modes, odd-frequency pairing develops a divergent low-frequency profile, which we interpret as the unambiguous self-conjugated Majorana signature. Our results help understand Majorana and trivial Andreev states from a superconducting correlations perspective in Majorana devices.

cond-mat.supr-con↗

arXiv2025

Odd-frequency superconducting pairing due to multiple Majorana edge modes in driven topological superconductors

Majorana zero modes have been shown to be the simplest quasiparticles exhibiting pure odd-frequency pairing, an effect that has so far been theoretically established in the static regime. In this work, we investigate the formation of Majorana modes and odd-frequency pairing in $p$-wave spin-polarized superconductors under a time-dependent drive. We first show that the driven system hosts multiple Majorana modes emerging at zero and $π$, whose formation can be controlled by an appropriate tuning of the drive frequency and chemical potential, in agreement with previous studies. Then we explore the induced pair correlations and find that odd-frequency spin-polarized $s$-wave pairing is broadly induced, acquiring large values in the presence of Majorana modes. We discover that, while odd-frequency pairing is proportional to $\sim1/ω$ in the presence of Majorana zero modes, it is proportional to $\sim 1/(ω-π\hbar/T)$ in the presence of Majorana $π$ modes, where $T$ is the periodicity of the drive. Furthermore, we find that the amount of odd-frequency pairing becomes larger when multiple Majorana modes appear but the overall divergent profile as a function of frequency remains. We also show that the divergent odd-frequency pairing is robust against scalar disorder. Notably, we establish a spectral bulk-boundary correspondence between the amount of boundary odd-$ω$ pairing and the bulk topological invariants in driven chiral systems, which we show to be protected by chiral symmetry and is thus robust against disorder. Our work thus paves the way for understanding the emergent pair correlations in driven topological superconductors

cond-mat.supr-con↗