arXiv subjects

Jorge Cayao

Publications and source records attributed to Jorge Cayao.

At least 55 records · Page 3Linked to original sources

arXiv2024

Floquet engineering Higgs dynamics in time-periodic superconductors

Higgs modes emerge in superconductors as collective excitations of the order parameter amplitude when periodically driven by electromagnetic radiation. In this work, we develop a Floquet approach to study Higgs modes in superconductors under time-periodic driving, where the dynamics of the order parameter is captured by anomalous Floquet Green's functions. We show that the Floquet description is particularly powerful as it allows one to exploit the time-periodic nature of the driving, thus considerably reducing the complexity of the time-dependent problem. Interestingly, the Floquet approach is also enlightening because it naturally offers a physical explanation for the renormalized steady-state order parameter as a result of photon processes between Floquet sidebands. We demonstrate the usefulness of Floquet engineering Higgs modes in time-periodic $s$-wave superconductors.

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arXiv2024

Controllable odd-frequency Cooper pairs in multi-superconductor Josephson junctions

We consider Josephson junctions formed by multiple superconductors with distinct phases and explore the formation of nonlocal or inter-superconductor pair correlations. We find that the multiple superconductor nature offers an additional degree of freedom that broadens the classification of pair symmetries, enabling nonlocal even- and odd-frequency pairings that can be highly controlled by the superconducting phases and the energy of the superconductors. Specially, when the phase difference between two superconductors is $π$, their associated nonlocal odd-frequency pairing is the only type of inter-superconductor pair correlations. Finally, we show that these nonlocal odd-frequency Cooper pairs dominate the nonlocal conductance via crossed Andreev reflections, which constitutes a direct evidence of odd-frequency pairing.

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arXiv2024

Theory of Majorana zero modes in unconventional superconductors

Majorana fermions are spin-1/2 neutral particles that are their own antiparticles and were initially predicted by Ettore Majorana in particle physics but their observation still remains elusive. The concept of Majorana fermions has been borrowed into condensed matter physics, where, unlike particle physics, Majorana fermions emerge as zero-energy quasiparticles that can be engineered by combining electrons and holes and have therefore been coined Majorana zero modes. In this review, we provide a pedagogical explanation of the basic properties of Majorana zero modes in unconventional superconductors and their consequences in experimental observables, putting a special emphasis on the initial theoretical discoveries. In particular, we first show that Majorana zero modes are self-conjugated and emerge as a special type of zero energy surface Andreev bound states at the boundary of unconventional superconductors. We then explore Majorana zero modes in one-dimensional spin-polarized $p$-wave superconductors, where we address the formation of topological superconductivity and the physical realization in superconductor-semiconductor hybrids. In this part we highlight that Majorana quasiparticles appear as zero-energy edge states, exhibiting charge neutrality, spin-polarized, and spatial nonlocality as unique properties that can be already seen from their energies and wavefunctions. Next, we discuss analytically obtained Green's functions of $p$-wave superconductors and demonstrate that the emergence of Majorana zero modes is always accompanied by the formation of odd-frequency spin-triplet pairing as a unique result of the self-conjugate nature of Majorana zero modes. We finally address the signatures of Majorana zero modes in tunneling spectroscopy, including the anomalous proximity effect, and the phase-biased Josephson effect.

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arXiv2024

Nonlocality of Majorana bound states revealed by electron waiting times in a topological Andreev interferometer

The analysis of waiting times of electron transfers has recently become experimentally accessible owing to advances in noninvasive probes working in the short-time regime. We study electron waiting times in a topological Andreev interferometer: a superconducting loop with controllable phase difference connected to a quantum spin Hall edge, where the edge state helicity enables the transfer of electrons and holes into separate leads, with transmission controlled by the loop's phase difference $ϕ$. This setup features gapless Majorana bound states at $ϕ=π$. The waiting times for electron transfers across the junction are sensitive to the presence of the gapless states, but are uncorrelated for all $ϕ$. By contrast, at $ϕ=π$ the waiting times of Andreev-scattered holes show a strong correlation and the crossed (hole-electron) distributions feature a unique behavior. Both effects exclusively result from the nonlocal properties of Majorana bound states. Consequently, electron waiting times and their correlations could circumvent some of the challenges for detecting topological superconductivity and Majorana states beyond conductance signatures.

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arXiv2024

Enhancing the Josephson diode effect with Majorana bound states

We consider phase-biased Josephson junctions with spin-orbit coupling under external magnetic fields and study the emergence of the Josephson diode effect in the presence of Majorana bound states. We show that junctions having middle regions with Zeeman fields along the spin-orbit axis develop a low-energy Andreev spectrum that is asymmetric with respect to the superconducting phase difference $ϕ=π$, which is strongly influenced by Majorana bound states in the topological phase. This asymmetric Andreev spectrum gives rise to anomalous current-phase curves and critical currents that are different for positive and negative supercurrents, thus signaling the emergence of the Josephson diode effect. While this effect exists even in the trivial phase, it gets enhanced in the topological phase due to the spatial nonlocality of Majorana bound states. Our work thus establishes the utilization of topological superconductivity for enhancing the functionalities of Josephson diodes.

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arXiv2023

Topological superconductivity enhanced by exceptional points

Majorana zero modes (MZMs) emerge as edge states in topological superconductors and are promising for topological quantum computation, but their detection has so far been elusive. Here we show that non-Hermiticity can be used to obtain dramatically more robust MZMs. The enhanced properties appear as a result of an extreme instability of exceptional points to superconductivity, such that even a vanishingly small superconducting order parameter already opens a large energy gap, produces well-localized MZMs, and leads to strong superconducting pair correlations. Our work thus illustrates the large potential of enhancing topological superconductivity using non-Hermitian exceptional points.

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arXiv2023

Mitigating disorder-induced zero-energy states in weakly-coupled semiconductor-superconductor hybrid systems

Disorder has appeared as one of the main mechanisms to induce topologically trivial zero-energy states in superconductor-semiconductor systems, thereby challenging the detection of topological superconductivity and Majorana bound states. Here we demonstrate that, for disorder in any part of the system, the formation of disorder-induced trivial zero-energy states can to a large extent be mitigated by keeping the coupling between the semiconductor and superconductor weak. The only exception is strong disorder in the semiconductor, where instead the strong coupling regime is somewhat more robust against disorder. Furthermore, we find that the topological phase in this weak coupling regime is robust against disorder, with a large and well-defined topological gap which is highly beneficial for topological protection. Our work shows the advantages and disadvantages of weak and strong couplings under disorder, important for designing superconductor-semiconductor hybrid structures.

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arXiv2023

Exceptional degeneracies in non-Hermitian Rashba semiconductors

Exceptional points are spectral degeneracies of non-Hermitian systems where eigenvalues and eigenvectors coalesce, inducing unique topological phases that have no counterpart in the Hermitian realm. Here we consider a non-Hermitian system by coupling a two-dimensional semiconductor with Rashba spin-orbit coupling to a ferromagnet lead and show the emergence of highly tunable exceptional points along rings in momentum space. Interestingly, these exceptional degeneracies are the endpoints of lines formed by the eigenvalue coalescence at finite real energy, resembling the bulk Fermi arcs commonly defined at zero real energy. We then show that an in-plane Zeeman field provides a way to control these exceptional degeneracies although higher values of non-Hermiticity are required in contrast to the zero Zeeman field regime. Furthermore, we find that the spin projections also coalescence at the exceptional degeneracies and can acquire larger values than in the Hermitian regime. Finally, we demonstrate that the exceptional degeneracies induce large spectral weights, which can be used as a signature for their detection. Our results thus reveal the potential of systems with Rashba spin-orbit coupling for realizing non-Hermitian bulk phenomena.

cond-mat.mes-hall↗

arXiv2023

Bulk Bogoliubov Fermi arcs in non-Hermitian superconducting systems

We consider a non-Hermitian superconducting system by coupling a conventional superconductor to a ferromagnet lead and demonstrate the emergence of exceptional points when an external Zeeman field is applied. We discover that, depending on the non-Hermiticity and the Zeeman field, the exceptional points mark the ends of lines with zero real energy, thus giving rise to topologically protected and highly tunable bulk Fermi arcs, which we coin bulk Bogoliubov Fermi arcs due to their superconducting nature. We show that these bulk Bogoliubov Fermi arcs are the non-Hermitian counterparts of the Hermitian topological phase transition but are much more prevalent and also experimentally detectable through large spectral signatures.

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arXiv2023

Zero-frequency supercurrent susceptibility signatures of trivial and topological zero-energy states in nanowire junctions

We propose a method to distinguish between trivial and topological, Majorana, zero-energy states in both short and long superconductor-normal-superconductor junctions based on Rashba nanowires using phase-biased equilibrium transport measurements. In particular, we show how the sawtooth profile of the supercurrent, due to the Majorana oscillation suppression in the topological phase for sufficiently long superconductor regions, leads to a strong signal in its zero-frequency susceptibility for a phase difference of $ϕ=π$. This signal is notably insensitive to the chemical potential in the normal region, while trivial zero-energy states only causes signals in the susceptibility that is highly varying with the chemical potential, thus turning gating of the normal region into a simple experimental control knob. Furthermore, we obtain that, by tuning the junction transparency, critical currents in both short and long junctions undergo a reduction in the number of oscillations as a function of magnetic field only in the topological phase, an effect that find to be intimately linked to Majorana non-locality. Finally, we show that our results also hold at finite temperatures, thus highlighting their potential measurability under realistic experimental conditions.

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arXiv2022

Phase-tunable electron transport assisted by odd-frequency Cooper pairs in topological Josephson junctions

We consider a finite-size topological Josephson junction formed at the edge of a two-dimensional topological insulator in proximity to conventional superconductors and study the impact of Cooper pair symmetries on the electron transport. We find that, due to the finite junction size, electron transport is highly tunable by the superconducting phase difference $ϕ$ across the junction. At zero frequency and $ϕ=π$, the setup exhibits vanishing local Andreev reflection and perfect normal transmission due to the interplay of finite junction size and formation of topological Andreev bound states in the middle of the junction. We reveal that this striking behavior enables odd-frequency Cooper pairs to become the only type of pairing inside the topological junction that contribute to transport. Our paper thus offers a highly tunable detection scheme for odd-frequency Cooper pairs.

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arXiv2022

Robust topological superconductivity in weakly coupled nanowire-superconductor hybrid structures

We investigate the role of the coupling between a spin-orbit coupled semiconductor nanowire and a conventional $s$-wave superconductor on the emergence of the topological superconducting phase with Majorana bound states in an applied magnetic field. We show that when the coupling is strong, the topological phase transition point is very sensitive to the size of the superconductor and in order to reach the topological phase a strong magnetic field is required, which can easily be detrimental for superconductivity. Moreover, the induced energy gap separating the Majorana bound states and other quasi-particles in the topological phase is substantially suppressed compared to the gap at zero field. In contrast, in the weak coupling regime, we find that the situation is essentially the opposite, with the topological phase emerging at much lower magnetic fields and a sizable induced energy gap in the topological phase, that can also be controlled by the chemical potential of the superconductor. Furthermore, we show that the weak coupling regime does not generally allow for the formation of topologically trivial zero-energy states at the wire end points, in stark contrast to the strong coupling regime where such states are found for a wide range of parameters. Our results thus put forward the weak coupling regime as a promising route to mitigate the most unwanted problems present in nanowires for realizing topological superconductivity and Majorana bound states.

cond-mat.mes-hall↗

arXiv2022

Exceptional odd-frequency pairing in non-Hermitian superconducting systems

We first show the realization of exceptional points in a non-Hermitian superconducting system based on a conventional superconductor and then demonstrate that, surprisingly, the system hosts odd-frequency pairing, solely generated by the non-Hermiticity. While there is a coexistence of even- and odd-frequency pairs under general conditions, we find that the even-frequency term vanishes at the exceptional degeneracies, leaving only odd-frequency pairing. This exceptional odd-frequency pairing is directly given by the imaginary part of the eigenvalues at the exceptional points and can be measured from the spectral function. Our results thus put forward non-Hermitian systems as a powerful platform to realize odd-frequency superconducting pairing.

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arXiv2021

Disorder-robust $p$-wave pairing with odd frequency dependence in normal metal-conventional superconductor junctions

We investigate the induced superconducting pair correlations in junctions between a conventional spin-singlet $s$-wave superconductor and a disordered normal metal. Decomposing the pair amplitude based on its symmetries in the time domain, we demonstrate that the odd-time, or equivalently odd-frequency, spin-singlet $p$-wave correlations are both significant in size and entirely robust against random non-magnetic disorder. We find that these odd-frequency correlations can even be generated by disorder. Our results show that anisotropic odd-frequency pairing represent an important fraction of the proximity-induced correlations in disordered superconducting hybrid structures.

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arXiv2021

Efficient numerical method for evaluating normal and anomalous time-domain equilibrium Green's functions in inhomogeneous systems

In this work we develop EPOCH: Equilibrium Propagator by Orthogonal polynomial CHain, a computationally efficient method to calculate the time-dependent equilibrium Green's functions, including the anomalous Green's functions of superconductors, to capture the time-evolution in large inhomogeneous systems. The EPOCH method generalizes the Chebyshev wave-packet propagation method from quantum chemistry and efficiently incorporates the Fermi-Dirac statistics that is needed for equilibrium quantum condensed matter systems. The computational cost of EPOCH scales only linearly in the system degrees of freedom, generating an extremely efficient algorithm also for very large systems. We demonstrate the power of the EPOCH method by calculating the time-evolution of an excitation near a superconductor-normal metal interface in two and three dimensions, capturing transmission as well as normal and Andreev reflections.

cond-mat.mes-hall↗

arXiv2021

Distinguishing trivial and topological zero energy states in long nanowire junctions

The emergence of zero energy states in non-topological superconductors represents an inevitable problem that obscures the proper identification of zero energy Majorana bound states (MBSs) and prevents their use as topologically protected qubits. In this Research Letter we investigate long superconductor-normal-superconductor junctions where trivial zero energy states, robust over a large range of parameters, appear as a result of helicity and confinement in the normal region. We demonstrate that both equilibrium supercurrents and critical currents are sensitive to variations in the length of the superconductor regions in the topological phase hosting MBSs, but, remarkably, no such length dependence exists when robust, but trivial, zero energy states are present. This strikingly different response originates from the non-local nature of the MBSs and we, therefore, propose it as a simple protocol for distinguishing between trivial and topological zero energy states.

cond-mat.mes-hall↗

arXiv2021

Confinement-induced zero-bias peaks in conventional superconductor hybrids

Majorana bound states in topological superconductors have been predicted to appear in the form of zero-bias conductance peaks of height $2e^{2}/h$, which represents one of the most studied signatures so far. Here, we show that quasi zero-energy states, similar to Majorana bound states, can naturally form in any superconducting hybrid junction due to confinement effects, without relation to topology. Remarkably, these topologically trivial quasi zero-energy states produce zero-bias conductance peaks, thus mimicking the most representative Majorana signature. Our results put forward confinement as an alternative mechanism to explain the ubiquitous presence of trivial zero-bias peaks and quasi zero-energy states in superconductor hybrids.

cond-mat.mes-hall↗

arXiv2021

Floquet engineering bulk odd-frequency superconducting pairs

We introduce the concept of Floquet odd-frequency superconducting pairs and establish their emergence in time-periodic conventional superconductors, where the continuous time-translation invariance is broken. We show that these exotic Cooper pairs are possible because the Floquet modes in time-periodic systems provide an additional index (a Floquet index) that broadens the classification of pair symmetries, with no analog in the static regime. Our results thus put forward a different route for odd-frequency superconducting pairs and pave the way for Floquet engineered dynamical superconducting states.

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