arXiv subjects

Tudor D. Stanescu

Publications and source records attributed to Tudor D. Stanescu.

At least 19 recordsLinked to original sources

arXiv2026

A First-Principles Multiscale Framework for Topological Superconductivity

A microscopic understanding of topological superconductivity (TSC) in real materials requires a materials-informed approach that integrates first-principles electronic structure, superconductivity, and topology within a unified computational framework. Here, we develop such an approach by combining density functional theory, Wannier-based low-energy Hamiltonians, Bogoliubov-de Gennes theory, and Matsubara Green's-function-based Chern number calculations performed directly on realistic multiorbital superconducting Hamiltonians. We apply this framework to bulk-like and monolayer FeTeSe and to FeSe/GaAs heterostructures, enabling a unified investigation of both intrinsic and proximity-induced topological superconductivity. We identify key electronic-structure ingredients that promote robust TSC, including Rashba-active states near the Fermi level, strong induced superconducting pairing, and substantial orbital hybridization between spin-orbit-active and superconducting sectors. Guided by these design principles, we predict multiple topological superconducting phases in Fe-based materials and demonstrate that FeSe/GaAs heterostructures are particularly promising, exhibiting topological transitions at experimentally accessible chemical potentials and low Zeeman fields. Complementing the theoretical predictions, we demonstrate the growth and structural characterization of FeSe/GaAs heterostructures, establishing the experimental feasibility of the proposed materials platform. Our results provide a quantitative route for engineering and screening candidate Majorana materials and heterostructures directly from realistic electronic structures and establish a foundation for future materials-by-design approaches to topological superconductivity.

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arXiv2026

Rashba spin-orbit coupling and artificially engineered topological superconductors

One of the most important physical effects in condensed matter physics is the Rashba spin-orbit coupling (RSOC), introduced in seminal works by Emmanuel Rashba. In this article, we discuss, describe, and review (providing critical perspectives on) the crucial role of RSOC in the currently active research area of topological quantum computation. Most, if not all, of the current experimental topological quantum computing platforms use the idea of Majorana zero modes as the qubit ingredient because of their non-Abelian anyonic property of having an intrinsic quantum degeneracy, which enables nonlocal encoding protected by a topological energy gap. It turns out that RSOC is a crucial ingredient in producing a low-dimensional topological superconductor in the laboratory, and such topological superconductors naturally have isolated localized midgap Majorana zero modes. In addition, increasing the RSOC strength enhances the topological gap, thus enhancing the topological immunity of the qubits to decoherence. Thus, Rashba's classic work on SOC may lead not only to the realization of localized non-Abelian anyons, but also fault tolerant quantum computation.

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arXiv2026

Planar Josephson junction devices with narrow superconducting strips: Topological properties and optimization

We study the low-energy physics of planar Josephson junction structures realized in a quasi-two dimensional semiconductor system proximity-coupled to narrow superconducting films. Using both a recursive Green's function approach and an effective Hamiltonian approximation, we investigate the topological superconducting phase predicted to emerge in this type of system. We first characterize the effects associated with varying the electrostatic potentials applied within the unproximitized semiconductor regions. We then address the problem of optimizing the width of the superconductor films and identifying the optimal regimes characterized by large topological gap values. We find that structures with narrow superconducting films of widths ranging between about $100~$nm and $200~$nm can support topological superconducting phases with gaps up to $40\%$ of the parent superconducting gap, significantly larger than those characterizing the corresponding wide-superconductor structures. This work represents the first component of a proposed comprehensive strategy to address this optimization problem in planar Josephson junction structures and realize robust topological devices.

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arXiv2025

Topological invariant for finite systems in the presence of disorder

Topological invariants, rigorously defined only in the thermodynamic limit, have been generalized to topological indicators applicable to finite-size disordered systems. However, in many experimentally relevant situations, such as semiconductor-superconductor (SM-SC) hybrid nanowires hosting Majorana zero modes, the interplay between strong disorder and finite-size effects renders these indicators (e.g., the so-called topological visibility) biased and ill-defined, significantly limiting their usefulness. In this paper, we propose the topological invariant rigorously defined for an infinite system constructed by periodically repeating the original finite disordered system, as a topological indicator. Using the one-dimensional SM-SC hybrid nanowire as an example, we show that this general and transparent approach yields faithful topological indicators free from the biases affecting commonly used finite-size indicators, capturing the nature (topological or trivial) of the phase at generic points in parameter space, and providing a reliable tool for interpreting experimental results.

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arXiv2025

Fermion parity and quantum capacitance oscillation with partially separated Majorana and quasi-Majorana modes

In a recent experiment, flux dependent oscillations of the quantum capacitance were observed in a one dimensional spin-orbit coupled semiconductor superconductor heterostructure connected end to end via a quantum dot and threaded by a magnetic flux. In the topological superconducting phase of the heterostructure, the oscillations corresponding to different fermion parity sectors are shifted by half a period and can serve as a mechanism for fermion parity readout or fusion operations involving a pair of localized, well separated Majorana modes. In this work, we demonstrate that flux induced fermion parity dependent oscillations of the quantum capacitance in a disordered semiconductor superconductor quantum dot system can originate not only from topologically protected, spatially well separated Majorana zero modes (MZMs) localized at the wire ends, but also, generically, from partially separated Majorana modes with significant overlap, as well as from quasi-Majorana modes in the topologically trivial phase, which can be viewed as Andreev bound states whose constituent Majorana wave functions are slightly shifted relative to each other and have nonzero amplitude at opposite ends of the wire. Therefore, while the detection of flux dependent oscillations of quantum capacitance marks an important experimental advance, such observations alone do not constitute evidence of the presence of topological Majorana zero modes.

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arXiv2025

Disorder effects in planar semiconductor-superconductor structures: Majorana wires versus Josephson junctions

Disorder effects in hybrid semiconductor-superconductor (SM-SC) nanowires, widely recognized as the main obstacle to realizing stable Majorana zero modes (MZMs) in these structures, have been systematically investigated theoretically in recent years. However, there are no corresponding detailed studies of disorder effects in planar Josephson junction (JJ) structures, which represent a promising alternative to the Majorana nanowire platform. In this paper, we perform a numerical analysis of the low-energy physics of JJ structures based on an effective microscopic model that includes two types of disorder, charge impurities inside the semiconductor and roughness on the surface of the superconducting film. We consider different parameter regimes, including low and high chemical potential values, weak and strong effective SM-SC coupling strengths, and weak and strong disorder strengths. The results are benchmarked using disordered hybrid nanowires realized in planar SM-SC structures similar to those involved in the fabrication of Josephson junctions and having similar model parameters and disorder strengths. We find that the topological superconducting phase hosted by a JJ structure is, generally, more robust against disorder than the topological superconductivity realized in a hybrid nanowire with similar parameters. On the other hand, we find that operating the JJ in a regime characterized by large values of chemical potential results in huge finite-size effects that can destroy the stability of MZMs.

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arXiv2024

Stabilizing topological superconductivity in disordered spin-orbit coupled semiconductor-superconductor heterostructures

We investigate theoretically a one-dimensional semiconductor-superconductor (SM-SC) heterostructure with Rashba spin-orbit coupling and parallel Zeeman field in the presence of disorder generated by random charged impurities and identify the optimal regimes for realizing topological superconductivity and Majorana zero modes. Using a Green's function approach, we show that upon increasing the disorder strength the stable topological superconducting phase characterized by robust end-to-end Majorana correlations "migrates" toward larger values of the Zeeman field and can be stabilized by increasing the effective SM-SC coupling. Based on these findings, we propose a strategy for accessing a regime characterized by well-separated Majorana zero modes that is based on (a) enhancing the strength of the effective SM-SC coupling (e.g., through interface engineering) and (b) expanding the range of accessible Zeeman fields (e.g., by enhancing the gyromagnetic ratio or optimizing the parent superconductor, to enable the application of larger magnetic fields). While this strategy may still require some reduction of the disorder strength, this requirement is significantly less strict than the corresponding requirement in a strategy that focuses exclusively on disorder reduction.

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arXiv2023

Spectral properties, topological patches, and effective phase diagrams of finite disordered Majorana nanowires

We consider theoretically the physics of bulk topological superconductivity accompanied by boundary non-Abelian Majorana zero modes in semiconductor-superconductor (SM-SC) hybrid systems consisting of finite wires in the presence of correlated disorder arising from random charged impurities. We find the system to manifest a highly complex behavior due to the subtle interplay between finite wire length and finite disorder, leading to copious low-energy in-gap states throughout the wire and considerably complicating the interpretation of tunneling spectroscopic transport measurements used extensively to search for Majorana modes. The presence of disorder-induced low-energy states may lead to the non-existence of end Majorana zero modes even when tunneling spectroscopy manifests zero bias conductance peaks in local tunneling and signatures of bulk gap closing/reopening in the nonlocal transport. In short wires within the intermediate disorder regime, apparent topology may manifest in small ranges ("patches") of parameter values, which may or may not survive the long wire limit depending on various details. Because of the dominance of disorder-induced in-gap states, the system may even occasionally have an appropriate topological invariant without manifesting isolated end Majorana zero modes. We discuss our findings in the context of a recent breakthrough experiment from Microsoft reporting the simultaneous observations of zero bias conductance peaks in local tunneling and gap opening in nonlocal transport within small patches of parameter space. Based on our analysis, we believe that the disorder strength to SC gap ratio must decrease further for the definitive realization of non-Abelian Majorana zero modes in SM-SC devices.

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arXiv2022

Proximity-induced superconductivity generated by thin films: Effects of Fermi surface mismatch and disorder in the superconductor

We investigate the effects of disorder characterising a superconducting thin film on the proximity-induced superconductivity generated by the film (in, e.g., a semiconductor) based on the exact numerical analysis of a three-dimensional microscopic model. To make the problem numerically tractable, we use a recursive Green's function method in combination with a patching approach that exploits the short-range nature of the interface Green's function in the presence of disorder. As a result of the Fermi surface mismatch between the superconductor (SC) and the semiconductor (SM) in combination with the confinement-induced quantization of the transverse SC modes, the proximity effect induced by a clean SC film is typically one to three orders of magnitude smaller that the corresponding quantity for a bulk SC and exhibits huge thickness-dependent variations. The presence of disorder has competing effects: on the one hand it enhances the proximity-induced superconductivity and suppresses its strong thickness dependence, on the other hand it generates proximity-induced effective disorder in the SM. The effect of proximity-induced disorder on the topological superconducting phase and the associated Majorana modes is studied nonperturbatively.

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arXiv2021

Estimating disorder and its adverse effects in semiconductor Majorana nanowires

We use the available transport measurements in the literature to develop a dataset for the likely amount of disorder in semiconductor (InAs and InSb) materials which are used in fabricating the superconductor-semiconductor nanowire samples in the experimental search for Majorana zero modes. Using the estimated disorder in direct Majorana simulations, we conclude that the current level of disorder in semiconductor Majorana nanowires is at least an order of magnitude higher than that necessary for the emergence of topological Majorana zero modes. In agreement with existing results, we find that our estimated disorder leads to the occasional emergence of trivial zero modes, which can be post-selected and then further fine-tuned by varying system parameters (e.g., tunnel barrier), leading to trivial zero-bias conductance peaks in tunneling spectroscopy with $ \sim 2e^2/h $ magnitude. Most calculated tunnel spectra in these disordered systems, however, manifest essentially no significant features, which is also consistent with the current experimental status, where zero-bias peaks are found only occasionally in some samples under careful fine-tuning.

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arXiv2021

Enhanced topological superconductivity in spatially modulated planar Josephson junctions

We propose a semiconductor-superconductor hybrid device for realizing topological superconductivity and Majorana zero modes consisting of a planar Josephson junction structure with periodically modulated junction width. By performing a numerical analysis of the effective model describing the low-energy physics of the hybrid structure, we demonstrate that the modulation of the junction width results in a substantial enhancement of the topological gap and, consequently, of the robustness of the topological superconducting phase and associated Majorana zero modes. This enhancement is due to the formation of minibands with strongly renormalized effective parameters, including stronger spin-orbit coupling, generated by the effective periodic potential induced by the modulated structure. In addition to a larger topological gap, the proposed device supports a topological superconducting phase that covers a significant fraction of the parameter space, including the low Zeeman field regime, in the absence of a superconducting phase difference across the junction. Furthermore, the optimal regime for operating the device can be conveniently accessed by tuning the potential in the junction region using, for example, a top gate.

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arXiv2021

Charge impurity effects in hybrid Majorana nanowires

We address an outstanding problem that represents a critical roadblock in the development of the Majorana-based topological qubit using semiconductor-superconductor hybrid structures: the quantitative characterization of disorder effects generated by the unintentional presence of charge impurities within the hybrid device. Given that disorder can have far-reaching consequences for the Majorana physics, but is intrinsically difficult to probe experimentally in a hybrid structure, providing a quantitative theoretical description of disorder effects becomes essential. To accomplish this task, we develop a microscopic theory that (i) provides a quantitative characterization of the effective potential generated by a charge impurity embedded inside a semiconductor wire proximity-coupled to a superconductor layer by solving self-consistently the associated three-dimensional Schrödinger-Poisson problem, (ii) describes the low-energy physics of the hybrid structure in the presence of s-wave superconductivity, spin-orbit coupling, Zeeman splitting, and disorder arising from multiple charge impurities by using the results of (i) within a standard free fermion approach, and (iii) links the microscopic results to experimentally observable features by generating tunneling differential conductance maps as function of the control parameters (e.g., Zeeman field and chemical potential). We find that charge impurities lead to serious complications regarding the realization and observation of Majorana zero modes, which have direct implications for the development of Majorana-based qubits. More importantly, our work provides a clear direction regarding what needs to be done for progress in the field, including specific materials quality and semiconductor purity targets that must be achieved to create a topological qubit.

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arXiv2020

Electrostatic effects and topological superconductivity in semiconductor-superconductor-magnetic insulator hybrid wires

We investigate the impact of electrostatics on the proximity effect between a magnetic insulator and a semiconductor wire in semiconductor-superconductor-magnetic insulator hybrid structures. By performing self-consistent Schr$\ddot{\rm o}$dinger-Poisson calculations using an effective model of the hybrid system, we find that large effective Zeeman fields consistent with the emergence of topological superconductivity emerge within a large parameter window in wires with overlapping layers of magnetic insulator and superconductor, but not in non-overlapping structures. We show that this behavior is essentially the result of electrostatic effects determining the amplitude of the low-energy wave functions near the semiconductor-magnetic insulator interface.

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arXiv2020

Feasibility of measurement-based braiding in the quasi-Majorana regime of semiconductor-superconductor heterostructures

We discuss the feasibility of measurement-based braiding in semiconductor-superconductor (SM-SC) heterostructures in the so-called quasi-Majorana regime $-$ the topologically-trivial regime due to partially-separated Andreev bound states (ps-ABSs). These low energy ABSs consist of component Majorana bound states (quasi-Majorana modes) that are spatially separated by a length scale smaller than the length of the system, in contrast with the Majorana zero modes (MZMs), which are separated by the length of the wire. In the quasi-Majorana regime, the ZBCPs appear to be robust to various perturbations as long as the energy splitting of the ps-ABS is less than the typical width $\e_w$ of the low-energy conductance peaks $\e_w$. However, the feasibility of measurement-based braiding depends on a different energy scale $\e_m$. In this paper we show that it is possible to prepare the SM-SC system in the quasi-Majorana regime with energy splittings below the $\e_m$ threshold, so that measurement-based braiding is possible in principle. Starting with ps-ABSs with energy below $\e_m$, we identify the maximum amplitudes of different types of perturbations that are consistent with perturbation-induced energy splittings not exceeding the $\e_m$ limit. We argue that measurements generating perturbations larger than the threshold amplitudes appropriate for $\e_m$ cannot realize measurement-based braiding in SM-SC heterostructures in the quasi-Majorana regime. We find that, if possible at all, quantum computation using measurement-based braiding in the quasi-Majorana regime would be plagued with errors introduced by the measurement processes themselves, while such errors are significantly less likely in a scheme involving topological MZMs.

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arXiv2020

Enhanced topological protection in planar quasi-one-dimensional channels with periodically-modulated width

We study one dimensional (1D) and quasi-1D periodic structures as possible platforms for the emergence of Majorana bound states with enhanced robustness against disorder and system inhomogeneity. First, using a simple 1D model, we analytically derive the effective parameters characterizing the minibands generated by the periodic potential. We show that, for strong enough periodic potentials, the higher energy minibands hosting Majorana bound states have significant advantages compared to their counterparts in uniform systems, including increased topological gaps, enhanced robustness against disorder, and enlarged parameter space regions consistent with the presence of topological superconductivity. We identify the problem of engineering a strong enough periodic potential as a key roadblock to realizing efficient periodic 1D structures. To address this challenge, we propose an efficient implementation of the periodic potential based on quasi-1D channels realized in 2D semiconductor heterostructures proximity coupled to superconductor strips of periodically modulated width. Our numerical study of the modulated channel device shows excellent agreement with the simple 1D model, reveals a topological phase diagram that is quite insensitive to the details of the confining potential associated with screening by the superconductor, and demonstrates that engineering patterned 2D structures represents a powerful and versatile approach to realizing robust Majorana bound states.

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arXiv2020

Majorana versus Andreev bound state energy oscillations in a 1D semiconductor-superconductor heterostructure

The recent experimental observations of decaying energy oscillations in semiconductor-superconductor Majorana nanowires is in contrast with the typical expectations based on the presence of Majorana zero modes localized at the ends of the system, when the amplitude of the hybridization energy oscillations is predicted to increase with the applied magnetic field. These observations have been theoretically justified recently by considering a position-dependent, step-like spin-orbit coupling near end of the nanowire, which could arise due to the presence of tunnel gates in a standard tunneling conductance experiment. Here, we show that the window in parameter space where this phenomenology occurs is vanishingly small, when compared to the parameter region where Majorana oscillations increase in amplitude with the applied field. Further, including a position-dependent effective potential, which is also induced naturally near the end of the wire by, e.g., tunnel gates, practically removes the small window associated with decaying oscillations. Using extensive numerical calculations, we show that, as expected, increasing amplitude oscillations of the hybridization energy represent a generic property of topological Majorana zero modes, while decreasing amplitude oscillations are a generic property of low-energy trivial Andreev bound states that typically emerge in non-homogeneous systems. By averaging over several realistic parameter configurations, we identify robust features of the hybridization energy that can be observed in a typical differential conductance experiment without fine-tuning the control parameters.

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arXiv2019

Robust low-energy Andreev bound states in semiconductor-superconductor heterostructures: Importance of partial separation of component Majorana bound states

Robust topologically trivial low-energy Andreev bound states (ABSs) induced by position-dependent effective potentials have recently come under renewed focus, in light of a remarkable set of experiments observing robust quantized zero-bias conductance plateaus in semiconductor-superconductor heterostructures. We show that (1) the partial spatial separation of the wave functions of the component Majorana bound states (MBSs) is crucial for the creation and stability of topologically trivial near-zero-energy Andreev bound states, (2) the signs of the spin polarizations of the component MBSs can be either the same or opposite, depending on the profile of the inducing potential, and (3) the spin polarizations do not play a fundamental role in generating vastly different coupling strengths to local probes and/or ensuring the robustness of the near-zero-energy ABS. Consequently, in contrast to recent theoretical claims (Vuik et al., arXiv:1806.02801), we find that a robust, quantized zero-bias conductance plateau of height 2e^2/h measured in the topologically trivial regime necessarily requires partially separated ABSs (ps-ABSs), independent of the relative signs of the spin-polarizations. In addition, we show that (4) well-defined energy splitting oscillations involve MBSs characterized by exponential tails pointing toward each other, and (5) ps-ABSs generated by the tunnel barrier itself produce zero-bias conductance peaks with a characteristic width that increases strongly with the applied magnetic field. Finally, we propose (6) a quantitative scheme for analyzing the stability of Majorana modes based on probability distributions of splitting susceptibilities and show that a ps-ABS mode can be remarkably robust when judged based on its signature in a charge tunneling experiment, but, in essence, is topologically unprotected.

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arXiv2019

Subband occupation in semiconductor-superconductor nanowires

Subband occupancy (i.e. the number of occupied subbands or energy levels in the semiconductor) is a key physical parameter characterizing the topological properties of superconductor-semiconductor hybrid systems in the context of the search for non-Abelian Majorana zero modes. We theoretically study the subband occupation of semiconductor nanowire devices as function of the applied gate potential, the semiconductor-superconductor (SM-SC) work function difference, and the surface charge density by solving self-consistently the Schrödinger-Poisson equations for the conduction electrons of the semiconductor nanowire. Realistic surface charge densities, which are responsible for band bending, are shown to significantly increase the number of occupied subbands, making it difficult or impossible to reach a regime where only a few subbands are occupied. We also show that the energy separation between subbands is significantly reduced in the regime of many occupied subbands, with highly detrimental consequences for the realization and observation of robust Majorana zero modes. As a consequence, the requirements for the realization of robust topological superconductivity and Majorana zero modes should include a low value of the chemical potential, consistent with the occupation of only a few subbands. Finally, we show that the local density of states on the exposed nanowire facets provides a powerful tool for identifying a regime with many occupied subbands and is capable of providing additional critical information regarding the feasibility of Majorana physics in semiconductor-superconductor devices. In our work, we address both InAs/Al and InSb/Al superconductor-nanowire hybrid systems of current experimental interest.

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