Quantized Conductance of Majorana Zero Mode in the Vortex of the Topological Superconductor (Li _0.84 Fe _0.16 )OHFeSe

Abstract

The Majorana zero mode (MZM), which manifests as an exotic neutral excitation in superconductors, is the building block of topological quantum computing. It has recently been found in the vortices of several iron-based superconductors as a zero-bias conductance peak (ZBCP) in tunneling spectroscopy. In particular, a clean and robust MZM has been observed in the cores of free vortices in (Li0.84Fe0.16)OHFeSe. Here using scanning tunneling spectroscopy (STS), we demonstrate that Majorana-induced resonant Andreev reflection occurs between the STM tip and this zero-bias bound state, and consequently, the conductance at zero bias is quantized as 2e 2 /h. Our results present a hallmark signature of the MZM in the vortex of an intrinsic topological superconductor, together with its intriguing behavior.A peak at zero energy in tunneling spectroscopy is an important hallmark, but not sufficient proof, for identifying a MZM. For example, although the existence of a MZM is predicted at the ends of a strongly spin-orbital-coupled semiconductor nanowire in the presence of a proximate superconductor and a large Zeeman field [1-7], there are alternative interpretations for the experimental observations of zero energy peaks in these systems [8][9][10][11][12]. Morecompelling evidence for a MZM is that the zero-bias peak possesses the quantized universal conductance, G0 = 2e 2 /h, in tunneling experiments, due to resonant Andreev reflection and the particle-hole symmetry of the MZM [13]. At zero temperature, this Majorana-induced resonant Andreev reflection (MIRAR) would give the quantized conductance regardless of the coupling strength [13]; and at finite temperature, the quantized conductance may be observed when the tunneling coupling is sufficiently strong [1,14]. The quantized conductance of a ZBCP was first observed in the tunneling spectrum of a hybrid device between superconducting aluminum and an InSb nanowire [15]. It thus strongly supports the existence of MZMs in semiconductor nanowire devices, although the ultimate proof of Majorana physics would be a demonstration of non-Abelian statistics [16][17][18]. *

On the Remarkable Superconductivity of FeSe and Its Close Cousins

“…A clear prediction of proximitized topological surface states is the emergence of MZMs inside the cores of field-induced vortex lines [384]. A number of experimental groups have performed detailed STM experiments on the surfaces of FeTe 0.55 Se 0.45 and (Li 0.84 Fe 0.16 )OHFeSe and discussed the possibility of MZMs in the data [399,[406][407][408][409][410][411][412]. The topic is complicated, e.g., due to the sensitivity to instrumental resolution, unknown effects of impurity pinning sites [352], contributions from bulk states, quasi-particle poisoning, sample inhomogeneity, disordered vortex lattices, and the presence of other topologically trivial but contaminating low-energy vortex cores states.…”

Majorana corner and hinge modes in second-order topological insulator/superconductor heterostructures

“…In a later influential paper[30], Qi et al pointed out that quantum anomalous Hall insulator (QAHI)/SC heterostructures provide a simple realization of 2d chiral TSCs which harbor not only vortex-core MZMs, but also chiral Majorana edge modes. These two theoretical works have triggered a lot of experimental works on TI(QAHI)/SC heterostructures [28,29,[31][32][33][34][35][36][37][38][39][40][41], and remarkable progress in detecting vortex-core MZMs has been witnessed in recent years [28,29,40,41].Very recently, TIs and TSCs have been generalized to include their higher-order counterparts [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56]. Importantly, higher-order TIs (HOTIs) and TSCs (HOTSCs) have extended the conventional bulk-boundary correspondence.…”

“…In a later influential paper [30], Qi et al pointed out that quantum anomalous Hall insulator (QAHI)/SC heterostructures provide a simple realization of 2d chiral TSCs which harbor not only vortex-core MZMs, but also chiral Majorana edge modes. These two theoretical works have triggered a lot of experimental works on TI(QAHI)/SC heterostructures [28,29,[31][32][33][34][35][36][37][38][39][40][41], and remarkable progress in detecting vortex-core MZMs has been witnessed in recent years [28,29,40,41].…”

Scalable Majorana vortex modes in iron-based superconductors

“…Hence, while the tunneling peaks away from zero-bias voltage have been observed and indicated the CdGM modes (19), the tunneling observation of the ZBPs in the vortex cores (8) has provided the first clue of MVM existence. The further hint of the MVM existence is supported by the recent observation of the tunneling conductance plateau that is near the quantized value (20,21).…”