Zero-energy vortex bound state in the superconducting topological surface state of Fe(Se,Te)

Abstract

Majorana quasiparticles (MQPs) in condensed matter play an important role in strategies for topological quantum computing [1-5] but still remain elusive.Vortex cores of topological superconductors may accommodate MQPs that appear as the zero-energy vortex bound state (ZVBS) [6,7]. An iron-based superconductor Fe(Se,Te) possesses a superconducting topological surface state [8][9][10][11] that has been investigated by scanning tunneling microscopies to detect the ZVBS [12,13]. However, the results are still controversial [12,13]. Here, we performed spectroscopic-imaging scanning tunneling microscopy with unprecedentedly high energy resolution to clarify the nature of the vortex bound states in Fe(Se,Te). We found the ZVBS at 0±20 µeV suggesting its MQP origin, and revealed that some vortices host the ZVBS while others do not. The fraction of vortices hosting the ZVBS decreases with increasing magnetic field, while chemical and electronic quenched disorders are apparently unrelated to the ZVBS. These observations elucidate the conditions to achieve the ZVBS, and may lead to controlling MQPs. * tadashi.machida@riken.jp † hanaguri@riken.jp arXiv:1812.08995v2 [cond-mat.supr-con]

FeTe0.55Se0.45 van der Waals tunneling devices

“…Differential conductance dI/dV of bottom-electrode Device BE ( Figure 3) exhibits, at the lowest temperature, a hard superconducting gap with R N /R 0 ≈ 70. The sub-gap energy range is punctuated by in-gap states at bias voltage V ≈ ±0.7 meV, which we attribute to defects in the FeTe 0.55 Se 0.45 , as observed also in STM measurements [6]. In addition, this junction exhibits two noteworthy phenomena:…”

“…ARPES measurements identify four gaps: 1.7 meV and 2.5 meV for the α' and β hole bands at the Γ point, 1.9 meV for the topological surface band, and 4.2 meV for the γ electron band around the M point [31,10]. Similar gaps have been observed in STM measurements, although the exact energies of the quasiparticle peaks vary and not all studies observe features related to all gaps [11,4,5,6,7].…”

“…This has two important consequences: First, it makes these superconductors sensitive to interaction effects such as the BCS-BEC crossover [1,2]. Second, the ∆/E F ratio determines the energy separation between Caroli-de Gennes Matricon (CdGM) vortex-bound states [3], allowing the observation of discrete CdGM states in scanning * Equal contribution 1 arXiv:1907.09758v1 [cond-mat.supr-con] 23 Jul 2019 tunneling microscopy (STM) measurements [4,5,6,7,8]. Among these, some reports observe zero energy bound states interpreted as Majorana excitations [9,6,7], expected in view of ARPES signatures of a topological superconducting state on the surface of FeTe 0.55 Se 0.45 [10].…”

Atomic line defects and zero-energy end states in monolayer Fe(Te,Se) high-temperature superconductors

“…ZEBSs have been detected in the vortices of strong topological insulators and s-wave superconductor heterostructures by scanning tunneling microscopy 10 . Moreover, ZEBSs have been observed recently in a fraction of magnetic field induced vortices in the superconducting (SC) transition temperature (T c ) ~ 14.5 K bulk Fe(Te,Se) 11,12 , hosting the observed SC Dirac cone topological surface states [13][14][15][16] , as well as at the magnetic interstitial Fe impurities 17 where anomalous vortices may nucleate in zero external field 18 . The most advanced path currently is the nanowire/s-wave superconductor/Zeeman field hybrid structure [19][20][21] where a ZEBS has been detected at one end with a tunneling conductance close to the quantized value expected of a MZM [22][23][24][25] .…”

Majorana phase gate based on the geometric phase

“…So far, the experimental and theoretical studies have focused mostly on finding an optimal physical system that hosts the MZM [12-18] as well as on developing appropriate techniques which clearly confirm the existence of MZM therein [19][20][21]. Recent experimental results strongly support the presence of the MZM in superconductor-semiconductor hybrid nanostructures [22][23][24][25][26][27][28][29][30][31], in one-dimensional monoatomic chains deposited on the surface of superconductors [32][33][34][35][36][37], in the superconducting vortices [38][39][40][41] and in two-dimensional topological superconductors [42,43].The fundamental problem for quantum computing is to effectively implement the set of the universal gates which consists of the Hadamard gate, the Z gate and also the π/8-gate (phase-gate) [44]. The general scheme for building the former two gates is already well established via topologically protected braiding operations of MZMs [45][46][47][48][49][50][51][52][53][54][55][56].…”