The ω-SQUIPT as a tool to phase-engineer Josephson topological materials

Abstract

Multi-terminal superconducting Josephson junctions based on the proximity effect offer the bright opportunity to tailor non trivial quantum states in nanoscale weaklinks. These structures can realize exotic topologies in multidimensions [1] as, for example, artificial topological superconductors able to support Majorana bound states [2,3], and pave the way to emerging quantum technologies [4][5][6][7] and future quantum information schemes [8]. Here, we report the first realization of a three-terminal Josephson interferometer based on a proximized nanosized weak-link. Our tunneling spectroscopy measurements reveal transitions between gapped (i.e., insulating) and gapless (i.e., conducting) states, those being controlled by the phase configuration of the three superconducting leads connected to the junction. We demonstrate the topological nature of these transitions: a gapless state necessarily occurs between two gapped states of different topological index, very much like the interface between two insulators of different topology is necessarily conducting [9]. The topological numbers characterizing such gapped states are given by superconducting phase windings over the two loops forming the Josephson interferometer. Since these gapped states cannot be transformed to one another continuously withouth passing through a gapless condition, these are topologically protected. Our observation of the gapless state is pivotal for enabling phase engineering of more sophisticated artificial topological materials realizing Weyl points or the anomalous Josephson effect [1,[4][5][6][7]10].When two superconductors (S) are coupled through a normal metal (N), they realize a Josephson junction (JJ) and superconducting correlations are induced in the N region due to proximity effect [11][12][13][14][15][16]. As a consequence, the N metal acquires genuine superconducting-like properties such as the ability to sustain a supercurrent, and a gap in the density of states (DoS) whose amplitude can be controlled by the macroscopic phase difference between the S leads [12,[17][18][19]. This process therefore enables the N region to possess a character ranging from insulating-like (gapped state) to conductinglike (gapless state) [14,17].Although two-terminal JJs based on proximity effect have been at the focus of an intense research for several years [11][12][13][14][15][16][17][18][19][20][21] weak-links [10]. Yet, the impact and control over three superconducting phases acting on a nanosized N region have never been explored so far despite recent predictions of using multi-terminal JJs for tailoring and controlling exotic quantum states. [1,4,6,10]. Indeed, multi-terminal (>2) JJs allow the spin-orbit interaction to affect substantially the Andreev bound levels enabling the manipulation of electrons in sinarXiv:1603.00338v1 [cond-mat.mes-hall]

Coherent transport properties of a three-terminal hybrid superconducting interferometer

“…Figure 2 suggests that the gap closes at ≈ 0 /3, as confirmed by recent measurements [19]. Interestingly enough, to each gapped region it can be assigned a topological index defined by the pair of numbers obtained by the integration of the superconducting phase gradient over the left and right loop [19]. We note that our results agree well with the recent findings of Ref.…”

“…One of the main signatures of the three-terminal configuration, with respect to conventional two-terminal JJ, is the conducting gapless state observed for symmetric devices (e.g., at r C = r R = 1) in a large range of fluxes 0 /3 < < 2 0 /3. In the same range, this conducting state evolves in two additional gapped states by increasing the asymmetries of the interface resistances (e.g., at r C = 10 2 ,r R = 10) as also observed in the measurements of asymmetric ω-SQUIPTs [19]. As shown in can be drawn also for the fluctuations of the arm lengths affecting only weakly the main features of the DOS (shown in Fig.…”

“…This evolution is crucial to clarify the experimental observations reported in Ref. [19] in which the quasiparticle DOSs have been probed by tunneling spectroscopy (see Sec. V for details).…”

“…This continuous gapless region is the main hallmark of multiterminal JJs (recently observed experimentally in Ref. [19]), and it is a consequence of the crossing of the Andreev bound states at zero energy.…”

“…The ω-SQUIPT represent a useful tool to explore the nontrivial physics accessible in multiterminal Josephson junctions (JJs), in which the Andreev bound states can cross the Fermi level (zero-energy) [10] to tailor exotic quantum states [11][12][13][14][15], to simulate topological materials able to support Majorana bound states in the case of quasiballistic junctions with strong spin-orbit coupling [12,16], or to implement different kinds of Q-bits [17] or switchers [18]. The first ω-SQUIPT was realized [19] very recently with a diffusive three-terminal JJ. The experiment, in agreement with theoretical expectations, demonstrates that a superconducting-like gapped state is induced in the weak link and nontrivially controlled by an external magnetic field.…”