Destruction of global coherence in long superconducting nanocylinders

We present a novel route to realizing topological superconductivity using magnetic flux applied to a full superconducting shell surrounding a semiconducting nanowire core. In the destructive Little-Parks regime, reentrant regions of superconductivity are associated with integer number of phase windings in the shell. Tunneling into the core reveals a hard induced gap near zero applied flux, corresponding to zero phase winding, and a gapped region with a discrete zero-energy state around one applied flux quantum, Φ0 = h/2e, corresponding to 2π phase winding. Theoretical analysis indicates that in the presence of radial spin-orbit coupling in the semiconductor, the winding of the superconducting phase can induce a transition to a topological phase supporting Majorana zero modes. Realistic modeling shows a topological phase persisting over a wide range of parameters, and reproduces experimental tunneling conductance data. Further measurements of Coulomb blockade peak spacing around one flux quantum in full-shell nanowire islands shows exponentially decreasing deviation from 1e periodicity with device length, consistent with Majorana modes at the ends of the nanowire. arXiv:2003.13177v1 [cond-mat.mes-hall]

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“….) [30,42,81]. Throughout the extended range where T C vanishes, superconductivity is destroyed [31,32].…”

Section: Destructive Regimementioning

confidence: 99%

Flux-induced topological superconductivity in full-shell nanowires

Vaitiekėnas

Winkler

Heck

et al. 2020

Science

218

238

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“…Subsequent experiments showed a low-temperature phase with normal-state resistance, R N , around Φ 0 /2, but did not display fully recovered superconductivity at higher flux [22]. Several theoretical mod-els were proposed to interpret these different scenarios [23][24][25], but no consensus emerged.Here, we report a study of the Little-Parks effect in InAs nanowires with a thin epitaxial cylindrical Al shell, demonstrating the relation between destructive superconductivity and various experimentally controllable parameters. Excellent agreement with Ginzburg-Landau mean field theory is found as a function of flux, temperature, and current bias, using independently measured material and device parameters.…”

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confidence: 99%

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confidence: 99%

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Anomalous metallic phase in tunable destructive superconductors

Vaitiekėnas¹,

Krogstrup²,

Marcus³

2020

Phys. Rev. B

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The Little-Parks effect is a flux-dependent modulation of the superconducting transition temperature resulting from fluxoid quantization through holes in a multiply connected superconductor. In hollow superconducting cylinders with diameter smaller than the coherence length, flux-induced supercurrents can give rise to the destructive Little-Parks effect, characterized by repeated reentrant quantum phase transitions between superconducting and metallic phases. Here, we use axial and transverse magnetic fields to control the crossover between conventional and destructive Little-Parks regimes in nanowires with an epitaxial Al shell fully surrounding InAs core. The observed dependences on flux, transverse field, temperature, and current bias are in excellent agreement with theory. Near the crossover between the conventional and destructive regimes, an anomalous metal phase is found. The anomalous metallic phase is characterized by a field-controllable, temperatureindependent resistivity between adjacent superconducting lobes.Quantum phase transitions (QPT) [1, 2] in conventional superconductors serve as prototypes for related effects in more complex, strongly-correlated systems [3], including heavy-fermion materials [4] and high-temperature superconductors [5].While lowtemperature superconductors are well understood in bulk, new phenomena can arise in mesoscopic samples and reduced dimensionality [6,7]. For instance, in two-dimensional films, electrons theoretically condense into either a superconductor or insulator in the lowtemperature limit [8]. Yet, in many instances, an anomalous metallic state with finite temperature-independent resistance is found at low temperatures [9]. In onedimensional wires, incoherent phase slips can destroy superconductivity [10] or give rise to an anomalous metallic state [11], while coherent quantum phase slips can lead to superposition of quantum states enclosing different numbers of flux quanta [12], potentially useful as a qubit [13].Multiply connected superconductors provide an even richer platform for investigating phase transitions. Fluxoid quantization in units of Φ 0 = h/2e [14,15], reveals not only electron pairing but a complex macroscopic order parameter, ∆e iϕ [6,16]. The same physical mechanism underlies the Little-Parks effect, a periodic modulation of the transition temperature, T C , of a superconducting cylinder with magnetic flux period Φ 0 [17]. For hollow superconducting cylinders with diameter, d, smaller than the coherence length, the modulation amplitude can exceed zero-field transition temperature, T C0 , leading to a reentrant destruction of superconductivity near odd half-integer multiples of Φ 0 [18][19][20].Early experimental investigation of the destructive Little-Parks effect reported reentrant superconductivity interrupted by an anomalous-resistance phase around applied flux Φ 0 /2 [21]. Subsequent experiments showed a low-temperature phase with normal-state resistance, R N , around Φ 0 /2, but did not display fully recovered superconductivity at higher ...

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“…The maximum ∆T c (H ⊥ ) ≈ −28 mK is very close to the theoretical value ∆T c = −0.18ξ 0 lT c0 /R 2 = −26 mK for a thin Nb ring of radius R = 75 nm in the dirty limit 15 , where ξ 0 = 38 nm and l = 3.6 nm were chosen for the calculation 16 . The decay in the amplitude of ∆T c (H ⊥ ) with increasing magnetic field may originate from the slight imperfections in the arrangement of the holes in the triangular lattice 38 and/or the finite width of the circulating supercurrent path 39,40 .…”

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confidence: 99%