Subgap resonant quasiparticle transport in normal-superconductor quantum dot devices

Abstract

We report thermally activated transport resonances for biases below the superconducting energy gap in a carbon nanotube (CNT) quantum dot (QD) device with a superconducting Pb and a normal metal contact. These resonances are due to the superconductor's finite quasi-particle population at elevated temperatures and can only be observed when the QD life-time broadening is considerably smaller than the gap. This condition is fulfilled in our QD devices with optimized Pd/Pb/In multilayer contacts, which result in reproducibly large and "clean" superconducting transport gaps with a strong conductance suppression for subgap biases. We show that these gaps close monotonically with increasing magnetic field and temperature. The accurate description of the subgap resonances by a simple resonant tunneling model illustrates the ideal characteristics of the reported Pb contacts and gives an alternative access to the tunnel coupling strengths in a QD.Quantum phenomena in nanostructures with a superconductor (S) and a normal metal contact (N) coupled to low-dimensional electron systems like a quantum dot (QD) 1 have recently gained much attention due to potential applications in quantum technology. Especially prominent are transport phenomena at energies below the superconductor's energy gap, ∆, which typically comprise quasi-particle (QP) tunneling and Andreev processes due to Cooper pair transport. These processes result in a large variety of subgap features, for example Majorana Fermions, 2 which might be used for topological quantum computation, 3 Cooper pair splitting 4-8 as a source of entangled electrons, resonant and inelastic Andreev tunneling, 9 or Andreev bound states (ABSs) 10-13which can be implemented as Andreev qubits. 14,15 Recent experiments have highlighted the importance to understand in detail the QP excitations in such structures, which, for example, lead to additional subgap features, 16,17 or to a poisoning of the bound state parity lifetime. 18To identify subgap transport mechanisms, a transport gap much larger than the QD life time, ∆ Γ, is very beneficial -a regime which is not easily achieved in S-QD hybrid devices. In addition, a strong suppression of the QP conductance in the subgap regime is required, which is commonly known as a "clean gap". While the widely used superconductor Al 5-7 has yielded devices with good transport characteristics, long superconducting coherence lengths, ξ 0 , and more recently also clean gaps, 18-20it's small gap renders spectroscopic investigations difficult. S-QD devices based on the large-gap superconductor Nb allowed the observation of several fundamental transport processes 9,13,16,17,21 and new effects due to the large critical field.22 However, Nb has rather short coherence lengths and the devices often exhibit strongly suppressed or "soft" gaps 16,21,22 and complex magnetic field characteristics, 9,22 which make normal state control experiments difficult. In contrast, in the superconductor Pb one finds a large bulk coherence length of ξ 0 ∼ 90 nm, a supercondu...

Charge-vibration interaction effects in normal-superconductor quantum dots

“…According to our theory, the two kind of peaks are correlated and this property can be readily exploited for the spectroscopy of the vibrational modes. Similar inelastic processes were observed in recent experiments [144][145][146][147] suggesting that our proposal is within the reach of current research.…”

“…Finally, we remark that experiments with U as in Refs. [145,146,170] focused on devices with large asymmetry S N the normal lead basically acts as a spectroscopic probe of the Andreev spectra. In some ranges of parameters, the experimental results have been qualitatively explained in terms of a noninteracting model, viz.…”

Andreev bound states probed in three-terminal quantum dots

“…We have fabricated CNT three-terminal devices with a lead (Pb) based central ∼200 nm wide Pd/Pb/In (4.5-6/110/20 nm) S contact [47], two Pd N contacts and two side gates (SGs) using close to residue free-electron beam lithography [54]. The typical critical perpendicular field for these S contacts is ∼200 mT and we apply 400 mT for our normal-state experiments.…”

“…A scanning electron microscopy (SEM) image is shown in Fig. 1 (14) 195418-1 ©2017 American Physical Society a large energy gap and thus in a high relative spectroscopic resolution [47], crucial for our experiments. The paper is structured as follows: In Sec.…”

Nonequilibrium properties of graphene probed by superconducting tunnel spectroscopy

“…In a last step, the superconducting electrode was deposited. Here, we used an optimized three layer structure consisting of 5 nm Pd as wetting layer, 110 nm of Pb and 20 nm of In as a capping layer 31 . The Pd sticking layer was used to avoid oxidation at the hBN/Pb interface 32 and the In capping layer was used to prevent oxidation from the top.…”