We report the observation of two fundamental subgap transport processes through a quantum dot (QD) with a superconducting contact. The device consists of a carbon nanotube contacted by a Nb superconducting and a normal metal contact. First, we find a single resonance with position, shape, and amplitude consistent with the theoretically predicted resonant Andreev tunneling (AT) through a single QD level. Second, we observe a series of discrete replicas of resonant AT at a separation of ~145 μeV, with a gate, bias, and temperature dependence characteristic for boson-assisted, inelastic AT, in which energy is exchanged between a bosonic bath and the electrons. The magnetic field dependence of the replica's amplitudes and energies suggest that two different bosons couple to the tunnel process.

Andreev bound states (ABSs) are well-defined many-body quantum states that emerge from the hybridization of individual quantum dot (QD) states with a superconductor and exhibit very rich and fundamental phenomena. We demonstrate several electron transport phenomena mediated by ABSs that form on three-terminal carbon nanotube (CNT) QDs, with one superconducting (S) contact in the center and two adjacent normal-metal (N) contacts. Three-terminal spectroscopy allows us to identify the coupling to the N contacts as the origin of the Andreev resonance (AR) linewidths and to determine the critical coupling strengths to S, for which a ground state (or quantum phase) transition in such S-QD systems can occur. In addition, we ascribe replicas of the lowest-energy ABS resonance to transitions between the ABS and odd-parity excited QD states, a process we call excited state ABS resonances. In the conductance between the two N contacts we find a characteristic pattern of positive and negative differential subgap conductance, which we explain by considering two nonlocal processes, the creation of Cooper pairs in S by electrons from both N terminals, and a transport mechanism we call resonant ABS tunneling, possible only in multiterminal QD devices. In the latter process, electrons are transferred via the ABS without effectively creating Cooper pairs in S. The three-terminal geometry also allows spectroscopy experiments with different boundary conditions, for example by leaving S floating. Surprisingly, we find that, depending on the boundary conditions and the device parameters, the experiments either show single-particle Coulomb blockade resonances, ABS characteristics, or both in the same measurements, seemingly contradicting the notion of ABSs replacing the single-particle states as eigenstates of the QD. We qualitatively explain these results as originating from the finite time scale required for the coherent oscillations between the superposition states after a single-electron tunneling event. These experiments demonstrate that three-terminal experiments on a single complex quantum object can also be useful to investigate charge dynamics otherwise not accessible due to the very high frequencies.

Articles you may be interested inPermalloy and Co50Pd50 as ferromagnetic contacts for magnetoresistance measurements in carbon nanotubebased transport structuresWe report an improved fabrication scheme for carbon based nanospintronic devices and demonstrate the necessity for a careful data analysis to investigate the fundamental physical mechanisms leading to magnetoresistance. The processing with a low-density polymer and an optimised recipe allows us to improve the electrical, magnetic, and structural quality of ferromagnetic Permalloy contacts on lateral carbon nanotube (CNT) quantum dot spin valve devices, with comparable results for thermal and sputter deposition of the material. We show that spintronic nanostructures require an extended data analysis, since the magnetization can affect all characteristic parameters of the conductance features and lead to seemingly anomalous spin transport. In addition, we report measurements on CNT quantum dot spin valves that seem not to be compatible with the orthodox theories for spin transport in such structures. V C 2014 AIP Publishing LLC. [http://dx.

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...

Various promising qubit concepts have been put forward recently based on engineered superconductor (SC) subgap states like Andreev bound states, Majorana zero modes or the Yu-Shiba-Rusinov (Shiba) states. The coupling of these subgap states via a SC strongly depends on their spatial extension and is an essential next step for future quantum technologies. Here we investigate the spatial extension of a Shiba state in a semiconductor quantum dot coupled to a SC for the first time. With detailed transport measurements and numerical renormalization group calculations we find a remarkable more than 50 nm extension of the zero energy Shiba state, much larger than the one observed in very recent scanning tunneling microscopy (STM) measurements. Moreover, we demonstrate that its spatial extension increases substantially in magnetic field.Superconductor nanostructures are the most advanced platforms for quantum computational architectures thanks to the macroscopic coherent wavefunction and the robust protection by the superconducting gap. Recently, various novel qubit concepts like the Andreev (spin) qubits [1-5], Majorana box qubits [6-8], braiding with Majorana zero modes in a Majorana or a Shibachain [9-18] have been put forward or even implemented. All these qubits are based on their associated sub-gap states such as Andreev bound states [19], Majorana zero modes [18,[20][21][22][23][24][25][26] or Shiba states [27][28][29][30]. The Shiba state is formed when a magnetic adatom or its artificial version (quantum dot) is coupled to a superconductor and the localized magnetic moment creates a subgap state by binding an anti-aligned quasiparticle from the superconductor. Depending on the coupling strength between the superconductor and the magnetic moment, the ground state can be either the screened local moment with singlet character or the unscreened doublet states.The coupling of these sub-gap states via a superconductor is an essential next step towards 2-qubit operations or state engineering, e.g. an Andreev molecule [31][32][33] or a Majorana-chain, which consists of series of adatoms or quantum dots interlinked by the superconductor [9][10][11][12][13][14][15][16][17][18]. Obviously, the coupling between such subgap states strongly depends on their spatial extension into the superconductor, so it is required for these localized states to extend as much as possible.So far, the spatial extent and structure of the Shiba states was investigated by STM measurements on mag-netic adatoms deposited on the surface of a superconductor [34][35][36][37] and, interestingly, it revealed that the dimensionality plays a crucial role [36]. In a three dimensional isotropic s-wave superconductor, it was found that the Shiba states decay over a very short distance of the order of ∼ 1 nm [34,35], but extends one order of magnitude further, as far as ∼ 10 nm, if the impurity is placed on the surface of a two-dimensional superconductor [36,38].In this work, we investigate the spatial extension of the Shiba state formed when an artificial ...