Optimized fabrication and characterization of carbon nanotube spin valves

Samm, J.; Gramich, J.; Baumgärtner, A.; Weiß, Markus; Schönenberger, Christian

doi:10.1063/1.4874919

Cited by 30 publications

(45 citation statements)

References 29 publications

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“…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.…”

Section: Device Fabrication and Measurment Setupmentioning

confidence: 99%

Andreev bound states probed in three-terminal quantum dots

2017

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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.

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Section: Device Fabrication and Measurment Setupmentioning

confidence: 99%

Andreev bound states probed in three-terminal quantum dots

2017

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“…25,28,32,45,48 Alternatively, the efficiency can be improved using spin-filtering as in spin valves. 49,50 Typically all these systems are investigated assuming a very strong on-site Coulomb interaction. In the present paper, we consider the possibility of a weak Coulomb interaction which complicates the analysis as it introduces additional transport channels.…”

Section: -29mentioning

confidence: 99%

Double quantum dot Cooper-pair splitter at finite couplings

et al. 2016

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We consider the sub-gap physics of a hybrid double-quantum dot Cooper-pair splitter with large single-level spacings, in the presence of tunnelling between the dots and finite Coulomb intra-and inter-dot Coulomb repulsion. In the limit of a large superconducting gap, we treat the coupling of the dots to the superconductor exactly. We employ a generalized master-equation method which easily yields currents, noise and cross-correlators. In particular, for finite inter-and intra-dot Coulomb interaction, we investigate how the transport properties are determined by the interplay between local and nonlocal tunneling processes between the superconductor and the dots. We examine the effect of inter-dot tunneling on the particle-hole symmetry of the currents with and without spinorbit interaction. We show that spin-orbit interaction in combination with finite Coulomb energy opens the possibility to control the nonlocal entanglement and its symmetry (singlet/triplet). We demonstrate that the generation of nonlocal entanglement can be achieved even without any direct nonlocal coupling to the superconducting lead.

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“…The split gate consists of a pair of long, narrow ferromagnetic Permalloy (Py) strips which we call ferromagnetic sidegates (FSGs). The strips have a single domain magnetization with a switching field determined by the geometric width [6,7]. In addition, these FSGs can be used individually as electrical gates.…”

Section: Introductionmentioning

confidence: 99%

“…For nonmagnetic bulk materials, one typically finds a parabolic field dependence and a saturation, but it can also be linear to large magnetic fields, either due to disorder [1], or due to a linear dispersion relation [2], for example on the surface of a topological insulator [3]. Many MR effects and applications are known for magnetic materials [4,5], for example the anisotropic magnetoresistance (AMR) used to characterize magnetic contacts [6,7], or the tunneling magnetoresistance (TMR), in which electrons tunnel between two magnetic reservoirs, resulting in a resistance that depends on the relative orientation of the reservoir magnetizations. TMR typically shows hysteretic rectangular MR loops in up and down sweeps of B.…”

Section: Introductionmentioning

confidence: 99%

“…If not stated otherwise, the backgate voltage and dc bias are set to 0 for all presented experiments, while the electronic structure of the NW segment is electrically tuned by the FSGs. Two ∼200 nm wide Py strips with a large aspect ratio [7] are fabricated with the ends in close proximity to the NW segment between S and D. The strips are fabricated to be aligned in parallel to the external magnetic field, which results in an angle to the randomly oriented NW axis. The magnetic force microscopy image of an identical split-gate geometry in Fig.…”

Section: Introductionmentioning

confidence: 99%

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Magnetoresistance engineering and singlet/triplet switching in InAs nanowire quantum dots with ferromagnetic sidegates

et al. 2016

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We present magnetoresistance (MR) experiments on an InAs nanowire quantum dot device with two ferromagnetic sidegates (FSGs) in a split-gate geometry. The wire segment can be electrically tuned to a single dot or to a double dot regime using the FSGs and a backgate. In both regimes we find a strong MR and a sharp MR switching of up to 25% at the field at which the magnetizations of the FSGs are inverted by the external field. The sign and amplitude of the MR and the MR switching can both be tuned electrically by the FSGs. In a double dot regime close to pinch-off we find two sharp transitions in the conductance, reminiscent of tunneling MR (TMR) between two ferromagnetic contacts, with one transition near zero and one at the FSG switching fields. These surprisingly rich characteristics we explain in several simple resonant tunneling models. For example, the TMR-like MR can be understood as a stray-field controlled transitions between singlet and triplet double dot states. Such local magnetic fields are the key elements in various proposals to engineer novel states of matter and may be used for testing electron spin based Bell inequalities.

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Optimized fabrication and characterization of carbon nanotube spin valves

Cited by 30 publications

References 29 publications

Andreev bound states probed in three-terminal quantum dots

Andreev bound states probed in three-terminal quantum dots

Double quantum dot Cooper-pair splitter at finite couplings

Magnetoresistance engineering and singlet/triplet switching in InAs nanowire quantum dots with ferromagnetic sidegates

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