6,286 research outputs found
Andreev bound states probed in three-terminal quantum dots
We demonstrate several new electron transport phenomena mediated by Andreev
bound states (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
transition S-QD systems can occur. We ascribe replicas of the lowest-energy ABS
resonance to transitions between the ABS and odd-parity excited QD states, a
process called 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 novel mechanism called resonant ABS tunneling. In the latter,
electrons are transferred via the ABS without 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, 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.Comment: 15 pages, 16 figure
Resonant and inelastic Andreev tunneling observed on a carbon nanotube quantum dot
We report the observation of two fundamental sub-gap 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 eV, 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.Comment: 5 pages + 9 pages supplementary materia
Fork stamping of pristine carbon nanotubes onto ferromagnetic contacts for spin-valve devices
We present a fabrication scheme called 'fork stamping' optimized for the dry
transfer of individual pristine carbon nanotubes (CNTs) onto ferromagnetic
contact electrodes fabricated by standard lithography. We demonstrate the
detailed recipes for a residue-free device fabrication and in-situ current
annealing on suspended CNT spin-valve devices with ferromagnetic Permalloy (Py)
contacts and report preliminary transport characterization and
magnetoresistance experiments at cryogenic temperatures. This scheme can
directly be used to implement more complex device structures, including
multiple gates or superconducting contacts.Comment: 7 pages, 4 figures, submitted to IWEPNM 2015 conference proceedings
(physica status solidi (b)
Contact resistance dependence of crossed Andreev reflection
We show experimentally that in nanometer scaled superconductor/normal metal
hybrid devices and in a small window of contact resistances, crossed Andreev
reflection (CAR) can dominate the nonlocal transport for all energies below the
superconducting gap. Besides CAR, elastic cotunneling (EC) and nonlocal charge
imbalance (CI) can be identified as competing subgap transport mechanisms in
temperature dependent four-terminal nonlocal measurements. We demonstrate a
systematic change of the nonlocal resistance vs. bias characteristics with
increasing contact resistances, which can be varied in the fabrication process.
For samples with higher contact resistances, CAR is weakened relative to EC in
the midgap regime, possibly due to dynamical Coulomb blockade. Gaining control
of CAR is an important step towards the realization of a solid state entangler.Comment: 5 pages, 4 figures, submitted to PR
Assessing composition gradients in multifilamentary superconductors by means of magnetometry methods
We present two magnetometry-based methods suitable for assessing gradients in
the critical temperature and hence the composition of multifilamentary
superconductors: AC magnetometry and Scanning Hall Probe Microscopy. The
novelty of the former technique lies in the iterative evaluation procedure we
developed, whereas the strength of the latter is the direct visualization of
the temperature dependent penetration of a magnetic field into the
superconductor. Using the example of a PIT Nb3Sn wire, we demonstrate the
application of these techniques, and compare the respective results to each
other and to EDX measurements of the Sn distribution within the sub-elements of
the wire.Comment: 7 pages, 8 figures; broken hyperlinks are due to a problem with arXi
Permalloy-based carbon nanotube spin-valve
In this Letter we demonstrate that Permalloy (Py), a widely used Ni/Fe alloy,
forms contacts to carbon nanotubes (CNTs) that meet the requirements for the
injection and detection of spin-polarized currents in carbon-based spintronic
devices. We establish the material quality and magnetization properties of Py
strips in the shape of suitable electrical contacts and find a sharp
magnetization switching tunable by geometry in the anisotropic
magnetoresistance (AMR) of a single strip at cryogenic temperatures. In
addition, we show that Py contacts couple strongly to CNTs, comparable to Pd
contacts, thereby forming CNT quantum dots at low temperatures. These results
form the basis for a Py-based CNT spin-valve exhibiting very sharp resistance
switchings in the tunneling magnetoresistance, which directly correspond to the
magnetization reversals in the individual contacts observed in AMR experiments.Comment: 3 page
Finite bias Cooper pair splitting
In a device with a superconductor coupled to two parallel quantum dots (QDs)
the electrical tunability of the QD levels can be used to exploit non-classical
current correlations due to the splitting of Cooper pairs. We experimentally
investigate the effect of a finite potential difference across one quantum dot
on the conductance through the other completely grounded QD in a Cooper pair
splitter fabricated on an InAs nanowire. We demonstrate that the electrical
transport through the device can be tuned by electrical means to be dominated
either by Cooper pair splitting (CPS), or by elastic co-tunneling (EC). The
basic experimental findings can be understood by considering the energy
dependent density of states in a QD. The reported experiments add
bias-dependent spectroscopy to the investigative tools necessary to develop
CPS-based sources of entangled electrons in solid-state devices.Comment: 4 pages, 4 figure
Magnetoresistence engineering and singlet/triplet switching in InAs nanowire quantum dots with ferromagnetic sidegates
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 {\it 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 transition between singlet and a 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.Comment: 7 pages, 6 figure
Wet etch methods for InAs nanowire patterning and self-aligned electrical contacts
Advanced synthesis of semiconductor nanowires (NWs) enables their application
in diverse fields, notably in chemical and electrical sensing, photovoltaics,
or quantum electronic devices. In particular, Indium Arsenide (InAs) NWs are an
ideal platform for quantum devices, e.g. they may host topological Majorana
states. While the synthesis has been continously perfected, only few techniques
were developed to tailor individual NWs after growth. Here we present three wet
chemical etch methods for the post-growth morphological engineering of InAs NWs
on the sub-100 nm scale. The first two methods allow the formation of
self-aligned electrical contacts to etched NWs, while the third method results
in conical shaped NW profiles ideal for creating smooth electrical potential
gradients and shallow barriers. Low temperature experiments show that NWs with
etched segments have stable transport characteristics and can serve as building
blocks of quantum electronic devices. As an example we report the formation of
a single electrically stable quantum dot between two etched NW segments.Comment: 9 pages, 5 figure
Near-unity Cooper pair splitting efficiency
The two electrons of a Cooper pair in a conventional superconductor form a
singlet and therefore a maximally entangled state. Recently, it was
demonstrated that the two particles can be extracted from the superconductor
into two spatially separated contacts via two quantum dots (QDs) in a process
called Cooper pair splitting (CPS). Competing transport processes, however,
limit the efficiency of this process. Here we demonstrate efficiencies up to
90%, significantly larger than required to demonstrate interaction-dominated
CPS, and on the right order to test Bell's inequality with electrons. We
compare the CPS currents through both QDs, for which large apparent
discrepancies are possible. The latter we explain intuitively and in a
semi-classical master equation model. Large efficiencies are required to detect
electron entanglement and for prospective electronics-based quantum information
technologies.Comment: 4 page
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