12 research outputs found
Phase-dependent microwave response of a graphene Josephson junction
Gate-tunable Josephson junctions embedded in a microwave environment provide a promising platform to
in situ engineer and optimize novel superconducting quantum circuits. The key quantity for the circuit design
is the phase-dependent complex admittance of the junction, which can be probed by sensing a radio frequency
SQUID with a tank circuit. Here, we investigate a graphene-based Josephson junction as a prototype gate-tunable
element enclosed in a SQUID loop that is inductively coupled to a superconducting resonator operating at 3 GHz.
With a concise circuit model that describes the dispersive and dissipative response of the coupled system, we
extract the phase-dependent junction admittance corrected for self-screening of the SQUID loop. We decompose
the admittance into the current-phase relation and the phase-dependent loss, and as these quantities are dictated
by the spectrum and population dynamics of the supercurrent-carrying Andreev bound states, we gain insight to
the underlying microscopic transport mechanisms in the junction. We theoretically reproduce the experimental
results by considering a short, diffusive junction model that takes into account the interaction between the
Andreev spectrum and the electromagnetic environment, from which we estimate lifetimes on the order of
∼10 ps for nonequilibrium populations
Mobility enhancement in graphene by in situ reduction of random strain fluctuations
Microscopic corrugations are ubiquitous in graphene even when placed on
atomically flat substrates. These result in random local strain fluctuations
limiting the carrier mobility of high quality hBN-supported graphene devices.
We present transport measurements in hBN-encapsulated devices where such strain
fluctuations can be in situ reduced by increasing the average uniaxial strain.
When of uniaxial strain is applied to the graphene, an enhancement
of the carrier mobility by is observed while the residual doping
reduces by . We demonstrate a strong correlation between the mobility
and the residual doping, from which we conclude that random local strain
fluctuations are the dominant source of disorder limiting the mobility in these
devices. Our findings are also supported by Raman spectroscopy measurements
Global strain-induced scalar potential in graphene devices
By mechanically distorting a crystal lattice it is possible to engineer the
electronic and optical properties of a material. In graphene, one of the major
effects of such a distortion is an energy shift of the Dirac point, often
described as a scalar potential. We demonstrate how such a scalar potential can
be generated systematically over an entire electronic device and how the
resulting changes in the graphene work function can be detected in transport
experiments. Combined with Raman spectroscopy, we obtain a characteristic
scalar potential consistent with recent theoretical estimates. This direct
evidence for a scalar potential on a macroscopic scale due to deterministically
generated strain in graphene paves the way for engineering the optical and
electronic properties of graphene and similar materials by using external
strain
Compact SQUID realized in a double layer graphene heterostructure
Two-dimensional systems that host one-dimensional helical states are exciting
from the perspective of scalable topological quantum computation when coupled
with a superconductor. Graphene is particularly promising for its high
electronic quality, versatility in van der Waals heterostructures and its
electron and hole-like degenerate 0 Landau level. Here, we study a compact
double layer graphene SQUID (superconducting quantum interference device),
where the superconducting loop is reduced to the superconducting contacts,
connecting two parallel graphene Josephson junctions. Despite the small size of
the SQUID, it is fully tunable by independent gate control of the Fermi
energies in both layers. Furthermore, both Josephson junctions show a skewed
current phase relationship, indicating the presence of superconducting modes
with high transparency. In the quantum Hall regime we measure a well defined
conductance plateau of 2 an indicative of counter propagating edge
channels in the two layers. Our work opens a way for engineering topological
superconductivity by coupling helical edge states, from graphene's
electron-hole degenerate 0 Landau level via superconducting contacts.Comment: 38 pages, 12 figure
Out-of-plane corrugations in graphene based van der Waals heterostructures
Two dimensional materials are usually envisioned as flat, truly 2D layers.
However out-of-plane corrugations are inevitably present in these materials. In
this manuscript, we show that graphene flakes encapsulated between insulating
crystals (hBN, WSe2), although having large mobilities, surprisingly contain
out-of-plane corrugations. The height fluctuations of these corrugations are
revealed using weak localization measurements in the presence of a static
in-plane magnetic field. Due to the random out-of-plane corrugations, the
in-plane magnetic field results in a random out-of-plane component to the local
graphene plane, which leads to a substantial decrease of the phase coherence
time. Atomic force microscope measurements also confirm a long range height
modulation present in these crystals. Our results suggest that phase coherent
transport experiments relying on purely in-plane magnetic fields in van der
Waals heterostructures have to be taken with serious care
Edge Contacts to Atomically Precise Graphene Nanoribbons.
Bottom-up-synthesized graphene nanoribbons (GNRs) are an emerging class of designer quantum materials that possess superior properties, including atomically controlled uniformity and chemically tunable electronic properties. GNR-based devices are promising candidates for next-generation electronic, spintronic, and thermoelectric applications. However, due to their extremely small size, making electrical contact with GNRs remains a major challenge. Currently, the most commonly used methods are top metallic electrodes and bottom graphene electrodes, but for both, the contact resistance is expected to scale with overlap area. Here, we develop metallic edge contacts to contact nine-atom-wide armchair GNRs (9-AGNRs) after encapsulation in hexagonal boron-nitride (h-BN), resulting in ultrashort contact lengths. We find that charge transport in our devices occurs via two different mechanisms: at low temperatures (9 K), charges flow through single GNRs, resulting in quantum dot (QD) behavior with well-defined Coulomb diamonds (CDs), with addition energies in the range of 16 to 400 meV. For temperatures above 100 K, a combination of temperature-activated hopping and polaron-assisted tunneling takes over, with charges being able to flow through a network of 9-AGNRs across distances significantly exceeding the length of individual GNRs. At room temperature, our short-channel field-effect transistor devices exhibit on/off ratios as high as 3 × 105 with on-state current up to 50 nA at 0.2 V. Moreover, we find that the contact performance of our edge-contact devices is comparable to that of top/bottom contact geometries but with a significantly reduced footprint. Overall, our work demonstrates that 9-AGNRs can be contacted at their ends in ultra-short-channel FET devices while being encapsulated in h-BN
Engineered graphene Josephson junctions probed by quantum interference effects
After the introduction in Ch.1. the theoretical background of the investigated physical phenomena is given in Ch.2. In Ch.3 the fabrication of the nano structures is described and some of basic properties of graphene based Josephson junction with superconducting molybdenum-rhenium contacts are summarized. In the end of the chapter the applied methods for processing the data are explained. Afterwards the results of the supercurrent transport in superlattices of graphene and boron-nitride are summarized, which reveal the existence of van Hove singularities and satellite Dirac points in the band structure. In Ch.5 different realized approaches to create a helical quantum Hall state in graphene are described using the engineering of van der Waals heterostructures. The potential coupling of the helical states to superconductors was investigated. In the last experimental chapter, Ch.6, we focus on the magnetic field dependence of a double layer graphene heterostructure and show by current-phase relation measurements, that highly transparent superconducting modes exists within both graphene layers. In Ch.7 a summary of the thesis is given and possibilities of future experiments are sketched
Experimental demonstration of the suppression of optical phonon splitting in 2D materials by Raman spectroscopy
Raman spectroscopy is one of the most extended experimental techniques to investigate thin-layered 2D materials. For a complete understanding and modeling of the Raman spectrum of a novel 2D material, it is often necessary to combine the experimental investigation to density functional theory calculations. We provide the experimental proof of the fundamentally different behavior of polar 2D vs 3D systems regarding the effect of the dipole - dipole interactions, which in 2D systems ultimately lead to the absence of optical phonons splitting, otherwise present in 3D materials. We demonstrate that non-analytical corrections (NACs) should not be applied to properly model the Raman spectra of few-layered 2D materials, such as WSe2 and h-BN, corroborating recent theoretical predictions (Sohier et al 2017 Nano Lett. 17 3758-63). Our findings are supported by measurements performed on tilted samples that allow increasing the component of photon momenta in the plane of the flake, thus unambiguously setting the direction of an eventual NAC. We also investigate the influence of the parity of the number of layers and of the type of layer-by-layer stacking on the effect of NACs on the Raman spectra
Experimental demonstration of the suppression of optical phonon splitting in 2D materials by Raman spectroscopy
Raman spectroscopy is one of the most extended experimental techniques to investigate thin-layered 2D materials. For a complete understanding and modeling of the Raman spectrum of a novel 2D material, it is often necessary to combine the experimental investigation to density functional theory calculations. We provide the experimental proof of the fundamentally different behavior of polar 2D vs 3D systems regarding the effect of the dipole − dipole interactions, which in 2D systems ultimately lead to the absence of optical phonons splitting, otherwise present in 3D materials. We demonstrate that non-analytical corrections (NACs) should not be applied to properly model the Raman spectra of few-layered 2D materials, such as WSe2 and h-BN, corroborating recent theoretical predictions (Sohier et al 2017 Nano Lett. 17 3758–63). Our findings are supported by measurements performed on tilted samples that allow increasing the component of photon momenta in the plane of the flake, thus unambiguously setting the direction of an eventual NAC. We also investigate the influence of the parity of the number of layers and of the type of layer-by-layer stacking on the effect of NACs on the Raman spectra
New method of transport measurements on van der Waals heterostructures under pressure
The interlayer coupling, which has a strong influence on the properties of van der Waals heterostructures, strongly depends on the interlayer distance. Although considerable theoretical interest has been demonstrated, experiments exploiting a variable interlayer coupling on nanocircuits are scarce due to the experimental difficulties. Here, we demonstrate a novel method to tune the interlayer coupling using hydrostatic pressure by incorporating van der Waals heterostructure based nanocircuits in piston-cylinder hydrostatic pressure cells with a dedicated sample holder design. This technique opens the way to conduct transport measurements on nanodevices under pressure using up to 12 contacts without constraints on the sample at the fabrication level. Using transport measurements, we demonstrate that a hexagonal boron nitride capping layer provides a good protection of van der Waals heterostructures from the influence of the pressure medium, and we show experimental evidence of the influence of pressure on the interlayer coupling using weak localization measurements on a transitional metal dichalcogenide/graphene heterostructure