68 research outputs found
Standardization of surface potential measurements of graphene domains
We compare the three most commonly used scanning probe techniques to obtain a
reliable value of the work function in graphene domains of different thickness.
The surface potential (SP) of graphene is directly measured in Hall bar
geometry via a combination of electrical functional microscopy and spectroscopy
techniques, which enables calibrated work function measurements of graphene
domains with values ~4.55+/-0.02 eV and ~4.44+/-0.02eV for single- and
bi-layer, respectively. We demonstrate that frequency-modulated Kelvin probe
force microscopy (FM-KPFM) provides more accurate measurement of the SP than
amplitude-modulated (AM)-KPFM. The discrepancy between experimental results
obtained by different techniques is discussed. In addition, we use FM-KPFM for
contactless measurements of the specific components of the device resistance.
We show a strong non-Ohmic behavior of the electrode-graphene contact
resistance and extract the graphene channel resistivity
Private Colleges, State Aid, and the Establishment Clause
Using local scanning electrical techniques we study edge effects in side-gated Hall bar nanodevices made of epitaxial graphene. We demonstrate that lithographically defined edges of the graphene channel exhibit hole conduction within the narrow band of similar to 60-125 nm width, whereas the bulk of the material is electron doped. The effect is the most pronounced when the influence of atmospheric contamination is minimal. We also show that the electronic properties at the edges can be precisely tuned from hole to electron conduction by using moderate strength electrical fields created by side-gates. However, the central part of the channel remains relatively unaffected by the side-gates and retains the bulk properties of graphene.Funding Agencies|NMS under the IRD Graphene Project (NPL); EMRP</p
Quantifying dynamics and interactions of individual spurious low-energy fluctuators in superconducting circuits
Understanding the nature and dynamics of material defects in superconducting circuits is of paramount importance for improving qubit coherence and parameter stability and much needed for implementing large-scale quantum computing. Here we present measurements on individual highly coherent environmental two-level systems (TLS). We trace the spectral diffusion of specific TLS and demonstrate that it originates from the TLS coupling to a small number of low energy incoherent fluctuators. From the analysis of these fluctuations, we access the relevant parameters of low energy fluctuators: Dipole moments, switching energies, and, more importantly, interaction energies. Our approach opens up the possibility of deducing the macroscopic observables in amorphous glassy media from direct measurements of local fluctuator dynamics at the microscopic level- A route towards substantiating commonly accepted, but so far phenomenological, models for the decohering environment
A prototype of RK/200 quantum Hall array resistance standard on epitaxial graphene
Epitaxial graphene on silicon carbide is a promising material for the next generation of quantum Hall re-
sistance standards. Single Hall bars made of graphene have already surpassed their state-of-the-art GaAs
based counterparts as an RK/2 (RK = h/e^2) standard, showing at least the same precision and higher break-
down current density. Compared to single devices, quantum Hall arrays using parallel or series connection
of multiple Hall bars can offer resistance values spanning several orders of magnitude and (in case of parallel
connection) significantly larger measurement currents, but impose strict requirements on uniformity of the
material. To evaluate the quality of the available material, we have fabricated arrays of 100 Hall bars con-
nected in parallel on epitaxial graphene. One out of four devices has shown quantized resistance that matched
the correct value of RK/200 within the measurement precision of 1e-4 at magnetic fields between 7 and 9
Tesla. The defective behaviour of other arrays is attributed mainly to non-uniform doping. This result con-
firms the acceptable quality of epitaxial graphene, pointing towards the feasibility of well above 90% yield
of working Hall bars
Influence of impurity spin dynamics on quantum transport in epitaxial graphene
Experimental evidence from both spin-valve and quantum transport measurements
points towards unexpectedly fast spin relaxation in graphene. We report
magnetotransport studies of epitaxial graphene on SiC in a vector magnetic
field showing that spin relaxation, detected using weak-localisation analysis,
is suppressed by an in-plane magnetic field, , and thereby
proving that it is caused at least in part by spinful scatterers. A
non-monotonic dependence of effective decoherence rate on
reveals the intricate role of scatterers' spin dynamics in forming the
interference correction to conductivity, an effect that has gone unnoticed in
earlier weak localisation studie
Disorder induced Dirac-point physics in epitaxial graphene from temperature-dependent magneto-transport measurements
We report a study of disorder effects on epitaxial graphene in the vicinity
of the Dirac point by magneto-transport. Hall effect measurements show that the
carrier density increases quadratically with temperature, in good agreement
with theoretical predictions which take into account intrinsic thermal
excitation combined with electron-hole puddles induced by charged impurities.
We deduce disorder strengths in the range 10.2 31.2 meV, depending on
the sample treatment. We investigate the scattering mechanisms and estimate the
impurity density to be cm for our samples.
An asymmetry in the electron/hole scattering is observed and is consistent with
theoretical calculations for graphene on SiC substrates. We also show that the
minimum conductivity increases with increasing disorder potential, in good
agreement with quantum-mechanical numerical calculations.Comment: 6 pages, 3 figure
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