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1. Nitrogen-doped graphene sheets grown by chemical vapor deposition A significant advance toward achieving practical
applications of graphene as a two-dimensional material in nanoelectronics
would be provided by successful synthesis of both n-type and p-type doped
graphene. However, reliable doping and a thorough understanding of carrier
transport in the presence of charged impurities governed by ionized donors or
acceptors in the graphene lattice are still lacking. Here we report
experimental realization of few-layer nitrogen-doped (N-doped) graphene
sheets by chemical vapor deposition of organic molecule 1,3,5-triazine on Cu
metal catalyst. When reducing the growth temperature, the atomic percentage
of nitrogen doping is raised from 2.1% to 5.6%. With increasing doping
concentration, N-doped graphene sheet exhibits a crossover from p-type to
n-type behavior accompanied by a strong enhancement of electron-hole
transport asymmetry, manifesting the influence of incorporated nitrogen
impurities. In addition, by analyzing the data of X-ray photoelectron
spectroscopy, Raman spectroscopy, and electrical measurements, we show that
pyridinic and pyrrolic N impurities play an important role in determining the
transport behavior of carriers in our N-doped graphene sheets. [Please see Y.-F.
Lu et al. ACS Nano 7, 6522 (2013) for details.] 2. Controllable disorder in a hybrid nanoelectronic system: realization of a superconducting diode We have studied a hybrid nanoelectronic
system which consists of an AlGaAs/GaAs
two-dimensional electron gas (2DEG) in close proximity (~70 nm) to an Al
superconducting nanofilm. By tuning the current through the Al film, we can
change the conductance of the 2DEG and furthermore vary the effective
disorder in the Al superconducting film in a controllable way. When a high
current is injected into the film, screening which couples the Al film and
the 2DEG results in a collapse of anti-symmetric behavior in the
current-voltage characteristics, V(I) ∼ -V(-I), which holds true in a conventional
superconductor. Our results may open a new avenue of experimentally realizing
a superconducting diode. [Please see S.-T. Lo et
al. Sci. Rep. 3, 2274 (2013) for details.] 3. Fractional quantum Hall effect in a high Landau level in bilayer graphene The fractional quantum Hall effect is a canonical
example of electron-electron interactions producing new ground states in
many-body systems. Most fractional quantum Hall studies have focussed on the lowest Landau level, whose fractional
states are successfully explained by the composite fermion model. In the
widely studied GaAs-based system, the composite fermion picture is thought to
become unstable for the N≥2 Landau level, where competing many-body phases
have been observed. Here we report magneto-resistance measurements of
fractional quantum Hall states in the N=2 Landau level (filling factors
4<|v|<8) in bilayer graphene. In contrast with recent observations of
particle-hole asymmetry in the N=0/N=1 Landau levels of bilayer graphene, the
fractional quantum Hall states we observe in the N=2 Landau level obey
particle-hole symmetry within the fully symmetry-broken Landau level.
Possible alternative ground states other than the composite fermions are
discussed. [Please see G. Diankov et al., Nat. Commun. 7, 13908 (2016) for
details.] 4. Temperature dependence of electron density and electron–electron interactions in graphene We report carrier density measurements and
electron-electron (e-e) interactions in monolayer epitaxial graphene grown on
SiC. The temperature (T)-independent carrier
density determined from the Shubnikov-de Haas (SdH) oscillations clearly demonstrates that the observed
logarithmic temperature dependence of the Hall slope in our system must be
due to e-e interactions. Since the electron density determined from
conventional SdH measurements does not depend on
e-e interactions based on Kohn's theorem, SdH
experiments appear to be more reliable compared with the classical Hall
effect when one studies the T dependence of the carrier density in the
low T regime. On the other hand, the logarithmic T dependence of the
Hall slope δRxy/δB
can be used to probe e-e interactions even when the conventional conductivity
method is not applicable due to strong electron-phonon scattering. [Please
see C. W. Liu et al., 2D Mater. 4, 025007 (2017) for
details.] 5. Non-saturating magnetoresistance in graphene We report large, non-saturating magnetoresistance (MR)
of ~140% in single layer chemical vapor deposition (CVD) graphene with an
h-BN capping layer at room temperature at B = 9 T. Based on the
classical model developed by Parish and Littlewood, our results show that the
MR is proportional to the average mobility <μ> and decreases
with increasing temperature. In contrast, in a large-area, extremely
homogenous single layer epitaxial graphene (EG) device, the MR is saturating
and is inversely proportional to <μ>, which is consistent with
the finite resistance network picture. By comparing the results obtained from
CVD graphene with an h-BN capping layer with those from the EG device, we
show that the non-saturating linear characteristics come from multi-channel
current paths in a two-dimensional plane due to the intrinsic grain
boundaries and domains of CVD graphene by capping an h-BN layer that increase
the <μ> of CVD graphene. Our results on CVD graphene with an
h-BN capping layer pave the way for industrial schemes of graphene-based and
air-stable magnetic field sensors with a linear, large response at room
temperature. [Please see C. Chuang et al., Carbon 136, 211 (2018) for details.] 6. Disorder-Induced 2D Superconductivity in a NbTiN Film Grown on Si We report on the growth and characterization of a
niobium titanium nitride (NbTiN) film on a Si
substrate prepared by ultrahigh vacuum sputtering. We show that the
superconducting transition temperature is lower than those of high-quality NbTiN films. Interestingly, even though the
zero-temperature Ginzburg-Landau coherence length (=9.77 nm) is significantly
shorter than the film thickness (=86 nm), we are still able to observe the Berezinskii-Kosterlitz-
Oxford He3/He4 Dilution Cryostat System Top-loading He3 System
Probe & TO-five MOTIC Microscope
Turbo Pumping Station 15 T superconductor magnet
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