454 research outputs found
Anomalous thermoelectric transport of Dirac particles in graphene
We report a thermoelectric study of graphene in both zero and applied
magnetic fields. As a direct consequence of the linear dispersion of massless
particles, we find that the Seebeck coefficient Sxx diverges with 1 /, where
n2D is the carrier density. We observe a very large Nernst signal Sxy (~ 50
uV/K at 8 T) at the Dirac point, and an oscillatory dependence of both Sxx and
Sxy on n2D at low temperatures. Our results underscore the anomalous
thermoelectric transport in graphene, which may be used as a highly sensitive
probe for impurity bands near the Dirac point
Gate Tunable Dissipation and "Superconductor-Insulator" Transition in Carbon Nanotube Josephson Transistors
Dissipation is ubiquitous in quantum systems, and its interplay with
fluctuations is critical to maintaining quantum coherence. We experimentally
investigate the dissipation dynamics in single-walled carbon nanotubes coupled
to superconductors. The voltage-current characteristics display gate-tunable
hysteresis, with sizes that perfectly correlate with the normal state
resistance RN, indicating the junction undergoes a periodic modulation between
underdamped and overdamped regimes. Surprisingly, when a device's Fermi-level
is tuned through a local conductance minimum, we observe a gate-controlled
transition from superconducting-like to insulating-like states, with a
"critical" R_N value of about 8-20 kohm.Comment: Figures revised to improve clarity. Accepted for publication by
Physical Review Letter
Quantum Transport and Field Induced Insulating States in Bilayer Graphene pnp Junctions
We perform transport measurements in high quality bilayer graphene pnp
junctions with suspended top gates. At a magnetic field B=0, we demonstrate
band gap opening by an applied perpendicular electric field, with an On/Off
ratio up to 20,000 at 260mK. Within the band gap, the conductance decreases
exponentially by 3 orders of magnitude with increasing electric field, and can
be accounted for by variable range hopping with a gate-tunable density of
states, effective mass, and localization length. At large B, we observe quantum
Hall conductance with fractional values, which arise from equilibration of edge
states between differentially-doped regions, and the presence of an insulating
state at filling factor {\nu}=0. Our work underscores the importance of bilayer
graphene for both fundamental interest and technological applications.Comment: 4 figures, to appear in Nano Lett. Minor typos correcte
Electrical Transport in High Quality Graphene pnp Junctions
We fabricate and investigate high quality graphene devices with contactless,
suspended top gates, and demonstrate formation of graphene pnp junctions with
tunable polarity and doping levels. The device resistance displays distinct
oscillations in the npn regime, arising from the Fabry-Perot interference of
holes between the two pn interfaces. At high magnetic fields, we observe
well-defined quantum Hall plateaus, which can be satisfactorily fit to
theoretical calculations based on the aspect ratio of the device.Comment: to appear in a special focus issue in New Journal of Physic
Anomalous thermoelectric transport of Dirac particles in graphene
We report a thermoelectric study of graphene in both zero and applied
magnetic fields. As a direct consequence of the linear dispersion of massless
particles, we find that the Seebeck coefficient Sxx diverges with 1 /, where
n2D is the carrier density. We observe a very large Nernst signal Sxy (~ 50
uV/K at 8 T) at the Dirac point, and an oscillatory dependence of both Sxx and
Sxy on n2D at low temperatures. Our results underscore the anomalous
thermoelectric transport in graphene, which may be used as a highly sensitive
probe for impurity bands near the Dirac point
- …