499 research outputs found
Josephson current through a single Anderson impurity coupled to BCS leads
We investigate the Josephson current J(\phi) through a quantum dot embedded
between two superconductors showing a phase difference \phi. The system is
modeled as a single Anderson impurity coupled to BCS leads, and the functional
and the numerical renormalization group frameworks are employed to treat the
local Coulomb interaction U. We reestablish the picture of a quantum phase
transition occurring if the ratio between the Kondo temperature T_K and the
superconducting energy gap \Delta or, at appropriate T_K/\Delta, the phase
difference \phi or the impurity energy is varied. We present accurate zero- as
well as finite-temperature T data for the current itself, thereby settling a
dispute raised about its magnitude. For small to intermediate U and at T=0 the
truncated functional renormalization group is demonstrated to produce reliable
results without the need to implement demanding numerics. It thus provides a
tool to extract characteristics from experimental current-voltage measurements.Comment: version accepted for publication in PR
Real Time Electron Tunneling and Pulse Spectroscopy in Carbon Nanotube Quantum Dots
We investigate a Quantum Dot (QD) in a Carbon Nanotube (CNT) in the regime
where the QD is nearly isolated from the leads. An aluminum single electron
transistor (SET) serves as a charge detector for the QD. We precisely measure
and tune the tunnel rates into the QD in the range between 1 kHz and 1 Hz,
using both pulse spectroscopy and real - time charge detection and measure the
excitation spectrum of the isolated QD.Comment: 12 pages, 5 figure
Electric Field Control of Soliton Motion and Stacking in Trilayer Graphene
The crystal structure of a material plays an important role in determining
its electronic properties. Changing from one crystal structure to another
involves a phase transition which is usually controlled by a state variable
such as temperature or pressure. In the case of trilayer graphene, there are
two common stacking configurations (Bernal and rhombohedral) which exhibit very
different electronic properties. In graphene flakes with both stacking
configurations, the region between them consists of a localized strain soliton
where the carbon atoms of one graphene layer shift by the carbon-carbon bond
distance. Here we show the ability to move this strain soliton with a
perpendicular electric field and hence control the stacking configuration of
trilayer graphene with only an external voltage. Moreover, we find that the
free energy difference between the two stacking configurations scales
quadratically with electric field, and thus rhombohedral stacking is favored as
the electric field increases. This ability to control the stacking order in
graphene opens the way to novel devices which combine structural and electrical
properties
Band Structure Mapping of Bilayer Graphene via Quasiparticle Scattering
A perpendicular electric field breaks the layer symmetry of Bernal-stacked
bilayer graphene, resulting in the opening of a band gap and a modification of
the effective mass of the charge carriers. Using scanning tunneling microscopy
and spectroscopy, we examine standing waves in the local density of states of
bilayer graphene formed by scattering from a bilayer/trilayer boundary. The
quasiparticle interference properties are controlled by the bilayer graphene
band structure, allowing a direct local probe of the evolution of the band
structure of bilayer graphene as a function of electric field. We extract the
Slonczewski-Weiss-McClure model tight binding parameters as
eV, eV, and eV.Comment: 12 pages, 4 figure
Coupling carbon nanotube mechanics to a superconducting circuit
The quantum behaviour of mechanical resonators is a new and emerging field
driven by recent experiments reaching the quantum ground state. The high
frequency, small mass, and large quality-factor of carbon nanotube resonators
make them attractive for quantum nanomechanical applications. A common element
in experiments achieving the resonator ground state is a second quantum system,
such as coherent photons or superconducting device, coupled to the resonators
motion. For nanotubes, however, this is a challenge due to their small size.
Here, we couple a carbon nanoelectromechanical (NEMS) device to a
superconducting circuit. Suspended carbon nanotubes act as both superconducting
junctions and moving elements in a Superconducting Quantum Interference Device
(SQUID). We observe a strong modulation of the flux through the SQUID from
displacements of the nanotube. Incorporating this SQUID into superconducting
resonators and qubits should enable the detection and manipulation of nanotube
mechanical quantum states at the single-phonon level
Performance of Dual-Purpose Cows on a Native Pasture-\u3ci\u3eArachis pintoi\u3c/i\u3e Association in the Humid Tropics of Mexico
Native grasslands (NG) are the main feed supply of dual-purpose (DP) cows of the Mexican humid tropics. NG comprise about 85% of Paspalum, Axonopus and Cynodon species, about 5% of native legumes, mainly of Desmodium, and the remaining 10% are narrow and broad leafed weeds. Arachis pintoi (AP) is a persistent grazing tolerant tropical legume. In association with sown grasses, it has improved dry matter (DM) yield, nutritive quality of forage, and milk yield up to 9 kg/cow/day (González et al., 1996). The objective was to determine if productive performance of DP cows could be improved by the introduction of AP into NG grassland
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