193 research outputs found
Excited state spectroscopy in carbon nanotube double quantum dots
We report on low temperature measurements in a fully tunable carbon nanotube
double quantum dot. A new fabrication technique has been used for the top-gates
in order to avoid covering the whole nanotube with an oxide layer as in
previous experiments. The top-gates allow us to form single dots, control the
coupling between them and we observe four-fold shell filling. We perform
inelastic transport spectroscopy via the excited states in the double quantum
dot, a necessary step towards the implementation of new microwave-based
experiments.Comment: 16 pages, 6 figures, submitted to nanoletter
Electronic excitation spectrum of metallic carbon nanotubes
We have studied the discrete electronic spectrum of closed metallic nanotube
quantum dots. At low temperatures, the stability diagrams show a very regular
four-fold pattern that allows for the determination of the electron addition
and excitation energies. The measured nanotube spectra are in excellent
agreement with theoretical predictions based on the nanotube band structure.
Our results permit the complete identification of the electron quantum states
in nanotube quantum dots.Comment: 4 pages, 3 figure
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
Electronic Transport Spectroscopy of Carbon Nanotubes in a Magnetic Field
We report magnetic field spectroscopy measurements in carbon nanotube quantum
dots exhibiting four-fold shell structure in the energy level spectrum. The
magnetic field induces a large splitting between the two orbital states of each
shell, demonstrating their opposite magnetic moment and determining transitions
in the spin and orbital configuration of the quantum dot ground state. We use
inelastic cotunneling spectroscopy to accurately resolve the spin and orbital
contributions to the magnetic moment. A small coupling is found between
orbitals with opposite magnetic moment leading to anticrossing behavior at zero
field.Comment: 7 pages, 4 figure
Ferromagnetism in thin-film Cr-doped topological insulator Bi2Se3
We report on the observation of ferromagnetism in epitaxial thin films of the topological insulator compound Bi2Se3 with chromium doping. The structural, magnetic, and magnetoelectrical properties of Bi2Se3 were investigated for Cr concentrations up to 10%. For a Cr content up to similar to 5% the films are of good crystalline quality, with the lattice parameter a decreasing and the lattice parameter c increasing with increasing Cr concentration. The Curie temperature reached a maximum T-C=20K for 5.2% Cr. Well-defined ferromagnetic hysteresis in the magnetization and in the magnetoresistance was also observed in these films. (C) 2012 American Institute of Physics. [doi:10.1063/1.3688043
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
- …