1,152 research outputs found
Spin-valley blockade in carbon nanotube double quantum dots
We present a theoretical study of the Pauli or spin-valley blockade for
double quantum dots in semiconducting carbon nanotubes. In our model we take
into account the following characteristic features of carbon nanotubes: (i)
fourfold (spin and valley) degeneracy of the quantum dot levels, (ii) the
intrinsic spin-orbit interaction which is enhanced by the tube curvature, and
(iii) valley-mixing due to short-range disorder, i.e., substitutional atoms,
adatoms, etc. We find that the spin-valley blockade can be lifted in the
presence of short-range disorder, which induces two independent random (in
magnitude and direction) valley-Zeeman-fields in the two dots, and hence acts
similarly to hyperfine interaction in conventional semiconductor quantum dots.
In the case of strong spin-orbit interaction, we identify a parameter regime
where the current as the function of an applied axial magnetic field shows a
zero-field dip with a width controlled by the interdot tunneling amplitude, in
agreement with recent experiments.Comment: 15 pages, 6 figures, 2 tables; v2: published versio
Variational wave functions for homogenous Bose systems
We study variational wave functions of the product form, factorizing
according to the wave vectors k, for the ground state of a system of bosons
interacting via positive pair interactions with a positive Fourier transform.
Our trial functions are members of different orthonormal bases in Fock space.
Each basis contains a quasiparticle vacuum state and states with an arbitrary
finite number of quasiparticles. One of the bases is that of Valatin and Butler
(VB), introduced fifty years ago and parametrized by an infinite set of
variables determining Bogoliubov's canonical transformation for each k. In
another case, inspired by Nozi\`eres and Saint James the canonical
transformation for k=0 is replaced by a shift in the creation/annihilation
operators. For the VB basis we prove that the lowest energy is obtained in a
state with ~sqrt{volume} quasiparticles in the zero mode. The number of k=0
physical particles is of the order of the volume and its fluctuation is
anomalously large, resulting in an excess energy. The same fluctuation is
normal in the second type of optimized bases, the minimum energy is smaller and
is attained in a vacuum state. Associated quasiparticle theories and questions
about the gap in their spectrum are also discussed
Spin-orbit-induced strong coupling of a single spin to a nanomechanical resonator
We theoretically investigate the deflection-induced coupling of an electron
spin to vibrational motion due to spin-orbit coupling in suspended carbon
nanotube quantum dots. Our estimates indicate that, with current capabilities,
a quantum dot with an odd number of electrons can serve as a realization of the
Jaynes-Cummings model of quantum electrodynamics in the strong-coupling regime.
A quantized flexural mode of the suspended tube plays the role of the optical
mode and we identify two distinct two-level subspaces, at small and large
magnetic field, which can be used as qubits in this setup. The strong intrinsic
spin-mechanical coupling allows for detection, as well as manipulation of the
spin qubit, and may yield enhanced performance of nanotubes in sensing
applications.Comment: 5 pages, 3 figures + appendix; published versio
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