246 research outputs found
Electronic States in Silicon Quantum Dots: Multivalley Artificial Atoms
Electronic states in silicon quantum dots are examined theoretically, taking
into account a multivalley structure of the conduction band. We find that (i)
exchange interaction hardly works between electrons in different valleys. In
consequence electrons occupy the lowest level in different valleys in the
absence of Hund's coupling when the dot size is less than 10 nm. High-spin
states are easily realized by applying a small magnetic field. (ii) When the
dot size is much larger, the electron-electron interaction becomes relevant in
determining the electronic states. Electrons are accommodated in a valley,
making the highest spin, to gain the exchange energy. (iii) In the presence of
intervalley scattering, degenerate levels in different valleys are split. This
could result in low-spin states. These spin states in multivalley artificial
atoms can be observed by looking at the magnetic-field dependence of peak
positions in the Coulomb oscillation.Comment: 18 pages, 5 figure
Magnetically-induced reconstructions of the ground state in a few-electron Si quantum dot
We report unexpected fluctuations in the positions of Coulomb blockade peaks
at high magnetic fields in a small Si quantum dot. The fluctuations have a
distinctive saw-tooth pattern: as a function of magnetic field, linear shifts
of peak positions are compensated by abrupt jumps in the opposite direction.
The linear shifts have large slopes, suggesting formation of the ground state
with a non-zero angular momentum. The value of the momentum is found to be well
defined, despite the absence of the rotational symmetry in the dot.Comment: 5 pages, 4 figures, accepted to PR
Double-dot charge transport in Si single electron/hole transistors
We studied transport through ultra-small Si quantum dot transistors
fabricated from silicon-on-insulator wafers. At high temperatures, 4K<T<100K,
the devices show single-electron or single-hole transport through the
lithographically defined dot. At T<4K, current through the devices is
characterized by multidot transport. From the analysis of the transport in
samples with double-dot characteristics, we conclude that extra dots are formed
inside the thermally grown gate oxide which surrounds the lithographically
defined dot.Comment: 4 pages, 5 figures, to appear in Appl. Phys. Let
Highly skewed current-phase relation in superconductor-topological insulator-superconductor Josephson junctions
Three-dimensional topological insulators (TI's) in proximity with
superconductors are expected to exhibit exotic phenomena such as topological
superconductivity (TSC) and Majorana bound states (MBS), which may have
applications in topological quantum computation. In
superconductor-TI-superconductor Josephson junctions, the supercurrent versus
the phase difference between the superconductors, referred to as the
current-phase relation (CPR), reveals important information including the
nature of the superconducting transport. Here, we study the induced
superconductivity in gate-tunable Josephson junctions (JJs) made from
topological insulator BiSbTeSe2 with superconducting Nb electrodes. We observe
highly skewed (non-sinusoidal) CPR in these junctions. The critical current, or
the magnitude of the CPR, increases with decreasing temperature down to the
lowest accessible temperature (T ~ 20 mK), revealing the existence of
low-energy modes in our junctions. The gate dependence shows that close to the
Dirac point the CPR becomes less skewed, indicating the transport is more
diffusive, most likely due to the presence of electron/hole puddles and charge
inhomogeneity. Our experiments provide strong evidence that superconductivity
is induced in the highly ballistic topological surface states (TSS) in our
gate-tunable TI- based JJs. Furthermore, the measured CPR is in good agreement
with the prediction of a model which calculates the phase dependent eigenstate
energies in our system, considering the finite width of the electrodes as well
as the TSS wave functions extending over the entire circumference of the TI
- …