276 research outputs found
Electrical control over single hole spins in nanowire quantum dots
Single electron spins in semiconductor quantum dots (QDs) are a versatile
platform for quantum information processing, however controlling decoherence
remains a considerable challenge. Recently, hole spins have emerged as a
promising alternative. Holes in III-V semiconductors have unique properties,
such as strong spin-orbit interaction and weak coupling to nuclear spins, and
therefore have potential for enhanced spin control and longer coherence times.
Weaker hyperfine interaction has already been reported in self-assembled
quantum dots using quantum optics techniques. However, challenging fabrication
has so far kept the promise of hole-spin-based electronic devices out of reach
in conventional III-V heterostructures. Here, we report gate-tuneable hole
quantum dots formed in InSb nanowires. Using these devices we demonstrate Pauli
spin blockade and electrical control of single hole spins. The devices are
fully tuneable between hole and electron QDs, enabling direct comparison
between the hyperfine interaction strengths, g-factors and spin blockade
anisotropies in the two regimes
Quasiparticle Interference on the Surface of Topological Crystalline Insulator Pb(1-x)Sn(x)Se
Topological crystalline insulators represent a novel topological phase of
matter in which the surface states are protected by discrete point
group-symmetries of the underlying lattice. Rock-salt lead-tin-selenide alloy
is one possible realization of this phase which undergoes a topological phase
transition upon changing the lead content. We used scanning tunneling
microscopy (STM) and angle resolved photoemission spectroscopy (ARPES) to probe
the surface states on (001) PbSnSe in the topologically
non-trivial (x=0.23) and topologically trivial (x=0) phases. We observed
quasiparticle interference with STM on the surface of the topological
crystalline insulator and demonstrated that the measured interference can be
understood from ARPES studies and a simple band structure model. Furthermore,
our findings support the fact that PbSnSe and PbSe have
different topological nature.Comment: 5 pages, 4 figure
Valley-spin blockade and spin resonance in carbon nanotubes
Manipulation and readout of spin qubits in quantum dots made in III-V
materials successfully rely on Pauli blockade that forbids transitions between
spin-triplet and spin-singlet states. Quantum dots in group IV materials have
the advantage of avoiding decoherence from the hyperfine interaction by
purifying them with only zero-spin nuclei. Complications of group IV materials
arise from the valley degeneracies in the electronic bandstructure. These lead
to complicated multiplet states even for two-electron quantum dots thereby
significantly weakening the selection rules for Pauli blockade. Only recently
have spin qubits been realized in silicon devices where the valley degeneracy
is lifted by strain and spatial confinement. In carbon nanotubes Pauli blockade
can be observed by lifting valley degeneracy through disorder. In clean
nanotubes, quantum dots have to be made ultra-small to obtain a large energy
difference between the relevant multiplet states. Here we report on
low-disorder nanotubes and demonstrate Pauli blockade based on both valley and
spin selection rules. We exploit the bandgap of the nanotube to obtain a large
level spacing and thereby a robust blockade. Single-electron spin resonance is
detected using the blockade.Comment: 31 pages including supplementary informatio
Disentangling the effects of spin-orbit and hyperfine interactions on spin blockade
We have achieved the few-electron regime in InAs nanowire double quantum
dots. Spin blockade is observed for the first two half-filled orbitals, where
the transport cycle is interrupted by forbidden transitions between triplet and
singlet states. Partial lifting of spin blockade is explained by spin-orbit and
hyperfine mechanisms that enable triplet to singlet transitions. The
measurements over a wide range of interdot coupling and tunneling rates to the
leads are well reproduced by a simple transport model. This allows us to
separate and quantify the contributions of the spin-orbit and hyperfine
interactions.Comment: 5 pages, 4 figure
Suppression of Zeeman gradients by nuclear polarization in double quantum dots
We use electric dipole spin resonance to measure dynamic nuclear polarization
in InAs nanowire quantum dots. The resonance shifts in frequency when the
system transitions between metastable high and low current states, indicating
the presence of nuclear polarization. We propose that the low and the high
current states correspond to different total Zeeman energy gradients between
the two quantum dots. In the low current state, dynamic nuclear polarization
efficiently compensates the Zeeman gradient due to the -factor mismatch,
resulting in a suppressed total Zeeman gradient. We present a theoretical model
of electron-nuclear feedback that demonstrates a fixed point in nuclear
polarization for nearly equal Zeeman splittings in the two dots and predicts a
narrowed hyperfine gradient distribution
Termination dependent topological surface states of the natural superlattice phase BiSe
We describe the topological surface states of BiSe, a compound in the
infinitely adaptive Bi-BiSe natural superlattice phase series,
determined by a combination of experimental and theoretical methods. Two
observable cleavage surfaces, terminating at Bi or Se, are characterized by
angle resolved photoelectron spectroscopy and scanning tunneling microscopy,
and modeled by ab-initio density functional theory calculations. Topological
surface states are observed on both surfaces, but with markedly different
dispersions and Kramers point energies. BiSe therefore represents the
only known compound with different topological states on differently terminated
surfaces.Comment: 5 figures references added Published in PRB:
http://link.aps.org/doi/10.1103/PhysRevB.88.08110
Electric and Magnetic Tuning Between the Trivial and Topological Phases in InAs/GaSb Double Quantum Wells
Among the theoretically predicted two-dimensional topological insulators,
InAs/GaSb double quantum wells (DQWs) have a unique double-layered structure
with electron and hole gases separated in two layers, which enables tuning of
the band alignment via electric and magnetic fields. However, the rich
trivial-topological phase diagram has yet to be experimentally explored. We
present an in situ and continuous tuning between the trivial and topological
insulating phases in InAs/GaSb DQWs through electrical dual-gating.
Furthermore, we show that an in-plane magnetic field shifts the electron and
hole bands relatively to each other in momentum space, functioning as a
powerful tool to discriminate between the topologically distinct states
Spectroscopy of spin-orbit quantum bits in indium antimonide nanowires
Double quantum dot in the few-electron regime is achieved using local gating
in an InSb nanowire. The spectrum of two-electron eigenstates is investigated
using electric dipole spin resonance. Singlet-triplet level repulsion caused by
spin-orbit interaction is observed. The size and the anisotropy of
singlet-triplet repulsion are used to determine the magnitude and the
orientation of the spin-orbit effective field in an InSb nanowire double dot.
The obtained results are confirmed using spin blockade leakage current
anisotropy and transport spectroscopy of individual quantum dots.Comment: 5 pages, supplementary material available at
http://link.aps.org/supplemental/10.1103/PhysRevLett.108.16680
Chaotic spin-dependent electron dynamics in a field-driven double dot potential
We study the nonlinear classical dynamics of an electron confined in a double
dot potential and subjected to a spin-orbit coupling and a constant external
magnetic field. It is shown that due to the spin orbit coupling, the energy can
be transferred from the spin to the orbital motion. This naturally heats up the
orbital motion which, due to the presence of the separatrix line in the phase
space of the system, results in a motion of the electron between the dots. It
is shown that depending on the strength of the spin orbit coupling and the
energy of the system, the electronic orbital motion undergoes a transition from
the regular to the chaotic regime.Comment: 15 pages, 5 figure
Nanoscale spin rectifiers controlled by the Stark effect
The control of orbital and spin state of single electrons is a key ingredient
for quantum information processing, novel detection schemes, and, more
generally, is of much relevance for spintronics. Coulomb and spin blockade (SB)
in double quantum dots (DQDs) enable advanced single-spin operations that would
be available even for room-temperature applications for sufficiently small
devices. To date, however, spin operations in DQDs were observed at sub-Kelvin
temperatures, a key reason being that scaling a DQD system while retaining an
independent field-effect control on the individual dots is very challenging.
Here we show that quantum-confined Stark effect allows an independent
addressing of two dots only 5 nm apart with no need for aligned nanometer-size
local gating. We thus demonstrate a scalable method to fully control a DQD
device, regardless of its physical size. In the present implementation we show
InAs/InP nanowire (NW) DQDs that display an experimentally detectable SB up to
10 K. We also report and discuss an unexpected re-entrant SB lifting as a
function magnetic-field intensity
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