39 research outputs found
Remarks on the tight-binding model of graphene
We address a simple but fundamental issue arising in the study of graphene,
as well as of other systems that have a crystalline structure with more than
one atom per unit cell. For these systems, the choice of the tight-binding
basis is not unique. For monolayer graphene two bases are widely used in the
literature. While the expectation values of operators describing physical
quantities should be independent of basis, the form of the operators may depend
on the basis, especially in the presence of disorder or of an applied magnetic
field. Using the inappropriate form of certain operators may lead to erroneous
physical predictions. We discuss the two bases used to describe monolayer
graphene, as well as the form of the most commonly used operators in the two
bases. We repeat our analysis for the case of bilayer graphene.Comment: 15 pages, 4 figure
Tunable Nb Superconducting Resonator Based on a Constriction Nano- SQUID Fabricated with a Ne Focused Ion Beam
Hybrid superconducting-spin systems offer the potential to combine highly coherent atomic quantum systems with the scalability of superconducting circuits. To fully exploit this potential requires a high-quality-factor microwave resonator, tunable in frequency and able to operate at magnetic fields optimal for the spin system. Such magnetic fields typically rule out conventional
Al
-based Josephson-junction devices that have previously been used for tunable high-
Q
microwave resonators. The larger critical field of
Nb
allows microwave resonators with large field resilience to be fabricated. Here we demonstrate how constriction-type weak links, patterned in parallel into the central conductor of a
Nb
coplanar resonator with a neon focused ion beam, can be used to implement a frequency-tunable resonator. We study transmission through two such devices and show how they realize high-quality-factor, tunable, field-resilient devices that hold promise for future applications coupling to spin system
Strain-gradient position mapping of semiconductor quantum dots
COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESWe introduce a nondestructive method to determine the position of randomly distributed semiconductor quantum dots (QDs) integrated in a solid photonic structure. By setting the structure in an oscillating motion, we generate a large stress gradient across the QDs plane. We then exploit the fact that the QDs emission frequency is highly sensitive to the local material stress to map the position of QDs deeply embedded in a photonic wire antenna with an accuracy ranging from +/- 35 nm down to +/- 1 nm. In the context of fast developing quantum technologies, this technique can be generalized to different photonic nanostructures embedding any stress-sensitive quantum emitters.1181116COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES88887.059630/2014-00The authors wish to thank E. Gautier for the FIB cut and images. Sample fabrication was carried out in the Upstream Nanofabrication Facility (PTA) and CEA LETI MINATEC/DOPT clean rooms. P.-L. de Assis was financially supported by Agence Nationale de la Recherche (Project No. ANR-11-BS10-011) and CAPES Young Talents Fellowship Grant No. 88887.059630/2014-00, and D. Tumanov by a doctoral scholarship from the Rhône-Alpes Region
Dispersively detected Pauli Spin-Blockade in a Silicon Nanowire Field-Effect Transistor
We report the dispersive readout of the spin state of a double quantum dot
formed at the corner states of a silicon nanowire field-effect transistor. Two
face-to-face top-gate electrodes allow us to independently tune the charge
occupation of the quantum dot system down to the few-electron limit. We measure
the charge stability of the double quantum dot in DC transport as well as
dispersively via in-situ gate-based radio frequency reflectometry, where one
top-gate electrode is connected to a resonator. The latter removes the need for
external charge sensors in quantum computing architectures and provides a
compact way to readout the dispersive shift caused by changes in the quantum
capacitance during interdot charge transitions. Here, we observe Pauli
spin-blockade in the high-frequency response of the circuit at finite magnetic
fields between singlet and triplet states. The blockade is lifted at higher
magnetic fields when intra-dot triplet states become the ground state
configuration. A lineshape analysis of the dispersive phase shift reveals
furthermore an intradot valley-orbit splitting of 145 eV.
Our results open up the possibility to operate compact CMOS technology as a
singlet-triplet qubit and make split-gate silicon nanowire architectures an
ideal candidate for the study of spin dynamics
Superconducting Nanocircuits for Topologically Protected Qubits
For successful realization of a quantum computer, its building blocks
(qubits) should be simultaneously scalable and sufficiently protected from
environmental noise. Recently, a novel approach to the protection of
superconducting qubits has been proposed. The idea is to prevent errors at the
"hardware" level, by building a fault-free (topologically protected) logical
qubit from "faulty" physical qubits with properly engineered interactions
between them. It has been predicted that the decoupling of a protected logical
qubit from local noises would grow exponentially with the number of physical
qubits. Here we report on the proof-of-concept experiments with a prototype
device which consists of twelve physical qubits made of nanoscale Josephson
junctions. We observed that due to properly tuned quantum fluctuations, this
qubit is protected against magnetic flux variations well beyond linear order,
in agreement with theoretical predictions. These results demonstrate the
feasibility of topologically protected superconducting qubits.Comment: 25 pages, 5 figure
Bound states at impurities in a two-dimensional metal
We present an experimental study of the local electronic
properties of atomic-size impurities in an \chem{ErSi_{2}} layer grown
on the Si(111) surface, which behaves as a quasi–two-dimensional
(2D) metal. From scanning tunneling spectroscopy measurements at \un{K}, we show that the silicide 2D state probed on defect-free
areas becomes localized at impurities. For two different silicide
point-defects, a resonance is found in the conductance spectra, at
voltages above the 2D state band edge. This resonance is
interpreted as a bound state within a repulsive impurity potential
model. Spectroscopy on additional extra-impurities obtained after
Co deposition is also reported
Confinement of Bloch waves in
Two-dimensional \chem{YSi_2} or \chem{ErSi_2} layers on
\chem{Si(111)} surface present two surface states in the vicinity
of the Fermi level and form a two-dimensional surface electron
gas. We have performed density functional theory (DFT)
calculations of a realistic one-dimensional nanostructure of
\chem{YSi_2} on \chem{Si(111)} to study confinement effects of
this electron gas. The calculated square modulus of the wave
function shows complex modulations related to the quantum
interference patterns observed by scanning tunneling microscopy
(STM). For each quantised state, the modulation involves at least
three components consistent with the scattering of Bloch waves. A
Fourier analysis of the real space modulations is used to
construct the surface states dispersion curves. They are compared
to the direct calculation of the ideal \chem{YSi_2/Si(111)}
surface electronic structure and to the curves deduced
from conductance images in STM experiments
Strain-Gradient Position Mapping of Semiconductor Quantum Dots
International audienceIn the context of fast developing quantum technologies, locating single quantum objects embedded in solid or fluid environment while keeping their properties unchanged is a crucial requirement as well as a challenge. Such ``quantum microscopes'' have been demonstrated already for NV-centers embedded in diamond [1], and for single atoms within an ultracold gas [2]. In this work, we demonstrate a new method to determine non-destructively the position of randomly distributed semiconductor quantum dots (QDs) deeply embedded in a solid photonic waveguide. By setting the wire in an oscillating motion, we generate large stress gradients across the QDs plane. We then exploit the fact that the QDs emission frequency is highly sensitive to the local material stress [3-5] to infer their positions with an accuracy ranging from +/- 35 nm down to +/-1 nm for close-to-axis QDs