551 research outputs found
On-chip SQUID measurements in the presence of high magnetic fields
We report a low temperature measurement technique and magnetization data of a
quantum molecular spin, by implementing an on-chip SQUID technique. This
technique enables the SQUID magnetometery in high magnetic fields, up to 7
Tesla. The main challenges and the calibration process are detailed. The
measurement protocol is used to observe quantum tunneling jumps of the S=10
molecular magnet, Mn12-tBuAc. The effect of transverse field on the tunneling
splitting for this molecular system is addressed as well.Comment: 7 pages, 3 figure
Landau-Zener tunneling of a single Tb3+ magnetic moment allowing the electronic read-out of a nuclear spin
A multi-terminal device based on a carbon nanotube quantum dot was used at
very low tem- perature to probe a single electronic and nuclear spin embedded
in a bis-phthalocyanine Terbium (III) complex (TbPc2). A spin-valve signature
with large conductance jumps was found when two molecules were strongly coupled
to the nanotube. The application of a transverse field separated the magnetic
signal of both molecules and enabled single-shot read-out of the Terbium
nuclear spin. The Landau-Zener (LZ) quantum tunneling probability was studied
as a function of field sweep rate, establishing a good agreement with the LZ
equation and yielding the tunnel splitting \Delta. It was found that ?
increased linearly as a function of the transverse field. These studies are an
essential prerequisite for the coherent manipulation of a single nuclear spin
in TbPc2.Comment: 7 pages, 6 figures, to appear in PR
Interférométrie supraconductrice dans un nanotube de carbone
Les progrès des techniques de nano-fabrication permettent aujourd'hui de connecter électriquement des molécules individuelles avec des contacts faiblement résistifs. Les nanotubes de carbone ont été parmi les premiers objets moléculaires à avoir pu bénéficier de ces avancées. Les circuits obtenus sont des conducteurs unidimensionnels quasi-idéaux. La transmission entre le nanotube et des électrodes supraconductrices, par exemple, est telle qu'à très basse température, un courant supraconducteur peut circuler au travers du nanotube. Dans cet article, nous présentons une expérience d'interférence de deux courants supraconducteurs circulant dans deux portions d'un même nanotube de carbone. Grâce à la miniaturisation ultime apportée par le nanotube, une telle interférométrie quantique permet de sonder avec précision des champs magnétiques à l'échelle nanométrique et ouvre la voie à la magnétométrie d'objets quantiques uniques comportant un petit nombre de spins tels des aimants moléculaires uniques
Dynamics and Dissipation induced by Single-Electron Tunneling in Carbon Nanotube Nanoelectromechanical Systems
We demonstrate the effect of single-electron tunneling (SET) through a carbon
nanotube quantum dot on its nanomechanical motion. We find that the frequency
response and the dissipation of the nanoelectromechanical system (NEMS) to SET
strongly depends on the electronic environment of the quantum dot, in
particular on the total dot capacitance and the tunnel coupling to the metal
contacts. Our findings suggest that one could achieve quality factors of
10 or higher by choosing appropriate gate dielectrics and/or by improving
the tunnel coupling to the leads
Optimizing the flux coupling between a nanoSQUID and a magnetic particle using atomic force microscope nanolithography
We present results of Niobium based SQUID magnetometers for which the
weak-links are engineered by the local oxidation of thin films using an Atomic
Force Microscope (AFM). Firstly, we show that this technique allows the
creation of variable thickness bridges with 10 nm lateral resolution. Precise
control of the weak-link milling is offered by the possibility to realtime
monitor weak-link conductance. Such a process is shown to enhance the magnetic
field modulation hence the sensitivity of the magnetometer. Secondly, AFM
lithography is used to provide a precise alignment of NanoSQUID weak-links with
respect to a ferromagnetic iron dot. The magnetization switching of the
near-field coupled particle is studied as a junction of the applied magnetic
field direction
Molecular Spin Qudits for Quantum Algorithms
Presently, one of the most ambitious technological goals is the development
of devices working under the laws of quantum mechanics. One prominent target is
the quantum computer, which would allow the processing of information at
quantum level for purposes not achievable with even the most powerful computer
resources. The large-scale implementation of quantum information would be a
game changer for current technology, because it would allow unprecedented
parallelised computation and secure encryption based on the principles of
quantum superposition and entanglement. Currently, there are several physical
platforms racing to achieve the level of performance required for the quantum
hardware to step into the realm of practical quantum information applications.
Several materials have been proposed to fulfil this task, ranging from quantum
dots, Bose-Einstein condensates, spin impurities, superconducting circuits,
molecules, amongst others. Magnetic molecules are among the list of promising
building blocks, due to (i) their intrinsic monodispersity, (ii) discrete
energy levels (iii) the possibility of chemical quantum state engineering, and
(iv) their multilevel characteristics, leading to the so called Qudits (d > 2),
amongst others. Herein we review how a molecular multilevel nuclear spin qubit
(or qudit, where d = 4), known as TbPc2, gathers all the necessary requirements
to perform as a molecular hardware platform with a first generation of
molecular devices enabling even quantum algorithm operations.Comment: Chem. Soc. Rev., 2017, Advance Articl
First order quantum phase transition in the Kondo regime of a superconducting carbon nanotube quantum dot
We study a carbon nanotube quantum dot embedded into a SQUID loop in order to
investigate the competition of strong electron correlations with proximity
effect. Depending whether local pairing or local magnetism prevails, a
superconducting quantum dot will respectively exhibit positive or negative
supercurrent, referred to as a 0 or Josephson junction. In the regime of
strong Coulomb blockade, the 0 to transition is typically controlled by a
change in the discrete charge state of the dot, from even to odd. In contrast,
at larger tunneling amplitude the Kondo effect develops for an odd charge
(magnetic) dot in the normal state, and quenches magnetism. In this situation,
we find that a first order 0 to quantum phase transition can be triggered
at fixed valence when superconductivity is brought in, due to the competition
of the superconducting gap and the Kondo temperature. The SQUID geometry
together with the tunability of our device allows the exploration of the
associated phase diagram predicted by recent theories. We also report on the
observation of anharmonic behavior of the current-phase relation in the
transition regime, that we associate with the two different accessible
superconducting states. Our results ultimately reveal the spin singlet nature
of the Kondo ground state, which is the key process in allowing the stability
of the 0-phase far from the mixed valence regime.Comment: 10 pages, 6 figures in main text, 4 figures in appendi
Electronic transport properties of double-wall carbon nanotubes
We studied the discretized electronic spectra of double-wall carbon nanotube (DWCNT) quantum dots (QDs) in the Coulomb-blockade regime. At low temperatures, the stability diagrams show a clear and regular eight-electron periodicity, which is due to the nonzero intershell couplings. Furthermore, the electronic charging energy, the energy level spacing, and the intershell coupling strengths of the measured DWCNT QDs were determined
Contacting individual Fe(110) dots in a single electron-beam lithography step
We report on a new approach, entirely based on electron-beam lithography
technique, to contact electrically, in a four-probe scheme, single
nanostructures obtained by self-assembly. In our procedure, nanostructures of
interest are localised and contacted in the same fabrication step. This
technique has been developed to study the field-induced reversal of an internal
component of an asymmetric Bloch domain wall observed in elongated structures
such as Fe(110) dots. We have focused on the control, using an external
magnetic field, of the magnetisation orientation within N\'eel caps that
terminate the domain wall at both interfaces. Preliminary magneto-transport
measurements are discussed demonstrating that single Fe(110) dots have been
contacted.Comment: 5 page
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