20 research outputs found
Room-temperature tunnel current amplifier and experimental setup for high resolution electronic spectroscopy in millikelvin STM experiments
The spectroscopic resolution of tunneling measurements performed with a
scanning tunneling microscope is ultimately limited by the temperature at which
the experiment is performed. To take advantage of the potential high
spectroscopic resolution associated with operating an STM in a dilution
refrigerator we have designed a room temperature tunnel current amplifier
having very small back-action on the tunnel contact and allowing to nearly
reach the predicted energy resolution. This design is a modification of the
standard op-amp based tip-biasing current-voltage converter which implements
differential voltage sensing and whose back action on the tip voltage is only
~2 V rms for a 14 MV/A transimpedance and 22 kHz bandwidth.Comment: Available at http://www-spht.cea.fr/articles/s06/03
Plasmon scattering approach to energy exchange and high frequency noise in nu=2 quantum Hall edge channels
Inter-edge channel interactions in the quantum Hall regime at filling factor
nu= 2 are analyzed within a plasmon scattering formalism. We derive analytical
expressions for energy redistribution amongst edge channels and for high
frequency noise, which are shown to fully characterize the low energy plasmon
scattering. In the strong interaction limit, the predictions for energy
redistribution are compared with recent experimental data and found to
reproduce most of the observed features. Quantitative agreement can be achieved
by assuming 25 % of the injected energy is lost towards other degrees of
freedom, possibly the additional gapless excitations predicted for smooth edge
potentials.Comment: 4 pages, 4 figure
Manipulating Fock states of a harmonic oscillator while preserving its linearity
We present a new scheme for controlling the quantum state of a harmonic
oscillator by coupling it to an anharmonic multilevel system (MLS) with first
to second excited state transition frequency on-resonance with the oscillator.
In this scheme that we call "ef-resonant", the spurious oscillator Kerr
non-linearity inherited from the MLS is very small, while its Fock states can
still be selectively addressed via an MLS transition at a frequency that
depends on the number of photons. We implement this concept in a circuit-QED
setup with a microwave 3D cavity (the oscillator, with frequency 6.4 GHz and
quality factor QO=2E-6) embedding a frequency tunable transmon qubit (the MLS).
We characterize the system spectroscopically and demonstrate selective
addressing of Fock states and a Kerr non-linearity below 350 Hz. At times much
longer than the transmon coherence times, a non-linear cavity response with
driving power is also observed and explained.Comment: 8 pages, 5 figure
Bi-layer Kinetic Inductance Detectors for space observations between 80-120 GHz
We have developed Lumped Element Kinetic Inductance Detectors (LEKID)
sensitive in the frequency band from 80 to 120~GHz. In this work, we take
advantage of the so-called proximity effect to reduce the superconducting gap
of Aluminium, otherwise strongly suppressing the LEKID response for frequencies
smaller than 100~GHz. We have designed, produced and optically tested various
fully multiplexed arrays based on multi-layers combinations of Aluminium (Al)
and Titanium (Ti). Their sensitivities have been measured using a dedicated
closed-circle 100 mK dilution cryostat and a sky simulator allowing to
reproduce realistic observation conditions. The spectral response has been
characterised with a Martin-Puplett interferometer up to THz frequencies, and
with a resolution of 3~GHz. We demonstrate that Ti-Al LEKID can reach an
optical sensitivity of about ~ (best pixel), or
~ when averaged over the whole array. The optical
background was set to roughly 0.4~pW per pixel, typical for future space
observatories in this particular band. The performance is close to a
sensitivity of twice the CMB photon noise limit at 100~GHz which drove the
design of the Planck HFI instrument. This figure remains the baseline for the
next generation of millimetre-wave space satellites.Comment: 7 pages, 9 figures, submitted to A&
Un AFM-STM cryogénique pour la physique mésoscopique
Electronic spectroscopy based on electron tunneling gives access to the electronic Density of States (DoS) in conductive materials, and thus provides detailed information about their electronic properties. During this thesis work, we have developed a microscope in order to perform spatially resolved (10 nm) tunneling spectroscopy, with an unprecedented energy resolution (10 µeV), on individual nanocircuits. This machine combines an Atomic Force Microscope (AFM mode) together with a Scanning Tunneling Spectroscope (STS mode), and functions at very low temperatures (30mK). In the AFM mode, the sample topography is recorded using a piezoelectric quartz tuning fork, which allows locating and imaging nanocircuits. Tunneling can then be performed on conductive areas of the circuit. With this microscope, we have measured the local DoS in a hybrid Superconductor-Normal metal-Superconductor (S-N-S) structure. In such circuit, the electronic properties of N and S are modified by the superconducting proximity effect. In particular, for short N wires, we have observed a minigap in the DoS of the N wire, independent of position. Moreover, when varying the superconducting phase difference between the S electrodes, we have measured the modification of the minigap, and its disappearance when the phase difference equals p. Our experimental results for the DoS, and its dependences (with phase, position, N length) are quantitatively accounted for by the quasiclassical theory of superconductivity. Some predictions of this theory are observed for the first time.La spectroscopie électronique basée sur l'effet tunnel donne accès à la densité d'états des électrons (DoS) dans les matériaux conducteurs, et renseigne ainsi en détail sur leurs propriétés électroniques. Au cours de cette thèse, nous avons développé un microscope permettant d'effectuer la spectroscopie tunnel résolue spatialement (10 nm) de nanocircuits individuels, avec une résolution en énergie inégalée (10 µeV). Cet appareil combine les fonctions de Microscopie par Force Atomique (mode AFM) et de spectroscopie Tunnel locale (mode STM), et fonctionne à 30 mK. Dans le mode AFM, la topographie de l'échantillon est imagée grâce à un diapason en quartz piézoélectrique, ce qui permet de repérer les circuits. La spectroscopie tunnel peut ensuite être faite sur les zones conductrices. Avec ce microscope, nous avons mesuré la DoS locale dans une structure hybride Supraconducteur-métal Normal-Supraconducteur (S-N-S). Dans un tel circuit, les propriétés électroniques de N et de S sont modifiées par l'effet de proximité supraconducteur. Notamment, pour des fils N courts, nous avons pu observer -comme prédit- la présence d'un gap dans sa DoS, indépendant de la position dans la structure : le “minigap”. De plus, en modulant la phase supraconductrice entre les deux S, nous avons mesuré la modification de ce gap, et sa disparition lorsque la différence de phase vaut π. Nos résultats expérimentaux pour la DoS, ainsi que ses dépendances en phase, en position, et en longueur de N sont en accord quantitatif avec les prédictions de la théorie quasiclassique de la supraconductivité. Certaines de ces prédictions sont observées pour la première fois
Design of a Cooper-Pair Box Electrometer for Application to Solid-State and Astroparticle Physics
International audienceWe describe the design and principle of operation of a fast and sensitive electrometer operated at millikelvin temperatures, which aims at replacing conventional semiconducting charge amplifiers in experiments needing low back action or high sensitivity. This electrometer consists of a Cooper-pair box (CPB) coupled to a microwave resonator, which converts charge variations to resonance frequency shifts. By analyzing in detail its sensitivity to various parameters, we find that the resonator nonlinearity induced by the CPB can be exploited to improve sensitivity. Using conventional nanofabrication and measurement techniques, a charge sensitivity down to 10−7e/Hz with a megahertz bandwidth can be reached, which outperforms by 1 order of magnitude existing single-charge electrometers operated in the linear regime and opens up alternative possibilities in several fields such as mesoscopic and particle physics
Absence of a dissipative quantum phase transition in Josephson junctions: Theory
We obtain the reduced density matrix of a resistively shunted Josephson junction (RSJ), using the stochastic Liouville equation method in imaginary time - an exact numerical scheme based on the Feynman-Vernon influence functional. For all parameters looked at, we find a shunted junction is more superconducting than the same unshunted junction. We find no trace of Schmid's superconducting-insulating quantum phase transition long believed to occur in the RSJ. This work confirms theoretically a similar conclusion drawn in 2020 by Murani et al., based on experimental observations. We reveal that predictions of an insulating junction in previous works were due to considering Ohmic environments with no UV cutoff