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H. Pothier

Publications and source records attributed to H. Pothier.

36 records · Page 2Linked to original sources
arXiv2007

Josephson junctions as detectors for non-Gaussian noise

Non-Gaussian fluctuations of the electrical current can be detected with a Josephson junction placed on-chip with the noise source. We present preliminary measurements with an NIS junction as a noise source, and a Josephson junction in the thermal escape regime as a noise detector. It is shown that the Josephson junction detects not only the average noise, which manifests itself as an increased effective temperature, but also the noise asymmetry. A theoretical description of the thermal escape of a Josephson junction in presence of noise with a non-zero third cumulant is presented, together with numerical simulations when the noise source is a tunnel junction with Poisson noise. Comparison between experiment and theory is discussed.

cond-mat.mes-hall↗
arXiv2007

Current to frequency conversion in a Josephson circuit

The voltage oscillations which occur in an ideally current-biased Josephson junction, were proposed to make a current standard for metrology. We demonstrate similar oscillations in a more complex Josephson circuit derived from the Cooper pair box: the quantronium. When a constant current I is injected in the gate capacitor of this device, oscillations develop at the frequency fB=I/2e, with e the electron charge. We detect these oscillations through the sidebands induced at multiples of fB in the spectrum of a microwave signal reflected on the circuit, up to currents I exceeding 100 pA. We discuss the potential interest of this current to frequency conversion experiment for metrology.

cond-mat.mes-hall↗
arXiv2007

Electron heating in metallic resistors at sub-Kelvin temperature

In the presence of Joule heating, the electronic temperature in a metallic resistor placed at sub-Kelvin temperatures can significantly exceed the phonon temperature. Electron cooling proceeds mainly through two processes: electronic diffusion to and from the connecting wires and electron-phonon coupling. The goal of this paper is to present a general solution of the problem, in a form that can easily be used in practical situations. As an application, we compute two quantities that depend on the electronic temperature profile: the second and the third cumulant of the current noise at zero frequency, as a function of the voltage across the resistor. We also consider time dependent heating, an issue relevant for experiments in which current pulses are used, for instance in time-resolved calorimetry experiments.

cond-mat.mes-hall↗
arXiv2007

Measurement of the current-phase relation of superconducting atomic contacts

We have probed the current-phase relation of an atomic contact placed with a tunnel junction in a small superconducting loop. The measurements are in quantitative agreement with the predictions of a resistively shunted SQUID model in which the Josephson coupling of the contact is calculated using the independently determined transmissions of its conduction channels.

cond-mat.mes-hall↗
arXiv2007

Voltage-induced Shapiro steps in a superconducting multi-terminal structure

When a superconducting tunnel junction at a finite voltage is irradiated with microwaves, the interplay between the alternating Josephson current and the ac field gives rise to steps in the dc current known as Shapiro steps. In this work we predict that in a mesoscopic structure connected to several superconducting terminals one can induce Shapiro-like steps in the absence of any external radiation simply by tuning the voltages of the leads. To illustrate this effect we make quantitative predictions for a three-terminal structure which comprises a diffusive superconductor-normal metal-superconductor junction and a tunneling probe, a set-up which can be realized experimentally.

cond-mat.supr-con↗
arXiv2006

Superconducting atomic contacts under microwave irradiation

We have measured the effect of microwave irradiation on the dc current-voltage characteristics of superconducting atomic contacts. The interaction of the external field with the ac supercurrents leads to replicas of the supercurrent peak, the well known Shapiro resonances. The observation of supplementary fractional resonances for contacts containing highly transmitting conduction channels reveals their non-sinusoidal current-phase relation. The resonances sit on a background current which is itself deeply modified, as a result of photon assisted multiple Andreev reflections. The results provide firm support for the full quantum theory of transport between two superconductors based on the concept of Andreev bound states.

cond-mat.mes-hall↗
arXiv2006

Bloch Oscillations in a Josephson Circuit

Bloch oscillations predicted to occur in current-biased single Josephson junctions have eluded direct observation up to now. Here, we demonstrate similar Bloch oscillations in a slightly richer Josephson circuit, the quantronium. The quantronium is a Bloch transistor with two small junctions in series, defining an island, in parallel with a larger junction. In the ground state, the microwave impedance of the device is modulated periodically with the charge on the gate capacitor coupled to the transistor island. When a current flows across this capacitor, the impedance modulation occurs at the Bloch frequency, which yields Bloch sidebands in the spectrum of a reflected continuous microwave signal. We have measured this spectrum, and compared it to predictions based on a simple model for the circuit. We discuss the interest of this experiment for metrology and for mesoscopic physics.

cond-mat.supr-con↗
arXiv2005

Effect of Magnetic Impurities on Energy Exchange between Electrons

In order to probe quantitatively the effect of Kondo impurities on energy exchange between electrons in metals, we have compared measurements on two silver wires with dilute magnetic impurities (manganese) introduced in one of them. The measurement of the temperature dependence of the electron phase coherence time on the wires provides an independent determination of the impurity concentration. Quantitative agreement on the energy exchange rate is found with a theory by Göppert et al. that accounts for Kondo scattering of electrons on spin-1/2 impurities.

cond-mat.mes-hall↗
arXiv2004

Intensity of Coulomb Interaction between quasiparticles in diffusive metallic wires

The energy dependence and intensity of Coulomb interaction between quasiparticles in metallic wires is obtained from two different methods: determination of the temperature dependence of the phase coherence time from the magnetoresistance, and measurements of the energy distribution function in out-of-equilibrium situations. In both types of experiment, the energy dependence of the Coulomb interaction is found to be in excellent agreement with theoretical predictions. In contrast, the intensity of the interaction agrees closely with theory only with the first method, whereas an important discrepancy is found using the second one. Different explanations are proposed, and results of a test experiment are presented.

cond-mat.mes-hall↗
arXiv2003

Dephasing of Electrons in Mesoscopic Metal Wires

We have extracted the phase coherence time $τ_ϕ$ of electronic quasiparticles from the low field magnetoresistance of weakly disordered wires made of silver, copper and gold. In samples fabricated using our purest silver and gold sources, $τ_ϕ$ increases as $T^{-2/3}$ when the temperature $T$ is reduced, as predicted by the theory of electron-electron interactions in diffusive wires. In contrast, samples made of a silver source material of lesser purity or of copper exhibit an apparent saturation of $τ_ϕ$ starting between 0.1 and 1 K down to our base temperature of 40 mK. By implanting manganese impurities in silver wires, we show that even a minute concentration of magnetic impurities having a small Kondo temperature can lead to a quasi saturation of $τ_ϕ$ over a broad temperature range, while the resistance increase expected from the Kondo effect remains hidden by a large background. We also measured the conductance of Aharonov-Bohm rings fabricated using a very pure copper source and found that the amplitude of the $h/e$ conductance oscillations increases strongly with magnetic field. This set of experiments suggests that the frequently observed ``saturation'' of $τ_ϕ$ in weakly disordered metallic thin films can be attributed to spin-flip scattering from extremely dilute magnetic impurities, at a level undetectable by other means.

cond-mat.mes-hall↗
arXiv2003

Density of states in a superconductor carrying a supercurrent

We have measured the tunneling density of states (DOS) in a superconductor carrying a supercurrent or exposed to an external magnetic field. The pair correlations are weakened by the supercurrent, leading to a modification of the DOS and to a reduction of the gap. As predicted by the theory of superconductivity in diffusive metals, we find that this effect is similar to that of an external magnetic field.

cond-mat.supr-con↗
arXiv2003

Magnetic-field-dependent quasiparticle energy relaxation in mesoscopic wires

In order to find out if magnetic impurities can mediate interactions between quasiparticles in metals, we have measured the effect of a magnetic field B on the energy distribution function f(E) of quasiparticles in two silver wires driven out-of-equilibrium by a bias voltage U. In a sample showing sharp distributions at B=0, no magnetic field effect is found, whereas in the other sample, rounded distributions at low magnetic field get sharper as B is increased, with a characteristic field proportional to U. Comparison is made with recent calculations of the effect of magnetic-impurities-mediated interactions taking into account Kondo physics.

cond-mat.mes-hall↗
arXiv2002

Rabi oscillations, Ramsey fringes and spin echoes in an electrical circuit

We present a superconducting tunnel junction circuit which behaves as a controllable atom, and whose ground and first excited state form an effective spin 1/2. By applying microwave pulses, we have performed on this circuit experiments demonstrating the controlled manipulation of the spin : Rabi precession, Ramsey interferences, and spin echoes.

cond-mat.mes-hall↗
arXiv2002

Manipulating the Quantum State of an Electrical Circuit

We have designed and operated a superconducting tunnel junction circuit that behaves as a two-level atom: the ``quantronium''. An arbitrary evolution of its quantum state can be programmed with a series of microwave pulses, and a projective measurement of the state can be performed by a pulsed readout sub-circuit. The measured quality factor of quantum coherence Qphi=25000 is sufficiently high that a solid-state quantum processor based on this type of circuit can be envisioned.

cond-mat.mes-hall↗
arXiv2000

Probing interactions in mesoscopic gold wires

We have measured in gold wires the energy exchange rate between quasiparticles, the phase coherence time of quasiparticles and the resistance vs. temperature, in order to probe the interaction processes which are relevant at low temperatures. We find that the energy exchange rate is higher than expected from the theory of electron-electron interactions, and that it has a different energy dependence. The dephasing time is constant at temperatures between 8 K and 0.5 K, and it increases below 0.5 K. The magnetoresistance is negative at large field scales, and the resistance decreases logarithmically with increasing temperatures, indicating the presence of magnetic impurities, probably Fe. Whereas resistivity and phase coherence measurements can be attributed to magnetic impurities, the question is raised whether these magnetic impurities could also mediate energy exchanges between quasiparticles.

cond-mat.mes-hall↗
arXiv1999

Comparison of energy and phase relaxation in metallic wires

We have measured the phase coherence time, $τ_ϕ,$ in long, narrow wires of Au, Ag, and Cu, over the temperature range 40 mK-6 K. In the Cu and Au wires, $τ_ϕ$ saturates at low temperature. In the Ag wire, $τ_ϕ$ continues to increase down to the lowest temperatures measured; moreover, its temperature dependence below about 1 K is consistent with theoretical predictions of Altshuler, Aronov and Khmelnitskii published in 1982. These results cast doubt on recent assertions that saturation of $τ_ϕ$ at low temperature is a universal phenomenon in metal wires. We compare these results with those of recent experiments on energy relaxation in similar metallic wires. The results of the two experiments are strongly correlated, suggesting that a single (unknown) mechanism is the cause of the enhanced phase and energy relaxation observed in some samples.

cond-mat.mes-hall↗
arXiv1999

Energy redistribution between quasiparticles in mesoscopic silver wires

We have measured with a tunnel probe the energy distribution function of quasiparticles in silver diffusive wires connected to two large pads (``reservoirs''), between which a bias voltage was applied. From the dependence in energy and bias voltage of the distribution function we have inferred the energy exchange rate between quasiparticles. In contrast with previously obtained results on copper and gold wires, these data on silver wires can be well interpreted with the theory of diffusive conductors either solely, or associated with another mechanism, possibly the coupling to two-level systems.

cond-mat.mes-hall↗