Hall detection of time-reversal symmetry breaking under AC electric driv ing

In a four terminal sample microscopic time-reversibility leads to symmetry relations between resistance measurements where the role of current and voltage leads are exchanged. These reciprocity relations are a manifestation of general Onsager-Casimir symmetries in equilibrium systems. We investigate experimentally the validity of time reversal symmetry in a $GaAs/Ga_{1-x}Al_xAs$ Hall bar irradiated by an external AC field, at zero magnetic field. For inhomogeneous AC fields we find strong deviations from reciprocity relations and show that their origin can be understood from the the billiard model of a Hall junction. Under homogeneous irradiation the symmetry is more robust, indicating that time-reversal symmetry is preserved

Spatial Constraint Corrections to the Elasticity of dsDNA Measured with Magnetic Tweezers

In this paper, we have studied, within a discrete WLC model, the spatial constraints in magnetic tweezers used in single molecule experiments. Two elements are involved: first, the fixed plastic slab on which is stuck the initial strand, second, the magnetic bead which pulls (or twists) the attached molecule free end. We have shown that the bead surface can be replaced by its tangent plane at the anchoring point, when it is close to the bead south pole relative to the force. We are led to a model with two parallel repulsive plates: the fixed anchoring plate and a fluctuating plate, simulating the bead, in thermal equilibrium with the system. The bead effect is a slight upper shift of the elongation, about four times smaller than the similar effect induced by the fixed plate. This rather unexpected result, has been qualitatively confirmed within the soluble Gaussian model. A study of the molecule elongation versus the countour length exhibits a significant non-extensive behaviour. The curve for short molecules (with less than 2 kbp) is well fitted by a straight line, with a slope given by the WLC model, but it does not go through the origin. The non-extensive offset gives a 15% upward shift to the elongation of a 2 kbp molecule stretched by a 0.3 pN force.Comment: 28 pages, 6 figures An explanatory figure has been added. The physical interpretation of the results has been made somewhat more transparen

Emission Noise and High Frequency Cut-Off of the Kondo Effect in a Quantum Dot

By coupling on chip a carbon nanotube to a quantum noise detector, a superconductor-insulator-superconductor junction, via a resonant circuit, we measure the emission noise of a carbon nanotube quantum dot in the Kondo regime. The signature of the Kondo effect in the current noise is measured for different ratios of the Kondo temperature over the measured frequency and for different asymmetries of the coupling to the contacts, and compared to finite frequency quantum noise calculations. Our results point towards the existence of a high frequency cut-off of the electronic emission noise associated with the Kondo resonance. This cut-off frequency is of the order of a few times the Kondo temperature when the electronic system is close to equilibrium, which is the case for a strongly asymmetric coupling. On the other hand, this cut-off is shifted to lower frequency in a symmetric coupling situation, where the bias voltage drives the Kondo state out-of-equilibrium. We then attribute the low frequency cut-off to voltage induced spin relaxation.Comment: 5 pages, 3 figures and appendi

Measurements of flux dependent screening in Aharonov-Bohm rings

In order to investigate the effect of electronic phase coherence on screening we have measured the flux dependent polarizability of isolated mesoscopic rings at 350 MHz. At low temperature (below 100 mK) both non-dissipative and dissipative parts of the polarizability exhibit flux oscillations with a period of half a flux quantum in a ring. The sign and amplitude of the effect are in good agreement with recent theoretical predictions. The observed positive magneto-polarizability corresponds to an enhancement of screening when time reversal symmetry is broken. The effect of electronic density and temperature are also measured.Comment: 4 pages, revtex, 4 figures, to appear in Phys. Rev. Let

Coupling between Rydberg states and Landau levels of electrons trapped on liquid helium

We investigate the coupling between Rydberg states of electrons trapped on a liquid Helium surface and Landau levels induced by a perpendicular magnetic field. We show that this realises a prototype quantum system equivalent to an atom in a cavity, where their coupling strength can be tuned by a parallel magnetic field. We determine experimentally the renormalisation of the atomic transition energies induced by the coupling to the cavity, which can be seen as an analogue of the Lamb shift. When the coupling is sufficiently strong the transition between the ground and first excited Rydberg states splits into two resonances corresponding to dressed states with vacuum and one photon in the cavity. Our results are in quantitative agreement with the energy shifts predicted by the effective atom in a cavity model where all parameters are known with high accuracy

0-π\pi quantum transition in a carbon nanotube Josephson junction: universal phase dependence and orbital degeneracy

We investigate experimentally the supercurrent in a clean carbon nanotube quantum dot, close to orbital degeneracy, connected to superconducting leads in a regime of strong competition between local electronic correlations and superconducting proximity effect. For an odd occupancy of the dot and intermediate coupling to the reservoir, the Kondo effect can develop in the normal state and screen the local magnetic moment of the dot. This leads to singlet-doublet transitions that strongly affect the Josephson effect in a single-level quantum dot: the sign of the supercurrent changes from positive to negative (0 to $\pi$-junction). In the regime of strongest competition between the Kondo effect and proximity effect, meaning that the Kondo temperature equals the superconducting gap, the magnetic state of the dot undergoes a first order quantum transition induced by the superconducting phase difference across the junction. This is revealed experimentally by anharmonic current-phase relations. In addition, the very specific electronic configuration of clean carbon nanotubes, with two nearly orbitally degenerated states, leads to different physics depending whether only one or both quasi-degenerate upper levels of the dots participate to transport, which is determined by their occupancy and relative widths. When the transport of Cooper pairs takes place through only one of these levels, we find that the phase diagram of the phase-dependent 0-$\pi$ transition is a universal characteristic of a discontinuous level-crossing quantum transition at zero temperature. In the case were two levels participate to transport, the nanotube Josephson current exhibits a continuous 0-$\pi$ transition, independent of the superconducting phase, revealing a different physical mechanism of the transition.Comment: 14 pages, 12 figure