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Department of Physics University of Basel Klingelbergstrasse 82 CH-4056 Basel, Switzerland |

e-mail: | Charles.Doiron@unibas.ch |

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arXiv:0810.5718, submitted to Physical Review B

We investigate a superconducting single-electron transistor capacitively coupled to a nanomechanical oscillator and focus on the double Josephson quasiparticle resonance. The existence of two coherent Cooper pair tunneling events is shown to lead to pronounced backaction effects. Measuring the current and the shot noise provides a direct way of gaining information on the state of the oscillator. In addition to an analytical discussion of the linear-response regime we discuss and compare results of higher-order approximation schemes and a fully numerical solution. We find that cooling of the mechanical resonator is possible, and that there are driven and bistable oscillator states at low couplings. Finally, we also discuss the frequency dependence of the charge noise of the superconducting single electron transistor

Phys. Rev. Lett.

We propose a way to measure the momentum p of a nanomechanical oscillator. The p detector is based on two tunnel junctions in an Aharonov-Bohm-type setup. One of the tunneling amplitudes depends on the motion of the oscillator, the other one not. Although the coupling between the detector and the oscillator is assumed to be linear in the position x of the oscillator, it turns out that the finite-frequency noise output of the detector will in general contain a term proportional to the momentum spectrum of the oscillator. This is a true quantum phenomenon, which can be realized in practice if the phase of the tunneling amplitude of the detector is tuned by the Aharonov-Bohm flux to a p-sensitive value.

Phys. Rev. B

We consider position measurements using the cross-correlated output of two tunnel junction position detectors. Using a fully quantum treatment, we calculate the equation of motion for the density matrix of the coupled detector-detector-mechanical oscillator system. After discussing the presence of a bound on the peak-to-background ratio in a position measurement using a single detector, we show how one can use detector cross-correlations to overcome this bound. We analyze two different possible experimental realizations of the cross-correlation measurement and show that in both cases the maximum cross-correlated output is obtained when using twin detectors and applying equal bias to each tunnel junction.

Phys. Rev. B

We investigate electrical transport through a single-electron transistor coupled to a nanomechanical oscillator. Using a combination of a master-equation approach and a numerical Monte Carlo method, we calculate the average current and the current noise in the strong-coupling regime, studying deviations from previously derived analytic results valid in the limit of weak-coupling. After generalizing the weak-coupling theory to enable the calculation of higher cumulants of the current, we use our numerical approach to study how the third cumulant is affected in the strong-coupling regime. In this case, we find an interesting crossover between a weak-coupling transport regime where the third cumulant heavily depends on the frequency of the oscillator to one where it becomes practically independent of this parameter. Finally, we study the spectrum of the transport noise and show that the two peaks found in the weak-coupling limit merge on increasing the coupling strength. Our calculation of the frequency-dependence of the noise also allows to describe how transport-induced damping of the mechanical oscillations is affected in the strong-coupling regime.

Phys. Rev. B

In a quantizing magnetic field, the two-dimensional electron (2DEG) gas has a rich phase diagram with broken translational symmetry phases such as Wigner, bubble, and stripe crystals. In this paper, we derive a method to get the dynamical matrix of these crystals from the density response function computed in the Generalized Random Phase Approximation (GRPA). We discuss the validity of our method by extracting the elastic coefficients from this GRPA dynamical matrix and comparing them with those obtained by a direct calculation of the deformation energy of the crystal in the Hartree-Fock approximation.

Phys. Rev. B 76, 161306(R) (2007)

We study fluctuations and topological melting transitions of quantum Hall stripes near half filling of intermediate Landau levels. Taking the stripe state to be an anisotropic Wigner crystal (AWC) allows us to identify the quantum Hall nematic state conjectured in previous studies of the two-dimensional (2D) electron gas as an anisotropic hexatic. The transition temperature from the AWC to the quantum Hall nematic state is explicitly calculated, and a tentative phase diagram for the 2D electron gas near half filling is suggested.

Phys. Rev. Lett.

We study the collective states formed by two-dimensional electrons in Landau levels (LLs) of index n < 2 near half-filling. By numerically solving the static Hartree-Fock (HF) equations for a set of oblique two-dimensional lattices, we find that the stripe state is in fact an anisotropic Wigner crystal (AWC), and determine its precise structure for varying values of the filling factor. Calculating the elastic energy, we find that the shear modulus of the AWC is small but finite (nonzero) within the HF approximation. This implies, in particular, that the long-wavelength magnetophonon mode in the stripe state vanishes like q^{3/2} much as in an ordinary Wigner crystal, and not like q^{5/2} as was found in previous studies where the energy of shear deformations was neglected.

Physica E

In the Hartree–Fock approximation and at total filling factor ν=4N+1, the ground state of the two-dimensional electron gas in a double quantum well system in a quantizing magnetic field is, in some range of interlayer distances, a coherent striped phase. This stripe phase has one-dimensional coherent channels that support charged excitations in the form of pseudospin solitons. In this work, we compute the transport gap of the coherent striped phase due to the creation of soliton–antisoliton pairs using a supercell microscopic unrestricted Hartree–Fock approach. We study the energy gap as a function of interlayer distance and tunneling amplitude. Our calculations confirm that the soliton–antisoliton excitation energy is lower than the corresponding Hartree–Fock electron–hole pair energy.

Phys. Rev. B

In some range of interlayer distances, the ground state of the two-dimensional electron gas at filling factor ν=4N+1 with N=0,1,2,... is a coherent stripe phase in the Hartree-Fock approximation. This phase has one-dimensional coherent channels that support charged excitations in the form of pseudospin solitons. In this work, we compute the transport gap of the coherent striped phase due to the creation of soliton-antisoliton pairs using a supercell microscopic unrestricted Hartree-Fock approach. We study this gap as a function of interlayer distance and tunneling amplitude. Our calculations confirm that the soliton-antisoliton excitation energy is lower than the corresponding Hartree-Fock electron-hole pair energy. We compare our results with estimates of the transport gap obtained from a field-theoretic model valid in the limit of slowly varying pseudospin textures.

Phys. Rev. B

In Landau levels N>1, the ground state of the two-dimensional electron gas (2DEG) in a perpendicular magnetic field evolves from a Wigner crystal for small filling ν^{*} of the partially filled Landau level, into a succession of bubble states with increasing number of guiding centers per bubble as ν^{*} increases, to a modulated stripe state near ν^{*}= 0.5. In this work, we show that these first-order phase transitions between the bubble states lead to measurable discontinuities in several physical quantities such as the density of states and the magnetization of the 2DEG. We discuss in detail the behavior of the collective excitations of the bubble states and show that their spectra have higher-energy modes besides the pinned phonon mode. The frequencies of these modes, at small wave vector **k**, have a discontinuous evolution as a function of filling factor that should be measurable in, for example, microwave absorption experiments.

Annual Meeting of the DPG, Berlin, Germany. March 25, 2008.

University of Nottingham. Nottingham, United Kingdom. October 19, 2006.

APS March Meeting, New Orleans, USA. March 12, 2008

16th Intl. Conference on Electronic Properties of Two-Dimensional Systems (EP2DS-16), Albuquerque, USA. July 14 2005.

Canadian Association of Physicists's 2004 Meeting. Winnipeg, Canada. June 17, 2004.

FNST 2008 workshop, Basel, Switzerland. January 7, 2008.

ICN+T 2006, Basel, Switzerland. August 2, 2006.

Annual Meeting of the DPG, Dresden, Germany. March 29, 2006.

Annual Meeting of the Swiss Physical Society, Lausanne, Switzerland. February 14, 2006.

Annual meeting of the Regroupement Québecois sur les Matériaux de Pointe, Montréal, Canada, June 28, 2004.