arXiv subjects

F. Pistolesi

Publications and source records attributed to F. Pistolesi.

At least 19 recordsLinked to original sources

arXiv2025

Evidence for atomic-scale vibron-mediated electron bunching

Due to the Coulomb blockade effect, electrons rarely bunch during transport, a phenomenon observed only in a few specially engineered mesoscopic configurations. In this work, we introduce an atomically resolved shot-noise study to demonstrate the possibility of electron bunching through vibrational coupling which takes place in an atomically sized nano-electro-mechanical system. Using tunnelling spectroscopy, we observe signatures of vibron-assisted tunnelling on an Fe impurity in Bi$_2$Se$_3$. Notably, simultaneous shot-noise measurements at the centre of the vibrating impurity reveal super-Poissonian noise. In the absence of alternative sources of super-Poissonian noise, this implies vibronic-coupling-induced bunching of electrons during the tunnelling process through the impurity, as theoretically predicted decades ago. As a future outlook, if coherence between electrons can be implemented, vibron-mediated electron bunching at single atomic sites may be exploited as a local injection source of $N$-paired electrons.

cond-mat.str-el↗

arXiv2022

Nonlinear nanomechanical resonators approaching the quantum ground state

An open question in mechanics is whether mechanical resonators can be made nonlinear with vibrations approaching the quantum ground state. This requires engineering a mechanical nonlinearity far beyond what has been realized thus far. Here we discovered a mechanism to boost the Duffing nonlinearity by coupling the vibrations of a nanotube resonator to single-electron tunneling and by operating the system in the ultrastrong coupling regime. Remarkably, thermal vibrations become highly nonlinear when lowering the temperature. The average vibration amplitude at the lowest temperature is 13 times the zero-point motion, with approximately 42% of the thermal energy stored in the anharmonic part of the potential. Our work paves the way for realizing mechanical Schrodinger cat states [1], mechanical qubits [2, 3], and quantum simulators emulating the electron-phonon coupling [4].

cond-mat.mes-hall↗

arXiv2022

Nanomechanical vibrational response from electrical mixing measurements

Driven nanomechanical resonators based on low-dimensional materials are routinely and efficiently detected with electrical mixing measurements. However, the measured signal is a non-trivial combination of the mechanical eigenmode displacement and an electrical contribution, which makes the extraction of the driven mechanical response challenging. Here, we report a simple yet reliable method to extract solely the driven mechanical vibrations by eliminating the contribution of pure electrical origin. This enables us to measure the spectral mechanical response as well as the driven quadratures of motion. We further show how to calibrate the measured signal into units of displacement. Additionally, we utilize the pure electrical contribution to directly determine the effective mass of the measured mechanical mode. Our method marks a key step forward in the study of nanoelectromechanical resonators based on low-dimensional materials in both the linear and the nonlinear regime.

cond-mat.mes-hall↗

arXiv2021

Photon-Emission Statistics induced by Electron Tunnelling in Plasmonic Nanojunctions

We investigate the statistics of photons emitted by tunneling electrons in a single electronic level plasmonic nanojunction. We compute the waiting-time distribution of successive emitted photons $w(τ)$. When the cavity damping rate $κ$ is larger than the electronic tunneling rate $Γ$, we show that in the photon-antibunching regime, $w(τ)$ indicates that the average delay-time between two successive photon emission events is given by $1/Γ$. This is in contrast with the usually considered second-order correlation function of emitted photons, $g^{(2)}(τ)$, which displays the single time scale $1/κ$. Our analysis shows a relevant example for which $w(τ)$ gives independent information on the photon-emission statistics with respect to $g^{(2)}(τ)$, leading to a physical insight on the problem. We discuss how this information can be extracted from experiments even in presence of a non-perfect photon detection yield.

cond-mat.mes-hall↗

arXiv2021

Proposal for a nanomechanical qubit

Mechanical oscillators have been demonstrated with very high quality factors over a wide range of frequencies. These also couple to a wide variety of fields and forces, making them ideal as sensors. The realization of a mechanically-based quantum bit could therefore provide an important new platform for quantum computation and sensing. Here we show that by coupling one of the flexural modes of a suspended carbon nanotube to the charge states of a double quantum dot defined in the nanotube, it is possible to induce sufficient anharmonicity in the mechanical oscillator so that the coupled system can be used as a mechanical quantum bit. This can however only be achieved when the device enters the ultrastrong coupling regime. We discuss the conditions for the anharmonicity to appear, and we show that the Hamiltonian can be mapped onto an anharmonic oscillator, allowing us to work out the energy level structure and how decoherence from the quantum dot and the mechanical oscillator are inherited by the qubit. Remarkably, the dephasing due to the quantum dot is expected to be reduced by several orders of magnitude in the coupled system. We outline qubit control, readout protocols, the realization of a CNOT gate by coupling two qubits to microwave cavity, and finally how the qubit can be used as a static force quantum sensor.

cond-mat.mes-hall↗

arXiv2020

Two-level system as topological actuator for nanomechanical modes

We investigate theoretically the dynamics of two quasidegenerate mechanical modes coupled through an open quantum two-level system. A mean-field approach shows that by engineering the retarded response of the two-level system with a coherent drive, the non-Hermitian mechanical spectrum exhibits an exceptional degeneracy point where the two modes coalesce. We show that this degeneracy can be exploited to manipulate the vectorial polarization of the mechanical oscillations. We find that adiabatically varying the detuning and the intensity of the drive induces a rotation of the mechanical polarization, which enables the topological and chiral actuation of one mode from the other. This topological manifestation of the degeneracy is further supported by quantum-jump Monte Carlo simulations to account for the strong quantum fluctuations due to the spontaneous emission of the two-level system. Our presentation focuses on a promising realization based on flexural modes of a carbon-nanotube cantilever coupled to a single-molecule electric dipole irradiated by a laser.

cond-mat.mes-hall↗

arXiv2018

Bistability of a slow mechanical oscillator coupled to a laser-driven two-level system

It has been recently proposed that single molecule spectroscopy could be employed to detect the motion of nano-mechanical resonators. Estimates of the coupling constant (g) between the molecular two-level system and the oscillator indicate that it can reach values much larger than the mechanical resonating pulsation (omega_m) and the two-level system linewidth (Gamma). Other experimental realization of the same system are also approching this strong coupling regim. In this paper we investigate the behavior of the system in the limit for slow mechanical oscillator omega_m << Gamma}. We find that, for sufficiently large coupling, the system undergoes a bistability reminiscent of that observed in optical cavities coupled to mechanical resonators.

cond-mat.mes-hall↗

arXiv2018

Bistability and Displacement Fluctuations in a Quantum Nano-mechanical Oscillator

Remarkable features have been predicted for the mechanical fluctuations at the bistability transition of a classical oscillator coupled capacitively to a quantum dot [Phys. Rev. Lett. 115, 206802 (2015)]. These results have been obtained in the regime $ω$0 $\ll$ kBT $\ll$ $Γ$, where $ω$0, T , and $Γ$ are the mechanical resonating frequency, the temperature, and the tunneling rate, respectively. A similar behavior could be expected in the quantum regime of $Γ$ $\ll$ kBT $\ll$ $ω$0. We thus calculate the energy and displacement fluctuation spectra and study their behavior as a function of the electro-mechanical coupling constant when the system enters the Frank-Condon regime. We find that, in analogy with the classical case, the energy fluctuation spectrum and the displacement spectrum widths show a maximum for values of the coupling constant at which a mechanical bistability establishes.

cond-mat.mes-hall↗

arXiv2015

Tunable spin-polaron state in a singly clamped semiconducting carbon nanotube

We consider a semiconducting carbon nanotube (CNT) laying on a ferromagnetic insulating sub-strate with one end depassing the substrate and suspended over a metallic gate. We assume that the polarised substrate induces an exchange interaction acting as a local magnetic field for the electrons in the non-suspended CNT side. Generalizing the approach of I. Snyman and Yu.V. Nazarov [Phys. Rev. Lett. 108, 076805 (2012)] we show that one can generate electrostatically a tun-able spin-polarized polaronic state localized at the bending end of the CNT. We argue that at low temperatures manipulation and detection of the localised quantum spin state is possible.

cond-mat.mes-hall↗

arXiv2012

Quantum current noise from a Born-Markov master equation

We present a formalism for calculating the non-symmetrized quantum current noise within the Born-Markov approximation for the master equation. The formalism is particularly well suited to obtaining the current noise for quantum transport in mesoscopic devices such as a superconducting single electron transistor (SSET). As an example of the method, we obtain explicit results for the double Josephson-quasiparticle (DJQP) resonance in a SSET. Our calculations reveal the asymmetries that develop in the current noise as well as clarifying the behavior at high frequencies. Our findings are consistent with recent measurements of the asymmetry in the current noise spectrum.

cond-mat.mes-hall↗

arXiv2011

Detection of ultrafast oscillations in Superconducting Point-Contacts by means of Supercurrent Measurements

We present a microscopic calculation of the nondissipative current through a superconducting quantum point contact coupled to a mechanical oscillator. Using the non-equilibrium Keldysh Green function approach, we determine the current-phase relation. The latter shows that at certain phases, the current is sharply suppressed. These dips in the current-phase relation provide information about the oscillating frequency and coupling strength of the mechanical oscillator. We also present an effective two-level model from which we obtain analytical expressions describing the position and width of the dips. Our findings are of relevance for nanomechanical resonators based on superconducting materials.

cond-mat.mes-hall↗

arXiv2010

Charge noise at Cooper-pair resonances

We analyze the charge dynamics of a superconducting single-electron transistor (SSET) in the regime where charge transport occurs via Cooper-pair resonances. Using an approximate description of the system Hamiltonian, in terms of a series of resonant doublets, we derive a Born-Markov master equation describing the dynamics of the SSET. The average current displays sharp peaks at the Cooper-pair resonances and we find that the charge noise spectrum has a characteristic structure which consists of a series of asymmetric triplets of peaks. The strongest feature in the charge noise spectrum is the triplet of peaks centered at zero frequency which has a peak spacing equal to the level separation within the doublets and is similar to the triplet in the spectrum of a driven, damped, two-level system. We also explore the back-action that the SSET charge noise would have on an oscillator coupled to the island charge, measurement of which provides a way of probing the charge noise spectrum.

cond-mat.mes-hall↗

arXiv2009

Resonant magneto-conductance through a vibrating nanotube

We address the electronic resonant transport in presence of a transverse magnetic field through the single level of a suspended carbon nanotube acting as a quantum oscillator. We predict a negative magneto-conductance with a magnetic-field induced narrowing of the resonance line and a reduction of the conductance peak when the nanotube is asymmetrically contacted to the leads. At finite bias voltage we study the threshold for phonon-assisted transport.

cond-mat.mes-hall↗

arXiv2008

Coulomb blockade for an oscillating tunnel junction

We consider a tunnel junction formed between a fixed electrode and an oscillating one. Accumulation of the charge on the junction capacitor induces a force on the nano-mechanical oscillator. The junction is voltage biased and connected in series with an impedance $Z(ω)$. We discuss how the picture of Coulomb blockade is modified by the presence of the oscillator. Quantum fluctuations of the mechanical oscillator modify the $I$-$V$ characteristics particularly in the strong Coulomb blockade limit. We show that the oscillator can be taken into account by a simple modification of the effective impedance of the circuit. We discuss in some details the case of a single inductance $Z(ω)=iLω$ and of a constant resistance $Z(ω)=R$. With little modifications the theory applies also to incoherent transport in Josephson junctions in the tunneling limit.

cond-mat.mes-hall↗

arXiv2006

Distortion blockade in classical nano-electromechanical resonator

We consider a single electron transistor where the central island can oscillate. It has been shown that for weak coupling of the elastic and electric degrees of freedom the position of the island fluctuates with a small variation of the current through the device. In this paper we consider the strong coupling limit. We show that the system undergoes a static mechanical instability that is responsible for the opening of a gap in the current voltage characteristics even at the degeneracy point. We provide an analytical description of the transition point. We also discuss how the mechanical nature of the suppression of the current can be probed experimentally by a slow modulation of the gate voltage.

cond-mat.mes-hall↗

arXiv2006

Dynamics and Current Fluctuations in AC driven Charge Shuttle

The behavior of a charge shuttle under a pure AC field has been recently considered theoretically and experimentally. If the system presents an asymmetry in the tunneling amplitudes the device acts as a nano-electromechanical rectifier, transforming a pure AC voltage field into a direct curren. In this paper we first review the model and the appearance of the rectifying effect for bias voltages below the threshold of self-oscillation. We discuss in some details the dynamics of the central island that, like the current, presents strong dependence on the forcing AC field frequency. In presence of both a constant and a small oscillating bias voltage we analyze the transition from the static to self-oscillating solution. We then consider current fluctuations (full counting statistics) for periodic motion of the grain. We explicitly evaluate the current noise numerically and we find that it shows clear signatures of correlated transport at certain locking frequencies. In the adiabatic limit we obtain a simple expression for the full-counting statistics and calculate explicitly the first four moments.

cond-mat.mes-hall↗

arXiv2006

Frequency dispersion of photon-assisted shot noise in mesoscopic conductors

We calculate the low-frequency current noise for AC biased mesoscopic chaotic cavities and diffusive wires. Contrary to what happens for the admittance, the frequency dispersion is not dominated by the electric response time (the "RC" time of the circuit), but by the time that electrons need to diffuse through the structure (dwell time or diffusion time). Frequency dispersion of noise stems from fluctuations of the Fermi distribution function that preserve charge neutrality. Our predictions can be verified with present experimental technology.

cond-mat.mes-hall↗

arXiv2006

Superharmonic Josephson relation at 0-/$π$-junction transition

Critical current was recently measured near the transition from 0 to $π$-contact in superconductor/ferromagnet/superconductor Josephson junctions. Contrary to expectations, it does not vanish at the transition point. It shows instead a tiny, though finite, minimum. The observation of fractional Shapiro steps reenforces the idea that the vanishing of the main sinusoidal term in the Josephson relation gives room to the next harmonics. Within quasiclassical approach we calculate the Josephson relation taking into account magnetic scattering. We find that the observed minimum is compatible with the value of the second harmonics expected from the theory.

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