Nanoscale resonators that oscillate at high frequencies are useful in many measurement applications.We studied a high-quality mechanical resonator made from a suspended carbon nanotube driven into motion by applying a periodic radio frequency potential using a nearby antenna. Single-electron charge fluctuations created periodic modulations of the mechanical resonance frequency. A quality factor exceeding 10 5 allows the detection of a shift in resonance frequency caused by the addition of a single-electron charge on the nanotube. Additional evidence for the strong coupling of mechanical motion and electron tunneling is provided by an energy transfer to the electrons causing mechanical damping and unusual nonlinear behavior. We also discovered that a direct current through the nanotube spontaneously drives the mechanical resonator, exerting a force that is coherent with the high-frequency resonant mechanical The combination of a high resonance frequency and a small mass also makes nanomechanical resonators attractive for a fundamental study of mechanical motion in the quantum limit [6, 7, 8, 9]. For a successful observation of quantum motion of a macroscopic object, a high-frequency nanoscale resonator must have low dissipation (which implies a high quality-factor Q), and a sensitive detector with minimum back-action (i.e. quantum limited) [10, 11]. Here, we demonstrate a dramatic backaction that strongly couples a quantum dot detector to the resonator dynamics of a carbon nanotube, and which, in the limit of strong feedback, spontaneously excites large amplitude resonant mechanical motion.Nanomechanical resonators have been realized by etching down larger structures. In small devices, however, surfaces effects impose a limit on the quality-factor [2]. Alternatively, suspended carbon nanotubes can be used to avoid surface damage from the (etching) fabrication process. We recently developed a mechanical resonator based on an ultra-clean carbon nanotube with high resonance frequencies of several 100 MHz and a Q exceeding 10 5 [12]. Here, we exploit this resonator to explore a strong coupling regime between single electron tunneling and nanomechanical motion. We followed the pioneering approaches in which aluminium single electron transistors were used as position detectors [6, 7, 8] and AFM cantilevers as resonators [13,14,15]; however, our experiment is in the limit of much stronger electro-mechanical coupling, achieved by embedding a quantum dot detector in the nanomechanical resonator itself.Our device consists of a nanotube suspended across a trench that makes electrical contact to two metal electrodes ( Fig. 1). Electrons are confined in the nanotube by Schottky barriers at the Pt metal contacts, forming a quantum dot in the suspended segment. The nanotube growth is the last step in the fabrication process, yielding ultra-clean devices [16], as demonstrated by the four-fold shell-filling of the Coulomb peaks (Fig. 1C). All measurements were performed at a temperature of 20 mK with an electron temperat...

We have observed the transversal vibration mode of suspended carbon nanotubes at millikelvin temperatures by measuring the single-electron tunneling current. The suspended nanotubes are actuated contact-free by the radio frequency electric field of a nearby antenna; the mechanical resonance is detected in the time-averaged current through the nanotube. Sharp, gate-tunable resonances due to the bending mode of the nanotube are observed, combining resonance frequencies of up to nu(0) = 350 MHz with quality factors above Q = 10(5), much higher than previously reported results on suspended carbon nanotube resonators. The measured magnitude and temperature dependence of the Q factor shows a remarkable agreement with the intrinsic damping predicted for a suspended carbon nanotube. By adjusting the radio frequency power on the antenna, we find that the nanotube resonator can easily be driven into the nonlinear regime.

We demonstrate how molecular quantum states of coupled semiconductor quantum dots are coherently probed and manipulated in transport experiments. The applied method probes quantum states by the virtual cotunneling of two electrons and hence resolves the sequences of molecular states simultaneously. This result is achieved by weakly probing the quantum system through parallel contacts to its constituting quantum dots. The overlap of the dots' wave functions and, in turn, the splitting of molecular states are adjusted by the direct influence of coupling electrodes.

Understanding the interplay between many-body phenomena and non-equilibrium in systems with entangled spin and orbital degrees of freedom is a central objective in nano-electronics. We demonstrate that the combination of Coulomb interaction, spin-orbit coupling and valley mixing results in a particular selection of the inelastic virtual processes contributing to the Kondo resonance in carbon nanotubes at low temperatures. This effect is dictated by conjugation properties of the underlying carbon nanotube spectrum at zero and finite magnetic field. Our measurements on a clean carbon nanotube are complemented by calculations based on a new approach to the nonequilibrium Kondo problem which well reproduces the rich experimental observations in Kondo transport.

We investigate quantum dots in clean single-wall carbon nanotubes with ferromagnetic PdNi-leads in the Kondo regime. In most odd Coulomb valleys the Kondo resonance exhibits a pronounced splitting, which depends on the tunnel coupling to the leads and an external magnetic field B, and only weakly on gate voltage. Using numerical renormalization group calculations, we demonstrate that all salient features of the data can be understood using a simple model for the magnetic properties of the leads. The magnetoconductance at zero bias and low temperature depends in a universal way on gµB(B − Bc)/kBTK, where TK is the Kondo temperature and Bc the external field compensating the splitting.PACS numbers: 73.23. Hk, 73.63.Fg, 72.15.Qm, The Kondo effect resulting from the exchange interaction of a single spin with a bath of conduction electrons [1], is one of the archetypical phenomena of many-body physics. Its competition with ferromagnetism and possible applications in spintronics [2] have raised wide interest in the past few years. The Kondo effect in quantum dots [3,4] has, in recent experiments, been investigated in the presence of ferromagnetic (FM) leads [5][6][7]. It was found that the Kondo resonance, usually observed at zero bias in the odd Coulomb blockade (CB) valleys, is split into two peaks at finite bias [5]. The splitting consists of a term depending logarithmically on gate voltage [6,7], and, as demonstrated here, a second term nearly independent of gate voltage. These phenomena were predicted theoretically [8][9][10][11], attributing the splitting of the Kondo resonance to a tunneling induced exchange field, which results from the magnetic polarization of the leads. So far no detailed and quantitative comparison of the measured conductance with the theory has been undertaken to verify whether the simplistic description of FM leads used in Refs. 8-11 has quantitative predictive power. The latter would be needed for future spintronics applications that exploit the lead-induced local spin splitting, e.g., spin filtering.In this Letter we present low-temperature transport measurements of a single wall carbon nanotube quantum dot with PdNi leads. We concentrate on the less studied gate-independent contribution of the exchange splitting of the Kondo resonance and attribute it to the saturation magnetization of the contact material. We show that the evolution of the conductance with magnetic field and gate voltage can be understood within a simple model for the magnetization and polarization in the FM leads, by presenting numerical renormalization group (NRG) calculations for this model, using parameters extracted from experiment. Moreover, by comparing resonances of different transparency, we demonstrate a universal scaling of the magnetic field dependence of the Kondo conductance, proving that the magnetization of the leads can indeed be viewed as an exchange field, which acts analogously to an external magnetic field. Experimental setup.-The nanotubes are grown by chemical vapor deposition on a highly doped ...