Abstract: We consider a combined nanomechanical-supercondcuting device that allows the Cooper pair tunneling to interfere with the mechanical motion of the middle superconducting island. Coupling of mechanical oscillations of a superconducting island between two superconducting leads to the electronic tunneling generates a supercurrent that is modulated by the oscillatory motion of the island. This coupling produces alternating finite and vanishing supercurrent as function of the superconducting phases. Current peaks are sensitive to the superconducting phase shifts relative to each other. The proposed device may be used to study the nanoelectromechanical coupling in case of superconducting electronics.
Abstract: We report Pauli spin blockade in a carbon nanotube double quantum dot defined by tunnel barriers at the contacts and a structural defect in the nanotube. We observe a pronounced current suppression for negative source-drain bias voltages, which is investigated for both symmetric and asymmetric coupling of the quantum dots to the leads. The measured differential conductance agrees well with a theoretical model of a double quantum dot system in the spin-blockade regime, which allows us to estimate the occupation probabilities of the relevant singlet and triplet states. This work shows that effective spin-to-charge conversion in nanotube quantum dots is feasible and opens the possibility of single-spin readout in a material that is not limited by hyperfine interaction with nuclear spins.
Abstract: The dynamics of a single spin embedded in a tunnel junction between ferromagnetic contacts is strongly affected by the exchange coupling to the tunneling electrons. Moment reversal of the local spin induced by the bias voltage across the junction is shown to have a measurable effect on the tunneling current. Furthermore, the frequency of a harmonic bias voltage is picked up by the local spin dynamics and transferred back to the current, generating a double frequency component.
Abstract: The dynamics of a single spin embedded in the tunnel junction (quantum point contact) between ferromagnets is addressed. Using the Keldysh technique, we derive a quantum Langevin equation. As a consequence of the spin-polarization in the leads, the spin displays a rich and unusual dynamics. Parallel configured and equally strong magnetic moments in the leads yield an ordinary spin precession with a Larmor frequency given by the effective magnetic field. Unequal and/or non-parallel configured magnetization, however, causes nutation of the spin in addition to the precession. Our predictions may be directly tested for macroscopic spin clusters.
Abstract: We study a scheme for electrical detection, using electron spin resonance, of coherent vibrations in a molecular single electron level trapped near a conduction channel. Both equilibrium spin currents and nonequilibrium spin and charge currents are investigated. Inelastic side-band antiresonances corresponding to the vibrational modes appear in the electron spin resonance spectrum.
Abstract: The dynamics of a single spin that is embedded in a tunnel junction between ferromagnetic contacts is strongly affected by the exchange coupling to the tunneling electrons. By using time-dependent equations of motion for the spin, which is under the influence of a spin-polarized tunneling current, it is shown that the magnetic field induced by bias voltage pulses allows for a subnanosecond switching of the local spin and the possibility of spin reversal is illustrated. Furthermore, it is shown that the time evolution of the Larmor frequency sharply peaks around the spin-flip event and it is argued that this feature can be used as an indicator for the spin flip.
Abstract: We address Fano-like interference effects in scanning tunneling microscopy (STM) measurements of nanoscale systems, e.g. two-level systems. Common for these systems is that second order tunneling contributions give rise to interference effects that cause suppressed transmission through the system for certain energies. The suppressed transmission is measurable either in the differential conductance or in the bias voltage derivative thereof.
Abstract: Impurities that are present on the surface of a metal often have
internal degrees of freedom. Inelastic scattering due to
impurities can be revealed by observing local features seen in
the tunneling current with
scanning
tunneling microscope (STM). We consider
localized vibrational modes coupled to the electronic
structure of a surface. We argue that
vibrational modes of impurities produce Fermi momentum $k_F$
oscillations in second derivative of
current with respect to voltage $\partial^2I(\bfr, V)/\partial V^2$. These
oscillations are similar to the well known Friedel
oscillations of screening charge on the surface.
We propose to measure inelastic scattering generated by the
presence of the vibrational modes
with STM by imaging the $\partial^2I/\partial V^2$
oscillations on the metal surface.
Abstract: We propose a mechanism to use scanning tunneling microscopy (STM) for direct measurements of the two-electron singlet-triplet exchange splitting $J$ in diatomic molecular systems, unsing the coupling between the molecule and the substrate electrons. The different pathways for electrons lead to interference effects and generate kinks in the differential conductance at the energies for the singlet and triplet states. These features are related to Fano resonance due to the branched electron wave functions. The ratio between the tunneling amplitudes through the two atoms can be modulated by spatial movements of the tip along the surface.
Abstract: It is theoretically demonstrated that parallel weakly tunnel coupled quantum dots exhibit non-equilibrium blockade regimes caused by a full occupation in the spin triplet state, in analogy to the Pauli spin blockade in serially weakly coupled quantum dots. Charge tends to accumulate in the two-electron triplet for bias voltages that support transitions between the singlet and three-electron states.
Abstract: In a two-level system, constituted by two serially coupled single level quantum dots, coupled to external leads we find that the current is suppressed in one direction of biasing caused by a fully occupied two-electron triplet state in the interacting region. The efficiency of the current suppression is governed by the ratio between the interdot tunnelling rate and the level off-set. In the opposite bias direction, the occupation of the two-electron triplet is lifted which allows a larger current to flow through the system, where the conductance is provided by transitions between one-electron states and two-electron singlet states. Is is also shown that a finite ferromagnetic interdot exchange interaction provides an extended range of the current suppression, while an anti-ferromagnetic exchange leads to a decreased range of the blockade regime.
Abstract: Weakly coupled quantum dots in the Pauli spin blockade regime are considered with respect to spin-dependent transport. By attaching one half-metallic and one non-magnetic lead, the Pauli spin blockade if formed by a pure triplet state with spin moment $S=1$ or $S=-1$. Furthermore, additional spin blockade regimes emerge because of full occupation in states with opposite spin to that of the half-metallic lead.
Abstract: We consider fluctuations of the electronic spin due to coupling to nuclear spin. Noise spectroscopy of an electronic spin can be revealed in the Scanning tunneling Microscope (STM). We argue that the noise spectroscopy of electronic spin can reveal the nuclear spin dynamics due to hyperfine coupling. tunneling current develops satellites of the main lines at Larmor frequency and at zero frequency due to hyperfine coupling. We also address the role of the rf field that is at or near the resonance with the nuclear hyperfine field. This approach is similar to Electron Nuclear Double Resonance (ENDOR), in that is allows one to detect nuclear spin dynamics indirectly through its effect on electronic spin.
Abstract: The dynamics of a single spin embedded in a tunneling junction is
studied. Within a nonequilibrium Keldysh Green's function technique,
we derive a quantum Langevin equation describing the spin dynamics.
At high temperature limit, it reduces to a Bloch equation, for which
the spin relaxation rate as determined by the temporal fluctuation
is linearly proportional to the temperature. In the opposite limit,
the relaxation rate depends on the applied voltage, in contrast to
the case of a spin in an equilibrium environment. We also show that
spin-flip transition processes during the electron tunneling
converts the applied electric field (i.e., voltage bias) into an
effective magnetic field. Consequently, the dynamics of the spin,
otherwise precessing along the static magnetic field, will have
either a frequency shift proportional to the dc bias or a magnetic
resonance driven indirectly by an ac electric field at the Larmor
frequency $\omega_{L}$. An experiment to measure this effect is also
proposed.
