arXiv subjects

J. Fransson

Publications and source records attributed to J. Fransson.

At least 73 records · Page 4Linked to original sources

arXiv2009

Spin Inelastic Electron Tunneling Spectroscopy on Local Spin Adsorbed on Surface

The recent experimental conductance measurements taken on magnetic impurities on metallic surfaces, using scanning tunneling microscopy technique and suggesting occurrence of inelastic scattering processes, are theoretically addressed. We argue that the observed conductance signatures are caused by transitions between the spin states which have opened due to e.g. exchange coupling between the local spins and the tunneling electrons, and are directly interpretable in terms of inelastic transitions energies. Feasible measurements using spin-polarized scanning tunneling microscopy that would enable new information about the excitation spectrum of the local spins are discussed.

cond-mat.mes-hall↗

arXiv2008

Detection of the spin character of Fe(001) surface states by scanning tunneling microscopy: A theoretical proposal

We consider the magnetic structure on the Fe(001) surface and theoretically study the scanning tunneling spectroscopy using a spin-polarized tip (SP-STM). We show that minority-spin surface states induce a strong bias dependence of the tunneling differential conductance which largely depends on the orientation of the magnetization in the SP-STM tip relative to the easy magnetization axis in the Fe(001) surface. We propose to use this effect in order to determine the spin character of the Fe(001) surface states. This technique can be applied also to other magnetic surfaces in which surface states are observed.

cond-mat.mtrl-sci↗

arXiv2008

Vibrational coherence in electron spin resonance in nanoscale oscillators

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 non-equilibrium spin- and charge currents are investigated. Inelastic side-band anti-resonances corresponding to the vibrational modes appear in the electron spin resonance spectrum.

cond-mat.mes-hall↗

arXiv2008

Vibrating Superconducting Island in a Josephson Junction

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 generate a supercurrent which 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.

cond-mat.mes-hall↗

arXiv2008

Detection of spin reversal and nutations through current measurements

The dynamics of a single spin embedded in a the 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.

cond-mat.mes-hall↗

arXiv2008

Pauli spin blockade in carbon nanotube double quantum dots

We report Pauli spin blockade in an impurity defined carbon nanotube double quantum dot. 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.

cond-mat.mes-hall↗

arXiv2008

Subnanosecond switching of local spin-exchange coupled to ferromagnets

The dynamics of a single spin embedded in a the tunnel junction between ferromagnetic contacts is strongly affected by the exchange coupling to the tunneling electrons. Using time-dependent equation of motion for the spin under influence of the spin-polarized tunneling current, it is shown that the magnetic field induced by bias voltage pulses allows for sub-nanosecond 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.

cond-mat.mes-hall↗

arXiv2007

Spin Dynamics in a Tunnel Junction between Ferromagnets

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.

cond-mat.mes-hall↗

arXiv2007

Detection of exchange interaction in STM measurements through Fanolike interference effects

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.

cond-mat.mes-hall↗

arXiv2007

Surface imaging of inelastic Friedel oscillations

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 $\partial^2I(\br, V)/\partial V^2$. 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.

cond-mat.mes-hall↗

arXiv2007

Exchange interaction and Fano resonances in diatomic molecular systems

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.

cond-mat.mes-hall↗

arXiv2006

Formation of pure two-electron triplet states in weakly coupled quantum dots attached to ferromagnetic leads

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 $S_z=1$ or -1. Furthermore, additional spin blockade regimes emerge because of full occupation in states with opposite spin to that of the half-metallic lead.

cond-mat.mes-hall↗

arXiv2006

Non-equilibrium triplet blockade in parallel coupled quantum dots

It is demonstrated that parallel weakly 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.

cond-mat.mes-hall↗

arXiv2006

Nuclear Spin Noise and STM Noise Spectroscopy

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 Tunnelling Microscope (STM). We argue that the noise spectroscopy of electronic spin can reveal the nuclear spin dynamics due to hyperfine coupling. Tunnelling 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.

cond-mat.mes-hall↗

arXiv2005

Pauli spin blockade in weakly coupled quantum dots

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.

cond-mat.mes-hall↗

arXiv2005

Angular Conductance Resonances of Quantum Dots Non-Collinearly Coupled to Ferromagnetic Leads

The zero bias conductance of quantum dots coupled to ferromagnetic leads is investigated. In the strong coupling regime, it is found that the conductance is a non-monotonic function of the angle between the magnetisation directions in the two contacts. This behaviour is an effect of the presence of the leads which induces an angle dependent spin split of the quantum dot states, and spin flip transitions between the quantum dot states whenever the magnetisation directions of the leads are non-collinear which enhances the current density at the chemical potential. In the weak coupling regime, the system reverts to normal spin valve character.

cond-mat.mes-hall↗

arXiv2005

Non-equilibrium theory for strongly coupled quantum dot with arbitrary on-site correlation strength

An analytical expression for the current through a single level quantum dot for arbitrary strength of the on-site electron-electron interaction is derived beyond standard mean-field theory. By describing the localised states in terms of many-body operators, the employed diagrammatic technique for strong coupling enables inclusion of electron correlation effects into the description of the local dynamics, which provides transport properties that are consistent with recent experimental data.

cond-mat.mes-hall↗

arXiv2005

Signatures of band-like tunnelling in granular nanowires

We explore the problem of tunneling through disorderd nanowires, comprised of a random distribution of metallic grains, by means of a many-body model that captures the essential physics of the system. The random configuration of grains gives rise to a smooth band-like set of states, which mediates current flow through the nanowire. Analytical and numerical calculations show the characteristic signature of this unusual band-like transport to be a quadratic variation of the current as a function of the applied voltage (i.e. $I\sim V^2$), a variation that is clearly observed in experimental studies of Pt/C composite nanowires.

cond-mat.mes-hall↗