Excitation of spin waves by tunneling electrons in ferromagnetic and antiferromagnetic spin-1/2 Heisenberg chains

14 páginas, 14 figuras.-- PACS number(s): 68.37.Ef, 72.25.−b, 73.23.−b, 75.30.DsExcitation of finite chains of magnetic atoms adsorbed on a surface by tunneling electrons from a scanning tunneling microscope tip is studied using a Heisenberg Hamiltonian description of the magnetic couplings along the chain and a strong coupling approach to inelastic tunneling. The excitation probability of the magnetic levels is very high and the excitation spectra in chains of different lengths are very similar. The excitations in finite chains can be considered as spin waves quantized in the finite object. The energy and momentum spectra of the spin waves excited in the idealized infinite chain by tunneling electrons are determined from the results on the finite chains. Both ferromagnetic and antiferromagnetic couplings are considered, leading to very different results. In particular, in the antiferromagnetic case, excitations linked to the entanglement of the chain ground state are evidenced.Peer reviewe

Magnetic reversal of a quantum nanoferromagnet

When the external magnetic field applied to a ferromagnetically coupled atomic chain is reversed suddenly, the magnetization of the chain switches, due to the reversal of all the atomic magnetic moments in the chain. The quantum processes underlying the magnetization switching and the time required for the switching are analyzed for model magnetic chains adsorbed on a surface at 0 K. The sudden field reversal brings the chain into an excited state that relaxes towards the system ground state via interactions with the substrate electrons. Different mechanisms are outlined, ranging from the global stepwise rotation of the chain macrospin induced by spin-flip collisions with substrate electrons in the pure Heisenberg chain (Néel-Brown process) to a correlation-mediated direct switching process in the presence of strong magnetic anisotropies in short chains (the global spin of the chain reverses in a single electron interaction). The processes for magnetization switching induced by electrons tunneling from a scanning tunneling microscope tip are also analyzed. © 2013 American Physical Society.Peer Reviewe

Extremely long-lived magnetic excitations in supported Fe chains

We report on a theoretical study of the lifetime of the first excited state of spin chains made of an odd number of Fe atoms on Cu2N/Cu(100). Yan et al (Nat. Nanotech. 10, 40 (2015)) recently observed very long lifetimes in the case of Fe3 chains. We consider the decay of the first excited state induced by electron-hole pair creation in the substrate. For a finite magnetic field, the two lowest-lying states in the chain have a quasi-N\'eel state structure. Decay from one state to the other strongly depends on the degree of entanglement of the local spins in the chain. The entanglement in the chain accounts for the long lifetimes that increase exponentially with chain length. Despite their apparently very different properties, the behaviour of odd and even chains is governed by the same kind of phenomena, in particular entanglement effects. The present results account quite well for the lifetimes recently measured by Yan et al on Fe3Comment: 21 page

R-matrix calculation of electron collisions with electronically excited O2 molecules

Low-energy electron collisions with O$_2$ molecules are studied using the fixed-bond R-matrix method. In addition to the O$_2$ ${X}^3\Sigma_{g}^-$ ground state, integrated cross sections are calculated for elecron collisions with the ${a}^1\Delta_{g}$ and ${b}^1\Sigma_{g}^+$ excited states of O$_2$ molecules. 13 target electronic states of O$_2$ are included in the model within a valence configuration interaction representations of the target states. Elastic cross sections for the ${a}^1\Delta_{g}$ and ${b}^1\Sigma_{g}^+$ excited states are similar to the cross sections for the ${X}^3\Sigma_{g}^-$ ground state. As in case of excitation from the ${X}^3\Sigma_{g}^-$ state, the O$_2^-$ $\Pi_u$ resonance makes the dominant contribution to excitation cross sections from the ${a}^1\Delta_{g}$ and ${b}^1\Sigma_{g}^+$ states. The magnitude of excitation cross sections from the ${a}^1\Delta_{g}$ state to the ${b}^1\Sigma_{g}^+$ state is about 10 time larger than the corresponding cross sections from the ${X}^3\Sigma_{g}^-$ to the ${b}^1\Sigma_{g}^+$ state. For this ${a}^1\Delta_{g}$ $\to$ ${b}^1\Sigma_{g}^+$ transition, our cross section at 4.5 eV agrees well with the available experimental value. These results should be important for models of plasma discharge chemistry which often requires cross sections between the excited electronic states of O$_2$.Comment: 26 pages, 10 figure

Correlation-mediated processes for electron-induced switching between Néel states of Fe antiferromagnetic chains

The controlled switching between two quasistable Néel states in adsorbed antiferromagnetic Fe chains has recently been achieved by Loth et al. using tunneling electrons from an STM tip. In order to rationalize their data, we evaluate the rate of tunneling electron-induced switching between the Néel states. Good agreement is found with the experiment, permitting us to identify three switching mechanisms: (i) low STM voltage direct electron-induced transitions, (ii) intermediate STM voltage switching via spin-wave-like excitation, and (iii) high STM voltage transitions mediated by domain-wall formation. Spin correlations in the antiferromagnetic chains are the switching driving force, leading to a marked chain-size dependence. © 2013 American Physical Society.M.Y. and X. C. acknowledge financial support through Spain’s MINECO under Contract No. TEC2009-06986.Peer Reviewe

Inelastic effects in electron transport studied with wave packet propagation

A time-dependent approach is used to explore inelastic effects during electron transport through few-level systems. We study a tight-binding chain with one and two sites connected to vibrations. This simple but transparent model gives insight about inelastic effects, their meaning and the approximations currently used to treat them. Our time-dependent approach allows us to trace back the time sequence of vibrational excitation and electronic interference, the ibrationally introduced time delay and the electronic phase shift. We explore a full range of parameters going from weak to strong electron-vibration coupling, from tunneling to contact, from one-vibration description to the need of including all vibrations for a correct description of inelastic effects in transport. We explore the validity of single-site resonant models as well as its extension to more sites via molecular orbitals and the conditions under which multi-orbital, multi-vibrational descriptions cannot be simplified. We explain the physical meaning of the spectral features in the second derivative of the electron current with respect to the bias voltage. This permits us to nuance the meaning of the energy value of dips and peaks. Finally, we show that finite-band effects lead to electron back-scattering off the molecular vibrations in the regime of high-conductance, although the drop in conductance at the vibrational threshold is rather due to the rapid variation of the vibronic density of states.Comment: 38 pages, 14 figure

Tunneling electron induced rotation of a copper phthalocyanine molecule on Cu(111)

The rates of a hindered molecular rotation induced by tunneling electrons are evaluated using scattering theory within the sudden approximation. Our approach explains the excitation of copper phthalocyanine molecules (CuPc) on Cu(111) as revealed in a recent measurement of telegraph noise in a scanning tunneling microscopy experiment. A complete explanation of the experimental data is performed by computing the geometry of the adsorbed system, its electronic structure, and the energy transfer between tunneling electrons and the molecule's rotational degree of freedom. The results unambiguously show that tunneling electrons induce a frustrated rotation of the molecule. In addition, the theory determines the spatial distribution of the frustrated rotation excitation, confirming the striking dominance of two out of four molecular lobes in the observed excitation process. This lobe selectivity is attributed to the different hybridizations with the underlying substrate. © 2013 American Physical Society.J.S., A.S., C.A.B., and R.M. gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft through the SFB616 ‘Energy Dissipation at Surfaces.’ N.L. is supported by the ICT-FET Integrated Project AtMol (http://www.atmol.eu). M.C.C. thanks the Studienstiftung desdeutschen Volkes.Peer Reviewe

Isotope effect for associative detachment: H(D)−+H(D)→H2(D2)+e

We report experimental and theoretical results for associative detachment (AD) of D−+D→D2+e−. We compare these data to our previously published results for H−+H→H2+e−. The measurements show no significant isotope effect in the total cross section. This is to be contrasted with previously published experimental and theoretical work which has found a significant isotope effect in diatomic systems for partial AD cross sections, i.e., as a function of the rotational and vibrational levels of the final molecule formed. Our work implies that though the rovibrational distribution of flux is different for AD of H− + H and D− + D, the total flux for these two systems is essentially the same when summed over all possible final channels

Optimization of a Langmuir-Taylor detector for lithium

This paper describes the construction and optimization of a Langmuir-Taylor detector for lithium, using a rhenium ribbon. The absolute detection probability of this very sensitive detector is measured and the dependence of this probability with oxygen pressure and surface temperature is studied. Sources of background signal and their minimization are also discussed in details. And a comparison between our data concerning the response time of the detector and literature values is given. A theoretical analysis has been made: this analysis supports the validity of the Saha-Langmuir law to relate the ionization probability to the work function. Finally, the rapid variations of the work function with oxygen pressure and temperature are explained by a chemical equilibrium model.Comment: 11 pages, 7 figures, to appear in Rev. Sci. Instru