Unoccupied Quantum Research Articles

The role of electron confinement in Pd films for the oscillatory magnetic anisotropy in an adjacent Co layer

We demonstrate the interplay between quantum well states in Pd and the magnetic anisotropy in Pd/Co/Cu (0 0 1) by combined scanning tunneling spectroscopy (STS) and magneto optical Kerr effect (MOKE) measurements. Low temperature scanning tunneling spectroscopy reveals occupied and unoccupied quantum well states (QWS) in atomically flat Pd films on Co/Cu (0 0 1). These states give rise to sharp peaks in the differential conductance spectra. A quantitative analysis of the spectra reveals the electronic dispersion of the Pd (0 0 1) d-band (-type) along the -X direction. In situ MOKE experiments on Pd/Co/Cu (1, 1, 13) uncover a periodic variation of the in-plane uniaxial magnetic anisotropy as a function of Pd thickness with a period of 6 atomic layers Pd. STS shows that QWS in Pd cross the Fermi level with the same periodicity of 6 atomic layers. Backed by previous theoretical work we ascribe the variation of the magnetic anisotropy in Co to QWS in the Pd overlayer. Our results suggest a novel venue towards tailoring uniaxial magnetic anisotropy of ferromagnetic films by exploiting QWS in an adjacent material with large spin–orbit coupling.

Comparative analysis of the effects of electron and hole capture on the power characteristics of a semiconductor quantum-well laser

The operating characteristics of a semiconductor quantum-well laser calculated using three models are compared. These models are (i) a model not taking into account differences between the electron and hole parameters and using the electron parameters for both types of charge carriers; (ii) a model, which does not take into account differences between the electron and hole parameters and uses the hole parameters for both types of charge carriers; and (iii) a model taking into account the asymmetry between the electron and hole parameters. It is shown that, at the same velocity of electron and hole capture into an unoccupied quantum well, the laser characteristics, obtained using the three models, differ considerably. These differences are due to a difference between the filling of the electron and hole subbands in a quantum well. The electron subband is more occupied than the hole subband. As a result, at the same velocities of electron and hole capture into an empty quantum well, the effective electron-capture velocity is lower than the effective hole-capture velocity. Specifically, it is shown that for the laser structure studied the hole-capture velocity of 5 × 105 cm/s into an empty quantum well and the corresponding electron-capture velocity of 3 × 106 cm/s into an empty quantum well describe the rapid capture of these carriers, at which the light–current characteristic of the laser remains virtually linear up to high pump-current densities. However, an electron-capture velocity of 5 × 105 cm/s and a corresponding hole-capture velocity of 8.4 × 104 cm/s describe the slow capture of these carriers, causing significant sublinearity in the light–current characteristic.

Interplay between Electronic Properties and Interatomic Spacing in Artificial Gold Chains on NiAl(110)

Monatomic Au chains with 2.89 and 4.08 A interatomic spacing were constructed on a NiAl(110) surface, using atom manipulation techniques with a scanning tunneling microscope. Differential conductance spectra taken on the two chains revealed a series of unoccupied quantum well states that exhibit the parabolic dispersion of a free-electron-like band. The effective electron mass of the band, a measure of the coupling strength between the Au atomic orbitals, was found to be two times smaller in the compact (0.5 me) than that in the loosely packed chain (0.94 me). Moreover, the band onset shifted toward the Fermi level (1.5 versus 0.75 eV) with decreasing distance between the Au atoms. The interrelation between geometric and electronic properties of the two chains was analyzed with density functional theory calculations and tight binding modeling. In the compact chain, the large orbital overlap results in a relatively steep parabolic electron band with strong Au 6sp character. The band flattens out as the int...

Unoccupied electronic structure and relaxation dynamics of Pb/Si(1 1 1)

The unoccupied electronic structure of epitaxial Pb films on Si(111) is analyzed by angle-resolved two-photon photoemission in the Γ¯→M¯ direction close to the Brillouin zone center. The experimental results are compared to density functional theory calculations and we focus on the nature of the interaction of the 6pz states with the Si substrate. The experimentally obtained dispersion E(k||) of the unoccupied quantum well states is weaker than expected for freestanding films, in good agreement with their occupied counterparts. Following E(k||) of quantum well states as a function of momentum at different energies, which are degenerate and non-degenerate with the Si conduction band, we observe no influence of the Si bulk band and conclude a vanishing direct interaction of the Pb 6pz states with the Si band. However, the momentum range at which mixing of 6pz and 6px,y derived subbands is found to occur in the presence of the Si substrate is closer to Γ¯ than in the corresponding freestanding film, which indicates a substrate-mediated enhancement of the mixing of these states. Additional femtosecond time-resolved measurements show a constant relaxation time of hot electrons in unoccupied quantum well states as a function of parallel electron momentum which supports our conclusion of a px,y mediated interaction of the pz states with the Si conduction band.

Spin-orbit coupling in unoccupied quantum well states: Experiment and theory for Co/Cu(001)

The influence of spin-orbit coupling in the unoccupied $s{p}_{z}$ quantum well states in ultrathin Co films is investigated by two-photon photoemission spectroscopy with linearly polarized light. The asymmetry of the experimental two-photon photoemission intensities upon magnetization reversal is as large as 10%, which is in striking contrast to expectations based on the weakness of spin-orbit coupling in the observed quantum well states. The magnetic dichroic signal depends strongly on the polarization of the incident light as well as on the spin polarization of the quantum well states. These findings are explained by detailed electronic structure calculations and one-photon photoemission calculations. Our conclusions are supported by analytical considerations. In this way, a detailed picture of spin-orbit coupling in the unoccupied Co/Cu(001) quantum well states is provided.

Lifetimes of quantum well states and resonances in Pb overlayers on Cu(111)

We present results of calculations of the lifetimes of excited electrons (holes) in quantum well states and quantum well resonances in Pb overlayers supported on Cu(111). Many-body decay via inelastic energy relaxation and one-electron decay via energy-conserving one-electron transfer into the substrate are considered. One-electron energies and wave functions have been computed for different coverages of the Pb overlayer (from 1 to 18 monolayers) by using a one-dimensional pseudopotential for the entire overlayer-substrate system in the framework of density functional theory within the local density approximation. The elastic (energy-conserving resonant electron transfer) contribution to the total lifetime broadening of quantum well resonances has been calculated within the wave packet propagation method. The inelastic electron-electron (many-body) contribution to the lifetime broadening of both occupied and unoccupied quantum well states has been evaluated using $GW$ approximation. The decay mechanisms of both quantum well states and quantum well resonances in thick overlayers are discussed.

Magnetic Dichroism from Optically Excited Quantum Well States

We demonstrate magnetic dichroism from optically excited states in two-photon photoemission. Using ultrathin cobalt films grown on Cu(001), we observe unoccupied quantum well states which give rise to a sizable intensity change in photoemission under magnetization reversal. The simultaneous comparison of both circular and linear magnetic dichroism in the same system permits us to check fundamental symmetry requirements and allows us to explicitly elucidate the common origin of both effects. Based on our observations we argue that the observed effect is related to spin-orbit coupling in the intermediate quantum well states.

Ultrafast electron dynamics in Pb/Si(111) investigated by two-photon photoemission

Ultrafast electron dynamics in Pb/Si(111) quantum well structures have been investigated by femtosecond time-resolved two-photon photoemission spectroscopy. Three unoccupied quantum well states (or subbands) as well as an image potential state are identified and analyzed as a function of Pb thickness. Using 1.9 eV photon energy for optical excitation electron-hole pairs are formed in the Pb film and in the Si substrate. The hot electron population in the quantum well states exhibits a biexponential decay and both decay times increase when the energy difference to the Fermi level is reduced. The slower decay component with decay times of 130 and 900 fs is attributed to delayed filling of the states in the adlayer due to electronic relaxation in the Si(111) substrate. The faster decay (30--140 fs) is caused by electron-electron scattering in the Pb film. At a first glance, the extracted decay times increase toward the Fermi level as predicted by the Fermi-liquid theory. However, by investigation of the quantized electronic structure more detailed insight into scattering among defined subbands has been gained. The rise of the excited electron population in the lowest unoccupied quantum well state is delayed by 70 fs which occurs concomitantly with a decay in the second lowest state. We describe this simultaneous decay and rise in the two adjacent subbands by coupled rate equations and attribute it to intersubband scattering. Thereby, we demonstrate the importance of intersubband decay in low-dimensional metallic systems in addition to intraband scattering processes considered in Fermi-liquid theory and derived descriptions.

Spin polarization of Ag/Fe(100) quantum well empty states

Unoccupied quantum well states in Ag/Fe(100) have been investigated by means of spin polarized inverse photoemission. These states, confined to the noble metal overlayer, have been found to be polarized, showing a predominant minority spin character. The results are discussed in terms of the different confinement conditions for the two spin states.