Session W27: Invited Session: Electrons, Spins, and Collective Modes in Nanofilms

Plasmons under extreme dimensional confinement

In our studies, we explore how surface and bulk plasmons emerge under extreme dimensional confinement, i.e., dimensions that are orders of magnitude smaller than those employed in `nanoplasmonics'. Atomically-smooth ultrathin Mg films were epitaxially grown on Si(111), allowing for atomically-precise tuning of the plasmon response.\footnote{M.M. \"{O}zer, E.J. Moon, A.G. Eguiluz, and H.H. Weitering, Phys. Rev. Lett. \textbf{106}, 197601 (2011).} While the single-particle states in these 3-12 monolayer (ML) thick films consist of a series of two-dimensional subbands, the bulk-plasmon response is like that of a thin slice carved from bulk Mg subject to quantum-mechanical boundary conditions. Remarkably, this bulk-like behavior persists all the way down to 3 ML. In the 3-12 ML thickness range, bulk loss spectra are dominated by the n=1 and n=2 normal modes, consistent with the excitation of plasmons involving quantized electronic subbands. The collective response of the thinnest films is furthermore characterized by a thickness-dependent spectral weight transfer from the high-energy collective modes to the low-energy single-particle excitations, until the bulk plasmon ceases to exist below 3 ML. Surface- and multipole plasmon modes even persist down to 2 ML. These results are striking manifestations of the role of quantum confinement on plasmon resonances in precisely controlled nanostructures. They furthermore suggest the intriguing possibility of tuning resonant plasmon frequencies via precise dimensional control.   

Radial band structure of a ultrathin liquid metal film

Understanding the properties of strongly disordered materials has been one of the long standing and most challenging problems in condensed matter physics. One major difficulty lies in the failure of the band structure concept to describe electronic properties due to the lack of the periodicity, as notorious for electrons in liquid metals without any well-established and well-tested alternative until now. In this work, we experimentally, using angle-resolved photoelectron spectroscopy, establish the formation of an intriguing ``double radial band structure'' in a strongly disordered electronic system of a liquid metal Pb. A monolayer Pb film was formed on Si(111) with an unusually low melting temperature and its detailed band dispersions and Fermi contours were mapped throughout the melting process. Furthermore, we introduce the way to understand this characteristic band structure based on an old theoretical idea proposed in 1962 invoking the coherent radial scattering of electrons, which can be widely encountered in wave scatterings within disordered media. In conclusion, liquid metals, or possibly other strongly disordered electronic systems, have well defined radial bandstructures through the coherent radial scattering of electrons and the radial correlation of atoms.   

Ultrafast mass transport during decay of gigantic Pb mesas on Ni(111)

We have studied the initial growth of Pb films on Ni(111) at elevated temperatures of 424 K and 474 K. Quantum Well States (QWS's) have been found to be responsible for the morphology of these Pb films on Ni(111). The delicate balance between surface energies, elastic energies and QWS's is initially tilted towards QWS's, as discrete layer heights are observed. First, a strong preference for 5 and 7 layers high, flat topped Pb islands is observed, showing several striking similarities with Pb films on Si(111) and on Cu(111). Key examples of these will be discussed. When the character of the rough film gradually changes from 2D to 3D, the balance between these forces becomes more and more dominated by interfacial energies. A tipping point is reached by very slowly heating the surface to about 520 K. As the energetic balance is tipped for good in favor of the interface free energy, the electronically stabilized, extended, about 40 layers high mesas suddenly decay towards compact hemispheric structures. The spectacular speed at which the transition takes place (billions of atoms move over several microns during a few milliseconds!) is many orders of magnitude larger than what is expected, based on arguments involving thermally activated behavior on atomic scales. I will discuss peculiarities of the wetting layer and its changes, which appear to coincide with the ultrafast transition of the film morphology. With a widespread interest in nanostructures in general, our results illustrate the generic need to characterize all aspects of nanostructures, both structural and electronic, since small excursions away from equilibrium can have dramatic consequences. T.R.J. Bollmann, R. van Gastel, H.J.W. Zandvliet and B. Poelsema; Phys. Rev. Lett. 107, 116101 (2011); T.R.J. Bollmann, R. van Gastel, H.J.W. Zandvliet and B. Poelsema; New J. Phys. 13, 103025 (2011).   

Regulating spin and Fermi surface topology of a quantum metal film by the surface (interface) monatomic layer

Spin and current controls in solids have been one of the central issues in researches of electron and spin transport. Nowadays, electronics/spintronics deals with nanometer- or atomic-scale structures and miniaturization of these systems implies emergence of various quantum phenomena, intimately linked to the formation of electronic states different from those of the corresponding bulk materials. For example, valence electrons of films with the thickness comparable to the electron wavelength form discrete quantum-well states (QWSs) under opportune conditions of confinement (quantum size effect). Furthermore, the size reduction also increases the surface/volume ratio and a film possibly changes its electronic (spin) properties by the surface effect. Concerning metal films, the quantum size effect requires the thickness in a range of nanometers and the length corresponds to several tens of atoms, indicating the very large ratio of a surface (interface) monatomic layer to film atomic layers. Thus, we have been interested in combining the quantum size effects and the surface effect on the metal films to induce new physical phenomena. In the present talk, two research cases are shown. 1) Instead of isotropic two-dimensional in-plane states expected for an isolated metal film, quasi-one-dimensional quantized states were measured by photoemission spectroscopy in an epitaxial Ag(111) ultra thin film, prepared on an array of atomic chains [1]. 2) High-resolution spin-resolved photoemission and magneto-transport experiments of ultrathin Ag(111) films, covered with a /3$\times$/3-Bi/Ag surface ordered alloy, were performed. The surface state (SS) bands, spin-split by the Rashba interaction, selectively couple to the originally spin-degenerate QWS bands in the metal film, making the spin-dependent hybridization [2,3]. Magnetoconductance of the films, measured in situ by the micro-four-point probe method as a function of the applied magnetic field [4], has shown that the formation of the Rashba-type surface alloy reduces the spin-relaxation time in the ultrathin film significantly [5]. These results demonstrate that spin and Fermi surface topology of a quantum metal film can be regulated by the surface (interface) monatomic layer.\\[0pt] [1] T. Okuda, Y. Takeichi, K. He, A. Harasawa, A. Kakizaki, and I. Matsuda, Phys. Rev. B 80, 113409 (2009).\\[0pt] [2] K. He, T. Hirahara, T. Okuda, S. Hasegawa, A. Kakizaki, and I. Matsuda, Phys. Rev. Lett. 101, 107604 (2008).\\[0pt] [3] K. He, Y. Takeichi, M. Ogawa, T. Okuda, P. Moras, D. Topwal, A. Harasawa, T. Hirahara, C. Carbone, A. Kakizaki, and I. Matsuda, Phys. Rev. Lett. 104, 156805 (2010).\\[0pt] [4] N. Miyata, R. Hobara, H. Narita, T. Hirahara, S. Hasegawa, and I. Matsuda, Japanese Journal of Applied Physics 50, 036602 (2011).\\[0pt] [5] N. Miyata, H. Narita, M. Ogawa, A. Harasawa, R. Hobara, T. Hirahara, P. Moras, D.Topwal, C.Carbone, S.Hasegawa, and I. Matsuda, Phys. Rev. B, 83, 195305 (2011).   

Quantum Oscillations of Surface Electronic Structure: Inter-relation Between Quantum Well States, Work Functions and Surface Energies

Quantum size effects (QSE) in ultra-thin metallic film has been a topic of intense investigations. Of particular interests are the inter-relationship between the quantum well states (QWS), work function (W) and surface energy (E{\_}s). In ultrathin Pb films on semiconductors, quantum oscillations of E{\_}s as a function of layer thickness (L) have been investigated by various experimental methods which have all yielded identical results. Experimental studies of work function, however, took a longer journey. Photoemission can probe the work function for an uniform film but in this case uniform film only exists for certain thicknesses. Scanning tunneling microscopy, can probe ``local'' properties for all thicknesses, but the very existence of QWS in these films profoundly affects the measured tunneling decay constant $\kappa $. Consequently, L-dependence of $\kappa $ also depends on the bias voltage. It was then discovered that at a very low sample bias ($\vert $Vs$\vert <$ 0.03 V) the measured $\kappa $ vs. L accurately reflects the quantum size effect on the work function [1]. With this last obstacle removed, we are able to simultaneously measure the W vs. L, E{\_}s vs. L and to correlate these quantities with the measured QWS locations, yielding the quantitative phase relationship between the quantum oscillations of work function and surface energy. To our surprise, instead of a predicted $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 4$} $ wavelength phase shift, we find that the quantum oscillations of these two quantities are exactly in-phase. A new model is proposed.