Surface Elementary Excitations Research Articles

Phase Transition in Frustrated Magnetic Thin Film-Physics at Phase Boundaries.

In this review, we outline some principal theoretical knowledge of the properties of frustrated spin systems and magnetic thin films. The two points we would like to emphasize: (i) the physics in low dimensions where exact solutions can be obtained; (ii) the physics at phase boundaries where interesting phenomena can occur due to competing interactions of the two phases around the boundary. This competition causes a frustration. We will concentrate our attention on magnetic thin films and phenomena occurring near the boundary of two phases of different symmetries. Two-dimensional (2D) systems are in fact the limiting case of thin films with a monolayer. Naturally, we will treat this case at the beginning. We begin by defining the frustration and giving examples of frustrated 2D Ising systems that we can exactly solve by transforming them into vertex models. We will show that these simple systems already contain most of the striking features of frustrated systems such as the high degeneracy of the ground state (GS), many phases in the GS phase diagram in the space of interaction parameters, the reentrance occurring near the boundaries of these phases, the disorder lines in the paramagnetic phase, and the partial disorder coexisting with the order at equilibrium. Thin films are then presented with different aspects: surface elementary excitations (surface spin waves), surface phase transition, and criticality. Several examples are shown and discussed. New results on skyrmions in thin films and superlattices are also displayed. By the examples presented in this review we show that the frustration when combined with the surface effect in low dimensions gives rise to striking phenomena observed in particular near the phase boundaries.

Phase and group velocities of the surface elementary excitations on the superfluid helium–II surface at low temperatures

Phase and group velocities of the surface elementary excitations on the superfluid helium–II surface at low temperatures

Studying creation of bulk elementary excitation by heaters in superfluid helium-II at low temperatures

In this paper, the obtained experimental results concerning creation of bulk elementary excitations (BEEs) in isotopically pure liquid 4He at low temperatures ∼60 mK are discussed. Positive rotons’ (R+-rotons) creation by a pulsed heater was studied. Signals were recorded for the following quantum processes: quantum evaporation of 4He-atoms from the free liquid-helium surface by the BEEs of the liquid helium-II, and BEEs reflection from the free surface back into the bulk liquid. Typical signals are shown, and ratios of signal amplitudes are evaluated. For long heater pulses from 5 to 10 μs, appearance of the second atomic cloud consisting of evaporated 4He-atoms was observed in addition to the first atomic cloud. It is thought that the first atomic cloud of the evaporated helium atoms consists of very fast 4He-atoms with energies ∼35 K evaporated by positive rotons with the special energies ∼17 K (∼2ER∼2×8.6 K with ER representing the roton minimum energy) corresponding to the third non-dispersive Zakharenko wave. The second cloud of slower 4He-atoms was created by surface elementary excitations (SEEs or ripplons) possessing the special energies ∼7.15 K representing the binding energy. It was assumed that such SEEs can be created by phonons incoming to the liquid surface with special energies ∼6.2 K corresponding to the first non-dispersive Zakharenko wave, which can interact at the liquid surface with the same phonons already reflected from the surface for long heater pulses. Also, some pulsed-heater characteristics were studied in order to better understand the features of such heaters in low temperature experiments.

Electron States and Elementary Excitations at Semiconductor Surfaces Controlled by Adsorption

Adsorption on a doped semiconductor surface often induces a gradual formation of a carrier-depletion or carrier-accumulation layer at the surface, which has a great influence on the surface electronic excitations often coupled with surface optical phonons in compound semiconductors. Firstly, we give a brief survey of formation of depletion layers at n-type GaAs and InSb surfaces and of accumulation layers at n-type InAs surfaces. Secondly, we describe how the subband structure varies in an accumulation-layer formation process at an n-type InAs(110) surface, with special emphasis on the growing influence of nonparabolicity of the conduction-band dispersion. Finally, we elucidate the evolution of surface elementary excitations, namely, two coupled plasmon-phonon modes at the surface and a surface optical-phonon mode involving screening charges, in a depletion-layer formation process at an n-type GaAs(110) surface. Our systematic analysis provides a clear explanation of the change in the electron energy-loss spectrum.

Ellipsometry of guided wave polaritons at solid surfaces

A new method is proposed for studying surfaces in the region of resonance interaction of electromagnetic radiation with surface elementary excitations. In this method the sample surface is illuminated through the ATR prism having a thin metallic layer on its base, and the ellipsometric angles of the reflected beam are measured. We have studied ellipsometry in measuring refraction and absorption indices at the surface. A substantial improvement of sensitivity was demonstrated.

Electronic collective modes and instabilities on semiconductor surfaces. I

A Green's-function theory of electronic collective modes is presented which leads to a practical scheme for a microscopic determination of surface elementary excitations in conducting as well as nonconducting solids. Particular emphasis is placed on semiconductor surfaces where the jellium approximation is not valid, due to the importance of density fluctuations on a microscopic scale (reflected in the local-field effects). Starting from the Bethe-Salpeter equation for the two-particle Green's function of the surface system, an equation of motion for the electron-hole pair is obtained. Its solutions determine the energy spectra, lifetimes, and amplitudes of the surface elementary excitations, i.e., surface plasmons, excitons, polaritons, and magnons. Exchange and correlation effects are taken into account through the random-phase and time-dependent Hartree-Fock (screened electron-hole attraction) approximations. The formalism is applied to the study of electronic (charge- and spin-density) instabilities at covalent semiconductor surfaces. Quantitative calculations for an eight-layer Si(111) slab display an instability of the ideal paramagnetic surface with respect to spin-density waves with wavelength nearly corresponding to (2\ifmmode\times\else\texttimes\fi{}1) and (7\ifmmode\times\else\texttimes\fi{}7) superstructures.

Charge- and spin-density wave instabilities on ideal semiconductor surfaces

A microscopic theory of surface elementary excitations is presented, with particular emphasis on semiconductors. Within the framework of this formalism we study the conditions for the appearance of electronic (charge- and spin-density wave) instabilities at surfaces. Quantitative calculations for a Si(111) slab, based on a self-consistent pseudopotential bandstructure, display a pronounced effect of the excitonic (electron-hole) and RPA local-field many-body interactions. Our results show that the ideal paramagnetic surface is unstable with respect to SDW's with wave vectors nearly corresponding to (2×1) and (7×7) superstructures. This is in accordance with recent total energy calculations based on local spin-density pseudopotentials.