The scaling of the single-pulse laser threshold fluence for dielectric breakdown with respect to pulse duration and material band gap energy was investigated in the subpicosecond pulse regime using oxide films ͑TiO 2 , Ta 2 O 5 , HfO 2 , Al 2 O 3 , and SiO 2 ͒. A phenomenological model attributes the pulse duration dependence to the interplay of multiphoton ionization, impact ionization, and subpicosecond electron decay out of the conduction band. The observed linear scaling of the breakdown fluence with band gap energy can be explained within the framework of this model by invoking the band gap dependence of the multiphoton absorption coefficient from Keldysh photoionization theory. The power exponent of the observed dependence of the breakdown threshold fluence F th on pulse duration p , F th ϰ p , is independent of the material and is attributed to photoionization seeded avalanche ionization.

We give experimental and theoretical evidence of the Rashba effect at the magnetic rare-earth metal surface Gd(0001). The Rashba effect is substantially enhanced and the Rashba parameter changes its sign when a metal-oxide surface layer is formed. The experimental observations are quantitatively described by ab initio calculations that give a detailed account of the near-surface charge density gradients causing the Rashba effect. Since the sign of the Rashba splitting depends on the magnetization direction, the findings open up new opportunities for the study of surface and interface magnetism.PACS numbers: 71.70. Ej, A key issue in condensed-matter research aiming at future spintronic devices [1] is to control and manipulate the electron spin in a two-dimensional electron gas (2DEG) of semiconductor systems without the need of applying an external magnetic field. Rashba had realized early on [2] that this can be achieved by an electric field which acts as a magnetic field in the rest frame of a moving electron. The interaction between the spin s of a moving electron of momentumhk with an electric field oriented along the z-axis e z is described by the Rashba HamiltonianThe Rashba parameter α R is proportional to the electric field and depends on the effective, material-dependent spin-orbit coupling (SOC) strength. In nonmagnetic systems the Rashba effect lifts the spin-degeneracy of the energy dispersion ǫ(k) of an electronic state, and the energy difference between ǫ ↑ (k) and ǫ ↓ (k) is called Rashba splitting ∆ǫ(k) = α R |k|. Even though spintronic research currently focuses on spin-polarized electrons in semiconductors [3,4], it is important to explore the Rashba effect in other material classes as well. A necessary condition for the Rashba effect to occur is the absence of inversion symmetry and, while in the proposed FET-type spin transistor [5] a gate voltage must be applied to break inversion symmetry of the 2DEG, this condition is naturally fulfilled by the structural inversion asymmetry (SIA) existing at any crystal surface or interface. Owing to SIA, electrons in a two-dimensional surface or interface state experience an effective crystal potential gradient perpendicular to their plane of propagation, hereby optimizing (e z × k) in Eq. (1). One should expect that the Rashba effect is a general surface and interface phenomenon, but up to now Rashba splittings have only been observed for surface states at Au(111) [6,7] and W(110) [8,9]. Recently relativistic density functional theory (DFT) calculations were able to reproduce the observed splitting of the Au sp-like surface state [10] and the analogy to a 2DEG has been pointed out [11]. Yet, it is still a challenging task to give a physical picture of the Rashba effect from the electronic structure point of view.This Letter presents the first experimental and theoretical evidence of a Rashba splitting of exchange-split two-dimensional electron states. Using the surface state of ferromagnetic Gd metal as example we report on the novel finding of a k-depe...

Coherent spin dynamics in the THz domain coupled to a coherent phonon is observed in the time-resolved second harmonic response of the Gd(0001) ferromagnetic metal surface. An LO phonon of 2.9 THz is excited by a transient charge displacement at the surface caused by resonant absorption of a fs laser pulse in the exchange-split surface state. This lattice vibration modulates the interlayer distance inducing a coherent variation of the exchange interaction between spins in adjacent layers. The resulting magnetization dynamics is considered as optical magnon wave packets coupled to the phonon.

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