Reiner Bormann

Phase formation in mechanically alloyed Nb‐Al powders

Nb‐Al alloys have been prepared by mechanical alloying of elemental crystalline powders and investigated with x‐ray diffraction and transmission electron microscopy. Crystalline equilibrium phases form for Al‐rich alloys. Amorphous phase formation is observed for the Nb50Al50 composition. Mechanically alloyed powders with more than 70 at. % Nb exhibit the metastable bcc (A2) solid solution which transforms after thermal annealing into the A15 phase. Formation of the different phases can be understood by considering the free‐energy curves of the amorphous and bcc phases as calculated from the phase diagram and additional thermodynamic data.

Ultrafast transmission electron microscopy using a laser-driven field emitter: Femtosecond resolution with a high coherence electron beam

We present the development of the first ultrafast transmission electron microscope (UTEM) driven by localized photoemission from a field emitter cathode. We describe the implementation of the instrument, the photoemitter concept and the quantitative electron beam parameters achieved. Establishing a new source for ultrafast TEM, the Göttingen UTEM employs nano-localized linear photoemission from a Schottky emitter, which enables operation with freely tunable temporal structure, from continuous wave to femtosecond pulsed mode. Using this emission mechanism, we achieve record pulse properties in ultrafast electron microscopy of 9Å focused beam diameter, 200fs pulse duration and 0.6eV energy width. We illustrate the possibility to conduct ultrafast imaging, diffraction, holography and spectroscopy with this instrument and also discuss opportunities to harness quantum coherent interactions between intense laser fields and free-electron beams.

An ultrafast electron microscope gun driven by two-photon photoemission from a nanotip cathode

We experimentally and numerically investigate the performance of an advanced ultrafast electron source, based on two-photon photoemission from a tungsten needle cathode incorporated in an electron microscope gun geometry. Emission properties are characterized as a function of the electrostatic gun settings, and operating conditions leading to laser-triggered electron beams of very low emittance (below 20 nm mrad) are identified. The results highlight the excellent suitability of optically driven nano-cathodes for the further development of ultrafast transmission electron microscopy.

An ultrafast nanotip electron gun triggered by grating-coupled surface plasmons

We demonstrate multiphoton photoelectron emission from gold nanotips induced by nanofocusing surface plasmons, resonantly excited on the tip shaft by a grating coupler. The tip is integrated into an electron gun assembly, which facilitates control over the spatial emission sites and allows us to disentangle direct grating emission from plasmon-triggered apex emission. The nanoscale source size of this electron gun concept enables highly coherent electron pulses with applications in ultrafast electron imaging and diffraction.

Ultrafast sublattice pseudospin relaxation in graphene probed by polarization-resolved photoluminescence

Electronic states in 2D materials can exhibit pseudospin degrees of freedom, which allow for unique carrier-field interaction scenarios. Here, we investigate ultrafast sublattice pseudospin relaxation in graphene by means of polarization-resolved photoluminescence spectroscopy. Comparison with microscopic Boltzmann simulations allows to determine a lifetime of the optically aligned pseudospin distribution of 12± 2 fs. This experimental approach extends the toolbox of graphene pseudospintronics, providing novel means to investigate pseudospin dynamics in active devices or under external fields. 1 ar X iv :1 70 2. 00 96 2v 1 [ co nd -m at .m es -h al l] 3 F eb 2 01 7 Present-day electronic devices process, store, and transport information based on charge carriers. Recent developments in spintronics [1–3] have extended these capabilities by additionally making use of the electronic spin. Moreover, depending on the local environment, carriers can be equipped with additional pseudospin degrees of freedom, including sublattice, valley, and layer pseudospin, which may be exploited in future information technology. These angular momentum components exhibit rich physical phenomena, not unlike the ones observed for the intrinsic spin of electrons [4–7]. For example, the valley pseudospin is exploited in valleytronics [7] by manipulating the occupation of degenerate but inequivalent chiral electron states. In particular, in hexagonal 2D materials such as transition metal dichalcogenides (TMDCs), selection rules enable a direct optical manipulation of the carrier pseudospin [6–10]. Manifestations of valley pseudospin polarization in TMDCs include the valley-spin analogue of the spin Hall effect [10, 11], and the valley Zeeman effect [12]. Even single-layer graphene as the most simple 2D material displays valley and sublattice pseudospin [13]. While, similar to the case of TMDCs, the valley pseudospin distinguishes the occupation within the Dirac cones at the K and K ′ points, the sublattice pseudospin controls the relative phase of the electron wave function on the two hexagonal sublattices [14]. In carrier momentum space, sublattice pseudospin is collinear with the carrier momentum relative to the Dirac point [4, 13]. Sublattice pseudospin conservation enables Klein tunneling in graphene, i. e. the counter-intuitive carrier transmission through infinitely high potential barriers [15–17]. In graphene, coupling between carriers and optical fields [18] is governed by sublattice pseudospin selection rules [19], so that the initial carrier populations created by linearly polarized interband excitation exhibit a pronounced angularly asymmetric population within the Dirac cones [20]. However, intrinsic carrier scattering mechanisms are expected to rapidly destroy the optically imprinted pseudospin alignment. Ultrafast pseudospin relaxation has been addressed in a series of polarization-resolved transient optical spectroscopy experiments [21–28], elucidating the role of carrier momentum isotropization in carrier thermalization and cooling processes [29–38]. Although it is challenging to observe the fastest relaxation dynamics, most of the previous investigations suggest that sublattice pseudospin relaxation is complete within 50 fs to 150 fs [22–24]. Theoretical models predict even faster dynamics [39], and a precise relation between energetic and momentum relaxation timescales has not

Strong-Field Photoemission from Metallic Nanotips

Nonlinear photoemission from single nanostructures is investigated over a broad wavelength range in the near- and mid-infrared. The field enhancement at a nanotip apex and mid-infrared excitation enable sub-cycle dynamics, where electrons are ejected from the near-field within a fraction of the optical half-cycle. The implications of this field-driven acceleration are studied via photoemission spectrocopy and numerical simulations.

Site-selective laser-triggered electron emission in a field emitter geometry

Laser-driven metal needle emitters offer great potential for low emittance pulsed electron sources as required in ultrafast transmission electron microscopy. Here, we experimentally and theoretically study site-selective photoelectron emission in a field emitter geometry.

Confined electron emission with femtosecond timing: nonlinearity, localization, enhancement

The local extraction of electrons from metal nanotips is an essential component of both scanning tunneling microscopes and transmission or scanning electron microscopes based on field emission cathodes. Laser-induced electron emission from sharp tip structures is a prerequisite for equipping such methods with ultrafast temporal resolution. In this paper, recent experiments on femtosecond electron emission from sharp gold tips are discussed. Based on far-field and near-field characterization, confined multiphoton electron emission from the apex is demonstrated. The effective nonlinearity can be tuned by the application of an additional static bias voltage.

Strong-Field Photoelectron Emission From Metal Nanostructures

Photoelectron emission from metallic nanotips is studied experimentally and theoretically in the strong-field regime. The passage from multiphoton to tunnel emission is clearly resolved, and explained in terms of a one-dimensional quantum mechanical treatment.