We demonstrate the compression of 95 keV, space-charge-dominated electron bunches to sub-100 fs durations. These bunches have sufficient charge (200 fC) and are of sufficient quality to capture a diffraction pattern with a single shot, which we demonstrate by a diffraction experiment on a polycrystalline gold foil. Compression is realized by means of velocity bunching by inverting the positive space-charge-induced velocity chirp. This inversion is induced by the oscillatory longitudinal electric field of a 3 GHz radio-frequency cavity. The arrival time jitter is measured to be 80 fs.

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RF Compression of sub-relativistic space-charge-dominated electron bunches for single-shot femtosecond electron diffraction

(i) R. W. Walker: Some burning problems in combustion chemistry (68 pp.). The major features of this article are the initiation steps for alkanes, alkenes, aromatics, and aldehydes, the reactions of O, with ethyl radicals, and the formation of aromatics, polycyclic aromatics, and soot by cyclisation reactions of reactive intermediates, especially allyl and propargyl type radicals. A lot of actual kinetic data are given and dicussed in detail. (ii) J. L. Durant Jr.: Reactions of NH„ species (47 pp.). The chemistry of NH, species contains a wide variety of processes ranging from very low temperatures in the troposphere up to NO, formation at very high temperatures in combustion processes. Therefore, several reviews have been published in the past. The present review is focused on reactions of NH and NH2 with O, 02 and NO from a theoretical point of view. The thermochemistry, potential energy surfaces, including different electronic states, reaction paths, and the dynamics of some selected processes are discussed in a compact manner. (iii) S. M. Pimblott, N. J. B. Green: Recent advances in the kinetics of radiolytic processes (58 pp.). The main topics are theoretical models for the energy loss (LET) of the radiation, the distribution of ionization and excitation processes, followed by chemical reactions in the spurs and later at the time scale the transition from an inhomogeneous to the homoge-

Surfaces and Interfaces of Solid Materials

When electrons in a solid are excited by light, they can alter the free energy landscape and access phases of matter that are out of reach in thermal equilibrium. This accessibility becomes important in the presence of phase competition, when one state of matter is preferred over another by only a small energy scale that, in principle, is surmountable by the excitation. Here, we study a layered compound, LaTe3, where a small lattice anisotropy in the a–c plane results in a unidirectional charge density wave (CDW) along the c axis1,2. Using ultrafast electron diffraction, we find that, after photoexcitation, the CDW along the c axis is weakened and a different competing CDW along the a axis subsequently emerges. The timescales characterizing the relaxation of this new CDW and the reestablishment of the original CDW are nearly identical, which points towards a strong competition between the two orders. The new density wave represents a transient non-equilibrium phase of matter with no equilibrium counterpart, and this study thus provides a framework for discovering similar states of matter that are ‘trapped’ under equilibrium conditions. Short pulses of light shift the balance between two competing charge density wave phases, allowing the weaker one to manifest transiently while suppressing the stronger one. This shows that competing phases can be tuned in a non-equilibrium setting.

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Light-induced charge density wave in LaTe 3

Upon excitation with an intense laser pulse, a symmetry-broken ground state can undergo a non-equilibrium phase transition through pathways different from those in thermal equilibrium. The mechanism underlying these photoinduced phase transitions has long been researched in the study of condensed matter systems1, but many details in this ultrafast, non-adiabatic regime still remain to be clarified. To this end, we investigate the light-induced melting of a unidirectional charge density wave (CDW) in LaTe3. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a fast (approximately 1 picosecond) recovery of the CDW amplitude is followed by a slower re-establishment of phase coherence. This longer timescale is dictated by the presence of topological defects: long-range order is inhibited and is only restored when the defects annihilate. Our results provide a framework for understanding other photoinduced phase transitions by identifying the generation of defects as a governing mechanism. Three different ultrafast probes investigate a non-adiabatic phase transition and find substantial evidence of topological defects inhibiting the reformation of the equilibrium phase.

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Evidence for topological defects in a photoinduced phase transition

Upon excitation with an intense ultrafast laser pulse, a symmetry-broken ground state can undergo a non-equilibrium phase transition through pathways dissimilar from those in thermal equilibrium. Determining the mechanism underlying these photo-induced phase transitions (PIPTs) has been a long-standing issue in the study of condensed matter systems. To this end, we investigate the light-induced melting of a unidirectional charge density wave (CDW) material, LaTe$_3$. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a quick ($\sim\,$1$\,$ps) recovery of the CDW amplitude is followed by a slower reestablishment of phase coherence. This longer timescale is dictated by the presence of topological defects: long-range order (LRO) is inhibited and is only restored when the defects annihilate. Our results provide a framework for understanding other PIPTs by identifying the generation of defects as a governing mechanism.

Disentangling amplitude and phase dynamics of a charge density wave in a photo-induced phase transition.

A tracing of the phase-ordering kinetics of a charge density wave system demonstrates the potential of ultrafast low-energy electron diffraction for studying phase transitions and ordering phenomena at surfaces and in low-dimensional systems.

Phase ordering of charge density waves traced by ultrafast low-energy electron diffraction

We introduce ultrafast low-energy electron diffraction (ULEED) in backscattering for the study of structural dynamics at surfaces. Using a tip-based source of ultrashort electron pulses, we investigate the optically-driven transition between charge-density wave phases at the surface of 1T-TaS2. Employing spot-profile analysis enabled by the large transfer width of the instrument, we resolve the phase-ordering kinetics in the nascent incommensurate charge-density wave phase. We attribute the observed power-law scaling of the correlation length to the annihilation of topological defects resembling edge-dislocations of the charge-ordered lattice. Our work opens up the study of a wide class of structural transitions at surfaces and in low-dimensional systems.

S. Schramm

Papers

Phase ordering of charge density waves traced by ultrafast low-energy electron diffraction

Ultrafast low-energy electron diffraction traces phase-ordering kinetics of charge density waves