The effect of dimensionality on materials properties has become strikingly evident with the recent discovery of graphene. Charge ordering phenomena can be induced in one dimension by periodic distortions of a material's crystal structure, termed Peierls ordering transition. Charge-density waves can also be induced in solids by strong coulomb repulsion between carriers, and at the extreme limit, Wigner predicted that crystallization itself can be induced in an electrons gas in free space close to the absolute zero of temperature. Similar phenomena are observed also in higher dimensions, but the microscopic description of the corresponding phase transition is often controversial, and remains an open field of research for fundamental physics. Here, we photoinduce the melting of the charge ordering in a complex three-dimensional solid and monitor the consequent charge redistribution by probing the optical response over a broad spectral range with ultrashort laser pulses. Although the photoinduced electronic temperature far exceeds the critical value, the charge-density wave is preserved until the lattice is sufficiently distorted to induce the phase transition. Combining this result with ab initio electronic structure calculations, we identified the Peierls origin of multiple charge-density waves in a three-dimensional system for the first time.ultrafast broadband spectroscopy | electron-lattice interactions | optical spectral weight C harge ordering phenomena occurring upon symmetry breaking are important in solids as they give rise to current and spin flow patterns in promising materials such as organic conductors (1), multilayered graphene (2) and transition metal oxides (3). The possibility to investigate the microscopic steps through which such ordering transition occurs also gives the opportunity to speculate on more general aspects of critical phenomena. Chargedensity waves (CDWs) (4, 5), sandpile automata (6), and Josephson arrays (7) have been investigated in relation to the scale invariance of self-organized critical phenomena (8), of which avalanches are dramatic manifestations (9). In one dimension, Peierls demonstrated that at low temperature an instability can be induced by the coupling between carriers and a periodic lattice distortion. Such an instability triggers a charge ordering phenomenon and a metal-insulator phase transition, called Peierls transition, occurs (10). Like for Bardeen-Cooper-Schrieffer (BCS) superconductors, such an electron-phonon interaction-driven transition is expected to be second order (10). Although this situation is fairly established in monodimensional organic materials (11), increased hybridization leading to higher dimensionality of a solid perturbs this scenario and makes the assessment of the microscopic origin of charge localization phenomena more difficult (12)(13)(14).Contrary to other low-dimensional CDW (15) systems studied so far by time-resolved spectroscopies (16-19), Lu 5 Ir 4 Si 10 presents a complex three-dimensional structure with several substructures suc...

The strength of the electron-phonon coupling parameter and its evolution throughout a solid's phase diagram often determines phenomena such as superconductivity, charge-and spin-density waves. Its experimental determination relies on the ability to distinguish thermally activated phonons from those emitted by conduction band electrons, which can be achieved in an elegant way by ultrafast techniques. Separating the electronic from the out-of-equilibrium lattice subsystems, we probed their re-equilibration by monitoring the transient lattice temperature through femtosecond X-ray diffraction in La2−xSrxCuO4 single crystals with x=0.1 and 0.21. The temperature dependence of the electron-phonon coupling is obtained experimentally and shows similar trends to what is expected from the ab-initio calculated shape of the electronic density-of-states near the Fermi energy. This study evidences the important role of band effects in the electron-lattice interaction in solids, in particular in superconductors.

Imaging of flux vortices in high quality MgB2 single crystals has been successfully performed in a commercial Field Emission Gun-based Transmission Electron Microscope. In Cryo-Lorentz Microscopy, the sample quality and the vortex lattice can be monitored simultaneously, allowing one to relate microscopically the surface quality and the vortex dynamics. Such a vortex motion ultimately determines the flow resistivity, ρ f , the knowledge of which is indispensable for practical applications such as superconducting magnets or wires for Magnetic Resonance Imaging. The observed patterns have been analyzed and compared with other studies by Cryo-Lorentz Microscopy or Bitter decoration. We find that the vortex lattice arrangement depends strongly on the surface quality obtained during the specimen preparation, and tends to form an hexagonal Abrikosov lattice at a relatively low magnetic field. Stripes or gossamer-like patterns, recently suggested as potential signatures of an unconventional behavior of MgB2, were not observed.In superconductors, identifying the relationship between the transverse force felt by the triangular array of vortices in response to a transport current and the pinning force resulting from defects or inhomogeneities is of capital importance. This is because flux motion induces a longitudinal resistive voltage, which is a source of energy dissipation and ultimately hinders a material's performance in applications [1]. Indeed, in type-II superconductors, the superconducting state is not completely destroyed when an external magnetic field exceeds the lower critical one, H c1 . The external field partially penetrates into the material in the form of midget microscopic filaments called vortices. As a first approximation (Bardeen-Stephen model [2]), the core of each vortex, where superconductivity is supressed, is modeled by a cylinder with a radius given by the coherence length ξ, and is surrounded by circling supercurrents over a distance corresponding to the London penetration depth λ. It is assumed that the core of each vortex is a conventional metallic state inside of which the energy dissipation is dominated by impurity scattering [3]. Each vortex carries a magnetic flux equal to Φ 0 = h/(2e), where h is the Planck constant and e the elementary charge [4]. Due to their mutual repulsion, in a defect free superconductor, vortices tend to form a 2D close-packed triangular array surrounded by an hexagonal pattern of other vortices, called the Abrikosov lattice [5]. Cryo-Lorentz Transmission Electron Microscopy (Cryo-LTEM) allows the direct observation of quantized flux lines and so is a key technique for understanding the flux flow resistivity associated with the viscous motion of the vortices [6]. It is with the out-of focus imaging of Lorenz mode that individual flux quanta can be imaged, as well as superconducting or magnetic domains (Fig. 1, Fresnel mode) [7]. Specifically, flux lines can be imaged thanks to the deflection imparted to the electrons by the magnetic flux associated with each vorte...

In this communication we report on the performance of a fs-resolved transmission electron microscope, installed at the École Polytechnique Fédérale de Lausanne (EPFL), in the Laboratory for Ultrafast Microscopy and Electron Scattering (LUMES). The microscope, constructed by Integrated Dynamic Electron Solutions (IDES), is a variant of the Dynamic Transmission Electron Microscope developed at Lawrence Livermore National Laboratory capable of probing photoinitiated processes in materials on femtosecond timescales with2 Ångström resolution. Ultrafast temporal resolution is achieved by generating a photoelectron probe using 266nm laser pulses that are optically delayed relative to infrared pump pulses hitting the sample. To meet the energy, duration and repetition rate requirements we use a prototype laser system from KMLabs that delivers 80fs pulses a ta tunable repetition rate, 200kHz to 2MHz,with an energy of 1.55eV per photon and an average power of 3 W. The microscope is a modified JEOL JEM-2100TEM equipped with IDES constructed laser port and C0 lens sections that enable two pulsed laser beams to enter the column; an ultraviolet beam to illuminate the LaB6 cathode, generating electron pulses, and an infrared beam to stimulate excitations in the sample figure 1). In stroboscopic operation, to avoid space-charge effects and achieve femtosecond time resolution, every bunch of electrons should contain as few as 1 electron. This poses limitations in terms of integration time however this is overcome by the high repetition rate of the laser system and the improved coupling into the condenser system provided by the C0 lens. It is of capital importance to control the amount and spatial distribution of the charge photoemitted from the LaB6 tip. To do this, the optical set-up is designed in order to match the UV spot size to the flat surface of the cathode (50 µm); this limits the emission area of the tip, without the relying on high Wehnelt bias settings. The emitted electrons are then coupled in the column via an additional electromagnetic lens (C0lens) located just below the electron gun. This solution allows optimal coupling of the photoemitted electrons to the condenser system. High resolution images on a gold nanoparticle test sample have been collected to demonstrate that the modifications to the electron-optical system did not limit the spatial resolution of the microscope ( gure 2). The energy spread of the electron beam has been also characterized via the post column Gatan imaging filter. We notice here that the addition of the C0 lens allows us to efficiently couple electrons emitted from the lament and conserve the source brightness. Because the C0 lens allows much higher throughput, all the electrons emitted from the filament can be coupled to the standard TEM optics, producing currents as high as few microamps on the sample and detector. Thus lower lament heating currents can be used while maintaining reasonable signals on the detector, reducing the energy spread of the source and increasing spectroscopic ...

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