Local Real-Space View of the Achiral 1T-TiSe2 2 x 2 x 2 Charge Density Wave

The transition metal dichalcogenide 1T-TiSe2-two-dimensional layered material undergoing a commensurate 2 × 2 × 2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD) below ≈200 K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial Ti atoms allows us to go beyond the usual spatial resolution of STM and to intimately probe the threedimensional character of the PLD. Furthermore, the inversion-symmetric achiral nature of the CDW in the z direction is revealed, contradicting the claimed existence of helical CDW stacking and associated chiral order. This study paves the way to a simultaneous real-space probing of both charge and structural reconstructions in CDW compounds

Enhanced skyrmion metastability under applied strain in FeGe

Mechanical straining of skyrmion hosting materials has previously demonstrated increased phase stability through the expansion of the skyrmion equilibrium pocket. Additionally, metastable skyrmions can be generated via rapid field cooling to form significant skyrmion populations at low temperatures. Using small-angle x-ray scattering and x-ray holographic imaging on a thermally strained 200-nm-thick FeGe lamella, we observe temperature-induced strain effects on the structure and metastability of the skyrmion lattice. We find that in this sample orientation ( H ∥ [ ¯ 1 1 0 ] ) with no strain, metastable skyrmions produced by field cooling through the equilibrium skyrmion pocket vanish from the sample upon dropping below the well-known helical reorientation temperature. However, when strain is applied along the [ 1 1 0 ] axis, and this procedure is repeated, a substantial volume fraction of metastable skyrmions persist upon cooling below this temperature down to 100 K. Additionally, we observe a large number of skyrmions retained after a complete magnetic field polarity reversal, implying that the metastable energy barrier protecting skyrmions from decay is enhanced

Structural dynamics during laser induced ultrafast demagnetization

The mechanism underlying femtosecond laser pulse induced ultrafast magnetization dynamics remains elusive despite two decades of intense research on this phenomenon. Most experiments focused so far on characterizing magnetization and charge carrier dynamics, while first direct measurements of structural dynamics during ultrafast demagnetization were reported only very recently. We here present our investigation of the infrared laser pulse induced ultrafast demagnetization process in a thin Ni film, which characterizes simultaneously magnetization and structural dynamics. This is achieved by employing femtosecond time resolved X-ray resonant magnetic reflectivity (tr-XRMR) as probe technique. The experimental results reveal unambiguously that the sub-picosecond magnetization quenching is accompanied by strong changes in non-magnetic X-ray reflectivity. These changes vary with reflection angle and changes up to 30$\%$ have been observed. Modeling the X-ray reflectivity of the investigated thin film, we can reproduce these changes by a variation of the apparent Ni layer thickness of up to 1$\%$. Extending these simulations to larger incidence angles we show that tr-XRMR can be employed to discriminate experimentally between currently discussed models describing the ultrafast demagnetization phenomenon

Robustness of the charge-ordered phases in IrTe2{\mathrm{IrTe}}_{2} against photoexcitation

We present a time-resolved angle-resolved photoelectron spectroscopy study of IrTe2, which undergoes two first-order structural and charge-ordered phase transitions on cooling below 270 K and below 180 K. The possibility of inducing a phase transition by photoexcitation with near-infrared femtosecond pulses is investigated in the charge-ordered phases. We observe changes of the spectral function occurring within a few hundreds of femtoseconds and persisting up to several picoseconds, which we interpret as a partial photoinduced phase transition (PIPT). The necessary time for photoinducing these spectral changes increases with increasing photoexcitation density and reaches time scales longer than the rise time of the transient electronic temperature. We conclude that the PIPT is driven by a transient increase of the lattice temperature following the energy transfer from the electrons. However, the photoinduced changes of the spectral function are small, which indicates that the low- temperature phase is particularly robust against photoexcitation. We suggest that the system might be trapped in an out-of-equilibrium state, for which only a partial structural transition is achieved

Comparison of earthquake-triggered turbidites from the Saguenay (Eastern Canada) and Reloncavi (Chilean margin) Fjords: implications for paleoseismicity and sedimentology

International audienceHigh-resolution seismic profiles along with physical and sedimentological properties of sediment cores from the Saguenay (Eastern Canada) and Reloncavi (Chile) Fjords allowed the identification of several decimeter to meter-thick turbidites. In both fjords, the turbidites were associated with large magnitude historic and pre-historic earthquakes including the 1663 AD (M > 7) earthquake in the Saguenay Fjord, and the 1960 (M 9.5), 1837 (M ~ 8) and 1575 AD major Chilean subduction earthquakes in the Reloncavi Fjord. In addition, a sand layer with exoscopic characteristics typical of a tsunami deposit was observed immediately above the turbidite associated with the 1575 AD earthquake in the Reloncavi Fjord and supports both the chronology and the large magnitude of that historic earthquake. In the Saguenay Fjord, the earthquake-triggered turbidites are sometimes underlying a hyperpycnite associated with the rapid breaching and draining of a natural dam formed by earthquake-triggered landslides. Similar hyperpycnal floods were also recorded in historical and continental geological archives for the 1960 and 1575 AD Chilean subduction earthquakes, highlighting the risk of such flood events several weeks or months after main earthquake. In both fjords, as well as in other recently recognized earthquake-triggered turbidites, the decimeter-to meter-thick normally-graded turbidites are characterized by a homogeneous, but slightly fining upward tail. Finally, this paper also emphasizes the sensitivity of fjords to record historic and pre-historic seismicity

Robustness of the charge-ordered phases in IrTe2_2 against photoexcitation

We present a time-resolved angle-resolved photoelectron spectroscopy study of IrTe$_2$, which undergoes two first-order structural and charge-ordered phase transitions on cooling below 270 K and below 180 K. The possibility of inducing a phase transition by photoexcitation with near-infrared femtosecond pulses is investigated in the charge-ordered phases. We observe changes of the spectral function occuring within a few hundreds of femtoseconds and persisting up to several picoseconds, which we interpret as a partial photoinduced phase transition (PIPT). The necessary time for photoinducing these spectral changes increases with increasing photoexcitation density and reaches timescales longer than the rise time of the transient electronic temperature. We conclude that the PIPT is driven by a transient increase of the lattice temperature following the energy transfer from the electrons. However, the photoinduced changes of the spectral function are small, which indicates that the low temperature phase is particularly robust against photoexcitation. We suggest that the system might be trapped in an out-of-equilibrium state, for which only a partial structural transition is achieved.Comment: 8 pages, 5 figures, accepted for publication in Phys. Rev.

Robust Perpendicular Skyrmions and their Surface-Confinement

Magnetic skyrmions are two-dimensional magnetization swirls that stack in the form of tubes in the third dimension and which are proposed as prospective information carriers for nonvolatile memory devices due to their unique topological properties. From resonant elastic X-ray scattering measurements on Cu2OSeO3 with an in-plane magnetic field, we find that a state of perpendicularly ordered skyrmions forms, in stark contrast to the well-studied bulk state. The surface state is stable over a wide temperature range, unlike the bulk state in out-of-plane fields which is confined to a narrow region of the temperature-field phase diagram. In contrast to ordinary skyrmions found in the bulk, the surface state skyrmions result from the presence of magnetic interactions unique to the surface which stabilize them against external perturbations. The surface guiding makes the robust state particular interesting for racetracklike devices, ultimately allowing for much higher storage densities due to the smaller lateral footprint of the perpendicular skyrmions

Effect of surface polishing and oxidization induced strain on electronic order at the Verwey transition in Fe3O4

International audienceFollowing the controversy between two previous publications (Lorenzo et al 2008 Phys. Rev. Lett. 101 226401 and Garcia et al 2009 Phys. Rev. Lett. 102 176405), we report on the influence of mechanical polishing, and subsequent sample storage, on the electronic order at the Verwey transition of highly pure magnetite, Fe3O4, by resonant x-ray scattering. Contrary to expectations, mechanically polishing the surface induces an inhomogeneous micron deep dead layer, probably of powdered Fe3O4. In addition, we have found that polishing the sample immediately before the experiment influences and favors the appearance of long range order electronic correlations, whereas samples polished well in advance have their electronic order quenched. Conversely, lattice distortions associated with the Verwey transition appear less affected by the surface state. We conclude that mechanical polishing induces stresses at the surface that may propagate into the core of the single crystal sample. These strains relax with time, which affects the different order parameters, as measured by x-ray resonant diffraction