The technological success of phase-change materials in the field of data storage and functional systems stems from their distinctive electronic and structural peculiarities on the nanoscale. Recently, superlattice structures have been demonstrated to dramatically improve the optical and electrical performances of these chalcogenide based phase-change materials. In this perspective, unravelling the atomistic structure that originates the improvements in switching time and switching energy is paramount in order to design nanoscale structures with even enhanced functional properties. This study reveals a high-resolution atomistic insight of the [GeTe/Sb 2 Te 3 ] interfacial structure by means of Extended X-Ray Absorption Fine Structure spectroscopy and Transmission Electron Microscopy. Based on our results we propose a consistent novel structure for this kind of chalcogenide superlattices.The need for fast and efficient management of information stimulates research on materials that can be switched on nanometer length scales and sub-nanosecond time scales. Phase-Change materials (PCMs) possess a unique property portfolio, which is ideally suited for memory device applications [1][2][3][4][5][6] . A PCM is identified by its ability of switching rapidly and reversibly between a crystalline and an amorphous state, where the amorphous state is obtained by melting the crystalline state followed by rapid quenching. These two states significantly differ in their properties, such as the optical reflectivity as well as the electrical conductivity. The phase transformation is in general triggered by thermal heating, or by either electrical and optical pulses of different time duration and amplitude. The large contrast in reflectivity between these two states lays at the base of already working PCM-based optical rewritable media devices-like DVDs or Blu-Ray Disc-where information is encoded as amorphous marks in a crystalline background. The contrast in resistivity could be exploited in the next generation of electronic solid-state memories based on PCMs, which might replace the current leading storage technologies, namely FLASH and magnetic disks. Furthermore, these materials could be employed in displays or data visualization applications by combining both their optical and electronic property modulations 7 . Hence, a lot of interest and effort is currently devoted to uncover the complex physical origin of the high contrast between the two phases [8][9][10]
We investigate the evolution of both the occupied and unoccupied electronic structure in representative compounds of the infinitely adaptive superlattice series (Sb 2 ) m -Sb 2 Te 3 (m = 0-3) by means of angle-resolved photoemission spectroscopy and time-delayed two-photon photoemission, combined with first-principles band-structure calculations. We discover that the topological nature of the surface states and their spin texture are robust, with dispersions evolving from linear (Dirac-like) to parabolic (Rashba-like) along the series, as the materials evolve from semiconductors to semimetals. Our findings provide a promising strategy for engineering the topological states with the desired flexibility needed for realizing different quantum phenomena and spintronics applications. Three-dimensional topological insulators (TIs) are materials where the topological properties of the insulating bulk band structure guarantee the existence of metallic surface states [1,2]. These topological surface states (TSS) are characterized by a Dirac-like energy-momentum dispersion and by a helical spin-momentum texture [3,4] that protects them against backscattering and localization [5,6]. TIs are promising materials for spintronics and provide an exceptional playground to study novel physical phenomena [7,8]. Possible applications depend on the ability to control the electronic properties of the TSS. Common strategies for achieving such a control involve chemical substitution and adsorbate doping [9][10][11][12][13]. Alternatively, it has been proposed that multilayer heterostructures could be used to obtain artificial TIs with tunable properties defined by the stacking sequences of the constituent two-dimensional building blocks [14][15][16].In this Rapid Communication, we demonstrate the possibility of TSS band-structure engineering in the naturally occurring homologous series of topological superlattices (Sb 2 ) m -(Sb 2 Te 3 ) n composed of Sb 2 bilayers (BLs) and Sb 2 Te 3 quintuple layers (QLs) [17,18]. This series enables a systematic investigation of the evolution of topological properties as a function of the stacking sequence of the constituent building blocks. Our results show that the topological states are remarkably robust and that their dispersion can be tuned in the explored range (m = 0-3; n = 1), and in bulk Sb, with an unchanging strong Z 2 index ν 0 = 1.All (Sb 2 ) m -(Sb 2 Te 3 ) n compounds display layered crystal structures produced by ordered stacking of Sb 2 Te 3 QLs and antimony BLs along the c axis of the hexagonal unit cell. This provides for an "infinitely adaptive series" of * marco.grioni@epfl.ch distinct compounds between the end member compositions Sb 2 Te 3 (m = 0) and Sb (n = 0) [17][18][19][20] that are known to be a TI and a topological semimetal, respectively [21][22][23][24][25][26][27]. Early experiments assessed the topological nature of bulk Sb and Sb-Bi alloys [21,22]. By contrast, the experimental observation of the predicted TSS in Sb 2 Te 3 has been delayed by the intrinsic p doping ...
In the past few years, we have been witnessing an increased interest for studying materials properties under non-equilibrium conditions. Several well established spectroscopies for experiments in the energy domain have been successfully adapted to the time domain with sub-picosecond time resolution. Here we show the realization of high resolution resonant inelastic X-ray scattering (RIXS) with a stable ultrashort X-ray source such as an externally seeded free electron laser (FEL). We have designed and constructed a RIXS experimental endstation that allowed us to successfully measure the d-d excitations in KCoF3 single crystals at the cobalt M2,3-edge at FERMI FEL (Elettra-Sincrotrone Trieste, Italy). The FEL-RIXS spectra show an excellent agreement with the ones obtained from the same samples at the MERIXS endstation of the MERLIN beamline at the Advanced Light Source storage ring (Berkeley, USA). We established experimental protocols for performing time resolved RIXS experiments at a FEL source to avoid X ray-induced sample damage, while retaining comparable acquisition time to the synchrotron based measurements. Finally, we measured and modelled the influence of the FEL mixed electromagnetic modes, also present in externally seeded FELs, and the beam transport with ~120 meV experimental resolution achieved in the presented RIXS setup.
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