42 research outputs found
Coherent phonon modes of crystalline and amorphous Ge2Sb2Te5 thin films: A fingerprint of structure and bonding
Copyright © 2015 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Applied Physics, Volume 117, article 025306, and may be found at http://dx.doi.org/10.1063/1.4905617Femtosecond optical pump-probe measurements have been made upon epitaxial, polycrystalline, and amorphous thin films of Ge2Sb2Te5 (GST). A dominant coherent optical phonon mode of 3.4 THz frequency is observed in time-resolved anisotropic reflectance (AR) measurements of epitaxial films, and is inferred to have 3-dimensional T2-like character based upon the dependence of its amplitude and phase on pump and probe polarization. In contrast, the polycrystalline and amorphous phases exhibit a comparatively weak mode of about 4.5 THz frequency in both reflectivity (R) and AR measurements. Raman microscope measurements confirm the presence of the modes observed in pump-probe measurements, and reveal additional modes. While the Raman spectra are qualitatively similar for all three phases of GST, the mode frequencies are found to be different within experimental error, ranging from 3.2 to 3.6 THz and 4.3 to 4.7 THz, indicating that the detailed crystallographic structure has a significant effect upon the phonon frequency. While the lower frequency (3.6 THz) mode of amorphous GST is most likely associated with GeTe4 tetrahedra, modes in epitaxial (3.4 THz) and polycrystalline (3.2 THz) GST could be associated with either GeTe6 octahedra or Sb-Te bonds within defective octahedra. The more polarizable Sb-Te bonds are the most likely origin of the higher frequency (4.3-4.7 THz) mode, although the influence of Te-Te bonds cannot be excluded. The effect of high pump fluence, which leads to irreversible structural changes, has been explored. New modes with frequency of 3.5/3.6 THz in polycrystalline/amorphous GST may be associated with Sb2Te3 or GeTe4 tetrahedra, while a 4.2 THz mode observed in epitaxial GST may be related to segregation of Sb.Engineering and Physical Sciences Research Council (EPSRC
Coherent ultrafast spin-dynamics probed in three dimensional topological insulators
Topological insulators are candidates to open up a novel route in spin based
electronics. Different to traditional ferromagnetic materials, where the
carrier spin-polarization and magnetization are based on the exchange
interaction, the spin properties in topological insulators are based on the
coupling of spin- and orbit interaction connected to its momentum. Specific
ways to control the spin-polarization with light have been demonstrated: the
energy momentum landscape of the Dirac cone provides spin-momentum locking of
the charge current and its spin. The directionality of spin and momentum, as
well as control with light has been demonstrated. Here we demonstrate a
coherent femtosecond control of spin-polarization for states in the valence
band at around the Dirac cone.Comment: 14 pages, 4 figure
Observation of T2-like coherent optical phonons in epitaxial Ge2Sb2Te5/GaSb(001) films (journal article)
The dataset associated with this article is in ORE at http://hdl.handle.net/10871/13760The phonon spectrum of Ge2Sb2Te5 is a signature of its crystallographic structure and underlies the phase transition process used in memory applications. Epitaxial materials allow coherent optical phonons to be studied in femtosecond anisotropic reflectance measurements. A dominant phonon mode with frequency of 3.4 THz has been observed in epitaxial Ge2Sb2Te5 grown on GaSb(001). The dependence of signal strength upon pump and probe polarization is described by a theory of transient stimulated Raman scattering that accounts for the symmetry of the crystallographic structure through use of the Raman tensor. The 3.4 THz mode has the character of the 3 dimensional T2 mode expected for the Oh point group, confirming that the underlying crystallographic structure is cubic. New modes are observed in both Ge2Sb2Te5 and GaSb after application of large pump fluences, and are interpreted as 1 and 2 dimensional modes associated with segregation of Sb.The authors gratefully acknowledge financial support from Engineering and Physical Sciences Research Council grant EP/F015046/
Gate-tunable black phosphorus spin valve with nanosecond spin lifetimes
Two-dimensional materials offer new opportunities for both fundamental
science and technological applications, by exploiting the electron spin. While
graphene is very promising for spin communication due to its extraordinary
electron mobility, the lack of a band gap restricts its prospects for
semiconducting spin devices such as spin diodes and bipolar spin transistors.
The recent emergence of 2D semiconductors could help overcome this basic
challenge. In this letter we report the first important step towards making 2D
semiconductor spin devices. We have fabricated a spin valve based on ultra-thin
(5 nm) semiconducting black phosphorus (bP), and established fundamental spin
properties of this spin channel material which supports all electrical spin
injection, transport, precession and detection up to room temperature (RT).
Inserting a few layers of boron nitride between the ferromagnetic electrodes
and bP alleviates the notorious conductivity mismatch problem and allows
efficient electrical spin injection into an n-type bP. In the non-local spin
valve geometry we measure Hanle spin precession and observe spin relaxation
times as high as 4 ns, with spin relaxation lengths exceeding 6 um. Our
experimental results are in a very good agreement with first-principles
calculations and demonstrate that Elliott-Yafet spin relaxation mechanism is
dominant. We also demonstrate that spin transport in ultra-thin bP depends
strongly on the charge carrier concentration, and can be manipulated by the
electric field effect
Symmetry and topology in antiferromagnetic spintronics
Antiferromagnetic spintronics focuses on investigating and using
antiferromagnets as active elements in spintronics structures. Last decade
advances in relativistic spintronics led to the discovery of the staggered,
current-induced field in antiferromagnets. The corresponding N\'{e}el
spin-orbit torque allowed for efficient electrical switching of
antiferromagnetic moments and, in combination with electrical readout, for the
demonstration of experimental antiferromagnetic memory devices. In parallel,
the anomalous Hall effect was predicted and subsequently observed in
antiferromagnets. A new field of spintronics based on antiferromagnets has
emerged. We will focus here on the introduction into the most significant
discoveries which shaped the field together with a more recent spin-off
focusing on combining antiferromagnetic spintronics with topological effects,
such as antiferromagnetic topological semimetals and insulators, and the
interplay of antiferromagnetism, topology, and superconductivity in
heterostructures.Comment: Book chapte
Nucleation of lateral compositional modulation in InGaP epitaxial films grown on (001) GaAs
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A comparative transport study of Bi2Se3 and Bi 2Se3/yttrium iron garnet
Bilayers of 20 quintuple layer Bi2Se3 on 30 nm thick yttrium iron garnet (YIG) have been grown with molecular beam epitaxy in conjunction with pulsed laser deposition. The presence of the ferri-magnetic insulator YIG causes additional scattering to the surface states of the Bi 2Se3 topological insulator layer, as indicated by the temperature dependence of the resistivity. From the two-channel analysis of the Hall data, we find that the surface contribution in the bilayer samples is greatly reduced. Furthermore, the weak antilocalization effect from the surface states is clearly suppressed due to the presence of the YIG layer. © 2014 AIP Publishing LLC