39 research outputs found
Magnetooptical determination of a topological index
When a Dirac fermion system acquires an energy-gap, it is said to have either
trivial (positive energy-gap) or non-trivial (negative energy-gap) topology,
depending on the parity ordering of its conduction and valence bands. The
non-trivial regime is identified by the presence of topological surface or
edge-state dispersing in the energy gap of the bulk and is attributed a
non-zero topological index. In this work, we show that such topological indices
can be determined experimentally via an accurate measurement of the effective
velocity of bulk massive Dirac fermions. We demonstrate this analytically
starting from the Bernevig-Hughes-Zhang Hamiltonian (BHZ) to show how the
topological index depends on this velocity. We then experimentally extract the
topological index in Pb1-xSnxSe and Pb1-xSnxTe using infrared magnetooptical
Landau level spectroscopy. This approach is argued to be universal to all
material classes that can be described by a BHZ-like model and that host a
topological phase transition.Comment: Accepted for publication in Nature Partner Journal Quantum Material
A novel ferroelectric Rashba semiconductor
Fast, reversible, and low-power manipulation of the spin texture is crucial for next generation spintronic devices like non-volatile bipolar memories, switchable spin current injectors or spin field effect transistors. Ferroelectric Rashba semiconductors (FERSC) are the ideal class of materials for the realization of such devices. Their ferroelectric character enables an electronic control of the Rashba-type spin texture by means of the reversible and switchable polarization. Yet, only very few materials are established to belong to this class of multifunctional materials. Here, PbGeTe is unraveled as a novel FERSC system down to nanoscale. The ferroelectric phase transition and concomitant lattice distortion are demonstrated by temperature dependent X-ray diffraction, and their effect on electronic properties are measured by angle-resolved photoemission spectroscopy. In few nanometer-thick epitaxial heterostructures, a large Rashba spin-splitting is exhibiting a wide tuning range as a function of temperature and Ge content. This work defines PbGeTe as a high-potential FERSC system for spintronic applications
Persistence of structural distortion and bulk band Rashba splitting in SnTe above its ferroelectric critical temperature
The ferroelectric semiconductor -SnTe has been regarded as a
topological crystalline insulator and the dispersion of its surface states has
been intensively measured with angle-resolved photoemission spectroscopy
(ARPES) over the last decade. However, much less attention has been given to
the impact of the ferroelectric transition on its electronic structure, and in
particular on its bulk states. Here, we investigate the low-energy electronic
structure of -SnTe with ARPES and follow the evolution of the
bulk-state Rashba splitting as a function of temperature, across its
ferroelectric critical temperature of about K. Unexpectedly, we
observe a persistent band splitting up to room temperature, which is consistent
with an order-disorder contribution to the phase transition that requires the
presence of fluctuating local dipoles above . We conclude that no
topological surface state can occur at the (111) surface of SnTe, at odds with
recent literature.Comment: 26 pages, 8 figure
Field-induced ultrafast modulation of Rashba coupling at room temperature in ferroelectric -GeTe(111)
Rashba materials have appeared as an ideal playground for spin-to-charge
conversion in prototype spintronics devices. Among them, -GeTe(111) is
a non-centrosymmetric ferroelectric (FE) semiconductor for which a strong
spin-orbit interaction gives rise to giant Rashba coupling. Its room
temperature ferroelectricity was recently demonstrated as a route towards a new
type of highly energy-efficient non-volatile memory device based on switchable
polarization. Currently based on the application of an electric field, the
writing and reading processes could be outperformed by the use of femtosecond
(fs) light pulses requiring exploration of the possible control of
ferroelectricity on this timescale. Here, we probe the room temperature
transient dynamics of the electronic band structure of -GeTe(111) using
time and angle-resolved photoemission spectroscopy (tr-ARPES). Our experiments
reveal an ultrafast modulation of the Rashba coupling mediated on the fs
timescale by a surface photovoltage (SPV), namely an increase corresponding to
a 13 % enhancement of the lattice distortion. This opens the route for the
control of the FE polarization in -GeTe(111) and FE semiconducting
materials in quantum heterostructures.Comment: 31 pages, 12 figure
Collective topological spin dynamics in a correlated spin glass
The interplay between spin-orbit interaction (SOI) and magnetic order is
currently one of the most active research fields in condensed matter physics
and leading the search for materials with novel and tunable magnetic and spin
properties. Here we report on a variety of unexpected and unique observations
in thin multiferroic \GeMnTe films. The ferrimagnetic order in this
ferroelectric semiconductor is found to reverse with current pulses six orders
of magnitude lower as for typical spin-orbit torque systems. Upon a switching
event, the magnetic order spreads coherently and collectively over macroscopic
distances through a correlated spin-glass state. Lastly, we present a novel
methodology to controllably harness this stochastic magnetization dynamics,
allowing us to detect spatiotemporal nucleation of topological spin textures we
term ``skyrmiverres''.Comment: 26 pages, 10 figures, 2 table
Ultra-steep side facets in multi-faceted SiGe/Si(001) Stranski-Krastanow islands
For the prototypical Ge/Si(001) system, we show that at high growth temperature a new type of Stranski-Krastanow islands is formed with side facets steeper than {111} and high aspect ratio. Nano-goniometric analysis of the island shapes reveals the presence of six new facet groups in addition to those previously found for dome or barn-shaped islands. Due to the highly multi-faceted island shape and high aspect ratio, the new island types are named "cupola" islands and their steepest {12 5 3} side facet is inclined by 68°to the substrate surface. Assessing the relative stability of the new facets from surface area analysis, we find that their stability is similar to that of {113} and {15 3 23} facets of dome islands. The comparison of the different island shapes shows that they form a hierarchical class of geometrical structures, in which the lower aspect ratio islands of barns, domes and pyramids are directly derived from the cupola islands by successive truncation of the pedestal bases without facet rearrangements. The results underline the key role of surface faceting in the process of island formation, which is as crucial for understanding the island's growth evolution as it is important for device applications
Field-induced ultrafast modulation of Rashba coupling at room temperature in ferroelectric alpha-GeTe(111)
Rashba materials have appeared as an ideal playground for spin-to-charge conversion in prototype spintronics devices. Among them, α-GeTe(111) is a non-centrosymmetric ferroelectric semiconductor for which a strong spin-orbit interaction gives rise to giant Rashba coupling. Its room temperature ferroelectricity was recently demonstrated as a route towards a new type of highly energy-efficient non-volatile memory device based on switchable polarization. Currently based on the application of an electric field, the writing and reading processes could be outperformed by the use of femtosecond light pulses requiring exploration of the possible control of ferroelectricity on this timescale. Here, we probe the room temperature transient dynamics of the electronic band structure of α-GeTe(111) using time and angle-resolved photoemission spectroscopy. Our experiments reveal an ultrafast modulation of the Rashba coupling mediated on the fs timescale by a surface photovoltage, namely an increase corresponding to a 13% enhancement of the lattice distortion. This opens the route for the control of the ferroelectric polarization in α-GeTe(111) and ferroelectric semiconducting materials in quantum heterostructures.Rashba materials have appeared as an ideal playground for spin-to-charge conversion in prototype spintronics devices. Among them, α-GeTe(111) is a non-centrosymmetric ferroelectric semiconductor for which a strong spin-orbit interaction gives rise to giant Rashba coupling. Its room temperature ferroelectricity was recently demonstrated as a route towards a new type of highly energy-efficient non-volatile memory device based on switchable polarization. Currently based on the application of an electric field, the writing and reading processes could be outperformed by the use of femtosecond light pulses requiring exploration of the possible control of ferroelectricity on this timescale. Here, we probe the room temperature transient dynamics of the electronic band structure of α-GeTe(111) using time and angle-resolved photoemission spectroscopy. Our experiments reveal an ultrafast modulation of the Rashba coupling mediated on the fs timescale by a surface photovoltage, namely an increase corresponding to a 13% enhancement of the lattice distortion. This opens the route for the control of the ferroelectric polarization in α-GeTe(111) and ferroelectric semiconducting materials in quantum heterostructures
Epitaxial Metal Halide Perovskites by InkjetâPrinting on Various Substrates
Metalâhalideâperovskites revolutionized the field of thinâfilm semiconductor technology, due to their favorable optoelectronic properties and facile solution processing. Further improvements of perovskite thinâfilm devices require structural coherence on the atomic scale. Such perfection is achieved by epitaxial growth, a method that is based on the use of highâend deposition chambers. Here epitaxial growth is enabled via a â1000 times cheaper device, a single nozzle inkjet printer. By printing, singleâcrystal microâ and nanostructure arrays and crystalline coherent thin films are obtained on selected substrates. The heteroâepitaxial structures of methylammonium PbBr3 grown on lattice matching substrates exhibit similar luminescence as bulk single crystals, but the crystals phase transitions are shifted to lower temperatures, indicating a structural stabilization due to interfacial lattice anchoring by the substrates. Thus, the inkjetâprinting of metalâhalide perovskites provides improved material characteristics in a highly economical way, as a future cheap competitor to the highâend semiconductor growth technologies.DFG, 404984854, Bleifreie Perovksite fĂŒr die RöntgendetektionDFG, 399073171, GRK 2495: Energiekonvertierungssysteme: von Materialien zu Bauteile