20 research outputs found
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
Two-dimensional electron gas at the (001) surface of ferromagnetic EuTiO3
Studies on oxide quasi-two-dimensional electron gas (q2DEG) have been a playground for the discovery of novel and sometimes unexpected phenomena, like the reported magnetism at the surface of SrTiO3 (001) and at the interface between nonmagnetic LaAlO3 and SrTiO3 band insulators. However, magnetism in this system is weak and there is evidence of a nonintrinsic origin. Here, by using in situ high-resolution angle-resolved photoemission, we demonstrate that ferromagnetic EuTiO3, the magnetic counterpart of SrTiO3 in the bulk, hosts a q2DEG at its (001) surface. This is confirmed by density functional theory calculations with Hubbard U terms in the presence of oxygen divacancies in various configurations, all of them leading to a spin-polarized q2DEG related to the ferromagnetic order of Eu-4f magnetic moments. The results suggest EuTiO3(001) as a new material platform for oxide q2DEGs, characterized by broken inversion and time-reversal symmetries
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
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
Hedgehog Spin-texture and Berry's Phase tuning in a Magnetic Topological Insulator
Understanding and control of spin degrees of freedom on the surfaces of
topological materials are key to future applications as well as for realizing
novel physics such as the axion electrodynamics associated with time-reversal
(TR) symmetry breaking on the surface. We experimentally demonstrate
magnetically induced spin reorientation phenomena simultaneous with a
Dirac-metal to gapped-insulator transition on the surfaces of manganese-doped
Bi2Se3 thin films. The resulting electronic groundstate exhibits unique
hedgehog-like spin textures at low energies, which directly demonstrate the
mechanics of TR symmetry breaking on the surface. We further show that an
insulating gap induced by quantum tunnelling between surfaces exhibits spin
texture modulation at low energies but respects TR invariance. These spin
phenomena and the control of their Fermi surface geometrical phase first
demonstrated in our experiments pave the way for the future realization of many
predicted exotic magnetic phenomena of topological origin.Comment: 38 pages, 18 Figures, Includes new text, additional datasets and
interpretation beyond arXiv:1206.2090, for the final published version see
Nature Physics (2012
α-GeTe a (GeMn) Te polovodiče: nová paradigma pro spintroniku
GeTe je nejjednodušší známý binární feroelektrický polovodič s úzkým pásmem. Nižší než 700 K předpokládá nesentrosymmetrickou rohomedickou strukturu, ve které je vytvořen elektrický dipól vzhledem k relativní Ge / Te podtlakový posun ve směru [111]. Feroelektrická polarizace je výsledkem asymetrických poloh Ge a Te atomů v tomto směru a zajišťuje, že systém má dobře definovanou osu pro rozdělení symetrie, což vede k obří rozebírání rozebíratelné struktury pásma Rashby. Zpráva o prvotřídních výpočtech naznačuje, že velké rozměry mřížky zodpovědné za feroelektrické uspořádání jsou také nejvýznamnější složkou pro obra Rashba-spin-dělení. Zkontrolujeme experimentální ověření tohoto obřího rozštěpení typu Rashba a zobrazíme hlavní výsledky (GeMn) Te jsou nové paralelní multiferoické polovodiče s magnetoelektickými vlastnostmi, nabízející široké možnosti pro navrhování spintronických materiálů.GeTe is the simplest known binary ferroelectric semiconductor with a narrow band gap. Below 700 K it assumes a non-centrosymmetric rhombohedral structure in which an electric dipole is formed due to a relative Ge/Te sublattice displacement along the [111] direction. Ferroelectric polarization results from asymmetric positions of Ge and Te atoms along that direction and ensures that the system possess a well-defined axis for symmetry breaking, resulting in a giant Rashba-type spin splitting of the bulk band structure. We report on first-principle calculations which indicate that the large lattice distortion responsible for the ferroelectric order is also the most significant ingredient for the giant Rashba-type spin-splitting. We review the experimental verification of this giant Rashba-type splitting and show the main results proving that (GeMn)Te is a new paradigm multiferroic semiconductor with magnetoelectic properties, offering broad opportunities for spintronics materials design
Spin Polarization and Attosecond Time Delay in Photoemission from Spin Degenerate States of Solids
Poďla Eisenbud-Wigner-Smithovho modelu, časové meškanie po absorpcii svetla je možné vysvetliť pomocou fǎzového posunu. V tejto práci sme ukázali že tento posun je možné merať v spinovo a uhlovo rozlýšenej fotoemisii na 111 povrchu Cu. Tieto merania sme modelovali novým modelom ktorý umožnǐ vysvetliť tento fenomén v budúcich meraniach.After photon absorption, electrons from a dispersive band of a solid require a finite time in the photoemission process before being photoemitted as free particles, in line with recent attosecond-resolved photoemission experiments. According to the Eisenbud-Wigner-Smith model, the time delay is due to a phase shift of different transitions that occur in the process. Such a phase shift is also at the origin of the angular dependent spin polarization of the photoelectron beam, observable in spin degenerate systems without angular momentum transfer by the incident photon. We propose a semiquantitative model which permits us to relate spin and time scales in photoemission from condensed matter targets and to better understand spin- and angle-resolved photoemission spectroscopy (SARPES) experiments on spin degenerate systems. We also present the first experimental determination by SARPES of this time delay in a dispersive band, which is found to be greater than 26 as for electrons emitted from the sp-bulk band of the model system Cu(111)
Spin-resolved electronic structure of ferroelectric alpha-GeTe and multiferroic Ge1-xMnxTe
Germanium telluride features special spin-electric effects originating from spin-orbit coupling and symmetry breaking by the ferroelectric lattice polarization, which opens up many prospectives for electrically tunable and switchable spin electronic devices. By Mn doping of the alpha-GeTe host lattice, the system becomes a multiferroic semiconductor possessing magnetoelectric properties in which the electric polarization, magnetization and spin texture are coupled to each other. Employing spin- and angle-resolved photoemission spectroscopy in bulk- and surface-sensitive energy ranges and by varying dipole transition matrix elements, we disentangle the bulk, surface and surface-resonance states of the electronic structure and determine the spin textures for selected parameters. From our results we derive a comprehensive model of the a-GeTe surface electronic structure which fits to experimental data and first principle theoretical predictions and we discuss the unconventional evolution of the Rashba-type spin splitting upon manipulation by external B- and E-fields