18 research outputs found
Unconventional spin Hall effects in nonmagnetic solids
Direct and inverse spin Hall effects lie at the heart of novel applications
that utilize spins of electrons as information carriers, allowing generation of
spin currents and detecting them via the electric voltage. In the standard
arrangement, applied electric field induces transverse spin current with
perpendicular spin polarization. Although conventional spin Hall effects are
commonly used in spin-orbit torques or spin Hall magnetoresistance experiments,
the possibilities to configure electronic devices according to specific needs
are quite limited. Here, we investigate unconventional spin Hall effects that
have the same origin as conventional ones, but manifest only in low-symmetry
crystals where spin polarization, spin current and charge current are not
enforced to be orthogonal. Based on the symmetry analysis for all 230 space
groups, we have identified crystal structures that could exhibit unusual
configurations of charge-to-spin conversion. The most relevant geometries have
been explored in more detail; in particular, we have analyzed the collinear
components yielding transverse charge and spin current with spin polarization
parallel to one of them, as well as the longitudinal ones, where charge and
spin currents are parallel. In addition, we have demonstrated that
unconventional spin Hall effect can be induced by controllable breaking the
crystal symmetries by an external electric field, which opens a perspective for
external tuning of spin injection and detection by electric fields. The results
have been confirmed by density functional theory calculations performed for
various materials relevant for spintronics. We are convinced that our findings
will stimulate further computational and experimental studies of unconventional
spin Hall effects
Quantum computation of silicon electronic band structure
Development of quantum architectures during the last decade has inspired
hybrid classical-quantum algorithms in physics and quantum chemistry that
promise simulations of fermionic systems beyond the capability of modern
classical computers, even before the era of quantum computing fully arrives.
Strong research efforts have been recently made to obtain minimal depth quantum
circuits which could accurately represent chemical systems. Here, we show that
unprecedented methods used in quantum chemistry, designed to simulate molecules
on quantum processors, can be extended to calculate properties of periodic
solids. In particular, we present minimal depth circuits implementing the
variational quantum eigensolver algorithm and successfully use it to compute
the band structure of silicon on a quantum machine for the first time. We are
convinced that the presented quantum experiments performed on cloud-based
platforms will stimulate more intense studies towards scalable electronic
structure computation of advanced quantum materials
Collinear Rashba-Edelstein effect in non-magnetic chiral materials
Efficient generation and manipulation of spin signals in a given material
without invoking external magnetism remain one of the challenges in
spintronics. The spin Hall effect (SHE) and Rashba-Edelstein effect (REE) are
well-known mechanisms to electrically generate spin accumulation in materials
with strong spin-orbit coupling (SOC), but the exact role of the strength and
type of SOC, especially in crystals with low symmetry, has yet to be explained.
In this study, we investigate REE in two different families of non-magnetic
chiral materials, elemental semiconductors (Te and Se) and semimetallic
disilicides (TaSi and NbSi), using an approach based on density
functional theory (DFT). By analyzing spin textures across the full Brillouin
zones and comparing them with REE magnitudes calculated as a function of
chemical potential, we link specific features in the electronic structure with
the efficiency of the induced spin accumulation. Our findings show that
magnitudes of REE can be increased by: (i) the presence of purely radial
(Weyl-type) spin texture manifesting as the parallel spin-momentum locking,
(ii) high spin polarization of bands along one specific crystallographic
direction, (iii) low band velocities. By comparing materials possessing the
same crystal structures, but different strengths of SOC, we conclude that
larger SOC may indirectly contribute to the enhancement of REE. It yields
greater spin-splitting of bands along specific crystallographic directions,
which prevents canceling the contributions from the oppositely spin-polarized
bands over wider energy regions and helps maintain larger REE magnitudes. We
believe that these results will be useful for designing spintronics devices and
may aid further computational studies searching for efficient REE in materials
with different symmetries and SOC strengths
Ferroelectric control of the spin texture in germanium telluride
The electrical manipulation of spins in semiconductors, without magnetic
fields or auxiliary ferromagnetic materials, represents the holy grail for
spintronics. The use of Rashba effect is very attractive because the
k-dependent spin-splitting is originated by an electric field. So far only tiny
effects in two-dimensional electron gases (2DEG) have been exploited. Recently,
GeTe has been predicted to have bulk bands with giant Rashba-like splitting,
originated by the inversion symmetry breaking due to ferroelectric
polarization. In this work, we show that GeTe(111) surfaces with inwards or
outwards ferroelectric polarizations display opposite sense of circulation of
spin in bulk Rashba bands, as seen by spin and angular resolved photoemission
experiments. Our results represent the first experimental demonstration of
ferroelectric control of the spin texture in a semiconductor, a fundamental
milestone towards the exploitation of the non-volatile electrically switchable
spin texture of GeTe in spintronic devices.Comment: 18 pages, 4 figure
Analogs of Rashba-Edelstein effect from density functional theory
Studies of structure-property relationships in spintronics are essential for
the design of materials that can fill specific roles in devices. For example,
materials with low symmetry allow unconventional configurations of
charge-to-spin conversion which can be used to generate efficient spin-orbit
torques. Here, we explore the relationship between crystal symmetry and
geometry of the Rashba-Edelstein effect (REE) that causes spin accumulation in
response to an applied electric current. Based on a symmetry analysis performed
for 230 crystallographic space groups, we identify classes of materials that
can host conventional or collinear REE. Although transverse spin accumulation
is commonly associated with the so-called 'Rashba materials', we show that the
presence of specific spin texture does not easily translate to the
configuration of REE. More specifically, bulk crystals may simultaneously host
different types of spin-orbit fields, depending on the crystallographic point
group and the symmetry of the specific -vector, which, averaged over the
Brillouin zone, determine the direction and magnitude of the induced spin
accumulation. To explore the connection between crystal symmetry, spin texture,
and the magnitude of REE, we perform first-principles calculations for
representative materials with different symmetries. We believe that our results
will be helpful for further computational and experimental studies, as well as
the design of spintronics devices.Comment: 10 pages, 5 figure
The role of defects in graphene on the H-terminated SiC surface: Not quasi-free-standing any more
Intercalation of H between the SiC surface and graphene is known to largely reduce the graphene-substrate interaction thus leaving a so called quasi-free-standing graphene monolayer (QFG) which preserves most of the properties of free-standing graphene (FG). Here, we investigate via large-scale density functional theory (DFT) based calculations point defects in FG and QFG in the form of single vacancies passivated by additional H atoms. For QFG our results reveal that the intercalated H layer interacts strongly with the defects attracting unsaturated C atoms but repelling the H-passivated ones thus leading to large reconstructions which, in turn, may induce drastic changes on the electronic and magnetic properties when compared against FG. We conclude that QFG with defect concentrations larger than 0.3% cannot be regarded in general as quasi-free-standing any more. © 2014 Elsevier Ltd. All rights reserved.J.S. acknowledges Polish Ministry of Science and Higher Education for supporting the postdoctoral stay at the ICMM-CSIC in the frame of the programme Mobility Plus. J.I.C acknowledges support from the Spanish Ministry of Innovation and Science under contract No. MAT2010-18432.Peer Reviewe
Ferroelectric control of charge-to-spin conversion in WTe2
Ferroelectric materials hold great potential for alternative memories and computing, but several challenges need to be overcome before bringing the ideas to applications. In this context, the recently discovered link between electric polarization and spin textures in some classes of ferroelectrics expands the perspectives of the design of devices that could simultaneously benefit from ferroelectric and spintronic properties. Here, we explore the concept of nonvolatile ferroelectric control of charge-to-spin conversion in semimetallic WTe2, which may provide a way for nondestructive readout of the polar state. Based on the first-principles simulations, we show that the Rashba-Edelstein effect (REE) that converts electric currents into spin accumulation switches its sign upon the reversal of the electric polarization. The numerical values of REE, calculated for the first time for both bulk and bilayer WTe2, demonstrate that the conversion is sizable, and may remain large even at room temperature. The ferroelectric control of spin transport in nonmagnetic materials provides functionalities similar to multiferroics and allows the design of memories or logic-in-memory devices that combine ferroelectric writing of information at low power with the spin-orbit readout of state
Quantum computation of silicon electronic band structure
Development of quantum architectures during the last decade has inspired hybrid classical-quantum algorithms in physics and quantum chemistry that promise simulations of fermionic systems beyond the capability of modern classical computers, even before the era of quantum computing fully arrives. Strong research efforts have been recently made to obtain minimal depth quantum circuits which could accurately represent chemical systems. Here, we show that unprecedented methods used in quantum chemistry, designed to simulate molecules on quantum processors, can be extended to calculate properties of periodic solids. In particular, we present minimal depth circuits implementing the variational quantum eigensolver algorithm and successfully use it to compute the band structure of silicon on a quantum machine for the first time. We are convinced that the presented quantum experiments performed on cloud-based platforms will stimulate more intense studies towards scalable electronic structure computation of advanced quantum materials. This journal i