668 research outputs found
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
Giant spin Hall Effect in two-dimensional monochalcogenides
One of the most exciting properties of two dimensional materials is their
sensitivity to external tuning of the electronic properties, for example via
electric field or strain. Recently discovered analogues of phosphorene,
group-IV monochalcogenides (MX with M = Ge, Sn and X = S, Se, Te), display
several interesting phenomena intimately related to the in-plane strain, such
as giant piezoelectricity and multiferroicity, which combine ferroelastic and
ferroelectric properties. Here, using calculations from first principles, we
reveal for the first time giant intrinsic spin Hall conductivities (SHC) in
these materials. In particular, we show that the SHC resonances can be easily
tuned by combination of strain and doping and, in some cases, strain can be
used to induce semiconductor to metal transitions that make a giant spin Hall
effect possible even in absence of doping. Our results indicate a new route for
the design of highly tunable spintronics devices based on two-dimensional
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
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
Advanced modeling of materials with PAOFLOW 2.0:New features and software design
Recent research in materials science opens exciting perspectives to design novel quantum materials and devices, but it calls for quantitative predictions of properties which are not accessible in standard first principles packages. PAOFLOW, is a software tool that constructs tight-binding Hamiltonians from self consistent electronic wavefunctions by projecting onto a set of atomic orbitals. The electronic structure provides numerous materials properties that otherwise would have to be calculated via phenomenological models. In this paper, we describe recent re-design of the code as well as the new features and improvements in performance. In particular, we have implemented symmetry operations for unfolding equivalent k-points, which drastically reduces the runtime requirements of first principles calculations, and we have provided internal routines of projections onto atomic orbitals enabling generation of real space atomic orbitals. Moreover, we have included models for non-constant relaxation time in electronic transport calculations, doubling the real space dimensions of the Hamiltonian as well as the construction of Hamiltonians directly from analytical models. Importantly, PAOFLOW has been now converted into a Python package, and is streamlined for use directly within other Python codes. The new object oriented design treats PAOFLOW's computational routines as class methods, providing an API for explicit control of each calculation.</p
Using Incentives and Social Information to Promote Energy Conservation Behavior
Improving the efficiency in the domestic energy consumption has become a showpiece of how behavioral economics can be applied to the field of environmental economics. This study builds upon the literature by providing subjects with individual and social energy performance information at group level in a controlled field experiment setting. We aim to test whether extrinsic incentives accentuate or crowd out the intrinsic motivation to save energy and how heterogeneity in environmental attitudes also impacts on electricity conservation. Besides, we test for the persistence of energy-saving habits after the information is removed. Results suggest that the provision of individual feedback and social information increase energy conserving behavior, with this being most effective among those who signaled in a previous stage preferences for pro-environmental and sustainable living. However, treatment variations indicate that subjects overall fail to maintain “good habits” once the intervention stops, with exception of pro-environmental subjects who continue to consume less electricity in the post-intervention phase. Furthermore, our findings indicate that rewarding groups in a competitive environment may create perverse long-run effects. While providing individual and social information could improve both consumer welfare and energy demand forecasting, the timescale, frequency, and mechanism undertaken require careful scrutiny and planning if these potential benefits are to be maximized and undesirable side effects prevented
Observation of Two New Excited Ξb0 States Decaying to Λb0 K-π+
Two narrow resonant states are observed in the Λb0K-π+ mass spectrum using a data sample of proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the LHCb experiment and corresponding to an integrated luminosity of 6 fb-1. The minimal quark content of the Λb0K-π+ system indicates that these are excited Ξb0 baryons. The masses of the Ξb(6327)0 and Ξb(6333)0 states are m[Ξb(6327)0]=6327.28-0.21+0.23±0.12±0.24 and m[Ξb(6333)0]=6332.69-0.18+0.17±0.03±0.22 MeV, respectively, with a mass splitting of Δm=5.41-0.27+0.26±0.12 MeV, where the uncertainties are statistical, systematic, and due to the Λb0 mass measurement. The measured natural widths of these states are consistent with zero, with upper limits of Γ[Ξb(6327)0]<2.20(2.56) and Γ[Ξb(6333)0]<1.60(1.92) MeV at a 90% (95%) credibility level. The significance of the two-peak hypothesis is larger than nine (five) Gaussian standard deviations compared to the no-peak (one-peak) hypothesis. The masses, widths, and resonant structure of the new states are in good agreement with the expectations for a doublet of 1D Ξb0 resonances
Observation of the Decay Λ0b→Λ+cτ−¯ν
The first observation of the semileptonic b-baryon decay Λb0→Λc+τ-ν¯τ, with a significance of 6.1σ, is reported using a data sample corresponding to 3 fb-1 of integrated luminosity, collected by the LHCb experiment at center-of-mass energies of 7 and 8 TeV at the LHC. The τ- lepton is reconstructed in the hadronic decay to three charged pions. The ratio K=B(Λb0→Λc+τ-ν¯τ)/B(Λb0→Λc+π-π+π-) is measured to be 2.46±0.27±0.40, where the first uncertainty is statistical and the second systematic. The branching fraction B(Λb0→Λc+τ-ν¯τ)=(1.50±0.16±0.25±0.23)% is obtained, where the third uncertainty is from the external branching fraction of the normalization channel Λb0→Λc+π-π+π-. The ratio of semileptonic branching fractions R(Λc+)B(Λb0→Λc+τ-ν¯τ)/B(Λb0→Λc+μ-ν¯μ) is derived to be 0.242±0.026±0.040±0.059, where the external branching fraction uncertainty from the channel Λb0→Λc+μ-ν¯μ contributes to the last term. This result is in agreement with the standard model prediction
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