1,669 research outputs found
The emergence of classical behavior in magnetic adatoms
A wide class of nanomagnets shows striking quantum behavior, known as quantum
spin tunneling (QST): instead of two degenerate ground states with opposite
magnetizations, a bonding-antibonding pair forms, resulting in a splitting of
the ground state doublet with wave functions linear combination of two
classically opposite magnetic states, leading to the quenching of their
magnetic moment. Here we study how QST is destroyed and classical behavior
emerges in the case of magnetic adatoms, as the strength of their coupling,
either to the substrate or to each other, is increased. Both spin-substrate and
spin-spin coupling renormalize the QST splitting to zero allowing the
environmental decoherence to eliminate superpositions between classical states,
leading to the emergence of spontaneous magnetization.Comment: 5 pages, 4 figure
Electronic Structure of gated graphene and graphene ribbons
We study the electronic structure of gated graphene sheets. We consider both
infinite graphene and finite width ribbons. The effect of Coulomb interactions
between the electrically injected carriers and the coupling to the external
gate are computed self-consistently in the Hartree approximation. We compute
the average density of extra carriers, , the number of occupied
subbands and the density profiles as a function of the gate potential . We
discuss quantum corrections to the classical capacitance and we calculate the
threshold above which semiconducting armchair ribbons conduct. We find
that the ideal conductance of perfectly transmitting wide ribbons is
proportional to the square root of the gate voltage.Comment: 8 pages, 6 figure
Optical spin transfer in ferromagnetic semiconductors
Circularly polarized laser pulses that excite electron-hole pairs across the
band gap of (III,Mn)V ferromagnetic semiconductors can be used to manipulate
and to study collective magnetization dynamics. The initial spin orientation of
a photocarrier in a (III,V) semiconductors is determined by the polarization
state of the laser. We show that the photocarrier spin can be irreversibly
transferred to the collective magnetization, whose dynamics can consequently be
flexibly controlled by suitably chosen laser pulses. As illustrations we
demonstrate the feasibility of all optical ferromagnetic resonance and optical
magnetization reorientation.Comment: 8 pages, 3 figure
Space station integrated propulsion and fluid systems study
The program study was performed in two tasks: Task 1 addressed propulsion systems and Task 2 addressed all fluid systems associated with the Space Station elements, which also included propulsion and pressurant systems. Program results indicated a substantial reduction in life cycle costs through integrating the oxygen/hydrogen propulsion system with the environmental control and life support system, and through supplying nitrogen in a cryogenic gaseous supercritical or subcritical liquid state. A water sensitivity analysis showed that increasing the food water content would substantially increase the amount of water available for propulsion use and in all cases, the implementation of the BOSCH CO2 reduction process would reduce overall life cycle costs to the station and minimize risk. An investigation of fluid systems and associated requirements revealed a delicate balance between the individual propulsion and fluid systems across work packages and a strong interdependence between all other fluid systems
Magnetic and orbital blocking in Ni nanocontacts
We address the fundamental question of whether magneto-resistance (MR) of
atomic-sized contacts of Nickel is very large because of the formation of a
domain wall (DW) at the neck. Using {\em ab initio} transport calculations we
find that, as in the case of non-magnetic electrodes, transport in Ni
nanocontacts depends very much on the orbital nature of the electrons. Our
results are in agreement with several experiments in the average value of the
conductance. On the other hand, contrary to existing claims, DW scattering does
{\em not} account for large MR in Ni nanocontacts.Comment: 5 pages, 3 Figure
Internationalisation des élites académiques suisses au 20ème siècle : convergences et contrastes
A partir d'une base de données originale sur les professeurs de droit et de sciences économiques des universités suisses sur l'ensemble du XXe siècle, cet article rend compte des diverses dynamiques d'internationalisation de ces élites. Trois enseignements majeurs peuvent être tirés de nos analyses. D'abord, d'un point de vue diachronique, il est possible de diviser le XXe siècle en trois phases historiques : une internationalité forte des élites académiques au début du siècle, une nationalisation ou « relocalisation » suite à la Première Guerre mondiale, puis une « ré-internationalisation » à partir des années 1960 et de manière accélérée depuis les années 1980. Ensuite, les professeurs de sciences économiques, en terme de nationalités ou de lieu de formation, sont plus cosmopolites et ont moins d'ancrage local que leurs homologues juristes. Enfin, la prédominance germanique parmi les professeurs des universités suisses au début du siècle, qui s'explique autant par une internationalité d'« excellence » que de « proximité », laisse place, surtout en sciences économiques, à une montée de l'influence des Etats-Unis, révélatrice d'un effritement de l'internationalité de « proximité »
Emergence of quasiparticle Bloch states in artificial crystals crafted atom-by-atom
The interaction of electrons with a periodic potential of atoms in
crystalline solids gives rise to band structure. The band structure of existing
materials can be measured by photoemission spectroscopy and accurately
understood in terms of the tight-binding model, however not many experimental
approaches exist that allow to tailor artificial crystal lattices using a
bottom-up approach. The ability to engineer and study atomically crafted
designer materials by scanning tunnelling microscopy and spectroscopy (STM/STS)
helps to understand the emergence of material properties. Here, we use atom
manipulation of individual vacancies in a chlorine monolayer on Cu(100) to
construct one- and two-dimensional structures of various densities and sizes.
Local STS measurements reveal the emergence of quasiparticle bands, evidenced
by standing Bloch waves, with tuneable dispersion. The experimental data are
understood in terms of a tight-binding model combined with an additional
broadening term that allows an estimation of the coupling to the underlying
substrate.Comment: 7 figures, 12 pages, main text and supplementary materia
Magneto-optical Kerr effect in spin split two-dimensional massive Dirac materials
Two-dimensional (2D) massive Dirac electrons possess a finite Berry curvature, with Chern number 1/2, that entails both a quantized dc Hall response and a subgap full-quarter Kerr rotation. The observation of these effects in 2D massive Dirac materials such as gapped graphene, hexagonal boron nitride or transition metal dichalcogenides (TMDs) is obscured by the fact that Dirac cones come in pairs with opposite sign Berry curvatures, leading to a vanishing Chern number. Here, we show that the presence of spin-orbit interactions, combined with an exchange spin splitting induced either by diluted magnetic impurities or by proximity to a ferromagnetic insulator, gives origin to a net magneto-optical Kerr effect in such systems. We focus on the case of TMD monolayers and study the dependence of Kerr rotation on frequency and exchange spin splitting. The role of the substrate is included in the theory and found to critically affect the results. Our calculations indicate that state-of-the-art magneto-optical Kerr spectroscopy can detect a single magnetic impurity in diluted magnetic TMDs.We thank Allan H MacDonald, Elaine Li, Alejandro Molina-Sanchez and Joao C G Henriques for fruitful discussions. GC acknowledges Fundacao para a Ciencia e a Tecnologia (FCT) for Grant No. SFRH/BD/138806/2018. GC and JF-R acknowledge financial support from FCT through Grant No. P2020-PTDC/FIS-NAN/4662/2014. NMRP acknowledges financial support from European Commission through project 'Graphene-Driven Revolutions in ICT and Beyond' (Ref. No. 785219), FCT in the framework of Strategic Financing (Ref. No. UID/FIS/04650/2019), and COMPETE2020, PORTUGAL2020, FEDER and FCT for Grants No. PTDC/FIS-NAN/3668/2013, No. POCI-01-0145-FEDER-028114, No. POCI-01-0145-FEDER-029265 and No. PTDC/NANOPT/29265/2017. JF-R acknowledges FCT for Grant No. UTAP-EXPL/NTec/0046/2017, as well as Generalitat Valenciana funding Prometeo2017/139 and MINECO-Spain (Grant No. MAT201678625-C2)
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