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A scanning tunneling microscope is used to explore the evolution of electron and Cooper-pair transport across single Mn-phthalocyanine molecules adsorbed on Pb(111) from tunneling to contact ranges. Normal-metal as well as superconducting tips give rise to a gradual transition of the Bardeen-Cooper-Schrieffer energy gap in the tunneling range into a zero-energy resonance close to and at contact. Supporting transport calculations show that in the normal-metal–superconductor junctions this resonance reflects the merging of in-gap Yu-Shiba-Rusinov states as well as the onset of Andreev reflection. For the superconductor-superconductor contacts, the zero-energy resonance is rationalized in terms of a finite Josephson current that is carried by phase-dependent Andreev and Yu-Shiba-Rusinov levels. Financial support by the Deutsche Forschungsgemeinschaft through Grant No. KR 2912/10-1, the FCT (Projects No. PTDC/FIS-NAN/4662/2014 and No. P2020-PTDC/FISNAN/3668/2014), and the MINECO-Spain (MAT2016-78625-C2) is acknowledged. J.L.L. acknowledges financial support from the ETH Zurich Postdoctoral Fellowship program.

Exchange interactions with itinerant electrons are known to act as a relaxation mechanism for individual local spins. The same exchange interactions induce the so-called RKKY indirect exchange interaction between two otherwise decoupled local spins. Here, we show that both the spin relaxation and the RKKY coupling can be seen as the dissipative and reactive response to the coupling of the local spins with the itinerant electrons. We thereby predict that the spin relaxation rates of magnetic nanostructures of exchanged coupled local spins, such as nanoengineered spin chains, have an oscillatory dependence on kFd, where kF is the Fermi wave number and d is the interspin distance, very much like the celebrated oscillations in the RKKY interaction. We demonstrate that both T1 and T2 can be enhanced or suppressed, compared to the single-spin limit, depending on the interplay between the Fermi surface and the nanostructure geometrical arrangement. Our results open a route to engineer spin relaxation and decoherence in atomically designed spin structures., J.F.R. acknowledges financial supported by MEC-Spain (Grant No. FIS2013-47328-C2-2-P) and Generalitat Valenciana (Grant No. ACOMP/2010/070), Prometeo. This work is funded by ERDF funds through the Portuguese Operational Program for Competitiveness and InternationalizationÐ COMPETE 2020, and National Funds through FCT-The Portuguese Foundation for Science and Technology, under the project PTDC/FIS-NAN/4662/2014 (Project. No. 016656). F.D. acknowledges support from Spanish Government through Grant No. MAT2015-66888-C3-2-R and Basque Government, Grant No. IT986-16.

We present a theoretical study of the dynamics of H atoms adsorbed on graphene bilayers with Bernal stacking. First, through extensive density functional theory calculations, including van der Waals Interactions, we obtain the activation barriers involved in the desorption and migration processes of a single H atom. These barriers, along with attempt rates and the energetics of H pairs, are used as input parameters in kinetic Monte Carlo simulations to study the time evolution of an initial random distribution of adsorbed H atoms. The simulations reveal that, at room temperature, H atoms occupy only one sublattice before they completely desorb or form clusters. This sublattice selectivity in the distribution of H atoms may last for sufficiently long periods of time upon lowering the temperature down to 0 ◦C. The final fate of the H atoms, namely, desorption or cluster formation, depends on the actual relative values of the activation barriers which can be tuned by doping. In some cases, a sublattice selectivity can be obtained for periods of time experimentally relevant even at room temperature. This result shows the possibility for observation and applications of the ferromagnetic state associated with such distribution, This work was supported by MINECO under Grant Nos. FIS2013-47328 and FIS2012-37549, by CAM under Grant Nos. S2013/MIT-3007, P2013/MIT-2850, and by Generalitat Valenciana under Grant PROMETEO/2012/011

We analyze the transport properties of a double quantum dot device with both dots coupled to perfect conducting leads and to a finite chain of N noninteracting sites connecting both of them. The interdot chain strongly influences the transport across the system and the local density of states of the dots. We study the case of a small number of sites, so that Kondo box effects are present, varying the coupling between the dots and the chain. For odd N and small coupling between the interdot chain and the dots, a state with two coexisting Kondo regimes develops: the bulk Kondo due to the quantum dots connected to leads and the one produced by the screening of the quantum dot spins by the spin in the finite chain at the Fermi level. As the coupling to the interdot chain increases, there is a crossover to a molecular Kondo effect, due to the screening of the molecule (formed by the finite chain and the quantum dots) spin by the leads. For even N the two Kondo temperatures regime does not develop and the physics is dominated by the usual competition between Kondo and antiferromagnetism between the quantum dots. We finally study how the transport properties are affected as N is increased. For the study we used exact multiconfigurational Lanczos calculations and finite-U slave-boson mean-field theory at T=0. The results obtained with both methods describe qualitatively and also quantitatively the same physics. We acknowledge financial support from the Brazilian agencies FAPERJ (CNE) and CNPq, the Spanish MCYT (Grants No. FIS2009-10325 and No. FIS2012-35880), CONICET (Argentina), Universidad de Alicante, and PUC–Rio de Janeiro.

We study the conduction band spin splitting that arises in transition metal dichalcogenide (TMD) semiconductor monolayers such as MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$ due to the combination of spin-orbit coupling and lack of inversion symmetry. Two types of calculation are done. First, density functional theory (DFT) calculations based on plane waves and pseudo potentials that yield large splittings, between 3 and 30 meV. Second, we derive a tight-binding model, that permits to address the atomic origin of the splitting. The basis set of the model is provided by the maximally localized Wannier orbitals, obtained from the DFT calculation, and formed by 11 atomic-like orbitals corresponding to 5$d$ and 3$p$ orbitals of the transition metal (W,Mo) and chalcogenide (S,Se) atoms respectively. In the resulting Hamiltonian we can independently change the atomic spin-orbit coupling constant of the two atomic species at the unit cell, which permits to analyse their contribution to the spin splitting at the high symmetry points. We find that ---in contrast to the valence band--- both atoms give comparable contributions to the conduction band splittings. Given that these materials are most often $n-$doped, our findings are important for developments in TMD spintronics., Comment: 8 pages, 4 figures

The conductance of atomic-sized metallic point contacts is shown to be strongly voltage dependent due to quantum interference with impurities even in samples with low impurity concentrations. Transmission through these small contacts depends not only on the local atomic structure at the contact but also on the distribution of impurities or defects within a coherence length of the contact. In contrast with other mesoscopic systems we show that transport through atomic contacts is coherent even at room temperature. The use of a scanning tunneling microscope ~STM! makes it possible to fabricate one atom contacts of gold whose transmission can be controlled by manipulation of the contact allowing inelastic spectroscopy in such small contacts. The conductance of atomic-sized contacts has been extensively studied in relation with the question of conductance quantization. Experimentally, the contacts are fabricated stretching a metallic contact using a scanning tunneling microscope 1 ~STM! or a mechanically controlled break junction ~MCBJ!. 2 Theoretically, the point contact has been modeled as a constriction for free electrons, 3,4 or a tight-binding model using different atomic arrangements. 5,6 Electronic transport in these nanoscopic size structures is coherent, and the conductance at zero bias voltage is given by the Landauer formula 7

We study single electron transport through a graphene quantum dot with magnetic adsorbates. We focus on the relation between the spin order of the adsorbates and the linear conductance of the device. The electronic structure of the graphene dot with magnetic adsorbates is modeled through numerical diagonalization of a tight-binding model with an exchange potential. We consider several mechanisms by which the adsorbate magnetic state can influence transport in a single electron transistor: by tuning the addition energy, by changing the tunneling rate and, in the case of spin polarized electrodes, through magnetoresistive effects. Whereas the first mechanism is always present, the others require that the electrode has either an energy or spin dependent density of states. We find that graphene dots are optimal systems to detect the spin state of a few magnetic centers., 7 pages, 5 Figures, PRB accepted

Two-dimensional insulators with time-reversal symmetry can have two topologically different phases, the quantum spin Hall and the normal phase. The former is revealed by the existence of conducting edge states that are topologically protected. Here we show that the reaction to impurity, in bulk, is radically different in the two phases and can be used as a marker for the topological phase. Within the context of the Kane-Mele model for graphene, we find that strictly normalizable in-gap impurity states only occur in the quantum spin Hall phase and carry a dissipationless current whose chirality is determined by the spin and pseudospin of the residing electron. This work has been financially supported by MEC-Spain (Grant Nos. FIS2010-21883-C02-01, FIS2009-08744, and CONSOLIDER CSD2007-0010) as well as Generalitat Valenciana, grant no. Prometeo 2011-12.

We propose cotunneling as the microscopic mechanism that makes possible inelastic electron tunneling spectroscopy of magnetic atoms in surfaces for a wide range of systems, including single magnetic adatoms, molecules, and molecular stacks. We describe electronic transport between the scanning tip and the conducting surface through the magnetic system (MS) with a generalized Anderson model, without making use of effective spin models. Transport and spin dynamics are described with an effective cotunneling Hamiltonian in which the correlations in the magnetic system are calculated exactly and the coupling to the electrodes is included up to second order in the tip MS and MS substrate. In the adequate limit our approach is equivalent to the phenomenological Kondo exchange model that successfully describes the experiments. We apply our method to study in detail inelastic transport in two systems, stacks of cobalt phthalocyanines and a single Mn atom on Cu2N. Our method accounts for both the large contribution of the inelastic spin exchange events to the conductance and the observed conductance asymmetry. This work was supported by MEC-Spain (MAT07-67845, FIS2010-21883-C02-01, Grants JCI-2008-01885 and CONSOLIDER CSD2007-00010) and Generalitat Valenciana (ACOMP/2010/070).

A scanning tunneling microscope can probe the inelastic spin excitations of a single magnetic atom in a surface via spin-flip assisted tunneling in which transport electrons exchange spin and energy with the atomic spin. If the inelastic transport time, defined as the average time elapsed between two inelastic spin flip events, is shorter than the atom spin relaxation time, the STM current can drive the spin out of equilibrium. Here we model this process using rate equations and a model Hamiltonian that describes successfully spin flip assisted tunneling experiments, including a single Mn atom, a Mn dimer and Fe Phthalocyanine molecules. When the STM current is not spin polarized, the non-equilibrium spin dynamics of the magnetic atom results in non-monotonic $dI/dV$ curves. In the case of spin polarized STM current, the spin orientation of the magnetic atom can be controlled parallel or anti-parallel to the magnetic moment of the tip. Thus, spin polarized STM tips can be used both to probe and to control the magnetic moment of a single atom., Comment: 15 pages, 12 figures

(1) Departamento de F`isica Aplicada, Universidad de Alicante, San Vicente del Raspeig, Spain(2) Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco E28049, Madrid, Spain(Dated: March 31, 2010)We present a new mechanism for dissipationless persistent charge current. Two dimensionaltopological insulators hold dissipationless spin currents in their edges so that, for a given spinorientation, a net charge current flows which is exactly compensated by the counter-flow of theopposite spin. Here we show that ferromagnetic order in the edge upgrades the spin currents intopersistent charge currents, without applied fields. For that matter, we study an interacting graphenezigzag ribbon with spin-orbit coupling. We find three electronic phases with magnetic edges thatcarry currents reaching 0.4nA, comparable to persistent currents in metallic rings, for the small spinorbit coupling in graphene. One of the phases is a valley half-metal.

We show how hydrogenation of graphene nanoribbons at small concentrations can open venues toward carbon-based spintronics applications regardless of any specific edge termination or passivation of the nanoribbons. Density-functional theory calculations show that an adsorbed H atom induces a spin density on the surrounding π orbitals whose symmetry and degree of localization depends on the distance to the edges of the nanoribbon. As expected for graphene-based systems, these induced magnetic moments interact ferromagnetically or antiferromagnetically depending on the relative adsorption graphene sublattice, but the magnitude of the interactions are found to strongly vary with the position of the H atoms relative to the edges. We also calculate, with the help of the Hubbard model, the transport properties of hydrogenated armchair semiconducting graphene nanoribbons in the diluted regime and show how the exchange coupling between H atoms can be exploited in the design of novel magnetoresistive devices. This work has been financially supported by MICINN of Spain (Grants No. MAT07-67845 and CONSOLIDER No. CSD2007-00010). D.S. acknowledges financial support from Consejo Superior de Investigaciones Científicas (CSIC).

An electronic phase with coexisting magnetic and ferroelectric order is predicted for graphene ribbons with zigzag edges. The electronic structure of the system is described with a mean-field Hubbard model that yields results very similar to those of density functional calculations. Without further approximations, the mean-field theory is recasted in terms of a BCS wave function for electron-hole pairs in the edge bands. The BCS coherence present in each spin channel is related to spin-resolved electric polarization. Although the total electric polarization vanishes, due to an internal phase locking of the BCS state, strong magnetoelectric effects are expected in this system. The formulation naturally accounts for the two gaps in the quasiparticle spectrun, Δ0 and Δ1, and relates them to the intraband and interband self-energies. This work has been financially supported by MEC-Spain (Grant No. MAT2007-65487 and Ramon y Cajal Program), by Generalitat Valenciana (No. Accomp07-054), by Consolider (No. CSD2007-0010), and, in part, by FEDER funds.

We investigate coupling of localized spins in a semiconductor quantum dot embedded in a microcavity. The lowest cavity mode and the quantum dot exciton are coupled and close in energy, forming a polariton. The fermions forming the exciton interact with localized spins via exchange. Exact diagonalization of a Hamiltonian in which photons, spins, and excitons are treated quantum mechanically shows that a single polariton induces a sizable indirect anisotropic exchange interaction between spins. At sufficiently low temperatures strong ferromagnetic correlations show up without an appreciable increase in exciton population. In the case of a (Cd,Mn)Te quantum dot, Mn-Mn ferromagnetic coupling is still significant at 1 K: spin-spin correlation around 3 for exciton occupation smaller than 0.3. We find that the interaction mediated by photon-polaritons is 10 times stronger than the one induced by a classical field for equal Rabi splitting. G.C. and J.F.R. acknowledge financial support from the Ramón y Cajal Program (MCyT, Spain) and from CAV, Grant No. GV05/152. Partial financial support by the Spanish MCYT (Grant Nos. MAT2002-04429-C03, MAT2003-08109-C02-01, and FIS200402356), the Brazilian agencies FAPERJ, CAPES, and CNPq, and the Argentinian UBACYT and CONICET.

In this paper we develop the excitonic theory of Kerr rotation angle in a In this paper, we develop the excitonic theory of the Kerr rotation angle in a two-dimensional (2D) transition metal dichalcogenide at zero magnetic field. The finite Kerr angle is induced by the interplay between spin-orbit splitting and proximity exchange coupling due to the presence of a ferromagnet. We compare the excitonic effect with the single-particle theory approach. We show that the excitonic properties of the 2D material lead to a dramatic change in the frequency dependence of the optical response function. We also find that the excitonic corrections enhance the optical response by a factor of 2 in the case of MoS2 in proximity to a cobalt thin film., N.M.R.P. acknowledges support from the European Commission through the project "Graphene-Driven Revolutions in ICT and Beyond" (Ref. No. 785219), and the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Financing UID/FIS/04650/2019. In addition, N.M.R.P. acknowledges COMPETE2020, PORTUGAL2020, FEDER, and the Portuguese Foundation for Science and Technology (FCT) through Projects No. PTDC/FIS-NAN/3668/2013, No. POCI-01-0145-FEDER-028114, No. POCI-01-0145FEDER-029265, No. PTDC/NAN-OPT/29265/2017, and No. POCI-01-0145-FEDER-02888. G.C. acknowledges Fundacao para a Ciencia e a Tecnologia (FCT) for Grant No. SFRH/BD/138806/2018. G.C. and J.F.-R. acknowledge financial support from FCT through Grant No. P2020-PTDC/FIS-NAN/4662/2014. J.F.-R. acknowledges financial support from FCT for the P2020-PTDC/FISNAN/4662/2014, P2020-PTDC/FIS-NAN/3668/2014, and the UTAP EXPL/NTec/0046/2017 projects, as well as Generalitat Valenciana funding Prometeo 2017/139 and MINECO Spain (Grant No. MAT2016-78625-C2).