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During the fracture of nanocontacts gold spontaneously forms freely suspended chains of atoms, which is not observed for the isoelectronic noble metals Ag and Cu. Au also differs from Ag and Cu in forming reconstructions at its low-index surfaces. Using mechanically controllable break junctions we show that all the 5d metals that show similar reconstructions (Ir, Pt, and Au) also form chains of atoms, while both properties are absent in the 4d neighbor elements (Rh, Pd, and Ag), indicating a common origin for these two phenomena. A competition between s and d bonding is proposed as an explanation. This work is part of the research program of de “Stichting FOM,” which is financially supported by NWO, and has been supported by a European Community Marie Curie Fellowship.

We discuss Fermi-edge singularity effects on the linear and nonlinear transient response of an electron gas in a doped semiconductor. We use a bosonization scheme to describe the low energy excitations, which allows to compute the time and temperature dependence of the response functions. Coherent control of the energy absorption at resonance is analyzed in the linear regime. It is shown that a phase-shift appears in the coherent control oscillations, which is not present in the excitonic case. The nonlinear response is calculated analytically and used to predict that four wave-mixing experiments would present a Fermi-edge singularity when the exciting energy is varied. A new dephasing mechanism is predicted in doped samples that depends linearly on temperature and is produced by the low-energy bosonic excitations in the conduction band., Comment: long version; 9 pages, 4 figures

Keywords: RAYLEIGH-SCATTERING ; FEMTOSECOND Note: Univ Autonoma Madrid, Dept Fis Teor Mat Condensada, E-28049 Madrid, Spain. Swiss Fed Inst Technol, EPFL, Dept Phys, IMO, CH-105 Lausanne, Switzerland. Fernandez-Rossier, J, Univ Calif San Diego, Dept Phys, 9500 Gilman Dr, La Jolla, CA 92092 USA.ISI Document Delivery No.: 289YTTimes Cited: 3Cited Reference Count: 12Cited References:BIRKEDAL D, 1998, PHYS REV LETT, V81, P2372FERNANDEZROSSIER J, 1999, J PHYS-CONDENS MAT, V11, P6013GARRO N, COMMUNICATIONGARRO N, 1999, PHYS REV B, V60, P4497GURIOLI M, 1997, PHYS REV LETT, V78, P3205HAACKE S, 1997, PHYS REV LETT, V78, P2228HAACKE S, 2000, PHYS REV B, V61, P5109HACKE S, 1998, OSA TRENDS OPTICS PH, V18, P64KIRA M, 1999, PHYS REV LETT, V82, P3544LANGBEIN W, 1999, PHYS REV LETT, V82, P1040WANG HL, 1995, PHYS REV LETT, V74, P3065WOERNER M, 1998, PHYS REV LETT, V81, P4208 Reference LOEQ-ARTICLE-2000-028View record in Web of Science Record created on 2007-08-31, modified on 2017-05-12

Scanning tunneling microscopy permits us to image the Kondo resonance of a single magnetic atom adsorbed on a metallic surface. When the magnetic impurity is placed at the focus of an elliptical quantum corral, a Kondo resonance has been recently observed both on top of the impurity and on top of the focus where no magnetic impurity is present. This projection of the Kondo resonance to a remote point on the surface is referred to as quantum mirage. We present a quantum mechanical theory for the quantum mirage inside an ideal quantum corral and predict that the mirage will occur in corrals with shapes other than elliptical. J.F.R. and D.P. acknowledge Spanish Ministry of Education (MEC) for postdoctoral research support and Grant No. FPU/AP98. Work supported in part by MEC under Contract No. PB96-0085.

In this work we present a theoretical description of the transient response of the Fermi Edge Singularity (FES). We study the linear and the nonlinear response of an n-doped QW to laser pulses in the Coherent Control (CC) and Four Wave Mixing (FWM) Configurations. By means of a bosonization formalism we calculate the FWM signal emitted by the sample when it is excited by pulses spectrally peaked around the FES and we show that the long time behavior of the nonlinear signal is very similar to the linear case., Conference paper (13 EP2DS)

We discuss light-heavy hole beats observed in transient optical experiments in GaAs quantum wells in terms of a free-boson coherent state model. This approach is compared with descriptions based on few-level representations. Results lead to an interpretation of the beats as due to classical electromagnetic interference. The boson picture correctly describes photon excitation of extended states and accounts for experiments involving coherent control of the exciton density and Rayleigh scattering beating., Comment: 4 pages, no figures. Accepted for publication in Solid State Communications

We develop a theory to calculate exciton binding energies of both two- and three-dimensional spin polarized exciton gases within a mean field approach. Our method allows the analysis of recent experiments showing the importance of the polarization and intensity of the excitation light on the exciton luminescence of GaAs quantum wells. We study the breaking of the spin degeneracy observed at high exciton density $(5 \ \ 10^{10} cm ^2)$. Energy level splitting betwen spin +1 and spin -1 is shown to be due to many-body inter-excitonic exchange while the spin relaxation time is controlled by intra-exciton exchange., Comment: Revtex, 4 figures sent by fax upon request by e-mail

Problems encountered in the description of ion-induced compositional changes in non-monatomic solids, on the basis of the binary collision approach (BCA), are reported. The ingredients of a collisional model responsible for eventual ambiguity of computer simulation results are pointed out. The conclusion is made that application of the BCA to ion-induced composition build-up problem can require preliminary parametrization of the collisional model used for a given target composition. Financial support from DGICyT (project PS89-0065), A NATO grant, tile Spanish MAE (in collaboration with Sweden) and a postdoctoral grant to VK from the Spanish MEC are acknowledged.

Transmission electron microscopy (TEM) in situ ion implantation is a convenient way to study radiation damage, but it is biased by the proximity of the free surfaces of the electron transparent thin sample. In this work this bias was investigated by performing irradiation of Fe in thin foil and bulk form with ions of energies between 50 keV and 100 keV using molecular dynamics simulations. The damage resulting from the subsequent displacement cascades differs significantly between the two sample geometries. The most remarkable difference is in the resulting 〈1 0 0〉 vacancy loops. Both their size and frequency are much greater in thin films, with loops reaching 4 nm in size. This is due to an imbalance between the number of vacancies and self-interstitials produced, since the faster self-interstitials can escape to the surfaces and remain there as ad-atoms. In addition, the self-interstitial clusters are smaller for thin foils and there is a larger dispersion of the induced damage in terms of defect number, defect clustering and defect morphology. The study discusses the impact of these results on the study of radiation effects during in situ experiments. MJA thanks the UA for support through an institutional fellowship. The research leading to these results is partly funded by the European Atomic Energy Community’s (Euratom) Seventh Framework Programme FP7/2007–2013 under Grant agreement No. 604862 (MatISSE project) and in the framework of the EERA (European Energy Research Alliance) Joint Programme on Nuclear Materials and the Generalitat Valenciana PROMETEO2012/011. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 under Grant agreement No. 633053.

Semiconducting ferromagnet-nonmagnet interfaces in van der Waals heterostructures present a unique opportunity to investigate magnetic proximity interactions dependent upon a multitude of phenomena including valley and layer pseudospins, moiré periodicity, or exceptionally strong Coulomb binding. Here, we report a charge-state dependency of the magnetic proximity effects between MoSe2 and CrBr3 in photoluminescence, whereby the valley polarization of the MoSe2 trion state conforms closely to the local CrBr3 magnetization, while the neutral exciton state remains insensitive to the ferromagnet. We attribute this to spin-dependent interlayer charge transfer occurring on timescales between the exciton and trion radiative lifetimes. Going further, we uncover by both the magneto-optical Kerr effect and photoluminescence a domain-like spatial topography of contrasting valley polarization, which we infer to be labyrinthine or otherwise highly intricate, with features smaller than 400 nm corresponding to our optical resolution. Our findings offer a unique insight into the interplay between short-lived valley excitons and spin-dependent interlayer tunneling, while also highlighting MoSe2 as a promising candidate to optically interface with exotic spin textures in van der Waals structures., One advantage of van der Waals materials is the ability to combine different materials in layers to form new heterostructures. Here, the authors investigate heterostructures of CrBr3 and MoSe2, and find that the ferromagnetism of CrBr3 enhances the valley dependent optical response of the MoSe2.

Pt nanocontacts, like those formed in mechanically controlled break junctions, are shown to develop spontaneous local magnetic order. Our density functional calculations predict that a robust local magnetic order exists in the atoms presenting low coordination, i. e., those forming the atom-sized neck. In contrast to previous work, we thus find that the electronic transport can be spin-polarized, although the net value of the conductance still agrees with available experimental information. Experimental implications of the formation of this new type of nanomagnet are discussed., Comment: 4 pages, 3 figures

14 pags., 7 figs., 2 tabs.-- Open Access funded by Creative Commons Atribution Licence 4.0, The irradiation of silica with ions of specific energy larger than ~0.1 MeV/u produces very high electronic excitations that induce permanent changes in the physical, chemical and structural properties and give rise to defects (colour centres), responsible for the loss of sample transparency at specific bands. This type of irradiation leads to the generation of nanometer-sized tracks around the ion trajectory. In situ optical reflection measurements during systematic irradiation of silica samples allowed us to monitor the irradiation-induced compaction, whereas ex situ optical absorption measurements provide information on colour centre generation. In order to analyse the results, we have developed and validated an atomistic model able to quantitatively explain the experimental results. Thus, we are able to provide a consistent explanation for the size of the nanotracks, the velocity and thresholding effects for track formation, as well as, the colour centre yield per ion and the colour centre saturation density. In this work we will discuss the different processes involved in the permanent modification of silica: collective atomic motion, bond breaking, pressure-driven atom rearrangement and ultra-fast cooling. Despite the sudden lattice energy rise is the triggering and dominant step, all these processes are important for the final atomic configuration., The authors acknowledge the computer resources and technical assistance provided by CESVIMA (UPM), funding by Spanish MINECO through project ENE2015-70300-C3-3-R, funding by EUROfusion Consortium through project AWP15-ENR-01/CEA-02 and funding by Madrid Region (CAM) through project Technofusion (II)-CM (S2013/MAE-2745). E.M.B. thanks support from PICT-2014-0696 (ANPCyT), and SeCTyP-UNCuyo grant 2016-0003.

A scanning tunneling microscope (STM) can probe the inelastic spin excitations of single magnetic atoms in a surface via spin-flip assisted tunneling. A particular and intriguing case is the Mn dimer case. We show here that the existing theories for inelastic transport spectroscopy do not explain the observed spin transitions when both atoms are equally coupled to the STM tip and the substrate. The hyperfine coupling to the nuclear spins is shown to lead to a finite excitation amplitude, but the physical mechanism leading to the large inelastic signal observed is still unknown. We discuss some other alternatives that break the symmetry of the system and allows for larger excitation probabilities., 4 pages, 2 figures

A (II,Mn)VI diluted magnetic semiconductor quantum dot with an integer number of electrons controlled with a gate voltage is considered. We show that a single conduction band electron is able to induce a spontaneous collective magnetization of the Mn spins, overcoming the short range antiferromagnetic interactions. The carrier mediated ferromagnetism in the dot survives at temperatures above 1 Kelvin, two orders of magnitude larger than the Curie temperature for the same material in bulk. The magnetic behavior of the dot depends dramatically on the parity of the number of injected electrons., Comment: 4 pages, 3 figures

Ab-initio quantum transport calculations show that short NiO chains suspended in Ni nanocontacts present a very strong spin-polarization of the conductance. The generalized gradient approximation we use here predicts a similiar polarization of the conductance as the one previously computed with non-local exchange, confirming the robustness of the result. Their use as nanoscopic spinvalves is proposed., Comment: 2 pages, 1 figure; accepted in JMMM (Proceedings of ICM'06, Kyoto)

We study the optically-induced coupling between spins mediated by polaritons in a planar micro-cavity. In the strong coupling regime, the vacuum Rabi splitting introduces anisotropies in the spin coupling. Moreover, due to their photon-like mass, polaritons provide an extremely long spin coupling range. This suggests the realization of two-qubit all-optical quantum operations within tens of picoseconds with spins localized as far as hundreds of nanometers apart., Comment: 5 pages, 3 figures

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).

Nanometer-sized metallic necks have the unique ability to sustain extreme uniaxial loads (about 20 times greater than the bulk material). We present an experimental and theoretical study of the electronic transport properties under such extreme conditions. Conductance measurements on gold and aluminum necks show a strikingly different behavior: While gold shows the expected conductance decrease with increasing elastic elongation of the neck, aluminum necks behave in the opposite way. We have performed first-principles electronic-structure calculations which reproduce this behavior, showing that it is an intrinsic property of the bulk band structure under high uniaxial strain. This work has been supported by the Dirección General de Investigación Científica y Tecnológica of Spain (DGICYT) under Grants No. MAT95-1542, No. PB95-0202, and No. PB95-0061.

We present a theoretical analysis of a spin-dependent multicomponent condensate in two dimensions. The case of a condensate of resonantly photoexcited excitons having two different spin orientations is studied in detail. The energy and the chemical potentials of this system depend strongly on the spin polarization . When electrons and holes are located in two different planes, the condensate can be either totally spin polarized or spin unpolarized, a property that is measurable. The phase diagram in terms of the total density and electron-hole separation is discussed., 4 pages, 3 figures, Accepted for publication in Physical Review Letters

We have observed a large spin splitting between "spin" $+1$ and $-1$ heavy-hole excitons, having unbalanced populations, in undoped GaAs/AlAs quantum wells in the absence of any external magnetic field. Time-resolved photoluminescence spectroscopy, under excitation with circularly polarized light, reveals that, for high excitonic density and short times after the pulsed excitation, the emission from majority excitons lies above that of minority ones. The amount of the splitting, which can be as large as 50% of the binding energy, increases with excitonic density and presents a time evolution closely connected with the degree of polarization of the luminescence. Our results are interpreted on the light of a recently developed model, which shows that, while intra-excitonic exchange interaction is responsible for the spin relaxation processes, exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors., Revtex, four pages; four figures, postscript file Accepted for publication in Physical Review B (Rapid Commun.)

A photo-excited II-VI semiconductor nanocrystal doped with a few Mn spins is considered. The effects of spin-exciton interactions and the resulting multi-spin correlations on the photoluminescence are calculated by numerical diagonalization of the Hamiltonian, including exchange interaction between electrons, holes and Mn spins, as well as spin-orbit interaction. The results provide a unified description of recent experiments of photoluminesnce of dots with one and many Mn atoms as well as optically induced ferromagnetism in semimagnetic nanocrystals., 5 pages, 3 figures

We show that a quasi-two dimensional condensate of optically active excitons emits coherent light even in the absence of population inversion. This allows an unambiguous and clear experimental detection of the condensed phase. We prove that, due to the exciton–photon coupling, quantum and thermal fluctuations do not destroy condensation at finite temperature. Suitable conditions to achieve condensation are temperatures of a few K for typical exciton densities and the use of a pulsed and preferably circularly polarized, laser. Work supported in part by MEC of Spain under Contract Number PB96-0085, by the Fundación Ramón Areces and by the U.S. Army Research Office under Contract Number DAAH04-96-1-0183.

We address the interplay between stacking and interlayer exchange for ferromagnetically ordered CrI$_3$, both for bilayers and bulk. Whereas bulk CrI$_3$ is ferromagnetic, both magneto-optical and transport experiments show that interlayer exchange for CrI$_3$ bilayers is antiferromagnetic. Bulk CrI$_3$ is known to assume two crystal structures, rhombohedral and monoclinic, that differ mostly in the stacking between monolayers. Below 210-220 Kelvin, bulk CrI$_3$ orders in a rhombohedral phase. Our density functional theory calculations show a very strong dependence of interlayer exchange and stacking. Specifically, the ground states of both bulk and free-standing CrI$_3$ bilayers are ferromagnetic for the rhombohedral phase. In contrast, the energy difference between both configurations is more than one order of magnitude smaller for the monoclinic phase, and eventually becomes antiferromagnetic when either positive strain or on-site Hubbard interactions ($U \geq 3$) are considered. We also explore the interplay between interlayer hybrydization and stacking, using a Wannier basis, and between interlayer hybrydization and relative magnetic alignment for CrI$_3$ bilayers, that helps to account for the very large tunnel magnetoresistance obvserved in recent experiments., Comment: 5 pages, 3 figures. New version of the manuscript