Vibron-assisted spin excitation in a magnetically anisotropic molecule.

The electrical control and readout of molecular spin states are key for high-density storage. Expectations are that electrically-driven spin and vibrational excitations in a molecule should give rise to new conductance features in the presence of magnetic anisotropy, offering alternative routes to study and, ultimately, manipulate molecular magnetism. Here, we use inelastic electron tunneling spectroscopy to promote and detect the excited spin states of a prototypical molecule with magnetic anisotropy. We demonstrate the existence of a vibron-assisted spin excitation that can exceed in energy and in amplitude a simple excitation among spin states. This excitation, which can be quenched by structural changes in the magnetic molecule, is explained using first-principles calculations that include dynamical electronic correlations.

Atomic-scale spin sensing with a single molecule at the apex of a scanning tunneling microscope.

Recent advances in scanning probe techniques rely on the chemical functionalization of the probe-tip termination by a single molecule. The success of this approach opens the prospect of introducing spin sensitivity through functionalization by a magnetic molecule. We used a nickelocene-terminated tip (Nc-tip), which offered the possibility of producing spin excitations on the tip apex of a scanning tunneling microscope (STM). When the Nc-tip was 100 picometers away from point contact with a surface-supported object, magnetic effects could be probed through changes in the spin excitation spectrum of nickelocene. We used this detection scheme to simultaneously determine the exchange field and the spin polarization of iron atoms and cobalt films on a copper surface with atomic-scale resolution.

A model for individual quantal nano-skyrmions.

A quantal model description of a discrete localized skyrmion singularity embedded in a ferromagnetic environment is proposed. It allows discussing the importance of various parameters in the appearance of a quantal skyrmion singularity. Analysis of the skyrmion reveals a few specific quantal properties: presence of a whole series of skyrmion states, non-classical nature of the local spins, presence of superposition states and presence of extra-skyrmion states due to the quantization of the central spin of the singularity. The interaction of an electron, tunneling or substrate, with the skyrmion is also described allowing a new view at the origin of the skyrmion stability as well as the possibility to discriminate between ferromagnetic and skyrmion phase in an inelastic electron tunneling spectroscopy experiment, based on the skyrmion quantal properties.

Controlled spin switching in a metallocene molecular junction.

The active control of a molecular spin represents one of the main challenges in molecular spintronics. Up to now spin manipulation has been achieved through the modification of the molecular structure either by chemical doping or by external stimuli. However, the spin of a molecule adsorbed on a surface depends primarily on the interaction between its localized orbitals and the electronic states of the substrate. Here we change the effective spin of a single molecule by modifying the molecule/metal interface in a controlled way using a low-temperature scanning tunneling microscope. A nickelocene molecule reversibly switches from a spin 1 to 1/2 when varying the electrode-electrode distance from tunnel to contact regime. This switching is experimentally evidenced by inelastic and elastic spin-flip mechanisms observed in reproducible conductance measurements and understood using first principle calculations. Our work demonstrates the active control over the spin state of single molecule devices through interface manipulation.

Risk factors to develop multicentric lesions of the lower genital tract.

Purpose ofinvestigation: To analyze which are the risk factors in developing multicentric lesions of lower genital tract. MATERIALS AND METHODS: A prospective study of 1,011 patients was conducted at the low genital tract pathology clinic of Sant Joan de Deu Hospital between 2003-2011. A complete assessment of cervix, vagina, and vulva was carried out including HPV-DNA testing, cytology study, colposcopy, and biopsy in case of atypical findings. The statistical analysis was done with SPSS v.19 software. Differences between groups were considered statistically significant atp < 0.05. RESULTS: Twenty-two patients presented multicentric lesions (2.2%). The average age was 43 years. Most of the lesions were bicentric affecting cervix and vagina and cervix and vulva. Only in two cases (9%) there were three sites of genital neoplasia. The authors found four cervical cancer, 17 high grade, and one low grade lesions of the cervix. Eighteen vaginal intraepithelial neoplasia (VAIN), six high grade, 14 low grade, and four vulvar intraepithelial neoplasia (VIN) were found. HPV infection, age > 35 years, multiparity, contraceptive method, immunodeficiency, and level of studies were significantly correlated with multicentric lesions. High percentage of affected margins were found. VIN cases were treated with surgical excision and in two cases microinvasion was found. VAIN III cases were treated with surgical excision or with laser and one case progressed to vaginal cancer. Recurrence after treatment was 27%. CONCLUSION: Age, multiparity, contraceptive method, immunodeficiency, and level of studies were significantly correlated with multicentric lesion. Multicentric lesions had an increased risk of recurrence and progression to cancer.

Reversible 2D Phase Transition Driven By an Electric Field: Visualization and Control on the Atomic Scale.

We report on a reversible structural phase transition of a two-dimensional system that can be locally induced by an external electric field. Two different structural configurations may coexist within a CO monolayer on Cu(111). The balance between the two phases can be shifted by an external electric field, causing the domain boundaries to move, increasing the area of the favored phase controllable both in location and size. If the field is further enhanced new domains nucleate. The arrangement of the CO molecules on the Cu surface is observed in real time and real space with atomic resolution while the electric field driving the phase transition is easily varied over a broad range. Together with the well-known molecular manipulation of CO adlayers, our findings open exciting prospects for combining spontaneous long-range order with man-made CO structures such as "molecule cascades" or "molecular graphene". Our new manipulation mode permits us to bridge the gap between fundamental concepts and the fabrication of arbitrary atomic patterns in large scale, by providing unprecedented insight into the physics of structural phase transitions on the atomic scale.

Relationship between smoking, HPV infection, and risk of Cervical cancer.

PURPOSE OF INVESTIGATION: To document the relationship between smoking and HPV infection, and the risk of developing preinvasive lesions and cervical carcinoma. MATERIALS AND METHODS: Prospective, cross-sectional descriptive study. A total of 1,007 patients were recruited among women seen at the cervical pathology clinic of Sant Joan de Déu University Hospital in Barcelona (Spain) between January 2003 and March 2011. Patients were asked specifically about their smoking habits. Statistical analyses were done with SPSS v.19 software. Differences between groups were considered statistically significant at p < 0.05. RESULTS: In patients studied, 48.7% were smokers. The average number of cigarettes per day among smoking patients was 7.07 (1-40). In the of patients with HPV infection, 53% were smokers versus 37% of patients without HPV infection (p < 0.05). The average number of cigarettes per day among patients with HPV infection was 7.64 cigarettes/day versus 5.55 cigarettes/day among patients without HPV infection (p < 0.05). In the patients with high-risk HPV genotypes infection, 54.5% were smokers versus 43.2% of patients without high-risk HPV infection (p < 0.05). Risk of HPV infection increases 1.905 times among smoking patients versus no smoking patients (OR = 1.905, CI 95% (1.426-2.545), p < 0.05). Among patients with changes associated to HPV and atypical cells, there were 29.2% and 14.4% of smokers, respectively, versus 45.5%, 55.6%, and 48.6% of smokers among patients with grade 1 cervical intraepithelial neoplasia (CIN 1), CIN 2-3, and carcinoma, respectively (p < 0.05). Risk of CIN 2-3 or cervical carcinoma cervical increases 1.642 times among smoking patients versus no smoking ones (OR = 1.642, CI 95% (1.325-1.884), p < 0.05). CONCLUSIONS: Smoking interferes in the increase of HPV infection prevalence and in an increased risk of CIN and cervical carcinoma. Risk also increases with more cigars smoked per day.

Excitation of bond-alternating spin-1/2 Heisenberg chains by tunnelling electrons.

Inelastic electron tunneling spectra (IETS) are evaluated for spin-1/2 Heisenberg chains showing different phases of their spin ordering. The spin ordering is controlled by the value of the two different Heisenberg couplings on the two sides of each of the chain's atoms (bond-alternating chains). The perfect anti-ferromagnetic phase, i.e. a unique exchange coupling, marks a topological quantum phase transition (TQPT) of the bond-alternating chain. Our calculations show that the TQPT is recognizable in the excited states of the chain and hence that IETS is in principle capable of discriminating the phases. We show that perfectly symmetric chains, such as closed rings mimicking infinite chains, yield the same spectra on both sides of the TQPT and IETS cannot reveal the nature of the spin phase. However, for finite size open chains, both sides of the TQPT are associated with different IETS spectra, especially on the edge atoms, thus outlining the transition.

Energetics and stability of dangling-bond silicon wires on H passivated Si(100).

We evaluate the electronic, geometric and energetic properties of quasi 1D wires formed by dangling-bonds on Si(100)-H(2 × 1). The calculations are performed with density functional theory (DFT). Infinite wires are found to be insulating and Peierls distorted, however finite wires develop localized electronic states that can be of great use for atomic scale devices. The ground state solution of finite wires does not correspond to a geometrical distortion but rather to an antiferromagnetic ordering. For the stability of wires, the presence of abundant H atoms in nearby Si atoms can be a problem. We have evaluated the energy barriers for intradimer and intrarow diffusion, finding all of them about 1 eV or larger, even in the case where a H impurity is already sitting on the wire. These results are encouraging for using dangling-bond wires in future devices.

A theoretical rationalization of a total inelastic electron tunneling spectrum: the comparative cases of formate and benzoate on Cu(111).

Inelastic electron tunneling spectroscopy (IETS) performed with the scanning tunneling microscope (STM) has been deemed as the ultimate tool for identifying chemicals at the atomic scale. However, direct IETS-based chemical analysis remains difficult due to the selection rules that await a definite understanding. We present IETS simulations of single formate and benzoate species adsorbed in the same upright bridge geometry on a (111)-cleaved Cu surface. In agreement with measurements on a related substrate, the simulated IET-spectra of formate/Cu(111) clearly resolve one intense C-H stretching mode whatever the tip position in the vicinity of the molecular fragment. At variance, benzoate/Cu(111) has no detectable IET signal. The dissimilar IETS responses of chemically related molecules--formate and benzoate adsorbates--permit us to unveil another factor that complements the selection rules, namely the degree of the vacuum extension of the tunneling active states perturbed by the vibrations. As a consequence, the lack of a topmost dangling bond orbital is entirely detrimental for STM-based inelastic spectroscopy but not for STM elastic imaging.

Orbital specific chirality and homochiral self-assembly of achiral molecules induced by charge transfer and spontaneous symmetry breaking.

We study the electronic mechanisms underlying the induction and propagation of chirality in achiral molecules deposited on surfaces. Combined scanning tunneling microscopy and ab initio electronic structure calculations of Cu-phthalocyanines adsorbed on Ag(100) reveal the formation of chiral molecular orbitals in structurally undistorted molecules. This effect shows that chirality can be manifest exclusively at the electronic level due to asymmetric charge transfer between molecules and substrate. Single molecule chirality correlates with attractive van der Waals interactions, leading to the propagation of chirality at the supramolecular level. Ostwald ripening provides an efficient pathway for complete symmetry breaking and self-assembly of homochiral supramolecular layers.

Graphene on Ru(0001): contact formation and chemical reactivity on the atomic scale.

Graphene on Ru(0001) is contacted with Au tips of a cryogenic scanning tunneling microscope. The formation and conductance of single-atom contacts vary within the moiré unit cell. Density functional calculations reveal that elastic distortions of the graphene lattice occur at contact due to a selectively enhanced chemical reactivity of C atoms at hollow sites of Ru(0001). Concomitant quantum transport calculations indicate that the graphene-Ru distance determines the conductance variations.

Formation of dispersive hybrid bands at an organic-metal interface.

An electronic band with quasi-one-dimensional dispersion is found at the interface between a monolayer of a charge-transfer complex (TTF-TCNQ) and a Au(111) surface. Combined local spectroscopy and numerical calculations show that the band results from a complex mixing of metal and molecular states. The molecular layer folds the underlying metal states and mixes with them selectively, through the TTF component, giving rise to anisotropic hybrid bands. Our results suggest that, by tuning the components of such molecular layers, the dimensionality and dispersion of organic-metal interface states can be engineered.

Adsorption of N2O on Cu(100): a combined scanning tunneling microscopy and density functional theory study.

The adsorption of N(2)O on Cu(100) has been studied by using scanning tunneling microscopy (STM). In the first molecular layer N(2)O forms a densely packed c(3 x 2) structure, in which the molecules occupy two different adsorption sites. The bonding strength of this layer is found to be very weak as revealed by a low desorption temperature and the formation of misalignments and defects. Density functional theory (DFT) finds a stable c(3 x 2) structure in which the molecules are considerably bent due to charge transfer. In model calculations for a 2 x 2 hollow phase we show that in order to reach the chemisorbed, bent configuration, the molecules have to pass an activation barrier. In the experimentally accessible range, this is apparently not possible and the molecules remain in a stable physisorbed state.

Reducing the molecule-substrate coupling in C60-based nanostructures by molecular interactions.

Codeposition of C60 and the three-dimensional molecular hydrocarbon 1,3,5,7-tetraphenyladamantane (TPA) on Au(111) leads to the spontaneous formation of molecular nanostructures in which each fullerene is locked into a specific orientation by three surrounding TPA. Scanning tunneling spectroscopy shows that the electronic coupling of C60 with the surface is significantly reduced in these nanostructures, enhancing the free-molecule properties. As evidenced by density functional theory simulations, the nanostructures are stabilized by 18 local electrostatic forces between C60 and TPA, resulting in a lifting of the C60 cage from the surface.

Long-range repulsive interaction between molecules on a metal surface induced by charge transfer.

The adsorption of a molecular electron donor on Au(111) is characterized by the spontaneous formation of a superlattice of monomers spaced several nanometers apart. The coverage-dependent molecular pair distributions obtained from scanning tunneling microscopy data reveal an intermolecular long-range repulsive potential, which decreases as the inverse of the molecular separation. Density functional theory calculations show a charge accumulation in the molecules due to electron donation into the metal. Our results suggest that electrostatic repulsion between molecules persists on the surface of a metal.

Electron transport via local polarons at interface atoms.

Electronic transport is profoundly modified in the presence of strong electron-vibration coupling. We show that in certain situations, the electron flow takes place only when vibrations are excited. By controlling the segregation of boron in semiconducting Si(111)-square root 3 x square root 3 R 30 degrees surfaces, we create a type of adatom with a dangling-bond state that is electronically decoupled from any other electronic state. However, probing this state with scanning tunnelling microscopy at 5 K yields high currents. These findings are rationalized by ab-initio calculations that show the formation of a local polaron in the transport process.

Spontaneous formation of triptycene supramolecules on surfaces.

In the limit of weak molecular interaction with an inorganic surface, noncovalent interactions between molecules dominate the nucleation and thin-film growth. Here, we report on the formation of three-dimensional triptycene clusters with a particularly stable structure. Once formed at the early stage of molecular adsorption, the clusters are stable for all temperatures until desorption. Furthermore, the clusters diffuse and nucleate as individual entities, therefore constituting building blocks for the later thin-film formation. High resolution scanning tunneling microscopy images indicate that the cluster is stabilized by C-H-pi interactions. The formation of such molecular structures at a surface is possible because the three-dimensional structure of the triptycene molecule leads to a very weak and mobile adsorption state. These results show that it is possible to investigate complex pathways in the formation of three-dimensional supramolecules at surfaces using a scanning tunneling microscope.

Inelastic spectroscopy identification of STM-induced benzene dehydrogenation.

Inelastic electron tunneling spectroscopy (IETS) performed with the scanning tunneling microscope (STM) has been deemed as the ultimate tool for identifying chemicals on the atomic scale. However, IETS-based chemical analysis is error-prone due to the numerous degrees of freedom of chemisorbed molecular systems. First-principles simulations of IETS are presented that, by quantitative comparison with the experimental spectra, permit one to determine the final products of an STM-induced reaction on chemisorbed benzene. Our simulations reveal that IETS possesses an enhanced sensitivity to atomic structure as compared to topographic imaging due to both its energy and space resolution.