Enhanced quantum coherence in exchange coupled spins via singlet-triplet transitions.

Manipulation of spin states at the single-atom scale underlies spin-based quantum information processing and spintronic devices. These applications require protection of the spin states against quantum decoherence due to interactions with the environment. While a single spin is easily disrupted, a coupled-spin system can resist decoherence by using a subspace of states that is immune to magnetic field fluctuations. Here, we engineered the magnetic interactions between the electron spins of two spin-1/2 atoms to create a "clock transition" and thus enhance their spin coherence. To construct and electrically access the desired spin structures, we use atom manipulation combined with electron spin resonance (ESR) in a scanning tunneling microscope. We show that a two-level system composed of a singlet state and a triplet state is insensitive to local and global magnetic field noise, resulting in much longer spin coherence times compared with individual atoms. Moreover, the spin decoherence resulting from the interaction with tunneling electrons is markedly reduced by a homodyne readout of ESR. These results demonstrate that atomically precise spin structures can be designed and assembled to yield enhanced quantum coherence.

Origin of Perpendicular Magnetic Anisotropy and Large Orbital Moment in Fe Atoms on MgO.

We report on the magnetic properties of individual Fe atoms deposited on MgO(100) thin films probed by x-ray magnetic circular dichroism and scanning tunneling spectroscopy. We show that the Fe atoms have strong perpendicular magnetic anisotropy with a zero-field splitting of 14.0±0.3 meV/atom. This is a factor of 10 larger than the interface anisotropy of epitaxial Fe layers on MgO and the largest value reported for Fe atoms adsorbed on surfaces. The interplay between the ligand field at the O adsorption sites and spin-orbit coupling is analyzed by density functional theory and multiplet calculations, providing a comprehensive model of the magnetic properties of Fe atoms in a low-symmetry bonding environment.

Magnetism in single metalloorganic complexes formed by atom manipulation.

The magnetic properties of molecular structures can be tailored by chemical synthesis or bottom-up assembly at the atomic scale. We used scanning tunneling microscopy to study charge and spin transfer in individual complexes of transition metals with the charge acceptor, tetracyanoethylene (TCNE). The complexes were formed on a thin insulator, Cu2N on Cu(100), by manipulation of individual atoms and molecules. The Cu2N layer decouples the complexes from Cu electron density, enabling direct imaging of the TCNE molecular orbitals as well as spin-flip inelastic electron tunneling spectroscopy. Results were obtained at low temperature down to 1 K and in magnetic fields up to 7 T in order to resolve splitting of spin states in the complexes. We also performed spin-polarized density functional theory calculations to compare with the experimental data. Our results indicate that charge transfer to TCNE leads to a change in spin magnitude, Kondo resonance, and magnetic anisotropy for the metal atoms.

Spin excitations of a Kondo-screened atom coupled to a second magnetic atom.

Screening the electron spin of a magnetic atom via spin coupling to conduction electrons results in a strong resonant peak in the density of states at the Fermi energy, the Kondo resonance. We show that magnetic coupling of a Kondo atom to another unscreened magnetic atom can split the Kondo resonance into two peaks. Inelastic spin excitation spectroscopy with scanning tunneling microscopy is used to probe the Kondo effect of a Co atom, supported on a thin insulating layer on a Cu substrate, that is weakly coupled to a nearby Fe atom to form an inhomogeneous dimer. The Kondo peak is split by interaction with the non-Kondo atom, but can be reconstituted with a magnetic field of suitable magnitude and direction. Quantitative modeling shows that this magnetic field results in a spin-level degeneracy in the dimer, which enables the Kondo effect to occur.

Single-atom spin-flip spectroscopy.

We demonstrate the ability to measure the energy required to flip the spin of single adsorbed atoms. A low-temperature, high-magnetic field scanning tunneling microscope was used to measure the spin excitation spectra of individual manganese atoms adsorbed on Al2O3 islands on a NiAl surface. We find pronounced variations of the spin-flip spectra for manganese atoms in different local environments.

Information transport and computation in nanometre-scale structures.

We discuss two examples of novel information-transport and processing mechanisms in nanometre-scale structures. The local modulation and detection of a quantum state can be used for information transport at the nanometre length-scale, an effect we call a 'quantum mirage'. We demonstrate that, unlike conventional electronic information transport using wires, the quantum mirage can be used to pass multiple channels of information through the same volume of a solid. We discuss a new class of nanometre-scale structures called 'molecule cascades', and show how they may be used to implement a general-purpose binary-logic computer in which all of the circuitry is at the nanometre length-scale.

Molecule cascades.

Carbon monoxide molecules were arranged in atomically precise configurations, which we call "molecule cascades," where the motion of one molecule causes the subsequent motion of another, and so on in a cascade of motion similar to a row of toppling dominoes. Isotopically pure cascades were assembled on a copper (111) surface with a low-temperature scanning tunneling microscope. The hopping rate of carbon monoxide molecules in cascades was found to be independent of temperature below 6 kelvin and to exhibit a pronounced isotope effect, hallmarks of a quantum tunneling process. At higher temperatures, we observed a thermally activated hopping rate with an anomalously low Arrhenius prefactor that we interpret as tunneling from excited vibrational states. We present a cascade-based computation scheme that has all of the devices and interconnects required for the one-time computation of an arbitrary logic function. Logic gates and other devices were implemented by engineered arrangements of molecules at the intersections of cascades. We demonstrate a three-input sorter that uses several AND gates and OR gates, as well as the crossover and fan-out units needed to connect them.

Scattering theory of Kondo mirages and observation of single Kondo atom phase shift.

We explain the origin of the Kondo mirage seen in recent quantum corral scanning tunneling microscope experiments with a scattering theory of electrons on the surfaces of metals. Our theory, combined with experimental data, provides a direct observation of a single Kondo atom phase shift. The Kondo mirage observed at the empty focus of an elliptical quantum corral is shown to arise from multiple electron bounces off the corral wall adatoms. We demonstrate our theory with direct quantitive comparison to experimental data.

The family and disease management in Hispanic and European-American patients with type 2 diabetes.

OBJECTIVE: To determine the relationship between the characteristics of families involved in disease management and the self-care practices of Hispanic and European-American (EA) patients with type 2 diabetes. RESEARCH DESIGN AND METHODS: A total of 74 Hispanic patients and 113 EA patients with type 2 diabetes recruited from managed care settings were assessed on three domains of family life (family structure/organization, family world view, and family emotion management [four scales]) and five areas of disease management (biological, general health and function status, emotional tone, quality of life, and behavioral [seven scales]). Analyses assessed the independent associations of patient sex, family, and sex by family interactions with disease management. RESULTS: Both sex and the three domains of family life were related to disease management, but the results varied by ethnic group. For EA patients, sex, family world view, and family emotion management were related to disease management (scores for Family Coherence were negatively associated with HbA1c level and depression, and poor scores for Conflict Resolution were linked with high depression); for Hispanic patients, sex and family structure/organization were related to disease management (high scores for Organized Cohesiveness were associated with good diet and exercise, and high scores for Family Sex-Role Traditionalism were related to high quality of life). No significant interactions with sex occurred. CONCLUSIONS: Characteristics of the family setting in which disease management takes place are significantly linked to patient self-care behavior, and these linkages vary by patient ethnicity. A family's multiple independent dimensions provide multiple targets for intervention, and differences in family norms, structures, and emotion management should be considered to ensure that interventions are compatible with the setting of disease management.

Disease management status: a typology of Latino and Euro-American patients with type 2 diabetes.

The number of indicators of chronic disease management is large and no practical framework is available to guide selection for use in intervention programs. In addition, it is often difficult to integrate multiple disease management indicators into a comprehensive picture of patient functioning. The authors present a heuristic framework for creating a profile of disease management and an empirically based descriptive typology of disease management behavior. From the literature, they identified 5 domains of disease management indicators: biologic, general health and functional status, emotional tone, quality of life, and behavioral. They selected 11 scales and assessed 187 patients with type 2 diabetes. Five statistically replicable patient disease management types were derived from exploratory and confirmatory cluster analyses and a descriptive narrative was created for each: balanced (33%), problematic (6%), coasters (34%), discouraged (16%), and distressed (11%). The typology describes different presentations of disease management that can be linked with patient-tailored interventions for patients with type 2 diabetes.

The family and type 2 diabetes: a framework for intervention.

Four broad groups of factors have been linked with self-management behavior in type 2 diabetes over time: (1) characteristics of patients, (2) amount and management of stress, (3) characteristics of providers and provider-patient relationships, and (4) characteristics of the social network/context in which disease management takes place. Of these four, social network/context has received the least amount of study and has been described in terms not easily applicable to intervention. In this paper, we identified the social network/context of diabetes management as residing within the family. We defined the family for clinical purposes, reviewed the literature concerning what is known about the link between properties of the family context of care and outcomes in type 2 diabetes and other chronic diseases, and identified areas of family life that are relevant to diabetes management. This information was then used to demonstrate how a family context of care can serve as a clinical framework for integrating all four groups of factors that affect disease management. Implications of this approach for practice and research are described.

Confinement of electrons to quantum corrals on a metal surface.

A method for confining electrons to artificial structures at the nanometer lengthscale is presented. Surface state electrons on a copper(111) surface were confined to closed structures (corrals) defined by barriers built from iron adatoms. The barriers were assembled by individually positioning iron adatoms with the tip of a 4-kelvin scanning tunneling microscope (STM). A circular corral of radius 71.3 A was constructed in this way out of 48 iron adatoms. Tunneling spectroscopy performed inside of the corral revealed a series of discrete resonances, providing evidence for size quantization. STM images show that the corral's interior local density of states is dominated by the eigenstate density expected for an electron trapped in a round two-dimensional box.