D. M. Eigler

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 � 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 corrals interior local density of states is dominated by the eigenstate density expected for an electron trapped in a round two-dimensional box.

Atomic and Molecular Manipulation with the Scanning Tunneling Microscope

The prospect of manipulating matter on the atomic scale has fascinated scientists for decades. This fascination may be motivated by scientific and technological opportunities, or from a curiosity about the consequences of being able to place atoms in a particular location. Advances in scanning tunneling microscopy have made this prospect a reality; single atoms can be placed at selected positions and structures can be built to a particular design atom-by-atom. Atoms and molecules may be manipulated in a variety of ways by using the interactions present in the tunnel junction of a scanning tunneling microscope. Some of these recent developments and some of the possible uses of atomic and molecular manipulation as a tool for science are discussed.

Quantum mirages formed by coherent projection of electronic structure

Image projection relies on classical wave mechanics and the use of natural or engineered structures such as lenses or resonant cavities. Well-known examples include the bending of light to create mirages in the atmosphere, and the focusing of sound by whispering galleries. However, the observation of analogous phenomena in condensed matter systems is a more recent development, facilitated by advances in nanofabrication. Here we report the projection of the electronic structure surrounding a magnetic Co atom to a remote location on the surface of a Cu crystal; electron partial waves scattered from the real Co atom are coherently refocused to form a spectral image or ‘quantum mirage’. The focusing device is an elliptical quantum corral, assembled on the Cu surface. The corral acts as a quantum mechanical resonator, while the two-dimensional Cu surface-state electrons form the projection medium. When placed on the surface, Co atoms display a distinctive spectroscopic signature, known as the many-particle Kondo resonance, which arises from their magnetic moment. By positioning a Co atom at one focus of the ellipse, we detect a strong Kondo signature not only at the atom, but also at the empty focus. This behaviour contrasts with the usual spatially-decreasing response of an electron gas to a localized perturbation.

Bistability in Atomic-Scale Antiferromagnets

Structured Memories High-density magnetic memory is generally produced by using ferromagnetic materials. As the density increases and the memory elements become closer together, stray fields can result in cross-talk and a corruption of the stored information. Antiferromagetic structures, however, are expected to be relatively insensitive to magnetic fields and so should, in principle, allow the elements to be packed in even closer. Loth et al. (p. 196) carried out low-temperature experiments to construct antiferromagnetic structures atom by atom. Electrical switching of the magnetic states was observed, and data could be robustly stored on the structure for several hours, albeit at low temperature. Atomically engineered antiferromagnets consisting of a few atoms exhibit stable magnetic states at low temperature. Control of magnetism on the atomic scale is becoming essential as data storage devices are miniaturized. We show that antiferromagnetic nanostructures, composed of just a few Fe atoms on a surface, exhibit two magnetic states, the Néel states, that are stable for hours at low temperature. For the smallest structures, we observed transitions between Néel states due to quantum tunneling of magnetization. We sensed the magnetic states of the designed structures using spin-polarized tunneling and switched between them electrically with nanosecond speed. Tailoring the properties of neighboring antiferromagnetic nanostructures enables a low-temperature demonstration of dense nonvolatile storage of information.

Measurement of fast electron spin relaxation times with atomic resolution.

In a Spin The relaxation dynamics of electron spins in solid-state systems is of crucial importance for their usage in quantum computation and information storage. The interaction of the spin with its local environment results in lifetimes in the pico- to microsecond range. Thus, high temporal and spatial resolutions are needed to measure the relaxation time with atomic precision. Loth et al. (p. 1628, see the cover; see the Perspective by Morgenstern) used a scanning tunneling microscope with a spin-polarized tip to monitor the electron spin relaxation times of individual atoms adsorbed on a surface. A spin was excited by a pump signal, and its state read out after a variable time delay with a weak probe pulse that produced a spin-sensitive tunneling current. This general technique may be applicable to other systems with fast dynamics. Scanning tunneling microscopy is used to monitor the fast relaxation dynamics of an atomic spin adsorbed on a surface. Single spins in solid-state systems are often considered prime candidates for the storage of quantum information, and their interaction with the environment the main limiting factor for the realization of such schemes. The lifetime of an excited spin state is a sensitive measure of this interaction, but extending the spatial resolution of spin relaxation measurements to the atomic scale has been a challenge. We show how a scanning tunneling microscope can measure electron spin relaxation times of individual atoms adsorbed on a surface using an all-electronic pump-probe measurement scheme. The spin relaxation times of individual Fe-Cu dimers were found to vary between 50 and 250 nanoseconds. Our method can in principle be generalized to monitor the temporal evolution of other dynamical systems.

Off-Resonance Conduction Through Atomic Wires

The electrical resistance of wires consisting of either a single xenon atom or two xenon atoms in series was measured and calculated on the basis of an atom-jellium model. Both the measurement and the calculation yielded a resistance of 105 ohms for the single-xenon atom system and 107 ohms for the two-xenon atom system. These resistances greatly exceeded the 12,900-ohm resistance of an ideal one-dimensional conduction channel because conduction through the xenon atoms occurs through the tail of the xenon 6s resonance, which lies far above the Fermi level. This conduction process in an atom-sized system can now be understood in terms of the electronic states of individual atoms.

IMPURITY-INDUCED BOUND EXCITATIONS ON THE SURFACE OF BI2SR2CACU2O8

We have probed the effects of atomic-scale impurities on superconductivity in Bi_{2}Sr_{2}CaCu_{2}O_{8} by performing low-temperature tunneling spectroscopy measurements with a scanning tunneling microscope. Our results show that non-magnetic defect structures at the surface create localized low-energy excitations in their immediate vicinity. The impurity-induced excitations occur over a range of energies including the middle of the superconducting gap, at the Fermi level. Such a zero bias state is a predicted feature for strong non-magnetic scattering in a d-wave superconductor.

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.

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.

Quantum interference in 2D atomic-scale structures

Abstract Electrons occupying surface states on the close-packed faces of the noble metals form a two-dimensional (2D) electron gas that is accessible to the scanning tunneling microscope (STM). Using a cryogenic STM, we have observed quantum mechanical interference patterns arising from 2D electrons on the surface of Cu. These interference patterns can be artificially controlled by arranging individual Fe atoms into “quantum corrals” on the Cu surface. Quantum corrals behave qualitatively like 2D hard-wall boxes, but a quantitative understanding is obtained within a multiple scattering formalism. The scattering here is characterized by a complex phase shift which can be extracted from the electronic density pattern near a quantum corral.