Within a Kondo lattice, the strong hybridization between electrons localized in real space (r-space) and those delocalized in momentum-space (k-space) generates exotic electronic states called 'heavy fermions'. In URu(2)Si(2) these effects begin at temperatures around 55 K but they are suddenly altered by an unidentified electronic phase transition at T(o) = 17.5 K. Whether this is conventional ordering of the k-space states, or a change in the hybridization of the r-space states at each U atom, is unknown. Here we use spectroscopic imaging scanning tunnelling microscopy (SI-STM) to image the evolution of URu(2)Si(2) electronic structure simultaneously in r-space and k-space. Above T(o), the 'Fano lattice' electronic structure predicted for Kondo screening of a magnetic lattice is revealed. Below T(o), a partial energy gap without any associated density-wave signatures emerges from this Fano lattice. Heavy-quasiparticle interference imaging within this gap reveals its cause as the rapid splitting below T(o) of a light k-space band into two new heavy fermion bands. Thus, the URu(2)Si(2) 'hidden order' state emerges directly from the Fano lattice electronic structure and exhibits characteristics, not of a conventional density wave, but of sudden alterations in both the hybridization at each U atom and the associated heavy fermion states.

The miniaturization of magnetic devices toward the limit of single atoms calls for appropriate tools to study their magnetic properties. We demonstrate the ability to measure magnetization curves of individual magnetic atoms adsorbed on a nonmagnetic metallic substrate with use of a scanning tunneling microscope with a spin-polarized tip. We can map out low-energy magnetic interactions on the atomic scale as evidenced by the oscillating indirect exchange between a Co adatom and a nanowire on Pt(111). These results are important for the understanding of variations that are found in the magnetic properties of apparently identical adatoms because of different local environments.

Ruderman-Kittel-Kasuya-Yosida interaction [1][2][3] is an indirect magnetic coupling between localized spins in a non-magnetic host mediated by conduction electrons. In diluted systems it is often the dominating magnetic interaction and has played a key part in the development of giant magnetoresistance devices 4,5 , drives ferromagnetism in heavy rare-earth elements 6 as well as in diluted magnetic semiconductors 7 and gives rise to complex magnetic phases such as spin glasses 8 . For bulk systems, an isotropic and continuous model of Ruderman-Kittel-KasuyaYosida interaction is often sufficient. However, it can be misleading in magnetic nanostructures consisting of separate magnetic atoms adsorbed on the surface of a non-magnetic material. Here, an atomically precise map of the magnetic coupling between individual adatoms in pairs is measured and directly compared with first-principles calculations, proving that Ruderman-Kittel-Kasuya-Yosida interaction is strongly directional. By investigating adatom triplets of different shapes we demonstrate that the map can serve to tailor the magnetism of larger nanostructures.Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction is ubiquitous in solid-state systems containing diluted magnetic moments in a conducting non-magnetic host. It becomes dominant whenever there is a sufficiently strong exchange coupling between the localized moments and the conduction electrons. Then, the spins of the conduction electrons, which are on average unpolarized, are forced into a preferred direction in the vicinity of each moment. This preferential direction oscillates with increasing distance from the moment. A second localized moment will interact with this spin-density oscillation and perceive either a ferromagnetic or an antiferromagnetic coupling to the first, depending on their distance. Therefore, RKKY interaction is also called indirect magnetic exchange. The first indications of indirect magnetic exchange through conduction electrons came with the research on diluted bulk alloys, where a dependence of the interaction strength on the distance between the moments with an oscillation period of half the Fermi wavelength was proposed in an isotropic and continuous model [1][2][3] . Direct experimental evidence for RKKY-like coupling of magnetic layers through transition-metal layers was obtained by spatial-averaging techniques [9][10][11][12] . There have been theoretical [13][14][15] and experimental 16 hints that accurate RKKY models have to take into account the topology of the Fermi surface and the discrete distribution of magnetic moments on the atomic lattice, but a direct experimental proof was hampered by the spatial averaging, which provides only fragmentary information on the distribution. However, as magnetic devices are becoming smaller and approaching the limit of nanostructures built by separate atoms, knowledge of the RKKY interaction on the atomic scale is essential.By using scanning tunnelling spectroscopy, it became possible to investigate magnetic interactions in atom ...

Using scanning tunneling spectroscopy in an ultrahigh vacuum at low temperature (T=0.3 K) and high magnetic fields (B

We present low-temperature scanning tunneling spectroscopy measurements on Mn acceptors in InAs in comparison with tight-binding calculations. We find a strong (001)-mirror asymmetry of the bound hole wave function close to the (110) surface. In addition, multiple acceptor-related peaks are observed and are attributed to a spin-orbit splitting of the acceptor level. Because of the p-d exchange interaction the local density of states near the acceptors is enhanced in the valence band and suppressed in the conduction band. We also observe signs of anisotropic scattering of the conduction band states by neutral acceptors.