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.

We have carried out detailed experimental studies of the exchange bias effect of a series of CoO/Co(111) textured bilayers with different Co layer thickness, using the magneto-optical Kerr effect, SQUID magnetometry, polarized neutron reflectivity, x-ray diffraction, and atomic force microscopy. All samples exhibit a pronounced asymmetry of the magnetic hysteresis at the first magnetization reversal as compared to the second reversal. Polarized neutron reflectivity measurements show that the first reversal occurs via nucleation and domain wall motion, while the second reversal is characterized by magnetization rotation. Off-specular diffuse spin-flip scattering indicates the existence of interfacial magnetic domains. All samples feature a small positive exchange bias just below the blocking temperature, followed by a dominating negative exchange bias field with decreasing temperature.

We present a new method to engineer the charge carrier mobility and its directional asymmetry in epitaxial graphene by using metal cluster superlattices self-assembled onto the moiré pattern formed by graphene on Ir(111). Angle-resolved photoemission spectroscopy reveals threefold symmetry in the band structure associated with strong renormalization of the electron group velocity close to the Dirac point giving rise to highly anisotropic Dirac cones. We further find that the cluster superlattice also affects the spectral-weight distribution of the carbon bands as well as the electronic gaps between graphene states. DOI: 10.1103/PhysRevLett.105.246803 PACS numbers: 73.20.Àr, 73.21.Cd, 73.22.Pr, 79.60.Ài Graphitic materials have attracted strong scientific interest because they exhibit exotic phenomena such as superconductivity or the anomalous quantum Hall effect [1]. Graphene (gr) is the building block of these materials; it is wrapped up into fullerenes, rolled up into carbon nanotubes, or stacked into 3D graphite. It presents a model system to investigate the influence of many-body interactions on the electron dynamics in these materials. In addition, the exceptional electronic mobility makes graphene a candidate material for next generation electronic devices [2]. Freestanding graphene is a zero-gap semiconductor. Because most electronic applications require a gap between valence and conduction bands, considerable effort has been spent to induce and control the opening of such a band gap [3][4][5][6].A related, and for applications equally relevant, issue is the ability to tailor the band dispersion. The speed with which information can be transmitted along a graphene layer depends on the charge carrier group velocity. Close to the K points, the bands of freestanding graphene have a linear dispersion well described by the relativistic Dirac equation for massless neutrinos. The resulting Dirac cones are trigonally warped due to the chiral nature of graphene charge carriers in the equivalent A and B carbon sublattices [7]. The ability to increase and tailor this anisotropy would open a manifold of new applications. Several theoretical studies suggest that this goal can be reached by applying an external periodic potential with nanometer period giving rise to highly anisotropic Dirac cones [8][9][10][11].Epitaxial graphene layers grown on lattice mismatched close-packed metal substrates, such as Pt (111) Here we demonstrate with angle-resolved photoemission spectroscopy (ARPES) that an Ir cluster superlattice grown on the gr=Irð111Þ-(9:32 AE 0:15 Â 9:32 AE 0:15) moiré structure [13,15] gives rise to significant group velocity and Dirac cone asymmetries. We attribute this to a much stronger periodic potential caused by the cluster superlattice than by the moiré itself. H decorated gr=Irð111Þ has been reported to give rise to band gap opening but not to the Dirac cone asymmetries reported here [6]. We thereby confirm theoretical predictions and present a new way to tailor the directionality of carrier mobilit...

We present design and performance of a scanning tunneling microscope (STM) that operates at temperatures down to 10 mK providing ultimate energy resolution on the atomic scale. The STM is attached to a dilution refrigerator with direct access to an ultra high vacuum chamber allowing in situ sample preparation. High magnetic fields of up to 14 T perpendicular and up to 0.5 T parallel to the sample surface can be applied. Temperature sensors mounted directly at the tip and sample position verified the base temperature within a small error margin. Using a superconducting Al tip and a metallic Cu(111) sample, we determined an effective temperature of 38 ± 1 mK from the thermal broadening observed in the tunneling spectra. This results in an upper limit for the energy resolution of ΔE = 3.5 kBT = 11.4 ± 0.3 μeV. The stability between tip and sample is 4 pm at a temperature of 15 mK as demonstrated by topography measurements on a Cu(111) surface.

We investigate the effects of Na adsorption on the electronic structure of bare and Ir cluster superlattice covered epitaxial graphene on Ir(111) using angle-resolved photoemission spectroscopy and scanning tunneling microscopy. At Na saturation coverage a massive charge migration from sodium atoms to graphene raises the graphene Fermi level by about 1.4 eV relative to its neutrality point. We find that Na is adsorbed on top of the graphene layer and when coadsorbed onto an Ir cluster superlattice it results in the opening of a large bandgap of Δ Na/Ir/G = 740 meV comparable to the one of Ge and with preserved high group velocity of the charge carriers.