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Target control of directed networks, which aims to control only a target subset instead of the entire set of nodes in large natural and technological networks, is an outstanding challenge faced in various real world applications. We address one fundamental issue regarding this challenge, i.e., for a given target subset, how to allocate a minimum number of control sources which provide input signals to the network nodes. This issue remains open in general networks with loops. We show that the issue is essentially a path cover problem and can be further converted into a maximum network flow problem. A method termed `Maximum Flow based Target Path-cover' (MFTP) with complexity $O(|V|^{1/2}|E|)$ in which $|V|$ and $|E|$ denote the number of network nodes and edges is proposed. It is also rigorously proven to provide the minimum number of control sources on arbitrary directed networks, whether loops exist or not. We anticipate that this work would serve wide applications in target control of real-life networks, as well as counter control of various complex systems which may contribute to enhancing system robustness and resilience.

The generation of intense electron beams with low emittance is key to both the production of coherent x rays from free electron lasers, and electron pulses with large transverse coherence length used in ultrafast electron diffraction. These beams are generated today by photoemission from disordered polycrystalline surfaces. We show that the use of single crystal surfaces with appropriate electronic structures allows us to effectively utilize the physics of photoemission to generate highly directed electron emission, thus reducing the emittance of the electron beam being generated.

The study of ultrathin oxide film as templates for magnetic nanostructures is presented. The ultrathin oxide thin films are investigated for the surface morphology, electronic properties, surface terminations, and ferroelectric properties by the combination of various local and non-local surface science techniques. Ultrathin oxide films, only a few unit cells in thickness, have significant advantages over bulk oxides, such as atomically defined, unreconstructed and stoichiometric surfaces. The high structural quality makes them suitable templates for the fabrication of metal-oxide heterostructures with unprecedented interface quality. This is expected to enable the studies of fundamental interactions between the oxide and metallic adsorbates, such as the theoretically predicted magnetoelectric coupling based on interfacial Fe-Ti bonds. Two kinds of oxide films, BaTiO3 and Cr2O3, are discussed. Scanning tunneling microscopy is utilized to investigate their structure and electronic properties. Firstly, studies of magnetism of Fe nanoclusters deposited on BaTiO3 by X-ray absorption and photoelectron spectroscopy demonstrate that the oxidization of Fe is largely suppressed by the molecular beam epitaxy growth at low temperature so that even isolated Fe atoms on the BaTiO 3 have significant magnetic moments. As a second example, the growth and characterization of ultrathin chromia film are discussed. The thin chromia films exhibit highly ordered and atomically smooth surfaces. The electronic structure is locally explored with scanning tunneling spectroscopy, to resolve surface termination and polarization. The presented studies on the ultrathin oxide films are expected to advance the fundamental understanding of interface effects in multiferroics and may help improving magneto-electric effects. In a third example, the formation of magnetic Co nanoclusters on boron nitride nanotemplates is analyzed. The analytic models and Monte-Carlo simulations for the analysis of the c

First-principle calculations are employed to show that the magnetic structure of small atomic clusters of Co, formed on a crystalline W(110) surface and containing 3–12 atoms, strongly deviates from the usual stable ferromagnetism of Co in other systems. The clusters are ferri-, ferro- or non-magnetic, depending on cluster size and geometry. We determine the atomic Co moments and their relative alignment, and show that antiferromagnetic spin alignment in the Co clusters is caused by hybridization with the tungsten substrate and band filling. This is in contrast with the typical strong ferromagnetism of bulk Co alloys, and ferromagnetic coupling in Fe/W(110) clusters.