Promoting Atoms into Delocalized Long-Living Magnetically Modified State Using Atomic Force Microscopy

Abstract

We report on a low-temperature AFM manipulation of Co atoms in UHV on an oxidized copper surface in which the manipulated atom is kept delocalized above several surface unit cells over macroscopic times. The manipulation employed, in addition to the ubiquitous shortrange tip-generated chemical forces, also longrange forces generated via Friedel oscillations of the metal charge density due to Co nanostructures prearranged on the surface by lateral manipulation. We show that our manipulation protocol requires mechanical control of the spin state of the Co atom.Keywords: AFM manipulation, magnetic atoms, mechanical control of spin, Friedel oscillations.Scanning probe microscopies (SPM), such as scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM), have played a key role in constructing nanostructures with atomic dimensions. STM and NC-AFM have a number of unique abilities such as scanning surfaces with atomic resolution, 1 providing information on the chemical identity of the scanned atoms, 2 and perhaps the most fascinating ability of performing nano-manipulations.3-5 Both lateral 6,7 and vertical 8-10 atomic manipulations have been performed to build bottom-up nanostructures on surfaces either by moving the manipulated atoms directly 6,11 or by exchanging pairs of atoms.9,12 All these manipulations have been performed with non-magnetic atoms on high-symmetry (111) and (001) substrates and have strongly relied on short-range site-specific chemical interactions. 4 Here we show that by harnessing both short-(≤5Å) and long-range (≈ 5 -40Å) interactions exerted on magnetic atoms, novel manipulation protocols can be designed whereby in a controlled way the manipulated atoms can be kept in a magnetically modified state which is imaged over macroscopic times as delocalized above several surface unit cells.Friedel oscillations 13,14 of metal surface charge density due to adatoms or adatom superstructures adsorbed on it may provide a natural mechanism of long-range interaction forces exerted on an adatom which is manipulated by an SPM tip. However, combining these rather weak (in the range of meV 13,14 ) interactions with the short range chemical interaction of the adatom with the substrate (and possibly the tip) is challenging as the latter interaction is rather strong (typically in the eV range). Existence of strong short-range 1 forces is essential in trapping the adatoms in the desired positions when building nanostructures, as otherwise they would be undesirably highly mobile at finite temperatures and be easily moved by the tip. However, as will be demonstrated here, if the short-range interaction provides a highly corrugated potential energy surface (PES), which contains smooth plateaus separated by deep minima, then the long-range interaction due to the surrounding adatoms provided by the Friedel oscillations, may have profound implications on the manipulated adatom.One example of such a system with the required form of the PES is a cobalt adatom adsorbed on the p(2 × 1) phase...

Subatomic-scale resolution with SPM: Co adatom on p(2 × 1)Cu(110):O

“…Scanning probe microscopies (SPM), such as scanning tunneling microscopy (STM) and non-contact atomic force microscopy (NC-AFM), have played a key role in imaging surfaces and constructing nanostructures with atomic dimensions. STM and NC-AFM have a number of unique abilities such as scanning surfaces with unprecedented atomic resolution [1], providing information on the chemical identity of the scanned atoms [2], and performing both lateral and vertical nano-manipulations [3][4][5][6].…”

“…Transition metal 3d adatoms are especially suitable for tracing subatomic features, as the unpaired electrons in the d shells may provide an easy way to observe deviations from closed-shell spherical atoms, especially if fairly weakly bonded to the surface. As shown below, such a situation may be realized in cobalt adatom manipulation on an oxidized p(2 × 1)Cu(110):O copper surface [5], which we use here as a case study. Spin states of the related system of Co adatoms on a black phosphorus surface and their manipulation have recently been studied as a model system for single-atom spin memory [17].…”

“…Recent low-temperature NC-AFM manipulation experiments in ultra high vacuum of Co atoms deposited onto the p(2 × 1)Cu(110):O oxidized copper surface [5] have demonstrated the possibility of very peculiar manipulation where the Co atoms could be kept delocalized over macroscopic times above several unit cells in a spin state mechanically modified by the tip of the AFM apparatus, figure 1. When Co atoms are deposited on the p(2 × 1)Cu(110):O surface, they assume a very stable oxygen-bonded position, figure 1, akin to the between-O position occupied in the c(6×2) phase under high-oxygen load [18,19].…”

“…The mechanical energy of the AFM tip may laterally manipulate the atom on the surface into other positions, such as the on-top-Cu, position. The activation barrier for this manipulation is very high, ≈2 eV, and therefore this is a rare event that, nevertheless, was experimentally realized [5]. Once that happens, the state of the Co adatom is predicted to undergo a [Ar]d 8 →[Ar]d 7 modification and the Co atom can be laterally manipulated into a long-lived smeared-out elliptical trajectory, figure 1.…”

Cyclic Single Atom Vertical Manipulation on a Nonmetallic Surface

“…Sugimoto and co-workers introduced a key mechanism for manipulation of this type: the “vertical interchange” protocol. Subsequently, a number of other vertical manipulation strategies have been reported. ,,, However, using the same tip to manipulate different types of atoms on a semiconducting surface repeatedly during the same experimental run (and hence with the same tip) still remains elusive. The atomic manipulation process described below represents a new and important addition to the class of vertical manipulation methods, with an ability to manipulate a wide class of different atoms individually with the same tip within the same experimental run.…”