Engineering the Eigenstates of Coupled Spin- 1/2 Atoms on a Surface

Abstract

Quantum spin networks having engineered geometries and interactions are eagerly pursued for quantum simulation and access to emergent quantum phenomena such as spin liquids. Spin-1/2 centers are particularly desirable because they readily manifest coherent quantum fluctuations. Here we introduce a controllable spin-1/2 architecture consisting of titanium atoms on a magnesium oxide surface. We tailor the spin interactions by atomic-precision positioning using a scanning tunneling microscope (STM), and subsequently perform electron spin resonance (ESR) on individual atoms to drive transitions into and out of quantum eigenstates of the coupled-spin system. Interactions between the atoms are mapped over a range of distances extending from highly anisotropic dipole coupling, to strong exchange coupling. The local magnetic field of the magnetic STM tip serves to precisely tune the superposition states of a pair of spins. The precise control of the spin-spin interactions and ability to probe the states of the coupled-spin network by addressing individual spins will enable exploration of quantum many-body systems based on networks of spin-1/2 atoms on surfaces.Building networks of spin-1/2 objects with adjustable interactions represents a versatile approach for quantum simulation of model Hamiltonians [1, 2] because it provides direct experimental access to quantum emergent phenomena, such as topologically generated gapped excitations [3], spin liquids [4] and anyon excitations [5]. However, the precise control of spin interactions and integration beyond a few spins, while maintaining the ability to address individual spins, remains notoriously challenging [6]. Atomically engineered spin networks on surfaces, such as coupled atomic dimers, chains [7,8], ladders [9] and arrays [10], provide a bottom-up realization of tailored spin systems, by using STM to position and address individual atoms [9,11]. Atoms with large spin S generally exhibit strong magnetocrystalline anisotropy that results in Ising-like interactions [4,12]. In contrast, quantum fluctuations scale in proportion to 1/S, so they are maximal for the smallest possible spin, S = 1/2 [4].Spins interact via exchange and dipolar interactions. At the scale of a few coupled spins, shortrange exchange coupling can give rise to magnetic ordering such as magnetic bistability [9,13] and

Single-atom electron paramagnetic resonance in a scanning tunneling microscope driven by a radio-frequency antenna at 4 K

“…Spin-polarized STM tips were prepared by repeatedly picking up of Fe atoms from the surface (on the order of 10); the resulting spin polarization was confirmed by conductivity spectra on TiH O showing a pronounced step around zero DC bias that is otherwise absent (see inset in Fig. 2c) [21].…”

“…It is known that residual hydrogen gas in the vacuum chamber tends to hydrogenate the Ti atoms, forming TiH [44]. [6,7,21,22]. d d Τ spectra (see Fig.…”

“…Following early attempts [18,19], Baumann et al [20] were the first to convincingly demonstrate EPR of single atoms on a surface using STM. In these experiments [20][21][22][23], the authors studied single Fe, Ti, and Cu atoms on a thin insulating MgO layer grown on Ag (100) (see Fig. 1a).…”

Electrical spin manipulation in graphene nanostructures

“…With this approach, the application of an off-plane electric field significantly larger than in conventional field effect transistor geometries is possible. On the other hand, STM can be used to carry out electrically driven spin paramagnetic resonance of individual atoms [41][42][43] and coupled spin 1/2 atoms [44]. Therefore, the electrical manipulation of localized spin states in graphene seems within reach with state-of-the-art surface scanning probes.…”

Enhanced lifetimes of spin chains coupled to chiral edge states

“…One natural question that emerges when talking about magnetic atoms on surfaces is the role of magnetic anisotropy and the spin size. Most of the magnetic atoms used for assembly engineered spin structures do not behave as spin S=1/2 systems and they suffer some magnetocrystalline anisotropy [2,[4][5][6][7][8]. We have analyzed the relaxation and decoherence times of anisotropic S>1/2 spin chains, including both longitudinal and transverse anisotropy in the local spin Hamiltonian:…”