The ability to manipulate single atoms has opened up the door to constructing interesting and useful quantum structures from the ground up 1 . On the one hand, nanoscale arrangements of magnetic atoms are at the heart of future quantum computing and spintronic devices 2,3 ; on the other hand, they can be used as fundamental building blocks for the realization of textbook many-body quantum models 4 , illustrating key concepts such as quantum phase transitions, topological order or frustration as a function of system size. Here, we use low-temperature scanning tunnelling microscopy to construct arrays of magnetic atoms on a surface, designed to behave like spin-1/2 XXZ Heisenberg chains in a transverse field, for which a quantum phase transition from an antiferromagnetic to a paramagnetic phase is predicted in the thermodynamic limit 5. Site-resolved measurements on these finite-size realizations reveal a number of sudden ground state changes when the field approaches the critical value, each corresponding to a new domain wall entering the chains. We observe that these state crossings become closer for longer chains, suggesting the onset of critical behaviour. Our results present opportunities for further studies on quantum behaviour of many-body systems, as a function of their size and structural complexity.Since the birth of quantum mechanics, lattice spin systems 6 have represented a natural starting point for understanding collective quantum dynamics. Today, scanning tunnelling microscopy (STM) techniques allow one to experimentally build and probe realizations of exchange-coupled lattice spins in different geometries [7][8][9] . In linear arrangements, quantum effects are strongest 10 and notions such as quantum phase transitions 11 are most easily understood, the simplest illustration being the Ising model in a transverse field 12,13 . In this work, using STM, we construct finite-size versions of a model in the same universality class, namely the spin-1/2 XXZ chain in a transverse field 5 , which has previously been realized in the bulk material Cs 2 CoCl 4 (refs 14,15). Our set-up allows us to probe the chains with single-spin resolution while tuning an externally applied transverse field through the critical regime.The chains are created by manipulating Co atoms evaporated onto a Cu 2 N/Cu(100) surface (see Methods), which provides efficient decoupling for the magnetic d-shell electrons from the underlying bulk electrons 7 . Employing inelastic electron tunnelling spectroscopy (IETS) 16,17 at sufficiently low temperature (330 mK) allows us to determine the magnetic anisotropy vector of each atom 18 as well as the strength of the exchange coupling between neighbouring atoms 19 . It was previously demonstrated that Co atoms on this surface behave as spin S = 3/2 objects experiencing a strong uniaxial hard-axis anisotropy pointing inplane, perpendicular to the bond with the neighbouring N atoms 20 . As a result, the m z = ±3/2 states split off approximately 5.5 meV above the m z = ±1/2 doublet (see Fig. 1a)...
The advent of devices based on single dopants, such as the single atom transistor 1 , the single spin magnetometer 2,3 and the single atom memory 4 , motivates the quest for strategies that permit to control matter with atomic precision. Manipulation of individual atoms by means of low-temperature scanning tunnelling microscopy 5 provides ways to store data in atoms, encoded either into their charge state 6,7 , magnetization state 8-10 or lattice position 11 . A defining challenge at this stage is the controlled integration of these individual functional atoms into extended, scalable atomic circuits. Here we present a robust digital atomic scale memory of up to 1 kilobyte (8,000 bits) using an array of individual surface vacancies in a chlorine terminated Cu(100) surface. The memory can be read and rewritten automatically by means of atomic scale markers, and offers an areal density of 502 Terabits per square inch, outperforming state-of-the-art hard disk drives by three orders of magnitude. Furthermore, the chlorine vacancies are found to be stable at temperatures up to 77 K, offering prospects for expanding large-scale atomic assembly towards ambient conditions.Since the first demonstration of atom manipulation, 25 years ago 5 , the preferred approach for assembling atomic arrangements has been the lateral positioning of atoms or molecules evaporated onto a flat metal surface, most notably the (111) crystal surface of copper [12][13][14][15] . While ideal for experiments comprising up to several hundreds of constituents, the absence of a large-scale defect-free detectable grid on this surface prohibits the construction of architectures involving correlated lattice-placement of atoms separated by more than a few nanometres. Moreover, thermal motion of the adatoms restricts the technique to temperatures below 10 K. As we demonstrate below, we find that manipulation of missing atoms in a surface (vacancies) 16 , as opposed to additional atoms atop, permits a dramatic leap forward in our capability to build functional devices on the atomic scale.To this purpose, we take advantage of the self-assembly of chlorine atoms on the Cu(100) surface [17][18][19][20] , forming a flat two-dimensional lattice with several convenient properties. First, it provides large areas of a perfect template grid, with a controllable coverage of vacancies. Second, the chlorine lattice remains stable up to a large density of vacancies and up to relatively high temperature (77 K). And third, critical for our purpose, the precise location of the vacancies can be manipulated by STM with a very high level of control (and without the need to pick-up atoms with the tip, i.e. vertical atom manipulation). As we show below, these properties allow us to position thousands of vacancies at predefined atomic sites in a reasonable timeframe.The chlorinated copper surface is prepared in ultra-high vacuum through the evaporation of anhydrous CuCl 2 powder heated to 300 °C onto a clean Cu(100) crystal surface. The crystal is pre-heated to 100-150 °C prior...
A system of two exchange-coupled Kondo impurities in a magnetic field gives rise to a rich phase space hosting a multitude of correlated phenomena. Magnetic atoms on surfaces probed through scanning tunnelling microscopy provide an excellent platform to investigate coupled impurities, but typical high Kondo temperatures prevent field-dependent studies from being performed, rendering large parts of the phase space inaccessible. We present a study of pairs of Co atoms on insulating Cu2N/Cu(100), which each have a Kondo temperature of only 2.6 K. The pairs are designed to have interaction strengths similar to the Kondo temperature. By applying a sufficiently strong magnetic field, we are able to access a new phase in which the two coupled impurities are simultaneously screened. Comparison of differential conductance spectra taken on the atoms to simulated curves, calculated using a third-order transport model, allows us to independently determine the degree of Kondo screening in each phase.
The inelastic portion of the tunnel current through an individual magnetic atom grants unique access to read out and change the atom’s spin state, but it also provides a path for spontaneous relaxation and decoherence. Controlled closure of the inelastic channel would allow for the latter to be switched off at will, paving the way to coherent spin manipulation in single atoms. Here, we demonstrate complete closure of the inelastic channels for both spin and orbital transitions due to a controlled geometric modification of the atom’s environment, using scanning tunneling microscopy (STM). The observed suppression of the excitation signal, which occurs for Co atoms assembled into chains on a Cu2N substrate, indicates a structural transition affecting the d z 2 orbital, effectively cutting off the STM tip from the spin-flip cotunneling path.
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