Colloquium: Atomic spin chains on surfaces

Abstract

Magnetism at low dimensions is a thriving field of research with exciting opportunities in technology. In the present Colloquium, we focus on the properties of 1-D magnetic systems on solid surfaces. From the emulation of 1-D quantum phases to the potential realization of Majorana edge states, spin chains are unique systems to study. The advent of scanning tunnelling microscope (STM) based techniques has permitted us to engineer spin chains in an atom-by-atom fashion via atom manipulation and to access their spin states on the ultimate atomic scale. Here, we present the current state of research on spin correlations and dynamics of atomic spin chains as studied by the STM. After a brief review of the main properties of spin chains on solid surfaces, we classify spin chains according to the coupling of their magnetic moments with the holding substrate. This classification scheme takes into account that the nature and lifetimes of the spinchain excitation intrinsically depend on the holding substrate. We first show the interest of using insulating layers on metals, which generally results in an increase in the spin state's lifetimes such that their quantized nature gets evident and they are individually accessible. Next, we show that the use of semiconductor substrates promises additional control through the tunable electron density via doping. When the coupling to the substrate is increased for spin chains on metals, the substrate conduction electron mediated interactions can lead to emergent exotic phases of the coupled spin chain-substrate conduction electron system. A particularly interesting example is furnished by superconductors. Magnetic impurities induce states in the superconducting gap. Due to the extended nature of the spin chain, the in-gap states develop into bands that can lead to the emergence of 1-D topological superconductivity and, consequently to the appearance of Majorana edge states. Finally, we give an outlook on the use of spin chains in spintronics, quantum communication, quantum computing, quantum simulations and quantum sensors.

Identification of topological superconductivity in magnetic impurity systems using bulk spin polarization

“…The notion of topology as a classifier uses nonlocal properties, such as the Berry phase, and is thus fundamentally different from the traditional Landau-Ginzburg paradigm for phase transitions using local order parameters [6,7]. In terms of realizing topological superconductivity, different platforms have already been proposed, such as spin-orbit-coupled nanowires in proximity to superconductors or nanostructures created by depositing magnetic atoms on the surface of superconductors [8][9][10][11][12][13][14][15][16][17][18][19].…”

Controlling in-gap end states by linking nonmagnetic atoms and artificially-constructed spin chains on superconductors

“…However, for all investigated systems so far, the experiments suffer from a concurrence of the expected location of the Majorana bound state with the location of the termination of the chain. This termination is intrinsically different from the inner part of the chain, as the end atoms have a different coordination 17 . Thereby, these atoms tend to have a different bonding length and, thus, a different hybridization with the substrate as compared to those in the interior of the chain.…”

Impurity-induced resonant spinon zero modes in Dirac quantum spin liquids

“…We now consider the potential experimental signatures of these spinon zero modes. In particular, scanning tunnel spectroscopic techniques have been demonstrated to be very well suited to detect quantum spin excitations [74], as demonstrated in a variety of experiments showing atomic-scale magnons [82], quantum critical transitions [83], and quantum transitions in nanomagnets [72]. In particular, two different techniques can be used to probe magnetic excitations with scanning tunnel microscopy (STM): Inelastic spectroscopy [61,70,84] and electrically driven paramagnetic resonance [85][86][87][88][89][90][91][92][93][94].…”

“…Along this line, recent experimental advances have demonstrated the possibility of single-atom manipulation in a variety of systems by means of scanning probe techniques [59][60][61][62][63][64][65][66][67][68][69][70][71][72][73]. This motivates the question of whether single-atom manipulation [74] can detect unique features of quantum spin-liquid states.…”