Explore the vast range of titles available.

Coherent control of individual atomic and molecular spins on surfaces has recently been demonstrated by using electron spin resonance (ESR) in a scanning tunneling microscope (STM). Here, a combined experimental and modeling study of the ESR of a single hydrogenated Ti atom that is exchange‐coupled to a Fe adatom positioned 0.6–0.8 nm away by means of atom manipulation is presented. Continuous wave and pulsed ESR of the Ti spin show a Rabi rate with two contributions, one from the tip and the other from the Fe, whose spin interactions with Ti are modulated by the radio‐frequency electric field. The Fe contribution is comparable to the tip, as revealed by its dominance when the tip is retracted, and tunable using a vector magnetic field. The new ESR scheme allows on‐surface individual spins to be addressed and coherently controlled without the need for magnetic interaction with a tip. This study establishes a feasible implementation of spin‐based multi‐qubit systems on surfaces.

Fractionalization is a phenomenon in which strong interactions in a quantum system drive the emergence of excitations with quantum numbers that are absent in the building blocks. Outstanding examples are excitations with charge e /3 in the fractional quantum Hall effect 1 , 2 , solitons in one-dimensional conducting polymers 3 , 4 and Majorana states in topological superconductors 5 . Fractionalization is also predicted to manifest itself in low-dimensional quantum magnets, such as one-dimensional antiferromagnetic S  = 1 chains. The fundamental features of this system are gapped excitations in the bulk 6 and, remarkably, S  = 1/2 edge states at the chain termini 7 – 9 , leading to a four-fold degenerate ground state that reflects the underlying symmetry-protected topological order 10 , 11 . Here, we use on-surface synthesis 12 to fabricate one-dimensional spin chains that contain the S  = 1 polycyclic aromatic hydrocarbon triangulene as the building block. Using scanning tunnelling microscopy and spectroscopy at 4.5 K, we probe length-dependent magnetic excitations at the atomic scale in both open-ended and cyclic spin chains, and directly observe gapped spin excitations and fractional edge states therein. Exact diagonalization calculations provide conclusive evidence that the spin chains are described by the S  = 1 bilinear-biquadratic Hamiltonian in the Haldane symmetry-protected topological phase. Our results open a bottom-up approach to study strongly correlated phases in purely organic materials, with the potential for the realization of measurement-based quantum computation 13 . Using scanning tunnelling microscopy and spectroscopy, fractional edge excitations are observed in nanographene spin chains, enabling the potential to study strongly correlated phases in purely organic materials.

Nuclear spins serve as sensitive probes in chemistry1 and materials science2 and are promising candidates for quantum information processing3–6. NMR, the resonant control of nuclear spins, is a powerful tool for probing local magnetic environments in condensed matter systems, which range from magnetic ordering in high-temperature superconductors7,8 and spin liquids9 to quantum magnetism in nanomagnets10,11. Increasing the sensitivity of NMR to the single-atom scale is challenging as it requires a strong polarization of nuclear spins, well in excess of the low polarizations obtained at thermal equilibrium, as well as driving and detecting them individually4,5,12. Strong nuclear spin polarization, known as hyperpolarization, can be achieved through hyperfine coupling with electron spins2. The fundamental mechanism is the conservation of angular momentum: an electron spin flips and a nuclear spin flops. The nuclear hyperpolarization enables applications such as in vivo magnetic resonance imaging using nanoparticles13, and is harnessed for spin-based quantum information processing in quantum dots14 and doped silicon15–17. Here we polarize the nuclear spins of individual copper atoms on a surface using a spin-polarized current in a scanning tunnelling microscope. By employing the electron–nuclear flip-flop hyperfine interaction, the spin angular momentum is transferred from tunnelling electrons to the nucleus of individual Cu atoms. The direction and magnitude of the nuclear polarization is controlled by the direction and amplitude of the current. The nuclear polarization permits the detection of the NMR of individual Cu atoms, which is used to sense the local magnetic environment of the Cu electron spin.

The properties of quantum systems interacting with their environment, commonly called open quantum systems, can be affected strongly by this interaction. Although this can lead to unwanted consequences, such as causing decoherence in qubits used for quantum computation 1 , it can also be exploited as a probe of the environment. For example, magnetic resonance imaging is based on the dependence of the spin relaxation times of protons 2 in water molecules in a host's tissue 3 . Here we show that the excitation energy of a single spin, which is determined by magnetocrystalline anisotropy and controls its stability and suitability for use in magnetic data-storage devices 4 , can be modified by varying the exchange coupling of the spin to a nearby conductive electrode. Using scanning tunnelling microscopy and spectroscopy, we observe variations up to a factor of two of the spin excitation energies of individual atoms as the strength of the spin's coupling to the surrounding electronic bath changes. These observations, combined with calculations, show that exchange coupling can strongly modify the magnetic anisotropy. This system is thus one of the few open quantum systems in which the energy levels, and not just the excited-state lifetimes, can be renormalized controllably. Furthermore, we demonstrate that the magnetocrystalline anisotropy, a property normally determined by the local structure around a spin, can be tuned electronically. These effects may play a significant role in the development of spintronic devices 5 in which an individual magnetic atom or molecule is coupled to conducting leads. The spin excitation energy and the magnetic anisotropy of individual atoms can be modified by varying the exchange coupling of the atomic spin to metallic leads.

The interaction of nuclei with nonzero spin with electron spins creates small electronic energy. With a scanning tunneling microscope tip, Willke et al. measured these hyperfine interactions for iron and titanium atoms that were manipulated on a magnesium oxide surface. The tip was also used to measure electron paramagnetic resonance spectra. The hyperfine structure of single atoms was sensitive to the binding site of the atom as well as its position relative to other magnetic atoms. Science , this issue p. 336 Atom manipulation and spin sensing with scanning tunneling microscopy reveal details underlying hyperfine interactions. Taking advantage of nuclear spins for electronic structure analysis, magnetic resonance imaging, and quantum devices hinges on knowledge and control of the surrounding atomic-scale environment. We measured and manipulated the hyperfine interaction of individual iron and titanium atoms placed on a magnesium oxide surface by using spin-polarized scanning tunneling microscopy in combination with single-atom electron spin resonance. Using atom manipulation to move single atoms, we found that the hyperfine interaction strongly depended on the binding configuration of the atom. We could extract atom- and position-dependent information about the electronic ground state, the state mixing with neighboring atoms, and properties of the nuclear spin. Thus, the hyperfine spectrum becomes a powerful probe of the chemical environment of individual atoms and nanostructures.

Nanographenes with zigzag edges are predicted to manifest non-trivial π-magnetism resulting from the interplay of concurrent electronic effects, such as hybridization of localized frontier states and Coulomb repulsion between valence electrons. This provides a chemically tunable platform to explore quantum magnetism at the nanoscale and opens avenues towards organic spintronics. The magnetic stability in nanographenes is thus far greatly limited by the weak magnetic exchange coupling, which remains below the room-temperature thermal energy. Here, we report the synthesis of large rhombus-shaped nanographenes with zigzag peripheries on gold and copper surfaces. Single-molecule scanning probe measurements show an emergent magnetic spin singlet ground state with increasing nanographene size. The magnetic exchange coupling in the largest nanographene (C70H22, containing five benzenoid rings along each edge), determined by inelastic electron tunnelling spectroscopy, exceeds 100 meV or 1,160 K, which outclasses most inorganic nanomaterials and survives on a metal electrode.Open-shell nanographenes are promising for quantum technologies, but their magnetic stability has remained limited by weak exchange coupling. Now, two large rhombus-shaped nanographenes with zigzag peripheries, one with 48 carbon atoms and the other with 70, have been synthesized on gold and copper surfaces. The 70-carbon compound exhibits a large magnetic exchange coupling exceeding 100 meV.

Odd- and even-layer variations in magnetization occur in two-dimensional chromium triiodide The discovery of ferromagnetic order in monolayers of two different materials, CrI 3 ( 1 ) and Cr 2 Ge 2 Te 6 ( 2 ), has added ferromagnetism to the electronic properties displayed by two-dimensional (2D) crystals. Characterization of magnetic 2D crystals has relied on magneto-optical methods ( 1 , 3 ) such as the Kerr effect or magnetic circular dichroism that can interrogate small sample volumes. However, these probes do not provide an absolute measurement of the magnetic moment density and have very limited spatial resolution. On page 973 of this issue, Thiel et al. ( 4 ) use sensors based on nitrogen vacancy (NV)–center scanning magnetometry ( 5 ) to map the absolute magnetic moments of 2D crystals of CrI 3 with a resolution of a few tens of nanometers and show how the anomalous interlayer spin interactions vary with number of layers.

Shrinking magnets: a new spin on the Kondo effect Magnetism in metals such as iron is typically considered an intrinsic property of the material. But as with many physical effects, such general pictures can break down once you reduce the size of the system to the nanoscale. Calvo et al . report a striking manifestation of such a change: they find that when the diameter of a magnetic wire is reduced to atomic dimensions, the material's magnetic properties are strongly altered, to the point where magnetism can even be eliminated. This is an unexpected realization of the so-called Kondo effect, for which one usually requires two different species of atoms; it also highlights vividly the need to take into account atomic-scale geometry when investigating the properties of magnetic nanostructures. Magnetism in metals is typically considered an intrinsic property of the material. But when the diameter of a magnetic wire is reduced to atomic dimensions, the material's magnetic properties are strongly altered, to the point where magnetism can even be eliminated. This is an unexpected realization of the so-called Kondo effect, and highlights the need to take into account atomic-scale geometry when investigating the properties of magnetic nanostructures. Iron, cobalt and nickel are archetypal ferromagnetic metals. In bulk, electronic conduction in these materials takes place mainly through the s and p electrons, whereas the magnetic moments are mostly in the narrow d -electron bands, where they tend to align. This general picture may change at the nanoscale because electrons at the surfaces of materials experience interactions that differ from those in the bulk. Here we show direct evidence for such changes: electronic transport in atomic-scale contacts of pure ferromagnets (iron, cobalt and nickel), despite their strong bulk ferromagnetism, unexpectedly reveal Kondo physics, that is, the screening of local magnetic moments by the conduction electrons below a characteristic temperature 1 . The Kondo effect creates a sharp resonance at the Fermi energy, affecting the electrical properties of the system; this appears as a Fano–Kondo resonance 2 in the conductance characteristics as observed in other artificial nanostructures 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 . The study of hundreds of contacts shows material-dependent log-normal distributions of the resonance width that arise naturally from Kondo theory 12 . These resonances broaden and disappear with increasing temperature, also as in standard Kondo systems 4 , 5 , 6 , 7 . Our observations, supported by calculations, imply that coordination changes can significantly modify magnetism at the nanoscale. Therefore, in addition to standard micromagnetic physics, strong electronic correlations along with atomic-scale geometry need to be considered when investigating the magnetic properties of magnetic nanostructures.

Controlling quantum interference near avoided energy-level crossings is crucial for fast and reliable coherent manipulation in quantum information processing. However, achieving tunable quantum interference in atomically-precise engineered structures remains challenging. Here, we demonstrate electrical control of quantum interference using atomic spins on an insulating film in a scanning tunneling microscope. Using bias voltages applied across the tunnel junction, we modulate the atomically-confined magnetic interaction between the probe tip and surface atoms with a strong electric field, and drive the spin state rapidly through the energy-level anticrossing. This all-electrical manipulation allows us to achieve Landau-Zener-Stückelberg-Majorana (LZSM) interferometry on both single spins and pairs of interacting spins. The LZSM pattern exhibits multiphoton resonances, and its asymmetry suggests that the spin dynamics is influenced by spin-transfer torque of tunneling electrons. Multi-level LZSM spectra measured on coupled spins with tunable interactions show distinct interference patterns depending on their many-body energy landscapes. These results open new avenues for all-electrical quantum manipulation in spin-based quantum processors in the strongly driven regime. Control of quantum interference in engineered atomic-scale systems could enable precise manipulation of quantum states, however it has remained challenging. Here the authors demonstrate electrically tunable quantum interference in a system of Ti atoms on MgO surface, using a scanning probe microscope setup.

A series of breakthroughs is making the fabrication of single-atom devices possible. Their behaviour is controlled by the quantum state of single dopants, and they hold promise for applications such as quantum bits, magnetometers and memories.