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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.

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.

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.

We report stable room-temperature ferromagnetism in commercially available MoS 2 powder with a nominal purity greater than 98%. In order to assess the origin of the unexpected ferromagnetic signal, we carried out thorough characterization of the samples, by a combination of X-ray diffraction, Raman spectroscopy, electron microscopy, X-ray photoelectron spectroscopy and superconducting quantum interference device magnetometry. Using secondary ion mass spectrometry, we infer that up to 1.6% of a pool of different external dopants, including 0.8% of Fe and others, are present in the MoS 2 samples. We find very low value of magnetic moment per unit formula that, together with the small density of magnetic dopants, and the room-temperature magnetic order, leads us to conclude that ferromagnetism is not hosted at the MoS 2 crystal but can be ascribed to secondary phase of transition metal atoms’ clusters that aggregate. Our results stress the need of a careful characterization of transition metal dichalcogenides in the study of magnetism and spintronics involving either nominally pure MoS 2 as a diamagnetic semiconductor substrate or as a host material for diluted magnetic alloying.

The production of single-walled inorganic nanotubes is challenging due to the energetic favorability of multi-walled structures during synthesis. CrI 3 , a layered ferromagnetic insulator, has gained significant attention as the first stand-alone monolayer ferromagnet, sparking interest in two-dimensional magnetic materials. Here, we report the synthesis of high-quality, monolayer CrI 3 nanotubes encapsulated within multiwalled carbon nanotubes (MWCNTs), ranging from 2 to 10 nm with an average diameter of 5.3 nm, as well as a smaller amount of CrI 3 nanorods. Through aberration-corrected transmission electron microscopy, X-ray magnetic circular dichroism (XMCD) spectroscopy, and first-principles calculations, we explored the fundamental physics and magnetism of these 1D van der Waals heterostructures. These findings pave the way towards the exploration of non-collinear magnetic states in tubular geometries, driven by the interplay of magnetic anisotropy and curvature. Producing single-walled inorganic nanotubes is challenging because their multi-walled counterparts are favored during synthesis. Here, the authors produce one-dimensional single-walled CrI 3 encapsulated within multi-walled carbon nanotubes and explore their magnetic properties.

Unlocking the potential of topological order in many-body spin systems has been a key goal in quantum materials research. Despite extensive efforts, the quest for a versatile platform enabling site-selective spin manipulation, essential for tuning and probing diverse topological phases, has persisted. Here we utilize on-surface synthesis to construct spin-1/2 alternating-exchange Heisenberg chains by covalently linking Clar’s goblets—nanographenes each hosting two antiferromagnetically coupled spins. Using scanning tunnelling microscopy, we exert atomic-scale control over chain lengths, parities and exchange-coupling terminations, and probe their magnetic response via inelastic tunnelling spectroscopy. Our investigation confirms the gapped nature of bulk excitations in the chains, known as triplons. Their dispersion relation is extracted from the spatial variation of tunnelling spectral amplitudes. Depending on the parity and termination of chains, we observe varying numbers of in-gap spin-1/2 edge excitations, reflecting the degeneracy of distinct topological ground states in the thermodynamic limit. By monitoring interactions between these edge spins, we identify the exponential decay of spin correlations. Our findings present a phase-controlled many-body platform, opening avenues toward spin-based quantum devices. Scanning probe microscopy experiments realize the alternating-exchange spin-1/2 Heisenberg model via magnetic nanographene chains. They control odd- to even-Haldane phase transitions and monitor spin–spin correlations and triplon dispersion.

Antiferromagnetic Heisenberg chains exhibit two distinct types of excitation spectrum: gapped for integer-spin chains and gapless for half-integer-spin chains. However, in finite-length half-integer-spin chains, quantization induces a gap, requiring precise control over sufficiently long chains to study its evolution. Here we create length-controlled spin-1/2 Heisenberg chains by covalently linking Olympicenes—Olympic-ring-shaped magnetic nanographenes. With large exchange interactions, tunable lengths and negligible magnetic anisotropy, this system is ideal for investigating length-dependent spin excitations, probed via inelastic electron tunnelling spectroscopy. We observe a power-law decay of the lowest excitation energy with length L , following a 1/ L dependence in the large- L regime, consistent with theory. For L  = 50, a V-shaped excitation continuum confirms a gapless behaviour in the thermodynamic limit. Additionally, low-bias current maps reveal the standing wave of a single spinon in odd-numbered chains. Our findings provide evidence for the realization of a one-dimensional analogue of a gapless spin liquid within an artificial graphene lattice. Open-shell nanographenes are used to fabricate length-controlled antiferromagnetic spin-1/2 Heisenberg chains. It is revealed that the spin excitation spectra evolve from gapped to gapless following a power-law dependence on chain length, along with the visualization of the standing waves of confined single spinons.

Quantum spin tunneling and Kondo effect are two very different quantum phenomena that produce the same effect on quantized spins, namely, the quenching of their magnetization. However, the nature of this quenching is very different so that quantum spin tunneling and Kondo effect compete with each other. Importantly, both quantum spin tunneling and Kondo effect produce very characteristic features in the spectral function that can be measured by means of single spin scanning tunneling spectroscopy and allows to probe the crossover from one regime to the other. We model this crossover, and the resulting changes in transport, using a non-perturbative treatment of a generalized Anderson model including magnetic anisotropy that leads to quantum spin tunneling. We predict that, at zero magnetic field, integer spins can feature a split-Kondo peak driven by quantum spin tunneling.