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Screening of exfoliation conditions (dispersion solvent, prewetting solvent, microwave power, microwave irradiation time, MoS2–NMP ratio, and % yield); UV–vis analyses, optical and electronic microscopy analyses; thermogravimetric analysis of bulk and exfoliated MoS2, X-ray photoelectron spectroscopy analysis, electron transport measurements.

A quantum spin liquid (QSL) is an elusive state of spin matter characterized by the absence of long-range magnetic order, even at zero temperature, and by the presence of exotic quasiparticle excitations. In spite of their relevance for quantum communication, topological quantum computation and the understanding of strongly correlated systems, like high-temperature superconductors, the unequivocal experimental identification of materials behaving as QSLs remains challenging. Here, we present a novel 2D heterometallic magnetically-diluted oxalate complex meeting all key requirements to become a QSL: a low spin ground state, determined by spin-orbit coupling, a magnetically-frustrated triangular lattice due to the presence of antiferromagnetic correlations , strongly suppressed direct exchange interactions and the presence of equivalent interfering superexchange paths between Co centres. A combination of specific heat and ac magnetic susceptibility measurements in a wide range of frequencies and temperatures show the presence of strong antiferromagnetic correlations concomitant with no signs of magnetic ordering down to 15 milliKelvin. These results show that magnetically diluted oxalates are therefore appealing QSL candidates as well as versatile systems to chemically fine tune key aspects of a QSL, like magnetic frustration and superexchange paths geometries.

Hybrid systems based on superconducting planar resonators strongly coupled to magnetic molecules are promising candidates for future large-scale quantum processors. Superconducting microwave resonators allow the strong confinement of the electromagnetic fields mode volume, which is crucial for reaching a strong and coherent coupling with the spin qubits. However, achieving control over a few or single spins remains challenging. In this sense, two main issues need to be overcome. First, the cavity field effective volume needs to be further concentrated to maximize the addressability of a single spin in a controlled way. Second, as the microwave field is localized in a nanoscopic region, developing a method to deliver the magnetic sample with sufficient spatial accuracy is crucial. As a first approach, I. Gimeno et al. showed that reducing the width of a superconducting transmission line locally leads to an enhancement of the microwave magnetic field and, therefore, of the achieved coupling. Utilizing dip pen nanolithography, they can place nanodeposits in a controlled way, reducing the number of coupled spins by several orders of magnitude. However, new approaches must be developed to achieve strong coupling to a single spin. We propose to downscale and reach the single-spin sensitivity limit by replacing the constriction of the superconducting resonator with a carbon nanotube (CNT). This approach presents two significant advantages: first, it concentrates further the microwave magnetic volume, increasing the coupling rate to a single spin; second, as CNT can be directly functionalized with molecular species, it allows addressing a single magnetic molecule in a localized position. Preliminary results of a hybrid superconducting/CNT resonator are presented, where the controlled positioning of the carbon nanotube on the nanometer scale has been achieved by dielectrophoresis. Moreover, optimized superconducting lumped element resonators (LERs) have been developed and co

A quantum spin liquid (QSL) is an elusive state of matter characterized by the absence of long-range magnetic order, even at zero temperature, and by the presence of exotic quasiparticle excitations. In spite of their relevance for quantum communication, topological quantum computation and the understanding of strongly correlated systems, like high-temperature superconductors, the unequivocal experimental identification of materials behaving as QSLs remains challenging. Here, we present a novel 2D heterometallic oxalate complex formed by high-spin Co(II) ions alternating with diamagnetic Rh(III) in a honeycomb lattice. This complex meets the key requirements to become a QSL: a spin ½ ground state for Co(II), determined by spin–orbit coupling and crystal field, a magnetically-frustrated triangular lattice due to the presence of antiferromagnetic correlations, strongly suppressed direct exchange interactions and the presence of equivalent interfering superexchange paths between Co centres. A combination of electronic paramagnetic resonance, specific heat and ac magnetic susceptibility measurements in a wide range of frequencies and temperatures shows the presence of strong antiferromagnetic correlations concomitant with no signs of magnetic ordering down to 15 mK. These results show that bimetallic oxalates are appealing QSL candidates as well as versatile systems to chemically fine tune key aspects of a QSL, like magnetic frustration and superexchange path geometries.

We study experimentally how crystal size influences magnetic ordering in arrays of molecular nanomagnets coupled by dipolar interactions (Fig. 1, left). Compressed fluids techniques have been applied to synthesize crystals of Mn6 molecules (spin S = 12) with typical sizes ranging from 28 um down to 220 nm (Fig. 1, centre). The onset of ferromagnetic order and the spin thermalization rates have been studied by means of micro-SQUID ac susceptibility measurements. We find (Fig. 1, right) that the ordered phase remains ferromagnetic, as in bulk, but the critical temperature Tc decreases with crystal size. Simple magnetostatic energy calculations, supported by Monte Carlo simulations, account for the drop in Tc in terms of the minimum attainable energy for finite-size magnetic domains limited by the crystal boundaries. Frequency-dependent susceptibility measurements give access to the spin dynamics. Although magnetic relaxation remains dominated by individual spin flips, the magnetic order leads to very long spin thermalization time scales. The results show that size influences the magnetism of dipolar systems with as many as 1011 spins and are relevant for the interpretation of quantum simulations performed on finite lattices.

Molecule-based spin crossover (SCO) materials display likely one of the most spectacular switchable processes. The SCO involves reversible changes in their physicochemical properties (i.e. optical, magnetic, electronic, and elastic) that are coupled with the spin-state change under an external perturbation (i.e. temperature, light, magnetic field, or the inclusion/release of analytes). Although very promising for their future integration into electronic devices, most SCO compounds show two major drawbacks: (i) their intrinsic low conductance and (ii) the unclear mechanism connecting the spin-state change and the electrical conductivity. Herein, we report the controlled single-crystal-to-single-crystal temperature-induced transformation in a robust metal–organic framework, [Fe2(H0.67bdt)3]·9H2O (1), being bdt2– = 1,4-benzeneditetrazolate, exhibiting a dynamic spin-state change concomitant with an increment in the anisotropic electrical conductance. Compound 1 remains intact during the SCO process even after approximately a 15% volume reduction. The experimental findings are rationalized by analyzing the electronic delocalization of the frontier states by means of density-functional theory calculations. The results point to a correlation between the spin-state of the iron and the electronic conductivity of the 3D structure. In addition, the reversibility of the process is proved.

The most widespread method for the synthesis of 2D–2D heterostructures is the direct growth of one material on top of the other. Alternatively, flakes of different materials can be manually stacked on top of each other. Both methods typically involve stacking 2D layers through van der Waals forces—such that these materials are often referred to as van der Waals heterostructures—and are stacked one crystal or one device at a time. Here we describe the covalent grafting of 2H-MoS2 flakes onto graphene monolayers embedded in field-effect transistors. A bifunctional molecule featuring a maleimide and a diazonium functional group was used, known to connect to sulfide- and carbon-based materials, respectively. MoS2 flakes were exfoliated, functionalized by reaction with the maleimide moieties and then anchored to graphene by the diazonium groups. This approach enabled the simultaneous functionalization of several devices. The electronic properties of the resulting heterostructure are shown to be dominated by the MoS2–graphene interface.

The most widespread method for the synthesis of 2D-2D heterostructures is the direct growth of one material on top of the other. Alternatively, one can manually stack flakes of different materials. Both methods are limited to one crystal/device at a time and involve interfacing the 2D materials through van der Waals forces, to the point that all these materials are known as van der Waals heterostructures. Synthetic chemistry is the paradigm of atomic-scale control, yet its toolbox remains unexplored for the construction of 2D-2D heterostructures. Here, we describe how to covalently connect 2H-MoS2 flakes to several single-layer graphene field-effect transistors simultaneously, and show that the final electronic properties of the MoS2-graphene heterostructure are dominated by the molecular interface. We use a bifunctional molecule with two chemically orthogonal anchor points, selective for sulfides and carbon-based materials: On one hand, we exploit the chemistry of maleimide described by our group to functionalize sulfide-based materials under mild conditions. On the other hand, we use a diazonium salt, which is the most common method for the covalent modification graphene. The solvents play a key role to activate a specific part of the molecule for the covalent anchoring. In order to measure the electrical properties of the heterostructure, we have fabricated a number of f-MoS2@graphene field effect transistors (FET) on Si/SiO2 substrates. The electronic properties of the functionalized devices are dominated by the chemical interface, resulting in p-doped devices in which the charge mobility is conserved after reaction, and the degree of doping can be controlled by increasing the degree of functionalization. The results described here show the power of the chemical approach to build functional 2D-2D heterostructures beyond van der Waals. Figure 1: Experimental procedure for the production of MoS2@Graphene covalent heterostructures.

Magnetic molecules have been proposed as versatile building blocks for quantum computing and molecular spintronics devices. The molecular spin can be used to encode quantum information in qubits or even perform logic operations as quantum gates with unmatched reproducibility and scalability. In spintronics, that same molecular spin can be used to generate spin currents in molecular based spin filters, spin switches or spin valves in carbon-nanotube/molecule hybrids, among other applications. Several strategies have been followed to couple the magnetic molecules to carbon nanotubes: direct physisorption of the molecules, covalent bonding or encapsulation of the magnetic molecules. We have developed the synthesis of mechanically interlocked rotaxane-like SWCNT derivatives (MINTs), in which the ring-closing metathesis of a U-shape molecule around SWCNTs is templated. In particular, we fabricated Cu2+ and Co2+ metalloporphyrin dimer rings mechanically interlocked around carbon nanotubes to form magnetic MINTs (mMINT). Magnetic porphyrins are selected due to their recently proved suitability as qubits, even preserving their magnetic properties and quantum coherence on surfaces. The mechanical bond places the porphyrin magnetic cores in close contact with the SWCNT without disturbing the molecular spin nor the carbon nanotube structure. The magnetic properties of the metallic dimers are preserved upon formation the mechanically interlocked hybrid.

Spin-crossover complexes embedded in nanodevices experience effects that are absent in the bulk that can modulate, quench and even suppress the spin-transition. In this work we explore, by means of state-of-the-art quantum chemistry calculations, different aspects of the integration of SCO molecules on active nanodevices, such as the geometry and energetics of the interaction with the substrate, extension of the charge transfer between the substrate and SCO molecule, impact of the applied external electric field on the spin-transition, and sensitivity of the transport properties on the local conditions of the substrate. We focus on the recently reported encapsulation of Fe(II) spin-crossover complexes in single-walled carbon nanotubes, with new measurements that support the theoretical findings. Even so our results could be useful to many other systems where SCO phenomena take place at the nanoscale, the spin-state switching is probed by an external electric field or current, or the substrate is responsible for the quenching of the SCO mechanism.

[EN] Efficient and low cost detection of harmful volatile organic compounds (VOCs) is a major health and environmental need in industrialized societies. For this, tailor-made porous coordination polymers are emerging as promising molecular sensing materials thanks to their responsivity to a wide variety of external stimuli and could be used to complement conventional sensors. Here, a non-porous crystalline 1D Fe(ii) coordination polymer acting as a porous acetonitrile host is presented. The desorption of interstitial acetonitrile is accompanied by magneto-structural transitions easily detectable in the optical and electronic properties of the material. This structural switch and therefore its (opto)electronic readout are reversible under exposure of the crystal to acetonitrile vapor. This simple and robust iron-based coordination polymer could be ideally suited for the construction of multifunctional sensor devices for volatile acetonitrile and potentially for other organic compounds.

The erasure of a bit of information is an irreversible operation whose minimal entropy production of kB ln 2 is set by the Landauer limit1. This limit has been verified in a variety of classical systems, including particles in traps2,3 and nanomagnets4. Here, we extend it to the quantum realm by using a crystal of molecular nanomagnets as a quantum spin memory and showing that its erasure is still governed by the Landauer principle. In contrast to classical systems, maximal energy efficiency is achieved while preserving fast operation owing to its high-speed spin dynamics. The performance of our spin register in terms of energy–time cost is orders of magnitude better than existing memory devices to date. The result shows that thermodynamics sets a limit on the energy cost of certain quantum operations and illustrates a way to enhance classical computations by using a quantum system.

We analyse the electrical response of narrow graphene nanogaps in search for transport signatures stemming from spin-polarized edge states. We find that the electrical transport across graphene nanogaps having perfectly defined zigzag edges does not carry any spin-related signature. We also analyse the magnetic and electrical properties of nanogaps whose electrodes have wedges that possibly occur in the currently fabricated nanogaps. These wedges can host spin polarized wedge low-energy states due to the bipartite nature of the graphene lattice. We find that these spin-polarized low-energy modes give rise to low-voltage signatures in the differential conductance and to distinctive features in the stability diagrams. These are caused by fully spin-polarized currents.

Embedding a magnetic electroactive molecule in a three-terminal junction allows for the fast and local electric field control of magnetic properties desirable in spintronic devices and quantum gates. Here, we provide an example of this control through the reversible and stable charging of a single all-organic neutral diradical molecule. By means of inelastic electron tunnel spectroscopy we show that the added electron occupies a molecular orbital distinct from those containing the two radical electrons, forming a spin system with three antiferromagnetically coupled spins. Changing the redox state of the molecule therefore switches on and off a parallel exchange path between the two radical spins through the added electron. This electrically controlled gating of the intramolecular magnetic interactions constitutes an essential ingredient of a single-molecule quantum gate.

The magnetic properties of a nanoscale system are inextricably linked to its local environment. In adatoms on surfaces and inorganic layered structures, the exchange interactions result from the relative lattice positions, layer thicknesses, and other environmental parameters. Here, we report on a sample-dependent sign inversion of the magnetic exchange coupling between the three unpaired spins of an organic triradical molecule embedded in a three-terminal device. This ferro-to-antiferromagnetic transition is due to structural distortions and results in a high-to-low spin ground-state change in a molecule traditionally considered to be a robust high-spin quartet. Moreover, the flexibility of the molecule yields an in situ electric tunability of the exchange coupling via the gate electrode. These findings open a route to the controlled reversal of the magnetic states in organic molecule-based nanodevices by mechanical means, electrical gating, or chemical tailoring.

Organic radicals are neutral, purely organic molecules exhibiting an intrinsic magnetic moment due to the presence of an unpaired electron in the molecule in its ground state. This property, added to the low spin–orbit coupling and weak hyperfine interactions, make neutral organic radicals good candidates for molecular spintronics insofar as the radical character is stable in solid state electronic devices. Here we show that the paramagnetism of the polychlorotriphenylmethyl radical molecule in the form of a Kondo anomaly is preserved in two- and three-terminal solid-state devices, regardless of mechanical and electrostatic changes. Indeed, our results demonstrate that the Kondo anomaly is robust under electrodes displacement and changes of the electrostatic environment, pointing to a localized orbital in the radical as the source of magnetism. Strong support to this picture is provided by density functional calculations and measurements of the corresponding nonradical species. These results pave the way toward the use of all-organic neutral radical molecules in spintronics devices and open the door to further investigations into Kondo physics.

We show that the dynamic magnetic susceptibility and the superparamagnetic blocking temperature of an Fe8 single molecule magnet oscillate as a function of the magnetic field Hx applied along its hard magnetic axis. These oscillations are associated with quantum interferences, tuned by Hx, between different spin tunneling paths linking two excited magnetic states. The oscillation period is determined by the quantum mixing between the ground S=10 and excited multiplets. These experiments enable us to quantify such mixing. We find that the weight of excited multiplets in the magnetic ground state of Fe8 amounts to approximately 11.6%. © 2013 American Physical Society.

We report the magnetic behavior of the one-dimensional (1D) cyanido-bridged chain complex [Tb(pzam) 3(H 2O)M(CN) 8] •H 2O, where M = W(V). The system shows qualitatively similar magnetic behavior with its already reported M = Mo(V): a broad anomaly in the specific heat ascribed to the magnetic interactions, a transition to three-dimensional magnetic order at T C = 1.15 K, and comparable magnetization and susceptibility. However, substituting the Mo(V) ion by the larger W(V) causes a drastic change in the symmetry of the Tb(III) g tensor, whereby the magnetic interaction between the Tb(III) and M(V) changes from Ising type into an anisotropic XY exchange. We analyze the data in terms of theoretical predictions for the 1D XYZ Hamiltonian and we find an excellent agreement between the theory and experimental data (J x = 1.89 K, J y = 2J x, J z = 0). © 2012 American Physical Society.

We report the synthesis, structures and magnetic properties of a series of chromium(iii) metal-centered triangle (or "star") clusters, [Cr 4RC(CH2O)32(4,4′- R′2-bipy)3Cl6] [R = Et, R′ = H (2); R = HOCH2, R′ = H (3); R = Et, R′ = tBu (4)], prepared by two-step solvothermal reactions starting from [CrCl 3(thf)3]. The product of the first stage of this reaction is the salt [Cr(bipy)2Cl2]2[Cr 2Cl8(MeCN)2] (1). In the absence of the diimine, a different family of tetrametallics is isolated: the butterfly complexes [Cr4EtC(CH2O)32NH(C(R) NH)22Cl6] (R = Me (5), Et (6), Ph (7)] where the chelating N-acetimidoylacetamidine NH(C(R)NH)2 ligands are formed in situ via condensation of the nitrile solvents (RCN) under solvothermal conditions. Magnetic measurements show the chromium stars to have an isolated S = 3 ground state, arising from antiferromagnetic coupling between the central and peripheral metal ions, analogous to the well-known Fe(iii) stars. Bulk antiferromagnetic ordering is observed at 0.6 K. The butterfly complexes have a singlet ground state, with a low-lying S = 1 first excited state, due to dominant wing-body antiferromagnetic coupling. © 2011 The Royal Society of Chemistry

We report the synthesis, structures and magnetic properties of a series of chromium(III) metal-centered triangle (or >star>) clusters, [Cr 4{RC(CH2O)3}2(4,4′- R′2-bipy)3Cl6] [R = Et, R′ = H (2); R = HOCH2, R′ = H (3); R = Et, R′ = tBu (4)], prepared by two-step solvothermal reactions starting from [CrCl 3(thf)3]. The product of the first stage of this reaction is the salt [Cr(bipy)2Cl2]2[Cr 2Cl8(MeCN)2] (1). In the absence of the diimine, a different family of tetrametallics is isolated: the butterfly complexes [Cr4{EtC(CH2O)3}2{NH(C(R) NH)2}2Cl6] (R = Me (5), Et (6), Ph (7)] where the chelating N-acetimidoylacetamidine NH(C(R)NH)2 ligands are formed in situ via condensation of the nitrile solvents (RCN) under solvothermal conditions. Magnetic measurements show the chromium stars to have an isolated S = 3 ground state, arising from antiferromagnetic coupling between the central and peripheral metal ions, analogous to the well-known Fe(III) stars. Bulk antiferromagnetic ordering is observed at 0.6 K. The butterfly complexes have a singlet ground state, with a low-lying S = 1 first excited state, due to dominant wing-body antiferromagnetic coupling. © 2011 The Royal Society of Chemistry.

La motivación de este trabajo es el estudio del papel que las interacciones dipolares juegan en cristales de moléculas imán y en algún otro tipo de materiales magnéticos. Las moléculas imán (SMM de sus siglas en inglés) han sido objeto de un amplio estudio en los últimos años debido a su potencial como memorias magnéticas y a que muestran fascinantes fenómenos cuánticos, como el efecto túnel de espín. Cada una de estas moléculas se comportaría como un bit de información. Por tanto, la densidad de información en medios magnéticos podría incrementarse enormemente. Las interacciones dipolares son dominantes en algunos de estos materiales, algo que no es frecuente en la naturaleza ya que estas suelen estar enmascaradas por las interacciones de canje, mucho más intensas. Las interacciones dipolares dan lugar a toda una gama de fenómenos que van desde el orden magnético dipolar, comportamientos de vidrio de espín hasta fenómenos cuánticos como las transiciones de fase cuánticas. Estas últimas se producen gracias a la gran debilidad de las interacciones dipolares que permiten a las débiles fluctuaciones de punto cero jugar un papel fundamental a muy bajas temperaturas. El objetivo de esta tesis es el estudio de todos estos fenómenos. Capítulo 1: En el capítulo 1 se describen las principales características de las interacciones dipolares. Capítulo 2: En el segundo capítulo se describe el marco teórico utilizado para el análisis de los tiempos de relajación. Capítulo 3: En el tercer capítulo se describen las técnicas experimentales utilizadas en esta tesis. Las condiciones experimentales requeridas están condicionadas por las escalas de energía típicas de las interacciones dipolares. Capítulo 4: El estudio del orden magnético en imanes moleculares puramente dipolares está seriamente dificultado no sólo por la escasez de estos materiales, sino también por la relajación magnética extremadamente lenta hacia el equilibrio térmico que se produce normalmente a muy bajas temperatur

The counterion makes a difference: A square 2D array of cobalt citrate cubanes linked by bridging six-coordinate cobalt atoms (see graphic) displays collective magnetic behavior, which changes with subtle differences in the crystalline surroundings. The magnetic properties of two solid modifications, both synthesized by using wet chemistry techniques, reveal two different relaxation processes in each system at low temperatures, each with its own timescale.

A material in which superconducting and magnetic properties coexist is synthesized by the intercalation of single-molecule magnets into the layered structure of a group V metal dichalcogenide. A molecule-based/solid-state hybrid strategy is here employed, proving as a promising chemical approach for preparing new materials in which superconductivity coexists with different molecule-intrinsic functionalities.

We show that a crystal of mesoscopic Fe8 single-molecule magnets is an experimental realization of the quantum Ising model in a transverse field, with dipolar interactions. Quantum annealing has enabled us to explore the quantum and classical phase transitions between the paramagnetic and ferromagnetic phases, at thermodynamical equilibrium. The phase diagram and critical exponents that we obtain agree with expectations for the mean-field universality class.