Quantum phase transition in a single-molecule quantum dot

Perfect Zeeman Anisotropy in Rotationally Symmetric Quantum Dots with Strong Spin-Orbit Interaction

In nanoscale structures with rotational symmetry, such as quantum rings, the orbital motion of electrons combined with a spin-orbit interaction can produce a very strong and anisotropic Zeeman effect. Since symmetry is sensitive to electric fields, ring-like geometries provide an opportunity to manipulate magnetic properties over an exceptionally wide range. In this work, we show that it is possible to form rotationally symmetric confinement potentials inside a semiconductor quantum dot, resulting in electron orbitals with large orbital angular momentum and strong spin-orbit interactions. We find complete suppression of Zeeman spin splitting for magnetic fields applied in the quantum dot plane, similar to the expected behavior of an ideal quantum ring. Spin splitting reappears as orbital interactions are activated with symmetry-breaking electric fields. For two valence electrons, representing a common basis for spin-qubits, we find that modulating the rotational symmetry may offer new prospects for realizing tunable protection and interaction of spin-orbital states.

Single-molecule magnet behavior in heterometallic decanuclear [Ln2Fe8] (Ln = Y, Dy, Ho, Tb, Gd) coordination clusters

Five decanuclear lanthanide-iron clusters, formulated as [Ln2Fe8(hmp)10(μ2-OH)4(μ3-OH)2(μ4-O)4(H2O)6]·6ClO4·xH2O (x ≈ 8, Ln = Y for 1; x ≈ 6, Ln = Dy for 2; x ≈ 6, Ln = Ho for 3; x ≈ 7, Ln = Tb for 4; x ≈ 7, Ln = Gd for 5, Hhmp = 2-(hydroxymethyl)pyridine), have been synthesized and structurally characterized. Single-crystal structural analysis reveals that the cluster consists of six face-sharing defective cubane units. Dynamic magnetic investigations indicated that cluster 2 exhibits single-molecule magnet behavior under a zero dc field eliciting an effective energy barrier of Ueff = 17.76 K and a pre-exponential factor of τ0 = 7.93 × 10-8 s. Investigation of the performance of a series of FeIII-DyIII SMMs indicates that the relatively low energy barrier in 2 is associated with the weak ferromagnetic coupling between FeIII and DyIII ions, while the strength of ferromagnetic interaction in these clusters is mainly related to the bond distances between DyIII and O atoms coordinated to FeIII ions. Clusters 3 and 4 exhibit similar dual relaxation pathways under their respective optimal external applied dc field, where the direct relaxation process occurs in the low-frequency area, which impedes the extraction of the Ueff, while the secondary relaxation process appears at a higher frequency, which is probably a connection with intermolecularly driven relaxation. Our findings offer a magneto-structural correlation model for further investigating the single-molecule magnet behavior in lanthanide-iron systems.

Shaping Electronic Flows with Strongly Correlated Physics

Nonequilibrium quantum transport is of central importance in nanotechnology. Its description requires the understanding of strong electronic correlations that couple atomic-scale phenomena to the nanoscale. So far, research in correlated transport has focused predominantly on few-channel transport, precluding the investigation of cross-scale effects. Recent theoretical advances enable the solution of models that capture the interplay between quantum correlations and confinement beyond a few channels. This problem is the focus of this study. We consider an atomic impurity embedded in a metallic nanosheet spanning two leads, showing that transport is significantly altered by tuning only the phase of a single local hopping parameter. Furthermore─depending on this phase─correlations reshape the electronic flow throughout the sheet, either funneling it through the impurity or scattering it away from a much larger region. This demonstrates the potential for quantum correlations to bridge length scales in the design of nanoelectronic devices and sensors.

Correlation anisotropy driven Kosterlitz-Thouless-type quantum phase transition in a Kondo simulator

The precise manipulation of the quantum states of individual atoms/molecules adsorbed on metal surfaces is one of the most exciting frontiers in nanophysics, enabling us to realize novel single molecular logic devices and quantum information processing. Herein, by modeling an iron phthalocyanine molecule adsorbed on the Au(111) surface with a two-impurity Anderson model, we demonstrate that the quantum states of such a system could be adjusted by the uniaxial magnetic anisotropy D_z. For negative D_z, the ground state is dominated by a parallel configuration of the z component of local spins, whereas it turns to be an antiparallel one when D_z becomes positive. Interestingly, we found that these two phases are separated by a Kosterlitz-Thouless-type quantum phase transition, which is confirmed by the critical behaviors of the transmission coefficient and the local magnetic moment. Both phases are associated with spin correlation anisotropy, thus move against the Kondo effect. When the external magnetic field is applied, it first plays a role in compensating for the effect of D_z, and then it contributes significantly to the Zeeman effect for positive D_z, accompanied by the reappearance and the splitting of the Kondo peak, respectively. For fixed negative D_z, only the Zeeman behavior is revealed. Our results provide deep insights into the manipulation of the quantum phase within a single molecular junction.

Construction and physical properties of low-dimensional structures for nanoscale electronic devices

Over the past decades, construction of nanoscale electronic devices with novel functionalities based on low-dimensional structures, such as single molecules and two-dimensional (2D) materials, has been rapidly developed. To investigate their intrinsic properties for versatile functionalities of nanoscale electronic devices, it is crucial to precisely control the structures and understand the physical properties of low-dimensional structures at the single atomic level. In this review, we provide a comprehensive overview of the construction of nanoelectronic devices based on single molecules and 2D materials and the investigation of their physical properties. For single molecules, we focus on the construction of single-molecule devices, such as molecular motors and molecular switches, by precisely controlling their self-assembled structures on metal substrates and charge transport properties. For 2D materials, we emphasize their spin-related electrical transport properties for spintronic device applications and the role that interfaces among 2D semiconductors, contact electrodes, and dielectric substrates play in the electrical performance of electronic, optoelectronic, and memory devices. Finally, we discuss the future research direction in this field, where we can expect a scientific breakthrough.

Co3Gd4 Cage as Magnetic Refrigerant and Co3Dy3 Cage Showing Slow Relaxation of Magnetisation

Two structurally dissimilar 3d-4f cages having the formulae [(Co^III)₃Gd₄(μ₃-OH)₂(CO₃) (O₂C^tBu)₁₁(teaH)₃]·5H₂O (1) and [(Co^III)₃Dy₃(μ₃-OH)₄(O₂C^tBu)₆(teaH)₃]·(NO₃)₂·H₂O (2) have been isolated under similar reaction conditions and stoichiometry of the reactants. The most important factor for structural diversity seems to be the incorporation of one μ₃-carbonate anion in 1 and not in 2. Co atoms are in a +3 oxidation state in both complexes, as shown by the Bond Valence Sum (BVS) calculations and bond lengths, and as further supported by magnetic measurements. Co₃Gd₄ displays a significant magnetocaloric effect (−∆S_m = 25.67 J kg⁻¹ K⁻¹), and Co₃Dy₃ shows a single molecule magnet (SMM) behavior.

Iron phthalocyanine on Au(111) is a "non-Landau" Fermi liquid

The paradigm of Landau’s Fermi liquid theory has been challenged with the finding of a strongly interacting Fermi liquid that cannot be adiabatically connected to a non-interacting system. A spin-1 two-channel Kondo impurity with anisotropy D has a quantum phase transition between two topologically different Fermi liquids with a peak (dip) in the Fermi level for D < D_c (D > D_c). Extending this theory to general multi-orbital problems with finite magnetic field, we reinterpret in a unified and consistent fashion several experimental studies of iron phthalocyanine molecules on Au(111) that were previously described in disconnected and conflicting ways. The differential conductance shows a zero-bias dip that widens when the molecule is lifted from the surface (reducing the Kondo couplings) and is transformed continuously into a peak under an applied magnetic field. We reproduce all features and propose an experiment to induce the topological transition.

Single molecules on metal surfaces are paradigmatic systems for the study of many-body phenomena. Here, the authors show that several spectroscopic experiments on iron phthalocyanine on Au(111) surface can be described in a unified way in terms of a strongly interacting topologically non-trivial (non-Landau) Fermi liquid.

Atomically resolved single-molecule triplet quenching

The nonequilibrium triplet state of molecules plays an important role in photocatalysis, organic photovoltaics, and photodynamic therapy. We report the direct measurement of the triplet lifetime of an individual pentacene molecule on an insulating surface with atomic resolution by introducing an electronic pump-probe method in atomic force microscopy. Strong quenching of the triplet lifetime is observed if oxygen molecules are coadsorbed in close proximity. By means of single-molecule manipulation techniques, different arrangements with oxygen molecules were created and characterized with atomic precision, allowing for the direct correlation of molecular arrangements with the lifetime of the quenched triplet. Such electrical addressing of long-lived triplets of single molecules, combined with atomic-scale manipulation, offers previously unexplored routes to control and study local spin-spin interactions.

Increasing the Hilbert space dimension using a single coupled molecular spin

Quantum technologies are expected to introduce revolutionary changes in information processing in the near future. Nowadays, one of the main challenges is to be able to handle a large number of quantum bits (qubits), while preserving their quantum properties. Beyond the usual two-level encoding capacity of qubits, multi-level quantum systems are a promising way to extend and increase the amount of information that can be stored in the same number of quantum objects. Recent work (Kues et al. 2017), has shown the possibility to use devices based on photonic integrated circuits to entangle two qudits (with "d" being the number of available states). In the race to develop a mature quantum technology with real-world applications, many possible platforms are being investigated, including those that use photons, trapped ions, superconducting and silicon circuits and molecular magnets. In this work, we present the electronic read-out of a coupled molecular multi-level quantum systems, carried by a single Tb₂Pc₃ molecular magnet. Owning two magnetic centres, this molecular magnet architecture permits a 16 dimensions Hilbert space, opening the possibility of performing more complex quantum algorithms.

Fluorine-Induced Surface Metallization for Ammonia Synthesis under Photoexcitation up to 1550 nm

The first observation of surface metallization of TiO_2-x induced by fluoride ions is presented. The emerging metallic states are contributed by the 3d orbital of surface Ti and the 2p orbital of surface bridging F, which are intrinsically originated from the strong electron repulsion between F^- and adjacent Ti³⁺ . The metalized TiO_2-x with reduced work function and downward band bending possesses high electron-donating power to supported Ru species via atomic-scale ohmic contacts, exhibiting unprecedented photocatalytic performances for ammonia synthesis across the entire solar spectrum region (200-1550 nm) at room temperature. Mechanism and kinetic analysis revealed that the loaded Ru could behave as efficient electron sinks to accumulate photogenerated electrons and that the metallic surface markedly enhanced the dissociation of H₂ and N₂ by the hot electrons generated by the visible or even infrared light irradiation.

Evolution and universality of two-stage Kondo effect in single manganese phthalocyanine molecule transistors

The Kondo effect offers an important paradigm to understand strongly correlated many-body physics. Although under intensive study, some of the important properties of the Kondo effect, in systems where both itinerant coupling and localized coupling play significant roles, are still elusive. Here we report the evolution and universality of the two-stage Kondo effect, the simplest form where both couplings are important using single molecule transistor devices incorporating Manganese phthalocyanine molecules. The Kondo temperature T* of the two-stage Kondo effect evolves linearly against effective interaction of involved two spins. Observed Kondo resonance shows universal quadratic dependence with all adjustable parameters: temperature, magnetic field and biased voltages. The difference in nonequilibrium conductance of two-stage Kondo effect to spin 1/2 Kondo effect is also identified. Messages learned in this study fill in directive experimental evidence of the evolution of two-stage Kondo resonance near a quantum phase transition point, and help in understanding sophisticated molecular electron spectroscopy in a strong correlation regime.

The Kondo effect can serve as a powerful paradigm to understand strongly correlated many-body processes in physics. Here, Guo et al. utilize single molecule transistor devices as a testbed to study multi-level Kondo correlation and show electrical gate evolution and the universality of the two-stage Kondo effect.

A Gd@C82 single-molecule electret

Electrets are dielectric materials that have a quasi-permanent dipole polarization. A single-molecule electret is a long-sought-after nanoscale component because it can lead to miniaturized non-volatile memory storage devices. The signature of a single-molecule electret is the switching between two electric dipole states by an external electric field. The existence of these electrets has remained controversial because of the poor electric dipole stability in single molecules. Here we report the observation of a gate-controlled switching between two electronic states in Gd@C₈₂. The encapsulated Gd atom forms a charged centre that sets up two single-electron transport channels. A gate voltage of ±11 V (corresponding to a coercive field of ~50 mV Å^-1) switches the system between the two transport channels with a ferroelectricity-like hysteresis loop. Using density functional theory, we assign the two states to two different permanent electrical dipole orientations generated from the Gd atom being trapped at two different sites inside the C₈₂ cage. The two dipole states are separated by a transition energy barrier of 11 meV. The conductance switching is then attributed to the electric-field-driven reorientation of the individual dipole, as the coercive field provides the necessary energy to overcome the transition barrier.

Quantum Quasi-Monte Carlo Technique for Many-Body Perturbative Expansions

High order perturbation theory has seen an unexpected recent revival for controlled calculations of quantum many-body systems, even at strong coupling. We adapt integration methods using low-discrepancy sequences to this problem. They greatly outperform state-of-the-art diagrammatic Monte Carlo simulations. In practical applications, we show speed-ups of several orders of magnitude with scaling as fast as 1/N in sample number N; parametrically faster than 1/sqrt[N] in Monte Carlo simulations. We illustrate our technique with a solution of the Kondo ridge in quantum dots, where it allows large parameter sweeps.

Room temperature single electron transistor based on a size-selected aluminium cluster

Single electron transistors (SETs) are powerful devices to study the properties of nanoscale objects. However, the capabilities of placing a nano-object between electrical contacts under pristine conditions are lacking. Here, we developed a versatile two point contacting approach that tackles this challenge, which is demonstrated by constructing in situ a prototypical SET device consisting of a single aluminium cluster of 66 ± 5 atoms, deposited directly in a gold nanogap using an innovative cluster beam deposition technique. The gate driven conductance measurements demonstrate Coulomb blockade oscillations at room temperature correlating with an extracted charging energy of 0.14 eV, which is five times larger than k_BT at 300 K. Our work provides a model SET device platform to probe the quantum features of nano-objects with high precision.

Electric Field Control of Molecular Charge State in a Single-Component 2D Organic Nanoarray

Quantum dots (QD) with electric-field-controlled charge state are promising for electronics applications, e.g., digital information storage, single-electron transistors, and quantum computing. Inorganic QDs consisting of semiconductor nanostructures or heterostructures often offer limited control on size and composition distribution as well as low potential for scalability and/or nanoscale miniaturization. Owing to their tunability and self-assembly capability, using organic molecules as building nanounits can allow for bottom-up synthesis of two-dimensional (2D) nanoarrays of QDs. However, 2D molecular self-assembly protocols are often applicable on metals surfaces, where electronic hybridization and Fermi level pinning can hinder electric-field control of the QD charge state. Here, we demonstrate the synthesis of a single-component self-assembled 2D array of molecules [9,10-dicyanoanthracene (DCA)] that exhibit electric-field-controlled spatially periodic charging on a noble metal surface, Ag(111). The charge state of DCA can be altered (between neutral and negative), depending on its adsorption site, by the local electric field induced by a scanning tunneling microscope tip. Limited metal-molecule interactions result in an effective tunneling barrier between DCA and Ag(111) that enables electric-field-induced electron population of the lowest unoccupied molecular orbital (LUMO) and, hence, charging of the molecule. Subtle site-dependent variation of the molecular adsorption height translates into a significant spatial modulation of the molecular polarizability, dielectric constant, and LUMO energy level alignment, giving rise to a spatially dependent effective molecule-surface tunneling barrier and likelihood of charging. This work offers potential for high-density 2D self-assembled nanoarrays of identical QDs whose charge states can be addressed individually with an electric field.

Various Structural Types of Cyanide-Bridged FeIII-MnIII Bimetallic Coordination Polymers (CPs) and Polynuclear Clusters Based-on A New mer-Tricyanoiron(III)Building Block: Synthesis, Crystal Structures, and Magnetic Properties

Four cyanide-bridged Fe^III–Mn^III complexes {[Fe(qxcq)(CN)₃][Mn(L¹)(H₂O)]}[Mn(L¹)(H₂O)(CH₃OH)](ClO₄)·1.5MeOH·0.5H₂O (L¹ = N,N′-bis(3-methoxy-5-bromosalicylideneiminate) (2), {[Fe(qxcq)(CN)₃][Mn(L²)]}₂·0.5H₂O (L² = N,N′-ethylene-bis(3-ethoxysalicylideneiminate)) (3), [Fe(qxcq)(CN)₃][Mn(L³)] (L³ = bis(acetylacetonato)ethylenediimine) (4), [Fe(qxcq)(CN)₃][Mn(L⁴)]·1.5MeOH·0.5CH₃CN·0.25H₂O (L⁴ = N,N′-(1,1,2,2-tetramethylethylene)bis(salicylideneiminate)) (5), were prepared by assembling a new structurally characterized mer-tricyanoiron(III) molecular precursor (Ph₄P)[Fe(qxcq)(CN)₃]·0.5H₂O (qxcq⁻ = 8-(2-quinoxaline-2-carboxamido)quinoline anion) (1) and the corresponding manganese(III) Schiff base compound. Complexes 2and 3containa cyanide-bridged heterobimetallic dinuclear entity, which can be further dimerized by self-complementary H-bond interactions through the coordinated water molecule from one complex and the free O₄unit from the adjacent complex. Complexes 4 and 5 area one-dimensional coordination polymer (CP) comprised of the repeated [Mn(Schiffbase)-Fe(qxcq)(CN)₃] units. Complex 4 shows a linear-chain conformation with two trans-located cyano groups bridgingthe neighboring Mn units, while complex 5 is a zigzag-like 1D CP, where the two cyano groups in cis configurationfunction as bridges. In bothcomplexes 4 and 5, the inter-chain π–πstack interactions within the aromaticrings of cyanide precursor extend the 1D chain into the supermolecular 2D networks. The magnetic property has been experimentally studied and theoretically fitted over the four Fe(III)-Mn(III) complexes, revealing the antiferromagnetic interaction in complexes 2 and 4 and the unusual ferromagnetic coupling in complexes 3 and 5 between the Fe(III) ion and the Mn(III) ion bridged by the cyano group. Furthermore, the different magnetic coupling nature has been analyzed on the basis of the magneto-structure correlation of the mer-tricyanometallate-based Fe(III)-Mn(III) magnetic system.

Tunable giant magnetoresistance in a single-molecule junction

Controlling electronic transport through a single-molecule junction is crucial for molecular electronics or spintronics. In magnetic molecular devices, the spin degree-of-freedom can be used to this end since the magnetic properties of the magnetic ion centers fundamentally impact the transport through the molecules. Here we demonstrate that the electron pathway in a single-molecule device can be selected between two molecular orbitals by varying a magnetic field, giving rise to a tunable anisotropic magnetoresistance up to 93%. The unique tunability of the electron pathways is due to the magnetic reorientation of the transition metal center, resulting in a re-hybridization of molecular orbitals. We obtain the tunneling electron pathways by Kondo effect, which manifests either as a peak or a dip line shape. The energy changes of these spin-reorientations are remarkably low and less than one millielectronvolt. The large tunable anisotropic magnetoresistance could be used to control electronic transport in molecular spintronics.

Molecular electronics or spintronics relies on manipulating the electronic transport through microscopic molecule structures. Here the authors demonstrate the selective electron pathway in single-molecule device by magnetic field which enables a tunable anisotropic magnetoresistance up to 93%.

Quantum impurity models coupled to Markovian and non-Markovian baths

We develop a method to study quantum impurity models, small interacting quantum systems bilinearly coupled to an environment, in the presence of an additional Markovian quantum bath, with a generic nonlinear coupling to the impurity. We aim at computing the evolution operator of the reduced density matrix of the impurity, obtained after tracing out all the environmental degrees of freedom. First, we derive an exact real-time hybridization expansion for this quantity, which generalizes the result obtained in the absence of the additional Markovian dissipation and which could be amenable to stochastic sampling through diagrammatic Monte Carlo. Then, we obtain a Dyson equation for this quantity and we evaluate its self-energy with a resummation technique known as the noncrossing approximation. We apply this novel approach to a simple fermionic impurity coupled to a zero temperature fermionic bath and in the presence of Markovian pump, losses, and dephasing.

Synthesis of a series of hetero-multi-spin Ln2Cu3 complexes based on a methyl-pyrazole nitronyl nitroxide radical with slow magnetic relaxation behaviors

Multinuclear hetero-tri-spin complexes based on a methyl-pyrazole nitronyl nitroxide radical, namely, [Ln2Cu3(hfac)12(4-NIT-MePyz)4] (Ln = Gd(1), Tb(2), Dy(3); hfac = hexafluoroacetylacetone; 4-NIT-MePyz = 2-{4-(1-methyl)-pyrazolyl}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) have been successfully obtained through a one-pot reaction of the radical ligand (4-NIT-MePyz) with Cu(hfac)2 and Ln(hfac)3. These 2p-3d-4f complexes exhibit five-nuclear structures with the sequence [Cu-Rad-Ln-Rad-Cu-Rad-Ln-Rad-Cu], in which each 4-NIT-MePyz radical acting as a bidentate bridging ligand is coordinated to one Ln(hfac)3 unit through one oxygen atom of the NO groups and to one Cu(hfac)2 unit with one nitrogen atom from the pyrazole ring. For complex 1, based on the spin Hamiltonian calculations and MAGPACK program, it is concluded that there exist ferromagnetic couplings between GdIII and NIT radicals, as well as between CuII and free radicals with J1 = 6.8(1) and J3 = 1.3(2) cm-1, respectively, and antiferromagnetic interactions between radical and radical with J2 = -2.8(5) cm-1. Complex 2 shows frequency-dependent out-of-phase signals under a zero or 2000 Oe dc field indicating single-molecule magnetic behavior.

Direct Probe of the Seebeck Coefficient in a Kondo-Correlated Single-Quantum-Dot Transistor

We report on the first measurement of the Seebeck coefficient in a tunnel-contacted and gate-tunable individual single-quantum dot junction in the Kondo regime, fabricated using the electromigration technique. This fundamental thermoelectric parameter is obtained by directly monitoring the magnitude of the voltage induced in response to a temperature difference across the junction, while keeping a zero net tunneling current through the device. In contrast to bulk materials and single molecules probed in a scanning tunneling microscopy (STM) configuration, investigating the thermopower in nanoscale electronic transistors benefits from the electric tunability to showcase prominent quantum effects. Here, striking sign changes of the Seebeck coefficient are induced by varying the temperature, depending on the spin configuration in the quantum dot. The comparison with numerical renormalization group (NRG) calculations demonstrates that the tunneling density of states is generically asymmetric around the Fermi level in the leads, both in the cotunneling and Kondo regimes.

Slow relaxation of the magnetization observed in mononuclear Ln-radical compounds with D4d geometry configurations

The combination of LnIII ions (GdIII, TbIII or DyIII) and a pyrazole nitronyl nitroxide radical results in three isomorphous complexes, namely, [Ln(hfac)3(NIT-Pyz)]2 (Ln = Gd(1), Tb(2), Dy(3); hfac = hexafluoroacetylacetone; NIT-Pyz = 2-{3-pyrazolyl}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide). Single crystal X-ray diffraction studies revealed that all of them are composed of two crystallographically independent mononuclear systems, in which the central LnIII ions are coordinated by three hfac and one bidentate chelating NIT-Pyz radical. The central LnIII ions are all in square antiprism geometry (D4d) polyhedron configurations. Based on the spin Hamiltonian calculations, there exist antiferromagnetic couplings in the GdIII-NIT radical system in complex 1. Complexes 2 and 3 show frequency-dependent out-of-phase signals in a zero field indicating single-molecule magnetic behavior. Moreover, Tb's complex (2) shows a single thermal relaxation process with an energy barrier of 26 K. For Dy's complex (3), the Orbach and Raman processes both contribute to the magnetic relaxation behaviors.

Heterometallic Heptanuclear [Cu5Ln2] (Ln = Tb, Dy, and Ho) Single-Molecule Magnets Organized in One-Dimensional Coordination Polymeric Network

The reaction of a multisite coordination ligand, LH₃, with Cu(II) salts and Ln(NO₃)₃·nH₂O in a 1:2:1 stoichiometric ratio in the presence of triethylamine was found to afford a series of one-dimensional heterometallic [{Cu₅Ln₂(L)₂(μ₃-OH)₄(ClO₄)(NO₃)₃(OH₂)₅}(ClO₄)₂(H₂O)_x]_∞ [Ln = Tb(1), Dy(2) and Ho(3), x = 4.25, 5.5, and 5 for 1-3, respectively] coordination polymers. Complexes 1-3 have been characterized by single crystal X-ray crystallography. The detailed study of the magnetic properties has also been performed and compared with the parent [Cu₅Ln₂] molecular analogues. The ac susceptibility measurements for complexes 1-3 confirm their SMM behavior with the following characteristics: Δ_eff/k_B = 23.4 K, τ₀ = 1.1 × 10^-6 s and Δ_eff/k_B = 27.9 K, τ₀ = 6.6 × 10^-7 s under 0 and 1200 Oe dc fields, respectively for 1; Δ_eff/k_B = 8.3 K, τ₀ = 3.1 × 10^-6 s for 2 under 0 dc field. For 3, the fast QTM below 4 K prevents the estimation of the SMM energy barrier. Remarkably, the magnetic and SMM properties of the previously reported molecular [Cu₅Ln₂] analogues are preserved after their assembly in these coordination networks.

Tuning magnetoresistance in molybdenum disulphide and graphene using a molecular spin transition

Coupling spins of molecular magnets to two-dimensional (2D) materials provides a framework to manipulate the magneto-conductance of 2D materials. However, with most molecules, the spin coupling is usually weak and devices fabricated from these require operation at low temperatures, which prevents practical applications. Here, we demonstrate field-effect transistors based on the coupling of a magnetic molecule quinoidal dithienyl perylenequinodimethane (QDTP) to 2D materials. Uniquely, QDTP switches from a spin-singlet state at low temperature to a spin-triplet state above 370 K, and the spin transition can be electrically transduced by both graphene and molybdenum disulphide. Graphene-QDTP shows hole-doping and a large positive magnetoresistance ( ~ 50%), while molybdenum disulphide-QDTP demonstrates electron-doping and a switch to large negative magnetoresistance ( ~ 100%) above the magnetic transition. Our work shows the promise of spin detection at high temperature by coupling 2D materials and molecular magnets.

Engineering a coupling between magnetic molecules and conducting materials at room temperature could help the development of spintronic devices. Loh et al. show that the spin state of QDTP molecules deposited on graphene and MoS₂ couples to their electronic structure, affecting magnetotransport.

Kondo blockade due to quantum interference in single-molecule junctions

Molecular electronics offers unique scientific and technological possibilities, resulting from both the nanometre scale of the devices and their reproducible chemical complexity. Two fundamental yet different effects, with no classical analogue, have been demonstrated experimentally in single-molecule junctions: quantum interference due to competing electron transport pathways, and the Kondo effect due to entanglement from strong electronic interactions. Here we unify these phenomena, showing that transport through a spin-degenerate molecule can be either enhanced or blocked by Kondo correlations, depending on molecular structure, contacting geometry and applied gate voltages. An exact framework is developed, in terms of which the quantum interference properties of interacting molecular junctions can be systematically studied and understood. We prove that an exact Kondo-mediated conductance node results from destructive interference in exchange-cotunneling. Nonstandard temperature dependences and gate-tunable conductance peaks/nodes are demonstrated for prototypical molecular junctions, illustrating the intricate interplay of quantum effects beyond the single-orbital paradigm.

Kondo effect in double quantum dots with ferromagnetic RKKY interaction

We study Kondo effect in parallel-coupled double quantum dots with ferromagnetic Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction, by using the exact and nonperturbative hierarchical equations of motion approach. We construct the phase diagram in the parameter plane of the inter-dot coupling t and the ferromagnetic exchange interaction J (J  -  4t ²/U plane). Three different ground states, the Kondo singlet, spin singlet and S  =  1 Kondo, are determined in the diagram. We find the ferromagnetic coupling will raise the Kondo peak in the area of finite hopping t which is called 'J-enhanced Kondo effect'. Another enhancement of the Kondo effect by t ('t-enhanced Kondo effect') at J  =  0 is also presented. By checking the electron-electron interaction self-energy and magnetic susceptibility, we verify the ground state at large J is an under-screening Kondo state composed of a Fermi liquid with a residual spin 1/2, which is consistent with the 'singular Fermi liquid state' in the literature.

Large Magnetoresistance at Room Temperature in Organic Molecular Tunnel Junctions with Nonmagnetic Electrodes

We report room-temperature resistance changes of up to 30% under weak magnetic fields (0.1 T) for molecular tunnel junctions composed of oligophenylene thiol molecules, 1-2 nm in length, sandwiched between gold contacts. The magnetoresistance (MR) is independent of field orientation and the length of the molecule; it appears to be an interface effect. Theoretical analysis suggests that the source of the MR is a two-carrier (two-hole) interaction at the interface, resulting in spin coupling between the tunneling hole and a localized hole at the Au/molecule contact. Such coupling leads to significantly different singlet and triplet transmission barriers at the interface. Even weak magnetic fields impede spin relaxation processes and thus modify the ratio of holes tunneling via the singlet state versus the triplet state, which leads to the large MR. Overall, the experiments and analysis suggest significant opportunities to explore large MR effects in molecular tunnel junctions based on widely available molecules.

Metallic, magnetic and molecular nanocontacts

Scanning tunnelling microscopy and break-junction experiments realize metallic and molecular nanocontacts that act as ideal one-dimensional channels between macroscopic electrodes. Emergent nanoscale phenomena typical of these systems encompass structural, mechanical, electronic, transport, and magnetic properties. This Review focuses on the theoretical explanation of some of these properties obtained with the help of first-principles methods. By tracing parallel theoretical and experimental developments from the discovery of nanowire formation and conductance quantization in gold nanowires to recent observations of emergent magnetism and Kondo correlations, we exemplify the main concepts and ingredients needed to bring together ab initio calculations and physical observations. It can be anticipated that diode, sensor, spin-valve and spin-filter functionalities relevant for spintronics and molecular electronics applications will benefit from the physical understanding thus obtained.

Fidelity Susceptibility Perspective on the Kondo Effect and Impurity Quantum Phase Transitions

The Kondo effect is a ubiquitous phenomenon appearing at low temperature in quantum confined systems coupled to a continuous bath. Efforts in understanding and controlling it have triggered important developments across several disciplines of condensed matter physics. A recurring pattern in these studies is that the suppression of the Kondo effect often results in intriguing physical phenomena such as impurity quantum phase transitions or non-Fermi-liquid behavior. We show that the fidelity susceptibility is a sensitive indicator for such phenomena because it quantifies the sensitivity of the system's state with respect to its coupling to the bath. We demonstrate the power of the fidelity susceptibility approach by using it to identify the crossover and quantum phase transitions in the one and two impurity Anderson models. The feasibility of measuring fidelity susceptibility in condensed matter as well as ultracold quantum gases experiments opens exciting new routes to diagnose the Kondo problem and impurity quantum phase transitions.

Transport Spectroscopy of a Spin-Coherent Dot-Cavity System

Quantum engineering requires controllable artificial systems with quantum coherence exceeding the device size and operation time. This can be achieved with geometrically confined low-dimensional electronic structures embedded within ultraclean materials, with prominent examples being artificial atoms (quantum dots) and quantum corrals (electronic cavities). Combining the two structures, we implement a mesoscopic coupled dot-cavity system in a high-mobility two-dimensional electron gas, and obtain an extended spin-singlet state in the regime of strong dot-cavity coupling. Engineering such extended quantum states presents a viable route for nonlocal spin coupling that is applicable for quantum information processing.

Towards a full ab initio theory of strong electronic correlations in nanoscale devices

In this paper I give a detailed account of an ab initio methodology for describing strong electronic correlations in nanoscale devices hosting transition metal atoms with open d- or f-shells. The method combines Kohn-Sham density functional theory for treating the weakly interacting electrons on a static mean-field level with non-perturbative many-body methods for the strongly interacting electrons in the open d- and f-shells. An effective description of the strongly interacting electrons in terms of a multi-orbital Anderson impurity model is obtained by projection onto the strongly correlated subspace properly taking into account the non-orthogonality of the atomic basis set. A special focus lies on the ab initio calculation of the effective screened interaction matrix U for the Anderson model. Solution of the effective Anderson model with the one-crossing approximation or other impurity solver techniques yields the dynamic correlations within the strongly correlated subspace giving rise e.g. to the Kondo effect. As an example the method is applied to the case of a Co adatom on the Cu(0 0 1) surface. The calculated low-bias tunnel spectra show Fano-Kondo lineshapes similar to those measured in experiments. The exact shape of the Fano-Kondo feature as well as its width depend quite strongly on the filling of the Co 3d-shell. Although this somewhat hampers accurate quantitative predictions regarding lineshapes and Kondo temperatures, the overall physical situation can be predicted quite reliably.

Chiral biomolecule based dodecanuclear dysprosium(iii)–copper(ii) clusters: structural analyses and magnetic properties

Biomolecule pyroglutamic acid has been proficiently harnessed for synthesizing a family of three isostructural M₄Cu₈ dodecanuclear symmetric clusters, which after structural characterization were subjected to magnetic analyses.

Single molecule magnet behaviour in a rare trinuclear {CrIIIDyIII2} methoxo-bridged complex

A new neutral hetero-trinuclear {Cr^III–DyIII2} coordination complex exhibiting single molecule magnet behaviour has been synthesized.

Transport properties of individual C60-molecules

Electrical and thermal transport properties of C60 molecules are investigated with density-functional-theory based calculations. These calculations suggest that the optimum contact geometry for an electrode terminated with a single-Au atom is through binding to one or two C-atoms of C60 with a tendency to promote the sp(2)-hybridization into an sp(3)-type one. Transport in these junctions is primarily through an unoccupied molecular orbital that is partly hybridized with the Au, which results in splitting the degeneracy of the lowest unoccupied molecular orbital triplet. The transmission through these junctions, however, cannot be modeled by a single Lorentzian resonance, as our results show evidence of quantum interference between an occupied and an unoccupied orbital. The interference results in a suppression of conductance around the Fermi energy. Our numerical findings are readily analyzed analytically within a simple two-level model.