Spin Transition Research Articles

Single-molecule electron spin resonance by means of atomic force microscopy

Understanding and controlling decoherence in open quantum systems is of fundamental interest in science, whereas achieving long coherence times is critical for quantum information processing1. Although great progress was made for individual systems, and electron spin resonance (ESR) of single spins with nanoscale resolution has been demonstrated2–4, the understanding of decoherence in many complex solid-state quantum systems requires ultimately controlling the environment down to atomic scales, as potentially enabled by scanning probe microscopy with its atomic and molecular characterization and manipulation capabilities. Consequently, the recent implementation of ESR in scanning tunnelling microscopy5–8 represents a milestone towards this goal and was quickly followed by the demonstration of coherent oscillations9,10 and access to nuclear spins11 with real-space atomic resolution. Atomic manipulation even fuelled the ambition to realize the first artificial atomic-scale quantum devices12. However, the current-based sensing inherent to this method limits coherence times12,13. Here we demonstrate pump–probe ESR atomic force microscopy (AFM) detection of electron spin transitions between non-equilibrium triplet states of individual pentacene molecules. Spectra of these transitions exhibit sub-nanoelectronvolt spectral resolution, allowing local discrimination of molecules that only differ in their isotopic configuration. Furthermore, the electron spins can be coherently manipulated over tens of microseconds. We anticipate that single-molecule ESR-AFM can be combined with atomic manipulation and characterization and thereby paves the way to learn about the atomistic origins of decoherence in atomically well-defined quantum elements and for fundamental quantum-sensing experiments.

Substituent effect on the spin transition of Mixed-Valence Cyanide-Bridged {FeIII2FeII} complexes

We synthesized three isostructural mixed-valence {FeIII2FeII} trinuclear complexes {[(Tp4-Me)FeIII(CN)3]2FeII(1-(4-R-phenyl)-1H-imidazole)}·Sol (1: R = Br, Sol = 10·H2O; 2: R = Cl, Sol = 2 CH3OH; 3: R = F, Sol = 2 CH3OH; Tp4-Me = tri(4-methyl-pyrazol-1-yl)borate) and characterized their structures and magnetic and spectroscopic properties. The linear trinuclear structures of complexes 1–––3 are isomorphic. Magnetic susceptibility measurements and variable-temperature IR analyses revealed that the magnetic behavior of the complexes can be finely tuned by the substituents, which significantly influence the ligand field strength. The complexes exhibit complete one-step spin-crossover (SCO) behavior with T1/2↑ = 146 K, T1/2↓ = 142 K for 1; T1/2↑ = 155 K, T1/2↓ = 150 K for 2; T1/2↑ = 147 K, T1/2↓ = 144 K for 3, respectively. Photomagnetic study indicates that all of them show the light-induced spin state transition from low-spin to high spin states by applying 946 nm light for 1 and 808 nm light for 2 and 3. Variable-temperature structural analysis demonstrates that the three complexes exhibit π···π interactions between the intermolecular benzene ring and the pyrazole ring. Our study demonstrates that the modification of substituent ligand can significantly affect the intermolecular interactions of these trinuclear complexes and the SCO behaviour of them.

Photoexcitation dynamics of azide ion bound ferric myoglobin probed by femtosecond infrared spectroscopy

AbstractThe photoexcitation dynamics of N3−‐bound ferric myoglobin (MbN3) were investigated after exciting MbN3 in D2O at 283 K with a 575 nm pulse by probing the anti‐symmetric stretching mode of the azide. Global fitting of the overall time‐resolved spectra revealed that thermal relaxation of two stretching bands proceeded with a time constant of 6 ps, and that a new absorption band formed and decayed with time constants of 0.6 and 23 ps, respectively. The new absorption near 2040 cm−1 was attributed to the high‐spin species 2.4 kJ/mol above the low‐spin species, as the excited low‐spin relaxes thermally via the high‐spin species. However, this absorption could also arise from deligated N3̄ remaining within the protein. The decay of this absorption can be interpreted as either spin transition of the high‐spin species into the low‐spin species or geminate recombination of the dissociated N3̄. The implications of both interpretations are discussed.

Local probe-induced structural isomerization in a one-dimensional molecular array

Synthesis of one-dimensional molecular arrays with tailored stereoisomers is challenging yet has great potential for application in molecular opto-, electronic- and magnetic-devices, where the local array structure plays a decisive role in the functional properties. Here, we demonstrate the construction and characterization of dehydroazulene isomer and diradical units in three-dimensional organometallic compounds on Ag(111) with a combination of low-temperature scanning tunneling microscopy and density functional theory calculations. Tip-induced voltage pulses firstly result in the formation of a diradical species via successive homolytic fission of two C-Br bonds in the naphthyl groups, which are subsequently transformed into chiral dehydroazulene moieties. The delicate balance of the reaction rates among the diradical and two stereoisomers, arising from an in-line configuration of tip and molecular unit, allows directional azulene-to-azulene and azulene-to-diradical local probe structural isomerization in a controlled manner. Furthermore, our theoretical calculations suggest that the diradical moiety hosts an open-shell singlet with antiferromagnetic coupling between the unpaired electrons, which can undergo an inelastic spin transition of 91 meV to the ferromagnetically coupled triplet state.

Improving Electrocatalytic Oxygen Evolution through Local Field Distortion in Mg/Fe Dual‐site Catalysts

AbstractTransition metal single atom electrocatalysts (SACs) with metal‐nitrogen‐carbon (M−N−C) configuration show great potential in oxygen evolution reaction (OER), whereby the spin‐dependent electrons must be allowed to transfer along reactants (OH−/H2O, singlet spin state) and products (O2, triplet spin state). Therefore, it is imperative to modulate the spin configuration in M−N−C to enhance the spin‐sensitive OER energetics, which however remains a significant challenge. Herein, we report a local field distortion induced intermediate to low spin transition by introducing a main‐group element (Mg) into the Fe−N−C architecture, and decode the underlying origin of the enhanced OER activity. We unveil that, the large ionic radii mismatch between Mg2+ and Fe2+ can cause a FeN4 in‐plane square local field deformation, which triggers a favorable spin transition of Fe2+ from intermediate (dxy2dxz2dyz1dz21, 2.96 μB) to low spin (dxy2dxz2dyz2, 0.95 μB), and consequently regulate the thermodyna‐mics of the elementary step with desired Gibbs free energies. The as‐obtained Mg/Fe dual‐site catalyst demonstrates a superior OER activity with an overpotential of 224 mV at 10 mA cm−2 and an electrolysis voltage of only 1.542 V at 10 mA cm−2 in the overall water splitting, which outperforms those of the state‐of‐the‐art transition metal SACs.

Improving Electrocatalytic Oxygen Evolution through Local Field Distortion in Mg/Fe Dual-site Catalysts.

Transition metal single atom electrocatalysts (SACs) with metal-nitrogen-carbon (M-N-C) configuration show great potential in oxygen evolution reaction (OER), whereby the spin-dependent electrons must be allowed to transfer along reactants (OH- /H2 O, singlet spin state) and products (O2 , triplet spin state). Therefore, it is imperative to modulate the spin configuration in M-N-C to enhance the spin-sensitive OER energetics, which however remains a significant challenge. Herein, we report a local field distortion induced intermediate to low spin transition by introducing a main-group element (Mg) into the Fe-N-C architecture, and decode the underlying origin of the enhanced OER activity. We unveil that, the large ionic radii mismatch between Mg2+ and Fe2+ can cause a FeN4 in-plane square local field deformation, which triggers a favorable spin transition of Fe2+ from intermediate (dxy 2 dxz 2 dyz 1 dz2 1 , 2.96 μB ) to low spin (dxy 2 dxz 2 dyz 2 , 0.95 μB ), and consequently regulate the thermodyna-mics of the elementary step with desired Gibbs free energies. The as-obtained Mg/Fe dual-site catalyst demonstrates a superior OER activity with an overpotential of 224 mV at 10 mA cm-2 and an electrolysis voltage of only 1.542 V at 10 mA cm-2 in the overall water splitting, which outperforms those of the state-of-the-art transition metal SACs.

Enhancement of microwave fields in pulse EPR of quantum paraelectrics

The pulse electron paramagnetic resonance (EPR) is widely used in different branches of material and life sciences, including promising applications in quantum information processing and quantum sensing. Here, we study the effect of the high polarizability of KTaO3 and SrTiO3 quantum paraelectrics on local electric and magnetic field components of microwaves (MW) at Fe3+ and Mn2+ paramagnetic ions. The measurements are performed with a commercial EPR spectrometer using dielectric and split-ring resonators. It is found that the power of MW pulses used in coherent spin manipulation at nanoseconds timescale decreases to milliwatts as compared to the tens–hundreds of watts usually used for spins in conventional materials. The amplification of MW fields is related to the very high dielectric permittivity (up to 25 000 in SrTiO3) of quantum paraelectrics at GHz frequencies and temperatures below 20 K. This creates the large induced polarization and, thus, huge displacement current and in turn the secondary MW magnetic field. Numerical simulations support the observation of the enhanced magnetic MW field in the high-permittivity sample. The low MW power for excitation of spin transitions in quantum paraelectrics eliminates the requirement of expensive high-power MW equipment. This approach also allows to globally control spin qubits in tandem with integrated devices based on conventional semiconductor MW circuits working at mW powers. It is suggested that quantum paraelectrics can also be used as substrates for deposition of nanoparticles or films of other materials, which would be manipulated by the low-power MW pulses.

Phase transition, structural, optical and thermoelectric properties of spin coated CuxS thin films

Copper sulphide (CuxS) thin films were deposited by the spin coating method using copper acetate (Cu₂(CH₃COO)₄) and thiourea (CH₄N₂S) as precursors. This simple technique was shown to produce thin films with high quality in terms of tunable stoichiometry and physical properties. In fact, the room temperature structural, optical and thermoelectric properties were studied. It is observed that the annealing during 1 h at 250 °C and 300 °C results in the transition from the covellite (CuS) to the chalcocite (Cu2S) and djurleite (Cu1.95S) phases, respectively. Optical measurements show the formation of absorbing films in the visible range with a transmittance lower than 50 % for all phases. The covellite phase is found to be thermoelectrically interesting with a power factor of 12.3 μWK−2m−1, while the conductivity of the djurleite phase (281.7 Ω−1cm−1) needs to be improved to match its high Seebeck coefficient (21.3 μVK−1).

Design and Processing as Ultrathin Films of a Sublimable Iron(II) Spin Crossover Material Exhibiting Efficient and Fast Light-Induced Spin Transition.

Materials based on spin crossover (SCO) molecules have centered the attention in molecular magnetism for more than 40 years as they provide unique examples of multifunctional and stimuli-responsive materials, which can be then integrated into electronic devices to exploit their molecular bistability. This process often requires the preparation of thermally stable SCO molecules that can sublime and remain intact in contact with surfaces. However, the number of robust sublimable SCO molecules is still very scarce. Here, we report a novel example of this kind. It is based on a neutral iron(II) coordination complex formulated as [Fe(neoim)2], where neoimH is the ionogenic ligand 2-(1H-imidazol-2-yl)-9-methyl-1,10-phenanthroline. In the first part, a comprehensive study, which covers the synthesis and magnetostructural characterization of the [Fe(neoim)2] complex as a bulk microcrystalline material, is reported. Then, in the second part, we investigate the suitability of this material to form thin films through high-vacuum sublimation. Finally, the retainment of all present SCO capabilities in the bulk when the material is processed is thoroughly studied by means of X-ray absorption spectroscopy. In particular, a very efficient and fast light-induced spin transition (LIESST effect) has been observed, even for ultrathin films of 15 nm.

Dirac cohomology for the BGG category [formula omitted

We study Dirac cohomology HDg,h(M) for modules belonging to category O of a finite dimensional complex semisimple Lie algebra. We start by studying the generalized infinitesimal character decomposition of M⊗S, with S being a spin module of h⊥. As a consequence, “Vogan’s conjecture” holds, and we prove a nonvanishing result for HDg,h(M) while we show that in the case of a Hermitian symmetric pair (g,k) and an irreducible unitary module M∈O, Dirac cohomology coincides with the nilpotent Lie algebra cohomology with coefficients in M. In the last part, we show that the higher Dirac cohomology and index introduced by Pandžić and Somberg satisfy nice homological properties for M∈O.

Manipulating Li2O2 deposition morphology by surface spin modulation of cobalt-based spinel oxide catalysts in lithium−oxygen batteries

The lithium peroxide (Li2O2) morphology manipulated by the distribution of Li2O2 nucleation sites dramatically affects the electrochemical performance of lithium − oxygen batteries (LOBs). Cathode surface spin modulation engineering is adopted to obtain ideal Li2O2 deposition morphology, as it can effectively manipulate the nucleation site distribution. Herein, Cu1-ZnCo2O4 (Cu0.15Zn0.85Co2O4) electrocatalysts are elaborately constructed by introducing an appropriate amount of Cu2+ into the ZnCo2O4 tetrahedral sites, which successfully regulates the spin state of Co sites under the action of co-top oxygen connection. Density functional theory calculations reveal that the moderate Co dz2 orbital occupancy is obtained via spin regulation, causing the optimum adsorption capability of Cu1-ZnCo2O4 towards oxygen-containing intermediate. On the basis, the distribution of Li2O2 nucleation sites is modulated to promote the uniform deposition of small-size Li2O2 agglomerates, which possess a large contact area with the cathodic surface and a certain void structure, greatly improving reversible deposition and decomposition kinetics. As a result, LOBs catalyzed by Cu1-ZnCo2O4 offer a large specific capacity of 12984.9 mAh/g and an extended cycle stability of over 400 cycles. This work emphasizes that spin regulation engineering efficiently affects the distribution of Li2O2 nucleation sites, achieving the Li2O2 morphology modulation in LOBs.

Field-free magnetization switching through modulation of zero-field spin–orbit torque efficacy

To make spin–orbit torque magnetic random access memory (SOT-MRAM) practical, current-induced magnetization switching without an external bias field is essential. Given that the CoFeB/MgO structure has already been used in typical spin-transfer torque-MRAM for its high tunneling magnetoresistance, leveraging a similar material system to achieve field-free SOT switching is of great importance. In this work, we systematically investigate the field-free switching mechanism in CoFeB/W/CoFeB T-type structures, where the two CoFeB layers are in-plane and perpendicularly magnetized, respectively. Initial SOT characterization shows a sizable zero-field SOT efficacy (χHx=0) for such T-type devices. Furthermore, field-free angle-dependent SOT measurement confirms the parallel relationship between the built-in bias field and the magnetization of the in-plane magnetized CoFeB. Based on thorough verification and exclusion of other potential mechanisms, the Néel orange-peel effect emerges as the dominant origin for such a built-in bias field, where a positive correlation between the deposited film surface roughness and χHx=0 is found. Based on this discovery, the field-free switching efficacy in T-type structures is further optimized via film roughness tuning and examined with pillar-shaped devices. Our results provide insights into the tentative approach to improve field-free switching using T-type devices and the feasibility of downscaling.

Bistable Spin-Crossover Nanoparticles for Molecular Electronics.

The field of spin-crossover complexes is rapidly evolving from the study of the spin transition phenomenon to its exploitation in molecular electronics. Such spin transition is gradual in a single-molecule, while in bulk it can be abrupt, showing sometimes thermal hysteresis and thus a memory effect. A convenient way to keep this bistability while reducing the size of the spin-crossover material is to process it as nanoparticles (NPs). Here, the most recent advances in the chemical design of these NPs and their integration into electronic devices, paying particular attention to optimizing the switching ratio are reviewed. Then, integrating spin-crossover NPs over 2D materials is focused to improve the endurance, performance, and detection of the spin state in these hybrid devices.

Hofmann-Type Cyanide Bridged Coordination Polymers for Advanced Functional Nanomaterials.

Since the discovery of Hofmann clathrates of inorganic cyanide bridged coordination polymers (Hofmann-type CN-CPs), extensive research is done to understand their behavior during spin transitions caused by guest molecules or external stimuli. Lately, research on their nanoscale architectures for sensors and switching devices is of interest. Their potential is reported for producing advanced functional inorganic materials in two-dimensional (2D) morphology using a scalable solid-state thermal treatment method. For instance, but not restricted to, alloys, carbides, chalcogenides, oxides, etc. Simultaneously, their in situ crystallization at graphene oxide (GO) nanosheet surfaces, followed by a subsequent self-assembly to build layered lamellar structures, is reported providing hybrid materials with a variety of uses. Hence, an overview of the most recent developments is presented here in the synthesis of nanoscale structures, including thin films and powders, using Hofmann-type CN-CPs. Also thoroughly demonstrated are the most recent synthetic ideas with the modest control over the size and shape of nanoscale particles. Additionally, in order to create new functional hybrid materials for electrical and energy applications, their thermal decomposition in various environments and hybridization with GO and other guest molecules is examined. This review article also conveyed their spin transition, astounding innovative versatile adhesives, and structure features.

In Situ Study of the Magnetic Field Gradient Produced by a Miniature Bi-Planar Coil for Chip-Scale Atomic Devices.

The miniaturization of quantum sensors is a popular trend for the development of quantum technology. One of the key components of these sensors is a coil which is used for spin modulation and manipulation. The bi-planar coils have the advantage of producing three-dimensional magnetic fields with only two planes of current confinement, whereas the traditional Helmholtz coils require three-dimensional current distribution. Thus, the bi-planar coils are compatible with the current micro-fabrication process and are quite suitable for the compact design of the chip-scale atomic devices that require stable or modulated magnetic fields. This paper presents a design of a miniature bi-planar coil. Both the magnetic fields produced by the coils and their inhomogeneities were designed theoretically. The magnetic field gradient is a crucial parameter for the coils, especially for generating magnetic fields in very small areas. We used a NMR (Nuclear Magnetic Resonance) method based on the relaxation of 131Xe nuclear spins to measure the magnetic field gradient in situ. This is the first time that the field inhomogeneities of the field of such small bi-planar coils have been measured. Our results indicate that the designed gradient caused error is 0.08 for the By and the Bx coils, and the measured gradient caused error using the nuclear spin relaxation method is 0.09±0.02, suggesting that our method is suitable for measuring gradients. Due to the poor sensitivity of our magnetometer under a large Bz bias field, we could not measure the Bz magnetic field gradient. Our method also helps to improve the gradients of the miniature bi-planar coil design, which is critical for chip-scale atomic devices.

Perspective from a Hubbard U-density corrected scheme towards a spin crossover-mediated change in gas affinity.

By employing a recently proposed Hubbard U density-corrected scheme within density functional theory, we provide design principles towards the design of materials exhibiting a spin crossover-assisted gas release. Small molecular fragments are used as case study to identify two main mechanisms behind the change in binding energy upon spin transitions. The feasibility of the proposed mechanism in porous crystals is assessed by correlating the change in binding energy of CO2, CO, N2, and H2, upon spin crossover, with the adiabatic energy difference associated with the spin state change of the square-planar metal in Hofmann-type clathrates (M = Fe, Mn, Ni). A few promising cases are identified for the adsorption of intermediate ligand field strength molecules such as N2 and H2. The latter stands out as the most original result as the strong interaction in low spin, as expected from a Kubas mechanism, results in a large change in binding energy. This work provides a general perspective towards the engineering of open-metal site frameworks exhibiting local environments designed to have a spin crossover upon adsorption of specific gas molecules.

Electric control of spin transitions at the atomic scale

Electric control of spins has been a longstanding goal in the field of solid state physics due to the potential for increased efficiency in information processing. This efficiency can be optimized by transferring spintronics to the atomic scale. We present electric control of spin resonance transitions in single TiH molecules by employing electron spin resonance scanning tunneling microscopy (ESR-STM). We find strong bias voltage dependent shifts in the ESR signal of about ten times its line width. We attribute this to the electric field in the tunnel junction, which induces a displacement of the spin system changing the g-factor and the effective magnetic field of the tip. We demonstrate direct electric control of the spin transitions in coupled TiH dimers. Our findings open up new avenues for fast coherent control of coupled spin systems and expands on the understanding of spin electric coupling.

Substituent group tuned structural dimensionalities and magnetic properties of V-shaped bis-pyridyl ligands based Fe(II) coordination polymers

A series of V-shaped thioether-based bis-pyridyl ligands containing different substituent groups have been successfully employed to assemble Fe(II)-based complexes by using H-tube diffusion method, resulting in six new coordination polymers with different structure types, [Fe(L1)(SCN)2]n·1.5nCH3OH (1), [Fe(L1)(SeCN)2]n·nCH3OH·nH2O (2), [Fe(L2)(SCN)2]n (3), [Fe(L2)(SeCN)2]n (4), [Fe(L3)(SCN)2]n (5), [Fe(L3)(SeCN)2]n (6) (L1 = 1,3-bis((pyridin-4-ylthio)methyl)benzene, L2 = 4,4′-(((2,4,6-trimethyl-1,3-phenylene)bis(methylene))bis(sulfanediyl))dipyridine, L3 = 4,4′-(((perchloro-1,3-phenylene)bis(methylene))bis(sulfanediyl))dipyridine). All the complexes have been structurally characterized by elemental analysis, FT-IR spectroscopy, single crystal and powder X-ray diffraction. The single crystal structure analysis revealed the one-dimensional (1D) chain for complexes 1–4, in which the different conformation the ligand adopted was found, and two-dimensional (2D) grid-like network for 5 and 6, respectively. Investigation of the temperature dependent magnetic susceptibilities discovered the always high spin state of the Fe(II) ion in all the compounds within the whole temperature range of 10-300 K, except complex 2 with the obvious temperature dependent color change from yellow (293 K) to violet (100 K). The thermo- and photomagnetic property study for complex 2 showed the incomplete spin transition behavior with interesting light-induced excited spin state trapping effect (LIESST). The current results indicated that the steric effect and/or the electronegativity of the substitute groups on the ligand and the conformation of the ligand could play an important role on the topologic structure of the target products, therefore putting obvious influence on spin state of the central Fe(II) ion.

Colossal magnetoresistance in the multiple wave vector charge density wave regime of an antiferromagnetic Dirac semimetal.

Colossal negative magnetoresistance is a well-known phenomenon, notably observed in hole-doped ferromagnetic manganites. It remains a major research topic due to its potential in technological applications. In contrast, topological semimetals show large but positive magnetoresistance, originated from the high-mobility charge carriers. Here, we show that in the highly electron-doped region, the Dirac semimetal CeSbTe demonstrates similar properties as the manganites. CeSb0.11Te1.90 hosts multiple charge density wave modulation vectors and has a complex magnetic phase diagram. We confirm that this compound is an antiferromagnetic Dirac semimetal. Despite having a metallic Fermi surface, the electronic transport properties are semiconductor-like and deviate from known theoretical models. An external magnetic field induces a semiconductor metal-like transition, which results in a colossal negative magnetoresistance. Moreover, signatures of the coupling between the charge density wave and a spin modulation are observed in resistivity. This spin modulation also produces a giant anomalous Hall response.

Sulfone/Carbonyl-Based Donor-Acceptor Fluorescent Dyes: Synthesis, Structures, Photophysical Properties and Cell Imaging.

Electron-accepting units play vital roles in constructing donor-acceptor (D-A) conjugated organic optoelectronic materials; the electronic structures and functions of the acceptors need to be carefully unveiled to controllably tailor the optoelectronic properties. We have synthesized two D-A conjugated organic fluorophores, TPA-SO and TPA-CO, with similar molecular skeletons based on sulfone- or carbonyl-containing polycyclic aromatic acceptors. Both TPA-SO and TPA-CO display obvious solvent polarity-dependent photophysical properties and large Stokes shift of over 100 nm for strong intramolecular charge transfer processes. Experimental evidence indicates that the sulfone group in TPA-SO merely serves as a strong electron-withdrawing unit. TPA-SO shows yellowish-green emission with a peak at 542 nm and an absolute photoluminescence quantum yield (PLQY) of 98 % in solution, whereas the carbonyl group in TPA-CO can act as both an electron-withdrawing unit and spin transition convertor, so TPA-CO displays red emission with a low absolute PLQY of 0.32 % in solution. Impressively, upon going from solution to aggregate state, TPA-SO nanoparticles keep a high PLQY of 9.5 % and moderate biocompatibility, thus they are good nano-agents for cellular fluorescence imaging. The results reveal that the inherent acceptor characteristic acts as a crucial effect in the photophysical properties and applications of the organic fluorophores.