Yan Duan

Enhancing coherence in molecular spin qubits via atomic clock transitions

Quantum computing is an emerging area within the information sciences revolving around the concept of quantum bits (qubits). A major obstacle is the extreme fragility of these qubits due to interactions with their environment that destroy their quantumness. This phenomenon, known as decoherence, is of fundamental interest. There are many competing candidates for qubits, including superconducting circuits, quantum optical cavities, ultracold atoms and spin qubits, and each has its strengths and weaknesses. When dealing with spin qubits, the strongest source of decoherence is the magnetic dipolar interaction. To minimize it, spins are typically diluted in a diamagnetic matrix. For example, this dilution can be taken to the extreme of a single phosphorus atom in silicon, whereas in molecular matrices a typical ratio is one magnetic molecule per 10,000 matrix molecules. However, there is a fundamental contradiction between reducing decoherence by dilution and allowing quantum operations via the interaction between spin qubits. To resolve this contradiction, the design and engineering of quantum hardware can benefit from a ‘bottom-up’ approach whereby the electronic structure of magnetic molecules is chemically tailored to give the desired physical behaviour. Here we present a way of enhancing coherence in solid-state molecular spin qubits without resorting to extreme dilution. It is based on the design of molecular structures with crystal field ground states possessing large tunnelling gaps that give rise to optimal operating points, or atomic clock transitions, at which the quantum spin dynamics become protected against dipolar decoherence. This approach is illustrated with a holmium molecular nanomagnet in which long coherence times (up to 8.4 microseconds at 5 kelvin) are obtained at unusually high concentrations. This finding opens new avenues for quantum computing based on molecular spin qubits.

Construction of a general library for the rational design of nanomagnets and spin qubits based on mononuclear f-block complexes. The polyoxometalate case.

This paper belongs to a series of contributions aiming at establishing a general library that helps in the description of the crystal field (CF) effect of any ligand on the splitting of the J ground states of mononuclear f-element complexes. Here, the effective parameters associated with the oxo ligands (effective charges and metal-ligand distances) are extracted from the study of the magnetic properties of the first two families of single-ion magnets based on lanthanoid polyoxometalates (POMs), formulated as [Ln(W5O18)2](9-) and [Ln(β2-SiW11O39)2](13-) (Ln = Tb, Dy, Ho, Er, Tm, Yb). This effective CF approach provides a good description of the lowest-lying magnetic levels and the associated wave functions of the studied systems, which is fully consistent with the observed magnetic behavior. In order to demonstrate the predictive character of this model, we have extended our model in a first step to calculate the properties of the POM complexes of the early 4f-block metals. In doing so, [Nd(W5O18)2](9-) has been identified as a suitable candidate to exhibit SMM behavior. Magnetic experiments have confirmed such a prediction, demonstrating the usefulness of this strategy for the directed synthesis of new nanomagnets. Thus, with an effective barrier of 51.4 cm(-1) under an applied dc field of 1000 Oe, this is the second example of a Nd(3+)-based single-ion magnet.

Magnetization Relaxation in a Three‐Dimensional Ligated Cobalt Phosphonate Containing Ferrimagnetic Chains

Paramagnetic chain systems that show magnetization relaxation have been of great interest since the discovery by Caneschi et al. of the first single-chain magnet (SCM) in a magnetically isolated chain compound containing alternating [Co ACHTUNGTRENNUNG(hfac)2] and nitronyl–nitroxide radicals. Such systems are of potential importance not only in molecular spintronics and quantum computing but also because they raise the Tc with respect to single-molecule magnets (SMMs). [2] In order to observe the SCM behavior, the system requires a strong easy axis anisotropy and strong intrachain interactions (J) with negligible interchain (J’) ones. It is noted that the majority of compounds with SCM-like behaviors are heteroor homo-spin systems bridged by organic radicals, azide, oximate, carboxylate, cyanide, and oxalate . Only two examples, [Co ACHTUNGTRENNUNG(H2L1) ACHTUNGTRENNUNG(H2O)] (L =4-MeC6H4-CH2N ACHTUNGTRENNUNG(CPO3H2)2)[10] and [MnACHTUNGTRENNUNG(TPP)O2PHPh]· ACHTUNGTRENNUNG(H2O) (TPP=meso-tetraphenylporphyrin), present the O-P-O bridges, in which the Co or Mn ions are antiferromagnetically coupled, thus resulting in spin canting structures with SCM behaviors. Another interesting phenomenon, discovered recently by Ishida and co-workers, is found when 1 D SCM-like systems undergo a phase transition to a 3D magnetic order, a very large coercivity could appear and increase pronouncedly at low temperature. Thus, the introduction of an interchain interaction can improve magnetic hardness and avoid magnetization loss at zero field. As a class of important organic–inorganic hybrid materials, metal phosphonates have potential applications in many fields, including catalysis and separation, photochemistry, magnetism, intercalation chemistry, and biotechnology. Usually, these materials display layered or pillared layered structures where the inorganic layers are separated by organic groups. By introducing additional functional groups, a secondary Nand/or O-donor co ACHTUNGTRENNUNGli ACHTUNGTRENNUNGgand, or organic templates, compounds with chain structures can also be obtained. To our knowledge, twoor three-dimensional metal phosphonates embedded with isolated magnetic chains have rarely been reported. We have been very interested in studying the structural– magnetic relationships in low dimensional metal phosphonates—especially those with chain structures. More recently, particular efforts have been devoted to the syntheses of metal phosphonates with layer or framework structures which contain isolated inorganic chains, through the introduction of a long and flexible second organic ligand. As a result of these efforts, herein we present a novel mixed ligated cobalt phosphonate [Co3ACHTUNGTRENNUNG(pna)2(L) ACHTUNGTRENNUNG(H2O)2] (1) (pnaH3 =6phosphonic nicotinic acid, L= 1,4-bis(imidazol-1-ylmethyl)benzene; Scheme 1), which shows a rare 3,8-connected net

Influence of the covalent grafting of organic radicals to graphene on its magnetoresistance

Graphene was obtained by direct exfoliation of graphite in o-dichlorobenzene (oDCB) or benzylamine, and further functionalized with 4,4′-[(1,3-dioxo-1,3-propanediyl)bis(oxy)]bis[2,2,6,6-tetramethyl-1-piperidinyloxy] (1-TEMPO) organic radicals by using the Bingel–Hirsch cyclopropanation reaction. Here, the use of different solvents permits variation of the density of radicals anchored to the carbon layers. Covalent grafting is unambiguously demonstrated by TGA, μ-Raman, XPS and EPR measurements, which also rule out spurious physisorption. Our transport measurements indicate that the conduction mechanism varies as a function of the density of radicals grafted to the carbon layers. Moreover, the presence of paramagnetic moieties influences the magnetoresistive behaviour of the system by the appearance of a low field magnetoresistance (LFMR) effect. Finally, the derivatization of graphene with diamagnetic ethylmalonate molecules, by following an analogous route, permits us to discard the chemical derivatization, which is responsible for the observed differences in the MR response that must be rather ascribed to the grafting of organic spin carriers.

Cobalt Clusters with Cubane-Type Topologies Based on Trivacant Polyoxometalate Ligands

Four novel cobalt-substituted polyoxometalates having cobalt cores exhibiting cubane or dicubane topologies have been synthesized and characterized by IR, elemental analysis, electrochemistry, UV-vis spectroscopy, X-ray single-crystal analysis, and magnetic studies. The tetracobalt(II)-substituted polyoxometalate [Co4(OH)3(H2O)6(PW9O34)](4-) (1) consists of a trilacunary [B-α-PW9O34](9-) unit which accommodates a cubane-like {Co(II)4O4} core. In the heptacobalt(II,III)-containing polyoxometalates [Co7(OH)6(H2O)6(PW9O34)2](9-) (2), [Co7(OH)6(H2O)4(PW9O34)2]n(9n-) (3), and [Co7(OH)6(H2O)6(P2W15O56)2](15-) (4), dicubane-like {Co(II)6Co(III)O8} cores are encapsulated between two heptadentate [B-α-PW9O34](9-) (in 2 and 3) or [α-P2W15O56](15-) (in 4) ligands. While 1, 2, and 4 are discrete polyoxometalates, 3 exhibits a polymeric, chain-like structure that results from the condensation of polyoxoanions of type 2. The magnetic properties of these complexes have been fitted according to an anisotropic exchange model in the low-temperature regime and discussed on the basis of ferromagnetic interactions between Co(2+) ions with angles Co-L-Co (L = O, OH) close to orthogonality and weakly antiferromagnetic interactions between Co(2+) ions connected through central diamagnetic Co(3+) ion. Moreover, we will show the interest of the unique spin structures provided by these cubane and dicubane cobalt topologies in molecular spintronics (molecular spins addressed though an electric field) and quantum computing (spin qu-gates).

Rational Design of Lanthanoid Single‐Ion Magnets: Predictive Power of the Theoretical Models

We report two new single-ion magnets (SIMs) of a family of oxydiacetate lanthanide complexes with D3 symmetry to test the predictive capabilities of complete active space ab initio methods (CASSCF and CASPT2) and the semiempirical radial effective charge (REC) model. Comparison of the theoretical predictions of the energy levels, wave functions and magnetic properties with detailed spectroscopic and magnetic characterisation is used to critically discuss the limitations of these theoretical approaches. The need for spectroscopic information for a reliable description of the properties of lanthanide SIMs is emphasised.

Single ion magnets based on lanthanoid polyoxomolybdate complexes

Polyoxometalate (POM) chemistry has recently offered excellent examples of single ion magnets (SIMs) and molecular spin qubits. Compared with conventional coordination compounds, POMs provide rigid and highly symmetric coordination sites. However, all POM-based SIMs reported to date exhibit a very limited range of possibilities for chemical processability. We present herein two new families of POM-based SIMs which are soluble in organic solvents: [Ln(β-Mo8O26)2]5- {LnIII = Tb, Dy, Ho, Er, Tm and Yb} and the functionalised POMs [Ln{Mo5O13(OMe)4NNC6H4-p-NO2}2]3- {LnIII = Tb, Dy, Ho, Er, Yb and Nd}. In addition, these two families represent the first SIMs based on polyoxomolybdates. A magneto-structural analysis of these families is presented, which is based on an effective crystal field model, and compared with the results reported in analogous lanthanoid SIMs based on polyoxotungstates.

A decacobalt(II) cluster with triple-sandwich structure obtained by partial reductive hydrolysis of a pentacobalt(II/III) Weakley-type polyoxometalate

Partial reductive hydrolysis of a penta-CoII/III cluster [Co(H2O)2(CoIIIW9O34)(PW9O34)]12- (1) leads to the formation of [Co2{Co3(H2O)(Co(OH)2W7O26)(PW9O34)}2]22- (2). This polyoxometalate is made up of two capping [PW9O34]9- units and two bridging [W7O26]10- units that assemble to encapsulate a novel deca-CoII cluster core comprising octahedral and tetrahedral CoII ions.

A ferroelectric iron(II) spin crossover material

A dual-function material in which ferroelectricity and spin crossover coexist in the same temperature range has been obtained. Our synthetic strategy allows the construction of acentric crystal structures in a predictable way and is based on the high directionality of hydrogen bonds. The well-known iron(II) spin crossover complex [Fe(bpp)2 ]2+ (bpp=2,6-bis(pyrazol-3-yl)pyridine), a four-fold noncentrosymmetric H-bond donor, was combined with a disymmetric H-bond acceptor such as the isonicotinate (isonic) anion to afford [Fe(bpp)2 ](isonic)2 ⋅2 H2 O. This low-spin iron(II) compound crystallizes in the acentric nonpolar I4‾ space group and shows piezoelectricity and SHG properties. Upon dehydration, it undergoes a single-crystal to single-crystal structural rearrangement to a monoclinic polar Pc phase that is ferroelectric and exhibits spin crossover.

Hydrogen-bonded networks of [Fe(bpp)2]2+ spin crossover complexes and dicarboxylate anions: structural and photomagnetic properties

The paper reports the syntheses, crystal structures, thermal and (photo)magnetic properties of spin crossover salts of formula [Fe(bpp)2](C6H8O4)·4H2O (1·4H2O), [Fe(bpp)2](C8H4O4)·2CH3OH·H2O (2·2MeOH·H2O) and [Fe(bpp)2](C8H4O4)·5H2O (2·5H2O) (bpp = 2,6-bis(pyrazol-3yl)pyridine; C6H8O4 = adipate dianion; C8H4O4 = terephthalate dianion). The salts exhibit an intricate network of hydrogen bonds between low-spin iron(ii) complexes and carboxylate dianions, with solvent molecules sitting in the voids. Desolvation is accompanied by a low-spin (LS) to high-spin (HS) transformation in the materials. The dehydrated phase 2 undergoes a two-step transition with a second step showing thermal hysteresis (T1/2↑ = 139 K and T1/2↓ = 118 K). 2 displays a quantitative LS to HS photomagnetic conversion, with a T(LIESST) value of 63 K.