Stephen Hill

Florida State University, Tallahassee, Florida, United States

Are you Stephen Hill?

Claim your profile

Publications (105)391.79 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The relationship between electronic structure and zero-field splitting dictates key design parameters for magnetic molecules. In particular, to enable the directed synthesis of new electronic spin based qubits, developing complexes where zero-field splitting energies are invariant to structural changes is a critical challenge. Toward those ends, we report three salts of a new compound, a four-coordinate iron(II) complex [Fe(C3S5)2]2− ([(18-crown-6)K]+ (1), Ph4P+ (2), Bu4N+ (3)) with a continuous structural variation in a single parameter, the dihedral angle (θd) between the two C3S52− ligands, as a function of counterion (θd = 89.98(4)° for 1 to 72.41(2)° for 3). Electron paramagnetic resonance data for 1–3 reveal zero-field splitting parameters that are unusually robust to the structural variation. Mössbauer spectroscopic measurements indicate that the structural variation in θd primarily affects the highest-energy 3d-orbitals (dxz and dyz) of the iron(II) ion. These orbitals have the smallest impact on the zero-field splitting parameters, thus the distortion has a minor effect on D and E. These results represent the first part of a directed effort to understand how spin state energies may be fortified against structural distortions for future applications of qubits in non-crystalline environments.
    No preview · Article · Dec 2015 · Chemical Science
  • [Show abstract] [Hide abstract]
    ABSTRACT: Controlling and understanding transitions between molecular spin states allows selection of the most suitable ones for qubit encoding. Here we present a detailed investigation of single crystals of a polynuclear Cr8Zn molecular wheel using 241 GHz Electron Paramagnetic Resonance (EPR) spectroscopy in high magnetic field. Continuous wave spectra are well reproduced by spin Hamiltonian calculations, which evidence that transitions in correspondence to a well-defined anti-crossing involve mixed states with different total spin. We studied, by means of spin echo experiments, the temperature dependence of the dephasing time (T2) down to 1.35 K. These results are reproduced by considering both hyperfine and intermolecular dipolar interactions, evidencing that the dipolar contribution is completely suppressed at the lowest temperature. Overall, these results shed light on the effects of the decoherence mechanisms, whose understanding is crucial to exploit chemically engineered molecular states as a resource for quantum information processing.
    No preview · Article · Dec 2015 · Journal of Physical Chemistry Letters
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The syntheses and properties of four magnetically-supramolecular oligomers of triangular Mn3 units are reported: dimeric [Mn6O2(O2CMe)8(CH3OH)2(pdpd)2] (3) and [Mn6O2(O2CMe)8(py)2(pdpd)2](ClO4)2 (4), and tetrameric [Mn12O4(O2CR)12(pdpd)6](ClO4)4 (R = Me (5), tBu (6)). They were all obtained employing 3-phenyl-1,5-di(pyridin-2-yl)pentane-1,5-dione dioxime (pdpdH2), either in direct synthesis reactions involving oxidation of MnII salts or in metathesis reactions with the preformed complex [Mn3O(O2CMe)6(py)3](ClO4) (1); complex 6 was then obtained by carboxylate substitution on complex 5. Complexes 3 and 4 contain two [MnIII2MnII(μ3-O)]6+ and [MnIII3(μ3-O)]7+ units, respectively, linked by two pdpd2− groups. Complexes 5 and 6 contain four [MnIII3(μ3-O)]7+ units linked by six pdpd2− groups into a rectangular tetramer [MnIII3]4. Solid-state dc magnetic susceptibility studies showed that the Mn3 subunits in 3 and 4 have a ground-state spin of S = 3/2 and S = 2, respectively, while the Mn3 subunits in 5 and 6 possess an S = 6 ground state. Complexes 5 and 6 exhibit frequency-dependent out-of-phase (χ′′M) ac susceptibility signals indicating 5 and 6 to be tetramers of Mn3 single-molecule magnets (SMMs). High-frequency EPR studies of a microcrystalline powder sample of 5·2CH2Cl2 provided precise spin Hamiltonian parameters of D = −0.33 cm−1, |E| = 0.03 cm−1, B04 = −8.0 × 10−5 cm−1, and g = 2.0. Magnetization vs. dc field sweeps on a single crystal of 5·xCH2Cl2 gave hysteresis loops below 1 K that exhibit exchange-biased quantum tunneling of magnetization (QTM) steps with a bias field of 0.19 T. Simulation of the loops determined that each Mn3 unit is exchange-coupled to the two neighbors linked to it by the pdpd2− linkers, with an antiferromagnetic inter-Mn3 exchange interaction of J/kB = −0.011 K (Ĥ = −2JŜi·Ŝj convention). The work demonstrates a rational approach to synthesizing magnetically-supramolecular aggregates of SMMs as potential multi-qubit systems for quantum computing.
    Full-text · Article · Nov 2015 · Chemical Science
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Monometallic complexes based on 3d transition metal ions in certain axial coordination environments can exhibit appreciably enhanced magnetic anisotropy, important for memory applications, due to stabilisation of an unquenched orbital moment. For high-spin trigonal bipyramidal Ni(II), if competing structural distortions can be minimised, this may result in an axial anisotropy that is at least an order of magnitude stronger than found for orbitally non-degenerate octahedral complexes. Broadband, high-field EPR studies of [Ni(MDABCO)2Cl3]ClO4 (1) confirm an unprecedented axial magnetic anisotropy, which pushes the limits of the familiar spin-only description. Crucially, compared to complexes with multidentate ligands that encapsulate the metal ion, we see only a very small degree of axial symmetry breaking. 1 displays field-induced slow magnetic relaxation, which is rare for monometallic Ni(II) complexes due to efficient spin–lattice and quantum tunnelling relaxation pathways.
    Full-text · Article · Sep 2015 · Chemical Science
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: [Mn3O(O2CMe)3(dpd)3/2)]2(I3)2 has been obtained from the reaction of 1,3-di(pyridin-2-yl)propane-1,3-dione dioxime (dpdH2) with triangular [Mn(III)3O(O2CMe)(py)3](ClO4). It comprises two [Mn(III)3O](7+) triangular units linked covalently by three dioximate ligands into a [Mn3]2 dimer. Solid state dc and ac magnetic susceptibility measurements reveal that each Mn3 subunit of the dimer is a separate single-molecule magnet (SMM) with an S = 6 ground state and that the two SMM units are very weakly ferromagnetically exchange coupled. High-frequency EPR spectroscopy on a single crystal displays signal splittings indicative of quantum superposition/entanglement of the two SMMs, and parallel studies on MeCN/toluene (1:1) frozen solutions reveal the same spectral features. The dimer thus retains its structure and inter-Mn3 coupling upon dissolution. This work establishes that covalently linked molecular oligomers of exchange-coupled SMMs can be prepared that retain their oligomeric nature and attendant inter-SMM quantum mechanical coupling in solution, providing a second phase for their study and demonstrating the feasibility of using solution methods for their deposition on surfaces and related substrates for study.
    Full-text · Article · Jun 2015 · Journal of the American Chemical Society
  • [Show abstract] [Hide abstract]
    ABSTRACT: A complex exhibiting valence delocalization was prepared from 3,5-bis(2-pyridyl)-1,2,4,6-thiatriazinyl (Py2TTA·), an inherently redox active pincer-type ligand, coordinated to iron (Fe(Py2TTA)Cl2 (1)). Complex 1 can be prepared via two routes, either from the reaction of the neutral radical with FeCl2 or by treatment of the anionic ligand (Py2TTA-) with FeCl3, demonstrating its unique redox behaviour. Electrochemical studies, solution absorption and solid-state diffuse reflectance measurements along with X-ray crystallography were carried out to elucidate the molecular and solid-state properties. Temperature- and field-dependent Mössbauer spectroscopy coupled with magnetic measurements revealed that 1 exhibits an isolated S = 5/2 ground spin state for which the low-temperature magnetic behaviour is dominated by exchange interactions between neighbouring molecules. This ground state is rationalized on the basis of DFT calculations that predict the presence of strong electronic interactions between the redox active ligand and metal. This interaction leads to the delocalization of β electron density over the two redox active centres and highlights the difficulty in assigning formal charges to 1.
    No preview · Article · May 2015 · Dalton Transactions
  • [Show abstract] [Hide abstract]
    ABSTRACT: Molecular magnetism research incorporates fundamental concepts of coordination and organometallic chemistry and merges them with physics with one of the highest profile current topics being molecules that exhibit bistability, known as “Single Molecule Magnets” (SMMs). Recent strides in designing new generations of tiny magnets that retain their memory effect at higher temperatures provide promising evidence that they may be useful for new generations of nanoscale electronic devices and computers. The overarching goal of our research in this area is the use of discrete building units to control molecular architecture and the resulting magnetic properties. The implementation of this strategy is helpful for gleaning valuable information about how magnetic properties are affected by electronic as well as geometric structure as well as weak and strong chemical interactions. This talk will cover various topics including combining specific metals of the periodic table to enhance the magnetic properties. Experiments are being conducted to test the validity of several independent theoretical studies that predict that specific metal combinations will lead to higher temperature magnets with cyanide bridges. Experimental results are correlated with theory and augmented by the use of special techniques involving neutrons and high frequency and high-field spectroscopies.
    No preview · Conference Paper · Mar 2015
  • [Show abstract] [Hide abstract]
    ABSTRACT: The development and study of molecular nanomagnets has witnessed tremendous progress in recent years, with potential applications on the horizon [1]. Of particular interest are so-called single-molecule magnets (SMMs) that display slow magnetic relaxation below a characteristic blocking temperature, TB, due to the combination of a large magnetic moment and appreciable spin-orbit (SO) anisotropy. Early efforts aimed at increasing TB focused primarily on maximizing the molecular spin ground state, S, associated with polynuclear clusters. However, this becomes challenging for large clusters whilst simultaneously maintaining the molecular anisotropy. Thus, a more direct route to increasing TB involves optimization of the magnetic anisotropy, albeit for simpler molecules in which one can exert considerable synthetic control over the ligand field (LF). This approach resulted in the first SMM comprising a single lanthanide ion in 2003 [2]. Since then, much effort has been directed towards increasing the magnetic anisotropy of mononuclear species. Here, we discuss two series of hexa-coordinate VIII complexes: one consisting of trans-dicyanide VIII building blocks with equatorial chelating ligands; the other involving a tridentate scorpionate ligand, with three halides completing the coordination sphere. Multi-high-field/frequency (up to 35 T and 450 GHz) EPR was employed to determine the magnetic anisotropy of these complexes. Members of the Jahn-Teller elongated cyanovanadate series can be described by an effective spin-only Hamiltonian with positive D parameters in the +3 to +6 cm‑1 range, and negligible rhombicity. On the other hand, one of the scorpionate complexes was found to have a very large easy-axis-type zero-field-splitting on the order of ‑40 cm‑1; this complex also possesses a non-zero, albeit small E value of 1.1 cm-1. The large magnetic anisotropy is thought to arise from the trigonal distortion imposed by the rigid scorpionate ligand, which also partially suppresses the Jahn-Teller physics, thereby giving rise to a considerable orbital contribution to the magnetic ground state. [1] Molecular Magnets - Physics and Applications, Springer Series on Nanoscience and Technology, eds J. Bartolomé, F. Luis, J. F. Fernández (Springer, 2014). [2] N. Ishikawa et al., J. Am. Chem. Soc. 125, 8694 (2003).
    No preview · Conference Paper · Mar 2015
  • Stephen M Winter · Stephen Hill · Richard T Oakley
    [Show abstract] [Hide abstract]
    ABSTRACT: Recent developments in stable radical chemistry have afforded "heavy atom" radicals, neutral open-shell (S = 1/2) molecular species containing heavy p-block elements (S, Se), which display solid-state magnetic properties once considered exclusive to conventional metal-based magnets. These highly spin-delocalized radicals do not associate in the solid state and yet display extensive networks of close intermolecular interactions. Spin density on the heavy atoms allows for increased isotropic and spin-orbit mediated anisotropic exchange effects. Structural variations induced by chemical modification and physical pressure, coupled with ab-initio methods to estimate exchange energies, have facilitated the development of predictive structure/property relationships. These results, coupled with detailed theoretical analyses and magnetic resonance spectroscopic measurements, have provided insight into the magnetic structure of ferromagnetic and spin-canted antiferromagnetic ordered materials as well as an understanding of the importance of spin-orbit coupling contributions to magnetic hysteresis and anisotropy. Isotropic and anisotropic ferromagnetic exchange can also be enhanced indirectly by the incorporation of heavy atoms into nonspin-bearing sites, where they can contribute to multi-orbital spin-orbit coupling.
    No preview · Article · Mar 2015 · Journal of the American Chemical Society
  • [Show abstract] [Hide abstract]
    ABSTRACT: A new "offset" analogue of the classical [Mn6O2]-core oxime-bridged single-molecule magnets is introduced with a modified stacking arrangement of the [Mn3O] units. Studies of the magnetic properties reveal antiferromagnetic exchange interactions, a spin S = 4 ground state and population of low-lying excited states. Slow relaxation of the magnetization can be detected, with a corresponding energy barrier of 35.8 K. Interpretation of these features is supported with high-frequency EPR studies, quantifying the easy-axis type magnetic anisotropy, leading to a biaxial system. Redox properties investigated by cyclic and differential pulse voltammetry reveal multiple irreversible redox processes.
    No preview · Article · Jan 2015 · Inorganic Chemistry
  • [Show abstract] [Hide abstract]
    ABSTRACT: The combination of high-pressure ferromagnetic resonance (FMR) and an ab-initio scheme suitable for calculation of spin-orbit mediated anisotropic exchange interactions in molecular materials provides insights into the role of spin-orbit coupling (SOC) in a Se-based organic ferromagnet. FMR measurements reveal a continuous increase in the magnetic anisotropy with increasing pressure (up to 2.2 GPa), in excellent agreement with calculations based on the known pressure dependence of the structure. The large value of anisotropic exchange terms in this heavy atom organic ferromagnet emphasizes the important role of SOC in a wide range of organics where this effect is usually considered to be small.
    No preview · Article · Jan 2015 · Physical Review B
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: High-field electron paramagnetic resonance spectroscopy shows that the structurally distorted Mn(III) ion in Na5[Mn(l-tart)2]·12H2O (1; l-tart = l-tartrate) has a significant negative axial zero-field splitting and a small rhombic anisotropy (∼1% of D). Alternating-current magnetic susceptibility measurements demonstrate that 1, which contains isolated Mn(III) centers, displays slow relaxation of its magnetization under an applied direct-current magnetic field.
    Full-text · Article · Dec 2014 · Inorganic Chemistry
  • Source

    Full-text · Dataset · Jul 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report a room temperature study on the electrical response of field-effect transistors (FETs) based on few-layered MoSe2, grown by a chemical vapor transport technique, mechanically exfoliated onto SiO2. In contrast to previous reports on MoSe2 FETs electrically contacted with Ni, MoSe2 FETs electrically contacted with Ti display ambipolar behavior with current on to off ratios up to 106 for both hole and electron channels when applying a small excitation voltage. A rather small hysteresis is observed when sweeping the back-gate voltage between positive and negative values, indicating the near absence of charge “puddles”. For both channels the Hall-effect indicates Hall mobilities μ_H ≈ 250 cm^2/Vs which are comparable to the corresponding field-effect mobilities, i.e. μ_FE 150 to 200 cm2/Vs evaluated through the conventional two-terminal field-effect configuration. Therefore, our results suggest that MoSe2 could be a good candidate for p-n junctions composed of a single atomic layer and for low-power, complimentary logic applications.
    Full-text · Article · Jul 2014 · ACS Nano
  • [Show abstract] [Hide abstract]
    ABSTRACT: The fluorinated oxobenzo-bridged bisdithiazolyl radical FBBO was recently observed to undergo a pressure-induced Mott insulator-to-metal transition, suggesting a novel organic system for studying Mott physics. This report describes the electronic structure of this material in relation to the observed magnetic response at low pressures. Through analysis of antiferromagnetic resonance measurements, we identify a layered antiferromagnetic ordered phase below TN=13 K at ambient pressure, which requires strong ferromagnetic coupling between nearest neighbours. The origin of such coupling is elucidated from both molecular and solid-state electronic-structure calculations, which suggest a minimal two-orbital model with strong Hund's-rule coupling. This layered phase is partially frustrated by a second-nearest-neighbor antiferromagnetic coupling, which drives a magnetic phase transition at elevated pressure. On the basis of the two-orbital model, we suggest the pressure-induced Mott transition to proceed via rehybridization of the frontier molecular orbitals, resulting in a half-filled insulator to quarter-filled metal crossover.
    No preview · Article · Jun 2014 · Physical Review B
  • [Show abstract] [Hide abstract]
    ABSTRACT: Enabling the rational synthesis of molecular candidates for quantum information processing requires design principles that minimize electron spin decoherence. Here we report a systematic investigation of decoherence via the synthesis of two series of paramagnetic coordination complexes. These complexes, [M(C2O4)3](3-) (M = Ru, Cr, Fe) and [M(CN)6](3-) (M = Fe, Ru, Os), were prepared and interrogated by pulsed electron paramagnetic resonance (EPR) spectroscopy to assess quantitatively the influence of the magnitude of spin (S = (1)/2, (3)/2, (5)/2) and spin-orbit coupling (ζ = 464, 880, 3100 cm(-1)) on quantum decoherence. Coherence times (T2) were collected via Hahn echo experiments and revealed a small dependence on the two variables studied, demonstrating that the magnitudes of spin and spin-orbit coupling are not the primary drivers of electron spin decoherence. On the basis of these conclusions, a proof-of-concept molecule, [Ru(C2O4)3](3-), was selected for further study. The two parameters establishing the viability of a qubit are a long coherence time, T2, and the presence of Rabi oscillations. The complex [Ru(C2O4)3](3-) exhibits both a coherence time of T2 = 3.4 μs and the rarely observed Rabi oscillations. These two features establish [Ru(C2O4)3](3-) as a molecular qubit candidate and mark the viability of coordination complexes as qubit platforms. Our results illustrate that the design of qubit candidates can be achieved with a wide range of paramagnetic ions and spin states while preserving a long-lived coherence.
    No preview · Article · May 2014 · Journal of the American Chemical Society
  • [Show abstract] [Hide abstract]
    ABSTRACT: A family of axially distorted mononuclear vanadium (III) compounds of general formula, A[L3VX3] (3-9 ) (X = F, Cl or Br, A+ = Et4N+, nBu4N+ or PPN+ , L3 = Tp or Tp* (Tp = tris(-1-pyrazolyl)borohydride), Tp* = tris(3,5-dimethyl-1-pyrazolyl)borohydride)), were studied (Figure 1). Replacement of the Tp ligand in 3 with the stronger π-donor Tp* results in a near doubling of the magnitude of the axial zero-field splitting parameter Dz (Dz = -16.0 cm-1 in 3 , and -30.0 cm-1 in PPN[Tp*VCl3] 4 )as determined by magnetic measurements. Such findings support the notion that control of the axial crystal field distortion is an excellent way to control single-ion anisotropy. High Field-High Frequency EPR measurements on 4 revealed an even higher D value, -40.0 cm-1.
    No preview · Conference Paper · Mar 2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This chapter provides a detailed overview of some of the primary spectroscopic methods that have contributed to the current understanding of molecular nanomagnets (MNs). These include: electron paramagnetic resonance (EPR); optical spectroscopy, including magnetic and X-ray magnetic circular dichroism (MCD/XMCD); inelastic neutron scattering (INS); and muon spin rotation (μ +SR). For each technique, a historical survey of the most important discoveries is provided, up to and including the most recent developments. Each section gives an introduction to the theoretical principles underpinning the techniques, as well as a description of experimental requirements and protocols. A common theme among the described spectroscopies is the fact that state-of-the-art measurements typically have to be performed at major research facilities such as synchrotrons (terahertz EPR and XMCD), high magnetic field laboratories (EPR), and accelerator facilities or reactors (INS and μ +SR). Details of such facilities are given where appropriate. Forefront issues that are addressed in the chapter include: the fundamental properties of both mono- and poly-nuclear single-molecule magnets (SMMs); the deployment of MNs in quantum information processing applications; the addressing of individual magnetic molecules on surfaces or in devices; the probing of spin dynamics in MNs using EPR, INS, and μ +SR; and studies of long-range magnetic ordering in MN crystals. An extensive list of references is provided. The chapter is intended for physicists, chemists, and materials scientists, particularly junior researchers who are just starting work in the field.
    Full-text · Chapter · Jan 2014
  • Stephen Hill · Enrique del Barco

    No preview · Article · Dec 2013 · Polyhedron
  • Junjie Liu · Stephen Hill
    [Show abstract] [Hide abstract]
    ABSTRACT: We examine the magnetization quantum tunneling (MQT) behavior expected for a single-molecule magnet (SMM) with improper rotational symmetry. The simplest possible realization is the [Ni-II(hmp)(dmb)Cl](4) cubane complex that crystallizes in the I41/a space group, resulting in S-4 molecular point-group symmetry. A mapping is performed of the energy-level diagram obtained via exact diagonalization of a multi-spin Hamiltonian onto that of a giant-spin model which assumes ferromagnetic coupling and a spin S = 4 ground state. The results are compared with a similar analysis for a C-3 symmetric Mn-3 SMM (S = 6 ground state). In the even rotational case (Ni-4), the time-reversal invariance associated with the spin-orbit interaction gives rise to a zero-field spin-Hamiltonian that possesses an additional mirror plane perpendicular to the S-4 axis, which is not a symmetry element of the molecular point-group. This conclusion applies quite generally to any molecule with improper rotational symmetry (S-q, with q even), including the more widely studied Mn-12 SMM. The combined Ni-4 and Mn-3 studies lead to some interesting predictions concerning MQT selection rules in molecules with even versus odd rotational symmetries. We conclude by considering a case with essentially no symmetry at all, by deliberately distorting the high-symmetry Ni-4 molecule. In this case, finite gaps are found at all intersections in the energy-level diagram, indicating a complete absence of MQT selection rules.
    No preview · Article · Dec 2013 · Polyhedron

Publication Stats

1k Citations
391.79 Total Impact Points

Institutions

  • 2009-2015
    • Florida State University
      • Department of Physics
      Tallahassee, Florida, United States
    • University of Central Florida
      • Department of Physics
      Orlando, Florida, United States
  • 2010-2014
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 2003-2012
    • University of Florida
      • • Department of Chemistry
      • • Department of Physics
      Gainesville, FL, United States