Multiple Ground States Research Articles

Enhanced fluorescence with nanosecond dynamics in the solid state of metal ion complexes of alkoxy salophens.

Salophen (N,N'-bis-(salicylidene)-o-phenylenediamine) is a Schiff base, which is weakly emissive in solution, but strongly emissive in the solid state. The trend is reversed in its aluminium complex (SalAl+), which is strongly emissive in solution, but feebly emissive in the solid state. Thus, SalAl+ is not attractive for application as a solid state emitter. In the present study, salophen derivatives containing -OCH3 and OC2H5 groups have been studied with the intent to perturb the structure and control the emission properties. The newly synthesized salophen derivatives are feebly emissive with ultrafast lifetime in solution. The mutiexponential nature of their fluorescence decay is ascribed to multiple ground state conformers of the enol and keto tautomers. In the solid phase, the ligands exhibit an order of magnitude increase in emission. Complexation with Al3+ (and Zn2+) does not bring about any significant increase in emissivity in solution, possibly because rotation/vibration of the alkoxy groups (with a contribution from axial water molecules) now provides the major nonradiative deactivation pathway. Such pathways are blocked in the solid state, leading to an increase in emission of up to 50 times in the aluminium complex. Thus, introduction of small functionalities is found to have profound effects on the suitability of a molecule as a solid state emitter.

Diversity of Secondary Structure in Catalytic Peptides with β-Turn-Biased Sequences.

X-ray crystallography has been applied to the structural analysis of a series of tetrapeptides that were previously assessed for catalytic activity in an atroposelective bromination reaction. Common to the series is a central Pro-Xaa sequence, where Pro is either l- or d-proline, which was chosen to favor nucleation of canonical β-turn secondary structures. Crystallographic analysis of 35 different peptide sequences revealed a range of conformational states. The observed differences appear not only in cases where the Pro-Xaa loop-region is altered, but also when seemingly subtle alterations to the flanking residues are introduced. In many instances, distinct conformers of the same sequence were observed, either as symmetry-independent molecules within the same unit cell or as polymorphs. Computational studies using DFT provided additional insight into the analysis of solid-state structural features. Select X-ray crystal structures were compared to the corresponding solution structures derived from measured proton chemical shifts, 3J-values, and 1H–1H-NOESY contacts. These findings imply that the conformational space available to simple peptide-based catalysts is more diverse than precedent might suggest. The direct observation of multiple ground state conformations for peptides of this family, as well as the dynamic processes associated with conformational equilibria, underscore not only the challenge of designing peptide-based catalysts, but also the difficulty in predicting their accessible transition states. These findings implicate the advantages of low-barrier interconversions between conformations of peptide-based catalysts for multistep, enantioselective reactions.

Evolution of thermodynamic properties and inelastic neutron scattering intensities for spin-12antiferromagnetic quantum rings

This study examines the increasing complexity in the magnetic properties of small $n$ = 3, 4, 5, 6 spin-1/2 quantum rings. Using an exact diagonalization of the isotropic Heisenberg Hamiltonian with nearest and next-nearest neighbor interactions, the energy eigenstates, magnetic specific heat capacity, magnetic susceptibility, and inelastic neutron scattering structure factors are determined for variable next-nearest neighbor interactions. Here, it is shown that the presence of a complex spin-mixing, multiple ground states, and non-zero ground states greatly complicate the spin Hamiltonian. Overall, the energy eigenstates and structure factor intensities are presented in closed form, while the thermodynamic properties detail the effect of a crossing interaction in the rings. The goal of this work is to provide insight into the evolution of the magnetic properties and spin excitations within these systems.

Magnetic ground state of an individual Fe(2+) ion in strained semiconductor nanostructure.

Single impurities with nonzero spin and multiple ground states offer a degree of freedom that can be utilized to store the quantum information. However, Fe2+ dopant is known for having a single nondegenerate ground state in the bulk host semiconductors and thus is of little use for spintronic applications. Here we show that the well-established picture of Fe2+ spin configuration can be modified by subjecting the Fe2+ ion to high strain, for example, produced by lattice mismatched epitaxial nanostructures. Our analysis reveals that high strain induces qualitative change in the ion energy spectrum and results in nearly doubly degenerate ground state with spin projection Sz=±2. We provide an experimental proof of this concept using a new system: a strained epitaxial quantum dot containing individual Fe2+ ion. Magnetic character of the Fe2+ ground state in a CdSe/ZnSe dot is revealed in photoluminescence experiments by exploiting a coupling between a confined exciton and the single-iron impurity. We also demonstrate that the Fe2+ spin can be oriented by spin-polarized excitons, which opens a possibility of using it as an optically controllable two-level system free of nuclear spin fluctuations.

19F NMR and DFT Analysis Reveal Structural and Electronic Transition State Features for RhoA‐Catalyzed GTP Hydrolysis

AbstractMolecular details for RhoA/GAP catalysis of the hydrolysis of GTP to GDP are poorly understood. We use 19F NMR chemical shifts in the MgF3− transition state analogue (TSA) complex as a spectroscopic reporter to indicate electron distribution for the γ‐PO3− oxygens in the corresponding TS, implying that oxygen coordinated to Mg has the greatest electron density. This was validated by QM calculations giving a picture of the electronic properties of the transition state (TS) for nucleophilic attack of water on the γ‐PO3− group based on the structure of a RhoA/GAP‐GDP‐MgF3− TSA complex. The TS model displays a network of 20 hydrogen bonds, including the GAP Arg85′ side chain, but neither phosphate torsional strain nor general base catalysis is evident. The nucleophilic water occupies a reactive location different from that in multiple ground state complexes, arising from reorientation of the Gln‐63 carboxamide by Arg85′ to preclude direct hydrogen bonding from water to the target γ‐PO3− group.

(19)F NMR and DFT Analysis Reveal Structural and Electronic Transition State Features for RhoA-Catalyzed GTP Hydrolysis.

Molecular details for RhoA/GAP catalysis of the hydrolysis of GTP to GDP are poorly understood. We use 19F NMR chemical shifts in the MgF3 − transition state analogue (TSA) complex as a spectroscopic reporter to indicate electron distribution for the γ‐PO3 − oxygens in the corresponding TS, implying that oxygen coordinated to Mg has the greatest electron density. This was validated by QM calculations giving a picture of the electronic properties of the transition state (TS) for nucleophilic attack of water on the γ‐PO3 − group based on the structure of a RhoA/GAP‐GDP‐MgF3 − TSA complex. The TS model displays a network of 20 hydrogen bonds, including the GAP Arg85′ side chain, but neither phosphate torsional strain nor general base catalysis is evident. The nucleophilic water occupies a reactive location different from that in multiple ground state complexes, arising from reorientation of the Gln‐63 carboxamide by Arg85′ to preclude direct hydrogen bonding from water to the target γ‐PO3 − group.

Modeling magneto-optical trapping of CaF molecules

Magneto-optical trapping forces for molecules are far weaker than for alkali atoms because the photon scattering rate is reduced when there are multiple ground states, and because of optical pumping into dark states. The force is further reduced when the upper state has a much smaller Zeeman splitting than the lower state. We use a rate model to estimate the strength of the trapping and damping forces in a magneto-optical trap (MOT) of CaF molecules, using either the A$^{2}\Pi_{1/2}$ - X$^{2}\Sigma^{+}$ transition or the B$^{2}\Sigma^{+}$ - X$^{2}\Sigma^{+}$ transition. We identify a new mechanism of magneto-optical trapping that arises when, in each beam of the MOT, two laser components with opposite polarizations and different detunings address the same transition. This mechanism produces a strong trapping force even when the upper state has little or no Zeeman splitting. It is the main mechanism responsible for the trapping force when the A$^{2}\Pi_{1/2}$ - X$^{2}\Sigma^{+}$ transition is used.

Glass-Like Slow Dynamics in a Colloidal Solid with Multiple Ground States.

We study the phase-ordering dynamics of a 2D model colloidal solid using molecular dynamics simulations. The colloid particles interact with each other with a Hamaker potential modified by the presence of equatorial "patches" of attractive and repulsive regions. The total interaction potential between two such colloids is, therefore, strongly directional and has a 3-fold symmetry. Working in the canonical ensemble, we determine the phase diagram in the density-temperature plane. We obtain three distinct crystalline ground states, viz., a low density honeycomb solid, a rectangular solid at intermediate density, and finally a high-density triangular structure. We show that when cooled rapidly from the liquid phase along iso-chores, the system undergoes a transition to a "strong glass", while slow cooling gives rise to crystalline phases. We claim that geometrical frustration arising from the presence of many competing crystalline ground states causes glassy ordering and dynamics in this solid. Our results may be easily confirmed by suitable experiments on patchy colloids.

The structural, electronic and magnetic properties of Ga8−xMnxAs8 clusters

We systematically investigate the ground-state magnetic properties of Ga8−xMnxAs8 clusters (x=0, 2, 4, 6, and 8) within the framework of density functional theory (DFT) using a strategy that successively adopts the particle swarm optimization (CALYPSO) code and fixed spin-moment (FSM) method. The results show that for Ga8−xMnxAs8 in the ground states or low-lying isomers, Mn atoms tend to assemble at the core of the clusters and the ferrimagnetic Mn–Mn couplings are identified for Ga8−xMnxAs8 (x=4, 6, and 8), while Ga8As8 and Ga6Mn2As8 are nonmagnetic. The possibility of multiple ground states of Ga8−xMnxAs8 (x=4, 6, and 8) is also demonstrated. The binding energy and LUMO–HOMO gap analysis show that Ga8−xMnxAs8 clusters with large x are more likely synthesized and exhibit stronger chemical reactivity.

Oxygen-driven anisotropic transport in ultra-thin manganite films

Transition metal oxides have a range of unique properties due to coupling of charge, spin, orbital and lattice degrees of freedom and nearly degenerate multiple ground states. These properties make them interesting for applications and for fundamental investigations. Here we report a new phase with abnormal transport anisotropy in La0.7Sr0.3MnO3 ultra-thin films under large tensile strain. This anisotropy is absent in films under smaller tensile strain or compressive strain. Furthermore, thickness and magnetic-field-dependent experiments suggest that the tensile-strain-induced two-dimensional character is crucial for the observed phenomena. X-ray absorption spectroscopy results indicate that this anisotropy is likely driven by O 2p orbital, which hybridizes with Mn 3d. Ab initio calculations confirm this result. Our results may help to understand the anisotropic transport behaviour observed in other systems.

Models of three-dimensional fractional topological insulators

Time-reversal invariant three-dimensional topological insulators can be defined fundamentally by a topological field theory with a quantized axion angle $\ensuremath{\theta}$ of 0 or $\ensuremath{\pi}$. It was recently shown that fractional quantized values of $\ensuremath{\theta}$ are consistent with time-reversal invariance if deconfined, gapped, fractionally charged bulk excitations appear in the low-energy spectrum due to strong correlation effects, leading to the concept of a fractional topological insulator. These fractionally charged excitations are coupled to emergent gauge fields, which ensure that the microscopic degrees of freedom, the original electrons, are gauge-invariant objects. A first step towards the construction of microscopic models of fractional topological insulators is to understand the nature of these emergent gauge theories and their corresponding phases. In this work, we show that low-energy effective gauge theories of both Abelian or non-Abelian type are consistent with a fractional quantized axion angle if they admit a Coulomb phase or a Higgs phase with gauge group broken down to a discrete subgroup. The Coulomb phases support gapless but electrically neutral bulk excitations while the Higgs phases are fully gapped. The Higgs and non-Abelian Coulomb phases exhibit multiple ground states on boundaryless spatial three-manifolds with nontrivial first homology, while the Abelian Coulomb phase has a unique ground state. The ground-state degeneracy receives an additional contribution on manifolds with boundary due to the induced boundary Chern-Simons term.

Static disorder and local structure in zinc(II) isonicotinate, a quartzlike metal–organic framework

Using a combination of Rietveld and reverse Monte Carlo (RMC) refinement of neutron total scattering data, we find that the 10 K structure of zinc(II) isonicotinate shows strong evidence of static disorder. This disorder takes the form of transverse displacements of the isonicotinate ligand and results in elongated atomic displacement parameters and dampening of the experimental G(r). We analyse the RMC configurations using an approach derived from geometric algebra. Complications regarding the inclusion of hydrogenous guest molecules within the pore structure are discussed. This study highlights the way in which structural flexibility can give rise to multiple low-energy ground states in metal-organic framework materials.

Magnetic properties of Co1+yAl2−yO4–SiO2 nanocomposites synthesized by sol–gel method

Nanocomposites made of magnetic spinel oxide Co1+yAl2−yO4 particles dispersed in an amorphous SiO2 matrix were synthesized by a sol–gel technique. Samples with various mass fractions of Co1+yAl2−yO4 in composite x, and with various Co concentrations in spinel y were studied. The grain sizes were below 100 nm with a large size distribution. The paper represents thorough investigation of magnetic properties, but special attention was given to peculiar effects and low temperature magnetic frozen states. Magnetic measurements reveal multiple magnetic ground states at the lowest temperature. The frequency dependent ac susceptibility measurements indicate spin-glass behavior of these ground states at low temperatures. The samples, with higher concentration x of Co1+yAl2−yO4 in SiO2, showed very similar Curie–Weiss paramagnetic behavior with antiferromagnetically interacting Co ions (θ ≈ −100 K). Other samples, with lower x, showed complex behavior, spin-glass magnetic freezing at the lowest temperatures and lower absolute value of θ and consequently lower exchange constant.

Emerging Chirality in Artificial Spin Ice

Artificial spin ice, made up of planar nanostructured arrays of simple ferromagnetic bars, is a playground for rich physics associated with the spin alignment of the bars and spin texture associated with the magnetic frustration at the bar vertices. The phase diagram is exotic, showing magnetic monopole-like defects and liquid and solid phases of spins arranged in loop states with predicted chiral order. We show that magnetotransport measurements in connected honeycomb structures yield the onset of an anomalous Hall signal at 50 kelvin. The temperature scale can be attributed to the long-range dipolar ice phase. The topological Hall signal arises because chiral loops form at the sample edges, indicating a generic route to exotic states via nanoarray edge structure.

On the space of symmetric operators with multiple ground states

We study the homology structure of the filtration of the space of self-adjoint operators by the multiplicity of the ground state. We consider only operators acting on a finite-dimensional complex or real Hilbert space, but infinite-dimensional generalizations are easy to guess.

On the Nature of the Multiple Ground States of the MMX Mixed-Valence Chain Compound, [PtII/III2(n-PenCS2)4I]∞

We present a comprehensive study of the temperature dependence of the crystal structure using single-crystal X-ray diffraction and diffuse scattering, and electrical transport and magnetic properties as well as some optical properties at room temperature to elucidate the origin and the form of multiple ground states demonstrated in a previous study of the heat-capacity of the MMX chain compound, [Pt(II/III)(2)(n-PenCS(2))(4)I](∞). The present results confirm the presence of the two phase transitions, one reversible of first order at 207 K and the other nonreversible monotropic at 324 K, separating the low temperature (LT), room temperature (RT), and high temperature (HT) phases. The unit cell displays a 3-fold periodicity of -Pt-Pt-I- in the RT and HT phases because of the structural disorder which is exhibited by the dithiocarboxylato groups and the n-pentyl groups belonging to the central diplatinum unit. In addition, for the HT-phase all the dimers show this disorder. This compound undergoes a metal-semiconductor transition at T(M-S) = 235 K. The presence of diffuse streaks corresponding to 2-fold -Pt-Pt-I- periodicity in the HT and RT phases indicates dynamic valence ordering of the type -Pt(2+)-Pt(2+)-I(-)-Pt(3+)-Pt(3+)-I(-)-or-Pt(2+)-Pt(3+)-I(-)-Pt(3+)-Pt(2+)-I(-)-. For the LT-phase the diffuse scattering is condensed into clear Bragg diffraction peaks while keeping the 3-fold periodicity. This fact suggests further localization through dimerization of charges and spins confirming the diamagnetic state in the magnetic susceptibility and the low electrical conduction below 207 K. The present results are further discussed in relation to those of previous studies on the homologues, [Pt(II/III)(2)(RCS(2))(4)I](∞), R = methyl, ethyl, n-propyl, and n-butyl.

Scattering theory of multilevel atoms interacting with arbitrary radiation fields

We present a generic transfer matrix approach for the description of the interaction of atoms possessing multiple ground state and excited state sublevels with light fields. This model allows us to treat multilevel atoms as classical scatterers in light fields modified by, in principle, arbitrarily complex optical components such as mirrors, resonators, dispersive and dichroic elements, and filters. We verify our formalism for two prototypical sub-Doppler cooling mechanisms and show that it agrees with the standard literature.

Structure and Photoinduced Electron Transfer in DNA Hairpin Conjugates Possessing a Tethered 5′-Pyrenecarboxamide

The structure, spectroscopy, and photophysical behavior of a series of hairpin-forming conjugates possessing a 5'-tethered N-alkylpyrenecarboxamide chromophore have been investigated. Comparison of the NMR spectra of the conjugates and analogs lacking the tethered pyrene indicates that the pyrene does not behave as an end-capping group but rather is intercalated between the two terminal hairpin base pairs. An intercalated structure is also consistent with the thermodynamic parameters for hairpin formation and the steady state and transient spectral properties of the conjugates. Quenching of the pyrene fluorescence and transient absorption spectra is observed only when guanine is located in one of the two terminal base pairs and is attributed to hole injection from singlet pyrene to guanine. The fast component of the transient decay is more rapid when guanine is located in the second vs first base pair, consistent with an intercalated but not an end-capped geometry. Spectral broadening of ultraviolet, fluorescence, and transient absorption spectra is attributed to multiple ground state conformations.

Spin-driven ferroelectricity in the multiferroic compounds of [formula omitted

Neutron diffraction studies under magnetic field as well as under hydrostatic pressure for microscopic magnetism on multiferroic RMn 2 O 5 ( R = rare-earth , Bi, and Y) are reviewed to discuss about the relevance between the magnetic property and dielectric property in detail. Without any external fields, a series of RMn 2 O 5 shows successive magnetic phase transitions of incommensurate–commensurate–incommensurate as temperature decreases, in which the dielectric phase transitions concomitantly occur. At the lowest temperature phase of HoMn 2 O 5 , the magnetic transition from the incommensurate phase to the commensurate one is induced by applying both magnetic field and hydrostatic pressure. At this field-induced magnetic transition, a spontaneous electric polarization simultaneously induced, indicating that the ferroelectricity in this material is magnetically controlled. Competition of multiple magnetic ground states due to intensional magnetic frustration in this system can be easily tuned by applying external fields, which give rise to the rich variety of spin-driven dielectric transitions.

Metric gravity theories and cosmology: II. Stability of a ground state in f(R) theories

In the second part of the investigation of metric nonlinear gravity theories, we study a fundamental criterion of viability of any gravity theory: the existence of a stable ground-state solution being either Minkowski, de Sitter or anti-de Sitter space. Stability of the ground state is independent of which frame is physical. In general, a given theory has multiple ground states and splits into independent physical sectors. The fact that all L = f(gαβ, Rμν) gravity theories (except some singular cases) are dynamically equivalent to Einstein gravity plus a massive spin-2 and a massive scalar field allows us to investigate the stability problem using methods developed in general relativity. These methods can be directly applied to L = f(R) theories wherein the spin-2 field is absent. Furthermore for these theories which have anti-de Sitter space as the ground state we prove a positive-energy theorem allowing to define the notion of conserved total gravitational energy in the Jordan frame (i.e., for the fourth-order equations of motion). As is shown in 13 examples of specific Lagrangians the stability criterion works effectively without long computations whenever the curvature of the ground state is determined. An infinite number of gravity theories have a stable ground state and further viability criteria are necessary.