Bis(1,2,3-trimethylindenyl)iron(III) 2,3-dicyanonaphtho-1,4-quinonide, a non-metallocene, charge-transfer salt metamagnet with complementary donor-acceptor geometries

Bis(1,2,3-trimethylindenyl)iron(III) can be paired with 2,3-dicyanonaphtho-1,4-quinone to give an air-stable pi-stacked metamagnet with T(c) = 4.1 K, the first example of a non-metallocene CT salt magnet in this class. A single crystal X-ray structure of the Fe(1,2,3-Me(3)C(9)H(4))(2)[DCNQ] salt indicates that it consists of stacks of alternating donors and acceptors, in which the DCNQ ring systems are aligned with indenyl groups above and below, and the long axes of the DCNQ and the indenyl ligand are parallel to each other. This arrangement suggests that the magnetic properties arise from favorable interactions due to geometric similarities between the donor and acceptor, and not from a unique property of the donor itself.     

The solution to a conjecture of Tits on the subgroup generated by the squares of the generators of an Artin group

Let A be an Artin group with standard generating set {σ _s :s∈S}. Tits conjectured that the only relations in A amongst the squares of the generators are consequences of the obvious ones, namely that σ _s ² and σ _t ² commute whenever σ _s and σ _t commute, for s,t∈S. In this paper we prove Tits’ conjecture for all Artin groups. In fact, given a number m _s ≥2 for each s∈S, we show that the elements {T _s =σ _s ^ms :s∈S} generate a subgroup that has a finite presentation in which the only defining relations are that T _s and T _t commute if σ _s and σ _t commute. Oblatum 21-III-2000 & 1-XII-2000?Published online: 5 March 2001     

Temperature hysteretic effect and its influence on colossal magnetoresistance of La0.33Nd0.33Ca0.33MnO3

Electrical resistance (R) measurements of a bulk La_0.33Nd_0.33Ca_0.33MnO₃ perovskite in magnetic fields up to 40 kOe have revealed anomalous temperature hysteretic effects both in 0 Oe and 20 kOe magnetic fields. The sharp peak observed in the R vs. T plot indicates the occurrence of metal-to-insulator (M-I) transition at a temperature of T _MI=110 K and 140 K, for cooling and warming paths, respectively. An applied magnetic field of 20 kOe reduces the resistance and shifts T _MI to 160 K and 185 K for cooling and warming, respectively. We have observed a much higher resistance in the cooling path than in the warming path leading to the hysteretic resistance ratio (R _cool/R _warm) of 200 at 110 K and 1.8 at 160 K for 0 Oe and 20 kOe, respectively. Record values of colossal magnetoresistance (CMR) have been achieved. The CMR value reaches nearly 99% in the temperature ranges of 90 K to 140 K and 90 K to 170 K for 20 kOe and 40 kOe magnetic fields in the cooling mode, respectively. The observed unusual behavior is attributed to the co-existence of La-rich and Nd-rich domains assumed to be distributed randomly in the compound.     

ac Conductivity of mixed spinel NiAl0.7Cr0.7Fe0.6O4

ac Conductivity measurements are carried out across the metal to insulator transition in NiAl_0.7Cr_0.7Fe_0.6O₄. The low frequency data is analyzed using Summerfield scaling theory for hopping conductivity. The exponent of the scaling behavior has significantly different values in the conducting and insulating regimes. The hopping frequency and the zero frequency conductivity are found to increase with temperature, slowly in the metallic regime and rapidly in the insulating regime.     

Two-Dimensional Artin Groups with CAT(0) Dimension Three

We exhibit 3-generator Artin groups which have finite two-dimensional Eilenberg-Mac Lane spaces, but which do not act properly discontinuously by semi-simple isometries on a two-dimensional CAT(0) complex. We prove that infinitely many of these groups are the fundamental groups of compact, non-positively curved 3-complexes. These examples show that the geometric dimension of a CAT(0) group may be strictly less than its CAT(0) dimension.     

Single and dual beam optical switching of resonance energy transfer

Through photonic mechanisms based on near-field coupling, laser radiation can engage with resonant energy transfer in a variety of suitably designed materials and molecular structures. Energy that has been acquired, through the initial absorption of resonant laser light, undergoes transfer between chromophores only on the throughput of off-resonant light, the process known as laser-assisted resonance energy transfer. The comprehensive results that are presented here extend and generalize the theory for both single and dual beam configurations, producing results that are applicable to media of various types including doped crystals, heterogeneous multichromophore solids, and solutions. The detailed principles, here explained in terms of both energetics and optical selection rule criteria, are specifically illustrated for a variety of materials. It is shown how general application of the theory can facilitate the elucidation of experiments, by clearly interpreting the effects of laser polarization manipulation. On further analysis of the photophysical mechanisms it is also demonstrated that such effects represent an entirely practicable basis for optical switching and logic gate operation. The additional polarization selectivity afforded by a two-beam setup proves to allow the most complete system control. With such a configuration, there is considerable promise for the realization of new optically driven logic and molecular devices.     

Quasi-isometrically embedded subgroups of braid and diffeomorphism groups

We show that a large class of right-angled Artin groups (in particular, those with planar complementary defining graph) can be embedded quasi-isometrically in pure braid groups and in the group of area preserving diffeomorphisms of the disk fixing the boundary (with respect to the -norm metric); this extends results of Benaim and Gambaudo who gave quasi-isometric embeddings of and for all . As a consequence we are also able to embed a variety of Gromov hyperbolic groups quasi-isometrically in pure braid groups and in the group . Examples include hyperbolic surface groups, some HNN-extensions of these along cyclic subgroups and the fundamental group of a certain closed hyperbolic 3-manifold.

     

On the theory of slowing down gracefully

We discuss the transport of a tracer particle through the Bose?CEinstein condensate of a Bose gas. The particle interacts with the atoms in the Bose gas through two-body interactions. In the limiting regime where the particle is very heavy and the Bose gas is very dense, but very weakly interacting (??mean-field limit??), the dynamics of this system corresponds to classical Hamiltonian dynamics. We show that, in this limit, the particle is decelerated by emission of gapless modes into the condensate (Cerenkov radiation). For an ideal gas, the particle eventually comes to rest. In an interacting Bose gas, the particle is decelerated until its speed equals the propagation speed of the Goldstone modes of the condensate. This is a model of ??Hamiltonian friction??. It is also of interest in connection with the phenomenon of ??decoherence?? in quantum mechanics. This note is based on work we have carried out in collaboration with D Egli, I M Sigal and A Soffer.     

Theory of Directed Electronic Energy Transfer

The migration of electronic energy between molecules or chromophores in molecular solids is a well-studied phenomenon. The ability to exert control over the directionality of this transfer, by a variety of methods involving applied electrical or optical fields, holds promise for advances in fields including nanoelectronics and energy harvesting materials. In this paper, we review in detail a number of methods for directing energy transfer, also identifying potential applications.