Coupled multiferroic domain switching in the canted conical spin spiral system Mn2GeO4.

Despite remarkable progress in developing multifunctional materials, spin-driven ferroelectrics featuring both spontaneous magnetization and electric polarization are still rare. Among such ferromagnetic ferroelectrics are conical spin spiral magnets with a simultaneous reversal of magnetization and electric polarization that is still little understood. Such materials can feature various multiferroic domains that complicates their study. Here we study the multiferroic domains in ferromagnetic ferroelectric Mn2GeO4 using neutron diffraction, and show that it features a double-Q conical magnetic structure that, apart from trivial 180o commensurate magnetic domains, can be described by ferromagnetic and ferroelectric domains only. We show unconventional magnetoelectric couplings such as the magnetic-field-driven reversal of ferroelectric polarization with no change of spin-helicity, and present a phenomenological theory that successfully explains the magnetoelectric coupling. Our measurements establish Mn2GeO4 as a conceptually simple multiferroic in which the magnetic-field-driven flop of conical spin spirals leads to the simultaneous reversal of magnetization and electric polarization.

Multiple species of noninteracting molecules adsorbed on a Bethe lattice.

A simple method, previously used to calculate the equilibrium concentration of dimers adsorbed on a Bethe lattice as a function of the dimer activity, is generalized to solve the problem of a Bethe lattice in contact with a reservoir containing a mixture of molecules. The molecules may have arbitrary sizes and shapes consistent with the geometry of the lattice and the molecules do not interact with one another except for the hard-core restriction that two molecules cannot touch the same site. We obtain a set of simultaneous nonlinear equations, one equation for each species of molecule, which determines the equilibrium concentration of each type of molecule as a function of the (arbitrary) activities of the various species. Surprisingly, regardless of the number of species, the equilibrium concentrations are given explicitly in terms of the solution of a single equation in one unknown which can be solved numerically, if need be. Some numerical examples show that increasing the activity of one species need not necessarily decrease the equilibrium concentration of all other species. We also calculate the adsorption isotherm of an "annealed" Bethe lattice consisting of two types of sites which differently influence the activity of an adsorbed molecule. We prove that if the reservoir contains a finite number of molecular species, regions of two different polymer densities cannot simultaneously exist on the lattice. The widely used Guggenheim theory of mixtures, which can also be construed as a theory of adsorption, assumes for simplicity that the molecules in the mixture are composed of elementary units, which occupy sites of a lattice of coordination number q . Guggenheim's analysis relies on approximate combinatorial formulas which become exact on a Bethe lattice of the same coordination number, as we show in an appendix. Our analysis involves no combinatorics and relies only on recognizing the statistical independence of certain quantities. Despite the nominal equivalence of the two approaches, the easily visualized properties of the Bethe lattice enable one to solve some apparently difficult problems by quite elementary methods.

Order parameters and phase diagram of multiferroic RMn2O5.

The generic magnetic phase diagram of multiferroic RMn2O5 (with R=Y, Ho, Tb, Er, Tm), which allows different sequences of ordered magnetic structures for different R's and different control parameters, is described using order parameters which explicitly incorporate the magnetic symmetry. A phenomenological magnetoelectric coupling is used to explain why some of these magnetic phases are also ferroelectric. Several new experiments, which can test this theory, are proposed.

Direct transition from a disordered to a multiferroic phase on a triangular lattice.

We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet RbFe(MoO4)(2). Ferroelectricity is observed only when the magnetic structure has chirality and breaks inversion symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that chiral magnetic order can give rise to a pseudoelectric field, whose temperature dependence agrees with experiment.

Dimer statistics on a Bethe lattice.

We discuss the exact solutions of various models of the statistics of dimer coverings of a Bethe lattice. We reproduce the well-known exact result for noninteracting hard-core dimers by both a very simple geometrical argument and a general algebraic formulation for lattice statistical problems. The algebraic formulation enables us to discuss loop corrections for finite dimensional lattices. For the Bethe lattice we also obtain the exact solution when either (a) the dimers interact via a short-range interaction or (b) the underlying lattice is anisotropic. We give the exact solution for a special limit of dimers on a Bethe lattice in a quenched random potential in which we consider the maximal covering of dimers on random clusters at site occupation probability p. Surprisingly the partition function for "maximal coverage" on the Bethe lattice is identical to that for the statistics of branched polymers when the activity for a monomer unit is set equal to -p. Finally we give an exact solution for the number of residual vacancies when hard-core dimers are randomly deposited on a one dimensional lattice.

Spin waves in the frustrated kagomé lattice antiferromagnet KFe3(OH)6(SO4)2.

The spin wave excitations of the S=5/2 kagomé lattice antiferromagnet KFe3(OH)6(SO4)2 have been measured using high-resolution inelastic neutron scattering. We directly observe a flat mode which corresponds to a lifted "zero energy mode," verifying a fundamental prediction for the kagomé lattice. A simple Heisenberg spin Hamiltonian provides an excellent fit to our spin wave data. The antisymmetric Dzyaloshinskii-Moriya interaction is the primary source of anisotropy and explains the low-temperature magnetization and spin structure.

1/d expansion for k-core percolation.

The physics of k-core percolation pertains to those systems whose constituents require a minimum number of k connections to each other in order to participate in any clustering phenomenon. Examples of such a phenomenon range from orientational ordering in solid ortho-para H2 mixtures to the onset of rigidity in bar-joint networks to dynamical arrest in glass-forming liquids. Unlike ordinary (k = 1) and biconnected (k = 2) percolation, the mean field k > or = 3-core percolation transition is both continuous and discontinuous, i.e., there is a jump in the order parameter accompanied with a diverging length scale. To determine whether or not this hybrid transition survives in finite dimensions, we present a 1/d expansion for k-core percolation on the d-dimensional hypercubic lattice. We show that to order 1/d3 the singularity in the order parameter and in the susceptibility occur at the same value of the occupation probability. This result suggests that the unusual hybrid nature of the mean field k-core transition survives in high dimensions.

Magnetically driven ferroelectric order in Ni3V2O8.

We show that long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition in Ni3V2O8. The temperature and magnetic-field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry using Landau theory for continuous phase transitions, which shows that the spin structure alone can break spatial inversion symmetry leading to ferroelectric order. This phenomenological theory explains our experimental observation that the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter.

Magnetic inversion symmetry breaking and ferroelectricity in TbMnO3.

TbMnO3 is an orthorhombic insulator where incommensurate spin order for temperature T(N)<41 K is accompanied by ferroelectric order for T<28 K. To understand this, we establish the magnetic structure above and below the ferroelectric transition using neutron diffraction. In the paraelectric phase, the spin structure is incommensurate and longitudinally modulated. In the ferroelectric phase, however, there is a transverse incommensurate spiral. We show that the spiral breaks spatial inversion symmetry and can account for magnetoelectricity in TbMnO3.

Cell proliferation rates in an artificial tissue-engineered environment.

Worldwide, and particularly in Europe, Japan and the USA, cardiovascular disease is a major killer. It can be treated using tissue or organ transplant surgery, but donor organs may be scarce. Tissue engineering is the integration of engineering principles and biology to produce satisfactory synthetic replacement body parts, using viable cells in a suitable matrix, for regenerative medicine. The aim of this study was to measure and compare cell proliferation kinetics after different time intervals of myofibroblasts in a synthetic matrix, thus to be able to deduce the period that a transplanted-cell population can be expected to survive in a tissue-engineered environment. Porcine aortic wall cells were grown in a porous sponge scaffold, that later could be fashioned into aortic or heart valve substitutes. Freshly acquired cells were seeded on identical sponges and were grown under normal culture conditions for a period of 4 weeks. Seeding concentration was a million cells per sponge. Cells progressively populated the sponges, both covering the surface and infiltrating the depth of the matrix, via sponge pores. Samples were taken at 1 week and at 4 weeks, and the rate of cell proliferation was determined by the metaphase arrest technique. Specimens were also taken for light and electron microscopy to determine whether these transplanted cells were capable of synthesizing their own extracellular matrix.

Competing magnetic phases on a kagomé staircase.

We present thermodynamic and neutron data on Ni3V2O8, a spin-1 system on a kagomé staircase. The extreme degeneracy of the kagomé antiferromagnet is lifted to produce two incommensurate phases at finite T--one amplitude modulated, the other helical--plus a commensurate canted antiferromagnet for T-->0. The H-T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni3V2O8 thus provides an elegant example of order from subleading interactions in a highly frustrated system.

Unusual symmetries in the Kugel-Khomskii Hamiltonian.

The Kugel-Khomskii Hamiltonian for cubic titanates describes spin and orbital superexchange interactions between d(1) ions having threefold degenerate t(2g) orbitals. Since orbitals do not couple along "inactive" axes, perpendicular to the orbital planes, the total number of electrons in |alpha> orbitals in any such plane and the corresponding total spin are both conserved. A Mermin-Wagner construction shows that there is no long-range spin ordering at nonzero temperatures. Inclusion of spin-orbit coupling allows such ordering, but even then the excitation spectrum is gapless due to a continuous symmetry. Thus, the observed order and gap require more symmetry breaking terms.

Heart valve and arterial tissue engineering.

In the industrialized world, cardiovascular disease alone is responsible for almost half of all deaths. Many of the conditions can be treated successfully with surgery, often using transplantation techniques; however, autologous vessels or human-donated organs are in short supply. Tissue engineering aims to create specific, matching grafts by growing cells on appropriate matrices, but there are many steps between the research laboratory and the operating theatre. Neo-tissues must be effective, durable, non-thrombogenic and non-immunogenic. Scaffolds should be bio-compatible, porous (to allow cell/cell communication) and amenable to surgery. In the early days of cardiovascular tissue engineering, autologous or allogenic cells were grown on inert matrices, but patency and thrombogenicity of grafts were disappointing. The current ethos is toward appropriate cell types grown in (most often) a polymeric matrix that degrades at a rate compatible with the cells' production of their own extracellular matrical proteins, thus gradually replacing the graft with a living counterpart. The geometry is crucial. Computer models have been made of valves, and these are used as three-dimensional patterns for mass-production of implant scaffolds. Vessel walls have integral connective tissue architecture, and application of physiological level mechanical forces conditions bio-engineered components to align in precise orientation. This article reviews the concepts involved and successes achieved to date.

Prolonged exercise induces angiogenesis and increases cerebral blood volume in primary motor cortex of the rat.

Plastic changes in motor cortex capillary structure and function were examined in three separate experiments in adult rats following prolonged exercise. The first two experiments employed T-two-star (T(2)*)-weighted and flow-alternating inversion recovery (FAIR) functional magnetic resonance imaging to assess chronic changes in blood volume and flow as a result of exercise. The third experiment used an antibody against the CD61 integrin expressed on developing capillaries to determine if motor cortex capillaries undergo structural modifications. In experiment 1, T(2)*-weighted images of forelimb regions of motor cortex were obtained following 30 days of either repetitive activity on a running wheel or relative inactivity. The proton signal intensity was markedly reduced in the motor cortex of exercised animals compared with that of controls. This reduction was not attributable to alterations of vascular iron levels. These results are therefore most consistent with increased capillary perfusion or blood volume of forelimb regions of motor cortex. FAIR images acquired during experiment 2 under normocapnic and hypercapnic conditions indicated that resting cerebral blood flow was not altered under normal conditions but was elevated in response to high levels of CO(2), suggesting that prolonged exercise increases the size of a capillary reserve. Finally, the immunohistological data indicated that exercise induces robust growth of capillaries (angiogenesis) within 30 days from the onset of the exercise regimen. Analysis of other regions failed to find any changes in perfusion or capillary structure suggesting that this motor activity-induced plasticity may be specific to motor cortex.These data indicate that capillary growth occurs in motor areas of the cerebral cortex as a robust adaptation to prolonged motor activity. In addition to capillary growth, the vascular system also experiences heightened flow under conditions of activation. These changes are chronic and observable even in the anesthetized animal and are measurable using noninvasive techniques.

Rat pancreatic acinar cells express a cytosolic phospholipase D1b isoform that is not regulated by cholecystokinin.

Evidence for the presence of a regulated phospholipase D (PLD) activity in pancreatic acinar cells is conflicting. Such knowledge is important because signal-activated PLD has been implicated in, amongst other things, regulated exocytosis. In this study, freshly isolated rat pancreatic acini were used to identify PLD transcripts by RT-PCR, to assess the presence and subcellular localization of PLD protein by Western blotting and to evaluate the presence of secretagogue-regulated PLD activity by means of the PLD-catalysed transphosphatidylation reaction. Transcripts of PLD1b and PLD2, but not PLD1a, were present in acinar cells. Moreover, a specific anti-human PLD1 antibody demonstrated the expression of substantial amounts of PLD1 protein. Intriguingly, however, the distribution pattern of acinar PLD1 seen following subcellular fractionation was clearly atypical in that immunoreactivity occurred predominantly in the acinar cytosol. Pretreatment of intact acini with a phorbol ester (4beta-phorbol 12-myristate 13-acetate, PMA) to activate PLD1 protein kinase C (PKC) dependently did not change the subcellular distribution of PLD1. Similarly, pretreatment of a broken cell preparation of acini with guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) to activate PLD via small GTPases and PMA also did not influence this distribution. In the presence of ethanol, cholecystokinin-(26-33)-peptide amide (CCK8) did not increase the amount of radiolabelled phosphatidylethanol (PtdEth) in intact acini prelabelled with either o-[32P]phosphate or [3H]myristic acid. Similarly, an increased cytosolic Ca2+ concentration evoked by the specific inhibitor of the endoplasmic reticulum Ca2+-ATPase, thapsigargin, did not stimulate acinar PLD activity whereas high-level PKC activation with PMA elicited slight stimulation. In contrast, all three stimuli are known to increase PLD activity readily in Chinese hamster ovary (CHO) cells expressing the rat pancreatic acinar cell CCKA receptor. Finally, the combination of PMA and GTPgammaS did not increase PLD activity following homologous reconstitution of acinar cytosol and membranes, whereas the same manoeuvre resulted in marked stimulation of PLD activity in CHO cells. Heterologous reconstitution experiments revealed that PLD activity in CHO membranes was stimulated readily in the presence of acinar cytosol, indicating that the acinar cytosol contains the necessary factors for PMA/GTPgammaS-induced stimulation of membrane PLD activity. In contrast, CHO cell cytosol did not confer PMA/GTPgammaS-stimulation of PLD activity on acinar membranes, in agreement with the predominantly cytosolic localization of acinar PLD. The present findings show that rat pancreatic acinar cells express a cytosolic PLD1 isoform that is not regulated by the physiologically important secretagogue CCK.

Two-stage rotational disordering of a molecular crystal surface: C60.

We propose a two-stage mechanism for the rotational surface disordering phase transition of a molecular crystal, as realized in C60 fullerite. Our study, based on Monte Carlo simulations, uncovers the existence of a new intermediate regime, between a low-temperature ordered (2x2) state, and a high-temperature (1x1) disordered phase. In the intermediate regime there is partial disorder, strongest for a subset of particularly frustrated surface molecules. These concepts and calculations provide a coherent understanding of experimental observations, with possible extension to other molecular crystal surfaces.

Recurrent intracranial Masson's vegetant intravascular hemangioendothelioma. Case report and review of the literature.

In the central nervous system, recurrence of intracranial Masson's vegetant intravascular hemangioendothelioma (MVIH) is rare. To the authors' knowledge, only three recurrent intracranial cases have been reported. The authors report the case of a 75-year-old woman with a recurrent left-sided cerebellopontine angle and middle cranial fossa MVIH. When the patient was 62 years of age, she underwent preoperative embolization and subtotal resection of the intracranial lesion followed by postoperative radiotherapy. She was well and free from disease until 9 years postoperatively when she became symptomatic. At 71 years of age, the patient again underwent preoperative embolization and near-gross-total resection of the lesion. Follow-up imaging performed 15 months later revealed tumor recurrence, and she underwent stereotactic gamma knife radiosurgery. At a 2.75-year follow-up review, the patient's imaging studies revealed stable residual tumor. This case report is unique in that it documents the clinical and pathological features, surgical and postoperative treatment, and long-term follow-up review of a patient with recurrent intracranial MVIH and suggests that this unusual vascular lesion is a slow-growing benign tumor rather than a reactive process. Because the pathological composition of the lesion may resemble an angiosarcoma, understanding this benign vascular neoplasm is crucial so that an erroneous diagnosis of malignancy is not made and unnecessary adjuvant therapy is not given.

Quantum theory of chiral interactions in cholesteric liquid crystals.

The effective chiral interaction between molecules arising from long-range quantum interactions between fluctuating charge moments is analyzed in terms of a simple model of chiral molecules. This model is based on the approximations that (a) the dominant excited states of a molecule form a band whose width is small compared to the average energy of excitation above the ground state and (b) biaxial orientational correlation between adjacent molecules can be neglected. Previous treatments of quantum chiral interactions have been based on a multipole expansion of the effective interaction energy within second-order perturbation theory. We consider a system consisting of elongated molecules and, although we invoke the expansion in terms of coordinates transverse to the long axis of constituent molecules, we treat the longitudinal coordinate exactly. Such an approximation is plausible for molecules in real liquid crystals. The macroscopic cholesteric wave vector Q (Q=2 pi/P, where P is the pitch) is obtained via Q=h/K(2), where K2 is the Frank elastic constant for twist and h is the torque field which we calculate from the effective chiral interaction kappa(IJ)a(I)xa(J) x R(IJ), where the unit vector a(I) specifies the orientation of molecule I and R(IJ) is the displacement of molecule I relative to molecule J. We identify two distinct physical limits depending on whether one or both of the interacting molecules are excited in the virtual state. When both molecules are excited, we regain the R(-8)(IJ) dependence of kappa(IJ) on intermolecular separation found previously by Van der Meer et al. [J. Chem. Phys. 65, 3935 (1976)]. The two-molecule, unlike the one-molecule term, can be interpreted in terms of a superposition of pairwise interactions between individual atoms (or local chiral centers) on the two molecules. Contributions to kappa(IJ) when one molecule is excited in the virtual state are of order R(-7)(IJ) for helical molecules which are assumed not to have a global dipole moment, but whose atoms possess a dipole moment. It is shown that for a helical molecule Q can have either the same or the opposite sign as the chiral pitch of an individual molecule, depending on the details of the anisotropy of the atomic polarizability. The one-molecule mechanism can become important when the local atomic dipoles become sizable, although biaxial correlations (ignored here) should then be taken into account. Our results suggest how the architecture of molecular dipole moments might be adjusted to significantly influence the macroscopic pitch.

Surface enhanced Raman spectroscopic monitor of P. acnes lipid hydrolysis in vitro.

Surface enhanced Raman spectroscopy (SERS) at a silver microelectrode was used to monitor bacterial hydrolysis of triglycerides in lipid mixtures that model sebaceous gland secretions. Mixtures of wax esters, squalene, triolein, and triisostearin were used as model skin secretions. The transformation was followed in vitro as changes in the SERS caused by hydrolysis of triglyceride to fatty acid. The fatty acid was adsorbed as its carboxylate, which is readily identified by the characteristic band at ca. 1395 cm(-1). Co-adsorption of propionate was also observed. The technique can also confirm the presence of bacteria by detection of short chain carboxylic acids released as products of fermentation during the growth of these cells.