4,707 research outputs found
Dynamic elastic properties and magnetic susceptibility across the austenite-martensite transformation in site-disordered ferromagnetic Ni-Fe-Al alloy
Besides permitting an accurate determination of the
ferromagnetic-to-paramagnetic phase transition temperature and the
characteristic temperatures for the beginning and end of the growth of
martensite (austenite) phase at the expense of austenite (martensite) phase
while cooling (heating), the results of an extensive ac susceptibility, sound
velocity and internal friction investigation of the thermoelastic martensitic
transformation in melt-quenched (site-disordered) Ni55Fe20Al25 alloy provide a
clear experimental evidence for the following. Irreversible thermoelastic
changes (thermal hysteresis) occur in the austenite phase in the premartensitic
regime. In the heating cycle, the system retains the "memory" of the initiation
and subsequent growth of the martensitic phase (at the expense of the parent
austenite phase) that had taken place during the cooling cycle in the
austenite-martensite phase coexistence region. We report and discuss these
novel findings in this communication.Comment: 5 figure
Chirality of Knots and and Chern-Simons Theory
Upto ten crossing number, there are two knots, and whose
chirality is not detected by any of the known polynomials, namely, Jones
invariants and their two variable generalisations, HOMFLY and Kauffman
invariants. We show that the generalised knot invariants, obtained through
Chern-Simons topological field theory, which give the known polynomials
as special cases, are indeed sensitive to the chirality of these knots.Comment: 15 pages + 7 diagrams (available on request
Representations of Composite Braids and Invariants for Mutant Knots and Links in Chern-Simons Field Theories
We show that any of the new knot invariants obtained from Chern-Simons theory
based on an arbitrary non-abelian gauge group do not distinguish isotopically
inequivalent mutant knots and links. In an attempt to distinguish these knots
and links, we study Murakami (symmetrized version) -strand composite braids.
Salient features of the theory of such composite braids are presented.
Representations of generators for these braids are obtained by exploiting
properties of Hilbert spaces associated with the correlators of Wess-Zumino
conformal field theories. The -composite invariants for the knots are given
by the sum of elementary Chern-Simons invariants associated with the
irreducible representations in the product of representations (allowed by
the fusion rules of the corresponding Wess-Zumino conformal field theory)
placed on the individual strands of the composite braid. On the other hand,
composite invariants for links are given by a weighted sum of elementary
multicoloured Chern-Simons invariants. Some mutant links can be distinguished
through the composite invariants, but mutant knots do not share this property.
The results, though developed in detail within the framework of
Chern-Simons theory are valid for any other non-abelian gauge group.Comment: Latex, 25pages + 16 diagrams available on reques
Quantum phase transitions in bilayer SU(N) anti-ferromagnets
We present a detailed study of the destruction of SU(N) magnetic order in
square lattice bilayer anti-ferromagnets using unbiased quantum Monte Carlo
numerical simulations and field theoretic techniques. We study phase
transitions from an SU(N) N\'eel state into two distinct quantum disordered
"valence-bond" phases: a valence-bond liquid (VBL) with no broken symmetries
and a lattice-symmetry breaking valence-bond solid (VBS) state. For finite
inter-layer coupling, the cancellation of Berry phases between the layers has
dramatic consequences on the two phase transitions: the N\'eel-VBS transition
is first order for all accesible in our model, whereas the N\'eel-VBL
transition is continuous for N=2 and first order for N>= 4; for N=3 the
N\'eel-VBL transition show no signs of first-order behavior
Knot invariants from rational conformal field theories
A framework for studying knot and link invariants from any rational conformal
field theory is developed. In particular, minimal models, superconformal models
and models are studied. The invariants are related to the invariants
obtained from the Wess-Zumino models associated with the coset representations
of these models. Possible Chern-Simons representation of these models is also
indicated. This generalises the earlier work on knot and link invariants from
Chern-Simons theories.Comment: 18pages+6 figures (available on request through email
Two-dimensional frustrated spin systems in high magnetic fields
We discuss our numerical results on the properties of the S = 1/2 frustrated
J1-J2 Heisenberg model on a square lattice as a function of temperature and
frustration angle phi = atan(J2/J1) in an applied magnetic field. We cover the
full phase diagram of the model in the range -pi <= phi <= pi. The discussion
includes the parameter dependence of the saturation field itself, and addresses
the instabilities associated with it. We also discuss the magnetocaloric effect
of the model and show how it can be used to uniquely determine the effective
interaction constants of the compounds which were investigated experimentally.Comment: 4 pages, 5 figures, proceedings of RHMF 200
Stereochemical control of peptide folding
Stereochemically constrained amino acid residues that strongly favour specific backbone conformations may be used to nucleate and stabilize specific secondary structures in designed peptides. An overview of the use of αα-dialkyl amino acids in stabilizing helical structures in synthetic peptides is presented, with an emphasis on work carried out in the authors laboratory. α-Aminoisobutyric acid (Aib) and related achiral homologs facilitate stable helix formation in oligopeptides as exemplified by a large number of crystal structure determinations in the solid state. The ability to design conformationally rigid helical modules has been exploited in attempts to design structurally well characterized helix-linker-helix, using potential nonhelical linking segments. β-Hairpin design has been approached by exploiting the tendency of 'prime turns' to nucleate hairpin formation. The use of nucleating DPro-Gly segments has resulted in the generation of several well characterized β-hairpin structures, including the crystallographic observation of β-hairpin in a synthetic apolar octapeptide. Extensions of this approach to three stranded β-sheets and larger structures containing multiple DPro-Gly segments appear readily possible
Spin correlations and exchange in square lattice frustrated ferromagnets
The J1-J2 model on a square lattice exhibits a rich variety of different
forms of magnetic order that depend sensitively on the ratio of exchange
constants J2/J1. We use bulk magnetometry and polarized neutron scattering to
determine J1 and J2 unambiguously for two materials in a new family of vanadium
phosphates, Pb2VO(PO4)2 and SrZnVO(PO4)2, and we find that they have
ferromagnetic J1. The ordered moment in the collinear antiferromagnetic ground
state is reduced, and the diffuse magnetic scattering is enhanced, as the
predicted bond-nematic region of the phase diagram is approached.Comment: 4 pages, 4 figure
Three-Manifold Invariants from Chern-Simons Field Theory with Arbitrary Semi-Simple Gauge Groups
Invariants for framed links in obtained from Chern-Simons gauge field
theory based on an arbitrary gauge group (semi-simple) have been used to
construct a three-manifold invariant. This is a generalization of a similar
construction developed earlier for SU(2) Chern-Simons theory. The procedure
exploits a theorem of Lickorish and Wallace and also those of Kirby, Fenn and
Rourke which relate three-manifolds to surgeries on framed unoriented links.
The invariant is an appropriate linear combination of framed link invariants
which does not change under Kirby calculus. This combination does not see the
relative orientation of the component knots. The invariant is related to the
partition function of Chern-Simons theory. This thus provides an efficient
method of evaluating the partition function for these field theories. As some
examples, explicit computations of these manifold invariants for a few
three-manifolds have been done.Comment: 24 pages Latex file, 26 eps files, included generalisations for
arbitrary semi-simple group, corrected typo
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