346 research outputs found
Excitonic pairing between nodal fermions
We study excitonic pairing in nodal fermion systems characterized by a
vanishing quasiparticle density of states at the pointlike Fermi surface and a
concomitant lack of screening for long-range interactions. By solving the gap
equation for the excitonic order parameter, we obtain a critical value of the
interaction strength for a variety of power-law interactions and densities of
states. We compute the free energy and analyze possible phase transitions, thus
shedding further light on the unusual pairing properties of this peculiar class
of strongly correlated systems.Comment: 9 pages, 7 figures, minor revisions made, final versio
Linear independence of localized magnon states
At the magnetic saturation field, certain frustrated lattices have a class of
states known as "localized multi-magnon states" as exact ground states. The
number of these states scales exponentially with the number of spins and
hence they have a finite entropy also in the thermodynamic limit
provided they are sufficiently linearly independent. In this article we present
rigorous results concerning the linear dependence or independence of localized
magnon states and investigate special examples. For large classes of spin
lattices including what we called the orthogonal type and the isolated type as
well as the kagom\'{e}, the checkerboard and the star lattice we have proven
linear independence of all localized multi-magnon states. On the other hand the
pyrochlore lattice provides an example of a spin lattice having localized
multi-magnon states with considerable linear dependence.Comment: 23 pages, 6 figure
Magnetic excitations in the spin-1/2 triangular-lattice antiferromagnet CsCuBr
We report on high-field electron spin resonance (ESR) studies of magnetic
excitations in the spin-1/2 triangular-lattice antiferromagnet CsCuBr.
Frequency-field diagrams of ESR excitations are measured for different
orientations of magnetic fields up to 25 T. We show that the substantial
zero-field energy gap, K, observed in the low-temperature
excitation spectrum of CsCuBr [Zvyagin , Phys. Rev. Lett. 112,
077206 (2014)], is present well above . Noticeably, the transition into
the long-range magnetically ordered phase does not significantly affect the
size of the gap, suggesting that even below the high-energy spin dynamics
in CsCuBr is determined by short-range-order spin correlations. The
experimental data are compared with results of model spin-wave-theory
calculations for spin-1/2 triangle-lattice antiferromagnet.Comment: 6 pages, 9 figure
XY checkerboard antiferromagnet in external field
Ordering by thermal fluctuations is studied for the classical XY
antiferromagnet on a checkerboard lattice in zero and finite magnetic fields by
means of analytical and Monte Carlo methods. The model exhibits a variety of
novel broken symmetries including states with nematic ordering in zero field
and with triatic order parameter at high fields.Comment: 6 page
Fermi Surface Measurements on the Low Carrier Density Ferromagnet Ca1-xLaxB6 and SrB6
Recently it has been discovered that weak ferromagnetism of a dilute 3D
electron gas develops on the energy scale of the Fermi temperature in some of
the hexaborides; that is, the Curie temperature approximately equals the Fermi
temperature. We report the results of de Haas-van Alphen experiments on two
concentrations of La-doped CaB6 as well as Ca-deficient Ca1-dB6 and
Sr-deficient Sr1-dB6. The results show that a Fermi surface exists in each case
and that there are significant electron-electron interactions in the low
density electron gas.Comment: 4 pages, 5 figures, submitted to PR
Collinear N\'eel-type ordering in partially frustrated lattices
We consider two partially frustrated S = 1/2 antiferromagnetic spin systems
on the triangular and pentagonal lattices. In an elementary plaquette of the
two lattices, one bond has exchange interaction strength () whereas all other bonds have exchange interaction strength unity. We show
that for less than a critical value , collinear
N\'eel-type ordering is possible in the ground state. The ground state energy
and the excitation spectrum have been determined using linear spin wave theory
based on the Holstein-Primakoff transformation.Comment: Four pages, LaTeX, Four postscripts figures, Phys. Rev. B58, 73
(1998
Lifetime of Gapped Excitations in a Collinear Quantum Antiferromagnet
We demonstrate that local modulations of magnetic couplings have a profound
effect on the temperature dependence of the relaxation rate of optical magnons
in a wide class of antiferromagnets in which gapped excitations coexist with
acoustic spin waves. In a two-dimensional collinear antiferromagnet with an
easy-plane anisotropy, the disorder-induced relaxation rate of the gapped mode,
Gamma_imp=Gamma_0+A(TlnT)^2, greatly exceeds the magnon-magnon damping,
Gamma_m-m=BT^5, negligible at low temperatures. We measure the lifetime of
gapped magnons in a prototype XY antiferromagnet BaNi2(PO4)2 using a
high-resolution neutron-resonance spin-echo technique and find experimental
data in close accord with the theoretical prediction. Similarly strong effects
of disorder in the three-dimensional case and in noncollinear antiferromagnets
are discussed.Comment: 4.5 pages + 2.5 pages supplementary material, published versio
Fabrication of Net-Shape Functionally Graded Composites by Electrophoretic Deposition and Sintering: Modeling and Experimentation
It is shown that electrophoretic deposition (EPD) sintering is a technological sequence that is capable of producing net-shape bulk functionally graded materials (FGM). By controlling the shape of the deposition electrode, components of complex shapes can be obtained. To enable sintering net-shape capabilities, a novel optimization algorithm and procedure for the fabrication of net-shape functionally graded composites by EPD and sintering has been developed. The initial shape of the green specimen produced by EPD is designed in such a way that the required final shape is achieved after sintering-imposed distortions. The optimization is based on a special innovative iteration procedure that is derived from the solution of the inverse sintering problem: the sintering process is modeled in the “backward movie” regime using the continuum theory of sintering incorporated into a finite-element code. The experiments verifying the modeling approach include the synthesis by EPD of Al2O3/ZrO2 3-D (FGM) structures. In order to consolidate green parts shaped by EPD, post-EPD sintering is used. The fabricated deposits are characterized by optical and scanning electron microscopy. The experimentally observed shape change of the FGM specimen obtained by EPD and sintering is compared with theoretical predictions
Field induced transitions in a kagome antiferromagnet
The thermal order by disorder effect in magnetic field is studied for a
classical Heisenberg antiferromagnet on the kagome lattice. Using analytical
arguments we predict a unique H-T phase diagram for this strongly frustrated
magnet: states with a coplanar and a uniaxial triatic order parameters
respectively at low and high magnetic fields and an incompressible collinear
spin-liquid state at a one-third of the saturation field. We also present the
Monte Carlo data which confirm existence of these phases.Comment: 4 pages, 2 figures, accepted versio
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