323 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
Instability of antiferromagnetic magnons in strong fields
We predict that spin-waves in an ordered quantum antiferromagnet (AFM) in a
strong magnetic field become unstable with respect to spontaneous two-magnon
decays. At zero temperature, the instability occurs between the threshold field
and the saturation field . As an example, we investigate the
high-field dynamics of a Heisenberg antiferromagnet on a square lattice and
show that the single-magnon branch of the spectrum disappears in the most part
of the Brillouin zone.Comment: RevTeX, 4 pages, 3 figures, accepted to PR
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
Interplay of Spin and Lattice Degrees of Freedom in the Frustrated Antiferromagnet CdCr_2O_4: High-field and Temperature Induced Anomalies of the Elastic Constants
Temperature and magnetic field studies of the elastic constants of the
chromium spinel CdCr_2O_4 show pronounced anomalies related to strong
spin-phonon coupling in this frustrated antiferromagnet. A detailed comparison
of the longitudinal acoustic mode propagating along the [111] direction with
theory based on an exchange-striction mechanism leads to an estimate of the
strength of the magneto-elastic interaction. The derived spin-phonon coupling
constant is in good agreement with previous determinations based on infrared
absorption. Further insight is gained from intermediate and high magnetic field
experiments in the field regime of the magnetization plateau. The role of the
antisymmetric Dzyaloshinskii-Moriya interaction discussed and we compare the
spin-phonon coupling in CdCr_2O_4 in both the ordered and disordered states.Comment: 12 pages, 8 figures; Appendix added,To appear in Phys Rev.
Enhanced magnetocaloric effect in frustrated magnets
The magnetothermodynamics of strongly frustrated classical Heisenberg
antiferromagnets on kagome, garnet, and pyrochlore lattices is examined. The
field induced adiabatic temperature change (dT/dH)_S is significantly larger
for such systems compared to ordinary non-frustrated magnets and also exceeds
the cooling rate of an ideal paramagnet in a wide range of fields. An
enhancement of the magnetocaloric effect is related to presence of a
macroscopic number of soft modes in frustrated magnets below the saturation
field. Theoretical predictions are confirmed with extensive Monte Carlo
simulations.Comment: 7 page
Magnetocaloric effect in integrable spin-s chains
We study the magnetocaloric effect for the integrable antiferromagnetic
high-spin chain. We present an exact computation of the Gr\"uneisen parameter,
which is closely related to the magnetocaloric effect, for the quantum spin-s
chain on the thermodynamical limit by means of Bethe ansatz techniques and the
quantum transfer matrix approach. We have also calculated the entropy S and the
isentropes in the (H,T) plane. We have been able to identify the quantum
critical points H_c^{(s)}=2/(s+1/2) looking at the isentropes and/or the
characteristic behaviour of the Gr\"uneisen parameter.Comment: 6 pages, 3 figure
Excitonic order at strong-coupling: pseudo-spins, doping, and ferromagnetism
A tight binding model is introduced to describe the strong interaction limit
of excitonic ordering. At stoichiometry, the model reduces in the strong
coupling limit to a pseudo-spin model with approximate U(4) symmetry. Excitonic
order appears in the pseudo-spin model as in-plane pseudo-magnetism. The U(4)
symmetry unifies all possible singlet and triplet order parameters describing
such states. Super-exchange, Hunds-rule coupling, and other perturbations act
as anisotropies splitting the U(4) manifold, ultimately stabilizing a
paramagnetic triplet state. The tendency to ferromagnetism with doping
(observed experimentally in the hexaborides) is explained as a spin-flop
transition to a different orientation of the U(4) order parameter. The physical
mechanism favoring such a reorientation is the enhanced coherence (and hence
lower kinetic energy) of the doped electrons in a ferromagnetic background
relative to the paramagnet. A discussion of the physical meaning of various
excitonic states and their experimental consequences is also provided.Comment: 16 pages, 5 figure
Localized-magnon states in strongly frustrated quantum spin lattices
Recent developments concerning localized-magnon eigenstates in strongly
frustrated spin lattices and their effect on the low-temperature physics of
these systems in high magnetic fields are reviewed. After illustrating the
construction and the properties of localized-magnon states we describe the
plateau and the jump in the magnetization process caused by these states.
Considering appropriate lattice deformations fitting to the localized magnons
we discuss a spin-Peierls instability in high magnetic fields related to these
states. Last but not least we consider the degeneracy of the localized-magnon
eigenstates and the related thermodynamics in high magnetic fields. In
particular, we discuss the low-temperature maximum in the isothermal entropy
versus field curve and the resulting enhanced magnetocaloric effect, which
allows efficient magnetic cooling from quite large temperatures down to very
low ones.Comment: 21 pages, 10 figures, invited paper for a special issue of "Low
Temperature Physics " dedicated to the 70-th anniversary of creation of
concept "antiferromagnetism" in physics of magnetis
Field induced ordering in highly frustrated antiferromagnets
We predict that an external field can induce a spin order in highly
frustrated classical Heisenberg magnets. We find analytically stabilization of
collinear states by thermal fluctuations at a one-third of the saturation field
for kagome and garnet lattices and at a half of the saturation field for
pyrochlore and frustrated square lattices. This effect is studied numerically
for the frustrated square-lattice antiferromagnet by Monte Carlo simulations
for classical spins and by exact diagonalization for . The field induced
collinear states have a spin gap and produce magnetization plateaus.Comment: 4 pages, new analytical proof the order by disorder by thermal
fluctuations is adde
- …