1,546 research outputs found
Classical and quantum theories of proton disorder in hexagonal water ice
It has been known since the pioneering work of Bernal, Fowler and Pauling
that common, hexagonal (Ih) water ice is the archetype of a frustrated material
: a proton-bonded network in which protons satisfy strong local constraints -
the "ice rules" - but do not order. While this proton disorder is well
established, there is now a growing body of evidence that quantum effects may
also have a role to play in the physics of ice at low temperatures. In this
Article we use a combination of numerical and analytic techniques to explore
the nature of proton correlations in both classical and quantum models of ice
Ih. In the case of classical ice Ih, we find that the ice rules have two,
distinct, consequences for scattering experiments - singular "pinch points",
reflecting a zero-divergence condition on the uniform polarization of the
crystal, and broad, asymmetric features, coming from its staggered
polarisation. In the case of the quantum model, we find that the collective
quantum tunnelling of groups of protons can convert states obeying the ice
rules into a quantum liquid, whose excitations are birefringent, emergent
photons. We make explicit predictions for scattering experiments on both
classical and quantum ice Ih, and show how the quantum theory can explain the
"wings" of incoherent inelastic scattering observed in recent neutron
scattering experiments [Bove et al., Phys. Rev. Lett. 103, 165901 (2009)].
These results raise the intriguing possibility that the protons in ice Ih could
form a quantum liquid at low temperatures, in which protons are not merely
disordered, but continually fluctuate between different configurations obeying
the ice rules.Comment: 33 pages (21 in main text), 13 figures (9 in main text), expanded
discussion of experiment with new subsection on thermodynamic
Ordered ground states of kagome magnets with generic exchange anisotropy
There is a growing family of rare-earth kagome materials with dominant
nearest-neighbor interactions and strong spin orbit coupling. The low symmetry
of these materials makes theoretical description complicated, with six distinct
nearest-neighbor coupling parameters allowed. In this Article, we ask what
kinds of classical, ordered, ground states can be expected to occur in these
materials, assuming generic (i.e. non-fine-tuned) sets of exchange parameters.
We use symmetry analysis to show that there are only five distinct classical
ground state phases occurring for generic parameters. The five phases are: (i)
a coplanar, 2-fold degenerate, state with vanishing magnetization (), (ii) a noncoplanar, 2-fold degenerate, state with magnetization
perpendicular to the kagome plane (), (iii) a coplanar, 6-fold
degenerate, state with magnetization lying within the kagome plane (-coplanar), (iv) a noncoplanar, 6-fold degenerate, state with magnetization
lying within a mirror plane of the lattice (-noncoplanar), (v) a
noncoplanar, 12-fold degenerate, state with magnetization in an arbitrary
direction (-noncoplanar). All five are translation invariant
() states. Having found the set of possible ground states, the
ground state phase diagram is obtained by comparing numerically optimized
energies for each possibility as a function of the coupling parameters. The
state noncoplanar is extremely rare, occupying of the
full phase diagram, so for practical purposes there are four main ordered
states likely to occur in anisotropic kagome magnets with dominant nearest
neighbor interactions. These results can aid in interpreting recent experiments
on ``tripod kagome'' systems RASbO, as well as materials
closer to the isotropic limit such as Cr- and Fe- jarosites
Ground state phase diagram of dipolar-octupolar pyrochlores
The dipolar-octupolar pyrochlore oxides RMO (R=Ce, Sm, Nd)
represent an important opportunity in the search for three dimensional Quantum
Spin Liquid (QSL) ground states. Their low energy physics is governed by an
alluringly simple XYZ Hamiltonian, enabling theoretical description with only a
small number of free parameters. Meanwhile, recent experiments on Ce
pyrochlores strongly suggest QSL physics. Motivated by this, we present here a
complete analysis of the ground state phase diagram of dipolar-octupolar
pyrochlores. Combining cluster mean field theory, variational arguments and
exact diagonalization we find multiple U(1) QSL phases which together occupy a
large fraction of the parameter space. These results give a comprehensive
picture of the ground state physics of an important class of QSL candidates and
support the possibility of a QSL ground state in CeZrO and
CeSnO
Reentrance of disorder in the anisotropic shuriken Ising model
For a material to order upon cooling is common sense. What is more seldom is
for disorder to reappear at lower temperature, which is known as reentrant
behavior. Such resurgence of disorder has been observed in a variety of
systems, ranging from Rochelle salts to nematic phases in liquid crystals.
Frustration is often a key ingredient for reentrance mechanisms. Here we shall
study a frustrated model, namely the anisotropic shuriken lattice, which offers
a natural setting to explore an extension of the notion of reentrance between
magnetic disordered phases. By tuning the anisotropy of the lattice, we open a
window in the phase diagram where magnetic disorder prevails down to zero
temperature. In this region, the competition between multiple disordered ground
states gives rise to a double crossover where both the low- and
high-temperature regimes are less correlated than the intervening classical
spin liquid. This reentrance of disorder is characterized by an entropy
plateau, a multi-step Curie law crossover and a rather complex diffuse
scattering in the static structure factor. Those results are confirmed by
complementary numerical and analytical methods: Monte Carlo simulations,
Husimi-tree calculations and an exact decoration-iteration transformation.Comment: 16 pages, 13 figure
Living on the edge : ground-state selection in quantum spin-ice pyrochlores
The search for new quantum phases, especially in frustrated magnets, is
central to modern condensed matter physics. One of the most promising places to
look is in rare-earth pyrochlore magnets with highly-anisotropic exchange
interactions, materials closely related to the spin ices Ho2Ti2O7 and Dy2Ti2O7.
Here we establish a general theory of magnetic order in these materials. We
find that many of their most interesting properties can be traced back to the
accidental degeneracies where phases with different symmetry meet. These
include the ordered ground state selection by fluctuations in Er2Ti2O7, the
dimensional-reduction observed in Yb2Ti2O7, and the absence of magnetic order
in Er2Sn2O7.Comment: A long-paper version of this preprint, "Living on the Edge", appears
as arXiv:1603.09466 [accepted for publication in Physical Review B]. The text
of v2 is otherwise unchanged from v1 (Submitted on 14 Nov 2013
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