19 research outputs found
Multiferroicity in spin ice: towards a magnetic crystallography of Tb2Ti2O7 in a field
We combine two aspects of magnetic frustration, multiferroicity and emergent
quasi-particles in spin liquids, by studying magneto-electric monopoles. Spin
ice offers to couple these emergent topological defects to external fields, and
to each other, in unusual ways, making possible to lift the degeneracy
underpinning the spin liquid and to potentially stabilize novel forms of charge
crystals, opening the path to a "magnetic crystallography". In developing the
general phase diagram including nearest-neighbour coupling, Zeeman energy,
electric and magnetic dipolar interactions, we uncover the emergence of a
bi-layered crystal of singly-charged monopoles, whose stability, remarkably, is
strengthened by an external [110] magnetic field. Our theory is able to account
for the ordering process of Tb2Ti2O7 in large field for reasonably small
electric energy scales.Comment: 10 pages, 10 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
Curie-law crossover in spin liquids
The Curie-Weiss law is widely used to estimate the strength of frustration in
frustrated magnets. However, the Curie-Weiss law was originally derived as an
estimate of magnetic correlations close to a mean-field phase transition, which
-- by definition -- is absent in spin liquids. Instead, the susceptibility of
spin liquids is known to undergo a Curie-law crossover between two magnetically
disordered regimes. Here, we study the generic aspect of the Curie-law
crossover by comparing a variety of frustrated spin models in two and three
dimensions, using both classical Monte Carlo simulations and analytical Husimi
tree calculations. Husimi tree calculations fit remarkably well the simulations
for all temperatures and almost all lattices. We also propose a Husimi Ansatz
for the reduced susceptibility , to be used in complement to the
traditional Curie-Weiss fit in order to estimate the Curie-Weiss temperature
. Applications to materials are discussed.Comment: 26 pages, 15 figure
Spin ice under pressure: symmetry enhancement and infinite order multicriticality
We study the low-temperature behaviour of spin ice when uniaxial pressure
induces a tetragonal distortion. There is a phase transition between a Coulomb
liquid and a fully magnetised phase. Unusually, it combines features of
discontinuous and continuous transitions: the order parameter exhibits a jump,
but this is accompanied by a divergent susceptibility and vanishing domain wall
tension. All these aspects can be understood as a consequence of an emergent
SU(2) symmetry at the critical point. We map out a possible experimental
realisation
A Three Dimensional Kasteleyn Transition: Spin Ice in a [100] Field
We examine the statistical mechanics of spin-ice materials with a [100]
magnetic field. We show that the approach to saturated magnetisation is, in the
low-temperature limit, an example of a 3D Kasteleyn transition, which is
topological in the sense that magnetisation is changed only by excitations that
span the entire system. We study the transition analytically and using a Monte
Carlo cluster algorithm, and compare our results with recent data from
experiments on Dy2Ti2O7.Comment: 4 pages, 5 figure
The Classical Heisenberg Model on the Centred Pyrochlore Lattice
The centred pyrochlore lattice is a novel geometrically frustrated lattice,
realized in the metal-organic framework Mn(ta) (arXiv:2203.08780) where the
basic unit of spins is a five site centred tetrahedron. Here, we present an
in-depth theoretical study of the classical Heisenberg model on this
lattice, using a combination of mean-field analytical methods and Monte Carlo
simulations. We find a rich phase diagram with low temperature states
exhibiting ferrimagnetic order, partial ordering, and a highly degenerate spin
liquid with distinct regimes of low temperature correlations. We discuss in
detail how the regime displaying broadened pinch points in its spin structure
factor is consistent with an effective description in terms of a fluid of
interacting charges. We also show how this picture holds in two dimensions on
the analogous centred kagome lattice and elucidate the connection to the
physics of thin films in () dimensions. Furthermore, we show that a
Coulomb phase can be stabilized on the centred pyrochlore lattice by the
addition of further neighbour couplings. This demonstrates the centred
pyrochlore lattice is an experimentally relevant geometry which naturally hosts
emergent gauge fields in the presence of charges at low energies.Comment: 29 pages, 9 figures, resubmission to SciPost with minor revision
Schwinger boson theory of the J1,J2=J3 kagome antiferromagnet
We study the kagome antiferromagnet for quantum spin-1/2 with first J1,
second J2 and third J3 neighbour exchanges, along the J2 = J3 = J line. We use
Schwinger-boson mean-field theory for the precise determination of the phase
diagram, and two different rewritings of the Hamiltonian to build an intuition
about the origin of the transitions. The spin liquid obtained at J = 0 remains
essentially stable over a large window, up to J = 1/3, because it is only
weakly frustrated by the J term. Then at J = 1/2, the intermediate Z2 spin
liquid condenses into a long-range chiral order because of the change of nature
of local magnetic fluctuations. As a side benefit, our Hamiltonian rewriting
offers an exact solution for the ground state of our model on a Husimi cactus.Comment: Last version before publication in PR
Magnetic-Moment Fragmentation and Monopole Crystallization
The Coulomb phase, with its dipolar correlations and pinch-point-scattering
patterns, is central to discussions of geometrically frustrated systems, from
water ice to binary and mixed-valence alloys, as well as numerous examples of
frustrated magnets. The emergent Coulomb phase of lattice-based systems has
been associated with divergence-free fields and the absence of long-range
order. Here, we go beyond this paradigm, demonstrating that a Coulomb phase can
emerge naturally as a persistent fluctuating background in an otherwise ordered
system. To explain this behavior, we introduce the concept of the fragmentation
of the field of magnetic moments into two parts, one giving rise to a magnetic
monopole crystal, the other a magnetic fluid with all the characteristics of an
emergent Coulomb phase. Our theory is backed up by numerical simulations, and
we discuss its importance with regard to the interpretation of a number of
experimental results
Topological Sector Fluctuations and Curie Law Crossover in Spin Ice
At low temperatures, a spin ice enters a Coulomb phase - a state with
algebraic correlations and topologically constrained spin configurations. In
Ho2Ti2O7, we have observed experimentally that this process is accompanied by a
non-standard temperature evolution of the wave vector dependent magnetic
susceptibility, as measured by neutron scattering. Analytical and numerical
approaches reveal signatures of a crossover between two Curie laws, one
characterizing the high temperature paramagnetic regime, and the other the low
temperature topologically constrained regime, which we call the spin liquid
Curie law. The theory is shown to be in excellent agreement with neutron
scattering experiments. On a more general footing, i) the existence of two
Curie laws appears to be a general property of the emergent gauge field for a
classical spin liquid, and ii) sheds light on the experimental difficulty of
measuring a precise Curie-Weiss temperature in frustrated materials; iii) the
mapping between gauge and spin degrees of freedom means that the susceptibility
at finite wave vector can be used as a local probe of fluctuations among
topological sectors.Comment: 10 pages, 5 figure