1,463 research outputs found
Spin liquid phase in a spatially anisotropic frustrated antiferromagnet
We explore the effect of the third nearest-neighbors on the magnetic
properties of the Heisenberg model on an anisotropic triangular lattice. We
obtain the phase diagram of the model using Schwinger-boson mean-field theory.
Competition between N\'eel, spiral and collinear magnetically ordered phases is
found as we vary the on the ratios of the nearest, J1, next-nearest, J2, and
third-nearest, J_3, neighbor exchange couplings. A spin liquid phase is
stabilized between the spiral and collinear ordered states when J2/J1 < 1.8 for
rather small J3/J1 < 0.1. The lowest energy two-spinon dispersions relevant to
neutron scattering experiments are analyzed and compared to semiclassical
magnon dispersions finding significant differences in the spiral and collinear
phases between the two approaches. The results are discussed in the context of
the anisotropic triangular materials: Cs2CuCl4 and Cs2CuBr4 and layered organic
materials, kappa-(BEDT-TTF)2X and Y[Pd(dmit)2]2.Comment: 11 pages, 9 figure
Spin liquid phase due to competing classical orders in the semiclassical theory of the Heisenberg model with ring exchange on an anisotropic triangular lattice
Linear spin wave theory shows that ring exchange induces a quantum disordered
region in the phase diagram of the title model. Spin wave spectra show that
this is a direct manifestation of competing classical orders. A spin liquid is
found in the `Goldilocks zone' of frustration, where the quantum fluctuations
are large enough to cause strong competition between different classical
orderings but not strong enough to stabilize spiral order. We note that the
spin liquid phases of -(BEDT-TTF) and [Pd(dmit)] are
found in this Goldilocks zone.Comment: 5 pages, 3 figure
Emergent particles and gauge fields in quantum matter
I give a pedagogical introduction to some of the many particles and gauge
fields that can emerge in correlated matter. The standard model of materials is
built on Landau's foundational principles: adiabatic continuity and spontaneous
symmetry breaking. These ideas lead to quasiparticles that inherit their
quantum numbers from fundamental particles, Nambu-Goldstone bosons, the
Anderson-Higgs mechanism, and topological defects in order parameters. I then
describe the modern discovery of physics beyond the standard model. Here,
quantum correlations (entanglement) and topology play key roles in defining the
properties of matter. This can lead to fractionalised quasiparticles that carry
only a fraction of the quantum numbers that define fundamental particles. These
particles can have exotic properties: for example Majorana fermions are their
own antiparticles, anyons have exchange statistics that are neither bosonic nor
fermionic, and magnetic monopoles do not occur in the vacuum. Gauge fields
emerge naturally in the description of highly correlated matter and can lead to
gauge bosons. Relationships to the standard model of particle physics are
discussed.Comment: Pedagogical review submitted to Contemporary Physics; 50 pages, 20
figures. Minor corrections to previous postin
Fast, accurate enthalpy differences in spin crossover crystals from DFT+U
Spin crossover materials are bi-stable systems with potential applications as molecular scale electronic switches, actuators, thermometers, barometers, and displays. However, calculating the enthalpy difference, ΔH, between the high spin and low spin states has been plagued with difficulties. For example, many common density functional theory (DFT) methods fail to even predict the correct sign of ΔH, which determines the low temperature state. Here, we study a collection of Fe(II) and Fe(III) materials, where ΔH\ua0has been measured, which has previously been used to benchmark density functionals. The best performing hybrid functional, TPSSh, achieves a mean absolute error compared to experiment of 11 kJ mol−1\ua0for this set of materials. However, hybrid functionals scale badly in the solid state; therefore, local functionals are preferable for studying crystalline materials, where the most interesting spin crossover phenomena occur. We show that both the Liechtenstein and Dudarev DFT+U methods are a little more accurate than TPSSh. The Dudarev method yields a mean absolute error of 8 kJ mol−1\ua0for\ua0Ueff\ua0= 1.6 eV. However, the mean absolute error for both TPSSh and DFT+U is dominated by a single material, for which the two theoretical methods predict similar enthalpy differences—if this is excluded from the set, then DFT+U achieves chemical accuracy. Thus, DFT+U is an attractive option for calculating the properties of spin crossover crystals, as its accuracy is comparable to that of meta-hybrid functionals, but at a much lower computational cost
Unconventional superconductivity near a flat band in organic and organometallic materials
We study electron correlation driven superconductivity on a decorated
honeycomb lattice (DHL), which has a low-energy flat band. On doping, we find
singlet superconductivity with extended-s, extended-d and f-wave symmetry
mediated by magnetic exchange. f-wave singlet pairing is enabled by the lattice
decoration. The critical temperature is predicted to be significantly higher
than on similar lattices lacking flat bands. We discuss how high-temperature
superconductivity could be realized in the DHL materials such as Rb3TT. 2 H2O
and Mo3S7(dmit)3.Comment: 4 pages, 4 figures + Supplemental materia
On the origin of electrical conductivity in the bio-electronic material melanin
The skin pigment melanin is one of a few bio-macromolecules that display electrical and photo-conductivity in the solid-state. A model for melanin charge transport based on amorphous semiconductivity has been widely accepted for 40 years. In this letter, we show that a central pillar in support of this hypothesis, namely experimental agreement with a hydrated dielectric model, is an artefact related to measurement geometry and non-equilibrium behaviour. Our results cast significant doubt on the validity of the amorphous semiconductor model and are a reminder of the difficulties of electrical measurements on low conductivity, disordered organic materials. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3688491
The effect of irradiation-induced disorder on the conductivity and critical temperature of the organic superconductor -(BEDT-TTF)Cu(SCN)
We have introduced defects into clean samples of the organic superconductor
-(BEDT-TTF)Cu(SCN) in order to determine their effect on the
temperature dependence of the conductivity and the critical temperature . We find a violation of Matthiessen's rule that can be explained by a model
of the conductivity involving a defect-assisted interlayer channel which acts
in parallel with the band-like conductivity. We observe an unusual dependence
of on residual resistivity which is not consistent with the
generalised Abrikosov-Gor'kov theory for an order parameter with a single
component, providing an important constraint on models of the superconductivity
in this material
On the validity of the linear speed selection mechanism for fronts of the nonlinear diffusion equation
We consider the problem of the speed selection mechanism for the one
dimensional nonlinear diffusion equation . It has been
rigorously shown by Aronson and Weinberger that for a wide class of functions
, sufficiently localized initial conditions evolve in time into a monotonic
front which propagates with speed such that . The lower value is that predicted
by the linear marginal stability speed selection mechanism. We derive a new
lower bound on the the speed of the selected front, this bound depends on
and thus enables us to assess the extent to which the linear marginal selection
mechanism is valid.Comment: 9 pages, REVTE
New exact fronts for the nonlinear diffusion equation with quintic nonlinearities
We consider travelling wave solutions of the reaction diffusion equation with
quintic nonlinearities . If the parameters
and obey a special relation, then the criterion for the existence of a
strong heteroclinic connection can be expressed in terms of two of these
parameters. If an additional restriction is imposed, explicit front solutions
can be obtained. The approach used can be extended to polynomials whose highest
degree is odd.Comment: Revtex, 5 page
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