1,030 research outputs found
Spin-charge separation at small lengthscales in the 2D t-J model
We consider projected wavefunctions for the 2D model. For various
wavefunctions, including correlated Fermi-liquid and Luttinger-type
wavefunctions we present the static charge-charge and spin-spin structure
factors. Comparison with recent results from a high-temperature expansion by
Putikka {\it et al.} indicates spin-charge separation at small lengthscales.Comment: REVTEX, 5 pages, 5 figures hardcopies availabl
Upper Critical Field in a Spin-Charge Separated Superconductor
It is demonstrated that the spatial decay of the pair propagator in a
Luttinger liquid with spin charge separation contains a logarithmic correction
relative to the free fermi gas result in a finite interval between the spin and
charge thermal lengths. It is argued that similar effects can be expected in
higher dimensional systems with spin charge separation and that the temperature
dependence of the upper critical field curve is a probe of this
effect.Comment: 3 pages, postscript file (compressed and uuencoded
Valence Bonds in Random Quantum Magnets: Theory and Application to YbMgGaO4
We analyze the effect of quenched disorder on spin-1/2 quantum magnets in
which magnetic frustration promotes the formation of local singlets. Our
results include a theory for 2d valence-bond solids subject to weak bond
randomness, as well as extensions to stronger disorder regimes where we make
connections with quantum spin liquids. We find, on various lattices, that the
destruction of a valence-bond solid phase by weak quenched disorder leads
inevitably to the nucleation of topological defects carrying spin-1/2 moments.
This renormalizes the lattice into a strongly random spin network with
interesting low-energy excitations. Similarly when short-ranged valence bonds
would be pinned by stronger disorder, we find that this putative glass is
unstable to defects that carry spin-1/2 magnetic moments, and whose residual
interactions decide the ultimate low energy fate. Motivated by these results we
conjecture Lieb-Schultz-Mattis-like restrictions on ground states for
disordered magnets with spin-1/2 per statistical unit cell. These conjectures
are supported by an argument for 1d spin chains. We apply insights from this
study to the phenomenology of YbMgGaO, a recently discovered triangular
lattice spin-1/2 insulator which was proposed to be a quantum spin liquid. We
instead explore a description based on the present theory. Experimental
signatures, including unusual specific heat, thermal conductivity, and
dynamical structure factor, and their behavior in a magnetic field, are
predicted from the theory, and compare favorably with existing measurements on
YbMgGaO and related materials.Comment: v2: Stylistic revisions to improve clarity. 22 pages, 8 figures, 2
tables main text; 13 pages, 3 figures appendice
The role of local structure in dynamical arrest
Amorphous solids, or glasses, are distinguished from crystalline solids by
their lack of long-range structural order. At the level of two-body structural
correlations, glassformers show no qualitative change upon vitrifying from a
supercooled liquid. Nonetheless the dynamical properties of a glass are so much
slower that it appears to take on the properties of a solid. While many
theories of the glass transition focus on dynamical quantities, a solid's
resistance to flow is often viewed as a consequence of its structure. Here we
address the viewpoint that this remains the case for a glass. Recent
developments using higher-order measures show a clear emergence of structure
upon dynamical arrest in a variety of glass formers and offer the tantalising
hope of a structural mechanism for arrest. However a rigorous fundamental
identification of such a causal link between structure and arrest remains
elusive. We undertake a critical survey of this work in experiments, computer
simulation and theory and discuss what might strengthen the link between
structure and dynamical arrest. We move on to highlight the relationship
between crystallisation and glass-forming ability made possible by this deeper
understanding of the structure of the liquid state, and emphasize the potential
to design materials with optimal glassforming and crystallisation ability, for
applications such as phase-change memory. We then consider aspects of the
phenomenology of glassy systems where structural measures have yet to make a
large impact, such as polyamorphism (the existence of multiple liquid states),
aging (the time-evolution of non-equilibrium materials below their glass
transition) and the response of glassy materials to external fields such as
shear.Comment: 70 page
Deconfined Quantum Criticality, Scaling Violations, and Classical Loop Models
Numerical studies of the N\'eel to valence-bond solid phase transition in 2D
quantum antiferromagnets give strong evidence for the remarkable scenario of
deconfined criticality, but display strong violations of finite-size scaling
that are not yet understood. We show how to realise the universal physics of
the Neel-VBS transition in a 3D classical loop model (this includes the
interference effect that suppresses N\'eel hedgehogs). We use this model to
simulate unprecedentedly large systems (of size ). Our results are
compatible with a direct continuous transition at which both order parameters
are critical, and we do not see conventional signs of first-order behaviour.
However, we find that the scaling violations are stronger than previously
realised and are incompatible with conventional finite-size scaling over the
size range studied, even if allowance is made for a weakly/marginally
irrelevant scaling variable. In particular, different determinations of the
anomalous dimensions and yield very
different results. The assumption of conventional finite-size scaling gives
estimates which drift to negative values at large , in violation of
unitarity bounds. In contrast, the behaviour of correlators on scales much
smaller than is consistent with large positive anomalous dimensions.
Barring an unexpected reversal in behaviour at still larger sizes, this implies
that the transition, if continuous, must show unconventional finite-size
scaling, e.g. from a dangerously irrelevant scaling variable. Another
possibility is an anomalously weak first-order transition. By analysing the
renormalisation group flows for the non-compact model (-component
Abelian Higgs model) between two and four dimensions, we give the simplest
scenario by which an anomalously weak first-order transition can arise without
fine-tuning of the Hamiltonian.Comment: 20 pages, 19 figure
Spin-orbit coupling and electron spin resonance for interacting electrons in carbon nanotubes
We review the theoretical description of spin-orbit scattering and electron
spin resonance in carbon nanotubes. Particular emphasis is laid on the effects
of electron-electron interactions. The spin-orbit coupling is derived, and the
resulting ESR spectrum is analyzed both using the effective low-energy field
theory and numerical studies of finite-size Hubbard chains and two-leg Hubbard
ladders. For single-wall tubes, the field theoretical description predicts a
double peak spectrum linked to the existence of spin-charge separation. The
numerical analysis basically confirms this picture, but also predicts
additional features in finite-size samples.Comment: 19 pages, 4 figures, invited review article for special issue in J.
Phys. Cond. Mat., published versio
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