33 research outputs found
Effect of Dirac Spinons on ARPES signatures of Herbertsmithe
The spinon continues to be an elusive elementary excitation of frustrated
antiferromagnets. To solidify evidence for its existence, we address the
question of what will be the Angle Resolved Photoemission Spectroscopy (ARPES)
signatures of single crystal samples of Herbertsmithite assuming it is
described by the Dirac spin liquid state. In particular, we show that the
electron spectral function will have a linear in energy dependence near
specific wave vectors and that this dependence is expected even after
fluctuations to the mean field values are taken into account. Observation of
this unique signature in ARPES will provide very strong evidence for the
existence of spinons in greater than one dimension.Comment: 10 page
Boson features in STM spectra of cuprate superconductors: Weak-coupling phenomenology
We derive the shape of the high-energy features due to a weakly coupled boson
in cuprate superconductors, as seen experimentally in Bi_2 Sr_2 Ca_1 Cu_2 O_8+x
(BSCCO) by Lee et al. [Nature (London) 442, 546 (2006)]. A simplified model is
used of d-wave Bogoliubov quasiparticles coupled to Einstein oscillators with a
momentum-independent electron-boson coupling and an analytic fitting form is
derived, which allows us (a) to extract the boson mode's frequency and (b) to
estimate the electron-boson coupling strength. We further calculate the maximum
possible superconducting gap due to an Einstein oscillator with the extracted
electron-boson coupling strength, which is found to be less than 0.2 times of
the observed gap indicating at the observed boson's non-dominant role in the
superconductivity's mechanism. The extracted momentum-independent
electron-boson coupling parameter (that we show a posteriori to indeed be in
the weak-coupling regime) is then to be interpreted as an (band-structure
detail dependent weighted) average over the Brillouin zone of the actual
momentum-dependent electron-boson coupling in BSCCO.Comment: 6.5 page
N\'eel to valence-bond solid transition on the honeycomb lattice: Evidence for deconfined criticality
We study a spin-1/2 SU(2) model on the honeycomb lattice with
nearest-neighbor antiferromagnetic exchange that favors N\'eel order, and
competing 6-spin interactions which favor a valence bond solid (VBS) state
in which the bond-energies order at the "columnar" wavevector . We present quantum Monte-Carlo evidence for a direct
continuous quantum phase transition between N\'eel and VBS states, with
exponents and logarithmic violations of scaling consistent with those at
analogous deconfined critical points on the square lattice. Although this
strongly suggests a description in terms of deconfined criticality, the
measured three-fold anisotropy of the phase of the VBS order parameter shows
unusual near-marginal behaviour at the critical point.Comment: published version with extensive T > 0 data; author list rearranged
to reflect these new result
Sign-problem-free Monte Carlo simulation of certain frustrated quantum magnets
We introduce a Quantum Monte Carlo (QMC) method which efficiently simulates
in a sign-problem-free way a broad class of frustrated models with
competing antiferromagnetic interactions. Our scheme uses the basis of total
spin eigenstates of clusters of spins to avoid the severe sign problem faced by
other QMC methods. We also flag important limitations of the new method, and
comment on possibilities for further progress.Comment: 6 pages + appendix with supplemental informatio
Transitions to valence-bond solid order in a honeycomb lattice antiferromagnet
We use Quantum Monte-Carlo methods to study the ground state phase diagram of
a S=1/2 honeycomb lattice magnet in which a nearest-neighbor antiferromagnetic
exchange J (favoring N\'eel order) competes with two different multi-spin
interaction terms: a six-spin interaction Q_3 that favors columnar valence-bond
solid (VBS) order, and a four-spin interaction Q_2 that favors staggered VBS
order. For Q_3 ~ Q_2 >> J, we establish that the competition between the two
different VBS orders stabilizes N\'eel order in a large swathe of the phase
diagram even when J is the smallest energy-scale in the Hamiltonian. When Q_3
>> (Q_2,J) (Q_2 >> (Q_3,J)), this model exhibits at zero temperature phase
transition from the N\'eel state to a columnar (staggered) VBS state. We
establish that the N\'eel-columnar VBS transition is continuous for all values
of Q_2, and that critical properties along the entire phase boundary are
well-characterized by critical exponents and amplitudes of the non-compact CP^1
(NCCP^1) theory of deconfined criticality, similar to what is observed on a
square lattice. However, a surprising three-fold anisotropy of the phase of the
VBS order parameter at criticality, whose presence was recently noted at the
Q_2=0 deconfined critical point, is seen to persist all along this phase
boundary. We use a classical analogy to explore this by studying the critical
point of a three-dimensional XY model with a four-fold anisotropy field which
is known to be weakly irrelevant at the three-dimensional XY critical point. In
this case, we again find that the critical anisotropy appears to saturate to a
nonzero value over the range of sizes accessible to our simulations.Comment: 14 page
Notes on resummation-based quantum Monte Carlo vis-\`a-vis sign-problematic Heisenberg models on canonical geometrically frustrated lattices
We show here that a direct application of resummation-based quantum Monte
Carlo (QMC) -- implemented recently for sign-problem-free SU(2)-symmetric
Hamiltonians in the stochastic series expansion (SSE) framework -- does not
reduce the sign problem for frustrated SU(2)-symmetric Heisenberg
antiferromagnets on canonical geometrically frustrated lattices composed of
triangular motifs such as the triangular lattice. In the process, we
demonstrate that resummation-based updates do provide an ergodic sampling of
the SSE-based QMC configurations which can be an issue when using the standard
SSE updates, however, severely limited by the sign problem as previously
mentioned. The notions laid out in these notes may be useful in the design of
better algorithms for geometrically frustrated magnets.Comment: 13 pages, 5 figures in one-column preprint forma