40 research outputs found
Impurity-doped Kagome Antiferromagnet: A Quantum Dimer Model Approach
The doping of quantum Heisenberg antiferromagnets on the kagome lattice by
non-magnetic impurities is investigated within the framework of a generalized
quantum dimer model (QDM) describing a) the valence bond crystal (VBC), b) the
dimer liquid and c) the critical region on equal footing. Following the
approach by Ralko et al. [Phys. Rev. Lett. 101, 117204 (2008)] for the square
and triangular lattices, we introduce the (minimal) extension of the QDM on the
Kagome lattice to account for spontaneous creation of mobile S=1/2 spinons at
finite magnetic field. Modulations of the dimer density (at zero or finite
magnetic field) and of the local field-induced magnetization in the vicinity of
impurities are computed using Lanczos Exact Diagonalization techniques on small
clusters (48 and 75 sites). The VBC is clearly revealed from its pinning by
impurities, while, in the dimer liquid, crystallization around impurities
involves only two neighboring dimers. We also find that a next-nearest-neighbor
ferromagnetic coupling favors VBC order. Unexpectedly, a small size
spinon-impurity bound state appears in some region of the the dimer liquid
phase. In contrast, in the VBC phase the spinon delocalizes within a large
region around the impurity, revealing the weakness of the VBC confining
potential. Lastly, we observe that an impurity concentration as small as 4%
enhances dimerization substantially. These results are confronted to the
Valence Bond Glass scenario [R.R.P. Singh, Phys. Rev. Lett. 104, 177203 (2010)]
and implications to the interpretation of the Nuclear Magnetic Resonance
spectra of the Herbertsmithite compound are outlined.Comment: Extended version. 9 pages, 11 figure
Generic mixed columnar-plaquette phases in Rokhsar-Kivelson models
We revisit the phase diagram of Rokhsar-Kivelson models, which are used in
fields such as superconductivity, frustrated magnetism, cold bosons, and the
physics of Josephson junction arrays. From an extended height effective theory,
we show that one of two simple generic phase diagrams contains a mixed phase
that interpolates continuously between columnar and plaquette states. For the
square lattice quantum dimer model we present evidence from exact
diagonalization and Green's function Monte Carlo techniques that this scenario
is realised, by combining an analysis of the excitation gaps of different
symmetry sectors with information on plaquette structure factors. This presents
a natural framework for resolving the disagreement between previous studies.Comment: 4 pages, 5 figure
Multiorbital kinetic effects on charge ordering of frustrated electrons on the triangular lattice
The role of the multiorbital effects on the emergence of frustrated
electronic orders on the triangular lattice at half filling is investigated
through an extended spinless fermion Hubbard model. By using two complementary
approaches, unrestricted Hartree-Fock and exact diagonalizations, we unravel a
very rich phase diagram controlled by the strength of both local and off-site
Coulomb interactions and by the interorbital hopping anisotropy ratio .
Three robust unconventional electronic phases, a pinball liquid, an inverse
pinball liquid, and a large-unit-cell droplet
phase, are found to be generic in the triangular geometry, being controlled by
the band structure parameters. The latter are also stabilized in the isotropic
limit of our microscopic model, which recovers the standard SU(2) spinful
extended single-band Hubbard model.Comment: 10 pages, 6 figure
Quantum Melting of Valence Bond Crystal Insulators and Novel Supersolid Phase at Commensurate Density
Bosonic and fermionic Hubbard models on the checkerboard lattice are studied
numerically for infinite on-site repulsion. At particle density n=1/4 and
strong nearest-neighbor repulsion, insulating Valence Bond Crystals (VBC) of
resonating particle pairs are stabilized. Their melting into
superfluid/metallic phases under increasing hopping is investigated at T=0K.
More specifically, we identify a novel and unconventional commensurate VBC
supersolid region, precursor to the melting of the bosonic crystal. Hardcore
bosons (spins) are compared to fermions (electrons), as well as positive to
negative (frustrating) hoppings.Comment: 4 pages, 5 figures; added references, improved content; fitting with
PRL forma
Resonating color state and emergent chromodynamics in the kagome antiferromagnet
We argue that the spin-wave breakdown in the Heisenberg kagome
antiferromagnet signals an instability of the ground state and leads, through
an emergent local constraint, to a quantum dynamics described by a gauge theory
similar to that of chromodynamics. For integer spins, we show that the quantum
fluctuations of the gauge modes select the sqrt(3)xsqrt(3) Neel state with an
on-site moment renormalized by color resonances. We find non-magnetic
low-energy excitations that may be responsible for a deconfinement "transition"
at experimentally accessible temperatures which we estimate.Comment: 4 pages, 4 figures, v2: printable figs, v3: publ. versio
Synthesis and Exploration of Dopamine and 4â HPAA Analogs for Norcoclaurine Synthase
Precursor directed biosynthesis (PDB) presents a useful approach for modifying large scale drug and drug lead mole cules . Large molecular structures are synthetically modified as part of total synthesis approach, and often result in l ow yield and expensive r eagents. Of particular interest are plant alkaloid drugs, many of which come from tetrahydroisoquinoline (THI) precursor. We present an organic synthesis of novel dopamine and 4 - hydroxyphenacetaldehyde (4-HPAA) analogs as precursors for enzymatic conversion to tetrahydroisoquinoline analogs using norcoclaurine synthase (NCS) . Modifications for investigation include halogenation of the 2 and 5 positions of the aromatic ring and variation of atoms in the ethylamine chain of the dopamine molecule. Modifications for 4- HPAA include exploration of different amino acids as starting materials for THI conversion. Exploration of THI analogs a llows for investigation of complex molecules such as, berberine, sanguinarine, galanthamine, and other plant alkaloid drugs. Precursor directed biosynthesis presents an interesting way to incorporate structural modifications for large drug candidates without total synthetic approach
Role of quantum fluctuations on spin liquids and ordered phases in the Heisenberg model on the honeycomb lattice
Motivated by the rich physics of honeycomb magnetic materials, we obtain the
phase diagram and analyze magnetic properties of the spin-1/2 and spin-1
J1-J2-J3 Heisenberg model on the honeycomb lattice. Based on the SU(2) and
SU(3) symmetry representations of the Schwinger boson approach, which treats
disordered spin liquids and magnetically ordered phases on an equal footing, we
obtain the complete phase diagrams in the (J2,J3)plane. This is achieved using
a fully unrestricted approach which does not assume any pre-defined Ansatze.
For S=1/2, we find a quantum spin liquid (QSL) stabilized between the N\'eel,
spiral and collinear antiferromagnetic phases in agreement with previous
theoretical work. However, by increasing S from 1/2 to 1, the QSL is quickly
destroyed due to the weakening of quantum fluctuations indicating that the
model already behaves as a quasi-classical system. The dynamical structure
factors and temperature dependence of the magnetic susceptibility are obtained
in order to characterize all phases in the phase diagrams. Moreover, motivated
by the relevance of the single-ion anisotropy, D, to various S=1 honeycomb
compounds, we have analyzed the destruction of magnetic order based on a SU(3)
representation of the Schwinger bosons. Our analysis provides a unified
understanding of the magnetic properties of honeycomb materials realizing the
J1-J2-J3 Heisenberg model from the strong quantum spin regime at S=1/2 to the
S=1 case. Neutron scattering and magnetic susceptibility experiments can be
used to test the destruction of the QSL phase when replacing S=1/2 by S=1
localized moments in certain honeycomb compounds.Comment: 12 pages, 6 figure
Statistical transmutation in doped quantum dimer models
We prove a "statistical transmutation" symmetry of doped quantum dimer models
on the square, triangular and kagome lattices: the energy spectrum is invariant
under a simultaneous change of statistics (i.e. bosonic into fermionic or
vice-versa) of the holes and of the signs of all the dimer resonance loops.
This exact transformation enables to define duality equivalence between doped
quantum dimer Hamiltonians, and provides the analytic framework to analyze
dynamical statistical transmutations. We investigate numerically the doping of
the triangular quantum dimer model, with special focus on the topological Z2
dimer liquid. Doping leads to four (instead of two for the square lattice)
inequivalent families of Hamiltonians. Competition between phase separation,
superfluidity, supersolidity and fermionic phases is investigated in the four
families.Comment: 3 figure
Competing supersolids of Bose-Bose mixtures in a triangular lattice
We study the ground state properties of a frustrated two-species mixture of
hard-core bosons on a triangular lattice, as a function of tunable amplitudes
for tunnelling and interactions. By combining three different methods, a
self-consistent cluster mean-field, exact diagonalizations and effective
theories, we unravel a very rich and complex phase diagram. More specifically,
we discuss the existence of three original mixture supersolids: (i) a
commensurate with frozen densities and supersolidity in spin degrees of
freedom, in a regime of strong interspecies interactions; and (ii) when this
interaction is weaker, two mutually competing incommensurate supersolids.
Finally, we show how these phases can be stabilized by a quantum fluctuation
enhancement of peculiar insulating parent states.Comment: 6 pages, 9 figure