241 research outputs found
Tunable Electron Interactions and Fractional Quantum Hall States in Graphene
The recent discovery of fractional quantum Hall states in graphene raises the
question of whether the physics of graphene and its bilayer offers any
advantages over GaAs-based materials in exploring strongly-correlated states of
two-dimensional electrons. Here we propose a method to continuously tune the
effective electron interactions in graphene and its bilayer by the dielectric
environment of the sample. Using this method, the charge gaps of prominent FQH
states, including \nu=1/3 or \nu=5/2 states, can be increased several times, or
reduced all the way to zero. The tunability of the interactions can be used to
realize and stabilize various strongly correlated phases in the FQH regime, and
to explore the transitions between them.Comment: 4.2 pages, 5 figure
Internal screening and dielectric engineering in magic-angle twisted bilayer graphene
Magic-angle twisted bilayer graphene (MA-tBLG) has appeared as a tunable
testing ground to investigate the conspiracy of electronic interactions, band
structure, and lattice degrees of freedom to yield exotic quantum many-body
ground states in a two-dimensional Dirac material framework. While the impact
of external parameters such as doping or magnetic field can be conveniently
modified and analyzed, the all-surface nature of the quasi-2D electron gas
combined with its intricate internal properties pose a challenging task to
characterize the quintessential nature of the different insulating and
superconducting states found in experiments. We analyze the interplay of
internal screening and dielectric environment on the intrinsic electronic
interaction profile of MA-tBLG. We find that interlayer coupling generically
enhances the internal screening. The influence of the dielectric environment on
the effective interaction strength depends decisively on the electronic state
of MA-tBLG. Thus, we propose the experimental tailoring of the dielectric
environment, e.g. by varying the capping layer composition and thickness, as a
promising pursuit to provide further evidence for resolving the hidden nature
of the quantum many-body states in MA-tBLG.Comment: 9 pages, 3 figures, supplemental material included (8 figures
Functional renormalization group study of an eight-band model for the iron arsenides
We investigate the superconducting pairing instabilities of eight-band models
for the iron arsenides. Using a functional renormalization group treatment, we
determine how the critical energy scale for superconductivity depends on the
electronic band structure. Most importantly, if we vary the parameters from
values corresponding to LaFeAsO to SmFeAsO, the pairing scale is strongly
enhanced, in accordance with the experimental observation. We analyze the
reasons for this trend and compare the results of the eight-band approach to
those found using five-band models.Comment: 11 pages, 10 figure
Thermal Hall Conductivity as a Probe of Gap Structure in Multi-band Superconductors: The Case of
The sign and profile of the thermal Hall conductivity gives
important insights into the gap structure of multi-band superconductors. With
this perspective, we have investigated and the thermal
conductivity in which display large
peak anomalies in the superconducting state. The anomalies imply that a large
hole-like quasiparticle (qp) population exists below the critical temperature
. We show that the qp mean-free-path inferred from
reproduces the observed anomaly in , providing a consistent
estimate of a large qp population. Further, we demonstrate that the hole-like
signal is consistent with a theoretical scenario where despite potentially
large gap variations on the electron pockets, the minimal homogeneous gap of
the superconducting phase resides at a hole pocket. Implications for probing
the gap structure in the broader class of pnictide superconductors are
discussed.Comment: 5 pages, 4 figures. Orientation significantly updated from previous
(0811.4668v1) reflecting new theoretical understanding of experimental
results and physical implications. Introduction, discussion, and figures
updated including additional figure for model calculatio
Bound states in two-dimensional spin systems near the Ising limit: A quantum finite-lattice study
We analyze the properties of low-energy bound states in the transverse-field
Ising model and in the XXZ model on the square lattice. To this end, we develop
an optimized implementation of perturbative continuous unitary transformations.
The Ising model is studied in the small-field limit which is found to be a
special case of the toric code model in a magnetic field. To analyze the XXZ
model, we perform a perturbative expansion about the Ising limit in order to
discuss the fate of the elementary magnon excitations when approaching the
Heisenberg point.Comment: 21 pages, 18 figures, published versio
Relevance of the Heisenberg-Kitaev model for the honeycomb lattice iridates A_2IrO_3
Combining thermodynamic measurements with theoretical density functional and
thermodynamic calculations we demonstrate that the honeycomb lattice iridates
A2IrO3 (A = Na, Li) are magnetically ordered Mott insulators where the
magnetism of the effective spin-orbital S = 1/2 moments can be captured by a
Heisenberg-Kitaev (HK) model with Heisenberg interactions beyond
nearest-neighbor exchange. Experimentally, we observe an increase of the
Curie-Weiss temperature from \theta = -125 K for Na2IrO3 to \theta = -33 K for
Li2IrO3, while the antiferromagnetic ordering temperature remains roughly the
same T_N = 15 K for both materials. Using finite-temperature functional
renormalization group calculations we show that this evolution of \theta, T_N,
the frustration parameter f = \theta/T_N, and the zig-zag magnetic ordering
structure suggested for both materials by density functional theory can be
captured within this extended HK model. Combining our experimental and
theoretical results, we estimate that Na2IrO3 is deep in the magnetically
ordered regime of the HK model (\alpha \approx 0.25), while Li2IrO3 appears to
be close to a spin-liquid regime (0.6 < \alpha < 0.7).Comment: Version accepted for publication in PRL. Additional DFT and
thermodynamic calculations have been included. 6 pages of supplementary
material include
Doping Evolution of Oxygen K-edge X-ray Absorption Spectra in Cuprate Superconductors
We study oxygen K-edge x-ray absorption spectroscopy (XAS) and investigate
the validity of the Zhang-Rice singlet (ZRS) picture in overdoped cuprate
superconductors. Using large-scale exact diagonalization of the three-orbital
Hubbard model, we observe the effect of strong correlations manifesting in a
dynamical spectral weight transfer from the upper Hubbard band to the ZRS band.
The quantitative agreement between theory and experiment highlights an
additional spectral weight reshuffling due to core-hole interaction. Our
results confirm the important correlated nature of the cuprates and elucidate
the changing orbital character of the low-energy quasi-particles, but also
demonstrate the continued relevance of the ZRS even in the overdoped region.Comment: Original: 5 pages, 4 figures. Replaced: 6 pages and 4 figures, with
updated title and conten
Signatures of a gearwheel quantum spin liquid in a spin- pyrochlore molybdate Heisenberg antiferromagnet
We theoretically investigate the low-temperature phase of the recently
synthesized LuMoON material, an extraordinarily rare
realization of a three-dimensional pyrochlore Heisenberg
antiferromagnet in which Mo are the magnetic species. Despite a
Curie-Weiss temperature () of K, experiments have
found no signature of magnetic ordering spin freezing down to
K. Using density functional theory, we find that the compound
is well described by a Heisenberg model with exchange parameters up to third
nearest neighbors. The analysis of this model via the pseudofermion functional
renormalization group method reveals paramagnetic behavior down to a
temperature of at least , in agreement with the
experimental findings hinting at a possible three-dimensional quantum spin
liquid. The spin susceptibility profile in reciprocal space shows
momentum-dependent features forming a "gearwheel" pattern, characterizing what
may be viewed as a molten version of a chiral noncoplanar incommensurate spiral
order under the action of quantum fluctuations. Our calculated reciprocal space
susceptibility maps provide benchmarks for future neutron scattering
experiments on single crystals of LuMoON.Comment: Published version. Main paper (6 pages, 3 figures) + Supplemental
Material (4 pages, 3 figures, 1 table
- …