8 research outputs found
Kondo Phase in Twisted Bilayer Graphene -- A Unified Theory for Distinct Experiments
A number of interesting physical phenomena have been discovered in
magic-angle twisted bilayer graphene (MATBG), such as superconductivity,
correlated gapped and gapless phases, etc. The gapped phases are believed to be
symmetry-breaking states described by mean-field theories, whereas gapless
phases exhibit features beyond mean field. This work, combining poor man's
scaling, numerical renormalization group, and dynamic mean-field theory,
demonstrates that the gapless phases are the heavy Fermi liquid state with some
symmetries broken and the others preserved. We adopt the recently proposed
topological heavy fermion model for MATBG with effective local orbitals around
AA-stacking regions and Dirac fermions surrounding them. At zero temperature
and most non-integer fillings, the ground states are found to be heavy Fermi
liquids and exhibit Kondo resonance peaks. The Kondo temperature is found
at the order of 1meV. A higher temperature than will drive the system
into a metallic LM phase where disordered LM's and a Fermi liquid coexist. At
integer fillings , is suppressed to zero or a value weaker
than RKKY interaction, leading to Mott insulators or symmetry-breaking states.
This theory offers a unified explanation for several experimental observations,
such as zero-energy peaks and quantum-dot-like behaviors in STM, the
Pomeranchuk effect, and the saw-tooth feature of inverse compressibility, etc.
For future experimental verification, we predict that the Fermi surface in the
gapless phase will shrink upon heating - as a characteristic of the heavy Fermi
liquid. We also conjecture that the heavy Fermi liquid is the parent state of
the observed unconventional superconductivity because the Kondo screening
reduces the overwhelming Coulomb interaction (~60meV) to a rather small
effective interaction (~1meV) comparable to possible weak attractive
interactions.Comment: DMFT calculations for the THF model and discussions on possible
symmetry-breaking states are adde
Hierarchical Liouville-space approach for accurate and universal characterization of quantum impurity systems
A hierarchical equations of motion (HEOM) based numerical approach is
developed for accurate and efficient evaluation of dynamical observables of
strongly correlated quantum impurity systems. This approach is capable of
describing quantitatively Kondo resonance and Fermi liquid characteristics,
achieving the accuracy of latest high-level numerical renormalization group
approach, as demonstrated on single-impurity Anderson model systems. Its
application to a two-impurity Anderson model results in differential
conductance versus external bias, which correctly reproduces the continuous
transition from Kondo states of individual impurity to singlet spin-states
formed between two impurities. The outstanding performance on characterizing
both equilibrium and nonequilibrium properties of quantum impurity systems
makes the HEOM approach potentially useful for addressing strongly correlated
lattice systems in the frame work of dynamical mean field theory.Comment: 5 pages, 4 figures, to appear in PR