114 research outputs found
Sign problem free quantum Monte-Carlo study on thermodynamic properties and magnetic phase transitions in orbital-active itinerant ferromagnets
The microscopic mechanism of itinerant ferromagnetism is a long-standing
problem due to the lack of non-perturbative methods to handle strong magnetic
fluctuations of itinerant electrons. We have non-pertubatively studied
thermodynamic properties and magnetic phase transitions of a two-dimensional
multi-orbital Hubbard model exhibiting ferromagnetic ground states. Quantum
Monte-Carlo simulations are employed, which are proved in a wide density region
free of the sign problem usually suffered by simulations for fermions. Both
Hund's coupling and electron itinerancy are essential for establishing the
ferromagnetic coherence. No local magnetic moments exist in the system as a
priori, nevertheless, the spin channel remains incoherent showing the
Curie-Weiss type spin magnetic susceptibility down to very low temperatures at
which the charge channel is already coherent exhibiting a weakly
temperature-dependent compressibility. For the SU(2) invariant systems, the
spin susceptibility further grows exponentially as approaching zero temperature
in two dimensions. In the paramagnetic phase close to the Curie temperature,
the momentum space Fermi distributions exhibit strong resemblance to those in
the fully polarized state. The long-range ferromagnetic ordering appears when
the symmetry is reduced to the Ising class, and the Curie temperature is
accurately determined. These simulations provide helpful guidance to searching
for novel ferromagnetic materials in both strongly correlated -orbital
transition metal oxide layers and the -orbital ultra-cold atom optical
lattice systems.Comment: 17 pages, 17 figure
Detecting edge degeneracy in interacting topological insulators through entanglement entropy
The existence of degenerate or gapless edge states is a characteristic
feature of topological insulators, but is difficult to detect in the presence
of interactons. We propose a new method to obtain the degeneracy of the edge
states from the perspective of entanglement entropy, which is very useful to
identify interacting topological states. Employing the determinant quantum
Monte Carlo technique, we investigate the interaction effect on two
representative models of fermionic topological insulators in one and two
dimensions, respectively. In the two topologically nontrivial phases, the edge
degeneracies are reduced by interactions but remain to be nontrivial.Comment: 6 pages, 4 figure
One-dimensional Quantum Spin Dynamics of Bethe String States
Quantum dynamics of strongly correlated systems is a challenging problem.
Although the low energy fractional excitations of one dimensional integrable
models are often well-understood, exploring quantum dynamics in these systems
remains challenging in the gapless regime, especially at intermediate and high
energies. Based on the algebraic Bethe ansatz formalism, we study spin dynamics
in a representative one dimensional strongly correlated model, {\it i.e. }, the
antiferromagnetic spin- XXZ chain with the Ising anisotropy, via
the form-factor formulae. Various excitations at different energy scales are
identified crucial to the dynamic spin structure factors under the guidance of
sum rules. At small magnetic polarizations, gapless excitations dominate the
low energy spin dynamics arising from the magnetic-field-induced
incommensurability. In contrast, spin dynamics at intermediate and high
energies is characterized by the two- and three-string states, which are
multi-particle excitations based on the commensurate N\'eel ordered background.
Our work is helpful for experimental studies on spin dynamics in both condensed
matter and cold atom systems beyond the low energy effective Luttinger liquid
theory. Based on an intuitive physical picture, we speculate that the dynamic
feature at high energies due to the multi-particle anti-bound state excitations
can be generalized to non-integrable spin systems.Comment: 15 pages, to appear in Phys. Rev.
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