77 research outputs found
Multiperiodic magnetic structures in Hubbard superlattices
We consider fermions in one-dimensional superlattices (SL's), modeled by
site-dependent Hubbard-U couplings arranged in a repeated pattern of repulsive
(i.e., U>0) and free (U=0) sites. Density Matrix Renormalization Group (DMRG)
diagonalization of finite systems is used to calculate the local moment and the
magnetic structure factor in the ground state. We have found four regimes for
magnetic behavior: uniform local moments forming a spin-density wave (SDW),
`floppy' local moments with short-ranged correlations, local moments on
repulsive sites forming long-period SDW's superimposed with short-ranged
correlations, and local moments on repulsive sites solely with long-period
SDW's; the boundaries between these regimes depend on the range of electronic
densities, rho, and on the SL aspect ratio. Above a critical electronic
density, rho_{uparrow downarrow}, the SDW period oscillates both with rho and
with the spacer thickness. The former oscillation allows one to reproduce all
SDW wave vectors within a small range of electronic densities, unlike the
homogeneous system. The latter oscillation is related to the exchange
oscillation observed in magnetic multilayers. A crossover between regimes of
`thin' to `thick' layers has also been observed.Comment: 9 two-column pages, 10 figure
Size and shape of Mott regions for fermionic atoms in a two-dimensional optical lattice
We investigate the harmonic-trap control of size and shape of Mott regions in
the Fermi Hubbard model on a square optical lattice. The use of Lanczos
diagonalization on clusters with twisted boundary conditions, followed by an
average over 50-80 samples, drastically reduce finite-size effects in some
ground state properties; calculations in the grand canonical ensemble together
with a local-density approximation (LDA) allow us to simulate the radial
density distribution. We have found that as the trap closes, the atomic cloud
goes from a metallic state, to a Mott core, and to a Mott ring; the coverage of
Mott atoms reaches a maximum at the core-ring transition. A `phase diagram' in
terms of an effective density and the on-site repulsion is proposed, as a guide
to maximize the Mott coverage. We also predict that the usual experimentally
accessible quantities, the global compressibility and the average double
occupancy (rather, its density derivative) display detectable signatures of the
core-ring transition. Some spin correlation functions are also calculated, and
predict the existence N\'eel ordering within Mott cores and rings.Comment: 5 pages, 6 figure
Fermi-surface Reconstruction in the Repulsive Fermi-Hubbard Model
One of the fundamental questions about the high temperature cuprate
superconductors is the size of the Fermi surface (FS) underlying the
superconducting state. By analyzing the single particle spectral function for
the Fermi Hubbard model as a function of repulsion and chemical potential
, we find that the Fermi surface in the normal state reconstructs from a
large Fermi surface matching the Luttinger volume as expected in a Fermi
liquid, to a Fermi surface that encloses fewer electrons that we dub the
"Luttinger Breaking" (LB) phase, as the Mott insulator is approached. This
transition into a non-Fermi liquid phase that violates the Luttinger count, is
a continuous phase transition at a critical density in the absence of any other
broken symmetry. We obtain the Fermi surface contour from the spectral weight
and from an analysis of the poles and zeros of the
retarded Green's function , calculated using
determinantal quantum Monte Carlo and analytic continuation methods.We discuss
our numerical results in connection with experiments on Hall measurements,
scanning tunneling spectroscopy and angle resolved photoemission spectroscopy
Short-Range Correlations and Cooling of Ultracold Fermions in the Honeycomb Lattice
We use determinantal quantum Monte Carlo simulations and numerical
linked-cluster expansions to study thermodynamic properties and short-range
spin correlations of fermions in the honeycomb lattice. We find that, at half
filling and finite temperatures, nearest-neighbor spin correlations can be
stronger in this lattice than in the square lattice, even in regimes where the
ground state in the former is a semimetal or a spin liquid. The honeycomb
lattice also exhibits a more pronounced anomalous region in the double
occupancy that leads to stronger adiabatic cooling than in the square lattice.
We discuss the implications of these findings for optical lattice experiments.Comment: 5 pages, 4 figure
Finite-temperature properties of strongly correlated fermions in the honeycomb lattice
We study finite-temperature properties of strongly interacting fermions in the honeycomb lattice using numerical linked-cluster expansions and determinantal quantum Monte Carlo simulations. We analyze a number of thermodynamic quantities, including the entropy, the specific heat, uniform and staggered spin susceptibilities, short-range spin correlations, and the double occupancy at and away from half filling. We examine the viability of adiabatic cooling by increasing the interaction strength for homogeneous as well as for trapped systems. For the homogeneous case, this process is found to be more efficient at finite doping than at half filling. That, in turn, leads to an efficient adiabatic cooling in the presence of a trap, which, starting with even relatively high entropies, can drive the system to have a Mott insulating phase with substantial antiferromagnetic correlations
Fermions in 3D Optical Lattices: Cooling Protocol to Obtain Antiferromagnetism
A major challenge in realizing antiferromagnetic (AF) and superfluid phases
in optical lattices is the ability to cool fermions. We determine the equation
of state for the 3D repulsive Fermi-Hubbard model as a function of the chemical
potential, temperature and repulsion using unbiased determinantal quantum Monte
Carlo methods, and we then use the local density approximation to model a
harmonic trap. We show that increasing repulsion leads to cooling, but only in
a trap, due to the redistribution of entropy from the center to the metallic
wings. Thus, even when the average entropy per particle is larger than that
required for antiferromagnetism in the homogeneous system, the trap enables the
formation of an AF Mott phase.Comment: 4 pages; 5 figures; also see supplementary material in 2 pages with 1
figur
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