70,096 research outputs found
Numerical Studies of the two-leg Hubbard ladder
The Hubbard model on a two-leg ladder structure has been studied by a
combination of series expansions at T=0 and the density-matrix renormalization
group. We report results for the ground state energy and spin-gap
at half-filling, as well as dispersion curves for one and two-hole
excitations. For small both and show a dramatic drop near
, which becomes more gradual for larger . This
represents a crossover from a "band insulator" phase to a strongly correlated
spin liquid. The lowest-lying two-hole state rapidly becomes strongly bound as
increases, indicating the possibility that phase separation may
occur. The various features are collected in a "phase diagram" for the model.Comment: 10 figures, revte
Critical domain-wall dynamics of model B
With Monte Carlo methods, we simulate the critical domain-wall dynamics of
model B, taking the two-dimensional Ising model as an example. In the
macroscopic short-time regime, a dynamic scaling form is revealed. Due to the
existence of the quasi-random walkers, the magnetization shows intrinsic
dependence on the lattice size . A new exponent which governs the
-dependence of the magnetization is measured to be .Comment: 10pages, 4 figure
Networking Effects on Cooperation in Evolutionary Snowdrift Game
The effects of networking on the extent of cooperation emerging in a
competitive setting are studied. The evolutionary snowdrift game, which
represents a realistic alternative to the well-known Prisoner's Dilemma, is
studied in the Watts-Strogatz network that spans the regular, small-world, and
random networks through random re-wiring. Over a wide range of payoffs, a
re-wired network is found to suppress cooperation when compared with a
well-mixed or fully connected system. Two extinction payoffs, that characterize
the emergence of a homogeneous steady state, are identified. It is found that,
unlike in the Prisoner's Dilemma, the standard deviation of the degree
distribution is the dominant network property that governs the extinction
payoffs.Comment: Changed conten
Cosmic clocks: A Tight Radius - Velocity Relationship for HI-Selected Galaxies
HI-Selected galaxies obey a linear relationship between their maximum
detected radius Rmax and rotational velocity. This result covers measurements
in the optical, ultraviolet, and HI emission in galaxies spanning a factor of
30 in size and velocity, from small dwarf irregulars to the largest spirals.
Hence, galaxies behave as clocks, rotating once a Gyr at the very outskirts of
their discs. Observations of a large optically-selected sample are consistent,
implying this relationship is generic to disc galaxies in the low redshift
Universe. A linear RV relationship is expected from simple models of galaxy
formation and evolution. The total mass within Rmax has collapsed by a factor
of 37 compared to the present mean density of the Universe. Adopting standard
assumptions we find a mean halo spin parameter lambda in the range 0.020 to
0.035. The dispersion in lambda, 0.16 dex, is smaller than expected from
simulations. This may be due to the biases in our selection of disc galaxies
rather than all halos. The estimated mass densities of stars and atomic gas at
Rmax are similar (~0.5 Msun/pc^2) indicating outer discs are highly evolved.
The gas consumption and stellar population build time-scales are hundreds of
Gyr, hence star formation is not driving the current evolution of outer discs.
The estimated ratio between Rmax and disc scale length is consistent with
long-standing predictions from monolithic collapse models. Hence, it remains
unclear whether disc extent results from continual accretion, a rapid initial
collapse, secular evolution or a combination thereof.Comment: 14 pages, 7 figures, 3 in colour. Published in MNRAS. This v2
corrects wrong journal in the references section (all instances of
"Astrophysics and Space Sciences" should have been ApJ). The Posti+2017 has
also been updated. An erratum has been submitted to MNRA
Ground state and finite temperature signatures of quantum phase transitions in the half-filled Hubbard model on a honeycomb lattice
We investigate ground state and finite temperature properties of the
half-filled Hubbard model on a honeycomb lattice using quantum monte carlo and
series expansion techniques. Unlike the square lattice, for which magnetic
order exists at T=0 for any non-zero , the honeycomb lattice is known to
have a semi-metal phase at small and an antiferromagnetic one at large .
We investigate the phase transition at T=0 by studying the magnetic
structureU_c/tC(T)U>U_cU <
U_cUC(T)U \approx U_c$.Comment: 11 pages, 19 figure
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