26,041 research outputs found
Differential recruitment of brain networks following route and cartographic map learning of spatial environments.
An extensive neuroimaging literature has helped characterize the brain regions involved in navigating a spatial environment. Far less is known, however, about the brain networks involved when learning a spatial layout from a cartographic map. To compare the two means of acquiring a spatial representation, participants learned spatial environments either by directly navigating them or learning them from an aerial-view map. While undergoing functional magnetic resonance imaging (fMRI), participants then performed two different tasks to assess knowledge of the spatial environment: a scene and orientation dependent perceptual (SOP) pointing task and a judgment of relative direction (JRD) of landmarks pointing task. We found three brain regions showing significant effects of route vs. map learning during the two tasks. Parahippocampal and retrosplenial cortex showed greater activation following route compared to map learning during the JRD but not SOP task while inferior frontal gyrus showed greater activation following map compared to route learning during the SOP but not JRD task. We interpret our results to suggest that parahippocampal and retrosplenial cortex were involved in translating scene and orientation dependent coordinate information acquired during route learning to a landmark-referenced representation while inferior frontal gyrus played a role in converting primarily landmark-referenced coordinates acquired during map learning to a scene and orientation dependent coordinate system. Together, our results provide novel insight into the different brain networks underlying spatial representations formed during navigation vs. cartographic map learning and provide additional constraints on theoretical models of the neural basis of human spatial representation
Coherent Perfect Absorbers: Time-reversed Lasers
We show that an arbitrary body or aggregate can be made perfectly absorbing
at discrete frequencies if a precise amount of dissipation is added under
specific conditions of coherent monochromatic illumination. This effect arises
from the interaction of optical absorption and wave interference, and
corresponds to moving a zero of the elastic S-matrix onto the real wavevector
axis. It is thus the time-reversed process of lasing at threshold. The effect
is demonstrated in a simple Si slab geometry illuminated in the 500-900 nm
range. Coherent perfect absorbers are novel linear optical elements, absorptive
interferometers, which may be useful for controlled optical energy transfer.Comment: 4 pages, 4 figure
Finite temperature phase diagram of a spin-polarized ultracold Fermi gas in a highly elongated harmonic trap
We investigate the finite temperature properties of an ultracold atomic Fermi
gas with spin population imbalance in a highly elongated harmonic trap.
Previous studies at zero temperature showed that the gas stays in an exotic
spatially inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid
state at the trap center; while moving to the edge, the system changes into
either a non-polarized Bardeen-Cooper-Schriffer superfluid () or a fully
polarized normal gas (), depending on the smallness of the spin
polarization , relative to a critical value . In this work, we show how
these two phase-separation phases evolve with increasing temperature, and
thereby construct a finite temperature phase diagram. For typical interactions,
we find that the exotic FFLO phase survives below one-tenth of Fermi degeneracy
temperature, which seems to be accessible in the current experiment. The
density profile, equation of state, and specific heat of the polarized system
have been calculated and discussed in detail. Our results are useful for the
on-going experiment at Rice University on the search for FFLO states in
quasi-one-dimensional polarized Fermi gases.Comment: 9 pages and 8 figures; Published version in Phys. Rev.
Exact few-body results for strongly correlated quantum gases in two dimensions
The study of strongly correlated quantum gases in two dimensions has
important ramifications for understanding many intriguing pheomena in solid
materials, such as high- superconductivity and the fractional quantum
Hall effect. However, theoretical methods are plagued by the existence of
significant quantum fluctuations. Here, we present two- and three-body exact
solutions for both fermions and bosons trapped in a two-dimensional harmonic
potential, with an arbitrary -wave scattering length. These few-particle
solutions link in a natural way to the high-temperature properties of
many-particle systems via a quantum virial expansion. As a concrete example,
using the energy spectrum of few fermions, we calculate the second and third
virial coefficients of a strongly interacting Fermi gas in two dimensions, and
consequently investigate its high-temperature thermodynamics. Our thermodynamic
results may be useful for ongoing experiments on two-dimensional Fermi gases.
These exact results also provide an unbiased benchmark for quantum Monte Carlo
simulations of two-dimensional Fermi gases at high temperatures.Comment: 11 pages, 6 figure
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