1,216 research outputs found
Magnon pairing in quantum spin nematic
Competing ferro- and antiferromagnetic exchange interactions may lead to the
formation of bound magnon pairs in the high-field phase of a frustrated quantum
magnet. With decreasing field, magnon pairs undergo a Bose-condensation prior
to the onset of a conventional one-magnon instability. We develop an analytical
approach to study the zero-temperature properties of the magnon-pair
condensate, which is a bosonic analog of the BCS superconductors.
Representation of the condensate wave-function in terms of the coherent bosonic
states reveals the spin-nematic symmetry of the ground-state and allows one to
calculate various static properties. Sharp quasiparticle excitations are found
in the nematic state with a small finite gap. We also predict the existence of
a long-range ordered spin-nematic phase in the frustrated chain material
LiCuVO4 at high fields.Comment: 5 pages, final versio
A Simple Model for the Checkerboard Pattern of Modulated Hole Densities in Underdoped Cuprates
A simple model is proposed as a possible explanation for the checkerboard
pattern of modulations in the hole density observed in recent tunneling
experiments on underdoped cuprates. Two assumptions are made; first, an
enhanced hole density near the acceptor dopants and secondly short range
correlations in the positions of these dopants caused by their electrostatic
and anisotropic elastic interactions. Together these can lead to a structure
factor in qualitative agreement with experiment.Comment: 4 pages, 4 figures; Fig.3 and Fig.4(c) added, typos corrected,
references adde
Exact low-temperature behavior of kagome antiferromagnet at high fields
Low-energy degrees of freedom of a spin-1/2 kagome antiferromagnet in the
vicinity of the saturation field are mapped to a hard-hexagon model on a
triangular lattice. The latter model is exactly solvable. The presented mapping
allows to obtain quantitative description of the magnetothermodynamics of a
quantum kagome antiferromagnet up to exponentially small corrections as well as
predict the critical behavior for the transition into a magnon crystal state.
Analogous mapping is presented for the sawtooth chain, which is mapped onto a
model of classical hard dimers on a chain.Comment: 5 pages, 2 figures, replaced with accepted versio
Thermometry and Refrigeration in a Two-Component Mott Insulator of Ultracold Atoms
Interesting spin Hamiltonians can be realized with ultracold atoms in a
two-component Mott insulator (2CMI). It was recently demonstrated that the
application of a magnetic field gradient to the 2CMI enables new techniques of
thermometry and adiabatic cooling. Here we present a theoretical description
which provides quantitative analysis of these two new techniques. We show that
adiabatic reduction of the field gradient is capable of cooling below the Curie
or N\'eel temperature of certain spin ordered phases.Comment: 5 pages, 5 figures (v4): Added journal referenc
Spin gradient thermometry for ultracold atoms in optical lattices
We demonstrate spin gradient thermometry, a new general method of measuring
the temperature of ultracold atoms in optical lattices. We realize a mixture of
spins separated by a magnetic field gradient. Measurement of the width of the
transition layer between the two spin domains serves as a new method of
thermometry which is observed to work over a broad range of lattice depths and
temperatures, including in the Mott insulator regime. We demonstrate the
thermometry in a system of ultracold rubidium atoms, and suggest that
interesting spin physics can be realized in this system. The lowest measured
temperature is 1 nK, indicating that the system has reached the quantum regime,
where insulating shells are separated by superfluid layers.Comment: 5 pages, 3 figures, minor edits for clarit
Spin gradient demagnetization cooling of ultracold atoms
A major goal of ultracold atomic physics is quantum simulation of spin
Hamiltonians in optical lattices. Progress towards this goal requires the
attainment of extremely low temperatures. Here we demonstrate a new cooling
method which consists of applying a time-varying magnetic field gradient to a
spin mixture of ultracold atoms. We have used this method to prepare isolated
spin distributions at positive and negative spin temperatures of +/-50
picokelvin. The spin system can also be used to cool other degrees of freedom,
and we have used this coupling to reduce the temperature of an apparently
equilibrated sample of rubidium atoms in a Mott insulating state to 350
picokelvin. These are the lowest temperatures ever measured in any system.Comment: 4 pages, 4 figures; (v4) Shortened, added journal re
Lattice gas description of pyrochlore and checkerboard antiferromagnets in a strong magnetic field
Quantum Heisenberg antiferromagnets on pyrochlore and checkerboard lattices
in a strong external magnetic field are mapped onto hard-core lattice gases
with an extended exclusion region. The effective models are studied by the
exchange Monte Carlo simulations and by the transfer matrix method. The
transition point and the critical exponents are obtained numerically for a
square-lattice gas of particles with the second-neighbor exclusion, which
describes a checkerboard antiferromagnet. The exact structure of the magnon
crystal state is determined for a pyrochlore antiferromagnet.Comment: 11 pages, accepted versio
Fluctuation properties of strength function associated with the giant quadrupole resonance in 208Pb
We performed fluctuation analysis by means of the local scaling dimension for
the strength function of the isoscalar (IS) giant quadrupole resonance (GQR) in
208Pb where the strength function is obtained by the shell model calculation
including 1p1h and 2p2h configurations. It is found that at almost all energy
scales, fluctuation of the strength function obeys the Gaussian orthogonal
ensemble (GOE) random matrix theory limit. This is contrasted with the results
for the GQR in 40Ca, where at the intermediate energy scale about 1.7 MeV a
deviation from the GOE limit was detected. It is found that the physical origin
for this different behavior of the local scaling dimension is ascribed to the
difference in the properties of the damping process.Comment: 10 pages, 14 figures, submitted to Physical Review
The Co‐Occurring Development of Executive Function Skills and Receptive Vocabulary in Preschool‐Aged Children: A Look at the Direction of the Developmental Pathways
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106081/1/icd1829.pd
Bragg Scattering as a Probe of Atomic Wavefunctions and Quantum Phase Transitions in Optical Lattices
We have observed Bragg scattering of photons from quantum degenerate
Rb atoms in a three-dimensional optical lattice. Bragg scattered light
directly probes the microscopic crystal structure and atomic wavefunction whose
position and momentum width is Heisenberg-limited. The spatial coherence of the
wavefunction leads to revivals in the Bragg scattered light due to the atomic
Talbot effect. The decay of revivals across the superfluid to Mott insulator
transition indicates the loss of superfluid coherence.Comment: 5 pages, 4 figure
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