25,528 research outputs found
Gapless Fermions and Quantum Order
Using 2D quantum spin-1/2 model as a concrete example, we studied the
relation between gapless fermionic excitations (spinons) and quantum orders in
some spin liquid states. Using winding number, we find the projective symmetry
group that characterizes the quantum order directly determines the pattern of
Fermi points in the Brillouin zone. Thus quantum orders provide an origin for
gapless fermionic excitations.Comment: 23 pages. LaTeX. Homepage http://dao.mit.edu/~we
An Ultra-Low-Power Oscillator with Temperature and Process Compensation for UHF RFID Transponder
This paper presents a 1.28MHz ultra-low-power oscillator with temperature and process compensation. It is very suitable for clock generation circuits used in ultra-high-frequency (UHF) radio-frequency identification (RFID) transponders. Detailed analysis of the oscillator design, including process and temperature compensation techniques are discussed. The circuit is designed using TSMC 0.18μm standard CMOS process and simulated with Spectre. Simulation results show that, without post-fabrication calibration or off-chip components, less than ±3% frequency variation is obtained from –40 to 85°C in three different process corners. Monte Carlo simulations have also been performed, and demonstrate a 3σ deviation of about 6%. The power for the proposed circuitry is only 1.18µW at 27°C
Escape of Ionizing Radiation from High Redshift Galaxies
We model the escape of ionizing radiation from high-redshift galaxies using
high-resolution Adaptive Mesh Refinement N-body + hydrodynamics simulations.
Our simulations include time-dependent and spatially-resolved transfer of
ionizing radiation in three dimensions, including effects of dust absorption.
For galaxies of total mass M > 10^11 Msun and star formation rates SFR ~ 1-5
Msun/yr, we find angular averaged escape fractions of 0.01-0.03 over the entire
redshift interval studied (3<z<9). In addition, we find that the escape
fraction varies by more than an order of magnitude along different
lines-of-sight within individual galaxies, from the largest values near
galactic poles to the smallest along the galactic disk. The escape fraction
declines steeply at lower masses and SFR. We show that the low values of escape
fractions are due to a small fraction of young stars located just outside the
edge of HI disk. We compare our predicted escape fraction of ionizing photons
with previous results, and find a general agreement with both other simulation
results and available direct detection measurements at z ~ 3. We also compare
our simulations with a novel method to estimate the escape fraction in galaxies
from the observed distribution of neutral hydrogen column densities along the
lines of sights to long duration gamma-ray bursts. Using this method we find
escape fractions of the GRB host galaxies of 2-3%, consistent with our
theoretical predictions. [abridged]Comment: submitted to Ap
Scaling of Anisotropic Flows and Nuclear Equation of State in Intermediate Energy Heavy Ion Collisions
Elliptic flow () and hexadecupole flow () of light clusters have
been studied in details for 25 MeV/nucleon Kr + Sn at large
impact parameters by Quantum Molecular Dynamics model with different potential
parameters. Four parameter sets which include soft or hard equation of state
(EOS) with/without symmetry energy term are used. Both number-of-nucleon ()
scaling of the elliptic flow versus transverse momentum () and the scaling
of versus have been demonstrated for the light clusters
in all above calculation conditions. It was also found that the ratio of
keeps a constant of 1/2 which is independent of for all the
light fragments. By comparisons among different combinations of EOS and
symmetry potential term, the results show that the above scaling behaviors are
solid which do not depend the details of potential, while the strength of flows
is sensitive to EOS and symmetry potential term.Comment: 5 pages, 5 figure
A scheme for demonstration of fractional statistics of anyons in an exactly solvable model
We propose a scheme to demonstrate fractional statistics of anyons in an
exactly solvable lattice model proposed by Kitaev that involves four-body
interactions. The required many-body ground state, as well as the anyon
excitations and their braiding operations, can be conveniently realized through
\textit{dynamic}laser manipulation of cold atoms in an optical lattice. Due to
the perfect localization of anyons in this model, we show that a quantum
circuit with only six qubits is enough for demonstration of the basic braiding
statistics of anyons. This opens up the immediate possibility of
proof-of-principle experiments with trapped ions, photons, or nuclear magnetic
resonance systems.Comment: 4 pages, 3 figure
Topological surface states in three-dimensional magnetic insulators
An electron moving in a magnetically ordered background feels an effective
magnetic field that can be both stronger and more rapidly varying than typical
externally applied fields. One consequence is that insulating magnetic
materials in three dimensions can have topologically nontrivial properties of
the effective band structure. For the simplest case of two bands, these "Hopf
insulators" are characterized by a topological invariant as in quantum Hall
states and Z_2 topological insulators, but instead of a Chern number or parity,
the underlying invariant is the Hopf invariant that classifies maps from the
3-sphere to the 2-sphere. This paper gives an efficient algorithm to compute
whether a given magnetic band structure has nontrivial Hopf invariant, a
double-exchange-like tight-binding model that realizes the nontrivial case, and
a numerical study of the surface states of this model.Comment: 4 pages, 2 figures; published versio
Charge and spin fractionalization in strongly correlated topological insulators
We construct an effective topological Landau-Ginzburg theory that describes
general SU(2) incompressible quantum liquids of strongly correlated particles
in two spatial dimensions. This theory characterizes the fractionalization of
quasiparticle quantum numbers and statistics in relation to the topological
ground-state symmetries, and generalizes the Chern-Simons, BF and hierarchical
effective gauge theories to an arbitrary representation of the SU(2) symmetry
group. Our main focus are fractional topological insulators with time-reversal
symmetry, which are treated as generalizations of the SU(2) quantum Hall
effect.Comment: 8 pages, published versio
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