9,933 research outputs found
The t-J model on a semi-infinite lattice
The hole spectral function of the t-J model on a two-dimensional
semi-infinite lattice is calculated using the spin-wave and noncrossing
approximations. In the case of small hole concentration and strong
correlations, , several near-boundary site rows appear to be depleted
of holes. The reason for this depletion is a deformation of the magnon cloud,
which surrounds the hole, near the boundary. The hole depletion in the boundary
region leads to a more complicated spectral function in the boundary row in
comparison with its bulk shape.Comment: 8 pages, 5 figure
High resolution Ge/Li/ spectrometer reduces rate-dependent distortions at high counting rates
Modified spectrometer system with a low-noise preamplifier reduces rate-dependent distortions at high counting rates, 25,000 counts per second. Pole-zero cancellation minimizes pulse undershoots due to multiple time constants, baseline restoration improves resolution and prevents spectral shifts
Temperature behavior of the magnon modes of the square lattice antiferromagnet
A spin-wave theory of short-range order in the square lattice Heisenberg
antiferromagnet is formulated. With growing temperature from T=0 a gapless mode
is shown to arise simultaneously with opening a gap in the conventional
spin-wave mode. The spectral intensity is redistributed from the latter mode to
the former. For low temperatures the theory reproduces results of the modified
spin-wave theory by M.Takahashi, J.E.Hirsch et al. and without fitting
parameters gives values of observables in good agreement with Monte Carlo
results in the temperature range 0 <= T < 0.8J where J is the exchange
constant.Comment: 12 pages, 2 figure
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The Performance of US Bond Mutual Funds
We evaluate the performance of the US bond mutual fund industry using a comprehensive sample of bond funds over a long time period from January 1998 to February 2017. In this one study, we evaluate bond fund selectivity, market timing and performance persistence. We evaluate bond funds relative to their self-declared benchmarks and in terms of both gross-of-fee returns and net-of-fee returns. We document considerable abnormal performance among funds both to the fund (gross returns) and to the investor (net returns). Bond fund performance is found to be superior in the post financial crisis period. However, past strong performance cannot be relied upon to predict future performance. Finally, while some funds exhibit market timing ability; we find a predominance of negative market timing among US bond mutual funds
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Multi-asset class mutual funds: Can they time the market? Evidence from the US, UK and Canada
The importance of asset allocation decisions in wealth management is well established. However, given its importance it is perhaps surprising that so little attention has been paid to the question of whether professional fund managers are skilful at timing market movement across asset classes over time. The timing literature has tended to concentrate on the timing skill of single asset class funds. Using data on US, UK and Canadian multi-asset class funds, we apply two alternative methodologies to identify the asset class timing abilities of managers. Overall, whether we apply a returns-based method or a holdings-based testing approach, we find evidence of only a tiny minority of funds with asset class timing ability
One-loop approximation for the Heisenberg antiferromagnet
We use the diagram technique for spin operators to calculate Green's
functions and observables of the spin-1/2 quantum Heisenberg antiferromagnet on
a square lattice. The first corrections to the self-energy and interaction are
taken into account in the chain diagrams. The approximation reproduces main
results of Takahashi's modified spin-wave theory [Phys. Rev. B 40, 2494 (1989)]
and is applicable in a wider temperature range. The energy per spin calculated
in this approximation is in good agreement with the Monte Carlo and
small-cluster exact-diagonalization calculations in the range 0 <= T < 1.2J
where J is the exchange constant. For the static uniform susceptibility the
agreement is good for T < 0.6J and becomes somewhat worse for higher
temperatures. Nevertheless the approximation is able to reproduce the maximum
in the temperature dependence of the susceptibility near T = 0.9J.Comment: 15 pages, 6 ps figure
AMATEUR BOXER BIOMECHANICS AND PUNCH FORCE
The current study investigates the correlation between punch biomechanics and punch force in amateur male boxers (n=39). A Hybrid III 50th percentile male dummy was used to gather punch force values. TrackEye Motion Analysis (TEMA) was used to measure the velocity of each boxer’s punch. Lower body force values were determined using the Functional Assessment of Biomechanics (FAB) system. Two types of punches, hooks and straights, were analyzed. It was determined that punch forces correlated more strongly to hand velocity than to lower body forces. Punch force correlated to hand velocity with R2 values of 0.380 and 0.391 for hook and straight punches, respectively (
Applications of BGP-reflection functors: isomorphisms of cluster algebras
Given a symmetrizable generalized Cartan matrix , for any index , one
can define an automorphism associated with of the field of rational functions of independent indeterminates It is an isomorphism between two cluster algebras associated to the
matrix (see section 4 for precise meaning). When is of finite type,
these isomorphisms behave nicely, they are compatible with the BGP-reflection
functors of cluster categories defined in [Z1, Z2] if we identify the
indecomposable objects in the categories with cluster variables of the
corresponding cluster algebras, and they are also compatible with the
"truncated simple reflections" defined in [FZ2, FZ3]. Using the construction of
preprojective or preinjective modules of hereditary algebras by Dlab-Ringel
[DR] and the Coxeter automorphisms (i.e., a product of these isomorphisms), we
construct infinitely many cluster variables for cluster algebras of infinite
type and all cluster variables for finite types.Comment: revised versio
Fixed-parameter tractability of multicut parameterized by the size of the cutset
Given an undirected graph , a collection of
pairs of vertices, and an integer , the Edge Multicut problem ask if there
is a set of at most edges such that the removal of disconnects
every from the corresponding . Vertex Multicut is the analogous
problem where is a set of at most vertices. Our main result is that
both problems can be solved in time , i.e.,
fixed-parameter tractable parameterized by the size of the cutset in the
solution. By contrast, it is unlikely that an algorithm with running time of
the form exists for the directed version of the problem, as
we show it to be W[1]-hard parameterized by the size of the cutset
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