36,619 research outputs found
Unemployment and the Productivity Slowdown: A Labour Supply Perspective
Many OECD economies suffered a productivity slowdown beginning in the early 1970s. However, the increase in unemployment that followed this slowdown was more pronounced in European economies relative to the US. In this paper we present an efficiency wage model, which enables us to identify five channels through which the productivity slowdown can affect workers’ effort incentives. We argue that this model can explain the different trends in unemployment across countries over this period in the face of a similar slowdown in productivity. We also demonstrate how the link between growth and unemployment depends upon labour market institutions in such a way that we can reconcile the mixed empirical results observed in the literature.technical progress; endogenous growth; unemployment; efficiency wages
Highly frustrated spin-lattice models of magnetism and their quantum phase transitions: A microscopic treatment via the coupled cluster method
We outline how the coupled cluster method of microscopic quantum many-body
theory can be utilized in practice to give highly accurate results for the
ground-state properties of a wide variety of highly frustrated and strongly
correlated spin-lattice models of interest in quantum magnetism, including
their quantum phase transitions. The method itself is described, and it is
shown how it may be implemented in practice to high orders in a systematically
improvable hierarchy of (so-called LSUB) approximations, by the use of
computer-algebraic techniques. The method works from the outset in the
thermodynamic limit of an infinite lattice at all levels of approximation, and
it is shown both how the "raw" LSUB results are themselves generally
excellent in the sense that they converge rapidly, and how they may accurately
be extrapolated to the exact limit, , of the truncation
index , which denotes the {\it only} approximation made. All of this is
illustrated via a specific application to a two-dimensional, frustrated,
spin-half -- model on a honeycomb lattice with
nearest-neighbor and next-nearest-neighbor interactions with exchange couplings
and , respectively, where both
interactions are of the same anisotropic type. We show how the method can
be used to determine the entire zero-temperature ground-state phase diagram of
the model in the range of the frustration parameter and
of the spin-space anisotropy parameter. In particular,
we identify a candidate quantum spin-liquid region in the phase space
An efficient non-linear Feshbach engine
We investigate a thermodynamic cycle using a Bose-Einstein condensate with
nonlinear interactions as the working medium. Exploiting Feshbach resonances to
change the interaction strength of the BEC allows us to produce work by
expanding and compressing the gas. To ensure a large power output from this
engine these strokes must be performed on a short timescale, however such
non-adiabatic strokes can create irreversible work which degrades the engine's
efficiency. To combat this, we design a shortcut to adiabaticity which can
achieve an adiabatic-like evolution within a finite time, therefore
significantly reducing the out-of-equilibrium excitations in the BEC. We
investigate the effect of the shortcut to adiabaticity on the efficiency and
power output of the engine and show that the tunable nonlinearity strength,
modulated by Feshbach resonances, serves as a useful tool to enhance the
system's performance.Comment: 8 pages, 5 figures. To Appear New J. Phys. Focus on Shortcuts to
Adiabaticit
Spin-1/2 - Heisenberg model on a cross-striped square lattice
Using the coupled cluster method (CCM) we study the full (zero-temperature)
ground-state (GS) phase diagram of a spin-half () -
Heisenberg model on a cross-striped square lattice. Each site of the square
lattice has 4 nearest-neighbour exchange bonds of strength and 2
next-nearest-neighbour (diagonal) bonds of strength . The bonds
are arranged so that the basic square plaquettes in alternating columns have
either both or no bonds included. The classical () version of the model has 4 collinear phases when and
can take either sign. Three phases are antiferromagnetic (AFM), showing
so-called N\'{e}el, double N\'{e}el and double columnar striped order
respectively, while the fourth is ferromagnetic. For the quantum model
we use the 3 classical AFM phases as CCM reference states, on top of which the
multispin-flip configurations arising from quantum fluctuations are
incorporated in a systematic truncation hierarchy. Calculations of the
corresponding GS energy, magnetic order parameter and the susceptibilities of
the states to various forms of valence-bond crystalline (VBC) order are thus
carried out numerically to high orders of approximation and then extrapolated
to the (exact) physical limit. We find that the model has 5 phases,
which correspond to the four classical phases plus a new quantum phase with
plaquette VBC order. The positions of the 5 quantum critical points are
determined with high accuracy. While all 4 phase transitions in the classical
model are first order, we find strong evidence that 3 of the 5 quantum phase
transitions in the model are of continuous deconfined type
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