10,544 research outputs found
Time-reversal-symmetry-broken quantum spin Hall effect
Quantum spin Hall (QSH) state of matter is usually considered to be protected
by time-reversal (TR) symmetry. We investigate the fate of the QSH effect in
the presence of the Rashba spin-orbit coupling and an exchange field, which
break both inversion and TR symmetries. It is found that the QSH state
characterized by nonzero spin Chern numbers persists when the
TR symmetry is broken. A topological phase transition from the TR
symmetry-broken QSH phase to a quantum anomalous Hall phase occurs at a
critical exchange field, where the bulk band gap just closes. It is also shown
that the transition from the TR symmetry-broken QSH phase to an ordinary
insulator state can not happen without closing the band gap.Comment: 5 pages, 5 figure
Evolution of worldwide stock markets, correlation structure and correlation based graphs
We investigate the daily correlation present among market indices of stock
exchanges located all over the world in the time period Jan 1996 - Jul 2009. We
discover that the correlation among market indices presents both a fast and a
slow dynamics. The slow dynamics reflects the development and consolidation of
globalization. The fast dynamics is associated with critical events that
originate in a specific country or region of the world and rapidly affect the
global system. We provide evidence that the short term timescale of correlation
among market indices is less than 3 trading months (about 60 trading days). The
average values of the non diagonal elements of the correlation matrix,
correlation based graphs and the spectral properties of the largest eigenvalues
and eigenvectors of the correlation matrix are carrying information about the
fast and slow dynamics of correlation of market indices. We introduce a measure
of mutual information based on link co-occurrence in networks, in order to
detect the fast dynamics of successive changes of correlation based graphs in a
quantitative way.Comment: 8 pages, 11 figure
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