299 research outputs found
Frustration-driven QPT in the 1D extended anisotropic Heisenberg model
By using Density Matrix Renormalization Group (DMRG) technique we study the
1D extended anisotropic Heisenberg model. We find that starting from the
ferromagnetic phase, the system undergoes two quantum phase transitions (QPTs)
induced by frustration. By increasing the next-nearest-neighbor (NNN)
interaction, the ground state of the system changes smoothly from a completely
polarized state to a NNN correlated one. On the contrary, letting the in-plane
interaction to be greater than the out-of-plane one, the ground state changes
abruptly.Comment: 4 pages, 4 figures, to be presented at CSMAG-07 Kosice, Slovakia,
July 200
Does corruption matter for sources of foreign direct investment?
The paper provides a cross-country empirical analysis of the impact of corruption on foreign direct investment flows. The gravity model augmented with joint effects of corruption in the origin and destination countries determines differentiated patterns of investment flows between countries with various level of control of corruption. The estimates point towards greater investment flows between countries with good control of corruption. Moreover, if control of corruption in the destination country improves, investment flows from cleaner countries rise more than they do from countries with a higher incidence of corruption. The resulting changes in composition of investment volumes towards more investment from cleaner countries may further reinforce the strengthening of economic and political institutions that keep corruption in check
Efficiency of photodesorption of Rb atoms collected on polymer organic film in vapor-cell
The efficiency of photodesorption of Rb atoms previously collected on polymer
organic film has been studied in detail. This study was carried out in a Pyrex
glass cell of which the inner surface was covered with (poly)dimethylsiloxane
(PDMS) film and illuminated by a powerful flash lamp. The desorption dynamic of
the Rb atoms density in the cell caused by the illumination and percentage of
desorbed atoms was studied by using of Rb resonance lamp and free running diode
laser as sources of probing light. It was determined that 85 percent collected
chemical active Rb atoms and stored during 16 seconds in the closed cell, 75
percent in the pumped cell can be desorbed by single flash of the lamp. The
number of stored atoms decays with a characteristic time of 60 min in isolated
cell and with a time 12.4 minutes in a pumped cell. We believe that this
efficient method of collection and fast realization of atoms or molecules could
be used for enhancement of sensitivity of existed sensors for the trace
detection of various elements (including toxic or radioactive ones) which is
important to environmental applications, medicine or in geology. The effect
might help to construct an efficient light-driven atomic source for a
magneto-optical trap in a case of extremely low vapor density or very weak flux
of atoms, such as artificial radioactive alkali atoms.Comment: 9 pages, 9 figures; accepted for publication in EPJ
Metal-insulator transition in copper oxides induced by apex displacements
High temperature superconductivity has been found in many kinds of compounds
built from planes of Cu and O, separated by spacer layers. Understanding why
critical temperatures are so high has been the subject of numerous
investigations and extensive controversy. To realize high temperature
superconductivity, parent compounds are either hole-doped, such as
{LaCuO} (LCO) with Sr (LSCO), or electron doped, such as
{NdCuO} (NCO) with Ce (NCCO). In the electron doped cuprates, the
antiferromagnetic phase is much more robust than the superconducting phase.
However, it was recently found that the reduction of residual out-of-plane
apical oxygens dramatically affects the phase diagram, driving those compounds
to a superconducting phase. Here we use a recently developed first principles
method to explore how displacement of the apical oxygen (A-O) in LCO affects
the optical gap, spin and charge susceptibilities, and superconducting order
parameter. By combining quasiparticle self-consistent GW (QS\emph{GW}) and
dynamical mean field theory (DMFT), that LCO is a Mott insulator; but small
displacements of the apical oxygens drive the compound to a metallic state
through a localization/delocalization transition, with a concomitant maximum
-wave order parameter at the transition. We address the question whether NCO
can be seen as the limit of LCO with large apical displacements, and elucidate
the deep physical reasons why the behaviour of NCO is so different than the
hole doped materials. We shed new light on the recent correlation observed
between T and the charge transfer gap, while also providing a guide towards
the design of optimized high-Tc superconductors. Further our results suggest
that strong correlation, enough to induce Mott gap, may not be a prerequisite
for high-Tc superconductivity
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