6,520 research outputs found
Revealing the pure confinement effect in glass-forming liquids by dynamic mechanical analysis
Many molecular glass forming liquids show a shift of the glass transition Tg
to lower temperatures when the liquid is confined into mesoporous host
matrices. Two contrary explanations for this effect are given in literature:
First, confinement induced acceleration of the dynamics of the molecules leads
to an effective downshift of Tg increasing with decreasing pore size. Secondly,
due to thermal mismatch between the liquid and the surrounding host matrix,
negative pressure develops inside the pores with decreasing temperature, which
also shifts Tg to lower temperatures. Here we present novel dynamic mechanical
analysis measurements of the glass forming liquid salol in Vycor and Gelsil
with pore sizes of d = 2.6, 5.0 and 7.5 nm. The dynamic complex elastic
susceptibility data can be consistently described with the assumption of two
relaxation processes inside the pores: A surface induced slowed down relaxation
due to interaction with rough pore interfaces and a second relaxation within
the core of the pores. This core relaxation time is reduced with decreasing
pore size d, leading to a downshift of Tg in perfect agreement with recent DSC
measurements
Lattice Properties of PbX (X = S, Se, Te): Experimental Studies and ab initio Calculations Including Spin-Orbit Effects
During the past five years the low temperature heat capacity of simple
semiconductors and insulators has received renewed attention. Of particular
interest has been its dependence on isotopic masses and the effect of spin-
orbit coupling in ab initio calculations. Here we concentrate on the lead
chalcogenides PbS, PbSe and PbTe. These materials, with rock salt structure,
have different natural isotopes for both cations and anions, a fact that allows
a systematic experimental and theoretical study of isotopic effects e.g. on the
specific heat. Also, the large spin-orbit splitting of the 6p electrons of Pb
and the 5p of Te allows, using a computer code which includes spin-orbit
interaction, an investigation of the effect of this interaction on the phonon
dispersion relations and the temperature dependence of the specific heat and on
the lattice parameter. It is shown that agreement between measurements and
calculations significantly improves when spin-orbit interaction is included.Comment: 25 pages, 12 Figures, 1 table, submitted to PR
Electron-hole asymmetry in Co- and Mn-doped SrFe2As2
Phase diagram of electron and hole-doped SrFe2As2 single crystals is
investigated using Co and Mn substitution at the Fe-sites. We found that the
spin-density-wave state is suppressed by both dopants, but the superconducting
phase appears only for Co (electron)-doping, not for Mn (hole)-doping. Absence
of the superconductivity by Mn-doping is in sharp contrast to the hole-doped
system with K-substitution at the Sr sites. Distinct structural change, in
particular the increase of the Fe-As distance by Mn-doping is important to have
a magnetic and semiconducting ground state as confirmed by first principles
calculations. The absence of electron-hole symmetry in the Fe-site-doped
SrFe2As2 suggests that the occurrence of high-Tc superconductivity is sensitive
to the structural modification rather than the charge doping.Comment: 7 pages, 6 figure
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