5,069 research outputs found
Quantum oscillation studies of the Fermi surface of iron-pnictide superconductors
This paper reviews quantum oscillation studies of iron-pnictide
superconductors and related materials. These measurements give unique
information regarding the full three dimensional topology of the Fermi surfaces
and the renormalisation of the quasi-particle masses. The review will cover
measurements of the 122 arsenide end members, XFe_2As_2 (X = Ba, Sr, Ca) which
have a spin density wave ground state, but will concentrate on the phosphide
end members (LaFePO and XFe2P2) which have a paramagnetic ground state and a
Fermi surface topology which is similar to the higher Tc superconducting
iron-pnictides. All three of the 122 phosphides become superconducting when P
is partially substituted by As, and for the BaFe2(As1-xPx)2 series de Haas-van
Alphen oscillations are observable for 0:42 \leq x \leq 1 with Tc up to 25 K.
The results show the changes in the Fermi surface topology and the increase in
the mass renormalisation as the correlations which induce superconductivity
develop.Comment: to appear in Reports on Progress in Physic
A General Expression for Symmetry Factors of Feynman Diagrams
The calculation of the symmetry factor corresponding to a given Feynman
diagram is well known to be a tedious problem. We have derived a simple formula
for these symmetry factors. Our formula works for any diagram in scalar theory
( and interactions), spinor QED, scalar QED, or QCD.Comment: RevTex 11 pages with 10 figure
Generation of electron spin polarization in disordered organic semiconductors
The generation mechanisms of electron spin polarization (ESP) of charge
carriers (electrons and holes, called "doublets") in doublet-doublet
recombination and triplet-doublet quenching in disordered organic
semiconductors are analyzed in detail. The ESP is assumed to result from
quantum transitions between the states of the spin Hamiltonian of the pair of
interacting particles. The value of the ESP is essentially determined by the
mechanism of relative motion of particles. In our work we have considered the
cage and free diffusion models. The effect of possible attractive
spin-independent interactions between particles is also analyzed. Estimation
with obtained formulas shows that the proposed mechanisms can lead to a fairly
strong ESP much larger than the thermal one (at room temperatures)Comment: 10 pages, 3 figure
Magnetic field effects on electron-hole recombination in disordered organic semiconductors
Characteristic properties of magnetic field effects on spin selective
geminate and bulk electron-hole polaron pair (PP) recombination are analyzed in
detail within the approach based on the stochastic Liouville equation. Simple
expressions for the magnetic field (B) dependence of recombination yield and
rate are derived within two models of relative PP motion: free diffusion and
diffusion in the presence of well (cage). The spin evolution of PPs is
described taking in account the relaxation induced by hyperfine interaction,
anisotropic part of the Zeeman interaction induced, as well as -mechanism. A large variety of the -dependences of the recombination yield
and rate is obtained depending on the relative weights of
above-mentioned mechanisms. The proposed general method and derived particular
formulas are shown to be quite useful for the analysis of recent experimental
results.Comment: 12 pages, 3 figure
Destruction of chain-superconductivity in YBa_2Cu_4O_8 in a weak magnetic field
We report measurements of the temperature dependent components of the
magnetic penetration depth {\lambda}(T) in single crystal samples of
YBa_2Cu_4O_8 using a radio frequency tunnel diode oscillator technique. We
observe a downturn in {\lambda}(T) at low temperatures for currents flowing
along the b and c axes but not along the a axis. The downturn in {\lambda}_b is
suppressed by a small dc field of ~0.25 T. This and the zero field anisotropy
of {\lambda}(T) likely result from proximity induced superconducting on the CuO
chains, however we also discuss the possibility that a significant part of the
anisotropy might originate from the CuO2 planes.Comment: 5 page
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