277 research outputs found
Pressure dependence of the Shubnikov-de Haas oscillation pectrum of beta''-(BEDT-TTF)4(NH4)[Cr(C2O4)3].DMF
The Shubnikov-de Haas (SdH) oscillation spectra of the
beta''-(BEDT-TTF)4(NH4)[Cr(C2O4)\_3].DMF organic metal have been studied in
pulsed magnetic fields of up to either 36 T at ambient pressure or 50 T under
hydrostatic pressures of up to 1 GPa. The ambient pressure SdH oscillation
spectra can be accounted for by up to six fundamental frequencies which points
to a rather complex Fermi surface (FS). A noticeable pressure-induced
modification of the FS topology is evidenced since the number of frequencies
observed in the spectra progressively decreases as the pressure increases.
Above 0.8 GPa, only three compensated orbits are observed, as it is the case
for several other isostructural salts of the same family at ambient pressure.
Contrary to other organic metals, of which the FS can be regarded as a network
of orbits, no frequency combinations are observed for the studied salt, likely
due to high magnetic breakdown gap values or (and) high disorder level
evidenced by Dingle temperatures as large as about 7 K.Comment: To be published in European Physical Journal
Crystal structure, Fermi surface calculations and Shubnikov-de Haas oscillations spectrum of the organic metal -(BETS)HgBr(CHCl) at low temperature
The organic metal \theta_4_4_6_5$Cl) is known to
undergo a phase transition as the temperature is lowered down to about 240 K.
X-ray data obtained at 200 K indicate a corresponding modification of the
crystal structure, the symmetry of which is lowered from quadratic to
monoclinic. In addition, two different types of cation layers are observed in
the unit cell. The Fermi surface (FS), which can be regarded as a network of
compensated electron and hole orbits according to band structure calculations
at room temperature, turns to a set of two alternating linear chains of orbits
at low temperature. The field and temperature dependence of the Shubnikov-de
Haas oscillations spectrum have been studied up to 54 T. Eight frequencies are
observed which, in any case, points to a FS much more complex than predicted by
band structure calculations at room temperature, even though some of the
observed Fourier components might be ascribed to magnetic breakdown or
frequency mixing. The obtained spectrum could result from either an interaction
between the FS's linked to each of the two cation layers or to an eventual
additional phase transition in the temperature range below 200 K.Comment: accepted for publication in Solid State Science
A stereodivergent asymmetric approach to difluorinated aldonic acids
A (bromodifluoromethyl)alkyne has been deployed in a stereoselective route to difluorinated aldonic acid analogues, in which a Sharpless asymmetric dihydroxylation reaction and diastereoisomer separation set the stage for phenyl group oxidation
Damping of field-induced chemical potential oscillations in ideal two-band compensated metals
The field and temperature dependence of the de Haas-van Alphen oscillations
spectrum is studied for an ideal two-dimensional compensated metal. It is shown
that the chemical potential oscillations, involved in the frequency
combinations observed in the case of uncompensated orbits, are strongly damped
and can even be suppressed when the effective masses of the electron- and
hole-type orbits are the same. When magnetic breakdown between bands occurs,
this damping is even more pronounced and the Lifshits-Kosevich formalism
accounts for the data in a wide field range.Comment: 11 pages, 10 figures, to appear in PR
Indication for the coexistence of closed orbit and quantum interferometer with the same cross section in the organic metal (ET)4(H3O)[Fe(C2O4)3].C6H4Cl2: Persistence of SdH oscillations above 30 K
Shubnikov-de Haas (SdH) and de Haas-van Alphen (dHvA) oscillations spectra of
the quasi-two dimensional charge transfer salt
"-(ET)(HO)[Fe(CO)]CHCl have been
investigated in pulsed magnetic fields up to 54 T. The data reveal three basic
frequencies F, F and F, which can be interpreted on the basis
of three compensated closed orbits at low temperature. However a very weak
thermal damping of the Fourier component F, with the highest amplitude, is
evidenced for SdH spectra above about 6 K. As a result, magnetoresistance
oscillations are observed at temperatures higher than 30 K. This feature, which
is not observed for dHvA oscillations, is in line with quantum interference,
pointing to a Fermi surface reconstruction in this compound.Comment: published in Eur. Phys. J. B 71 203 (2009
Analytical treatment of the dHvA frequency combinations due to chemical potential oscillations in an idealized two-band Fermi liquid
de Haas-van Alphen oscillation spectrum is studied for an idealized
two-dimensional Fermi liquid with two parabolic bands in the case of canonical
(fixed number of quasiparticles) and grand canonical (fixed chemical potential)
ensembles. As already reported in the literature, oscillations of the chemical
potential in magnetic field yield frequency combinations that are forbidden in
the framework of the semiclassical theory. Exact analytical calculation of the
Fourier components is derived at zero temperature and an asymptotic expansion
is given for the high temperature and low magnetic field range. A good
agreement is obtained between analytical formulae and numerical computations.Comment: 10 pages, 4 figure
Recent developments in the determination of the amplitude and phase of quantum oscillations for the linear chain of coupled orbits
De Haas-van Alphen oscillations are studied for Fermi surfaces (FS)
illustrating the model proposed by Pippard in the early sixties, namely the
linear chain of orbits coupled by magnetic breakdown. This FS topology is
relevant for many multiband quasi-two dimensional (q-2D) organic metals such as
-(BEDT-TTF)Cu(NCS) and
-(BEDT-TTF)CoBr(CHCl) which are considered in
detail. Whereas the Lifshits-Kosevich model only involves a first order
development of field- and temperature-dependent damping factors, second order
terms may have significant contribution on the Fourier components amplitude for
such q-2D systems at high magnetic field and low temperature. The strength of
these second order terms depends on the relative value of the involved damping
factors, which are in turns strongly dependent on parameters such as the
magnetic breakdown field, effective masses and, most of all, effective
Land\'{e} factors. In addition, the influence of field-dependent Onsager phase
factors on the oscillation spectra is considered.Comment: arXiv admin note: text overlap with arXiv:1304.665
Multiple Quantum Oscillations in the de Haas van Alphen Spectra of the Underdoped High Temperature Superconductor YBa_2Cu_3O_6.5
By improving the experimental conditions and extensive data accumulation, we
have achieved very high-precision in the measurements of the de Haas-van Alphen
effect in the underdoped high-temperature superconductor
YBaCuO. We find that the main oscillation, so far believed
to be single-frequency, is composed of three closely spaced frequencies. We
attribute this to bilayer splitting and warping of a single quasi-2D Fermi
surface, indicating that \emph{c}-axis coherence is restored at low temperature
in underdoped cuprates. Our results do not support the existence of a larger
frequency of the order of 1650 T reported recently in the same compound [S.E.
Sebastian {\it et al}., Nature {\bf 454}, 200 (2008)]
Upper critical magnetic field in K0.83Fe1.83Se2 and Eu0.5K0.5Fe2As2 single crystals
The H-T phase diagrams of single crystalline electron-doped K0.83Fe1.83Se2
(KFS1), K0.8Fe2Se2 (KFS2) and hole-doped Eu0.5K0.5Fe2As2 (EKFA) have been
deduced from tunnel diode oscillator-based contactless measurements in pulsed
magnetic fields up to 57 T for the inter-plane (H//c) and in-plane (H//ab)
directions. The temperature dependence of the upper critical magnetic field
Hc2(T) relevant to EFKA is accounted for by the Pauli model including an
anisotropic Pauli paramagnetic contribution (\mu_BHp=114 T for H//ab and 86 T
for H//c). This is also the case of KFS1 and KFS2 for H//ab whereas a
significant upward curvature, accounted for by a two-gap model, is observed for
H//c. Despite the presence of antiferromagnetic lattice order within the
superconducting state of the studied compounds, no influence of magnetic
ordering on the temperature dependence of Hc2(T) is observed.Comment: 9 pages, 5 figures. arXiv admin note: text overlap with
arXiv:1104.561
Onecut-dependent Nkx6.2 transcription factor expression is required for proper formation and activity of spinal locomotor circuits.
In the developing spinal cord, Onecut transcription factors control the diversification of motor neurons into distinct neuronal subsets by ensuring the maintenance of Isl1 expression during differentiation. However, other genes downstream of the Onecut proteins and involved in motor neuron diversification have remained unidentified. In the present study, we generated conditional mutant embryos carrying specific inactivation of Onecut genes in the developing motor neurons, performed RNA-sequencing to identify factors downstream of Onecut proteins in this neuron population, and employed additional transgenic mouse models to assess the role of one specific Onecut-downstream target, the transcription factor Nkx6.2. Nkx6.2 expression was up-regulated in Onecut-deficient motor neurons, but strongly downregulated in Onecut-deficient V2a interneurons, indicating an opposite regulation of Nkx6.2 by Onecut factors in distinct spinal neuron populations. Nkx6.2-null embryos, neonates and adult mice exhibited alterations of locomotor pattern and spinal locomotor network activity, likely resulting from defective survival of a subset of limb-innervating motor neurons and abnormal migration of V2a interneurons. Taken together, our results indicate that Nkx6.2 regulates the development of spinal neuronal populations and the formation of the spinal locomotor circuits downstream of the Onecut transcription factors
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