53 research outputs found
Gércei alginit hatása savanyú homoktalaj termékenységére = Effect of gérce alginit on the fertility of an acid sandy soil
Strong coupling between Eu2+ spins and Fe2As2 layers in EuFe1.9Co0.1As2 observed with NMR
A combination of x-ray diffraction, magnetization, and 75As nuclear magnetic
resonance (NMR) experiments were performed on single-crystal EuFe1.9Co0.1As2.
The strength of the hyperfine interaction between the 75As nuclei and the
Eu^(2+) 4f states suggests a strong coupling between the Eu^(2+) moments and
the Fe1.9Co0.1As2 layers. Such a strong interlayer coupling may be due to an
indirect exchange interaction between the localized Eu^(2+) 4f moments,
mediated by the Fe 3d conduction electrons. Magnetic susceptibility as well as
75As-NMR measurements reveal a decrease of the SDW transition temperature to
T_SDW = 120 K as a result of Co doping. A change of the slope in the
temperature dependence of the NMR frequency of the 75As lower-satellite line
was observed at 225 K. At the same temperature also a change of the satellite
line shape was found. These changes of the NMR spectra may be caused by the
formation of a nematic phase below 225 K in EuFe1.9Co0.1As2.Comment: 8 pages, 7 figure
Giant microwave absorption in fine powders of superconductors
Enhanced microwave absorption, larger than that in the normal state, is
observed in fine grains of type-II superconductors (MgB and KC)
for magnetic fields as small as a few of the upper critical field. The
effect is predicted by the theory of vortex motion in type-II superconductors,
however its direct observation has been elusive due to skin-depth limitations;
conventional microwave absorption studies employ larger samples where the
microwave magnetic field exclusion significantly lowers the absorption. We show
that the enhancement is observable in grains smaller than the penetration
depth. A quantitative analysis on KC in the framework of the
Coffey--Clem (CC) theory explains well the temperature dependence of the
microwave absorption and also allows to determine the vortex pinning force
constant
Muon Spin Rotation Study of the Intercalated Graphite Superconductor CaC6 at Low Temperatures
Muon spin rotation (μSR) experiments were performed on the intercalated graphite CaC6 in the normal and superconducting state down to 20mK. In addition, AC magnetization measurements were carried out resulting in an anisotropic upper critical field , from which the coherence lengths ξ ab (0)=36.3(1.5)nm and ξ c (0)=4.3(7)nm were estimated. The anisotropy parameter increases monotonically with decreasing temperature. Asingle isotropic s-wave description of superconductivity cannot account for this behavior. From magnetic field dependent μSR experiments, the absolute value of the in-plane magnetic penetration depth λ ab (0)=78(3)nm was determined. The temperature dependence of the superfluid density ρ s (T) is slightly better described by a two-gap than a single-gap mode
Generalized Elliott-Yafet theory of electron spin relaxation in metals: the origin of the anomalous electron spin life-time in MgB2
The temperature dependence of the electron spin relaxation time in MgB2 is
anomalous as it does not follow the temperature dependence of the resistivity
above 150 K, it has a maximum around 400 K, and it decreases for higher
temperatures. This violates the well established Elliot-Yafet theory of
electron spin relaxation in metals. We show that the anomaly occurs when the
quasi-particle scattering rate (in energy units) becomes comparable to the
energy difference between the conduction- and a neighboring band. We find that
the anomalous behavior is related to the unique band structure of MgB and
the large electron-phonon coupling. The saturating spin-lattice relaxation can
be regarded as the spin transport analogue of the Ioffe-Regel criterion of
electron transport
Magnetic resonance in the antiferromagnetic and normal state of NH_3K_3C_60
We report on the magnetic resonance of NH_3K_3C_60 powders in the frequency
range of 9 to 225 GHz. The observation of an antiferromagnetic resonance below
the phase transition at 40 K is evidence for an antiferromagnetically ordered
ground state. In the normal state, above 40 K, the temperature dependence of
the spin-susceptibilty measured by ESR agrees with previous static measurements
and is too weak to be explained by interacting localized spins in an insulator.
The magnetic resonance line width has an unusual magnetic-field dependence
which is large and temperature independent in the magnetically ordered state
and decreases rapidly above the transition. These observations agree with the
suggestion that NH_3K_3C_60 is a metal in the normal state and undergoes a
Mott-Hubbard metal to insulator transition at 40 K.Comment: 4 pages, 5 figures. Submitted to Phys. Rev.
Anisotropy of superconducting MgB2 as seen in electron spin resonance and magnetization data
We have observed the conduction electron spin resonance (CESR) in fine
powders of MgB2 both in the superconducting and normal states. The Pauli
susceptibility is chi_s=2.0*10^{-5} emu/mole in the temperature range of 450 to
600 K. The spin relaxation rate has an anomalous temperature dependence. The
CESR measured below T_c at several frequencies suggests that MgB_2 is a
strongly anisotropic superconductor with the upper critical field, H_c2,
ranging between 2 and 16 T. The high-field reversible magnetization data of a
randomly oriented powder sample are well described assuming that MgB_2 is an
anisotropic superconductor with H_c2^{ab} / H_{c2}^{c} \approx 6--9.Comment: 4 pages, 4 eps figure
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