233 research outputs found
Dielectricly Enhanced T c in Underdoped Cuprates
It is proposed to create a multilayer structure in which an underdoped copper-oxide high-temperature superconductor is sandwiched between high-dielectric-constant insulator layers such as ferro- or ferri-electrics, thereby reducing the Coulomb repulsion between the intrinsically present clusters or stripes in the CuO2 layers of the pseudogap phase. This should lead to an increase in the size of such clusters, resulting in smaller distances between them and coherence at higher temperature, i.e., a higher T c with a smaller pseudogap (T ∗−T c
^25Mg NMR study of the MgB_2 superconductor
^25Mg NMR spectra and nuclear spin-lattice relaxation time, T_1, have been
measured in polycrystalline ^25MgB_2 with a superconducting transition
temperature T_c = 39.0 K in zero magnetic field. From the first order and
second order quadrupole perturbed NMR spectrum a quadrupole coupling frequency
nu_Q = 222(1.5) kHz is obtained. T_1T = 1090(50) sK and Knight shift K_c =
242(4) ppm are temperature independent in the normal conducting phase. The
^25Mg Korringa ratio equals to 0.95 which is very close to the ideal value of
unity for s-electrons. The comparison of the experimental nu_Q, T_1T, and K_c
with the corresponding values obtained by LDA calculations shows an excellent
agreement for all three quantities.Comment: 4 pages including 4 eps-figures, revtex
Electron paramagnetic resonance studies of the insulating ferromagnetic manganite Nd_0.8Pb_0.2MnO_3 above the transition temperature
Single crystals of Nd_{1-x}Pb_{x}MnO_{3} with x=0.2 are grown by high
temperature solution growth technique using PbO-PbF_{2} flux. Magnetization
studies on the samples show a transition to a ferromagnetic state below T_c ~
125 K and the resistivity measurements show it to be an insulator throughout
the temperature range 50 - 300 K. Electron Paramagnetic Resonance studies have
been performed for T > T_{C} with a view to comparing the results with those on
metallic ferromagnetic manganites. The temperature dependence of various
parameters like g-value, linewidth and intensity has been studied in the
temperature range 150 - 300 K. It is found that they behave in a manner similar
to that exhibited by metallic ferromagnetic manganites.Comment: 8 pages, 4 figure
Negative Oxygen Isotope Effect on the Static Spin Stripe Order in La_(2-x)Ba_xCuO_4 (x = 1/8)
Large negative oxygen-isotope (16O/18O) effects (OIE's) on the static
spin-stripe ordering temperature T_so and the magnetic volume fraction V_m were
observed in La_(2-x)Ba_xCuO_4 (x = 1/8) by means of muon spin rotation
experiments. The corresponding OIE exponents were found to be alpha_(T_so) =
-0.57(6) and alpha_(V_m) = -0.71(9), which are sign reversed to alpha_(T_c) =
0.46(6) measured for the superconducting transition temperature T_c. This
indicates that the electron-lattice interaction is involved in the stripe
formation and plays an important role in the competition between bulk
superconductivity and static stripe order in the cuprates.Comment: 5 pages, 4 figure
Correlation between the transition temperature and the superfluid density in BCS superconductor NbB_2+x
The results of the muon-spin rotation experiments on BCS superconductors
NbB_2+x (x = 0.2, 0.34) are reported. Both samples, studied in the present
work, exhibit rather broad transitions to the superconducting state, suggesting
a distribution of the volume fractions with different transition temperatures
(T_c)'s. By taking these distributions into account, the dependence of the
inverse squared zero-temperature magnetic penetration depth (\lambda_0^{-2}) on
T_c was reconstructed for temperatures in the range 1.5K<T_c<8.0K.
\lambda_0^{-2} was found to obey the power law dependence \lambda_0^{-2}\propto
T_c^{3.1(1)} which appears to be common for some families of BCS
superconductors as, {\it e.g.}, Al doped MgB_2 and high-temperature cuprate
superconductors as underdoped YBa_2Cu_3O_{7-\delta}.Comment: 9 pages, 7 figures. Accepted for publication in Phys. Rev.
Tuning the structural instability of SrTiO_3 by Eu doping: The phase diagram of Sr_1-xEu_xTiO_3
The phase diagram of Sr_1-xEu_xTiO_3 is determined experimentally by electron
paramagnetic resonance and resistivity measurements and analyzed theoretically
within the self-consistent phonon approximation as a function of x
([0.03-1.0]). The transition temperature of the structural instability of the
system increases nonlinearly to higher temperatures with increasing x. This is
interpreted theoretically by a substantial alteration in the dynamics caused by
a change in the double-well potential from broad and shallow to narrow and
deep.Comment: 15 pages, 5 figure
Electron Spin-Lattice Relaxation of Er3+ ions in Er0.01Y0.99Ba2Cu3Ox
The temperature dependence of the electron spin-lattice relaxation SLR was
studied in Er0.01Y0.99Ba2Cu3Ox compounds. The data derived from the electron
spin resonance ESR and SLR measurements were compared to those from inelastic
neutron scattering studies. SLR of Er3+ ions in the temperature range from 20 K
to 65 K can be explained by the resonant phonon relaxation process with the
involvement of the lowest excited crystalline-electric-field electronic states
of Er3+. These results are consistent with a local phase separation effects.
Possible mechanisms of the ESR line broadening at lower temperatures are
discussed. Keywords: YBCO; EPR; ESR; Electron spin-lattice relaxation time, T ;
Crystalline-electric-fieldComment: 6 pages, 4 figure
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