656 research outputs found
NMR Evidence for Charge Inhomogeneity in Stripe Ordered La_{1.8-x}Eu_{0.2}Sr_{x}CuO_4
We report ^{17}O Nuclear Magnetic Resonance (NMR) results in the stripe
ordered La_{1.8-x}Eu_{0.2}Sr_{x}CuO_4 system. Below a temperature T_q ~ 80K,
the local electric field gradient (EFG) and the absolute intensity of the NMR
signal of the planar O site exhibit a dramatic decrease. We interpret these
results as microscopic evidence for a spatially inhomogeneous charge
distribution, where the NMR signal from O sites in the domain walls of the spin
density modulation are wiped out due to large hyperfine fields, and the
remaining signal arises from the intervening Mott insulating regions.Comment: 4 pages, to appear in Phys. Rev. Let
Disorder-induced Spin Gap in the Zigzag Spin-1/2 Chain Cuprate Sr_{0.9}Ca_{0.1}CuO_2
We report a comparative study of 63Cu Nuclear Magnetic Resonance spin lattice
relaxation rates, T_1^{-1}, on undoped SrCuO_2 and Ca doped
Sr_{0.9}Ca_{0.1}CuO_2 spin chain compounds. A temperature independent T_1^{-1}
is observed for SrCuO_2 as expected for an S=1/2 Heisenberg chain.
Surprisingly, we observe an exponential decrease of T_1^{-1} for T < 90,K in
the Ca-doped sample evidencing the opening of a spin gap. The data analysis
within the J_1-J_2 Heisenberg model employing density-matrix renormalization
group calculations suggests an impurity driven small alternation of the
J_2-exchange coupling as a possible cause of the spin gap.Comment: 4 pages, 4 figure
Precise Pressure Dependence of the Superconducting Transition Temperature of FeSe: Resistivity and ^77Se--NMR Study
We report the precise pressure dependence of FeSe from a resistivity
measurement up to 4.15 GPa. Superconducting transition temperature (T_c)
increases sensitively under pressure, but shows a plateau between 0.5-1.5 GPa.
The maximum T_c, which is determined by zero resistance, is 21 K at
approximately 3.5 GPa. The onset value reaches ~37 K at 4.15 GPa. We also
measure the nuclear spin-lattice relaxation rate 1/T_1 under pressure using
77Se--NMR measurement. 1/T_1 shows that bulk superconductivity is realized in
the zero-resistance state. The pressure dependence of 1/T_1T just above T_c
shows a plateau as well as the pressure dependence of T_c, which gives clear
evidence of the close relationship between 1/T_1T and T_c. Spin fluctuations
are suggested to contribute to the mechanism of superconductivity.Comment: 4pages, 6figures: to be published in J. Phys. Soc. Jpn. Vol.78 No.6
(2009
Spin Gap in the Single Spin-1/2 Chain Cuprate SrCaCuO
We report Cu nuclear magnetic resonance and muon spin rotation
measurements on the S=1/2 antiferromagnetic Heisenberg spin chain compound
SrCaCuO. An exponentially decreasing spin-lattice
relaxation rate 1/T indicates the opening of a spin gap. This behavior is
very similar to what has been observed for the cognate zigzag spin chain
compound SrCaCuO, and confirms that the occurrence of a
spin gap upon Ca doping is independent of the interchain exchange coupling
. Our results therefore generally prove the appearance of a spin gap in an
antiferromagnetic Heisenberg spin chain induced by a local bond disorder of the
intrachain exchange coupling . A low temperature upturn of 1/T evidences
growing magnetic correlations. However, zero field muon spin rotation
measurements down to 1.5 K confirm the absence of magnetic order in this
compound which is most likely suppressed by the opening of the spin gap.Comment: 5 pages, 4 figure
Recommended from our members
How to catch more prey with less effective traps: explaining the evolution of temporarily inactive traps in carnivorous pitcher plants.
Carnivorous Nepenthes pitcher plants capture arthropods with specialized slippery surfaces. The key trapping surface, the pitcher rim (peristome), is highly slippery when wetted by rain, nectar or condensation, but not when dry. As natural selection should favour adaptations that maximize prey intake, the evolution of temporarily inactive traps seems paradoxical. Here, we show that intermittent trap deactivation promotes 'batch captures' of ants. Prey surveys revealed that N. rafflesiana pitchers sporadically capture large numbers of ants from the same species. Continuous experimental wetting of the peristome increased the number of non-recruiting prey, but decreased the number of captured ants and shifted their trapping mode from batch to individual capture events. Ant recruitment was also lower to continuously wetted pitchers. Our experimental data fit a simple model that predicts that intermittent, wetness-based trap activation should allow safe access for 'scout' ants under dry conditions, thereby promoting recruitment and ultimately higher prey numbers. The peristome trapping mechanism may therefore represent an adaptation for capturing ants. The relatively rare batch capture events may particularly benefit larger plants with many pitchers. This explains why young plants of many Nepenthes species additionally employ wetness-independent, waxy trapping surfaces
- …