108 research outputs found
Quantum critical dynamics of a S = 1/2 antiferromagnetic Heisenberg chain studied by 13C-NMR spectroscopy
We present a 13C-NMR study of the magnetic field driven transition to
complete polarization of the S=1/2 antiferromagnetic Heisenberg chain system
copper pyrazine dinitrate Cu(C_4H_4N_2)(NO_3)_2 (CuPzN). The static local
magnetization as well as the low-frequency spin dynamics, probed via the
nuclear spin-lattice relaxation rate 1/T_1, were explored from the low to the
high field limit and at temperatures from the quantum regime (k_B T << J) up to
the classical regime (k_B T >> J). The experimental data show very good
agreement with quantum Monte Carlo calculations over the complete range of
parameters investigated. Close to the critical field, as derived from static
experiments, a pronounced maximum in 1/T_1 is found which we interpret as the
finite-temperature manifestation of a diverging density of zero-energy magnetic
excitations at the field-driven quantum critical point.Comment: 5 pages, 4 figure
Field-induced coupled superconductivity and spin density wave order in the Heavy Fermion compound CeCoIn5
The high field superconducting state in CeCoIn5 has been studied by
transverse field muon spin rotation measurements with an applied field parallel
to the crystallographic c-axis close to the upper critical field Hc2 = 4.97 T.
At magnetic fields >= 4.8 T the muon Knight shift is enhanced and the
superconducting transition changes from second order towards first order as
predicted for Pauli-limited superconductors. The field and temperature
dependence of the transverse muon spin relaxation rate sigma reveal
paramagnetic spin fluctuations in the field regime from 2 T < H < 4.8 T. In the
normal state close to Hc2 correlated spin fluctuations as described by the self
consistent renormalization theory are observed. The results support the
formation of a mode-coupled superconducting and antiferromagnetically ordered
phase in CeCoIn5 for H directed parallel to the c-axis.Comment: 5 paes, 4 figure
1H-NMR study of the spin dynamics of fine superparamagnetic nanoparticles
We report a broadband 1H-NMR study of the temperature spin dynamics of nearly monodisperse dextran-coated \u3b3-Fe2O3 magnetic nanoparticles. We observed a maximum in T1 121(T) that decreases in amplitude and shifts toward higher temperatures with increasing field. We suggest that this is related to the progressive superparamagnetic spin blocking of the ferrite core. The data can be explained by assuming a single electronic spin-spin correlation time and introducing a field-dependent distribution of anisotropy energy barriers
Coexistence of the spin-density-wave and superconductivity in the (Ba,K)Fe2As2
The relation between the spin-density-wave (SDW) and superconducting order is
a central topic in current research on the FeAs-based high Tc superconductors.
Conflicting results exist in the LaFeAs(O,F)-class of materials, for which
whether the SDW and superconductivity are mutually exclusive or they can
coexist has not been settled. Here we show that for the (Ba,K)Fe2As2 system,
the SDW and superconductivity can coexist in an extended range of compositions.
The availability of single crystalline samples and high value of the energy
gaps would make the materials a model system to investigate the high Tc
ferropnictide superconductivity.Comment: 4 pages, 5 figure
Commensurate Spin Density Wave in LaOFeAs: A Local Probe Study
We present a detailed study on the magnetic order in the undoped mother
compound LaOFeAs of the recently discovered Fe-based superconductor
LaOFFeAs. In particular, we present local probe measurements of the
magnetic properties of LaOFeAs by means of Fe M\"ossbauer spectroscopy
and muon spin relaxation in zero external field along with magnetization and
resistivity studies. These experiments prove a commensurate static magnetic
order with a strongly reduced ordered moment of 0.25(5) at the iron
site below T_N = 138 K, well separated from a structural phase transition at
T_N = 156 K. The temperature dependence of the sublattice magnetization is
determined and compared to theory. Using a four-band spin density wave model
both, the size of the order parameter and the quick saturation below T_N are
reproduced.Comment: 5 pages, 5 figure
Electronic properties of LaOFFeAs in the normal state probed by NMR/NQR
We report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear
quadrupole resonance (NQR) measurements on powders of the new LaO1-xFxFeAs
superconductor for x = 0 and x = 0.1 at temperatures up to 480 K, and compare
our measured NQR spectra with local density approximation (LDA) calculations.
For all three nuclei in the x = 0.1 material, it is found that the local Knight
shift increases monotonically with an increase in temperature, and scales with
the macroscopic susceptibility, suggesting a single magnetic degree of freedom.
Surprisingly, the spin lattice relaxation rates for all nuclei also scale with
one another, despite the fact that the form factors for each site sample
different regions of q-space. This result suggests a lack of any q-space
structure in the dynamical spin susceptibility that might be expected in the
presence of antiferromagnetic correlations. Rather, our results are more
compatible with simple quasi-particle scattering. Furthermore, we find that the
increase in the electric field gradient at the As cannot be accounted for by
LDA calculations, suggesting that structural changes, in particular the
position of the As in the unit cell, dominate the NQR response.Comment: 17 pages, 6 figure
Enhancement of the activity of phenoxodiol by cisplatin in prostate cancer cells
Phenoxodiol is a novel isoflav-3-ene, currently undergoing clinical trials, that has a broad in vitro activity against a number of human cancer cell lines. Phenoxodiol alone inhibited DU145 and PC3 in a dose- and time-dependent manner with IC50 values of 8±1 and 38±9 μM, respectively. The combination of phenoxodiol and cisplatin was synergistic in DU145, and additive in PC3, as assessed by the Chou–Talalay method. Carboplatin was also synergistic in combination with phenoxodiol in DU145 cells. The activity of the phenoxodiol and cisplatin combination was confirmed in vivo using a DU145 xenograft model in nude mice. Pharmacokinetic data from these mice suggest that the mechanism of synergy may occur through a pharmacodynamic mechanism. An intracellular cisplatin accumulation assay showed a 35% (P<0.05) increase in the uptake of cisplatin when it was combined in a ratio of 1 μM: 5 μM phenoxodiol, resulting in a 300% (P<0.05) increase in DNA adducts. Taken together, our results suggest that phenoxodiol has interesting properties that make combination therapy with cisplatin or carboplatin appealing
The structure and function of Alzheimer's gamma secretase enzyme complex
The production and accumulation of the beta amyloid protein (Aβ) is a key event in the cascade of oxidative and inflammatory processes that characterizes Alzheimer’s disease (AD). A multi-subunit enzyme complex, referred to as gamma (γ) secretase, plays a pivotal role in the generation of Aβ from its parent molecule, the amyloid precursor protein (APP). Four core components (presenilin, nicastrin, aph-1, and pen-2) interact in a high-molecular-weight complex to perform intramembrane proteolysis on a number of membrane-bound proteins, including APP and Notch. Inhibitors and modulators of this enzyme have been assessed for their therapeutic benefit in AD. However, although these agents reduce Aβ levels, the majority have been shown to have severe side effects in pre-clinical animal studies, most likely due to the enzymes role in processing other proteins involved in normal cellular function. Current research is directed at understanding this enzyme and, in particular, at elucidating the roles that each of the core proteins plays in its function. In addition, a number of interacting proteins that are not components of γ-secretase also appear to play important roles in modulating enzyme activity. This review will discuss the structural and functional complexity of the γ-secretase enzyme and the effects of inhibiting its activity
- …