527 research outputs found
X-ray induced persistent photoconductivity in Si-doped AlGaAs
We demonstrate that X-ray irradiation can be used to induce an
insulator-metal transition in Si-doped AlGaAs, a
semiconductor with {\it DX} centers. The excitation mechanism of the {\it DX}
centers into their shallow donor state was revealed by studying the
photoconductance along with fluorescence. The photoconductance as a function of
incident X-ray energy exhibits an edge both at the Ga and As K-edge, implying
that core-hole excitation of Ga and As are efficient primary steps for the
excitation of {\it DX} centers. A high quantum yield () suggests that
the excitation is indirect and nonlocal, due to secondary electrons, holes, and
fluorescence photons.Comment: 3 pages of text, 6 figures. An error in Fig.5 was detected, so we
corrected i
Hysteresis of Backflow Imprinted in Collimated Jets
We report two different types of backflow from jets by performing 2D special
relativistic hydrodynamical simulations. One is anti-parallel and
quasi-straight to the main jet (quasi-straight backflow), and the other is bent
path of the backflow (bent backflow). We find that the former appears when the
head advance speed is comparable to or higher than the local sound speed at the
hotspot while the latter appears when the head advance speed is slower than the
sound speed bat the hotspot. Bent backflow collides with the unshocked jet and
laterally squeezes the jet. At the same time, a pair of new oblique shocks are
formed at the tip of the jet and new bent fast backflows are generated via
these oblique shocks. The hysteresis of backflow collisions is thus imprinted
in the jet as a node and anti-node structure. This process also promotes
broadening of the jet cross sectional area and it also causes a decrease in the
head advance velocity. This hydrodynamic process may be tested by observations
of compact young jets.Comment: 9 pages, 5 figures, accepted for publication in ApJ
A Comparison of the High-Frequency Magnetic Fluctuations in Insulating and Superconducting La2-xSrxCuO4
Inelastic neutron scattering performed at a spallation source is used to make
absolute measurements of the dynamic susceptibility of insulating La2CuO4 and
superconducting La2-xSrxCuO4 over the energy range 15<EN<350 meV. The effect of
Sr doping on the magnetic excitations is to cause a large broadening in
wavevector and a substantial change in the spectrum of the local spin
fluctuations. Comparison of the two compositions reveals a new energy scale of
22 meV in La1.86Sr0.14CuO4.Comment: RevTex, 7 Pages, 4 postscript figure
String Picture of a Frustrated Quantum Magnet and Dimer Model
We map a geometrically frustrated Ising system with transversal field
generated quantum dynamics to a strongly anisotropic lattice of non-crossing
elastic strings. The combined effect of frustration, quantum and thermal spin
fluctuations is explained in terms of a competition between intrinsic lattice
pinning of strings and topological defects in the lattice. From this picture we
obtain analytic results for correlations and the phase diagram which agree
nicely with recent simulations.Comment: 4 pages, 2 figure
Scaling of magnetic fluctuations near a quantum phase transition
We use inelastic neutron scattering to measure the magnetic fluctuations in a
single crystal of the heavy fermion alloy CeCu_5.9Au_0.1 close to the
antiferromagnetic quantum critical point. The energy and temperature-dependent
spectra obey (E/T) scaling at Q near (1,0,0). The neutron data and earlier bulk
susceptibility are consistent with the form 1/X ~ f(Q)+(-iE+bT)^a, with an
anomalous exponent a=0.8. We confirm the earlier observation of quasi-low
dimensionality and show how both the magnetic fluctuations and the
thermodynamics can be understood in terms of a quantum Lifshitz point.Comment: Latex file with two postscript figure
CeRuSn: heavy fermions emerging from a Kondo-insulating state
The combination of low-temperature specific-heat and
nuclear-magnetic-resonance (NMR) measurements reveals important information of
the ground-state properties of CeRuSn, which has been proposed as a
rare example of a tetragonal Kondo-insulator (KI). The NMR
spin-latticerelaxation rate deviates from the Korringa law below 100 K
signaling the onset of an energy gap K. This gap is
stable against magnetic fields up to 10 T. Below 10 K, however, unusual
low-energy excitations of in-gap states are observed, which depend strongly on
the field H. The specific heat C detects these excitations in the form of an
enhanced Sommerfeld coefficient : In zero field,
increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at
J/molK. This maximum is shifted to higher temperatures with
increasing field suggesting a residual density of states at the Fermi level
developing a spin gap . A simple model, based on two narrow
quasiparticle bands located at the Fermi level - which cross the Fermi level in
zero field at 0.022 states/meV f.u. - can account qualitatively as well as
quantitatively for the measured observables. In particular, it is demonstrated
that fitting our data of both specific heat and NMR to the model, incorporating
a Ce magnetic moment of , leads to
the prediction of the field dependence of the gap. Our measurements rule out
the presence of a quantum critical point as the origin for the enhanced
in CeRuSn and suggest that this arises rather from correlated,
residual in-gap states at the Fermi level. This work provides a fundamental
route for future investigations into the phenomenon of narrow-gap formation in
the strongly correlated class of systemComment: 11 pages, 13 figure
Microscopic and Macroscopic Signatures of Antiferromagnetic Domain Walls
Magnetotransport measurements on small single crystals of Cr, the elemental
antiferromagnet, reveal the hysteretic thermodynamics of the domain structure.
The temperature dependence of the transport coefficients is directly correlated
with the real-space evolution of the domain configuration as recorded by x-ray
microprobe imaging, revealing the effect of antiferromagnetic domain walls on
electron transport. A single antiferromagnetic domain wall interface resistance
is deduced to be of order at a
temperature of 100 K.Comment: 3 color figure
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