4,261 research outputs found
Formation of hydrogen impurity states in silicon and insulators at low implantation energies
The formation of hydrogen-like muonium (Mu) has been studied as a function of
implantation energy in intrinsic Si, thin films of condensed van der Waals
gases (N2, Ne, Ar, Xe), fused and crystalline quartz and sapphire. By varying
the initial energy of positive muons (mu+) between 1 and 30 keV the number of
electron-hole pairs generated in the ionization track of the mu+ can be tuned
between a few and several thousand. The results show the strong suppression of
the formation of those Mu states that depend on the availability of excess
electrons. This indicates, that the role of H-impurity states in determining
electric properties of semiconductors and insulators depends on the way how
atomic H is introduced into the material.Comment: 4 pages, 4 enscapulated postscript figures, uses revtex4 twocolumn
style to be published in Physical Review Letter
Charge degree of freedom and single-spin fluid model in YBa_2Cu_4O_8
We present a 17O nuclear magnetic resonance study in the stoichiometric
superconductor YBa_2Cu_4O_8. A double irradiation method enables us to show
that, below around 180 K, the spin-lattice relaxation rate of plane oxygen is
not only driven by magnetic, but also significantly by quadrupolar
fluctuations, i.e. low-frequency charge fluctuations. In the superconducting
state, on lowering the temperature, the quadrupolar relaxation diminishes
faster than the magnetic one. These findings show that, with the opening of the
pseudo spin gap, a charge degree of freedom of mainly oxygen character is
present in the electronic low-energy excitation spectrum.Comment: 4 pages, 3 figures, REVTE
Depth dependent spin dynamics of canonical spin glass films: A low-energy muon spin rotation study
We have performed depth dependent muon spin rotation/relaxation studies of
the dynamics of single layer films of {\it Au}Fe and {\it Cu}Mn spin glasses as
a function of thickness and of its behavior as a function of distance from the
vacuum interface (5-70 nm). A significant reduction in the muon spin relaxation
rate as a function of temperature with respect to the bulk material is observed
when the muons are stopped near (5-10 nm) the surface of the sample. A similar
reduction is observed for the whole sample if the thickness is reduced to e.g.
20 nm and less. This reflects an increased impurity spin dynamics (incomplete
freezing) close to the surface although the freezing temperature is only
modestly affected by the dimensional reduction
Quantum information processing using strongly-dipolar coupled nuclear spins
Dipolar coupled homonuclear spins present challenging, yet useful systems for
quantum information processing. In such systems, eigenbasis of the system
Hamiltonian is the appropriate computational basis and coherent control can be
achieved by specially designed strongly modulating pulses. In this letter we
describe the first experimental implementation of the quantum algorithm for
numerical gradient estimation on the eigenbasis of a four spin system.Comment: 5 pages, 5 figures, Accepted in PR
Relativity and EPR Entanglement: Comments
Recent experiment by Zhinden et al (Phys. Rev {\bf A} 63 02111, 2001)
purports to test compatibility between relativity and quantum mechanics in the
classic EPR setting. We argue that relativity has no role in the EPR argument
based solely on non-relativistic quantum formalism. It is suggested that this
interesting experiment may have significance to address fundamental questions
on quantum probability.Comment: 6 pages, no figure; Submitted to Phys. Rev.
Measurement of the spatial extent of inverse proximity in a Py/Nb/Py superconducting trilayer using low-energy muon-spin rotation
The authors acknowledge the financial support of the EPSRC (Grant No. EP/J01060X).Muon-spin rotation has been used to observe directly the spatial variation of the magnetic flux density near the ferromagnetic-superconducting interface in a permalloy-niobium trilayer. Above the superconducting transition temperature Tc the profile of the induced magnetic flux density within the niobium layer has been determined. Below Tc there is a significant reduction of the induced flux density, predominantly near the ferromagnetic-superconducting interfaces. We are uniquely able to determine the magnitude and spatial variation of this reduction in induced magnetization due to the presence of the Cooper pairs, yielding the magnitude and length scale associated with this phenomenon. Both are inconsistent with a simple Meissner screening and indicate the existence of another mechanism, the influence of which is localized within the vicinity of the ferromagnetic interface.Publisher PDFPeer reviewe
Coronary stenosis vasomotion during dynamic exercise before and after PTCA
Coronary vasomotion was evaluated in eight patients (age 50 ± 8 years) with coronary disease before and 3·3 ± 1·9 months after successful percutaneous transluminal coronary angioplasly (PTCA). Luminal area of a normal and a stenotic coronary artery was determined before and after PTCA using biplane quantitative coronary arteriography. Patients were studied at rest, during supine bicycle exercise and 5 mm after 1·6 mg sublingual nitroglycerin. Workloads before and after PTCA were identical. Percentage diameter stenosis decreased from 78% to 24% (P < 0·001) after PTCA. Mean pulmonary artery pressure increased during exercise from 21 to 40 mmHg (P < 0·001) before and from 19 to 34 mmHg (P < 0·001) after PTCA. Peak exercise pulmonary artery mean pressure was significantly (P < 0·05) lower after PTCA. Normal coronary arteries showed a minimal increase in mean luminal area before (+2%; NS) as well as after (+ 6%; NS) PTCA. Nitroglycerin produced dilation of the normal vessel segment to a similar extent pre- (+27%; P < 0·001) and post- (+31%; P < 0·001) PTCA. In contrast, stenotic vessel segments showed coronary vasoconstriction during exercise before PTCA (−28%; P < 0·01); after PTCA, exercise-induced vasoconstriction of the diseased segment was minimal (−4%; NS). Nitroglycerin was associated with vasodi lation of the stenotic vessel segment before (+17%; NS) as well as after (+26%; P <0·005) PTCA. Thus, exercise-induced coronary vasoconstriction of stenotic coronary arteries is observed before as well as after PTCA, but vasoconstriction after PTCA is significantly less than before PTCA. Coronary vasomotion appears to be modified in a positive way by PTCA, but the exact mechanism remains unclea
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