Near-zero field microwave

In favorable cases, superconductivity is a sensitive and useful screen for desired crystals. Near zero field microwave absorption (95) can be used to examine very small samples of only a few micrograms. This technique has value in the search for new superconducting phases. In early attempts to identify the superconducting phase in the Pb-Sr-Y-Cu-O system, both superconducting and non-supercon-ducting crystals were obtained. Individual crystals were examined for superconductivity using near-zero field microwave absorption. Then X-ray diffraction was used to establish the structure and stoichiometry of the superconducting phase. 

At low microwave fields transitions involving only a few photons are observed, at high static fields, near the field of the avoided crossing. As the microwave field is increased more transitions are observed, at progressively lower static fields, until at the highest microwave fields the sequence of resonant transitions extends to zero static field. As shown by the scale at the top of Fig. 10.9 the 18s —> (16,3) transition nearest zero static field corresponds to the absorption of 28 photons. 

In our picture of microwave ionization the n dependence of the ionization fields comes from the rate limiting step between the bluest n and reddest + 1 Stark states. It would be most desirable to study this two level system in detail, but in Na this pair of Stark levels is almost hopelessly enmeshed in all the other levels. In K, however, there is an analogous pair of levels which is experimentally much more attractive [17,18]. The K energy levels are shown in Fig. 6. All are m = 0 levels, and we are interested in the 18s level and the Stark level labelled (16,3). We label the Stark states as (n, k) where n is the principal quantum number and k is the zero field state to which the Stark state is adiabatically connected. As shown in Fig. 6, the (16, k) Stark states have very nearly linear Stark shifts and the 18s state has only a very small second order Stark shift, which is barely visible on the scale of Fig. 6. The 18s and (16,3) states have an avoided crossing at a field of 753 V/cm due to the coupling produced by the finite size of the K-" core [19]. 

MHz of [Rh(bpy)3](0104)3, in the phosphorescent triplet state, upon switching on the microwave power. The oscillations occur as the microwave pulse duration is increased. Photoexcitation is near 320 nm, detection is at 456 nm temperature is 1.4 K. b Optically detected echo amplitude decay for the 2320 MHz zero-field transition of [Rh(bpy)3] (0104)3 as obtained by applying a n/2-T-n-T-nl2 pulse sequence when increasing 2r... 

Although the initial measurements of ODMR 86-88) were made in the conventional manner, (fixed microwave frequency source and variable externally-applied Zeeman field) nearly all measurements on the aromatic amino acid triplet states have been made at zero applied field using a variable frequency microwave source. Consequently, we will focus on the zero-field experimental method in this section. 

The synthesis of macroscopic amounts of C o and C70 (fullerenes) has stimuiated a variety of studies on their chemical and physical properties. We recently demonstrated that C o and C70 become conductive when doped with alkali metals. Here we describe iow-temperature studies of potassium-doped both as films and bulk samples, and demonstrate that this material becomes superconducting, Superconductivity is demonstrated by microwave, resistivity and Melssner-effect measurements. Both polycrystalline powders and thin-flim samples were studied. A thin film showed a resistance transition with an onset temperature of 16 K and essentially zero resistance near 5 K. Bulk samples showed a well-defined Meissner effect and magnetic-field-dependent microwave absorption beginning at 18 K. The onset of superconductivity at 18 K is the highest yet observed for a molecular superconductor. 

K. Park, Radiation-induced zero-resistance state at low magnetic fields and near half-filling of the lowest Landau level, Phys. Rev. B 69, 201301 (2004) M. G. Vavilov et al.. Magnetotransport in a two-dimensional electron gas at large filling factors, Phys. Rev. B 69, 035303 (2004) I. A. Dmitriev et al.. Oscillatory ac conductivity and photoconductivity of a two-dimensional electron gas Quasiclassical transport beyond the Boltzmann equation, Phys. Rev. B 70, 165305 (2004) J. Inarrea and G. Platero, Theoretical approach to microwave-radiation-induced zero-resistance states in 2D electron systems, Phys. Rev. Lett. 94, 016806 (2005). 

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