A new quantum fluid at high magnetic fields in the marginal charge-density-wave system α\alpha-(BEDT-TTF)2M_2MHg(SCN)4_4 (where M=M=~K and Rb)

Single crystals of the organic charge-transfer salts $\alpha$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ have been studied using Hall-potential measurements ($M=$K) and magnetization experiments ($M$ = K, Rb). The data show that two types of screening currents occur within the high-field, low-temperature CDW$_x$ phases of these salts in response to time-dependent magnetic fields. The first, which gives rise to the induced Hall potential, is a free current (${\bf j}_{\rm free}$), present at the surface of the sample. The time constant for the decay of these currents is much longer than that expected from the sample resistivity. The second component of the current appears to be magnetic (${\bf j}_{\rm mag}$), in that it is a microscopic, quasi-orbital effect; it is evenly distributed within the bulk of the sample upon saturation. To explain these data, we propose a simple model invoking a new type of quantum fluid comprising a CDW coexisting with a two-dimensional Fermi-surface pocket which describes the two types of current. The model and data are able to account for the body of previous experimental data which had generated apparently contradictory interpretations in terms of the quantum Hall effect or superconductivity.Comment: 13 pages, 11 figure

Coded-aperture imaging in nuclear medicine

Coded-aperture imaging is a technique for imaging sources that emit high-energy radiation. This type of imaging involves shadow casting and not reflection or refraction. High-energy sources exist in x ray and gamma-ray astronomy, nuclear reactor fuel-rod imaging, and nuclear medicine. Of these three areas nuclear medicine is perhaps the most challenging because of the limited amount of radiation available and because a three-dimensional source distribution is to be determined. In nuclear medicine a radioactive pharmaceutical is administered to a patient. The pharmaceutical is designed to be taken up by a particular organ of interest, and its distribution provides clinical information about the function of the organ, or the presence of lesions within the organ. This distribution is determined from spatial measurements of the radiation emitted by the radiopharmaceutical. The principles of imaging radiopharmaceutical distributions with coded apertures are reviewed. Included is a discussion of linear shift-variant projection operators and the associated inverse problem. A system developed at the University of Arizona in Tucson consisting of small modular gamma-ray cameras fitted with coded apertures is described

Experimental observation of Frohlich superconductivity in high magnetic fields

Resistivity and irreversible magnetisation data taken within the high-magnetic-field CDWx phase of the quasi-two-dimensional organic metal alpha-(BEDT-TTF)2KHg(SCN)4 are shown to be consistent with a field-induced inhomogeneous superconducting phase. In-plane skin-depth measurements show that the resistive transition on entering the CDWx phase is both isotropic and representative of the bulk.Comment: ten pages, four figure

Study of Chromium-Frit-Type Coatings for High-Temperature Protection of Molybdenum

The achievement of more compact and efficient power plants for aircraft is dependent, among other factors, on the perfection of heat-resisting materials that are superior to those in current use. Molybdenum is one of the high-melting metals (melting point, 4750 F). It is fairly abundant and also can be worked into many of the shapes required in modern power plants. To permit its widespread use at elevated temperatures, however, some means must first be found to prevent its rapid oxidation. The application of a protective coating is one method that might be used to achieve this goal. In the present work, a number of chromium-frit-type coatings were studied. These were bonded to molybdenum specimens by firing in controlled atmospheres to temperatures in the range of 2400 to 2700 F

Effects of rapid thermal annealing on device characteristics of InGaAs/GaAs quantum dot infrared photodetectors

In this work, rapid thermal annealing was performed on InGaAs/GaAs quantum dot infrared photodetectors (QDIPs) at different temperatures. The photoluminescence showed a blueshifted spectrum in comparison with the as-grown sample when the annealing temperature was higher than 700 °C, as a result of thermal interdiffusion of the quantum dots (QDs). Correspondingly, the spectral response from the annealed QDIP exhibited a redshift. At the higher annealing temperature of 800 °C, in addition to the largely redshifted photoresponse peak of 7.4 µm (compared with the 6.1 µm of the as-grown QDIP), a high energy peak at 5.6 µm (220 meV) was also observed, leading to a broad spectrum linewidth of 40%. This is due to the large interdiffusion effect which could greatly vary the composition of the QDs and thus increase the relative optical absorption intensity at higher energy. The other important detector characteristics such as dark current, peak responsivity, and detectivity were also measured. It was found that the overall device performance was not affected by low annealing temperature, however, for high annealing temperature, some degradation in device detectivity (but not responsivity) was observed. This is a consequence of increased dark current due to defect formation and increased ground state energy. © 2006 American Institute of Physic

S=1/2 Kagome antiferromagnets Cs2_2Cu3MF_3MF_{12}$ with M=Zr and Hf

Magnetization and specific heat measurements have been carried out on Cs$_2$Cu$_3$ZrF$_{12}$ and Cs$_2$Cu$_3$HfF$_{12}$ single crystals, in which Cu$^{2+}$ ions with spin-1/2 form a regular Kagom\'{e} lattice. The antiferromagnetic exchange interaction between neighboring Cu$^{2+}$ spins is $J/k_{\rm B}\simeq 360$ K and 540 K for Cs$_2$Cu$_3$ZrF$_{12}$ and Cs$_2$Cu$_3$HfF$_{12}$, respectively. Structural phase transitions were observed at $T_{\rm t}\simeq 210$ K and 175 K for Cs$_2$Cu$_3$ZrF$_{12}$ and Cs$_2$Cu$_3$HfF$_{12}$, respectively. The specific heat shows a small bend anomaly indicative of magnetic ordering at $T_\mathrm{N}= 23.5$ K and 24.5 K in Cs$_2$Cu$_3$ZrF$_{12}$ and Cs$_2$Cu$_3$HfF$_{12}$, respectively. Weak ferromagnetic behavior was observed below $T_\mathrm{N}$. This weak ferromagnetism should be ascribed to the antisymmetric interaction of the Dzyaloshinsky-Moriya type that are generally allowed in the Kagom\'{e} lattice.Comment: 6 pages, 4 figure. Conference proceeding of Highly Frustrated Magnetism 200

Interplay between Fermi surface topology and ordering in URu2_{2}Si2_2 revealed through abrupt Hall coefficient changes in strong magnetic fields

Temperature- and field-dependent measurements of the Hall effect of pure and 4 % Rh-doped URu$_{2}$Si$_{2}$ reveal low density (0.03 hole/U) high mobility carriers to be unique to the `hidden order' phase and consistent with an itinerant density-wave order parameter. The Fermi surface undergoes a series of abrupt changes as the magnetic field is increased. When combined with existing de Haas-van Alphen data, the Hall data expose a strong interplay between the stability of the `hidden order,' the degree of polarization of the Fermi liquid and the Fermi surface topology.Comment: 4 pages, 4 figures, Accepted to Phys. Rev. Let

Critical State Behaviour in a Low Dimensional Metal Induced by Strong Magnetic Fields

We present the results of magnetotransport and magnetic torque measurements on the alpha-(BEDT-TTF)2KHg(SCN)4 charge-transfer salt within the high magnetic field phase, in magnetic fields extending to 33 T and temperatures as low as 27 mK. While the high magnetic field phase (at fields greater than ~ 23 T) is expected, on theoretical grounds, to be either a modulated charge-density wave phase or a charge/spin-density wave hybrid, the resistivity undergoes a dramatic drop below ~ 3 K within the high magnetic field phase, falling in an approximately exponential fashion at low temperatures, while the magnetic torque exhibits pronounced hysteresis effects. This hysteresis, which occurs over a broad range of fields, is both strongly temperature-dependent and has several of the behavioural characteristics predicted by critical-state models used to describe the pinning of vortices in type II superconductors in strong magnetic fields. Thus, rather than exhibiting the usual behaviour expected for a density wave ground state, both the transport and the magnetic properties of alpha-(BEDT-TTF)2KHg(SCN)4, at high magnetic fields, closely resembles those of a type II superconductor

Re-entrant hidden order at a metamagnetic quantum critical end point

Magnetization measurements of URu2Si2 in pulsed magnetic fields of 44 T reveal that the hidden order phase is destroyed before appearing in the form of a re-entrant phase between ~ 36 and 39 T. Evidence for conventional itinerant electron metamagnetism at higher temperatures suggests that the re-entrant phase is created in the vicinity of a quantum critical end point.Comment: 8 pages, including 3 figures (Physical Review Letters, in press) a systematic error in the field calibration has been fixed since the original submission of this manuscrip