V. V. Dmitriev

Current sensing noise thermometry using a low Tc DC SQUID preamplifier

We describe here the design and performance of a current sensing noise thermometer using a low Tc DC SQUID as the front end amplifier. The DC SQUID is used to measure the thermal noise current in a resistor and the temperature is then obtained from the Nyquist formula. The thermometer is fast, absolute and precise and is usable over a wide temperature range below 4.2 K, in principle down to well below 1 mK. The excellent energy sensitivity of the DC SQUID, operated at fixed temperature, enables the use of a relatively large noise resistor, in the mΩ range. This requires relatively short averaging times when measuring the spectrum of noise fluctuations. We have shown that it is possible to determine absolute temperature with a precision of 1% in a measuring time of 10 seconds with an amplifier noise temperature, TN, of the order of 30 µK, and to an accuracy better than 0.3%. The percentage precision is independent of temperature for temperatures much greater than TN. Our method of heat sinking the noise resistor ensures proper cooling of the electrons. We incorporate a fixed point device for checking the gain calibration. We have cooled the thermometer successfully to below 1 mK, achieving a minimum electron temperature of 300 µK. We present the results of a preliminary comparison with a 3He melting curve thermometer (MCT) above 4.5 mK, and with a platinum NMR thermometer down to the lowest temperatures.

[Ultrastructure of resting cells of some non-spore-forming bacteria].

Using electron microscopy (ultrathin sections and freeze-fractures), we investigated the ultrastructure of the resting cells formed in cultures of Micrococcus luteus, Arthrobacter globiformis, and Pseudomonas aurantiaca under conditions of prolonged incubation (up to 9 months). These resting cells included cystlike forms that were characterized by a complex cell structure and the following ultrastructural properties: (i) a thickened or multiprofiled cell wall (CW), typically made up of a layer of the preexisting CW and one to three de novo synthesized murein layers; (ii) a thick, structurally differentiated capsule; (iii) the presence of large intramembrane particles (d = 180–270 Å), occurring both on the PF and EF faces of the membrane fractures of M. luteus and A. globiformis; (iv) a peculiar structure of the cytoplasm, which was either fine-grained or lumpy (coarse-grained) in different parts of the cell population; and (v) a condensed nucleoid. Intense formation of cystlike cells occurred in aged (2- to 9-month-old) bacterial cultures grown on diluted complex media or on nitrogen-, carbon-, and phosphorus-limited synthetic media, as well as in cell suspensions incubated in media with sodium silicate. The general morphological properties, ultrastructural organization, and physiological features of cystlike cells formed during the developmental cycle suggest that constitutive dormancy is characteristic of non-spore-forming bacteria.

Catastrophic Relaxation in 3He-B at 0.4Tc

We have observed a rapid decrease in the lifetime of a homogeneously precessing domain in 3He-B at a temperature of 0.4Tc. This phenomenon does not depend on pressure, magnetic field, or on the presence of 4He covering the surface of the chamber. We suppose this effect is the result of a magnetic relaxation through some unknown channel. Catastrophic relaxation may possibly be connected to some new phase transition in 3He.

Polar Phase of Superfluid (3)He in Anisotropic Aerogel.

We report the first observation of the polar phase of superfluid (3)He. This phase appears in (3)He confined in a new type of aerogel with a nearly parallel arrangement of strands which play the role of ordered impurities. Our experiments qualitatively agree with theoretical predictions and suggest that in other systems with unconventional Cooper pairing (e.g., in unconventional superconductors) similar phenomena may be found in the presence of anisotropic impurities.

Orbital glass and spin glass states of 3He-A in aerogel

AbstractGlass states of superfluid A-like phase of 3He in aerogel induced by random orientations of aerogel strands are investigated theoretically and experimentally. In anisotropic aerogel with stretching deformation two glass phases are observed. Both phases represent the anisotropic glass of the orbital ferromagnetic vector Ηthe orbital glass (OG). The phases differ by the spin structure: the spin nematic vector

Changes in the Fine Structure of Microbial Cells Induced by Chaotropic Salts

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Fine Structure of Mummified Cells of Microorganisms Formed under the Influence of a Chemical Analogue of the Anabiosis Autoinducer

can be either in the ordered spin nematic (SN) state or in the disordered spin-glass (SG) state. The first phase (OG-SN) is formed under conventional cooling from normal 3He. The second phase (OG-SG) is metastable, being obtained by cooling through the superfluid transition temperature, when large enough resonant continuous radio-frequency excitation is applied. NMR signature of different phases allows us to measure the parameter of the global anisotropy of the orbital glass induced by deformation.

Ultrastructural Characteristics of Natural Forms of Microorganisms Isolated from Permafrost Grounds of Eastern Siberia by the Method of Low-Temperature Fractionation

Nucleation of vortices in units of one quantum has been observed with c.w. NMR in a rotating cylinder filled with 3He-B. During acceleration a new vortex is created each time the counterflow velocity at the perimeter reaches a critical value vc(T, p). The measured vc resembles the calculated velocity vcb(T, p) of the bulk superflow instability, but is smaller by a factor with power law dependence on the superfluid coherence length. This indicates that a nucleation event takes place whenever the flow exceeds vcb locally at the nucleation site and the nucleation barrier vanishes.