We have measured the initial magnetic suppression of the B phase of superfluid ^He at pressures from zero to 29 bars. This suppression, which is asymptotically quadratic in the field strength, is always significantly greater than the weak-coupling prediction, even at zero pressure. We also provide the first values of the .4-phase specific-heat jump for pressures below the polycritical point. Microscopic models of normal liquid ^He do not conform to these experimental constraints. Consequently, it is possible that the conventional identification of the order parameter of superfluid ^HQ-A is incorrect.
A spin cluster glass behavior and a complicated exchange bias effect are observed in high quality BiFeO(3) nanocrystals grown by a hydrothermal method. The dynamic properties of the spin clusters investigated by measuring the frequency dependences of ac susceptibility show that the relaxation process can be described using a power law with the glass transition temperature T(g) = 57 K, relaxation time constant τ(0) = 4.4 × 10(-10) s, and critical exponent zv = 10.3 ± 1.9, consistent with a three-dimensional Ising spin glass. The exchange bias field (H(EB)) varies non-monotonically with temperature and achieves a minimum at T(g). The abnormal shift of hysteresis loops above T(g) may be interpreted in terms of a Malozemoff's random-field model with a framework of antiferromagnetic core/spin-cluster shell structure and a two-dimensional diluted antiferromagnet in a field (2D-DAFF) model, respectively. The exchange anisotropy of the BiFeO(3) nanocrystals will shed light on a possible application for magnetism related nanosized devices.
A general method is proposed to evaluate the uncertainty of Josephson voltage standard measurements exclusive of the uncertainty contributed by the device under test. The method is based on a statistical evaluation of the results of measurements of a short circuit under conditions that exactly duplicate the procedure used for normal calibrations. It gives a rigorous Type A evaluation of many uncertainty components that have typically been considered Type B uncertainties and often assigned worst-case values. Uncertainty components for frequency and leakage are not detected by short-circuit measurements and are separately evaluated. The method is applied to the results of an interlaboratory comparison and used to generate a statement of equivalence.
The solidification of 4 He in Vycor glass and in sintered Ag powder has been studied between 0.8 and 2.5 K. At 0.8 K, elevation of the melting pressure by 12 bars is seen in Vycor and by 0.3 bar in the Ag sinter. In each material a range of melting pressures, corresponding to different pore sizes, is observed. The location of the \ transition in the liquid in Vycor is determined up to solidification pressure.PACS numbers: 67.40. Kh, 67.80.Gb Several studies have been made of the solidification and superfluidity of 4 He in confined geometries. 1 " 3 Experiments on helium in Vycor glass have indicated that it remained a liquid at pressures of ~-15 bars above the bulk melting pressure. 2 Recently Beamish et al 3 (BHTE) have observed solidification and the superfluid transition of helium in Vycor from changes in sound velocity. However, an accurate phase diagram has not been available because of uncertainties in the presence of the helium in the Vycor.We report measurements of pressure versus temperature along isochores and of the cooling rate (related to the heat capacity) of helium in Vycor. We identify solidification, melting, and the X transition and obtain a phase diagram up to P = 60 bars and T = 2.5 K. There is a range of pressures where both solid and liquid may exist, which, at r = 0.8 K, extends from 36.3 to 38.0 bars. Similar behavior is seen in the Ag sinter, but at only ~ 0.3 bar above bulk melting. The X transition of helium in Vycor has been followed to above solidification pressure, made possible by supercooling of the liquid.The cell is depicted in Fig. 1 (inset). Measurement of the pressure of 4 He in Vycor above bulk melting is difficult because of the long time constant required for communication to a transducer. We obtained a short time constant by grinding the Vycor into a powder with grains ^ 74 pm (200 mesh sieve) thereby reducing the distance for pressure communication. 4 The powder loosely filled the cell to a filling factor of 0.5, allowing individual grains to move with the bulk helium surrounding them and to transmit pressure to the capacitive transducer. 5 Bulk helium existed in the spaces between grains which were ~ 10 4 times the dimension of the 60-A pores in the Vycor.A layer of 700-A sintered Ag powder, pressed to a filling factor of 0.5, provided thermal contact to the helium. The cell was mounted on the still of a dilution refrigerator with a weak thermal link to allow slow cooling of the cell with a time constant ~ 500 times that for equilibrium within the cell. Temperatures were measured with a carbon resistor calibrated against the 4 He melting pressure. 6 Cooling along the bulk melting curve at T = 2 K was at a rate \dT/dt\^4 mK/min, with the cell reaching 0.8 K in ~5 h. The cooling rate is related to the heat current from the cell Q by ti-ll dt = Ceff dT dt where C/ and c s are the specific heats of the liquid 20.0 20.2 20.4 20.6 20.8 21.0 21.2 V(cm 3 /mole) FIG. 1. Upper-right inset, the cell: (a) thermal link, (b) heater, (c) thermometer, (d) Ag sinter, (e) Vycor powder...
Abstract-NIST recently reported the first application of a quantum ac Josephson Voltage Standard for calibration of thermal transfer standards in the 1 kHz to 10 kHz frequency range. This paper describes preliminary work on extending its frequency calibration range up to 100 kHz by correcting systematic errors due to voltage leads. A ground loop created by the dc blocks, a previously unaccounted for source of high frequency systematic error, has been identified and its effects partially mitigated.
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