Search Results (18 total)

We present the performances and the strain sensitivity of the first spherical gravitational wave detector equipped with a capacitive transducer and readout by a low noise two-stage SQUID amplifier and operated at a temperature of 5 K. We characterized the detector performance in terms of thermal and electrical noise in the system output signal. We measured a peak strain sensitivity of 1.5×10^-20  Hz^-1/2 at 2942.9 Hz. A strain sensitivity of better than 5×10^-20  Hz^-1/2 has been obtained over a bandwidth of 30 Hz. We expect an improvement of more than 1 order of magnitude when the detector will operate at 50 mK. Our results represent the first step towards the development of an ultracryogenic omnidirectional detector sensitive to gravitational radiation in the 3 kHz range.

We report on experiments performed to probe quantum coherence in a system consisting of an rf-SQUID in which the Josephson junction is replaced by a small loop containing two junctions in parallel. At temperatures of the order of 10 mK the system may develop three potential energy wells, which modify the usual two well energy profile and thereby verify the qubit manipulation strategy. The appearance of the third potential well can be interpreted as evidence of a butterfly catastrophe, namely, a catastrophe expected for a system described by four control parameters and one state variable.

We report the present results of CUORICINO, a search for neutrinoless double-beta (0νββ) decay of ¹³⁰Te. The detector is an array of 62 TeO₂ bolometers with a total active mass of 40.7 kg. The array is cooled by a dilution refrigerator shielded from environmental radioactivity and energetic neutrons, operated at ∼8  mK in the Gran Sasso Underground Laboratory. No evidence for (0νββ) decay was found and a new lower limit, T_1/2^0ν≥1.8×10²⁴  yr (90% C.L.) is set, corresponding to ⟨m_ν⟩≤0.2 to 1.1 eV, depending on the theoretical nuclear matrix elements used in the analysis.

The formation of “hot” (8 eV) electrons under excess electron drift in a moderate electrostatic field through solid xenon has been experimentally proved by observation of secondary electrons emitted from the photocathode. At T=77  K and U=1000  V one drifting electron produces about 20 (172 nm) photons, the efficiency of electric field–to–vacuum ultraviolet emission conversion is 15% tending to grow with temperature. A self-sustained electric discharge has been generated in solid Xe using a three-electrode cell with a zinc cathode.

We elucidate the melting process of highly magnetized solid ³He by observing the magnetization profile and the liquid-solid interface simultaneously. Clear enhancements of magnetization and magnetization gradients at the interface of both the solid and the liquid were observed during melting. These measurements provide a mesoscopic confirmation of the melting scenario of Castaing and Nozières, and explain the long delay before the instability sets in: The magnetization gradient in the liquid leads to an initial suppression of the melting instability, in accordance with our extension of the stability analysis of Puech et al. This resolves the discrepancy between theory and experiment.

A melting instability has been observed during rapid melting of highly magnetized solid ³He. The instability occurred only if the solid is grown at low initial temperature and in high magnetic field, i.e., with high magnetization, and if the solid is melted sufficiently rapidly. After the instability of the interface occurred, the solid formed many cellular dendrites, directed parallel to the magnetic field. This is the first observation where a clear influence is seen of the magnetic field and the magnetization on a growth and melting process. The instability is attributed to a Mullins-Sekerka–type instability due to the magnetization gradient at the interface in the solid.

The most important features of the proposed spherical gravitational wave detectors are closely linked with their symmetry. Hollow spheres share this property with solid ones, considered in the literature so far, and constitute an interesting alternative for the realization of an omnidirectional gravitational wave detector. In this paper we address the problem of how a hollow elastic sphere interacts with an incoming gravitational wave and find an analytical solution for its normal mode spectrum and response, as well as for its energy absorption cross sections. It appears that this shape can be designed having relatively low resonance frequencies (∼ 200 Hz) yet keeping a large cross section, so its frequency range overlaps with the projected large interferometers. We also apply the obtained results to discuss the performance of a hollow sphere as a detector for a variety of gravitational wave signals.

We present the first measurements of the viscosity of superfluid ³He in magnetic fields up to 15 T, at temperatures down to 2 mK along the melting curve, using an assumption for the temperature dependence of the normal density. At fields higher than 4 T, the viscosity shows a minimum in the A₁ phase, and reaches a local maximum at the A₂ transition which becomes global at 14.6 T. The sharp decrease just below the A₁ transition can be understood within the Ginzburg-Landau framework and is consistent with both theory and experiments in lower fields. At temperatures below the minimum, the viscosity in the A₁ phase is dominated by a term with T^-2 dependence. Additionally, we have calculated the temperature dependence of the viscosity tensor in the low temperature limit using the method of temperature asymptotes.

Images of ³He crystals at temperatures down to 1 mK have been obtained by means of a new type of optical cryostat, relying on the use of a compact charge-coupled device camera inside a nuclear demagnetization refrigerator. During growth from the superfluid ³He-A phase, some crystals have shown second and third roughening, i.e., the presence of (100) and (211) facets in addition to the (110) facets previously observed. The presence of these new facets is in accordance with the Kosterlitz-Thouless theory of the roughening transition.

We have measured the behavior of a vibrating-wire viscometer in superfluid ³He in the A₁ and A₂ phases in magnetic fields up to 9.2 T and at various pressures. In reduced units the viscosity in the A₁ phase is, within our accuracy, independent of magnetic field and displays a minimum as a function of temperature for sufficiently high magnetic fields. Slightly below the A₂ transition the viscosity shows an anomalous dip, which could indicate a new phase or a textural transformation. We offer a preliminary explanation for the observed anomalies.

The polarization dependence of the viscosity of spin-polarized liquid ³He has been determined in the temperature range 75<T<300 mK and the polarization range Δ<0.35. We find an increase in the viscosity, which can be described as η∝1+αΔ² with α=4.0±1.5, at a pressure of 27 bars. This increase is expected from the ratio of the cross sections for s-wave and p-wave scattering. It is rather surprising, however, that there is no temperature dependence of α.

A new method to measure the magnetization dependence of the viscosity in polarized liquid ³He is presented. The magnetization is obtained by "brute-force polarization" at 45 mK in magnetic fields up to 11 T; it is subsequently destroyed by saturation of the NMR signal. Our result, a relative increase of the viscosity of (3±1.5)×10^-3 at 3.9% polarization and a pressure of 30 bars, disagrees with a prediction based on the "nearly metamagnetic" model.

We have explored some of the consequences of the nearly metamagnetic model of liquid ³He due to Bedell and Sanchez-Castro. The most interesting result is for the superfluid transition. The superfluid phase shows a reentrant behavior with the transition temperature reaching a maximum of ∼ 14 mK at 25 bars and 35% polarization. We propose an experimental procedure for reaching this region of the phase diagram and discuss a number of experiments to explore the reentrant behavior.

For temperatures below phase separation of 1000 ppm ³He in solid ⁴He, we measure a heat capacity γT for a pressure between melting of pure ³He and ⁴He. Together with the confined sample geometry, this results in liquid ³He droplets (ϕ∼10³ Å) either dilute or pure depending on the phase diagram topology which is discussed theoretically. In the case of a pure ³He droplet, an anomalously high effective mass m^* / m=10 is found, which could be explained by paramagnon effects enhanced by the confined geometry.

With use of compressional cooling, ordering in solid ³He is observed and identified by a null in the pressurization rate for reversible compressions in magnetic fields up to 7.2 T. At this field the ordering is above 3 mK. Entropies at T_A deduced from (P_A2-P_A1) / (T_A2-T_A1) are shown to be lower than expected from the various models. Irreversible magnetic heating at high fields which limits the effective cooling power is also observed.

In liquid ³He in a Pomeranchuk cell, spin polarizations of 5-20% have been obtained. The method involved production of polarized solid by Pomeranchuk cooling in magnetic fields from 0.03 to 3 T, followed by rapid decompression to a pressure between 2.9 and 0.2 MPa. Relaxation times in the liquid range from 1 to 5 min, increasing with decreasing pressure and with increasing field.

We describe superconducting-quantum-interference-device susceptibility measurements on several dilute natural and monoisotopic AuYb alloys down to 7 mK. Whereas Au¹⁷⁴Yb obeys a Curie law in the whole range of temperature, a very unusual case of a Van Vleck-like behavior due to an electronuclear singlet ground state is reported for Au¹⁷¹Yb.