Xavier Gratens

The Schenberg spherical gravitational wave detector: the first commissioning runs

Here we present a status report of the first spherical antenna project equipped with a set of parametric transducers for gravitational detection. The Mario Schenberg, as it is called, started its commissioning phase at the Physics Institute of the University of Sao Paulo, in September 2006, under the full support of FAPESP. We have been testing the three preliminary parametric transducer systems in order to prepare the detector for the next cryogenic run, when it will be calibrated. We are also developing sapphire oscillators that will replace the current ones thereby providing better performance. We also plan to install eight transducers in the near future, six of which are of the two-mode type and arranged according to the truncated icosahedron configuration. The other two, which will be placed close to the sphere equator, will be mechanically non-resonant. In doing so, we want to verify that if the Schenberg antenna can become a wideband gravitational wave detector through the use of an ultra-high sensitivity non-resonant transducer constructed using the recent achievements of nanotechnology.

Status Report of the Schenberg Gravitational Wave Antenna

Here we present a status report of the Schenberg antenna. In the past three years it has gone to a radical upgrading operation, in which we have been installing a 1K pot dilution refrigerator, cabling and amplifiers for nine transducer circuits, designing a new suspension and vibration isolation system for the microstrip antennas, and developing a full set of new transducers, microstrip antennas, and oscillators. We are also studying an innovative approach, which could transform Schenberg into a broadband gravitational wave detector.

Magnetic tuning of all-organic binary alloys between two stable radicals

Mixtures of 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (F4BImNN) and 2-(benzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (BImNN) crystallize as solid solutions (alloys) across a wide range of binary compositions. (F4BImNN)(x)(BImNN)((1-x)) with x < 0.8 gives orthorhombic unit cells, while x ≥ 0.9 gives monoclinic unit cells. In all crystalline samples, the dominant intermolecular packing is controlled by one-dimensional (1D) hydrogen-bonded chains that lead to quasi-1D ferromagnetic behavior. Magnetic analysis over 0.4-300 K indicates ordering with strong 1D ferromagnetic exchange along the chains (J/k = 12-22 K). Interchain exchange is estimated to be 33- to 150-fold weaker, based on antiferromagnetic ordered phase formation below Néel temperatures in the 0.4-1.2 K range for the various compositions. The ordering temperatures of the orthorhombic samples increase linearly as (1 - x) increases from 0.25 to 1.00. The variation is attributed to increased interchain distance corresponding to decreased interchain exchange, when more F4BImNN is added into the orthorhombic lattice. The monoclinic samples are not part of the same trend, due to the different interchain arrangement associated with the phase change.

Magnetization steps in Sn1-xEuxTe: Eu-Eu exchange and Eu distribution

The magnetization of Sn1-x Eux Te, with x = 0.011 and 0.042, was measured at 20 mK in magnetic fields up to 90 kOe. Magnetization steps (MSTs) from pairs and triplets were observed. The MSTs give J /kB = -0.311±0.006 K for the dominant Eu-Eu exchange constant. Comparisons of the magnetization curves with numerical simulations indicate that, instead of being distributed randomly, the Eu ions tend to bunch together. A phenomenological approach which uses the concept of a local Eu concentration xL is quite successful in describing the data for these two samples.

High-field magnetization in the quantum spin magnet NiCl24SC(NH2)2

The field-induced antiferromagnetic phase transitions in NiCl24SC(NH2)2 have been determined in fields up to 18 T and temperatures down to 0.016 K. This is an S=1 system with a singlet ground state separated from an excited doublet by D/kB. In zero field this spin-gap system remains nonmagnetic down to T=0, because the small antiferromagnetic exchange interaction (J) is “subcritical” (2zJ<D). However, a magnetic field applied along the uniaxial direction lowers one of the excited energy levels, thus creating a condition for the establishment of long-range order. At T=0 this happens between B1=2.11±0.01 T and B2=12.11±0.03 T. The onset of this long-range order has been treated as Bose–Einstein condensation of magnons. The phase-boundaries B1(T) and B2(T) determined follow power-law dependence, Bi(T)−Bi(0)∝Tφ, with φ=2.6±0.2 for both.

Magnetization steps in Pb1−xCexS at 20 mK ☆

Abstract The magnetization of Pb 1− x Ce x S single crystals was measured at 20 mK and magnetic field upto 90 kOe. Magnetization steps from two types of pairs were observed. The first has an isotropic exchange interaction with J=−0.32 K. The second type of pairs gives a structured step which can be explained by an anisotropic exchange interaction. Cluster statistics suggests that both types are nearest-neighbor pairs.

The Schenberg data acquisition and analysis: results from its first commissioning run

The Mario Schenberg gravitational wave detector has started its commissioning phase at the Physics Institute of the University of Sao Paulo. We have collected almost 200 h of data from the instrument in order to check out its behavior and performance. We have also been developing a data acquisition system for it under a VXI System. Such a system is composed of an analog-to-digital converter and a GPS receiver for time synchronization. We have been building the software that controls and sets up the data acquisition. Here we present an overview of the Mario Schenberg detector and its data acquisition system, some results from the first commissioning run and solutions for some problems we have identified.

Distant exchange interactions in Cd1-xMnxS from magnetization steps method

Three new distant neighbor (DN) antiferromagnetic exchange constants have been measured in wurtzite Cd1–xMnxS (x = 0.0087 and x = 0.0278) using the magnetization steps method in the millikelvin regime. The second (J(2)), third (J(3)), and fourth (J(4)) largest exchange constants (after the two first neighbor exchange interactions J1 and J1′) have been measured. The results are J(2) = 300 ± 10 mK, J(3) = 173 ± 10 mK, and J(4) = 55 ± 10 mK. No significant change of the exchange constant values has been observed as a function of the Mn concentration. The mapping of the DN exchange interactions has been investigated within a sphere of radius 2 times the nearest neighbors distance. The only way to identify the J constants was using the number coordination Zn of the different DN classes: J(2) is associated to DN classes with Zn = 6, J(3) with Zn = 12, and J(4) is ascribed to a group of three DN classes with J values around J(4). The magnitude of the J constants is not in agreement with theoretical predictions o...

2F5/2 Manifold Splitting of Ce3+ in PbCeX (X= Te, Se, S): A Magnetic Susceptibility Study

The magnetic susceptibility of Pb1-xCexX (X = S, Se and Te) crystals with several Cerium concentrations ranging from x = 0.006 to 0.036 has been measured between 2 K and 300 K. The experimental susceptibility curves were found to be consistent with a 2F5/2 lowest manifold for Ce3+ ions; the cubic crystal-field splitting values of 2F5/2 were estimated to be about 340 K, 440 K and 540 K for Pb1-xCexTe, Pb1-xCexSe, and Pb1-xCexS, respectively. For all the studied samples, it was found that the 7 doublet lies below the 8 quadruplet. These results confirm that Ce3+ ions substitute Pb2+ in the host crystals. Furthermore, the effective Landé factors were determined by X-band (~9.5 GHz) Electron Paramagnetic Measurements (EPR) to be g = 1.333, 1.364, and 1.402 for Ce ions in PbX, X = S, Se, and Te, respectively.