A Metallic Magnetic Calorimeter for Hard X-Ray and Gamma Ray Spectrometry

The CEA/LNHB is responsible for the determination and publication of atomic and nuclear data such as X-ray and gamma ray emission probabilities. In order to reduce uncertainties on the determination of these data, a high energy resolution associated with a good intrinsic detection efficiency is required. Hence taking into account these two aspects, we are developing cryogenic detectors, especially metallic magnetic calorimeters (MMCs) for photon spectrometry from few keV up to 200 keV. A MMC using a meander pick-up coil made of niobium thin films has been optimized. The gold absorber (diameter: 1.1 mm, thickness: 335 μm) has an intrinsic detection efficiency larger than 70% for photons from few keV up to 100 keV. From an energy spectrum obtained with a ¹³³Ba multi-gamma source, we have characterized this first detector. The energy resolution is 320 eV and 560 eV respectively at 30 keV and 357 keV. Possible improvements of the performance of the detector are discussed.     

Optimization of Large Multiple Coil Systems for Pulsed Magnets

Very high pulsed magnetic fields can be generated more economically using a system of multiple coils, with a high-energy, limited-power pulse generator providing the background field for a smaller inner coil, energized in its turn, but for a much shorter pulse duration, with a very high-power, limited-energy generator. Because of the increased number of parameters in the design of multi-coils, systematic insight into their mutual dependence is helpful in order to converge to an optimized design. In this paper we will discuss strategies to determine the optimum choice for the design of inner- and outer-coil and how to optimize their design in relation to the type of pulse generator used. In particular, we will consider energy-limited capacitor banks and power-limited supplies. The approach will use scaling arguments and modeling tools as the ‘Pulsed Magnet Design Software’ (PMDS) package, developed at the Katholieke Universiteit Leuven and Huazhong University of Science and Technology. Optimization of coil systems is demonstrated with the example of the successful 87 T pulsed dual-coil system in Dresden.     

Transverse Magnetic Field Distribution in the Vortex State of Noncentrosymmetric Superconductor with O Symmetry

We investigate the magnetic field distribution inside a Type II superconductor which has point group symmetry O such as Li₂Pt₃B. The absence of inversion symmetry as a departure from perfect cubic group O _h causes a magnetization collinear with the phase gradient associated with the order parameter, and a component of current collinear with the local magnetic field. In the vortex state, we predict, by solving the Maxwell equation, a local magnetic field transverse to the vortex lines. The probability distribution of this transverse field is also obtained for the prospective muon-spin-rotation measurements.     

Properties of Superconducting Rhenium as an Absorber for Magnetic Calorimeters

A still puzzling problem in the development of low temperature micro-calorimeters for the measurement of the ¹⁸⁷rhenium β-spectrum is the understanding of the thermalization of energetic electrons in the superconducting rhenium absorber. We studied metallic magnetic calorimeters (MMC) with single crystal rhenium absorbers and paramagnetic Au:Er temperature sensors. The energy released into the detector leads to a change of magnetization of the paramagnetic sensor located in a weak magnetic field. A SQUID with meander shaped inductance is used to read out this change. This setup allows the study of several properties of the superconducting absorber. The transition to the superconducting state is studied by measuring the magnetic flux expelled by the rhenium sample. The resistivity of rhenium above T _c can be estimated from the measurement of the spectral power density of the Johnson noise. Furthermore the quasiparticle lifetime can be investigated through the analysis of the shape of detector signals caused by intrinsic β-decays and the absorption of X-rays. We present the data obtained in these experiments and discuss the physical quantities which can be derived from these.      

Development of Micro-SQUID Magnetometers for Investigation of Quantum Tunneling of Magnetization in Nanometer-Size Magnetic Materials

We report development of micro superconducting quantum interference device (μ-SQUID) magnetometers for investigation of quantum tunneling of magnetization in μm- and nm-size magnetic materials. Both high- and low-temperature superconductor (HTS and LTS) based μ-SQUID magnetometers were fabricated and a three dimensional magnetic coil system was constructed for this purpose. The HTS-μ-SQUIDs with a hole of 4×9 μm² work at temperatures between 4.2 and 70 K and in magnetic fields up to 120 mT. A magnetization measurement of a ferrimagnetic micro-crystal was carried out at 35 K with an accuracy of 10^?9 emu. The development of LTS-μ-SQUIDs has been started in order to study much smaller magnetic materials in a mK temperature range. We present a preliminary result on the LTS-μ-SQUID with a hole of 1×1 μm². The critical current as a function of applied magnetic field shows the SQUID modulation at 4.2 K and up to 30 mT. The heat release associated with the present measurement method is estimated to be on the order of several microwatts.     

3He NMR with a 100 μm Planar Nb Coil

A superconducting 100 m planar coil has been tested for low field cw NMR. The nine-turn 5 nH coil is connected via a planar 1:8 step-up transformer to an external parallel capacitor and resonated at 0.8 MHz to give a Q of 350. The tank circuit voltage is measured with a liquid He temperature GaAs MESFET preamplifier. Test experiments have been conducted by immersing the Si chip with the planar pick-up coil in liquid and solid ³ He to measure their cw NMR absorptions in the temperature range 0.05 – 0.5 K.     

Neutron Scattering at Highest Magnetic Fields at the Helmholtz Centre Berlin

The Helmholtz Centre Berlin (HZB), formerly Hahn-Meitner Institute is a user facility for the study of structure and dynamics with neutrons and synchrotron radiation with special emphasis on experiments under extreme conditions. Neutron scattering is uniquely suited to study magnetic properties on a microscopic length scale, because neutrons have comparable wavelengths and, due to their magnetic moment, they interact with the atomic magnetic moments. At HZB a dedicated instrument for neutron scattering at extreme fields is under construction, the Extreme Environment Diffractometer ExED. It is projected according to the “time-of-flight” principle for elastic and inelastic neutron scattering and for the special geometric constraints of analysing samples in a high field magnet. The new magnet will not only allow for novel experiments, it will be at the forefront of development in magnet technology itself. The design of the magnet will follow the Series Connected Hybrid System Technology (SCH) developed at the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida. To compromise between the needs of the magnet design for highest fields and the concept of the neutron instrument, the magnetic field will be generated by means of a coned solenoid with horizontal field orientation. By using resistive insert coils, which are mounted in the room temperature bore of a superconducting cable-in-conduit (CIC) magnet, fields above 30 Tesla can be obtained in a geometry optimised for the demands of neutron scattering.     

The EBIT Calorimeter Spectrometer: A New, Permanent User Facility at the LLNL EBIT

The EBIT Calorimeter Spectrometer (ECS) is currently being completed and will be installed at the EBIT facility at the Lawrence Livermore National Laboratory in October 2007. The ECS will replace the smaller XRS/EBIT microcalorimeter spectrometer that has been in almost continuous operation since 2000. The XRS/EBIT was based on a spare laboratory cryostat and an engineering model detector system from the Suzaku/XRS observatory program. The new ECS spectrometer was built to be a low maintenance, high performance implanted silicon microcalorimeter spectrometer with 4 eV resolution at 6 keV, 32 detector channels, 10 μs event timing, and capable of uninterrupted acquisition sessions of over 60 hours at 50 mK. The XRS/EBIT program has been very successful, producing many results on topics such as laboratory astrophysics, atomic physics, nuclear physics, and calibration of the spectrometers for the National Ignition Facility. The ECS spectrometer will continue this work into the future with improved spectral resolution, integration times, and ease-of-use. We designed the ECS instrument with TES detectors in mind by using the same highly successful magnetic shielding as our laboratory TES cryostats. This design will lead to a future TES instrument at the LLNL EBIT. Here we discuss the legacy of the XRS/EBIT program, the performance of the new ECS spectrometer, and plans for a future TES instrument.      

CeRu2Al10: Anomalous Magnetic Ordering and Its Field Stability

The intermetallic compound CeRu₂Al₁₀ orders in a new crystal structure type that is cage-like on account of its peculiar atomic arrangement and large interatomic distances—especially concerning the environment of the rare-earth element Ce. In previous work we showed that anomalous thermal, electronic, and magnetic properties of this compound coalesce into a phase transition at T ^*=27 K, which is exceptionally high for a cerium intermetallic compound. T ^* has been characterized through various temperature-dependent properties which suggest a multi-parameter nature of the ordering at T ^*. Here we report on continued investigations into this compound and focus in particular on the response to applied magnetic fields. Whereas transport properties in the ordered region are receptive to magnetic fields, the transition itself turns out to be more robust and is insensitive to static fields up to 14 T.     

Magnetic Forces Relaxation Step Change Characteristics of YBCO Bulk with Mechanical Impact Load over a NdFeB Guideway

Magnetic force relaxation of YBCO bulk above the NdFeB permanent magnet guideway (PMG) with impact load has been investigated. An experimental setup has been built using a single YBCO bulk and a symmetrical center NdFeB PMG. There are two experimental methods: the case of magnetic levitation force relaxation measuring perturbed with impact load in zero-field-cooling (ZFC); and the case of magnetic levitation and guidance force relaxation measuring synchronously perturbed with impact load in field-cooling (FC). The results show that there is magnetic levitation and guidance forces relaxation step change at the time of the impact load. Two times impact loads are applied for each method. The first step change range is much larger than the second one. The Bean critical model and Anderson–Kim theory are used to analyze it.     

Dynamic Simulation of High Temperature Superconductors Above a Spinning Circular Permanent Magnetic Guideway

Before a high temperature superconducting (HTS) magnetic levitation (Maglev) vehicle system can be fully applied and operational, the study of its dynamic characteristics is necessary. With the developed HTS Maglev dynamic measurement system (SCML-03), with a circular permanent magnet guideway (PMG) of 1.5 m in diameter, the vehicle’s translational motion above a PMG can be effectively simulated with the PMG allowed to rotate freely. Levitation force measurements of a high temperature superconductor (HTSC) array of seven YBa₂Cu₃O_7−x bulks were carried out above regular (linear) and a simulated (circular) PMG. The levitation force above a linear PMG segment and a circular PMG segment in the static state is found to be in good agreement with each other. The levitation force in the dynamic state is found to slowly attenuate since the presence of a rotating circular PMG below the HTS array is found to be analogous to the application of an AC external magnetic field.     

Magnetic Rare-Earth Impurity Resonance Bound States in Iron-Based Superconductors

Starting from an Anderson model for two-orbital model of electron and hole conduction bands which hybridized with the localized impurity spin, we investigate the effect of magnetic impurities on the local quasiparticle density of states (LDOS) in iron-based superconductors. We consider extended s-wave (s ⁺⁻) superconducting gap symmetry with higher harmonic correction. The impurity-induced bound states are a probe for the nodal structure of the extended s-wave symmetry in ferropnictides.     

Observation of the Semiconductor-Semimetal and Semimetal-Semiconductor Transitions in Bi Quantum Wires Induced by Anisotropic Deformation and Magnetic Field

The semimetal-semiconductor transition is observed in glass-coated quantum single-crystal bismuth wires with diameters less than 70 nm due to the quantum size effect. It is found that elastic deformation of Bi nanowires (10 $\bar{1}$ 1) oriented along the wire axis with the semiconductor dependence R(T) leads to the approaching of L and T bands and to the semiconductor-semimetal transition; as a result, Shubnikov-de Haas oscillations appear on the magnetoresistance dependences R(H). It is shown that strong magnetic field and elastic deformation are the tools to control gap size in quantum bismuth wires, which is principal for their practical use in particular in thermoelectricity.     

Superconducting Vortex Pinning with Magnetic Dots: Does Size and Magnetic Configuration Matter?

The pinning of superconducting vortices in type-II superconductors has been studied for a long time due to the wide variety of unusual flux flow phenomena and more importantly, for its relevance in applications, since vortex pinning is one of the essential parameters controlling the enhancement of critical currents. A case of particular interest is the use of artificial magnetic pinning centers, since they can be fabricated to match well the characteristic length scales relevant for superconductivity and their magnetization offers another degree of freedom to influence the pinning properties. This article reviews our work on the role of the size and separation of the magnetic dots. Furthermore, we also show that the magnetic configuration can influence significantly the pinning strength, through the magnetic stray fields penetrating the superconductor, which can be drastically different.     

Application of an Electro-Magnetic Induction Technique for the Magnetization up to 100 T in a Vertical Single-turn Coil System

The system was developed for the magnetization measurement in the vertical single-turn coil (V-STC) system at ISSP, which can generate magnetic fields over 100 T in a semi-destructive manner. We have adjusted the electro-magnetic induction method to our V-STC. The new system was applied to the manganite with the perovskite-type structure Bi_1/2Ca_1/2MnO₃. The total magnetization process was obtained up to 105 T in excellent quality comparable to those obtained by the non-destructive long pulse magnet.     

Properties of Tungsten Thin Films Produced with the RF-Sputtering Technique

For the purpose of building very sensitive light and phonon detectors, as e.g. applied in the Dark Matter (DM) experiment CRESST (Cryogenic Rare Event Search with Superconducting Thermometers), transition edge sensors (TESs) in combination with a massive absorber crystal are used. To ensure high sensitivity of the detectors, low heat capacities, i.e. low working temperatures of about 10 mK are aimed at. Therefore, TESs made of tungsten thin films exhibiting the alpha-tungsten (α-W) phase with transition temperatures of T _c =10–15 mK are required. We have produced tungsten thin films with T _c in the range of 25–55 mK by rf-sputtering. To decouple the thermometer production from the choice of the target material and to avoid heating cycles of the absorber crystal, a composite design for detector production is applied. The composite design includes fabrication of the TES on a separate substrate and then attaching of this separate TES to a massive absorber crystal by gluing. For this purpose small sapphire substrates are used for the deposition of the TES. Properties of tungsten thin films grown with the rf-sputtering technique as well as first results of composite detectors built with these films acting as TESs will be presented.      

Dynamic Alignment of Single-Walled Carbon Nanotubes in Pulsed Magnetic Fields

We have used linear dichroism spectroscopy to measure the dynamic alignment of single-walled carbon nanotubes (SWNT) in pulsed magnetic fields up to 55 T. We make use of the fact that SWNTs absorb light only when the electric-field vector is oriented parallel to the tube axis. SWNTs thus produce a polarization dependent change of the optical transmission, that permits precise measurements of their orientation. In order to distinguish the influence of different mechanisms governing the alignment such as the external magnetic field, Brownian motion or the tube length, we have systematically varied parameters such as the viscosity of the aqueous solution and the sample temperature.     

Magnetic Orders of Highly Frustrated Spinel, ZnCr2O4 in Magnetic Fields up to 400 T

We investigated the magnetization process of the highly frustrated spinel ZnCr₂O₄ using the Faraday rotation method at 6 K under the ultra-high magnetic fields up to 400 T by the electro-magnetic flux compression method. We successfully observed the full magnetization process in this material. We found some anomalies which show new phase transitions at 240 T, 290 T, 350 T, and 390 T.     

Fabrication of Cryogenic Readout Circuits with n-type GaAs-JFETs for Low Temperature Detectors

We present the design and fabrication of cryogenic readout integrated circuits (ROICs) for Superconducting Tunneling Junctions using the SONY n-type GaAs-JFETs, which have good current-voltage characteristics and low noise performance even at <1 K. In order to fabricate the ROICs, we have designed simple GaAs-JFET amplifiers based on the measurement results of the GaAs-JFETs at <4.2 K: source follower amplifier, common source amplifier, and two types of cascode amplifiers. The obtainable gain of the cascode active load amplifiers is >100. These amplifiers were fabricated with the other circuit elements as an integrated circuit. We also show initial test results of the cascode active load amplifier.     

The structure and function of ribonuclease A upon interacting with carbon nanotubes

Ribonucleic acid (RNA) is an important genetic material whose lifetime is most often determined by the rate of its enzymatic degradation. Our studies showed that multi-walled carbon nanotubes and single-walled carbon nanotubes functionalized with carboxylic groups interacted with ribonuclease A (RNase A) and caused the reduction of its activity by changing its conformation, thus protecting RNA from enzymatic cleavage. The results showed that RNase A was less active on the carbon nanotube surface than in free solution, and the activity was decreased further on larger carbon nanotubes, suggesting that carbon nanotubes with various surface modifications may be useful in RNA extraction, purification, and manipulation.