Method for the preparation of a sample comprising highly polarized nuclear spins and uses and devices for such a method

Method for the preparation of a sample comprising highly polarized nuclear spins, comprising at least the following steps: a) bringing a polarization system, which using an electrochemical reaction can be converted into an EPR-active radical form thereof, in contact or close proximity with said sample; b) in situ electrochemically forming an EPR-active radical form of said polarization system; c) applying dynamic nuclear polarization techniques in the presence of a magnetic field by applying electromagnetic irradiation with a frequency adapted to transfer spin polarization from the electrons of the polarization system to the nuclear spins of the sample leading to a highly polarized state thereof. d) in situ electrochemically converting said polarization system, which is paramagnetic and comprises an EPR-active radical into a diamagnetic form without an EPR-active radical.

Power line communications

Emmanuel Marthe

With the deregulation of the electricity and telecommunications markets, a great deal of new operators providing some innovative services have appeared. In the telecommunications' field, the promise of broadband Internet access with easy installation procedure at low cost may help to win new customers. The Power Line Communications (PLC) technology is a way to fulfil this function. PLC uses the low voltage (LV) and the medium voltage (MV) networks as communication support. It consists in an additional signal carrying data information and superimposed to the 50 Hz power wave. As PLC is easy to install, it can be used to extend the Internet coverage to areas that are still badly covered by the other broadband technologies, or to provide high speed Internet access to every power socket of a building. This thesis, done as a part of research project called Digital Power Line Access, is devoted to the study of electromagnetic compatibility (EMC) aspects involved with the PLC. There are still a certain number of problems due to the fact that PLC, which is a quite recent technology, has not been regulated yet. Among these problems, all the aspects associated with the radiation created by the transmission of PLC signals over unshielded conductors and their potential disturbances are very significant. The aim of this thesis is the analysis of PLC data transmission over the low voltage network and the related EMC problems, mainly the radiation due to the transmission of PLC signals. The measurement and the analysis of these radiations lead us to develop a method to reduce these radiations. The mitigation of radiation created by the use of PLC within buildings is still an important EMC challenge, because there are no possibilities for network conditioning. The first two chapters of this thesis present a state of the art of PLC, its regulation and the related EMC problems. The second chapter describes the working of PLC and emphasizes the hostile nature of its environment. The problems (filtering, attenuation, generation of noise,...) caused by the use of a signal in the [1 – 30 MHz] band over a channel designed for a 50 Hz power wave are presented. On the other hand, the various technologies used to allow the PLC to work (suitable modulation scheme, use of repeater,...) are illustrated. An analysis of the different PLC concepts and of the successful and unfruitful commercial deployment ends this chapter. In the third chapter, we present the technical criterion (use of the PLC as a multi-purpose port: as a main port and as a telecommunication port) that we have to consider and the evolution of the conducted and radiated limits that we have to respect. The pollution of the radio amateur band, which could be the result of a massive deployment of PLC, is the subject of tremendous discussions and proves that if according to some persons the current limits are too permissive, for the PLC manufacturer they are already too restrictive. The coexistence of the PLC with other broadband technologies (xDSL, CA-TV, Wireless,...) is also addressed and we show that only the coexistence with an alternative of the DSL – VDSL – could be problematic. The main original contributions of this thesis are presented in chapters 4 to 6. We concentrated on the problems related to the electromagnetic radiations associated with PLC signals, especially in indoor environment. The reason for this is that the emission of electromagnetic noise, which can interfere with services such as public and amateur radio, remains an obstacle to the development of PLC. Furthermore, to the best of our knowledge, the mitigation of indoor radiation has not been addressed in the literature. In chapter 4, we present a complete characterization of the problems resulting from the use of PLC in an indoor environment. Two approaches are presented for the measurement of electromagnetic fields. The first one consists in the injection of a reference signal in the network and the measurement of the resulting radiations. The second one deals with the measurements of interferences due to the work of real PLC equipments. These two approaches enlight several aspects. The main aspect is the strong dispersion of the measurements, resulting from the influence of the position of the observation point and of the power network load. Measurements on real PLC networks show excessive radiations, which confirm that the limits are, at this time, too restrictive. We also present and discuss PLC radiated emissions in typical indoor environments. In particular, we analyze the field levels resulting from PLC signals with magnitudes given by EN55022/CISPR22 limits and compare them with various proposals for PLC radiated field limits. Thus, we show that the resulting fields exceed, for a wide range of frequencies, some of the current emission limits such as the German NB30, which is considered as a very strict limit. In this same chapter, we present a technique for the analysis of power line communication (PLC) signals, based on the formalism of the scattering parameters. This technique allows an efficient and fast characterization of any transmission medium of a power network. The analysis of the variations of the magnitude and phase of the scattering parameters gives useful insights on the increasing of the losses with frequency and confirm the need of a technique to reduce impedance mismatch at both injection and reception ports. The chapter 5 presents the modelling of the PLC environment and the computation of its radiations using a commercial code, the Numerical Electromagnetics Code (NEC), which solves the electric field integral equation in the frequency domain using the method of moments. We present some comparisons between the NEC simulations and the experimental data for the total magnetic field presented in the previous chapter. The simulations of simple models give results in good agreement with the experimental data and therefore allow the validation of certain principles. Unfortunately, the simulations on a model of increasing complexity give less satisfactory results. Because of the difficulty to respect the topology of the network and to take into account all of the components of the network coupled with the constraints of NEC, it is extremely difficult to obtain accurate results on the complete PLC band. The strong electromagnetic radiations appeared as the major problem of this technology. Therefore, the development of a method of mitigation was the logical continuation of this work, especially because there is still no solution to this problem. We developed a technique, which is presented in chapter 6, aiming at reducing the electromagnetic field radiated by indoor PLC signals. This technique is the major original contribution of this thesis and led us to file a patent. The central idea is to take advantage of the additional ground conductor and inject a signal similar to the PLC signal into the ground-neutral line, but with a reversed phase. As a result, the overall antenna mode current is significantly reduced, as well as the radiated electromagnetic fields. The method is specifically developed for indoor applications and applies to uni-directional and bi-directional data transmission systems with an implementation using switches. Experimental data obtained on a test network have shown that the use of the proposed technique implies a significant reduction (up to 20 dB) of the PLC electromagnetic radiation in the considered frequency range. It is also shown that the performance of the proposed technique increases when the ground-neutral line is terminated by an impedance equal to the one of the PLC modem. Other tests performed on 230 V network confirm this trend but show that further work is needed in this direction. This work could consist in the development of a system allowing the synchronisation of the real PLC signal and the auxiliary signal according to the behavior of the PLC channel, in order to optimise the performances for any frequency used by the PLC.

EPFL2005

Development of a Millikelvin Scanning Tunneling Microscope for Applications in Ultra High Vacuum and High Magnetic Fields

Maximilian Assig

In this thesis we discuss the realization of a scanning tunneling microscope (STM) operating at temperatures below 20 mK. This is accomplished by attaching the STM to a dilution refrigerator. We can apply high magnetic fields up to 14 T perpendicular and 0.5 T parallel to the sample surface. A full ultra high vacuum (UHV) preparation chamber is attached to the cryostat allowing in situ preparation of the samples to investigate. This setup allows us to investigate low temperature phase transitions in solids and offers the possibility for ultimate energy resolution tunneling spectroscopy measurements on the atomic scale. The concept of realizing the mK-STM is given. A main point of concern was the design of the STM unit. The choice of materials should be well adapted for the use at very low temperatures and in high magnetic fields. A special concern are thermal expansion coefficients and thermal conductivity. In addition, we analyzed the variation of the tip sample distance for a given external excitation for our STM design and two variations of the Besoke-STM by using a one dimensional mass spring model. This gives us the possibility to extract an optimization criterion for our design to limit the response of the tip sample distance to vibrations that are transmitted to the STM. The external damping mechanism, which ensures a low mutual vibration level of the STM as well as the grounding concept for the electronics is explained. To characterize the STM at operating temperatures of 1.5 K, 800 mK and 15 mK we measured topography images of Au(111) and Cu(111) surfaces. This allows us to extract the stability between tip and sample. A statistical analysis of the noise in the tunneling current without activated feedback loop yields a stability of 2.7±0.1 pm in a frequency range f ≈ 0.03 – 25 Hz at 800 mK. To verify if tip and sample are cooled efficiently, we measured the temperature at their positions with additional thermometers. We achieved 17 mK at the tip and 20 mK at the sample position, respectively. The energy resolution of the STM setup was verified by taking tunneling spectra between a superconductor and a normal conducting metal. These spectra could be fitted using the BCS theory of superconductivity. From those ts we can extract the upper limit of the effective temperature of the electrons which is Teff = 87 ± 3 mK for our best measurement. This corresponds to an energy resolution of 3.5 kbTeff = 26 ± 1 µeV. Further, the tunneling conductance between two superconductors was investigated. A sub-gap structure, which can be related to Andreev reflections in asymmetric tunneling junction was observed. In addition, at zero bias voltage a peak due to the Josephson effect can be measured. Side peaks with intensities that scale with this zero bias conductance peak appear most probably due to the interaction of Cooper pairs with the electromagnetical environment in which the junction is placed. Investigating the dependence of the conductance between a superconducting tip and a normal conducting sample on externally applied magnetic fields leads to the appearance of shoulders in the coherence peaks. These shoulders shift linearly with the external magnetic field. Such an effect has been measured for planar tunnel junctions and can be explained with the lifting of the spin degeneracy of the quasi-particle density of states in a superconductor. It was observed for the first time by Meservey, Tedrow and Fulde. The fact, that we are now able to prepare STM tips showing the Meservey-Tedrow-Fulde effect enables us to measure the absolute spin polarization at EF with atomic resolution.

EPFL2011

An apparatus for pulsed ESR and DNP experiments using optically excited triplet states down to liquid helium temperatures

Arnaud Comment, Tim Rolf Eichhorn, Martin Haag, Sami Jannin, Jacques Van Der Klink

In standard Dynamic Nuclear Polarization (DNP) electron spins are polarized at low temperatures in a strong magnetic field and this polarization is transferred to the nuclear spins by means of a microwave field. To obtain high nuclear polarizations cryogenic equipment reaching temperatures of 1 K or below and superconducting magnets delivering several Tesla are required. This equipment strongly limits applications in nuclear and particle physics where beams of particles interact with the polarized nuclei, as well as in neutron scattering science. The problem can be solved using short-lived optically excited triplet states delivering the electron spin. The spin is polarized in the optical excitation process and both the cryogenic equipment and magnet can be simplified significantly. A versatile apparatus is described that allows to perform pulsed dynamic nuclear polarization experiments at X-band using short-lived optically excited triplet sates. The efficient He-4 flow cryostat that cools the sample to temperatures between 4 K and 300 K has an optical access with a coupling stage for a fiber transporting the light from a dedicated laser system. It is further designed to be operated on a neutron beam. A combined pulse ESR/DNP spectrometer has been developed to observe and characterize the triplet states and to perform pulse DNP experiments. The ESR probe is based on a dielectric ring resonator of 7 mm inner diameter that can accommodate cubic samples of 5 mm length needed for neutron experiments. NMR measurements can be performed during DNP with a coil integrated in the cavity. With the presented apparatus a proton polarization of 0.5 has been achieved at 0.3 T. (C) 2013 Elsevier Inc. All rights reserved.