Cryogenic single chip electron spin resonance detectors

Methods based on the electron spin resonance (ESR) phenomenon are used to study paramagnetic systems at temperatures that ranges from 1000 to below 1 K. Commercially available spectrometers achieve spin sensitivities in the order of 10^(10) spins/¿Hz at room temperature on sample with volumes in the order of few µl. This results can be improved by cooling the system at cryogenic temperatures, where the larger magnetization of paramagnetic samples cause the detected signal to increase. Furthermore operation at high field (frequency) turns as well in an improved spin sensitivity. For what it concern the spin sensitivity operation at cryogenic temperature and high frequency are thus beneficial. In 2008 the group of Dr. G. Boero proposed a novel detection method based on the integration of all the element responsible for the sensitivity on a single silicon chip. The methodology allowed to study sample with nanoliter scale volume with spin sensitivity that were at least 2 orders of magnitude better than the best commercial spectrometer. The proposed method has performance that are comparable with the one obtained on similar scales with micro-resonator based spectroscopy tool. During this thesis I have investigated the possibility of extending the use of the detection method from frequency that goes from 20 to 200 GHz and temperatures that range from 77 to 4 K. In this frame several domains were touched. First of all the design of CMOS silicon oscillators operating at frequency which are closed to the most modern technology frequency limit. The lack of model valid for the target frequencies and the needs of limiting the power consumption for matching the limited cooling power of cryogenic systems, made the subject a challenging and interesting research topic itself. The study produces a remarkable result of a system operating at about 170 GHz with a power consumption of about 3 mW at room temperature and about 1.5 mW at 4 K. With the realized devices the first measurements of integrated silicon CMOS LC oscillators at temperature below 77 K were performed. From this measurements we could confirm the presence of expected effect, such as minimum power consumption reduction and oscillator frequency increase. In addition to that, by measuring the frequency-bias characteristic, it's been noticed a succession of smooth region and sharp transitions. This jumps are tentatively attributed to the random telegraph signal (RTS) effect that is supposed to be the main responsible for the flicker noise in sub-micrometer MOS devices. Since the impact of RTS on the performance of highly scaled transistor performance is expected to grow with the technology scale down, measurement methods based on LC oscillator, that shows better sensitivity if compared with nowadays employed methods, might allow to better understand the mechanism governing the effect and to develop technological strategy for lowering the impact on the future CMOS technology node. The realized devices have finally demonstrated ESR performances that are comparable with the most recent publication done with miniaturized resonators on mass-limited samples. In fact sensitivity of about 10^(7) spins/¿Hz at 50 GHz and 300 K and of about 10^(6) spins/¿Hz at 28 GHz and 4 K, at least 3 orders of magnitude better then commercially available state of the art devices, have been proven.

Electron Spin Resonance detectors from 400 MHz to 360 GHz

Alessandro Valentino Matheoud

Methods based on the electron spin resonance (ESR) phenomenon are non-invasive tools adopted to investigate paramagnetic systems at temperatures ranging from above 1000 K to below 1 K. Since 2008, the group of Dr. Boero has been working on a detection technique based on the integration of ESR sensors on single chips. The proposed methodology allowed to study samples in the nanoliter scale and reach a spin sensitivity at least two orders of magnitude better than the best commercially available spectrometers. The detection principle can be summarized as follows. An ESR sensitive sample is placed in close proximity to the planar inductor of an LC oscillator operating at microwave frequency. In presence of a suitable static magnetic field, the ESR phenomenon takes place. It causes a variation in the sample magnetization which translates to a variation of the inductance, leading to both a frequency shift of the oscillator (frequency detection) and a variation of the oscillation amplitude (amplitude detection). Consequently, the ESR phenomenon may be detected by tracking the operating point of the oscillator. In this thesis, I investigate the application of the aforementioned detection principle in the range from 400 MHz to 360 GHz. Firstly, a semi-integrated solution operating from 400 MHz to 610 MHz is developed for an industrial application (CTI project). In such context, the originality of the work stands in the implementation of a completely standalone portable scanner for contactless inspection which may also be used for ferromagnetic (FMR) applications and zero-field measurements. Secondly, a set of single-chip ESR detectors working from 10 GHz to 146 GHz and based on CMOS technologies are characterized from 300 K down to 10 K. Here, an ESR experiment at a frequency as high as 360 GHz can be performed thanks to the fourth harmonic signal generated by a 90 GHz detector. Conversely, the 10 GHz detector shows the best noise performance and allows to achieve the record distance resolution of 0.3 pm when used as a proximity sensor. After that, the possibility of using integrated technologies based on HEMTs is investigated so as to overcome the main limitations of the CMOS based detectors: (1) the high power consumption which denies their use below 10 K and (2) the saturation issue due to the magnitude of the intrinsic microwave magnetic field produced by the oscillators. In this context, two HEMT oscillators working at 11 GHz and 25 GHz are realized. In particular, the former achieves the record minimum power consumption (90 uW at 300 K and 4 uW below 30 K) currently reported in the literature for oscillators working in the same frequency range. Also, the proposed sensor achieves a minimum microwave magnetic field of less than 1 uT at 300 K and less than 0.1 uT below 30 K, i.e., orders of magnitude below the values achieved with previous CMOS detectors. Furthermore, an analytical model is carried out in order to estimate the minimum achievable power consumption for an LC single-ended Colpitts oscillator based on any single FET. Lastly, the DC characterization of a standing alone HEMT transistor is provided from 300 K down to 1.4 K, ranging from the standard Ids-Vs-Vds curves to the extraction of both the number of carriers and their effective mobility. The former comes from the analysis of the Shubnikov-de-Haas oscillations whereas the latter is calculated by means of Hall-effect based experiments.

EPFL2019

Multi-frequency high-pressure electron spin resonance study of low dimensonal organic conductors

Bálint Náfrádi

Electronic and magnetic properties of quasi-one dimensional ormore widely low-dimensional systems aswell as their related correlated electron phenomena have been at the very frontier of condensed matter physics for quite some time. The reduced dimensionality in these systems offers a unique possibility of direct comparison with model calculations. Furthermore the competing ordered ground states in strongly correlated low-dimensional systems lead to rich phase diagrams comprising many electronic phases, such as superconductivity, charge/spin densitywaves, different electric charge distributions, or a novel formof metalicity. These phases are generically sensitive to a variety of parameters, such as temperature, magnetic field, pressure, a certain degree of irregularity, etc. In this thesis, we employ Electron Spin Resonance (ESR) spectroscopy using strong magnetic fields, high microwave frequencies and high hydrostatic pressure to study a recently discovered family of quasi-one-dimensional organic charge transfer salts, the δ-(EDT-TTF-CONMe2)2X; family to gain insight into the complex physics of strongly correlated low-dimensional compounds. ESR is a highly efficient technique to study organic conductors. This can be easily understood if one realizes that ESR is observable inmost organic conductorswhile only very few "ordinary" metals show detectable ESR signals, even at low temperatures. Indeed, this feature results principally from the electronic low-dimensionality of these systems which controls the spin relaxation process. This tendency is even more reinforced by the fact that most organic molecules only contain light chemical elements for which the effect of the spin-orbit coupling on the ESR linewidth remains small. For this reason, ESR spectroscopy is certainly amajor tool to investigate the paramagnetic state of organic conductors. Moreover, in samples where a magnetic order is present, the large frequency bandwidth of the spectrometers used in the experimental setup (4 GHz–420 GHz) makes the resonance signal accessible at low temperatures in the ordered state, where the resonance frequencywould be too high for conventional ESRspectrometers. Our device hence represents an efficient tool to probe different magnetic ground states and, in particular, the antiferromagnetic state, which is most commonly found in organic salts. Organic conductors exhibit a high degree of sensitivity with respect to magnetic fields and applied pressure. To combine the high sensitivity of the ESR method with the sensitivity and thus fine tuneability of these organic systems by magnetic field and pressure, we build a unique high-field, high-frequency and high-pressure ESR spectrometerwhich is well suited to the study of organic materials. Compared to other devices of its kind, the ESR spectrometer built in our laboratory as a part of this thesis enables a wide parameter space to be explored: magnetic fields from 0 – 16 T, microwave frequencies in the 105 – 420 GHz range and by making use of a custom-made pressure cell, the sample applied pressure can be elevated up to 1.6GPa. To the best of our knowledge, the addition of such a pressure regulating unit to an ESR is unique. Through varying the applied pressure, it can be used to investigate the competition of different ground states: bymodifying the lattice parameters, it can tune the interactions without introducing disorder. The first Part of the thesis is dedicated to a general overview about the physical properties of quasi-one-dimensional organic systems. It also contains a more specific introduction to the crystallographic and electronic properties of the δ-(EDT-TTF-CONMe2)2X family of charge transfer salts. In Part II, we focus on the theoretical description of electronic correlations with a special emphasis on lowdimensional electron systems and on the effect of dimensionality. The objective of this part is to assemble the necessary theoretical notions to understand the phenomena observed in the system under investigation. We show that Coulomb-repulsion in general leads to antiferromagnetism, and give a brief phenomenological description of the antiferromagnetically ordered state. Then, a standard description of the physicalprinciples of Electron Spin Resonance follows. We also give a briefdescription of antiferromagnetic resonance (AFMR) and Conduction Electron Spin Resonance (CESR) with a special focus on quasi-one-dimensional situations. Part III presents the technical details of the ESR spectrometer developed as a part of this thesis. The final Part describes the experimental observations collected during the thesis. In Chapter 9, the phase transitions at atmospheric pressure of a sample of δ-(EDT-TTF-CONMe2)2AsF6 as a function of temperature are presented and discussed. We found a spin-chain behavior at high temperatures and a complicated antiferromagnetic structure below TN=8 K. In the intermediate temperature range we found a suppression of the susceptibility what we explained by the opening of a spin-pseudogap. In Chapter 10, the same procedure is performed for a δ-(EDT-TTF-CONMe2)2Br sample and the findings are really similar to the case of the AsF6 salt. Finally, in Chapter 11, we describe and evaluate measurements performed under high hydrostatic pressure on the δ-(EDT-TTF-CONMe2)2Br sample. We followed the deconfinement transition of the salt and then identified a dimensional crossover from a quasi-one dimensional metal to a more conventional higher dimensional conducting state. At the end of the thesis we drown a general pressure temperature phase diagram of this family of charge transfer salts.

EPFL2008

Frequency jumps in single chip microwave LC oscillators

Giovanni Boero, Marco Grisi, Gabriele Gualco

We report on the experimental observation of oscillation frequency jumps in microwave LC oscillators fabricated using standard complementary metal-oxide-semiconductor technologies. The LC oscillators, operating at a frequency of about 20 GHz, consist of a single turn planar coil, a metal-oxide-metal capacitor, and two cross-coupled metal-oxide-semiconductor field effect transistors used as negative resistance network. At 300 K as well as at 77 K, the oscillation frequency is a continuous function of the oscillator bias voltage. At 4 K, frequency jumps as large as 30 MHz are experimentally observed. This behavior is tentatively attributed to the emission and capture of single electrons from defects and dopant atoms. (C) 2014 AIP Publishing LLC.