Novel Instrumentation and Methods for Chemical Applications of Dissolution Dynamic Nuclear Polarization

One of the major disadvantages of Nuclear Magnetic Resonance (NMR) is its low sensitivity, mainly due to a very low spin polarization. Since 2003, Dissolution Dynamic Nuclear Polarization (D-DNP) provides a way of overcoming this drawback in solution by increasing polarization by factors exceeding 10'000-fold. In a D-DNP experiment, the high electron spin polarization at low temperatures is transferred to other nuclei (like 1H and 13C) by microwave irradiation, and the frozen sample is subsequently dissolved to room temperature while preserving part of the polarization. This thesis presents novel conceptual and instrumental developments in D-DNP that enable a better hyperpolarization of several nuclear spins. More specifically, after a brief introduction to NMR and D-DNP in Part A, we will discuss in Part B new strategies for improving microwave irradiation and implementing Longitudinal Detection Electron Spectroscopy Resonance (LODESR) in situ, c.a. at low temperature and high magnetic field. We will see how Cross Polarization can be made fully compatible with D-DNP with an original double resonance circuit developed in our laboratory (1H and another nucleus like 13C, 15N, 29Si or 129Xe). Finally, we will show how the problem of low field transfer of hyperpolarized solutions can be overcome by the use of a novel concept of a magnetic tunnel that sustains the magnetic field and prevent excessive losses of our hyperpolarized "ambitions". Finally, new applications in D-DNP that were either enabled or better realized by these new strategies are briefly presented in Chapter C.

Scanning Tunneling Microscopy using Superconducting Tips to Probe Absolute Spin Polarization

Matthias Alexander Eltschka

The interplay of superconductivity and magnetism is investigated for systems with dimensions ranging from the mesoscopic to the atomic scale by scanning tunneling microscopy (STM) at millikelvin temperatures and by numerical calculations. Based on geometrically confined superconductors in magnetic fields, a novel STM approach is introduced to quantitatively probe the spin polarization of tunneling electrons. In the first part of this work, the effects of magnetic fields and geometrical confinement are probed for superconducting vanadium STM tips. Due to the unique confinement ranging from the atomic to the mesoscopic scale, the superconducting properties of the STM tips vary considerably from their bulk counterparts. To analyze the experimentally determined magnetic field dependence of several V tips, the superconductivity is numerically calculated for modeled cone geometries with various opening angles. The numerical approach based on a one-dimensional Usadel equation leads to a direct correlation between the opening angle ¿ and the order of the superconducting phase transition. First order phase transitions occur when the opening angle is smaller than a critical value (¿ < ¿c), whilst larger opening angles (¿ > ¿c) result in second order phase transitions. The comparison of experimental findings and numerical results reveals the existence of first and second order quantum phase transitions in the V STM tips. In addition, the numerical calculations also explain experimentally observed broadening effects of the superconducting spectra by the specific tip geometry. In the second part, the superconducting V tips are employed in a novel approach to quantitatively probe the spin polarization of tunneling electrons on the nanoscale. For this purpose, the Meservey-Tedrow-Fulde technique is transferred to STM in order to combine their virtues, such as the quantitative probing capability of the spin polarization, the precise control at the atomic scale and the well-defined vacuum tunnel barrier. To demonstrate the capabilities of the new technique, the local spin structure is resolved for a magnetic Co nanoisland, where spin polarizations ranging from -56% up to 65% were found, depending on the local position. Furthermore, the spin polarization P strongly varies with the tip-to-sample distance z (dP/dz ¿ 10%/Å), which is described by the different decays of the spin-up and spin-down wave functions into the vacuum tunnel barrier. The final part describes the local interaction between isolated magnetic moments and the superconducting ground state. Copper phthalocyanine molecules on the superconducting V(100) surface induce bound states within the superconducting gap due to the magnetic coupling and the Coulomb potentials. Spatially resolved measurements reveal the non-isotropic structure of the spectral weights that is explained by the adsorption site on the 5x1 reconstruction of the V(100) surface. The quasi-particle excitations are not only observed on the magnetic molecule but also occur in its close vicinity. With increasing distance from the molecular structure, the intensities of the bound states decay within the distance x ¿ ±30Å and show periodic oscillations at the same time. Comparing the experimental findings to a one-dimensional model suggests the presence of a complicated scattering potential, which can be simplified by assuming two point scatterers within the molecular structure.

EPFL2015

Dynamic Nuclear Polarization and Other Magnetic Ideas at EPFL

Geoffrey Bodenhausen, Aurélien Bornet, Roberto Buratto, Marc Anthony Caporini, Diego Carnevale, Srinivas Chinthalapalli, Sami Jannin, Daniele Mammoli, Pascal Miéville, Jonas Milani, Angel Joaquin Perez Linde, Martial Rey, Nicola Salvi, Simone Ulzega, Shutao Wang

Although nuclear magnetic resonance (NMR) can provide a wealth of information, it often suffers from a lack of sensitivity. Dynamic Nuclear Polarization (DNP) provides a way to increase the polarization and hence the signal intensities in NMR spectra by transferring the favourable electron spin polarization of paramagnetic centres to the surrounding nuclear spins through appropriate microwave irradiation. In our group at EPFL, two complementary DNP techniques are under investigation: the combination of DNP with magic angle spinning at temperatures near 100 K ('MAS-DNP'), and the combination of DNP at 1.2 K with rapid heating followed by the transfer of the sample to a high-resolution magnet ('dissolution DNP'). Recent applications of MAS-DNP to surfaces, as well as new developments of magnetization transfer of H-1 to C-13 at 1.2 K prior to dissolution will illustrate the work performed in our group. A second part of the paper will give an overview of some 'non-enhanced' activities of our laboratory in liquid- and solid-state NMR.

Schweizerische Chemische Gesellschaft2012

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.

Academic Press Inc Elsevier Science2013

Scanning Tunneling Microscopy using Superconducting Tips to Probe Absolute Spin Polarization

Matthias Alexander Eltschka

EPFL2015