DNP-NMR Literature Blog

Dynamic Nuclear Polarization in III–V Semiconductors

G. Kaur and G. Denninger, Dynamic Nuclear Polarization in III-V Semiconductors, Appl. Magn. Reson., 2010, 39(1-2), 185-204

http://dx.doi.org/10.1007/s00723-010-0155-7

We report on electron spin resonance, nuclear magnetic resonance and Overhauser shift experiments on two of the most commonly used III–V semiconductors, GaAs and InP. Localized electron centers in these semiconductors have extended wavefunctions and exhibit strong electron–nuclear hyperfine coupling with the nuclei in their vicinity. These interactions not only play a critical role in electron and nuclear spin relaxation mechanisms, but also result in transfer of spin polarization from the electron spin system to the nuclear spin system.

This transfer of polarization, known as dynamic nuclear polarization (DNP), may result in an enhancement of the nuclear spin polarization by several orders of magnitude under suitable conditions. We determine the critical range of doping concentration and temperature conducive to DNP effects by studying these semiconductors with varying doping concentration in a wide temperature range. We show that the electron spin system in undoped InP exhibits electric current-induced spin polarization. This is consistent with model predictions in zinc-blende semiconductors with strong spin–orbit effects.

Amplification of Picosecond Pulses in a 140-GHz Gyrotron-TravelingWave Tube

H.J. Kim et al., Amplification of Picosecond Pulses in a 140-GHz Gyrotron Travelling Wave Tube, Phys. Rev. Lett., 105(13), 135101-135104

http://dx.doi.org/10.1103/PhysRevLett.105.135101

An experimental study of picosecond pulse amplification in a gyrotron-traveling wave tube (gyro- TWT) has been carried out. The gyro-TWT operates with 30 dB of small signal gain near 140 GHz in the HE06 mode of a confocal waveguide. Picosecond pulses show broadening and transit time delay due to two distinct effects: the frequency dependence of the group velocity near cutoff and gain narrowing by the finite gain bandwidth of 1.2 GHz.

Experimental results taken over a wide range of parameters show good agreement with a theoretical model in the small signal gain regime. These results show that in order to limit the pulse broadening effect in gyrotron amplifiers, it is crucial to both choose an operating frequency at least several percent above the cutoff of the waveguide circuit and operate at the center of the gain spectrum with sufficient gain bandwidth.

Amplification of Picosecond Pulses in a 140-GHz Gyrotron-TravelingWave Tube

H.J. Kim et al., Amplification of Picosecond Pulses in a 140-GHz Gyrotron Travelling Wave Tube, Phys. Rev. Lett., 105(13), 135101-135104

http://dx.doi.org/10.1103/PhysRevLett.105.135101

An experimental study of picosecond pulse amplification in a gyrotron-traveling wave tube (gyro- TWT) has been carried out. The gyro-TWT operates with 30 dB of small signal gain near 140 GHz in the HE06 mode of a confocal waveguide. Picosecond pulses show broadening and transit time delay due to two distinct effects: the frequency dependence of the group velocity near cutoff and gain narrowing by the finite gain bandwidth of 1.2 GHz.

Experimental results taken over a wide range of parameters show good agreement with a theoretical model in the small signal gain regime. These results show that in order to limit the pulse broadening effect in gyrotron amplifiers, it is crucial to both choose an operating frequency at least several percent above the cutoff of the waveguide circuit and operate at the center of the gain spectrum with sufficient gain bandwidth.

Hyperpolarizing Gases via Dynamic Nuclear Polarization and Sublimation

A. Comment et al., Hyperpolarizing Gases via Dynamic Nuclear Polarization and Sublimation, Phys. Rev. Lett., 2010, 105(1), 018104-018107.

http://dx.doi.org/10.1103/PhysRevLett.105.018104

A high throughput method was designed to produce hyperpolarized gases by combining low temperature dynamic nuclear polarization with a sublimation procedure. It is illustrated by applications to 129Xe nuclear magnetic resonance in xenon gas, leading to a signal enhancement of 3 to 4 orders of magnitude compared to the room-temperature thermal equilibrium signal at 7.05 T.

Hyperpolarizing Gases via Dynamic Nuclear Polarization and Sublimation

A. Comment et al., Hyperpolarizing Gases via Dynamic Nuclear Polarization and Sublimation, Phys. Rev. Lett., 2010, 105(1), 018104-018107.

http://dx.doi.org/10.1103/PhysRevLett.105.018104

A high throughput method was designed to produce hyperpolarized gases by combining low temperature dynamic nuclear polarization with a sublimation procedure. It is illustrated by applications to 129Xe nuclear magnetic resonance in xenon gas, leading to a signal enhancement of 3 to 4 orders of magnitude compared to the room-temperature thermal equilibrium signal at 7.05 T.

High-Field Dynamic Nuclear Polarization for Solid and Solution Biological NMR

A.B. Barnes et al., High-Field Dynamic Nuclear Polarization for Solid and Solution Biological NMR, Appl. Magn. Reson., 2008, 4(3), 237-263.

http://dx.doi.org/10.1007/s00723-008-0129-1

Dynamic nuclear polarization (DNP) results in a substantial nuclear polarization enhancement through a transfer of the magnetization from electrons to nuclei. Recent years have seen considerable progress in the development of DNP experiments directed towards enhancing sensitivity in biological nuclear magnetic resonance (NMR). This review covers the applications, hardware, polarizing agents, and theoretical descriptions that were developed at the Francis Bitter Magnet Laboratory at Massachusetts Institute of Technology for high-field DNP experiments.
In frozen dielectrics, the enhanced nuclear polarization developed in the vicinity of the polarizing agent can be efficiently dispersed to the bulk of the sample via 1H spin diffusion. This strategy has been proven effective in polarizing biologically interesting systems, such as nanocrystalline peptides and membrane proteins, without leading to paramagnetic broadening of the NMR signals. Gyrotrons have been used as a source of high-power (5–10 W) microwaves up to 460 GHz as required for the DNP experiments. Other hardware has also been developed allowing in situ microwave irradiation integrated with cryogenic magic-angle-spinning solid-state NMR. Advances in the quantum mechanical treatment are successful in describing the mechanism by which new biradical polarizing agents yield larger enhancements at higher magnetic fields. Finally, pulsed methods and solution experiments should play a prominent role in the future of DNP.

High-Field Dynamic Nuclear Polarization for Solid and Solution Biological NMR

A.B. Barnes et al., High-Field Dynamic Nuclear Polarization for Solid and Solution Biological NMR, Appl. Magn. Reson., 2008, 4(3), 237-263.

http://dx.doi.org/10.1007/s00723-008-0129-1

Dynamic nuclear polarization (DNP) results in a substantial nuclear polarization enhancement through a transfer of the magnetization from electrons to nuclei. Recent years have seen considerable progress in the development of DNP experiments directed towards enhancing sensitivity in biological nuclear magnetic resonance (NMR). This review covers the applications, hardware, polarizing agents, and theoretical descriptions that were developed at the Francis Bitter Magnet Laboratory at Massachusetts Institute of Technology for high-field DNP experiments.
In frozen dielectrics, the enhanced nuclear polarization developed in the vicinity of the polarizing agent can be efficiently dispersed to the bulk of the sample via 1H spin diffusion. This strategy has been proven effective in polarizing biologically interesting systems, such as nanocrystalline peptides and membrane proteins, without leading to paramagnetic broadening of the NMR signals. Gyrotrons have been used as a source of high-power (5–10 W) microwaves up to 460 GHz as required for the DNP experiments. Other hardware has also been developed allowing in situ microwave irradiation integrated with cryogenic magic-angle-spinning solid-state NMR. Advances in the quantum mechanical treatment are successful in describing the mechanism by which new biradical polarizing agents yield larger enhancements at higher magnetic fields. Finally, pulsed methods and solution experiments should play a prominent role in the future of DNP.

Dynamic Nuclear Polarization at High Magnetic Fields

Thorsten Maly et al., Dynamic nuclear polarization at high magnetic fields, J. Chem. Phys., 2008, 128(5), 052211-19

http://dx.doi.org/10.1063/1.283358

Dynamic nuclear polarization (DNP) is a method that permits NMR signal intensities of solids and liquids to be enhanced significantly, and is therefore potentially an important tool in structural and mechanistic studies of biologically relevant molecules. During a DNP experiment, the large polarization of an exogeneous or endogeneous unpaired electron is transferred to the nuclei of interest (I) by microwave (μw) irradiation of the sample. The maximum theoretical enhancement achievable is given by the gyromagnetic ratios (γe/γl), being ∼ 660 for protons. In the early 1950s, the DNP phenomenon was demonstrated experimentally, and intensively investigated in the following four decades, primarily at low magnetic fields.
This review focuses on recent developments in the field of DNP with a special emphasis on work done at high magnetic fields ( ≥ 5 T), the regime where contemporary NMR experiments are performed. After a brief historical survey, we present a review of the classical continuous wave (cw) DNP mechanisms—the Overhauser effect, the solid effect, the cross effect, and thermal mixing. A special section is devoted to the theory of coherent polarization transfer mechanisms, since they are potentially more efficient at high fields than classical polarization schemes. The implementation of DNP at high magnetic fields has required the development and improvement of new and existing instrumentation. Therefore, we also review some recent developments in μw and probe technology, followed by an overview of DNP applications in biological solids and liquids. Finally, we outline some possible areas for future developments.

Dynamic Nuclear Polarization at High Magnetic Fields

Thorsten Maly et al., Dynamic nuclear polarization at high magnetic fields, J. Chem. Phys., 2008, 128(5), 052211-19

http://dx.doi.org/10.1063/1.283358

Dynamic nuclear polarization (DNP) is a method that permits NMR signal intensities of solids and liquids to be enhanced significantly, and is therefore potentially an important tool in structural and mechanistic studies of biologically relevant molecules. During a DNP experiment, the large polarization of an exogeneous or endogeneous unpaired electron is transferred to the nuclei of interest (I) by microwave (μw) irradiation of the sample. The maximum theoretical enhancement achievable is given by the gyromagnetic ratios (γe/γl), being ∼ 660 for protons. In the early 1950s, the DNP phenomenon was demonstrated experimentally, and intensively investigated in the following four decades, primarily at low magnetic fields.
This review focuses on recent developments in the field of DNP with a special emphasis on work done at high magnetic fields ( ≥ 5 T), the regime where contemporary NMR experiments are performed. After a brief historical survey, we present a review of the classical continuous wave (cw) DNP mechanisms—the Overhauser effect, the solid effect, the cross effect, and thermal mixing. A special section is devoted to the theory of coherent polarization transfer mechanisms, since they are potentially more efficient at high fields than classical polarization schemes. The implementation of DNP at high magnetic fields has required the development and improvement of new and existing instrumentation. Therefore, we also review some recent developments in μw and probe technology, followed by an overview of DNP applications in biological solids and liquids. Finally, we outline some possible areas for future developments.

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