Category Archives: Research Article

Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization

A. Lesage et al., Surface enhanced NMR Spectroscopy by Dynamic Nuclear Polarization, J. Am. Chem. Soc., 2010, 132(44), 15459-15461

http://dx.doi.org/10.1021/ja104771z

It is shown that surface NMR spectra can be greatly enhanced using dynamic nuclear polarization. Polarization is transferred from the protons of the solvent to the rare nuclei (here carbon-13 at natural isotopic abundance) at the surface, yielding at least a 50-fold signal enhancement for surface species covalently incorporated into a silica framework.

EPR detected polarization transfer between GD3+ and protons at low temperature and 3.3T: The first step of dynamic nuclear polarization

Nagarajan V. et al., EPR detected polarization transfer between Gd3+ and protons at low temperature and 3.3 T: The first step of dynamic nuclear polarization, J. Chem. Phys., 2010, 132, 214504

http://dx.doi.org/10.1063/1.3428665

Electron-electron double resonance pulsed electron paramagnetic resonance (EPR) at 95 GHz (3.3 T) is used to follow the dynamics of the electron spin polarization during the first stages of dynamic nuclear polarization in solids. The experiments were performed on a frozen solution of Gd+3 (S=7/2) in water/glycerol. Focusing on the central |−1/2>→|+1/2> transition we measured the polarization transfer from the Gd3+ electron spin to the adjacent 1H protons.

The dependence of the echo detected EPR signal on the length of the microwave irradiation at the EPR “forbidden” transition corresponding to an electron and a proton spin flip is measured for different powers, showing dynamics on the microsecond to millisecond time scales. A theoretical model based on the spin density matrix formalism is suggested to account for this dynamics. The central transition of the Gd3+ ion is considered as an effective S=1/2 system and is coupled to 1H (I=1/2) nuclei. Simulations based on a single electron-single nucleus four level system are shown to deviate from the experimental results and an alternative approach taking into account the more realistic multinuclei picture is shown to agree qualitatively with the experiments.

EPR detected polarization transfer between GD3+ and protons at low temperature and 3.3T: The first step of dynamic nuclear polarization

Nagarajan V. et al., EPR detected polarization transfer between Gd3+ and protons at low temperature and 3.3 T: The first step of dynamic nuclear polarization, J. Chem. Phys., 2010, 132, 214504

http://dx.doi.org/10.1063/1.3428665

Electron-electron double resonance pulsed electron paramagnetic resonance (EPR) at 95 GHz (3.3 T) is used to follow the dynamics of the electron spin polarization during the first stages of dynamic nuclear polarization in solids. The experiments were performed on a frozen solution of Gd+3 (S=7/2) in water/glycerol. Focusing on the central |−1/2>→|+1/2> transition we measured the polarization transfer from the Gd3+ electron spin to the adjacent 1H protons.

The dependence of the echo detected EPR signal on the length of the microwave irradiation at the EPR “forbidden” transition corresponding to an electron and a proton spin flip is measured for different powers, showing dynamics on the microsecond to millisecond time scales. A theoretical model based on the spin density matrix formalism is suggested to account for this dynamics. The central transition of the Gd3+ ion is considered as an effective S=1/2 system and is coupled to 1H (I=1/2) nuclei. Simulations based on a single electron-single nucleus four level system are shown to deviate from the experimental results and an alternative approach taking into account the more realistic multinuclei picture is shown to agree qualitatively with the experiments.

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.

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.

Have a question?

If you have questions about our instrumentation or how we can help you, please contact us.