Volume 50, Nº 5 (2019)

Spin dynamics of quasi two-dimensional triangular-lattice antiferromagnet delafossite 3R-AgFeO₂ have been investigated by electron spin resonance. The divergence of the temperature-dependent linewidth has been analyzed in the frame of two possible scenarios: (1) critical broadening due to slowing down of spins on approaching the Néel temperature T_N from above and (2) in terms of a Berezinski–Kosterlitz–Thouless formalism on triangular lattice due to magnetic Z₂-vortex–antivortex pairing. It has been found that the latter model gives better description of the experimental data indicating traces of Z₂-vortices formation.

Microstrip transmission line (MTL) resonators are widely used as radio-frequency (RF) transceiver coils in high-field magnetic resonance imaging (MRI). Typically, discrete capacitors are used to tune the MTL resonators to the Larmor frequency and to match to the 50 Ω characteristic impedance of the RF chain. The cost, availability, and labor-intensive work of soldering capacitors on each coil contribute significantly to the expense of RF coil arrays for MRI; therefore, a manufacturing method with lower cost and fewer processing steps is desirable. The additive manufacturing method of rapid prototyping offers a new method to build custom-designed MTL resonators with reduced fabrication steps and, potentially, cost. This feasibility study explores fused deposition modelling to 3D print the MTL resonator structure simultaneously with matching/tuning capacitors and conductors. Typical low-cost 3D printers are capable of printing only polymers, not metal and polymer printing in one machine. In this work, a low-cost 3D printer was modified by adding the capability to print conductive ink and used to print MTL resonators with monolithic parallel-plate capacitors. These integrated capacitors eliminate the repetitive work of soldering, and tuning is achieved by trimming the capacitor plates. In addition, 3D printing allows unconventional designs that minimize the amount of dielectric below the microstrip and, therefore, losses in the substrate. Resulting signal-to-noise ratio values using ink conductors are within 30% of those achieved with copper despite a resistivity that is two orders of magnitude higher. This performance gap can be addressed using newer inks that have much lower resistivity.

Recently, we have demonstrated the possibility to transfer the light-induced hyperpolarization occurring on select ¹³C nuclei of photosynthetic cofactors due to the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect into the proton pool via inverse ¹³C–¹H cross-polarization Bielytskyi et al. (J Magn Reson 293: 82–91, 2018). Such approach allowed us to observe the selective response from the protons belonging to photochemically active cofactors in their native protein environment. In the present study, we demonstrate that also ¹⁵N nuclei can be used as a source of polarization in a similar type of experiments. We present the 2D photo-CIDNP ¹⁵N–¹H heteronuclear correlation (HETCOR) experiment acquired on uniformly ¹⁵N-labeled quinone-blocked reaction center (RC) from Rhodobacter sphaeroides R26. Obtained ¹H chemical shifts match with previously observed ones on selectively ¹³C-labeled RC from Rhodobacter sphaeroides WT. We expect that using ¹⁵N as a source of polarization in potential heteronuclear spin-torch experiments could be advantageous for systems where the procedure for selective ¹³C labeling is not yet established.

Spontaneous maser emission provides the practical possibility for a spectral edition in methyl chloroacetate. Methyl and methylene resonances opposite to conventional nuclear magnetic resonance spectra are clearly separated within the time domain of free induction decay. Associated characteristic time delay, which represents the appropriate time from completion of the radiofrequency pulse to the signal maximum, can be properly broken on a random and a nonrandom part. Overall, the time delay is distinctively shorter about 0.296 s for methyl and longer about 0.489 s for methylene peaks, enabling spectra edition. Although maser emission reaches maximum for pulse rotation angle at 180°, it can be observed in a significantly broader range. The maser emission reveals random intensity, and its scattering range depends on the pulse rotation angle and is most limited when rotation angle is within direct vicinity of 180°. The spontaneous maser emission is triggered by thermal Nyquist noise through radiation damping. If the pulse rotation angle is sufficiently small, the amplified spin noise is observed.