15 research outputs found
Surface structure and stacking of the commensurate charge density wave phase of
Full text linkBy quantitative low-energy electron diffraction (LEED) we investigate the extensively studied commensurate charge density wave (CDW) phase of trigonal tantalum disulphide (1T−TaS2), which develops at low temperatures with a (13×13)R13.9∘ periodicity. A full-dynamical analysis of the energy dependence of diffraction spot intensities reveals the entire crystallographic surface structure, i.e., the detailed atomic positions within the outermost two trilayers consisting of 78 atoms as well as the CDW stacking. The analysis is based on an unusually large data set consisting of spectra for 128 inequivalent beams taken in the energy range 20–250 eV and an excellent fit quality expressed by a best-fit Pendry R factor of R=0.110. The LEED intensity analysis reveals that the well-accepted model of star-of-David-shaped clusters of Ta atoms for the bulk structure also holds for the outermost two TaS2 trilayers. Specifically, in both layers the clusters of Ta atoms contract laterally by up to 0.25 Å and also slightly rotate within the superstructure cell, causing respective distortions as well as heavy bucklings (up to 0.23 Å) in the adjacent sulfur layers. Most importantly, our analysis finds that the CDWs of the first and second trilayers are vertically aligned, while there is a lateral shift of two units of the basic hexagonal lattice (6.71 Å) between the second and third trilayers. The results may contribute to a better understanding of the intricate electronic structure of the reference compound 1T−TaS2 and guide the way to the analysis of complex structures in similar quantum materials
Evidence for reduced periodic lattice distortion within the Sb-terminated surface layer of the kagome metal CsV 3 Sb 5
Full text linkThe discovery of the kagome metal CsV 3 Sb 5 sparked broad interest, due to the coexistence of a charge density wave (CDW) phase and possible unconventional superconductivity in the material. In this Letter, we use low-energy electron diffraction (LEED) with a µ m -sized electron beam to explore the periodic lattice distortion at the antimony-terminated surface in the CDW phase. We recorded high-quality backscattering diffraction patterns in ultrahigh vacuum from multiple cleaved samples. Unexpectedly, we did not find superstructure reflexes at intensity levels predicted from dynamical LEED calculations for the reported 2 × 2 × 2 bulk structure. Our results suggest that in CsV 3 Sb 5 the periodic lattice distortion accompanying the CDW is less pronounced at Sb-terminated surfaces than in the bulk. Published by the American Physical Society 2025European Research Council http://dx.doi.org/10.13039/501100000781Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Max Planck School of Photonics http://dx.doi.org/10.13039/50110002273
Two-electron two-nucleus effective Hamiltonian and the spin diffusion barrier
Full text linkDynamic nuclear polarization (DNP) involves a polarization transfer from unpaired electrons to hyperfine coupled nuclei and can increase the sensitivity of nuclear magnetic resonance (NMR) signals by several orders of magnitude. The hyperfine coupling is considered to suppress nuclear dipolar flip-flop transitions, hindering the transport of nuclear hyperpolarization into the bulk (\u27\u27spin-diffusion barrier\u27\u27). Possible polarization-transfer pathways leading to DNP and subsequent spin diffusion between hypershifted nuclei in a two-electron two-nucleus four-spin system are investigated. The Schrieffer-Wolff transformation is applied to characterize transitions that are only possible as second-order effects. An energy-conserving electron-nuclear four-spin flip-flop is identified, which combines an electron dipolar with a nuclear dipolar flip-flop process, describing spin diffusion close to electrons. The relevance of this process is supported by two-compartment model fits of HypRes-on experimental data. This suggests that all nuclear spins can contribute to the hyperpolarization of the bulk and the concept of a spin-diffusion barrier has to be reconsidered for samples with significant electron and nuclear dipolar couplings
Relaxation enhancement by microwave irradiation may limit dynamic nuclear polarization
Full text linkDynamic nuclear polarization enables the hyperpolarization of nuclear spins
beyond the thermal-equilibrium Boltzmann distribution. However, it is often
unclear why the experimentally measured hyperpolarization is below the
theoretical achievable maximum polarization. We report a (near-) resonant
relaxation enhancement by microwave (MW) irradiation, leading to a significant
increase in the nuclear polarization decay compared to measurements without MW
irradiation. For example, the increased nuclear relaxation limits the
achievable polarization levels to around 35% instead of hypothetical 60%,
measured in the DNP material TEMPO in 1H glassy matrices at 3.3 K and 7 T.
Applying rate-equation models to published build-up and decay data indicates
that such relaxation enhancement is a common issue in many samples when using
different radicals at low sample temperatures and high Boltzmann polarizations
of the electrons. Accordingly, quantification and a better understanding of the
relaxation processes under MW irradiation might help to design samples and
processes towards achieving higher nuclear hyperpolarization levels.better
understanding of the relaxation processes under MW irradiation might help to
design samples and processes towards achieving higher nuclear hyperpolarization
levels
Two-electron two-nucleus effective Hamiltonian and the spin diffusion barrier
No full textISSN:2375-254
Modelling and correcting the impact of RF pulses for continuous monitoring of hyperpolarized NMR
No full textISSN:2699-001
Relaxation enhancement by microwave irradiation may limit dynamic nuclear polarization
No full textISSN:1463-9084ISSN:1463-9076ISSN:1463-907
Electroplated waveguides to enhance DNP and EPR spectra of silicon and diamond particles
No full textISSN:2699-001
In vivo MRI of hyperpolarized silicon-29 nanoparticles
No full textISSN:0740-3194ISSN:1522-2594ISSN:1522-259
