Vasiliy N. Glazkov

We investigate thermoelectric transport in a single-walled carbon nanotube contacted by both nonmagnetic Au and ferromagnetic Ni electrodes, with Au–Au and Ni–Ni segments fabricated on the same individual nanotube. This geometry enables a direct comparison between the gate-tunable nanotube contribution and the contact-dependent contribution to the thermoelectric voltage. In the Au-contacted segment, the gate dependence of the thermovoltage qualitatively follows the behavior expected from the low-temperature Mott limit of Landauer transport, where the thermopower is governed by the energy derivative of the transmission function. In contrast, the Ni-contacted segment shows a finite saturated thermovoltage at negative gate voltages where the FET factor approaches zero, consistent with an additional gate-independent contribution associated with the Ni contacts. A second device reproduces this plateau at room temperature. Magnetic field measurements up to 15 T show that the plateau persists, although the Ni–Au contrast decreases with field. Estimates of the temperature difference indicate that the thermopower of the Ni electrodes can account for the plateau at 100 K but, under the stated assumptions, not for the larger room-temperature plateau, and ordinary contact barriers remain an alternative to spin-dependent transmission. These results suggest a contact-engineering strategy for enhancing thermoelectric response in one-dimensional conductors: the nanotube provides a strongly energy-dependent Landauer transmission, while ferromagnetic contacts may introduce spin-dependent electron–hole asymmetry.

An approach to determining and describing the distribution of the longitudinal and transverse components of the magnetic field in an axisymmetric laboratory electromagnet is proposed based on a polynomial expansion of the field induction taking into account the restrictions imposed by Maxwell’s equations. It is shown that the proposed approach allows obtaining high accuracy in describing the magnetic field distribution with a small number of measurements. It may also be of interest for refining the parameters of laboratory equipment and be used as a training task in laboratory practice when studying the properties of the electromagnetic field.

Nabokoite family compounds ACu$_7$(TeO$_4$)(SO$_4$)$_5$Cl (A=Na, K, Cs, Rb) are candidates for the experimental realization of highly-frustrated 2D square kagome lattice (SKL). Their magnetic subsystem includes SKL layers decorated by additional copper ions. All members of this family are characterized by quite high Curie-Weiss temperatures ($\sim 80-200$ K), but magnetic ordering was reported only for Na and K compounds at a much lower temperatures below 4 K. We report here results of the study of high-frequency ($\sim 10$ GHz) dielectric properties of this family of compounds. Our study revealed presence of the strong dielectric anomaly both in the real and imaginary parts of high-frequency dielectric permittivity for Na and K compounds approx. 100 and 26 K, correspondingly, presumably related to antiferroelectric ordering. Additionally, much weaker anomalies were observed at approximately 5K indicating possible interplay of magnetic and lattice degrees of freedom. We discuss possible relation between the structure rearrangements accompanying dielectric anomalies and a delayed magnetic ordering in the nabokoite family compounds.

We report results of a multi-frequency (0.8–60 GHz) electron spin resonance study of the spin dynamics in the quasi-2D square lattice antiferromagnet Ba2MnGe2O7 both in antiferromagnetically ordered and paramagnetic phases. We directly observe two zero-field gaps in the excitation spectrum of the ordered phase, the larger one being due to easy-plane anisotropy, and the smaller one indicates the presence of fourth-order in-plane anisotropy probably related to the multiferroic properties of this compound. We observe effects of hyperfine interaction on the electron spin resonance spectra in the antiferromagnetically ordered state, which turns out to be comparable with in-plane anisotropy. The hyperfine field strength is found from the observed low-temperature electron spin resonance data. The spin dynamics of the paramagnetic phase is characterized by strong broadening of the ESR absorption line, which can be ascribed to the vortex dynamics of a 2D magnet.

We report results of a multi-frequency (0.8–60 GHz) electron spin resonance study of the spin dynamics in the quasi-2D square lattice antiferromagnet Ba₂MnGe₂O₇ both in antiferromagnetically ordered and paramagnetic phases. We directly observe two zero-field gaps in the excitation spectrum of the ordered phase, the larger one being due to easy-plane anisotropy, and the smaller one indicates the presence of fourth-order in-plane anisotropy probably related to the multiferroic properties of this compound. We observe effects of hyperfine interaction on the electron spin resonance spectra in the antiferromagnetically ordered state, which turns out to be comparable with in-plane anisotropy. The hyperfine field strength is found from the observed low-temperature electron spin resonance data. The spin dynamics of the paramagnetic phase is characterized by strong broadening of the ESR absorption line, which can be ascribed to the vortex dynamics of a 2D magnet.