X- and Q-band EPR with cryogenic amplifiers independent of sample temperature

Inspired by the success of NMR cryoprobes, we recently reported a leap in X-band EPR sensitivity by equipping an ordinary EPR probehead with a cryogenic low-noise microwave amplifier placed closed to the sample in the same cryostat [Šimėnas et al. J. Magn. Reson. 322, 106876 (2021)]. Here, we explore, theoretically and experimentally, a more general approach, where the amplifier temperature is independent of the sample temperature. This approach brings a number of important advantages, enabling sensitivity improvement irrespective of sample temperature, as well as making it more practical to combine with ENDOR and Q-band resonators, where space in the sample cryostat is often limited. Our experimental realisation places the cryogenic preamplifier within an external closed-cycle cryostat, and we show CW and pulsed EPR and ENDOR sensitivity improvements at both X- and Q-bands with negligible dependence on sample temperature. The cryoprobe delivers signal-to-noise ratio enhancements that reduce the equivalent pulsed EPR measurement time by 16× at X-band and close to 5× at Q-band. Using the theoretical framework we discuss further improvements of this approach which could be used to achieve even greater sensitivity

Q-band EPR cryoprobe

Following the success of cryogenic EPR signal preamplification at X-band, we present a Q-band EPR cryoprobe compatible with a standard EPR resonator. The probehead is equipped with a cryogenic ultra low-noise microwave amplifier and its protection circuit that are placed close to the sample in the same cryostat. Our cryoprobe maintains the same sample access and tuning which is typical in Q-band EPR, as well as supports high-power pulsed experiments on typical samples. The performance of our setup is benchmarked against that of existing commercial and home-built Q-band spectrometers, using CW EPR and pulsed EPR/ENDOR experiments to reveal a significant sensitivity improvement which reduces the measurement time by a factor of about 40× at 6 K temperature at reduced power levels

Near-Surface Te+ 125 Spins with Millisecond Coherence Lifetime

Impurity spins in crystal matrices are promising components in quantum technologies, particularly if they can maintain their spin properties when close to surfaces and material interfaces. Here, we investigate an attractive candidate for microwave-domain applications, the spins of group-VI Te+125 donors implanted into natural Si at depths as shallow as 20 nm. We show that surface band bending can be used to ionize such near-surface Te to spin-active Te+ state, and that optical illumination can be used further to control the Te donor charge state. We examine spin activation yield, spin linewidth, and relaxation (T1) and coherence times (T2) and show how a zero-field 3.5 GHz "clock transition"extends spin coherence times to over 1 ms, which is about an order of magnitude longer than other near-surface spin systems

Baltic labour in the crucible of capitalist exploitation: reassessing 'post-communist' transformation

Thirty years after the fall of the Berlin Wall, this article re-assesses ‘post-communist’ transformation in the Baltic countries from the perspective of labour. The argument is based on a historical materialist approach focusing on the social relations of production as a starting point. It is contended that the uneven and combined unfolding of ‘post-communist’ transformation has subjected Baltic labour to doubly constituted exploitation processes. First, workers in Estonia, Latvia and Lithuania have suffered from extreme neoliberal restructuring of economic and employment relations at home. Second, migrant workers from Central and Eastern Europe in general, trying to escape exploitation at home, have faced another set of exploitative dynamics in host countries in Western Europe such as the UK. Nevertheless, workers have continued to challenge exploitation in Central and Eastern Europe and also in Western Europe, and have been active in extending networks of transnational solidarity across the continent

Reorientational dynamics of organic cations in perovskite-like coordination polymers

Here we report the dynamics of organic cations as guest molecules in a perovskite host-framework. The molecular motion of CH3NH3+ (MAFe), (CH3)2NH2+ (DMAFe) and (CH3)3NH+ (TrMAFe) in the cage formed by KFe(CN)63− units was studied using a combination of experimental methods: (i) thermal analysis, (ii) dielectric and electric studies, (iii) optical observations, (iv) EPR and 1H NMR spectroscopy and (v) quasielastic neutron scattering (QENS). In the case of MAFe and TrMAFe, the thermal analysis reveals one solid-to-solid phase transition (PT) and two PTs for the DMAFe crystal. A markedly temperature-dependent dielectric constant indicates the tunable and switchable properties of the complexes. Also, their semiconducting properties are confirmed by a dc conductivity measurement. The broadband dielectric relaxation is analyzed for the TrMAFe sample in the frequency range of 100 Hz–1 GHz. QENS shows that we deal rather with the localized motion of the cation than a diffusive one. Three models, which concern the simultaneous rotation of the CH3 and/or NH3 group, π-flips and free rotations of the organic cation, are used to fit the elastic incoherent structure factor. The 1H NMR spin–lattice relaxation time for all compounds under study, as well as the second moments, has been measured in a wide temperature range. In all studied samples, the temperature dependence of the second moment of the proton NMR line indicated the gradual evolution of the molecular movements from the rigid state up to a highly disordered on

Isostructural phase transition, quasielastic neutron scattering and magnetic resonance studies of a bistable dielectric ion-pair crystal [(CH3)2 NH2]2 KCr(CN)6

We have synthesised and characterised a novel organic–inorganic hybrid crystal, [(CH3)2NH2]2KCr(CN)6. The thermal DSC, TMA, DTG and DTA analyses indicate two solid-to-solid structural phase transitions (PTs). According to the X-ray diffraction experiments, the first PT at 220 K is isostructural, since it does not involve a change of the space group. This transition occurs between the states, where the (CH3)2NH2+ cations are orientationally disordered and ordered (frozen). The other reversible PT at 481 K leads to a melt-like phase similar to the one observed in plastic crystals or polar liquids. Dielectric spectroscopy has been used to characterise the switching properties of the dipole moments in the vicinity of the PTs. Continuous-wave electron paramagnetic resonance spectroscopy was employed to investigate the effect of ordering on the local environment of the Cr3+ ions. We have also applied the quasielastic neutron scattering (QENS) technique as well as 1H NMR spectroscopy to measure the dynamics of the (CH3)2NH2+ cations residing in the inorganic framework