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1. Pulsed EPR Signals from Triplets

   https://doi.org/10.1515/zpch-2016-0869  

   Bowman, Michael K.; Chen, Hanjiao; Maryasov, Alexander G. (January 2017, Zeitschrift für Physikalische Chemie)

   Abstract Pulsed EPR experiments have proven to be an important tool for measuring EPR spectra, kinetics and relaxation rates of free radicals and triplet molecules. The EPR frequencies and selection rules from CW-EPR spectra also govern pulsed EPR-experiments, but pulsed excitation provides much greater control over spin dynamics and allows clean separation and measurement of many properties of the spin system. Most pulsed EPR measurements of triplet molecules have been made in the selective pulse limit where only one EPR transition of a molecule is excited by microwave pulses and its EPR spectroscopy behaves like that of a radical with spin of 

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2. High-field/high-frequency electron spin resonances of Fe-doped $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ by terahertz generalized ellipsometry: Monoclinic symmetry effects

   https://doi.org/10.1103/PhysRevB.109.214106  

   Richter, Steffen; Knight, Sean; Bulancea-Lindvall, Oscar; Mu, Sai; Kühne, Philipp; Stokey, Megan; Ruder, Alexander; Rindert, Viktor; Ivády, Viktor; Abrikosov, Igor A; et al (June 2024, Physical Review B)

   We demonstrate detection and measurement of electron paramagnetic spin resonances (EPR) of iron defects in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$utilizing generalized ellipsometry at frequencies between 110 and 170 GHz. The experiments are performed on an Fe-doped single crystal in a free-beam configuration in reflection at ${45}^{\circ }$and magnetic fields between 3 and 7 T. In contrast with low-field, low-frequency EPR measurements, we observe all five transitions of the $s=5/2$high-spin state ${\mathrm{Fe}}^{3+}$simultaneously. We confirm that ferric ${\mathrm{Fe}}^{3+}$is predominantly found at octahedrally coordinated Ga sites. We obtain the full set of fourth-order monoclinic zero-field splitting parameters for both octahedrally and tetrahedrally coordinated sites by employing measurements at multiple sample azimuth rotations. The capability of high-field EPR allows us to demonstrate that simplified second-order orthorhombic spin Hamiltonians are insufficient, and fourth-order terms as well as consideration of the monoclinic symmetry are needed. These findings are supported by computational approaches based on density-functional theory for second-order and on ligand-field theory for fourth-order parameters of the spin Hamiltonian. Terahertz ellipsometry is a way to measure spin resonances in a cavity-free setup. Its possibility of varying the probe frequency arbitrarily without otherwise changing the experimental setup offers unique means of truly disentangling different components of highly anisotropic spin Hamiltonians. Published by the American Physical Society2024 

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3. Time domain measurement of electron spin relaxation at high fields and dynamic nuclear polarization at sub-millimeter wavelengths

   https://doi.org/10.1109/MWSYM.2017.8058878  

   Dubroca, Thierry; McKay, Johannes; Wang, Xiaoling; van Tol, Johan (June 2017, 2017 IEEE MTT-S International Microwave Symposium (IMS))

   We describe a 395 GHz pulsed electron paramagnetic resonance (EPR) setup, and initial results of relaxation measure-ments and cw EPR at these frequencies in samples used for liquid- and solid-state nuclear magnetic resonance enhanced by dynamic nuclear polarization (DNP). Depending on the amount of spin –orbit coupling, the spin lattice relaxation becomes significantly faster at higher fields and frequencies, which has consequences for some DNP applications at high fields and frequencies. We will dis-cuss the requirements for (sub)millimeter-wave sources and com-ponents for DNP and pulsed EPR at even higher frequencies and fields, as even higher magnetic fields will become available in the near future. 

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4. Theory of tunneling between two-dimensional electron layers driven by spin pumping: Adiabatic regime and beyond

   https://doi.org/10.1103/5wlm-qhrj  

   Ke, Modi; Asmar, Mahmoud M; Tse, Wang-Kong (September 2025, Physical Review B)

   Tunneling spectroscopy between parallel two-dimensional (2D) electronic systems provides a powerful method to probe the underlying electronic properties by measuring tunneling conductance. In this work, we present a theoretical framework for spin transport in 2D-to-2D tunneling systems, driven by spin pumping. This theory applies to a vertical heterostructure where two layers of metallic 2D electron systems are separated by an insulating barrier, with one layer exchange coupled to a magnetic layer driven at resonance. Utilizing a nonperturbative Floquet-Keldysh formalism, we derive general expressions for the tunneling spin and charge currents across a broad range of driving frequencies, extending beyond the traditional adiabatic pumping regime. At low frequencies, we obtain analytical results that recover the known behaviors in the adiabatic regime. However, at higher frequencies, our numerical findings reveal significant deviations in the dependence of spin and charge currents on both frequency and precession angle. This work offers fresh insights into the role of magnetization dynamics in tunneling transport, opening up new avenues for exploring nonadiabatic spin pumping phenomena. 

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5. Terahertz electron paramagnetic resonance generalized spectroscopic ellipsometry: The magnetic response of the nitrogen defect in 4H-SiC

   https://doi.org/10.1063/5.0082353  

   Schubert, Mathias; Knight, Sean; Richter, Steffen; Kühne, Philipp; Stanishev, Vallery; Ruder, Alexander; Stokey, Megan; Korlacki, Rafał; Irmscher, Klaus; Neugebauer, Petr; et al (March 2022, Applied Physics Letters)

   We report on terahertz (THz) electron paramagnetic resonance generalized spectroscopic ellipsometry (THz-EPR-GSE). Measurements of field and frequency dependencies of magnetic response due to spin transitions associated with nitrogen defects in 4H-SiC are shown as an example. THz-EPR-GSE dispenses with the need of a cavity, permits independently scanning field and frequency parameters, and does not require field or frequency modulation. We investigate spin transitions of hexagonal ( h) and cubic ( k) coordinated nitrogen including coupling with its nuclear spin (I = 1), and we propose a model approach for the magnetic susceptibility to account for the spin transitions. From the THz-EPR-GSE measurements, we can fully determine polarization properties of the spin transitions, and we can obtain the k coordinated nitrogen g and hyperfine splitting parameters using magnetic field and frequency dependent Lorentzian oscillator line shape functions. Magnetic-field line broadening presently obscures access to h parameters. We show that measurements of THz-EPR-GSE at positive and negative fields differ fundamentally and hence provide additional information. We propose frequency-scanning THz-EPR-GSE as a versatile method to study properties of spins in solid state materials. 

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