Enhancement of microwave fields in pulse EPR of quantum paraelectrics

The pulse electron paramagnetic resonance (EPR) is widely used in different branches of material and life sciences, including promising applications in quantum information processing and quantum sensing. Here, we study the effect of the high polarizability of KTaO3 and SrTiO3 quantum paraelectrics on local electric and magnetic field components of microwaves (MW) at Fe3+ and Mn2+ paramagnetic ions. The measurements are performed with a commercial EPR spectrometer using dielectric and split-ring resonators. It is found that the power of MW pulses used in coherent spin manipulation at nanoseconds timescale decreases to milliwatts as compared to the tens-hundreds of watts usually used for spins in conventional materials. The amplification of MW fields is related to the very high dielectric permittivity (up to 25 000 in SrTiO3) of quantum paraelectrics at GHz frequencies and temperatures below 20 K. This creates the large induced polarization and, thus, huge displacement current and in turn the secondary MW magnetic field. Numerical simulations support the observation of the enhanced magnetic MW field in the high-permittivity sample. The low MW power for excitation of spin transitions in quantum paraelectrics eliminates the requirement of expensive high-power MW equipment. This approach also allows to globally control spin qubits in tandem with integrated devices based on conventional semiconductor MW circuits working at mW powers. It is suggested that quantum paraelectrics can also be used as substrates for deposition of nanoparticles or films of other materials, which would be manipulated by the low-power MW pulses.