Electron paramagnetic resonance spectroscopy ofEr3+:Y2SiO5using a Josephson bifurcation amplifier: Observation of hyperfine and quadrupole structures

Abstract

We performed magnetic field and frequency tunable electron paramagnetic resonance spectroscopy of an Er 3+ doped Y2SiO5 crystal by observing the change in flux induced on a direct currentsuperconducting quantum interference device (dc-SQUID) loop of a tunable Josephson bifurcation amplifer. The observed spectra show multiple transitions which agree well with the simulated energy levels, taking into account the hyperfine and quadrupole interactions of 167 Er. The sensing volume is about 0.15 pl, and our inferred measurement sensitivity (limited by external flux noise) is approximately 1.5 × 10 4 electron spins for a 1 s measurement. The sensitivity value is two orders of magnitude better than similar schemes using dc-SQUID switching readout.Electron paramagnetic resonance (EPR) is the gold standard tool to characterize the spin and magnetic properties of a wide range of materials [1,2]. Over several decades, there has been numerous efforts to improve the sensitivity of EPR measurements, including magnetic resonance force microscopy [3,4] and using a single NV center in diamond [5,6]. In recent years, significant improvements were achieved by operating in cryogenic temperatures, where thermal noise is significantly reduced and the spins are highly polarized. In such cases the spins of interest are coupled to superconducting planar resonators [7,8] or 3D cavities [9], resulting in avoided crossing or reduction of resonator quality factor. By combining superconducting lumped-element resonators with small magnetic mode volume and quantumlimited parametric amplifiers, recently sensitivity values of pulsed EPR have reached ∼ 60 spins/ √ Hz [10]. However, these EPR schemes typically only allow magnetic field-dependent study, because the operating frequency range is limited to frequencies near the resonator's resonance. Frequency and magnetic field-dependent EPR spctroscopy can be advantageous when compared to conventional, single-frequency EPR, in the characterization of more complicated materials [11][12][13], e.g., anisotropic materials, systems with electron spin S > 1/2 (zerofield interactions), or systems with nuclear spin I > 0 (hyperfine and quadrupole interactions). Efforts to extend the frequency range include using a tunable resonator [8,13,14], a resonator with multiple narrowly spaced modes (e.g., a whispering gallery mode resonator [15,16]), or a broadband waveguide [12,17]. However, these schemes typically suffer from worse sensitivity or still limited frequency range.EPR can also be performed by measuring the magnetization from the polarization of spins using a sensitive magnetometer. This method allows both magnetic field and frequency-dependent measurements. One typ- * rangga.budoyo@lab.ntt.co.jp ical magnetometer is a direct current-superconducting quantum interference device (dc-SQUID) [18][19][20], with continuous wave (cw)-EPR sensitivities reaching ∼ 10 6 spins/ √ Hz [21]. The stated sensitivity was limited by measurements using switching readout to obtain the critical current of the SQUID, which i...

Transparent Sol-Gel Oxyfluoride Glass-Ceramics with High Crystalline Fraction and Study of RE Incorporation

“…For the composition with a lower concentration of dopant, the hyperfine structure is only partially resolved, due to the interaction of the electrons with nuclear spins. The low intensity of the hyperfine structure is due to the quantity of Er isotope nuclei with I = 7/2 (~23%), while the remaining nuclei (77%) with I = 0 do not produce any hyperfine interaction [58,61,62]. For the composition doped with 0.5 Er 3+ , the hyperfine structure is too broad to be resolved but its presence may be discerned from the shape of the resonance at g 1 = 11.2 which presents broader sidebands.…”

Electron Spin Resonance with up to 20 Spin Sensitivity Measured using a Superconducting Flux Qubit

“…Assuming δP sw = 2σP sw , a typical repetition time between 5 and 20 µs infers a sensitivity of about 20 spins in a 1 second measurement. This performance represents nearly three orders of magnitude improvement than the 10 4 spin sensitivity for the JBA ESR 7 . Direct comparison between the flux qubit ESR and JBA ESR can be made by comparing Fig.…”

“…However, to fully characterize more complicated spin systems (for example, anisotropic systems, or systems with hyperfine and quadrupole interactions), magnetic field-and frequencydependent ESR spectroscopy is required 3,4 . This can be achieved by using a broadband waveguide 3 , a tunable resonator 4 , or by using the magnetic flux detection capabilities of superconducting loop structures such as a direct current-superconducting quantum interference device (dc-SQUID) 5,6 , a frequency-tunable Josephson bifurcation amplifier (JBA) 7 , or a flux qubit 8,9 . Recently flux qubit ESR spectroscopy with sensitivity of 400 spins Hz −1/2 has been reported 9 .…”

Polarizing electron spins with a superconducting flux qubit

“…This means that our cooling scheme with the FQ is especially useful when we perform an ESR with the FQ: A sensitivity of ESR measurements is proportional to p ↓ − p ↑ [14,47], and so the sensitivity of ESR with our polarization scheme leads 10 times better than the conventional one without active cooling. Also, it is worth mentioning that the actual temperature of the electron spins in the dilution refrigerator might be 50 mK or more and not 10 mK [7,8] because T 1 of the electron spins is large [20]. Moreover, an interval between measurements in the standard ESR should be a few time larger than T 1 of the electron spins.…”

“…An improvement of the ESR sensitivity is important for practical applications. Therefore, superconducting circuits have often been used to detect the small number of electron spins [1][2][3][4][5][6][7][8]. By using a superconducting resonator, it is possible to measure only 12 spins with 1 s measurement time where the detection volume is around 6fl [9] where the frequency of the superconducting resonator is fixed.…”