Broadband electron spin resonance from 500 MHz to 40 GHz using superconducting coplanar waveguides

Abstract

We present non-conventional electron spin resonance (ESR) experiments based on microfabricated superconducting Nb thin film waveguides. A very broad frequency range, from 0.5 to 40 GHz, becomes accessible at low temperatures down to 1.6 K and in magnetic fields up to 1.4 T. This allows for an accurate inspection of the ESR absorption position in the frequency domain, in contrast to the more common observation as a function of magnetic field. We demonstrate the applicability of frequency-swept ESR on Cr 3+ atoms in ruby as well as on organic radicals of the Nitronylnitroxide family. Measurements between 1.6 and 30 K reveal a small frequency shift of the ESR and a resonance broadening below the critical temperature of Nb, which we both attribute to a modification of the magnetic field configuration due to the appearance of shielding supercurrents in the waveguide.PACS numbers: 87.80. Lg, 76.30.Rn, 84.40.Az, 07.57.Pt To improve the performance of electron spin resonance (ESR) systems the main strategy was the use resonant of cavities with higher and higher fields and frequencies. As a result, most modern instrumentation can operate only at a single frequency or in an extremely narrow frequency range [1]. However, this can be a serious hindrance when a frequency-dependent effect is to be observed, as needed to probe a complete phase diagram of some material or to reliably assess the distance and orientation of spin labels. The latter problem, in particular, is increasingly important in the domain of biological ESR, as it is fundamental to understand the structural conformation and dynamics of biological systems. Until now the main strategy to overcome such problems was to measure the response of the system at a few discrete and widely-spaced frequencies [2]. Another approach is to reduce the dimension of the ESR cavities to micrometer size while at the same time somewhat relaxing the resonance requirements [3][4][5]. In this way small sample quantities can be probed over a wider frequency range. Setups that offer the possibility to sweep both the magnetic field and the radiation frequency have so far mostly been realized in the high frequency region (quasi-optical, from 50 to several 100 GHz) [6][7][8], while lower frequencies have been inspected using a coupled antenna approach [9], tuneable cavities [10] or by placing the sample close to the center conductor of a coaxial line [11].In this letter we demonstrate a different approach which uses a microfabricated superconducting coplanar waveguide to generate the radio frequency (RF) field. The feasibility of such an approach was shown before by Schuster et al. focusing on high-cooperativity coupling of spin ensembles to superconducting cavities [12]. Similar devices are also used to study ferromagnetic resonances of various materials [13][14][15][16]. Using superconducting waveguides one can obtain higher RF fields (with same input power) and no ohmic heating of the sample, as will be shown later. We show that it is possible to probe both the frequency and field d...

Superconducting Pb stripline resonators in parallel magnetic field and their application for microwave spectroscopy

“…Here Pb with T c ≈ 7.2 K and a critical magnetic field of about B c ≈ 80 mT is a well suited basis material [39], as it can be processed for thin films rather easily. For the spectroscopic study of certain materials [21,22,[40][41][42], application of a magnetic field in the order of tens of mT is helpful, and therefore we address the microwave properties of Pb stripline resonators in finite magnetic field. This is not only relevant for spectroscopy, but like any exposure of a superconductor to a magnetic field is also an interesting issue on its own.…”

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

“…These values are slightly higher than the expected g ≈ 7.0 and 15 for the direction of B 0 [13,36,37]. These can be attributed to ≈ 10% enhancement of the magnetic field near the superconducting film due to Meissner effect, comparable to previous reports on superconducting waveguides [17].…”

“…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.…”

Metallic coplanar resonators optimized for low-temperature measurements

“…The observation can be explained by the magnetoresistance of copper at higher fields [35]. Planar structures (microstrip, stripline and coplanar) have already been employed for ESR experiments [15,[36][37][38][39][40][41][42][43][44][45][46]. Although microstrip and stripline resonators have also been successfully applied in ESR measurements [36-38, 41, 45, 47], coplanar configurations might be more appropriate candidates since they carry the signal non-dispersively in contrast to a microstrip configuration [45,48].…”

“…Superconducting planar resonators can be substantially limited by their critical fields [26,35], and broadband transmission line structures [15,44] have limitations in sensitivity. In order to test whether our metallic CPW resonators can be employed for ESR experiments, a 2,2-diphenyl-1-picrylhydrazyl (DPPH) sample (200×100 µm 2 ) was placed on the middle of the resonator meander line and fixed by a small amount of vacuum grease.…”