Broadband ESR from 500 MHz to 40 GHz using superconducting coplanar waveguides

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2 Broadband ESR from 500 MHz to 40 GHz using superconducting coplanar waveguides Martin Dressel 1. Physikalisches Institut, Universität Stuttgart, Germany Outline 1. Introduction ESR resonators 2. Strip line resonators basic concepts, superconductors 3. Transmission lines field-swept mode frequency-swept mode 4. Examples C. Clauss, L. Bogani, M. Scheffler 1. Physikalisches Institut Universität Stuttgart, Germany D. Bothner, R. Koelle, R. Kleiner Physikalisches Institut, Universität Tübingen

3 Introduction cavity resonator 3 In order to investigate frequency- and field-dependent effects different cavities have to be used at discrete frequencies. Enclosed metal cavities Advantage: high quality Q Disadvantage: for low frequencies the size becomes large: d λ/2, poor filling factor

4 Experimental Setup transmission line 4 Transmission line a flat strip acts as center conductor of a microwave transmission line Field pattern largely homogeneous, except for the field around the edges P.J.M. van Bentum et al. J.Magn. Res. 189, 104 (2007)

5 Experimental Setup transmission line 5 Coplanar transmission line 150 nm Nb on top of 330 µm sapphire T c = 9 K R 300 K / R 10 K = 6 Compact design by meander construction Large samples can be places on top as ground plate. The sample is put on top of the structure by letting it crystalize from solution. 1 mm organic radical (< 0.5 mg) in isopropyl NITPhoMe 2-(4'-methoxyphenyl)- 4,4,5,5-tetra-methylimidazoline-1-oxyl-3-oxide

6 Experimental Setup transmission line 6 Coplanar transmission line The structure with the sample is put into a gold plated brass sample holder box and connected to the in- and output feeds via silver paste. A microwave signal generator feeds a monochromatic signal via coaxial cables to the sample. 1 mm The transmittted power is measured with a power meter.

7 Experimental Setup superconducting stripline 7 Resonator Cutting a section of the transmission line (impedance mismatch) forms a resonator. Compact design by meander construction Large samples can be places on top as ground plate Superconducting strip lines yield very high quality factor M. Scheffler et al., J. Phys.: Conf. Ser. 400, (2012)

8 Experimental Setup superconducting stripline 8 Resonator Cutting a section of the transmission line (impedance mismatch) forms a resonator. Compact design by meander construction Large samples can be places on top as ground plate Superconducting strip lines yield very high quality factor. Q-factor of Pb stripline (T c = 7.2 K) and Ta sample (T c = 4.3 K) M. Scheffler et al., J. Phys.: Conf. Ser. 400, (2012)

9 Experimental Setup superconducting stripline 9 Vortex pinning Vortex motion causes energy dissipation Pinning by impurities or defects P.E. Gao et al., Supercond. Sci. Technol. 14, 729 (2001)

10 Experimental Setup superconducting stripline 10 Vortex pinning Film spin coated by photoresist Self-assembled microspheres of polysterene Illuminated by UV light UV D. Bothner et al., Supercond. Sci. Technol. 25, (2012)

11 Experimental Setup superconducting stripline 11 Vortex pinning Film spin coated by photoresist Self-assembled microspheres of polysterene Illuminated by UV light Undisturbed bulk material High density of pinning centers D. Bothner et al., Supercond. Sci. Technol. 25, (2012)

12 Experimental Setup superconducting stripline 12 Vortex pinning 150 nm Nb T c = 9 K on top of 330 µm sapphire polystyrene colloids D S = 770 nm serve as microlenses Langmuir-Blodgett deposition creates hexagonal array of antidots D a = 370 nm n h = 1.55 µm -2 Matching flux density B 1 = 3.4 mt which corresponds to one vortex per pinning site. D. Bothner et al., Supercond. Sci. Technol. 25, (2012)

13 Experimental Setup superconducting stripline 13 Vortex pinning Commensurability effects

14 Experimental Setup superconducting stripline 15 Stripline resonator 150 nm Nb T c = 9 K on top of 330 µm sapphire Resonance frequency f = 6.25 GHz width of center conductor 20 µm 60 µm gap to ground plate 8 µm 25 µm coupling gap 10 µm 30 µm using microlenses the film is perforated by a hexagonal hole array hole density 1.65 per µm 2 1 mm D. Bothner et al., Appl. Phys. Lett. 100, (2012)

15 Broadband ESR transmission line 16 Orientation ac magnetic field static magnetic field B RF B ext C. Clauss et al., Appl. Phys. Lett. 102, (2013)

16 Broadband ESR data analysis 17 Transmission spectra T = 1.6 K B = 0 T transmission decreases with increasing frequency mainly due to losses in waveguide detection limit

17 Broadband ESR data analysis 18 Transmission spectra T = 1.6 K B = 0 T and 100 mt only tiny changes when magnetic field is applied Normalized spectra T = 1.6 K In order to avoid large background Standing waves, box modes ESR absorption

18 Broadband ESR data analysis 19 Transmission spectra ESR absorption are visible 20 mt < B < 1.2 T The position of the absorption peak can be identified even above 33 GHz. The amplitude is increasing with field due to larger Zeeman splitting: larger difference in thermal occupancy Linear shift of absorption frequency with magnetic field: NITPhOMe g = ± 0.001

19 Broadband ESR NITPhOMe 22 Transmission spectra Field-swept spectra for various frequencies The shape of the absorption peak changes with frequency. Resonances can be observed in the range 0.5 GHz < f < 40 GHz Sensitivity: spins C. Clauss et al., Appl. Phys. Lett. 102, (2013)

20 Broadband ESR NITPhOMe 23 Transmission spectra Frequency-swept spectra for different temperatures Higher current in the superconducting state leads to larger active volume and more spins to contribute. Change baseline and frequency With decreasing temperature T < T c the absorption peak shifts to higher frequencies. In the superconducting state, the field is partially focused into the gaps of the structure and hence creates a locally enhanced field strength. C. Clauss et al., Appl. Phys. Lett. 102, (2013)

21 Broadband ESR ruby 24 Transmission spectra Normalized frequency-swept ESR spectra of ruby Cr 3+ 1 m s = 3/2 2 m s = ½ 3 m s = + 1/2 4 m s = + 3/2 B ext is oriented at 78 to c-axis, according to the Hamiltonian C. Clauss et al., Appl. Phys. Lett. 102, (2013)

22 Broadband ESR quantum criticality YbRh 2 Si 2 c B 25 YbRh 2 Si 2 Heavy Fermion compound localized 4f electrons and delocalized conduction electrons 10 T * competition of Kondo effect and RKKY interaction T (K) 1 NFL 0.1 AF LFL Power low in resistivity ρ dc (T) = ρ 0 + A ε T ε can be tuned by magnetic field afm order Fermi liquid behavior Non-Fermi liquid regime T* line (Hall effect, Fermi surface) B (T)

23 Broadband ESR quantum criticality 26 YbRh 2 Si 2 Heavy Fermion compound localized 4f electrons and delocalized conduction electrons competition of Kondo effect and RKKY interaction In general the local magnetic moments are screened in heavy Fermions no ESR But clear ESR in YbRh 2 Si 2 characteristics of local moments strong temperature dependence hints for change of g-factor and linewidth at T* J. Sichelschmidt et al., Phys. Rev. Lett. 91, (2003)

24 Broadband ESR quantum criticality YbRh 2 Si 2 c B 27 YbRh 2 Si 2 explore wide section of the low-temperature phase diagram by inserting the resonator in dilution refrigerator 1.5 GHz < f < 13 GHz T > 30 mk three coplanar resonators with fundamental modes: 1.50 GHz 2.07 GHz 2.65 GHz higher harmonics: up to 9 th mode T (K) T * AF LFL B (T)

25 Summary broadband ESR 28 Superconducting transmission lines Nb transmission lines, perforated in order to pin vortices compact design Nb Stripline resonators high sensitivity multiple frequencies due to higher harmonics Cr 3+ YbRh 2 Si 2

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