Microwave spectroscopy

Microwave spectroscopy is the spectroscopy method that employs microwaves, i.e. electromagnetic radiation at GHz frequencies, for the study of matter.

In molecular physics
Main article: Rotational spectroscopy

In the field of molecular physics, microwave spectroscopy is commonly used to probe the rotation of molecules.[1]

In condensed matter physics

In the field of condensed matter physics, microwave spectroscopy is used to detect dynamic phenomena of either charges or spins at GHz frequencies (corresponding to nanosecond time scales) and energy scales in the µeV regime. Matching to these energy scales, microwave spectroscopy on solids is often performed as a function of temperature (down to cryogenic regimes of a few K or even lower)[2] and/or magnetic field (with fields up to several T). Spectroscopy traditionally considers the frequency-dependent response of materials, and in the study of dielectrics microwave spectroscopy often covers a large frequency range. In contrast, for conductive samples as well as for magnetic resonance, experiments at a fixed frequency are common (using a highly sensitive microwave resonator),[3] but frequency-dependent measurements are also possible.[4]

Probing charges in condensed matter physics

For insulating materials (both solid and liquid),[5] probing charge dynamics with microwaves is a part of dielectric spectroscopy. Amongst the conductive materials, superconductors are a material class that is often studied with microwave spectroscopy, giving information about penetration depth (governed by the superconducting condensate),[3][6] energy gap (single-particle excitation of Cooper pairs), and quasiparticle dynamics.[7]

Another material class that has been studied using microwave spectroscopy at low temperatures are heavy fermion metals with Drude relaxation rates at GHz frequencies.[4]

Probing spins in condensed matter physics

Microwaves impinging on matter usually interact with charges as well as with spins (via electric and magnetic field components, respectively), with the charge response typically much stronger than the spin response. But in the case of magnetic resonance, spins can be directly probed using microwaves. For paramagnetic materials, this technique is called electron spin resonance (ESR) and for ferromagnetic materials ferromagnetic resonance (FMR).[8] In the paramagnetic case, such an experiment probes the Zeeman splitting, with a linear relation between the static external magnetic field and the frequency of the probing microwave field. A popular combination, as implemented in commercial X-band ESR spectrometers, is approximately 0.3 T (static field) and 10 GHz (microwave frequency) for a typical material with electron g-factor close to 2.

References
  1. Gordy, W. (1970). A. Weissberger (ed.). Microwave Molecular Spectra in Technique of Organic Chemistry. IX. New York: Interscience.
  2. Krupka, J.; et al. (1999). "Complex permittivity of some ultralow loss dielectric crystals at cryogenic temperatures". Meas. Sci. Technol. 10 (5): 387–392. Bibcode:1999MeScT..10..387K. doi:10.1088/0957-0233/10/5/308.
  3. Hardy, W. N.; et al. (1999). "Precision measurements of the temperature dependence of λ in YBa2Cu3O6.95: Strong evidence for nodes in the gap function". Phys. Rev. Lett. 70 (25): 3999–4002. Bibcode:1993PhRvL..70.3999H. doi:10.1103/PhysRevLett.70.3999. PMID 10054019.
  4. Scheffler, M.; et al. (2013). "Microwave spectroscopy on heavy-fermion systems: Probing the dynamics of charges and magnetic moments". Phys. Status Solidi B. 250 (3): 439–449. arXiv:1303.5011. Bibcode:2013PSSBR.250..439S. doi:10.1002/pssb.201200925. S2CID 59067473.
  5. Kaatze, U.; Feldman, Y. (2006). "Broadband dielectric spectrometry of liquids and biosystems". Meas. Sci. Technol. 17 (2): R17–R35. Bibcode:2006MeScT..17R..17K. doi:10.1088/0957-0233/17/2/R01.
  6. Hashimoto, K.; et al. (2009). "Microwave Penetration Depth and Quasiparticle Conductivity of PrFeAsO1−y Single Crystals: Evidence for a Full-Gap Superconductor". Phys. Rev. Lett. 102 (1): 017002. arXiv:0806.3149. Bibcode:2009PhRvL.102a7002H. doi:10.1103/PhysRevLett.102.017002. PMID 19257228. S2CID 41994664.
  7. Hosseini, A.; et al. (1999). "Microwave spectroscopy of thermally excited quasiparticles in YBa2Cu3O6.99". Phys. Rev. B. 60 (2): 1349–1359. arXiv:cond-mat/9811041. Bibcode:1999PhRvB..60.1349H. doi:10.1103/PhysRevB.60.1349. S2CID 119403711.
  8. Farle, M. (1998). "Ferromagnetic resonance of ultrathin metallic layers". Rep. Prog. Phys. 61 (7): 755–826. Bibcode:1998RPPh...61..755F. doi:10.1088/0034-4885/61/7/001.


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