Subterahertz spectroscopy at He-3 temperatures

Basov, D. N.; Dordevic, S. V.; Singley, E. J.; Padilla, Willie J.; Burch, Kenneth S.; Elenewski, Justin E.; Greene, Lh.; Morris, James S.; Schickling, Richard

doi:10.1063/1.1614855

Cited by 17 publications

(11 citation statements)

References 43 publications

(33 reference statements)

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“…Consequently, numerous superconducting materials have been studied with electrodynamic experiments in the infrared, THz, or microwave spectral range [1,10,11]. Most of these experiments were performed at temperatures of liquid 4 He, whereas only very few optical studies addressed temperatures below 1 K [13][14][15][16][17][18]. Experimental challenges for a long time precluded electrodynamic studies of superconductors at ultralow temperatures [19], and thus all superconductors with critical temperature T c well below 1 K could not be probed by optics, with microwave spectroscopy being particularly relevant (thermal energy k B T for 1 K corresponds to 86 µeV photon energy ω or 21 GHz).…”

Section: Introductionmentioning

confidence: 99%

Complete electrodynamics of a BCS superconductor with μeV energy scales: Microwave spectroscopy on titanium at mK temperatures

2018

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We performed resonant microwave measurements on superconducting titanium (Ti) down to temperatures of 40 mK, well below its critical temperature Tc ≈ 0.5 K. Our wide frequency range 3.3-40 GHz contains the zero-temperature energy gap 2∆0 and allows us to probe the full electrodynamics of the superconducting state, including excitations across the gap and the low-frequency responses of superfluid condensate and thermal quasiparticles. The observed behavior follows the predictions of the BCS-based Mattis-Bardeen formalism, which implies that superconducting Ti is in the dirty limit, in agreement with our determination of the scattering rate. We directly determine the temperature dependence of the energy gap, which is in accordance with BCS predictions, and 2∆0/kBTc ≈ 3.5 with ∆0 ≈ 75 µeV. We also evaluate the penetration depth, and we characterize the behavior of superconducting Ti in external magnetic field.

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Section: Introductionmentioning

confidence: 99%

Complete electrodynamics of a BCS superconductor with μeV energy scales: Microwave spectroscopy on titanium at mK temperatures

2018

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“…Therefore, if the metal is in the "dirty" regime defined by Γ(T c ) > 2∆(0), the ratio R s /R n exhibits a peak at 2∆. This property allows one to measure the optical gap [10]. Early studies indicate that boron-doped diamond films are in the dirty limit and display a highly symmetric wave function [11].…”

mentioning

confidence: 99%

Low-Energy Electrodynamics of Superconducting Diamond

Ortolani

Lupi²,

Baldassarre³

et al. 2006

Phys. Rev. Lett.

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Boron-doped diamond films can become superconducting with critical temperatures Tc well above 4 K. Here we first measure the reflectivity of such a film down to 5 cm −1 , by also using Coherent Synchrotron Radiation. We thus determine the optical gap 2∆, the field penetration depth λ, the range of action of the Ferrell-Glover-Tinkham sum rule, and the electron-phonon spectral function α 2 F (ω). We conclude that diamond behaves as a "dirty" BCS superconductor.PACS numbers: 74.78. Db, Diamond, with its extraordinary mechanical properties, excellent thermal conductivity, and large gap between the valence and the conduction band, is potentially a semiconductor more attractive than silicon for many applications. Therefore the transport properties of diamond films, deposited by Chemical Vapor Deposition (CVD), and doped by acceptors or donors, are being extensively explored in view of a possible, future diamondbased electronics. In this framework it has been discovered recently that heavily boron-doped diamond can also become a superconductor [1] below critical temperatures T c well above the liquid helium temperature [2], if the doping level is 2.5%.Strongly covalent bonds, high concentration of impurities, and high phonon frequencies make B-doped diamond much different from the conventional metals where the Bardeen-Cooper-Schrieffer (BCS) [3] theory of superconductivity holds. Indeed, the metallic properties of heavily B-doped diamond are now the subject of an intense theoretical investigation. If many authors suggest that B-doped diamond in the doping regime above ∼ 0.5% should be a degenerate metal [4,5], with a conduction band strongly broadened by disorder, others point out that the deep 0.37 eV level of the isolated Bacceptor [6] may prevent the merging of the B-like bands with the C valence band, and propose unconventional models for the metallization of diamond [7]. One thus may wonder whether diamond is anyhow a BCS material, eventually with a high degree of disorder, or an exotic superconductor like most of those discovered in the last two decades. The study of the electron-phonon interaction in metallic diamond, a likely candidate for the Cooper pairing mechanism, has also attracted considerable attention, since the high phonon frequencies make the adiabatic limit questionable and the covalent bonds may produce a very strong coupling costant, like in MgB 2 [8, 9]. Here we approach both this problems by first measuring the reflectivity of a superconducting diamond film, in the sub-Terahertz region down to 5 cm −1 where the gaps of superconductors are observed, and in the infrared region, where the signatures of the electron-phonon coupling appear. The sub-Terahertz frequencies have been reached, with the required signal-to-noise ratio, by use of Coherent Synchrotron Radiation.A basic feature of the superconducting state is the opening, for T < T c , of a gap E g in the electronic density of states. Correspondingly, if the Cooper pairs are in a spherically symmetric s state, the reflectivity becomes R s (ω)...

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“…On the experimental side, future studies should on the one hand attempt to reach lower temperatures 30,44,45 and frequencies 46,47 in particular to check if the crossover to a scattering rate quadratic in frequency that would indicate the elusive FL optical response 31,48-51 can be found. Furthermore, optical measurements on our MAD-grown samples at higher frequencies, in the infrared range, should be performed to allow direct comparison with previous studies of samples with somewhat lower RRR at infrared frequencies and to further investigate the nature of the non-Drude plateau.…”

Section: Discussionmentioning

confidence: 99%

THz conductivity of Sr$_{1-x}$Ca$_x$RuO$_3$

Geiger,

Pracht,

Dressel

et al. 2016

Preprint

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We investigate the optical conductivity of Sr1−xCaxRuO3 across the ferromagnetic to paramagnetic transition that occurs at x = 0.8. The thin films were grown by metalorganic aerosol deposition with 0 ≤ x ≤ 1 onto NdGaO3 substrates. We performed THz frequency domain spectroscopy in a frequency range from 3 cm −1 to 40 cm −1 (100 GHz to 1.4 THz) and at temperatures ranging from 5 K to 300 K, measuring transmittivity and phase shift through the films. From this we obtained real and imaginary parts of the optical conductivity. The end-members, ferromagnetic SrRuO3 and paramagnetic CaRuO3, show a strongly frequency-dependent metallic response at temperatures below 20 K. Due to the high quality of these samples we can access pronounced intrinsic electronic contributions to the optical scattering rate, which at 1.4 THz exceeds the residual scattering rate by more than a factor of three. Deviations from a Drude response start at about 0.7 THz for both end-members in a remarkably similar way. For the intermediate members a higher residual scattering originating in the compositional disorder leads to a featureless optical response, instead. The relevance of low-lying interband transitions is addressed by a calculation of the optical conductivity within density functional theory in the local density approximation (LDA).

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Subterahertz spectroscopy at He-3 temperatures

Cited by 17 publications

References 43 publications

Complete electrodynamics of a BCS superconductor with μeV energy scales: Microwave spectroscopy on titanium at mK temperatures

Complete electrodynamics of a BCS superconductor with μeV energy scales: Microwave spectroscopy on titanium at mK temperatures

Low-Energy Electrodynamics of Superconducting Diamond

THz conductivity of Sr$_{1-x}$Ca$_x$RuO$_3$

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