Direct observation of the superconducting gap in a thin film of titanium nitride using terahertz spectroscopy

Pracht, Uwe S.; Scheffler, Marc; Dressel, Martin; Kalok, David; Strunk, Christoph; Baturina, T. I.

doi:10.1103/physrevb.86.184503

Cited by 42 publications

(33 citation statements)

References 22 publications

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“…DC transport measurements were used to characterize T c . THz spectroscopy was applied in the past to confirm the BCS theory since it probes the energy range of the superconducting gap [26,27,29]. The experimental setup [26,27] is based on several backward wave oscillators as a powerful radiation sources to emit continuous-wave, coherent radiation which, in sum, can be tuned over the frequency range of 0.05 − 1.2 THz, corresponding a photon energy of 0.18 − 5 meV.…”

Section: Methodsmentioning

confidence: 99%

See 1 more Smart Citation

The Higgs mode in disordered superconductors close to a quantum phase transition

Sherman¹,

Pracht²,

Gorshunov³

et al. 2015

Nature Phys

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The concept of mass-generation via the Higgs mechanism was strongly inspired by earlier works on the Meissner-Ochsenfeld effect in superconductors. In quantum field theory, the excitations of longitudinal components of the Higgs field manifest as massive Higgs bosons. The analogous Higgs mode in superconductors has not yet been observed due to its rapid decay into particle-hole pairs. Following recent theories, however, the Higgs mode should decrease below the pairing gap 2∆ and become visible in two-dimensional systems close to the superconductor-insulator transition (SIT). For experimental verification, we measured the complex terahertz transmission and tunneling density of states (DOS) of various thin films of superconducting NbN and InO close to criticality. Comparing both techniques reveals a growing discrepancy between the finite 2∆ and the threshold energy for electromagnetic absorption which vanishes critically towards the SIT. We identify the excess absorption below 2∆ as a strong evidence of the Higgs mode in two dimensional quantum critical superconductors.The Higgs mechanism, which has great implications to recent developments in particle physics [1], originates in Anderson's pioneering work on symmetry breaking with gauge fields in superconductors [2]. A superconductor spontaneously breaks continuous U (1) symmetry and acquires the well-known Mexican hat potential with a degenerate circle of minima described by the order parameter Ψ = Ae iϕ , see Fig. 1a. Excitations from the ground state can be classified as transverse Nambu-Goldstone (phase) modes and massive longitudinal Higgs (amplitude) modes (see blue and red lines in Fig. 1a). In particle physics, the latter manifest themselves as the Higgs boson which was recently discovered at CERN [3]. Indications of a Higgs mode in correlated many-body systems have been found in one-dimensional charge-densitywave systems [4], quantum antiferromagnets [5] and twodimensional superfluid to Mott transition in cold atoms [6]. An amplitude mode, also named Higgs mode, was theoretically predicted for superconductors [7] and recently reported to be measured by pump-probe spectroscopy [8]. This amplitude mode describes pairing fluctuations, which are qualitatively distinct from the purely bosonic mode expected from the O(2) field theory. The Higgs-amplitude mode analogous to the highenergy Higgs Boson has not yet been observed in superconductors. A partial reason is that in homogeneous, BCS superconductors the Higgs mode is short-lived and decays to particle hole (Bogoliubov) pairs [9,10]. Nevertheless, collective modes were recently predicted to be significant in strongly disordered superconductors [11], and, in particular it was shown [12][13][14] that the Higgs mode softens but remains sufficiently sharp near a quantum critical point (QCP) in two dimensions since it is found to be a critical energy scale of the quantum phase transition. Hence, the Higgs mass can be reduced below twice the pairing gap, 2∆, making this mode experimentally visible. Such a critical...

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Section: Methodsmentioning

confidence: 99%

“…This robust approach is well established to study superconducting thin films. For more details see the methods section and, e.g., [26][27][28][29]. Fig.…”

mentioning

confidence: 99%

The Higgs mode in disordered superconductors close to a quantum phase transition

Sherman¹,

Pracht²,

Gorshunov³

et al. 2015

Nature Phys

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171

183

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“…A main feature of these films is their high kinetic inductance. We determine this quantity from L = R /π∆ 0 , where the sheet resistance R = ρ/t is measured just above T c and ∆ 0 = 1.76k B T c is the superconducting energy gap for TiN [25,26] predicted by BCS theory. A monotonic decrease in T c with decreasing film thickness is therefore linked to a monotonic increase in L , which is eventually limited by the thinnest film we can grow that still superconducts.…”

mentioning

confidence: 99%

Atomic layer deposition of titanium nitride for quantum circuits

Shearrow

Koolstra

Whiteley

et al. 2018

Applied Physics Letters

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Superconducting thin films with high intrinsic kinetic inductance are of great importance for photon detectors, achieving strong coupling in hybrid systems, and protected qubits. We report on the performance of titanium nitride resonators, patterned on thin films (9-110 nm) grown by atomic layer deposition, with sheet inductances of up to 234 pH/ . For films thicker than 14 nm, quality factors measured in the quantum regime range from 0.4 to 1.0 million and are likely limited by dielectric two-level systems. Additionally, we show characteristic impedances up to 28 kΩ, with no significant degradation of the internal quality factor as the impedance increases. These high impedances correspond to an increased single photon coupling strength of 24 times compared to a 50 Ω resonator, transformative for hybrid quantum systems and quantum sensing.

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“…2), in contrast to what is generally assumed for TiN films. 22,23 In the case of the mostdisordered superconducting film C, the picture is different. The measured curves are not predicted by the model.…”

mentioning

confidence: 99%

Electrodynamic response and local tunneling spectroscopy of strongly disordered superconducting TiN films

et al. 2013

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We have studied the electrodynamic response of strongly disordered superconducting TiN films using microwave resonators, where the disordered superconductor is the resonating element in a high-quality superconducting environment of NbTiN. We describe the response assuming an effective pair-breaking mechanism modifying the density of states and compare this to local tunneling spectra obtained using scanning tunneling spectroscopy. For the least disordered film (k F l = 8.7, R s = 13 ), we find good agreement, whereas for the most disordered film (k F l = 0.82, R s = 4.3 k ), there is a strong discrepancy, which signals the breakdown of a model based on uniform properties.

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Direct observation of the superconducting gap in a thin film of titanium nitride using terahertz spectroscopy

Cited by 42 publications

References 22 publications

The Higgs mode in disordered superconductors close to a quantum phase transition

The Higgs mode in disordered superconductors close to a quantum phase transition

Atomic layer deposition of titanium nitride for quantum circuits

Electrodynamic response and local tunneling spectroscopy of strongly disordered superconducting TiN films

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