Specific heat measurements of a superconducting<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>NbS</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>single crystal in an external magnetic field: Energy gap structure

Kačmarčı́k, J.; Pribulová, Z.; Marcenat, C.; Klein, Thierry; Rodière, Pierre; Cario, Laurent; Samuely, P.

doi:10.1103/physrevb.82.014518

Cited by 60 publications

(56 citation statements)

References 27 publications

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“…The agreement between our calculations and experiments is very good (apart from the empirical scaling discussed above), and conrms that the peak around 1.0 meV and the shoulder around 0.6 meV in the tunneling data taken at 1.8 K are to be associated with two distinct superconducting gaps on the Γ-centered and on the K-centered Fermi surface pockets. Our nding of two distinct superconducting gaps is also in line with the anomalous temperature dependence of the specic heat [50] and the pressure dependence of the upper critical eld [51].…”

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confidence: 85%

Origin of Superconductivity and Latent Charge Density Wave in NbS2

et al. 2017

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We elucidate the origin of the phonon-mediated superconductivity in 2H-NbS2 using the ab initio anisotropic Migdal-Eliashberg theory including Coulomb interactions. We demonstrate that superconductivity is associated with Fermi surface hot spots exhibiting an unusually strong electronphonon interaction. The electron-lattice coupling is dominated by low-energy anharmonic phonons, which place the system on the verge of a charge density wave instability. We also provide denitive evidence for two-gap superconductivity in 2H-NbS2, and show that the low-and high-energy peaks observed in tunneling spectra correspond to the Γ-and K-centered Fermi surface pockets, respectively. The present ndings call for further eorts to determine whether our proposed mechanism underpins superconductivity in the whole family of metallic transition metal dichalcogenides. PACS numbers: 74.70.Xa, 63.20.kd, 74.20.Fg, 74.25.Jb Transition metal dichalcogenides (TMDs) have generated considerable interest in recent years, since they provide an ideal playground for studying semiconductors, metals, and superconductors in two dimensions using the same structural template [13]. In the case of superconducting TMDs, one remarkable feature is that Cooper pair condensation usually coexists with a charge density wave (CDW) [4], raising the question on whether superconductivity and CDW co-operate or compete in these compounds [512].Within the family of superconducting TMDs, 2H-NbS 2 stands out as the only system for which a CDW phase has not been observed [13, 14]. This suggests that a comparative analysis of NbS 2 and other superconducting TMDs may help to clarify the interplay between the superconductive and the CDW instabilities in the entire family. 2H-NbS 2 is a phonon-mediated superconductor with a critical temperature T c = 5.7 K. Scanning tunneling spectroscopy (STS) measurements on this compound revealed two pronounced features in the density of states (DOS) at 0.53 meV and 0.97 meV below the critical temperature, providing strong indications of two-gap superconductivity [14]. However, so far microscopic calculations have considered only a single-gap scenario [15, 16].In this work we investigate the nature of the superconducting gap and the pairing mechanism in 2H-NbS 2 using the fully anisotropic ab initio Migdal-Eliashberg theory, and describe both electron-phonon and electronelectron interactions without any adjustable parameters. Our key nding is that a very signicant contribution to the superconducting pairing comes from the lowenergy anharmonic phonons with wavevectors near the line connecting the M and L points. These are the same phonons responsible for the CDW instability in other TMDs [8, 11, 1719], indicating that superconductivity in NbS 2 is intimately connected with a latent CDW. In agreement with the STS experiments of Ref. 14, we nd two distinct and anisotropic superconducting gaps.All calculations reported in this work were performed using density functional theory (DFT) in the local density approximation [20,21]. We employed...

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confidence: 85%

Origin of Superconductivity and Latent Charge Density Wave in NbS2

et al. 2017

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“…However, the temperature dependence of H c1 (and hence the gap distribution) is very similar to the one previously observed in 2H-dichalcogenides (NbSe 2 or NbS 2 [21], open symbols in Fig.5) despite very different electronic structures (see [6] and [23], respectively). Note also that, in those later systems, the multi-gap structure observed in magnetic measurements has been confirmed by both specific heat [24] and tunneling spectroscopy [25] measurements. The influence of the presence of a CDW leading to a strong k−dependence of the electron phonon coupling [26] (and hence of the gap) first seemed also to be excluded since this CDW is present only in NbSe 2 (and not NbS 2 ), but recent measurements clearly showed a strong softening of the phonon modes in some directions even in NbS 2 [27].…”

Section: B Gap Valuessupporting

confidence: 67%

Magnetic and thermodynamic properties of CuxTiSe2 single crystals

et al. 2017

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We present a detailed study of the phase diagram of copper intercalated TiSe2 single crystals, combining local Hall-probe magnetometry, tunnel diode oscillator technique (TDO), and specific heat and angle-resolved photoemission spectroscopy measurements. A series of the CuxTiSe2 samples from three different sources with various copper content x and superconducting critical temperatures Tc have been investigated. We first show that the vortex penetration mechanism is dominated by geometrical barriers enabling a precise determination of the lower critical field, Hc1. We then show that the temperature dependence of the superfluid density deduced from magnetic measurements (both Hc1 and TDO techniques) clearly suggests the existence of a small energy gap in the system, with a coupling strength 2∆s ∼ [2.4 − 2.8]kBTc, regardless of the copper content, in puzzling contradiction with specific heat measurements which can be well described by one single large gap 2∆ l ∼ [3.7 − 3.9]kBTc. Finally, our measurements reveal a non-trivial doping dependence of the condensation energy, which remains to be understood.

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“…The crystals had a cubic form with the edge of about 0.5 mm. The values of the critical temperature T c have been determined by the point-contactspectroscopy, resistivity and ac-calorimetry specific-heat measurements [20]. The first two techniques give T c = 7.4 ÷ 7.5 K and the specific heat measurements sensitive to the bulk of the sample yield from the entropy balance construction around the anomaly T c = 7.32 K, a reasonably close value.…”

Section: Methodsmentioning

confidence: 99%

Point-contact spectroscopy of the phononic mechanism of superconductivity in YB₆

Szabó

Girovský

Kačmarčı́k

et al. 2013

Supercond. Sci. Technol.

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Lortz et al. [Phys. Rev. B 73, 024512 (2006)] have utilized specific heat and resistivity measurements as "thermal spectroscopies" to deconvolve the spectrum of the electron-phonon interaction in YB6 assuming a major role of the low frequency phonon mode in mediating superconductivity. Here, we present direct point-contact spectroscopy studies of the superconducting interaction in this system. As a result the normalized superconducting gap reveals a strong coupling with 2∆/kBTc = 4 and moreover the spectra contain nonlinearities typical of the electron-phonon interaction at energies around 8 meV. The in-magnetic-field measurements evidence that the phonon features found in the second derivative of the current-voltage characteristics are due to the energy dependence of the superconducting energy gap as their energy position shrinks equally as the gap is closing. This is a direct proof that the superconducting coupling in the system is due to the low energy Einstein-like phonon mode associated with the yttrium ion vibrations in a perfect agreement with determinations from bulk measurements.

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Specific heat measurements of a superconductingNbS2single crystal in an external magnetic field: Energy gap structure

Cited by 60 publications

References 27 publications

Origin of Superconductivity and Latent Charge Density Wave in NbS2

Origin of Superconductivity and Latent Charge Density Wave in NbS2

Magnetic and thermodynamic properties of CuxTiSe2 single crystals

Point-contact spectroscopy of the phononic mechanism of superconductivity in YB₆

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Specific heat measurements of a superconductingNbS2single crystal in an external magnetic field: Energy gap structure

Cited by 60 publications

References 27 publications

Origin of Superconductivity and Latent Charge Density Wave in NbS2

Origin of Superconductivity and Latent Charge Density Wave in NbS2

Magnetic and thermodynamic properties of CuxTiSe2 single crystals

Point-contact spectroscopy of the phononic mechanism of superconductivity in YB6

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Product

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Point-contact spectroscopy of the phononic mechanism of superconductivity in YB₆