Enhancement of Pairing Interaction and Magnetic Fluctuations toward a Band Insulator in an Electron-Doped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>Li</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:mi>ZrNCl</mml:mi></mml:math>Superconductor

Kasahara, Yuichi; Kishiume, Tsukasa; Takano, T.; Kobayashi, K.; Matsuoka, Eiichi; Onodera, Hidetoshi; Kuroki, Kazuhiko; Taguchi, Y.; Iwasa, Yoshihiro

doi:10.1103/physrevlett.103.077004

Cited by 51 publications

(84 citation statements)

References 33 publications

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“…Kasahara et al (Y. Kasahara et al [636]) report the specific heat of -LixZrNCl for various x, and find for 0.10x0.40, 15 KTc10 K, that  as a function of magnetic field H rises quickly at first, and then more slowly. Such behavior is indicative of either two s-wave gaps (Bang [83])) or strong anisotropy/possible nodal behavior in a single gap (Wang et al [526]).…”

Section: Superconducting Properties That Impinge On Possible Ucs -Formmentioning

confidence: 99%

“…Such behavior is indicative of either two s-wave gaps (Bang [83])) or strong anisotropy/possible nodal behavior in a single gap (Wang et al [526]). The calculation of large 2/kBTc values (up to 4.5) by Kasahara et al [636], which they claimed as evidence for an increase in coupling strength with doping above x=0.07, was done using an empirical theory (Padamsee, Neighbor, and Shiffman [637]) that assumed an "isotopically [sic] gapped state," which is questionable based on the evidence from their own (H) data which implied strong anisotropy.…”

Section: Superconducting Properties That Impinge On Possible Ucs -Formmentioning

confidence: 99%

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Unconventional superconductivity

Stewart

2017

Advances in Physics

222

165

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Abstract:"Conventional" superconductivity, as used in this review, refers to electron-phonon coupled superconducting electron pairs described by BCS theory. Unconventional superconductivity refers to superconductors where the Cooper pairs are not bound together by phonon-exchange but instead by exchange of some other kind, e. g. spin fluctuations in a superconductor with magnetic order either coexistent or nearby in the phase diagram. Such unconventional superconductivity has been known experimentally since heavy fermion CeCu2Si2, with its strongly correlated 4f electrons, was discovered to superconduct below 0.6 K in 1979. Since the discovery of unconventional superconductivity in the layered cuprates in 1986, the study of these materials saw Tc jump to 164 K by 1994. Further progess in high temperature superconductivity would be aided by understanding the cause of such unconventional pairing. This review compares the fundamental properties of 9 unconventional superconducting classes of materialsfrom 4f-electron heavy fermions to organic superconductors to classes where only three known members exist to the cuprates with over 200 examples -with the hope that common features will emerge to help theory explain (and predict!) these phenomena. In addition, three new emerging classes of superconductors (topological, interfacial -e. g. FeSe on SrTiO3, and H2S under high pressure) are briefly covered, even though their "conventionality" is not yet fully determined.

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Section: Superconducting Properties That Impinge On Possible Ucs -Formmentioning

confidence: 99%

Section: Superconducting Properties That Impinge On Possible Ucs -Formmentioning

confidence: 99%

Unconventional superconductivity

Stewart

2017

Advances in Physics

222

165

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“…Despite Li x ZrNCl being nonmagnetic, in the low-doping limit, the magnetic susceptibility is enhanced in a way very similar way to that in the superconducting T c . The interacting magnetic susceptibility χ s is not constant, and strongly deviates from the constant free-electron-like behavior in 2D 23,24 ; in particular, the susceptibility χ s is enhanced at the low-doping regime.…”

Section: Introductionmentioning

confidence: 99%

“…These terms are doping independent, and are subtracted from the experimental data to obtain the spin susceptibility of the conduction electrons in the experimental Ref. 23 and the corresponding Supplemental Material:…”

Section: Appendix B: Changing the Exact Exchange And Range Separationmentioning

confidence: 99%

Spin susceptibility and electron-phonon coupling of two-dimensional materials by range-separated hybrid density functionals: Case study ofLixZrNCl

et al. 2016

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We investigate the capability of density functional theory (DFT) to appropriately describe the spin susceptibility, χs, and the intervalley electron-phonon coupling in LixZrNCl. At low doping, LixZrNCl behaves as a two-dimensional two-valley electron gas, with parabolic bands. In such a system, χs increases with decreasing doping because of the electron-electron interaction. We show that DFT with local functionals (LDA/GGA) is not capable of reproducing this behavior. The use of exact exchange in Hartree-Fock (HF) or in DFT hybrid functionals enhances χs. HF, B3LYP, and PBE0 approaches overestimate χs, whereas the range-separated HSE06 functional leads to results similar to those obtained in the random phase approximation (RPA) applied to a two-valley two-spin electron gas. Within HF, LixZrNCl is even unstable towards a ferromagnetic state for x < 0.16. The intervalley phonons induce an imbalance in the valley occupation that can be viewed as the effect of a pseudomagnetic field. Thus, similarly to what happens for χs, the electron-phonon coupling of intervalley phonons is enhanced by the electron-electron interaction. Only hybrid DFT functionals capture such an enhancement and the HSE06 functional reproduces the RPA results presented in M. Calandra et al. [Phys. Rev. Lett. 114, 077001 (2015)]. These results imply that the description of the susceptibility and electron-phonon coupling with a range-separated hybrid functional would be important also in other two-dimensional weakly doped semiconductors, such as transition-metal dichalcogenides and graphene.

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“…This is not necessary the case if approximated local exchange and correlation functionals are used. If a 4-component local exchange correlation kernel is adopted in the calculation, namely xc = xc (n, m, m v ) (27) where m v (r) = n v=1 (r) − n v=2 (r) is the valley magnetization and xc (n, m, m v ) = xc (n, m v , m), then the SU (4) symmetry can be preserved. The valley exchange-correlation enhancement is written as…”

Section: Absence Of Valley Susceptibility Enhancement In Local Spin Dmentioning

confidence: 99%

Universal Increase in the Superconducting Critical Temperature of Two-Dimensional Semiconductors at Low Doping by the Electron-Electron Interaction

2015

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In 2-dimensional multivalley semiconductors, at low doping, even a moderate electron-electron interaction enhances the response to any perturbation inducing a valley polarization. If the valley polarization is due to the electron-phonon coupling, the electron-electron interaction results in an enhancement of the superconducting critical temperature. By performing first principles calculations beyond DFT, we prove that this effect accounts for the unconventional doping-dependence of the superconducting transition-temperature (Tc) and of the magnetic susceptibility measured in LixZrNCl. Finally we discuss what are the conditions for a maximal Tc enahnacement in weakly doped 2-dimensional semiconductors.The quest for high T c superconductivity has mainly focused on strongly correlated materials in proximity of electronic instabilities like the Mott transition (cuprates [1]) or fragile magnetic states (iron pnictides [2, 3]). Heavily doped three dimensional (3D) covalently bonded semiconductors, like diamond [4], silicon [5] and SiC [6, 7] have been considered as an alternative, that, however, has lead, so far, to fairly low T c (< 10 K). In 3D, the density of states at the Fermi level slowly grows with doping. As T c increases with the density of states [8], a large number of carriers has to be introduced to achieve a large T c . This demanding requirement could be released in 2 dimensional (2D) semiconductors, such as transition metal dichalcogenides [9][10][11][12], cloronitrides [13,14] or other layered materials with massive Dirac fermions, where the doping can be controlled by intercalation [13,14] or field-effect [11,12,15]. In 2D, the density of states is a constant function of the Fermi energy ( F ) and, in principle, T c is expected to be insensitive on doping. Surprisingly, measurements on Li x ZrNCl [13,14,16], a weakly-doped multivalley 2D semiconductor, revealed that T c not only does not increase with doping but even decreases. Here we show that the e-e interaction is responsible for such a puzzling behavior. In particular, in a weakly-doped 2D multivalley semiconductor, e-e manybody effects enhance the response to any perturbation inducing a valley polarization. If the valley polarization is due to the electron-phonon coupling, the e-e interaction will lead to a large increase of T c . We demonstrate that this effect explains the high T c in Li x ZrNCl and its unconventional behavior [16] as a function of doping. Finally, by finding the conditions for a maximal Tc enhancement, we show how weakly-doped 2D semiconductors are an alternative route towards high Tc superconductivity.The electronic structure of multivalley semiconductors has minima (maxima) in the conduction (valence) band that are named valleys. In the low doping limit, the equivalent g v valleys are occupied by few electrons or holes and the electronic structure is described by the effective mass theory. The resulting model Hamiltonian is that of a multicomponent electron gas of mass m * and density n where the valley index plays the r...

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Enhancement of Pairing Interaction and Magnetic Fluctuations toward a Band Insulator in an Electron-DopedLixZrNClSuperconductor

Cited by 51 publications

References 33 publications

Unconventional superconductivity

Unconventional superconductivity

Spin susceptibility and electron-phonon coupling of two-dimensional materials by range-separated hybrid density functionals: Case study ofLixZrNCl

Universal Increase in the Superconducting Critical Temperature of Two-Dimensional Semiconductors at Low Doping by the Electron-Electron Interaction

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