1,005 research outputs found
The origin of the condensation energy scaling of iron-based superconductors
No full textThe relation between the condensation energy (CE) and T-c of a phase transition reveals a fundamental nature of the transition. In view of this, the recent experimental observation of the non-BCS scaling relation of the CE vs. T-c (Delta E similar to T-c(3.5)) with about forty different samples of the Fe-based superconductors (Xing J. et al., Phys. Rev. B, 89 (2014) 140503) was intriguing and strongly hinted at a non-BCS pairing mechanism. In this paper, we have studied the CE and Tc of the multiband BCS model and found that the observed anomalous scaling relation Delta E similar to T-c(3.5) is well reproduced by the two-band BCS model paired by a dominant repulsive interband interaction (V-inter > V-intra > 0). Our result implies that this seemingly non-BCS-like scaling behavior of Delta E similar to T-c(3.5), on the contrary to the common expectations, is in fact a strong experimental evidence that the pairing mechanism of the Fe-based superconductors is genuinely a BCS mechanism, meaning that the Cooper pairs are formed by the itinerant carriers glued by a pairing interaction. Copyright (C) EPLA, 201611Nsciescopu
Correlation of Fe-Based Superconductivity and Electron-Phonon Coupling in an FeAs/Oxide Heterostructure
No full textInterfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have
received considerable attention due to the possibility of enhancing preexisting superconductivity. Using
scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph
interaction with interfacial phonons in an iron-based superconductor Sr2VO3FeAs (Tc ≈ 33 K) made
of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with
systematically different average superconducting gaps due to the e-ph coupling locally modulated by O
vacancies in the VO2 layer, and supporting self-consistent momentum-dependent Eliashberg calculations
provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total
superconducting pairing.1
Pairing Mechanism of the FeSe-monolayer and related Systems: Dynamical Tuning of Pairing Cutoff Energy
No full textThere are a group of FeSe systems: FeSe/SrTiO3 monolayer system (Tc~60-100K) and other heavily electron-doped iron selenide (HEDIS) compounds such as AxFe2-ySe2 (A=K, Rb, Cs, Tl, etc.) (Tc~30-40K), (Li1-xFexOH)FeSe (Tc~40K), etc. These systems have all very high Tc (30K -100K) despite having only the electron Fermi surfaces (FSs) but no hole FS.
Here we propose a unifying pairing mechanism [1] based on a new concept: dynamical tuning of pairing cutoff energy. With this mechanism, I show how the incipient band without a Fermi surface can participate pairing interaction through RG process and the system forms the s++-wave state only with the electron pockets. In this way, the HEDIS system can achieve the maximum Tc, stored in the system, and yet avoid the detrimental impurity pair-breaking scattering. Our theory not only provides a unifying pairing mechanism for all HEDIS system, but also naturally explains the puzzling double dome structure of the phase diagram of Tc versus tuning parameter (doping, pressure, etc) [2, 3].1
Quantum Phase Transition of Correlated Iron-Based Superconductivity in LiFe1-xCoxAs
No full textThe interplay between unconventional Cooper pairing and quantum states associated with atomic scale defects is a frontier of research with many open questions. So far, only a few of the high-temperature superconductors allow this intricate physics to be studied in a widely tunable way. We use scanning tunneling microscopy to image the electronic impact of Co atoms on the ground state of the LiFe1-xCoxAs system. We observe that impurities progressively suppress the global superconducting gap and introduce low energy states near the gap edge, with the superconductivity remaining in the strong-coupling limit. Unexpectedly, the fully opened gap evolves into a nodal state before the Cooper pair coherence is fully destroyed. Our systematic theoretical analysis shows that these new observations can be quantitatively understood by the nonmagnetic Born-limit scattering effect in an s +/--wave superconductor, unveiling the driving force of the superconductor to metal quantum phase transition.11Nsciescopu
Pairing mechanism of heavily electron doped FeSe systems: dynamical tuning of the pairing cutoff energy
No full textWe studied the pairing mechanism of the heavily electron doped FeSe (HEDIS) systems, which
commonly have one incipient hole band—a band top below the Fermi level by a finite energy distance
εb—at Γ point and ordinary electron bands at Mpoints in Brillouin zone (BZ).We found that the
system allows two degenerate superconducting solutions with the exactly same Tc in clean limit: the
incipient she-gap (D-h ¹ 0,D+e ¹ 0) and se+e+-gap (Δh=0,D+e ¹ 0) solutions with different pairing
cutoffs,Λsf (spin fluctuation energy) and εb, respectively. The se+e+-gap solution, in which the system
dynamically renormalizes the original pairing cutoffΛsf toΛphys=εb (<Λsf), therefore actively
eliminates the incipient hole band from forming Cooper pairs, but without loss of Tc, becomes
immune to the impurity pair-breaking. As a result, the HEDIS systems, by dynamically tuning the
pairing cutoff and therefore selecting the se+e+-pairing state, can always achieve themaximumTc—the
Tc of the degenerate she solution in the ideal clean limit—latent in the original pairing interactions,
even in the dirty limit.11Ysciescopu
Optical properties of optimally doped single-crystal Ca8.5La1.5(Pt3As8)(Fe2As2)5”
No full textWe have measured the reflectivity of the optimally doped Ca8.5La1.5(Pt3As8)(Fe10As10) single crystal (Tc =
32.8K) over the broad frequency range from 40 cm−1 to 12 000 cm−1 and for temperatures from 8 K to 300 K. The
optical conductivity spectra of the low-frequency region (<1000 cm−1) in the normal state (80 K <T 300 K)
is well fitted with two Drude forms, which indicates the presence of multiple bands at the Fermi level. Decreasing
temperature below 80 K, this low-frequency Drude spectra develops a pseudogap (PG) hump structure at around
100 cm−1 and continuously evolves into the fully opened superconducting (SC) gap structure belowTc . Theoretical
calculations of the optical conductivity with the preformed Cooper pair model provide an excellent description
of the temperature evolution of the PG structure above Tc into the SC gap structure below Tc. The extracted two
SC gap sizes are S = 4.9 meV and L = 14.2 meV, suggesting Ca8.5La1.5(Pt3As8)(Fe10As10) as a multiple-11Nsciescopu
Frustration-driven C 4 symmetric order in a naturally-heterostructured superconductor Sr2VO3FeAs
No full textA subtle balance between competing interactions in iron-based superconductors (FeSCs) can be tipped by additional interfacial interactions in a heterostructure, often inducing exotic phases with unprecedented properties. Particularly when the proximity-coupled layer is magnetically active, rich phase diagrams are expected in FeSCs, but this has not been explored yet. Here, using high-accuracy As-75 and V-51 nuclear magnetic resonance measurements, we investigate an electronic phase that emerges in the FeAs layer below T-O similar to 155 K of Sr2VO3FeAs, a naturally assembled heterostructure of an FeSC and a Mottinsulating vanadium oxide. We find that frustration of the otherwise dominant Fe stripe and V Neel fluctuations via interfacial coupling induces a charge/orbital order in the FeAs layers, without either static magnetism or broken C-4 symmetry, while suppressing the Neel antiferromagnetism in the SrVO3 layers. These findings demonstrate that the magnetic proximity coupling stabilizes a hidden order in FeSCs, which may also apply to other strongly correlated heterostructures.1
Superconducting properties of the s±-wave state: Fe-based superconductors
No full textAlthough the pairing mechanism of Fe-based superconductors (FeSCs) has not yet been
settled with consensus with regard to the pairing symmetry and the superconducting (SC)
gap function, the vast majority of experiments support the existence of spin-singlet signchanging
s-wave SC gaps on multi-bands (s±-wave state). This multi-band s±-wave state is a
very unique gap state per se and displays numerous unexpected novel SC properties, such as
a strong reduction of the coherence peak, non-trivial impurity effects, nodal-gap-like nuclear
magnetic resonance signals, various Volovik effects in the specific heat (SH) and thermal
conductivity, and anomalous scaling behaviors with a SH jump and condensation energy
versus Tc, etc. In particular, many of these non-trivial SC properties can easily be mistaken
as evidence for a nodal-gap state such as a d-wave gap. In this review, we provide detailed
explanations of the theoretical principles for the various non-trivial SC properties of the
s±-wave pairing state, and then critically compare the theoretical predictions with experiments
on FeSCs. This will provide a pedagogical overview of to what extent we can coherently
understand the wide range of different experiments on FeSCs within the s±-wave gap model.11Ysciescopu
Anomalous scaling of ΔC versus Tc in the Fe-based superconductors: the S+/-wave pairing state mode
No full textThe strong power law behavior of the specific heat jump Delta C versus T-c(Delta C/T-c similar to T-c(alpha), alpha approximate to 2), first observed by Bud'ko et al (2009 Phys. Rev. B 79 220516), has been confirmed with several families of the Fe-based superconducting compounds with various dopings. We have tested a minimal two band BCS model to understand this anomalous behavior and showed that this non-BCS relation between Delta C versus Tc is a generic property of the multiband superconducting state paired by a dominant interband interaction (V-inter > V-intra) reflecting the relation Delta(h)/Delta(e) similar to root N-e/N-h near T-c, as in the S-+/--wave pairing state. We also found that this Delta C versus T-c power law can continuously change from the ideal BNC scaling to a considerable deviation by a moderate variation of the impurity scattering rate Gamma(0) (non-pair-breaking). As a result, our model provides a consistent explanation why the electron-doped Fe-based superconductors follow the ideal BNC scaling very well while the hole-doped systems often show varying degree of deviations.11Nsciescopu
Effect of strain-induced orbital splitting on the magnetic excitations in undoped cuprates
No full textWe investigate the magnetic excitations in view of the recent reports suggesting that the spin-wave energy may exhibit a significant dependence on the in-plane strain of a thin film of La2CuO4. The nature of dependence, as we find, can be explained naturally within a two-orbital model based on the d(x2)(-y2) and d(3z)(2-r2) orbitals. In particular, as the orbital-splitting energy between the d(x2)(-y2) and d(3z2-)(r2) orbitals increases with compressive strain, the zone-boundary spin-wave energy hardens. However, the hardening persists only until the orbital splitting reaches similar to 2 eV, beyond which there is no significant change. The behavior of zone-boundary spin-wave energy is explained in terms of the extent of hybridization between one of the exchange-split d(x2)(-y2) bands which is nearly half-filled and the d(3z2-)(r2) band. The role of second order antiferromagnetic superexchange process involving the interorbital hopping is also discussed.11Nsciescopu
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