Electron-like Fermi Surface Research Articles

Electronic structure of Ca10(Pt4As8)(Fe2−xPtxAs2)5with metallic Pt4As8layers: An angle-resolved photoemission spectroscopy study

We studied the low-lying electronic structure of the newly discovered iron-platinum-arsenide superconductor, Ca${}_{10}$(Pt${}_{4}$As${}_{8}$)(Fe${}_{2\ensuremath{-}x}$Pt${}_{x}$As${}_{2}$)${}_{5}$ (${T}_{c}=22$ K) with angle-resolved photoemission spectroscopy. We found that the Pt${}_{4}$As${}_{8}$ layer contributes to a small electronlike Fermi surface, indicative of metallic charge reservoir layers that are rare for iron based superconductors. Moreover, the electronic structure of the FeAs layers resembles those of other prototype iron pnictides to a large extent. However, there is only ${d}_{xy}$-orbital originated holelike Fermi surface near the zone center, which is rather strange for an iron pnictide superconductor with relatively high superconducting transition temperature; and the ${d}_{xz}$ and ${d}_{yz}$ originated bands are not degenerate at the zone center. Furthermore, all bands near the Fermi energy show negligible ${k}_{z}$ dependence, indicating the strong two-dimensional nature of this material. Our results indicate this material possesses a rather interesting electronic structure, which enriches our current knowledge of iron based superconductors.

Transport anomalies due to anisotropic interband scattering

Unexpected transport behavior can arise due to anisotropic single-particle scattering in multiband systems. Specifically, we show within a semiclassical Boltzmann approach beyond the relaxation-time approximation that anisotropic scattering between electronlike and holelike Fermi surfaces generically leads to negative transport times, which in turn cause negative magnetoresistance, an extremum in the Hall coefficient, and a reduction of the resistivity. The anisotropy required for this to occur decreases with increasing mismatch between the Fermi-surface radii.

Nodal gap structure in Fe-based superconductors due to the competition between orbital and spin fluctuations

To understand the origin of the nodal gap structure realized in BaFe$_2$(As,P)$_2$, we study the three-dimensional gap structure based on the three-dimensional ten-orbital Hubbard model with quadrupole interaction. In this model, strong spin and orbital fluctuations develop by using the random-phase-approximation. By solving the Eliashberg gap equation, we obtain the fully-gapped s-wave state with (without) sign reversal between hole-like and electron-like Fermi surfaces due to strong spin (orbital) fluctuations, so called the $s_\pm$-wave ($s_{++}$-wave) state. When both spin and orbital fluctuations strongly develop, which will be realized near the orthorhombic phase, we obtain the nodal s-wave state in the crossover region between $s_{++}$-wave and $s_\pm$-wave states. The obtained nodal s-wave state possesses the loop-shape nodes on electron-like Fermi surfaces, due to the competition between attractive and repulsive interactions in k-space. In contrast, the SC gaps on the hole-like Fermi surfaces are fully-gapped due to orbital fluctuations. The present study explains the main characters of the anisotropic gap structure in BaFe$_2$(As,P)$_2$ observed experimentally.

The substitution effects on electronic structure of iron selenide superconductors

The influence of a partial substitution with S, Te, Co, Ni and Cu atoms on the electronic structure of the FeSe superconductor has been investigated within the density functional theory. The results of the supercell calculations reveal distinct changes of electronic structures of the substituted FeSe systems, which can be responsible for their superconducting properties. The replacement of Se atoms by Te or S ones yields imperfect nesting between the holelike and electronlike Fermi surface (FS) sheets, which enhances magnetic fluctuations responsible for superconducting pairing, thus leading to higher values of the superconducting critical temperatures. Meanwhile, the substitutions with transition-metal atoms for iron sites make more substantial changes of the FSs topology, since the holelike cylinders shrink at the cost of an enlargement of the electronlike ones. Thus, the superconducting pairing, driven by the nesting between these sheets, weakens and superconductivity disappears for a small percentage of dopants. The results support the idea of spin-fluctuation mediated superconductivity in iron chalcogenides.

Electronic structure of vacancy-ordered iron-selenide K0.5Fe1.75Se2

The electronic structure of the vacancy-ordered K$_{0.5}$Fe$_{1.75}$Se$_2$ iron-selenide compound (278 phase) is studied using the first-principles density functional method. The ground state of the 278 phase is stripe-like antiferromagnetic, and its bare electron susceptibility shows a large peak around $(\pi, \pi)$ in the folded Brillouin zone. Near Fermi level, the density of states are dominated by the Fe-3d orbitals, and both electron-like and hole-like Fermi surfaces appear in the Brillouin zone. Unfolded band structure shows limited similarities to a hole doped 122 phase. With 0.1e electron doping, the susceptibility peak is quickly suppressed and broadened; while the two-dimensionality of the electron-like Fermi surfaces are greatly enhanced, resulting in a better nesting behavior. Our study should be relevant to the recently reported superconducting phase K$_{0.5+x}$Fe$_{1.75+y}$Se$_2$ with both $x$ and $y$ very tiny.

Band structure and Fermi surface of UPd3studied by soft x-ray angle-resolved photoemission spectroscopy

Soft x-ray angle-resolved photoemission experiments have been performed to investigate the band structure and Fermi surface of UPd${}_{3}$. We have observed three U 5$f$ derived bands whose bandwidths are very narrow. Two of these bands appear around 0.8 eV and are slightly separated by about 0.2 eV. The presence of two bands at nearly the same energy might be due to the presence of the two inequivalent U sites. The other U 5$f$ band appears around 1.8 eV. The energy separation between this band and the two nearly degenerate bands agrees with the 5${f}^{1}$-final-state multiplet model. In the vicinity of Fermi level, several highly dispersive bands, which form a holelike Fermi surface centered at the $\ensuremath{\Gamma}$ point and an electronlike Fermi surface at the K point, were observed. These dispersive valence bands are qualitatively explained by a band structure calculation that assumes a localized 5${f}^{2}$ configuration. All these experimental observations are consistent with the localized U 5$f$ nature.

High-resolution angle-resolved photoemission study of electronic structure and electron self-energy in palladium

In this study, we investigate the electronic structure and electron self-energy of palladium single crystals using polarization-dependent high-resolution angle-resolved photoemission spectroscopy. The observed Fermi surfaces and energy-band dispersions agree with those given by the band-structure calculation. A detailed comparison between the observed and theoretical band dispersions of the ${\ensuremath{\Sigma}}_{1}$ band forming the electronlike Fermi surface indicates an electron-electron coupling parameter of ${\ensuremath{\lambda}}_{ee}\ensuremath{\sim}0.02$. Near the Fermi level, a kink structure in the energy-band dispersion exists at $\ensuremath{\sim}\ensuremath{-}20$ meV, in agreement with the Debye energy. The electron-phonon coupling parameter is estimated to be ${\ensuremath{\lambda}}_{ep}=0.39\ifmmode\pm\else\textpm\fi{}0.05$ at 8 K for the ${\ensuremath{\Sigma}}_{1}$ band, which is consistent with the theoretical values of ${\ensuremath{\lambda}}_{ep}=0.35--0.41$. Furthermore, analyses of the self-energy indicate a possible contribution from the electron-paramagnon interaction in the energy range of $\ensuremath{-}50\ensuremath{\sim}\ensuremath{-}150$ meV. The evaluated electron-paramagnon coupling parameter is ${\ensuremath{\lambda}}_{em}\ensuremath{\sim}0.06$ for the ${\ensuremath{\Sigma}}_{1}$ band. We found that the magnitudes of ${\ensuremath{\lambda}}_{ep}$ and ${\ensuremath{\lambda}}_{em}$ depend on the Fermi surface points. The total effective mass enhancement factor is estimated to be $1+{\ensuremath{\lambda}}_{ep}+{\ensuremath{\lambda}}_{ee}+{\ensuremath{\lambda}}_{em}\ensuremath{\sim}1.5$ for the ${\ensuremath{\Sigma}}_{1}$ band, which is close to the values ${m}^{*}/{m}_{b}\ensuremath{\sim}1.5--1.7$ given by the de Haas--van Alphen and electron specific-heat measurements.

Orbital characters and near two-dimensionality of Fermi surfaces in NaFe1−xCoxAs

We report a comprehensive study of orbital characters and tridimensional nature of the electronic bands of 111-family in Fe-pnictides superconductors, NaFe1−xCoxAs (x = 0 and 0.05), with angle-resolved photoemission spectroscopy. We determined the orbital characters and the kz dependence of the low-energy electronic structures by tuning the polarization and the energy of the incident photons. We observed two nearly two-dimensional hole-like Fermi surfaces (FS) near the Brillouin zone (BZ) center and two electron-like FS near BZ corner. The bands near the Fermi level (EF) are mainly derived from the Fe 3dxy, 3dyz, and 3dzx orbitals.

Anomalous isotope effect near a 2.5 Lifshitz transition in a multi-band multi-condensate superconductor made of a superlattice of stripes

The doping dependent isotope effect on the critical temperature (Tc) is calculated for multi-band multi-condensate superconductivity near a 2.5 Lifshitz transition. We consider a superlattice of quantum stripes with finite hopping between stripes near a 2.5 Lifshitz transition for the appearance of a new sub-band making a circular electron-like Fermi surface pocket. We describe a particular type of BEC (Bose–Einstein Condensate) to BCS (Bardeen–Cooper–Schrieffer condensate) crossover in multi-band/multi-condensate superconductivity at a metal-to-metal transition that is quite different from the standard BEC–BCS crossover at an insulator-to-metal transition. The results show that the isotope coefficient strongly deviates from the standard BCS value 0.5, when the chemical potential is tuned at the 2.5 Lifshitz transition for the metal-to-metal transition. The critical temperature Tc shows a minimum due to the Fano antiresonance in the superconducting gaps and the isotope coefficient diverges at the point where a BEC coexists with a BCS condensate. In contrast Tc reaches its maximum and the isotope coefficient vanishes at the crossover from a polaronic condensate to a BCS condensate in the newly appearing sub-band.

Electron-like Fermi surface and in-plane anisotropy due to chain states in YBa2Cu3O7−δsuperconductors

We present magneto-transport calculations for YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) materials to show that the electron-like metallic chain state gives both the negative Hall effect and in-plane anisotropic large Nernst signal. We show that the inevitable presence of the metallic 1D CuO chain layer lying between the CuO2 bilayers in YBCO renders an electron-like Fermi surface in the doping range as wide as p=0.05 to overdoping. With underdoping, a pseudogap opening in the CuO2 state reduces its hole-carrier contribution, and therefore the net electron-like quasiparticles dominate the transport properties, and a negative Hall resistance commences. We also show that the observation of in-plane anisotropy in the Nernst signal - which was taken as a definite evidence of the electronic `nematic' pseudogap phase - is naturally explained by including the `quasi-uniaxial' metallic chain state. Finally, we comment on how the chain state can also lead to electron-like quantum oscillations.

Point-contact spectroscopy in Co-doped CaFe2As2: nodal superconductivity and topological Fermi surface transition

We performed point-contact Andreev-reflection spectroscopy measurements in Ca(Fe1−xCox)2As2 single crystals with effective x = 0.060 ± 0.005. The spectra of ab-plane contacts show a zero-bias maximum and broad shoulders at about 5–6 meV. Their fit with the three-dimensional Blonder–Tinkham–Klapwijk (BTK) model (making use of an analytical expression for the Fermi surface that mimics the one calculated from first principles) shows that this compound presents a large isotropic gap on the quasi-2D electronlike Fermi surface sheets and a smaller anisotropic (possibly nodal) gap on the 3D holelike Fermi surface pockets centered at the Z point in the Brillouin zone. These results nicely fit into the theoretical picture for the appearance of nodal superconductivity in 122 compounds.

A three-dimensional tight-binding model and magnetic instability of iron selenide [formula omitted

For a newly discovered iron-based high Tc superconducting parent material KFe2Se2, we present an effective three-dimensional five-orbital tight-binding model by fitting the band structures. The three t2g-symmetry orbitals of the five Fe 3d orbitals mainly contribute to the electron-like Fermi surface, which has a three-dimensional character, in agreement with recent angle-resolved photoemission spectroscopy experiments. To understand the groundstate magnetic properties, the two- and three-dimensional spin and charge susceptibilities within the random phase approximation are investigated. It obviously shows a sharp peak at wave vector q∼(π,π), indicating the magnetic instability of Néel-type antiferromagnetic rather than (π/2, π/2)-type or stripe (π, 0)-type antiferromagnetic ordering, while it exhibits a ferromagnetic instability between the nearest neighboring FeSe layers along c axis. The three-dimensional spin fluctuation may play an important role in pairing symmetry in superconducting materials AyFe2−xSe2(A=Rb,Cs,K, etc.).

Volovik effect in a highly anisotropic multiband superconductor: Experiment and theory

We present measurements of the specific heat coefficient \gamma(= C/T) in the low temperature limit as a function of an applied magnetic field for the Fe-based superconductor BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$. We find both a linear regime at higher fields and a limiting square root $H$ behavior at very low fields. The crossover from a Volovik-like $\sqrt{H}$ to a linear field dependence can be understood from a multiband calculation in the quasiclassical approximation assuming gaps with different momentum dependence on the hole- and electron-like Fermi surface sheets.

Role of degeneracy, hybridization, and nesting in the properties of multiorbital systems

To understand the role that degeneracy, hybridization, and nesting play in the magnetic and pairing properties of multiorbital Hubbard models we here study numerically two types of two-orbital models, both with holelike and electron-like Fermi surfaces (FS's) that are related by nesting vectors $(\ensuremath{\pi},0)$ and $(0,\ensuremath{\pi})$. In one case the bands that determine the FS's arise from strongly hybridized degenerate ${d}_{xz}$ and ${d}_{yz}$ orbitals, while in the other the two bands are determined by nondegenerate and nonhybridized $s$-like orbitals. Using a variety of techniques, in the weak-coupling regime it is shown that only the model with hybridized bands develops metallic magnetic order, while the other model exhibits an ordered excitonic orbital-transverse spin state that is insulating and does not have a local magnetization. However, both models display similar insulating magnetic stripe ordering in the strong-coupling limit. These results indicate that nesting is a necessary but not sufficient condition for the development of ordered states with finite local magnetization in multiorbital Hubbard systems; the additional ingredient appears to be that the nested portions of the bands need to have the same orbital flavor. This condition can be achieved via strong hybridization of the orbitals in weak coupling or via the FS reconstruction induced by the Coulomb interactions in the strong-coupling regime. This effect also affects the pairing symmetry as demonstrated by the study of the dominant pairing channels for the two models.

Unconventional superconducting gap inNaFe0.95Co0.05Asobserved by angle-resolved photoemission spectroscopy

We have performed high resolution angle-resolved photoemission measurements on superconducting electron-doped NaFe$_{0.95}$Co$_{0.05}$As ($T_{c}\sim$18 K). We observed a hole-like Fermi surface around the zone center and two electron-like Fermi surfaces around the M point which can be connected by the $Q=(\pi, \pi)$ wavevector, suggesting that scattering over the near-nested Fermi surfaces is important to the superconductivity of this "111" pnicitide. Nearly isotropic superconducting gaps with sharp coherent peaks are observed below $T_c$ on all three Fermi surfaces. Upon increasing temperature through $T_c$, the gap size shows little change while the coherence vanishes. Large ratios of $2\Delta/k_{B}T_{c}\sim8$ are observed for all the bands, indicating a strong coupling in this system. These results are not expected from a classical phonon-mediated pairing mechanism.

Absence of a Holelike Fermi Surface for the Iron-BasedK0.8Fe1.7Se2Superconductor Revealed by Angle-Resolved Photoemission Spectroscopy

We have performed an angle-resolved photoemission spectroscopy study of the new iron-based superconductor K(0.8)Fe(1.7)Se(2) (T(c)∼30 K). Clear band dispersion is observed with the overall bandwidth renormalized by a factor of 2.5 compared to our local density approximation calculations, indicating relatively strong correlation effects. Only an electronlike band crosses the Fermi energy, forming a nearly circular Fermi surface (FS) at M (π, 0). The holelike band at Γ sinks ∼90 meV below the Fermi energy, with an indirect band gap of 30 meV, to the bottom of the electronlike band. The observed FS topology in this superconductor favors (π, π) inter-FS scattering between the electronlike FSs at the M points, in sharp contrast to other iron-based superconductors which favor (π, 0) inter-FS scattering between holelike and electronlike FSs.

Common Fermi-surface topology and nodeless superconducting gap ofK0.68Fe1.79Se2and (Tl0.45K0.34)Fe1.84Se2superconductors revealed via angle-resolved photoemission

We carried out high resolution angle-resolved photoemission measurements on the electronic structure and superconducting gap of K_0.68Fe_1.79Se_2 (T_c=32 K) and (Tl_0.45K_0.34)Fe_1.84Se_2 (T_c=28 K) superconductors. In addition to the electron-like Fermi surface near M(\pi,\pi), two electron-like Fermi pockets are revealed around the zone center \Gamma(0,0) in K0.68Fe1.79Se_2. This observation makes the Fermi surface topology of K_0.68Fe_1.79Se_2 consistent with that of (Tl,Rb)_xFe_{2-y}Se_2 and (Tl,K)_xFe_{2-y}Se_2 compounds. A nearly isotropic superconducting gap (\Delta) is observed along the electron-like Fermi pocket near the M point in K_0.68Fe_1.79Se_2 (\Delta\sim 9 meV) and (Tl_0.45K_0.34)Fe_1.84Se_2 (\Delta\sim 8 meV). The establishment of a universal picture on the Fermi surface topology and superconducting gap in the A_xFe_2-ySe_2 (A=K, Tl, Cs, Rb and etc.) superconductors will provide important information in understanding the superconductivity mechanism of the iron-based superconductors.

Electronic structure of optimally doped pnictideBa0.6K0.4Fe2As2: a comprehensive angle-resolved photoemission spectroscopy investigation

The electronic structure of the Fe-based superconductorBa0.6K0.4Fe2As2 is studied by means of angle-resolved photoemission. We identify dispersive bandscrossing the Fermi level forming hole-like (electron-like) Fermi surfaces (FSs) aroundΓ (M) with nearly nested FS pockets connected by the antiferromagnetic wavevector.Compared to band structure calculation findings, the overall bandwidth is reduced by afactor of 2 and the low energy dispersions display even stronger mass renormalization.Using an effective tight banding model, we fitted the band structure and the FSs to obtainband parameters reliable for theoretical modeling and calculation of physical quantities.

Distinct Fermi Surface Topology and Nodeless Superconducting Gap in a(Tl0.58Rb0.42)Fe1.72Se2Superconductor

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the (Tl0.58Rb0.42)Fe1.72Se2 superconductor with a T(c) = 32 K. The Fermi surface topology consists of two electronlike Fermi surface sheets around the Γ point which is distinct from that in all other iron-based superconductors reported so far. The Fermi surface around the M point shows a nearly isotropic superconducting gap of ∼12 meV. The large Fermi surface near the Γ point also shows a nearly isotropic superconducting gap of ∼15 meV, while no superconducting gap opening is clearly observed for the inner tiny Fermi surface. Our observed new Fermi surface topology and its associated superconducting gap will provide key insights and constraints into the understanding of the superconductivity mechanism in iron-based superconductors.

Disorder-induced magneto-oscillations in bilayer graphene at high bias

The energy spectrum of biased bilayer graphene near the bottom has a ``Mexican-hat''-like shape. For the Fermi level within the Mexican hat, we predict that, apart from conventional magneto-oscillations that vanish with temperature, there are additional magneto-oscillations that are weakly sensitive to temperature. These oscillations are also insensitive to a long-range disorder. Their period in magnetic field scales with bias $V$ as ${V}^{2}$. The origin of these oscillations is the disorder-induced scattering between electronlike and holelike Fermi surfaces, specific to the Mexican hat.