Hole-like Fermi Surface Research Articles

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.

Fermiology of the strongly spin-orbit coupled superconductor Sn(1-x)In(x)Te: implications for topological superconductivity.

We have performed angle-resolved photoemission spectroscopy on the strongly spin-orbit coupled low-carrier density superconductor Sn(1-x)In(x)Te (x = 0.045) to elucidate the electronic states relevant to the possible occurrence of topological superconductivity, as recently reported for this compound based on point-contact spectroscopy. The obtained energy-band structure reveals a small holelike Fermi surface centered at the L point of the bulk Brillouin zone, together with a signature of a topological surface state, indicating that this material is a doped topological crystalline insulator characterized by band inversion and mirror symmetry. A comparison of the electronic states with a band-noninverted superconductor possessing a similar Fermi surface structure, Pb(1-x)Tl(x)Te, suggests that the anomalous behavior in the superconducting state of Sn(1-x)In(x)Te is related to the peculiar orbital characteristics of the bulk valence band and/or the presence of a topological surface state.

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.

Discerning electronic fingerprints of nodal and antinodal nestings and their phase coherences in doped cuprate superconductors

The complexity of competing orders in cuprates has recently been multiplied by a number of bulk evidences of charge ordering with wave vector that connects the antinodal region of the Fermi surface. This result contradicts many spectroscopic results of the nodal nesting. To resolve this issue, we carry out a unified study of the resulting electronic fingerprints of both nodal and antinodal nestings (NNs/ANs) and compare with angle-resolved photoemission, scanning tunneling spectroscopic data, as well as bulk-sensitive Hall-effect measurements. Our result makes several definitive distinctions between them in that while both nestings gap out the antinodal region, AN induces an additional quasiparticle gap below the Fermi level along the nodal direction, which is so far uncharted in spectroscopic data. Furthermore, we show that the Hall coefficient in the AN state obtains a discontinuous jump at the phase transition from an electronlike nodal pocket (negative value) to a large holelike Fermi surface (positive value), in contrast to a continuous transition in the available data. We conclude that individual NNs and ANs have difficulties in explaining all of the data. In this spirit, we study a possibility of coexisting NN and AN phases within a Ginsburg-Landau functional formalism. An interesting possibility of disorder pinned ``chiral'' charge ordering is finally discussed.

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.

From quantum oscillations to charge order in high-Tc copper oxides in high magnetic fields

This article constitutes an update made of numerous elements from an article by Vignolle et al. [C. R. Phys. 12 (2011) 446] published in the issue of C. R. Physique dedicated to superconductivity. By including this article to the present issue on physics in high magnetic field, we have aimed, in agreement with the editorial board of the review, offering a complete issue and also reporting on the last developments in the study of superconductors in high field. We review how experiments in very high magnetic fields over the last five years have given a new twist to the understanding of the normal state of hole-doped cuprate superconductors. The discovery of quantum oscillations in underdoped YBa2Cu3Oy and overdoped Tl2Ba2CuO6 + δ has proven the existence of a Fermi surface across the whole phase diagram, which had been a controversial issue for more than twenty years. However, the striking difference in oscillation frequency for the two compounds has revealed a very different Fermi surface topology. The observation of negative Hall and Seebeck coefficients in the underdoped materials has shown that the large hole-like Fermi surface of overdoped materials undergoes a reconstruction in the high field and low temperature limits for which quantum oscillation can be observed. This has been interpreted as evidence for a translational symmetry breaking due to some form of electronic (spin, charge, or orbital current) order. The angular dependence of the quantum oscillations has constrained the source of the Fermi-surface reconstruction to something other than a spin-density wave with moments perpendicular to the field. Finally, nuclear magnetic resonance studies have revealed that it is actually charge order, without spin order, which is induced in the copper oxide planes as soon as superconductivity is sufficiently weakened by the magnetic field. The results suggest that there is a generic competition between superconductivity and a charge-density-wave instability in high Tc cuprates.

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.

Effects of Ru substitution on electron correlations and Fermi-surface dimensionality in Ba(Fe1−xRux)2As2

We report a systematic angle-resolved photoemission spectroscopy study on Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ for a wide range of Ru concentrations (0.15 $\leq$ \emph{x} $\leq$ 0.74). We observed a crossover from two-dimension to three-dimension for some of the hole-like Fermi surfaces with Ru substitution and a large reduction in the mass renormalization close to optimal doping. These results suggest that isovalent Ru substitution has remarkable effects on the low-energy electron excitations, which are important for the evolution of superconductivity and antiferromagnetism in this system.

Neutron scattering studies of spin excitations in superconducting Rb0.82Fe1.68Se2

We use inelastic neutron scattering to show that superconducting (SC) rubidium iron selenide Rb0.82Fe1.68Se2 exhibits antiferromagnetic (AF) spin excitations near the in-plane wave vector Q = (PI, 0) identical to that for iron arsenide superconductors. Moreover, we find that these excitations change from incommensurate to commensurate with increasing energy, and occur at the expense of spin waves associated with the coexisting sqrt(5)\timessqrt(5) block AF phase. Since angle resolved photoemission experiments reveal no evidence for hole-like Fermi surface at Gamma(0, 0), our results suggest that the Q = (PI, 0) excitations in SC Rb0.82Fe1.68Se2 come from localized moments and may have a similar origin as the hourglass-like spin excitations in copper oxide superconductors.

Properties of edge states in a spin-triplet two-band superconductor

Motivated by Sr2RuO4 the magnetic properties of edge states in a two-band spin-triplet superconductor with electron- and hole-like Fermi surfaces are investigated assuming chiral p-wave pairing symmetry. The two bands correspond to the alpha-beta-bands of Sr2RuO4 and are modeled within a tight-binding model including inter-orbital hybridization and spin-orbit coupling effects. Including superconductivity the quasiparticle spectrum is determined by means of a self-consistent Bogolyubov-de Gennes calculation. While a full quasiparticle excitation gap appears in the bulk, gapless states form at the edges which produce spontaneous spin and/or charge currents. The spin current is the result of the specific band structure while the charge current originates from the superconducting condensate. Together they induce a small spin polarization at the edge. Furthermore onsite Coulomb repulsion is included to show that the edge states are unstable against the formation of a Stoner-like spin polarization of the edge states. Through spin-orbit coupling the current- and the correlation-induced magnetism are coupled to the orientation of the chirality of the superconducting condensate. We speculate that this type of phenomenon could yield a compensation of the magnetic fields induced by currents and also explain the negative result in the recent experimental search for chiral edge currents.

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.

Iron based superconductors: Pnictides versus chalcogenides

We present a brief review of the present day situation with studies of high-temperature superconductivity in iron pnictides and chalcogenides. Recent discovery of superconductivity with Tc>30K in AxFe2−x/2Se2 (A=K, Cs, Tl, etc) represents the major new step in the development of new concepts in the physics of Fe-based high-temperature superconductors. We compare LDA and ARPES data on the band structure and Fermi surfaces of novel superconductors and those of the previously studied FeAs superconductors, especially isostructural 122-superconductors like BaFe2As2. It appears that electronic structure of new superconductors is rather different from that of FeAs 122-systems. In particular, no nesting properties of electron and hole-like Fermi surfaces is observed, casting doubts on most popular theoretical schemes of Cooper pairing for these systems. Doping of novel materials is extremely important as a number of topological transitions of Fermi surface near the Γ point in the Brillouin zone are observed for different doping levels. The discovery of Fe vacancies ordering and antiferromagnetic (AFM) ordering at pretty high temperatures (TN>500K), much exceeding superconducting Tc makes these systems unique antiferromagnetic superconductors with highest TN observed up to now. This poses very difficult problems for theoretical understanding of superconductivity. We discuss the role of both vacancies and AFM ordering in transformations of band structure and Fermi surfaces, as well as their importance for superconductivity. In particular, we show that system remains metallic with unfolded Fermi surfaces quite similar to that in paramagnetic state. Superconducting transition temperature Tc of new superconductors is discussed within the general picture of superconductivity in multiple band systems. It is demonstrated that both in FeAs-superconductors and in new FeSe-systems the value of Tc correlates with the value of the total density of states (DOSs) at the Fermi level.

First-Principles Electronic Structure of Superconductor Ca4Al2O6Fe2P2: Comparison with LaFePO and Ca4Al2O6Fe2As2

We investigate the electronic structures of iron-based superconductors having perovskite-like blocking layers, %Ca$_4$Al$_2$O$_6$Fe$_2$(As$_{1-x}$P$_x$)$_2$ from first principles. Ca$_4$Al$_2$O$_6$Fe$_2$P$_2$ and Ca$_4$Al$_2$O$_6$Fe$_2$As$_2$ from first principles. Ca$_4$Al$_2$O$_6$Fe$_2$P$_2$ is found to have two hole-like Fermi surfaces around $\Gamma$, and one hole-like Fermi surface around M in the unfolded Brillouin zone. This is in contrast with LaFePO, where no Fermi surface is found around M. The relationship of their band structures and measured transition temperatures of superconductivity is discussed. The number of Fermi surfaces in Ca$_4$Al$_2$O$_6$Fe$_2$P$_2$ is also different from that of Ca$_4$Al$_2$O$_6$Fe$_2$As$_2$, in which only one Fermi surface is formed around $\Gamma$. Analysis using maximally localized Wannier functions clarifies that the differences between their band structures originate mainly from the pnictogen height. We then analyze the alloying effect on the electronic structure of Ca$_4$Al$_2$O$_6$Fe$_2$AsP. It is found that its electronic structure is similar to that of Ca$_4$Al$_2$O$_6$Fe$_2$P$_2$ and Ca$_4$Al$_2$O$_6$Fe$_2$As$_2$ with the average crystal structure, though Ca$_4$Al$_2$O$_6$Fe$_2$AsP contains the pnictogen height disorder. We calculate the generalized susceptibility for Ca$_4$Al$_2$O$_6$Fe$_2$(As$_{1-x}$P$_x$)$_2$ and clarify the factors determining its tendency.

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.

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.

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.

Electronic Structure of Novel Superconductor Ca4Al2O6Fe2As2

We have performed the first-principles electronic structure calculation for the novel superconductor Ca 4 Al 2 O 6 Fe 2 As 2 which has the smallest a lattice parameter and the largest As height from the Fe plane among the Fe–As superconductors. We find that one of the hole-like Fermi surfaces is missing around the Γ point compared to the case of LaFeAsO. Analysis using the maximally-localized-Wannier-function technique indicates that the x y orbital becomes more localized as the As–Fe–As angle decreases. This induces rearrangement of bands, which results in the change of the Fermi-surface topology of Ca 4 Al 2 O 6 Fe 2 As 2 from that of LaFeAsO. The strength of electron correlation is also evaluated using the constraint RPA method, and it turns out that Ca 4 Al 2 O 6 Fe 2 As 2 is more correlated than LaFeAsO.

Angle-Resolved Photoemission Spectroscopy of the Iron-Chalcogenide SuperconductorFe1.03Te0.7Se0.3: Strong Coupling Behavior and the Universality of Interband Scattering

We have performed angle-resolved photoemission spectroscopy of the iron-chalcogenide superconductor Fe1.03Te0.7Se0.3 to investigate the electronic structure relevant to superconductivity. We observed a holelike Fermi surface (FS) and an electronlike FS at the Brillouin zone center and corner, respectively, which are nearly nested by the Q∼(π,π) wave vector. We do not find evidence for the nesting instability with Q∼(π+δ,0) reminiscent of the antiferromagnetic order in the parent compound Fe1+yTe. We have observed an isotropic superconducting (SC) gap along the holelike FS with the gap size Δ of ∼4 meV (2Δ/kBTc ∼ 7), demonstrating the strong-coupling superconductivity. The observed similarity of low-energy electronic excitations between iron-chalcogenides and iron-arsenides strongly suggests that common interactions which involve Q∼(π,π) scattering are responsible for the SC pairing.

Three-dimensional electronic structure in highly doped NaxCoO2 studied by angle-resolved photoemission spectroscopy

We have investigated three-dimensional electronic structure for NaxCoO2 (x=0.77 and 0.65) by high-resolution angle-resolved photoemission spectroscopy to study the origin of antiferromagnetic (AF) transition of highly doped NaxCoO2(x>0.75). The a1g large hole-like Fermi surface (FS) in x=0.77 shows distinct three-dimensionality along the kz direction, and a three-dimensional small electron pocket appears around Γ point, indicating strong inter-layer electronic correlation. On the other hand, x=0.65 sample does not show three-dimensional behavior. This result indicates that transition of FS as a function of band filling is closely related to the occurrence of the magnetic transition in highly doped NaxCoO2.