Electronlike Fermi Surface Sheets Research Articles

Externally Controlled Magnetism and Band Gap in Twisted Bilayer Graphene.

We theoretically study the effects of electron-electron interaction in twisted bilayer graphene in atransverse dc electric field. When the twist angle is not very small, the electronic spectrum of the bilayer consists of four Dirac cones inherited from each graphene layer. An applied bias voltage leads to the appearance of two holelike and two electronlike Fermi surface sheets with perfect nesting among electron and hole components. Such a band structure is unstable with respect to the exciton band-gap opening due to the screened Coulomb interaction. The exciton order parameter is accompanied by spin-density-wave order. The gap depends on the twist angle and can be varied by a bias voltage. This result correlates well with recent transport measurements [J.-B. Liu etal., Sci. Rep. 5, 15285 (2015)SRCEC32045-232210.1038/srep15285]. Our proposal allows the coexistence of (i)an externally controlled semiconducting gap and (ii)a nontrivial multicomponent magnetic order. This is interesting for both fundamental research and applications.

Stable Weyl points, trivial surface states, and particle-hole compensation in WP2

A possible connection between extremely large magneto-resistance and the presence of Weyl points has garnered much attention in the study of topological semimetals. Exploration of these concepts in transition metal phosphide WP2 has been complicated by conflicting experimental reports. Here we combine angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to disentangle surface and bulk contributions to the ARPES intensity, the superposition of which has plagued the determination of the electronic structure in WP2. Our results show that while the hole- and electron-like Fermi surface sheets originating from surface states have different areas, the bulk-band structure of WP2 is electron-hole-compensated in agreement with DFT. Furthermore, the detailed band structure is compatible with the presence of at least 4 temperature-independent Weyl points, confirming the topological nature of WP2 and its stability against lattice distortions.

Strain effects on electronic structures of monolayer iron sulphide and selenide

Structural and electronic properties of single atomic layers of FeS and FeSe superconductors have been investigated from first principles within the density functional theory (DFT). For both systems the mixed single-stripe and block-checkerboard type antiferromagnetic ground states are predicted. Namely, the single-stripe and block-checkerboard orders are almost degenerate. The metal-insulator transition in a hypothetical block-checkerboard phase of FeS occurs only under significant tensile strain, i.e. for in-plane lattice parameters larger than 3.8 Å. Compressive strain leads to almost a complete disappearance of magnetism in FeS system, whereas tensile strain strengthens it. The electronic structure of FeS monolayer in nonmagnetic phase is similar to that of FeSe, exhibiting the (π,π) nesting between hole- and electron-like Fermi surface sheets. The findings presented here explain different characters of superconductivity in FeS and FeSe materials and encourage further experimental studies on FeS-based thin films and heterostructures.

Electronic structure and magnetism of RbGd2Fe4As4O2

Abstract The electronic structure, magnetism, and Fermi surface (FS) nesting of new superconductor RbGd2Fe4As4O2 (12442) were studied by the first-principes calculation. Its calculated ground state is the stripe antiferromagnetic state. The hole-like FSs will overlap with the electron-like FS sheets by the electron doping. Such significant FS nesting induces a peak of the bare susceptibility χ 0 ( q → ) at the X point, suggesting the parent compound RbGd2Fe4As4O2 is a self hole-doping induced superconductor.

Electronic Structure and Magnetism of the Multiband New Superconductor CaRbFe4As4

By first-principles calculations, we investigated the electronic structure, magnetism, and Fermi surface (FS) nesting of the newly discovered superconductor CaRbFe4As4. In the nonmagnetic (NM) state, there are ten bands crossing the Fermi level, which is more complicated than other FeAs-based superconductors, showing a multiband character. The FS consists of six holelike sheets around the Γ-point and four electronlike sheets near the Brillouin zone corner M-point. The holelike FSs will overlap with the electronlike FS sheets, if they are shifted by the vector \(\boldsymbol{{q}} = (\pi ,\pi ,0)\). Such FS nesting induces the bare susceptibility peak \(\chi _{0}(\boldsymbol{{q}})\) at the M-point. Fixed spin moment calculations indicate that CaRbFe4As4 has a strong tendency towards magnetism. Total energy calculations predicted that the ground state of CaRbFe4As4 is the stripe antiferromagnetic state. Therefore, CaRbFe4As4 is strongly similar to other FeAs-based superconductors, although it crystallized wit...

Self-hole-doping–induced superconductivity in KCa2Fe4As4F2

The electronic structure, magnetism, Fermi surface nesting, and bare susceptibility of the superconductor KCa2Fe4As4F2 are discussed in relation to the Fe-pnictide superconductors based on the first-principle calculations. There are ten bands crossing the Fermi level in the nonmagnetic (NM) states, resulting in six hole-like Fermi surface (FS) sheets along the line and four electron-like FS sheets along the line. It is more complicated than other FeAs-based superconductors, showing multi-bands character. Fixed spin moment calculations and the total energies of different magnetic phase indicate that KCa2Fe4As4F2 has a strong tendency towards magnetism, i.e., the stripe antiferromagnetic state. But the self-hole-doping suppressed the spin-density wave state, and then induced superconductivity in the parent compound. Such self-doping strategy may be used for future exploration of new superconductors.

De Haas–van Alphen effect and Fermi surface properties of single-crystal CrB2

We report the angular dependence of three distinct de Haas-van Alphen (dHvA) frequencies of the torque magnetization in the itinerant antiferromagnet CrB2 at temperatures down to 0.3K and magnetic fields up to 14T. Comparison with the calculated Fermi surface of nonmagnetic CrB2 suggests that two of the observed dHvA oscillations arise from electron-like Fermi surface sheets formed by bands with strong B-px,y character which should be rather insensitive to exchange splitting. The measured effective masses of these Fermi surface sheets display strong enhancements of up to a factor of two over the calculated band masses which we attribute to electron-phonon coupling and electronic correlations. For the temperature and field range studied, we do not observe signatures reminiscent of the heavy d-electron bands expected for antiferromagnetic CrB2. In view that the B-p bands are at the heart of conventional high-temperature superconductivity in the isostructural MgB2, we consider possible implications of our findings for nonmagnetic CrB2 and an interplay of itinerant antiferromagnetism with superconductivity.

Small and nearly isotropic hole-like Fermi surfaces in LiFeAs detected through de Haas–van Alphen effect

LiFeAs is unique among the arsenic based Fe-pnictide superconductors because it is the only nearly stoichiometric compound which does not exhibit magnetic order. This is at odds with electronic structure calculations which find a very stable magnetic state and predict cylindrical hole- and electron-like Fermi surface sheets whose geometry suggests spin fluctuations and a possible instability toward long-range ordering at the nesting vector. In fact, a complex magnetic phase diagram is indeed observed in the isostructural NaFeAs compound. Previous angle-resolved photoemission (ARPES) experiments revealed the existence of both hole and electron-like surfaces, but with rather distinct cross-sectional areas and an absence of the nesting that is thought to underpin both magnetic order and superconductivity in the pnictide family of superconductors. These ARPES observations were challenged by subsequent de Haas--van Alphen (dHvA) measurements which detected a few, electron-like Fermi surface sheets in rough agreement with the original band calculations. Here, we show a detailed dHvA study unveiling additional, small and nearly isotropic Fermi surface sheets in LiFeAs single crystals, which ought to correspond to hole-like orbits, as previously observed by ARPES. Therefore, our results reconcile the apparent discrepancy between ARPES and the previous dHvA results. The small size of these Fermi surface pockets suggests a prominent role for the electronic correlations in LiFeAs. The absence of gap nodes, in combination with the coexistence of quasi-two-dimensional and three-dimensional Fermi surfaces, favor an $s$-wave pairing symmetry for LiFeAs. But similar electron-like Fermi surfaces combined with very different hole pockets between LiFeAs and LiFeP suggest that the nodes in the gap function of LiFeP might be located on the hole pockets. This would be difficult to reconcile with the current understanding of the $s\ifmmode\pm\else\textpm\fi{}$ scenario.

Weak magnetism and the Mott state of vanadium in superconducting Sr2VO3FeAs

We report neutron scattering data and DFT calculations of the stoichiometric iron-arsenide superconductor Sr2VO3FeAs. Rietveld refinements of neutron powder patterns confirm the ideal composition without oxygen deficiencies. Experiments with polarized neutrons prove weak magnetic ordering in the V-sublattice of Sr2VO3FeAs at ~ 45 K with a probable propagation vector q = (1/8,1/8,0). The ordered moment of ~ 0.1 muB is too small to remove the V-3d bands from the Fermi level by magnetic exchange splitting, and much smaller than predicted from a recent LDA+U study. By using DFT calculations with a GGA+EECE functional we recover the typical quasi-nested Fermi-surface even without magnetic moment. From this we suggest that the V-atoms are in a Mott-state where the electronic correlations are dominated by on-site Coulomb-repulsion which shifts the V-3d states away from the Fermi energy. Our results are consistent with photoemission data and clearly reveal that Sr2VO3FeAs is a typical iron-arsenide superconductor with quasi-nested hole- and electron-like Fermi surface sheets, and constitutes no new paradigm. We suggest that intrinsic electron-doping through V3+/V4+ mixed valence is responsible for the absence of SDW ordering.

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.

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.

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.