Fermi surface of Ba1−xKxFe2As2 as probed by angle-resolved photoemission

Here we apply high resolution angle-resolved photoemission spectroscopy (ARPES) using a wide excitation energy range to probe the electronic structure and the Fermi surface topology of the Ba_1?xK_xFe₂As₂ (T_c = 32 K) superconductor. We find significant deviations in the low energy band structure from that predicted in calculations. A set of Fermi surface sheets with unexpected topology is detected at the Brillouin zone boundary. At the X-symmetry point the Fermi surface is formed by a shallow electron-like pocket surrounded by four hole-like pockets elongated in Γ?X and Γ?Y directions.     

Superconductivity in a simple model of the pseudogap state

An analysis is made of characteristics of the superconducting state (s-and d-pairing) using a simple, exactly solvable model of the pseudogap state produced by fluctuations of the short-range order (such as antiferromagnetic) based on a Fermi surface model with “hot” sections. It is shown that the superconducting gap averaged over these fluctuations is nonzero at temperatures higher than the mean-field superconducting transition temperature T _c over the entire sample. At temperatures T > T _c superconductivity evidently exists in isolated sections (“ drops”). Studies are made of the spectral density and the density of states in which superconducting characteristics exist in the range T > T _c however, in this sense the temperature T = T _c itself is no different in any way. These anomalies show qualitative agreement with various experiments using underdoped high-temperature superconducting cuprates.     

Superconductivity in W5SiB2 with the T2-phase structure studied by the specific heat measurement and band structure calculation

The superconducting state of W₅SiB₂ with the T₂-phase structure has been investigated by a specific heat measurement and a density of state calculation. The estimated ΔC_p/γT_c and 2Δ(0)/k_BT_c values are 1.49 and 3.32, which are close to 1.43 and 3.53 within the weak coupling regime. The electronic specific heat data clearly indicates the absence of gap nodes in the superconducting order parameter, suggesting an isotropic s-wave superconductor. From the density of state calculations, we found that W d-orbital plays an important role for the superconductivity in W₅SiB₂.     

Doping dependence of the superconducting gap by Andreev reflection in Au/La2−xSrxCuO4 point-contact junctions

We studied the doping dependence of the superconducting gap in La_2−xSr_xCuO₄ (LSCO) by means of Andreev reflection measurements in Au/LSCO point-contact junctions. The Andreev reflection features were found to disappear at T_c^A close to the bulk T_c. The fit of the conductance curves with the BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is used. The low-temperature dominant isotropic gap component Δ_s follows very well the T_c vs. x curve, while the gap-like features observed by angle-resolved photoemission spectroscopy and tunneling scale with T^∗. This confirms the different origin of these two energy scales at T<T_c.     

Unconventional superconductivity originating from disconnected Fermi surfaces in correlated superconductors : A case study for iron pnictides

We first generally summarize the effect of disconnected Fermi surfaces in spin fluctuation mediated superconductivity. We argue that disconnected Fermi surfaces are favorable in that the sign of the superconducting gap can be changed without nodal lines intersecting the Fermi surface. Then, as an example of actual materials that have disconnected Fermi surfaces, we focus on the iron-based high T_c superconductors. We construct a model that contains all of the five Fe d bands, and apply random-phase approximation. We find that multiple spin fluctuation modes develop due to the nesting between disconnected Fermi surfaces, and the superconductivity originating from the cooperation or competition between these multiple spin fluctuation modes depends on the lattice structure. In particular, the appearance of the Fermi surface around (π, π) in the unfolded Brillouin zone is sensitive to the pnictogen height h_Pn measured from the Fe plane, and the height can act as a switch between high T_c nodeless and low T_c nodal pairings. In the high T_c case, the superconducting gap is fully open on all of the five Fermi surfaces, but changes sign across the nesting vectors that bridge the disconnected Fermi surfaces.     

Near EF electronic structure of heavily boron-doped superconducting diamond

We have performed soft X-ray angle-resolved photoemission spectroscopy (SXARPES) of a heavily boron-doped superconducting diamond film (T_c=7.2 K) in order to study the electronic structure near the Fermi level (E_F). Careful determination of measured momentum space that across Γ point in the Brillouin zone (BZ) and increase of an energy resolution provide further spectroscopic evidence that E_F is located at the highly dispersive diamond-like bands, indicating that holes at the top of the diamond-like valence band play an essential role for the conducting properties of the heavily boron-doped superconducting diamond for this boron-doping region (effective carrier concentration of 1.6%). The SXARPES intensities at E_F were also mapped out over BZ to obtain experimental Fermi surface sheets and compared with calculations.     

Pairing symmetry in layered BiS2 compounds driven by electron-electron correlation

We investigate the pairing symmetry of layered BiS2 compomlds by assuming that electron-electron correlation is still important so that the pairing is rather short range. We lind that the extended .s-wave pairing symmetry always wins over d-wave when the pairing is confined between two short range sites up to next nearest neighbors. The pairing strength is peaked around the doping level ：r = 0.5. which is consistent with experimental observation. The extended s-wave pairing symmetry is very robust against spin orbital coupling because it is mainly determined by the structure of Fermi surfaces, Moreover. the extended s-wave pafiring can be distinguished from conventional swave pairing by measuring and comparing superconducting gaps of different Fermi surfaces.     

Bulk-sensitive spectroscopic studies on noncentrosymmetric superconducting system of Mg–Ir–B

We present the data from ultrahigh-resolution laser-excited photoemission spectroscopy on a newly discovered noncentrosymmetric superconductor Mg₁₀Ir₁₉B₁₆. We observed the evidences for the opening of superconducting gap where numerical fittings show that isotropic s-wave model can satisfactorily explain the behavior of spectra near the Fermi level. We also present the valence band photoemission spectrum and B K fluorescence spectrum to discuss the basic electronic structure of Mg₁₀Ir₁₉B₁₆.     

Analytical assessment of some characteristic ratios for <Emphasis Type="Italic">s</Emphasis>-wave superconductors

We evaluate some thermodynamic quantities and characteristic ratios that describe low- and high-temperature s-wave superconducting systems. Based on a set of fundamental equations derived within the conformal transformation method, a simple model is proposed and studied analytically. After including a one-parameter class of fluctuations in the density of states, the mathematical structure of the s-wave superconducting gap, the free energy difference, and the specific heat difference is found and discussed in an analytic manner. Both the zero-temperature limit T = 0 and the subcritical temperature range T ? T_c are discussed using the method of successive approximations. The equation for the ratio R₁, relating the zero-temperature energy gap and the critical temperature, is formulated and solved numerically for various values of the model parameter. Other thermodynamic quantities are analyzed, including a characteristic ratio R₂, quantifying the dynamics of the specific heat jump at the critical temperature. It is shown that the obtained model results coincide with experimental data for low-T_c superconductors. The prospect of application of the presented model in studies of high-T_c superconductors and other superconducting systems of the new generation is also discussed.     

Electronic band structure and Fermi surface for new layered superconductor LaO0.5F0.5BiS2 in comparison with parent phase LaOBiS2 from first principles

By means of ab initio calculations, we have probed the peculiarities of the electronic band structure and Fermi surface for the recently discovered layered superconductor LaO_0.5F_0.5BiS₂ in comparison with the parent phase LaOBiS₂. The electronic factors promoting the transition of LaOBiS₂ upon fluorine doping to superconducting state: inter-layer charge transfer, the evolution of the Fermi surface, and the dependence of the near-Fermi densities of states on x for LaO_{1 ? x} F _x BiS₂ are evaluated and discussed in comparison with the available experiments.     

Electronic structure reconstruction of CaFe2As2 in the spin density wave state

The electronic structure of CaFe₂As₂, a parent compound of iron-based superconductors, is studied with high-resolution angle-resolved photoemission spectroscopy. The electronic structure of CaFe₂As₂ in the paramagnetic state is consistent with that of density-functional theory calculations. We show that the electronic structure of this compound is significantly reconstructed when entering the spin density wave state. We could resolve two hole-like pockets and four electron-like pockets around the (0, 0) point, and one electron-like pocket surrounded with a pair of electron- and hole-like pockets around the (π, π) point in the spin density wave state. Therefore, the complicated Fermi surface topology and electronic structure near Fermi surface of CaFe₂As₂ illustrate that there exists unconventional electronic reconstruction in the spin density wave state, which cannot be explained by the band folding and Fermi surface nesting pictures.     

Pseudogap and symmetry of superconducting order parameter in cuprates

The phase diagram, nature of the normal state pseudogap, type of the Fermi surface, and behavior of the superconducting gap in various cuprates are discussed in terms of a correlated state with valence bonds. The variational correlated state, which is a band analogue of the Anderson (RVB) states, is constructed using local unitary transformations. Formation of valence bonds causes attraction between holes in the d-channel and corresponding superconductivity compatible with antiferromagnetic spin order. Our calculations indicate that there is a fairly wide range of doping with antiferromagnetic order in isolated CuO₂ planes. The shape of the Fermi surface and phase transition curve are sensitive to the value and sign of the hopping interaction t′ between diagonal neighboring sites. In underdoped samples, the dielectrization of various sections of the Fermi boundary, depending on the sign of t′, gives rise to a pseudogap detected in photoemission spectra for various quasimomentum directions. In particular, in bismuth-and yttrium-based ceramics (t′>0), the transition from the normal state of overdoped samples to the pseudogap state of underdoped samples corresponds to the onset of dielectrization on the Brillouin zone boundary near k=(0,π) and transition from “large” to “small” Fermi surfaces. The hypothesis about s-wave superconductivity of La-and Nd-based ceramics has been revised: a situation is predicted when, notwithstanding the d-wave symmetry of the superconducting order parameter, the excitation energy on the Fermi surface does not vanish at all points of the phase space owing to the dielectrization of the Fermi boundary at k _x=± k _y. The model with orthorhombic distortions and two peaks on the curve of T _c versus doping is discussed in connection with experimental data for the yttrium-based ceramic. Zh. éksp. Teor. Fiz. 115, 649–674 (February 1999)     

Zn-substitution effects on the low-energy quasiparticles in Bi2Sr2CaCu2O8+δ studied by angle-resolved photoemission spectroscopy

We have performed high-resolution angle-resolved photoemission spectroscopy on Bi₂Sr₂Ca(Cu_1−xZn_x)₂O_8+δ (x=0.0, 0.05) to study the Zn-substitution effects on the low-energy quasiparticles. We found that the Zn substitution causes the reduction of the quasiparticle intensity and the superconducting gap, while it does not affect the normal-state band dispersion, the Fermi surface, or the quasiparticle lifetime. This indicates that the Zn substitution locally destroys or hinders the superconducting pairing and as a result decreases the superfluid density, but does not affect the long-range coherence among quasiparticles.     

Geometry effects on the electronic properties of YBa2Cu3O7

We have performed electronic structure calculations for the high T _c compound YBa₂Cu₃O₇ with a 3% reduced unit-cell volume. The effect on band structure and Fermi surface is investigated. The predominant features are van Hoove singularities at the Fermi level. Keeping the experimental lattice parameters, nearly the same effects occur when replacing the local-density approximation by a generalized gradient correction scheme.     

Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments in Au/ La 2 - x Sr x CuO 4 point-contact junctions

We present new Au/La_2-xSr_xCuO₄ (LSCO) point-contact conductance measures as a function of voltage and temperature in samples with 0.08 ? x ? 0.2. Andreev reflection features disappear at about the bulk T _c , giving no evidence of gap for T > T _c . The fit of the normalized conductance at any T < T _c supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component follows the T _c ( x ) curve in contrast with recent angle-resolved photoemission spectroscopy and quasiparticle tunneling data. These results prove the separation between pseudogap and phase-coherence superconducting gap in LSCO at x 0.2. Received 14 June 2001     

The “pair” Fermi contour and high-temperature superconductivity

Superconducting pairing of holes with a large (on the order of doubled Fermi) total pair momentum and small relative motion momenta is considered taking into account the quasi-two-dimensional electronic structure of high-T _c cuprates with clearly defined nesting of the Fermi contour situated in an extended neighborhood of the saddle point of the electronic dispersion law (the momentum space region with a hyperbolic metric) and the arising of a spatially inhomogeneous (stripe) structure as a result of the redistribution of current carriers (holes) that restores regions with antiferromagnetic ordering. The superconducting energy gap and condensation energy were determined, and their dependences on the doping level were qualitatively studied. The energy gap was shown to exist in some hole concentration region limited on both sides. The superconducting state with a positive condensation energy appears in a narrower range of doping within this region. The reason for the arising of the superconducting state at a repulsive screened Coulomb interaction between holes is largely the redistribution of hole pairs in the momentum space related to the special features of the hyperbolic metric, which is responsible for the formation of the “pair” Fermi contour, and the renormalization of the kinetic energy of holes when the chemical potential changes because of the condensation of pairs. Hole pairs of the type under consideration exist not only in the condensate but also in the form of quasi-stationary states with very weak decay at temperatures substantially exceeding the superconducting transition temperature. The pseudogap region of the phase diagram of high-T _c cuprates is related to such states. The pairing mechanism under consideration allows not only the principal characteristics of the phase diagram but also key experimental data on high-T _c cuprate materials to be qualitatively explained.     

STM spectroscopy on superconducting FeSe1−xTex single crystals at 300 mK

We report cryogenic scanning tunneling spectroscopy measurements on single crystals of superconducting FeSe_1−xTe_x, at doping levels of x=0.5 and 0.7, with critical temperatures . Atomically resolved topographic images were obtained, showing large-scale density-of-state clustering which appears to have no periodicity and to vary with the doping. Conductance spectra taken at 300 mK showed a generally asymmetric V-shaped background, along with a sharp dip structure within . These spectra appeared to vary over ∼nm length scale, and not correlated with the topography. The overall spectral evolution versus temperature is consistent with the dip structure arising from a superconducting energy gap which closes above T_c, and with the spectral background having a non-superconducting origin. The persistence of finite zero-bias conductance down to 300 mK, well below T_c, indicates the presence of low-energy quasiparticles on parts of the Fermi surface. We discuss our data in light of some other recent spectroscopic measurements of FeSe_1−xTe_x, and in terms of its characteristic band structure.     

Unconventional pairing originating from disconnected Fermi surfaces in the iron-based superconductor

For the iron-based high T_c superconductor LaFeAsO_1?xF_x, we construct a minimal model, where all of the five Fe d bands turn out to be involved. We then investigate the origin of superconductivity with a five-band random-phase approximation by solving the Eliashberg equation. We conclude that the spin fluctuation modes arising from the nesting between the disconnected Fermi pockets realise, basically, an extended s-wave pairing, where the gap changes sign across the nesting vector.     

d-Wave superconductivity from correlated-hopping interactions determined by angle-resolved photoemission spectroscopy

Starting from a generalized Hubbard model with correlated-hopping interactions, we solve numerically two coupled integral equations within the Bardeen–Cooper–Schrieffer formalism, in order to study the doping effects on the critical temperature (_Tc$T_c$), d-wave superconducting gap, and the electronic specific heat. Within the mean-field approximation, we determine the single- and correlated-electron-hopping parameters for La_2 − xSr_xCuO₄ by using angle-resolved photoemission spectroscopy data. The resulting parametrized Hubbard model is able to explain the experimental _Tc$T_c$ variation with the doping level (x). Moreover, the observed power-law behavior of the superconducting specific heat is reproduced by this correlated-hopping Hubbard model without adjustable parameters.     

Direct photoemission spectroscopy and electronic properties of in situ grown,strained high-Tc and related oxide films

We have optimized laser ablation thin film deposition and transfer procedures within synchrotron vault, specifically to perform angle-integrated and angle-resolved photoemission spectroscopy (ARPES) on high-T_c and related films without cleaving the samples. However, the chain-containing phases like YBCO-123 easily loose surface oxygen and do not exhibit stable Fermi edge, hence are not suitable for ARPES studies. Direct in situ ARPES studies on strained LSCO-214 films show striking strain effects on the electronic structure. The Fermi surface (FS) of LSCO evolves with doping, yet changes even more significantly with strain. The strain changes the FS topology from hole-like to electron-like, and causes band dispersion along k_x and the Fermi level crossing before the Brillouin zone boundary, in sharp contrast to the ‘usual’ flat band remaining ≈30 meV below E_F measured on unstrained samples. The associated reduction of the density of states does not diminish the superconductivity; T_c is enhanced in all our strained samples. Implications for the evolving high-T_c theory and studies of nano-engineered film heterostructures are briefly discussed.