Adam Kaminski

Quasiparticles in the superconducting state of Bi(2)Sr(2)CaCu(2)O(8+delta)

Recent improvements in momentum resolution lead to qualitatively new angle-resolved photoemission spectroscopy results on the spectra of Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) along the (pi,pi) direction, where there is a node in the superconducting gap. We now see the intrinsic line shape, which indicates the presence of true quasiparticles at all Fermi momenta in the superconducting state, and lack thereof in the normal state. The region of momentum space probed here is relevant for charge transport, motivating a comparison of our results to conductivity measurements by infrared reflectivity.

Dirac node arcs in PtSn4

A combination of detailed photoelectron spectroscopy measurements and numerical simulations reveal the presence of so-called Dirac node arcs in the electronic structure of PtSn4.

Fermi Surface of Bi2Sr2CaCu2O8

We study the Fermi surface of Bi2Sr2CaCu2O8 using angle resolved photoemission spectroscopy (ARPES) with a momentum resolution of approximately 0.01 of the Brillouin zone. We show that, contrary to recent suggestions, the ARPES derived Fermi surface is a large hole barrel centered at (pi,pi), independent of the incident photon energy. We caution that the photon energy and k dependence of the matrix elements, if not properly accounted for, can lead to misinterpretation of ARPES intensities.

Importance of the Fermi-surface topology to the superconducting state of the electron-doped pnictide Ba(Fe1-xCox)₂As₂

We used angle resolved photoemission spectroscopy and thermoelectric power to study the poorly explored, highly overdoped side of the phase diagram of Ba(Fe1−xCox)2As2 high temperature superconductor. Our data demonstrate that several Lifshitz transitions topological changes of the Fermi surface occur for large x. Tc starts to decrease with doping when the cylindrical, central hole pocket changes to ellipsoids centering at the Z point, and goes to zero before these ellipsoids disappear around x = 0.2. Changes in thermoelectric power occur at similar x-values. Beyond this doping level the central pocket changes to electron-like and superconductivity does not exist. Our observations reveal the crucial importance of the underlying Fermiology in this class of materials. A necessary condition for superconductivity is the presence of the central hole pockets rather than perfect nesting between central and corner pockets.

Determination of the Fermi surface in high-T c superconductors by angle-resolved photoemission spectroscopy

We study the normal-state electronic excitations probed by angle-resolved photoemission spectroscopy (ARPES) in Bi1.6Pb0.4Sr2CuO6 (Bi2201) and Bi2Sr2CaCu2O8 divided by delta (Bi2212). Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously; it is a single large hole barrel centered about (pi,pi) in both materials.

Unusual Temperature Dependence of Band Dispersion in Ba ( Fe 1 − x Ru x ) 2 As 2 and its Consequences for Antiferromagnetic Ordering

We have performed detailed studies of the temperature evolution of the electronic structure in Ba(Fe(1-x)Ru(x))(2)As(2) using angle resolved photoemission spectroscopy. Surprisingly, we find that the binding energy of both hole and electron bands changes significantly with temperature in both pure and Ru substituted samples. The hole and electron pockets are well nested at low temperature in unsubstituted (BaFe(2)As(2)) samples, which likely drives the spin density wave and resulting antiferromagnetic order. Upon warming, this nesting is degraded as the hole pocket shrinks and the electron pocket expands. Our results demonstrate that the temperature dependent nesting may play an important role in driving the antiferromagnetic-paramagnetic phase transition.

Evidence for a competition between the superconducting state and the pseudogap state of (BiPb)2(SrLa)2CuO6+delta from muon spin rotation experiments.

The in-plane magnetic penetration depth lambda ab in optimally doped (BiPb)2(SrLa)2CuO6+delta (OP Bi2201) was studied by means of muon-spin rotation. The measurements of lambda ab(-2)(T) are inconsistent with a simple model of a d-wave order parameter and a uniform quasiparticle weight around the Fermi surface. The data are well described assuming the angular gap symmetry obtained in ARPES experiments [Phys. Rev. Lett. 98, 267004 (2007)], which suggest that the superconducting gap in OP Bi2201 exists only in segments of the Fermi surface near the nodes. The remaining parts of the Fermi surface, which are strongly affected by the pseudogap state, do not contribute significantly to the superconducting condensate.

Suppression of the antinodal coherence of superconducting (Bi,Pb)2(Sr,La)2CuO6+δ as revealed by muon spin rotation and angle-resolved photoemission

R. Khasanov, ∗ Takeshi Kondo, 3 M. Bendele, 4 Yoichiro Hamaya, A. Kaminski, S.L. Lee, S.J. Ray, and Tsunehiro Takeuchi 6 Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA Department of Crystalline Materials Science, Nagoya University, Nagoya 464-8603, Japan Physik-Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland School of Physics and Astronomy, University of St. Andrews, Fife, KY16 9SS, UK EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan

Dual character of the electronic structure of YBa2Cu4O8 : The conduction bands of CuO2 planes and CuO chains

We use microprobe angle-resolved photoemission spectroscopy (microARPES) to separately investigate the electronic properties of CuO2 planes and CuO chains in the high temperature superconductor, YBa2Cu4O8. For the CuO2 planes, a two-dimensional (2D) electronic structure is observed and, in contrast to Bi2Sr2CaCu2O8+delta, the bilayer splitting is almost isotropic and 50% larger, which strongly suggests that bilayer splitting has no direct effect on the superconducting properties. In addition, the scattering rate for the bonding band is about 1.5 times stronger than the antibonding band and is independent of momentum. For the CuO chains, the electronic structure is quasi-one-dimensional and consists of a conduction and insulating band. Finally, we find that the conduction electrons are well confined within the planes and chains with a nontrivial hybridization.

Time-reversal symmetry breaking? (reply): Superconductors

Kaminski et al. reply- There are two components of the circular dichroism (CD) signal in angle-resolved photoemission (ARPES) measurements. One is always present in crystals, regardless of any time-reversal symmetry considerations. This component, which we refer to as ‘geometric’, is antisymmetric about any symmetry plane of the crystal, and is therefore zero at that plane. But in underdoped samples of the high-temperature superconductor Bi2212, we find another component, which is non-zero at the symmetry plane below the pseudogap temperature. We attribute that component to time-reversal symmetry breaking. The objections of Borisenko et al. comprise three main points: the circular dichroism that we observe at the mirror plane is due to the superstructure of the Bi–O layer; our momentum accuracy is not as we stated; and the absence of dichroism in overdoped samples is due to a weaker influence of the superstructure because of an increased Fermi surface volume compared with underdoped samples.