S. Rosenkranz

Unconventional superconductivity in Ba0.6K0.4Fe2As2 from inelastic neutron scattering

A new family of superconductors containing layers of iron arsenide has attracted considerable interest because of their high transition temperatures (Tc), some of which are >50 K, and because of similarities with the high-Tc copper oxide superconductors. In both the iron arsenides and the copper oxides, superconductivity arises when an antiferromagnetically ordered phase has been suppressed by chemical doping. A universal feature of the copper oxide superconductors is the existence of a resonant magnetic excitation, localized in both energy and wavevector, within the superconducting phase. This resonance, which has also been observed in several heavy-fermion superconductors, is predicted to occur when the sign of the superconducting energy gap takes opposite values on different parts of the Fermi surface, an unusual gap symmetry which implies that the electron pairing interaction is repulsive at short range. Angle-resolved photoelectron spectroscopy shows no evidence of gap anisotropy in the iron arsenides, but such measurements are insensitive to the phase of the gap on separate parts of the Fermi surface. Here we report inelastic neutron scattering observations of a magnetic resonance below Tc in Ba0.6K0.4Fe2As2, a phase-sensitive measurement demonstrating that the superconducting energy gap has unconventional symmetry in the iron arsenide superconductors.

Evolution of the pseudogap from Fermi arcs to the nodal liquid

The pseudogap phase in the cuprates is a most unusual state of matter: it is a metal, but its Fermi surface is broken up into disconnected segments known as Fermi arcs. Using angle resolved photoemission spectroscopy, we show that the anisotropy of the pseudogap in momentum space and the resulting arcs depend only on the ratio T/T*(x), where T*(x) is the temperature below which the pseudogap first develops at a given hole doping x. In particular, the arcs collapse linearly with T/T* and extrapolate to zero extent as T goes to 0. This suggests that the T = 0 pseudogap state is a nodal liquid, a strange metallic state whose gapless excitations are located only at points in momentum space, just as in a d-wave superconductor.

Charge Melting and Polaron Collapse in La1.2Sr1.8Mn2O7

X-ray and neutron scattering measurements directly demonstrate the existence of polarons in the paramagnetic phase of optimally doped colossal magnetoresistive oxides. The polarons exhibit shortrange correlations that grow with decreasing temperature, but disappear abruptly at the ferromagnetic transition because of the sudden charge delocalization. The “melting” of the charge ordering as we cool through TC occurs with the collapse of the quasistatic polaron scattering, and provides important new insights into the relation of polarons to colossal magnetoresistance.

Crossover from coherent to incoherent electronic excitations in the normal state of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+delta}.

Angle resolved photoemission spectroscopy (ARPES) and resistivity measurements are used to explore the overdoped region of the high temperature superconductor Bi(2)Sr(2)CaCu(2)O(8+delta). We find evidence for a new crossover line in the phase diagram between a coherent metal phase, for lower temperatures and higher doping, and an incoherent metal phase, for higher temperatures and lower doping. The former is characterized by two well-defined spectral peaks in ARPES due to coherent bilayer splitting and superlinear behavior in the resistivity, whereas the latter is characterized by a single broad spectral feature in ARPES and a linear temperature dependence of the resistivity.

Observation of a d -wave nodal liquid in highly underdoped Bi 2 Sr 2 CaCu 2 O 8+ δ

High-temperature superconductivity in the cuprates arises when charge carriers are added to an insulator. Between these states lies the so-called nodal liquid at low temperature. Photoemission spectroscopy suggests that superconductivity evolves smoothly from this nodal-liquid state.

NEUTRON SCATTERING INVESTIGATION OF MAGNETIC BILAYER CORRELATIONS IN LA1.2SR1.8MN2O7 : EVIDENCE OF CANTING ABOVE TC

the change in sign of Mn-O bond compressibilities at the transition. This aspect of the problem is only now receiving the theoretical attention it deserves, even though de Gennes first considered it nearly forty years ago [10–12]. The work reported here is part of a general study linking the magnetic and transport properties of naturally layered manganites directly to the underlying magnetic correlations measured by neutron diffraction and spectroscopy. We find evidence that, although the magnetic correlations are predominantly ferromagnetic within the two-dimensional planes, there is much weaker ferromagnetic correlation between spins in neighboring layers within each bilayer. This observation is consistent with a canting of the spins in neighboring layers with a cant angle that is dependent on both magnetic field and temperature, becoming smaller as the temperature approaches TC. The correlation is not confined to short-lived clusters, as were the antiferromagnetic fluctuations observed by Perring et al [3], but involves the critical fluctuations that lead to ferromagnetic ordering. Our results imply that there is a delicate balance between competing double exchange and superexchange interactions in these compounds. In the critical regime below TC, the

Nondispersive Fermi Arcs and the Absence of Charge Ordering in the Pseudogap Phase of Bi2Sr2CaCu2O8+δ

The autocorrelation of angle resolved photoemission data from the high temperature superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) shows distinct peaks in momentum space which disperse with binding energy in the superconducting state, but not in the pseudogap phase. Although it is tempting to attribute a nondispersive behavior in momentum space to charge ordering, a deconstruction of the autocorrelation reveals that the nondispersive peaks arise from the tips of the Fermi arcs, which themselves do not change with binding energy.

Effect of fermi surface nesting on resonant spin excitations in Ba 1-xKxFe2As2

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba(1-x)K(x)Fe(2)As(2) over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s(±)-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight, caused by the weakening of electron-electron correlations.

Momentum anisotropy of the scattering rate in cuprate superconductors

We examine the momentum and energy dependence of the scattering rate of the high-temperature cuprate superconductors using angle-resolved photoemission spectroscopy. The scattering rate is of the form

Electron-Phonon Coupling and the Soft Phonon Mode in TiSe2

a+b\ensuremath{\omega}