G. Aeppli

Low energy excitations in superconducting La1.86Sr0.14CuO4.

We present magnetic neutron scattering and specific heat data on the high-[ital T][sub [ital c]] superconductor La[sub 1.86]Sr[sub 0.14]CuO[sub 4]. Both types of measurements show that even when the samples are superconducting, there are excitations with energies well below 3.5[ital k][sub [ital B]T[ital c]]. These excitations coincide with a magnetic response, [chi][prime][prime], whose amplitude is considerably below that observed at [ital T][sub [ital c]]=35[plus minus]1 K. At the same time, the wave-vector dependence of [chi][prime][prime] is identical to that for the normal state, which implies that the low frequency excitations in our crystals are not those associated with the nodes of a clean [ital d]-wave superconductor. However, the data are consistent with gapless superconductivity of the type induced by localized magnetic impurities, clearly observed in the specific heat measurements.

Scaling of magnetic fluctuations near a quantum phase transition

We use inelastic neutron scattering to measure the magnetic fluctuations in a single crystal of the heavy fermion alloy CeCu_5.9Au_0.1 close to the antiferromagnetic quantum critical point. The energy and temperature-dependent spectra obey (E/T) scaling at Q near (1,0,0). The neutron data and earlier bulk susceptibility are consistent with the form 1/X ~ f(Q)+(-iE+bT)^a, with an anomalous exponent a=0.8. We confirm the earlier observation of quasi-low dimensionality and show how both the magnetic fluctuations and the thermodynamics can be understood in terms of a quantum Lifshitz point.

Comparison of the high-frequency magnetic fluctuations in insulating and superconducting La2-xSrxCuO4.

Inelastic neutron scattering performed at a spallation source is used to make absolute measurements of the dynamic susceptibility of insulating La{sub 2}CuO{sub 4} and superconducting La{sub 1.86}Sr{sub 0.14}CuO{sub 4} over the energy range 15{le}{h_bar}{omega}{le}350 meV. The effect of Sr doping on the magnetic excitations is to cause a large broadening in wave vector and a substantial change in the spectrum of the local spin fluctuations. Comparison of the two compositions reveals a new energy scale {sq_bullet}{Gamma}=22{plus_minus}5 meV in La{sub 1.86}Sr{sub 0.14}CuO{sub 4}. {copyright} {ital 1996 The American Physical Society.}

Dispersive excitations in the high-temperature superconductor La2-xSrxCuO4

High-resolution neutron scattering experiments on optimally doped La2-xSrxCuO4 (x=0.16) reveal that the magnetic excitations are dispersive. The dispersion is the same as in YBa2Cu3O6.85, and is quantitatively related to that observed with charge sensitive probes. The associated velocity in La2-xSrxCuO4 is only weakly dependent on doping with a value close to the spin-wave velocity of the insulating (x=0) parent compound. In contrast with the insulator, the excitations broaden rapidly with increasing energy, forming a continuum at higher energy and bear a remarkable resemblance to multiparticle excitations observed in 1D S=1/2 antiferromagnets. The magnetic correlations are 2D, and so rule out the simplest scenarios where the copper oxide planes are subdivided into weakly interacting 1D magnets.

New Magnetic Coherence Effect in Superconducting La2-xSrxCuO4.

We have used inelastic neutron scattering to examine the magnetic fluctuations at intermediate frequencies in the simplest high temperature superconductor, La{sub 2{minus}{ital x}}Sr{sub {ital x}}CuO{sub 4}. The suppression of the low energy magnetic response in the superconducting state is accompanied by an increase in the response at higher energies. Just above a threshold energy of {approximately}7meV there is additional scattering present below {ital T}{sub {ital c}} which is characterized by an extraordinarily long coherence length, in excess of 50 A. {copyright} {ital 1996 The American Physical Society.}

Quantum and Classical Glass Transitions in LiHOxY1-xF4

When performed in the proper low-field, low-frequency limits, measurements of the dynamics and the nonlinear susceptibility in the model Ising magnet in a transverse field LiHo(x)Y(1-x)F(4) prove the existence of a spin-glass transition for x=0.167 and 0.198. The classical behavior tracks for the two concentrations, but the behavior in the quantum regime at large transverse fields differs because of the competing effects of quantum entanglement and random fields.

Spin correlations near the ferromagnetic‐to‐spin‐glass crossover (invited)

We have performed neutron scattering studies on two very different alloys which undergo transitions from ferromagnetic (FM) to spin‐glass (SG) states as the temperature is reduced. The alloys are EuxSr1−xS, a crystalline insulator, and (FexMn1−x)75P16B6Al3, an amorphous metal, and their FM‐SG multicritical points are at x≂0.50, T=4 K and x≂0.65, T=42 K respectively. In spite of the substantial differences between these materials, the neutron scattering data show that their spin correlations are remarkably similar. In particular, for the samples near the multicritical points, a single Lorentzian describes the magnetic scattering very well. Its width κ corresponds to a ferromagnetic correlation length ξ which, as T is reduced, first increases to a value indistinguishable from infinity, and then decreases to a finite value, as expected for a ferromagnet which evolves into a reentrant spin glass. As the Fe or Eu content is raised, the scattering function at low temperatures deviates increasingly from the Lore...

Dramatic Switching of Magnetic Exchange in a Classic Transition Metal Oxide: Evidence for Orbital Ordering

Spin correlations in metallic and insulating phases of V2O3 and its derivatives are investigated using magnetic neutron scattering. Metallic samples have incommensurate spin correlations varying little with hole doping. Paramagnetic insulating samples have, spin correlations only among near neighbors. The transition from either of these phases into the low temperature insulating antiferromagnetic phase is accompanied by an abrupt change of dynamic magnetic short range, order. Our results support the idea that the transition into the antiferromagnetic insulator is also an orbital ordering transition.

Neutron scattering studies of magnetic properties of actinide systems

Abstract In this article we review neutron scattering studies of the magnetic properties of actinide systems over the last 15 years, with particular emphasis on the work since 1984 and the reviews by Rossat-Mignod et al. and Buyers and Holden. In section 2 the results obtained on the actinide dioxides UO2, NpO2 and PuO2 at spallation sources and the recent observation of intermultiplet transitions in UPd3 and UPt3 are presented. In section 3 a discussion is given of magnetization densities, starting with the almost localized system such as PuSb and continuing to more recent work on itinerant intermetallic systems such as UNi2, UFe2 and PuFe2. In these latter systems the ratio μ L μ S of the orbital and spin moments may be determined by neutrons and compared to theory. In section 4 we present work on compounds with the NaCl crystal structure. This includes complex magnetic phase diagrams, as found for example in UAs and NpAs, the discussion of the results of critical scattering experiments, and neutron inelastic experiments on ferromagnets such as PuSb and UTe. In section 5 the work on single crystals of U-containing heavy fermions is presented. In these systems neutron scattering has been able to characterize the small magnetic moments and the nature of the magnetic correlations, which are frequency dependent. A summary of the significant progress and problems remaining is given in section 6.

Electronic phase-purity in La2-xSrxCuO4- delta.

A Comment on the Letter by A. Weidinger {ital et} {ital al}., Phys. Rev. Lett. 61, 102 (1989).