Professor Stephen Blundell

Abstract:

μSR has been used to probe the structure and stability of the flux vortex array in the organic superconductor κ-(BEDT-TTF)2Cu(SCN)2. At temperatures below 5 K and fields below 5 mT the internal field distribution is found to closely match that expected for a three dimensional (3D) Abrikosov flux line lattice (FLL). Careful studies in this 3D-FLL regime have enabled an improved measurement of the temperature dependence of the superconducting penetration depth to be made. A linear term is found in the temperature dependence of the penetration depth, suggesting the presence of line nodes in the gap parameter and d-wave pairing.

Abstract:

The shape of the Fermi surface of organic metals can be measured by recording angle-dependent magnetoresistance oscillations. We review this technique and develop a model for parametrizing the shape of the quasi-two-dimensional Fermi surface sections which often appear in organic metals. Using this model, we show that it is possible to extract more detail about the quasi-two-dimensional pocket shape from angle-dependent magnetoresistance oscillations than in the traditional approximation which assumes an elliptical Fermi surface shape. We also consider the implications for cyclotron resonance experiments.

Abstract:

The longitudinal muon spin relaxation in metals and semimetals is suggestive of a form of Korringa relaxation in which the hyperfine interaction between the muons and the conduction electrons plays a dominant rôle. We give an alternative derivation of the Korringa law and show how muons may thus be used to study interactions with conduction electrons at interstitial sites. The alternative derivation links the topic to the use of implanted muons both as probes of magnetic and correlated-electron systems and as proton analogues, modelling the behaviour of hydrogen impurity in metals and semimetals.

Abstract:

We report the results of a study of the metal-organic magnets M^II[N(CN)2]2, where M^II = Ni, Co and Mn, using bulk magnetization and muon-spin relaxation (μSR). Implanted muons are sensitive to the onset of long-range magnetic order in each of these materials and strong muon-spin relaxation is observed in the paramagnetic state due to low-frequency fluctuations of the electronic moments in the 10⁹-10¹⁰ Hz range. The size of the muon-spin relaxation in the paramagnetic state can be related to the magnitude of the transition-metal-ion moment. Very strongly damped oscillations are observed below the magnetic transition temperature in Co[N(CN)2]2.

Abstract:

The substitution of Ga(3+) into the Jahn--Teller distorted, antiferromagnetic perovskites LaMnO(3) and NdMnO(3) strongly affects both the crystal structures and resulting magnetic ordering. In both compounds the Ga(3+) and Mn(3+) cations are disordered over the six coordinate sites. La(2)GaMnO(6) is a ferromagnetic insulator (T(c) = 70 K); a moment per Mn cation of 2.08(5) mu(B) has been determined by neutron powder diffraction at 5 K. Bond length and displacement parameter data suggest Jahn--Teller distortions which are both coherent and incoherent with the Pnma space group symmetry of the perovskite structure (a = 5.51122(4) A, b = 7.80515(6) A, c = 5.52947(4) A) at room temperature. The coherent distortion is strongly suppressed in comparison with the parent LaMnO(3) phase, but the displacement ellipsoids suggest that incoherent distortions are significant and arise from local Jahn--Teller distortions. The preparation of the new phase Nd(2)GaMnO(6) has been found to depend on sample cooling rates, with detailed characterization necessary to ensure phase separation has been avoided. This compound also adopts the GdFeO(3)-type orthorhombically distorted perovskite structure (space group Pnma, a = 5.64876(1) A, b = 7.65212(2) A, c = 5.41943(1) A at room temperature). However, the B site substitution has a totally different effect on the Jahn--Teller distortion at the Mn(3+) centers. This phase exhibits a Q(2) mode Jahn--Teller distortion similar to that observed in LaMnO(3), although reduced in magnitude as a result of the introduction of Ga(3+) onto the B site. There is no evidence of a dynamic Jahn-Teller distortion. At 5 K a ferromagnetically ordered Nd(3+) moment of 1.06(6) mu(B) is aligned along the y-axis and a moment of 2.8(1) mu(B) per Mn(3+) is ordered in the xy plane making an angle of 29(2) degrees with the y-axis. The Mn(3+) moments couple ferromagnetically in the xz plane. However, along the y-axis the moments couple ferromagnetically while the x components are coupled antiferromagnetically. This results in a canted antiferromagnetic arrangement in which the dominant exchange is ferromagnetic. Nd(2)GaMnO(6) is paramagnetic above 40(5) K, with a paramagnetic moment and Weiss constant of 6.70(2) mu(B) and 45.9(4) K, respectively. An ordered moment of 6.08(3) mu(B) per Nd(2)GaMnO(6) formula unit was measured by magnetometry at 5 K in an applied magnetic field of 5 T.