F. L. Pratt

Magnetotransport studies of the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2 under pressure: the relationship between carrier effective mass and critical temperature

Magnetotransport measurements have been carried out on the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2 at temperatures down to 500 mK and in hydrostatic pressures up to 16.3 kbar. The observation of Shubnikov-de Haas and magnetic breakdown oscillations has allowed the pressure dependences of the area of the closed pocket of the Fermi surface and the carrier effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature Tc. The effective mass measured by the temperature dependence of the Shubnikov-de Haas oscillations is found to fall rapidly with increasing pressure up to a critical pressure Pc approximately=5 kbar. Above Pc a much weaker pressure dependence is observed; Tc also falls rapidly with pressure from 10.4 K at ambient pressure to zero at around Pc. This strongly suggests that the enhanced effective mass and the superconducting behaviour are directly connected in this organic superconductor. A simplified model of the band structure of kappa -(BEDT-TTF)2Cu(NCS)2 has been used to derive the bare band masses of the electrons from optical data. Comparisons of these parameters with cyclotron resonance data and the effective masses measured in the present experiments indicate that the greater part of the enhancement of the effective mass necessary for superconductivity in this material is due to quasiparticle interactions, with the electron-phonon interactions playing a secondary role. The dependence of Tc on the effective mass may be fitted satisfactorily to a suitably parametrized weak-coupling BCS expression, although this cannot be taken as a definitive proof of the nature of superconductivity in organic conductors.

Magnetic and non-magnetic phases of a quantum spin liquid

A quantum spin-liquid phase is an intriguing possibility for a system of strongly interacting magnetic units in which the usual magnetically ordered ground state is avoided owing to strong quantum fluctuations. It was first predicted theoretically for a triangular-lattice model with antiferromagnetically coupled S = 1/2 spins. Recently, materials have become available showing persuasive experimental evidence for such a state. Although many studies show that the ideal triangular lattice of S = 1/2 Heisenberg spins actually orders magnetically into a three-sublattice, non-collinear 120° arrangement, quantum fluctuations significantly reduce the size of the ordered moment. This residual ordering can be completely suppressed when higher-order ring-exchange magnetic interactions are significant, as found in nearly metallic Mott insulators. The layered molecular system κ-(BEDT-TTF)2Cu2(CN)3 is a Mott insulator with an almost isotropic, triangular magnetic lattice of spin-1/2 BEDT-TTF dimers that provides a prime example of a spin liquid formed in this way. Despite a high-temperature exchange coupling, J, of 250 K (ref. 6), no obvious signature of conventional magnetic ordering is seen down to 20 mK (refs 7, 8). Here we show, using muon spin rotation, that applying a small magnetic field to this system produces a quantum phase transition between the spin-liquid phase and an antiferromagnetic phase with a strongly suppressed moment. This can be described as Bose–Einstein condensation of spin excitations with an extremely small spin gap. At higher fields, a second transition is found that suggests a threshold for deconfinement of the spin excitations. Our studies reveal the low-temperature magnetic phase diagram and enable us to measure characteristic critical properties. We compare our results closely with current theoretical models, and this gives some further insight into the nature of the spin-liquid phase.

WIMDA: a muon data analysis program for the Windows PC

Abstract A package for analysing μSR data has been developed for the PC running Microsoft Windows operating systems (Windows 95, 98 and NT4). Some of the key points considered in the design of the program were that it should run on a typical Windows PC, that it should directly read the binary files produced by muon data acquisition systems, that it should be equally easy to analyse data on-line and off-line and that appropriate features should be included for both pulsed and continuous muon sources.

Muon spin relaxation investigation of magnetic ordering in the hybrid organic-inorganic perovskites [(CH3)2NH2]M(HCOO)3 (M=Ni,Co,Mn,Cu)

Muon spin relaxation measurements are reported on samples of dimethylammonium metal formates containing magnetic divalent nickel, cobalt, manganese, and copper ions. These hybrid organic-inorganic perovskites exhibit weak ferromagnetism and are, apart from the copper system, multiferroics with well separated magnetic and antiferroelectric transitions. We use muons to follow the sublattice magnetization, observing the effect of the spin reorientation transitions below TN and the criticality approaching TN. The multiferroic samples have three-dimensional antiferromagnetic interactions, but the copper sample shows quasi-one-dimensional behavior due to its Jahn-Teller distorted structure, with a ratio of its inter- and intrachain exchange constants j/J=0.037.

Organic ferromagnetism in the nitronyl nitroxides p-NPNN and 3-QNNN: MUSR, EPR and a.c. susceptibility studies

Following the discovery of organic ferromagnetism in p-NPNN, we report the observation of ferromagnetic ordering in a second nitronyl nitroxide radical system, 3-quinoyl nitroxide (3-QNNN), using the muon spin rotation (MUSR) technique. The onset of ferromagnetic order is indicated in the MUSR experiment by the appearance of a rotation signal in zero applied field for temperatures below ∼0.2 K. This signal corresponds to the precession of the muon spin in the internal field of the ferromagnetic state. We have measured the muon spin rotation and relaxation signals as a function of temperature, giving a measure of the magnetization and spin dynamical properties. The temperature dependence of the internal field is consistent with the S = 12 Weiss molecular field model with a Curie temperature of 0.21 K and a saturation internal field of 60 G. We also report additional studies of the magnetic properties of 3-QNNN and p-NPNN using EPR and a.c. susceptibility techniques. We find evidence for a canted ferromagnetic spin structure in 3-QNNN and a simpler uniform magnetic lattice in p-NPNN.

The effects of open sections of the Fermi surface on the physical properties of 2D organic molecular metals

Abstract We report two cases in which the quasi-one-dimensional (Q1D) sections of Fermi surface (FS) in ET salts contribute to the low temperature behaviour of the magnetoresistance (MR). In κ-(ET)2Cu(NCS)2, breakdown orbits are observed between the Q2D and Q1D sections of FS; the MR oscillations contain a probable contribution from the Stark quantum interference effect. The results for the FS parameters are compared with theoretical calculations and the effects of many-body interactions are deduced. In α-(ET)2KHg(NCS)4, nesting of the Q1D open orbits gives a SDW ground state which considerably modifies the FS and leads to a field-induced transition known as the ‘kink’ between 22 and 23 T. Measurements of MR as a function of orientation of the crystal in the magnetic field reveal that the changes in FS at the ‘kink’ are rather subtle.

Gradual destruction of magnetism in the superconducting family NaFe1−xCoxAs

The interplay and coexistence of superconducting, magnetic and structural order parameters in NaFe{1-x}Co{x}As has been studied using SQUID magnetometry, muon-spin rotation and synchrotron x-ray powder diffraction. Substituting Fe by Co weakens the ordered magnetic state through both a suppression of T_N and a reduction in the size of the ordered moment. Upon further substitution of Fe by Co the high sensitivity of the muon as a local magnetic probe reveals a magnetically disordered phase, in which the size of the moment continues to decrease and falls to zero around the same point at which the magnetically-driven structural distortion is no longer resolvable. Both magnetism and the structural distortion are weakened as the robust superconducting state is established.

Enhanced superfluid stiffness, lowered superconducting transition temperature, and field-induced magnetic state of the pnictide superconductor LiFeAs

F. L. Pratt, P. J. Baker, S. J. Blundell, T. Lancaster, H. J. Lewtas, P. Adamson, M. J. Pitcher, D. R. Parker, and S. J. Clarke ISIS Muon Facility, ISIS, Chilton, Oxon., OX11 0QX, United Kingdom Oxford University Department of Physics, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, United Kingdom (Dated: October 6, 2008)

Anisotropic local modification of crystal field levels in Pr-based pyrochlores : a muon-induced effect modeled using density functional theory.

Although muon spin relaxation is commonly used to probe local magnetic order, spin freezing, and spin dynamics, we identify an experimental situation in which the measured response is dominated by an effect resulting from the muon-induced local distortion rather than the intrinsic behavior of the host compound. We demonstrate this effect in some quantum spin ice candidate materials Pr(2)B(2)O(7) (B=Sn, Zr, Hf), where we detect a static distribution of magnetic moments that appears to grow on cooling. Using density functional theory we show how this effect can be explained via a hyperfine enhancement arising from a splitting of the non-Kramers doublet ground states on Pr ions close to the muon, which itself causes a highly anisotropic distortion field. We provide a quantitative relationship between this effect and the measured temperature dependence of the muon relaxation and discuss the relevance of these observations to muon experiments in other magnetic materials.

Probing magnetic order in LiMPO4(M=Ni, Co, Fe) and lithium diffusion in LixFePO4

Muon spin relaxation measurements are reported on three members of the LixMPO4 series. The magnetic properties of stoichiometric samples with M = Ni, Co, Fe, were investigated at low-temperature. In LiNiPO4 we observe different forms of the muon decay asymmetry in the commensurate and incommensurate antiferromagnetic phases, accompanied by a change in the temperature dependence of the muon oscillation frequency. In LiCoPO4 the form of the muon decay asymmetry indicates that the correlation between layers decreases as the Neel temperature is approached from below. LiFePO4 shows more conventional behaviour, typical for an antiferromagnet. Measurements on LixFePO4 with x = 0.8, 0.9, and 1 show evidence for lithium diffusion below ~250 K and muon diffusion dominating the form of the relaxation at higher temperature. The thermally activated form of the observed hopping rate suggests an activation barrier for lithium diffusion of ~ 100 meV and a diffusion constant of D_Li ~ 10^-10 - 10^-9 cm2/s at room temperature.