Stephen J. Blundell

Will Spin-Relaxation Times in Molecular Magnets Permit Quantum Information Processing?

Using X-band pulsed electron-spin resonance, we report the intrinsic spin-lattice (T1) and phase-coherence (T2) relaxation times in molecular nanomagnets for the first time. In Cr7M heterometallic wheels, with M=Ni and Mn, phase-coherence relaxation is dominated by the coupling of the electron spin to protons within the molecule. In deuterated samples T2 reaches 3 micros at low temperatures, which is several orders of magnitude longer than the duration of spin manipulations, satisfying a prerequisite for the deployment of molecular nanomagnets in quantum information applications.

Spin waves and revised crystal structure of honeycomb iridate Na2IrO3.

We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zigzag magnetic order. The onset of long-range magnetic order below T(N)=15.3  K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90° Ir-O-Ir bonds required for dominant Kitaev exchange.

Soft Chemical Control of Superconductivity in Lithium Iron Selenide Hydroxides Li1–xFex(OH)Fe1–ySe

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li(1-x)Fe(x)(OH)Fe(1-y)Se (x ∼ 0.2; 0.02 < y < 0.15) with a wide range of iron site vacancy concentrations in the iron selenide layers. This iron vacancy concentration is revealed as the only significant compositional variable and as the key parameter controlling the crystal structure and the electronic properties. Single crystal X-ray diffraction, neutron powder diffraction, and X-ray absorption spectroscopy measurements are used to demonstrate that superconductivity at temperatures as high as 40 K is observed in the hydrothermally synthesized samples when the iron vacancy concentration is low (y < 0.05) and when the iron oxidation state is reduced slightly below +2, while samples with a higher vacancy concentration and a correspondingly higher iron oxidation state are not superconducting. The importance of combining a low iron oxidation state with a low vacancy concentration in the iron selenide layers is emphasized by the demonstration that reductive postsynthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li(1-x)Fe(x)(OH) reservoir layer to fill vacancies in the selenide layer.

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.

Frustration of Magnetic and Ferroelectric Long-Range Order in Bi2Mn4/3Ni2/3O6

The slight incommensurate modulation of the structure of Bi(2)Mn(4/3)Ni(2/3)O(6) is sufficient to suppress the electrical polarization which arises in commensurate treatments of the structure, due to antiferroelectric coupling of local polar units of over 900 A(3). The incommensurate structure is produced by the competition between ferroelectric Bi lone pair-driven A site displacement, chemical order of Mn and Ni on the B site, and both charge and orbital order at these transition metals. The interplay between the frustrated polar Bi displacements and the frustrated spin order at the B site, induced by positional disorder, produces magnetodielectric coupling between the incommensurately modulated lattice and the spin-glass-like ground state with an unusual relationship between the magnetocapacitance and the applied field.

Angle-dependent magnetoresistance of the layered organic superconductor κ − ( ET ) 2 Cu ( NCS ) 2 : Simulation and experiment

The angle-dependences of the magnetoresistance of two different isotopic substitutions (deuterated and undeuterated) of the layered organic superconductor \kappa-(ET)2Cu(NCS)2 are presented. The angle dependent magnetoresistance oscillations (AMRO) arising from the quasi-one-dimensional (Q1D) and quasi-two-dimensional (Q2D) Fermi surfaces in this material are often confused. By using the Boltzman transport equation extensive simulations of the AMRO are made that reveal the subtle differences between the different species of oscillation. No significant differences are observed in the electronic parameters derived from quantum oscillations and AMRO for the two isotopic substitutions. The interlayer transfer integrals are determined for both isotopic substitutions and a slight difference is observed which may account for the negative isotope effect previously reported [1]. The success of the semi-classical simulations suggests that non-Fermi liquid effects are not required to explain the interlayer-transport in this system.

A study of the magnetoresistance of the charge-transfer salt at hydrostatic pressures of up to 20 kbar: evidence for a charge-density-wave ground state and the observation of pressure-induced superconductivity

The magnetoresistance of single crystals of the quasi-two-dimensional (Q2D) organic conductor has been studied at temperatures between 700 mK and 300 K in magnetic fields of up to 15 T and hydrostatic pressures of up to 20 kbar. Measurements of the resistivity using a direct-current van der Pauw technique at ambient pressure show that the material undergoes a metal-to-insulator transition at ; below this temperature the resistivity increases by more than five orders of magnitude as the samples are cooled to 4.2 K. If the current exceeds a critical value, the sample resistivity undergoes irreversible changes, and exhibits non-ohmic behaviour over a wide temperature range. Below 30 K, either an abrupt increase of the resistivity by two orders of magnitude or bistable behaviour is observed, depending on the size and/or direction of the measurement current and the sample history. These experimental data strongly suggest that the metal - insulator transition and complex resistivity behaviour are due to the formation of a charge-density wave (CDW) with a well-developed domain structure. The magnetotransport data recorded under hydrostatic pressure indicate that pressure has the effect of gradually reducing the CDW ordering temperature. At higher pressures, there is a pressure-induced transition from the CDW state to a metallic, superconducting state which occurs in two distinct stages. Firstly, a relatively small number of Q2D carriers are induced, evidence for which is seen in the form of the magnetoresistance and the presence of Shubnikov - de Haas oscillations; in spite of the low carrier density, the material then superconducts below a temperature of . Subsequently, at higher pressures, the CDW state collapses, resulting in Q1D behaviour of the magnetoresistance, and eventual suppression of the superconductivity.

Colossal magnetoresistance in ( x = 0.0, 0.1)

Magnetization and magnetotransport measurements have been used to study the composition dependence of the electronic properties of the Ruddlesden - Popper phases and . Although their behaviour differs in detail, both compounds show a colossal magnetoresistance (CMR) effect (> 10000% in 14 T) in the temperature range . However, neither material shows a transition to a ferromagnetic state above 4.2 K, and both materials have higher resistivities ( for ) than the metallic oxides previously found to show CMR. In view of the low conductivity and the absence of ferromagnetism, the CMR of these phases is not readily explained by a double-exchange mechanism.

Angle dependence of the upper critical field in the layered organic superconductor kappa-(BEDT-TTF)2Cu(NCS)2 (BEDT-TTFequiv bis(ethylene-dithio)tetrathiafulvalene)

We have performed detailed studies of the angle- and temperature-dependent resistive upper critical fields in the layered organic superconductor -(BEDT-TTF)2Cu(NCS)2. With the magnetic field lying in the conducting planes, our measurements show an upper critical field which comfortably exceeds the Pauli-paramagnetic limit in this material. We find no azimuthal angle dependence of the critical field, in spite of recent evidence that this material has gap nodes characteristic of d-wave superconductivity. We propose that the large critical fields may be due to a Fulde-Ferrell-Larkin-Ovchinnikov state which can exist in exactly in-plane fields because of the nature of the Fermi surface of -(BEDT-TTF)2Cu(NCS)2.

Local magnetism and spin correlations in the geometrically frustrated cluster magnet LiZn2Mo3O8.

LiZn2Mo3O8 has been proposed to contain S=½Mo3O13 magnetic clusters arranged on a triangular lattice with antiferromagnetic nearest-neighbor interactions. Here, microwave and terahertz electron spin resonance, Li7 nuclear magnetic resonance, and muon spin rotation spectroscopies are used to characterize the local magnetic properties of LiZn2Mo3O8. These results show the magnetism in LiZn2Mo3O8 arises from a single isotropic S=½electron per cluster and that there is no static long-range magnetic ordering down to T = 0.07 K. Further, there is evidence of gapless spin excitations with spin fluctuations slowing down as the temperature is lowered. These data indicate strong spin correlations, which, together with previous data, suggest a low-temperature resonating valence-bond state in LiZn2Mo3O8.