arXiv subjects

J. P. Attfield

Publications and source records attributed to J. P. Attfield.

34 records · Page 2Linked to original sources

arXiv2014

From Spin Glass to Quantum Spin Liquid Ground States in Molybdate Pyrochlores

We present new magnetic heat capacity and neutron scattering results for two magnetically frustrated molybdate pyrochlores: $S=1$ oxide Lu$_2$Mo$_2$O$_7$ and $S={\frac{1}{2}}$ oxynitride Lu$_2$Mo$_2$O$_5$N$_2$. Lu$_2$Mo$_2$O$_7$ undergoes a transition to an unconventional spin glass ground state at $T_f {\sim} 16$ K. However, the preparation of the corresponding oxynitride tunes the nature of the ground state from spin glass to quantum spin liquid. The comparison of the static and dynamic spin correlations within the oxide and oxynitride phases presented here reveals the crucial role played by quantum fluctuations in the selection of a ground state. Furthermore, we estimate an upper limit for a gap in the spin excitation spectrum of the quantum spin liquid state of the oxynitride of $Δ {\sim} 0.05$ meV or ${\fracΔ{|θ|}}\sim0.004$, in units of its antiferromagnetic Weiss constant $θ {\sim}-121$ K.

cond-mat.str-el↗

arXiv2013

Gapless spin liquid ground state in the S=1/2 vanadium oxyfluoride kagome antiferromagnet [NH4]2[C7H14N][V7O6F18]

The vanadium oxyfluoride [NH4]2[C7H14N][V7O6F18] (DQVOF) is a geometrically frustrated magnetic bilayer material. The structure consists of S=1/2 kagome planes of V4+ d1 ions with S=1 V3+ d2 ions located between the kagome layers. Muon spin relaxation measurements demonstrate the absence of spin freezing down to 40 mK despite an energy scale of 60 K for antiferromagnetic exchange interactions. From magnetization and heat capacity measurements we conclude that the S=1 spins of the interplane V3+ ions are weakly coupled to the kagome layers, such that DQVOF can be viewed as an experimental model for S=1/2 kagome physics, and that it displays a gapless spin liquid ground state.

cond-mat.str-el↗

arXiv2012

A MuSR study of the magnetoresistive ruthenocuprates RuSr2Nd1.8-xY0.2CexCu2O_{10-δ} (x = 0.95 and 0.80)

Zero field muon spin relaxation (ZF-μSR) has been used to study the magnetic properties of the underdoped giant magnetoresistive ruthenocuprates (GMR) RuSr2Nd1.8-xY0.2CexCu2O10-d (x = 0.95, 0.80). The results show a gradual loss of initial asymmetry A0 at the ruthenium spin transition temperature, TRu. At the same time the electronic relaxation rate, lambda shows a gradual increase with decreasing temperature below TRu. These results have been interpreted as evidence for Cu spin cluster formation below TRu. These magnetically ordered clusters grow as the temperature is decreased thus causing the initial asymmetry to decrease slowly. GMR is observed over a wide temperature range in the materials studied and the magnitude increases as the temperature is reduced from TRu to 4 K which suggests a relation between Cu spin cluster size and -MR.

cond-mat.supr-con↗

arXiv2009

Suppression of the Superconducting Transition in RFeAsO1-xFx for R = Tb, Dy, Ho

A suppression of superconductivity in the late rare earth RFeAsO1-xFx materials is reported. The maximum critical temperature (Tc) decreases from 51 K for R = Tb to 36 K for HoFeAsO0.9F0.1, which has been synthesised under 10 GPa pressure. This suppression is driven by a decrease in the Fe-As-Fe angle below an optimimum value of 110.6 degrees, as the angle decreases linearly with unit cell volume (V) across the RFeAsO1-xFx series. A crossover in electronic structure around this optimum geometry is evidenced by a change in sign of the compositional dTc/dV, from negative values for previously reported large R materials to positive for HoFeAsO0.9F0.1.

cond-mat.supr-con↗

arXiv2008

Metal-Insulator Transition and Orbital Order in PbRuO3

Anomalous low temperature electronic and structural behaviour has been discovered in PbRuO3. The structure (space group Pnma, a = 5.56314(1), b = 7.86468(1), c = 5.61430(1) A) and metallic conductivity at 290 K are similar to those of SrRuO3 and other ruthenate perovskites, but a sharp metal-insulator transition at which the resistivity increases by four orders of magnitude is discovered at 90 K. This is accompanied by a first order structural transition to an Imma phase (a = 5.56962(1), b = 7.74550(1), c = 5.66208(1) A at 25 K) that shows a coupling of Ru4+ 4d orbital order to distortions from Pb2+ 6s6p orbital hybridization. The Pnma to Imma transition is an unconventional reversal of the group-subgroup symmetry relationship. No long range magnetic order is evident down to 1.5 K. Electronic structure calculations show that hybridization of Pb 6s6p and Ru 4d orbitals and strong spin-orbit coupling stabilise this previously hidden ground state for ruthenate perovskites

cond-mat.str-el↗

arXiv2008

Limited local electron-lattice coupling in manganites

(Pr,Ca)MnO3 is the archetypal charge-ordered manganite, but in Pr0.48Ca0.52MnO3 we find (using convergent-beam electron diffraction and dark-field images) that the superlattice period is locally incommensurate with respect to the parent lattice, and that the superlattice orientation possesses significant local variations. This suggests that local electron-lattice coupling never overwhelmingly dominates the rich physics of manganites, even in the most extreme scenarios that produce the largest colossal magnetoresistance effects.

cond-mat.str-el↗

arXiv2007

Chemical Tuning of Positive and Negative Magnetoresistances, and Superconductivity in 1222-type Ruthenocuprates

High critical-temperature superconductivity and large (colossal) magnetoresistances are two important electronic conducting phenomena found in transition metal oxides. High-Tc materials have applications such as superconducting magnets for MRI and NMR, and magnetoresistive materials may find use in magnetic sensors and spintronic devices. Here we report chemical doping studies of RuSr2(R2-xCex)Cu2O10-d ruthenocuprates which show that a single oxide system can be tuned between superconductivity at high hole dopings and varied magnetoresistive properties at low doping levels. A robust variation of negative magnetoresistance with hole concentration is found in the RuSr2R1.8-xY0.2CexCu2O10-d series, while RuSr2R1.1Ce0.9Cu2O10-d materials show an unprecedented crossover from negative to positive magnetoresistance with rare earth (R) ion radius.

cond-mat.supr-con↗

arXiv2007

Induced antiferromagnetism and large magnetoresistances in RuSr2(Nd,Y,Ce)2Cu2O10-d ruthenocuprates

RuSr2(Nd,Y,Ce)2Cu2O10-d ruthenocuprates have been studied by neutron diffraction, magnetotransport and magnetisation measurements and the electronic phase diagram is reported. Separate Ru and Cu spin ordering transitions are observed, with spontaneous Cu antiferromagnetic order for low hole doping levels p, and a distinct, induced-antiferromagnetic Cu spin phase in the 0.02 < p < 0.06 pseudogap region. This ordering gives rise to large negative magnetoresistances which vary systematically with p in the RuSr2Nd1.8-xY0.2CexCu2O10-d series. A collapse of the magnetoresistance (MR) and magnetisation in the pre-superconducting region may signify the onset of superconducting fluctuations.

cond-mat.supr-con↗

arXiv2007

Very weak electron-phonon coupling and strong strain coupling in manganites

The coupling of the manganite stripe phase to the lattice and to strain has been investigated via transmission electron microscopy studies of polycrystalline and thin film manganites. In polycrystalline \PCMOfiftwo a lockin to $q/a^*=0.5$ in a sample with $x>0.5$ has been observed for the first time. Such a lockin has been predicted as a key part of the Landau CDW theory of the stripe phase. Thus it is possible to constrain the size of the electron-phonon coupling in the CDW Landau theory to between 0.04% and 0.05% of the electron-electron coupling term. In the thin film samples, films of the same thickness grown on two different substrates exhibited different wavevectors. The different strains present in the films on the two substrates can be related to the wavevector observed via Landau theory. It is demonstrated that the the elastic term which favours an incommensurate modulation has a similar size to the coupling between the strain and the wavevector, meaning that the coupling of strain to the superlattice is unexpectedly strong.

cond-mat.str-el↗

arXiv2007

Triplet Dimerization Crossover Driven by Magnetic Frustration in In2VO5

In2VO5, containing magnetically frustrated zig-zag chains, shows a remarkable magnetic crossover at 120 K between paramagnetic states with positive (17 K) and negative (-70 K) Weiss temperatures. Magnetic moment and entropy data show that the V4+ S = 1/2 spins condense into S = 1 triplet dimers below the crossover. A further freezing of the antiferromagnetically coupled triplet dimers into a global singlet state is observed at 2.5 K, with no long range magnetic order down to 0.42 K and in fields up to 9 T. No structural V-V dimerization is observed by high-resolution X-ray diffraction down to 10 K, but a subtle lattice anomaly evidences a spin-lattice coupling in the triplet dimer state. This is assigned to longitudinal oxygen displacement modes that reduce frustration within the chains and so couple to the spin dimer fluctuations.

cond-mat.str-el↗

arXiv2006

Absence of conventional charge ordering in Na0.5CoO2 from a high resolution neutron diffraction study

The structure of Na0.5CoO2, the low temperature insulator that separates the magnetic and superconducting regions in the NaxCoO2.yH2O phase diagram, is studied by high resolution powder neutron diffraction at temperatures between 10 and 300 K. Profile analysis confirms that it has an orthorhombic symmetry structure, space group Pnmm, consisting of layers of edge-sharing CoO6 octahedra in a triangular lattice, with Na ions occupying ordered positions in one-dimensional chains in the interleaving planes. The oxygen content is found to be stoichiometric within 1%, indicating that the Na concentration accurately determines the electronic doping. The Na ordering creates two distinct Co sites with different numbers of Na neighbours, but the difference in their Co-O bond distances and the derived bond valence sums is small.

cond-mat.str-el↗

arXiv2005

Charge order in Fe2OBO3: An LSDA+U study

Charge ordering in the low-temperature monoclinic structure of iron oxoborate (Fe2OBO3) is investigated using the local spin density approximation (LSDA)+U method. While the difference between t_{2g} minority occupancies of Fe^{2+} and Fe^{3+} cations is large and gives direct evidence for charge ordering, the static "screening" is so effective that the total 3d charge separation is rather small. The occupied Fe^{2+} and Fe^{3+} cations are ordered alternately within the chain which is infinite along the a-direction. The charge order obtained by LSDA+U is consistent with observed enlargement of the βangle. An analysis of the exchange interaction parameters demonstrates the predominance of the interribbon exchange interactions which determine the whole L-type ferrimagnetic spin structure.

cond-mat.str-el↗

arXiv2005

Weak charge-lattice coupling requires reinterpretation of stripes of charge order in La1-xCaxMnO3

Modulations in manganites attributed to stripes of charge/orbital/spin order are thought to result from strong electron-lattice interactions that lock the superlattice and parent lattice periodicities. Surprisingly in La1-xCaxMnO3(x>0.5, 90 K), convergent beam (3.6 nm spot) electron diffraction patterns rule out charge stacking faults and indicate a superlattice with uniform periodicity. Moreover, large area electron diffraction peaks are sharper than simulations with stacking faults. Since the electron-lattice coupling does not lock the two periodicities (to yield stripes) it may be too weak to strongly localise charge.

cond-mat.str-el↗

arXiv2003

Magneto-Transport Properties of Doped RuSr$_2$GdCu$_2$O$_8$

RuSr$_2$GdCu$_2$O$_8$, in which magnetic order and superconductivity coexist with $T_{Magnetic}$$\gg$$T_c$, is a complex material which poses new and important questions to our understanding of the interplay between magnetic and superconducting (SC) order. Resistivity, Hall effect and thermopower measurements on sintered ceramic RuSr$_2$GdCu$_2$O$_8$ are presented, together with results on a broad range of substituted analogues. The Hall effect and thermopower both show anomalous decreases below $T_{Magnetic}$ which may be explained within a simple two-band model by a transition from localized to more itinerant behavior in the RuO$_2$ layer at $T_{Magnetic}$.

cond-mat.supr-con↗

arXiv2001

Long Range Charge Ordering in Magnetite Below the Verwey Transition

The crystal structure of Fe3O4 below the 122 K Verwey transition has been refined using high-resolution X-ray and neutron powder diffraction data. The refinements give direct evidence for charge ordering (CO) over four independent octahedral Fe sites, two with a charge of +2.4 and the other two of +2.6. CO schemes consistent with our model do not meet the widely-accepted Anderson condition of minimum electrostatic repulsion. Instead we propose that CO is driven primarily by a [001] electronic instability, which opens a gap at the transition through a charge density wave mechanism.

cond-mat.str-el↗

arXiv2001

Magnetization, Structural and Nuclear Quadrupole Resonance Study of RuSr2EuCeCu2O10+d

We present the results from a magnetization, structural, and nuclear quadrupole resonance study of the ruthenate-cuprate, RuSr2R2-xCexCu2O10+d with x=1 and R=Eu. This compound is a superconductor for Ce doping in the range 0.4<=x<=0.8 and displays ferromagnetic order for 0.4<=x<=1.0. We show that the tilting and rotation of the RuO6 octahedra are essentially the same for x=1, x=0.6 and the superconducting antiferromagnet, RuSr2GdCu2O8. However, the moment per Ru in RuSr2EuCeCu2O10+d is comparable to that observed in RuSr2EuCu2O8. These results indicate that it is unlikely that the different magnetic order found in RuSr2R2-xCexCu2O10+d and RuSr2RCu2O8 (R=Y,Gd,Eu) is due to changes in structurally induced distortions of the RuO6 octahedra as found in the ruthenate compounds (e.g. Sr1-xCaxRuO3). We show that the Cu nuclear quadrupole resonance (NQR) data are similar to those observed in the single CuO2 layer superconductor, La1-xSrxCuO4, where the Cu spin-lattice relaxation rate is dominated by hyperfine coupling within the CuO2 layers and any additional hyperfine coupling from spin fluctuations in the RuO2 layers is small.

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