La2SrCr2O7: Controlling the tilting distortions of n = 2 Ruddlesden-Popper phases through A-site cation order

Inorganic Chemistry American Chemical Society 55 (2016) 8951–8960-

R Zhang, BM Abbett, G Read, F Lang, T Lancaster, TT Tran, PS Halasyamani, SJ Blundell, NA Benedek, M Hayward

Structural characterization by neutron diffraction, supported by magnetic, SHG, and μ(+)SR data, reveals that the n = 2 Ruddlesden-Popper phase La2SrCr2O7 adopts a highly unusual structural configuration in which the cooperative rotations of the CrO6 octahedra are out of phase in all three Cartesian directions (ΦΦΦz/ΦΦΦz; a(-)a(-)c(-)/a(-)a(-)c(-)) as described in space group A2/a. First-principles DFT calculations indicate that this unusual structural arrangement can be attributed to coupling between the La/Sr A-site distribution and the rotations of the CrO6 units, which combine to relieve the local deformations of the chromium-oxygen octahedra. This coupling suggests new chemical "handles" by which the rotational distortions or A-site cation order of Ruddlesden-Popper phases can be directed to optimize physical behavior. Low-temperature neutron diffraction data and μ(+)SR data indicate La2SrCr2O7 adopts a G-type antiferromagnetically ordered state below TN ∼ 260 K.

Antiferromagnetism in a family of S = 1 square lattice coordination polymers NiX2(pyz)2 (X = Cl, Br, I, NCS; pyz = Pyrazine).

Inorganic Chemistry American Chemical Society 55 (2016) 3515-3529

J Liu, P Goddard, J Singleton, J Brambleby, F Foronda, JS Möller, Y Kohama, S Ghannadzadeh, A Ardavan, SJ Blundell, T Lancaster, F Xiao, RC Williams, FL Pratt, PJ Baker, K Wierschem, SH Lapidus, KH Stone, PW Stephens, J Bendix, TJ Woods, KE Carreiro, HE Tran, CJ Villa, JL Manson

The crystal structures of NiX2(pyz)2 (X = Cl (1), Br (2), I (3) and NCS (4)) were determined at 298 K by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN4X2 units that are bridged by pyz ligands. The 2D layered motifs displayed by 1-4 are relevant to bifluoride-bridged [Ni(HF2)(pyz)2]ZF6 (Z = P, Sb) which also possess the same 2D layers. In contrast, terminal X ligands occupy axial positions in 1-4 and cause a staggering of adjacent layers. Long-range antiferromagnetic order occurs below 1.5 (Cl), 1.9 (Br and NCS) and 2.5 K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and g factor of 2, 3 and 4 are measured by electron spin resonance where no zero–field splitting was found. The magnetism of 1-4 crosses a spectrum from quasi-two-dimensional to three-dimensional antiferromagnetism. An excellent agreement was found between the pulsedfield magnetization, magnetic susceptibility and TN of 2 and 4. Magnetization curves for 2 and 4 calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. 3 is characterized as a three-dimensional antiferromagnet with the interlayer interaction (J⊥) slightly stronger than the interaction within the two-dimensional [Ni(pyz)2]2+ square planes (Jpyz).

Magnetic phase diagram of revised using muon-spin relaxation

Physical Review B (2016)

RC Williams, F Xiao, T Lancaster, R De Renzi, G Allodi, S Bordignon, PG Freeman, FL Pratt, JS M�ller, SJ Blundell, A Boothroyd, D Prabhakaran

� 2016 American Physical Society. We report the results of a muon-spin relaxation (?SR) investigation of La2-xSrxCoO4, an antiferromagnetic insulating series which has been shown to support charge ordered and magnetic stripe phases and an hourglass magnetic excitation spectrum. We present a revised magnetic phase diagram, which shows that the suppression of the magnetic ordering temperature is highly sensitive to small concentrations of holes. Distinct behavior within an intermediate x range (0.2?x0.6) suggests that the putative stripe ordered phase extends to lower x than previously thought. Further charge doping (0.67?x?0.9) prevents magnetic ordering for T1.5K.

Magnetization dynamics and frustration in the multiferroic double perovskite Lu2MnCoO6

Physical Review B - Condensed Matter and Materials Physics American Physicals Society 93 (2016) 134431

VS Zapf, BG Ueland, M Laver, M Lonsky, M Pohlit, J Müller, T Lancaster, JS Möller, SJ Blundell, J Singleton, J Mira, S Yañez-Vilar, MA Señarís-Rodríguez

We investigate the magnetic ordering and the magnetization dynamics (from kHz to THz time scales) of the double perovskite Lu2MnCoO6 using elastic neutron diffraction, muon spin relaxation, and micro-Hall magnetization measurements. This compound is known to be a type II multiferroic with the interesting feature that a ferromagneticlike magnetization hysteresis loop couples to an equally hysteretic electric polarization in the bulk of the material despite a zero-field magnetic ordering of the type ↑↑↓↓ along Co-Mn spin chains. Here we explore the unusual dynamics of this compound and find extremely strong fluctuations, consistent with the axial next-nearest-neighbor Ising (ANNNI) model for frustrated spin chains. We identify three temperature scales in Lu2MnCoO6 corresponding to the onset of highly fluctuating long-range order below TN = 50 ± 3 K identified from neutron scattering, the onset of magnetic and electric hysteresis, with change in kHz magnetic and electric dynamics below a 30 K temperature scale, and partial freezing of ∼MHz spin fluctuations in the muon spin relaxation data below 12 ± 3 K. Our results provide a framework for understanding the multiferroic behavior of this compound and its hysteresis and dynamics.

Robustness of superconductivity to competing magnetic phases in tetragonal FeS

Physical Review B American Physical Society 94 (2016)

FKK Kirschner, F Lang, CV Topping, PJ Baker, FL Pratt, SE Wright, DN Woodruff, SJ Clarke, S Blundell

We have determined the superconducting and magnetic properties of a hydrothermally synthesized powder sample of tetragonal FeS using muon spin rotation (μSR). The superconducting properties are entirely consistent with those of a recently published study, showing fully gapped behavior and giving a penetration depth of λab=204(3) nm. However, our zero-fieldμSR data are rather different and indicate the presence of a small, nonsuperconducting magnetic phase within the sample. These results highlight that sample-to-sample variations in magnetism can arise in hydrothermally prepared phases, but interestingly the superconducting behavior is remarkably insensitive to these variations.

Fourier space derivation of the demagnetization tensor for uniformly magnetized objects of cylindrical symmetry


F Lang, SJ Blundell

Quantum interference in graphene nanoconstrictions

Nano Letters American Chemical Society 16 (2016) 4210–4216-

P Gehring, H Sadeghi, S Sangtarash, CS Lau, J Liu, A Ardavan, JH Warner, CJ Lambert, GAD Briggs, JA Mol

We report quantum interference effects in the electrical conductance of chemical vapour deposited graphene nanoconstrictions fabricated using feedback controlled electroburning. The observed multi-mode Fabry-Pérot interferences can be attributed to reflections on potential steps inside the channel. Sharp anti-resonance features with a Fano line shape are observed. Theoretical modelling reveals that these Fano resonances are due to localised states inside the constriction, which couple to the delocalised states that also give rise to the Fabry-Pérot interference patterns. This study provides new insight into the interplay between two fundamental forms of quantum interference in graphene nanoconstrictions.

The parent Li(OH)FeSe phase of lithium iron hydroxide selenide superconductors

Inorganic Chemistry American Chemical Society 55 (2016) 9886–9891-

S Clarke, SJ Blundell, F Schild, DN Woodruff, SJ Cassidy, JN Blandy, AL Thompson, CV Topping

Lithiation of hydrothermally synthesized Li1-xFex(OH)Fe1–ySe turns on high temperature superconductivity when iron ions are displaced from the hydroxide layers by reductive lithiation to fill the vacancies in the iron selenide layers. Further lithiation results in reductive iron extrusion from the hydroxide layers which turns off superconductivity again as the stoichiometric composition Li(OH)FeSe is approached. The results demonstrate the twin requirements of stoichiometric FeSe layers and reduction of Fe below the +2 oxidation state as found in several iron selenide superconductors.

Magnetic phase diagram of La2−xSrxCoO4 revised using muon-spin relaxation

Physical Review B - Condensed Matter and Materials Physics American Physical Society 93 (2016)

RC Williams, F Xiao, T Lancaster, R De Renzi, G Allodi, S Bordignon, PG Freeman, FL Pratt, JS Moeller, SJ Blundell, A Boothroyd, D Prabhakaran

We report the results of a muon-spin relaxation ( μ SR ) investigation of La 2 − x Sr x CoO 4 , an antiferromagnetic insulating series which has been shown to support charge ordered and magnetic stripe phases and an hourglass magnetic excitation spectrum. We present a revised magnetic phase diagram, which shows that the suppression of the magnetic ordering temperature is highly sensitive to small concentrations of holes. Distinct behavior within an intermediate x range ( 0.2 ≤ x ≲ 0.6 ) suggests that the putative stripe ordered phase extends to lower x than previously thought. Further charge doping ( 0.67 ≤ x ≤ 0.9 ) prevents magnetic ordering for T ≳ 1.5 K .

Experimental and Theoretical Electron Density Analysis of Copper Pyrazine Nitrate Quasi-Low-Dimensional Quantum Magnets.

Journal of the American Chemical Society 138 (2016) 2280-2291

LHR Dos Santos, A Lanza, AM Barton, J Brambleby, WJA Blackmore, PA Goddard, F Xiao, RC Williams, T Lancaster, FL Pratt, SJ Blundell, J Singleton, JL Manson, P Macchi

The accurate electron density distribution and magnetic properties of two metal-organic polymeric magnets, the quasi-one-dimensional (1D) Cu(pyz)(NO3)2 and the quasi-two-dimensional (2D) [Cu(pyz)2(NO3)]NO3·H2O, have been investigated by high-resolution single-crystal X-ray diffraction and density functional theory calculations on the whole periodic systems and on selected fragments. Topological analyses, based on quantum theory of atoms in molecules, enabled the characterization of possible magnetic exchange pathways and the establishment of relationships between the electron (charge and spin) densities and the exchange-coupling constants. In both compounds, the experimentally observed antiferromagnetic coupling can be quantitatively explained by the Cu-Cu superexchange pathway mediated by the pyrazine bridging ligands, via a σ-type interaction. From topological analyses of experimental charge-density data, we show for the first time that the pyrazine tilt angle does not play a role in determining the strength of the magnetic interaction. Taken in combination with molecular orbital analysis and spin density calculations, we find a synergistic relationship between spin delocalization and spin polarization mechanisms and that both determine the bulk magnetic behavior of these Cu(II)-pyz coordination polymers.

Muon-spin relaxation study of the double perovskite insulators Sr2 BOsO6 (B  =  Fe, Y, ln).

Journal of physics. Condensed matter : an Institute of Physics journal 28 (2016) 076001-

RC Williams, F Xiao, IO Thomas, SJ Clark, T Lancaster, GA Cornish, SJ Blundell, W Hayes, AK Paul, C Felser, M Jansen

We present the results of zero-field muon-spin relaxation measurements made on the double perovskite insulators Sr2 BOsO6 (B = Fe,Y, In). Spontaneous muon-spin precession indicative of quasistatic long range magnetic ordering is observed in Sr2FeOsO6 within the AF1 antiferromagnetic phase for temperatures below [Formula: see text] K. Upon cooling below T2≈67 K the oscillations cease to be resolvable owing to the coexistence of the AF1 and AF2 phases, which leads to a broader range of internal magnetic fields. Using density functional calculations we identify a candidate muon stopping site within the unit cell, which dipole field simulations show to be consistent with the proposed magnetic structure. The possibility of incommensurate magnetic ordering is discussed for temperatures below TN = 53 K and 25 K for Sr2YOsO6 and Sr2InOsO6, respectively.

Detection of a Cooper-pair density wave in Bi<inf>2</inf>Sr<inf>2</inf>CaCu<inf>2</inf>O<inf>8+x</inf>

Nature 532 (2016) 343-347

MH Hamidian, SD Edkins, SH Joo, A Kostin, H Eisaki, S Uchida, MJ Lawler, EA Kim, AP Mackenzie, K Fujita, J Lee, JCS Davis

© 2016 Macmillan Publishers Limited. All rights reserved. The quantum condensate of Cooper pairs forming a superconductor was originally conceived as being translationally invariant. In theory, however, pairs can exist with finite momentum Q, thus generating a state with a spatially modulated Cooper-pair density. Such a state has been created in ultracold 6 Li gas but never observed directly in any superconductor. It is now widely hypothesized that the pseudogap phase of the copper oxide superconductors contains such a 'pair density wave' state. Here we report the use of nanometre-resolution scanned Josephson tunnelling microscopy to image Cooper pair tunnelling from a d-wave superconducting microscope tip to the condensate of the superconductor Bi2Sr2CaCu2O8+x. We demonstrate condensate visualization capabilities directly by using the Cooper-pair density variations surrounding zinc impurity atoms and at the Bi2Sr2CaCu2O8+x crystal supermodulation. Then, by using Fourier analysis of scanned Josephson tunnelling images, we discover the direct signature of a Cooper-pair density modulation at wavevectors QP ≈ (0.25, 0)2π/a0 and (0, 0.25)2π/a0 in Bi2Sr2CaCu2O8+x. The amplitude of these modulations is about five per cent of the background condensate density and their form factor exhibits primarily s or s′ symmetry. This phenomenology is consistent with Ginzburg-Landau theory when a charge density wave with d-symmetry form factor and wavevector QC = QP coexists with a d-symmetry superconductor; it is also predicted by several contemporary microscopic theories for the pseudogap phase.

Atomic-scale electronic structure of the cuprate d-symmetry form factor density wave state

Nature Physics 12 (2016) 150-156

MH Hamidian, SD Edkins, CK Kim, JC Davis, AP Mackenzie, H Eisaki, S Uchida, MJ Lawler, EA Kim, S Sachdev, K Fujita

© 2016 Macmillan Publishers Limited. Research on high-temperature superconducting cuprates is at present focused on identifying the relationship between the classic 'pseudogap'phenomenon and the more recently investigated density wave state. This state is generally characterized by a wavevector Q parallel to the planar Cu-O-Cu bonds along with a predominantly d-symmetry form factor (dFF-DW). To identify the microscopic mechanism giving rise to this state, one must identify the momentum-space states contributing to the dFF-DW spectral weight, determine their particle-hole phase relationship about the Fermi energy, establish whether they exhibit a characteristic energy gap, and understand the evolution of all these phenomena throughout the phase diagram. Here we use energy-resolved sublattice visualization of electronic structure and reveal that the characteristic energy of the dFF-DW modulations is actually the 'pseudogap' energy Δ1. Moreover, we demonstrate that the dFF-DW modulations at E=-Δ1 (filled states) occur with relative phase φ compared to those at E=Δ1 (empty states). Finally, we show that the conventionally defined dFF-DW Q corresponds to scattering between the 'hot frontier'regions of momentum-space beyond which Bogoliubov quasiparticles cease to exist. These data indicate that the cuprate dFF-DW state involves particle-hole interactions focused at the pseudogap energy scale and between the four pairs of 'hot frontier'regions in momentum space where the pseudogap opens.

Bimetallic MOFs (H3O)x[Cu(MF6)(pyrazine)2]·(4 - x)H2O (M = V^4+, x = 0; M = Ga^3+, x = 1): co-existence of ordered and disordered quantum spins in the V^4+ system.

Chemical Communications Royal Society of Chemistry 52 (2016) 12653-12656

JL Manson, JA Schlueter, KE Garrett, PA Goddard, T Lancaster, JS Möller, S Blundell, AJ Steele, I Franke, FL Pratt, J Singleton, J Bendix, SH Lapidus, M Uhlarz, O Ayala-Valenzuela, RD McDonald, M Gurak, C Baines

The title compounds are bimetallic MOFs containing [Cu(pyz)2]^2+ square lattices linked by MF6^n- octahedra. In each, only the Cu^2+ spins exhibit long-range magnetic order below 3.5 K (M = V^4+) and 2.6 K (M = Ga^3+). The V^4+ spins remain disordered down to 0.5 K.

Differential conductance and defect states in the heavy-fermion superconductor CeCoIn5

Physical Review B 93 (2016)

JS Van Dyke, JCS Davis, DK Morr

© 2016 American Physical Society. We demonstrate that the electronic band structure extracted from quasiparticle interference spectroscopy [Nat. Phys. 9, 468 (2013)1745-247310.1038/nphys2671] and the theoretically computed form of the superconducting gaps [Proc. Natl. Acad. Sci. USA 111, 11663 (2014)PNASA60027-842410.1073/pnas.1409444111] can be used to understand the dI/dV line shape measured in the normal and superconducting state of CeCoIn5 [Nat. Phys. 9, 474 (2013)1745-247310.1038/nphys2672]. In particular, the dI/dV line shape, and the spatial structure of defect-induced impurity states, reflects the existence of multiple superconducting gaps of dx2-y2 symmetry. These results strongly support a recently proposed microscopic origin of the unconventional superconducting state.

La2SrCr2O7F2: A Ruddlesden-Popper oxyfluoride containing octahedrally coordinated Cr(4+) centers

Inorganic Chemistry American Chemical Society 55 (2016) 3169–3174-

R Zhang, G Read, F Lang, T Lancaster, S Blundell, M Hayward

The low-temperature fluorination of the n = 2 Ruddlesden-Popper phase La2SrCr2O7 yields La2SrCr2O7F2 via a topochemical fluorine insertion reaction. The structure-conserving nature of the fluorination reaction means that the chromium centers of the initial oxide phase retain an octahedral coordination environment in the fluorinated product, resulting in a material containing an extended array of apex-linked Cr(4+)O6 units. Typically materials containing networks of octahedrally coordinated Cr(4+) centers can only be prepared at high pressure; thus, the preparation of La2SrCr2O7F2 demonstrates that low-temperature topochemical reactions offer an alternative synthesis route to materials of this type. Neutron diffraction, magnetization, and μ(+)SR data indicate that La2SrCr2O7F2 undergoes a transition to an antiferromagnetic state below TN ≈ 140 K. The structure-property relations of this phase and other Cr(4+) oxide phases are discussed.

Unconventional magnetism on a honeycomb lattice in studied by muon spin rotation

Physical Review B American Physical Society 94 (2016)

F Lang, PJ Baker, A-A Haghighirad, Y Li, D Prabhakaran, R Valentí, S Blundell

Muon spin rotation measurements have been performed on a powder sample ofα-RuCl3, a layered material in which Ru ions are arranged on a honeycomb lattice and which previously has been proposed to be close to a quantum spin liquid ground state. Our data reveal two distinct transitions at 11 and 14 K, which we interpret as originating from the onset of three-dimensional order and in-plane magnetic order, respectively. We identify, with the help of density functional theory calculations, likely muon stopping sites and combine these with dipolar field calculations to show that the two measured muon rotation frequencies are consistent with two inequivalent muon sites within a zigzag antiferromagnetic structure proposed previously.

Ordering gold nanoparticles with DNA origami nanoflowers

ACS Nano American Chemical Society 10 (2016) 7303–7306-

A Turberfield, R Schreiber, A Ardavan, I Santiago

Nanostructured materials, including plasmonic metamaterials made from gold and silver nanoparticles, provide access to new materials properties. The assembly of nanoparticles into extended arrays can be controlled through surface functionalization and the use of increasingly sophisticated linkers. We present a versatile way to control the bonding symmetry of gold nanoparticles by wrapping them in flower-shaped DNA origami structures. These ‘nanoflowers’ assemble into two-dimensonal gold nanoparticle lattices with symmetries that can be controlled through auxiliary DNA linker strands. Nanoflower lattices are true composites: interactions between the gold nanoparticles are mediated entirely by DNA, and the DNA origami will only fold into its designed form in the presence of the gold nanoparticles.

Control of the third dimension in copper-based square-lattice antiferromagnets

PHYSICAL REVIEW B 93 (2016) ARTN 094430

PA Goddard, J Singleton, I Franke, JS Moeller, T Lancaster, AJ Steele, CV Topping, SJ Blundell, FL Pratt, C Baines, J Bendix, RD McDonald, J Brambleby, MR Lees, SH Lapidus, PW Stephens, BW Twamley, MM Conner, K Funk, JF Corbey, HE Tran, JA Schlueter, JL Manson

Nanoscale depth-resolved polymer dynamics probed by the implantation of low energy muons

Polymer Elsevier 105 (2016) 516-525

FL Pratt, T Lancaster, PJ Baker, SJ Blundell, T Prokscha, E Morenzoni, A Suter, H Assender

The low energy muon (LEM) technique has been used to probe local changes in the dynamical spectrum of thin film polymer samples taking place as a function of the temperature and the implantation depth below the free surface. The studies have been made on samples of polydimethylsiloxane (PDMS) and polybutadiene (PB) using the transverse magnetic field (TF) configuration and diamagnetic probe muons. In PDMS evidence is found for suppression of the glass transition temperature near the surface, along with significantly modified dynamics in the near-surface region as well as at depths significantly below the surface. For PB the LEM technique reveals well-defined layers of dynamical and spatial inhomogeneity at depths of order 0.1–0.2 μm below the free surface. These inhomogeneous regions may be assigned to nanopores produced by solvent streaming during preparation of spin-cast films. A thermal annealing procedure is shown to significantly reduce the thickness of these inhomogeneous layers. These results demonstrate that using LEM in the TF configuration provides a promising new method for studying surface-modified local dynamics of polymers that is also able to reveal nanostructured buried layers in polymer films.