# Publications

## Effect of isoelectronic doping on honeycomb lattice iridate A_2IrO_3

ArXiv (0)

S Manni, S Choi, II Mazin, R Coldea, M Altmeyer, HO Jeschke, R Valenti, P Gegenwart

We have investigated experimentally and theoretically the series (Na$_{1-x}$Li$_{x}$)$_{2}$IrO$_{3}$. Contrary to what has been believed so far, only for $x\leq0.25$ the system forms uniform solid solutions. For larger Li content, as evidenced by powder X-ray diffraction, scanning electron microscopy and density functional theory calculations, the system shows a miscibility gap and a phase separation into an ordered Na$_{3}$LiIr$_2$O$_{6}$ phase with alternating Na$_3$ and LiIr$_2$O$_6$ planes, and a Li-rich phase close to pure Li$_{2}$IrO$_{3}$. For $x\leq 0.25$ we observe (1) an increase of $c/a$ with Li doping up to $x=0.25$, despite the fact that $c/a$ in pure Li$_{2}$IrO$_{3}$ is smaller than in Na$_{2}$IrO$_{3}$, and (2) a gradual reduction of the antiferromagnetic ordering temperature $T_{N}$ and ordered moment. The previously proposed magnetic quantum phase transition at $x\approx 0.7$ may occur in a multiphase region and its nature needs to be re-evaluated.

## Kitaev interactions between j=1/2 moments in honeycomb Na2IrO3 are large and ferromagnetic: insights from ab initio quantum chemistry calculations

ArXiv (0)

VM Katukuri, S Nishimoto, V Yushankhai, A Stoyanova, H Kandpal, S Choi, R Coldea, I Rousochatzakis, L Hozoi, JVD Brink

Na$_2$IrO$_3$, a honeycomb 5$d^5$ oxide, has been recently identified as a potential realization of the Kitaev spin lattice. The basic feature of this spin model is that for each of the three metal-metal links emerging out of a metal site, the Kitaev interaction connects only spin components perpendicular to the plaquette defined by the magnetic ions and two bridging ligands. The fact that reciprocally orthogonal spin components are coupled along the three different links leads to strong frustration effects and nontrivial physics. While the experiments indicate zigzag antiferromagnetic order in Na$_2$IrO$_3$, the signs and relative strengths of the Kitaev and Heisenberg interactions are still under debate. Herein we report results of ab initio many-body electronic structure calculations and establish that the nearest-neighbor exchange is strongly anisotropic with a dominant ferromagnetic Kitaev part, whereas the Heisenberg contribution is significantly weaker and antiferromagnetic. The calculations further reveal a strong sensitivity to tiny structural details such as the bond angles. In addition to the large spin-orbit interactions, this strong dependence on distortions of the Ir$_2$O$_2$ plaquettes singles out the honeycomb 5$d^5$ oxides as a new playground for the realization of unconventional magnetic ground states and excitations in extended systems.

## Electric field control of the magnetic chiralities in ferroaxial multiferroic RbFe(MoO4)2

arXiv [cond-mat.str-el] (2012) 1202.2317

AJ Hearmon, F Fabrizi, LC Chapon, RD Johnson, D Prabhakaran, PJ Brown, PG Radaelli

## Robust topological surface state against direct surface contamination

Physica E: Low-dimensional Systems and Nanostructures (0)

ZK Liu, YL Chen, JG Analytis, SK Mo, DH Lu, RG Moore, IR Fisher, Z Hussain, ZX Shen

## Spin dynamics of frustrated easy-axis triangular antiferromagnet 2H-AgNiO2 explored by inelastic neutron scattering

ArXiv (0)

EM Wheeler, R Coldea, E Wawrzynska, T Sorgel, M Jansen, MM Koza, J Taylor, P Adroguer, N Shannon

We report inelastic neutron scattering measurements of the spin dynamics in the layered hexagonal magnet 2H-AgNiO2 which has stacked triangular layers of antiferromagnetically-coupled Ni2+ spins (S=1) ordered in a collinear alternating stripe pattern. We observe a broad band of magnetic excitations above a small gap of 1.8 meV and extending up to 7.5 meV, indicating strongly dispersive excitations. The measured dispersions of the boundaries of the powder-averaged spectrum can be quantitatively explained by a linear spin-wave dispersion for triangular layers with antiferromagnetic nearest- and weak next-nearest neighbor couplings, a strong easy-axis anisotropy and additional weak inter-layer couplings. The resulting dispersion relation has global minima not at magnetic Bragg wavevectors but at symmetry-related soft points and we attribute this anomalous feature to the strong competition between the easy-axis anisotropy and the frustrated antiferromagnetic couplings. We have also calculated the quantum corrections to the dispersion relation to order 1/S in spin-wave theory by extending the work of Chubukov and Jolicoeur [Phys. Rev. B v46, 11137 (1992)] and find that the presence of easy-axis anisotropy significantly reduces the quantum renormalizations predicted for the isotropic model.

## Multiferroicity and spiral magnetism in FeVO$_4$ with quenched Fe orbital moments

ArXiv (0)

A Daoud-Aladine, B Kundys, C Martin, PG Radaelli, PJ Brown, C Simon, LC Chapon

FeVO$_4$ has been studied by heat capacity, magnetic susceptibility, electric polarization and single crystal neutron diffraction experiments. The triclinic crystal structure is made of \emph{S}-shaped clusters of six Fe$^{3+}$ ions, linked by VO$_4^{3-}$ groups. Two long-range magnetic ordering transitions occur at T$_{N1}$=22K and T$_{N2}$=15K. Both magnetic structures are incommensurate. That stable below T$_{N1}$ is collinear with amplitude modulated moments whereas below T$_{N2}$ the arrangement is non-collinear with a helicoidal modulation. Below T$_{N2}$, \fevo becomes weakly ferroelectric coincidentally with the loss of the collinearity of the magnetic structure. We conclude that \fevo provides another example of frustrated spiral magnet similar to the classical TbMnO$_3$ compound. However, \fevo has quenched orbital moments and a particular structure clarifying the respective role of anisotropy and magnetic frustration in this type of multiferroic materials.

## Magnetoelastic coupling across the metamagnetic transition in Ca$_{2-x}$Sr$_x$RuO$_4$ (0.2 < x < 0.5)

ArXiv (0)

J Baier, P Steffens, O Schumann, M Kriener, S Stark, H Hartmann, O Friedt, A Revcolevschi, PG Radaelli, S Nakatsuji, Y Maeno, JA Mydosh, T Lorenz, M Braden

The magnetoelastic coupling in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ and in Ca$_{1.5}$Sr$_{0.5}$RuO$_4$ has been studied combining high-resolution dilatometer and diffraction techniques. Both compounds exhibit strong anomalies in the thermal-expansion coefficient at zero and at high magnetic field as well as an exceptionally large magnetostriction. All these structural effects, which are strongest in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$, point to a redistribution of electrons between the different $t_{2g}$ orbitals tuned by temperature and magnetic field. The temperature and the field dependence of the thermal-expansion anomalies in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ yield evidence for a critical end-point lying close to the low-temperature metamagnetic transition; however, the expected scaling relations are not well fulfilled.

## Ground states of a frustrated spin-1/2 antifferomagnet: Cs_2CuCl_4 in a magnetic field

ArXiv (0)

MY Veillette, JT Chalker, R Coldea

We present detailed calculations of the magnetic ground state properties of Cs$_2$CuCl$_4$ in an applied magnetic field, and compare our results with recent experiments. The material is described by a spin Hamiltonian, determined with precision in high field measurements, in which the main interaction is antiferromagnetic Heisenberg exchange between neighboring spins on an anisotropic triangular lattice. An additional, weak Dzyaloshinkii-Moriya interaction introduces easy-plane anisotropy, so that behavior is different for transverse and longitudinal field directions. We determine the phase diagram as a function of field strength for both field directions at zero temperature, using a classical approximation as a first step. Building on this, we calculate the effect of quantum fluctuations on the ordering wavevector and components of the ordered moments, using both linear spinwave theory and a mapping to a Bose gas which gives exact results when the magnetization is almost saturated. Many aspects of the experimental data are well accounted for by this approach.

## Observation of extended scattering continua characteristic of spin fractionalization in the 2D frustrated quantum magnet Cs2CuCl4 by neutron scattering

ArXiv (0)

R Coldea, DA Tennant, Z Tylczynski

The magnetic excitations of the quasi-2D spin-1/2 Heisenberg antiferromagnet on an anisotropic triangular lattice Cs2CuCl4 are explored throughout the 2D Brillouin zone using inelastic neutron scattering. In the spin liquid phase above the transition to magnetic order extended excitation continua are observed, characteristic of fractionalization of S=1 spin waves into pairs of deconfined S=1/2 spinons and the hallmark of a resonating-valence-bond (RVB) state. The weak inter-layer couplings stabilize incommensurate spiral order at low temperatures and in this phase sharp magnons carrying a small part of the total scattering weight are observed at low energies below the continuum lower boundary. Linear spin-wave theory including one- and two-magnon processes can describe the sharp magnon excitation, but not the dominant continuum scattering, which instead is well described by a parameterized two-spinon cross-section. Those results suggest a cross-over in the nature of excitations from S=1 spin waves at low energies to S=1/2 spinons at medium to high energies, which could be understood if Cs2CuCl4 was in the close proximity of a transition between a fractional spin liquid and a magnetically-ordered phase.

## Order to disorder transition in the XY-like quantum magnet Cs2CoCl4 induced by noncommuting applied fields

ArXiv (0)

M Kenzelmann, R Coldea, DA Tennant, D Visser, M Hofmann, P Smeibidl, Z Tylczynski

We explore the effects of noncommuting applied fields on the ground-state ordering of the quasi-one-dimensional spin-1/2 XY-like antiferromagnet Cs2CoCl4 using single-crystal neutron diffraction. In zero field interchain couplings cause long-range order below T_N=217(5) mK with chains ordered antiferromagnetically along their length and moments confined to the (b,c) plane. Magnetic fields applied at an angle to the XY planes are found to initially stabilize the order by promoting a spin-flop phase with an increased perpendicular antiferromagnetic moment. In higher fields the antiferromagnetic order becomes unstable and a transition occurs to a phase with no long-range order in the (b,c) plane, proposed to be a spin liquid phase that arises when the quantum fluctuations induced by the noncommuting field become strong enough to overcome ordering tendencies. Magnetization measurements confirm that saturation occurs at much higher fields and that the proposed spin-liquid state exists in the region 2.10 < H_SL < 2.52 T || a. The observed phase diagram is discussed in terms of known results on XY-like chains in coexisting longitudinal and transverse fields.

## The properties of Haldane excitations and multi-particle states in the antiferromagnetic spin-1 chain compound CsNiCl

ArXiv (0)

M Kenzelmann, RA Cowley, WJL Buyers, Z Tun, R Coldea, M Enderle

We report inelastic time-of-flight and triple-axis neutron scattering measurements of the excitation spectrum of the coupled antiferromagnetic spin-1 Heisenberg chain system CsNiCl3. Measurements over a wide range of wave-vector transfers along the chain confirm that above T_N CsNiCl3 is in a quantum-disordered phase with an energy gap in the excitation spectrum. The spin correlations fall off exponentially with increasing distance with a correlation length xi=4.0(2) sites at T=6.2K. This is shorter than the correlation length for an antiferromagnetic spin-1 Heisenberg chain at this temperature, suggesting that the correlations perpendicular to the chain direction and associated with the interchain coupling lower the single-chain correlation length. A multi-particle continuum is observed in the quantum-disordered phase in the region in reciprocal space where antiferromagnetic fluctuations are strongest, extending in energy up to twice the maximum of the dispersion of the well-defined triplet excitations. We show that the continuum satisfies the Hohenberg-Brinkman sum rule. The dependence of the multi-particle continuum on the chain wave-vector resembles that of the two-spinon continuum in antiferromagnetic spin-1/2 Heisenberg chains. This suggests the presence of spin-1/2 degrees of freedom in CsNiCl3 for T < 12K, possibly caused by multiply-frustrated interchain interactions.

## Evolution of spin excitations in a gapped antiferromagnet from the quantum to the high-temperature limit

ArXiv (0)

M Kenzelmann, RA Cowley, WJL Buyers, R Coldea, M Enderle, DF McMorrow

We have mapped from the quantum to the classical limit the spin excitation spectrum of the antiferromagnetic spin-1 Heisenberg chain system CsNiCl3 in its paramagnetic phase from T=5 to 200K. Neutron scattering shows that the excitations are resonant and dispersive up to at least T=70K, but broaden considerably with increasing temperature. The dispersion flattens out with increasing temperature as the resonance energy Delta at the antiferromagnetic wave-vector increases and the maximum in the dispersion decreases. The correlation length xi between T=12 and 50K is in agreement with quantum Monte Carlo calculations. xi is also consistent with the single mode approximation, suggesting that the excitations are short-lived single particle excitations. Below T=12K where three-dimensional spin correlations are important, xi is shorter than predicted and the experiment is not consistent with the random phase approximation for coupled quantum chains. At T=200K, the structure factor and second energy moment of the excitation spectrum are in excellent agreement with the high-temperature series expansion.

## Mesoscopic and Microscopic Phase Segregation in Manganese Perovskites

ArXiv (0)

PG Radaelli, RM Ibberson, DN Argyriou, H Casalta, KH Andersen, S-W Cheong, JF Mitchell

Mesoscopic (500-2000 Angstrom) and microscopic (5-20 Angstrom) phase segregation with temperature and magnetic field was studied in the model manganite Pr0.7Ca0.3MnO3 by high-resolution neutron diffraction and inelastic neutron scattering. Intra-granular strain-driven mesoscopic segregation between two insulating phases, one of which is charge ordered (CO), sets in below the CO temperature in zero field. The CO phase orders antiferromagnetically, while the other insulating phase shows spin-glass behavior. After field-induced metallization, the CO phase coexists with a ferromagnetic metallic phase.

## Local Atomic Strain in ZnSe(1-x)Te(x) from High Real Space Resolution Neutron Pair Distribution Function Measurements

ArXiv (0)

PF Peterson, T Proffen, I-K Jeong, SJL Billinge, K-S Choi, MG Kanatzidis, PG Radaelli

High real-space resolution atomic pair distribution functions (PDFs) have been obtained from ZnSe(1-x)Te(x) using neutron powder diffraction. Distinct Zn-Se and Zn-Te nearest neighbor (nn) bonds, differing in length by delta_r= 0.14Angstroms, are resolved in the measured PDF allowing the evolution with composition of the individual bond-lengths to be studied. The local bond-lengths change much more slowly with doping than the average bond-length obtained crystallographically. The nn bond-length distributions are constant with doping but higher-neighbor pair distributions broaden significantly indicating that most of the strain from the alloying is accommodated by bond-bending forces in the alloy. The PDFs of alloys across the whole doping range are well fit using a model based on the Kirkwood potential. The resulting PDFs give excellent agreement with the measured PDFs over the entire alloy range with no adjustable parameters.

## Spin dynamics of the model 2D quantum antiferromagnet CFTD

ArXiv (0)

HM Ronnow, DF McMorrow, R Coldea, A Harrison, ID Youngson, TG Perring, G Aeppli, O Syljyasen, K Lefmann, C Rischel

The magnetic excitation spectrum in the two-dimensional (2D) S=1/2 Heisenberg antiferromagnet copper deuteroformate tetradeuterate (CFTD) has been measured for temperatures up to T\sim J/2, where J=6.31+-0.02 meV is the 2D exchange coupling. For T\ll J, a dispersion of the zone boundary energy is observed, which is attributed to a wavevector dependent quantum renormalization. At higher temperatures, spin-wave-like excitations persist, but are found to broaden and soften. By combining our data with numerical calculations, and with existing theoretical work, a consistent description of the behaviour of the model system is found over the whole temperature interval investigated.

## Magnetic and Electric properties of La_{1-delta}MnO_{3}

ArXiv (0)

SD Brion, F Ciorcas, G Chouteau, P Lejay, P Radaelli, C Chaillout

The magnetic phase diagram of $La_{1-\delta}MnO_{3}$ powdered samples have been studied as a function of $\delta$ in the low doping range. $La_{0.97}MnO_{3}$ has a canted magnetic structure at low temperature $(\theta \simeq 130\QTR{group}{{}^{\circ}})$. Above $T_{C}=118K$, it becomes a paramagnet with a huge effective magnetic moment, $\mu_{eff}=6.0\mu_{B}$, reflecting the presence of magnetoelastic polarons which are not affected by the magnetic field (up to 20T) nor the temperature $(1.2T_{C}<T<2.5T_{C})$. When $\delta$ is increased to $\delta =0.07$, polarons are still presen at high temperature, with a smaller size: $\mu_{eff}=5.8\mu_{B}$. The system becomes fully ferromagnetic below 170K but remains insulating down to the lowest temperature.

## The Quasi-1D S=1/2 Antiferromagnet Cs2CuCl4 in a Magnetic Field

ArXiv (0)

R Coldea, DA Tennant, RA Cowley, DF McMorrow, B Dorner, Z Tylczynski

Magnetic excitations of the quasi-1D S=1/2 Heisenberg antiferromagnet (HAF) Cs2CuCl4 have been measured as a function of magnetic field using neutron scattering. For T<0.62 K and B=0 T the weak inter-chain coupling produces 3D incommensurate ordering. Fields greater than Bc =1.66 T, but less than the field (~8 T) required to fully align the spins, are observed to decouple the chains, and the system enters a disordered intermediate-field phase (IFP). The IFP excitations are in agreement with the predictions of Muller et al. for the 1D S=1/2 HAF, and Talstra and Haldane for the related 1/r^2 chain (the Haldane-Shastry model). This behaviour is inconsistent with linear spin-wave theory.

## Reply to Comment by Borisenko et al. on article `A de Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAs'

ArXiv (0)

C Putzke, AI Coldea, I Guillamon, D Vignolles, A McCollam, D LeBoeuf, MD Watson, II Mazin, S Kasahara, T Terashima, T Shibauchi, Y Matsuda, A Carrington

Recently, Borisenko et al have posted a Comment (arXiv:1108.1159) where they suggest an alternative interpretation of our de Haas-van Alphen (dHvA) measurements on the superconductor LiFeAs. In our original paper (arXiv:1107.4375) we concluded that our measurements of the bulk Fermi surface were not consistent with the surface bands observed thus far by ARPES. Borisenko et al dispute this and suggest the two measurements are consistent if some of the orbits we observe are due to magnetic breakdown. We argue here that this scenario is inconsistent with the experimental data and therefore that our original conclusion stands.

## Interplay between localized and itinerant d electrons in a frustrated metallic antiferromagnet, 2H-AgNiO2

ArXiv (0)

AI Coldea, A Carrington, R Coldea, L Malone, AF Bangura, MD Johannes, II Mazin, EA Yelland, JG Analytis, JAAJ Perenboom, C Jaudet, D Vignolles, T Sorgel, M Jansen

We report the electronic and magnetic behaviour of the frustrated triangular metallic antiferromagnet 2H-AgNiO2 in high magnetic fields (54 T) using thermodynamic and transport measurements. Here localized d electrons are arranged on an antiferromagnetic triangular lattice nested inside a honeycomb lattice with itinerant d electrons. When the magnetic field is along the easy axis we observe a cascade of field-induced transitions, attributed to the competition between easy-axis anisotropy, geometrical frustration and coupling of the localized and itinerant system. The quantum oscillations data suggest that the Fermi surface is reconstructed by the magnetic order but in high fields magnetic breakdown orbits are possible. The itinerant electrons are extremely sensitive to scattering by spin fluctuations and a significant mass enhancement (~ 3) is found.

## Reciprocal space mapping of magnetic order in thick epitaxial MnSi films

arxiv (0)

B Wiedemann, A Chacon, SL Zhang, Y Khaydukov, T Hesjedal, O Soltwedel, T Keller, S Mühlbauer, T Adams, M Halder, C Pfleiderer, P Böni

We report grazing incidence small angle neutron scattering (GISANS) and complementary off-specular neutron reflectometry (OSR) of the magnetic order in a single-crystalline epitaxial MnSi film on Si(111) in the thick film limit. Providing a means of direct reciprocal space mapping, GISANS and OSR reveal a magnetic modulation perpendicular to the films under magnetic fields parallel and perpendicular to the film, where additional polarized neutron reflectometry (PNR) and magnetization measurements are in excellent agreement with the literature. Regardless of field orientation, our data does not suggest the presence of more complex spin textures, notably the formation of skyrmions. This observation establishes a distinct difference with bulk samples of MnSi of similar thickness under perpendicular field, in which a skyrmion lattice dominates the phase diagram. Extended x-ray absorption fine structure measurements suggest that small shifts of the Si positions within the unstrained unit cell control the magnetic state, representing the main difference between the films and thin bulk samples.