# Publications

## Proximity-induced odd-frequency superconductivity in a topological insulator

arxiv (0)

JA Krieger, A Pertsova, SR Giblin, M Döbeli, T Prokscha, CW Schneider, A Suter, T Hesjedal, AV Balatsky, Z Salman

At an interface between a topological insulator (TI) and a conventional superconductor (SC), superconductivity has been predicted to change dramatically and exhibit novel correlations. In particular, the induced superconductivity by an $s$-wave SC in a TI can develop an order parameter with a $p$-wave component. Here we present experimental evidence for an unexpected proximity-induced novel superconducting state in a thin layer of the prototypical TI, Bi$_2$Se$_3$, proximity-coupled to Nb. From depth-resolved magnetic field measurements below the superconducting transition temperature of Nb, we observe a local enhancement of the magnetic field in Bi$_2$Se$_3$ that exceeds the externally applied field, thus supporting the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state. Our experimental results are complemented by theoretical calculations supporting the appearance of an odd-frequency component at the interface which extends into the TI. This state is topologically distinct from the conventional Bardeen-Cooper-Schrieffer (BCS) state it originates from. To the best of our knowledge, these findings present a first observation of bulk odd-frequency superconductivity in a TI. We thus reaffirm the potential of the TI/SC interface as a versatile platform to produce novel superconducting states.

## Spin Jahn-Teller antiferromagnetism in CoTi$_2$O$_5$

Physical Review B American Physical Society (0)

FKK Kirschner, RD Johnson, F Lang, DD Khalyavin, P Manuel, T Lancaster, D Prabhakaran, SJ Blundell

We have used neutron powder diffraction to solve the magnetic structure of orthorhombic CoTi$_2$O$_5$, showing that the long-range ordered state below 26 K identified in our muon-spin rotation experiments is antiferromagnetic with propagation vector ${\bf k}=(\pm \frac{1}{2}, \frac{1}{2}, 0)$ and moment of 2.72(1)$\mu_{\rm B}$ per Co$^{2+}$ ion. This long range magnetic order is incompatible with the experimentally determined crystal structure because the imposed symmetry completely frustrates the exchange coupling. We conclude that the magnetic transition must therefore be associated with a spin Jahn-Teller effect which lowers the structural symmetry and thereby relieves the frustration. These results show that CoTi$_2$O$_5$ is a highly unusual low symmetry material exhibiting a purely spin-driven lattice distortion critical to the establishment of an ordered magnetic ground state.

## 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.

## A new spin-anisotropic harmonic honeycomb iridate

ArXiv (0)

KA Modic, TE Smidt, I Kimchi, NP Breznay, A Biffin, S Choi, RD Johnson, R Coldea, P Watkins-Curry, GT McCandless, F Gandara, Z Islam, A Vishwanath, JY Chan, A Shekhter, RD McDonald, JG Analytis

The physics of Mott insulators underlies diverse phenomena ranging from high temperature superconductivity to exotic magnetism. Although both the electron spin and the structure of the local orbitals play a key role in this physics, in most systems these are connected only indirectly --- via the Pauli exclusion principle and the Coulomb interaction. Iridium-based oxides (iridates) open a further dimension to this problem by introducing strong spin-orbit interactions, such that the Mott physics has a strong orbital character. In the layered honeycomb iridates this is thought to generate highly spin-anisotropic interactions, coupling the spin orientation to a given spatial direction of exchange and leading to strongly frustrated magnetism. The potential for new physics emerging from such interactions has driven much scientific excitement, most recently in the search for a new quantum spin liquid, first discussed by Kitaev \cite{kitaev_anyons_2006}. Here we report a new iridate structure that has the same local connectivity as the layered honeycomb, but in a three-dimensional framework. The temperature dependence of the magnetic susceptibility exhibits a striking reordering of the magnetic anisotropy, giving evidence for highly spin-anisotropic exchange interactions. Furthermore, the basic structural units of this material suggest the possibility of a new family of structures, the `harmonic honeycomb' iridates. This compound thus provides a unique and exciting glimpse into the physics of a new class of strongly spin-orbit coupled Mott insulators.

## Spin waves and revised crystal structure of honeycomb iridate Na2IrO3

Physical Review Letters 108 (2012) 127204

SK Choi, R Coldea, AN Kolmogorov, T Lancaster, II Mazin, SJ Blundell, PG Radaelli, Y Singh, P Gegenwart, KR Choi, S-W Cheong, PJ Baker, C Stock, J Taylor

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 zig-zag magnetic order. The onset of long-range magnetic order below 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 deg Ir-O-Ir bonds required for dominant Kitaev exchange.

## Magnetic structures of the anisotropic intermetallic compounds Er2CoGa8 and Tm2CoGa8

PHYSICAL REVIEW B AMER PHYSICAL SOC 82 (0) 10

RD Johnson, T Frawley, P Manuel, DD Khalyavin, C Adriano, C Giles, PG Pagliuso, PD Hatton

Two members of the isostructural R2CoGa8 intermetallic series, Er2CoGa8 and Tm2CoGa8, have been studied by powder neutron diffraction. Antiferromagnetic ordering of the rare-earth sublattices was confirmed to occur at 3.0 K and 2.0 K, respectively. Furthermore, determination of the critical exponent showed Er2CoGa8 to adopt a three-dimensional universality class. In spite of a common magnetic easy axis and similar structural characteristics, the antiferromagnetic structures were found to be different for the erbium- and thulium-based compounds. The corresponding magnetic space groups were determined to be P(2a)mmm’ and P(C)mmm. The difference in magnetic structures is discussed based on crystal electric field effects that are known to be prevalent in such materials.

## 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.

## Three-dimensional micromagnetic domain structure of MnAs films on GaAs(001): Experimental imaging and simulations

PHYSICAL REVIEW B AMERICAN PHYSICAL SOC 75 (0) 9

R Engel-Herbert, T Hesjedal, DM Schaadt

The micromagnetic domain structure of MnAs films on GaAs(001) has been systematically investigated by micromagnetic imaging and simulations. The magnetic force microscopy (MFM) contrast resulting from the stray field of the simulated three-dimensional domain patterns was calculated and found to be in excellent agreement with MFM experiments. By combining three-dimensional stray-field imaging by MFM with surface sensitive probing and micromagnetic simulations, we were able to derive a consistent picture of the micromagnetic structure of MnAs. For example, the origin of the comblike contrast observed through MFM was identified as a metastable domain configuration exhibiting a cross-tie wall.

## Ferroelectricity induced by acentric spin-density waves in YMn$_2$O$_5$

ArXiv (0)

LC Chapon, PG Radaelli, GR Blake, S Park, S-W Cheong

The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn$_{2}$O$_{5}$, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magneto-elastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature-dependence of the spontaneous polarization, in particular its sign reversal at the commensurate-incommensurate transition.

## Spin gaps and magnetic structure of NaxCoO2

ArXiv (0)

LM Helme, AT Boothroyd, R Coldea, D Prabhakaran, A Stunault, GJ McIntyre, N Kernavanois

We present two experiments that provide information on spin anisotropy and the magnetic structure of NaxCoO2. First, we report low-energy neutron inelastic scattering measurements of the zone-center magnetic excitations in the magnetically ordered phase of Na0.75CoO2. The energy spectra suggest the existence of two gaps, and are very well fitted by a spin-wave model with both in-plane and out-of-plane anisotropy terms. The gap energies decrease with increasing temperature and both gaps are found to have closed when the temperature exceeds the magnetic ordering temperature T_m~22 K. Secondly, we present neutron diffraction studies of Na0.85CoO2 with a magnetic field applied approximately parallel to the c axis. For fields in excess of ~8T a magnetic Bragg peak was observed at the (0,0,3) position in reciprocal space. We interpret this as a spin-flop transition of the A-type antiferromagnetic structure, and we show that the spin-flop field is consistent with the size of the anisotropy gap.

## Temperature-induced barium de-trapping from a double-well potential in Ba6Ge25

ArXiv (0)

M Schmidt, PG Radaelli, MJ Gutmann, SJL Billinge, N Hur, SW Cheong

The crystal structure of barium-germanium clathrate Ba6Ge25 was studied using neutron powder diffraction in the temperature range 20-300K. The compound was found to be cubic (S.G. P4_1 23) in the entire temperature range. However, the fully-ordered model of the crystal structure (no split sites) is marginal at room temperature, and clearly fails at low temperature. A much better description of the crystal structure below 250K is given in terms of two split Ba sites, with random occupancies, for two out of three types of cages present in the Ba6Ge25 structure. The Ba atoms were found to interact strongly with the Ge host. The separation of the split Ba sites grows with decreasing temperature, with a sudden increase on cooling through the 200-250K temperature range, accompanied by an expansion of the entire crystal structure. We propose a simple model for this transition, based on temperature-induced de- trapping of Ba from a deep double-well potential. This transition is associated with sizeable anomalies in the transport and magnetic properties. The most significant of these effects, that is, the drop in electrical conductivity on cooling, can be easily explained within our model through the enhanced structural disorder, which would affect the relaxation time for all portions of the Fermi surface. We suggest that the other anomalies (increase in the absolute value of the negative Seebeck coefficient, decrease in the magnetic susceptibility) can be explained within the framework of the one-electron semi- classical model, without any need to invoke exotic electron-electron interaction mechanisms.

## 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.

## 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.

## Epitaxial Heusler Alloys on III-V Semiconductors

John Wiley & Sons, Ltd (0)

T Hesjedal, KH Ploog

## Magnetic correlations in YBaCo4O7 probed by single-crystal neutron scattering

ArXiv (0)

P Manuel, LC Chapon, PG Radaelli, H Zheng, JF Mitchell

We have studied the frustrated system YBaCo4O7 generally described as an alternating stacking of Kagome and triangular layers of magnetic ions on a trigonal lattice, by single crystal neutron diffraction experiments above the Neel ordering transition. Experimental data reveals pronounced magnetic diffuse scattering, which is successfully modeled by direct Monte-Carlo simulations. Long-range magnetic correlations are found along the c-axis, due to the presence of corner-sharing bipyramids, creating quasi one-dimensional order at finite temperature. In contrast, in the Kagome layers ab-plane, the spin-spin correlation function -displaying a short-range 120 degrees configuration- decays rapidly as typically found in spin-liquids. YBaCo4O7 experimentally realizes a new class of two-dimensional frustrated systems where the strong out-of-plane coupling does not lift the in-plane degeneracy, but instead act as an external "field".

## Experimental Proof of a Magnetic Coulomb Phase

ArXiv (0)

T Fennell, PP Deen, AR Wildes, K Schmalzl, D Prabhakaran, AT Boothroyd, RJ Aldus, DF McMorrow, ST Bramwell

Spin ice materials are magnetic substances in which the spin directions map onto hydrogen positions in water ice. Recently this analogy has been elevated to an electromagnetic equivalence, indicating that the spin ice state is a Coulomb phase, with magnetic monopole excitations analogous to ice's mobile ionic defects. No Coulomb phase has yet been proved in a real magnetic material, as the key experimental signature is difficult to resolve in most systems. Here we measure the scattering of polarised neutrons from the prototypical spin ice Ho2Ti2O7. This enables us to separate different contributions to the magnetic correlations to clearly demonstrate the existence of an almost perfect Coulomb phase in this material. The temperature dependence of the scattering is consistent with the existence of deconfined magnetic monopoles connected by Dirac strings of divergent length.

## Structural phase transition and magnetism in hexagonal srmno

ArXiv (0)

A Daoud-Aladine, C Martin, LC Chapon, M Hervieu, KS Knight, M Brunelli, PG Radaelli

The structural and magnetic properties of the hexagonal four-layer form of SrMnO$_3$ have been investigated by combining magnetization measurements, electron diffraction and high-resolution synchrotron X-ray and neutron powder diffraction. Below 350K, there is subtle structural phase transition from hexagonal symmetry (space group $P6_3/mmc$) to orthorhombic symmetry (space group $C222_1$) where the hexagonal metric is preserved. The second-order phase transition involves a slight tilting of the corner-sharing Mn$_{2}$O$_{9}$ units composed of 2 face-sharing MnO$_6$ octahedra and the associated displacement of Sr$^{2+}$ cations. The phase transition is described in terms of symmetry-adapted displacement modes of the high symmetry phase. Upon further cooling, long range magnetic order with propagation vector $\mathbf{k}=(0,0,0)$ sets in below 300K. The antiferromagnetic structure, analyzed using representation theory, shows a considerably reduced magnetic moment indicating the crucial role played by direct exchange between Mn centers of the Mn$_{2}$O$_{9}$ units.

## Neutron Scattering Signatures of Magnon Weyl Points

ArXiv (0)

S Shivam, R Coldea, R Moessner, P McClarty

We study the inelastic neutron scattering cross section in the vicinity of touching points in magnon bands. Among the possible touching points are magnon Weyl points in three dimensional ordered magnets with significant spin-orbit coupling that are characterized by a linear dispersion in their vicinity. A Weyl point is topologically protected by its net chirality and here we show that this leads to a characteristic form for the dynamical structure factor. To address this question, we show that scattering intensities in the vicinity of arbitrary magnon two-band touching points are identical to expectation values of the pseudospin polarization along some direction $\hat{\boldsymbol{n}}$ in momentum space fixed by the magnetic Hamiltonian. This approach applied to Weyl points shows that they are singular points in the intensity of the form $\hat{\boldsymbol{n}}\cdot \delta\hat{\boldsymbol{k}}$ regardless of the magnetic ground state. We make specific predictions for the experimental signatures of such intensity singularities in several spin models hosting Weyl magnons applicable to candidate materials.

## Stress-induced semiautomatic wet chemical etching of MnAs nanodots on GaAs(001) substrates and their magnetic properties

Proc. 7th International Conference on Atomically Controlled Surfaces, Interfaces and Nanostructure (0)

Y Takagaki, E Wiebicke, T Hesjedal, L Daeweritz, KH Ploog

## A Simple and Novel Approach to Fabricating Microfluidic Components Actuated By Termoresponsive Hydrogels

(0)

ME Harmon, MX Tang, T Hesjedal, CW Frank