# Publications

## Magnetic ground state and magnon-phonon interaction in multiferroic h−YMnO3

Physical Review B American Physical Society 97 (2018) 134304-

SL Holm, A Kreisel, TK Schäffer, A Bakke, M Bertelsen, UB Hansen, M Retuerto, J Larsen, D Prabhakaran, PP Deen, Z Yamani, JO Birk, U Stuhr, C Niedermayer, AL Fennell, BM Andersen, K Lefmann

Inelastic neutron scattering has been used to study the magnetoelastic excitations in the multiferroic manganite hexagonal YMnO3. An avoided crossing is found between magnon and phonon modes close to the Brillouin zone boundary in the (a,b) plane. Neutron polarization analysis reveals that this mode has mixed magnon-phonon character. An external magnetic field along the c axis is observed to cause a linear field-induced splitting of one of the spin-wave branches. A theoretical description is performed, using a Heisenberg model of localized spins, acoustic phonon modes, and a magnetoelastic coupling via the single-ion magnetostriction. The model quantitatively reproduces the dispersion and intensities of all modes in the full Brillouin zone, describes the observed magnon-phonon hybridized modes, and quantifies the magnetoelastic coupling. The combined information, including the field-induced magnon splitting, allows us to exclude several of the earlier proposed models and point to the correct magnetic ground state symmetry, and provides an effective dynamic model relevant for the multiferroic hexagonal manganites.

## Pressure effect on magnetic susceptibility of LaCoO3

LOW TEMPERATURE PHYSICS 44 (2018) 328-333

AS Panfilov, GE Grechnev, IP Zhuravleva, AA Lyogenkaya, VA Pashchenko, BN Savenko, D Novoselov, D Prabhakaran, IO Troyanchuk

## Multi-band magnetotransport in exfoliated thin films of CuxBi2Se3

Journal of Physics: Condensed Matter IOP Publishing 30 (2018) 155302

JA Alexander-Webber, J Huang, J Beilsten-Edmands, P Cermak, C Drasar, RJ Nicholas, A Coldea

We report magnetotransport studies in thin (&lt;100 nm) exfoliated films of CuxBi2Se3 and we detect an unusual electronic transition at low temperatures. Bulk crystals show weak superconductivity with Tc ∼ 3.5 K and a possible electronic phase transition around 200K. Following exfoliation, superconductivity is supressed and a strongly temperature dependent multi-band conductivity is observed for T &lt; 30K. This transition between competing conducting channels may be enhanced due to the presence of electronic ordering, and could be affected by the presence of an effective internal stress due to Cu intercalation. By fitting to the weak antilocalisation conductivity correction at low magnetic fields we confirm that the low temperature regime maintains a quantum phase coherence length Lφ &gt; 100 nm indicating the presence of topologically protected surface states.

## Implications of bond disorder in a S=1 kagome lattice.

Scientific reports 8 (2018) 4745-4745

JL Manson, J Brambleby, PA Goddard, PM Spurgeon, JA Villa, J Liu, S Ghannadzadeh, F Foronda, J Singleton, T Lancaster, SJ Clark, IO Thomas, F Xiao, RC Williams, FL Pratt, SJ Blundell, CV Topping, C Baines, C Campana, B Noll

Strong hydrogen bonds such as F···H···F offer new strategies to fabricate molecular architectures exhibiting novel structures and properties. Along these lines and, to potentially realize hydrogen-bond mediated superexchange interactions in a frustrated material, we synthesized [H2F]2[Ni3F6(Fpy)12][SbF6]2 (Fpy = 3-fluoropyridine). It was found that positionally-disordered H2F+ ions link neutral NiF2(Fpy)4 moieties into a kagome lattice with perfect 3-fold rotational symmetry. Detailed magnetic investigations combined with density-functional theory (DFT) revealed weak antiferromagnetic interactions (J ~ 0.4 K) and a large positive-D of 8.3 K with ms = 0 lying below ms = ±1. The observed weak magnetic coupling is attributed to bond-disorder of the H2F+ ions which leads to disrupted Ni-F···H-F-H···F-Ni exchange pathways. Despite this result, we argue that networks such as this may be a way forward in designing tunable materials with varying degrees of frustration.

## LaSr3 NiRuO4 H4 : A 4d Transition-Metal Oxide-Hydride Containing Metal Hydride Sheets.

Angewandte Chemie (International ed. in English) (2018)

L Jin, M Lane, D Zeng, FKK Kirschner, F Lang, P Manuel, SJ Blundell, JE McGrady, MA Hayward

The synthesis of the first 4d transition metal oxide-hydride, LaSr3 NiRuO4 H4 , is prepared via topochemical anion exchange. Neutron diffraction data show that the hydride ions occupy the equatorial anion sites in the host lattice and as a result the Ru and Ni cations are located in a plane containing only hydride ligands, a unique structural feature with obvious parallels to the CuO2 sheets present in the superconducting cuprates. DFT calculations confirm the presence of S=1/2  Ni+ and S=0, Ru2+ centers, but neutron diffraction and μSR data show no evidence for long-range magnetic order between the Ni centers down to 1.8 K. The observed weak inter-cation magnetic coupling can be attributed to poor overlap between Ni 3dz2 and H 1s in the super-exchange pathways.

## High-Pressure Synthesis, Structures, and Properties of Trivalent A-Site-Ordered Quadruple Perovskites RMn7O12 (R = Sm, Eu, Gd, and Tb).

Inorganic chemistry 57 (2018) 5987-5998

L Zhang, N Terada, RD Johnson, DD Khalyavin, P Manuel, Y Katsuya, M Tanaka, Y Matsushita, K Yamaura, AA Belik

A-site-ordered quadruple perovskites RMn7O12 with R = Sm, Eu, Gd, and Tb were synthesized at high pressure and high temperature (6 GPa and ∼1570 K), and their structural, magnetic, and dielectric properties are reported. They crystallize in space group I2/ m at room temperature. All four compounds exhibit a high-temperature phase transition to the cubic Im3̅ structure at ∼664 K (Sm), 663 K (Eu), 657 K (Gd), and 630 K (Tb). They all show one magnetic transition at TN1 ≈ 82-87 K at zero magnetic field, but additional magnetic transitions below TN2 ≈ 12 K were observed in SmMn7O12 and EuMn7O12 at high magnetic fields. Very weak kinklike dielectric anomalies were observed at TN1 in all compounds. We also observed pyroelectric current peaks near 14 K and frequency-dependent sharp steps in dielectric constant (near 18-35 K)-these anomalies are probably caused by dielectric relaxation, and they are not related to any ferroelectric transitions. TbMn7O12 shows signs of nonstoichiometry expressed as (Tb1- xMn x)Mn7O12, and these samples exhibit negative magnetization or magnetization reversal effects of an extrinsic origin on zero-field-cooled curves in intermediate temperature ranges. The crystal structures of SmMn7O12 and EuMn7O12 were refined from neutron powder diffraction data at 100 K, and the crystal structures of GdMn7O12 and (Tb0.88Mn0.12)Mn7O12 were studied by synchrotron X-ray powder diffraction at 295 K.

## Direct observation of twisted surface Skyrmions in bulk crystals

Physical Review Letters American Physical Society 120 (2018) 227202

S Zhang, G van der Laan, WW Wang, A Haghighirad, T Hesjedal

Magnetic skyrmions in noncentrosymmetric helimagnets with Dn symmetry are Bloch-type magnetization swirls with a helicity angle of ±90∘. At the surface of helimagnetic thin films below a critical thickness, a twisted skyrmion state with arbitrary helicity angle has been proposed, however, its direct experimental observation has remained elusive. Here, we show that circularly polarized resonant elastic x-ray scattering is able to unambiguously measure the helicity angle of surface skyrmions, providing direct experimental evidence that a twisted skyrmion surface state also exists in bulk systems. The exact surface helicity angles of twisted skyrmions for both left- and right-handed chiral bulk Cu2OSeO3, in the single as well as in the multidomain skyrmion lattice state, are determined, revealing their detailed internal structure. Our findings suggest that a skyrmion surface reconstruction is a universal phenomenon, stemming from the breaking of translational symmetry at the interface.

## Microscopic effects of Dy doping in the topological insulator Bi2Te3

Physical Review B American Physical Society 97 (2018) 174427

LB Duffy, N-J Steinke, JA Krieger, AI Figueroa, K Kummer, T Lancaster, Giblin, FL Pratt, SJ Blundell, T Prokscha, A Suter, S Langridge, VN Strocov, Z Salman, G van der Laan, T Hesjedal

Magnetic doping with transition metal ions is the most widely used approach to break time-reversal symmetry in a topological insulator (TI)—a prerequisite for unlocking the TI’s exotic potential. Recently, we reported the doping of Bi2Te3 thin films with rare-earth ions, which, owing to their large magnetic moments, promise commensurately large magnetic gap openings in the topological surface states. However, only when doping with Dy has a sizable gap been observed in angle-resolved photoemission spectroscopy, which persists up to room temperature. Although disorder alone could be ruled out as a cause of the topological phase transition, a fundamental understanding of the magnetic and electronic properties of Dy-doped Bi2Te3 remained elusive. Here, we present an x-ray magnetic circular dichroism, polarized neutron reflectometry, muon-spin rotation, and resonant photoemission study of the microscopic magnetic and electronic properties. We find that the films are not simply paramagnetic but that instead the observed behavior can be well explained by the assumption of slowly fluctuating, inhomogeneous, magnetic patches with increasing volume fraction as the temperature decreases. At liquid helium temperatures, a large effective magnetization can be easily introduced by the application of moderate magnetic fields, implying that this material is very suitable for proximity coupling to an underlying ferromagnetic insulator or in a heterostructure with transition-metal-doped layers. However, the introduction of some charge carriers by the Dy dopants cannot be excluded at least in these highly doped samples. Nevertheless, we find that the magnetic order is not mediated via the conduction channel in these samples and therefore magnetic order and carrier concentration are expected to be independently controllable. This is not generally the case for transition-metal-doped topological insulators, and Dy doping should thus allow for improved TI quantum devices.

## Correlated oxygen displacements and phonon mode changes in LaCoO3 single crystal

PHYSICA B-CONDENSED MATTER 536 (2018) 597-599

VV Sikolenko, SL Molodtsov, M Izquierdo, IO Troyanchuk, D Karpinsky, SI Tiutiunnikov, E Efimova, D Prabhakaran, D Novoselov, V Efimov

## Magnetic edge states and coherent manipulation of molecular graphene nanoribbons

Nature Springer Nature 557 (2018) 691–695-

M Slota, A Keerthi, W Myers, E Tretyakov, M Baumgarten, A Ardavan, H Sadeghi, CJ Lambert, K Mullen, L Bogani

Graphene, a single-layer network of carbon atoms, shows outstanding electrical and mechanical properties, and graphene ribbons with nanometer-scale widths, should exhibit half-metallicity, quantum confinement and edge effects. Magnetic edges in graphene nanoribbons have undergone intense theoretical scrutiny, because their coherent manipulation would be a milestone for spintronic and quantum computing devices. Experimental investigations are however hampered by the fact that most nanoribbons do not have the required atomic control of the edges, and that the proposed graphene terminations are chemically unstable. Here we solve both of these problems, by using molecular graphene nanoribbons functionalized with stable spin-bearing radical groups. We observe the predicted delocalized magnetic edge states, and test present theoretical models about the spin dynamics and the spin-environment interactions. Comparison with a non graphitized reference material allows clear identification of fingerprint behaviours. We quantify the spin-orbit coupling parameters, define the interaction patterns, and unravel the spin decoherence channels. Even without any optimization, the spin coherence time is in the μs range at room temperature, and we perform quantum inversion operations between edge and radical spins. This new approach to problem of spins in well-defined electronic nanostructures offers a long awaited experimental testbed for the theory of magnetism in graphene nanoribbons. The observed coherence times open up encouraging perspectives for the use of magnetic nanoribbons in quantum spintronic devices.

## Reciprocal space tomography of 3D skyrmion lattice order in a chiral magnet

Proceedings of the National Academy of Sciences National Academy of Sciences 115 (2018) 6386-6391

S Zhang, G van der Laan, J Mueller, L Heinen, M Garst, A Bauer, H Berger, C Pfleiderer, T Hesjedal

It is commonly assumed that surfaces modify the properties of stable materials within the top few atomic layers of a bulk specimen only. Exploiting the polarization dependence of resonant elastic X-ray scattering to go beyond conventional diffraction and imaging techniques, we have determined the depth dependence of the full 3D spin structure of skyrmions—that is, topologically nontrivial whirls of the magnetization—below the surface of a bulk sample of Cu2OSeO3. We found that the skyrmions change exponentially from pure Néel- to pure Bloch-twisting over a distance of several hundred nanometers between the surface and the bulk, respectively. Though qualitatively consistent with theory, the strength of the Néel-twisting at the surface and the length scale of the variation observed experimentally exceed material-specific modeling substantially. In view of the exceptionally complete quantitative theoretical account of the magnetic rigidities and associated static and dynamic properties of skyrmions in Cu2OSeO3 and related materials, we conclude that subtle changes of the materials properties must exist at distances up to several hundred atomic layers into the bulk, which originate in the presence of the surface. This has far-reaching implications for the creation of skyrmions in surface-dominated systems and identifies, more generally, surface-induced gradual variations deep within a bulk material and their impact on tailored functionalities as an unchartered scientific territory.

## Imposing long-range ferromagnetic order in rare-earth doped magnetic topological-insulator heterostructures

Physical Review Materials American Physical Society 2 (2018) 054201

L Duffy, AJ Frisk, DM Burn, NJ Steinke, J Herrero-Martin, A Ernst, G van der Laan, T Hesjedal

The combination of topological properties and magnetic order can lead to new quantum states and exotic physical phenomena, such as the quantum anomalous Hall (QAH) effect. The size of the magnetic gap in the topological surface states, key for the robust observation of the QAH state, scales with the magnetic moment of the doped 3D topological insulator (TI). The pioneering transition-metal doped (Sb,Bi)2(Se,Te)3 thin films only allow for the observation of the QAH effect up to some 100 mK, despite the much higher magnetic ordering temperatures. On the other hand, high magnetic moment materials, such as rare-earth doped (Sb,Bi)2(Se,Te)3 thin films, show large moments but no long-range magnetic order. Proximity coupling and interfacial effects, multiplied in artificial heterostructures, allow for the engineering of the electronic and magnetic properties. Here, we show the successful growth of high-quality Dy:Bi2Te3/Cr:Sb2Te3 thin film heterostructures. Using x-ray magnetic spectroscopy we demonstrate that high transition temperature Cr:Sb2Te3 can introduce long-range magnetic order in high-moment Dy:Bi2Te3 - up to a temperature of 17 K - in excellent agreement with first-principles calculations, which reveal the origin of the long-range magnetic order in a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. Engineered magnetic TI heterostructures may be an ideal materials platform for observing the QAH effect at liquid He temperatures and above.

## Proposal for the detection of magnetic monopoles in spin ice via nanoscale magnetometry

Physical Review B American Physical Society 97 (2018) 140402(R)-

FKK Kirschner, F Flicker, A Yacoby, NY Yao, S Blundell

We present a proposal for applying nanoscale magnetometry to the search for magnetic monopoles in the spin ice materials holmium and dysprosium titanate. Employing Monte Carlo simulations of the dipolar spin ice model, we find that when cooled to below 1.5K these materials exhibit a sufficiently low monopole density to enable the direct observation of magnetic fields from individual monopoles. At these temperatures we demonstrate that noise spectroscopy can capture the intrinsic fluctuations associated with monopole dynamics, allowing one to isolate the qualitative e↵ects associated with both the Coulomb interaction between monopoles and the topological constraints implied by Dirac strings. We describe in detail three di↵erent nanoscale magnetometry platforms (muon spin rotation, nitrogen vacancy defects, and nanoSQUID arrays) that can be used to detect monopoles in these experiments, and analyze the advantages of each.

## The key ingredients of the electronic structure of FeSe

Annual Reviews of Condensed Matter Physics, Vol. 9, 125-146, 2018 (2018)

AI Coldea, MD Watson

FeSe is a fascinating superconducting material at the frontier of research in condensed matter physics. Here we provide an overview on the current understanding of the electronic structure of FeSe, focusing in particular on its low energy electronic structure as determined from angular resolved photoemission spectroscopy, quantum oscillations and magnetotransport measurements of single crystal samples. We discuss the unique place of FeSe amongst iron-based superconductors, being a multi-band system exhibiting strong orbitally-dependent electronic correlations and unusually small Fermi surfaces, prone to different electronic instabilities. We pay particular attention to the evolution of the electronic structure which accompanies the tetragonal-orthorhombic structural distortion of the lattice around 90 K, which stabilizes a unique nematic electronic state. Finally, we discuss how the multi-band multi-orbital nematic electronic structure has an impact on the understanding of the superconductivity, and show that the tunability of the nematic state with chemical and physical pressure will help to disentangle the role of different competing interactions relevant for enhancing superconductivity.

## Spin dynamics and exchange interactions in CuO measured by neutron scattering

Physical Review B American Physical Society 97 (2018) 144401

H Jacobsen, SM Gaw, AJ Princep, E Hamilton, S Tóth, RA Ewings, M Enderle, EM Hétroy Wheeler, D Prabhakaran, A Boothroyd

The magnetic properties of CuO encompass several contemporary themes in condensed matter physics, including quantum magnetism, magnetic frustration, magnetically-induced ferroelectricity and orbital currents. Here we report polarized and unpolarized neutron inelastic scattering measurements which provide a comprehensive map of the cooperative spin dynamics in the low temperature antiferromagnetic (AFM) phase of CuO throughout much of the Brillouin zone. At high energies $(E \gtrsim 100\,meV)$ the spectrum displays continuum features consistent with the des Cloizeax--Pearson dispersion for an ideal $S=\frac{1}{2}$ Heisenberg AFM chain. At lower energies the spectrum becomes more three-dimensional, and we find that a linear spin-wave model for a Heisenberg AFM provides a very good description of the data, allowing for an accurate determination of the relevant exchange constants in an effective spin Hamiltonian for CuO. In the high temperature helicoidal phase, there are features in the measured low-energy spectrum that we could not reproduce with a spin-only model. We discuss how these might be associated with the magnetically-induced multiferroic behavior observed in this phase.

## Manipulation of skyrmion motion by magnetic field gradients

Nature Communications Springer Nature 9 (2018) 2115

SL Zhang, WW Wang, DM Burn, H Peng, H Berger, A Bauer, C Pfleiderer, G van der Laan, T Hesjedal

Magnetic skyrmions are particle-like, topologically protected magnetisation entities that are promising candidates as information carriers in racetrack memory. The transport of skyrmions in a shift-register-like fashion is crucial for their embodiment in practical devices. Here, we demonstrate that chiral skyrmions in Cu2OSeO3 can be effectively manipulated under the influence of a magnetic field gradient. In a radial field gradient, skyrmions were found to rotate collectively, following a given velocity–radius relationship. As a result of this relationship, and in competition with the elastic properties of the skyrmion lattice, the rotating ensemble disintegrates into a shell-like structure of discrete circular racetracks. Upon reversing the field direction, the rotation sense reverses. Field gradients therefore offer an effective handle for the fine control of skyrmion motion, which is inherently driven by magnon currents. In this scheme, no local electric currents are needed, thus presenting a different approach to shift-register-type operations based on spin transfer torque.

## Folded superstructure and degeneracy-enhanced band gap in the weak-coupling charge density wave system 2H−TaSe2

Physical Review B American Physical Society 97 (2018)

Y Li, J Jiang, HF Yang, D Prabhakaran, ZK Liu, LX Yang, Y Chen

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we have mapped out the reconstructed electronic structure in the commensurate charge-density-wave (CDW) state of quasi-two-dimensional transition metal dichalcogenide 2H-TaSe2. The observation of the fine structure near Brillouin zone (BZ) center supplements the picture of Fermi surface folding in the 3×3 CDW state. In addition to the anisotropic CDW band gaps that energetically stabilize the system at the Fermi level in the first-order lock-in transition, we found band reconstruction at high binding energy, which can be well explained by the hybridization between main bands (MBs) and folded bands (FBs). Furthermore, in contrast to the perfectly nested quasi-one-dimensional system, triple-nesting-vector-induced CDW FBs increase the degeneracy of the band crossing and thus further enlarge the magnitude of band gap at certain momentum-energy positions. The visualization and modeling of CDW gaps in momentum-energy space reconciles the long-lasting controversy on the gap magnitude and suggests a weak-coupling Peierls physics in this system.

## Proposal for a micromagnetic standard problem for materials with Dzyaloshinskii-Moriya interaction

arxiv (2018)

D Cortes-Ortuno, M Beg, V Nehruji, L Breth, R Pepper, T Kluyver, G Downing, T Hesjedal, P Hatton, T Lancaster, R Hertel, O Hovorka, H Fangohr

Understanding the role of the Dzyaloshinskii-Moriya interaction (DMI) for the formation of helimagnetic order, as well as the emergence of skyrmions in magnetic systems that lack inversion symmetry, has found increasing interest due to the significant potential for novel spin based technologies. Candidate materials to host skyrmions include those belonging to the B20 group such as FeGe, known for stabilising Bloch-like skyrmions, interfacial systems such as cobalt multilayers or Pd/Fe bilayers on top of Ir(111), known for stabilising Neel-like skyrmions, and, recently, alloys with a crystallographic symmetry where anti-skyrmions are stabilised. Micromagnetic simulations have become a standard approach to aid the design and optimisation of spintronic and magnetic nanodevices and are also applied to the modelling of device applications which make use of skyrmions. Several public domain micromagnetic simulation packages such as OOMMF, MuMax3 and Fidimag already offer implementations of different DMI terms. It is therefore highly desirable to propose a so-called micromagnetic standard problem that would allow one to benchmark and test the different software packages in a similar way as is done for ferromagnetic materials without DMI. Here, we provide a sequence of well-defined and increasingly complex computational problems for magnetic materials with DMI. Our test problems include 1D, 2D and 3D domains, spin wave dynamics in the presence of DMI, and validation of the analytical and numerical solutions including uniform magnetisation, edge tilting, spin waves and skyrmion formation. This set of problems can be used by developers and users of new micromagnetic simulation codes for testing and validation and hence establishing scientific credibility.

## Pair density waves in superconducting vortex halos

PHYSICAL REVIEW B 97 (2018) ARTN 174510

Y Wang, SD Edkins, MH Hamidian, JCS Davis, E Fradkin, SA Kivelson

## Single-layer TiOx reconstructions on SrTiO3 (111): (√7 × √7)R19.1°, (√13 × √13)R13.9°, and related structures

Surface Science Elsevier 675 (2018) 36-41

TK Andersen, S Wang, M Castell, DD Fong, LD Marks

The atomic structures of two reconstructions, (√7 × √7)R19.1° and (√13 × √13)R13.9°, on the SrTiO3 (111) surface were determined using a combination of density functional theory and scanning tunneling microscopy data and APW + lo density functional theory minimizations and simulations. These reconstructions belong to the same structural family made up of an interconnected, single layer of edge-sharing TiO6 and TiO5[] octahedra. This family of reconstructions between 0.5 and 1.5 excess TiO2, representing the lowest-reported TiO2 coverages for reconstructions on this surface. This family is found to include the previously-solved (2 × 2)a reconstruction. They all follow a simple rule for surface composition, which serves as a tool for better understanding and predicting the structure of other reconstructions of arbitrary surface unit cell size on SrTiO3 (111). This reconstruction family and the calculations of surface energies for different hypothesis structures also shed light on the structure of Schottky defects observed on these reconstructed SrTO3 (111) surfaces.