# Publications

## Evidence for a J(eff)=0 ground state and defect-induced spin glass behavior in the pyrochlore osmate Y2Os2O7

PHYSICAL REVIEW B 99 (2019) ARTN 174442

NR Davies, CV Topping, H Jacobsen, AJ Princep, FKK Kirschner, MC Rahn, M Bristow, JG Vale, I da Silva, PJ Baker, CJ Sahle, Y-F Guo, D-Y Yan, Y-G Shi, SJ Blundell, DF McMorrow, AT Boothroyd

## Magnetic field-induced pair density wave state in the cuprate vortex halo.

Science (New York, N.Y.) 364 (2019) 976-980

SD Edkins, A Kostin, K Fujita, AP Mackenzie, H Eisaki, S Uchida, S Sachdev, MJ Lawler, E-A Kim, JC Séamus Davis, MH Hamidian

High magnetic fields suppress cuprate superconductivity to reveal an unusual density wave (DW) state coexisting with unexplained quantum oscillations. Although routinely labeled a charge density wave (CDW), this DW state could actually be an electron-pair density wave (PDW). To search for evidence of a field-induced PDW, we visualized modulations in the density of electronic states N(r) within the halo surrounding Bi2Sr2CaCu2O8 vortex cores. We detected numerous phenomena predicted for a field-induced PDW, including two sets of particle-hole symmetric N(r) modulations with wave vectors QP and 2Q P , with the latter decaying twice as rapidly from the core as the former. These data imply that the primary field-induced state in underdoped superconducting cuprates is a PDW, with approximately eight CuO2 unit-cell periodicity and coexisting with its secondary CDWs.

## Magnetic profile of proximity-coupled (Dy,Bi)2Te3/(Cr,Sb)2Te3 topological insulator heterostructures

Physical Review B: Condensed matter and materials physics American Physical Society 100 (2019) 054402

L Duffy, NJ Steinke, DB Burn, A Frisk, L Lari, B Kuerbanjiang, VK Lazarov, G Van Der Laan, S Langridge, T Hesjedal

<p>Magnetic topological insulators (TIs) are an ideal playground for the study of novel quantum phenomena building on time-reversal symmetry broken topological surface states. By combining different magnetic TIs in a heterostructure, their magnetic and electronic properties can be precisely tuned. Recently, we have combined high-moment Dy:Bi<sub>2</sub>Te<sub>3</sub>with high transition temperature Cr:Sb<sub>2</sub>Te<sub>3</sub> in a superlattice, and found, using x-ray magnetic circular dichroism (XMCD), that long-range magnetic order can be introduced in the Dy:Bi<sub>2</sub>Te<sub>3</sub> layers. Accompanying first-principles calculations indicated that the origin of the long-range magnetic order is a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. However, based on XMCD alone, which is either averaging over the entire thin film stack or is surface sensitive, this coupling scenario could not be fully confirmed. Here we use polarized neutron reflectometry (PNR), which is ideally suited for the detailed study of superlattices, to retrieve the magnetization in a layer- and interface-resolved way. We find that the magnetization is, in contrast to similar recent studies, homogeneous throughout the individual layers, with no apparent interfacial effects. This finding demonstrates that heterostructure engineering is a powerful way of controlling the magnetic properties of entire layers, with the effects of coupling reaching beyond the interface region.</p>

## Mott polaritons in cavity-coupled quantum materials

New Journal of Physics IOP Publishing 21 (2019) 073066

M Kiffner, J Coulthard, F Schlawin, A Ardavan, D Jaksch

We show that strong electron-electron interactions in quantum materials can give rise to electronic transitions that couple strongly to cavity fields, and collective enhancement of these interactions can result in ultrastrong effective coupling strengths. As a paradigmatic example we consider a Fermi-Hubbard model coupled to a single-mode cavity and find that resonant electron-cavity interactions result in the formation of a quasi-continuum of polariton branches. The vacuum Rabi splitting of the two outermost branches is collectively enhanced and scales with USD g_{\text{eff}}\propto\sqrt{2L} USD, where USD L USD is the number of electronic sites, and the maximal achievable value for USD g_{\text{eff}} USD is determined by the volume of the unit cell of the crystal. We find that USD g_{\text{eff}} USD for existing quantum materials can by far exceed the width of the first excited Hubbard band. This effect can be experimentally observed via measurements of the optical conductivity and does not require ultrastrong coupling on the single-electron level. Quantum correlations in the electronic ground state as well as the microscopic nature of the light-matter interaction enhance the collective light-matter interaction compared to an ensemble of independent two-level atoms interacting with a cavity mode.

## Revealing the nature of photoluminescence emission in the metal-halide double perovskite Cs2AgBiBr6

Journal of Materials Chemistry C Royal Society of Chemistry 7 (2019) 8350-8356

SJ Zelewski, JM Urban, A Surrente, DK Maude, A Kuc, L Schade, R Johnson, M Dollmann, P Nayak, H Snaith, P Radaelli, R Kudrawiec, R Nicholas, P Plochocka, M Baranowski

<p>Double perovskite crystals such as Cs<small><sub>2</sub></small>AgBiBr<small><sub>6</sub></small> are expected to overcome the limitation of classic hybrid organic–inorganic perovskite crystals related to the presence of lead and the lack of structural stability. Perovskites are ionic crystals in which the carriers are expected to strongly couple to lattice vibrations. In this work we demonstrate that the photoluminescence (PL) emission in Cs<small><sub>2</sub></small>AgBiBr<small><sub>6</sub></small> is strongly influenced by the strong electron–phonon coupling. Combining photoluminescence excitation (PLE) and Raman spectroscopy we show that the PL emission is related to a color center rather than a band-to-band transition. The broadening and the Stokes shift of the PL emission from Cs<small><sub>2</sub></small>AgBiBr<small><sub>6</sub></small> is well explained using a Franck–Condon model with a Huang–Rhys factor of <em>S</em> = 11.7 indicating a strong electron–phonon interaction in this material.</p>

## Local magnetism, magnetic order and spin freezing in the 'nonmetallic metal' FeCrAs.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019) 285803-285803

BM Huddart, MT Birch, FL Pratt, SJ Blundell, DG Porter, SJ Clark, W Wu, SR Julian, PD Hatton, T Lancaster

We present the results of x-ray scattering and muon-spin relaxation ([Formula: see text]SR) measurements on the iron-pnictide compound FeCrAs. Polarized non-resonant magnetic x-ray scattering results reveal the 120° periodicity expected from the suggested three-fold symmetric, non-collinear antiferromagnetic structure. [Formula: see text]SR measurements indicate a magnetically ordered phase throughout the bulk of the material below [Formula: see text] K. There are signs of fluctuating magnetism in a narrow range of temperatures above [Formula: see text] involving low-energy excitations, while at temperatures well below [Formula: see text] behaviour characteristic of freezing of dynamics is observed, likely reflecting the effect of disorder in our polycrystalline sample. Using density functional theory we propose a distinct muon stopping site in this compound and assess the degree of distortion induced by the implanted muon.

## Machine learning in electronic-quantum-matter imaging experiments.

Nature 570 (2019) 484-490

Y Zhang, A Mesaros, K Fujita, SD Edkins, MH Hamidian, K Ch'ng, H Eisaki, S Uchida, JCS Davis, E Khatami, E-A Kim

For centuries, the scientific discovery process has been based on systematic human observation and analysis of natural phenomena1. Today, however, automated instrumentation and large-scale data acquisition are generating datasets of such large volume and complexity as to defy conventional scientific methodology. Radically different scientific approaches are needed, and machine learning (ML) shows great promise for research fields such as materials science2-5. Given the success of ML in the analysis of synthetic data representing electronic quantum matter (EQM)6-16, the next challenge is to apply this approach to experimental data-for example, to the arrays of complex electronic-structure images17 obtained from atomic-scale visualization of EQM. Here we report the development and training of a suite of artificial neural networks (ANNs) designed to recognize different types of order hidden in such EQM image arrays. These ANNs are used to analyse an archive of experimentally derived EQM image arrays from carrier-doped copper oxide Mott insulators. In these noisy and complex data, the ANNs discover the existence of a lattice-commensurate, four-unit-cell periodic, translational-symmetry-breaking EQM state. Further, the ANNs determine that this state is unidirectional, revealing a coincident nematic EQM state. Strong-coupling theories of electronic liquid crystals18,19 are consistent with these observations.

## A low-temperature Kerr effect microscope for the simultaneous magneto-optic and magneto-transport study of magnetic topological insulators

Measurement Science and Technology IOP Publishing (2019)

J Liu, A Singh, J Llandro, L Duffy, MJ Applegate, T Hesjedal, CHW Barnes

Magneto-optical Kerr effect (MOKE) microscopy is a surface-sensitive probe of magnetisation with micron-sized lateral resolution. Here, we present a low-temperature, focused polar MOKE microscope for the simultaneous magnetooptical and magneto-transport measurements, which has a temperature range of 1.6-300 K and is equipped with a magnet capable of delivering a field of up to 9 T. In this microscope, all optical components are integrated in a free-standing probe, allowing for the straightforward incorporation into many non-optical cryostat systems. Two-dimensional magnetisation scans on patterned ferromagnetic [CoFeB/Pt]n films demonstrate a magnetisation sensitivity of 10 µrad (Kerr angle) and a spatial resolution of 2.2 µm. The combination of optical and electrical measurements provides complementary temperature-dependent information, as demonstrated by the study of magnetic topological insulator thin films with out-of-plane magnetic anisotropy. Using this complementary approach, we study the effects of a secondary phase in Cr and V co-doped Sb2Te3 thin films, which show a combination of weak antilocalization and anisotropic magnetoresistance effects above 70 K. Our results highlight the virtue of MOKE and electrical transport to optimise exotic topological magnetic materials, paving the way for energy-efficient spintronic devices.

## Cr2Te3 thin films for integration in magnetic topological insulator heterostructures

Scientific Reports Nature 9 (2019) 10793

DM Burn, L Duffy, R Fujita, S Zhang, AI Figueroa, J Herrero-Martin, G Van Der Laan, T Hesjedal

Chromium telluride compounds are promising ferromagnets for proximity coupling to magnetic topological insulators (MTIs) of the Cr-doped (Bi,Sb)2(Se,Te)3 class of materials as they share the same elements, thus simplifying thin film growth, as well as due to their compatible crystal structure. Recently, it has been demonstrated that high quality (001)-oriented Cr2Te3 thin films with perpendicular magnetic anisotropy can be grown on c-plane sapphire substrate. Here, we present a magnetic, and soft xray absorption spectroscopy study of the chemical and magnetic properties of Cr2Te3 thin films. X-ray magnetic circular dichroism (XMCD) measured at the Cr L2,3 edges gives information about the local electronic and magnetic structure of the Cr ions. We further demonstrate the overgrowth of Cr2Te3(001) thin films by high-quality Crdoped Sb2Te3 films. The magnetic properties of the layers have been characterized and our results provide a starting point for refining the physical models of the complex magnetic ordering in Cr2Te3 thin films, and their integration into advanced MTI heterostructures for quantum device applications.

## FeTi$_2$O$_5$: a spin Jahn-Teller transition tuned by cation substitution

Physical Review B American Physical Society (0)

F Lang, L Jowitt, D Prabhakaran, RD Johnson, SJ Blundell

We have used muon-spin rotation, heat capacity and x-ray diffraction measurements in combination with density functional theory and dipole field calculations to investigate the crystal and magnetic structure of FeTi$_2$O$_5$. We observe a long range ordered state below 41.8(5) K with indications of significant correlations existing above this temperature. We determine candidate muon stopping sites in this compound, and find that our data are consistent with the spin Jahn-Teller driven antiferromagnetic ground state with $\boldsymbol{k}$=(1/2,1/2,0) reported for CoTi$_2$O$_5$. By comparing our data with calculated dipolar fields we can restrict the possible moment size and directions of the Fe$^{2+}$ ions.

## Topological Lifshitz transitions and Fermi arc manipulation in Weyl semimetal NbAs.

Nature communications 10 (2019) 3478-

HF Yang, LX Yang, ZK Liu, Y Sun, C Chen, H Peng, M Schmidt, D Prabhakaran, BA Bernevig, C Felser, BH Yan, YL Chen

Surface Fermi arcs (SFAs), the unique open Fermi-surfaces (FSs) discovered recently in topological Weyl semimetals (TWSs), are unlike closed FSs in conventional materials and can give rise to many exotic phenomena, such as anomalous SFA-mediated quantum oscillations, chiral magnetic effects, three-dimensional quantum Hall effect, non-local voltage generation and anomalous electromagnetic wave transmission. Here, by using in-situ surface decoration, we demonstrate successful manipulation of the shape, size and even the connections of SFAs in a model TWS, NbAs, and observe their evolution that leads to an unusual topological Lifshitz transition not caused by the change of the carrier concentration. The phase transition teleports the SFAs between different parts of the surface Brillouin zone. Despite the dramatic surface evolution, the existence of SFAs is robust and each SFA remains tied to a pair of Weyl points of opposite chirality, as dictated by the bulk topology.

## Evidence for a vestigial nematic state in the cuprate pseudogap phase.

Proceedings of the National Academy of Sciences of the United States of America 116 (2019) 13249-13254

S Mukhopadhyay, R Sharma, CK Kim, SD Edkins, MH Hamidian, H Eisaki, S-I Uchida, E-A Kim, MJ Lawler, AP Mackenzie, JCS Davis, K Fujita

The CuO2 antiferromagnetic insulator is transformed by hole-doping into an exotic quantum fluid usually referred to as the pseudogap (PG) phase. Its defining characteristic is a strong suppression of the electronic density-of-states D(E) for energies |E| < [Formula: see text], where [Formula: see text] is the PG energy. Unanticipated broken-symmetry phases have been detected by a wide variety of techniques in the PG regime, most significantly a finite-Q density-wave (DW) state and a Q = 0 nematic (NE) state. Sublattice-phase-resolved imaging of electronic structure allows the doping and energy dependence of these distinct broken-symmetry states to be visualized simultaneously. Using this approach, we show that even though their reported ordering temperatures T DW and T NE are unrelated to each other, both the DW and NE states always exhibit their maximum spectral intensity at the same energy, and using independent measurements that this is the PG energy [Formula: see text] Moreover, no new energy-gap opening coincides with the appearance of the DW state (which should theoretically open an energy gap on the Fermi surface), while the observed PG opening coincides with the appearance of the NE state (which should theoretically be incapable of opening a Fermi-surface gap). We demonstrate how this perplexing phenomenology of thermal transitions and energy-gap opening at the breaking of two highly distinct symmetries may be understood as the natural consequence of a vestigial nematic state within the pseudogap phase of Bi2Sr2CaCu2O8.

## Unveiling Electronic Correlation and the Ferromagnetic Superexchange Mechanism in the van der Waals Crystal CrSiTe_{3}.

Physical review letters 123 (2019) 047203-

J Zhang, X Cai, W Xia, A Liang, J Huang, C Wang, L Yang, H Yuan, Y Chen, S Zhang, Y Guo, Z Liu, G Li

The recent discovery of intrinsic ferromagnetic order in the atomically thin van der Waals crystal CrXTe_{3} (X=Si, Ge) stimulates intensive studies on the nature of low-dimensional magnetism because the presence of long-range magnetic order in two-dimensional systems with continuous symmetry is strictly prohibited by thermal fluctuations. By combining advanced many-body calculations with angle-resolved photoemission spectroscopy we investigate CrSiTe_{3} single crystals and unveil the pivotal role played by the strong electronic correlations at both high- and low-temperature regimes. Above the Curie temperature (T_{c}), Coulomb repulsion (U) drives the system into a charge transfer insulating phase. In contrast, below T_{c} the crystal field arranges the Cr-3d orbitals such that the ferromagnetic superexchange profits, giving rise to the bulk ferromagnetic ground state with which the electronic correlations compete. The excellent agreement between theory and experiment establishes CrSiTe_{3} as a prototype low-dimensional crystal with the cooperation and interplay of electronic correlation and ferromagnetism.

## Selective probing of magnetic order on Tb and Ir sites in stuffed Tb2Ir2O7  using resonant x-ray scattering.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019) 344001-344001

C Donnerer, MC Rahn, E Schierle, RS Perry, LSI Veiga, G Nisbet, SP Collins, D Prabhakaran, AT Boothroyd, DF McMorrow

We study the magnetic structure of the 'stuffed' (Tb-rich) pyrochlore iridate Tb2+x Ir2-x O7-y  (x ∼ 0.18), using resonant elastic x-ray scattering (REXS). In order to disentangle contributions from Tb and Ir magnetic sublattices, experiments were performed at the Ir L 3 and Tb M 5 edges, which provide selective sensitivity to Ir 5d and Tb 4f  magnetic moments, respectively. At the Ir L 3 edge, we found the onset of long-range [Formula: see text] magnetic order below [Formula: see text] K, consistent with the expected signal of all-in all-out (AIAO) magnetic order. Using a single-ion model to calculate REXS cross-sections, we estimate an ordered magnetic moment of [Formula: see text] at 5 K. At the Tb M 5 edge, long-range [Formula: see text] magnetic order appeared below  ∼[Formula: see text] K, also consistent with an AIAO magnetic structure on the Tb site. Additional insight into the magnetism of the Tb sublattice is gleaned from measurements at the M 5 edge in applied magnetic fields up to 6 T, which is found to completely suppress the Tb AIAO magnetic order. In zero applied field, the observed gradual onset of the Tb sublattice magnetisation with temperature suggests that it is induced by the magnetic order on the Ir site. The persistence of AIAO magnetic order, despite the greatly reduced ordering temperature and moment size compared to stoichiometric Tb2Ir2O7, for which [Formula: see text] K and [Formula: see text], indicates that stuffing could be a viable means of tuning the strength of electronic correlations, thereby potentially offering a new strategy to achieve topologically non-trivial band crossings in pyrochlore iridates.

## Spin-orbit excitons in CoO

PHYSICAL REVIEW B 100 (2019) ARTN 075143

PM Sarte, M Songvilay, E Pachoud, RA Ewings, CD Frost, D Prabhakaran, KH Hong, AJ Browne, Z Yamani, JP Attfield, EE Rodriguez, SD Wilson, C Stock

## Muon-spin relaxation and AC magnetometry study of the ferrimagnet LaSr2Cr2SbO9

Journal of Solid State Chemistry Elsevier 279 (2019) 120935

EC Hunter, P Battle, S Blundell, E al.

AC susceptibility and muon spin relaxation data have been collected from a polycrystalline sample of LaSr2Cr2SbO9, a perovskite-like compound wherein the unequal distribution of Cr3+ and Sb5+ cations over two crystallographically-distinct six-coordinate B sites results in the onset of ferrimagnetism at ∼150 K. The data are used to elucidate the dynamics of the ferrimagnetic domain walls, thought to lie in Sb-rich regions, and suggest an energy barrier of ∼0.13 eV to their motion. The muon data confirm that the ferrimagnetic order is a true bulk phenomenon. The behaviour of this material is briefly compared to that of Ni2+- based relaxor ferromagnets.

## Manifold of spin states and dynamical temperature effects in LaCoO3: Experimental and theoretical insights

PHYSICAL REVIEW B 100 (2019) ARTN 054306

M Feygenson, D Novoselov, S Pascarelli, R Chernikov, O Zaharko, F Porcher, D Karpinsky, A Nikitin, D Prabhakaran, A Sazonov, V Sikolenko

## Magnetic monopole noise

Nature Springer Nature 571 (2019) 234-239

R Dusad, F Kirschner, JC Hoke, BR Roberts, A Eyal, F Flicker, GM Luke, S Blundell, J Davis

Magnetic monopoles1-3 are hypothetical elementary particles with quantized magnetic charge. In principle, a magnetic monopole can be detected by the quantized jump in magnetic flux that it generates upon passage through a superconducting quantum interference device (SQUID)4. Following the theoretical prediction that emergent magnetic monopoles should exist in several lanthanide pyrochlore magnetic insulators5,6, including Dy2Ti2O7, the SQUID technique has been proposed for their direct detection6. However, this approach has been hindered by the high number density and the generation-recombination fluctuations expected of such thermally generated monopoles. Recently, theoretical advances have enabled the prediction of the spectral density of magnetic-flux noise from monopole generation-recombination fluctuations in these materials7,8. Here we report the development of a SQUID-based flux noise spectrometer and measurements of the frequency and temperature dependence of magnetic-flux noise generated by Dy2Ti2O7 crystals. We detect almost all of the features of magnetic-flux noise predicted for magnetic monopole plasmas7,8, including the existence of intense magnetization noise and its characteristic frequency and temperature dependence. Moreover, comparisons of simulated and measured correlation functions of the magnetic-flux noise indicate that the motions of magnetic charges are strongly correlated. Intriguingly, because the generation-recombination time constant for Dy2Ti2O7 is in the millisecond range, magnetic monopole flux noise amplified by SQUID is audible to humans.

## Expanding the Lorentz Concept in magnetism

New Journal of Physics IOP Publishing 21 (2019) 073063

GJ Bowden, G Van Der Laan, T Hesjedal, RJ Hicken

In 1878, the Dutch physicist Hendrik Antoon Lorentz first addressed the calculation of the local electric field at an atomic site in a ferroelectric material, generated by all the other electric dipoles within the sample. This calculation, which applies equally well to ferromagnets, is taught in Universities around the World. Here we demonstrate that the Lorentz concept can be used to speed up calculations of the local dipolar field in square, circular, and elliptical shaped monolayers and thin films, not only at the center of the film, but across the sample. Calculations show that long elliptical and rectangular films should exhibit the narrowest ferromagnetic resonance linewidth. In addition, discrete dipole calculations show that the Lorentz cavity field (u 0M/3) does not hold in tetragonal films. Depending on the ratio (b/a), the local field can be either less/greater than (u 0M/3): an observation that has implications for ferromagnetic resonance. 3D simple cubic (SC) systems are also examined. For example, while most texts discuss the Lorentz cavity field in terms of a Lorentz sphere, the Lorentz cavity field still holds when a Lorentz sphere is replaced by a the Lorentz cube, but only in cubic SC, FCC and BCC systems. Finally, while the primary emphasis is on the discrete dipole-dipole interaction, contact is made with the continuum model. For example, in the continuous SC dipole model, just one monolayer is required to generate the Lorentz cavity field. This is in marked contrast to the discrete dipole model, where a minimum of five adjacent monolayers is required.

## Chiral topological semimetal with multifold band crossings and long Fermi arcs

NATURE PHYSICS 15 (2019) 759-+

NBM Schroter, D Pei, MG Vergniory, Y Sun, K Manna, F De Juan, JA Krieger, V Suess, M Schmidt, P Dudin, B Bradlyn, TK Kim, T Schmitt, C Cacho, C Felser, VN Strocov, Y Chen