# Scientific Highlights

### Observation of two types of fractional excitation in the Kitaev honeycomb magnet

Quantum spin liquid is a disordered but highly entangled magnetic state with fractional spin excitations. The ground state of an exactly solved Kitaev honeycomb model is perhaps its clearest example. Under a magnetic field, a spin flip in this model fractionalizes into two types of anyon, a quasiparticle with more complex exchange statistics than standard fermions or bosons: a pair of gauge fluxes and a Majorana fermion. Here, we demonstrate this kind of fractionalization in the Kitaev paramagnetic state of the honeycomb magnet α-RuCl_{3}. The spin excitation gap determined by nuclear magnetic resonance consists of the predicted Majorana fermion contribution following the cube of the applied magnetic field, and a finite zero-field contribution matching the predicted size of the gauge flux gap. The observed fractionalization into gapped anyons survives in a broad range of temperatures and magnetic fields, which establishes α-RuCl

_{3}as a unique platform for future investigations of anyons.

**Facility:**SINQ

**Reference:**N. Jansa et al, Nature Physics, adv online publication (May 2018)

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### Topological quantum phase transition in the Ising-like antiferromagnetic spin chain BaCo_{2}V_{2}O_{8}

Since the seminal ideas of Berezinskii, Kosterlitz and Thouless, topological excitations have been at the heart of our understanding of a whole novel class of phase transitions. In most cases, those transitions are controlled by a single type of topological objects. There are, however, some situations, still poorly understood, where two dual topological excitations fight to control the phase diagram and the transition. Finding experimental realizations of such cases is thus of considerable interest. We show here that this situation occurs in BaCo_{2}V

_{2}O

_{8}, a spin-1/2 Ising-like quasi-one-dimensional antiferromagnet, when subjected to a uniform magnetic field transverse to the Ising axis. Using neutron scattering experiments, we measure a drastic modification of the quantum excitations beyond a critical value of the magnetic field. This quantum phase transition is identified, through a comparison with theoretical calculations, to be a transition between two different types of solitonic topological object, which are captured by different components of the dynamical structure factor.

**Facility:**SINQ

**Reference:**Q. Faure et al, Nature Physics, adv online publication (May 2018)

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### Searching for New Physics with b → sτ^{+}τ^{-}

In recent years, intriguing hints for the violation of lepton flavor universality (LFU) have been accumulated in semileptonic B decays, both in the charged-current transitions b → cl^{-}ν

^{-}

_{l}(i.e., R

_{D}, R

_{D∗}, and R

_{J/Ψ}and the neutral-current transitions b → sl

^{+}l

^{-}(i.e., R

_{K}and R

_{K∗}. Hints for LFU violation in R

_{D(∗)}and R

_{J/Ψ}point at large deviations from the standard model (SM) in processes involving tau leptons. Moreover, LHCb has reported deviations from the SM expectations in b → sμ

^{+}μ

^{-}processes as well as in the ratios R

_{K}and R

_{K∗}, which together point at new physics (NP) affecting muons with a high significance. These hints for NP suggest the possibility of huge LFU-violating effects in b → sτ

^{+}τ

^{-}transitions. In this Letter, we predict the branching ratios of B → Kτ

^{+}τ

^{-}, B → K

^{∗}τ

^{+}τ

^{-}, and B

_{s}→ Φτ

^{+}τ

^{-}, taking into account NP effects in the Wilson coefficients C

^{ττ}

_{9(')}and C

^{ττ}

_{10(')}. Assuming a common NP explanation of R

_{D}, R

_{D(∗)}, and R

_{J/Ψ}, we show that a very large enhancement of b → sτ

^{+}τ

^{-}processes, of around 3 orders of magnitude compared to the SM, can be expected under fairly general assumptions. We find that the branching ratios of B

_{s}→ τ

^{+}τ

^{-}, B

_{s}→ Φτ

^{+}τ

^{-}, and B → K

^{(∗)}τ

^{+}τ

^{-}under these assumptions are in the observable range for LHCb and Belle II.

### Dirac and Chiral Quantum Spin Liquids on the Honeycomb Lattice in a Magnetic Field

Motivated by recent experimental observations in α-RuCl_{3}, we study the Κ-Γ model on the honeycomb lattice in an external magnetic field. By a slave-particle representation and variational Monte Carlo calculations, we reproduce the phase transition from zigzag magnetic order to a field-induced disordered phase. The nature of this state depends crucially on the field orientation. For particular field directions in the honeycomb plane, we find a gapless Dirac spin liquid, in agreement with recent experiments on α-RuCl

_{3}. For a range of out-of-plane fields, we predict the existence of a Kalmeyer- Laughlin-type chiral spin liquid, which would show an integer-quantized thermal Hall effect.

### Experimental signatures of emergent quantum electrodynamics in Pr_{2}Hf_{2}O_{7}

In a quantum spin liquid, the magnetic moments of the con- stituent electron spins evade classical long-range order to form an exotic state that is quantum entangled and coherent over macroscopic length scales. Such phases offer promis- ing perspectives for device applications in quantum information technologies, and their study can reveal new physics in quantum matter. Quantum spin ice is an appealing proposal of one such state, in which the fundamental ground state properties and excitations are described by an emergent U(1) lattice gauge theory. This quantum-coherent regime has quasiparticles that are predicted to behave like magnetic and electric monopoles, along with a gauge boson playing the role of an artificial photon. However, this emergent lattice quantum electrodynamics has proved elusive in experiments. Here we report neutron scattering measurements of the rare-earth pyrochlore magnet Pr_{2}Hf

_{2}O

_{7}that provide evidence for a quantum spin ice ground state. We find a quasi-elastic structure factor with pinch points - a signature of a classical spin ice - that are partially suppressed, as expected in the quantum-coherent regime of the lattice field theory at finite tem- perature. Our result allows an estimate for the speed of light associated with magnetic photon excitations. We also reveal a continuum of inelastic spin excitations, which resemble predictions for the fractionalized, topological excitations of a quantum spin ice. Taken together, these two signatures sug- gest that the low-energy physics of Pr

_{2}Hf

_{2}O

_{7}can be described by emergent quantum electrodynamics. If confirmed, the observation of a quantum spin ice ground state would consti- tute a concrete example of a three-dimensional quantum spin liquid—a topical state of matter that has so far mostly been explored in lower dimensionalities.

**Reference:**R. Sibille et al, Nature Physics, adv online publication (April 2018)

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### Quantitative 3D determination of self-assembled structures on nanoparticles using small angle neutron scattering

The ligand shell (LS) determines a number of nanoparticles’ properties. Nanoparticles’ cores can be accurately characterized; yet the structure of the LS, when composed of mixture of molecules, can be described only qualitatively (e.g., patchy, Janus, and random). Here we show that quantitative description of the LS' morphology of monodisperse nanoparticles can be obtained using small-angle neutron scattering (SANS), measured at multiple contrasts, achieved by either ligand or solvent deuteration. Three-dimensional models of the nanoparticles' core and LS are generated using an ab initio reconstruction method. Characteristic length scales extracted from the models are compared with simulations. We also characterize the evolution of the LS upon thermal annealing, and investigate the LS morphology of mixed-ligand copper and silver nanoparticles as well as gold nanoparticles coated with ternary mixtures. Our results suggest that SANS combined with multiphase modeling is a versatile approach for the characterization of nanoparticles' LS.**Facility:**SINQ

**Reference:**Z. Luo et al, Nature Communications 9, 1343 (2018)

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### No-Go Theorem for Nonstandard Explanations of the τ → K_{S}πν_{τ} CP Asymmetry

The CP asymmetry in τ → K_{S}πν

_{τ}, as measured by the BABAR collaboration, differs from the standard model prediction by 2.8 σ. Most nonstandard interactions do not allow for the required strong phase needed to produce a nonvanishing CP asymmetry, leaving only new tensor interactions as a possible mechanism. We demonstrate that, contrary to previous assumptions in the literature, the crucial interference between vector and tensor phases is suppressed by at least 2 orders of magnitude due to Watson’s final-state- interaction theorem. Furthermore, we find that the strength of the relevant CP-violating tensor interaction is strongly constrained by bounds from the neutron electric dipole moment and D—D

^{¯}. These observations together imply that it is extremely difficult to explain the current τ → K

_{S}πν

_{τ}measurement in terms of physics beyond the standard model originating in the ultraviolet.

### Magnetic Field Control of Cycloidal Domains and Electric Polarization in Multiferroic BiFeO_{3}

The magnetic field induced rearrangement of the cycloidal spin structure in ferroelectric monodomain single crystals of the room-temperature multiferroic BiFeO_{3}is studied using small-angle neutron scattering. The cycloid propagation vectors are observed to rotate when magnetic fields applied perpendicular to the rhombohedral (polar) axis exceed a pinning threshold value of ∼5T. In light of these experimental results, a phenomenological model is proposed that captures the rearrangement of the cycloidal domains, and we revisit the microscopic origin of the magnetoelectric effect. A new coupling between the magnetic anisotropy and the polarization is proposed that explains the recently discovered magnetoelectric polarization perpendicular to the rhombohedral axis.

**Facility:**SINQ

**Reference:**S. Bordacs et al, Physical Review Letters 120, 147203 (2018)

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### Dipolar Spin Ice States with a Fast Monopole Hopping Rate in CdEr_{2}X_{4} (X = Se, S)

Excitations in a spin ice behave as magnetic monopoles, and their population and mobility control the dynamics of a spin ice at low temperature. CdEr_{2}Se

_{4}is reported to have the Pauling entropy characteristic of a spin ice, but its dynamics are three orders of magnitude faster than the canonical spin ice Dy

_{2}Ti

_{2}O

_{7}. In this Letter we use diffuse neutron scattering to show that both CdEr

_{2}Se

_{4}and CdEr

_{2}S

_{4}support a dipolar spin ice state—the host phase for a Coulomb gas of emergent magnetic monopoles. These Coulomb gases have similar parameters to those in Dy

_{2}Ti

_{2}O

_{7}, i.e., dilute and uncorrelated, and so cannot provide three orders faster dynamics through a larger monopole population alone. We investigate the monopole dynamics using ac susceptometry and neutron spin echo spectroscopy, and verify the crystal electric field Hamiltonian of the Er

^{3+}ions using inelastic neutron scattering. A quantitative calculation of the monopole hopping rate using our Coulomb gas and crystal electric field parameters shows that the fast dynamics in CdEr

_{2}X

_{4}(X = Se, S) are primarily due to much faster monopole hopping. Our work suggests that CdEr

_{2}X

_{4}offer the possibility to study alternative spin ice ground states and dynamics, with equilibration possible at much lower temperatures than the rare earth pyrochlore examples.

**Facility:**SINQ

**Reference:**S. Gao et al, Physical Review Letters 120, 137201 (2018)

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### Spin-liquid-like state in a spin-1/2 square-lattice antiferromagnet perovskite induced by d^{10} – d^{0} cation mixing

A quantum spin liquid state has long been predicted to arise in spin-1/2 Heisenberg square-lattice antiferromagnets at the boundary region between Néel (nearest-neighbor interaction dominates) and columnar (next-nearest-neighbor interaction dominates) antiferromagnetic order. However, there are no known compounds in this region. Here we use d^{10}– d

^{0}cation mixing to tune the magnetic interactions on the square lattice while simultaneously introducing disorder. We find spin-liquid-like behavior in the double perovskite Sr

_{2}Cu(Te

_{0.5}W

_{0.5})O

_{6}, where the isostructural end phases Sr

_{2}CuTeO

_{6}and Sr

_{2}CuWO

_{6}are Néel and columnar type antiferromagnets, respectively. We show that magnetism in Sr

_{2}Cu(Te

_{0.5}W

_{0.5})O

_{6}is entirely dynamic down to 19 mK. Additionally, we observe at low temperatures for Sr

_{2}Cu(Te

_{0.5}W

_{0.5})O

_{6}- similar to several spin liquid candidates—a plateau in muon spin relaxation rate and a strong T-linear dependence in specific heat. Our observations for Sr

_{2}Cu(Te

_{0.5}W

_{0.5})O

_{6}highlight the role of disorder in addition to magnetic frustration in spin liquid physics.

**Facility:**SμS

**Reference:**O. Mustonen et al, Nature Communications 9, 1085 (2018)

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### Multi-q Mesoscale Magnetism in CeAuSb_{2}

We report the discovery of a field driven transition from a single-q to multi-q spin density wave (SDW) in the tetragonal heavy fermion compound CeAuSb_{2}. Polarized along

**c**, the sinusoidal SDW amplitude is 1.8(2)μ

_{B}/Ce for T<<T

_{N}=6.25(10)K with a wave vector q

_{1}=(η,η,1/2) [η=0.136(2)]. For

**H**||

**c**, harmonics appearing at 2q

_{1}evidence a striped magnetic texture below μ

_{0}H

_{1}=2.78(1) T. Above H

_{1}, these are replaced by coupled harmonics at q

_{1}+q

_{2}=(2η,0,0)+c

^{*}until μ

_{0}H

_{2}=5.42(5)T, where satellites vanish and magnetization nonlinearly approaches saturation at 1.64(2)μ

_{B}/Ce for μ

_{0}H≈7T.

**Facility:**SINQ

**Reference:**G.G. Marcus et al, Physical Review Letters 120, 097201 (2018)

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### Quasistatic antiferromagnetism in the quantum wells of SmTiO_{3}/SrTiO_{3} heterostructures

High carrier density quantum wells embedded within a Mott insulating matrix present a rich arena for exploring unconventional electronic phase behavior ranging from non-Fermi-liquid transport and signatures of quantum criticality to pseudogap formation. Probing the proposed connection between unconventional magnetotransport and incipient electronic order within these quantum wells has however remained an enduring challenge due to the ultra-thin layer thicknesses required. Here we address this challenge by exploring the magnetic properties of high-density SrTiO_{3}quantum wells embedded within the antiferromagnetic Mott insulator SmTiO

_{3}via muon spin relaxation and polarized neutron reflectometry measurements. The one electron per planar unit cell acquired by the nominal d

^{0}band insulator SrTiO

_{3}when embedded within a d

^{1}Mott SmTiO

_{3}matrix exhibits slow magnetic fluctuations that begin to freeze into a quasistatic spin state below a critical temperature T

^{*}. The appearance of this quasistatic well magnetism coincides with the previously reported opening of a pseudogap in the tunneling spectra of high carrier density wells inside this film architecture. Our data suggest a common origin of the pseudogap phase behavior in this quantum critical oxide heterostructure with those observed in bulk Mott materials close to an antiferromagnetic instability.

### Crystal-to-Crystal Transition of Ultrasoft Colloids under Shear

Ultrasoft colloids typically do not spontaneously crystallize, but rather vitrify, at high concentrations. Combining in situ rheo–small-angle-neutron-scattering experiments and numerical simulations we show that shear facilitates crystallization of colloidal star polymers in the vicinity of their glass transition. With increasing shear rate well beyond rheological yielding, a transition is found from an initial bcc-dominated structure to an fcc-dominated one. This crystal-to-crystal transition is not accompanied by intermediate melting but occurs via a sudden reorganization of the crystal structure. Our results provide a new avenue to tailor colloidal crystallization and the crystal-to-crystal transition at the molecular level by coupling softness and shear.**Facility:**SINQ

**Reference:**J. Ruiz-Franco et al, Physical Review Letters 120, 078003 (2018)

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### Three Dimensional Polarimetric Neutron Tomography of Magnetic Fields

Through the use of Time-of-Flight Three Dimensional Polarimetric Neutron Tomography (ToF 3DPNT) we have for the first time successfully demonstrated a technique capable of measuring and reconstructing three dimensional magnetic field strengths and directions unobtrusively and non-destructively with the potential to probe the interior of bulk samples which is not amenable otherwise. Using a pioneering polarimetric set-up for ToF neutron instrumentation in combination with a newly developed tailored reconstruction algorithm, the magnetic field generated by a current carrying solenoid has been measured and reconstructed, thereby providing the proof-of-principle of a technique able to reveal hitherto unobtainable information on the magnetic fields in the bulk of materials and devices, due to a high degree of penetration into many materials, including metals, and the sensitivity of neutron polarisation to magnetic fields. The technique puts the potential of the ToF time structure of pulsed neutron sources to full use in order to optimise the recorded information quality and reduce measurement time.

### Low-Field Bi-Skyrmion Formation in a Noncentrosymmetric Chimney Ladder Ferromagnet

The real-space spin texture and the relevant magnetic parameters were investigated for an easy-axis non-centrosymmetric ferromagnet Cr_{11}Ge

_{19}with Nowotny chimney ladder structure. Using Lorentz transmission electron microscopy,we report the formation of bi-Skyrmions,i.e., pairs of spin vortices with opposite magnetic helicities. The quantitative evaluation of the magnetocrystalline anisotropy and Dzyaloshinskii-Moriya interaction (DMI) proves that the magnetic dipolar interaction plays a more important role than the DMI on the observed bi-Skyrmion formation. Notably, the critical magnetic field value required for the formation of bi-Skyrmions turned out to be extremely small in this system, which is ascribed to strong easy-axis anisotropy associated with the characteristic helix crystal structure. The family of Nowotny chimney ladder compounds may offer a unique material platform where two distinctive Skyrmion formation mechanisms favoring different topological spin textures can become simultaneously active.

**Facility:**SINQ

**Reference:**R. Takagi et al, Physical Review Letters 120, 037203 (2018)

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