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The superconducting and magnetic properties of Fe y Se 0.25 Te 0.75 single crystals ͑0.9Յ y Յ 1.1͒ were studied by means of x-ray diffraction, superconducting quantum interference device magnetometry, muon-spin rotation, and elastic neutron diffraction. The samples with y Ͻ 1 exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for y Ն 1 but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the Fe y Se 1−x Te x can be tuned not only by varying the Se/Te ratio but also by changing the Fe content.

Superconductivity in layered copper oxide compounds emerges when charge carriers are added to antiferromagnetically ordered CuO(2) layers. The carriers destroy the antiferromagnetic order, but strong spin fluctuations persist throughout the superconducting phase and are intimately linked to superconductivity. Neutron scattering measurements of spin fluctuations in hole-doped copper oxides have revealed an unusual 'hour-glass' feature in the momentum-resolved magnetic spectrum that is present in a wide range of superconducting and non-superconducting materials. There is no widely accepted explanation for this feature. One possibility is that it derives from a pattern of alternating spin and charge stripes, and this idea is supported by measurements on stripe-ordered La(1.875)Ba(0.125)CuO(4) (ref. 15). Many copper oxides without stripe order, however, also exhibit an hour-glass spectrum. Here we report the observation of an hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the family of superconducting copper oxides. Our system has stripe correlations and is an insulator, which means that its magnetic dynamics can conclusively be ascribed to stripes. The results provide compelling evidence that the hour-glass spectrum in the copper oxide superconductors arises from fluctuating stripes.

Sr 2 CuTeO 6 presents an opportunity for exploring low-dimensional magnetism on a square lattice of S ¼ 1=2 Cu 2þ ions. We employ ab initio multireference configuration interaction calculations to unravel the Cu 2þ electronic structure and to evaluate exchange interactions in Sr 2 CuTeO 6 . The latter results are validated by inelastic neutron scattering using linear spin-wave theory and series-expansion corrections for quantum effects to extract true coupling parameters. Using this methodology, which is quite general, we demonstrate that Sr 2 CuTeO 6 is an almost ideal realization of a nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling of 7.18(5) meV. DOI: 10.1103/PhysRevLett.117.237203 Mott insulators are a subject of intense interest due to the observation of many different quantum phenomena [1,2]. In low-dimensional systems, frustration and quantum fluctuations can destroy long-range magnetic order giving rise to quantum paramagnetic phases such as valence-bond solids with broken lattice symmetry or spin liquids, where symmetry is conserved but with possible new collective behaviors involving emergent gauge fields and fractional excitations [3][4][5]. The spin-1=2 frustrated square lattice with nearest-neighbor (NN) J 1 and nextnearest neighbor J 2 exchange interactions is one of the simplest models for valence-bond solids and spin liquids [4,6]. Yet, despite the many theoretical efforts, experimental realizations of the J 1 -J 2 model have been rather scarce. The double perovskite oxides are particularly interesting as magnetic interactions can be tuned by changing structure, stoichiometry, and cation order [7,8].In the search for a quantum magnet with weak exchange energies, Sr 2 CuTeO 6 has been proposed [9,10].The tetragonal crystal structure of the double perovskite Sr 2 CuTeO 6 [11] consists of corner sharing CuO 6 and TeO 6 octahedra that are rotated in a staggered fashion about the c axis; see Figs. 1(a) and 1(b). The CuO 6 octahedra are elongated along the c axis, effectively resulting in the ground state of a Cu 2þ (3d 9 ) ion having a hole in the inplane d x 2 −y 2 orbital, where z is along the c axis. This could eventually result in quasi-2D magnetism in Sr 2 CuTeO 6 with dominant intraplane exchange interactions. In the basal ab plane, the exchange that couples the Cu 2þ ions is the super-superexchange interaction mediated through the bridging TeO 6 octahedra as shown in Fig. 1(b), which is expected to reduce the coupling strength in Sr 2 CuTeO 6 .Magnetic susceptibility and heat capacity measurements on Sr 2 CuTeO 6 indicate a quasi-2D magnetic behavior, suggesting that it is a realization of the square-lattice J 1 -J 2 model [10]. More recently, neutron diffraction measurements on Sr 2 CuTeO 6 have shown it to order in a Néel antiferromagnetic (AFM) structure below T N ≃ 29 K with moments in the ab plane [12]; see Fig. 1(a). The ordered moment at 1.5 K was found to be reduced to 0.69ð6Þμ B , from the classical value of 1 μ B [12], indicating a renormalization by q...

We have used inelastic neutron scattering and muon-spin rotation to compare the low energy magnetic excitations in single crystals of superconducting Fe(1.01)Se(0.50)Te(0.50) and non-superconducting Fe(1.10)Se(0.25)Te(0.75). We confirm the existence of a spin resonance in the superconducting phase of Fe(1.01)Se(0.50)Te(0.50), at an energy of 7 meV and a wavevector of (1/2, 1/2, 0). The non-superconducting sample exhibits two incommensurate magnetic excitations at (1/2, 1/2, 0) ± (0.18, - 0.18, 0) which rise steeply in energy, but no resonance is observed at low energies. A strongly dispersive low energy magnetic excitation is also observed in Fe(1.10)Se(0.25)Te(0.75) close to the commensurate antiferromagnetic ordering wavevector (1/2 - δ, 0, 1/2), where δ≈0.03. The magnetic correlations in both samples are found to be quasi-two-dimensional in character and persist well above the magnetic (Fe(1.10)Se(0.25)Te(0.75)) and superconducting (Fe(1.01)Se(0.50)Te(0.50)) transition temperatures.

In vortex-like spin arrangements, multiple spins can combine into emergent multipole moments. Such multipole moments have broken space-inversion and time-reversal symmetries, and can therefore exhibit linear magnetoelectric (ME) activity. Three types of such multipole moments are known: toroidal; monopole; and quadrupole moments. So far, however, the ME activity of these multipole moments has only been established experimentally for the toroidal moment. Here we propose a magnetic square cupola cluster, in which four corner-sharing square-coordinated metal-ligand fragments form a noncoplanar buckled structure, as a promising structural unit that carries an ME-active multipole moment. We substantiate this idea by observing clear magnetodielectric signals associated with an antiferroic ME-active magnetic quadrupole order in the real material Ba(TiO)Cu 4 (PO 4 ) 4 . The present result serves as a useful guide for exploring and designing new ME-active materials based on vortex-like spin arrangements.

Here we report further investigation on the thermodynamic, and transport properties, and the first 2

Ferroic magnetic quadrupole order exhibiting macroscopic magnetoelectric activity is discovered in the novel compound A(TiO)Cu4(PO4)4 with A = Pb, which is in contrast with antiferroic quadrupole order observed in the isostructural compounds with A = Ba and Sr. Unlike the famous lone-pair stereochemical activity which often triggers ferroelectricity as in PbTiO3, the Pb 2+ cation in Pb(TiO)Cu4(PO4)4 is stereochemically inactive but dramatically alters specific magnetic interactions and consequently switches the quadrupole order from antiferroic to ferroic. Our firstprinciples calculations uncover a positive correlation between the degree of A-O bond covalency and a stability of the ferroic quadrupole order. I. INTORODUCTIONEarlier works demonstrated that the usage of specific elements with characteristic chemical properties is effective to realize desired ferroic order. For example, lonepair stereochemical activity of a heavier post-transition metal cation with an s 2 electron configuration such as Pb 2+ and Bi 3+ , which we call an s 2 -cation, is known as a driving force for ferroelectric order [1,2], as discussed in perovskite oxides PbTiO 3 [3], BiMnO 3 [4], and BiFeO 3 [5]. The stereochemically active s 2 -cations are surrounded by "hemidirected" local coordination, in which there is a void in the distribution of bonds to the ligands [1]. The origin for this directional bonding is explained by the hybridization between nominally empty metal p states with anti-bonding states formed by filled metal s states and ligand p states [2]. Such a hybridization is possible only when the inversion-symmetry at the cation site is broken. This is a driving force for off-center distortion and thus ferroelectric order.Another potential role of s 2 -cations is an impact on magnetism in insulating magnetic oxides. There, dominant magnetic superexchange interactions are usually mediated by O 2p orbitals near Fermi energy (E F ) [6]. As exemplified by the comparison between PbTiO 3 and BaTiO 3 [3], s 2 -cations tend to exhibit strong orbital hybridization with O ions, which should significantly affects superexchange interactions. Note that s 2 -cations are not necessarily stereochemically active; there are comparable number of compounds containing such cations located in "holodirected" local environment without a void in the * kentakimura@edu.k.u-tokyo.ac.jp ligand bond distribution [1]. In this case, substituting s 2 -cations can be a promising way of fine-tuning magnetic interactions without large distortion of the original structure.Among various ferroic orders, a particular class of magnetic order with broken space-inversion and time-reversal symmetries has recently attracted considerable interest because it can exhibit symmetry-dependent unique phenomena, such as magnetoelectric (ME) effects [7][8][9][10][11][12][13][14][15] and unconventional nonreciprocal electromagnetic responses [16][17][18]. From a symmetry point of view, it is known that ferroic order of magnetic multipole moments (toroidal, monopole, and quadrupole m...