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We demonstrate quantum critical scaling for an S=1/2 Heisenberg antiferromagnetic chain compound Cu(C4H4N2)(NO3)2 in a magnetic field around saturation, by analyzing previously reported magnetization [Y. Kono et al., Phys. Rev. Lett. 114, 037202 (2015)], thermal expansion [J. Rohrkamp et al., J. Phys.: Conf. Ser. 200, 012169 (2010)], and NMR relaxation data [H. Kühne et al., Phys. Rev. B 80, 045110 (2009)]. The scaling of magnetization is demonstrated through collapsing the data for a range of both temperature and field onto a single curve without making any assumption for a theoretical form. The data collapse is subsequently shown to closely follow the theoretically predicted scaling function without any adjustable parameters. Experimental boundaries for the quantum critical region could be drawn from the variable range beyond which the scaled data deviate from the theoretical function. Similarly to the magnetization, quantum critical scaling of the thermal expansion is also demonstrated. Further, the spin dynamics probed via NMR relaxation rate 1/T1 close to the saturation is shown to follow the theoretically predicted quantum critical behavior as 1/T1∝T-0.5 persisting up to temperatures as high as kBT...J, where J is the exchange coupling constant. [ABSTRACT FROM AUTHOR]

Following the early prediction of the skyrmion lattice (SkL)-a periodic array of spin vortices-it has been observed recently in various magnetic crystals mostly with chiral structure. Although non-chiral but polar crystals with Cnv symmetry were identified as ideal SkL hosts in pioneering theoretical studies, this archetype of SkL has remained experimentally unexplored. Here, we report the discovery of a SkL in the polar magnetic semiconductor GaV4S8 with rhombohedral (C3v) symmetry and easy axis anisotropy. The SkL exists over an unusually broad temperature range compared with other bulk crystals and the orientation of the vortices is not controlled by the external magnetic field, but instead confined to the magnetic easy axis. Supporting theory attributes these unique features to a new Néel-type of SkL describable as a superposition of spin cycloids in contrast to the Bloch-type SkL in chiral magnets described in terms of spin helices. [ABSTRACT FROM AUTHOR]

Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spin-1/2 particles is far from complete. The quantum square-lattice Heisenberg antiferromagnet, for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum. This quantum effect manifests itself for excitations propagating with the specific wavevector (π, 0). We use polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound Cu(DCOO)2⋅4D2O, a known realization of the quantum square-lattice Heisenberg antiferromagnet model. Our experiments reveal an isotropic excitation continuum at the anomaly, which we analyse theoretically using Gutzwiller-projected trial wavefunctions. The excitation continuum is accounted for by the existence of spatially extended pairs of fractional S = 1/2 quasiparticles, 2D analogues of 1D spinons. Away from the anomalous wavevector, these fractional excitations are bound and form conventional magnons. Our results establish the existence of fractional quasiparticles in the high-energy spectrum of a quasi-two-dimensional antiferromagnet, even in the absence of frustration. [ABSTRACT FROM AUTHOR]

We carried out ac magnetic susceptibility measurements and muon spin relaxation spectroscopy on the cubic double perovskite Ba2YMoO6, down to 50 mK. Below ~1K the muon relaxation is typical of a magnetic insulator with a spin-liquid type ground state, i.e. without broken symmetries or frozen moments. However, the ac susceptibility revealed a dilute-spin-glass-like transition below ~1 K. Antiferromagnetically coupled Mo5+ 4d1 electrons in triply degenerate t2g orbitals are in this material arranged in a geometrically frustrated fcc lattice. Bulk magnetic susceptibility data has previously been interpreted in terms of a freezing to a heterogeneous state with non-magnetic sites where 4d1 electrons have paired in spin-singlets dimers, and residual unpaired Mo5+ 4d1 electron spins. Based on the magnetic heat capacity data it has been suggested that this heterogeneity is the result of kinetic constraints intrinsic to the physics of the pure system (possibly due to topological overprotection) leading to a self-induced glass of valence bonds between neighbouring 4d1 electrons. The muon spin relaxation (μSR) unambiguously points to a heterogeneous state with a static arrangement of unpaired electrons in a background of (valence bond) dimers between the majority of Mo5+ 4d electrons. The ac susceptibility data indicate that the residual magnetic moments freeze into a dilute-spin-glass-like state. This is in apparent contradiction with the muon-spin decoupling at 50mK in fields up to 200mT, which indicates that, remarkably, the time scale of the field fluctuations from the residual moments is ~5 ns. Comparable behaviour has been observed in other geometrically frustrated magnets with spin-liquid-like behaviour and the implications of our observations on Ba2YMoO6 are discussed in this context. [ABSTRACT FROM AUTHOR]

Cuprates and other high-temperature superconductors consist of two-dimensional layers that are crucial to their properties. The dynamics of the quantum spins in these layers lie at the heart of the mystery of the cuprates. In bulk cuprates such as La2CuO4, the presence of a weak coupling between the two-dimensional layers stabilizes a three-dimensional magnetic order up to high temperatures. In a truly two-dimensional system however, thermal spin fluctuations melt long-range order at any finite temperature. Here, we measure the spin response of isolated layers of La2CuO4 that are only one-unit-cell-thick. We show that coherent magnetic excitations, magnons, known from the bulk order, persist even in a single layer of La2CuO4, with no evidence for more complex correlations such as resonating valence bond correlations. These magnons are, therefore, well described by spin-wave theory (SWT). On the other hand, we also observe a high-energy magnetic continuum in the isotropic magnetic response that is not well described by two-magnon SWT, or indeed any existing theories. [ABSTRACT FROM AUTHOR]

High-temperature superconductivity remains arguably the greatest enigma of condensed matter physics. The discovery of iron-based hightemperature superconductors [1, 2] has renewed the importance of understanding superconductivity in materials susceptible to magnetic order and fluctuations. Intriguingly, they show magnetic fluctuations reminiscent of superconducting (SC) cuprates [3], including a 'resonance' and an 'hourglass'-shaped dispersion [4], which provides an opportunity to gain new insights into the coupling between spin fluctuations and superconductivity. In this paper, we report inelastic neutron scattering data on Fe1+yTe0.7Se0.3 using excess iron concentration to tune between an SC (y = 0.02) and a non-SC (y = 0.05) ground state. We find incommensurate spectra in both the samples but discover that in the one that becomes SC, a constriction toward a commensurate hourglass-shape develops well above Tc. Conversely, a spin gap and a concomitant spectral weight shift happen below Tc. Our results imply that the hourglass-shaped dispersion is most likely a prerequisite for superconductivity, whereas the spin gap and shift of spectral weight are the consequences of superconductivity. We explain this observation by pointing out that an inward dispersion toward the commensurate wave vector is needed for the opening of a spin gap to lower the magnetic exchange energy and hence provide the necessary condensation energy for the SC state to emerge. [ABSTRACT FROM AUTHOR]

When viewed as an elementary particle, the electron has spin and charge. When binding to the atomic nucleus, it also acquires an angular momentum quantum number corresponding to the quantized atomic orbital it occupies. Even if electrons in solids form bands and delocalize from the nuclei, in Mott insulators they retain their three fundamental quantum numbers: spin, charge and orbital. The hallmark of one-dimensional physics is a breaking up of the elementary electron into its separate degrees of freedom. The separation of the electron into independent quasi-particles that carry either spin (spinons) or charge (holons) was first observed fifteen years ago. Here we report observation of the separation of the orbital degree of freedom (orbiton) using resonant inelastic X-ray scattering on the one-dimensional Mott insulator Sr2CuO3. We resolve an orbiton separating itself from spinons and propagating through the lattice as a distinct quasi-particle with a substantial dispersion in energy over momentum, of about 0.2 electronvolts, over nearly one Brillouin zone. [ABSTRACT FROM AUTHOR]

The excitations responsible for producing high-temperature superconductivity in the copper oxides have yet to be identified. Two promising candidates are collective spin excitations and phonons. A recent argument against spin excitations is based on their inability to explain structures observed in electronic spectroscopies such as photoemission and optical conductivity. Here, we use inelastic neutron scattering to demonstrate that collective spin excitations in optimally doped La2−xSrxCuO4 are more structured than previously thought. The excitations have a two-component structure with a low-frequency component strongest around 18 meV and a broader component peaking near 40–70 meV. The second component carries most of the spectral weight and its energy matches structures observed in photoemission in the range 50–90 meV. Our results demonstrate that collective spin excitations can explain features of electronic spectroscopies and are therefore likely to be strongly coupled to the electron quasiparticles. [ABSTRACT FROM AUTHOR]

The very low temperature properties of two pyrochlore compounds, Yb2Ti2O7 and Gd2Sn2O7, were investigated using an ensemble of microscopic and bulk techniques. In both compounds, a first order transition is evidenced, as well as spin dynamics persisting down to the 20 mK range. The transition however has a quite different character in the two materials: whereas that in Gd2Sn2O7 (at 1 K) is a magnetic transition towards long range order, that in Yb2Ti2O7 (at 0.24 K) is reminiscent of the liquid–gas transition, in the sense that it involves a 4 orders of magnitude drop of the spin fluctuation frequency, with no long range order. We attribute these unusual features to the frustration of the antiferromagnetic exchange interaction in the pyrochlore lattice. [ABSTRACT FROM AUTHOR]

Topological defects are found ubiquitously in various kinds of matter, such as vortices in type-II superconductors, and magnetic skyrmions in chiral ferromagnets. While knowledge on the static behavior of magnetic skyrmions is accumulating steadily, their dynamics under forced flow is still a widely open issue. Here, we report the deformation of the moving magnetic skyrmion lattice in MnSi under electric current flow observed using small-angle neutron scattering. A spatially inhomogeneous rotation of the skyrmion lattice, with an inverse rotation sense for opposite sample edges, is observed for current densities greater than a threshold value jt ~ 1 MA m−2 (106 A m−2). Our result show that skyrmion lattices under current flow experience significant friction near the sample edges due to pinning, this being a critical effect that must be considered for anticipated skyrmion-based applications at the nanoscale. Magnetic skyrmions are topological objects that have been recently extensively studied for their particular characteristics and a view to be used in spintronics devices. The authors present a Small Angle Neutron Scattering study of the deformation of magnetic skyrmion lattice propelled by an electric current and find that the skyrmions experience frictional movement at the edges of their sample providing better understanding of the motion of skyrmions. [ABSTRACT FROM AUTHOR]