This paper is concerned with the representation and recognition of the observed dynamics (i.e., excluding purely spatial appearance cues) of spacetime texture based on a spatiotemporal orientation ...analysis. The term "spacetime texture" is taken to refer to patterns in visual spacetime, (x,y,t), that primarily are characterized by the aggregate dynamic properties of elements or local measurements accumulated over a region of spatiotemporal support, rather than in terms of the dynamics of individual constituents. Examples include image sequences of natural processes that exhibit stochastic dynamics (e.g., fire, water, and windblown vegetation) as well as images of simpler dynamics when analyzed in terms of aggregate region properties (e.g., uniform motion of elements in imagery, such as pedestrians and vehicular traffic). Spacetime texture representation and recognition is important as it provides an early means of capturing the structure of an ensuing image stream in a meaningful fashion. Toward such ends, a novel approach to spacetime texture representation and an associated recognition method are described based on distributions (histograms) of spacetime orientation structure. Empirical evaluation on both standard and original image data sets shows the promise of the approach, including significant improvement over alternative state-of-the-art approaches in recognizing the same pattern from different viewpoints.
Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. ...Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb
Ti
O
is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state.
We investigate the temperature dependence of the spin dynamics in the pyrochlore magnet Nd2Zr2O7 by neutron scattering experiments. At low temperature, this material undergoes a transition towards an ..."all-in–all-out" antiferromagnetic phase and the spin dynamics encompass a dispersionless mode, characterized by a dynamical spin ice structure factor. Unexpectedly, this mode is found to survive above TN≈ 300 mK . Concomitantly, elastic correlations of the spin ice type develop. These are the signatures of a peculiar correlated paramagnetic phase which can be considered as a new example of Coulomb phase. Our observations near TN do not reproduce the signatures expected for a Higgs transition, but show reminiscent features of the "all-in–all-out" order superimposed on a Coulomb phase.
Co–Zn–Mn chiral cubic magnets display versatile magnetic skyrmion phases, including equilibrium phases stable far above and far below room temperature, and the facile creation of robust ...far‐from‐equilibrium skyrmion states. In this system, compositional disorder and magnetic frustration are key ingredients that have profound effects on the chiral magnetism. Reported here are studies of the magnetism in Co6.75Zn6.75Mn6.5 by magnetometry, small‐angle neutron scattering (SANS), magnetic diffuse neutron scattering and Lorentz transmission electron microscopy (LTEM). While features in magnetometry and LTEM often give standard indications for skyrmion formation, they are not readily observed from the measurements on this system. Instead, skyrmion lattice correlations are only revealed by SANS, and they are found to form an orientationally disordered structure in a minority fraction of the sample. The majority fraction of the sample always displays orientationally disordered helical spin correlations, which undergo further disordering along the radial direction on cooling below the critical temperature (Tc ≃ 102 K). The near‐complete suppression of the skyrmion phase, and the process of disordering on cooling, are attributed to competing magnetic interactions that dominate over the ferromagnetic interaction expected to favour chiral magnetism in this system. These competing interactions start to develop above Tc and become further enhanced towards low temperatures. The present observations of co‐existing and disordered magnetic correlations over multiple length scales are not unique to Co6.75Zn6.75Mn6.5 but are seemingly common to the family of Co–Zn–Mn compounds with finite Mn, and their accurate description presents a challenge for theoretical modelling. In addition, this study highlights a need for neutron instrumentation capable of the comprehensive measurement of magnetic correlations over expanded ranges of momentum transfer in such multiple‐length‐scale magnets.
In the frustrated chiral magnet Co6.75Zn6.75Mn6.5, small‐angle neutron scattering reveals that the mesoscale chiral magnetism displays strong disorder and the skyrmion phase is nearly entirely suppressed.
Abstract
A prototypical quasi-2D metallic compound, 1T-TaS
2
has been extensively studied due to an intricate interplay between a Mott-insulating ground state and a charge-density-wave order. In the ...low-temperature phase, 12 out of 13 Ta
4+
5
d
-electrons form molecular orbitals in hexagonal star-of-David patterns, leaving one 5
d-
electron with
S
= ½ spin free. This orphan quantum spin with a large spin-orbit interaction is expected to form a highly correlated phase of its own. And it is most likely that they will form some kind of a short-range order out of a strongly spin-orbit coupled Hilbert space. In order to investigate the low-temperature magnetic properties, we performed a series of measurements including neutron scattering and muon experiments. The obtained data clearly indicate the presence of the short-ranged phase and put the upper bound on ~0.4 µ
B
for the size of the magnetic moment, consistent with the orphan-spin scenario.
Complex disordered states--from liquids and glasses to exotic quantum matter--are ubiquitous in nature. Their key properties include finite entropy, power-law correlations and emergent organizing ...principles. In spin ice, spin correlations are determined by the 'ice rules' organizing principle that stabilizes a magnetic state with the same zero-point entropy as water ice. The entropy can be manipulated with great precision by an applied magnetic field: when directed along the three-fold crystallographic axis, the field produces a state of finite entropy, known as kagome ice. Here, we investigate the spin-ice material Ho2Ti2O7 by tilting the magnetic field slightly away from that axis. We thus realize a classic statistical system named after Kasteleyn, in which the entropy of a critical phase can be continuously tuned. Our neutron scattering experiments reveal how this process occurs at a microscopic level. PUBLICATION ABSTRACT
Excitations in a spin ice behave as magnetic monopoles, and their population and mobility control the dynamics of a spin ice at low temperature. CdEr2Se4 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 Dy2Ti2O7. In this Letter we use diffuse neutron scattering to show that both CdEr2Se4 and CdEr2S4 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 Dy2Ti2O7, 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 Er3+ 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 CdEr2X4 (X=Se, S) are primarily due to much faster monopole hopping. Our work suggests that CdEr2X4 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.
The emergence of Localized Surface Plasmon Resonance (LSPR) in nanocomposite films consisting of a hydrogen-free amorphous Carbon (a-C) matrix and Ag is considered theoretically and experimentally. ...While in theory it could be manifested for highly tetrahedral (>90% sp3) matrices, Auger electron spectroscopy and neutron reflectivity have shown that the incorporation of Ag into the a-C matrix induces severe graphitization that eliminates the LSPR; nonetheless, the dielectric damping of graphitized a-C, in combination with the π−π⁎ electronic transitions of carbon and the defect states introduced by Ag, cumulatively result in a strong broadband optical absorption in the near infrared, visible and UVA/UVB spectral ranges, revealed by optical reflectivity spectra, that coincides with the solar emission spectrum. The incorporation of Ag into a-C at room temperature thus proves to be an energy-efficient pathway for the controlled graphitization and the tailored optical absorption of novel nanocomposite films for solar photothermal applications.
•We provide the optical response of amorphous Carbon/Silver nanocomposite layers, whose absorbance is tailored to coincide with the solar emittance.•We identify the mechanism lying behind the atomical dispersion or nanoparticle formation of Ag into amorphous Carbon.•We manufacture a black carbon material at room temperature and without ion damage.
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