Many correlated electron materials, such as high-temperature superconductors 1 , geometrically frustrated oxides 2 and lowdimensional magnets 3,4 , are still objects of fruitful study because of the unique properties that arise owing to poorly understood many-body effects. Heavy-fermion metals 5 -materials that have high effective electron masses due to those effects-represent a class of materials with exotic properties, ranging from unusual magnetism, unconventional superconductivity and 'hidden' order parameters 6 . The heavy-fermion superconductor URu 2 Si 2 has held the attention of physicists for the past two decades owing to the presence of a 'hidden-order' phase below 17.5 K. Neutron scattering measurements indicate that the ordered moment is 0.03μ B , much too small to account for the large heat-capacity anomaly at 17.5 K. We present recent neutron scattering experiments that unveil a new piece of this puzzle-the spin-excitation spectrum above 17.5 K exhibits well-correlated, itinerant-like spin excitations up to at least 10 meV, emanating from incommensurate wavevectors. The large entropy change associated with the presence of an energy gap in the excitations explains the reduction in the electronic specific heat through the transition.The central issue in URu 2 Si 2 concerns the identification of the order parameter that explains the reduction in the specific heat coefficient, γ = C/T, and thus the change in entropy, through the transition at 17.5 K (ref. 6). Numerous speculations about the ground state have been advanced, from quadrupolar ordering 7 , to spin-density wave formation 8 , to 'orbital currents' 9 to account for the missing entropy. Here, we present cold-neutron time-offlight spectroscopy results that shed some light on the 'hiddenorder' (HO) in URu 2 Si 2 . We have carried out experiments above and below the ordering temperature to measure how the spin excitations evolve. It is clear from our data that above T 0 the spectrum is dominated by fast, itinerant-like spin excitations emanating from incommensurate wavevectors at positions located 0.4a* from the antiferromagnetic (AF) points. From the group velocity and temperature dependence of these modes, we surmise that these are heavy-quasiparticle excitations that form below the 'coherence temperature' and play a crucial role in the formation of the heavy-fermion and HO states. The gapping of these excitations, which corresponds to a loss of accessible states, accounts for the reduction in γ through the transition at 17.5 K. Figure 1 shows the excitation spectrum of URu 2 Si 2 at 1.5 K in the H00 plane. The characteristic gaps at ∼2 meV at the AF zone centre (1, 0, 0) and ∼4 meV at the incommensurate wavevectors (0.6, 0, 0) and (1.4, 0, 0) have been known for some time 10 . The incommensurate wavevector corresponds to a displacement of ∼0.4a * from the AF zone centres (that is, where h + k + l = an odd integer, and is thus forbidden in the body-centred-cubic chemical structure). A scenario for this modesoftening at the incommensurate positi...
We present single crystal neutron diffraction measurements on multiferroic LuFe2O4. Magnetic reflections are observed below transitions at 240 and 175 K indicating that the magnetic interactions in LuFe2O4 are 3-dimensional (3D) in character. The magnetic structure is refined as a ferrimagnetic spin configuration below the 240 K transition. Below 175 K a significant broadening of the magnetic peaks is observed along with the build up of a diffuse component to the magnetic scattering. 75.30.Kz, 28.20.Cz, 25.40.Dn Materials that offer the possibility of simultaneously controlling magnetic and electric degrees of freedom are the subject of intense interest [1]. Recently, multiferroic materials have been identified that show large coupling between electric and magnetic degrees of freedom. Ferroelectricity driven by either magnetic or charge ordering appears to be the origin of the large coupling and, hence, understanding the underlying electronic interactions is crucial for further insight into multiferroicity [1].LuFe 2 O 4 has attracted attention as a novel ferroelectric material where ferroelectricity is driven by the electronic process of charge ordering of Fe 2+ and Fe 3+ ions and for indications of coupling between electronic and magnetic degrees of freedom [2,3,4,5,6]. LuFe 2 O 4 is a member of the RFe 2 O 4 (R = rare earth element) family, the physical properties of which depend strongly on oxygen stoichiometry. For example, nearly stoichiometric YFe 2 O 4 exhibits three-dimensional (3D) magnetic order while oxygen deficient YFe 2 O 4 exhibits two-dimensional (2D) magnetic order [7]. LuFe 2 O 4 exhibits multiple phase transitions. 2D charge correlations are observed below 500 K, while below 320 K 3D charge order is established, roughly coinciding with the onset of ferroelectricity [2,8]. Magnetic order appears below 240 K and 2D ferrimagnetic order has been suggested by neutron scattering studies [9]. However, strong sample dependent behavior observed in other members of RFe 2 O 4 [7] suggests that unraveling the interesting behavior of LuFe 2 O 4 requires paying due attention to sample quality.In this letter we present extensive neutron diffraction measurements from 20 to 300 K on high quality single crystals of LuFe 2 O 4 . We report several new findings that provide information about the underlying magnetic interactions. First, our measurements indicate that below 240 K 3D magnetic correlations exist with magnetic intensity appearing at (1/3 1/3 L) where L may take on integer and 1/2-integer values. The magnetic structure is refined with a ferrimagnetic spin configuration with a propagation vector of (1/3 1/3 0). The magnetic intensity appearing on peaks where L is a 1/2-integer is a consequence of the charge ordering at ∼320 K. In addition, evidence is presented for a second transition at 175 K with significant changes in magnetic peak intensities and broadening of many reflections.Single crystals of LuFe 2 O 4 were grown by floatingzone-melting, using an oxygen partial pressure tuned by a CO/CO 2 mi...
Iron-based superconductivity develops near an antiferromagnetic order and out of a bad-metal normal state, which has been interpreted as originating from a proximate Mott transition. Whether an actual Mott insulator can be realized in the phase diagram of the iron pnictides remains an open question. Here we use transport, transmission electron microscopy, X-ray absorption spectroscopy, resonant inelastic X-ray scattering and neutron scattering to demonstrate that NaFe1−xCuxAs near x≈0.5 exhibits real space Fe and Cu ordering, and are antiferromagnetic insulators with the insulating behaviour persisting above the Néel temperature, indicative of a Mott insulator. On decreasing x from 0.5, the antiferromagnetic-ordered moment continuously decreases, yielding to superconductivity ∼x=0.05. Our discovery of a Mott-insulating state in NaFe1−xCuxAs thus makes it the only known Fe-based material, in which superconductivity can be smoothly connected to the Mott-insulating state, highlighting the important role of electron correlations in the high-Tc superconductivity.
Rafts, or functional domains, are transient nano- or mesoscopic structures in the exoplasmic leaflet of the plasma membrane, and are thought to be essential for many cellular processes. Using neutron diffraction and computer modelling, we present evidence for the existence of highly ordered lipid domains in the cholesterol-rich (32.5 mol%) liquid-ordered () phase of dipalmitoylphosphatidylcholine membranes. The liquid ordered phase in one-component lipid membranes has previously been thought to be a homogeneous phase. The presence of highly ordered lipid domains embedded in a disordered lipid matrix implies non-uniform distribution of cholesterol between the two phases. The experimental results are in excellent agreement with recent computer simulations of DPPC/cholesterol complexes [Meinhardt, Vink and Schmid (2013). Proc Natl Acad Sci USA 110(12): 4476–4481], which reported the existence of nanometer size domains in a liquid disordered lipid environment.
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