Ho Phases Research Articles

The surface state of URu 2Si 2

The ‘hidden-order’ (HO) transition of URu 2Si 2 remains a puzzle after 25 years of research. Using high-resolution angle-resolved photoemission spectroscopy (ARPES) we found that a hole-like band around Γ having its band maximum at E = −35 meV, and previously thought to be a bulk band of the system, is indeed a surface state not related to the HO phase transition. Here we present our detailed investigations to assign that state to a surface feature, and discuss on the possible origins of this surface band.

Note on Anisotropy of Resistivity in Hidden Ordered State of URu2Si2

On an assumption that the HO is the antiferro-quadrupolar order with \Gamma_{5} (Q_{z(x+y)} and Q_{z(x-y)}) symmetry, the anisotropy in temperature dependence of resistivity (\rho_{c}\propto T^{2}, and \rho_{ab}\propto T) in the HO phase of URu_2Si_2 is explained.

New angles on the border of antiferromagnetism inNiS2andURu2Si2

Abstract Following the border of antiferromagnetism (AF) to zero temperature is a promising route to unconventional metallic and superconducting phases. Many interesting examples of antiferromagnetic quantum phase transitions can only be reached by pressure tuning. The range of quantitative experimental probes, which can be realised in a high-pressure environment is limited. However, advances have recently been made in neutron scattering, where elliptically shaped neutron guides now increase the beam intensity directed to mm size sample for high pressure studies. This has been demonstrated on the simple antiferromagnet NiS 2 . Neutron scattering also allows highly accurate measurements of the lattice constant via the Larmor diffraction technique, which proved extremely useful in studying the high-pressure phase diagram of the itinerant helimagnet MnSi. We now combined Larmor diffraction with conventional diffraction measurements to investigate the pressure–temperature phase diagram of URu 2 Si 2 up to 20 kbar. URu 2 Si 2 offers a further spectacular example for the presence of unconventional phases in the vicinity of antiferromagnetism. In this compound, antiferromagnetism is replaced below approximately 5 kbar by the mysterious “hidden order” (HO) and unconventional superconductivity. Our measurements allow the observation of magnetic order and changes in the a - and c -axis lattice constants across the phase transitions in the same experiment. The results contain clear indications of a first-order transition and strong differences between the AF phase and the HO phase in the coupling to the lattice.

Asymmetrically Substituted Benzene‐1,3,5‐tricarboxamides: Self‐Assembly and Odd–Even Effects in the Solid State and in Dilute Solution

Molecular organization: Chiral benzene tricarboxamides with methyl substituents at defined positions self-assemble into supramolecular polymers of preferred helicity by three-fold alpha-helical-type hydrogen bonding. The odd-even effect is operative and all derivatives are liquid crystalline showing a Col(ho) phase (see figure).Asymmetric benzene-1,3,5-tricarboxamides (aBTAs) comprising two n-octyl and one chiral methyl-alkyl side chain were synthesised and characterised. The influence of the position and the configuration of the chiral methyl group (methyl at the alpha, beta or gamma position) in the aliphatic side chains on the liquid-crystalline properties and the aggregation behaviour of the aBTAs was systematically studied and compared to symmetrical benzene-1,3,5-tricarboxamides (sBTAs). Solid-state characterisation (polarised optical microscopy, IR spectroscopy, X-ray diffraction and differential scanning calorimetry) revealed that all aBTAs show threefold, alpha-helical-type intermolecular hydrogen bonding between neighbouring molecules and exhibit a columnar hexagonal organisation from room temperature to well above 200 degrees C. Moving the chiral methyl group closer to the amide group stabilises the liquid-crystalline state, as evidenced by a higher clearing temperature and corresponding enthalpy. The self-assembly of dilute solutions of the aBTAs in methylcyclohexane ( approximately 10(-5) mol L(-1)) was investigated with circular dichroism (CD) spectroscopy. The sign of the Cotton effect demonstrated a pronounced odd-even effect, whereas the value of the molar ellipticity, Deltaepsilon, in the aBTAs was independent of the position of the methyl group. Subsequent temperature-dependent CD measurements showed that the aggregation of all aBTAs can quantitatively be described by the nucleation-growth model and that the stability of the aggregates increases when the chiral methyl group is closer to the amide moiety. The results presented herein illustrate that even small changes in the molecular structure of substituted benzene-1,3,5-tri-carboxamides affect their solid-state properties and their self-assembly behaviour in dilute solutions.

Satellite Holmium M-Edge Spectra from the Magnetic Phase via Resonant Soft X-Ray Scattering

Developing an expression of resonant x-ray scattering (RXS) amplitude which is convenient for investigating the contributions from the higher rank tensor on the basis of a localized electron picture, we analyze the RXS spectra from the magnetic phases of Ho near the M 4,5 absorption edges. At the M 5 edge in the uniform helical phase, the calculated spectra of the absorption coefficient, the RXS intensities at the first and second satellite spots capture the properties the experimental data possess, such as the spectral shapes and the peak positions. This demonstrates the plausibility of the adoption of the localized picture in this material and the effectiveness of the spectral shape analysis. The latter point is markedly valuable since the azimuthal angle dependence, which is one of the most useful informations RXS can provides, is lacking in the experimental conditions. Then, by focusing on the temperature dependence of the spectral shape at the second satellite spot, we expect that the spectrum is the...

Columnar mesophases and phase behaviors of novel polycatenar mesogens containing bi-1,3,4-oxadiazole

A new series of liquid-crystalline bi-1,3,4-oxadiazole derivatives (2,2′-bis(3,4,5-trialkoxyphenyl)-bi-1,3,4-oxadiazole, BOXD-T n, n=3, 4, 5, 6, 7, 8, 10, 14) were designed and synthesized. They have been confirmed to give rise to columnar mesophases. The columnar mesophases for BOXD-T n ( n=5, 6, 7, 8, 10) could be supercooled to −20 °C on the cooling runs. A room temperature Col ho phase was obtained for BOXD-T14. All BOXD-T n exhibit good fluorescence properties either in cyclohexane or in solid state.

Magnetic spin interactions observed by heat capacity measurements for layered compounds: Na LnTiO 4 ( Ln = Sm, Eu, Gd, Tb, Dy, Ho and Er)

Magnetic properties of a series of orthorhombic Na LnTiO 4 ( Ln = Sm, Eu, Gd, Tb, Dy, Ho and Er) characterized by heat capacity measurements are reported. Magnetic transitions are observed for all Na LnTiO 4 investigated except for Ln = Eu. The change in magnetic entropy closely agrees with the expected theoretical values except for Ln = Sm and Gd. Peak profiles in temperature dependence of heat capacity indicate that Ln = Sm, Tb, Dy, Ho and Er phases possess Ising square lattices. Furthermore, Ln = Gd also exhibits the similar property regardless of its S state; there seems some degree of anisotropy in its magnetic lattice. Magnetization and heat capacity data indicate that superexchange through oxygen is the dominant spin interaction in this system.

Magnetic excitations in the crystalline and quasicrystalline Zn–Mg–Ho phases

A comparative study has been carried out on magnetic excitations in the primitive- (p-) and face-centred- (f-) icosahedral quasicrystalline phases, as well as crystalline phase, in the Zn–Mg–Ho system. The magnetic excitation spectrum in both the quasicrystalline phases has only a broad quasielastic peak centred at the elastic position; the signal for the positive energy-transfer (ℏω) side is found to be almost temperature independent in a wide temperature range 1.5 <T <200K. The magnetic excitation spectrum in the crystalline Zn65.2Mg27.9Ho6.9 phase is quite similar to that of the quasicrystalline phases at high temperatures; however, well-defined inelastic peaks due to crystalline-electric-field (CEF) splitting levels appear as the temperature is decreased. The appearance of the sharp CEF peaks confirms well-defined site symmetry around the rare-earth sites in the hexagonal phase, whereas the temperature-independent quasielastic signal suggests anomalous spin fluctuations in the quasicrystalline phases.

Gibbs free energy of formation of the LnMnO 3 (Ln = Yb, Ho, and Er) phases determined by the e.m.f. method

The Gibbs free energies of formation of the solid YbMnO 3, HoMnO 3, and ErMnO 3 phases in the (Ln + Mn + O) systems were determined by employing electrochemical cells with zirconia solid electrolyte in the temperature range (1150 to 1450) K. The results obtained in this study were used to derive the Gibbs free energy of formation of solid phases from the oxides. The respective equations took the following form: for YbMnO 3: ( Δ f G m ∘ ±410)/(J · mol −1) = −29,189 + 0.021( T/K), for HoMnO 3: ( Δ f G m ∘ ±730)/(J · mol −1) = −28,557 + 2.28( T/K), for ErMnO 3: ( Δ f G m ∘ ±480)/(J · mol −1) = −25,324 + 4.265( T/K).

Comparative Study of Planar Motion in Monomeric and Dimeric Discotics

Deuterium NMR spectra are reported in powder samples of discotic monomer and dimer as a function of temperature in their column Col(ho) phases. To simulate the observed powder patterns, a threefold jump model is used in the monomer, while in the related dimer the libration motion of the monomeric core is described using the infinitesimal jump method under a restricting potential due to the spacer. By comparing the diffusive rates for the two samples, it is concluded that the planar motion in the dimer is at least 30 times smaller than that of the monomer. This could lead to an enhancement of charge and energy transport in discotic dimer systems.

H2NMR angle-dependent spectral study of core dynamics in a discotic columnar phase

A detailed (2)H NMR study of a discotic monomer is reported in its columnar phase at 61.4 MHz. A simulation of angular-dependent spectral patterns and/or signal intensities of the aromatic deuterons is shown to provide information on the small step (or jump) diffusion rates in the low temperature region of the D(ho) phase, where the motion is in the intermediate motion regime. In the high end of this phase, the motion falls into the fast motion regime and spectral patterns become insensitive to the diffusion rate. In such a case, angular-dependent NMR signal intensities were used to give the required dynamic information. The diffusive motion about the columnar axis is found to be thermally activated with an activation energy of 69 kJ/mol. Deuteron spin-lattice relaxation rates were measured at 61.4 MHz and interpreted by a conventional rotational diffusion model in order to complement and compare with the above study when possible.

Mobility of side chains in a discotic liquid crystal studied by deuteron spin relaxation

A hexaether-substituted rufigallol mesogen is studied by measuring deuteron spin–lattice relaxation times T 1Z and T 1Q in the hexagonal D ho phase. Small-step rotations of discotic molecules within each column are described by the so-called ‘anisotropic viscosity’ model, giving rotational diffusion constants D α and D β for rotations of the disc normal around and perpendicular to the director. A libration model is used to describe the wagging motions in different side chains as seen by the first CD bonds, which diffuse in a circular arc within a defined apex angle. The chain libration rates and corresponding apex angles are also derived.

The Al–R–Mg (R=Gd, Dy, Ho) systems. Part II: Thermodynamic modelling of the binary and ternary systems

The thermodynamic modelling and optimisation of the Al–R, R–Mg and Al–R–Mg (R=Gd, Dy, Ho) systems has been carried out, the Al–Mg system being already optimised in literature. The Compound Energy Formalism (CEF) has been used to describe the thermodynamic functions of either solutions and stoichiometric phases present in the systems. In particular, the order/disorder relation between cI2-W (A2) and cP2-CsCl (B2) phases present in the systems has been taken into account and thermodynamically modelled. Moreover γ-(Mg,Al) and R5(Mg1-xAlx)24 (R=Dy, Ho) phases, related to the cI58-αMn (A12) type structure, have been modelled as different sublattice occupations of the same A12 phase. The thermodynamic modelling of the ternary systems is mainly based on the experimental investigation carried out in our laboratory and reported in a separate paper in this issue [1]. As a result, a complete description of the solid-solid and solid-liquid phase equilibria in the six binary and three ternary systems has been obtained.

Three-dimensional dosimetric evaluation of a conventional radiotherapy technique for treatment of nasopharyngeal carcinoma

Background and purpose: The aim of this study is to evaluate and delineate the deficiencies in conventional two-dimensional (2-D) radiotherapy planning of nasopharyngeal carcinoma (NPC) treatment and to explore the means for improvement of the existing treatment technique aiming at enhancing local tumor control and reducing treatment complications.Methods and materials: Ten patients with NPC sparing the skull base and without intracranial extension or cranial nerve(s) palsy were chosen in the present study. Two sets of CT images for Phases I and II of the radiotherapy treatment were taken with patient immobilized in the flexed-head and the extended-head positions, respectively. Based on the CT images and endoscopic findings, the gross tumor volume (GTV) was defined. The clinical target volume (CTV) circumscribing the GTV was defined according to Ho's (Halnan, K.E. (ed.) Treatment of Cancer. London: Chapman and Hall, 1982. pp. 249–268) description of the organs at risk of tumor infiltration. The planning target volume (PTV) was defined by adding a margin to the CTV which catered for geometrical inaccuracies. The field borders and shields were set at standard distances from certain bony landmarks and were drawn on the simulator radiograph. Data on the beams and shield arrangements were then transferred to the planning computer via a digitizer. By applying 3-D volumetric dose calculation using a commercial three-dimensional (3D) treatment planning computer, the dose–volume-histograms (DVHs) of GTV, CTV, PTV and critical normal organs were generated for both phases of Ho's treatment technique. The same patients were re-planned using a modified Ho's technique which used 3-D beams-eye-view (BEV) in placing the shielding blocks and the same set of DVHs were generated and compared with those obtained from Ho's technique.Results: The median volumes of GTV, CTV and PTV covered by the 95% isodose in Ho's phase I treatment were around 60%. The dose coverage was unsatisfactory in the superior and inferior and the posterolateral regions. In phase II treatment, the median volume of GTV, CTV and PTV covered by the 95% isodose were 99, 96 and 72%, respectively. Even though the dose coverage of the PTV in both phases of treatment were unsatisfactory, radiotherapy with the original Ho's technique had consistently produced good local control for NPC. However, there is potential room for enhancing the local control further because after modifying Ho's technique by using 3-D BEV customization of the treatment portals, the median volume of the target covered by the 95% isodose was defined as V95. The V95 of the PTV during the Phase II treatment was improved by 13%. The 90% of the volume of temporo-mandibular joints and parotid glands were both irradiated to 53 Gy and 43.6 Gy of the total prescribed dose of 66 Gy, respectively, in phase I and II treatments. With the addition of a hypothalamus-pituitary shield to Ho's technique, 50% of the volume of optic chiasma and temporal lobes received, respectively, 19.3 Gy and 4.5 Gy. However, small volume of the temporal lobes received a maximum dose (Dmax) of 62.8 Gy (95.2% of 66Gy). Most of the brainstem was shielded from the lateral portals but 5% of its volume received a dose ranging from 25.4 to 50.4Gy. The spinal cord (at C1/C2 level) received a Dmax of 40.8 Gy in phase I and of 4.8 Gy in phase II. After modifying Ho's technique by 3-D BEV customization of the treatment portals, the Dmax to the brainstem, the optic chiasma and the temporal lobes could be reduced by 8, 12 and 5%, respectively.Conclusions: Our study indicated that the dose-coverage of the PTV in Ho's radiotherapy technique for the early T-stage NPC was less than satisfactory in the superior and inferior and the posterolateral regions. However, in view of the excellent historical local tumor control with Ho's technique, we have to postulate that the present definition of CTV (and hence the PTV after adding margins to the CTV) lacks clinical significance and can be improved. It appears that the inclusion of the entire sphenoid sinus floor and both medial and lateral pterygoid muscles in the CTV is not necessary for maximal tumor control in the absence of clinical/radiological evidence of tumor infiltration of these organs. Ho's technique can be improved by using 3-D BEV to customize the treatment portals with multileaf collimators or blocks.

Magnetic Phase Transitions and Lattice Changes in Rare Earth Metals Studied by X-ray Diffraction under a Magnetic Field

The lattice change of the rare earth metals Dy, Ho and Er has been studied by x-ray diffraction under a magnetic field in the c-plane of a hexagonal crystal. Continuous and discontinuous changes of the c-lattice parameter accompanying magnetic phase transitions have been observed in these metals, and satellite reflections caused by the lattice modulation have been found in the distorted helix and helifan phases of Ho and the distorted helix phase of Dy. In Er, the 2nd- and 4th-order satellites appeared below 4.5 T at 35 K, and the 6th-order satellite has been observed above 4.5 T in addition to the 2nd- and 4th-order satellites. These results can be explained with the magnetoelastic effect and the magnetic structure calculated by the self-consistent mean field method, except for the result for Er above 4.5 T at 35 K, where a strongly distorted fan magnetic structure should he assumed to explain the presence of the 6th-order satellite of the lattice modulation.

Experimental solution of the local-field problem in discotic liquid crystals

Polarized light-absorption spectra are obtained for single-domain planar-oriented samples of the discotic Dho. Previously predicted spectral effects induced by resonant dipole-dipole interactions of the molecules are observed. New methods for determining the parameters of the local field in the D ho phase are developed which take into account the mixing of the molecular excitations. It is shown that the two-dimensional crystalline ordering of the molecular columns decreases the anisotropy of the local field for this phase.

Phase identification of triphenylene-based discotic monomer and its main chain polymers

A monomer, 2,3,6,7,10,11-hexakispentyloxy triphenylene (HPT) possesses a triphenylene core as a discotic mesogen. Polymers containing this discotic mesogen have been studied using wide-angle X-ray and electron diffraction. HPT is known to show a discotic liquid crystal phase, noted as D ho (h for hexagonal bidimensional lattice, o for ordered molecular spacing in each column). In this paper, however, HPT liquid crystalline phases, heated up from the crystalline state and cooled down from the isotropic state, were characterized in the diameter dimensions. In addition. the diameters of the columns are close to a parameter of two separate crystals. A core orientation was, therefore, proposed in the mesophase obtained by heating the crystalline. In order to distinguish these differences, the D ho phase was divided to include the D hcd and D hco phases. Molecular modeling was performed to help our understanding of the orientation. The D hcd and D hco phases were used to characterize the phases of the discotic polymeric analogs by comparing their column diameters to those of the monomers.

A re-entrant Col ho phase

The formation of a re-entrant columnar discotic hexagonal ordered phase (re-entrant Col ho phase) is reported for the asymmetrically substituted triphenylene 3,6,7,10,11-penta- pentyloxytriphenylene-2-yl pivaloate (pivaloate) and its structural and dynamic properties described. The re-entrant phase behaviour is strongly modified by doping the pivaloate with the electron acceptor 2,4,7-trinitrofluorenone (TNF).

Kinetics of destruction of Mo-S binary phases and crystal growth of rare-earth molybdenum chalcogenides

The kinetics of thermal destruction of Mo-S binary phases, and phase relations prevalent in the solid state in some chosen RE-Mo-S ternary systems, have been determined. A new method of crystal growth for the RE-containing Chevrel-type phases has been elaborated and successfully tested. Based on the results of crystal growth, a general description is given of the phase equilibria occurring in these systems in the presence of a liquid phase, and their evolution resulting from the thermal stability of each specific REMo6S8 phase. Some structural and transport properties of single crystals of Ce, Eu, Gd, Ho, Er, Yb and Lu phases are reviewed.

Superconductivity in rare earth cuprate perovskites

Superconducting cuprate perovskites isostructural with Ba 2YCu 3O 7 incorporating a variety of rare earth ions are reported. Resistivity and magnetic susceptibility measurements show transition temperatures in excess of 90K for several materials, including Nd, Sm, Gd, Dy, Ho, Er and Lu-containing phases. Interestingly, Ce and Tb, with a readily accessible 4+ state, form phases other than the defect perovskite which are not superconductors. However, Pr, which also has an accessible 4+ state, does give a nonsuperconducting defect perovskite isostructural with Ba 2YCu 3O 7. The observation of high temperature superconductivity in phases containing ions carrying magnetic moments is significant because it suggests that the magnetic moment on the rare earth ion interacts only weakly with the electrons making up the band crucial to superconductivity in these materials.