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We present a group theoretical analysis of several classes of organic superconductor. We predict that highly frustrated organic superconductors, such as κ-(ET)₂Cu₂(CN)₃ (where ET is BEDT-TTF, bis(ethylene-dithio)tetrathiafulvalene) and β'-[Pd(dmit)₂]₂X, undergo two superconducting phase transitions, the first from the normal state to a d-wave superconductor and the second to a d+id state. We show that the monoclinic distortion of κ-(ET)₂Cu(NCS)₂ means that the symmetry of its superconducting order parameter is different from that of orthorhombic κ-(ET)₂Cu[N(CN)₂]Br. We propose that β'' and θ phase organic superconductors have d_xy+s order parameters.

A Datta–Das spin field effect transistor (FET) made of a nonballistic quantum wire with a single transport channel is considered. Although there is no spin relaxation and the spin precession is not influenced by elastic scattering, successful spin FET operation can still be prevented by the conductance fluctuations. The necessary condition for the desired spin FET operation is obtained.

The aim of this work is to investigate electronic structure, magnetic properties and electrical resistivity of Fe₂V_1−xTi_xAl Heusler alloys. Numerical calculations give a pseudogap at the Fermi level for the majority-spin band of Fe₂TiAl and a magnetic moment larger than 0.9 μ_B, whereas the ground state of Fe₂VAl is calculated as a nonmagnetic semimetal with a very low total density of states at the Fermi level. In our calculations the remaining alloys of the Fe₂V_1−xTi_xAl series are nonmagnetic for x<0.1 and weakly magnetic for 0.1<x≤1. The magnetic moment μ of the series of Fe₂V_1−xTi_xAl compounds scales with the number of valence electrons and fits well to the Slater–Pauling curve. We also present a study of the electronic transport properties and magnetic susceptibility. The resistivities ρ(T) of Fe₂VAl and Fe₂V_0.9Ti_0.1Al are large and exhibit a negative temperature coefficient dρ/dT of the resistivity between 2 and 300 K. Below 20 K, ρ(T) also shows an activated character. The magnetic susceptibility of Fe₂VAl and Fe₂V_0.9Ti_0.1Al shows a maximum at ∼2 K which could reflect either the disorder effect or the hybridization gap, characteristic of Kondo insulators.

We consider the full solution of McMillan's molecular model of the smectic A phase within the mean-field approximation, expressing the free energy (or the effective one-particle mean-field energy) of the model in terms of an infinite set of orientational and translational order parameters. The general formalism reduces to the usual McMillan theory (hereafter referred to as McMillan's approximation) when second- and higher-order harmonics in the Fourier expansion are neglected, which leads to a description of the smectic phase in terms of the leading order parameters. The effects of such a truncation on the location of the tricritical nematic–smectic A point have been previously considered by Longa (1986 J. Chem. Phys.85 2974). A quantitative analysis to assess the relative importance of the neglected terms in the description of the smectic phase and its various transitions is reported. It is shown that McMillan's approximation underestimates both orientational and translational order, and leads to values of the transition entropies smaller than those resulting from the full expansion.

We report on the valence band electronic structure as well as structural, magnetic and electronic transport properties of a novel intermetallic YbNi_0.8Al_4.2. The compound crystallizes in the orthorhombic YNiAl₄-type structure (a = 4.049(2),b = 15.305(5) and c = 6.586(3) Å) with space group Cmcm. The valence band spectrum and magnetic data indicate an intermediate valency of Yb ions in the studied compound with a valence of about 2.66 at room temperature. The temperature dependences of electrical resistivity, Hall effect and thermoelectric power display characteristic features of Kondo lattice with a large Kondo temperature (∼1300 K), being consistent with the intermediate valence scenario. An analysis of magnetic data suggests the presence of strong ferromagnetic correlations in the investigated compound at low temperatures. In order to obtain information about the electron–hole analogy between the Ce and Yb compounds, we have compared the observed behaviour with that of the dense Kondo compound CeNiAl₄.

In this work we study the structural and electronic properties of Be chalcogenides (BeS, BeSe and BeTe) using two different methods: the full-potential linear augmented-plane wave (FP-LAPW) and the plane-wave pseudopotential (PPsPW). The exchange–correlation effects are treated in the local-density approximation (LDA) and the generalized-gradient approximation (GGA). We have evaluated the ground-state quantities such as equilibrium volume, bulk modulus and its pressure derivative as well as the elastic constants. Various structural phase transitions were considered here in order to confirm the most stable structure and to predict the phase transition under hydrostatic pressure. In addition we have studied the band structure and the density of states, which show a wide indirect band gap for these compounds. These results were in favourable agreement with previous theoretical works and the existing experimental data. To complete the fundamental characteristics of beryllium chalcogenide compounds we have analysed their bonding character in terms of charge transfer and the ionicity parameter. The latter is found to be in agreement with the charge transfer behaviour, which shows an important ionic localization.

The phases, ion crystal packing and thermal properties of the N-alkyl-N-methylpyrrolidinium and piperidinium bis(trifluoromethanesulfonyl)imide (PYR_1RTFSI and PIP_1RTFSI (subscript R = 1 for methyl and 2 for ethyl), respectively) salts are compared using powder and single-crystal x-ray diffraction (XRD) and differential scanning calorimetry (DSC). The crystal structure of PIP₁₂TFSI has been determined at 123 K. The salt crystallizes in the triclinic space group with Z = 8. Structural data are also reported for PYR₁₁TFSI at 153 K and PIP₁₂TFSI at 223 K. PIP₁₁TFSI has identical ion crystal packing to the analogous pyrrolidinium salt PYR₁₁TFSI. Since increasing the cation alkyl chain length to propyl or butyl (R = 3 or 4) reduces the melting point of the salts below room temperature, this study may provide valuable insight into why these pyrrolidinium and piperidinium salts form room-temperature ionic liquids.

Ageing dynamics of a re-entrant ferromagnet stage-2 Cu_0.8Co_0.2Cl₂ graphite intercalation compound has been studied using DC magnetization measurements. This compound undergoes successive transitions at the transition temperatures T_c (≈8.7 K) and T_RSG (≈3.3 K). The relaxation rate S_ZFC(t) exhibits a characteristic peak at t_cr below T_c. The peak time t_cr at constant t_w shows a local maximum around 5.5 K, indicating a slow dynamics arising from a frustrated nature of the ferromagnetic phase. It drastically increases with decreasing temperature below T_RSG. The spin configuration imprinted at the stop and wait process at a stop temperature T_s (<T_c) during the field-cooled ageing protocol becomes frozen on further cooling. On reheating, the memory of the ageing at T_s is retrieved as an anomaly of the thermoremnant magnetization at T_s. These results indicate the occurrence of the ageing phenomena in the ferromagnetic phase (T_RSG<T<T_c) as well as in the re-entrant spin glass phase (T<T_RSG).

Mn 3d electronic states in the dilute magnetic semiconductor Zn_1−xMn_xS (x = 0.1–0.3) are studied using soft x-ray emission (XES) measurements and density functional theory (DFT). Mn L_2,3 emission spectra of Zn_1−xMn_xS (x = 0.1–0.3) suggest that the Mn impurities do not form clusters in the host ZnS lattice, in agreement with previous models. A shift in the position of a Mn L₃ XES feature suggests a change in the nature of the hybridization between the Mn 3d_3/2 and S 3p states as a function of x. Our DFT calculations reproduce the weak interatomic exchange interaction, as well as the strong intra-atomic exchange splitting that is expected from observations of Zeeman splitting in such materials.

To microscopically elucidate the initial evolution of the electronic and magnetic states of a Kondo compound CeAl₂ (Néel temperature T_N∼3.8 K) from the antiferromagnetically ordered state with a spin density wave to a magnetic quantum critical point with the application of pressure P, we have carried out ²⁷Al nuclear quadrupole resonance and magnetic resonance measurements for P = 0 and 2.5 GPa. The Knight shift, which is proportional to the uniform susceptibility , exhibits a rapid increase below ∼50 K down to T_N for each pressure, indicating that the sufficiently localized f electron does not directly participate in the formation of the Fermi surface even at P = 2.5 GPa. The nuclear spin–lattice relaxation measurements and the analysis lead to the conclusion that the cf hybridized band with a rather large density of states at the Fermi level is formed below an onset temperature above T_N, the value of which increases with the application of pressure. The relaxation rate in the paramagnetic state is dominated by the generalized susceptibility that has peaks near the antiferromagnetic wavevector associated with the nesting properties of the Fermi surface of the underlying cf hybridized band. With decreasing temperature, also exhibits a significant increase larger than that of χ(0). The finite pressure of 2.5 GPa has the effect of reducing both χ(0) and χ(Q_AF) by about 20% in their magnitudes. Then, changes in the nesting condition with pressure are conjectured to play an important role in depressing the magnetic ordering, in addition to the increase in the extent of mixing J_cf between the localized f electrons and conduction electrons.

We have performed density functional calculations with on-site Coulomb repulsion corrections of systems that may be involved in the Verwey transition in magnetite (Fe₃O₄). We find that the lowest energy solution for the minority spin wavefunction in the cubic cell involves orbitally ordered Fe-d and O-p states, which breaks cubic symmetry. This leads to partial charge ordering that triggers a Jahn–Teller distortion and band-gap opening. Our results show this to be the essential mechanism of the Verwey transition. Applying ionic relaxation within a larger tetragonal cell, three patterns of charge ordering are compared and a Pmca pattern matching x-ray data is found to be the most stable.

We report a theoretical study of the influence of the Coulomb interaction on the equilibrium spin current in a quantum-dot spin valve, in which the quantum dot described by the Anderson impurity model is coupled to two ferromagnetic leads with noncollinear magnetizations. In the Kondo regime, electrons transmit through the quantum dot via higher-order virtual processes, in which the spin of either lead electrons or a localized electron on the quantum dot may reverse. It is found that the magnitude of the spin current decreases with increasing Coulomb interactions due to spin flip effects on the dot. However, the spatial direction of the spin current remains unchanged; it is determined only by the exchange coupling between two noncollinear magnetizations.

Magnetic susceptibilities of the first and third orders as well as the magnetization of the singlet paramagnet HoBa₂Cu₃O_x (x≈6.0) at low temperatures are studied experimentally using a SQUID magnetometer and compared with theoretical calculations. The magnetic behaviour of single-crystal HoBa₂Cu₃O_x at low temperature is found not to follow the one calculated on the basis of the known crystal field parameters. Different effects which may change the magnetic properties are analysed and discussed: low symmetry components of the crystal field due to some disorder in the oxygen subsystem, a noncompensated effective field from the ordered Cu subsystem acting on the Ho³⁺ ions, and others.

Pulsed-laser deposited (Pb,Sr)TiO₃ (PSrT) films on Pt/SiO₂/Si substrate at various ambient oxygen pressures (P_O2) are investigated in this work. Films deposited at P_O2 below 100 mTorr exhibit the (100) preferred orientation and a tetragonal structure with larger tetragonality. In addition, films deposited at 80 mTorr exhibit the most apparent ferroelectric properties in contrast to those deposited at 200 mTorr. Moreover, films deposited at higher P_O2 also exhibit longer lifetimes and higher breakdown fields due to their smaller leakage current density, in terms of the reduction of defects, compensation of oxygen vacancies (OVs), an improved interface and small cluster sizes. An energy band model reveals that fatigue properties of PSrT films are dominated by interfacial states at low P_O2 and by deep trapping states at high P_O2, which could be ascribed to OVs located at the interfaces and inside films, respectively.

Magnetic transport properties in Ti_1−xCo_xO₂ and Zn_1−xCo_xO magnetic semiconductors have been studied experimentally and theoretically. A linear relation of lnρ versus T^−1/2 (ρ is sheet resistance and T is temperature), which shows different slopes and intersections at different magnetic fields, was observed experimentally in the low temperature range. The spin-dependent variable range hopping model has been proposed by taking into account the electron–electron Coulomb interaction and the spin–spin exchange interaction in the same frame, which can well describe the observed magnetic transport properties in Ti_1−xCo_xO₂ and Zn_1−xCo_xO magnetic semiconductors.

The aim of the present study, an extension of a recent one (Bose Roy and Bose Roy 2005 J. Phys.: Condens. Matter 17 6193), is to assess and compare the curve-fitting utility of the isothermal unrealistic two-parameter equations of state for solids (EOS), proposed at different stages in the development of the EOS field, for the purposes of smoothing and interpolation of pressure–volume data, and extraction of accurate values of the isothermal bulk modulus and its pressure derivative. To this end, 21 such EOSs are considered, formulated by/labelled as Born–Mie (1920), Born–Mayer (1932), Bardeen (1938), Slater–Morse (1939), Birch–Murnaghan (1947), Pack–Evans–James (1948), Lagrangian (1951), Davydov (1956), Davis and Gordon (1967), Onat and Vaisnys (1967), Grover–Getting–Kennedy (1973), Brennan–Stacey (1979), Walzer–Ullmann–Pan'kov (1979), Rydberg (1981), Dodson (1987), Holzapfel (1991), Parsafar–Mason (1994), Shanker–Kushwah–Kumar (1997), Poirier–Tarantola (1998), Deng–Yan (2002) and Kun–Loa–Syassen (2003). Furthermore, all these EOSs are compared with our three-parameter EOS, as well as its two-parameter counterpart proposed in this work. We have applied all the EOS models, with no constraint on the parameters, to the accurate and model-independent isotherms of nine solids. The applicability has been assessed in terms of an unbiased composite test, comprising fitting accuracy, agreement of the fit parameters with experiment, stability of the fit parameters with variation in the compression/pressure ranges and on the basis of the number of wiggles of the data deviation curves about the fit parameters. Furthermore, a rigorous method is devised to scale the relative adequacy of the EOSs with respect to the test parameters. A number of remarkable findings emerge from the present study. Surprisingly, both the old EOSs, the Born–Mie and the Pack–Evans–James, are significantly better in their curve-fitting capability than the Birch–Murnaghan EOS which has been widely used and continues to be used for curve-fitting purposes as a standard EOS in the literature. The Born–Mayer as well as the Walzer–Ullmann–Pan'kov models also fit isotherms better than the Birch. The performance of the EOS based on the Rydberg potential—that has been rediscovered by Rose et al (1984 Phys. Rev. B 29 2963), and strongly promoted by Vinet et al (1989 J. Phys.: Condens. Matter 1 1941) as the so-called universal equation of state, and is currently used as a standard EOS along with that of the Birch—is very poor, on a comparative scale. Furthermore, the curve-fitting capability of our original three-parameter EOS, and more importantly its two-parameter counterpart, is superior to all the isothermal unrealistic two-parameter EOSs so far proposed in the literature.

The effects of Pr substitution in the structure and ferroelectric response for the Sr_1−xPr_xBi₂Ta₂O₉ (SBT-Pr) compound have been studied. Rietveld refinement of the x-ray diffraction patterns indicates that the Pr ion progressively replaces the Sr site in the A 2₁am space group structure. The solubility of Pr in solid solution is around 15%. The replacement induces a change in the crystal structure and, as a consequence, the dielectric properties are affected. The ferroelectric transition at T_m∼558 K is shifted to lower temperatures, T_m∼413 K for x = 0.15 composition. Apparently, the tilt angle (α) associated with the c-axis does not play an important role since it remains essentially constant. However, the rotation in the ab-plane (β) as well as the octahedral distortion observed are strongly related to the coupling between T_m and x. A relaxor-type transition is observed as Pr is increased, leading to polar microregions above the nominal ferroelectric transition. The local disorder induced by the Pr ion is confirmed by the continuous increase in the diffuseness coefficient according to Isupov's model. These facts hinder the displacement of the TaO₆ octahedra with respect to Bi₂O₂ along the polarization axis, decreasing the polarization values.

Density functional theory is used to study the interactions of Ti and Zr with the tips of open-ended single-wall carbon nanotubes. It is found that Ti or Zr atoms can saturate the dangling bonds of a tip to make it closed. Zr displays much stronger interaction with the contacted carbon atom than Ti. The Fermi energies of the hybrid systems increase dramatically, and the peak values of the density of states near the Fermi levels increase significantly. The field emission properties are discussed qualitatively.

The band structure in figure 2(b) and density of states in figure 3(b) for LDA +U calculation are incorrect and should be replaced by the new figures given (see pdf for details). This change does not affect the conclusions of the paper. W E Pickett is removed from the list of authors.