Charge and Anion Ordering in (TMTTF)2X Quasi-One-Dimensional Conductors

We present results of measurements of dielectric spectroscopy concerning the 4k_F charge density wave associated with the charge ordering (CO) of Wigner type in (TMTTF)₂X conductors. This CO transition is due to correlated electron interactions on the molecular chains and to the appropriate displacement of the anion chains (transition at q=0). Recent experiments on the substitution of hydrogen by deuterium in TMTTF molecules will be reported. In the vicinity of T_CO it has also been found the divergence of the relaxation time and the development of a low frequency shoulder in the frequency dependence of the imaginary part of the dielectric permittivity, which are typical features for classical ferroelectrics.     

Redistribution of Electronic Charge in (TMTTF)2ReO4: 13C NMR Investigation

¹³C NMR measurements were performed for one-dimensional organic conductors, (TMTTF)₂ReO₄. An intermediate charge-ordering (CO) phase has been found firstly for a TMTTF salt with a Td-symmetry counter anion: The NMR parameters indicate two inequivalent molecules with unequal electron densities below 225 K. Moreover, the spin-singlet transition associated with ReO₄ anion ordering was confirmed at around 158 K by ¹³C NMR measurements. A drastic change of NMR parameters below 158 K also indicates a redistribution of the electronic charge at the anion ordering temperature.     

Pressure Effect on BDA-TTP Conductors

The pressure-induced conducting behavior of the charge-transfer salts (BDA-TTP)₂X [BDA-TTP = 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene; X = ClO₄·H₂O, ClO₄, BF₄·H₂O, BF₄, and I₃] has been investigated. With increasing pressure, the transition temperature to insulator observed in the ClO₄·H₂O, ClO₄, BF₄·H₂O, and BF₄ salts increases. On the other hand, the resistance of the I₃ salt as a function of temperature at 10.0 kbar shows a drop attributable to a superconducting transition with an onset at 4.5 K. The superconducting transition temperature (T_c) can reach 8.2 K by applying a pressure of 10.3 kbar, but gradually decreases with further increases in the pressure.     

Magnetic and Pairing Correlation Functions and Interchain Coherence in Quasi-One-Dimensional Dimerized Organic Conductors

We study how magnetic and pairing correlation evolves with an increasing interchain hopping integral and decreasing dimerization of intrachain hopping integrals, by applying the density matrix renormalization group (DMRG) method to a three-chain extended Hubbard model at quarter filling for quasi-one-dimensional organic conductors, (TMTTF) ₂ X and (TMTSF) ₂ X. Magnetic correlation changes from weakly coupled chains of large-amplitude spin density waves to an interchain-coherence-developed spin density wave. Pairing correlation increases, though it still decays exponentially owing to a charge gap for parameters considered here.     

Superconductivity of Bi Confined in an Opal Host

Superconductivity is observed in a composite of rhombohedral crystalline bismuth nanoparticles imbedded in an insulating porous opal host via electrical transport and AC magnetic susceptibility. The onset of superconductivity in this system occurs in two steps, with upper transition temperature T _c,U =4.1 K and lower transition temperature of T _c,L =0.7 K, which we attribute to the granular nature of the composite. The transition at T _c,U is observed to split into two transitions with the application of a magnetic field, and these have upper critical fields extrapolated to T=0 K of H _c2,1(0)=0.7 T and H _c2,2(0)=1.0 T, corresponding to coherence lengths of ξ ₁(0)=21 nm and ξ ₂(0)=18 nm, respectively. We suggest that because of the lack of bulk-like states in the Bi nanoparticles due to confinement effects, superconductivity originates from surface states arising from Rashba spin-orbit scattering at the interface.     

Specific heat of quench-condensed hydrogen films

The specific heatC(X, T) of quench-condensed films of H₂ has been measured as a function of ortho concentration X with 0.28–2 at temperatures between 0.4 and 3.0 K. The films were condensed on evaporated gold substrates held at several temperaturesT. _cond between 1.0 and 3.5 K. The observed specific heat is attributed to orientational ordering of the ortho-H₂ molecules. For the films withX = 0.74 condensed atT _cond>2.5 K, there is a peak which indicates a bulk-like ordering transition. At temperatures below the peak, there is a large contribution toC, which is not present in bulk H₂, and which we attribute to short-range ordering size effects. AsT _cond is decreased below 2.5 K, the shape of the specific heat curve changes, and the peak at 1.5 K is replaced by a gradual rise with a sharp drop above 2.6 K. Despite this strong dependence ofC onT _cond, the entropy per molecule at 3 K is only weakly dependent onT _cond and comparable to that for bulk H₂. Film annealing at 3.4 K produces a change in the specific heat curve, and a study of this effect is presented. The ortho-para conversion rate of the films condensed at the various temperatures is found to be same as in bulk, well-annealed H₂. As in bulk H₂, the transition temperature inferred from the location of the specific heat peak or anomaly decreases withX. Unlike in bulk H₂, there is no temperature hysteresis inC for any of the quench-condensed films. This implies that the ordering transitions are not accompanied by a martensitic transformation.     

Preparation, Structure, and Properties of (Bi,Pb)2Sr2Ca2Cu3O10 + δ Ceramics with Si3N4 Additions

The microstructure, phase composition, texture, and superconducting properties (T _c, T _c, j _c(T), and R(T) at H= 0 and 5 mT) of (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 +} ceramics (sintering at 840°C for 36 h) with ultrafine Si₃N₄ additions (0.05–0.2 wt %) are studied. The introduction of 0.05–0.1 wt % Si₃N₄ is shown to reduce the width of the superconducting transition by 2–3 K and to raise the critical current density at temperatures below 95 K.     

Induced interaction between polypyrrole and SO2 via molecular sieve 13X

Electrical conductivity sensitivity and interaction mechanisms between polypyrrole (Ppy)/molecular sieve 13X composites and CO₂, CO, and SO₂ were investigated. Polypyrrole was synthesized and composites were fabricated by dispersing zeolite particles into the Ppy matrix, followed by compression into a pellet form. Effects of zeolite concentration, cation type, and cation concentration were investigated. The electrical conductivity in air of Ppy doped with naphthalene-2-sulfonic acid (β) sodium salt increases monotonically with the doping level. There are negligible negative electrical conductivity responses of Ppy and its composites when exposed to CO₂, and CO in contrast to definite positive responses towards SO₂ in which the interaction is irreversible. Undoped Ppy and doped Ppy composites at 10% (v/v) of zeolite 13X content possess the highest sensitivity to SO₂; beyond this volume fraction the sensitivity is reduced with increasing molecular sieve 13X content. The composites of unmodified 13X, with Na⁺ fully present within its cavity, give the highest electrical conductivity sensitivity towards SO₂. The sensitivity of Ppy/13X composite to SO₂ diminishes as the cation Na⁺ is exchanged to other alkali cations in this decreasing order: Cs⁺, K⁺, and Li⁺, in the opposite order to the binding energy between the gas and the cations. The sensitivity and interaction are also reduced with increasing amount of Li⁺ present.     

Influence of Uniaxial Strain on the Electronic Properties of (TMTSF)<Subscript>2</Subscript>ReO<Subscript>4</Subscript> Compounds

To study the influence of uniaxial strain on anion-orderings and superconductivity, we measured electrical resistance along the a (R_xx) and c*-axes (R_zz) under uniaxial strain on (TMTSF)₂ReO₄. Compared to the pressure-temperature phase diagrams under hydrostatic pressure, those under uniaxial strain depend on the direction along which the pressure is applied. The difference is particularly large when the uniaxial strain is applied along the c*-axis. The compression along only one direction diminishes (1/2,1/2,1/2) anion ordering transition similar to those under hydrostatic pressure. On the other hand, (0,1/2,1/2) anion ordering transition is completely suppressed and anomalous behavior which enhances non-metallic ground state is observed when the samples are compressed along the c*-axis. The superconducting transition of (TMTSF)₂ReO₄ is suppressed in R_zz under pressure along the c*-axis.     

Phase Transitions from Incoherent and from Coherent Metal Phases in Quasi-One-Dimensional Organic Conductors

We study effects of a preformed Mott gap and dimensionality on interchain one-particle coherence and spin-density-wave phase transitions in weakly-coupled dimerized quarter-filled Hubbard chains. A phase diagram is given, based on the two-loop perturbative renormalization-group (RG) approach together with the random phase approximation. Feedback effects of interchain processes on the umklapp process are examined by the 1 + expansion. We discuss relevance of the present result to the SDW phase transitions in the quasi-one-dimensional dimerized quarter-filled organic conductors, (TMTTF) ₂ X and (TMTSF) ₂ X.     

Strong Vortex Pinning in the Low–Temperature Superconducting Phase of (U1−xThx)Be13

We have found a sharp transition at T _c2 = 350 mK in the vortex creep rate of a single crystal of (U _1–x Th _x )Be ₁₃ with T _c = 523 mK (x = 0.0275). For T T _c2 , no creep of vortices is observed in a time scale of 10⁵ s, while for T _c2 < T < T _c , vortices creep at very high rates (30% of decay from a metastable configuration in the first 10⁵ s at T = 400 mK). The sharp transition occurs at the same temperature at which the second jump in the specific heat appears in these samples. Similar low levels of creep rates have been reported by us in the low–T superconducting phase of UPt ₃.¹     

Shelf Life of Fresh Beef Stored in Master Packages with Carbon Monoxide and High Levels of Carbon Dioxide

Beef steaks from Gluteus medius, Longissimus dorsi, Psoas major and ground beef from Triceps brachii were packed in master packages containing expanded polystyrene trays overwrapped with polyvinyl chloride film under an anoxic atmosphere containing 0.2% CO, 60.0% CO₂ and 39.8% N₂ (T₁) or 0.2% CO/99.8% CO₂ plus oxygen scavengers. After 28 days of storage at 1 and 4°C, the meat quality was evaluated. The visual and instrumental colour of the beef steaks and ground beef were similar to that of fresh meat. Pathogenic bacteria were not detected, and psychrotrophic bacterial counts were lower than 7.5 log CFU/g. Copyright © 2012 John Wiley & Sons, Ltd.     

Unusual electronic state of layered θ-(ET)2X studied by raman spectroscopy

Employing Raman spectroscopy, we have studied the electronic structure of the charge-ordering system, θ-(ET)₂X, the dihedral angle (ϕ) of which ranges from 100° to 121°. The most stable metal, θ-(ET)₂I₃, (ϕ=100°) has a uniform charge distribution in a whole temperature range. The compounds with ϕ>111° have an inhomogeneous charge distribution even above the CO transition, which implies that the charges have localized nature in a whole temperature range. These localized charges are long-range ordered in the CO phase, whereas they are fluctuating randomly or forming short-range ordered domains above the CO phase transition.     

YBCO Superconducting Film Coated on LaAlO<Subscript>3</Subscript> Substrate by TFA-MOD Process

Epitaxial YBCO film was coated on (001) LaAlO₃ single-crystal substrate by metalorganic deposition of metal trifluoroacetate precursors. From XRD and EDS analysis, it was observed that the film contains CuO or Cu₂BaO₂ segregation, but transport measurement found the segregation structure does not severely prevent the percolation of supercurrent-carrying material. The YBCO films performance is up to 2.4 MA/cm² with I_c of 137 A/cm-width at 77 K and 0 T, and superconducting transition temperature T_c reaches around 91.3 K with sharp transition temperature T_c of 0.36 K.     

Magnetism in Double Perovskites

Double-perovskite compounds with general formula ABB′O₆, have attracted a lot of attention in recent years due to a variety of properties exhibited by them. In this paper, we will review our recent study on a number of double-perovskite compounds, namely La-doped Sr₂FeMoO₆, Cr-based family of compounds, Sr₂CrXO₆ (X=W, Re, Os), characterized with spectacularly high ferromagnetic transition temperatures and the magneto-capacitive compound, La₂NiMnO₆, We will discuss the signature of hybridization-driven antiferromagnetism in La-doped Sr₂FeMoO₆, while the parent compound, Sr₂FeMoO₆, is a half-metallic ferromagnet. The magnetism in the 3d–5d double perovskite Sr₂CrXO₆ (X=W, Re, Os) is found to be driven by the interplay of the hybridization-driven mechanism and the superexchange, which resulted into progressive increase of T _c as one moves from W to Re to Os at the B′ site. Our work, in the context of La₂NiMnO₆, identifies its superexchange-driven microscopic origin being responsible for the near room-temperature insulating ferromagnetic behavior.     

Surface Superconducting State of Heavy-Fermion Superconductor CeIrIn5 Probed by CeIrIn5-Ag Junctions

The CeIrIn₅-Ag junctions of about 2×10^–9 cm^–2 area have been made using microfabrication techniques, and the surface superconducting state of CeIrIn₅, which has two characteristic temperatures T ₀ and T _c, has been investigated, where T ₀ and T _c are the transition temperature to zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. The temperature, below which superconducting anomalies are observed, varies from junction to junction, and yet it is always well above T _c=0.4 K. This result, together with no indication of transition at T _c, suggests that at least the surface of CeIrIn₅ is in the superconducting state above T _c. The data on the critical current I _c in superconducting anomalies point to the possibility to define a local transition temperature for each junction.     

Ultrasonic Anomaly Near the Charge Ordering Transition in Sr-Doped Nd0.3La0.2Ca0.5−x Sr x MnO3 Manganites

Sr doping in the charge-ordered compound Nd_0.3La_0.2Ca_0.5?x Sr _x MnO₃ has been systematically studied to examine its effect on ultrasonic velocity and elastic moduli as well as magnetic and electrical transport properties. DC electrical resistivity, ρ and AC susceptibility, χ′ measurements showed all samples exhibit metal-insulator (MI) behavior accompanied by ferromagnetic-paramagnetic (FM-PM) transition where the MI transition temperature, T _MI and FM-PM transition temperature, T _C increased with Sr content indicating the enhancement of double-exchange mechanism. Analysis of the resistivity change with respect to temperature, dlnρ/dT ^?1 versus T indicates onset of charge-ordering (CO) state where its CO transition temperature, T _CO decreased with Sr content indicating weakening of the CO state. On the other hand, both absolute longitudinal and shear velocities as well as elastic moduli measured at 80 K increased significantly with Sr doping indicating improvement in elastic properties, which is suggested to be due to the increase in formation of ferromagnetic domains. A longitudinal velocity anomaly characterized by a slope change around the vicinity of T _CO was observed for all samples. The longitudinal elastic anomaly is attributed to the Jahn–Teller (JT) effect of Mn³⁺ ions where analysis of the anomaly using the mean-field theory suggests involvement of the JT effect in the samples, which transforms from dynamic to static type with decreasing temperature. The elastic anomaly shifted down from 222 K (x=0) to 205 K (x=0.05) indicating that the static JT effect was weakened with Sr content.     

Unusual Electronic State of Layered θ-(ET)2 X Studied by Raman Spectroscopy

Employing Raman spectroscopy, we have studied the electronic structure of the charge-ordering system, θ−(ET)₂X, the dihedral angle (φ) of which ranges from 100 to 121°. The most stable metal, θ−(ET)₂I₃, (φ=100°) has a uniform charge distribution in a whole temperature range. The compounds with φ > 111° have an inhomogeneous charge distribution even above the CO transition, which implies that the charges have localized nature in a whole temperature range. These localized charges are long-range ordered in the CO phase, whereas they are fluctuating randomly or forming short-range ordered domains above the CO phase transition.     

Studies on Low-Field and Room-Temperature Magnetoresistance in La2/3(Ca1−x Sr x )1/3MnO3 Perovskites

The polycrystalline perovskites La_2/3(Ca_1?x Sr _x )_1/3MnO₃ (x=0,0.2,0.4,0.6,0.8,1) were successfully prepared by a modified method of solid-state reactions, in which the ingredient mixture with ethanol as a liquid milling medium to form suspension was milled by high-energy ball milling for 10 h and sintered in air at 1400 °C for 10 h. The microstructure, electrical transport, and low-field magnetoresistance (LFMR) of the perovskites were investigated to study the room-temperature magnetoresistance (RTMR) behavior. The results reveal that the metal-to-insulator transition temperature (T _MI) increased with increasing doping level x, and the peak values of the magnetoresistance (MR) near T _MI dropped with the more Ca²⁺ substituted. A single-phase La_2/3Ca_1/3MnO₃ showed T _MI at 263 K and the peak MR of 23 % in the applied field of 3 kOe near T _MI. The LFMR effect at room temperature could be obtained by controlling the doping Sr²⁺-substituted Ca²⁺ level. When x=0.29, transition temperature (T _MI) was 305.30 K, and the MR effect was recorded up to 12.9 % at 298.55 K and 3 kOe. Finally, the possible mechanism is discussed.     

Inducement of Itinerant Electron Transport in Charge-Ordered Pr<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> by Ba Doping

The effects of Ba ²⁺ doping on the electrical and magnetic properties of charge-ordered Pr_0.6Ca_0.4MnO₃ were investigated through electrical resistivity and AC susceptibility measurements. X-ray diffraction data analysis showed an increase in unit cell volume with increasing Ba ²⁺ content indicating the possibility of substituting Ba ²⁺ for the Ca-site. Electrical resistivity measurements showed insulating behavior and a resistivity anomaly at around 220 K. This anomaly is attributed to the existence of charge ordering transition temperature, \(T^{\mathrm {R}}_{\text {CO}}\) for the x = 0 sample. The Ba-substituted samples exhibited metallic to insulator transition (MI) behavior, with transition temperature, T _MI, increasing from ～98 K (x = 0.1) to ～122 K (x = 0.3). AC susceptibility measurements showed ferromagnetic to paramagnetic (FM-PM) transition for Ba-substituted samples with FM-PM transition temperature, T _c, increasing from ～121 K (x = 0.1) to ～170 K (x = 0.3), while for x = 0, an antiferromagnetic to paramagnetic transition behavior with transition temperature, T _N, ～170 K was observed. In addition, inverse susceptibility versus T plot showed a deviation from the Curie–Weiss behavior above T _c, indicating the existence of the Griffiths phase with deviation temperature, T _G, increasing from 160 K (x = 0.1) to 206 K (x = 0.3). Magnetoresistance, MR, behavior indicates intrinsic MR mechanism for x = 0.1 which changed to extrinsic MR for x > 0.2 as a result of Ba substitution. The weakening of charge ordering and inducement of ferromagnetic metallic (FMM) state as well as increase in both T _c and T _MI are suggested to be related to the increase of tolerance factor, τ, and increase of e _g ?electron bandwidth as average ionic radius at A-site, <r _A> increased with Ba substitution. The substitution may have reduced MnO₆ octahedral distortion and changed the Mn–O–Mn angle which, in turn, promotes itinerancy of charge carrier and enhanced double exchange mechanism. On the other hand, increase in A-site disorder, which is indicated by the increase in σ ² is suggested to be responsible for the widening of the difference between T _c and T _MI.