Showing papers on "Transition temperature published in 2011"

From a single-band metal to a high-temperature superconductor via two thermal phase transitions.

Abstract: The nature of the pseudogap phase of cuprate high-temperature superconductors is one of the most important unsolved problems in condensed matter physics. We studied the commencement of the pseudogap state at temperature T* using three different techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and time-resolved reflectivity) on the same optimally-doped Bi2201 crystals. We observe the coincident onset at T* of a particle-hole asymmetric antinodal gap, a non-zero Kerr rotation, and a change in the relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (T{sub c}), entangled in an energy-momentum dependent fashion with the pre-existing pseudogap features.

Impedance and Raman spectroscopic studies of (Na0.5Bi0.5)TiO3

Abstract: Polycrystalline powder of (Na0.5Bi0.5)TiO3 (NBT) was prepared by a high-temperature solid-state reaction route. Preliminary x-ray diffraction analysis carried out at room temperature showed the formation of a single phase compound with a rhombohedral crystal system. Scanning electron micrograph reveals the polycrystalline nature of the material with size anisotropy. Dielectric study showed an existence of diffuse phase transition around 300 °C. The ac conductivity spectrum obeyed the Jonscher power law. The temperature dependent pre-exponential factor (A) shows peak and frequency exponent (n) possesses a minimum at transition temperature. The bulk conductivity of the material indicates an Arrhenius type of thermally activated process with three different conduction mechanisms as different activation energies are observed. The hopping charge carriers dominate at low temperature, small polaron and oxygen vacancy dominates at intermediate temperature and ionic conduction at higher temperatures. Studies of impedance spectroscopy indicate that the dielectric relaxation is of non-Debye type. In situ high-temperature Raman spectroscopy shows discontinuous changes in the phonon frequencies across the rhombohedral–tetragonal transition. In addition, anomalous changes in the intensity and the linewidth of a lattice mode are found around 350 °C.

Temporal correlations of superconductivity above the transition temperature in La 2−x Sr x CuO 4 probed by terahertz spectroscopy

Abstract: Above the superconducting temperature for a given material, correlations between pairs of electrons are already present. Many experiments indicate that such correlations may exist up to 100 K above the transition. However, a temporal probe of the superconducting fluctuations suggests that correlations only exist within a narrow temperature range.

Structural, dielectric, magnetic, magnetodielectric and impedance spectroscopic studies of multiferroic BiFeO3–BaTiO3 ceramics

Abstract: Polycrystalline (1−x)BiFeO3–xBaTiO3 (x = 0.00, 0.10, 0.20 and 0.30) ceramics have been prepared via mixed oxide route. The effect of BaTiO3 substitution on the dielectric, ferroelectric and magnetic properties of the BiFeO3 multiferroic perovskite was studied. From XRD analysis it revealed that BaTiO3 substitution does not affect the crystal structure of the (1−x)BiFeO3–xBaTiO3 system up to x = 0.30. Improved dielectric properties were observed in the prepared system. An anomaly in the dielectric constant (ɛ) was observed in the vicinity of the antiferromagnetic transition temperature. Experimental results suggest that in the (1−x)BiFeO3–xBaTiO3 system, the increase of BaTiO3 concentration leads to the effective suppression of the spiral spin structure of BiFeO3, resulting in the appearance of net magnetization. The dependence of dielectric constant and loss tangent on the magnetic field is a evidence of magnetoelectric coupling in (1−x)BiFeO3–xBaTiO3 system. The impedance analysis suggests the presence of a temperature dependent electrical relaxation process in the material, which is almost similar for all the concentrations in the present studies. The electrical conductivity has been observed to increase with rise in temperature showing a typical negative temperature coefficient of the resistance (NTCR) behaviors analogous to a semiconductor and suggests a non-Debye type of electrical relaxation.

Measurements of the timescales for the mass transfer of water in glassy aerosol at low relative humidity and ambient temperature

Abstract: . The influence of glassy states and highly viscous solution phases on the timescale of aerosol particle equilibration with water vapour is examined. In particular, the kinetics of mass transfer of water between the condensed and gas phases has been studied for sucrose solution droplets under conditions above and below the glass transition relative humidity (RH). Above the glass transition, sucrose droplets are shown to equilibrate on a timescale comparable to the change in RH. Below the glass transition, the timescale for mass transfer is shown to be extremely slow, with particles remaining in a state of disequilibrium even after timescales of more than 10 000 s. A phenomenological approach for quantifying the time response of particle size is used to illustrate the influence of the glassy aerosol state on the kinetics of mass transfer of water: the time is estimated for the droplet to reach the halfway point from an initial state towards a disequilibrium state at which the rate of size change decreases below 1 nm every 10 000 s. This half-time increases above 1000 s once the particle can be assumed to have formed a glass. The measurements are shown to be consistent with kinetic simulations of the slow diffusion of water within the particle bulk. When increasing the RH from below to above the glass transition, a particle can return to equilibrium with the gas phase on a timescale of 10's to 100's of seconds, once again forming a solution droplet. This is considerably shorter than the timescale for the size change of the particle when glassy and suggests that the dissolution of the glassy core can proceed rapidly, at least at room temperature. Similar behaviour in the slowing of the mass transfer rate below the glass transition RH is observed for binary aqueous raffinose solution droplets. Mixed component droplets of sucrose/sodium chloride/water also show slow equilibration at low RH, illustrating the importance of understanding the role of the bulk solution viscosity on the rate of mass transfer with the gas phase, even under conditions that may not lead to the formation of a glass.

Unusual superconducting state at 49 K in electron-doped CaFe2As2 at ambient pressure.

Abstract: We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe2As2 via electron-doping by partial replacement of Ca by rare-earth. The superconducting transition observed suggests the possible existence of two phases: one starting at 49 K, which has a low critical field 5 T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe2As2 (or Ae122), where Ae = Ca, Sr, or Ba. In Ae122, hole-doping has been previously observed to generate superconductivity with a transition temperature (Tc) only up to 38 K and pressurization has been reported to produce superconductivity with a Tc up to 30 K. The unusual 49 K phase detected will be discussed.

Mean-field theory of a nematic liquid crystal doped with anisotropic nanoparticles

Abstract: In the framework of molecular mean-field theory we study the effect of nanoparticles embedded in nematic liquid crystals on the orientational ordering and nematic–isotropic phase transition. We show that spherically isotropic nanoparticles effectively dilute the liquid crystal medium and decrease the nematic–isotropic transition temperature. At the same time, anisotropic nanoparticles become aligned by the nematic host and, reciprocally, improve the liquid crystal alignment. The theory clarifies the microscopic origin of the experimentally observed shift of the isotropic–nematic phase transition and an improvement of the nematic order in composite materials. A considerable softening of the first order nematic–isotropic transition caused by strongly anisotropic nanoparticles is also predicted.

Room temperature homogeneous flow in a bulk metallic glass with low glass transition temperature

Abstract: We report a high entropy metallic glass of Zn20Ca20Sr20Yb20(Li0.55Mg0.45)20 via composition design that exhibiting remarkable homogeneous deformation without shear banding under stress at room temperature. The glass also shows properties such as low glass transition temperature (323 K) approaching room temperature, low density and high specific strength, good conductivity, polymerlike thermoplastic manufacturability, and ultralow elastic moduli comparable to that of bones. The alloy is thermally and chemically stable.

Dielectric, Ferromagnetic Characteristics, and Room-Temperature Magnetodielectric Effects in Double Perovskite La2CoMnO6 Ceramics

Abstract: The magnetic, dielectric, and magnetodielectric properties of La2CoMnO6 ceramics were evaluated together with the polarization-electric field hysteresis loops. The phase segregation consisting of ordered and disordered regions was determined in the present ceramics. The ordering of Co2+ and Mn4+ gave rise to ferromagnetic transition temperature as high as 210 K, while the disordering of Co3+ and Mn3+ resulted in low ferromagnetic transition temperature of 80 and 150 K. The relaxor-like behavior combined with a giant dielectric constant (∼105) was determined in La2CoMnO6 ceramics, which was attributed to the charge ordering of Co2+ and Mn4+. Owing to the mutual origin of magnetism and dielectric relaxation, La2CoMnO6 ceramics showed considerable magnetodielectric effects (∼0.8% at 10 kOe) at room temperature.

Synthesis and characterization of VO2-based thermochromic thin films for energy-efficient windows

Abstract: Thermochromic VO2 thin films have successfully been grown on SiO2-coated float glass by reactive DC and pulsed-DC magnetron sputtering. The influence of substitutional doping of V by higher valence cations, such as W, Mo, and Nb, and respective contents on the crystal structure of VO2 is evaluated. Moreover, the effectiveness of each dopant element on the reduction of the intrinsic transition temperature and infrared modulation efficiency of VO2 is discussed. In summary, all the dopant elements--regardless of the concentration, within the studied range-- formed a solid solution with VO2, which was the only compound observed by X-ray diffractometry. Nb showed a clear detrimental effect on the crystal structure of VO2. The undoped films presented a marked thermochromic behavior, specially the one prepared by pulsed-DC sputtering. The dopants effectively decreased the transition of VO2 to the proximity of room temperature. However, the IR modulation efficiency is markedly affected as a consequence of the increased metallic character of the semiconducting phase. Tungsten proved to be the most effective element on the reduction of the semiconducting-metal transition temperature, while Mo and Nb showed similar results with the latter being detrimental to the thermochromism.

Size Dependence of a Temperature-Induced Solid-Solid Phase Transition in Copper(I) Sulfide

Abstract: Determination of the phase diagrams for the nanocrystalline forms of materials is crucial for our understanding of nanostructures and the design of functional materials using nanoscale building blocks. The ability to study such transformations in nanomaterials with controlled shape offers further insight into transition mechanisms and the influence of particular facets. Here we present an investigation of the size-dependent, temperature-induced solid–solid phase transition in copper sulfide nanorods from low- to high-chalcocite. We find the transition temperature to be substantially reduced, with the high chalcocite phase appearing in the smallest nanocrystals at temperatures so low that they are typical of photovoltaic operation. Size dependence in phase transformations suggests the possibility of accessing morphologies that are not found in bulk solids under ambient conditions. These otherwise inaccessible crystal phases could enable higher-performing materials in a range of applications, including sens...

Phase transition sequence in sodium bismuth titanate observed using high-resolution x-ray diffraction

Abstract: High resolution powder x-ray diffraction patterns of Na0.5Bi0.5TiO3 at selected temperatures were examined to compare structural changes with observed piezoelectric thermal depoling temperatures. The depoling temperatures do not correlate with discrete phase transition temperatures, and therefore, a structural transition is not the origin of thermal depoling. Rather, a correlation is made with an increase in volume fraction of material which does not obey the long-range Cc space group. The origin of the thermal depoling behavior may be the loss of long-range ferroelectric order by a decreasing proportion of the Cc phase or the associated percolation of disordered nano-scale platelets.

Physical properties of FeSe0.5Te0.5 single crystals grown under different conditions

Abstract: We report on structural, magnetic, conductivity, and thermodynamic studies of FeSe0.5Te0.5 single crystals grown by self-flux and Bridgman methods. The lowest values of the susceptibility in the normal state, the highest transition temperature T c of 14.4 K, and the largest heat-capacity anomaly at T c were obtained for pure (oxygen-free) samples. The critical current density j c of 8.6 × 104 A/cm2 (at 2 K) achieved in pure samples is attributed to intrinsic inhomogeneity due to disorder at the anion sites. The samples containing an impurity phase of Fe3O4 show increased j c up to 2.3 × 105 A/cm2 due to additional pinning centers. The upper critical field $H_{c2}$ of ~500 kOe is estimated from the resistivity study in magnetic fields parallel to the c-axis using a criterion of a 50% drop of the normal state resistivity R n . The anisotropy of the upper critical field γ H c2 = H ab c2/H c2 c reaches a value ~6 at $T\longrightarrow T_c$ . Extremely low values of the residual Sommerfeld coefficient $\gamma_r$ of about 1 mJ/mol K2, compared to the normal state Sommerfeld coefficient γ n = 25 mJ/mol K2 for pure samples indicate a high volume fraction of the superconducting phase (up to 97%). The electronic contribution to the specific heat in the superconducting state is well described within a single-band BCS model with a temperature dependent gap Δ(0 K) = 27(1) K. A broad cusp-like anomaly in the electronic specific heat observed at low temperatures in samples with suppressed bulk superconductivity is ascribed to a splitting of the ground state of the Fe2+ ions at the 2c sites. This contribution is fully suppressed in the ordered state in samples with bulk superconductivity.

Electron star birth: a continuous phase transition at nonzero density.

Abstract: We show that charged black holes in anti--de Sitter spacetime can undergo a third-order phase transition at a critical temperature in the presence of charged fermions. In the low temperature phase, a fraction of the charge is carried by a fermion fluid located a finite distance from the black hole. In the zero temperature limit, the black hole is no longer present and all charge is sourced by the fermions. The solutions exhibit the low temperature entropy density scaling $s\ensuremath{\sim}{T}^{2/z}$ anticipated from the emergent IR criticality of recently discussed electron stars.

Effect of size reduction on the ferromagnetism of the manganite La1-xCaxMnO3 (x = 0.33)

Abstract: In this paper we report an investigation on the ferromagnetic state and the nature of ferromagnetic transition of nanoparticles of $\mathrm{La_{0.67}Ca_{0.33}MnO_3}$ using magnetic measurements and neutron diffraction. The investigation was made on nanoparticles with crystal size down to $15$ nm. The neutron data show that even down to a size of $15$ nm the nanoparticles show finite spontaneous magnetization ($M_S$) although the value is much reduced compared to the bulk sample. We observed a non-monotonic variation of the ferromagnetic to paramagnetic transition temperature $T_C$ with size $d$ and found that $T_C$ initially enhances on size reduction, but for $d < 50$ nm it decreases again. The initial enhancement in $T_C$ was related to an increase in the bandwidth that occured due to a compaction of the Mn-O bond length and a straightening of the Mn-O-Mn bond angle, as determined form the neutron data. The size reduction also changes the nature of the ferromagnetic to paramagnetic transition from first order to second order with critical exponents approaching mean field values. This was explained as arising from a truncation of the coherence length by the finite sample size.

Semiconductor-metal transition in thin VO2 films grown by ozone based atomic layer deposition

Abstract: Vanadium dioxide (VO2) has the interesting feature that it undergoes a reversible semiconductor-metal transition (SMT) when the temperature is varied near its transition temperature at 68°C.1 The variation in optical constants makes VO2 useful as a coating material for e.g. thermochromic windows,2 while the associated change in resistivity could be interesting for applications in microelectronics, e.g. for resistive switches and memories.3 Due to aggressive scaling and increasing integration complexity, atomic layer deposition (ALD) is gaining importance for depositing oxides in microelectronics. However, attempts to deposit VO2 by ALD result in most cases in the undesirable V2O5. In the present work, we demonstrate the growth of VO2 by using Tetrakis[EthylMethylAmino]Vanadium and ozone in an ALD process at only 150°C. XPS reveals a 4+ oxidation state for the vanadium, related to VO2. Films deposited on SiO2 are amorphous, but during a thermal treatment in inert gas at 450°C VO2(R) is formed as the first and only crystalline phase. The semiconductor-metal transition has been observed both with in-situ X-ray diffraction and resistivity measurements. Near a temperature of 67°C, the crystal structure changes from VO2(M1) below the transition temperature to VO2(R) above with a hysteresis of 12°C. Correlated to this phase change, the resistivity varies over more than 2 orders of magnitude.

Bilayer superfluidity of fermionic polar molecules: Many-body effects

Abstract: We study the BCS superfluid transition in a single-component fermionic gas of dipolar particles loaded in a tight bilayer trap, with the electric dipole moments polarized perpendicular to the layers. Based on the detailed analysis of the interlayer scattering, we calculate the critical temperature of the interlayer superfluid pairing transition when the layer separation is both smaller (dilute regime) and on the order or larger (dense regime) than the mean interparticle separation in each layer. Our calculations go beyond the standard BCS approach and include the many-body contributions resulting in the mass renormalization, as well as additional contributions to the pairing interaction. We find that the many-body effects have a pronounced effect on the critical temperature and can either decrease (in the very dilute limit) or increase (in the dense and moderately dilute limits) the transition temperature as compared to the BCS approach.

High-Pressure Synthesis and Correlation between Structure, Magnetic, and Dielectric Properties in LiNbO3-Type MnMO3 (M = Ti, Sn)

Abstract: LiNbO3-type MnMO3 (M = Ti, Sn) were synthesized under high pressure and temperature; their structures and magnetic, dielectric, and thermal properties were investigated; and their relationships were discussed. Optical second harmonic generation and synchrotron powder X-ray diffraction measurements revealed that both of the compounds possess a polar LiNbO3-type structure at room temperature. Weak ferromagnetism due to canted antiferromagnetic interaction was observed at 25 and 50 K for MnTiO3 and MnSnO3, respectively. Anomalies in the dielectric permittivity were observed at the weak ferromagnetic transition temperature for both the compounds, indicating the correlation between magnetic and dielectric properties. These results indicate that LiNbO3-type compounds with magnetic cations are new candidates for multiferroic materials.

Effect of the Addition of CaZrO3 and LiNbO3 on the Phase Transitions and Piezoelectric Properties of K0.5Na0.5NbO3 Lead-Free Ceramics

Abstract: The (1-x-y)K0.5Na0.5NbO3-xCaZrO3-yLiNbO3 (KNLNCZ-x-y) lead-free piezoelectric ceramics were fabricated by conventional technique, and the effect of the addition of CaZrO3 and LiNbO3 on the phase transitions and piezoelectric properties of the KNN-based ceramics was investigated. The addition of CaZrO3 will shift the orthorhombic-rhombohedral transition temperature of the ceramics to above room temperature, and with the increase of the CaZrO3 additives, a dielectric peak rises up near the orthorhombic-tetragonal phase transition of the ceramics resulting in the spread of the ferroelectric-paraelectric phase transition over a rather wide temperature range due to polar nanoregions arising from composition fluctuation. For x = 0.05, the addition of LiNbO3 is effective to shift the TR-O and TO-T downward and TC upward in KNLNCZ-0.05-y ceramics. Optimum piezoelectric properties (d33 = 216 pC/N and kp = 42.9%) were obtained for the KNLNCZ-x-y ceramics with x = 0.05 and y = 0.07 due to the decrease of TO-T to be around room temperature.

Rotational motion in LiBH4/LiI solid solutions.

Abstract: We investigated the localized rotational diffusion of the (BH4)− anions in LiBH4/LiI solid solutions by means of quasielastic and inelastic neutron scattering. The (BH4)− motions are thermally activated and characterized by activation energies in the order of 40 meV. Typical dwell times between jumps are in the picosecond range at temperatures of about 200 K. The motion is dominated by 90° reorientations around the 4-fold symmetry axis of the tetrahedraly shaped (BH4)− ions. As compared to the pure system, the presence of iodide markedly reduces activation energies and increases the rotational frequencies by more than a factor of 100. The addition of iodide lowers the transition temperature, stabilizing the disordered high temperature phase well below room temperature.

Conventional and inverse magnetocaloric effects in La0.45Sr0.55MnO3 nanoparticles

Abstract: The magnetocaloric effect of La0.45Sr0.55MnO3 nanoparticles was studied by dc magnetization measurements. A sample with mean particle size of about 140 nm exhibits both a conventional magnetocaloric effect around the Curie temperature (≈ 295 K) and a large inverse magnetocaloric effect around the antiferromagnetic-ferromagnetic transition temperature (≈ 200 K). The change of magnetic entropy increases monotonically with applied magnetic field and reaches the values of 5.51 J/kg K and − 2.35 J/kg K at 200 K and 295 K, respectively, in an applied field of 5 T. The antiferromagnetic-ferromagnetic transition is absent in a 36 nm size sample, which shows only a broad ferromagnetic transition around 340 K and a small change in magnetic entropy near room temperature. The results are discussed in terms of the entropy difference between the A-type antiferromagnetic ground state of La0.45Sr0.55MnO3 and the low moment ferromagnetic state. By comparing the results obtained on nanoparticles and bulk La0.45Sr0.55MnO3, ...

Oxygen deficiency and grain boundary-related giant relaxation in Ba(Zr,Ti)O3 ceramics

Abstract: The influence of the oxygen vacancies on the dielectric response of BaZr0.10Ti0.90O3 ceramics prepared by solid-state reaction and sintered at 1400 °C for 2 h was investigated. The as-sintered ceramic exhibits a giant relaxation with a shift of the transition temperature from ∼85 °C to above 170 °C in the frequency ranges of 1 Hz–100 kHz, with high losses above unity and two components in the complex impedance plot. A complex dielectric relaxation response, with at least two thermally activated defect mechanisms with activation energies of ∼0.2 eV below the transition temperature and ∼0.7 eV for higher temperatures in the range of 85 °C–170 °C was determined. The observed giant relaxation is an extrinsic effect related to the oxygen deficiency, inhomogeneous distributed in the ceramic grain and not to the relaxor behavior of this system. After a post-annealing treatment at 1000 °C for 50 h, the dielectric response is completely changed: the permittivity vs. temperature dependences present maxima located a...