Bulk Transition Research Articles

Metallic magnetism and change of conductivity in the nano to bulk transition of cobalt ferrite

Variations in conductivity with particle size have been observed in cobalt ferrite, when synthesized by solgel auto-combustion method. Impedance analysis reveals metallic and semiconducting behavior at room temperature for a particle size of 6 nm and 52 nm, respectively. Upon thermal activation, metallic to semiconducting phase transition has been observed as a function of particle size and vice-versa. Grainboundary Resistance (Rgb), increased drastically with particle size (19 MΩ for 6 nm and 259 MΩ for 52 nm) at room temperature. AC conductivity and dielectric constants exhibit similar metallic to semiconducting phase transition at 6 nm and semiconducting behavior at 52 nm with temperature in the selected frequencies. Enhanced magnetic moment with an increase in the grain size along with decreased coercivity (1444 G to 1146 G) reveals transition from single domain to multi-domain. Increased inter-particle interaction is responsible for metallicity at the nano level and on the contrary semiconductivity is attributed to bulk.

Real-space imaging of the Verwey transition at the (100) surface of magnetite

Effects of the Verwey transition on the (100) surface of magnetite were studied using scanning tunneling microscopy and spin polarized low-energy electron microscopy. On cooling through the transition temperature ${T}_{\mathrm{V}}$, the initially flat surface undergoes a rooflike distortion with a periodicity of $\ensuremath{\sim}0.5$ $\ensuremath{\mu}$m due to ferroelastic twinning within monoclinic domains of the low-temperature monoclinic structure. The monoclinic $c$ axis orients in the surface plane, along the ${[001]}_{c}$ directions. At the atomic scale, the charge-ordered ($\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$)$R{45}^{\ensuremath{\circ}}$ reconstruction of the (100) surface is unperturbed by the bulk transition, and is continuous over the twin boundaries. Time resolved low-energy electron microscopy movies reveal the structural transition to be first order at the surface, indicating that the bulk transition is not an extension of the Verwey-like ($\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$)$R{45}^{\ensuremath{\circ}}$ reconstruction. Although conceptually similar, the charge-ordered phases of the (100) surface and sub-${T}_{\mathrm{V}}$ bulk of magnetite are unrelated phenomena.

Antiferromagnetism in UO2thin epitaxial films

Thin films (250--4500 \AA{}) of epitaxial UO${}_{2}$ were produced by reactive sputtering on two different substrate materials: LaAlO${}_{3}$ and CaF${}_{2}$. Using the large enhancement present with resonant x-ray scattering using photons at the uranium $M$${}_{4}$ absorption edge, antiferromagnetic (AF) order was found in all films. The ordering temperature $T$${}_{N}$ is the same as the bulk, but the films show second-order (continuous) transitions in contrast to the first-order bulk transition. For LaAlO${}_{3}$-based films, an additional strong diffuse magnetic disorder is observed, which is reminiscent of the second-length scale, associated with structural disorder and/or strain. By using a formulation accounting for the strong absorption and coherent nature of the photons, the energy widths at the U $M$${}_{4}$ resonances can be related to the thickness of the AF region. The LaAlO${}_{3}$-based films do not order magnetically over more than $\ensuremath{\sim}$600 \AA{}, whereas the CaF${}_{2}$-based film orders throughout. Further, for thicker films ($>$1000 \AA{}) the fitting procedure shows that the AF order is located at the top of the LaAlO${}_{3}$-based film. This points to the formation in thicker films of a nonmagnetic layer of UO${}_{2}$ adjacent to the substrate, which may have tetragonal symmetry.

Rashba-type spin splitting at Au(111) beyond the Fermi level: the other part of the story

We present a combined experimental and theoretical study of spin–orbit-induced spin splittings in the unoccupied surface electronic structure of the prototypical Rashba system Au(111). Spin- and angle-resolved inverse-photoemission measurements reveal a Rashba-type spin splitting in the unoccupied part of the L-gap surface state. With increasing momentum parallel to the surface, the spectral intensity is lowered and the spin splitting vanishes as the surface state approaches the band-gap boundary. Furthermore, we observe significantly spin-dependent peak positions and intensities for transitions between unoccupied sp-like bulk bands. Possible reasons for this behavior are considered: initial and final-state effects as well as the transition itself, which is controlled by selection rules depending on the symmetry of the involved states. Based on model calculations, we identify the initial states as origin of the observed Rashba-type spin effects in bulk transitions.

Interface resonances in optical second-harmonic generation from oxide-covered Ge(111) and Ge(100)

Ge surfaces have been investigated with optical second-harmonic generation (SHG) spectroscopy in the range from 1.78 to 3.44 eV. The spectra reveal surface-specific resonances corresponding to the E1 and E1+Δ1 bulk transitions. The splitting between the surface E1 and E1+Δ1 resonances is found to be larger than the bulk value. It is suggested this is caused by surface-induced band bending through a Rashba effect. By probing metal-oxide-semiconductor structures it is found that contributions from electric-field-induced SHG from the space charge region are negligible for Ge within the probed spectral range. Strong second-harmonic resonances in the 2.6–3.2 eV range are observed and tentatively assigned to Ge–Ge bonds at the interface.

Chirald-wave superconducting state in the core of a doubly quantizeds-wave vortex in graphene

We show that the intrinsic chiral $d_{x^2-y^2} \pm id_{xy}$-wave superconducting pairing in doped graphene is significantly strengthened in the core region of a doubly quantized s-wave superconducting vortex produced in a graphene-superconductor hybrid structure. The chiral d-wave state is induced by proximity effect, which transfers the center-of-mass angular momentum of the s-wave vortex to the orbital angular momentum of the chiral d-wave Cooper pairs. The proximity effect is enhanced by the circular geometry of the vortex and we find a $[1 + (T-T_{c,J})^2]^{-1}$ temperature dependence for the chiral d-wave core amplitude, where $T_{c,J}$ is its intrinsic bulk transition temperature. We further propose to detect the chiral d-wave state by studying the temperature dependence of the low-energy local density of states in the vortex core, which displays a sudden radial change as function of the strength of the d-wave core state.

Microstructure-sensitive weighted probability approach for modeling surface to bulk transition of high cycle fatigue failures dominated by primary inclusions

A simulation-based probabilistic strategy is developed to characterize the surface to bulk transition of high cycle fatigue failures dominated by primary inclusions. The probability of fatigue crack initiation in the surface region is calculated by computing the expected number of critical fatigue hot spots in this region. This is done by considering the probability of inclusion-matrix debonding and the fatigue crack initiation potency of partially-debonded inclusions for a given load ratio and stress amplitude.A case study is presented whereby the surface initiation probability is studied in uniaxial strain-controlled cyclic loading simulations of round smooth specimens of the fine grained powder metallurgy (PM) processed Ni-base superalloy IN100. The fatigue crack initiation potency of partially-debonded nonmetallic inclusions is assessed by calculating the Fatemi–Socie (FS) critical plane parameter from generalized plain strain crystal plasticity finite element simulations. Idealized spherical ceramic inclusions with homogeneous linear elastic isotropic material properties are considered to isolate the FS parameter sensitivity to inclusions’ size, stress amplitude, and polycrystalline microstructure realization around the inclusion.

Low‐Temperature CO2‐Assisted Assembly of Cyclic Olefin Copolymer Ferroelectrets of High Piezoelectricity and Thermal Stability

A novel supercritical carbon dioxide (CO2)‐assisted low‐temperature assembly approach to fabricate cyclic olefin copolymer (COC) ferroelectrets is reported. The process takes advantage of the strong COC–CO2 interaction and associated severe depression of both bulk and surface glass transition temperature. Using a multilayer structural design to implement a bending mechanism in the ferroelectrets, the apparent compression modulus can be greatly reduced and tailored, leading to COC ferroelectrets with excellent piezoelectric activity. The COC ferroelectrets exhibit excellent thermal stability. The study presents a viable low‐cost technology to mass produce COC ferroelectrets with high piezoelectric activity that can be used in structurally and thermally demanding conditions. image

Field induced ferromagnetic phase transition and large magnetocaloric effect in Sm0.55Sr0.45MnO3 phase separated manganites

(a) Magnetic field and particle size dependent T C of nanometric (black and red dot) and bulk (blue and wine dot) measured upon cooling (full dots) and warming (open dots). (b) Temperature dependence magnetic entropy change Δ S at different magnetic field calculated from the magnetization data of S1250 bulk sample. • Large magnetocaloric effect is reported based on the first order phase transition. • The value of |Δ S M | reaches a maximum value of 8.48 J/kg K at 60 kOe field. • A large relative cooling power (∼574 J/kg) around T C –172 K is reported. • The adiabatic temperature change is found to be 2.38 K for magnetic field of 10 kOe. • Noticeable |Δ S M | at low field makes the system useful for magnetic refrigeration. This paper reports about the magnetocaloric effect and relative cooling power based on the first order magnetic phase transition in Sm 0.55 Sr 0.45 MnO 3 polycrystalline phase separated manganites. Upon 60 kOe applied magnetic field, the magnetic entropy change (|Δ S M |) of bulk sample reaches a maximum value of 8.48 J/kg-K with a large relative cooling power (RCP) value of 574 J/kg around its Curie temperature ( T C )–172 K after the correction for hysteretic losses caused by the first order magnetic phase transition. The corresponding adiabatic temperature change is 2.38 K for magnetic field of 10 kOe. The magnetic field induced change of entropy and specific heat vary with temperature and have their maximum around the first order magnetic phase transition. We also report the magnetic field dependence of the order of the ferromagnetic (FM) to paramagnetic (PM) phase transition in bulk and nanometric manganites. It has been shown that bulk to nanometric samples exhibit first order FM → PM phase transition under low magnetic field accompanied by magnetization with thermal hysteresis in the field cooled cooling and warming cycle. However, the samples exhibit a second order magnetic phase transition above a critical field H CR . All the signatures of the first-order magnetic phase transition in bulk and nanometric sample disappear above the critical field H CR . The magnetocaloric effect is thus modified by the field induced order of magnetic phase transition. The field induced paramagnetic to ferromagnetic transition is confirmed to be first order in nature from dc magnetization measurements and Arrott plots using a criteria given by Banerjee. The magnetic phase transition is also accompanied by a large change in resistivity with thermal hysteresis. The observed value of magnetic entropy change in bulk sample is much higher than the value generally observed in other perovskite manganites of comparable T C . This large change mainly originates from a sharp magnetization jump, associated with a first-order metamagnetic transition and coalescence of ferromagnetic clusters in the paramagnetic state. Such noticeable magnetic entropy change at low magnetic field makes this material useful for the application of active magnetic refrigerant (AMR) materials.

Optical properties of vanadium oxides-an analysis

In this study, the optical properties of bulk and thin films of VO2, V2O3, and V2O5, deposited on Al2O3 substrates, have been analyzed from infrared to vacuum ultraviolet range (up to 12 eV). Utilizing the available data of wavelength dependent optical constants of these materials in the literature, the energy corresponding to the peaks in the imaginary part of the dielectric function (e2–R spectra), have been interpreted and compared as a function of structure, polarization, and temperature. The energies corresponding to the peaks in reflectivity-energy (R–E) spectra are explained in terms of the Penn gap (Ep). Ep values for VO2 and V2O5 are close to the average of energies corresponding to the peaks (\( \bar{E} \)) while, their values are even closer in V2O3, reflecting the degree of anisotropy in the order of V2O3 < VO2 < V2O5. The first order reversible, insulator to metal phase transition (IMT) of both bulk and thin films of the V–O systems are studied as an effect of temperature change. The effective number of electrons, neff, participating in the optical transitions is described from the numerical integration using the well-known sum rule. The change in neff with respect to the energy of incident photons is also calculated and it is found that this change is consistent with the peaks observed in the e2–E spectra.

The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations

The hot Jupiter HD189733b is the most extensively observed exoplanet. Its atmosphere has been detected and characterised in transmission and eclipse spectroscopy, and its phase curve measured at several wavelengths. This paper brings together results of our campaign to obtain the complete transmission spectrum of the atmosphere of this planet from UV to IR with HST, using STIS, ACS and WFC3. We provide a new tabulation of the transmission spectrum across the entire visible and IR range. The radius ratio in each wavelength band was rederived to ensure a consistent treatment of the bulk transit parameters and stellar limb-darkening. Special care was taken to correct for, and derive realistic estimates of the uncertainties due to, both occulted and unocculted star spots. The combined spectrum is very different from the predictions of cloud-free models: it is dominated by Rayleigh scattering over the whole visible and near infrared range, the only detected features being narrow Na and K lines. We interpret this as the signature of a haze of condensate grains extending over at least 5 scale heights. We show that a dust-dominated atmosphere could also explain several puzzling features of the emission spectrum and phase curves, including the large amplitude of the phase curve at 3.6um, the small hot-spot longitude shift and the hot mid-infrared emission spectrum. We discuss possible compositions and derive some first-order estimates for the properties of the putative condensate haze/clouds. We finish by speculating that the dichotomy between the two observationally defined classes of hot Jupiter atmospheres, of which HD189733b and HD209458b are the prototypes, might not be whether they possess a temperature inversion, but whether they are clear or dusty. We also consider the possibility of a continuum of cloud properties between hot Jupiters, young Jupiters and L-type brown dwarfs.

Mass anomalous dimension in sextet QCD

We extend our previous study of the SU(3) gauge theory with ${N}_{f}=2$ flavors of fermions in the sextet representation of color. Our tool is the Schr\"odinger functional method. By changing the lattice action, we push the bulk transition of the lattice theory to stronger couplings and thus reveal the beta function and the mass anomalous dimension ${\ensuremath{\gamma}}_{m}$ over a wider range of coupling, out to ${g}^{2}\ensuremath{\simeq}11$. Our results are consistent with an infrared fixed point, but walking is not ruled out. Our main result is that ${\ensuremath{\gamma}}_{m}$ never exceeds 0.45, making the model unsuitable for walking technicolor. We use a novel method of extrapolation to the large-volume/continuum limit, tailored to near-conformal theories.

Thermotropic nematic order upon nanocapillary filling

Optical birefringence and light absorption measurements reveal four regimes for the thermotropic behavior of a nematogen liquid (7CB) upon sequential filling of parallel-aligned capillaries of 12 nm diameter in a monolithic, mesoporous silica membrane. No molecular reorientation is observed for the first adsorbed monolayer. In the film-condensed state (up to 1 nm thickness), a weak, continuous paranematic-to-nematic (P-N) transition is found, which is shifted by 10 K below the discontinuous bulk transition at T(IN)=305 K. The capillary-condensed state exhibits a more pronounced, albeit still continuous P-N reordering, located 4 K below T(IN). This shift vanishes abruptly upon complete filling of the capillaries. It could originate in competing anchoring conditions at the free inner surfaces and at the pore walls or result from the 10-MPa tensile pressure release associated with the disappearance of concave menisci in the confined liquid upon complete filling. The study documents that the thermo-optical properties of nanoporous systems (or single nanocapillaries) can be tailored over a surprisingly wide range simply by variation of the filling fraction with liquid crystals.

Ethylenediamine-Mediated Wurtzite Phase Formation in ZnS

The usual high temperature wurtzite phase of ZnS was successfully obtained at low temperature (170 °C) in the presence of ethylenediamine (EN) as the soft template. X-ray diffraction and Raman spectroscopy analysis confirmed the EN-mediated phase transformation (zinc blende to wurtzite) of ZnS. X-ray photoelectron spectroscopy (XPS) showed that all the samples were sulfur deficient. A high temperature X-ray diffraction (XRD) study showed that ZnS samples, both EN-mediated and without EN, retained their phases except small changes in the unit cell dimension. Besides the EN-mediated phase transition, morphology transformations from nearly spherical shape to nanorods are also observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coupling between EN molecules with ZnS is confirmed by Fourier transform infrared spectroscopy. A significant reduction in the phase transition temperature of ZnS has been achieved as compared to the bulk transition temperature (1020 °C). Mech...

The bulk transition of QCD with twelve flavors and the role of improvement

We study the SU(3) gauge theory with Nf=12 flavors in the fundamental representation by use of lattice simulations with staggered fermions. With a non-improved action we observe a chiral zero-temperature (bulk) transition separating a region at weak coupling, where chiral symmetry is realized, from a region at strong coupling where chiral symmetry is broken. With improved actions, a more complicated pattern emerges, and in particular two first order transitions in the chiral limit appear. We observe that at sufficiently strong coupling the next-to-nearest neighbor terms of the improved lattice action are no longer irrelevant and can indeed modify the pattern observed without improvement. Baryon number conservation can be realized in an unusual way, allowing for an otherwise prohibited oscillating term in the pseudoscalar channel. We discuss the phenomenon by means of explicit examples borrowed from statistical mechanics. Finally, these observations can also be useful when simulating other strongly coupled systems on the lattice, such as graphene.

Fluid–Fluid Transitions at Bulk Supercritical Conditions

We use three different polymer solvent mixture models to theoretically determine the existence of capillary-induced phase separation in simple pores under supercritical bulk conditions. These models undergo bulk demixing, due to quite different mechanisms, yet readily display supercritical transitions without the use of esoteric interactions in the capillary. The theoretical method used to analyze these systems is density functional theory. We find that capillary demixing is not reliant on the presence of a pure surface transition but may occur in the absence of the latter. This is shown by considering cases where the surface enhancement factor is too weak to cause demixing at a single surface or else the bulk conditions are supercritical to both bulk and surface transitions. This phenomenon may prove useful in applications involving adsorption from mixtures into porous particles.

Atypical dielectric behavior in sol–gel derived fine grain PZT/CeO2 nanocomposites

We present here dielectric and electrical modulus studies of sol–gel derived fine grain (100−x)PZT/xCeO2 nanocomposites. Dielectric response shows superimposed local maxima(s) in high frequency regimes, which, in low frequency regions seems to be overshadowed by high conductivity. Observed local maxima(s) were found to be in concurrence with the nadir(s) observed in the imaginary part of electrical modulus plotted on the temperature plane. In addition to this, calculated bulk and grain boundary capacitance exhibit maxima(s) in the measured temperature range displaying the existence of both bulk and grain boundary transition in nanocomposite formation. Therefore, a collective effect of these two processes has been thought to be a plausible mechanism for observed dielectric behavior of nanocomposites. Anomalous increase in bulk transition temperature of 2wt% ceria substituted nanocomposite was also observed. Possibilities of increase in transition temperature have also been discussed taking in to account the extrinsic size effect.

Nucleation rate of colour superconducting droplets in protoneutron stars

We analyse the nucleation of quark matter droplets under protoneutron star conditions. We adopt a two-phase framework in which the hadronic phase is described through a nonlinear Walecka model and the just deconfined matter by the MIT bag model including colour superconductivity. Surface tension and curvature energy are calculated self-consistently within the multiple reflection expansion formalism. We impose flavour conservation during the transition, which means that the just deconfined quark droplet is transiently out of equilibrium with respect to weak interactions. Our results show that trapped neutrinos slightly increase the critical density for deconfinement and that colour superconductivity significantly decreases such density at low temperatures. We also show that the nucleation rate is negligible for droplets larger than 100–200 fm and is huge for smaller droplets provided that the temperature is low enough. We compare our results with previous calculations using the Nambu–Jona–Lasinio model with colour superconductivity and the MIT bag model without colour superconductivity. We conclude that the deconfinement transition should be triggered instantaneously when a density slightly larger than the bulk transition density is reached at some layer of a protoneutron star. Since colour superconductivity lowers the transition density at low temperatures, the transition is likely to occur after significant cooling in a massive enough protoneutron star.

Metal insulator transition of bulk and nanocrystalline La1−xCaxMnO3 perovskite manganite materials through in-situ ultrasonic measurements

Abstract Bulk and nanocrystalline La 1 − x Ca x MnO 3 ( x = 0.75, 0.80 and 0.85) perovskite manganite samples were prepared by using the solid-state reaction and sonochemical reactor methods, respectively. The X-ray diffraction studies confirmed that the prepared bulk and nanoperovskite manganite samples were crystalline in nature with an orthorhombic crystal structure. The microscopic studies revealed that the particle sizes of the bulk and nanoperovskite samples were 0.75–0.90 μm and 120–160 nm, respectively. In addition, ultrasonic velocities and attenuations of the bulk and nanosamples were measured by using the in-situ ultrasonic through transmission method. The temperature-dependent ultrasonic parameters showed an interesting anomaly in both bulk and nanoperovskite samples. The observed marked softening and hardening in sound velocities or attenuation were related to metal–insulator phase transition temperature. In nanosamples, the metal insulator transition took place at a lower temperature compared with the corresponding bulk samples. The observed results confirmed that nanoperovskite samples have a broad transition compared with the corresponding bulk samples. The observed broad peak at transition temperature in nanoperovskite samples was used to confirm the nanocrystalline nature of perovskites.

Observation of dynamical spin-dependent electron interactions and screening in magnetic transitions via core-level multiplet-energy separations

The magnetic phase transitions for Gd(0001) grown on W(110) – a bulk transition at 293K and a surface transition about 85K above this – are found to influence the energy separation of the Gd 5s and 4s core-photoelectron doublets. The 5s doublet separation ΔE5s changes over a range of temperatures spanning these transitions, and decreases by a maximum of 60meV in this region, but then recovers its original value; the 4s doublet shows a smaller change in the reverse direction, which does not recover at high temperature. Some of these effects are semi-quantitatively understood from free-atom multiplet theory and from theoretical calculations based on all-electron LDA+U calculations including 4f electron correlation effects. However, the high-temperature behavior of the data also suggest a dynamical nature to these effects via spin-dependent electron screening that is influenced by magnetic fluctuations. Several avenues for studying such effects in a time-resolved manner in future experiments are discussed.