In-plane Disorder Research Articles

Evolution of Eu valence and superconductivity in layered Eu0.5La0.5FBiS2−xSex system

We have studied the effect of Se substitution on Eu valence in a layered ${\mathrm{Eu}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{FBiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ superconductor using a combined analysis of x-ray absorption near-edge structure (XANES) and x-ray photoelectron spectroscopy (XPS) measurements. Eu ${L}_{3}$-edge XANES spectra reveal that Eu is in the mixed valence state with coexisting ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$. The average Eu valence decreases sharply from $\ensuremath{\sim}2.3$ for $x=0.0$ to $\ensuremath{\sim}2.1$ for $x=0.4$. Consistently, Eu $3d$ XPS shows a clear decrease in the average valence by Se substitution. Bi $4f$ XPS indicates that effective charge carriers in the ${\mathrm{BiCh}}_{2}$ (Ch = S, Se) layers are slightly increased by Se substitution. On the basis of the present results it has been discussed that the metallic character induced by Se substitution in ${\mathrm{Eu}}_{0.5}{\mathrm{La}}_{0.5}{\mathrm{FBiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ is likely to be due to increased in-plane orbital overlap driven by reduced in-plane disorder that affects the carrier mobility.

Intrinsic Phase Diagram of Superconductivity in the BiCh2-Based System Without In-Plane Disorder

We have investigated the crystal structure and physical properties of LaO1-xFxBiSSe to reveal the intrinsic superconductivity phase diagram of the BiCh2-based layered compound family. From synchrotron X-ray diffraction and Rietveld refinements with anisotropic displacement parameters, we clearly found that the in-plane disorder in the BiSSe layer was fully suppressed for all x. In LaO1-xFxBiSSe, metallic conductivity and superconductivity are suddenly induced by electron doping even at x = 0.05 and 0.1 with a monoclinic structure. In addition, x (F concentration) dependence of the transition temperature (Tc) for x = 0.2-0.5 with a tetragonal structure shows an anomalously flat phase diagram. With these experimental facts, we have proposed the intrinsic phase diagram of the ideal BiCh2-based superconductors with less in-plane disorder.

Controlling Two-Step Phase Transitions and Dielectric Responses by A-Site Cations in Two Perovskite-like Coordination Polymers

Two new perovskite-like coordination polymers, A2[KFe(CN)6], were constructed by employing guanidinium and acetamidinium as A-site cations, respectively. Their cation-controlled two-step phase transitions as well as the relevant dielectric responses were uncovered by the combined techniques of the variable-temperature single-crystal X-ray structural analyses and dielectric measurements. With a similar size and shape, the A-site cations reveal similar two-step thermal-induced transitions on their motional dynamics, i.e., from a frozen order state to an in-plane rotational disorder state, and to a melt-like disorder state. However, the variation of the A-site cations on their symmetries and dipole moments makes noticeable impacts on the symmetry breaking, the critical temperatures, and the dielectric responses for the two-step structural phase transitions, i.e., the D3h nonpolar guanidinium results in an R3c ↔ R3m ↔ Fm3m transition, whereas the C2v polar acetamidinium results in a C2/m ↔ R3m ↔ Fm3m tra...

Bombarding Graphene with Oxygen Ions: Combining Effects of Incident Angle and Ion Energy To Control Defect Generation

Ion bombardment is a key physical process in the ion implantation and irradiation of graphene, with important implications for tuning graphene’s electronic properties and for understanding the material’s behavior in irradiative environment. Using molecular dynamics with a reactive force field, this work systematically investigates the influence of the incident angle on the generation of defects and vacancies during the bombardment process. It is found that larger incident angles (between the incident line and the surface of graphene) ranging from 70° to 90° are desired for substitution and single vacancy, whereas smaller incident angles ranging from 30° to 50° are favored for forming double vacancies, multiple vacancies, and in-plane disorder. Oxygen ions with the incident angle of 70° produce the highest probability of ion substitution, and the ions at 40–60 eV and 70° yield the highest quality of doping with minimum other defects. These results demonstrate that bombarding graphene along oblique directio...

Isotropic in-plane quenched disorder and dilution induce a robust nematic state in electron-doped pnictides

The phase diagram of electron-doped pnictides is studied varying the temperature, electronic density, and isotropic quenched disorder strength by means of computational techniques applied to a three-orbital ($xz$, $yz$, $xy$) spin-fermion model with lattice degrees of freedom. In experiments, chemical doping introduces disorder but in theoretical studies the relationship between electronic doping and the randomly located dopants, with their associated quenched disorder, is difficult to address. In this publication, the use of computational techniques allows us to study independently the effects of electronic doping, regulated by a global chemical potential, and impurity disorder at randomly selected sites. Surprisingly, our Monte Carlo simulations reveal that the fast reduction with doping of the N\'eel $T_N$ and the structural $T_S$ transition temperatures, and the concomitant stabilization of a robust nematic state, is primarily controlled by the magnetic dilution associated with the in-plane isotropic disorder introduced by Fe substitution. In the doping range studied, changes in the Fermi Surface produced by electron doping affect only slightly both critical temperatures.

The Correlation between the Energy Gap and the Pseudogap Temperature in Cuprates: The YCBCZO and LSHCO Case

The paper analyzes the influence of the hole density, the out-of-plane or in-plane disorder, and the isotopic oxygen mass on the zero temperature energy gap (2Δ(0))Y1-xCaxBa2Cu1-yZny3O7-δ(YCBCZO) andLa1.96-xSrxHo0.04CuO4(LSHCO) superconductors. It has been found that the energy gap is visibly correlated with the value of the pseudogap temperature (T⋆). On the other hand, no correlation between 2Δ(0) and the critical temperature (TC) has been found. The above results mean that the value of the dimensionless ratio2Δ0/kBTCcan vary very strongly together with the chemical composition, while the parameter2Δ(0)/kBT⋆does not change significantly. In the paper, the analytical formula which binds the zero temperature energy gap and the pseudogap temperature has been also presented.

Linewidth-narrowing phenomena with intersubband cavity polaritons

Absorption spectra of strongly coupled intersubband cavity polaritons have been measured, using a tunable midinfrared quantum cascade laser, with high angular and spectral resolution. Pronounced linewidth narrowing of the polaritons around the anticrossing was found, with polariton linewidths narrower (4.2 meV) than both the bare intersubband transition linewidth and empty cavity linewidth (6.2 and 6 meV, respectively), at room temperature. This is due to variations in the degree of spatial averaging of the in-plane quantum-well disorder as the polariton's extended coherence length is increased by the photonic coupling over the value corresponding to the bare intersubband transition coherence length.

Doping Dependence of the Effects of In-Plane Disorder on T c and the Pseudogap in Single Layer La214 and Double Layer Y123: a Comparative Study

High-T c emerges from an strongly electronically correlated normal state in hole doped cuprates. In this paper, the comparative effect of Zn on the superconducting transition temperature, T c , was studied for the La2−x Sr x Cu1−y Zn y O4 (Zn-La214) and YBa2(Cu1−y Zn y )3O7−δ (Zn-Y123) compounds as a function of hole concentration, p, and Zn content (y) in order to explore the interplay among different electronic ground states in different cuprate systems. Zn induced rate of suppression of T c , dT c (p)/dy, for Zn-La214 was found to be strongly p-dependent and showed a monotonic variation, except in the vicinity of p∼0.125 (i.e., near the 1/8th anomaly where the charge/spin stripe correlations are at their strongest). Magnitude of dT c (p)/dy decreased markedly around p∼0.125. The same feature, at a somewhat reduced scale, was also observed for Zn-Y123. We have also reviewed the p-dependent pseudogap energy scale, e g (p), which shows a quasilinear decrease with increasing p, without any noticeable feature at p∼0.125. The magnitude and the evolution of e g (p) are quite similar for both Zn-La214 and Zn-Y123 compounds even though T c and structural and electronic anisotropies are significantly different.

Role of in-plane and out-of-plane dilution in CeFeAsO: Charge doping versus disorder

We provide direct experimental evidence for the identical effect of the in-plane Fe$_{1-x}$Co$_{x}$ and of the out-of-plane O$_{1-x}$F$_{x}$ chemical dilutions on the itinerant spin-density-wave (SDW) magnetic phase in CeFeAsO. Remarkably, the suppression of SDW is not sensitive at all to the different kinds of disorder introduced in the two cases. Still, it is clearly shown that the sizeable in-plane disorder induced by the Fe$_{1-x}$Co$_{x}$ substitution is highly effective in suppressing $T_{\textrm{c}}$. Differently from what is observed in CeFeAsO$_{1-x}$F$_{x}$, the ordered magnetic phase of the Ce sublattice is preserved throughout the whole phase diagram in CeFe$_{1-x}$Co$_{x}$AsO ($x \leq 0.2$). An intriguing effect is encountered, whereby the magnetic coupling among Ce$^{3+}$ ions is enhanced by the superconducting phase.

Isotope exponent in disordered underdoped and overdoped La214

The effect of oxygen isotope substitution on Tc has been analyzed for heavily underdoped and overdoped La2-xSrxCu1-yZnyO4 compounds with different Zn contents in the CuO2 plane. The effect of Zn on the isotope exponent, α, was more pronounced in the case of the underdoped (x = 0.09) compounds compared to the overdoped (x = 0.22) ones. The variation of α with in-plane disorder can be described quite well within a model based solely on impurity induced Cooper pair-breaking in the case of the underdoped compounds. This model, on the other hand, fails to describe the behavior of α(y) for the overdoped samples, even though Zn still breaks pairs quite effectively at this hole (Sr) content. We discuss the implications of these finding in details by considering the Zn induced magnetism, stripe correlations, and possible changes in the superconducting order parameter as number of charge carriers in the CuO2 plane, p (≡ x), is varied.

Effect of 3d Metal (Co and Ni) Doping on the Superconductivity of FeSe$_{0.5}$Te$_{0.5}$

We report the effect of 3d metal Cobalt (Co) and Nickel (Ni) doping on the FeTe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> superconductor with the nominal composition range Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1 - x</sub> M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> ( M = Co, Ni and x = 0.00, 0.01 0.02, 0.05 , and 0.10). Samples are synthesized through standard solid state reaction route and all are crystallize in single phase tetragonal structure with space group P4/nmm. The lattice parameters ` <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> ', ` <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> ' and volume decrease with increase in Co and Ni content, although not monotonically. In fact the ` <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> ' lattice parameter is nearly unaffected for Co doped samples. The superconducting transition temperature ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> ) is measured from both DC and AC magnetic susceptibility, which decrease with increase in Ni or Co content. Both Co and Ni suppress <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Tc</i> and drive the system into the normal state. Interestingly, Ni suppresses the superconductivity much faster than the Co. This indicates less effect on in-plane Fe-Se distances and thus reduced disorder in case of Co when compared with Ni substation at Fe site in FeTe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> superconductor. It is concluded that in-plane disorder on superconducting Fe-Se and Fe-Se-Se planes directly affects superconductivity in FeTe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> superconductor.

Structural and Electrical Properties of Graphene Films Grown by Propane/Hydrogen CVD on 6H-SiC(0001)

We have grown graphene on SiC(0001) using propane-hydrogen CVD. In this work, we present the effects of growth pressure and temperature on structural and electrical properties. Structural characterizations evidence the formation of graphene with in-plane rotational disorder, except for low growth pressure and high growth temperature which lead to the formation of a (6Ö3´6Ö3)-30° interface between graphene and SiC. Electrical properties of samples presenting different graphene/SiC stacking and interfaces are compared and discussed.

Graphene/SiC Interface Control Using Propane-Hydrogen CVD on 6H-SiC(0001) and 3C-SiC(111)/Si(111)

We have grown graphene on 6H-SiC(0001) and 3C-SiC(111)/Si (111) using propane-hydrogen CVD. This contribution studies the effects of pressure on graphene growth and on its structural properties studied through low energy electron diffraction. We show that varying pressure allows to control the formation of graphene on a (6√3×6√3)-R30° interface reconstruction (low pressure) or graphene with in-plane rotational disorder (high pressure) on both 6H-SiC(0001) and 3C-SiC(111). The effects of the SiC morphology before graphene growth are discussed in order to explain the differences observed between polytypes.

Out-of-plane scattering from vertically asymmetric photonic crystal slab waveguides with in-plane disorder

We characterize optical wave propagation along line defects in two-dimensional arrays of air-holes in free-standing silicon slabs. The fabricated waveguides contain random variations in orientation of the photonic lattice elements which perturb the in-plane translational symmetry. The vertical slab symmetry is also broken by a tilt of the etched sidewalls. We discuss how these lattice imperfections affect out-of-plane scattering losses and introduce a mechanism for high-Q cavity excitation related to polarization mixing.

Impurity effect on superconducting gap and pseudogap

Co substituted Bi2212 samples of Bi 2.1Sr 1.9Ca(Cu 1−xCo x)O 8+ δ ( x = 0, 0.01, 0.02, 0.04, 0.06, and Tc = 85,71,52,35 K) were prepared, and their electronic density states were characterized by a low temperature STM. The experimental results revealed that superconducting gap-like features were rapidly destroyed, while pseudogap-like features were enhanced with increasing impurities, suggesting that the origin of the pseudogap is different from that of a superconducting gap. We suggested a two phase-like model to explain the behavior of a gap map and a gap distribution function. On the other hand, spatial gap and pseudogap inhomogeneity persisted in all Co concentrations. This indicates that the origin of inhomogeneity is not related to in-plane disorder or impurities.

Orientational ordering in the low-temperature stable phases of deuterated thiophene

The stable structures of deuterated thiophene C(4)D(4)S were investigated at 155 (phase III), 115 (phase IV), 100 and 1.5 K (phase V) by neutron powder diffraction. At 155 K, thiophene is orthorhombic with space group Pbnm. Although there is some degree of in-plane orientational disorder, molecules begin to order along two symmetrically equivalent main orientations. At 115 K the structure is incommensurate, with a wavevector q approximately 0.55a*. At 100 K and below, there is a doubling of the a cell parameter and the structure space group is P2(1). For the first time, it is shown that, unlike C(4)H(4)S, phase V of C(4)D(4)S is not an orientational glass: thiophene molecules are perfectly ordered and are oriented within the molecular plane along two alternating directions, corresponding to the two main orientations observed at 155 K. This ordering probably originates in the slowing down of the in-plane reorientational dynamics upon deuteration.

Synchronized and Desynchronized Phases of Exciton-Polariton Condensates in the Presence of Disorder

Condensation of exciton polaritons in semiconductor microcavities takes place despite in-plane disorder. Below the critical density, the inhomogeneity of the disorder limits the spatial extension of the ground state. Above the critical density, in the presence of weak disorder, this limitation is spontaneously overcome by the nonlinear interaction, resulting in an extended synchronized phase. In the case of strong disorder, several non-phase-locked condensates can be evidenced. The transition from a synchronized phase to a desynchronized phase is addressed by sampling the cavity disorder.

Thermal conductivity mapping of pyrolytic carbon and silicon carbide coatings on simulated fuel particles by time-domain thermoreflectance

Thermal conductivity of pyrolytic carbon and silicon carbide coatings on spherical particles has been mapped using time-domain thermoreflectance. The thermal conductivities measured for pyrolytic carbon ranged between 3.4 and 13.5 W/m K. The effect of porosity, pore-size distribution, anisotropy, in-plane disorder and domain sizes is discussed. A thermal conductivity of 168 W/m K was obtained for SiC. Mapping of the thermal conductivity of coated fuel particles provides useful data for modeling fuel performance during the operation of nuclear reactors.

Fabrication of 2D Ordered Films of Tobacco Mosaic Virus (TMV): Processing Morphology Correlations for Convective Assembly

Biological colloids, and in particular viruses, have demonstrated substantial potential as scaffolds for nanoparticle arrays. However, the large-area, low-cost, and rapid assembly of viruses, such as by traditional colloidal processing techniques, is not well-established. Systematic exploration of processing space (virus concentration, assembly speed, and substrate surface energy) for the convective assembly method enables the fabrication of films of rod-shaped viruses (tobacco mosaic virus, TMV) with a high degree of long-range order. Monolayer assemblies several centimeters in length are comprised of TMV aligned parallel to the direction of assembly. Increasing TMV concentration and reducing assembly speed resulted in well-ordered viral layering ( N = 2 to N = 12); however, the top virus layer exhibits varying degrees of in-plane disorder.

Structure and mechanical properties of pyrolytic carbon produced by fluidized bed chemical vapor deposition

Pyrolytic carbon was deposited on spherical particles using a multi-spout fluidized bed chemical vapor deposition reactor to fabricate TRISO fuel for the High Temperature Reactor (HTR). Modern techniques such as Raman spectroscopy and nanoindentation supported by porosimetry, scanning electron microscopy and transmission electron microscopy were employed to analyze the particle coatings directly. Raman spectroscopy and nanoindentation were given special attention due to their capacity to provide information on the internal structure of pyrolytic carbon and its mechanical properties without the necessity of complex sample preparation. The results obtained were used to study the relationship deposition conditions–microstructure–mechanical properties in more detail. Increasing the deposition temperature reduced the density and Young's modulus as porosity and in-plane disorder of carbon domains increased. There was also a change from a laminar microstructure of PyC to that containing more spherical particles. It appeared that anisotropy, domain size and level of graphitization (examined by Raman and TEM) had a strong influence on the mechanical properties. Clear differences were observed between acetylene and the acetylene/propylene mixture as precursor gases.