Hasil untuk "cond-mat.mtrl-sci"

Kavita Yadav, Dheeraj Ranaut, K. Mukherjee

Recent studies on weak ferromagnets Fe2-xMnxCrAl (0<x<1) reveal the presence of cluster glass (CG) and Griffiths phase (GP) below and above the ferromagnetic transition temperature (TC) [(2019) Sci. Rep. 9 15888]. In this work, the influence of these inhomogeneous phases on the critical behaviour (around TC) of the above-mentioned series of alloys has been investigated in detail. For the parent alloy Fe2CrAl, γ is estimated as ~ 1.34, which lies near to ordered 3D Heisenberg model, whereas obtained value of β ~ 0.273 does not belong to any universality class. With increment in Mn concentration, both exponents γ and \b{eta} increases, where γ and β approaches disordered 3D Heisenberg model and ordered 3D Heisenberg model, respectively. The observed deviation in γ and unconventional value of δ can be ascribed to the increment in GP with Mn-concentration. The trend noted for β can be attributed to the increment in CG regime with an increase in Mn-content. The self-consistency and reliability of the estimated exponents are verified by the Widom scaling relation and scaling equations of state. Our studies indicate that the critical phenomenon of Fe2-xMnxCrAl (0<x<1) alloys possibly belong to a separate class and is not described within the framework of any existing universal model.

StÉphane Clemmen, Artur Hermans, Eduardo Solano et al.

We report the fabrication of artificial unidimensional crystals exhibiting an effective bulk second-order nonlinearity. The crystals are created by cycling atomic layer deposition of three dielectric materials such that the resulting metamaterial is non-centrosymmetric in the direction of the deposition. Characterization of the structures by second-harmonic generation Maker-fringe measurements shows that the main component of their nonlinear susceptibility tensor is about 5 pm/V which is comparable to well-established materials and more than an order of magnitude greater than reported for a similar crystal [1-Alloatti et al, arXiv:1504.00101[cond-mat.mtrl- sci]]. Our demonstration opens new possibilities for second-order nonlinear effects on CMOS-compatible nanophotonic platforms.

Guangqian Ding, Guoying Gao, Kailun Yao

Improving the thermoelectric efficiency is one of the greatest challenges in materials science. The recent discovery of excellent thermoelectric performance in simple orthorhombic SnSe crystal offers new promise in this prospect [Zhao et al. Nature 508, 373 (2014)]. By calculating the thermoelectric properties of orthorhombic IV-VI compounds GeS,GeSe,SnS,and SnSe based on the first-principles combined with the Boltzmann transport theory, we show that the Seebeck coefficient, electrical conductivity, and thermal conductivity of orthorhombic SnSe are in agreement with the recent experiment. Importantly, GeS,GeSe,and SnS exhibit comparative thermoelectric performance compared to SnSe. Especially, the Seebeck coefficients of GeS,GeSe,and SnS are even larger than that of SnSe under the studied carrier concentration and temperature region. We also use the Cahill's model to estimate the lattice thermal conductivities at the room temperature. The large Seebeck coefficients, high power factors, and low thermal conductivities make these four orthorhombic IV-VI compounds promising candidates for high-efficient thermoelectric materials.

Xuanye Wang, K. Tantiwanichapan, R. Paiella et al.

B. Schneider, A. Lathuilière, V. Laversenne et al.

Steffi L. Colyer, S. Roberts, Jonathan B Robinson et al.

X. Z. Lu, X. G. Gong, H. J. Xiang

Interface engineering in perovskite oxide superlattices has developed into a flourishing field, enabling not only further tuning of the exceptional properties, but also giving access to emergent physical phenomena. Here, we reveal a new mechanism for enhancing the electric polarization by the interface-induced oxygen octahedral tilts in BaTiO3/CaTiO3 superlattices. By combining a novel genetic algorithm with density functional theory (DFT), we predict that the true ground states in 1:1 and 2:2 BaTiO3/CaTiO3 superlattices grown on SrTiO3 adopt Pc symmetry with a large proper electric polarization (32.8μC/cm2 for 1:1 and 35.8 μC/cm2 for 2:2 superlattices), which is even larger than that of bulk BaTiO3. The tilt of oxygen octahedron is found to play a key role for the enhancement of out-of-plane polarization in 1:1 superlattices because it reduces greatly the rotation of oxygen octahedron (out-of-phase) which significantly suppresses the out-of-plane polarization.

G. Trewartha, E. Preatoni, M. England et al.

N. Alexandrov

D. V. Kurmude, R. Dorik, V. D. M. K. M. J. D. R. Shengule

George S. Kliros

Graphene nanostructures exhibit an intrinsic advantage in relation to the gate delay in three-terminal devices and provide additional benefits when operate in the quantum capacitance limit. In this paper, we developed a simple model that captures the Fermi energy and temperature dependence of the quantum capacitance for monolayer and bilayer graphene devices. Quantum capacitance is calculated from the broadened density of states taking into account electron-hole puddles and possible finite lifetime of electronic states through a Gaussian broadening distribution. The obtained results are in agreement with many features recently observed in quantum capacitance measurements on both gated monolayer and bilayer graphene devices. The temperature dependence of the minimum quantum capacitance around the charge neutrality point is also investigated.

Luxmi, N. Srivastava, R. M. Feenstra et al.

The formation of graphene on the (0001) surface of SiC (the Si-face) is studied by atomic force microscopy, low-energy electron microscopy, and scanning tunneling microscopy/spectroscopy. The graphene forms due to preferential sublimation of Si from the surface at high temperature, and the formation has been studied in both high-vacuum and 1-atm-argon environments. In vacuum, a few monolayers of graphene forms at temperatures around 1400 C, whereas in argon a temperature of about 1600 C is required in order to obtain a single graphene monolayer. In both cases considerable step motion on the surface is observed, with the resulting formation of step bunches separated laterally by >10 microns. Between the step bunches, layer-by-layer growth of the graphene is found. The presence of a disordered, secondary graphitic phase on the surface of the graphene is also identified.

Xi-Zhou Qin, Wie-Liang Wang, Zhi-Bing Li

We obtained the exact expression of the electric potential in the space around a nanowall that is vertically mounted on a planar cathode. The system is designed as a cold field electron emitter or an electron tunneling line scanner. The finite cathode-anode distance has been taking into account. The analytical results are compared with that obtained by the finite- element method.

Prithwish Kumar Nandi, M. C. Valsakumar, Sharat Chandra et al.

We calculate properties like equilibrium lattice parameter, bulk modulus and monovacancy formation energy for nickel (Ni), iron (Fe) and chromium (Cr) using Kohn-Sham density functional theory (DFT). We compare relative performance of local density approximation (LDA) and generalized gradient approximation (GGA) for predicting such physical properties for these metals. We also make a relative study between two different flavors of GGA exchange correlation functional, namely, PW91 and PBE. These calculations show that there is a discrepancy between DFT calculations and experimental data. In order to understand this discrepancy in the calculation of vacancy formation energy, we introduce a correction for the surface intrinsic error corresponding to an exchange correlation functional using the scheme implemented by Mattsson et al. [Phys. Rev. B 73, 195123 (2006)] and compare the effectiveness of the correction scheme for Al and the 3d-transition metals.

Benito Santos, Elena Loginova, Arantzazu Mascaraque et al.

We have grown epitaxial films a few atomic layers thick of iron oxides on ruthenium. We characterize the growth by low energy electron microscopy. Using selected area diffraction and intensity vs. voltage spectroscopy, we detect two distinct phases which are assigned to wustite and magnetite. Spin polarized low energy electron microscopy shows magnetic domain patterns in the magnetite phase at room temperature.

P. M. Dinh, F. Fehrer, P. -G. Reinhard et al.

We study deposition dynamics of Na and Na$_2$ on an Ar substrate, both species neutral as well as charged. The system is modeled by a hierarchical approach describing the Na valence electrons by time-dependent density-functional theory while Na core, Ar atoms and their dynamical polarizability are treated by molecular dynamics. We explore effects of Na charge and initial kinetic energy of the impinging Na system. We find that neutral Na is captured into a loosely bound adsorbate state for sufficiently low impact energy. The charged monomers are more efficiently captured and the cation Na$^+$ even penetrates the surface layer. For charged dimers, we come to different final configurations depending on the process, direct deposit of Na$_2^+$ as a whole, or sequential deposit. In any case, charge dramatically amplifies the excitation of the matrix, in particular at the side of the Ar dipoles. The presence of a charge also enhances the binding to the surface and favours accumulation of larger compounds.

R. Gröger, V. Vitek

The breakdown of the Schmid law in bcc metals has been known for a long time. The asymmetry of shearing in the slip direction <111> in the positive and negative sense, respectively, commonly identified with the twinning-antitwinning asymmetry, is undoubtedly one of the reasons. However, effect of stress components other than the shear stress in the slip direction may be important. In this paper we investigate by atomistic modeling the effect of shear stresses perpendicular to the Burgers vector on the glide of a/2<111> screw dislocations. We show that these shear stresses can significantly elevate or reduce the critical resolved shear stress (CRSS) in the direction of the Burgers vector needed for the dislocation motion, i.e. the Peierls stress. This occurs owing to the changes of the core induced by these stresses. This effect may be the reason why slip systems with smaller Schmid factors may be preferred over that with the largest Schmid factor.

Cvita Gregov, I. Jukić, G. Marković

Bringard Aurelien, Perrey Stephane, B. Nicolas