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At least 343 records · Page 19Linked to original sources
arXiv2013

Transport in Suspended Monolayer and Bilayer Graphene Under Strain: A New Platform for Material Studies

We develop two types of graphene devices based on nanoelectromechanical systems (NEMS), that allows transport measurement in the presence of in situ strain modulation. Different mobility and conductance responses to strain were observed for single layer and bilayer samples. These types of devices can be extended to other 2D membranes such as MoS2, providing transport, optical or other measurements with in situ strain.

cond-mat.mtrl-sci↗
arXiv2013

Comment On "A modified Lennard-Jones-type equation of state for solids strictly satisfying the spinodal condition"

The cohesive energies of solids calculated using mGLJ EOS proposed by Sun Jiuxun (Sun Jiuxun, J. Phys.: Condens. Matter 17, L103 (2005)) are seen to be erroneous. Also we observed that the thermodynamic properties calculated using the mGLJ potential diverge for materials whose pressure derivative of bulk modulus at equilibrium is less than 5. Thus the mGLJ potential cannot be used in liquid state theories and molecular simulations to obtain thermodynamic properties.

cond-mat.mtrl-sci↗
arXiv2013

Correct nonlinearity and hysteresis of volt-ampere characteristics of spin valves, magnetic tunnel junctions and memristors

There are essential achievements in synthesis of interesting for creation of compact electronic memory switched by own current structures of spin valves and magnetic tunnel junctions with hysteretic current dependences of resistance. In the offered message the attention to discrepancy to physical principles of a hysteresis of resistance represented in publications is paid. It is schematically presented how the dependences of resistance on current should look not contradicting the energy conservation law for hysteresis dependence of resistance on current and corresponding volt-ampere characteristic.

cond-mat.mtrl-sci↗
arXiv2013

Charge-transfer from Regularized Symmetry-Adapted Perturbation Theory

The charge-transfer (CT) together with the polarization energy appears at second and higher orders in symmetry-adapted perturbation theory (SAPT). At present there is no theoretically compelling way of isolating the charge-transfer energy that is simultaneously basis-set independent and applicable for arbitrary intermolecular separation. We argue that the charge-transfer can be interpreted as a tunneling phenomenon, and can therefore be defined via regularized SAPT. This leads to a physically convincing, basis-independent definition of the charge-transfer energy that captures subtleties of the process, such as the asymmetry in the forward and backward charge transfer, as well as secondary transfer effects. With this definition of the charge-transfer the damping needed for polarization models can be determined with a level of confidence hitherto not possible.

physics.chem-ph↗
arXiv2013

Synthesis and structural ordering of nano-sized Ba3B'Nb2O9 (B'= Ca and Zn) powders

In this work we investigated the phase formation and structural ordering of Ba$_{3}$B$^{'}$Nb$_{2}$O$_{9}$ (B$^{'}$ $=$ Ca and Zn) perovskite powders obtained by a modified polymeric precursor route using X-ray powder diffraction and Raman spectroscopy. We obtained single phase complex perovskites at 900 $^o$C. Williamson-Hall analysis and scanning electron microscopy showed that the powders have crystallites and particles of nanometric size.

cond-mat.mtrl-sci↗
arXiv2013

Interface control of the magnetic chirality in TaN|CoFeB|MgO heterosctructures

Recent advances in the understanding of spin orbital effects in ultrathin magnetic heterostructures have opened new paradigms to control magnetic moments electrically. The Dzyaloshinskii-Moriya interaction (DMI) is said to play a key role in forming a Neel-type domain wall that can be driven by the spin Hall torque, a torque resulting from the spin current generated in a neighboring non-magnetic layer via the spin Hall effect. Here we show that the sign of the DMI, which determines the direction to which a domain wall moves with current, can be changed by modifying the adjacent non-magnetic layer. We find that the sense of rotation of a domain wall spiral is reversed when the Ta underlayer is doped with nitrogen in Ta|CoFeB|MgO heterostructures. The spin Hall angle of the Ta and nitrogen doped Ta underlayers carry the same sign, suggesting that the sign of the DMI is defined at the interface. Depending on the sense of rotation, spin transfer torque and spin Hall torque can either compete or assist each other, thus influencing the efficiency of moving domain walls with current.

cond-mat.mes-hall↗
arXiv2013

Influence of morphological inhomogeneity induced carrier diffusion on transient photocurrent pulse shape in organic thin films

The influence of film morphology induced carrier diffusion on the broadening of the time-of-flight transient photo-current pulse was investigated using Monte Carlo simulation in organic thin films. Assuming the Gaussian Disorder Model for the charge transport the simulation of the time-of-flight photo-current pulse shape was carried out for homogeneous and inhomogeneous films by varying the overall energetic disorder of the system. In the case of homogeneous system, the value of the tail broadening parameter (W) of the photocurrent pulse is found to decrease upon decreasing the energetic disorder. The observed behavior is explained by using the temporal evolution of carrier diffusion coefficient. In case of the inhomogeneous system, upon decreasing the overall energetic disorder of the system the value of W initially attained a maximum before it started to decrease. This is attributed to the morphology dependent carrier diffusion in the latter case. This study elicits the importance of the influence of the film morphology induced carrier diffusion on the experimentally measured shape of the time-of-flight transient photo-current pulses, which is found to be generally ignored.

cond-mat.mtrl-sci↗
arXiv2013

Substituent-level Tuning of Frontier Orbital Energy Levels in Phthalocyanine/C60 Donor-Acceptor Charge Transfer Pairs

We have calculated several low-lying Charge Transfer (CT) excited-state energies for four non-covalently bound dyads composed of a sulfonated-ZnPc coupled to C60. Our results show that the di- and tri-sulfonated systems yield a CT state as the lowest-energy excited state in the system. In contrast, an energy re-ordering for the tetra-sulfonated ZnPc system leads to local excitations lying lower in energy than the CT state, displaying a possible deactivation pathway obstructing charge separation. Since several different donor-acceptor relative orientations may co-exist at an organic heterojunction, we compare the energetics of a few low-lying CT states for the end-on geometry of a di-sulfonated system to its co-facial orientation counterpart. The calculated CT excitation energies are larger for the end-on orientation in comparison to the co-facial structure by ~1.5 eV, which results principally from a substantial decrease in exciton binding energy in going from the co-facial to the end-on orientation. Furthermore, changes in relative donor-acceptor orientation have a larger impact on the CT energies than changes in donor-acceptor distance. TDDFT calculations on the various sulfonated ZnPc donor molecules show a significant splitting of the Q-band for only one of the four donor systems. Our present calculations, in line with previous experimental studies, show that the systematic variation of chemical functional groups is a promising avenue for the substituent-level tuning of various physical properties of organic semiconductors.

cond-mat.mtrl-sci↗
arXiv2013

Chemical vapor deposition of carbon nanotubes on monolayer graphene substrates: reduced etching via suppressed catalytic hydrogenation using C2H4

In most envisioned applications, full utilization of a graphene-carbon nanotube (CNT) construct requires maintaining the integrity of the graphene layer during the CNT growth step. In this work, we exhibit an approach towards controlled CNT growth atop graphene substrates where the reaction equilibrium between source hydrocarbon decomposition and carbon saturation into and precipitation from the catalyst nanoparticles shifts towards CNT growth rather than graphene consumption. By utilizing C2H4 feedstock, we demonstrate that the low temperature growth permissible with this gas suppresses undesirable catalytic hydrogenation and dramatically reduces the etching of the graphene layer to exhibit graphene-CNT hybrids with continuous, undamaged structures.

cond-mat.mtrl-sci↗
arXiv2013

Spin reorientation behavior in YMn1-xMxO3 (M = Ti, Fe, Ga; x = 0, 0.1)

The structural and magnetic properties of polycrystalline YMn1-xMxO3 (M = Ti, Fe, Ga; x = 0, 0.1) have been studied by neutron powder diffraction and magnetic measurements to probe the effect of Mn site doping on the frustration behavior and magnetic structure of these compounds. The compounds are isostructural and crystallize with hexagonal structure in P63cm space group. We find that doping with these three ions, Ti4+ (d0), Fe3+ (d5) and Ga3+ (d10), influences both the TN and magnetic structure, unlike other Mn-site dopants reported previously. The magnetic structure of YMnO3 is described by considering a linear combination of irreducible representations Γ3 and Γ4 below TN ~ 75K and with decrease in temperature the ratio of Γ3 and Γ4 changes. The mixing ratio of these two irreducible representations remains constant on lowering of temperature in the Ga doped compounds. The magnetic structure is modified on doping with nonmagnetic ion Ti4+ (d0). It is described by the basis vectors of the irreducible representation Γ2 with moment 2.3μB at 6 K. On doping with Fe3+ (d5) the magnetic structure immediately below TN is explained by considering the Γ3 irreducible representation. On further lowering of temperature, a spin reorientation at ~ 35 K is observed. Below this temperature, the magnetic structure of YMn0.9Fe0.1O3 is explained by considering Γ3 representation with 51% mixing of Γ4. The ordered moments are found to be reduced from the expected value for a Mn3+ ion in all these compounds indicating the frustrated nature of these compounds. However, the frustration parameter, f is significantly reduced in the case of Ti doped compound with Γ2 representation.

cond-mat.mtrl-sci↗
arXiv2013

Mott-insulator to commensurate-solid transition in a 4He layer on α-graphyne: Pseudo-spin symmetry breaking under a particle-induced pseudo-magnetic field

Path-integral Monte Carlo calculations were performed to study the adsorption of $^4$He atoms on $α$-graphyne. We find that one $^4$He atom can be embedded onto the in-plane center of each hexagon of the graphyne. In the first $^4$He layer above the $^4$He-embedded graphyne surface, a Mott insulating state was observed at the areal density of 0.0706 Å$^{-2}$ with three $^4$He atoms occupying each hexagonal cell while the helium atoms form a commensurate triangular solid at a density of 0.0941 Å$^{-2}$. Here we show that the Ising pseudo-spin symmetry introduced for two degenerate configurations of three $^4$He atoms in a hexagonal cell can be broken by additional $^4$He atoms placed at the hexagon vertices and the Mott-insulator to commensurate-solid transition is a transition from a nonmagnetic spin liquid of frustrated antiferromagnets to a spin-aligned ferromagnet under a particle-induced pseudo-magnetic field.

cond-mat.mtrl-sci↗
arXiv2013

Strong microwave absorption observed in dielectric La1.5Sr0.5NiO4 nanoparticles

La$_{1.5}$Sr$_{0.5}$NiO$_4$ is well known to have a colossal dielectric constant ($\varepsilon_R>10^7$). The La$_{1.5}$Sr$_{0.5}$NiO$_4$ nanoparticle powder was prepared by a combinational method of solid state reaction and high-energy ball milling. Magnetic measurements show that the material has a very small magnetic moment and paramagnetic characteristic at room temperature. The mixture of the nanoparticle powder (40% vol.) and paraffin (60% vol.) coated in the form of flat layers of different thicknesses ($t$) exhibits strong microwave absorption resonances in the 4-18 GHz range. The reflection loss ($RL$) decreases with $t$ and reaches down to -36.7 dB for $t=3.0$ mm. The impedance matching ($|Z|=Z_0=377$ $Ω$), rather than the phase matching mechanism, is found responsible for the resonance observed in the samples with $1 Z_0$ at all frequencies and a reduced absorption. The influence of non-metal backing is also discussed. Our observation suggests that La$_{1.5}$Sr$_{0.5}$NiO$_4$ nanoparticles could be used as good fillers for high performance radar absorbing material.

cond-mat.mtrl-sci↗
arXiv2013

Detailed analysis of the Bi-O pockets problem in $Bi_2Sr_2Ca_2Cu_3O_{10}$

The Bi-O pockets problem, namely, the appearance in theoretical ab initio calculations of the electronic band structure of Bi-cuprates of a pocket of states at the Fermi energy ($E_F$) that is attributed to states belonging to the Bi-O plane is an issue that still calls for more study. The Bi-O pockets are in contradiction with experiments. We have investigated the possible reasons for the disagreement. We checked that by using the experimental lattice and internal parameters without any optimization procedure, the Bi-O pockets do not appear at $E_F$ in agreement with experiment. Nevertheless, as pointed out by R. Kouba et al. [{\em Phys. Rev. B} {\bf 60}, 9321 (1999)] optimization is compulsory to a band structure calculation that will describe appropriately the electronic properties. But starting with the experimental parameters a further optimization procedure previous to the actual ab initio calculation leads to the Bi-O pockets. Doping with 25% of Pb they disappear. From the several configurations that we have considered, we found two very simple ways in which the Bi-O pockets disappear without avoiding an optimization procedure previous to the calculation and without including a doping of any kind. In this paper, we report the effect of the slight displacement of the oxygen atom associated to the Sr-plane (O3) in the electronic properties of $Bi_2Sr_2Ca_2Cu_3O_{10}$ (Bi-2223) with tetragonal structure ($I4/mmm$) using the Local Density Approximation (LDA). The slight displacement is performed after the system has been optimized. We determined the intervals of the O3 atomic positions for which calculations of the band structures show that the Bi-O bands emerge towards higher energies in agreement with the experimental results, thereby solving the Bi-O pockets problem (continue).

cond-mat.str-el↗
arXiv2013

Optics of semiconductors and insulators: Role of local-field effects revised

We show that by nullifying the short-wave response to the long-wave excitation (local-field-effects), the adiabatic time-dependent density-functional theory (TDDFT) of optics of semiconductors and insulators can be brought into excellent agreement with experiment. This indicates that the wing elements [($\Gv,\0v)$ and $(\0v,\Gv)$, $\Gv\ne\0v$] of both the Kohn-Sham (KS) density-response function $χ^s$ and the exchange-correlation kernel $f^{xc}$ are greatly overestimated by the existing approximations to the static DFT and TDDFT, respectively, to the extent that zero is a better approximation for them than the corresponding values provided by current theories. The head element of $f^{xc}$ is thereby fixed by the static macroscopic dielectric constant $ε_M$. Our method yields accurate optical spectra including both the weakly and strongly bound excitons, while its computational cost is extremely low, since only the head element of the KS response matrix and the static dielectric constant are needed.

cond-mat.mtrl-sci↗
arXiv2013

Crack Formation in the Presence of an Electric Field in Droplets of Laponite Gel

When a colloidal gel dries through evaporation, cracks are usually formed, which often reveal underlying processes at work during desiccation. Desiccating colloid droplets of few hundred $μl$ size show interesting effects of pattern formation and cracking which makes this an active subject of current research. Since aqueous gels of clay are known to be strongly affected by an electric field, one may expect crack patterns to exhibit a field effect. In the present study we allow droplets of laponite gel to dry under a radial electric field. This gives rise to highly reproducible patterns of cracks, which depend on the strength, direction and time of exposure to the electric field. For a continuously applied DC voltage, cracks always appear on dissipation of a certain constant amount of energy. If the field is switched off before cracks appear, the observed results are shown to obey a number of empirical scaling relations, which enable us to predict the time of appearance and the number of cracks under specified conditions.

cond-mat.soft↗
arXiv2013

A field theoretic model for static friction

We present a field theoretic model for friction, where the friction coefficient between two surfaces may be calculated based on elastic properties of the surfaces. We assume that the geometry of contact surface is not unusual. We verify Amonton's laws to hold that friction force is proportional to the normal load.This model gives the opportunity to calculate the static coefficient of friction for a few cases, and show that it is in agreement with observed values. Furthermore we show that the coefficient of static friction is independent of apparent surface area in first approximation.

cond-mat.soft↗
arXiv2013

Structural, Vibrational, Elastic and Topological Properties of PaN Under Pressure

Electronic, structural, vibrational and elastic properties of PaN have been studied both at ambient and high pressures, using first principles methods with several commonly used parameterizations of the exchange-correlation energy. The generalized gradient approximation (GGA) reproduces the ground state properties satisfactorily. Under pressure PaN is found to undergo a structural transition from NaCl to the R-3m structure near 58 GPa. The high pressure behavior of the acoustic phonon branch along the (1,0,0) and (1,1,0) directions, and the C44 elastic constant are anomalous, which signals the structural transition. With GGA exchange-correlation, a topological transition in the charge density occurs near the structural transition which may be regarded as a quantum phase transition, where the order parameter obeys a mean field scaling law. However, the topological transition is absent when other exchange-correlation functionals are invoked (local density approximation (LDA) and hybrid functional). Therefore, this constitutes an example of GGA and LDA leading to qualitatively different predictions, and it is of great interest to examine experimentally whether this topological transition occurs.

cond-mat.mtrl-sci↗