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arXiv2008

Growth dynamics of vertically aligned single-walled carbon nanotubes from in situ measurements

An in situ optical absorbance measurement was used to study the growth dynamics of vertically aligned single-walled carbon nanotubes (VA-SWCNTs) synthesized by chemical vapor deposition of ethanol. The growth rate of the VA-SWCNT film was found to decay exponentially from an initial maximum, resulting in an effective growth time of approximately 15 minutes. Investigation of various growth conditions revealed an optimum pressure at which growth is maximized, and this pressure depends on the growth temperature. Below this optimum pressure the synthesis reaction is first-order, and the rate-limiting step is the arrival of ethanol at the catalyst. We also present a novel method for determining the burning temperature of low-mass materials, which combines the in situ absorbance measurement with controlled oxidation.

cond-mat.mtrl-sci↗
arXiv2007

Dynamics of trion formation in GaAs quantum wells

We propose a double channel mechanism for the formation of charged excitons (trions); they are formed through bi- and tri-molecular processes. This directly implies that both negatively and positively charged excitons coexist in a quantum well, even in the absence of excess carriers. The model is applied to a time-resolved photoluminescence experiment performed on a very high quality InGaAs quantum well sample, in which the photoluminescence contributions at the energy of the trion, exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. The unresolved discrepancy between the theoretical and experimental radiative decay time of the exciton in a doped semiconductor is explained.

cond-mat.mtrl-sci↗
arXiv2007

Eulerian conjugate stress and strain

New results are presented for the stress conjugate to arbitrary Eulerian strain measures. The conjugate stress depends on two arbitrary quantities: the strain measure f(V) and the corotational rate defined by the spin Ω. It is shown that for every choice of f there is a unique spin, called the f-spin, which makes the conjugate stress as close as possible to the Cauchy stress. The f-spin reduces to the logarithmic spin when the strain measure is the Hencky strain log(V). The formulation and the results emphasize the similarities in form of the Eulerian and Lagrangian stresses conjugate to the strains f(V) and f(U), respectively. Many of the results involve the solution to the equation AX-XA=Y, which is presented in a succinct format.

cond-mat.mtrl-sci↗
arXiv2007

Polarizability of molecular chains: does one need exact exchange?

Standard density functional approximations greatly over-estimate the static polarizability of longchain polymers, but Hartree-Fock or exact exchange calculations do not. Simple self-interaction corrected (SIC) approximations can be even better than exact exchange, while their computational cost can scale only linearly with the number of occupied orbitals.

physics.atm-clus↗
arXiv2009

Ground and excited-state fermions in a 1D double-well, exact and time-dependent density-functional solutions

Two of the most popular quantum mechanical models of interacting fermions are compared to each other and to potentially exact solutions for a pair of contact-interacting fermions trapped in a 1D double-well potential, a model of atoms in a quasi-1D optical lattice or electrons of a Hydrogen molecule in a strong magnetic field. An exact few-body Hamiltonian is solved numerically in momentum space yielding a highly-correlated eigenspectrum. Additionally, approximate ground-state energies are obtained using both density functional theory (DFT) functional and 2-site Hubbard models. A 1D adiabatic LDA kernel is constructed for use in time-dependent density functional theory (TDDFT), and the resulting excited-state spectrum is compared to the exact and Hubbard results. DFT is shown to give accurate results for wells with small separations but fails to describe localization of opposite spin fermions to different sites. A locally cognizant (LC) density functional based on an effective local fermion number would provide a solution to this problem, and an approximate treatment presented here compares favorably with the exact and Hubbard results. The TDDFT excited-state spectrum is accurate in the small parameter regime with non-adiabatic effects accounting for any deviations. As expected, the ground-state Hubbard model outperforms DFT at large separations but breaks down at intermediate separations due to improper scaling to the united-atom limit. At strong coupling, both Hubbard and TDDFT methods fail to capture the appropriate energetics.

cond-mat.other↗
arXiv2008

Path to finding the critical thickness for memory in thin ferroelectric films

The finite screening length by real metallic electrodes, albeit very small (<1A), results in finite depolarizing field that tends to split the film into domains. In very thin ferroelectric films the domain structure reduces to sinusoidal distribution of polarization considered first in the 1980s. We discuss the phase transition between this structure and a single domain state and show that it is first order, if it exists at all. The alternative possibility is that the single domain state at zero bias voltage would be metastable for all temperatures in most systems. This scenario defines a path towards solution to a problem of finding parameters of a system that can sustain the ferroelectric memory over a desired period of time.

cond-mat.mtrl-sci↗
arXiv2007

Atomic structure analysis of nanocrystalline Boehmite AlO(OH)

Nanocrystalline n-AlO(OH) (Disperal P2(R) of Sasol) was investigated by means of the atomic pair distribution function (PDF). The PDF is derived from powder diffraction data, an ideally resolved PDF is obtained from a synchrotron source which provides a large maximal scattering vector length Qmax > 300 nm-1. Here, however, we were able to reveal atomic structure details of the about 4 nm particles from in-house diffraction data (Qmax = 130 nm-1): PDF least squares model refinements show that n-AlO(OH) is of the layered Boehmite structure type (oC16, Cmcm). But the structure is uniformly distorted in domains of ca. 2 nm size within the nano particles. The hydrogen bonds between the layers are widened up significantly by +13 pm, accounting for the higher reactivity when compared to microcrystalline Boehmite. Our results from only one "nanocrystallographic" experiment are consistent with a trend which was found via the Rietveld technique on a series of AlO(OH) of different crystallite size (Bokhimi et al., 2001). In addition, the PDF contains information on structural distortion as a function of (coherence) domain size within the nano particles.

cond-mat.mtrl-sci↗
arXiv2007

Specific heat of the mixed spin-1/2 and spin-S Ising model with a rope ladder structure

The mixed spin-1/2 and spin-S (S>1/2) Ising model on a rope ladder is examined by combining two exact analytical methods. By the decoration-iteration mapping transformation, this mixed-spin system is firstly transformed to a simple spin-1/2 Ising model on the two-leg ladder, which is then exactly solved by the standard transfer-matrix method. The thermal variations of the zero-field specific heat are discussed in particular.

cond-mat.stat-mech↗
arXiv2007

Brittle fracture down to femto-Joules - and below

We analyze large sets of energy-release data created by stress-induced brittle fracture in a pure sapphire crystal at close to zero temperature where stochastic fluctuations are minimal. The waiting-time distribution follows that observed for fracture in rock and for earthquakes. Despite strong time correlations of the events and the presence of large-event precursors, simple prediction algorithms only succeed in a very weak probabilistic sense. We also discuss prospects for further cryogenic experiments reaching close to single-bond sensitivity and able to investigate the existence of a transition-stress regime.

cond-mat.mtrl-sci↗
arXiv2008

Frustration-driven successive metamagnetic transitions in TbB4

Resonant magnetic x-ray diffraction experiments on the Shastry-Sutherland lattice TbB$_4$ were carried out under strong pulsed magnetic fields up to 30 T. TbB$_4$ exhibits a multi-step magnetization process above 16 T when magnetic fields are applied along the c-axis. We examined the intensity of the 010 magnetic reflection as a function of magnetic field and found that the magnetization plateau phases are accompanied by large XY components of magnetic moments, in contrast to normal fractional magnetization plateau phases. The magnetization was calculated using a simple spin model deduced from the above result. Finally we propose that frustration is the key to understanding the observed multi-step magnetization.

cond-mat.other↗
arXiv2008

Theory of Electromotive Force Induced by Domain Wall Motion

We formulate a theory on the dynamics of conduction electrons in the presence of moving magnetic textures in ferromagnetic materials. We show that the variation of local magnetization in both space and time gives rise to topological fields, which induce electromotive forces on the electrons. Universal results are obtained for the emf induced by both transverse and vortex domain walls traveling in a magnetic film strip, and their measurement may provide clear characterization on the motion of such walls.

cond-mat.mtrl-sci↗
arXiv2008

Reducing the magnetic susceptibility of parts in a magnetic gradiometer

In this paper we report a detailed investigation of a number of different materials commonly used in precision instrumentation in the view of using them as critical components in the magnetic gradiometer. The materials requirement inside a magnetic gradiometer is stringent because the presence of magnetic susceptible material will introduce intrinsic errors into the device. Many commercial grade non-magnetic materials still have unacceptably high levels of volume magnetic susceptibility between 0.001 and 0.0001. It is shown here that machining with steel tools can further increase the susceptibility by up to an order of magnitude. The ability of an acid wash to remove this contamination is also reported. Washing in acid is shown to reduce the variation of volume susceptibility in several commercial grade plastics which already have low values of susceptibility.

cond-mat.mtrl-sci↗
arXiv2007

Thermal Modeling of high power LED modules

This paper presents a study of accuracy issues in thermal modeling of high power LED modules on system level. Both physical as well as numerical accuracy issues are addressed. Incorrect physical assumptions may result in seemingly correct, but erroneous results. It is therefore important to motivate the underlying key physical assumptions of a thermal model. In this paper thermal measurements are used to calibrate a computational fluid dynamics (CFD) model of a high power LED module model at a reference application condition, and to validate it at other application conditions.

cond-mat.mtrl-sci↗
arXiv2007

Spiral growth, two-dimensional nucleation, and the Ehrlich-Schwoebel effect

Frank's prediction of the spiral growth mode in 1949 defined a pivotal moment in the history of crystal growth. In recent decades the topic has received less attention, and instead we have seen a resurrection of two-dimensional nucleation theory in the context of growth experiments on defect-free homoepitaxial thin films. In particular, the key role of interlayer transport controlled by step edge barriers of the Ehrlich-Schwoebel type in shaping the morphology of multilayer films has been increasingly recognized. After a brief review of the classical theory, this paper reports on a recent study of spiral growth in the presence of step edge barriers. Our key observation is that step edge barriers lead to unconventionally shaped spiral hillocks that display the same characteristic ever-steepening height profiles as wedding cakes formed during growth by two-dimensional nucleation. This prediction was verified experimentally by inducing screw dislocations through ion bombardment of the Pt(111) surface, thus creating a homoepitaxial growth system on which spiral hillocks and wedding cakes coexist.

cond-mat.mtrl-sci↗
arXiv2007

Molecular Dynamics Simulation of the Hydrogen Isotope Sputtering of Graphite

We used a molecular dynamics simulation with the modified Brenner reactive empirical bond order potential to investigate the erosion of a graphite surface due to the incidence of hydrogen, deuterium, and tritium atoms. Incident particles cause pressure on the graphite surface, and the chemical bond between graphene layers then generates heat to erode the graphite surface. We evaluated the speed of surface destruction by calculating the pseudo-radial distribution function. The speed of surface destruction due to incident hydrogen isotopes was higher than that due to hydrogen atoms. The surface destruction increased exponentially and its decay time constant was a power function of the incident energy. We measured the erosion yield, which indicated a steady state for the graphite erosion. The erosion yield flux in the steady state increased linearly with the incident energy. The erosion yield flux was almost independent of the type of incident particle, and the erosion yield start time was smaller for hydrogen isotopes than for hydrogen atoms.

cond-mat.mtrl-sci↗
arXiv2007

Numerical Improvement of the Discrete Element Method applied to Shear of Granular Media

We present a detailed analysis of the bounds on the integration step in Discrete Element Method (DEM) for simulating collisions and shearing of granular assemblies. We show that, in the numerical scheme, the upper limit for the integration step, usually taken from the average time $t_c$ of one contact, is in fact not sufficiently small to guarantee numerical convergence of the system during relaxation. In particular, we study in detail how the kinetic energy decays during the relaxation stage and compute the correct upper limits for the integration step, which are significantly smaller than the ones commonly used. In addition, we introduce an alternative approach, based on simple relations to compute the frictional forces, that converges even for integration steps above the upper limit.

cond-mat.mtrl-sci↗