tag:blogger.com,1999:blog-24196718006348048222018-03-06T08:12:09.748-08:00cond-mat.mtrl-sci - Materials ScienceSite for <a href="http://communitypeerreview.blogspot.com/">Community Peer Review</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.comBlogger5836125tag:blogger.com,1999:blog-2419671800634804822.post-7814741987370348912013-08-06T00:03:00.005-07:002013-08-06T00:03:02.030-07:001304.5873 (Matteo Nicoli et al.)<h2 class="title"><a href="http://arxiv.org/abs/1304.5873">Dimensional fragility of the Kardar-Parisi-Zhang universality class</a> [<a href="http://arxiv.org/pdf/1304.5873">PDF</a>]</h2>Matteo Nicoli, Rodolfo Cuerno, Mario Castro<a name='more'></a><blockquote class="abstract">We assess the dependence on substrate dimensionality of the asymptotic scaling behavior of a whole family of equations that feature the basic symmetries of the Kardar-Parisi-Zhang (KPZ) equation. Even for cases in which, as expected from universality arguments, these models display KPZ values for the critical exponents and limit distributions, their behavior deviates from KPZ scaling for increasing system dimensions. Such a fragility of KPZ universality contradicts naive expectations, and questions straightforward application of universality principles for the continuum description of experimental systems.</blockquote>View original: <a href="http://arxiv.org/abs/1304.5873">http://arxiv.org/abs/1304.5873</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-82234279338295357202013-08-06T00:03:00.003-07:002013-08-06T00:03:01.460-07:001308.1024 (S. Masys et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.1024">A first-principles study of structural and elastic properties of bulk<br /> SrRuO3</a> [<a href="http://arxiv.org/pdf/1308.1024">PDF</a>]</h2>S. Masys, V. Jonauskas<a name='more'></a><blockquote class="abstract">We present a first-principles investigation of structural and elastic properties of experimentally observed phases of bulk SrRuO3 - namely orthorhombic, tetragonal, and cubic - by applying density functional theory (DFT) approximations. At first, we focus our attention on the accuracy of calculated lattice constants in order to find out DFT approaches that best represent the crystalline structure of SrRuO3, since many important physical quantities crucially depend on change in volume. Next, we evaluate single-crystal elastic constants, mechanical stability, and macroscopic elastic parameters trying to at least partially compensate for the existing lack of information about these fundamental features of SrRuO3. Finally, we analyze the anomalous behavior of low-temperature orthorhombic phase under C44 related shear deformation. It turns out that at critical strain values the system exhibits a distinct deviation from the initial behavior which results in an isosymmetric phase transition. Moreover, under C44 related shear deformation tetragonal SrRuO3 becomes mechanically unstable raising an open question of what makes it experimentally observable at high temperatures.</blockquote>View original: <a href="http://arxiv.org/abs/1308.1024">http://arxiv.org/abs/1308.1024</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-61730020826929888772013-08-06T00:03:00.001-07:002013-08-06T00:03:00.381-07:001308.0613 (Pinchao Zhang et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0613">Database optimization for empirical interatomic potential models</a> [<a href="http://arxiv.org/pdf/1308.0613">PDF</a>]</h2>Pinchao Zhang, Dallas Trinkle<a name='more'></a><blockquote class="abstract">Weighted least squares fitting to a database of quantum mechanical calculations can determine the optimal parameters of empirical potential models. While algorithms exist to provide optimal potential parameters for a given fitting database of structures and their structure property functions, and to estimate prediction errors using Bayesian sampling, defining an optimal fitting database based on potential predictions remains elusive. A testing set of structures and their structure property functions provides an empirical measure of potential transferability. Here, we propose an objective function for fitting databases based on testing set errors. The objective function allows the optimization of the weights in a fitting database, the assessment of the inclusion or removal of structures in the fitting database, or the comparison of two different fitting databases. To showcase this technique, we consider an example Lennard-Jones potential for Ti, where modeling multiple complicated crystal structures is difficult for a radial pair potential. The algorithm finds different optimal fitting databases, depending on the objective function of potential prediction error for a testing set.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0613">http://arxiv.org/abs/1308.0613</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-24566504337898476052013-08-06T00:02:00.047-07:002013-08-06T00:02:59.348-07:001308.0633 (Adam T. Neal et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0633">Magneto-Transport in MoS2: Phase Coherence, Spin Orbit Scattering and<br /> the Hall Factor</a> [<a href="http://arxiv.org/pdf/1308.0633">PDF</a>]</h2>Adam T. Neal, Han Liu, Jiangjiang Gu, Peide D. Ye<a name='more'></a><blockquote class="abstract">We have characterized phase coherence length, spin orbit scattering length, and the Hall factor in n-type MoS2 2D crystals via weak localization measurements and Hall-effect measurements. Weak localization measurements reveal a phase coherence length of ~50 nm at T = 400 mK for a few-layer MoS2 film, decreasing as T^-1/2 with increased temperatures. Weak localization measurements also allow us, for the first time without optical techniques, to estimate the spin orbit scattering length to be 430 nm, pointing to the potential of MoS2 for spintronics applications. Via Hall-effect measurements, we observe a low temperature Hall mobility of 311 cm2/Vs at T = 1 K which decreases as a power law with a characteristic exponent {\gamma}=1.5 from 10 K to 60 K. At room temperature, we observe Hall mobility of 24 cm2/Vs. By determining the Hall factor for MoS2 to be 1.35 at T = 1 K and 2.4 at room temperature, we observe drift mobility of 420 cm2/Vs and 56 cm2/Vs at T = 1 K and room temperature, respectively.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0633">http://arxiv.org/abs/1308.0633</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-52637598816265948642013-08-06T00:02:00.045-07:002013-08-06T00:02:58.394-07:001308.0685 (V. A. Stephanovich et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0685">The exact solution of the diffusion trapping model of defect profiling<br /> with variable energy positrons</a> [<a href="http://arxiv.org/pdf/1308.0685">PDF</a>]</h2>V. A. Stephanovich, J. Dryzek<a name='more'></a><blockquote class="abstract">We report an exact analytical solution of so-called positron diffusion trapping model. This model have been widely used for the treatment of the experimental data for defect profiling of the adjoin surface layer using the variable energy positron (VEP) beam technique. Hovewer, up to now this model could be treated only numerically with so-called VEPFIT program. The explicit form of the solutions is obtained for the realistic cases when defect profile is described by a discreet step-like function and continuous exponential-like function. Our solutions allow to derive the analytical expressions for typical positron annihilation characteristics including the positron lifetime spectrum. Latter quantity could be measured using the pulsed, slow positron beam. Our analytical results are in good coincidence with both the VEPFIT numerics and experimental data. The presented solutions are easily generalizable for defect profiles of other shapes and can be well used for much more precise treatment of above experimental data.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0685">http://arxiv.org/abs/1308.0685</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-78633093293648988822013-08-06T00:02:00.043-07:002013-08-06T00:02:57.343-07:001308.0705 (Krzysztof Rościszewski et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0705">A possibility of high spin hole states in doped CoO$_2$ layered systems</a> [<a href="http://arxiv.org/pdf/1308.0705">PDF</a>]</h2>Krzysztof Rościszewski, Andrzej M. Oleś<a name='more'></a><blockquote class="abstract">We introduce and investigate an effective five-band model for $t_{2g}$ and $e_g$ electrons to describe doped cobalt oxides with Co$^{3+}$ and Co$^{4+}$ ions in two-dimensional CoO$_2$ triangular lattice layers, as in Na$_{1-x}$CoO$_2$. The effective Hamiltonian includes anisotropic kinetic energy (due to both direct Co-Co and indirect Co-O-Co hoppings), on-site Coulomb interactions parameterized by intraorbital Hubbard repulsion $U$ and full Hund's exchange tensor, crystal-field terms and Jahn-Teller static distortions. We study it using correlated wave functions on $6\times 6$ clusters with periodic boundary conditions. The computations indicate low S=0 spin to high S=2 spin abrupt transition in the undoped systems when increasing strength of the crystal field, while intermediate S=1 spins are not found. Surprisingly, for the investigated realistic Hamiltonian parameters describing low spin states in CoO$_2$ planes, doping generates high $S=\frac{5}{2}$ spins at Co$^{4+}$ ions that are pairwise bound into singlets, seen here as pairs of up and down spins. It is found that such singlet pairs self-organize at higher doping into lines of spins with coexisting antiferromagnetic and ferromagnetic bonds, forming stripe-like structures. The ground states are insulating within the investigated range of doping because computed HOMO-LUMO gaps are never small enough.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0705">http://arxiv.org/abs/1308.0705</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-27274029325345310932013-08-06T00:02:00.041-07:002013-08-06T00:02:56.476-07:001308.0714 (William Paul et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0714">FIM tips in SPM: Apex orientation and temperature considerations on atom<br /> transfer and diffusion</a> [<a href="http://arxiv.org/pdf/1308.0714">PDF</a>]</h2>William Paul, David Oliver, Yoichi Miyahara, Peter Grütter<a name='more'></a><blockquote class="abstract">Atoms transferred to W(111) and W(110) tip apices from the Au(111) surface during tunneling and approach to mechanical contact experiments in STM are characterized in FIM at room temperature and at 158 K. The different activation energies for diffusion on the (111) and (110) tip planes and the experiment temperature are shown to be important factors controlling the extent of changes to the atomic structure of the tip. W(111) tips are much better suited to scanning probe studies which require the characterization of an atomically defined tip and subsequent verification of its integrity in FIM. The statistics of the observed spikes in the tunneling current when the tips are approached to Au(111) are interpreted using a simple model of adatoms diffusing through the STM junction.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0714">http://arxiv.org/abs/1308.0714</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-15481093155203610322013-08-06T00:02:00.039-07:002013-08-06T00:02:55.532-07:001308.0732 (Mohamed B Elgindi et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0732">On the Solvability of an Euler Graphene Beam Subject to Axial<br /> Compressive Load</a> [<a href="http://arxiv.org/pdf/1308.0732">PDF</a>]</h2>Mohamed B Elgindi, Dongming Wei, Tarek M Elgindi<a name='more'></a><blockquote class="abstract">In this paper we formulate the equilibrium equation of a beam made of graphene material subjected to some boundary conditions and acted upon by axial compression and nonlinear lateral constrains as a fourth-order nonlinear boundary value problem. We also formulate the nonlinear eigenvalue for buckling analysis of the beam. We verify the solvability of the buckling problem as an asymptotic expansion in a ratio of the elastoplastic parameters, that the spectrum is bounded away from zero and contains a discrete infinite sequence of eigenvalues.We also verify, for certain ranges of the lateral forces, the solvability of the general equations using energy methods and a suitable iteration scheme.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0732">http://arxiv.org/abs/1308.0732</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-76770797591600008122013-08-06T00:02:00.037-07:002013-08-06T00:02:54.497-07:001308.0767 (Cheng Gong et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0767">Band alignment of two-dimensional transition metal dichalcogenides:<br /> application in tunnel field effect transistors</a> [<a href="http://arxiv.org/pdf/1308.0767">PDF</a>]</h2>Cheng Gong, Hengji Zhang, Weihua Wang, Luigi Colombo, Robert M. Wallace, Kyeongjae Cho<a name='more'></a><blockquote class="abstract">Tunnel field effect transistors (TFETs) based on vertical stacking of two dimensional materials are of interest for low-power logic devices. The monolayer transition metal dichalcogenides (TMDs) with sizable band gaps show promise in building p-n junctions (couples) for TFET applications. Band alignment information is essential for realizing broken gap junctions with excellent electron tunneling efficiencies. Promising couples composed of monolayer TMDs are suggested to be VIB-MeX2 (Me= W, Mo; X= Te, Se) as the n-type source and IVB-MeX2 (Me = Zr, Hf; X= S, Se) as the p-type drain by density functional theory calculations.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0767">http://arxiv.org/abs/1308.0767</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-19503157105342428132013-08-06T00:02:00.035-07:002013-08-06T00:02:53.844-07:001308.0770 (Kazuhiko Seki et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0770">Influence of Energetic Disorder on the Open-circuit Voltage in Organic<br /> Bulkheterojunction Composites</a> [<a href="http://arxiv.org/pdf/1308.0770">PDF</a>]</h2>Kazuhiko Seki, Kazuhiro Marumoto, M. Tachiya<a name='more'></a><blockquote class="abstract">Under open-circuit condition, the current is not extracted and the photogenerated carriers in principle disappear only by recombination. We study the open-circuit voltage, VOC, and transient photovoltage under the influence of bulk recombination in a medium with energetic disorder by using the multiple trapping (MT) model. The key parameter in the MT model is the dispersion parameter, {\alpha}, given by the ratio of thermal energy to the characteristic energy of trap states. We show that VOC depends linearly on the logarithm of the light intensity and the slope depends on {\alpha} of the MT model. Under the continuous irradiation of light, photovoltage response to the weak perturbation by a pulsed light obeys pseudofirst-order decay. The rate as a function of VOC is independent of the dispersion parameter. While, the rate obeys power law as a function of the light intensity and the exponent is given by 1/(1 + {\alpha}), which reduces to 1/2 in the absence of energetic disorder.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0770">http://arxiv.org/abs/1308.0770</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-85585101752891209282013-08-06T00:02:00.033-07:002013-08-06T00:02:52.791-07:001308.0794 (Natalia N. Rozhkova et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0794">Shungite as loosely packed fractal nets of graphene-based quantum dots</a> [<a href="http://arxiv.org/pdf/1308.0794">PDF</a>]</h2>Natalia N. Rozhkova, Elena F. Sheka<a name='more'></a><blockquote class="abstract">The current paper presents the first attempt to recreate the origin of shungite carbon at microscopic level basing on knowledge accumulated by graphene science. The main idea of our approach is that different efficacy of chemical transformation of primarily generated graphene flakes (aromatic lamellae in geological literature), subjected to oxidation/reduction reactions in aqueous environment, lays the foundation of the difference in graphite and shungite derivation under natural conditions. Low-efficient reactions in the case of graphite do not prevent from the formation of large graphite layers while high-efficient oxidation/reduction transforms the initial graphene flakes into those of reduced graphene oxide of ~1nm in size. Multistage aggregation of these basic structural units, attributed to graphene quantum dots of shungite, leads to the fractal structure of shungite carbon thus exhibiting it as a new allotrope of natural carbon. The suggested microscopic view has finds a reliable confirmation when analyzing the available empirical data.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0794">http://arxiv.org/abs/1308.0794</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-14672311460716009592013-08-06T00:02:00.031-07:002013-08-06T00:02:52.114-07:001308.0823 (H. Schenck et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0823">Vector chiral phases in frustrated 2D XY model and quantum spin chains</a> [<a href="http://arxiv.org/pdf/1308.0823">PDF</a>]</h2>H. Schenck, V. L. Pokrovsky, T. Nattermann<a name='more'></a><blockquote class="abstract">The phase diagram of the frustrated 2D XY model is calculated analytically. The chiral (Ising) transition is described by three independent critical exponents which are calculated in $d=5/2-\epsilon$ dimensions. Vortex interaction is short range on small and logarithmic on large scales, if compared with the chiral correlation length $\xi$. The vortex unbinding transitions is triggered by the increase of $\xi$ and occurs before the chiral transition takes place. In a narrow region close to the Lifshitz point a reentrant quasi-ferromagnetic phase appears. Application to antiferromagnetic quantum spin chains and multiferroics are discussed.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0823">http://arxiv.org/abs/1308.0823</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-41998773258087368152013-08-06T00:02:00.029-07:002013-08-06T00:02:51.335-07:001308.0826 (A. Barfuss et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0826">Elemental Topological Insulator with a Tunable Fermi Level: Strained<br /> α-Sn on InSb(001)</a> [<a href="http://arxiv.org/pdf/1308.0826">PDF</a>]</h2>A. Barfuss, L. Dudy, M. R. Scholz, H. Roth, P. Höpfner, C. Blumenstein, G. Landolt, J. H. Dil, N. C. Plumb, M. Radovic, A. Bostwick, E. Rotenberg, A. Fleszar, G. Bihlmayer, D. Wortmann, G. Li, W. Hanke, R. Claessen, J. Schäfer<a name='more'></a><blockquote class="abstract">We report on the epitaxial fabrication and electronic properties of a topological phase in strained \alpha-Sn on InSb. The topological surface state forms in the presence of an unusual band order not based on direct spin-orbit coupling, as shown in density functional and GW slab-layer calculations. Angle-resolved photoemission including spin detection probes experimentally how the topological spin-polarized state emerges from the second bulk valence band. Moreover, we demonstrate the precise control of the Fermi level by dopants.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0826">http://arxiv.org/abs/1308.0826</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-17024977113595516372013-08-06T00:02:00.027-07:002013-08-06T00:02:50.482-07:001308.0866 (B. Y. Sun et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0866">Negative differential transmission in graphene</a> [<a href="http://arxiv.org/pdf/1308.0866">PDF</a>]</h2>B. Y. Sun, M. W. Wu<a name='more'></a><blockquote class="abstract">By using the Kubo linear response theory with the Keldysh Green function approach, we investigate the mechanism leading to the negative differential transmission in system with the equilibrium electron density much smaller than the photon-excited one. It is shown that the negative differential transmission can appear at low probe-photon energy (in the order of the scattering rate) or at high energy (much larger than the scattering rate). For the low probe-photon energy case, the negative differential transmission is found to come from the increase of the intra-band conductivity due to the large variation of electron distribution after the pumping. As for the high probe-photon energy case, the negative differential transmission is shown to tend to appear with the hot-electron temperature being closer to the equilibrium one and the chemical potential higher than the equilibrium one but considerably smaller than half of the probe-photon energy. We also show that this negative differential transmission can come from both the inter- and the intra-band components of the conductivity. Especially, for the inter-band component, its contribution to the negative differential transmission is shown to come from both the Hartree-Fock self-energy and the scattering. Furthermore, the influence of the Coulomb-hole self-energy is also addressed.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0866">http://arxiv.org/abs/1308.0866</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-87440545941967225162013-08-06T00:02:00.025-07:002013-08-06T00:02:49.514-07:001308.0873 (Yi-Guo Xu et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0873">Vacancy trapping behaviors of hydrogen atoms in Ti3SiC2: a<br /> first-principles study</a> [<a href="http://arxiv.org/pdf/1308.0873">PDF</a>]</h2>Yi-Guo Xu, Xue-Dong Ou, Xi-Ming Rong<a name='more'></a><blockquote class="abstract">The behaviors of hydrogen (H) in MAX phase material Ti3SiC2 have been investigated using first-principles method. We show that a single H atom prefers to stay 1.01 {\AA} down of the Si vacancy with solution energy of about -4.07 eV, lowerthan that in bulk Ti3SiC2. Multi H atoms exhibit a repulsive interaction at the Si vacancy. And up to five H atoms can be trapped by a Si vacancy without H2 molecules formation. These results suggest the strong vacancy trapping characteristic of H atoms in Ti3SiC2. Meanwhile, the barrier for H diffusion from an interstitial site to a vacancy is 1.17 eV, which is much larger than that in metals, indicating that to some extent H atoms can not easily migrate or aggregate to form bubble in Ti3SiC.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0873">http://arxiv.org/abs/1308.0873</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-33628033850017448652013-08-06T00:02:00.023-07:002013-08-06T00:02:48.463-07:001308.0913 (Yu. A. Freiman et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0913">Sound velocities in H_2, He, and H_2-He solid mixtures under pressure</a> [<a href="http://arxiv.org/pdf/1308.0913">PDF</a>]</h2>Yu. A. Freiman, Alexei Grechnev, S. M. Tretyak, A. F. Goncharov, C. -s. Zha, R. J. Hemley<a name='more'></a><blockquote class="abstract">Bulk and shear sound velocities in solid hydrogen, helium and hydrogen-helium mixtures under pressure are calculated using two complementary approaches: semi-empirical lattice dynamics based on the many-body intermolecular potentials and density-functional theory in generalized gradient approximation. The thermodynamics of the He-H$_2$ mixture has been treated within the additive-volume approximation. Our results are compared with the first-principle diffusion Monte-Carlo calculation of Alavi {\it et al.} (Science {\bf 269}, 1252 (1995)) and the semi-empirical work of Duffy {\it et al.} (Science {\bf 263}, 1590 (1994)) which employs the two-body Young-Ross potential. It is shown that the stiffer the potential is, the more the sound velocity is overestimated. Comparison with the data obtained from the observation of global oscillations of Jupiter is also made.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0913">http://arxiv.org/abs/1308.0913</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-29969173832891271082013-08-06T00:02:00.021-07:002013-08-06T00:02:47.408-07:001308.0927 (Z. V. Pchelkina et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0927">Ab initio investigation of the exchange interactions in<br /> Bi$_2$Fe$_4$O$_9$: The Cairo pentagonal lattice compound</a> [<a href="http://arxiv.org/pdf/1308.0927">PDF</a>]</h2>Z. V. Pchelkina, S. V. Streltsov<a name='more'></a><blockquote class="abstract">We present the \emph{ab initio} calculation of the electronic structure and magnetic properties of Bi$_2$Fe$_4$O$_9$. This compound crystallizes in the orthorhombic crystal structure with the Fe$^{3+}$ ions forming the Cairo pentagonal lattice implying strong geometric frustration. The neutron diffraction measurements reveal nearly orthogonal magnetic configuration, which at first sight is rather unexpected since it does not minimize the total energy of the pair of magnetic ions coupled by the Heisenberg exchange interaction. Here we calculate the electronic structure and exchange integrals of Bi2Fe4O9 within the LSDA+U method. We obtain three different in-plane (J3=36 K, J4=73 K, J5=23 K) and two interplane (J1=10 K, J2=12 K) exchange parameters. The derived set of exchange integrals shows that the realistic description of Bi2Fe4O9 needs a more complicated model than the ideal Cairo pentagonal lattice with only two exchange parameters in the plane. However, if one takes into account only two largest exchange integrals, then according to the ratio x\equiv J3/J4=0.49<\sqrt{2} (a critical parameter for the ideal Cairo pentagonal lattice, see. Ref.~1) the ground state should be the orthogonal magnetic configuration in agreement with experiment. The microscopic origin of different exchange interactions is also discussed.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0927">http://arxiv.org/abs/1308.0927</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-15671738807120228702013-08-06T00:02:00.019-07:002013-08-06T00:02:46.450-07:001308.0934 (Nuala M. Caffrey et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0934">Atomic-scale inversion of spin polarization at an<br /> organic-antiferromagnetic interface</a> [<a href="http://arxiv.org/pdf/1308.0934">PDF</a>]</h2>Nuala M. Caffrey, Paolo Ferriani, Simone Marocchi, Stefan Heinze<a name='more'></a><blockquote class="abstract">Using first-principles calculations, we show that the magnetic properties of a two-dimensional antiferromagnetic transition-metal surface are modified on the atomic scale by the adsorption of small organic molecules. We consider benzene (C6H6), cyclooctatetraene (C8H8) and a small transition metal - benzene complex (BzV) adsorbed on a single atomic layer of Mn deposited on the W(110) surface -- a surface which exhibits a nearly antiferromagnetic alignment of the magnetic moments in adjacent Mn rows. Due to the spin-dependent hybridization of the molecular pz orbitals with the d states of the Mn monolayer there is a significant reduction of the magnetic moments in the Mn film. Furthermore, the spin-polarization at this organic-antiferromagnetic interface is found to be modulated on the atomic scale, both enhanced and inverted, as a result of the molecular adsorption. We show that this effect can be resolved by spin-polarized scanning tunneling microscopy (SP-STM). Our simulated SP-STM images display a spatially-dependent spin-resolved vacuum charge density above an adsorbed molecule -- i.e., different regions above the molecule sustain different signs of spin polarization. While states with s and p symmetry dominate the vacuum charge density in the vicinity of the Fermi energy for the clean magnetic surface, we demonstrate that after a molecule is adsorbed those d-states, which are normally suppressed due to their symmetry, can play a crucial role in the vacuum due to their interaction with the molecular orbitals. We also model the effect of small deviations from perfect antiferromagnetic ordering, induced by the slight canting of magnetic moments due to the spin spiral ground state of Mn/W(110).</blockquote>View original: <a href="http://arxiv.org/abs/1308.0934">http://arxiv.org/abs/1308.0934</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-78833008799001248322013-08-06T00:02:00.017-07:002013-08-06T00:02:45.130-07:001308.0937 (Gowrishankar Seshadri et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0937">Optimum Size of Nanorods for Heating Application</a> [<a href="http://arxiv.org/pdf/1308.0937">PDF</a>]</h2>Gowrishankar Seshadri, Rochish Thaokar, Anurag Mehra<a name='more'></a><blockquote class="abstract">Magnetic nanoparticles (MNP's) have become increasingly important in heating applications such as hyperthermia treatment of cancer due to their ability to release heat when a remote external alternating magnetic field is applied. It has been shown that the heating capability of such particles varies significantly with the size of particles used. In this paper, we theoretically evaluate the heating capability of rod-shaped MNP's and identify conditions under which these particles display highest efficiency. For optimally sized monodisperse particles, the power generated by rod-shaped particles is found to be equal to that generated by spherical particles. However, for particles which have a dispersion in size, rod-shaped particles are found to be more effective in heating as a result of the greater spread in the power density distribution curve. Additionally, for rod-shaped particles, a dispersion in the radius of the particle contributes more to the reduction in loss power when compared to a dispersion in the length. We further identify the optimum size, i.e the radius and length of nanorods, given a bi-variate log-normal distribution of particle size in two dimensions.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0937">http://arxiv.org/abs/1308.0937</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-64474745302871785412013-08-06T00:02:00.015-07:002013-08-06T00:02:44.451-07:001308.0963 (Martin Jesenko et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0963">Closure and commutability results for Gamma-limits and the geometric<br /> linearization and homogenization of multi-well energy functionals</a> [<a href="http://arxiv.org/pdf/1308.0963">PDF</a>]</h2>Martin Jesenko, Bernd Schmidt<a name='more'></a><blockquote class="abstract">Under a suitable notion of equivalence of integral densities we prove a $\Gamma$-closure theorem for integral functionals: The limit of a sequence of $\Gamma$-convergent families of such functionals is again a $\Gamma$-convergent family. Its $\Gamma$-limit is the limit of the $\Gamma$-limits of the original problems. This result not only provides a common basic principle for a number of linearization and homogenization results in elasticity theory. It also allows for new applications as we exemplify by proving that geometric linearization and homogenization of multi-well energy functionals commute.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0963">http://arxiv.org/abs/1308.0963</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-4904894941881744982013-08-06T00:02:00.013-07:002013-08-06T00:02:43.418-07:001308.0967 (Julia A. Mundy et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0967">Hetero-epitaxial EuO Interfaces Studied by Analytic Electron Microscopy</a> [<a href="http://arxiv.org/pdf/1308.0967">PDF</a>]</h2>Julia A. Mundy, Daniel Hodash, Alexander Melville, Rainer Held, Thomas Mairoser, David A. Muller, Lena F. Kourkoutis, Andreas Schmehl, Darrell G. Schlom<a name='more'></a><blockquote class="abstract">With nearly complete spin polarization, the ferromagnetic semiconductor europium monoxide could enable next-generation spintronic devices by providing efficient ohmic spin injection into silicon. Spin injection is greatly affected by the quality of the interface between the injector and silicon. Here, we use atomic-resolution scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy to directly image and chemically characterize a series of EuO|Si and EuO|YAlO3 interfaces fabricated using different growth conditions. We identify the presence of europium silicides and regions of disorder at the EuO|Si interfaces, imperfections that could significantly reduce spin injection efficiencies via spin-flip scattering.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0967">http://arxiv.org/abs/1308.0967</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-38016743130470533362013-08-06T00:02:00.011-07:002013-08-06T00:02:42.125-07:001308.0969 (Kurt Stokbro et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0969">First principles study of pentacene on Au(111)</a> [<a href="http://arxiv.org/pdf/1308.0969">PDF</a>]</h2>Kurt Stokbro, Søren Smidstrup<a name='more'></a><blockquote class="abstract">We investigate the atomic and electronic structure of a single layer of pentacene on the Au(111) surface using density functional theory. To find the candidate structures we strain match the pentacene crystal geometry with the Au(111) surface, in this way we find pentacene overlayer structures with a low strain. We show that the geometries obtained with this approach has lower energy than previous proposed surface geometries of pentacene on Au(111). We also show that the geometry and workfunction of the obtained structures are in excellent agreement with experimental data.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0969">http://arxiv.org/abs/1308.0969</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-91897273702443692802013-08-06T00:02:00.009-07:002013-08-06T00:02:38.865-07:001308.0976 (D. Wang et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0976">Unified description of laser induced magnetization dynamics in<br /> ferromagnetic metals</a> [<a href="http://arxiv.org/pdf/1308.0976">PDF</a>]</h2>D. Wang, A. J. Schellekens, B. Koopmans<a name='more'></a><blockquote class="abstract">Laser induced ultrafast demagnetization in ferromagnetic metals was discovered almost 20 years ago, but currently there is still lack of consensus on the microscopic mechanism responsible for the corresponding transfer of angular momentum and energy between electron, lattice and spin subsystems. A distinct, but intrinsically correlated phenomenon occurring on a longer timescale is the magnetization oscillation after the ultrafast demagnetization process, if a magnetic field is applied to tilt the magnetization vector away from its easy direction. Based on a microscopic three temperature model, which adapts the Elliott-Yafet scattering between electrons and phonons to explain the ultrafast demagnetization process, a unified theoretical description of both phenomena is proposed. In our unified description, the ultrafast demagnetization is mediated by the Elliott-Yafet scattering, while the resultant change of anisotropy is responsible for the following magnetization oscillation. Theoretical prediction with plausible parameters are in excellent agreement with experimental data measured using time resolved magneto-optical Kerr effect in an in-plane magnetized Pt/Co/Pt thin film with perpendicular interface anisotropy. This agreement between theory and experiment provides more insight into the microscopic mechanism behind the ultrafast demagnetization process. Furthermore, the model proposed here could also be used to study the magnetization dynamics triggered by the modification of anisotropy field through ultrafast laser heating.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0976">http://arxiv.org/abs/1308.0976</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-74545781664683063462013-08-06T00:02:00.007-07:002013-08-06T00:02:37.922-07:001308.0986 (Jonathan Chico et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0986">Thermally driven domain wall motion in Fe on W(1 1 0)</a> [<a href="http://arxiv.org/pdf/1308.0986">PDF</a>]</h2>Jonathan Chico, Corina Etz, Lars Bergqvist, Olle Eriksson, Jonas Fransson, Anna Delin, Anders Bergman<a name='more'></a><blockquote class="abstract">It has recently been shown that domain walls in ferromagnets can be moved in the presence of thermal gradients. In this work we study the motion of narrow domain walls in low-dimensional systems when subjected to thermal gradients. The system chosen is a monolayer of Fe on W(1 1 0) which is known to exhibit a large anisotropy while having a soft exchange, resulting in a very narrow domain wall. The study is performed by means of atomistic spin dynamics simulations coupled to first-principles calculations. By subjecting the systems to a thermal gradient we observe a temperature dependent movement of the domain wall as well as changes of the spatial magnetization profile of the system. The thermal gradient always makes the domain wall move towards the hotter region of the sample with a velocity proportional to the gradient. The material specific study is complemented by model simulations to discern the interplay between the thermal gradient, magnetic anisotropy and the exchange interaction, and shows that the larger DW velocities are found for materials with low magnetic anisotropy. The relatively slow DW motion of the Fe/W(1 1 0) system is hence primarily caused by its large magnetic anisotropy.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0986">http://arxiv.org/abs/1308.0986</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0tag:blogger.com,1999:blog-2419671800634804822.post-73822209857507234402013-08-06T00:02:00.005-07:002013-08-06T00:02:36.975-07:001308.0993 (U. Atxitia et al.)<h2 class="title"><a href="http://arxiv.org/abs/1308.0993">Controlling the Polarity of the Transient Ferromagnetic-Like State in<br /> Ferrimagnets</a> [<a href="http://arxiv.org/pdf/1308.0993">PDF</a>]</h2>U. Atxitia, J. Barker, R. W. Chantrell, O. Chubykalo-Fesenko<a name='more'></a><blockquote class="abstract">After the application of an ultrashort laser pulse, the antiferromagnetic alignment in rare earth-transition metal alloys can temporarily become ferromagnetic with the rare-earth polarity. Proposed models merely describe this effect, without showing the route for its manipulation. Here we use extensive atomistic spin model simulations and micromagnetic theory for ferrimagnets at elevated temperatures to predict that the polarity of this transient ferromagnetic-like state can be controlled by initial temperature. We show that this arises because the magnetic response of each lattice has a different temperature dependence, at low temperatures the transition metal responds faster than the rare earth, while at high temperatures this role is interchanged. Our findings contribute to the physical understanding and control of this state and thus open new perspectives for its use in ultrafast magnetic devices.</blockquote>View original: <a href="http://arxiv.org/abs/1308.0993">http://arxiv.org/abs/1308.0993</a>C.P.R.http://www.blogger.com/profile/13598012384534951656noreply@blogger.com0