Single-crystal growth and superconducting properties of LiFeAs

Lee, Bumsung,Khim, Seunghyun,Kim, Jung Soo,Stewart, G. R.,Kim, Kee Hoon Editions de Physique 2010 Europhysics letters Vol.91 No.6

<P>We report the successful growth of high-quality single crystals of LiFeAs with lateral sizes up to 5×5 mm<SUP>2</SUP> by the Sn-flux method. Electrical resistivity studies reveal that the superconducting onset temperature is 18.2 K with a transition width less than 1.1 K and the ratio of room temperature to residual resistivity is about 24. Bulk superconductivity is supported by perfect shielding in the magnetic susceptibility and a clear jump in the specific heat <I>C</I><SUB>p</SUB>, resulting in Δ<I>C</I><SUB>p</SUB>/<I>T</I> ≈ 20.0 mJ/mol K<SUP>2</SUP>. Upper-critical-field slopes of d<I>H</I><SUB>c2</SUB><SUP><I>c</I></SUP>/d<I>T</I> ≈−1.39 and d<I>H</I><SUB>c2</SUB><SUP><I>ab</I></SUP>/d<I>T</I> ≈−2.99 T/K near <I>T</I><SUB>c</SUB> predict zero-temperature upper critical fields of <I>H</I><SUB>c2</SUB><SUP><I>c</I></SUP>(0) ≈ 17.2 and <I>H</I><SUB>c2</SUB><SUP><I>ab</I></SUP>(0) ≈ 36.9 T and coherence lengths of ξ<SUB><I>ab</I></SUB>=4.4 and ξ<SUB><I>c</I></SUB>=2.0 nm in a single-band model. This result points to a modest superconducting anisotropy about 2.3 in LiFeAs.</P>

Reply to the Comment by A. Gadomski

Choi, M. Y.,Choi, H.,Fortin, J.-Y.,Choi, J. Editions de Physique 2010 Europhysics letters Vol.89 No.4

Stochastic resonance of a flexible chain crossing over a barrier

Asfaw, Mesfin,Sung, Wokyung Editions de Physique 2010 Europhysics letters Vol.90 No.3

<P>We study the stochastic resonance (SR) of a flexible polymer surmounting a bistable-potential barrier. Due to the flexibility that can enhance crossing rate and change chain conformations at the barrier, the SR behaviors manifest many features of the <I>bona fide</I> entropic SR, such as the power amplification peaks at optimal chain lengths and elastic constants as well as the optimal noise strengths. The pronounced peaks that emerge depending on the chain lengths and conformation states suggest novel means of manipulating biopolymers, such as efficient separation methods using undulating channels.</P>

Effects of high-order fields of a tightly focused laser pulse on relativistic nonlinear Thomson scattered radiation by a relativistic electron

Lee, K.,Chung, S. Y.,Park, S. H.,Jeong, Y. U.,Kim, D. Editions de Physique 2010 Europhysics letters Vol.89 No.6

<P>The effects of high-order fields of a tightly focused laser beam on the generation of relativistic nonlinear Thomson scattered radiations by a relativistic electron were investigated through numerical simulations. The high-order terms of the laser fields obtained by a series expansion in a diffraction angle were found to strongly affect the nonlinear Thomson scattered radiations by an increase in the transverse acceleration when electrons co-propagate with the laser pulse: the spectral range was broadened by a large factor and the angular power was enhanced by seven orders of magnitude compared to the corresponding values for a paraxial Gaussian laser beam. For electron energies higher than 200 MeV, the scaling exponent of the peak angular power with respect to the initial electron energy was also found to increase by a factor of 2.5 compared to the case of the paraxial laser beam.</P>

Synchronization of toggle switches coupled through a common inhibitor

Nguyen, Cuong,Han, Seung Kee Editions de Physique 2010 Europhysics letters Vol.90 No.1

<P>We have investigated the synchronization of discrete jump events arising from two coupled toggle switches. The toggle switches, composed of positive-feedback loops with bistability, were coupled through two branches of double negative-feedback loops induced by a common inhibitor. From a bifurcation analysis of the coupled feedback systems, we show that there are two different mechanisms of synchronization. One is based on the merging of bistabilities and the other on the suppression of intrinsic bistability by the strong bistability induced by the common inhibitor. The dependence of the synchronization on the strength of the double negative-feedback loop is presented.</P>

Valence band of graphite oxide

Jeong, Hae Kyung,Yang, Cheolsoo,Kim, Bong Soo,Kim, Ki-jeong Editions de Physique 2010 Europhysics letters Vol.92 No.3

<P>We investigated the valence band structure of graphite oxide by photoelectron spectroscopy at the Pohang Accelerator Laboratory, Korea. The typical <I>sp</I><SUP>2</SUP> hybridization states found in graphite were also seen in graphite oxide. However, the π state disappeared near the Fermi level because of bonding between the π and oxygen-related states originating from graphite oxide, indicating electron transfer from graphite to oxygen and resulting in a downward shift of the highest occupied molecular orbital (HOMO) state to higher binding energies. The band gap opening increased to about 1.8 eV, and additional oxygen-related peaks were observed at 8.5 and 27 eV. The electronic states of graphite were also found in graphite oxide. Thus, graphite oxide has an electronic structure similar to that of pristine graphite except for the states near the Fermi level and oxygen-related states.</P>

Are better conductors more rigid?

Eom, Young-Ho,Jeong, Hawoong,Orland, Henri,Yi, Juyeon Editions de Physique 2006 Europhysics letters Vol.76 No.2

<P>The variation of the bending stiffness of various materials is studied from the point of view of the electronic band characteristics. As far as the electronically generated bending stiffness κ<SUB><I>e</I></SUB> (which we refer to as electro-stiffness) is concerned, the relevant factors are the orbital overlap <I>t</I>, the gap width <I>u</I> between the valence band and the conduction band, and the electron filling fraction γ. A perturbative calculation leads to the approximate expression κ<SUB><I>e</I></SUB> ∼ <I>t</I><SUP>2</SUP>/&surd;<I>u</I><SUP>2</SUP> + <I>t</I><SUP>2</SUP>. This shows that materials with a large overlap and narrow band gap should be stiffer. The electro-stiffness also depends on the electron filling-fraction. We find that κ<SUB><I>e</I></SUB>(γ) ≤ κ<SUB><I>e</I></SUB>(1/2). These kinds of behavior are confirmed by numerical calculations. In addition, we study the variation in the projected length of flexible molecules under a voltage bias. The nonlinear variation of the bending rigidity is shown to give rise to a length contraction or dilation, depending on the voltage bias.</P>

Orbital ordering and enhanced magnetic frustration of strained BiMnO₃ thin films

Yang, C.-H.,Koo, T. Y.,Lee, S.-H.,Song, C.,Lee, K.-B.,Jeong, Y. H. Editions de Physique 2006 Europhysics letters Vol.74 No.2

<P>Epitaxial thin films of multiferroic perovskite BiMnO<SUB>3</SUB> were synthesized on SrTiO<SUB>3</SUB> substrates, and orbital ordering and magnetic properties of the thin films were investigated. The ordering of the Mn<SUP>3+</SUP> <I>e</I><SUB><I>g</I></SUB> orbitals at a wave vector (¼¼¼) was detected by Mn <I>K</I>-edge resonant X-ray scattering. This peculiar orbital order inherently contains magnetic frustration. While bulk BiMnO<SUB>3</SUB> is known to exhibit simple ferromagnetism, the frustration enhanced by in-plane compressive strains in the films brings about cluster-glass-like properties.</P>

The effect of nano-sized superparamagnetic particles on the dielectric constant of polyimide

Kang, J. H.,Park, C. E.,Jeong, Y. H. Editions de Physique 2006 Europhysics letters Vol.74 No.5

<P>We propose that dynamically induced internal fields be utilized in obtaining materials with low dielectric constant at high frequencies. We also demonstrate the validity of the proposal by measuring the effective dielectric constant of polyimide dispersed with nano-sized Ni<SUB>0.6</SUB>Zn<SUB>0.4</SUB>Fe<SUB>2</SUB>O<SUB>4</SUB> particles. The dielectric constant of a polyimide composite measured at 1 MHz increases as a function of Ni<SUB>0.6</SUB>Zn<SUB>0.4</SUB>Fe<SUB>2</SUB>O<SUB>4</SUB> contents when the diameter of the embedded particles is approximately 20 nm. On the other hand, the measured dielectric constant of a composite containing superparamagnetic particles of diameter approximately 6 nm showed much smaller value than that of pure polyimide. These contrasting behaviors indicate that the induced internal fields are in operation, in reducing the dielectric constant, for a material with superparamagnetic particles.</P>

Landau damping effects on the low-energy bremsstrahlung process in a quantum plasma

Kim, Hwa-Min,Jung, Young-Dae Editions de Physique 2007 Europhysics letters Vol.78 No.3

<P>The Landau damping effects on the low-energy bremsstrahlung process are investigated for both the soft- and hard-photon radiations in a quantum plasma composed of electrons and holes. The impact parameter method is applied to the motion of the projectile in order to investigate the variation of the bremsstrahlung cross-section as a function of the effective Debye length, impact parameter, and radiation photon energy. The results show that the Landau damping effects significantly enhance the bremsstrahlung radiation cross-section. It is important to note that the domain of the Landau damping effects is extended with increasing the radiation photon energy.</P>