arXiv subjects

J. S. Kim

Publications and source records attributed to J. S. Kim.

At least 55 records · Page 3Linked to original sources

arXiv2013

Young stellar population and ongoing star formation in the HII complex Sh2-252

In this paper an extensive survey of the star forming complex Sh2-252 has been undertaken with an aim to explore its hidden young stellar population as well as to understand the structure and star formation history. This complex is composed of five embedded clusters associated with the sub-regions A, C, E, NGC 2175s and Teu 136. Using 2MASS-NIR and Spitzer-IRAC, MIPS photometry we identified 577 young stellar objects (YSOs), of which, 163 are Class I, 400 are Class II and 14 are transition disk YSOs. Spatial distribution of the candidate YSOs shows that they are mostly clustered around the sub-regions in the western half of the complex, suggesting enhanced star formation activity towards its west. Using the spectral energy distribution and optical colour-magnitude diagram based age analyses, we derived probable evolutionary status of the sub-regions of Sh2-252. Our analysis shows that the region A is the youngest (~ 0.5 Myr), the regions B, C and E are of similar evolutionary stage (~ 1-2 Myr) and the clusters NGC 2175s and Teu 136 are slightly evolved (~ 2-3 Myr). Morphology of the region in the 1.1 mm map shows a semi-circular shaped molecular shell composed of several clumps and YSOs bordering the western ionization front of Sh2-252. Our analyses suggest that next generation star formation is currently under way along this border and that possibly fragmentation of the matter collected during the expansion of the HII region as one of the major processes responsible for such stars. We observed the densest concentration of YSOs (mostly Class I, ~ 0.5 Myr) at the western outskirts of the complex, within a molecular clump associated with water and methanol masers and we suggest that it is indeed a site of cluster formation at a very early evolutionary stage, sandwiched between the two relatively evolved CHII regions A and B.

astro-ph.SR↗

arXiv2013

Constraints on the R-Parity-violating minimal supersymmetric standard model with neutrino masses from multilepton studies at the LHC

In a recent paper, we proposed a hierarchical ansatz for the lepton-number-violating trilinear Yukawa couplings of the R-parity-violating minimal supersymmetric standard model. As a result, the number of free parameters in the lepton-number-violating sector was reduced from 36 to 6. Neutrino oscillation data fixes these six parameters, which also uniquely determines the decay modes of the lightest supersymmetric particle and thus governs the collider signature at the LHC. A typical signature of our model consists of multiple leptons in the final state and significantly reduced missing transverse momentum compared to models with R-parity conservation. In this work, we present exclusion limits on our model based on multilepton searches performed at the Large Hadron Collider with 7 TeV center-of-mass energy in 2011 while accommodating a 125 GeV Higgs.

hep-ph↗

arXiv2012

$a-b$ Plane Point Contact Spectroscopy measurements of optimally Cobalt doped Ba-122 iron-pnictide superconductors

Point contact spectroscopy (PCS) is a technique which can reveal the size and symmetry of a superconducting gap ($Δ$) and is especially useful for new materials such as the iron-based superconductors. PCS is usually employed in conjunction with the extended Blonder-Tinkham-Klapwijk (BTK) model which is used to extract information such as the existence of nodes in $Δ$ from PCS data obtained on unconventional superconductors. The BTK model uses a dimensionless parameter $Z$ to quantify the barrier strength across a normal metal - superconductor junction. We have used a unique feature of PCS which allows variation of $Z$ to obtain crucial information about $Δ$. We report our $Z$-dependent, $a-b$ plane PCS measurements on single crystals of the iron-based superconductor BaFe$_{2-x}$Co$_{x}$As$_{2}$. Our measurements show that BaFe$_{2-x}$Co$_{x}$As$_{2}$ ($x$=0.148) is a superconductor with two gaps which does not contain any nodes. The $Z$ dependent point contact spectra rule out a pure $d$ symmetry and the gaps at optimal doping have negligible anisotropy.

cond-mat.supr-con↗

arXiv2012

Specific Heat vs Field of LiFe1-xCuxAs

LiFeAs is one of the new class of iron superconductors with a bulk onset Tc in the 15-17 K range. We report on the specific heat characterization of single crystal material prepared from self-flux growth techniques with significantly improved properties, including a much decreased residual gamma (proportional to C/T as T->0) in the superconducting state. Thus, in contrast to previous explanations of a finite residual gamma in LiFeAs being due to intrinsic states in the superconducting gap, the present work shows that such a finite residual gamma in LiFeAs is instead a function of sample quality. Further, since LiFeAs has been characterized as nodeless with multiple superconducting gaps, we report here on its specific heat properties in zero and applied magnetic fields to compare to similar results on nodal iron superconductors. For comparison, we also investigate LiFe0.98Cu0.02As, which has the reduced Tc of 9 K and Hc2 of 15 T. Interestingly, although presumably both LiFeAs and LiFe0.98Cu0.02As are nodeless, they clearly show a non-linear dependence of the electronic density of states (proportional to the specific heat gamma) at the Fermi energy in the mixed state with applied field similar to the Volovik effect for nodal superconductors. However, rather than nodal behavior, the satisfactory comparison with a recent theory for gamma(H) for a two isotropic gap superconductor in the presence of impurities argues for nodeless behavior. Thus, in terms of specific heat in magnetic field, LiFeAs can serve as the prototypical multiband, nodeless iron superconductor.

cond-mat.supr-con↗

arXiv2012

Microscopic coexistence of superconductivity and antiferromagnetism in underdoped Ba(Fe1-xRux)2As2

We use $^{75}$As nuclear magnetic resonance (NMR) to investigate the local electronic properties of Ba(Fe$_{1-x}$Ru$_{x}$)$_2$As$_2$ ($x =$ 0.23). We find two phase transitions, to antiferromagnetism at $T_N \approx$ 60 K and to superconductivity at $T_C \approx$ 15 K. Below $T_N$, our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of 0.4 $μ_B$/Fe. The spin-lattice relaxation rate $1/^{75}T_1$ is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below $T_C$, $1/^{75}T_1$ on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity.

cond-mat.supr-con↗

arXiv2012

The Effects of Post-Thermal Annealing on the Emission Spectra of GaAs/AlGaAs Quantum Dots grown by Droplet Epitaxy

We fabricated GaAs/AlGaAs quantum dots by droplet epitaxy method, and obtained the geometries of the dots from scanning transmission electron microscopy data. Post-thermal annealing is essential for the optical activation of quantum dots grown by droplet epitaxy. We investigated the emission energy shifts of the dots and underlying superlattice by post-thermal annealing with photoluminescence and cathodoluminescence measurements, and specified the emissions from the dots by selectively etching the structure down to a lower layer of quantum dots. We studied the influences of the degree of annealing on the optical properties of the dots from the peak shifts of the superlattice, which has the same composition as the dots, since the superlattice has uniform and well-defined geometry. Theoretical analysis provided the diffusion length dependence of the peak shifts of the emission spectra.

cond-mat.mtrl-sci↗

arXiv2012

Specific Heat Discontinuity DeltaC vs Tc in Annealed Ba(Fe1-xCox)2As2

The low temperature specific heat of annealed single crystal samples of Ba(Fe1-xCox)2As2 with compositions spanning the entire superconducting phase diagram was measured. Effort was made to discover the best annealing schedule to maximize Tc and minimize transition width in these samples. Values of deltaC/Tc normalized to 100% superconducting volume fractions varied proportionally to Tc^alpha. Within a rather narrow error bar of +-0.15, the exponent alpha was the same (approximately 2) over a range of compositions (0.055 < x < 0.15) around the optimal concentration, x=0.08, where Tc is the maximum in the phase diagram. Thus, whether the superconductivity was coexistent with magnetism (underdoped) or not (overdoped) did not affect the non-BCS variation (alpha nearly 2 instead of 0.8-0.9 for BCS superconductors) of deltaC/Tc with Tc. The annealed samples in the present work, with increased deltaC/Tc and Tc values compared to previous results for the Ba(Fe1-xCox)2As2 alloy system, suggest that the intrinsic value of the exponent alpha for the iron superconductors, when the samples are annealed, with fewer defects, may indeed be even further from the BCS value of 0.8-0.9 than previously thought.

cond-mat.supr-con↗

arXiv2012

Testing neutrino masses in the R-parity violating minimal supersymmetric standard model with LHC results

Within the R-parity violating minimal supersymmetric standard model (MSSM), we use a hierarchical ansatz for the lepton-number violating trilinear Yukawa couplings by relating them to the corresponding Higgs-Yukawa couplings. This ansatz reduces the number of free parameters in the lepton-number violating sector from 36 to 6. Baryon-number violating terms are forbidden by imposing the discrete gauge symmetry Baryon Triality. We fit the lepton-number violating parameters to the most recent neutrino oscillation data, including the mixing angle theta13 found by Daya Bay. We find that we obtain phenomenologically viable neutrino masses and mixings only in the case of normal ordered neutrino masses and that the lepton-number violating sector is unambiguously determined by neutrino oscillation data. We discuss the resulting collider signals for the case of a neutralino as well as a scalar tau lightest supersymmetric particle. We use the ATLAS searches for multi-jet events and large transverse missing momentum in the 0, 1 and 2 lepton channel with 7 TeV center-of-mass energy in order to derive exclusion limits on the parameter space of this R-parity violating supersymmetric model.

hep-ph↗

arXiv2012

Specific Heat To Hc2: Evidence for Nodes or Deep Minima in the Superconducting Gap of Under- and Overdoped Ba(Fe1-xCox)2As2

Low temperature specific heat, C, in magnetic fields up to Hc2 is reported for underdoped Ba(Fe0.955Co0.045)2As2 (Tc=8 K) and for three overdoped samples Ba(Fe1-xCox)2As2 (x=0.103, 0.13, and 0.15, Tc=17.2, 16.5, and 11.7 K respectively). Previous measurements of thermal conductivity (as a function of temperature and field) and penetration depth on comparable composition samples gave some disagreement as to whether there was fully gapped/nodal behavior in the under-/overdoped materials respectively. The present work shows that the measured behavior of the specific heat gamma (proportional to C/T as T->0, i. e. a measure of the electronic density of states at the Fermi energy) as a function of field approximately obeys gamma proportional to H**(0.5 +- 0.1), similar to the Volovik effect for nodal superconductors, for both the underdoped and the most overdoped Co samples. However, for the two overdoped compositions x=0.103 and 0.13, the low field (H < 10 T) data show a Volovik-like behavior of gamma proportional to H**(0.3-0.4), followed by an inflection point, followed at higher fields by gamma proportional to H**1. We argue that within the 2-band theory of superconductivity, an inflection point may occur if the interband coupling is dominant.

cond-mat.supr-con↗

arXiv2012

CuBr2-A New Multiferroic Material with High Critical Temperature

A new multiferroic material, CuBr2, is reported for the first time. CuBr2 has not only a high transition temperature (close to liquid nitrogen temperature) but also low dielectric loss and strong magnetoelectric coupling. These findings reveal the importance of anion effects in the search for the high temperature multiferroics materials among these low-dimensional spin systems.

cond-mat.str-el↗

arXiv2012

Electron spin resonance in Eu based Fe pnictides

The phase diagrams of EuFe$_{2-x}$Co$_x$As$_2$ $(0 \leq x \leq 0.4)$ and EuFe$_2$As$_{2-y}$P$_y$ $(0 \leq y \leq 0.43)$ are investigated by Eu$^{2+}$ electron spin resonance (ESR) in single crystals. From the temperature dependence of the linewidth $ΔH(T)$ of the exchange narrowed ESR line the spin-density wave (SDW) $(T < T_{\rm SDW})$ and the normal metallic regime $(T > T_{\rm SDW})$ are clearly distinguished. At $T > T_{\rm SDW}$ the isotropic linear increase of the linewidth is driven by the Korringa relaxation which measures the conduction-electron density of states at the Fermi level. For $T < T_{\rm SDW}$ the anisotropy probes the local ligand field, while the coupling to the conduction electrons disappears. With increasing substitution $x$ or $y$ the transition temperature $T_{\rm SDW}$ decreases linearly accompanied by a linear decrease of the Korringa-relaxation rate from 8 Oe/K at $x=y=0$ down to 3 Oe/K at the onset of superconductivity at $x \approx 0.2$ or at $y \approx 0.3$, above which it remains nearly constant. Comparative ESR measurements on single crystals of the Eu diluted SDW compound Eu$_{0.2}$Sr$_{0.8}$Fe$_2$As$_2$ and superconducting (SC) Eu$_{0.22}$Sr$_{0.78}$Fe$_{1.72}$Co$_{0.28}$As$_2$ corroborate the leading influence of the ligand field on the Eu$^{2+}$ spin relaxation in the SDW regime as well as the Korringa relaxation in the normal metallic regime. Like in Eu$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ a coherence peak is not detected in the latter compound at $T_{\rm c}=21$ K, which is in agreement with the expected complex anisotropic SC gap structure.

cond-mat.supr-con↗

arXiv2012

CP asymmetries in the supersymmetric trilepton signal at the LHC

In the CP-violating Minimal Supersymmetric Standard Model, we study the production of a neutralino-chargino pair at the LHC. For their decays into three leptons, we analyze CP asymmetries which are sensitive to the CP phases of the neutralino and chargino sector. We present analytical formulas for the entire production and decay process, and identify the CP-violating contributions in the spin correlation terms. This allows us to define the optimal CP asymmetries. We present a detailed numerical analysis of the cross sections, branching ratios, and the CP observables. For light neutralinos, charginos, and squarks, the asymmetries can reach several 10%. We estimate the discovery potential for the LHC to observe CP violation in the trilepton channel.

hep-ph↗

arXiv2011

Measurement Limits to 134Cs Concentration in Soil

We investigate the caesium concentrations in soils in mountain areas near Gori nuclear power plant in Korea, focusing on the measurement limits to the 134Cs. In order to lower the minimum detectable amount (MDA) of activity for the 134Cs, we have used the ammonium molybdophosphase (AMP) precipitation method to get rid of the 40K existing in natural radioactivity, which reduces the MDA activity about ten times smaller than those without the AMP precipitation method. The MDA results for the 134Cs were found to be in the range between 0.015 and 0.044 Bq/kg-dry weight. In order to diminish the background, we also have measured a part of the soil samples in Yangyang, a small town in the east coast of Korea. However, it turns out that in order to detect the 134Cs in the samples the MDA should be reduced to the level of mBq/kg-dry weight.

physics.med-ph↗

arXiv2011

Evolution of transport properties of BaFe2-xRuxAs2 in a wide range of isovalent Ru substitution

The effects of isovalent Ru substitution at the Fe sites of BaFe2-xRuxAs2 are investigated by measuring resistivity and Hall coefficient on high-quality single crystals in a wide range of doping (0 < x < 1.4). Ru substitution weakens the antiferromagnetic (AFM) order, inducing superconductivity for relatively high doping level of 0.4 < x < 0.9. Near the AFM phase boundary, the transport properties show non-Fermi-liquid-like behaviors with a linear-temperature dependence of resistivity and a strong temperature dependence of Hall coefficient with a sign change. Upon higher doping, however, both of them recover conventional Fermi-liquid behaviors. Strong doping dependence of Hall coefficient together with a small magnetoresistance suggest that the anomalous transport properties can be explained in terms of anisotropic charge carrier scattering due to interband AFM fluctuations rather than a conventional multi-band scenario.

cond-mat.supr-con↗

arXiv2011

Volovik effect in a highly anisotropic multiband superconductor: experiment and theory

We present measurements of the specific heat coefficient γ(= C/T) in the low temperature limit as a function of an applied magnetic field for the Fe-based superconductor BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$. We find both a linear regime at higher fields and a limiting square root $H$ behavior at very low fields. The crossover from a Volovik-like $\sqrt{H}$ to a linear field dependence can be understood from a multiband calculation in the quasiclassical approximation assuming gaps with different momentum dependence on the hole- and electron-like Fermi surface sheets.

cond-mat.supr-con↗

arXiv2011

Specific Heat Discontinuity, deltaC, at Tc in BaFe2(As0.7P0.3)2 - Consistent with Unconventional Superconductivity

We report the specific heat discontinuity, deltaC/Tc, at Tc = 28.2 K of a collage of single crystals of BaFe2(As0.7P0.3)2 and compare the measured value of 38.5 mJ/molK**2 with other iron pnictide and iron chalcogenide (FePn/Ch) superconductors. This value agrees well with the trend established by Bud'ko, Ni and Canfield who found that deltaC/Tc ~ a*Tc**2 for 14 examples of doped Ba1-xKxFe2As2 and BaFe2-xTMxAs2, where the transition metal TM=Co and Ni. We extend their analysis to include all the FePn/Ch superconductors for which deltaC/Tc is currently known and find deltaC/Tc ~ a*Tc**1.9 and a=0.083 mJ/molK**4. A comparison with the elemental superconductors with Tc>1 K and with A-15 superconductors shows that, contrary to the FePn/Ch superconductors, electron-phonon-coupled conventional superconductors exhibit a significantly different dependence of deltaC on Tc, namely deltaC/Tc ~ Tc**0.9. However deltaC/gamma*Tc appears to be comparable in all three classes (FePn/Ch, elemental and A-15) of superconductors with, e. g., deltaC/gamma*Tc=2.4 for BaFe2(As0.7P0.3)2. A discussion of the possible implications of these phenomenological comparisons for the unconventional superconductivity believed to exist in the FePn/Ch is given.

cond-mat.supr-con↗

arXiv2011

Specific heat in KFe2As2 in zero and applied magnetic field

The specific heat down to 0.08 K of the iron pnictide superconductor KFe2As2 was measured on a single crystal sample with a residual resistivity ratio of ~650, with an onset Tc determined by specific heat of 3.7 K. The zero field normal state specific heat divided by temperature, C/T, was extrapolated from above Tc to T=0 by insisting on agreement between the extrapolated normal state entropy at Tc, Sn(Tc), and the measured superconducting state entropy at Tc, Ss(Tc), since for a second order phase transition the two entropies must be equal. This extrapolation would indicate that this rather clean sample of KFe2As2 exhibits non-Fermi liquid behavior, i. e. C/T increases at low temperatures, in agreement with the reported non-Fermi liquid behavior in the resistivity. However, specific heat as a function of magnetic field shows that the shoulder feature around 0.7 K, which is commonly seen in KFe2As2 samples, is not evidence for a second superconducting gap as has been previously proposed but instead is due to an unknown magnetic impurity phase, which can affect the entropy balance and the extrapolation of the normal state specific heat. This peak (somewhat larger in magnitude) with similar field dependence is also found in a less pure sample of KFe2As2, with a residual resistivity ratio of only 90 and an onset Tc=3.1 K. These data, combined with the measured normal state specific heat in field to suppress superconductivity, allow the conclusion that an increase in the normal state specific heat as T->0 is in fact not seen in KFe2As2, i. e. Fermi liquid behavior is observed.

cond-mat.supr-con↗

arXiv2011

New Young Star Candidates in CG4 and Sa101

The CG4 and Sa101 regions together cover a region of ~0.5 square degree in the vicinity of a "cometary globule" that is part of the Gum Nebula. There are seven previously identified young stars in this region; we have searched for new young stars using mid- and far-infrared data (3.6 to 70 microns) from the Spitzer Space Telescope, combined with ground-based optical data and near-infrared data from the Two-Micron All-Sky Survey (2MASS). We find infrared excesses in all 6 of the previously identified young stars in our maps, and we identify 16 more candidate young stars based on apparent infrared excesses. Most (73%) of the new young stars are Class II objects. There is a tighter grouping of young stars and young star candidates in the Sa101 region, in contrast to the CG4 region, where there are fewer young stars and young star candidates, and they are more dispersed. Few likely young objects are found in the "fingers" of the dust being disturbed by the ionization front from the heart of the Gum Nebula.

astro-ph.SR↗