arXiv subjects

J. Li

Publications and source records attributed to J. Li.

At least 505 records · Page 28Linked to original sources

arXiv2008

Structure and Magnetic Order in the NdFeAs(O,F) Superconductor System

The transition temperature Tc~26 K of the recently discovered superconductor LaFeAs(O,F) has been demonstrated to be extremely sensitive to the lanthanide ion, reaching 55 K for the Sm containing oxypnictides. Therefore, it is important to determine how the moment on the lanthanide affects the overall magnetism in these systems. Here we report a neutron diffraction study of the Nd oxypnictides. Long ranged antiferromagnetic order is apparent in NdFeAsO below 1.96 K. Rietveld refinement shows that both Fe and Nd magnetic ordering are required to describe the observed data with the staggered moment 1.55(4) Bohr magneton per Nd and 0.9(1) Bohr magneton per Fe at 0.3 K. The other structural properties such as the tetragonal-orthorhombic distortion are found to be very similar to those in LaFeAsO. Neither the magnetic ordering nor the structural distortion occur in the superconducting sample NdFeAsO0.80F0.20 at any temperatures down to 1.5 K.

cond-mat.supr-con↗

arXiv2008

Time-dependent CP Asymmetries in $B^0\to K^0_Sρ^0γ$ Decays

We report the first measurement of CP-violation parameters in B^0 -> K_S^0ρ^0γdecays based on 657 million B\bar B pairs collected with the Belle detector at the KEKB asymmetric-energy collider. We measure the time-dependent CP violating parameter S_{K_S^0ρ^0γ}= 0.11 +/- 0.33(stat.)^{+0.05}_{-0.09}(syst.). We also obtain the effective direct CP violating parameter A_eff=0.05 +/- 0.18(stat.) +/- 0.06(syst.) for m_{K_Sπ^+π^-}<1.8 GeV/c^2 and 0.6 GeV/c^2<m_{π^+π^-}<0.9 GeV/c^2.

hep-ex↗

arXiv2008

Dust Emission from the Perseus Molecular Cloud

Using far-infrared emission maps taken by IRAS and Spitzer and a near-infrared extinction map derived from 2MASS data, we have made dust temperature and column density maps of the Perseus molecular cloud. We show that the emission from transiently heated very small grains and the big grain dust emissivity vary as a function of extinction and dust temperature, with higher dust emissivities for colder grains. This variable emissivity can not be explained by temperature gradients along the line of sight or by noise in the emission maps, but is consistent with grain growth in the higher density and lower temperature regions. By accounting for the variations in the dust emissivity and VSG emission, we are able to map the temperature and column density of a nearby molecular cloud with better accuracy than has previously been possible.

astro-ph↗

arXiv2008

Anomalous Rashba spin-orbit interaction in InAs/GaSb quantum wells

We investigate theoretically the Rashba spin-orbit interaction in InAs/GaSb quantum wells(QWs). We find that the Rashba spin-splitting (RSS) depends sensitively on the thickness of the InAs layer. The RSS exhibits nonlinear behavior for narrow InAs/GaSb QWs and the oscillating feature for wide InAs/GaSb QWs. The nonlinear and oscillating behaviors arise from the weakened and enhanced interband coupling. The RSS also show asymmetric features respect to the direction of the external electric field.

cond-mat.mes-hall↗

arXiv2007

A possible disk mechanism for the 23d QPO in Mkn~501

Optically thin two-temperature accretion flows may be thermally and viscously stable, but acoustically unstable. Here we propose that the O-mode instability of a cooling-dominated optically thin two-temperature inner disk may explain the 23-day quasi-periodic oscillation (QPO) period observed in the TeV and X-ray light curves of Mkn~501 during its 1997 high state. In our model the relativistic jet electrons Compton upscatter the disk soft X-ray photons to TeV energies, so that the instability-driven X-ray periodicity will lead to a corresponding quasi-periodicity in the TeV light curve and produce correlated variability. We analyse the dependence of the instability-driven quasi-periodicity on the mass (M) of the central black hole, the accretion rate ($\rm{\dot{M}}$) and the viscous parameter ($α$) of the inner disk. We show that in the case of Mkn~501 the first two parameters are constrained by various observational results, so that for the instability occurring within a two-temperature disk where $α=0.05-1.0$, the quasi-period is expected to lie within the range of 8 to 100 days, as indeed the case. In particular, for the observed 23-day QPO period our model implies a viscosity coefficient $α\leq 0.28$, a sub-Eddington accretion rate $\dot{M} \simeq 0.02 \dot{M}_{\rm Edd}$ and a transition radius to the outer standard disk of $r_0 \sim 60 r_g$, and predicts a period variation $δP/P \sim 0.23$ due to the motion of the instability region.

astro-ph↗

arXiv2007

A GEM TPC End-Panel Pre-Prototype

A GEM TPC end panel pre-prototype was constructed for a large LC-TPC prototype to test its basic design philosophy and some of its engineering details. Its interim test results are presented.

physics.ins-det↗

arXiv2007

Limits on WIMP-nucleon interactions with CsI(Tl) crystal detectors

The Korea Invisible Mass Search(KIMS) experiment presents new limits on WIMP-nucleon cross section using the data from an exposure of 3409 kgd taken with low background CsI(Tl) crystals at Yangyang underground laboratory. The most stringent limit on the spin dependent interaction for pure proton case is obtained. The DAMA signal region for both spin independent and spin dependent interactions for the WIMP mass higher than 20 GeV/c^2are excluded by the single experiment with crystal scintillators.

astro-ph↗

arXiv2007

Double Spin Asymmetries A_NN and A_SS at sqrt{s}=200 GeV in Polarized Proton-Proton Elastic Scattering at RHIC

We present the first measurements of the double spin asymmetries A_NN and A_SS at sqrt{s}=200 GeV, obtained by the pp2pp experiment using polarized proton beams at the Relativistic Heavy Ion Collider (RHIC). The data were collected in the four momentum transfer t range 0.01<|t|<0.03 (GeV/c)^2. The measured asymmetries, which are consistent with zero, allow us to estimate upper limits on the double helicity-flip amplitudes phi_2 and phi_4 at small t as well as on the difference Delta(sigma_T) between the total cross sections for transversely polarized protons with antiparallel or parallel spin orientations.

nucl-ex↗

arXiv2007

Demagnetization Protocols for Frustrated Interacting Nanomagnet Arrays

We report a study of demagnetization protocols for frustrated arrays of interacting single domain permalloy nanomagnets by rotating the arrays in a changing magnetic field. The most effective demagnetization is achieved by not only stepping the field strength down while the sample is rotating, but by combining each field step with an alternation in the field direction. By contrast, linearly decreasing the field strength or stepping the field down without alternating the field direction leaves the arrays with a larger remanent magnetic moment. These results suggest that non-monotonic variations in field magnitude around and below the coercive field are important for the demagnetization process.

cond-mat.mtrl-sci↗

arXiv2006

Simultaneous readout of two charge qubits

We consider a system of two solid state charge qubits, coupled to a single read-out device, consisting of a single-electron transistor (SET). The conductance of each tunnel junction is influenced by its neighboring qubit, and thus the current through the transistor is determined by the qubits' state. The full counting statistics of the electrons passing the transistor is calculated, and we discuss qubit dephasing, as well as the quantum efficiency of the readout. The current measurement is then compared to readout using real-time detection of the SET island's charge state. For the latter method we show that the quantum efficiency is always unity. Comparing the two methods a simple geometrical interpretation of the quantum efficiency of the current measurement appears. Finally, we note that full quantum efficiency in some cases can be achieved measuring the average charge of the SET island, in addition to the average current.

cond-mat.mes-hall↗

arXiv2006

Magnetic field induced orientation of superconducting MgB$_2$ crystallites determined by X-ray diffraction

X-ray diffraction studies of fine polycrystalline samples of MgB$_2$ in the superconducting state reveal that crystals orient with their \emph{c}-axis in a plane normal to the direction of the applied magnetic field. The MgB$_2$ samples were thoroughly ground to obtain average grain size 5 - 10 $μ$m in order to increase the population of free single crystal grains in the powder. By monitoring Bragg reflections in a plane normal to an applied magnetic field we find that the powder is textured with significantly stronger (\emph{0,0,l}) reflections in comparison to (\emph{h,k,0}), which remain essentially unchanged. The orientation of the crystals with the \emph{ab}-plane parallel to the magnetic field at all temperatures below $T_c$ demonstrates that the sign of the torque under magnetic field does not alter, in disagreement with current theoretical predictions.

cond-mat.supr-con↗

arXiv2006

Improved Constraints on D0-D0bar Mixing in D0 -> K+ pi- Decays from the Belle Detector

We report the results of a search for D0-D0bar mixing in D0 -> K+ pi- decays based on 400 fb^{-1} of data accumulated by the Belle detector at KEKB. Both assuming CP conservation and allowing for CP violation, we fit the decay-time distribution for the mixing parameters x' and y', as well as for the parameter R_D, the ratio of doubly-Cabibbo-suppressed decays to Cabibbo-favored decays. The 95% confidence level region in the (x'^2,y') plane is obtained using a frequentist method. Assuming CP conservation, we find x'^2<0.72 x 10^{-3} and -9.9 x 10^{-3}<y'<6.8 x 10^{-3} at the 95% confidence level; these are the most stringent constraints on the mixing parameters to date. The no-mixing point (0,0) has a confidence level of 3.9%. Assuming no mixing, we measure R_D=(0.377+-0.008+-0.005)%.

hep-ex↗

arXiv2006

Small-polaron coherent conduction in lightly doped ReTiO$_{3+ δ/2}$ (Re=La or Nd) thin films prepared by Pulsed Laser Deposition

LaTiO$_{3+δ/2}$, NdTiO$_{3+δ/2}$, and Nd$_{1-x}$Sr$_{x}$TiO$_{3+δ/2}$ thin films have been epitaxially grown on (100)SrTiO$_{3}$ and (100)LaAlO$_{3}$ single crystal substrates by using the pulsed laser deposition technique. The oxygen pressure during deposition has been carefully controlled to ensure that the films are lightly doped in the metal side near the metal-Mott-insulator boundary. A steep drop of the effective carrier number at low temperatures has been observed in some of the films, which may correspond to a gradually opening of the Spin Density Wave (SDW) gap due to the antiferromagnetic spin fluctuations. At elevated temperatures, a thermally induced decrease of the Hall coefficient can also be clearly observed. In spite of the fact that these films were prepared from different materials in varied deposition conditions, the temperature dependence of their resistance can all be almost perfectly fitted by a small-polaron coherent conduction model ($R_s(T)=R_s(0)+Cω_α/\sinh^2(T_ω/T)$). Careful investigation on the fitting parameters implies that the frequency of the phonon coupled to the electrons may be partially related to the lattice distortion induced by the mismatch strain of the substrates.

cond-mat.str-el↗

arXiv2006

Large magnetoresistance in La2/3Ca1/3MnO3 thin films induced by metal masked ion damage technique

e have developed a simple process to obtain large magnetoresistance (MR) in perovskite manganite thin films by a combination of focused ion beam (FIB) milling and 120 keV H$_{2}^{+}$ ion implantation. Metal slits about 70 nm in width were printed by 30 kV focused Ga ion beam nanolithography on a 4 mm track, and the materials in these slits are then irradiated by the accelerated H$_{2}^{+}$ ions. Using this method, in a magnetic field of 5 T we can get a MR${>}$60% over a 230 K temperature scope, with a maximum value of 95% at around 70 K. This technique is very promising in terms of its simplicity and flexibility of fabrication and has potential for high-density integration.

cond-mat.mtrl-sci↗

arXiv2006

Artificial "spin ice" in a geometrically frustrated lattice of nanoscale ferromagnetic islands

We report an artificial geometrically frustrated magnet based on an array of lithographically fabricated single-domain ferromagnetic islands. The islands are arranged such that the dipole interactions create a two-dimensional analogue to spin ice. Images of the magnetic moments of individual elements in this correlated system allow us to study the local accommodation of frustration. We see both ice-like short-range correlations and an absence of long-range correlations, behaviour which is strikingly similar to the lowtemperature state of spin ice. These results demonstrate that artificial frustrated magnets can provide an uncharted arena in which the physics of frustration can be directly visualized.

cond-mat.dis-nn↗

arXiv2005

First limit on WIMP cross section with low background CsI(Tl) crystal detector

The Korea Invisible Mass Search (KIMS) collaboration has been carrying out WIMP search experiment with CsI(T$\ell$)crystal detectors at the YanYang Underground Laboratory. A successful reduction of the internal background of the crystal is done and a good pulse shape discrimination is achieved. We report the first result on WIMP search obtained with 237 kg$\cdot$days data using one full-size CsI(T$\ell$)crystal of 6.6 kg mass.

astro-ph↗

arXiv2005

Pair-produced heavy particle topologies:MSSM neutralino properties at the LHC from gluino/squark cascade decays

Processes of the form $pp\to anything \to X_i X_j \to x\bar{x} + y\bar{y} (+ \slashchar{E})$ are studied via a technique that may be viewed as an adaptation of time-honoured Dalitz plot analyses. $X_i$ and $X_j$ are new heavy states (with $i,j=1...n$), which may be identical or distinct; and $x\bar{x}$ and $y\bar{y}$ are necessarily distinct Standard Model (SM) fermion pairs whose invariant masses can be measured. A Dalitz-like plot of said invariant masses, $M(x\bar{x})$ {\it vs.$$} $M(y\bar{y})$, exhibits a topology connected to the masses and specific decay chains of $X_i$ and $X_j$. Aside from relatively minor details, observed patterns consist of a collection of box and wedge shapes. This collection is model-dependent: comparison of the observed pattern to the possibilities for a specific model yields information on which new particle pair combinations are actually being produced, information beyond that extractable from conventional one-dimensional invariant mass distributions. The technique is illustrated via application to the Minimal Supersymmetric Standard Model (MSSM) process $pp \to \tilde{g}\tilde{g},\tilde{g}\tilde{q},\tilde{q}\tilde{q} \to \widetildeχ_i^0 \widetildeχ_j^0 \to e^+ e^- + μ^+ μ^- (+ \slashchar{E})$. Here the heavy states are neutralinos $\widetildeχ_i^0$ ($i=2,3,4$) -- note $\widetildeχ_1^0$ is excluded -- which are produced in gluino/squark ($\tilde{g}$/$\tilde{q}$) cascade decay chains. Even with fairly modest expectations for the LHC performance during the first few years, this method still provides substantial insight into the neutralino mass spectrum and couplings if gluino/squark masses are relatively low ($\simeq 400 \hbox{GeV}$).

hep-ph↗

arXiv2005

Parity assignments in 172,174Yb using polarized photons and the K quantum number in rare earth nuclei

The 100 % polarized photon beam at the High Intensity gamma-ray Source (HIgS) at Duke University has been used to determine the parity of six dipole excitations between 2.9 and 3.6 MeV in the deformed nuclei 172,174 Yb in photon scattering (g,g') experiments. The measured parities are compared with previous assignments based on the K quantum number that had been assigned in Nuclear Resonance Fluorescence (NRF) experiments by using the Alaga rules. A systematic survey of the relation between gamma-decay branching ratios and parity quantum numbers is given for the rare earth nuclei.

nucl-ex↗