Results for "Pengcheng Dai"

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Antiferromagnetic order and spin dynamics in iron-based superconductorsMar 08 2015Jun 18 2015High-transition temperature (high-$T_c$) superconductivity in the iron pnictides/chalcogenides emerges from the suppression of the static antiferromagnetic order in their parent compounds, similar to copper oxides superconductors. This raises a fundamental ... More
Structure and composition of the superconducting phase in alkali iron selenide K$_y$Fe$_{1.6+x}$Se$_2$Apr 14 2014We use neutron diffraction to study the temperature evolution of the average structure and local lattice distortions in insulating and superconducting potassium iron selenide K$_y$Fe$_{1.6+x}$Se$_2$. In the high temperature paramagnetic state, both materials ... More
Stripe Antiferromagnetism and Disorder in the Mott Insulator NaFe$_{1-x}$Cu$_{x}$As ($x \lesssim 0.5$)Aug 19 2019Aug 23 2019Neutron scattering measurements have demonstrated that the heavily Cu-doped NaFe$_{1-x}$Cu$_{x}$As compound behaves like a Mott insulator exhibiting both real space Fe-Cu stripes, as well as antiferromagnetism below a N\'eel temperature for $x\lesssim ... More
Toward the Mott state with Magnetic Cluster Formation in Heavily Cu-Doped NaFe$_{1-x}$Cu$_{x}$AsMar 28 2019Apr 01 2019Recent neutron scattering measurements indicate that NaFe$_{1-x}$Cu$_{x}$As forms an antiferromagnetic stripe phase near $x\approx 0.5$ in a Mott insulating state. This copper concentration is well in excess of that required for superconductivity, $x ... More
Longitudinal spin excitations and magnetic anisotropy in antiferromagnetically ordered BaFe2As2Sep 29 2013We report on a spin-polarized inelastic neutron scattering study of spin waves in the antiferromagnetically ordered state of BaFe2As2. Three distinct excitation components are identified, with spins fluctuating along the c-axis, perpendicular to the ordering ... More
Spin gap and magnetic resonance in superconducting BaFe$_{1.9}$Ni$%_{0.1}$As$_{2}$Feb 04 2009We use neutron spectroscopy to determine the nature of the magnetic excitations in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ ($T_{c}=20$ K). Above $T_{c}$ the excitations are gapless and centered at the commensurate antiferromagnetic wave vector ... More
Neutron Studies of the Iron-based Family of High TC Magnetic SuperconductorsFeb 01 2009Feb 09 2009We review neutron scattering investigations of the crystal structures, magnetic structures, and spin dynamics of the iron-based RFe(As,P)O (R=La, Ce, Pr, Nd), (Ba,Sr,Ca)Fe2As2, and Fe1+x(Te-Se) systems. On cooling from room temperature all the undoped ... More
Orbital selective neutron spin resonance in underdoped superconducting NaFe$_\textbf{0.985}$Co$_\textbf{0.015}$AsMar 09 2017We use neutron scattering to study the electron-doped superconducting NaFe$_{0.985}$Co$_{0.015}$As ($T_c=14$ K), which has co-existing static antiferromagnetic (AF) order ($T_N=31$ K) and exhibits two neutron spin resonances ($E_{r1}\approx 3.5$ meV and ... More
P-moves between pants-block decompositions of 3-manifoldsOct 03 2018A pants-block decomposition of a 3-manifold is similar to a triangulation of a 3-manifold in many aspects. In this paper we show that any two pants-block decompositions of a 3-manifold are related by a finite sequence of moves which are called P-moves. ... More
Optimization of Spatial Dose Distribution for Controlling Sidewall Shape in Electron-beam LithographySep 07 2015Electron-beam (e-beam) lithography is widely employed in fabrication of 2-D patterns and 3-D structures. A certain type or shape of the sidewall in the remaining resist profile may be desired in an application, e.g., an undercut for lift-off and a vertical ... More
On Nontrivial Zeros of Riemann Zeta FunctionApr 17 2017Jul 17 2018Let {\Xi} be a function relating to the Riemann zeta function with . In this paper, we construct a function containing and {\Xi} , and prove that satisfies a nonadjoint boundary value problem to a nonsingular differential equation if is any nontrivial ... More
Observation of Magnetic Moments in the Superconducting State of YBa$_2$Cu$_3$O$_{6.6}$Feb 04 2001Neutron Scattering measurements for YBa$_2$Cu$_3$O$_{6.6}$ have identified small magnetic moments that increase in strength as the temperature is reduced below $T^\ast$ and further increase below $T_c$. An analysis of the data shows the moments are antiferromagnetic ... More
Incommensurate Magnetic Fluctuations in YBa2Cu3O6.6Jul 10 1997We use inelastic neutron scattering to demonstrate that at low temperatures, the low frequency magnetic fluctuations in YBa_2Cu_3O_{6.6} ($T_c=62.7$ K) are incommensurate, being found at positions displaced by $\pm\delta$ ($0.057\pm 0.006$ r.l.u.) along ... More
Local orthorhombic lattice distortions in the paramagnetic tetragonal phase of superconducting NaFe$_{1-x}$Ni$_x$AsJul 14 2018Understanding the interplay between nematicity, magnetism and superconductivity is pivotal for elucidating the physics of iron-based superconductors. Here we use neutron scattering to probe magnetic and nematic orders throughout the phase diagram of NaFe$_{1-x}$Ni$_x$As, ... More
Impact of the first order antiferromagnetic phase transition on the paramagnetic spin excitations and nematic phase of SrFe$_2$As$_2$Apr 14 2019Understanding the nature of the electronic nematic phase in iron pnictide superconductors is important for elucidating its impact on high-temperature superconductivity. Here we use transport and inelastic neutron scattering to study spin excitations and ... More
Magnetism and its microscopic origin in iron-based high-temperature superconductorsSep 03 2012High-temperature superconductivity in the iron-based materials emerges from, or sometimes coexists with, their metallic or insulating parent compound states. This is surprising since these undoped states display dramatically different antiferromagnetic ... More
Incommensurate magnetic structure in the orthorhombic perovskite ErMnO_3Jul 03 2007Aug 02 2007By combining dielectric, specific heat, and magnetization measurements and high-resolution neutron powder diffraction, we have investigated the thermodynamic and magnetic/structural properties of the metastable orthorhombic perovskite ErMnO_3 prepared ... More
Higher gradients estimates in Morrey spaces for weak solutions to linear ultraparabolic equationsApr 25 2014The aim of this paper is to consider the linear ultraparabolic equation with bounded and VMO coefficients $a_{ij} (z)$. Assume that the operator $L_0$ obtained by freezing the coefficients $a_{ij}(z)$ at any point ${z_0} \in {\mathbb{R}^{N + 1}}$ is hypoelliptic. ... More
A Syntactic Neural Model for General-Purpose Code GenerationApr 06 2017We consider the problem of parsing natural language descriptions into source code written in a general-purpose programming language like Python. Existing data-driven methods treat this problem as a language generation task without considering the underlying ... More
Evolution of the resonance and incommensurate spin fluctuations in superconducting YBa$_2$Cu$_3$O$_{6+x}$Nov 01 2000Polarized and unpolarized neutron triple-axis spectrometry was used to study the dynamical magnetic susceptibility $\chi^{\prime\prime}({\bf q},\omega)$ as a function of energy ($\hbar\omega$) and wave vector (${\bf q}$) in a wide temperature range for ... More
Magnetic Frustration and Iron-Vacancy Ordering in Iron-ChalcogenideMar 23 2011Apr 11 2011We show that the magnetic and vacancy orders in the 122 $(A_{1-y}Fe_{2-x}Se_2)$ iron-chalcogenides can be naturally derived from the $J_1-J_2-J_3$ model with $J_1$ being the ferromagnetic (FM) nearest neighbor exchange coupling and $J_{2}, J_3$ being ... More
Accelerating CNN Training by Sparsifying Activation GradientsAug 01 2019Gradients to activations get involved in most of the calculations during back propagation procedure of Convolution Neural Networks (CNNs) training. However, an important known observation is that the majority of these gradients are close to zero, imposing ... More
A Scalable Pipelined Dataflow Accelerator for Object Region Proposals on FPGA PlatformOct 26 2018Region proposal is critical for object detection while it usually poses a bottleneck in improving the computation efficiency on traditional control-flow architectures. We have observed region proposal tasks are potentially suitable for performing pipelined ... More
Anisotropic magnetic excitations of a frustrated bilinear-biquadratic spin model -- Implications for spin waves of detwinned iron pnictidesSep 10 2019Elucidating the nature of spin excitations is important to understanding the mechanism of superconductivity in the iron pnictides. Motivated by recent inelastic neutron scattering measurements in the nearly 100% detwinned BaFe$_{2}$As$_{2}$, we study ... More
Flat band magnetism and helical magnetic order in Ni-doped SrCo$_2$As$_2$Jul 22 2019A series of Sr(Co$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals was synthesized allowing a comprehensive phase diagram with respect to field, temperature, and chemical substitution to be established. Our neutron diffraction experiments revealed a helimagnetic ... More
Short-range polaron correlations in the ferromagnetic La(1-x)Ca(x)MnO(3)Aug 21 2000We use neutron scattering to demonstrate the presence of lattice polarons and their short-range correlations for several samples of La$_{1-x}$Ca$_x$MnO$_3$ in the Ca doping range $0.15\leq x\leq 0.3$. We establish the doping dependence of the orientation, ... More
Toward the Mott state with Magnetic Cluster Formation in Heavily Cu-Doped NaFe$_{1-x}$Cu$_{x}$AsMar 28 2019Recent neutron scattering measurements indicate that NaFe$_{1-x}$Cu$_{x}$As forms an antiferromagnetic stripe phase near $x\approx 0.5$ in a Mott insulating state. This copper concentration is well in excess of that required for superconductivity, $x ... More
Nematic crossover in BaFe$_2$As$_2$ under uniaxial stressJul 08 2015Raman scattering can detect spontaneous point-group symmetry breaking without resorting to single-domain samples. Here we use this technique to study $\mathrm{BaFe_2As_2}$, the parent compound of the "122" Fe-based superconductors. We show that an applied ... More
Unusual suppression of a spin resonance mode with magnetic field in underdoped NaFe$_{1-x}$Co$_x$As: Evidence for orbital-selective pairingJul 25 2018We use inelastic neutron scattering to study the fate of the two spin resonance modes in underdoped superconducting NaFe$_{1-x}$Co$_x$As ($x=0.0175$) under applied magnetic fields. While an applied in-plane magnetic field of $B=12$ T only modestly suppresses ... More
Spin-Charge Coupling in lightly doped Nd$_{2-x}$Ce$_{x}$CuO$_4$Nov 27 2004We use neutron scattering to study the influence of a magnetic field on spin structures of Nd$_2$CuO$_4$. On cooling from room temperature, Nd$_2$CuO$_4$ goes through a series of antiferromagnetic (AF) phase transitions with different noncollinear spin ... More
Magnons in Ferromagnetic Metallic ManganitesAug 21 2007Ferromagnetic (FM) manganites, a group of likely half-metallic oxides, are of special interest not only because they are a testing ground of the classical doubleexchange interaction mechanism for the colossal magnetoresistance, but also because they exhibit ... More
Direct observation of spin excitation anisotropy in the paramagnetic orthorhombic state of BaFe$_{2-x}$Ni$_x$As$_2$Feb 08 2018We use transport and inelastic neutron scattering measurements to investigate single crystals of iron pnictide BaFe$_{2-x}$Ni$_{x}$As$_{2}$ ($x=0,0.03$), which exhibit a tetragonal-to-orthorhombic structural transition at $T_s$ and stripe antiferromagnetic ... More
Nematic spin correlations in the tetragonal state of uniaxial strained BaFe2-xNixAs2Aug 12 2014Understanding the microscopic origins of electronic phases in high-transition temperature (high-Tc) superconductors is important for elucidating the mechanism of superconductivity. In the paramagnetic tetragonal phase of BaFe2-xTxAs2 (where T is Co or ... More
Magnetization in the Superconducting State of UPt$_3$ from Polarized Neutron DiffractionAug 14 2012Sep 18 2012The heavy fermion superconductor UPt$_3$ is thought to have odd-parity, a state for which the temperature dependence of the spin susceptibility is an important signature. In order to address conflicting reports from two different experiments, the NMR ... More
Resonance in Optimally Electron-Doped Superconductor Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-δ}$Mar 27 2007Mar 30 2007We use inelastic neutron scattering to probe magnetic excitations of an optimally electron-doped superconductor Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-\delta}$ above and below its superconducting transition temperature $T_c=25$ K. In addition to gradually opening ... More
Absence of long wavelength nematic fluctuations in LiFeAsNov 08 2016We investigated long-wavelength nematic fluctuations in an Fe-based superconductor LiFeAs near q=(0.05,0,0) by measuring temperature-dependent renormalization of acoustic phonons through inelastic neutron scattering. We found that the phonons have conventional ... More
Stripe Antiferromagnetism and Disorder in the Mott Insulator NaFe$_{1-x}$Cu$_{x}$As ($x \lesssim 0.5$)Aug 19 2019Neutron scattering measurements have demonstrated that the heavily Cu-doped NaFe$_{1-x}$Cu$_{x}$As compound behaves like a Mott insulator exhibiting both real space Fe-Cu stripes, as well as antiferromagnetism below a N\'eel temperature for $x\lesssim ... More
Two spatially separated phases in semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$Aug 05 2014We report neutron scattering and transport measurements on semiconducting Rb$_{0.8}$Fe$_{1.5}$S$_2$, a compound isostructural and isoelectronic to the well-studied $A_{0.8}$Fe$_{y}$Se$_2 (A=$ K, Rb, Cs, Tl/K) superconducting systems. Both resistivity ... More
Resonance in the electron-doped high-Tc superconductor Pr0.88LaCe0.12CuO(4-delta)Jul 05 2006In conventional superconductors, the interaction that pairs the electrons to form the superconducting state is mediated by lattice vibrations (phonons). In high-transition temperature (high-Tc) copper oxides, it is generally believed that magnetic excitations ... More
Crossover of the pairing symmetry from $s$- to $d$-wave in iron pnictide superconductorsFeb 25 2015One of the major themes in the physics of condensed matter is unconventional superconductivity in correlated electronic systems. The symmetry structure of Cooper pairs is thought to be the key to the understanding of the pairing mechanism of their superconductivity. ... More
Polarized Neutron Measurement of Magnetic Order in YBa2Cu3O6.45Feb 04 2004Orbital current order of the d-density-wave type (DDW) has been postulated to explain the pseudogap in the high temperature superconductors. We have performed neutron scattering experiments to search for this order and show here the results obtained on ... More
Direct observation of paramagnons in palladiumMay 24 2010We report an inelastic neutron scattering study of the spin fluctuations in the nearly-ferromagnetic element palladium. Dispersive over-damped collective magnetic excitations or ``paramagnons'' are observed up to 128 meV. We analyze our results in terms ... More
Iron based high transition temperature superconductorsMar 07 2014In a superconductor electrons form pairs and electric transport becomes dissipation-less at low temperatures. Recently discovered iron based superconductors have the highest superconducting transition temperature next to copper oxides. In this article, ... More
Spin excitation anisotropy in the paramagnetic tetragonal phase of BaFe2 As2Jun 26 2017We use neutron polarization analysis to study temperature dependence of the spin excitation anisotropy in BaFe$_2$As$_2$, which has a tetragonal-to-orthorhombic structural distortion at $T_s$ and antiferromagnetic (AF) phase transition at $T_N$ with ordered ... More
The effect of Cr impurity to superconductivity in electron-doped BaFe2-xNixAs2Sep 23 2014We use transport and magnetization measurements to study the effect of Cr-doping to the phase diagram of the electron-doped superconducting BaFe2-xNixAs2 iron pnictides. In principle, adding Cr to electron-doped BaFe2-xNixAs2 should be equivalent to the ... More
Optimal Control for Nonlinear Hybrid Systems via Convex RelaxationsFeb 14 2017Mar 19 2018This paper considers the optimal control for hybrid systems whose trajectories transition between distinct subsystems when state-dependent constraints are satisfied. Though this class of systems is useful while modeling a variety of physical systems undergoing ... More
Disentangling superconducting and magnetic orders in NaFe_1-xNi_xAs using muon spin rotationFeb 13 2018Muon spin rotation and relaxation studies have been performed on a "111" family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was characterized by obtaining the temperature and doping dependences of the local ordered magnetic moment ... More
Control Synthesis for Nonlinear Optimal Control via Convex RelaxationsOct 03 2016This paper addresses the problem of control synthesis for nonlinear optimal control problems in the presence of state and input constraints. The presented approach relies upon transforming the given problem into an infinite-dimensional linear program ... More
The connection between superconducting phase correlations and spin excitations in YBa$_2$Cu$_3$O$_{6.6}$: A magnetic field studyJun 28 2000One of the most striking universal properties of the high-transition-temperature (high-$T_c$) superconductors is that they are all derived from the hole-doping of their insulating antiferromagnetic (AF) parent compounds. From the outset, the intimate ... More
Anisotropic resistivity of Na$_{1-δ}$Fe$_{1-x}$Co$_x$AsAug 17 2012Temperature-dependent resistivity is studied in single crystals of iron-arsenide superconductor Na$_{1-\delta}$Fe$_{1-x}$Co$_x$As for electrical current directions along, $\rho_a (T)$, and transverse, $\rho_c (T)$, to the Fe-As layers. Doping with Co ... More
SPINBIS: Spintronics based Bayesian Inference System with Stochastic ComputingFeb 19 2019Bayesian inference is an effective approach for solving statistical learning problems, especially with uncertainty and incompleteness. However, Bayesian inference is a computing-intensive task whose efficiency is physically limited by the bottlenecks ... More
Fast Active Set Methods for Online Deconvolution of Calcium Imaging DataSep 02 2016Fluorescent calcium indicators are a popular means for observing the spiking activity of large neuronal populations, but extracting the activity of each neuron from raw fluorescence calcium imaging data is a nontrivial problem. We present a fast online ... More
Fast Active Set Methods for Online Deconvolution of Calcium Imaging DataSep 02 2016Nov 24 2016Fluorescent calcium indicators are a popular means for observing the spiking activity of large neuronal populations, but extracting the activity of each neuron from raw fluorescence calcium imaging data is a nontrivial problem. We present a fast online ... More
Electronic Raman study of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ from under-doping to over-dopingAug 22 2016We performed an electronic Raman scattering study of under-doped, optimally-doped and over-doped Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ samples, in the normal and superconducting states. We observe a quasi-elastic peak in the normal state in both the B$_{1g}$ ... More
Spin Susceptibility of the Topological Superconductor UPt3 from Polarized Neutron DiffractionMay 23 2017Experiment and theory indicate that UPt3 is a topological superconductor in an odd-parity state, based in part from temperature independence of the NMR Knight shift. However, quasiparticle spin-flip scattering near a surface, where the Knight shift is ... More
Close relationship between superconductivity and the bosonic mode in Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)AsOct 30 2013Since the discovery of high temperature superconductivity in the iron pnictides and chalcogenides in early 2008, a central issue has been the microscopic origin of the superconducting pairing. Although previous experiments suggest that the pairing may ... More
Common origin of the two types of magnetic fluctuations in iron chalcogenidesSep 20 2011We use inelastic neutron scattering to study the low energy spin excitations in moderately doped non-superconducting Fe$_{1.01}$Te$_{0.72}$Se$_{0.28}$. The spin excitations in this system contain components near (0.5,0,0) and (0.5,0.5,0) in a-b plane ... More
Direct observation of magnon-phonon coupling in yttrium iron garnetSep 12 2017The magnetic insulator yttrium iron garnet (YIG) with a ferrimagnetic transition temperature of $\sim$560 K has been widely used in microwave and spintronic devices. Anomalous features in the spin Seeback effect (SSE) voltages have been observed in Pt/YIG ... More
Resonance as a probe of the electron superconducting gap in BaFe1.9Ni0.1As2Aug 06 2009The discovery of high-transition temperature (high-Tc) superconductivity near antiferromagnetism in iron arsenides raised the possibility of an unconventional superconducting mechansim1-8. The observation of clear Fermi surfaces and nodeless superconducting ... More
Doping evoluton of antiferromagnetic order and structural distortion in LaFeAsO$_{1-x}$F$_x$Sep 28 2008Sep 30 2008We use neutron scattering to study the structural distortion and antiferromagnetic (AFM) order in LaFeAsO$_{1-x}$F$_{x}$ as the system is doped with fluorine (F) to induce superconductivity. In the undoped state, LaFeAsO exhibits a structural distortion, ... More
Evolution of London penetration depth with scattering in single crystals of K$_{1-x}$Na$_x$Fe$_2$As$_2$Jun 05 2014London penetration depth, $\lambda (T)$, was measured in single crystals of K$_{1-x}$Na$_x$Fe$_2$As$_2$, $x$=0 and 0.07, down to temperatures of 50~mK, $\sim T_c/50$. Isovalent substitution of Na for K significantly increases impurity scattering, with ... More
Microscopic spin interactions in the CMR manganitesOct 31 2001Using inelastic neutron scattering we measured the microscopic magnetic coupling associated with the ferromagnetic (FM) clusters of the ``colossal magnetoresistance'' (CMR) compound Pr$_{0.70}$Ca$_{0.30}$MnO$_3$. When the insulating to metal (I-M) transition ... More
Spin-pairing and penetration depth measurements from nuclear magnetic resonance in NaFe$_{0.975}$Co$_{0.025}$AsFeb 06 2013May 22 2013We have performed $^{75}$As nuclear magnetic resonance (NMR) Knight shift measurements on single crystals of NaFe$_{0.975}$Co$_{0.025}$As to show that its superconductivity is a spin-paired, singlet state consistent with predictions of the weak-coupling ... More
Magnon Damping by magnon-phonon coupling in Manganese PerovskitesApr 26 1999Inelastic neutron scattering was used to systematically investigate the spin-wave excitations (magnons) in ferromagnetic manganese perovskites. In spite of the large differences in the Curie temperatures ($T_C$s) of different manganites, their low-temperature ... More
Quantum Spin Excitations through the metal-to-insulator crossover in $Y Ba_2 Cu_3 O_{6+y}$Dec 14 2007We use inelastic neutron scattering to study the temperature dependence of the spin excitations of a detwinned superconducting YBa$_2$Cu$_3$O$_{6.45}$ ($T_c=48$ K). In contrast to earlier work on YBa$_2$Cu$_3$O$_{6.5}$ ($T_c=58$ K), where the prominent ... More
Electronic inhomogeneity and competing phases in electron-doped superconducting Pr0.88LaCe0.12CuO4Jan 06 2005We use neutron scattering to demonstrate that electron-doped superconducting Pr0.88LaCe0.12CuO4 in the underdoped regime is electronically phase separated in the ground state, showing the coexistence of a superconducting phase with a three-dimensional ... More
Effect of a magnetic field on the long-range magnetic order in insulating Nd2CuO4, nonsuperconducting and superconducting Nd1.85Ce0.15CuO4Aug 12 2003We have measured the effect of a c-axis aligned magnetic field on the long-range magnetic order of insulating Nd2CuO4, as-grown nonsuperconducting and superconducting Nd1.85Ce0.15CuO4. On cooling from room temperature, Nd2CuO4 goes through a series of ... More
High Energy Spin Excitations in Electron-Doped Superconducting Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-δ}$ with $T_c=21$ KApr 19 2006We use high-resolution inelastic neutron scattering to study the low-temperature magnetic excitations of electron-doped superconducting Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$ ($T_c=21\pm 1$ K) over a wide energy range (4 meV$\le\hbar\omega\le 330$ meV). ... More
Neutron spin resonance as a probe of Fermi surface nesting and superconducting gap symmetry in Ba$_{0.67}$K$_{0.33}$(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$Aug 18 2018We use inelastic neutron scattering to study energy and wave vector dependence of the superconductivity-induced resonance in hole-doped Ba$_{0.67}$K$_{0.33}$(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ ($x=0,0.08$ with $T_c\approx 37, 28$ K, respectively). In previous ... More
Polarized neutron scattering studies of magnetic excitations in electron-overdoped superconducting BaFe$_{1.85}$Ni$_{0.15}$As$_{2}$May 16 2012We use polarized inelastic neutron scattering to study low-energy spin excitations and their spatial anisotropy in electron-overdoped superconducting BaFe$_{1.85}$Ni$_{0.15}$As$_{2}$ ($T_c=14$ K). In the normal state, the imaginary part of the dynamic ... More
Spin-isotropic continuum of spin excitations in antiferromagnetically ordered Fe$_{1.07}$TeJan 11 2018Unconventional superconductivity typically emerges in the presence of quasi-degenerate ground states, and the associated intense fluctuations are likely responsible for generating the superconducting state. Here we use polarized neutron scattering to ... More
Magnetic anisotropy in hole-doped superconducting Ba 0.67K 0.33Fe 2As2 probed by polarized inelastic neutron scatteringJan 08 2013We use polarized inelastic neutron scattering (INS) to study spin excitations of optimally hole-doped superconductor Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_{2}$ ($T_c=38$ K). In the normal state, the imaginary part of the dynamic susceptibility, $\chi^{\prime\prime}(Q,\omega)$, ... More
Anomalous metamagnetism in the low carrier density Kondo lattice YbRh3Si7Mar 11 2018May 17 2018We report complex metamagnetic transitions in single crystals of the new low carrier Kondo antiferromagnet YbRh3Si7. Electrical transport, magnetization, and specific heat measurements reveal antiferromagnetic order at T_N = 7.5 K. Neutron diffraction ... More
Temperature dependence of the paramagnetic spin excitations in BaFe$_2$As$_2$Sep 27 2012We use inelastic neutron scattering to study temperature dependence of the paramagnetic spin excitations in iron pnictide BaFe$_2$As$_2$ throughout the Brillouin zone. In contrast to a conventional local moment Heisenberg system, where paramagnetic spin ... More
Magnetic Order in YBa$_2$Cu$_3$O$_{6+x}$ SuperconductorsMar 29 2002Polarized and unpolarized neutron diffraction has been used to search for magnetic order in YBa$_2$Cu$_3$O$_{6+x}$ superconductors. Most of the measurements were made on a high quality crystal of YBa$_2$Cu$_3$O$_{6.6}$. It is shown that this crystal has ... More
Destruction of Neel order and appearance of superconductivity in electron-doped cuprates by oxygen annealing processJun 20 2008We use thermodynamic and neutron scattering measurements to study the effect of oxygen annealing on the superconductivity and magnetism in Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-\delta}$. Although the transition temperature $T_c$ measured by susceptibility and ... More
Spin excitation anisotropy as a probe of orbital ordering in the paramagnetic tetragonal phase of superconducting BaFe1.904Ni0.096As2Aug 18 2013We use polarized neutron scattering to demonstrate that in-plane spin excitations in electron doped superconducting BaFe1.904Ni0.096As2 (Tc=19.8 K) change from isotropic to anisotropic in the tetragonal phase well above the antiferromagnetic (AF) ordering ... More
Magnetic field dependence of spin-lattice relaxation in the s$\pm$ state of Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$Sep 18 2011Apr 30 2012The spatially averaged density of states, <N(0)>, of an unconventional d-wave superconductor is magnetic field dependent, proportional to $H^{1/2}$, owing to the Doppler shift of quasiparticle excitations in a background of vortex supercurrents[1,2]. ... More
Magnetic Excitations and their energy change available to Superconducting Condensation in Optimally Doped YBa$_2$Cu$_3$O$_{6.95}$Aug 11 2006Understanding the magnetic excitations in high-transition temperature (high-$T_c$) copper oxides is important because they may mediate the electron pairing for superconductivity. By determining the wavevector ({\bf Q}) and energy ($\hbar\omega$) dependence ... More
Evolution of normal and superconducting properties of single crystals of Na$_{1-δ}$FeAs upon interaction with environmentOct 01 2011Iron-arsenide superconductor Na$_{1-\delta}$FeAs is highly reactive with the environment. Due to the high mobility of Na ions, this reaction affects the entire bulk of the crystals and leads an to effective stoichiometry change. Here we use this effect ... More
Doping evolution of antiferromagnetism and transport properties in the non-superconducting BaFe2-2xNixCrxAs2Feb 27 2015We report elastic neutron scattering and transport measurements on the Ni and Cr equivalently doped iron pnictide BaFe$_{2-2x}$Ni$_{x}$Cr$_{x}$As$_{2}$. Compared with the electron-doped BaFe$_{2-x}$Ni$_{x}$As$_{2}$, the long-range antiferromagnetic (AF) ... More
Spin waves throughout the Brillouin zone and magnetic exchange coupling in ferromagnetic metallic manganites La$_{1-x}$Ca$_{x}$MnO$_3$ ($x=0.25,0.30$)Feb 21 2007Using time-of-flight and triple-axis inelastic neutron spectroscopy, we determine spin wave excitations throughout the Brillouin zone for ferromagnetic manganites La$_{1-x}$Ca$_x$MnO$_3$ ($x=0.25,0.3$) in their low temperature metallic states. While spin ... More
Electronically smectic-like phase in a nearly half-doped manganiteMay 10 2005Aug 29 2005We use neutron scattering to study the spin and charge/orbital ordering (CO-OO) in the nearly half-doped perovskite manganite $Pr_{0.55}(Ca_{0.8}Sr_{0.2})_{0.45}MnO_3$ (PCSMO). On cooling from room temperature, PCSMO first enters into a CO-OO state below ... More
Antiferromagnetism-superconductivity competition in electron-doped cuprates triggered by oxygen reductionApr 03 2007We have performed a systematic angle-resolved photoemission study of as-grown and oxygen-reduced Pr$_{2-x}$Ce$_x$CuO$_4$ and Pr$_{1-x}$LaCe$_{x}$CuO$_4$ electron-doped cuprates. In contrast to the common belief, neither the band filling nor the band parameters ... More
Gluon Radiation of an Expanding Color Skyrmion in the Quark-Gluon PlasmaApr 02 2007The density of states and energy spectrum of the gluon radiation are calculated for the color current of an expanding hydrodynamic skyrmion in the quark gluon plasma with a semiclassical method. Results are compared with those in literatures.
Quantified separably injective spacesFeb 10 2014Apr 28 2016Let $X$, $Y$ be two Banach spaces. Let $\varepsilon\geq 0$. A mapping $f: X\rightarrow Y$ is said to be a standard $\varepsilon-$ isometry if $f(0)=0$ and $|\|f(x)-f(y)\|-\|x-y\||\leq \varepsilon$. In this paper we first show that if $Y^*$ has the point ... More
An iterative method for Kirchhoff type equations and its applicationsAug 21 2019In this short note, we propose an iterative method for finding nonnegative solutions of Kirchhoff type equations.
Lower order tensors in non-Kähler geometry and non-Kähler geometric flowMar 21 2015May 31 2016In recent years, Streets and Tian introduced a series of curvature flows to study non-K\"{a}hler geometry. In this paper, we study how to construct second order curvature flows in a uniform way, under some natural assumptions which holds in Streets and ... More
Ill-posedness of the incompressible magneto-hydrodynamics system in $\dot{B}_{\infty,\infty}^{-1}$Nov 16 2012Jul 25 2014We demonstrate that the three dimensional incompressible magneto-hydrodynamics (MHD) system is ill-posed in the largest scaling invariant space $\dot{B}_{\infty,\infty}^{-1}$. The construction method of initial data used in this paper is different from ... More
$L^p(Ω)$-Difference of One-Dimensional Stochastic Differential Equations with Discontinuous DriftApr 09 2014We consider a one-dimensional stochastic differential equations (SDE) with irregular coefficients. The purpose of this paper is to estimate the $L^p(\Omega)$-difference of SDEs using the norm of the difference of coefficients, where the discontinuous ... More
A Gel'fand-type spectral radius formula and stability of linear constrained switching systemsJul 01 2011Using ergodic theory, in this paper we present a Gel'fand-type spectral radius formula which states that the joint spectral radius is equal to the generalized spectral radius for a matrix multiplicative semigroup $\bS^+$ restricted to a subset that need ... More
The finite-step realizability of the joint spectral radius of a pair of $d\times d$ matrices one of which being rank-oneJun 05 2011We study the finite-step realizability of the joint/generalized spectral radius of a pair of real $d\times d$ matrices, one of which has rank 1. Then we prove that there always exists a finite-length word for which there holds the spectral finiteness ... More
Viewing the Welch bound inequality from the kernel trick viewpointMar 24 2014Apr 02 2014This brief note views to the Welch bound inequality using the idea of the kernel trick from the machine learning research area. From this angle, some novel insights of the inequality are obtained.
Asymptotic Joint Distribution of Extreme Sample Eigenvalues and Eigenvectors in the Spiked Population ModelApr 22 2013May 03 2013In this paper, we consider a data matrix $X_N\in\mathbb{R}^{N\times p}$ where all the rows are i.i.d. samples in $\mathbb{R}^p$ of mean zero and covariance matrix $\Sigma\in\mathbb{R}^{p\times p}$. Here the population matrix $\Sigma$ is of finite rank ... More
Eigenvalue, bifurcation, existence and nonexistence of solutions for Monge-Ampère equationsJul 27 2012In this paper we study the following eigenvalue boundary value problem for Monge-Amp\`{e}re equations: {equation} \{{array}{l} \det(D^2u)=\lambda^N f(-u)\,\, \text{in}\,\, \Omega, u=0,\,\text{on}\,\, \partial \Omega. {array}. {equation} We establish the ... More
Approximation to multifractional Riemann-Liouville Brownian sheetDec 08 2012Dec 25 2012In this paper, we first introduce multifrational Riemann-Liouville Brownian sheets. Then, we show a result of approximation in law of the multifractional Riemann-Liouville Brownian sheet. The construction of these approximations is based on a sequence ... More
Convergence in law to operator fractional Brownian motionsDec 31 2011In this paper, we provide two approximations in law of operator fractional Brownian motions. One is constructed by Poisson processes, and the other generalizes a result of Taqqu (1975).
Phase Separation, Competition, and Volume Fraction Control in NaFe$_{1-x}$Co$_x$AsJun 10 2014Oct 11 2014We report a detailed nuclear magnetic resonance (NMR) study by combined $^{23}$Na and $^{75}$As measurements over a broad range of doping to map the phase diagram of NaFe$_{1-x}$Co$_x$As. In the underdoped regime ($x \le$ 0.017), we find a magnetic phase ... More
Critical Quadrupole Fluctuations and Collective Modes in Iron Pnictide SuperconductorsOct 23 2014Jan 25 2016The multiband nature of iron pnictides gives rise to a rich temperature-doping phase diagram of competing orders and a plethora of collective phenomena. At low dopings, the tetragonal-to-orthorhombic structural transition is closely followed by a spin ... More
Doping influence of spin dynamics and magnetoelectric effect in hexagonal Y$_{0.7}$Lu$_{0.3}$MnO$_{3}$Aug 13 2013We use inelastic neutron scattering to study spin waves and their correlation with the magnetoelectric effect in Y$_{0.7}$Lu$_{0.3}$MnO$_3$. In the undoped YMnO$_3$ and LuMnO$_3$, the Mn trimerization distortion has been suggested to play a key role in ... More