arXiv subjects

Bo Gao

Publications and source records attributed to Bo Gao.

At least 127 records · Page 7Linked to original sources

arXiv2015

Growth and Characterization of Millimeter-sized Single Crystals of CaFeAsF

High-quality and sizable single crystals are crucial for studying the intrinsic properties of unconventional superconductors, which are lacking in the 1111 phase of the Fe-based superconductors. Here we report the successful growth of CaFeAsF single crystals with the sizes of 1-2 mm using the self-flux method. Owning to the availability of the high-quality single crystals, the structure and transport properties were investigated with a high reliability. The structure was refined by using the single-crystal x-ray diffraction data, which confirms the reports earlier on the basis of powder data. A clear anomaly associated with the structural transition was observed at 121 K from the resistivity, magnetoresistance, and magnetic susceptibility measurements. Another kink-feature at 110 K, most likely an indication of the antiferromagnetic transition, was also detected in the resistivity data. Our results supply a basis to propel the physical investigations on the 1111 phase of the Fe-based superconductors.

cond-mat.supr-con↗

arXiv2015

Coulomb Excitations for a Short Linear Chain of Metallic Shells

A self-consistent-field theory is given for the electronic collective modes of a chain containing a finite number, $N$, of Coulomb-coupled spherical two-dimensional electron gases (S2DE's) arranged with their centers along a straight line, simulating a narrow micro-ribbon of metallic shells. The separation between nearest-neighbor shells is arbitrary and because of the quantization of the electron energy levels due to their confinement to the spherical surface, all angular momenta $L$ of the Coulomb excitations and their projections $M$ on the quantization axis are coupled. However, for incoming light with a specific polarization, only one angular momentum quantum number is chosen. We show that when $N=3$ the next-nearest-neighbor Coulomb coupling is larger than its value if they are located at opposite ends of a right-angle triangle forming the triad. Additionally, the frequencies of the plasma excitations depend on the orientation of the line joining them with respect to the axis of quantization since the magnetic field generated from the induced oscillating electric dipole moment on one sphere can couple to the induced magnetic dipole moment on another.

cond-mat.mes-hall↗

arXiv2014

Multiscale quantum-defect theory and its application to atomic spectrum

We present a multiscale quantum-defect theory based on the first analytic solution for a two-scale long range potential consisting of a Coulomb potential and a polarization potential. In its application to atomic structure, the theory extends the systematic understanding of atomic Rydberg states, as afforded by the standard single-scale quantum-defect theory, to a much greater range of energies to include the first few excited states and even the ground state. Such a level of understanding has important implications not only on atomic structure, but also on the electronic structure of molecules and on atomic and molecular interactions and reactions. We demonstrate the theory by showing that it provides an analytic description of the energy variations of the standard Coulomb quantum defects for alkali-metal atoms.

physics.atom-ph↗

arXiv2014

Synthesis, Structural, and Transport Properties of Cr-doped BaTi_2As_2O

The interplay between unconventional superconductivity and the ordering of charge/spin density wave is one of the most vital issues in both condensed matter physics and material science. The Ti-based compound BaTi_2As_2O, which can be seen as the parent phase of superconducting BaTi_2Sb_2O, has a layered structure with a space group P4/mmm, similar to that of cuprate and iron-based superconductors. This material exhibits a charge density wave (CDW) ordering transition revealed by an anomaly at around 200 K in transport measurements. Here, we report the synthesis and systematical study of the physical properties in Cr-doped BaTi_{2-x}Cr_xAs_2O (x = 0 - 0.154), and demonstrate that the transition temperature of the CDW ordering is suppressed gradually by the doped Cr element. The magnetization measurements confirm the evolution of the CDW ordering transition. These observed behaviors are similar to that observed in iron-based superconductors, but no superconductivity emerges down to 2 K. In addition, the first-principles calculations are also carried out for well-understanding the nature of experimental observations.

cond-mat.supr-con↗

arXiv2014

Proton-hydrogen collision at cold temperatures

We study the proton-hydrogen collision in the energy range from 0 to 5 K where the hyperfine structure of the hydrogen atom is important. The proper multichannel treatment of the hyperfine structure is found to be crucial at cold temperatures compared to the elastic approximation used for higher temperatures. Both elastic and hyperfine-changing inelastic processes are investigated using the newly developed multichannel quantum-defect theory (MQDT) as well as the traditional coupled-channel numerical method. Cross section results from the two methods coincide well throughout the energy range. The MQDT with a few parameters provide an efficient and basically analytic description of the proton-hydrogen interaction in this energy regime.

physics.atom-ph↗

arXiv2014

Novel properties of graphene in the presence of energy gap: optics, transport and mobility studies

We review the transmission of Dirac electrons through a potential barrier in the presence of circularly polarized light. A different type of transmission is demonstrated and explained. Perfect transmission for nearly head-on collision in inffnite graphene is suppressed in gapped dressed states of electrons. We also present our results on enhanced mobility of hot Dirac electrons in nanoribbons and magnetoplasmons in graphene in the presence of the energy gap. The calculated carrier mobility for a graphene nanoribbon as a function of the bias field possesses a high threshold for entering the nonlinear transport regime. This threshold is a function of both extrinsic and intrinsic properties, such as lattice temperature, linear density, impurity scattering strength, ribbon width, and correlation length for the line-edge roughness. Analysis of non-equilibrium carrier distribution function confirms that the difference between linear and nonlinear transport is due to sweeping electrons from the right to left Fermi one through elastic scattering as well as moving electrons from low to high-energy ones through field-induced heating. The plasmons, as well as the electron-hole continuum are determined by both energy gap and the magnetic field, showing very specific features, which have been studied and discussed in details.

cond-mat.mes-hall↗

arXiv2014

Phase Diagram and Weak-link Behavior in Nd-doped CaFe_2As_2

The transport properties, phase diagram, and dopant distribution are investigated in systematically Nd doped CaFe_2As_2 single crystals. Coexistence of two superconducting (SC) phases with different critical transition temperature (T_c) is observed. The low-T_c phase emerges as x >= 0.031, and the T_c value increases to its maximum value of about 20 K at x = 0.083, the maximum doping level in our study. As x >= 0.060, the high-T_c phase with a T_c value of about 40 K is observed. The structure transition (STr) from tetragonal to orthorhombic phase vanishes suddenly around x = 0.060, where a new STr from tetragonal to collapsed tetragonal phase begins to turn up. Compared to the low-T_c phase, the end point of SC transition of the high-T_c phase is more sensitive to the magnetic field, showing a characteristic of Josephson weak-link behavior. Possible scenarios about this system are discussed based on our observations. We also find that the non-uniform SC properties cannot be attributed to the heterogeneous Nd distribution on the micro scale, as revealed by the detailed energy dispersive X-ray spectroscopy (EDS) measurements.

cond-mat.supr-con↗

arXiv2014

OS4M: Achieving Global Load Balance of MapReduce Workload by Scheduling at the Operation Level

The efficiency of MapReduce is closely related to its load balance. Existing works on MapReduce load balance focus on coarse-grained scheduling. This study concerns fine-grained scheduling on MapReduce operations, with each operation representing one invocation of the Map or Reduce function. By default, MapReduce adopts the hash-based method to schedule Reduce operations, which often leads to poor load balance. In addition, the copy phase of Reduce tasks overlaps with Map tasks, which significantly hinders the progress of Map tasks due to I/O contention. Moreover, the three phases of Reduce tasks run in sequence, while consuming different resources, thereby under-utilizing resources. To overcome these problems, we introduce a set of mechanisms named OS4M (Operation Scheduling for MapReduce) to improve MapReduce's performance. OS4M achieves load balance by collecting statistics of all Map operations, and calculates a globally optimal schedule to distribute Reduce operations. With OS4M, the copy phase of Reduce tasks no longer overlaps with Map tasks, and the three phases of Reduce tasks are pipelined based on their operation loads. OS4M has been transparently incorporated into MapReduce. Evaluations on standard benchmarks show that OS4M's job duration can be shortened by up to 42%, compared with a baseline of Hadoop.

cs.DC↗

arXiv2014

Multichannel quantum-defect theory for magnetic Feshbach resonances in heteronuclear group I systems

We present a multichannel quantum-defect theory for magnetic Feshbach resonances in the interaction of two heteronuclear group I atoms. The theory provides a unified and a uniform description of resonances in all partial waves, and enables the characterization of large number of resonances in terms of very few parameters. For the sample system of $^6$Li$^{40}$K, we present descriptions of all resonances in $aa$, $ab$, and $ba$ channels, in partial waves $s$ ($l=0$) through $h$ ($l=5$), and in a magnetic field of 0 through 1000 Gauss. All resonances, including those in nonzero partial waves, are fully characterized using the newly developed parametrization of Gao [Phy. Rev. A \textbf{84}, 022706 (2011)].

physics.atom-ph↗

arXiv2014

Improving the Load Balance of MapReduce Operations based on the Key Distribution of Pairs

Load balance is important for MapReduce to reduce job duration, increase parallel efficiency, etc. Previous work focuses on coarse-grained scheduling. This study concerns fine-grained scheduling on MapReduce operations. Each operation represents one invocation of the Map or Reduce function. Scheduling MapReduce operations is difficult due to highly screwed operation loads, no support to collect workload statistics, and high complexity of the scheduling problem. So current implementations adopt simple strategies, leading to poor load balance. To address these difficulties, we design an algorithm to schedule operations based on the key distribution of intermediate pairs. The algorithm involves a sub-program for selecting operations for task slots, and we name it the Balanced Subset Sum (BSS) problem. We discuss properties of BSS and design exact and approximation algorithms for it. To transparently incorporate these algorithms into MapReduce, we design a communication mechanism to collect statistics, and a pipeline within Reduce tasks to increase resource utilization. To the best of our knowledge, this is the first work on scheduling MapReduce workload at this fine-grained level. Experiments on PUMA [T+12] benchmarks show consistent performance improvement. The job duration can be reduced by up to 37%, compared with standard MapReduce.

cs.DC↗

arXiv2014

User recommendation in reciprocal and bipartite social networks -- a case study of online dating

Many social networks in our daily life are bipartite networks built on reciprocity. How can we recommend users/friends to a user, so that the user is interested in and attractive to recommended users? In this research, we propose a new collaborative filtering model to improve user recommendations in reciprocal and bipartite social networks. The model considers a user's "taste" in picking others and "attractiveness" in being picked by others. A case study of an online dating network shows that the new model has good performance in recommending both initial and reciprocal contacts.

cs.SI↗

arXiv2014

Multichannel quantum-defect theory for ion-atom interactions

We present a quantum theory of ion-atom interaction that is applicable at energies comparable to or smaller than the atomic hyperfine splitting and takes proper account of the effects of identical nuclei. The theory reveals the subtlety and the complexity of cold ion-atom interactions including the change of threshold behavior due to hyperfine splitting and the existence of a large number, and a variety of scattering resonances between hyperfine thresholds. We show how this complexity is described, efficiently and at a quantitative level, using a multichannel quantum-defect theory that we present here for ion-atom interactions. Such an efficient description is a key enabling element to understand few-body and many-body quantum systems involving ions.

physics.atom-ph↗

arXiv2014

Modeling Anisotropic Plasmon Excitations in Self-Assembled Fullerenes

The plasmon excitations in Coulomb-coupled spherical two-dimensional electron gases (S2DEGs) reveal an interesting dependence on the displacement vector between the centers of the spheres with respect to the axis of quantization for the angular momentum quantum number L. Specifically, plasmon modes for a bundle of three S2DEGs have been obtained within the random-phase approximation (RPA). The inter-sphere Coulomb interaction matrix elements and their symmetry properties were also investigated in detail. The case of a bundle gives an adequate picture of the way in which the Coulomb interaction depends on the orbital angular momentum quantum number L and its projection M. We concluded that the interaction between the S2DEGs aligned at an angle of 45o with the axis of quantization is negligible compared to the interaction along and perpendicular to the quantization axis, which are themselves unequal to each other. Consequently, the plasmon excitation frequencies reveal an interesting orientational anisotropic coupling to an external electromagnetic field probing the charge density oscillations. Our result on the spatial correlation may be experimentally observable. In this connection, there have already been some experimental reports pointing to a similar effect in nanoparticles

cond-mat.mes-hall↗

arXiv2013

Quantum-defect theory for $-1/r^4$ type of interactions

We present a quantum-defect theory (QDT) for the $-1/r^4$ type of long-range potential, as a foundation for a systematic understanding of charge-neutral quantum systems such as ion-atom, ion-molecule, electron-atom, and positron-atom interactions. The theory incorporates both conceptual and mathematical advances since earlier formulations of the theory. It also includes more detailed discussions of the concept of resonance spectrum and its representations, universal properties in charge-neutral quantum systems, and the QDT description of scattering resonances that is applicable to any $-1/r^n$ potential with $n>2$.

physics.atom-ph↗

arXiv2013

Ultracold collision in the presence of synthetic spin-orbit coupling

We present an analytic description of ultracold collision between two spin-$\frac{1}{2}$ fermions with isotropic spin-orbit coupling of the Rashba type. We show that regardless of how weak the spin-orbit coupling may be, the ultracold collision at sufficiently low energies is significantly modified, including the ubiquitous Wigner threshold behavior. We further show that the particles are preferably scattered into the lower-energy helicity state due to the break of parity conservation, thus establishing interaction with spin-orbit coupling as one mechanism for the spontaneous emergence of handedness.

physics.atom-ph↗

arXiv2013

Two-dimensional magnetotransport in Bi2Te2Se nanoplatelets

Single-crystalline Bi2Te2Se nanoplates with thicknesses between 8 and 30 nm and lateral sizes of several micrometers were synthesized by a vapour-solid growth method. Angle-dependent magnetoconductance measurements on individual nanoplates revealed the presence of a two-dimensional weak anti-localization effect. In conjunction with gate-dependent charge transport studies performed at different temperatures, evidence was gained that this effect originates from the topologically protected surface states of the nanoplates.

cond-mat.mes-hall↗

arXiv2012

Integral with respect to the $G$-Brownian local time

Let ${\mathscr L}$ be the local time of $G$-Brownian motion $B$. In this paper, we prove the existence of the quadratic covariation $ _{t}$ and the integral $\int_{\mathbb R}f(x){\mathscr L}(dx,t)$. Moreover, a sublinear version of the Bouleau-Yor identity $$ \int_{\mathbb R}f(x){\mathscr L}(dx,t)=- _{t} $$ is showed to hold under some suitable conditions. These allow us to write the Itô's formula for $C^1$-functions.

math.PR↗

arXiv2012

The Bouleau-Yor identity for a bi-fractional Brownian motion

Let $B$ be a bi-fractional Brownian motion with indices $H\in (0,1),K\in (0,1]$, $2HK=1$ and let ${\mathscr L}(x,t)$ be its local time process. We construct a Banach space ${\mathscr H}$ of measurable functions such that the quadratic covariation $[f(B),B]$ and the integral $\int_{\mathbb R}f(x){\mathscr L}(dx,t)$ exist provided $f\in {\mathscr H}$. Moreover, the Bouleau-Yor identity $$ [f(B),B]_t=-2^{1-K}\int_{\mathbb R}f(x){\mathscr L}(dx,t),\qquad t\geq 0, $$ holds for all $f\in {\mathscr H}$.

math.PR↗