arXiv subjects

Gang Chen

Publications and source records attributed to Gang Chen.

At least 685 records · Page 38Linked to original sources

arXiv2012

Magnetic orders and topological phases from f-d exchange in pyrochlore iridates

We study theoretically the effects of f-d magnetic exchange interaction in the R$_2$Ir$_2$O$_7$ pyrochlore iridates. The R$^{3+}$ f-electrons form localized magnetic doublets due to the crystal field environment, while the Ir$^{4+}$ d- electrons are more itinerant and feel a strong spin-orbit coupling. We construct and analyze a minimal model capturing this physics, treating the Ir subsystem using a Hubbard-type model. First neglecting the Hubbard interaction, we find Weyl semi-metal and Axion insulator phases induced by the f-d exchange. Next, we find that f-d exchange can cooperate with the Hubbard interaction to stabilize the Weyl semi-metal over a larger region of parameter space than when it is induced by d-electron correlations alone. Applications to experiments are discussed.

cond-mat.str-el↗

arXiv2012

Lifetime of sub-THz coherent acoustic phonons in a GaAs-AlAs superlattice

We measure the lifetime of the zone-center 340 GHz longitudinal phonon mode in a GaAs-AlAs superlattice excited and probed with femtosecond laser pulses. By comparing measurements conducted at room temperature and liquid nitrogen temperature we separate the intrinsic (phonon-phonon scattering) and extrinsic contributions to phonon relaxation. The estimated room temperature intrinsic lifetime of 0.95 ns is compared to available calculations and experimental data for bulk GaAs. We conclude that ~0.3 THz phonons are in the transition zone between Akhiezer and Landau-Rumer regimes of phonon-phonon relaxation at room temperature.

cond-mat.mes-hall↗

arXiv2012

Antimatter production in central Au+Au collisions at $\sqrt{s_{\rm{NN}}}$=200 GeV

We have used the dynamically constrained phase space coalescence model to investigate the production of light nuclei (anti-nuclei) based on the 1.134$\times 10^7$ hadronic final states generated by the PACIAE model for the 0-5% most central Au+Au collisions at $\sqrt{s_{\rm{NN}}}$=200 GeV with $|y| <$1 and $p_T<$5 acceptances. The STAR data of $\bar{_{\barΛ}^ 3H}$, ${_Λ^3 H}$, $^3{\bar{He}}$, and $^3{He}$ yields and ratios are well reproduced by the corresponding PACIAE results. The transverse momentum distribution of $\bar{^3{He}}$ (${^3{He}}$) and $\bar{_{\barΛ}^3H}$ (${_Λ^3H}$) is also given. It turned out that the transverse momentum distribution of light nuclei is close to that of the corresponding anti-nuclei.

nucl-th↗

arXiv2012

Thermodynamics of spin-orbit-coupled Bose-Einstein condensates

In this paper we develop a quantum field approach to reveal the thermodynamic properties of the trapped BEC with the equal Rashba and Dresselhaus spin-orbit couplings. In the experimentally-feasible regime, the phase transition from the separate phase to the single minimum phase can be well driven by the tunable temperature. Moreover, the critical temperature, which is independent of the trapped potential, can be derived exactly. At the critical point, the specific heat has a large jump and can be thus regarded as a promising candidate to detect this temperature-driven phase transition. In addition, we obtain the analytical expressions for the specific heat and the entropy in the different phases. In the single minimum phase, the specific heat as well as the entropy are governed only by the Rabi frequency. However, in the separate phase with lower temperature, we find that they are determined only by the strength of spin-orbit coupling. Finally, the effect of the effective atom interaction is also addressed. In the separate phase, this effective atom interaction affects dramatically on the critical temperature and the corresponding thermodynamic properties.

cond-mat.quant-gas↗

arXiv2012

Searching for Majorana Fermions in 2D Spin-orbit Coupled Fermi Superfluids at Finite Temperature

Recent experimental breakthrough in realizing spin-orbit (SO) coupling for cold atoms has spurred considerable interest in the physics of 2D SO coupled Fermi superfluids, especially topological Majorana fermions (MFs) which were predicted to exist at zero temperature. However, it is well known that long-range superfluid order is destroyed in 2D by the phase fluctuation at finite temperature and the relevant physics is the Berezinskii-Kosterlitz-Thouless (BKT) transition. In this Letter, we examine finite temperature effects on SO coupled Fermi gases and show that finite temperature is indeed necessary for the observation of MFs. MFs are topologically protected by a quasiparticle energy gap which is found to be much larger than the temperature. The restrictions to the parameter region for the observation of MFs have been obtained.

cond-mat.quant-gas↗

arXiv2012

Frustrated quantum critical theory of putative spin-liquid phenomenology in 6H-B-Ba3NiSb2O9

A recently discovered material, 6H-B-Ba3NiSb2O9 was found to display unusual low-temperature phenomenology, interpreted as a quantum spin liquid with spin S = 1 on a triangular lattice. We study a spin S = 1 quantum exchange model on an AB stacked triangular lattice near its quantum paramagnet-to-spiral magnet transition, driven by easy-plane single-ion anisotropy. We demonstrate that the frustrated inter- and intra-layer exchange interactions induce contour lines of low energy excitations that lead to a broad crossover regime of linear-temperature dependence of the specific heat. Based on this and a variety of other predictions, we argue that the observed phenomenology can be understood in terms of a conventional picture of a proximity to this frustrated critical point.

cond-mat.str-el↗

arXiv2012

Thermodynamics, geometrothermodynamics and critical behavior of (2+1)-dimensional black holes

In this paper, we study the properties of the (2+1)-dimensional black holes from the viewpoint of geometrothermodynamics. We show that the Legendre invariant metric of the (2+1)-dimensional black holes can produce correctly the behavior of the thermodynamic interaction and phase transition structure of the corresponding black hole configurations. We find that they are both curved and the curvature scalar gives the information about the phase transition point.

gr-qc↗

arXiv2012

LogBase: A Scalable Log-structured Database System in the Cloud

Numerous applications such as financial transactions (e.g., stock trading) are write-heavy in nature. The shift from reads to writes in web applications has also been accelerating in recent years. Write-ahead-logging is a common approach for providing recovery capability while improving performance in most storage systems. However, the separation of log and application data incurs write overheads observed in write-heavy environments and hence adversely affects the write throughput and recovery time in the system. In this paper, we introduce LogBase - a scalable log-structured database system that adopts log-only storage for removing the write bottleneck and supporting fast system recovery. LogBase is designed to be dynamically deployed on commodity clusters to take advantage of elastic scaling property of cloud environments. LogBase provides in-memory multiversion indexes for supporting efficient access to data maintained in the log. LogBase also supports transactions that bundle read and write operations spanning across multiple records. We implemented the proposed system and compared it with HBase and a disk-based log-structured record-oriented system modeled after RAMCloud. The experimental results show that LogBase is able to provide sustained write throughput, efficient data access out of the cache, and effective system recovery.

cs.DB↗

arXiv2012

Status Report of the DPHEP Study Group: Towards a Global Effort for Sustainable Data Preservation in High Energy Physics

Data from high-energy physics (HEP) experiments are collected with significant financial and human effort and are mostly unique. An inter-experimental study group on HEP data preservation and long-term analysis was convened as a panel of the International Committee for Future Accelerators (ICFA). The group was formed by large collider-based experiments and investigated the technical and organisational aspects of HEP data preservation. An intermediate report was released in November 2009 addressing the general issues of data preservation in HEP. This paper includes and extends the intermediate report. It provides an analysis of the research case for data preservation and a detailed description of the various projects at experiment, laboratory and international levels. In addition, the paper provides a concrete proposal for an international organisation in charge of the data management and policies in high-energy physics.

hep-ex↗

arXiv2012

Greedy Multiple Instance Learning via Codebook Learning and Nearest Neighbor Voting

Multiple instance learning (MIL) has attracted great attention recently in machine learning community. However, most MIL algorithms are very slow and cannot be applied to large datasets. In this paper, we propose a greedy strategy to speed up the multiple instance learning process. Our contribution is two fold. First, we propose a density ratio model, and show that maximizing a density ratio function is the low bound of the DD model under certain conditions. Secondly, we make use of a histogram ratio between positive bags and negative bags to represent the density ratio function and find codebooks separately for positive bags and negative bags by a greedy strategy. For testing, we make use of a nearest neighbor strategy to classify new bags. We test our method on both small benchmark datasets and the large TRECVID MED11 dataset. The experimental results show that our method yields comparable accuracy to the current state of the art, while being up to at least one order of magnitude faster.

cs.LG↗

arXiv2012

Direct Measurement of Room Temperature Non-diffusive Thermal Transport Over Micron Distances in a Silicon Membrane

The "textbook" phonon mean free path (MFP) of heat carrying phonons in silicon at room temperature is ~40 nm. However, a large contribution to the thermal conductivity comes from low-frequency phonons with much longer MFPs. We present a simple experiment demonstrating that room temperature thermal transport in Si significantly deviates from the diffusion model already at micron distances. Absorption of crossed laser pulses in a freestanding silicon membrane sets up a sinusoidal temperature profile that is monitored via diffraction of a probe laser beam. By changing the period of the thermal grating we vary the heat transport distance within the range ~1-10 μm. At small distances, we observe a reduction in the effective thermal conductivity indicating a transition from the diffusive to the ballistic transport regime for the low-frequency part of the phonon spectrum.

cond-mat.mtrl-sci↗

arXiv2012

Spinor formalism for massive fields with half-integral spin

In this article, we extend the %Weyl-van der Waerden spinor technique for calculating helicity amplitudes to general massive fields of half-integer spins. We find that the little group generators can be represented as first-order differential operators in the spinor formalism. We use the spinor forms of the generators to get the explicit form of the massive fields of any spin and any helicity. We also deal with the three-particle S-matrix by these spinor form generators, and find that we are able to extend the explicit form of the three-particle vertex obtained by Benincasa and Cachazo to the massive case. We present the explicit expressions for the amplitudes with external particles of the lowest helicities up to -3/2. Group theory, in the form of raising operators of the little group, then dictates other amplitudes with higher helicity in the same spin multiplets. The formalism allows, in principle, to determine the electromagnetic form-factors of charged particles of arbitrary helicities, without additional assumptions about the underlying lagrangian. We find that restrictions which follow from gauge and Lorentz invariance are nearly as restrictive as in the massless case.

hep-th↗

arXiv2012

Complexified Ward Identity in pure Yang-Mills theory at tree-level

Up until now, the BCFW technique has been a widely used method in getting the amplitudes in various theories. Usually, the vanishing of the boundary term is necessary for the efficiency of the method. However, there are also many kinds of amplitudes which will have boundary terms. Hence it will be nice if it is possible to get the boundary terms in an efficient manner. As is well-known, in gauge theory the Ward identity imposes constraints on the form of the amplitude. In [1], we studied the Ward identity with a pair of shifted lines and the implied recursion relations. In this article, we discuss the complexified Ward identity in more detail. In particular we give a proof of the complexified Ward identity directly from the Feynman rules in Feynman-Lorentz gauge. Furthermore, we give more examples in calculating the one-line off-shell currents using this technique.

hep-th↗

arXiv2012

Ward Identity implied recursion relations in Yang-Mills theory

The Ward identity in gauge theory constrains the behavior of the amplitudes. We discuss the Ward identity for amplitudes with a pair of shifted lines with complex momenta. This will induce a recursion relation identical to BCFW recursion relations at the finite poles of the complexified amplitudes. Furthermore, according to the Ward identity, it is also possible to transform the boundary term into a simple form, which can be obtained by a new recursion relation. For the amplitude with one off-shell line in pure Yang-Mills theory, we find this technique is effective for obtaining the amplitude even when there are boundary contributions.

hep-th↗

arXiv2012

Microscopic mechanism of low thermal conductivity in lead-telluride

The microscopic physics behind low lattice thermal conductivity of single crystal rocksalt lead telluride (PbTe) is investigated. Mode-dependent phonon (normal and umklapp) scattering rates and their impact on thermal conductivity were quantified by the first-principles-based anharmonic lattice dynamics calculations that accurately reproduce thermal conductivity in a wide temperature range. The low thermal conductivity of PbTe is attributed to the scattering of longitudinal acoustic phonons by transverse optical phonons with large anharmonicity, and small group velocity of the soft transverse acoustic phonons. This results in enhancing the relative contribution of optical phonons, which are usually minor heat carrier in bulk materials.

cond-mat.mtrl-sci↗

arXiv2012

Analytical solutions for the Rabi model

The Rabi model that describes the fundamental interaction between a two-level system with a quantized harmonic oscillator is one of the simplest and most ubiquitous models in modern physics. However, this model has not been solved exactly because it is hard to find a second conserved quantity besides the energy. Here we present a unitary transformation to map this unsolvable Rabi model into a solvable Jaynes-Cummings-like model by choosing a proper variation parameter. As a result, the analytical energy spectrums and wavefunctions including both the ground and the excited states can be obtained easily. Moreover, these explicit results agree well with the direct numerical simulations in a wide range of the experimental parameters. In addition, based on our obtained energy spectrums, the recent experimental observation of Bloch-Siegert in the circuit quantum electrodynamics with the ultrastrong coupling can be explained perfectly. Our results have the potential application in the solid-state quantum information processing.

quant-ph↗

arXiv2012

Majorana spin liquids and projective realization of SU(2) spin symmetry

We revisit the fermionic parton approach to S = 1/2 quantum spin liquids with SU(2) spin rotation symmetry, and the associated projective symmetry group (PSG) classification. We point out that the existing PSG classification is incomplete; upon completing it, we find spin liquid states with S=1 and S=0 Majorana fermion excitations coupled to a deconfined Z2 gauge field. The crucial observation leading us to this result is that, like space group and time reversal symmetries, spin rotations can act projectively on the fermionic partons; that is, a spin rotation may be realized by simultaneous SU(2) spin and gauge rotations. We show that there are only two realizations of spin rotations acting on fermionic partons: the familiar naive realization where spin rotation is not accompanied by any gauge transformation, and a single type of projective realization. We discuss the PSG classification for states with projective spin rotations. To illustrate these results, we show that there are four such PSGs on the two-dimensional square lattice. We study the properties of the corresponding states, finding that one -- with gapless Fermi points -- is a stable phase beyond mean-field theory. In this phase, depending on parameters, a small Zeeman magnetic field can open a partial gap for the Majorana fermion excitations. Moreover, there are nearby gapped phases supporting Z2 vortex excitations obeying non-Abelian statistics. We conclude with a discussion of various open issues, including the challenging question of where such S=1 Majorana spin liquids may occur in models and in real systems.

cond-mat.str-el↗

arXiv2012

Finite temperature Dicke phase transition of a Bose-Einstein condensate in an optical cavity

Dicke model predicts a quantum phase transition from normal to superradiant phases for a two-level atomic ensemble coupled with an optical cavity at zero temperature. In a recent pioneer experiment [Nature 464, 1301 (2010)], such a phase transition has been observed using a Bose-Einstein condensate (BEC) in an optical cavity. Compared with the original Dicke model, the experimental system features finite temperature and strong atom-photon nonlinear interaction in BEC. In this Letter, we develop a finite temperature theory for the Dicke phase transition of a BEC in an optical cavity, taking into account the atom-photon nonlinear interaction. In addition to explaining the experimentally observed transition from normal to superradiant phases at finite-temperature, we point it out that a new phase, the coexistence of normal and superradient phases, was also observed in the experiment. We show rich finite temperature phase diagrams existing in the experimental system by tuning various experimental parameters. We find that the specific heat of the BEC can serve as a powerful tool for probing various phases.

cond-mat.quant-gas↗