Xian Chen - Peking University

Xian Chen
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Xian Chen
Peking University

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Physics - Materials Science (14)
High Energy Astrophysical Phenomena (14)
General Relativity and Quantum Cosmology (11)
Cosmology and Nongalactic Astrophysics (10)
Astrophysics of Galaxies (9)
Quantum Physics (4)
Physics - Superconductivity (4)
Physics - Strongly Correlated Electrons (3)
Physics - Mesoscopic Systems and Quantum Hall Effect (3)
Mathematics - Optimization and Control (2)
Physics - Optics (2)
Instrumentation and Methods for Astrophysics (2)
Astrophysics (1)
Mathematical Physics (1)
Statistics - Theory (1)
Mathematics - Mathematical Physics (1)
High Energy Physics - Experiment (1)
Physics - Physics and Society (1)
Solar and Stellar Astrophysics (1)
Statistics - Applications (1)
Mathematics - Statistics (1)

Publications Authored By Xian Chen

Realizing long distance entanglement swapping with independent sources in the real-world condition is important for both future quantum network and fundamental study of quantum theory. Currently, demonstration over a few of tens kilometer underground optical fiber has been achieved. However, future applications demand entanglement swapping over longer distance with more complicated environment. Read More

Retrieving the mass of a gravitational-wave (GW) source is a difficult problem because it is degenerate with redshift. In astronomy three types of redshift exist, namely cosmological, Doppler, and gravitational redshift, but the latter two are often neglected in the analysis of GW data. Motivated by recent proposals that the binary black hole (BH) mergers detected by the Laser Interferometer GW Observatory (LIGO) could have happened in the vicinity of supermassive BHs (SMBHs), we study the effects of Doppler and gravitational redshifts on GWs. Read More

The formation of compact stellar-mass binaries is a difficult, but interesting problem in astrophysics. There are two main formation channels: In the field via binary star evolution, or in dense stellar systems via dynamical interactions. The Laser Interferometer Gravitational-Wave Observatory (LIGO) has detected black hole binaries (BHBs) via their gravitational radiation. Read More

Energy bandgap largely determines the optical and electronic properties of a semiconductor. Variable bandgap therefore makes versatile functionality possible in a single material. In layered material black phosphorus, the bandgap can be modulated by the number of layers; as a result, few-layer black phosphorus has discrete bandgap values that are relevant for opto-electronic applications in the spectral range from red, in monolayer, to mid-infrared in the bulk limit. Read More

The coalescence of two supermassive black holes (SMBHs) produces powerful gravitational-wave (GW) radiation and, if gas is present in the vicinity, also an electromagnetic (EM) counterpart. In the standard picture, an EM outburst will be produced when the binary "decouples" from the circum-binary disc and starts "squeezing" the disc inside the secondary orbit, resulting in its quick accretion on to the primary black hole. Here we use analytical arguments and numerical simulations to show that the disc within about $20~R_S$ of a SMBH survives the merger without being depleted. Read More

Authors: Demitri Muna, Michael Alexander, Alice Allen, Richard Ashley, Daniel Asmus, Ruyman Azzollini, Michele Bannister, Rachael Beaton, Andrew Benson, G. Bruce Berriman, Maciej Bilicki, Peter Boyce, Joanna Bridge, Jan Cami, Eryn Cangi, Xian Chen, Nicholas Christiny, Christopher Clark, Michelle Collins, Johan Comparat, Neil Cook, Darren Croton, Isak Delberth Davids, Éric Depagne, John Donor, Leonardo A. dos Santos, Stephanie Douglas, Alan Du, Meredith Durbin, Dawn Erb, Daniel Faes, J. G. Fernández-Trincado, Anthony Foley, Sotiria Fotopoulou, Søren Frimann, Peter Frinchaboy, Rafael Garcia-Dias, Artur Gawryszczak, Elizabeth George, Sebastian Gonzalez, Karl Gordon, Nicholas Gorgone, Catherine Gosmeyer, Katie Grasha, Perry Greenfield, Rebekka Grellmann, James Guillochon, Mark Gurwell, Marcel Haas, Alex Hagen, Daryl Haggard, Tim Haines, Patrick Hall, Wojciech Hellwing, Edmund Christian Herenz, Samuel Hinton, Renee Hlozek, John Hoffman, Derek Holman, Benne Willem Holwerda, Anthony Horton, Cameron Hummels, Daniel Jacobs, Jens Juel Jensen, David Jones, Arna Karick, Luke Kelley, Matthew Kenworthy, Ben Kitchener, Dominik Klaes, Saul Kohn, Piotr Konorski, Coleman Krawczyk, Kyler Kuehn, Teet Kuutma, Michael T. Lam, Richard Lane, Jochen Liske, Diego Lopez-Camara, Katherine Mack, Sam Mangham, Qingqing Mao, David J. E. Marsh, Cecilia Mateu, Loïc Maurin, James McCormac, Ivelina Momcheva, Hektor Monteiro, Michael Mueller, Roberto Munoz, Rohan Naidu, Nicholas Nelson, Christian Nitschelm, Chris North, Juan Nunez-Iglesias, Sara Ogaz, Russell Owen, John Parejko, Vera Patrício, Joshua Pepper, Marshall Perrin, Timothy Pickering, Jennifer Piscionere, Richard Pogge, Radek Poleski, Alkistis Pourtsidou, Adrian M. Price-Whelan, Meredith L. Rawls, Shaun Read, Glen Rees, Hanno Rein, Thomas Rice, Signe Riemer-Sørensen, Naum Rusomarov, Sebastian F. Sanchez, Miguel Santander-García, Gal Sarid, William Schoenell, Aleks Scholz, Robert L. Schuhmann, William Schuster, Peter Scicluna, Marja Seidel, Lijing Shao, Pranav Sharma, Aleksandar Shulevski, David Shupe, Cristóbal Sifón, Brooke Simmons, Manodeep Sinha, Ian Skillen, Bjoern Soergel, Thomas Spriggs, Sundar Srinivasan, Abigail Stevens, Ole Streicher, Eric Suchyta, Joshua Tan, O. Grace Telford, Romain Thomas, Chiara Tonini, Grant Tremblay, Sarah Tuttle, Tanya Urrutia, Sam Vaughan, Miguel Verdugo, Alexander Wagner, Josh Walawender, Andrew Wetzel, Kyle Willett, Peter K. G. Williams, Guang Yang, Guangtun Zhu, Andrea Zonca

The Astropy Project (http://astropy.org) is, in its own words, "a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages." For five years this project has been managed, written, and operated as a grassroots, self-organized, almost entirely volunteer effort while the software is used by the majority of the astronomical community. Read More

There has been considerable interest in exploiting the spin degrees of freedom of electrons for potential information storage and computing technologies. Topological insulators (TI), a class of quantum materials, have special gapless edge/surface states, where the spin polarization of the Dirac fermions is locked to the momentum direction. This spin-momentum locking property gives rise to very interesting spin-dependent physical phenomena such as the Edelstein and inverse Edelstein effects. Read More

We report European Very Long Baseline Interferometry Network (EVN) radio continuum observations of ASASSN-14li, one of the best studied tidal disruption events (TDEs) to date. At 1.7 GHz with ~12x6mas resolution, the emission is unresolved. Read More

We report a candidate centi-parsec supermassive black hole binary (SMBHB) in the radio-quiet quasar SDSS J0159+0105 at z=0.217. With a modified lomb-scargle code GLSdeDRW and the auto-correlation analysis ACF, we detect two significant (at P>99%) periodic signals at ~741 day and ~1500 day from the 8. Read More

This paper studies continuous-time Markov decision processes under the risk-sensitive average cost criterion. The state space is a finite set, the action space is a Borel space, the cost and transition rates are bounded, and the risk-sensitivity coefficient can take arbitrary positive real numbers. Under the mild conditions, we develop a new approach to establish the existence of a solution to the risk-sensitive average cost optimality equation and obtain the existence of an optimal deterministic stationary policy. Read More

A star wandering too close to a supermassive black hole (SMBH) will be tidally disrupted. Previous studies of such "tidal disruption event" (TDE) mostly focus on the stellar debris that are bound to the system, because they give rise to luminous flares. On the other hand, half of the stellar debris in principle are unbound and can stream to a great distance, but so far there is no clear evidence that this "unbound debris stream" (UDS) exists. Read More

We investigate all possible noise environments for controlled remote state preparation. We find that the optimal efficiency is not only dependent on the isolate decay rate but also dependent on the coupling term of environments. Such a coupling provides a clue to control the noisy quantum system for realistic quantum communication. Read More

Gravitational waves are a prediction of general relativity, and with ground-based detectors now running in their advanced configuration, we will soon be able to measure them directly for the first time. Binaries of stellar-mass black holes are among the most interesting sources for these detectors. Unfortunately, the many different parameters associated with the problem make it difficult to promptly produce a large set of waveforms for the search in the data stream. Read More

The kinetic energy of a star in orbit about a supermassive black hole is a significant fraction of its rest mass energy when its periapse is comparable to its tidal radius. Upon its destruction, a fraction of this energy is extracted and injected into the stellar debris, half of which becomes unbound from the black hole, with the fastest material moving at $\sim 0.03 c$. Read More

Statistical models for proteomics data often estimate protein fold changes between two samples, A and B, as the average peptide intensity from sample A divided by the average peptide intensity from sample B. Such average intensity ratios fail to take full advantage of the experimental design which eliminates unseen confounding variables by processing peptides from both samples under identical conditions. Typically this structure is exploited through the estimation of a protein ratio as the median ratio of matched peptide intensities. Read More

Motivated by recent experimental observation of an hydrostatic pressure induced transition from semiconductor to semimetal in black phosphorus [Chen et al. in arXiv:1504.00125], we present the first principles calculation on the pressure effect of the electronic structures of black phosphorus. Read More

Electron-electron and electron-phonon interactions are two major driving forces that stabilize various charge-ordered phases of matter. The intricate interplay between the two give rises to a peculiar charge density wave (CDW) state, which is also known as a Mott insulator, as the ground state of layered compound 1T-TaS2. The delicate balance also makes it possible to use external perturbations to create and manipulate novel phases in this material. Read More

Cold gas entering the central $1$ to $10^2$ pc of a galaxy fragments and condenses into clouds. The stability of the clouds determines whether they will be turned into stars or can be delivered to the central supermassive black hole (SMBH) to turn on an active galactic nucleus (AGN). The conventional criteria to assess the stability of these clouds, such as the Jeans criterion and Roche (or tidal) limit, are insufficient here, because they assume the dominance of self-gravity in binding a cloud, and neglect external agents, such as pressure and tidal forces, which are common in galactic nuclei. Read More

Development of new, high quality functional materials has been at the forefront of condensed matter research. The recent advent of two-dimensional black phosphorus has greatly enriched the material base of two-dimensional electron systems. Significant progress has been made to achieve high mobility black phosphorus two-dimensional electron gas (2DEG) since the development of the first black phosphorus field-effect transistors (FETs)$^{1-4}$. Read More

We demonstrated a large enhancement of Kerr electro-optic nonlinearity through cascaded Pockels effects in a domain inversion ferroelectric crystal. We designed a structure that can implement the cascaded Pockels effects and second-harmonic generation simultaneously. The energy coupling between the fundamental lights of different polarizations led to a large nonlinear phase shift, and thus an effective electro-optic nonlinear refractive index. Read More

Layered non-centrosymmetric bismuth tellurohalides are being examined as candidates for topological insulators. Pressure is believed to be essential for inducing and tuning topological order in these systems. Through electrical transport and Raman scattering measurements, we find superconductivity in two high-pressure phases of BiTeCl with the different normal state features, carrier characteristics, and upper critical field behaviors. Read More

The transformation stretch tensor plays an essential role in the evaluation of conditions of compatibility between phases and the use of the Cauchy-Born rule. This tensor is difficult to measure directly from experiment. We give an algorithm for the determination of the transformation stretch tensor from x-ray measurements of structure and lattice parameters. Read More

Superconductivity of high critical temperature ($T_{c}$) superconductors is usually realized through chemical dopant or application of pressure in a similar way to induce charge carriers of either electrons or holes into their parent compounds. For chemical doping, superconductivity behaves asymmetrically with the maximum $T_{c}$ often higher for optimal hole-doping than that of optimal electron-doping on the same parent compound. However, whether electron carriers could be in favour of higher $T_{c}$ than holes in such high-$T_{c}$ superconductors is unknown but attractive. Read More

Sagittarius A*, lying the Galactic Center $8$ kpc from Earth, hosts the closest supermassive black hole known to us. It is now inactive, but there are evidences indicating that about six million years ago it underwent a powerful outburst where the luminosity could have approached the Eddington limit. Motivated by the fact that in extragalaxies the supermassive black holes with similar masses and near-Eddington luminosities are usually strong X-ray emitters, we calculate here the X-ray luminosity of Sagittarius A*, assuming that the outburst was due to accretion of gas or tidal disruption of stars, both scenarios having been considered to trigger the previous outburst. Read More

We theoretically analyze the Einstein-Podolsky-Rosen (EPR) correlation, the quadrature squeezing, and the continuous-variable quantum teleportation when considering non-Gaussian entangled states generated by applying multiple-photon subtraction and multiple-photon addition to a two-mode squeezed vacuum state (TMSVs). Our results indicate that in the case of the multiple-photon-subtracted TMSVs with symmetric operations, the corresponding EPR correlation, the two-mode squeezing degree, the sum squeezing, and the fidelity of teleporting a coherent state or a squeezed vacuum state can be enhanced for any squeezing parameter r and these enhancements increase with the number of subtracted photons in the low-squeezing regime, while asymmetric multiple-photon subtractions will generally reduce these quantities. For the multiple-photon-added TMSVs, although it holds stronger entanglement, its EPR correlation, two-mode squeezing, sum squeezing, and the fidelity of a coherent state are always smaller than that of the TMSVs. Read More

Two-dimensional electron gases (2DEG) have been an important source of experimental discovery and conceptual development in condensed matter physics for decades. When combined with the unique electronic properties of two-dimensional (2D) crystals, rich new physical phenomena can be probed at the quantum level. Here, we create a new 2DEG in black phosphorus, a recent member of the two-dimensional atomic crystal family, using a gate electric field. Read More

The present work reports on a feasibility study commissioned by the Chinese Academy of Sciences of China to explore various possible mission options to detect gravitational waves in space alternative to that of the eLISA/LISA mission concept. Based on the relative merits assigned to science and technological viability, a few representative mission options descoped from the ALIA mission are considered. A semi-analytic Monte Carlo simulation is carried out to understand the cosmic black hole merger histories starting from intermediate mass black holes at high redshift as well as the possible scientific merits of the mission options considered in probing the light seed black holes and their coevolution with galaxies in early Universe. Read More

Observations of the innermost parsec surrounding Sgr A$^*$ ---the supermassive black hole in the center of our Galaxy--- have revealed a diversity of structures whose existence and characteristics apparently defy the fundamental principles of dynamics. In this article, we review the challenges to the dynamics theories that have been brought forth in the past two decades by the observations of the Galactic center (GC). We outline the theoretical framework that has been developed to reconcile the discrepancies between the theoretical predictions and the observational results. Read More

This paper deals with the unconstrained and constrained cases for continuous-time Markov decision processes under the finite-horizon expected total cost criterion. The state space is denumerable and the transition and cost rates are allowed to be unbounded from above and from below. We give conditions for the existence of optimal policies in the class of all randomized history-dependent policies. Read More

The ability to tune material properties using gate electric field is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as gate-electric-field induced superconductivity and metal-insulator transition. Here we describe an ionic field-effect transistor (termed "iFET"), which uses gate-controlled lithium ion intercalation to modulate the material property of layered atomic crystal 1T-TaS$_2$. Read More

Genetic recombination is one of the most important mechanisms that can generate and maintain diversity, and recombination information plays an important role in population genetic studies. However, the phenomenon of recombination is extremely complex, and hence simulation methods are indispensable in the statistical inference of recombination. So far there are mainly two classes of simulation models practically in wide use: back-in-time models and spatially moving models. Read More

The decoy-state method is widely used in practical quantum key distribution systems to replace ideal single photon sources with realistic light sources by varying intensities. Instead of active modulation, the passive decoy-state method employs built-in decoy states in a parametric down-conversion photon source, which can decrease the side channel information leakage in decoy state preparation and hence increase the security. By employing low dark count up-conversion single photon detectors, we have experimentally demonstrated the passive decoy-state method over a 50-km-long optical fiber and have obtained a key rate of about 100 bit/s. Read More

The existence of "S-stars" within a distance of 1" from SgrA$^*$ contradicts our understanding of star formation, due to the forbiddingly violent environment. A suggested possibility is that they form far and have been brought in by some fast dynamical process, since they are young. Nonetheless, all conjectured mechanisms either fail to reproduce their eccentricities --without violating their young age-- or cannot explain the problem of "inverse mass segregation": The fact that lighter stars (the S-stars) are closer to SgrA$^*$ and more massive ones, Wolf-Rayet (WR) and O-stars, are farther out. Read More

Two-dimensional crystals have emerged as a new class of materials with novel properties that may impact future technologies. Experimentally identifying and characterizing new functional two-dimensional materials in the vast material pool is a tremendous challenge, and at the same time potentially rewarding. In this work, we succeed in fabricating field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometers. Read More

Since 1996 we have known that the Galactic Center (GC) displays a core-like distribution of red giant branch (RGB) stars starting at ~ 10", which poses a theoretical problem, because the GC should have formed a segregated cusp of old stars. This issue has been addressed invoking stellar collisions, massive black hole binaries, and infalling star clusters, which can explain it to some extent. Another observational fact, key to the work presented here, is the presence of a stellar disk at the GC. Read More

The cofactor conditions, introduced in James and Zhang, are conditions of compatibility between phases in martensitic materials. They consist of three subconditions: i) the condition that the middle principal stretch of the transformation stretch tensor $\mathbf U$ is unity ($\lambda_2 = 1$), ii) the condition $\mathbf a \cdot \mathbf U\, \cof (\mathbf U^2 - \mathbf I)\mathbf n = 0$, where the vectors $\mathbf a$ and $\mathbf n$ are certain vectors arising in the specification of the twin system, and iii) the inequality ${\rm tr} \mathbf U^2 + \det \mathbf U^2 -(1/4) |\mathbf a|^2 |\mathbf n|^2\ge 2$. Together, these conditions are necessary and sufficient for the equations of the crystallographic theory of martensite to be satisfied for the given twin system but for any volume fraction f of the twins, $0 \le f \le 1$. Read More

Organic superconductors are unique materials with a crystal structure made primarily of a complex carbon based network, an element associated directly with life, which were postulated to have a high critical temperature, $T_{C}$, even above room temperature, from a theoretical viewpoint. Pressure plays an essential role in the study of superconductivity in such organic materials, including creation of the first organic superconductor as well as the achievement of the highest $T_{C}$ of 14.2 K for charge transfer salts and 38 K for metal-doped fullerides. Read More

Off-center stellar tidal disruption flares have been suggested to be a powerful probe of recoiling supermassive black holes (SMBHs) out of galactic centers due to anisotropic gravitational wave radiations. However, off-center tidal flares can also be produced by SMBHs in merging galaxies. In this paper, we computed the tidal flare rates by dual SMBHs in two merging galaxies before the SMBHs become self-gravitationally bounded. Read More

It has been suggested that an intermediate-massive black hole (IMBH) with mass $10^{3-5} M_\odot$ could fall into the galactic center (GC) and form a massive black hole binary (MBHB) with the central supermassive black hole, but current observations are not sensitive to constrain all mass and distance ranges. Motivated by the recent discovery that MBHBs could enhance the rate of tidal-disruption events (TDEs) of stellar objects, we investigate the prospect of using stellar-disruption rate to probe IMBHs in the GC. We incorporated the perturbation by an IMBH into the loss-cone theory and calculated the stellar-disruption rates in the GC. Read More

Supermassive black hole binaries (SMBHBs) are the products of frequent galaxy mergers. The coalescence of the SMBHBs is a distinct source of gravitational wave (GW) radiation. The detections of the strong GW radiation and their possible electromagnetic counterparts are essential. Read More

Numerical relativity simulations predict that coalescence of supermassive black hole (SMBH) binaries not only leads to a spin flip but also to a recoiling of the merger remnant SMBHs. In the literature, X-shaped radio sources are popularly suggested to be candidates for SMBH mergers with spin flip of jet-ejecting SMBHs. Here we investigate the spectral and spatial observational signatures of the recoiling SMBHs in radio sources undergoing black hole spin flip. Read More

Tidal stellar disruptions have traditionally been discussed as a probe of the single, massive black holes (MBHs) that are dormant in the nuclei of galaxies. In Chen et al. (2009), we used numerical scattering experiments to show that three-body interactions between bound stars in a stellar cusp and a non-evolving "hard" MBH binary will also produce a burst of tidal disruptions, caused by a combination of the secular "Kozai effect" and by close resonant encounters with the secondary hole. Read More

Supermassive black hole binaries (SMBHBs) are products of galaxy mergers, and are important in testing Lambda cold dark matter cosmology and locating gravitational-wave-radiation sources. A unique electromagnetic signature of SMBHBs in galactic nuclei is essential in identifying the binaries in observations from the IR band through optical to X-ray. Recently, the flares in optical, UV, and X-ray caused by supermassive black holes (SMBHs) tidally disrupting nearby stars have been successfully used to observationally probe single SMBHs in normal galaxies. Read More

Affiliations: 1Peking University, 2Peking University, 3Oxford
Category: Astrophysics

Supermassive black hole binaries (SMBHBs) are expected by the hierarchical galaxy formation model in $\Lambda$CDM cosmology. There is some evidence in the literature for SMBHBs in AGNs, but there are few observational constraints on the evolution of SMBHBs in inactive galaxies and gas-poor mergers. On the theoretical front, it is unclear how long is needed for a SMBHB in a typical galaxy to coalesce. Read More

Affiliations: 1University of Science and Technology of China, Hefei, PR China, 2Universite de Liege, Belgium, Euroland, 3Universite de Liege, Belgium, Euroland, 4Universite de Liege, Belgium, Euroland, 5Universite de Liege, Belgium, Euroland, 6Universite de Liege, Belgium, Euroland, 7Universite de Liege, Belgium, Euroland, 8Universite de Liege, Belgium, Euroland, 9Universite de Liege, Belgium, Euroland

The thermal conductivity $\kappa (H,T)$ of the new superconductor $MgB_2$ was studied as a function of the temperature and a magnetic field. No anomaly in the thermal conductivity $\kappa (H,T)$ is observed around the superconducting transition in absence or presence of magnetic fields up to 14 Tesla; upon that field the superconductivity of $MgB_2$ persisted. The thermal conductivity in zero-field shows a $T$-linear increase up to 50K. Read More