arXiv subjects

D. Wu

Publications and source records attributed to D. Wu.

At least 55 records · Page 3Linked to original sources

arXiv2020

Optical spectroscopy and ultrafast pump-probe study on Bi$_2$Rh$_3$Se$_2$: evidence for charge-density-wave order formation

The parkerite-type ternary chalcogenide Bi$_2$Rh$_3$Se$_2$ was discovered to be a charge density wave (CDW) superconductor. However, there was a debate on whether the observed phase transition at 240 K could be attributed to the formation of CDW order. To address the issue, we performed optical spectroscopy and ultrafast pump-probe measurements on single crystal samples of Bi$_2$Rh$_3$Se$_2$. Our optical conductivity measurement reveals clearly the formation of an energy gap with associated spectral change only at low energies, yielding strong evidence for a CDW phase transition at 240 K. Time resolved pump-probe measurement provides further support for the CDW phase transition. The amplitude and relaxation time of quasiparticles extracted from the photoinduced reflectivity show strong enhancement near transition temperature, yielding further evidence for the CDW energy gap formation. Additionally, a collective mode is identified from the oscillations in the pump-probe time delay at low temperature. This mode, whose frequency decreases gradually at elevated temperature, could be naturally attributed to the amplitude mode of CDW state.

cond-mat.str-el↗

arXiv2020

Photo-induced nonequilibrium response in underdoped YBa$_2$Cu$_3$O$_{6+x}$ probed by time-resolved terahertz spectroscopy

Intense laser pulses have recently emerged as a tool to tune between different orders in complex quantum materials. Among different light-induced phenomena, transient superconductivity far above the equilibrium transition temperature in cuprates is particularly attractive. Key to those experiments was the resonant pumping of specific phonon modes, which was believed to induce superconducting phase coherence by suppressing the competing orders or modifying the structure slightly. Here, we present a comprehensive study of photo-induced nonequilibrium response in underdoped YBa$_2$Cu$_3$O$_{6+x}$. We find that upon photo-excitations, Josephson plasma edge in superconducting state is initially removed accompanied by quasiparticle excitations, and subsequently reappears at frequency lower than the static plasma edge within short time. In normal state, an enhancement or weaker edge-like shape is indeed induced by pump pulses in the reflectance spectrum accompanied by simultaneous rises in both real and imaginary parts of conductivity. We compare the pump-induced effects between near- and mid-infrared excitations and exclude phonon pumping as a scenario for the photo-induced effects above. We further elaborate the transient responses in normal state are unlikely to be explained by photo-induced superconductivity.

cond-mat.supr-con↗

arXiv2019

Entanglement-Based Machine Learning on a Quantum Computer

Machine learning, a branch of artificial intelligence, learns from previous experience to optimize performance, which is ubiquitous in various fields such as computer sciences, financial analysis, robotics, and bioinformatics. A challenge is that machine learning with the rapidly growing "big data" could become intractable for classical computers. Recently, quantum machine learning algorithms [Lloyd, Mohseni, and Rebentrost, arXiv.1307.0411] was proposed which could offer an exponential speedup over classical algorithms. Here, we report the first experimental entanglement-based classification of 2-, 4-, and 8-dimensional vectors to different clusters using a small-scale photonic quantum computer, which is then used to implement supervised and unsupervised machine learning. The results demonstrate the working principle of using quantum computers to manipulate and classify high-dimensional vectors, the core mathematical routine in machine learning. The method can in principle be scaled to a larger number of qubits, and may provide a new route to accelerate machine learning.

quant-ph↗

arXiv2019

Photoinduced hidden CDW state and relaxation dynamics of 1T-TaS_2 probed by time-resolved terahertz spectroscopy

The dynamical properties of single crystal 1T-TaS$_{2}$ are investigated both in commensurate charge density wave state (CCDW state) and hidden charge density wave state (HCDW state). We develop a useful criterion in time-domain transmission terahertz measurement to judge whether the compound is driven into a metastable state or still in its virgin state. An increase of terahertz conductivity by two orders of magnitude from CCDW state to HCDW state is obtained by taking account of the penetration depth mismatch, which is in agreement with reported \emph{dc} transport measurement. Upon weak pumping, only transient processes with rapid decay dynamics are triggered in both CCDW and HCDW states. We compare the conductivity increases in terahertz frequency range between transient and HCDW states and suggest that fluctuated metallic domain walls may develop in the transient states.

cond-mat.str-el↗

arXiv2019

Layered semiconductor EuTe 4 with charge density wave order in square tellurium sheets

We report a novel quasi-two dimensional compound of EuTe4 hosting charge density waves (CDW) instability. The compound has a crystallographic structure in a orthorhombic space group Pmmn (No.59) with cell parameters a = 4.6347(2)Å, b = 4.5119(2)Å, c = 15.6747(10)Å at room temperature. The pristine structure contains consecutive near-square Te sheets separated by corrugated Eu-Te slabs. Upon cooling, the compound experiences a phase transition near 255 K. X-ray crystallographic analysis and transmission electron microscopy (TEM) measurements reveal strong structural distortions in the low temperature phase, showing a superstructure with a periodic formation of Te-trimers in the monolayer Te sheets, yielding evidence for the formation of CDW order. The charge transport properties show a semiconducting behavior in the CDW state. Density functional theory calculations reveals a Fermi surface nesting driven instability with a nesting vector in good agreement with the one observed experimentally. Our finding provides a promising system for the study of CDW driven 2D semiconducting mechanisms, which would shed a new light on exploring novel 2D semiconductors with collective electronic states.

cond-mat.str-el↗

arXiv2018

Distances of Galactic Supernova Remnants Using Red Clump Stars

We carry out a project to independently measure the distances of supernova remnants (SNRs) in the first quadrant of the Galaxy. In this project, red clump (RC) stars are used as standard candles and extinction probes to build the optical extinction (A$_V$) - distance(D) relation in each direction of extinction-known SNRs. 15 SNRs' distances are well determined. Among them, the distances of G65.8-0.5, G66.0-0.0 and G67.6+0.9 are given for the first time. We also obtain 32 upper/lower limits of distances, and the distances to G5.7-0.1, G15.1-1.6, G28.8+1.5 and G78.2+2.1 are constrained. Most of the distances measured by the RC method are consistent with previous results. The RC method provides an independent access to the distances of SNRs.

astro-ph.IM↗

arXiv2018

How does the stellar wind influence the radio morphology of a supernova remnant?

We simulate the evolutions of the stellar wind and the supernova remnant (SNR) originating from a runaway massive star in an uniform Galactic environment based on the three-dimensional magnetohydrodynamics models. Taking the stellar wind into consideration, we can explain the radio morphologies of many supernova remnants. The directions of the kinematic velocity of the progenitor, the magnetic field and the line of sight are the most important factors influencing the morphologies. If the velocity is perpendicular to the magnetic field, the simulation will give us two different unilateral SNRs and a bilateral symmetric SNR. If the velocity is parallel to the magnetic field, we can obtain a bilateral asymmetric SNR and a quasi-circular SNR. Our simulations show the stellar wind plays a key role in the radio evolution of a SNR, which implies the Galactic global density and magnetic field distribution play a secondary role in shaping a SNR.

astro-ph.GA↗

arXiv2018

In-flight performance of the DAMPE silicon tracker

DAMPE (DArk Matter Particle Explorer) is a spaceborne high-energy cosmic ray and gamma-ray detector, successfully launched in December 2015. It is designed to probe astroparticle physics in the broad energy range from few GeV to 100 TeV. The scientific goals of DAMPE include the identification of possible signatures of Dark Matter annihilation or decay, the study of the origin and propagation mechanisms of cosmic-ray particles, and gamma-ray astronomy. DAMPE consists of four sub-detectors: a plastic scintillator strip detector, a Silicon-Tungsten tracKer-converter (STK), a BGO calorimeter and a neutron detector. The STK is composed of six double layers of single-sided silicon micro-strip detectors interleaved with three layers of tungsten for photon conversions into electron-positron pairs. The STK is a crucial component of DAMPE, allowing to determine the direction of incoming photons, to reconstruct tracks of cosmic rays and to estimate their absolute charge (Z). We present the in-flight performance of the STK based on two years of in-flight DAMPE data, which includes the noise behavior, signal response, thermal and mechanical stability, alignment and position resolution.

astro-ph.IM↗

arXiv2018

Photo-induced new metastable state with modulated Josephson coupling strengths in Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$

Photoexcitations on a superconductor using ultrafast nir-infrared (NIR) pulses, whose energy is much higher than the superconducting energy gap, are expected to suppress/destroy superconductivity by breaking Cooper pairs and excite quasiparticles from occupied state to unoccupied state far above the Fermi level. This appears to be true only for small pumping fluence. Here we show that the intense NIR pumping has different effect. We perform an intense NIR pump, c-axis terahertz probe measurement on an electron-doped cuprate superconductor Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$ with T$_c$=22 K. The measurement indicates that, instead of destroying superconductivity or exciting quasiparticles, the intense NIR pump drives the system from an equilibrium superconducting state with uniform Josephson coupling strength to a new metastable superconducting phase with modulated Josephson coupling strengths below T$_c$.

cond-mat.supr-con↗

arXiv2018

Light induced sub-picosecond topological phase transition in MoTe2

Recent development of ultrashort laser pulses allows for optical control of structural and electronic properties of complex quantum materials. The layered transition metal dichalcogenide MoTe2, which can crystalize into several different structures with distinct topological and electronic properties, provides possibilities to control or switch between different phases. In this study we report a photo-induced sub-picosecond structural transition between the type-II Weyl semimetal phase and normal semimetal phase in bulk crystalline MoTe2 by using ultrafast pump-probe and time-resolved second harmonic generation spectroscopy. The phase transition is most clearly characterized by the dramatic change of the shear oscillation mode and the intensity loss of second harmonic generation. This work opens up new possibilities for ultrafast manipulation of the topological properties of solids, enabling potentially practical applications for topological switch device with ultrafast excitations.

cond-mat.mtrl-sci↗

arXiv2018

Internal alignment and position resolution of the silicon tracker of DAMPE determined with orbit data

The DArk Matter Particle Explorer (DAMPE) is a space-borne particle detector designed to probe electrons and gamma-rays in the few GeV to 10 TeV energy range, as well as cosmic-ray proton and nuclei components between 10 GeV and 100 TeV. The silicon-tungsten tracker-converter is a crucial component of DAMPE. It allows the direction of incoming photons converting into electron-positron pairs to be estimated, and the trajectory and charge (Z) of cosmic-ray particles to be identified. It consists of 768 silicon micro-strip sensors assembled in 6 double layers with a total active area of 6.6 m$^2$. Silicon planes are interleaved with three layers of tungsten plates, resulting in about one radiation length of material in the tracker. Internal alignment parameters of the tracker have been determined on orbit, with non-showering protons and helium nuclei. We describe the alignment procedure and present the position resolution and alignment stability measurements.

physics.ins-det↗

arXiv2017

Laser Driving Highly Collimated $γ$-ray Pulses for the Generation of $μ^-μ^+$ and $e^-e^+$ Pairs in $γ-γ$ Collider

A scheme to generate highly collimated $γ$-ray pulse is proposed for the production of muon and electron pairs in $γ-γ$ collider. The $γ$-ray pulse, with high conversion efficiency, can be produced as the result of electron phase-locked acceleration in longitudinal electric field through the interaction between an ultra-intense laser pulse and a narrow tube target. Numerical simulation shows that 18\% energy of a 10-PW laser pulse is transferred into the forward $γ$-rays in a divergence angle less than $ 3^\circ$. The $γ$-ray pulse is applied in $γ-γ$ collider, in which muon pairs can be produced and electron pairs can be enhanced by more than 3 orders of magnitude. This scheme, which could be realized with the coming 10PW class laser pulses, would allow the observation of a $γ-γ$ collider for electron and muon pairs in laboratory.

physics.plasm-ph↗

arXiv2017

Antiferromagnetic order in epitaxial FeSe films on SrTiO3

Single monolayer FeSe film grown on Nb-doped SrTiO$_3$(001) substrate shows the highest superconducting transition temperature (T$_C$ $\sim$ 100 K) among the iron-based superconductors (iron-pnictide), while T$_C$ of bulk FeSe is only $\sim$ 8 K. Antiferromagnetic spin fluctuations were believed to be crucial in iron-pnictides, which has inspired several proposals to understand the FeSe/SrTiO$_3$ system. Although bulk FeSe does not show the antiferromagnetic order, calculations suggest that the parent FeSe/SrTiO$_3$ films are AFM. Experimentally, due to lacking of direct probe, the magnetic state of FeSe/SrTiO$_3$ films remains mysterious. Here, we report the direct evidences of the antiferromagnetic order in the parent FeSe/SrTiO$_3$ films by the magnetic exchange bias effect measurements. The phase transition temperature is $\geq$ 140 K for single monolayer film. The AFM order disappears after electron doping.

cond-mat.supr-con↗

arXiv2017

Observation of Terahertz Radiation via the Two-Color Laser Scheme with Uncommon Frequency Ratios

In the widely-studied two-color laser scheme for terahertz (THz) radiation from a gas, the frequency ratio of the two lasers is usually fixed at $ω_2/ω_1=$1:2. We investigate THz generation with uncommon frequency ratios. Our experiments show, for the first time, efficient THz generation with new ratios of $ω_2/ω_1=$1:4 and 2:3. We observe that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization and the polarization adjustment becomes inefficient by rotating the other laser polarization; the THz energy shows similar scaling laws with different frequency ratios. These observations are inconsistent with multi-wave mixing theory, but support the gas-ionization model. This study pushes the development of the two-color scheme and provides a new dimension to explore the long-standing problem of the THz generation mechanism.

physics.plasm-ph↗

arXiv2017

Effects of tensor and $T=0$ pairing interactions on nuclear $β^+$/EC decay

The Hartree-Fock-Bogolyubov (HFB) plus proton-neutron quasiparticle random phase approximation (pnQRPA) approach based on Skyrme interaction is applied to study the nuclear $β^+$/EC decay for nuclei near the proton magic numbers $Z=$20, 28, and 50. With properly selected Skyrme interactions that include the tensor terms, and the $T=0$ pairing interaction, the experimental $β^+$/EC decay half-lives of these nuclei can be systematically reproduced quite well. It is shown that the tensor and $T=0$ pairing interactions play different roles in different nuclei. The specific effect is relevant to the configurations that contribute to the decay. The attractive and repulsive properties of tensor interaction for the GT state is also studied. The present results indicate that it works repulsively on the GT states dominated by the configurations from $πj_\gtrless$ to $νj_\gtrless$ orbits, while working attractively for those mainly composed of the configurations from $πj_\gtrless$ to $νj_\lessgtr$ orbits.

nucl-th↗

arXiv2017

The revised distance of supernova remnant G15.4+0.1

We measure the distance to the supernova remnant G15.4+0.1 which is likely associated with TeV source HESS J1818-154. We build the neutral hydrogen (HI) absorption and 13CO spectra to supernova remnant G15.4+0.1 by employing the data from the Southern Galactic Plane Survey (SGPS) and the HI/OH/Recombination line survey (THOR). The maximum absorption velocity of about 140 km/s constrains its lower limit distance of about 8.0 kpc. Further, the fact that the HI emission feature at about 95 km/s seems no corresponding absorption suggests that G15.4+0.1 has likely an upper limit distance of about 10.5 kpc. 13CO spectrum to the remnant supports our measurement. The new distance revises parameters on its associated pulsar wind nebula and TeV source.

astro-ph.HE↗

arXiv2017

Particle-in-cell simulations of laser-plasma interactions at solid densities and relativistic intensities: the role of atomic processes

Direct studies of intense laser-solid interactions is still of great challenges, because of the many coupled physical mechanisms, such as direct laser heating, ionization dynamics, collision among charged particles, and electrostatic or electromagnetic instabilities, to name just a few. Here, we present a full particle-in-cell simulation (PIC) framework, which enables us to calculate laser-solid interactions in a "first principle" way, covering almost "all" the coupled physical mechanisms. Apart from the mechanisms above, the numerical self-heating of PIC simulations, which usually appears in solid-density plasmas, is also well controlled by the proposed "layered-density" method. This method can be easily implemented into the state-of-the-art PIC codes. Especially, the electron heating/acceleration at relativistically intense laser-solid interactions in the presence of large scale pre-formed plasmas is re-investigated by this PIC code. Results indicate that collisional damping (even though it is very week) can significantly influence the electron heating/acceleration in front of the target. Furthermore the Bremsstrahlung radiation will be enhanced by $2\sim3$ times when the solid is dramatically heated and ionized. For the considered case, where laser is of intensity $10^{20}\ \text{W}/\text{cm}^2$ and pre-plasma in front of the solid target is of scale-length $10\ μ\text{m}$, collision damping coupled with ionization dynamics and Bremsstrahlung radiations is shown to lower the "cut-off" electron energy by $25\%$. In addition, the resistive electromagnetic fields due to Ohmic-heating also play a non-ignorable role and must be included in real laser-solid interactions.

physics.plasm-ph↗

arXiv2017

SOS-convex Semi-algebraic Programs and its Applications to Robust Optimization: A Tractable Class of Nonsmooth Convex Optimization

In this paper, we introduce a new class of nonsmooth convex functions called SOS-convex semialgebraic functions extending the recently proposed notion of SOS-convex polynomials. This class of nonsmooth convex functions covers many common nonsmooth functions arising in the applications such as the Euclidean norm, the maximum eigenvalue function and the least squares functions with $\ell_1$-regularization or elastic net regularization used in statistics and compressed sensing. We show that, under commonly used strict feasibility conditions, the optimal value and an optimal solution of SOS-convex semi-algebraic programs can be found by solving a single semi-definite programming problem (SDP). We achieve the results by using tools from semi-algebraic geometry, convex-concave minimax theorem and a recently established Jensen inequality type result for SOS-convex polynomials. As an application, we outline how the derived results can be applied to show that robust SOS-convex optimization problems under restricted spectrahedron data uncertainty enjoy exact SDP relaxations. This extends the existing exact SDP relaxation result for restricted ellipsoidal data uncertainty and answers the open questions left in [Optimization Letters 9, 1-18(2015)] on how to recover a robust solution from the semi-definite programming relaxation in this broader setting.

math.OC↗