arXiv subjects

Y. Zhang

Publications and source records attributed to Y. Zhang.

At least 235 records · Page 13Linked to original sources

arXiv2021

Metal-Insulator Transition and Emergent Gapped Phase in the Surface-Doped 2D Semiconductor 2H-MoTe$_2$

Artificially created two-dimensional (2D) interfaces or structures are ideal for seeking exotic phase transitions due to their highly tunable carrier density and interfacially enhanced many-body interactions. Here, we report the discovery of a metal-insulator transition (MIT) and an emergent gapped phase in the metal-semiconductor interface that is created in 2H-MoTe$_2$ via alkali-metal deposition. Using angle-resolved photoemission spectroscopy, we found that the electron-phonon coupling is strong at the interface as characterized by a clear observation of replica shake-off bands. Such strong electron-phonon coupling interplays with disorder scattering, leading to an Anderson localization of polarons which could explain the MIT. The domelike emergent gapped phase could then be attributed to a polaron extended state or phonon-mediated superconductivity. Our results demonstrate the capability of alkali-metal deposition as an effective method to enhance the many-body interactions in 2D semiconductors. The surface-doped 2H-MoTe$_2$ is a promising candidate for realizing polaronic insulator and high-$T_c$ superconductivity.

cond-mat.supr-con↗

arXiv2021

Probing the Symmetry Energy with the Spectral Pion Ratio

Many neutron star (NS) properties, such as the proton fraction within a NS, reflect the symmetry energy contributions to the Equation of State that dominate when neutron and proton densities differ strongly. To constrain these contributions at supra-saturation densities, we measure the spectra of charged pions produced by colliding rare isotope tin (Sn) beams with isotopically enriched Sn targets. Using ratios of the charged pion spectra measured at high transverse momenta, we deduce the slope of the symmetry energy to be $42 < L < 117$ MeV. This value is slightly lower but consistent with the $L$ values deduced from a recent measurement of the neutron skin thickness of $^{208}$Pb.

nucl-ex↗

arXiv2021

Quantum circuits with many photons on a programmable nanophotonic chip

Growing interest in quantum computing for practical applications has led to a surge in the availability of programmable machines for executing quantum algorithms. Present day photonic quantum computers have been limited either to non-deterministic operation, low photon numbers and rates, or fixed random gate sequences. Here we introduce a full-stack hardware-software system for executing many-photon quantum circuits using integrated nanophotonics: a programmable chip, operating at room temperature and interfaced with a fully automated control system. It enables remote users to execute quantum algorithms requiring up to eight modes of strongly squeezed vacuum initialized as two-mode squeezed states in single temporal modes, a fully general and programmable four-mode interferometer, and genuine photon number-resolving readout on all outputs. Multi-photon detection events with photon numbers and rates exceeding any previous quantum optical demonstration on a programmable device are made possible by strong squeezing and high sampling rates. We verify the non-classicality of the device output, and use the platform to carry out proof-of-principle demonstrations of three quantum algorithms: Gaussian boson sampling, molecular vibronic spectra, and graph similarity.

quant-ph↗

arXiv2021

Correlated Dirac Eigenvalues and Axial Anomaly in Chiral Symmetric QCD

We introduce novel relations between the derivatives [$\partial^{n}ρ(λ, m_{l})/\partial m_{l}^n$] of the Dirac eigenvalue spectrum [$ρ(λ, m_{l})$] with respect to the light sea quark mass ($m_{l}$) and the $(n+1)$-point correlations among the eigenvalues ($λ$) of the massless Dirac operator. Using these relations we present lattice QCD results for $\partial^{n}ρ(λ, m_{l})/\partial m_{l}^n$ ($n=1, 2, 3$) for $m_{l}$ corresponding to pion masses $m_π=160-55$ MeV, and at a temperature of about 1.6 times the chiral phase transition temperature. Calculations were carried out using (2+1) flavors of highly improved staggered quarks with the physical value of strange quark mass, three lattice spacings $a=0.12, 0.08, 0.06$ fm, and lattices having aspect ratios $4-9$. We find that $ρ(λ\to0, m_{l})$ develops a peaked structure. This peaked structure arises due to non-Poisson correlations within the infrared part of the Dirac eigenvalue spectrum, becomes sharper as $a\to0$, and its amplitude is proportional to $m_{l}^2$. We demonstrate that this $ρ(λ\to0,m_l)$ is responsible for the manifestations of axial anomaly in two-point correlation functions of light scalar and pseudoscalar mesons. After continuum and chiral extrapolations we find that axial anomaly remains manifested in two-point correlation functions of scalar and pseudoscalar mesons in the chiral limit.

hep-lat↗

arXiv2021

High-order phase-dependent asymmetry in the above-threshold ionization plateau

Above-threshold ionization spectra from cesium are measured as a function of the carrier-envelope phase (CEP) using laser pulses centered at 3.1 $μ$m wavelength. The directional asymmetry in the energy spectra of backscattered electrons oscillates three times, rather than once, as the CEP is changed from $0$ to $2π$. Using the improved strong-field approximation, we show that the unusual behavior arises from the interference of few quantum orbits. We discuss the conditions for observing the high-order CEP dependence, and draw an analogy with time-domain holography with electron wave packets.

physics.atom-ph↗

arXiv2021

Measurement of Absolute Single and Double Electron Capture Cross Sections for O6+ Ions Collision with CO2, CH4, H2 and N2

The absolute electron capture cross sections for single and double charge exchanges between the highly charged ions O6+ and CO2, CH4, H2, N2, the dominant collision processes in the solar wind, have been measured in the energy from 7 keV*q (2.63 keV/u) to 52 keV*q (19.5 keV/u). These measurements were carried out in the new experimental instrument set up at Fudan University, and the error of cross sections for single and double charge exchanges at the 1σ confidence level are about 11% and 16%, respectively. Limited agreement is achieved with single electron capture results calculated by the classical over-barrier model. These cross sections data are useful for simulation ion-neutral processes in astrophysical environments and to improve the present theoretical model of fundamental atomic processes.

physics.atom-ph↗

arXiv2021

Giant topological longitudinal circular photo-galvanic effect in the chiral multifold semimetal CoSi

The absence of mirror symmetry, or chirality, is behind striking natural phenomena found in systems as diverse as DNA and crystalline solids. A remarkable example occurs when chiral semimetals with topologically protected band degeneracies are illuminated with circularly polarized light. Under the right conditions, the part of the generated photocurrent that switches sign upon reversal of the light's polarization, known as the circular photogalvanic effect, is predicted to depend only on fundamental constants. The conditions to observe quantization are non-universal, and depend on material parameters and the incident frequency. In this work, we perform terahertz emission spectroscopy with tunable photon energy from 0.2 eV - 1.1 eV in the chiral topological semimetal CoSi. We identify a large longitudinal photocurrent peaked at 0.4 eV reaching $\sim$ 550 $μA/V^{2}$, which is much larger than the photocurrent in any chiral crystal reported in the literature. Using first-principles calculations we establish that the peak originates from topological band crossings, reaching 3.3$\pm$0.3 in units of the quantization constant. Our calculations indicate that the quantized CPGE is within reach in CoSi upon doping and increase of the hot-carrier lifetime. The large photo-conductivity suggests that topological semimetals could potentially be used as novel mid-infrared detectors.

cond-mat.mtrl-sci↗

arXiv2021

Itinerant ferromagnetism mediated by giant spin polarization of metallic ligand band in van der Waals magnet Fe5GeTe2

We investigate near-Fermi-energy (EF) element-specific electronic and spin states of ferromagnetic van der Waals (vdW) metal Fe5GeTe2. The soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurement provides spectroscopic evidence of localized Fe 3d band. We also find prominent hybridization between the localized Fe 3d band and the delocalized Ge/Te p bands. This picture is strongly supported from direct observation of the remarkable spin polarization of the ligand p bands near EF, using x-ray magnetic circular dichroism (XMCD) measurements. The strength of XMCD signal from ligand element Te shows the highest value, as far as we recognize, among literature reporting finite XMCD signal for none-magnetic element in any systems. Combining SX-ARPES and elemental selective XMCD measurements, we collectively point an important role of giant spin polarization of the delocalized ligand Te states for realizing itinerant long-range ferromagnetism in Fe5GeTe2. Our finding provides a fundamental elemental selective view-point for understanding mechanism of itinerant ferromagnetism in low dimensional compounds, which also leads insight for designing exotic magnetic states by interfacial band engineering in heterostructures.

cond-mat.str-el↗

arXiv2021

Is diffuse intracluster light a good tracer of the galaxy cluster matter distribution?

We explore the relation between diffuse intracluster light (central galaxy included) and the galaxy cluster (baryonic and dark) matter distribution using a sample of 528 clusters at $0.2\leq z \leq 0.35$ found in the Dark Energy Survey (DES) Year 1 data. The surface brightness of the diffuse light shows an increasing dependence on cluster total mass at larger radius, and appears to be self-similar with a universal radial dependence after scaling by cluster radius. We also compare the diffuse light radial profiles to the cluster (baryonic and dark) matter distribution measured through weak lensing and find them to be comparable. The IllustrisTNG galaxy formation simulation, TNG300, offers further insight into the connection between diffuse stellar mass and cluster matter distributions -- the simulation radial profile of the diffuse stellar component does not have a similar slope with the total cluster matter content, although that of the cluster satellite galaxies does. Regardless of the radial trends, the amount of diffuse stellar mass has a low-scatter scaling relation with cluster's total mass in the simulation, out-performing the total stellar mass of cluster satellite galaxies. We conclude that there is no consistent evidence yet on whether or not diffuse light is a faithful radial tracer of the cluster matter distribution. Nevertheless, both observational and simulation results reveal that diffuse light is an excellent indicator of the cluster's total mass.

astro-ph.GA↗

arXiv2020

Shadows in the Dark: Low-Surface-Brightness Galaxies Discovered in the Dark Energy Survey

We present a catalog of 23,790 extended low-surface-brightness galaxies (LSBGs) identified in $\sim 5000 °^2$ from the first three years of imaging data from the Dark Energy Survey (DES). Based on a single-component Sérsic model fit, we define extended LSBGs as galaxies with $g$-band effective radii $R_{eff}(g) > 2.5''$ and mean surface brightness $\barμ_{eff}(g) > 24.2 \,mag ȧrcsec^{-2}$. We find that the distribution of LSBGs is strongly bimodal in $(g-r)$ vs.\ $(g-i$) color space. We divide our sample into red ($g-i \geq 0.60$) and blue ($g-i<0.60$) galaxies and study the properties of the two populations. Redder LSBGs are more clustered than their blue counterparts and are correlated with the distribution of nearby ($z < 0.10$) bright galaxies. Red LSBGs constitute $\sim 33\%$ of our LSBG sample, and $\sim 30\%$ of these are located within 1 deg of low-redshift galaxy groups and clusters (compared to $\sim 8\%$ of the blue LSBGs). For nine of the most prominent galaxy groups and clusters, we calculate the physical properties of associated LSBGs assuming a redshift derived from the host system. In these systems, we identify 41 objects that can be classified as ultra-diffuse galaxies, defined as LSBGs with projected physical effective radii $R_{eff} > 1.5 \,kpc$ and central surface brighthness $μ_0(g) > 24.0\, mag \,arcsec^{-2}$. The wide-area sample of LSBGs in DES can be used to test the role of environment on models of LSBG formation and evolution.

astro-ph.GA↗

arXiv2020

Symmetry energy investigation with pion production from Sn+Sn systems

In the past two decades, pions created in the high density regions of heavy ion collisions have been predicted to be sensitive at high densities to the symmetry energy term in the nuclear equation of state, a property that is key to our understanding of neutron stars. In a new experiment designed to study the symmetry energy, the multiplicities of negatively and positively charged pions have been measured with high accuracy for central $^{132}$Sn+$^{124}$Sn, $^{112}$Sn+$^{124}$Sn, and $^{108}$Sn+$^{112}$Sn collisions at $E/A=270~\mathrm{MeV}$ with the S$π$RIT Time Projection Chamber. While the uncertainties of individual pion multiplicities are measured to 4\%, those of the charged pion multiplicity ratios are measured to 2\%. We compare these data to predictions from seven major transport models. The calculations reproduce qualitatively the dependence of the multiplicities and their ratios on the total neutron to proton number in the colliding systems. However, the predictions of the transport models from different codes differ too much to allow extraction of reliable constraints on the symmetry energy from the data. This finding may explain previous contradictory conclusions on symmetry energy constraints obtained from pion data in Au+Au system. These new results call for better understanding of the differences among transport codes, and new observables that are more sensitive to the density dependence of the symmetry energy.

nucl-ex↗

arXiv2020

Analysis and approximations of Dirichlet boundary control of Stokes flows in the energy space

We study Dirichlet boundary control of Stokes flows in 2D polygonal domains. We consider cost functionals with two different boundary control regularization terms: the $L^2$ norm and an energy space seminorm. We prove well-posedness and regularity results for both problems, develop finite element discretizations for both problems, and prove finite element error estimates for the latter problem. The motivation to study the energy space problem follows from our analysis: we prove that the choice of the control space ${\bm L}^2(Γ)$ can lead to an optimal control with discontinuities at the corners, even when the domain is convex. We observe this phenomenon in numerical experiments. This behavior does not occur in Dirichlet boundary control problems for the Poisson equation on convex polygonal domains, and may not be desirable in real applications. For the energy space problem, we derive the first order optimality conditions, and show that the solution of the control problem is more regular than the solution of the problem with the ${\bm L}^2(Γ)$ regularization. We also prove a priori error estimates for the control in the energy norm, and present several numerical experiments for both control problems on convex and nonconvex domains.

math.OC↗

arXiv2020

Influence of Electromagnetic Fields on Nuclear Processes

Although the energies associated with nuclear reactions are due primarily to interactions involving nuclear forces, the rates and probabilities associated with those reactions are effectively governed by electromagnetic forces. Charges in the local environment can modulate the Coulomb barrier, and thereby change the rates of nuclear processes. Experiments are presented in which low-temperature electrons are attached to high-density rotating neutrals to form negative ions. The steady-state quiescent rotating plasma generated here lends itself to first prove the principle that low temperature systems can yield MeV fusion particles. It allows the use of high density of neutrals interacting with the wall to yield gain greater than unity. It also demonstrates that instabilities can be avoided with high neutral densities. Collective dynamic interactions within this steady-state quiescent plasma result in an arrangement of negative charges that lowers the effective Coulomb barrier to nuclear reactions at a solid wall of reactants. MeV alpha particles are synchronously observed with externally imposed pulses as evidence of fusion being enabled by Coulomb fields. Impacts on fusion, the source of energy in the universe, will be discussed.

physics.plasm-ph↗

arXiv2020

QCD phase structure in strong magnetic fields

In this proceedings we discuss the natural connection between the reduction of neutral pion mass in the vacuum, and the magnetic catalysis as well as the reduction of transition temperature in the external magnetic field. We also present the first results on fluctuations of and correlations among conserved charges in strong magnetic fields from lattice QCD computations.

hep-lat↗

arXiv2020

Squeezed light from a nanophotonic molecule

Photonic molecules are composed of two or more optical resonators, arranged such that some of the modes of each resonator are coupled to those of the other. Such structures have been used for emulating the behaviour of two-level systems, lasing, and on-demand optical storage and retrieval. Coupled resonators have also been used for dispersion engineering of integrated devices, enhancing their performance for nonlinear optical applications. Delicate engineering of such integrated nonlinear structures is required for developing scalable sources of non-classical light to be deployed in quantum information processing systems. In this work, we demonstrate a photonic molecule composed of two coupled microring resonators on an integrated nanophotonic chip, designed to generate strongly squeezed light uncontaminated by noise from unwanted parasitic nonlinear processes. By tuning the photonic molecule to selectively couple and thus hybridize only the modes involved in the unwanted processes, suppression of parasitic parametric fluorescence is accomplished. This strategy enables the use of microring resonators for the efficient generation of degenerate squeezed light: without it, simple single-resonator structures cannot avoid contamination from nonlinear noise without significantly compromising pump power efficiency, and are thus limited to generating only weak degenerate squeezing. We use this device to generate 8(1) dB of broadband degenerate squeezed light on-chip, with 1.65(1) dB directly measured, which is the largest amount of squeezing yet reported from any nanophotonic source.

physics.optics↗

arXiv2020

Linear and nonlinear optical responses in the chiral multifold semimetal RhSi

Chiral topological semimetals are materials that break both inversion and mirror symmetries. They host interesting phenomena such as the quantized circular photogalvanic effect (CPGE) and the chiral magnetic effect. In this work, we report a comprehensive theoretical and experimental analysis of the linear and non-linear optical responses of the chiral topological semimetal RhSi, which is known to host multifold fermions. We show that the characteristic features of the optical conductivity, which display two distinct quasi-linear regimes above and below 0.4 eV, can be linked to excitations of different kinds of multifold fermions. The characteristic features of the CPGE, which displays a sign change at 0.4 eV and a large non-quantized response peak of around 160 $μ\textrm{A V}^{-2}$ at 0.7 eV, are explained by assuming that the chemical potential crosses a flat hole band at the Brillouin zone center. Our theory predicts that, in order to observe a quantized CPGE in RhSi, it is necessary to increase the chemical potential as well as the quasiparticle lifetime. More broadly our methodology, especially the development of the broadband terahertz emission spectroscopy, could be widely applied to study photo-galvanic effects in noncentrosymmetric materials and in topological insulators in a contact-less way and accelerate the technological development of efficient infrared detectors based on topological semimetals.

cond-mat.mtrl-sci↗

arXiv2020

Demonstration of electron cooling using a pulsed beam from an electrostatic electron cooler

Cooling of hadron beams is critically important in the next generation of hadron storage rings for delivery of unprecedented performance. One such application is the electron-ion collider presently under development in the US. The desire to develop electron coolers for operation at much higher energies than previously achieved necessitates the use of radio-frequency (RF) fields for acceleration as opposed to the conventional, electrostatic approach. While electron cooling is a mature technology at low energy utilizing a DC beam, RF acceleration requires the cooling beam to be bunched, thus extending the parameter space to an unexplored territory. It is important to experimentally demonstrate the feasibility of cooling with electron bunches and further investigate how the relative time structure of the two beams affects the cooling properties; thus, a set of four pulsed-beam cooling experiments was carried out by a collaboration of Jefferson Lab and Institute of Modern Physics (IMP). The experiments have successfully demonstrated cooling with a beam of electron bunches in both the longitudinal and transverse directions for the first time. We have measured the effect of the electron bunch length and longitudinal ion focusing strength on the temporal evolution of the longitudinal and transverse ion beam profile and demonstrate that if the synchronization can be accurately maintained, the dynamics are not adversely affected by the change in time structure.

physics.acc-ph↗

arXiv2020

Doping-controlled transition from excitonic insulator to semimetal in Ta$_2$NiSe$_5$

Excitonic insulator (EI) is an intriguing insulating phase of matter, where electrons and holes are bonded into pairs, so called excitons, and form a phase-coherent state via Bose-Einstein Condensation (BEC). Its theoretical concept has been proposed several decades ago, but the followed research is very limited, due to the rare occurrence of EI in natural materials and the lack of manipulating method of excitonic condensation. In this paper, we report the realization of a doping-controlled EI-to-semi-metal transition in Ta$_2$NiSe$_5$ using $in$-$situ$ potassium deposition. Combining with angle-resolved photoemission spectroscopy (ARPES), we delineate the evolution of electronic structure through the EI transition with unprecedented precision. The results not only show that Ta$ _2 $NiSe$ _5 $ (TNS) is an EI originated from a semi-metal non-interacting band structure, but also resolve two sequential transitions, which could be attributed to the phase-decoherence and pair-breaking respectively. Our results unveil the Bardeen-Cooper-Schrieffer (BCS)-BEC crossover behavior of TNS and demonstrate that its band structure and excitonic binding energy can be tuned precisely via alkali-metal deposition. This paves a way for investigations of BCS-BEC crossover phenomena, which could provide insights into the many-body physics in condensed matters and other many-body systems.

cond-mat.str-el↗