arXiv subjects

X. Liu

Publications and source records attributed to X. Liu.

At least 91 records · Page 5Linked to original sources

arXiv2021

Coulomb Effects on time-trajectory-resolved high-order harmonic generation

We study the effect of Coulomb potential on high-order harmonic generation (HHG) numerically and analytically. We focus on the influence of Coulomb potential on emission times of HHG associated with specific electron trajectories. By using a numerical procedure based on numerical solution of time-dependent Schrödinger equation (TDSE) in three dimensions, we extract the HHG emission times both for long and short electron trajectories. We compare TDSE predictions with those of a Coulomb-modified model arising from strong-field approximation (SFA). We show that the Coulomb effect induces earlier HHG emission times than those predicted by the general SFA model without considering the Coulomb potential. In particular, this effect influences differently on long and short electron trajectories and is more remarkable for low-energy harmonics than high ones. It also changes the HHG amplitudes for long and short electron trajectories. We validate our discussions with diverse laser parameters and forms of Coulomb potential. Our results strongly support a four-step model of HHG.

physics.atom-ph↗

arXiv2021

Projected sensitivities of the LUX-ZEPLIN (LZ) experiment to new physics via low-energy electron recoils

LUX-ZEPLIN (LZ) is a dark matter detector expected to obtain world-leading sensitivity to weakly interacting massive particles (WIMPs) interacting via nuclear recoils with a ~7-tonne xenon target mass. This manuscript presents sensitivity projections to several low-energy signals of the complementary electron recoil signal type: 1) an effective neutrino magnetic moment and 2) an effective neutrino millicharge, both for pp-chain solar neutrinos, 3) an axion flux generated by the Sun, 4) axion-like particles forming the galactic dark matter, 5) hidden photons, 6) mirror dark matter, and 7) leptophilic dark matter. World-leading sensitivities are expected in each case, a result of the large 5.6t 1000d exposure and low expected rate of electron recoil backgrounds in the $<$100keV energy regime. A consistent signal generation, background model and profile-likelihood analysis framework is used throughout.

hep-ex↗

arXiv2021

Laser-Induced Transient Magnons in Sr3Ir2O7 Throughout the Brillouin Zone

Although ultrafast manipulation of magnetism holds great promise for new physical phenomena and applications, targeting specific states is held back by our limited understanding of how magnetic correlations evolve on ultrafast timescales. Using ultrafast resonant inelastic x-ray scattering we demonstrate that femtosecond laser pulses can excite transient magnons at large wavevectors in gapped antiferromagnets, and that they persist for several picoseconds which is opposite to what is observed in nearly gapless magnets. Our work suggests that materials with isotropic magnetic interactions are preferred to achieve rapid manipulation of magnetism.

cond-mat.str-el↗

arXiv2021

Decoupling between propagating acoustic waves and two-level systems in hydrogenated amorphous silicon

Specific heat measurements of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition show a large density of two-level systems at low temperature. Annealing at 200 °C, well below the growth temperature, does not significantly affect the already-low internal friction or the sound velocity, but irreversibly reduces the non-Debye specific heat by an order of magnitude at 2 K, indicating a large reduction of the density of two-level systems. Comparison of the specific heat to the internal friction suggests that the two-level systems are uncharacteristically decoupled from acoustic waves, both before and after annealing. Analysis yields an anomalously low value of the coupling constant, which increases upon annealing but still remains anomalously low. The results suggest that the coupling constant value is lowered by the presence of hydrogen.

cond-mat.mtrl-sci↗

arXiv2021

Unraveling the orbital physics in a canonical orbital system KCuF$_3$

We explore the existence of the collective orbital excitations, orbitons, in the canonical orbital system KCuF$_3$. Using the Cu $L_3$-edge resonant inelastic X-ray scattering we show that the non-dispersive high-energy peaks result from the Cu$^{2+}$ $dd$ orbital excitations. These high-energy modes show good agreement with the {\it ab-initio} quantum chemistry calculation based on a single cluster, indicating that the $dd$ excitations are highly localized. At the same time, the low-energy excitations present clear dispersion. They match extremely well with the two-spinon continuum following the comparison with Mueller Ansatz calculations. The localized $dd$ excitations and the observation of the strongly dispersive magnetic excitations suggest that orbiton dispersion is below the resolution detection limit. Our results can reconcile with the strong {\it local} Jahn-Teller effect in KCuF$_3$, which predominantly drives orbital ordering.

cond-mat.str-el↗

arXiv2021

Quantum-enhanced radiometry via approximate quantum error correction

By exploiting the exotic quantum states of a probe, it is possible to realize efficient sensors that are attractive for practical metrology applications and fundamental studies. Similar to other quantum technologies, quantum sensing is suffering from noises and thus the experimental developments are hindered. Although theoretical schemes based on quantum error correction (QEC) have been proposed to combat noises, their demonstrations are prevented by the stringent experimental requirements, such as perfect quantum operations and the orthogonal condition between the sensing interaction Hamiltonian and the noise Lindbladians. Here, we report an experimental demonstration of a quantum enhancement in sensing with a bosonic probe with different encodings, by exploring the large Hilbert space of the bosonic mode and developing both the approximate QEC and the quantum jump tracking approaches. In a practical radiometry scenario, we attain a 5.3 dB enhancement of sensitivity, which reaches $9.1\times10^{-4}\,\mathrm{Hz}^{-1/2}$ when measuring the excitation population of a receiver mode. Our results demonstrate the potential of quantum sensing with near-term quantum technologies, not only shedding new light on the quantum advantage of sensing by revealing its difference from other quantum applications, but also stimulating further efforts on bosonic quantum technologies.

quant-ph↗

arXiv2021

Possible Evidence of a Universal Radio/X-ray Correlation in a near-Complete Sample of Hard X-ray Selected Seyfert Galaxies

Because the disc--jet coupling likely depends on various properties of sources probed, the sample control is always an important but challenging task. In this work, we re-analyzed the INTEGRAL hard X-ray-selected sample of Seyfert galaxies. We only consider sources that have measurements in black hole mass, and luminosities in radio and X-rays. Our sample includes 64 sources, consists of both bright AGNs and low-luminosity ones. We first find that, because of the similarity in the $L_{HX}/L_X$ distribution, the X-ray origin of radio-loud Seyferts may be the same to that of radio-quiet ones, where we attribute to the hot accretion flow (or similarly, the corona). We then investigate the connections between luminosities in radio and X-rays. Since our sample suffers a selection bias of a black hole mass $M_{BH}$ dependence on $L_X/L_{Edd}$, we focus on the correlation slope $ξ_X$ between the radio (at 1.4 GHz) and X-ray luminosities in Eddington unit, i.e. $(L_R/L_{Edd})\propto(L_X/L_{Edd})^{ξ_X}$. We classify the sources according to various properties, i.e. 1) Seyfert classification, 2) radio loudness, and 3) radio morphology. We find that, despite these differences in classification, all the sources in our sample are consistent with a universal correlation slope $ξ_X$, with $ξ_X=0.77\pm0.10$. This is unexpected, considering various possible radio emitters in radio-quiet systems. For the jet interpretation, our result may suggest a common/universal but to be identified jet launching mechanism among all the Seyfert galaxies, while properties like black hole spin and magnetic field strength only play secondary roles. We further estimate the jet production efficiency $η_{jet}$ of Seyfert galaxies, which is $η_{jet}\approx1.9^{+0.9}_{-1.5}\times10^{-4}$ on average. We also find that $η_{jet}$ increases as the system goes fainter.

astro-ph.HE↗

arXiv2021

Enhancing the sensitivity of the LUX-ZEPLIN (LZ) dark matter experiment to low energy signals

Two-phase xenon detectors, such as that at the core of the forthcoming LZ dark matter experiment, use photomultiplier tubes to sense the primary (S1) and secondary (S2) scintillation signals resulting from particle interactions in their liquid xenon target. This paper describes a simulation study exploring two techniques to lower the energy threshold of LZ to gain sensitivity to low-mass dark matter and astrophysical neutrinos, which will be applicable to other liquid xenon detectors. The energy threshold is determined by the number of detected S1 photons; typically, these must be recorded in three or more photomultiplier channels to avoid dark count coincidences that mimic real signals. To lower this threshold: a) we take advantage of the double photoelectron emission effect, whereby a single vacuum ultraviolet photon has a $\sim20\%$ probability of ejecting two photoelectrons from a photomultiplier tube photocathode; and b) we drop the requirement of an S1 signal altogether, and use only the ionization signal, which can be detected more efficiently. For both techniques we develop signal and background models for the nominal exposure, and explore accompanying systematic effects, including the dependence on the free electron lifetime in the liquid xenon. When incorporating double photoelectron signals, we predict a factor of $\sim 4$ sensitivity improvement to the dark matter-nucleon scattering cross-section at $2.5$ GeV/c$^2$, and a factor of $\sim1.6$ increase in the solar $^8$B neutrino detection rate. Dropping the S1 requirement may allow sensitivity gains of two orders of magnitude in both cases. Finally, we apply these techniques to even lower masses by taking into account the atomic Migdal effect; this could lower the dark matter particle mass threshold to $80$ MeV/c$^2$.

astro-ph.IM↗

arXiv2021

Strong Superexchange in a $d^{9-δ}$ Nickelate Revealed by Resonant Inelastic X-Ray Scattering

The discovery of superconductivity in a $d^{9-δ}$ nickelate has inspired disparate theoretical perspectives regarding the essential physics of this class of materials. A key issue is the magnitude of the magnetic superexchange, which relates to whether cuprate-like high-temperature nickelate superconductivity could be realized. We address this question using Ni L-edge and O K-edge spectroscopy of the reduced trilayer nickelate $d^{9-1/3}$ La4Ni3O8 and associated theoretical modeling. A magnon energy scale of ~80 meV resulting from a nearest-neighbor magnetic exchange of $J = 69(4)4$ meV is observed, proving that $d^{9-δ}$ nickelates can host a large superexchange. This value, along with that of the Ni-O hybridization estimated from our O K-edge data, implies that trilayer nickelates represent an intermediate case between the infinite-layer nickelates and the cuprates, and suggests that they represent a promising route towards higher-temperature nickelate superconductivity.

cond-mat.supr-con↗

arXiv2021

High ellipticity of harmonics from molecules in strong laser fields of small ellipticity

We study high-order harmonic generation (HHG) from aligned molecules in strong elliptically polarized laser fields numerically and analytically. Our simulations show that the spectra and polarization of HHG depend strongly on the molecular alignment and the laser ellipticity. In particular, for small laser ellipticity, large ellipticity of harmonics with high intensity is observed for parallel alignment, with forming a striking ellipticity hump around the threshold. We show that the interplay of the molecular structure and two-dimensional electron motion plays an important role here. This phenomenon can be used to generate bright elliptically-polarized EUV pulses.

physics.atom-ph↗

arXiv2021

A parsec-scale faint jet in the nearby changing-look Seyfert galaxy Mrk 590

Broad Balmer emission lines in active galactic nuclei (AGN) may display dramatic changes in amplitude, even disappearance and re-appearance in some sources. As a nearby galaxy at a redshift of z = 0.0264, Mrk 590 suffered such a cycle of Seyfert type changes between 2006 and 2017. Over the last fifty years, Mrk 590 also underwent a powerful continuum outburst and a slow fading from X-rays to radio wavelengths with a peak bolometric luminosity reaching about ten per cent of the Eddington luminosity. To track its past accretion and ejection activity, we performed very long baseline interferometry (VLBI) observations with the European VLBI Network (EVN) at 1.6 GHz in 2015. The EVN observations reveal a faint (~1.7 mJy) radio jet extending up to ~2.8 mas (projected scale ~1.4 pc) toward north, and probably resulting from the very intensive AGN activity. To date, such a parsec-scale jet is rarely seen in the known changing-look AGN. The finding of the faint jet provides further strong support for variable accretion as the origin of the type changes in Mrk 590.

astro-ph.HE↗

arXiv2021

Spin-transport in an organic semiconductor without free charge carrier involvement

We have experimentally tested the hypothesis of free charge carrier mediated spin-transport in the small molecule organic semiconductor Alq3 at room temperature. A spin current was pumped into this material by pulsed ferromagnetic resonance of an adjacent NiFe layer, while a charge current resulting from this spin current via the inverse spin-Hall effect (ISHE) was detected in a Pt layer adjacent on the other side of the Alq3 layer, confirming a pure spin current through the Alq3 layer. Charge carrier spin states in Alq3, were then randomized by simultaneous application of electron paramagnetic resonance (EPR). No influence of the EPR excitation on the ISHE current was found, implying that spin-transport is not mediated by free charge-carriers in Alq3.

cond-mat.mes-hall↗

arXiv2020

Introduction of Sr into Bi2Se3 thin films by molecular beam epitaxy

SrxBi2Se3 is a candidate topological superconductor but its superconductivity requires the intercalation of Sr by into the van-der-Waals gaps of Bi2Se3. We report the synthesis of SrxBi2Se3 thin films by molecular beam epitaxy, and we characterize their structural, vibrational and electrical properties. X-ray diffraction and Raman spectroscopy show evidence of substitutional Sr alloying into the structure, while transport measurements allow us to correlate the increasing Sr content with an increased n-type doping, but do not reveal superconductivity down to 1.5K. Our results suggest that Sr predominantly occupies sites within a quintuple layer, simultaneously substituting for Bi and as an interstitial. Our results motivate future density functional studies to further investigate the energetics of Sr substitution into Bi2Se3.

cond-mat.mtrl-sci↗

arXiv2020

Weak antilocalization beyond the fully diffusive regime in Pb1-xSnxSe topological quantum wells

We report the measurements and analysis of weak antilocalization (WAL) in Pb1-xSnxSe topological quantum wells in a new regime where the elastic scattering length is larger than the magnetic length. We achieve this regime through the development of high-quality epitaxy and doping of topological crystalline insulator (TCI) quantum wells. We obtain elastic scattering lengths that exceeds 100nm and become comparable to the magnetic length. In this transport regime, the Hikami-Larkin-Nagaoka model is no longer valid. We employ the model of Wittmann and Schmid to extract the coherence time from the magnetoresistance. We find that despite our improved transport characteristics, the coherence time may be limited by scattering channels that are not strongly carrier dependent, such as electron-phonon or defect scattering.

cond-mat.mes-hall↗

arXiv2020

Making Sense of the Robotized Pandemic Response: A Comparison of Global and Canadian Robot Deployments and Success Factors

From disinfection and remote triage, to logistics and delivery, countries around the world are making use of robots to address the unique challenges presented by the COVID-19 pandemic. Robots are being used to manage the pandemic in Canada too, but relative to other regions, we have been more cautious in our adoption -- this despite the important role that robots of Canadian origin are now playing on the global stage. This white paper discusses why this is the case, and argues that strategic investment and support for the Canadian robotics industry are urgently needed to bring the benefits of robotics home, where we have more control in shaping the future of this game-changing technology. Such investments will not only serve to support Canada's current pandemic response and post pandemic recovery, but will also prepare this country to weather future crises. Without such support, Canada risks falling behind other developed nations that are investing heavily in hardware automation at this time.

cs.CY↗

arXiv2020

Origin of Mechanical and Dielectric Losses from Two-Level Systems in Amorphous Silicon

Amorphous silicon contains tunneling two-level systems, which are the dominant energy loss mechanisms for amorphous solids at low temperatures. These two-level systems affect both mechanical and electromagnetic oscillators and are believed to produce thermal and electromagnetic noise and energy loss. However, it is unclear whether the two-level systems that dominate mechanical and dielectric losses are the same; the former relies on phonon-TLS coupling, with an elastic field coupling constant, $γ$, while the latter depends on a TLS dipole moment, $p_0$, which couples to the electromagnetic field. Mechanical and dielectric loss measurements as well as structural characterization were performed on amorphous silicon thin films grown by electron beam deposition with a range of growth parameters. Samples grown at 425 $^{\circ}$C show a large reduction of mechanical loss (34 times) and a far smaller reduction of dielectric loss (2.3 times) compared to those grown at room temperature. Additionally, mechanical loss shows lower loss per unit volume for thicker films, while dielectric loss shows lower loss per unit volume for thinner films. Analysis of these results indicate that mechanical loss correlates with atomic density, while dielectric loss correlates with dangling bond density, suggesting a different origin for these two energy dissipation processes in amorphous silicon.

cond-mat.mtrl-sci↗

arXiv2020

Characterization of High-Purity Germanium Detectors with Amorphous Germanium Contacts in Cryogenic Liquids

For the first time, planar high-purity germanium detectors with thin amorphous germanium contacts were successfully operated directly in liquid nitrogen and liquid argon in a cryostat at the Max-Planck-Institut fuer Physics in Munich. The detectors were fabricated at the Lawrence Berkeley National Laboratory and the University of South Dakota, using crystals grown at the University of South Dakota. They survived long-distance transportation and multiple thermal cycles in both cryogenic liquids and showed reasonable leakage currents and spectroscopic performance. Also discussed are the pros and cons of using thin amorphous semiconductor materials as an alternative contact technology in large-scale germanium experiments searching for physics beyond the Standard Model.

physics.ins-det↗

arXiv2020

AC Stark Shifts of Dark Resonances Probed with Ramsey Spectroscopy

The off-resonant AC Stark shift for coherent population trapping (CPT) resonances probed with Ramsey spectroscopy is investigated experimentally and theoretically. Measurements with laser-cooled $^{87}$Rb atoms show excellent quantitative agreement with a simple theory. The shift depends on the relative intensity of the two CPT light fields, but depends only weakly on the total intensity. Since the origin of the shift is through couplings of the interrogation light to off-resonant excited state hyperfine levels, the size and sign of the shift depend on the specific interrogation scheme. However, the simple theory shows that for several commonly used interrogation schemes the off-resonant shift goes to zero at specific values of this relative intensity.

physics.atom-ph↗