arXiv subjects

Z. Liu

Publications and source records attributed to Z. Liu.

At least 91 records · Page 5Linked to original sources

arXiv2022

A Muon Collider Facility for Physics Discovery

Muon colliders provide a unique route to deliver high energy collisions that enable discovery searches and precision measurements to extend our understanding of the fundamental laws of physics. The muon collider design aims to deliver physics reach at the highest energies with costs, power consumption and on a time scale that may prove favorable relative to other proposed facilities. In this context, a new international collaboration has formed to further extend the design concepts and performance studies of such a machine. This effort is focused on delivering the elements of a $\sim$10 TeV center of mass (CM) energy design to explore the physics energy frontier. The path to such a machine may pass through lower energy options. Currently a 3 TeV CM stage is considered. Other energy stages could also be explored, e.g. an s-channel Higgs Factory operating at 125 GeV CM. We describe the status of the R&D and design effort towards such a machine and lay out a plan to bring these concepts to maturity as a tool for the high energy physics community.

physics.acc-ph↗

arXiv2022

Study of filtered-x logarithmic recursive least $p$-power algorithm

For active impulsive noise control, a filtered-x recursive least $p$-power (FxRLP) algorithm is proposed by minimizing the weighted summation of the $p$-power of the \emph{a posteriori} errors. Since the characteristic of the target noise is investigated, the FxRLP algorithm achieves good performance and robustness. To obtain a better performance, we develop a filtered-x logarithmic recursive least $p$-power (FxlogRLP) algorithm which integrates the $p$-order moment with the logarithmic-order moment. Simulation results demonstrate that the FxlogRLP algorithm is superior to the existing algorithms in terms of convergence rate and noise reduction.

eess.SP↗

arXiv2021

Anomalous Magnetoresistance by Breaking Ice Rule in Bi2Ir2O7/Dy2Ti2O7 Heterostructure

While geometrically frustrated quantum magnets are known for a variety of exotic spin states that are of great interests of understanding emergent phenomena as well as enabling revolutionary quantum technologies, most of them are necessarily good insulators which are difficult to be integrated with modern electrical circuit that relies on moving charge carriers. The grand challenge of converting fluctuations and excitations of frustrated moments into electronic responses is finding ways to introduce charge carriers that interact with the localized spins without destroying the spin states. Here, we show that, by designing a Bi2Ir2O7/Dy2Ti2O7 heterostructure, the breaking of the spin ice rule in insulating Dy2Ti2O7 can lead to a charge response in the Bi2Ir2O7 conducting layer that can be detected as anomalous magnetoresistance. These results demonstrate a novel and feasible interfacial approach for electronically probing exotic spin states in insulating magnets, laying out a blueprint for the metallization of frustrated quantum magnets.

cond-mat.str-el↗

arXiv2021

Microscopic Theory of Superconducting Phase Diagram in Infinite-Layer Nickelates

Since the discovery of superconductivity in infinite-layer nickelates RNiO$_2$ (R=La, Pr, Nd), great research efforts have been paid to unveil its underlying superconducting mechanism. However, the physical origin of the intriguing hole-doped superconductivity phase diagram, characterized by a superconductivity dome sandwiched between two weak insulators, is still unclear. Here, we present a microscopic theory for electronic structure of nickelates from a fundamental model-based perspective. We found that the appearance of weak insulator phase in lightly and heavily hole-doped regime is dominated by Mottness and Hundness, respectively, exhibiting a unique orbital-selective doping originated from the competition of Hund interaction and crystal field splitting. Moreover, the superconducting phase can also be created in the "mixed" transition regime between Mott-insulator and Hund-induced insulator, exactly reproducing the experimentally observed superconducting phase diagram. Our findings not only demonstrate the orbital-dependent strong-correlation physics in Ni 3$d$ states, but also provide a unified understanding of superconducting phase diagram in hole-doped infinite-layer nickelates, which are distinct from the well-established paradigms in cuprates and iron pnictides.

cond-mat.str-el↗

arXiv2021

PANDA Phase One

The Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, provides unique possibilities for a new generation of hadron-, nuclear- and atomic physics experiments. The future antiProton ANnihilations at DArmstadt (PANDA or $\overline{\rm P}$ANDA) experiment at FAIR will offer a broad physics programme, covering different aspects of the strong interaction. Understanding the latter in the non-perturbative regime remains one of the greatest challenges in contemporary physics. The antiproton-nucleon interaction studied with PANDA provides crucial tests in this area. Furthermore, the high-intensity, low-energy domain of PANDA allows for searches for physics beyond the Standard Model, e.g. through high precision symmetry tests. This paper takes into account a staged approach for the detector setup and for the delivered luminosity from the accelerator. The available detector setup at the time of the delivery of the first antiproton beams in the HESR storage ring is referred to as the \textit{Phase One} setup. The physics programme that is achievable during Phase One is outlined in this paper.

hep-ex↗

arXiv2021

High-Resolution In-situ Synchrotron X-ray Studies of Inorganic Perovskite CsPbBr$_3$: New Symmetry Assignments and Structural Phase Transitions

Perovskite photovoltaic ABX$_3$ systems are being studied due to their high energy-conversion efficiencies with current emphasis placed on pure inorganic systems. In this work, synchrotron single-crystal diffraction measurements combined with second harmonic generation measurements reveal the absence of inversion symmetry below room temperature in CsPbBr$_3$. Local structural analysis by pair distribution function and X-ray absorption fine structure methods are performed to ascertain the local ordering, atomic pair correlations, and phase evolution in a broad range of temperatures. The currently accepted space group assignments for CsPbBr$_3$ are found to be incorrect in a manner that profoundly impacts physical properties. New assignments are obtained for the bulk structure: $Im$$\bar{3}$ (above $\sim$ 410 K), $P$2$_1$/$m$ (between $\sim$ 300 K and $\sim$ 410 K), and the polar group $Pm$ (below $\sim$ 300 K), respectively. The newly observed structural distortions exist in the bulk structure consistent with the expectation of previous photoluminescence and Raman measurements. High-pressure measurements reveal multiple low-pressure phases, one of which exists as a metastable phase at ambient pressure. This work should help guide research in the perovskite photovoltaic community to better control the structure under operational conditions and further improve transport and optical properties.

cond-mat.mtrl-sci↗

arXiv2021

A Versatile Post-Doping Towards Two-Dimensional Semiconductors

We have developed a simple and straightforward way to realize controlled post-doping towards 2D transition metal dichalcogenides (TMDs). The key idea is to use low-kinetic energy dopant beams and a high-flux chalcogen beam at the same time, leading to substitutional doping with controlled dopant densities. Atomic-resolution transmission electron microscopy has revealed that dopant atoms injected toward TMDs are incorporated substitutionally into the hexagonal framework of TMDs. Electronic properties of doped TMDs (Nb-doped WSe2) have shown drastic change, p-type action with more than two orders of magnitude increase in on current. Position-selective doping has also been demonstrated by the post-doping toward TMDs with a patterned mask on the surface. The post-doping method developed in this work can be a versatile tool for 2D-based next-generation electronics in the future.

cond-mat.mtrl-sci↗

arXiv2021

Superradiance of molecular nitrogen ions in strong laser fields

We perform a combined theoretical and experimental investigation of the superradiance in the quantum coherent system generated by strong laser fields. The semiclassical theory of superradiance that includes the superradiant temporal profile, character duration, time delay, intensity is derived. The experimental data and theoretical predictions of 391-nm forward emission as a function of nitrogen gas pressure are compared and show good agreement. Our results not only demonstrate that the time-delayed optical amplification inside the molecular nitrogen ions is superradiance, but also reveal the quantum optical properties of strong-field physics.

quant-ph↗

arXiv2021

Alignment facility and software for single crystal time of flight neutron spectroscopy

An instrument and software algorithm is described for the purpose of characterization of large single crystals at the Alignment Facility (ALF) of the ISIS spallation neutron source. We describe a method for both characterizing the quality of the sample and also aligning it in a particular scattering plane. We present a software package written for this instrument and demonstrate its utility by way of an example of the structural characterization of large singles crystals of Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$. We suggest extensions and modifications of characterization instruments for future improved beamlines. It is hoped that this software will be used by the neutron user community for pre characterizing large single crystals for spectroscopy experiments and that future facilities will include such a facility as part of the spectroscopy suite at spallation neutron sources.

cond-mat.mtrl-sci↗

arXiv2021

The cosmic muon and detector simulation framework of the Extreme Energy Events (EEE) experiment

This paper describes the simulation framework of the Extreme Energy Events (EEE) experiment. EEE is a network of cosmic muon trackers, each made of three Multi-gap Resistive Plate Chambers (MRPC), able to precisely measure the absolute muon crossing time and the muon integrated angular flux at the ground level. The response of a single MRPC and the combination of three chambers have been implemented in a GEANT4-based framework (GEMC) to study the telescope response. The detector geometry, as well as details about the surrounding materials and the location of the telescopes have been included in the simulations in order to realistically reproduce the experimental set-up of each telescope. A model based on the latest parametrization of the cosmic muon flux has been used to generate single muon events. After validating the framework by comparing simulations to selected EEE telescope data, it has been used to determine detector parameters not accessible by analysing experimental data only, such as detection efficiency, angular and spatial resolution.

physics.ins-det↗

arXiv2021

Spontaneous ferromagnetism induced topological phase transition in EuB6

The interplay between various symmetries and electronic bands topology is one of the core issues for topological quantum materials. Spontaneous magnetism, which leads to the breaking of time-reversal symmetry, has been proven to be a powerful approach to trigger various exotic topological phases. In this work, utilizing the combination of angle-resolved photoemission spectroscopy, magneto-optical Kerr effect microscopy and first-principles calculations, we present the direct evidence on the realization of the long-sought spontaneous time-reversal symmetry breaking induced topological phase transition in soft ferromagnetic EuB$_6$. We successfully disentangle the bulk band structure from complicated surface states, and reveal the hallmark band inversion occurring between two opposite-parity bulk bands below the phase transition temperature. Besides, our magneto-optical Kerr effect microscopy result confirms the simultaneous formation of magnetic domains in EuB$_6$, implying the intimate link between the topological phase transition and broken time-reversal symmetry therein. Our results demonstrate that EuB$_6$ provides a potential platform to study the interplay between the topological phases and tunable magnetic orders.

cond-mat.mtrl-sci↗

arXiv2021

AT 2019avd: A novel addition to the diverse population of nuclear transients

We report on SRG/eROSITA, ZTF, ASAS-SN, Las Cumbres, NEOWISE-R, and Swift XRT/UVOT observations of the unique ongoing event AT 2019avd, located in the nucleus of a previously inactive galaxy at $z=0.029$. eROSITA first observed AT 2019avd on 2020-04-28 during its first all sky survey, when it was detected as an ultra-soft X-ray source ($kT\sim 85$ eV) that was $\gtrsim 90$ times brighter in the $0.2-2$ keV band than a previous 3$σ$ upper flux detection limit (with no archival X-ray detection at this position). The ZTF optical light curve in the $\sim 450$ days preceding the eROSITA detection is double peaked, and the eROSITA detection coincides with the rise of the second peak. Follow-up optical spectroscopy shows the emergence of a Bowen fluorescence feature and high-ionisation coronal lines ([\ion{Fe}{X}] 6375 Å, [\ion{Fe}{XIV}] 5303 Å), along with persistent broad Balmer emission lines (FWHM$\sim 1400$ km s$^{-1}$). Whilst the X-ray properties make AT 2019avd a promising tidal disruption event (TDE) candidate, the optical properties are atypical for optically selected TDEs. We discuss potential alternative origins that could explain the observed properties of AT 2019avd, such as a stellar binary TDE candidate, or a TDE involving a super massive black hole binary.

astro-ph.HE↗

arXiv2021

New $α$-Emitting Isotope $^{214}$U and Abnormal Enhancement of $α$-Particle Clustering in Lightest Uranium Isotopes

A new $α$-emitting isotope $^{214}$U, produced by fusion-evaporation reaction $^{182}$W($^{36}$Ar, 4n)$^{214}$U, was identified by employing the gas-filled recoil separator SHANS and recoil-$α$ correlation technique. More precise $α$-decay properties of even-even nuclei $^{216,218}$U were also measured in reactions of $^{40}$Ar, $^{40}$Ca with $^{180, 182, 184}$W targets. By combining the experimental data, improved $α$-decay reduced widths $δ^2$ for the even-even Po--Pu nuclei in the vicinity of magic neutron number $N=126$ were deduced. Their systematic trends are discussed in terms of $N_{p}N_{n}$ scheme in order to study the influence of proton-neutron interaction on $α$ decay in this region of nuclei. It is strikingly found that the reduced widths of $^{214,216}$U are significantly enhanced by a factor of two as compared with the $N_{p}N_{n}$ systematics for the $84 \leq Z \leq 90$ and $N<126$ even-even nuclei. The abnormal enhancement is interpreted by the strong monopole interaction between the valence protons and neutrons occupying the $π1f_{7/2}$ and $ν1f_{5/2}$ spin-orbit partner orbits, which is supported by a large-scale shell model calculation.

nucl-ex↗

arXiv2020

Strongly modulated ultrafast demagnetization and magnetization precession dynamics in ferrimagnetic Gdx(CoFe)1-x alloys via 3d-4f intersublattice exchange coupling

Manipulation of the intersublattice interaction strengh (JRE-TM) in rare earth (RE)-transition metal (TM) alloys is a key issue to understand how efficiently the laser-induced angular momentum transfers from 3d to 4f spins and to have a better control of the ultrafast spin dynamics. In this work, the relationships between laser-induced demagnetization process and the intersublattice 3d-4f interaction for the GdCoFe alloys were systematically studied. The ultrafast two-stage demagnetization process could change into a one-stage mode as the angular momentum transferring channel between 3d and 4f spins is switched off, which could be modulated by JRE-TM. Furthermore, both the effective g-factor and damping constant deduced by the subsequently laser-induced magnetization precession process diverge at the angular momentum compensation point based on the ferromagnetic resonance method with the LLG equations. The results provide an alternative way to efficiently manipulate the ultrafast demagnetization time for practical applications.

cond-mat.mtrl-sci↗

arXiv2020

Light Quantum Control of Persisting Higgs Modes in Iron-Based Superconductors

The Higgs mechanism, i.e., spontaneous symmetry breaking of the quantum vacuum, is a cross-disciplinary principle, universal for understanding dark energy, antimatter and quantum materials, from superconductivity to magnetism. Yet, Higgs modes in one-band superconductors (SCs) are currently under debate due to their competition with charge-density fluctuations. A distinct Higgs mode, controllable by terahertz (THz) laser pulses, can arise in multi-band, unconventional SCs via strong {\em interband} Coulomb interaction, but is yet to be accessed. Here we both discover and demonstrate quantum control of such collective mode in iron-based high-temperature superconductors. Using two-pulse, phase coherent THz spectroscopy, we observe a tunable and coherent 2$Δ_{\mathrm{SC}}$ amplitude oscillation of the complex order parameter in such SC with coupled lower and upper bands. The nonlinear dependence of the amplitude mode oscillations on the THz driving fields is distinct from any one-band and conventional SC results: we observe a large nonlinear change of resonance strength, yet with a persisting mode frequency. We argue that this result provides compelling evidence for a transient coupling between the electron and hole amplitude modes via strong interband coherent interaction. To support this scenario, we perform quantum kinetic modeling of a hybrid Higgs mechanism without invoking extra disorder or phonons. In addition to distinguishing between collective modes and charge fluctuations, the light quantum control of multiband SCs can be extended to probe and manipulate many-body entanglement and hidden symmetries in different quantum materials.

cond-mat.supr-con↗

arXiv2020

Improved Limits on Millicharged Particles Using the ArgoNeuT Experiment at Fermilab

A search for millicharged particles, a simple extension of the standard model, has been performed with the ArgoNeuT detector exposed to the Neutrinos at the Main Injector beam at Fermilab. The ArgoNeuT Liquid Argon Time Projection Chamber detector enables a search for millicharged particles through the detection of visible electron recoils. We search for an event signature with two soft hits (MeV-scale energy depositions) aligned with the upstream target. For an exposure of the detector of $1.0$ $\times$ $10^{20}$ protons on target, one candidate event has been observed, compatible with the expected background. This search is sensitive to millicharged particles with charges between $10^{-3}e$ and $10^{-1}e$ and with masses in the range from $0.1$ GeV to $3$ GeV. This measurement provides leading constraints on millicharged particles in this large unexplored parameter space region.

hep-ex↗

arXiv2020

Strategies to reduce the environmental impact in the MRPC array of the EEE experiment

The Extreme Energy Events (EEE) Project employs Multi-gap Resistive Plate Chamber (MRPC) for studying the secondary cosmic ray muons in Extensive Air Showers. The array consists of about 60 tracking detectors, sparse on Italian territory and at CERN. The MRPCs are flowed with a gas mixture based on $C_2H_2F_4$ and $SF_6$, both of which are fluorinated greenhouse gases with a high environmental impact on the atmosphere. Due to the restrictions imposed by the European Union, these gases are being phased out of production and their cost is largely increasing. The EEE Collaboration started a campaign to reduce the gas emission from its array with the aim of containing costs and decreasing the experiment global warming impact. One method is to reduce the gas rate in each EEE detector. Another is to develop a gas recirculation system, whose a first prototype has been installed at one of the EEE stations located at CERN. Jointly a parallel strategy is focused on searching for environmental friendly gas mixtures which are able to substitute the standard mixture without affecting the MRPC performance. An overview and first results are presented here.

physics.ins-det↗

arXiv2020

Comprehensive Control of Metamagnetic Transition of Antiferromagnetic Mott Insulator Sr2IrO4 by in-situ Anisotropic Strain

Metamagnetism in antiferromagnets exhibits distinct critical behaviors and dynamics when invoking spin reversal and rotation. Here we show a 0.05% anisotropic strain suffices to in-situ modulate the metamagnetic critical field of the Mott insulator Sr2IrO4 by over 50%, enabling electrical switching of the transition. Resonant x-ray scattering and model simulation reveal that the transition is completely tuned from the spin-flop to spin-flip type as the strain introduces C4-symmetry-breaking magnetic anisotropy. Simultaneous transport study indicates the metamagnetic responses are reflected in the large elasto- and magnetoconductance, highlighting the active charge degree of freedom in the spin-orbit-coupled Mott state and its potential for spin-electronics.

cond-mat.str-el↗