arXiv subjects

Y. Lu

Publications and source records attributed to Y. Lu.

At least 37 records · Page 2Linked to original sources

arXiv2022

Pressure suppression of the excitonic insulator state in Ta2NiSe5 observed by optical conductivity

The layered chalcogenide Ta2NiSe5 has recently attracted much interest as a strong candidate for the long sought excitonic insulator (EI). Since the physical properties of an EI are expected to depend sensitively on the external pressure, it is important to clarify the pressure evolution of microscopic electronic state in Ta2NiSe5. Here we report the optical conductivity [s(w)] of Ta2NiSe5 measured at high pressures to 10 GPa and at low temperatures to 8 K. With cooling at ambient pressure, s(w) develops an energy gap of about 0.17 eV and a pronounced excitonic peak at 0.38 eV, as already reported in the literature. Upon increasing pressure, the energy gap becomes narrower and the excitonic peak is broadened. Above a structural transition at Ps~3 GPa, the energy gap becomes partially filled, indicating that Ta2NiSe5 is a semimetal after the EI state is suppressed by pressure. At higher pressures, s(w) exhibits metallic characteristics with no energy gap. The detailed pressure evolution of s(w) is presented, and discussed mainly in terms of a weakening of excitonic correlation with pressure.

cond-mat.str-el↗

arXiv2022

Nucleon-pair truncation of the shell model for medium-heavy nuclei

Background: Computationally tractable models of atomic nuclei is a long-time goal of nuclear structure physics. A flexible framework which easily includes excited states and many-body correlations is the configuration-interaction shell model (SM), but the exponential growth of the basis means one needs an efficient truncation scheme, ideally one that includes both deformation and pairing correlations. Purpose: We propose an efficient truncation scheme of the SM: starting from a pair condensate variationally defined by Hartree-Fock single-particle states and the particle-number conserved Bardeen-Cooper-Schrieffer (NBCS) approximation, we carry out projection of states with good angular momentum. Methods: After generating Hartree-Fock single-particle states with Kramers degeneracy in a SM space, we optimize the pair amplitudes in the NBCS by minimizing the energy, and then use linear algebra projection (LAP) of states with good angular momentum. Both NBCS and LAP are computationally fast. Results: Our calculations yield good agreement with full configuration-interaction SM calculations for low-lying states of transitional and rotational nuclei with axially symmetric and triaxial deformation in medium- and heavy-mass regions: $^{44,46,48}$Ti, $^{48,50}$Cr, $^{52}$Fe, $^{60,62,64}$Zn, $^{66,68}$Ge, $^{68}$Se, and $^{108,110}$Xe. We predict low-lying states of $^{112-114}\textrm{Ba}$ and $^{116-120}\textrm{Ce}$, nuclei difficult to reach by large-scale SM calculations. Conclusions: Both pair correlation and the configuration mixing between different intrinsic states play a key role in reproducing collectivity and shape coexistence, demonstrating the utility of this truncation scheme of the SM to study transitional and deformed nuclei.

nucl-th↗

arXiv2022

Varied magnetic phases in a van der Waals easy-plane antiferromagnet revealed by nitrogen-vacancy center microscopy

Interest in van der Waals materials often stems from a desire to miniaturise existing technologies by exploiting their intrinsic layered structure to create near atomically-thin components that do not suffer from surface defects. One appealing property is easily-switchable yet robust magnetic order, a quality only sparsely demonstrated in the case of in-plane anisotropy. In this work, we use widefield nitrogen-vacancy (NV) center magnetic imaging to measure the properties of individual flakes of CuCrP$_2$S$_6$, a multiferroic van der Waals magnet known to exhibit weak easy-plane anisotropy in the bulk. We chart the crossover between in-plane ferromagnetism in thin flakes down to the trilayer, and the bulk behaviour dominated by a low-field spin-flop transition. Further, by exploiting the directional dependence of NV center magnetometry, we are able to observe an instance of a predominantly out-of-plane ferromagetic phase near zero field, in contradiction with expectation and previous experiments on the bulk material. We attribute this to the presence of surface anisotropies arising from the sample preparation process or exposure to the ambient environment, which is expected to have more general implications for a broader class of weakly anisotropic van der Waals magnets.

cond-mat.mes-hall↗

arXiv2022

OMR-NPA: Optimized Matrix Representation of Nucleon Pair Approximation

We optimize the matrix representation of the nucleon-pair approximation (NPA) of the nuclear shell model. The NPA is a widely adopted truncation approach of the nuclear shell model and proves to be effective in describing low-lying states of medium-heavy and heavy nuclei. Due to simplified (yet flexible) commutators and absolute elimination of angular momentum coupling, the matrix representation provides a formalism for the $M$-scheme NPA more efficient than others as far as we know. It also enables the practicable organization and storage design for intermediate results, including generated collective pairs, matrix products, and matrix traces, so that further optimization is achieved by reducing repetitive matrix operations, which are the most time-consuming procedures in the matrix-represented $M$-scheme NPA. We also describe optimizations specified for the $M$-scheme NPA, realized by invoking the Wigner-Eckart theorem, time-reversal symmetry, and conjugate operation of spherical tensors. Our optimization makes the combination of matrix representation and NPA more profitable. Such an implementation denoted by optimized matrix representation of NPA (OMR-NPA) is publicly released with open source. Its performance is analyzed and compared against unoptimized NPA codes.

nucl-th↗

arXiv2021

Quantum microscopy with van der Waals heterostructures

Quantum microscopes based on solid-state spin quantum sensors have recently emerged as powerful tools for probing material properties and physical processes in regimes not accessible to classical sensors, especially on the nanoscale. Such microscopes have already found utility in a variety of problems, from imaging magnetism and charge transport in nanoscale devices, to mapping remanent magnetic fields from ancient rocks and biological organisms. However, applications of quantum microscopes have so far relied on sensors hosted in a rigid, three-dimensional crystal, typically diamond, which limits their ability to closely interact with the sample under study. Here we demonstrate a versatile and robust quantum microscope using quantum sensors embedded within a thin layer of a van der Waals (vdW) material, hexagonal boron nitride (hBN). To showcase the capabilities of this platform, we assemble several active vdW heterostructures, with an hBN layer acting as the quantum sensor. We demonstrate time-resolved, simultaneous temperature and magnetic imaging near the Curie temperature of a vdW ferromagnet as well as apply this unique microscope to map out charge currents and Joule heating in graphene. By enabling intimate proximity between sensor and sample, potentially down to a single atomic layer, the hBN quantum sensor represents a paradigm shift for nanoscale quantum sensing and microscopy. Moreover, given the ubiquitous use of hBN in modern materials and condensed matter physics research, we expect our technique to find rapid and broad adoption in these fields, further motivated by the prospect of performing in-situ chemical analysis and noise spectroscopy using advanced quantum sensing protocols.

cond-mat.mes-hall↗

arXiv2021

Spin dependent charge transfer in MoSe2/hBN/Ni hybrid structures

We present magneto-photoluminescence measurements in a hybrid 2D semiconductor/ferromagnetic structure consisting of MoSe2/hBN/Ni. When the Nickel layer is magnetized, we observe circularly polarized photoluminescence of the trion peak in MoSe2 monolayer under linearly polarized excitation. This build-up of circular polarization can reach a measured value of about 4% when the magnetization of Ni is saturated perpendicularly to the sample plane, and changes its sign when the magnetization is reversed. The circular polarization decreases when the hBN barrier thickness increases. These results are interpreted in terms of a spin-dependent charge transfer between the MoSe2 monolayer and the Nickel film. The build-up of circular polarization is observed up to 120 K, mainly limited by the trion emission that vanishes with temperature.

cond-mat.mtrl-sci↗

arXiv2021

Tree tensor-network real-time multiorbital impurity solver: Spin-orbit coupling and correlation functions in Sr$_2$RuO$_4$

We present a tree tensor-network impurity solver suited for general multiorbital systems. The network is constructed to efficiently capture the entanglement structure and symmetry of an impurity problem. The solver works directly on the real-time/frequency axis and generates spectral functions with energy-independent resolution of the order of one percent of the correlated bandwidth. Combined with an optimized representation of the impurity bath, it efficiently solves self-consistent dynamical mean-field equations and calculates various dynamical correlation functions for systems with off-diagonal Green's functions. For the archetypal correlated Hund's metal Sr$_2$RuO$_4$, we show that both the low-energy quasiparticle spectra related to the van Hove singularity and the high-energy atomic multiplet excitations can be faithfully resolved. In particular, we show that while the spin-orbit coupling has only minor effects on the orbital-diagonal one-particle spectral functions, it has a more profound impact on the low-energy spin and orbital response functions.

cond-mat.str-el↗

arXiv2021

Anomalous diamagnetism of electride electrons in transition metal silicides

Intermetallic silicide compounds, LaScSi and Y$_5$Si$_3$, known for being hydrogen (H) storage materials, are drawing attention as candidates for electrides in which anions are substituted by unbound electrons. It is inferred from a muon spin rotation experiment that the local field at the muon site (which is the same site as that for H) in these compounds exhibits a large negative shift under an external magnetic field, which is mostly independent of temperature. Such anomalous diamagnetism signals a unique property of electride electrons associated with transition metals. Moreover, the diamagnetic shift decreases with increasing H content, suggesting that the electride electrons existing coherently in the hollow interstitial positions are adsorbed by H to form hydride ions (H$^-$)

cond-mat.mtrl-sci↗

arXiv2021

Strong suppression of heat conduction in a laboratory replica of galaxy-cluster turbulent plasmas

Galaxy clusters are filled with hot, diffuse X-ray emitting plasma, with a stochastically tangled magnetic field whose energy is close to equipartition with the energy of the turbulent motions \cite{zweibel1997, Vacca}. In the cluster cores, the temperatures remain anomalously high compared to what might be expected considering that the radiative cooling time is short relative to the Hubble time \cite{cowie1977,fabian1994}. While feedback from the central active galactic nuclei (AGN) \cite{fabian2012,birzan2012,churazov2000} is believed to provide most of the heating, there has been a long debate as to whether conduction of heat from the bulk to the core can help the core to reach the observed temperatures \cite{narayan2001,ruszkowski2002,kunz2011}, given the presence of tangled magnetic fields. Interestingly, evidence of very sharp temperature gradients in structures like cold fronts implies a high degree of suppression of thermal conduction \cite{markevitch2007}. To address the problem of thermal conduction in a magnetized and turbulent plasma, we have created a replica of such a system in a laser laboratory experiment. Our data show a reduction of local heat transport by two orders of magnitude or more, leading to strong temperature variations on small spatial scales, as is seen in cluster plasmas \cite{markevitch2003}.

physics.plasm-ph↗

arXiv2021

Control of the metal-insulator transition in NdNiO$_3$ thin films through the interplay between structural and electronic properties

Heteroepitaxy offers a new type of control mechanism for the crystal structure, the electronic correlations, and thus the functional properties of transition-metal oxides. Here, we combine electrical transport measurements, high-resolution scanning transmission electron microscopy (STEM), and density functional theory (DFT) to investigate the evolution of the metal-to-insulator transition (MIT) in NdNiO$_3$ films as a function of film thickness and NdGaO$_3$ substrate crystallographic orientation. We find that for two different substrate facets, orthorhombic (101) and (011), modifications of the NiO$_6$ octahedral network are key for tuning the transition temperature $T_{\text{MIT}}$ over a wide temperature range. A comparison of films of identical thickness reveals that growth on [101]-oriented substrates generally results in a higher $T_{\text{MIT}}$, which can be attributed to an enhanced bond-disproportionation as revealed by the DFT+$U$ calculations, and a tendency of [011]-oriented films to formation of structural defects and stabilization of non-equilibrium phases. Our results provide insights into the structure-property relationship of a correlated electron system and its evolution at microscopic length scales and give new perspectives for the epitaxial control of macroscopic phases in metal-oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2021

Photoassociative Spectroscopy of $^{87}$Sr

We demonstrate photoassociation (PA) of ultracold fermionic $^{87}$Sr atoms. The binding energies of a series of molecular states on the $^1Σ^+_u$ $5s^2\,^1$S$_0+5s5p\,^1$P$_1$ molecular potential are fit with the semiclassical LeRoy-Bernstein model, and PA resonance strengths are compared to predictions based on the known $^1$S$_0+^1$S$_0$ ground state potential. Similar measurements and analysis were performed for the bosonic isotopes $^{84}$Sr and $^{86}$Sr, allowing a combined analysis of the long-range portion of the excited-state potential and determination of the $5s5p\,^1$P$_1$ atomic state lifetime of $5.20 \pm 0.02$ ns. The results enable prediction of PA rates across a wide range of experimental conditions.

physics.atom-ph↗

arXiv2021

Realizing Su-Schrieffer-Heeger topological edge states in Rydberg-atom synthetic dimensions

We demonstrate a platform for synthetic dimensions based on coupled Rydberg levels in ultracold atoms, and we implement the single-particle Su-Schrieffer-Heeger (SSH) Hamiltonian. Rydberg levels are interpreted as synthetic lattice sites, with tunneling introduced through resonant millimeter-wave couplings. Tunneling amplitudes are controlled through the millimeter-wave amplitudes, and on-site potentials are controlled through detunings of the millimeter waves from resonance. Using alternating weak and strong tunneling with weak tunneling to edge lattice sites, we attain a configuration with symmetry-protected topological edge states. The band structure is probed through optical excitation to the Rydberg levels from the ground state, which reveals topological edge states at zero energy. We verify that edge-state energies are robust to perturbation of tunneling-rates, which preserves chiral symmetry, but can be shifted by the introduction of on-site potentials.

physics.atom-ph↗

arXiv2020

Deterministic loading and phase shaping of microwaves onto a single artificial atom

Loading quantum information deterministically onto a quantum node is an important step towards a quantum network. Here, we demonstrate that coherent-state microwave photons, with an optimal temporal waveform, can be efficiently loaded onto a single superconducting artificial atom in a semi-infinite one-dimensional (1D) transmission-line waveguide. Using a weak coherent state (average photon number N<<1 with an exponentially rising waveform, whose time constant matches the decoherence time of the artificial atom, we demonstrate a loading efficiency of above 94% from 1D semi-free space to the artificial atom. We also show that Fock-state microwave photons can be deterministically loaded with an efficiency of 98.5%. We further manipulate the phase of the coherent state exciting the atom, enabling coherent control of the loading process. Our results open up promising applications in realizing quantum networks based on waveguide quantum electrodynamics (QED).

quant-ph↗

arXiv2020

Nucleon-pair approximation with matrix representation

In this paper, we propose an approach of the nucleon-pair approximation (NPA), in which the collective nucleon pairs are represented in terms of antisymmetric matrices, and commutations between nucleon pairs are given by using matrix multiplication that avoids angular-momentum couplings and recouplings. Therefore the present approach significantly simplifies the NPA computation. Furthermore, it is formulated on the same footing with and without isospin.

nucl-th↗

arXiv2020

Square Kilometre Array Science Data Challenge 1: analysis and results

As the largest radio telescope in the world, the Square Kilometre Array (SKA) will lead the next generation of radio astronomy. The feats of engineering required to construct the telescope array will be matched only by the techniques developed to exploit the rich scientific value of the data. To drive forward the development of efficient and accurate analysis methods, we are designing a series of data challenges that will provide the scientific community with high-quality datasets for testing and evaluating new techniques. In this paper we present a description and results from the first such Science Data Challenge (SDC1). Based on SKA MID continuum simulated observations and covering three frequencies (560 MHz, 1400MHz and 9200 MHz) at three depths (8 h, 100 h and 1000 h), SDC1 asked participants to apply source detection, characterization and classification methods to simulated data. The challenge opened in November 2018, with nine teams submitting results by the deadline of April 2019. In this work we analyse the results for 8 of those teams, showcasing the variety of approaches that can be successfully used to find, characterise and classify sources in a deep, crowded field. The results also demonstrate the importance of building domain knowledge and expertise on this kind of analysis to obtain the best performance. As high-resolution observations begin revealing the true complexity of the sky, one of the outstanding challenges emerging from this analysis is the ability to deal with highly resolved and complex sources as effectively as the unresolved source population.

astro-ph.IM↗

arXiv2020

Loss rates for high-$n$, $49\lesssim n \lesssim150$, 5sns($^{3}$S$_{1}$) Rydberg atoms excited in an $^{84}$Sr Bose-Einstein condensate

Measurements of the loss rates for strontium n$^{3}$S$_{1}$ Rydberg atoms excited in a dense BEC are presented for values of principal quantum number $n$ in the range $49\lesssim n \lesssim 150$ and local atom densities of $\sim1$ to $3\times10^{14}$cm$^{-3}$. Two main processes contribute to loss, associative ionization and state-changing. The relative importance of these two loss channels is investigated and their $n$- and density-dependences are discussed using a model in which Rydberg atom loss is presumed to involve a close collision between the Rydberg core ion and ground-state atoms. The present measurements are compared to earlier results obtained using rubidium Rydberg atoms. For both species the observed loss rates are sizable, $\sim10^{5}-10^{6}$s$^{-1}$, and limit the time scales over which measurements involving Rydberg atoms immersed in quantum degenerate gases can be conducted.

physics.atom-ph↗

arXiv2020

QCD2019 Workshop Summary

The topical workshop {\it Strong QCD from Hadron Structure Experiments} took place at Jefferson Lab from Nov. 6-9, 2019. Impressive progress in relating hadron structure observables to the strong QCD mechanisms has been achieved from the {\it ab initio} QCD description of hadron structure in a diverse array of methods in order to expose emergent phenomena via quasi-particle formation. The wealth of experimental data and the advances in hadron structure theory make it possible to gain insight into strong interaction dynamics in the regime of large quark-gluon coupling (the strong QCD regime), which will address the most challenging problems of the Standard Model on the nature of the dominant part of hadron mass, quark-gluon confinement, and the emergence of the ground and excited state hadrons, as well as atomic nuclei, from QCD. This workshop aimed to develop plans and to facilitate the future synergistic efforts between experimentalists, phenomenologists, and theorists working on studies of hadron spectroscopy and structure with the goal to connect the properties of hadrons and atomic nuclei available from data to the strong QCD dynamics underlying their emergence from QCD. These results pave the way for a future breakthrough extension in the studies of QCD with an Electron-Ion Collider in the U.S.

hep-ph↗

arXiv2020

Heteronuclear Rydberg molecules

We report the creation of heteronuclear ultralong-range Rydberg-molecule dimers by excitation of minority $^{88}\text{Sr}$ atoms to $5sns\,^3S_1$ Rydberg states ($n=31-39$) in a dense background of $^{84}\text{Sr}$. We observe an isotope shift of the $ν=0$ vibrational state over this range of $n$ and compare our measurements with a theoretical prediction and a simple scaling argument. With the appropriate choice of principal quantum number the isotope shift is sufficiently large to produce heteronuclear dimers with almost perfect fidelity. When the spectral selectivity is limited, we obtain a lower bound on the ratio of heteronuclear to homonuclear excitation probability of 30 to 1 by measuring the scaling of the molecular excitation rate with varying relative densities of $^{88}\text{Sr}$ and $^{84}\text{Sr}$ in the ultracold mixture.

physics.atom-ph↗