arXiv subjects

L. Zhou

Publications and source records attributed to L. Zhou.

At least 19 recordsLinked to original sources

arXiv2025

Lineshape-asymmetry-caused shift in atomic interferometers

We investigate the shift caused by asymmetry of spectroscopic lineshape in atomic interferometers, which has not previously been discussed in the scientific literature. This asymmetry arises because laser field is frequency-chirped not only during the free-evolution intervals of atoms, but also during the Ramsey pulses. As a result, the effective detuning from the working atomic transition during the pulses also depends on the chirping rate, which, in turn, leads to the lineshape-asymmetry-caused shift (LACS). It is shown that this shift has an inverse cubic dependence of $\propto 1/T^3$ on the duration of the interval between the Ramsey pulses $T$, which markedly contrasts with the $\propto 1/T^2$ dependence typical in atomic interferometry. Therefore, the metrological importance of this shift substantially increases for compact atomic interferometers with a short baseline. For example, for interferometers-gravimeters using two-photon transitions in rubidium atoms, at $T\sim 1$~ms we estimate the LACS shift and its variations at the level of 0.1-1~mGal, while for $T\sim 100$~$\mu$s this can reach a value of 0.1-1~Gal.

physics.atom-ph↗

arXiv2025

Quantum theory of magneto-optical trap

We present a quantum theory of a magneto-optical trap (MOT) from first principles based on the quantum kinetic equation for the atomic density matrix with taking into account the recoil effects caused by the interaction of atoms with the laser field. An efficient method for solving the quantum kinetic equation is proposed. It is shown that the steady-state solution describing the atoms in the MOT has a significantly non-equilibrium nature and can be described within the framework of a two-temperature distribution. The momentum distribution of cold atoms in the MOT depends on the magnetic field gradient and, in general, significantly differs from the momentum distribution of atoms in the optical molasses, which is usually used as an approximation to describe the MOT. We have also shown that with an increase in the magnetic field gradient, a spatial two-component distribution of atoms in the trap is formed even for a single particle approximation when interatomic interactions are neglected.

physics.atom-ph↗

arXiv2024

Resonant Raman scattering of surface phonon polaritons mediated by excitons in WSe$_2$ films

Surface phonon-polaritons propagating along interfaces of polar dielectrics coexist with excitons in many van der Waals heterostructures, so understanding their mutual interactions is of great interest. Here, we investigate the type I surface phonon polariton of hBN via low-temperature resonant-Raman spectroscopy in hBN/WSe2 heterostructures. The resonantly enhanced hBN surface phonon polariton (SPhP) Raman signal, when laser energy is such that the scattered photons have energy close to that of the WSe2 excitons, enables detailed characterization of type I SPhP in hBN even when hBN is one monolayer thick. We find that the measured bandwidth of the SPhP Raman signal depends on the thicknesses of the hBN layer. We are able explain the experimental data using transfer matrix method simulations of SPhP dispersions providing that we assume the Raman scattering to be momentum non-conserving, as could be the case if localized WSe2 exciton states participated in the process. We further show that resonant Raman scattering from SiO2 SPhP can also be mediated by WSe$_2$.

cond-mat.mes-hall↗

arXiv2024

Post-synthesis tuning of dielectric constant via ferroelectric domain wall engineering

A promising mechanism for achieving colossal dielectric constants is to use insulating internal barrier layers, which typically form during synthesis and then remain in the material. It has recently been shown that insulating domain walls in ferroelectrics can act as such barriers. One advantage domain walls have, in comparison to stationary interfaces, is that they can be moved, offering the potential of post-synthesis control of the dielectric constant. However, to date, direct imaging of how changes in domain wall pattern cause a change in dielectric constant within a single sample has not been realized. In this work, we demonstrate that changing the domain wall density allows the engineering of the dielectric constant in hexagonal-ErMnO3 single crystals. The changes of the domain wall density are quantified via microscopy techniques, while the dielectric constant is determined via macroscopic dielectric spectroscopy measurements. The observed changes in the dielectric constant are quantitatively consistent with the observed variation in domain wall density, implying that the insulating domain walls behave as 'ideal' capacitors connected in series. Our approach to engineer the domain wall density can be readily extended to other control methods, e.g., electric fields or mechanical stresses, providing a novel degree of flexibility to in-situ tune the dielectric constant.

cond-mat.mtrl-sci↗

arXiv2023

Antiferromagnetism and Ising Ground States in the Rare-earth Garnet Nd$_3$Ga$_5$O$_{12}$

In this paper, we investigate the low temperature magnetic properties of the rare-earth garnet compound Nd$_3$Ga$_5$O$_{12}$ in detail by means of magnetization, specific heat and magnetocaloric effect measurements. The magnetic thermal properties along with the crystal field calculations reveal that the Nd$^{3+}$ ions form into a frustrated hyper-kagome lattice with connected triangles have an Ising-like ground state with the easy axis along the local [100], [010] and [001] directions. Instead of a quantum spin liquid ground state, an antiferromagnetically ordered state is found below $T_{\mathrm{N}}=0.52~\rm K$. With applying field in the [111] direction, the antiferromagnetic order is suppressed at the critical field of $B_{\mathrm{c}}=0.75~\rm T$, and enhancement of the critical fluctuations with linear crossover behaviors is observed near the critical point.

cond-mat.str-el↗

arXiv2023

Successive magnetic orderings in the Ising spin chain magnet DyNi$_5$Ge$_3$

In this report, we investigated a new rare earth based one-dimensional Ising spin chain magnet~\DNG~by means of magnetization, specific heat and powder neutron diffraction measurements. Due to the crystalline electrical field splitting, the magnetic Dy ions share an Ising like ground doublet state. Owning to the local point symmetry, these Ising moments form into two canted magnetic sublattices, which were further confirmed by the angle-dependent magnetization measurement. In zero fields, two successive antiferromagnetic phase transitions were found at temperatures $T_{\mathrm{N1}}=6~\rm K$ and $T_{\mathrm{N2}}=5~\rm K$, respectively. Only part of the moments are statically ordered in this intermediate state between $T_{\mathrm{N1}}$ and $T_{\mathrm{N2}}$. Powder neutron diffraction experiments at different temperatures were performed as well. An incommensurate magnetic propagation vector of $\mathbf{k_{\rm m}}=(0.5,0.4,0.5)$ was identified. The refined spin configurations through the irreducible representation analysis confirmed that these Ising spins are canted in the crystal $ab$~plane.

cond-mat.str-el↗

arXiv2023

Epitaxial growth and scanning tunneling microscopy of LiV$_2$O$_4$ thin films on SrTiO$_3$(111)

LiV$_2$O$_4$ is a mixed-valent spinel oxide and one of a few transition-metal compounds to host a heavy fermion phase at low temperatures. While numerous experimental studies have attempted to elucidate how its 3$d$ electrons undergo giant mass renormalization, spectroscopic probes that may provide crucial hints, such as scanning tunneling microscopy (STM), remain to be applied. A prerequisite is atomically flat and pristine surfaces, which, in the case of LiV$_2$O$_4$, are difficult to obtain by cleavage of small, three-dimensional crystals. We report the epitaxial growth of LiV$_2$O$_4$ thin films with bulklike properties on SrTiO$_3$(111) via pulsed laser deposition and stable STM imaging of the LiV$_2$O$_4$(111) surface. The as-grown films were transferred $ex$ $situ$ to a room-temperature STM, where subsequent annealing with optional sputtering in ultrahigh vacuum enabled compact islands with smooth surfaces and a hexagonal 1$\times$1 atomic lattice to be resolved. Our STM measurements provide insights into growth mechanisms of LiV$_2$O$_4$ on SrTiO$_3$(111), as well as demonstrate the feasibility of performing surface-sensitive measurements of this heavy fermion compound.

cond-mat.mtrl-sci↗

arXiv2023

Small sample measurements at the low energy muon facility of Paul Scherrer Institute

The low energy muon spin rotation spectroscopy (LE-$\mu$SR) is primarily used to investigate thin films, surfaces, and interfaces of materials, which has matured at the Paul Scherrer Institute (PSI) and is routinely employed by users of the low energy muon (LEM) facility. However, because of the large beam spot and low implanted muons rate, LE-$\mu$SR measurements on small samples are difficult, requiring an optimal sample size of $25\times25$ mm$^2$. Recently, we have boosted our ability to measure small samples, down to $5\times5$ mm$^2$ area, by beam collimation and tuning. This achievement is crucial for the measurements of many magnetic and superconducting materials. Furthermore, we have devised a method that allows us to measure five small area samples mounted together on the same sample plate. We expect this method to further improve the efficient use of beam time at LEM.

physics.ins-det↗

arXiv2022

Recent Progress in Two-proton Radioactivity

During the last few decades, rare isotope beam facilities have provided unique data for studying the properties of nuclides located far from the beta-stability line. Such nuclei are often accompanied by exotic structures and radioactive modes, which represent the forefront of nuclear research. Among them, two-proton (2p) radioactivity is a rare decay mode found in a few highly proton-rich isotopes. The 2p decay lifetimes and properties of emitted protons hold invaluable information regarding the nuclear structures in the presence of a low-lying proton continuum; as such, they have attracted considerable research attention. In this review, we present some of the recent experimental and theoretical progress regarding the 2p decay, including technical innovations for measuring nucleon--nucleon correlations and developments in the models that connect their structural aspects with their decay properties. This impressive progress should play a significant role in elucidating the mechanism of these exotic decays, probing the corresponding components inside nuclei, and providing deep insights into the open quantum nature of dripline systems.

nucl-ex↗

arXiv2022

Visualizing heterogeneous dipole fields by terahertz light coupling in individual nano-junctions used in transmon qubits

The fundamental challenge underlying superconducting quantum computing is to characterize heterogeneity and disorder in the underlying quantum circuits. These nonuniform distributions often lead to local electric field concentration, charge scattering, dissipation and ultimately decoherence. It is particularly challenging to probe deep sub-wavelength electric field distribution under electromagnetic wave coupling at individual nano-junctions and correlate them with structural imperfections from interface and boundary, ubiquitous in Josephson junctions (JJ) used in transmon qubits. A major obstacle lies in the fact that conventional microscopy tools are incapable of measuring simultaneous at nanometer and terahertz, "nano-THz" scales, which often associate with frequency-dependent charge scattering in nano-junctions. Here we directly visualize interface nano-dipole near-field distribution of individual Al/AlO$_{x}$/Al junctions used in transmon qubits. Our THz nanoscope images show a remarkable asymmetry across the junction in electromagnetic wave-junction coupling response that manifests as "hot" vs "cold" cusp spatial electrical field structures and correlates with defected boundaries from the multi-angle deposition processes in JJ fabrication inside qubit devices. The asymmetric nano-dipole electric field contrast also correlates with distinguishing, "overshoot" frequency dependence that characterizes the charge scattering and dissipation at nanoscale, hidden in responses from topographic, structural imaging and spatially-averaged techniques. The real space mapping of junction dipole fields and THz charge scattering can be extended to guide qubit nano-fabrication for ultimately optimizing qubit coherence times.

cond-mat.mes-hall↗

arXiv2022

Exploring the intrinsic energy resolution of liquid scintillator to approximately 1 MeV electrons

We proposed a novel method for exploring the intrinsic energy resolution of a liquid scintillator (LAB + 2.5 g/L PPO + 3 mg/L bis-MSB) for approximately 1 MeV electrons. With the help of coincidence detection technology, single-energy electrons of Bi 207 were effectively selected. With careful measurement and analysis of the energy resolution of a small liquid scintillator detector, the intrinsic energy resolution to 976 keV electrons was extracted to be 1.83%. We used the wide-angle Compton coincidence (WACC) method to measure the luminescent nonlinearity of the liquid scintillator and found that it contributes only weakly to the intrinsic energy resolution of electrons. Such an unexpected large intrinsic energy resolution may come from fluctuations in energy transfer processes.

physics.ins-det↗

arXiv2022

Resonance Fluorescence from a two-level artificial atom strongly coupled to a single-mode cavity

We experimentally demonstrate the resonance fluorescence of a two-level artificial atom strongly coupled to a single-mode cavity field. The effect was theoretically predicted thirty years ago by Savage [Phys. Rev. Lett. 63, 1376 (1989)]. The system consists of a superconducting qubit circuit and a one-dimensional transmission line resonator. In addition, a one-dimensional transmission line strongly coupled to the atom serves as an open space. The effect takes place, when a microwave field is applied to the cavity, which in turn is resonantly coupled to the atom. The fluorescence spectrum is measured via the emission into the transmission line. We find that the central peak is determined by the atom spontaneous emission to the open space and the widths of side peaks are largely determined by the coherent interaction between the atom and the cavity, that is, the fluorescence spectrum here is very different from that of the Mollow triplet. We also derive analytical form for the spectrum. Our experimental results agree well with theoretical calculations.

quant-ph↗

arXiv2022

GOODS-ALMA 2.0: Starbursts in the main sequence reveal compact star formation regulating galaxy evolution prequenching

Compact star formation appears to be generally common in dusty star-forming galaxies (SFGs). However, its role in the framework set by the scaling relations in galaxy evolution remains to be understood. In this work we follow up on the galaxy sample from the GOODS-ALMA 2.0 survey, an ALMA blind survey at 1.1mm covering a continuous area of 72.42arcmin$^2$ using two array configurations. We derived physical properties, such as star formation rates, gas fractions, depletion timescales, and dust temperatures for the galaxy sample built from the survey. There exists a subset of galaxies that exhibit starburst-like short depletion timescales, but they are located within the scatter of the so-called main sequence of SFGs. These are dubbed starbursts in the main sequence and display the most compact star formation and they are characterized by the shortest depletion timescales, lowest gas fractions, and highest dust temperatures of the galaxy sample, compared to typical SFGs at the same stellar mass and redshift. They are also very massive, accounting for $\sim 60\%$ of the most massive galaxies in the sample ($\log (M_{\rm{*}}/M_{\odot}) > 11.0$). We find trends between the areas of the ongoing star formation regions and the derived physical properties for the sample, unveiling the role of compact star formation as a physical driver of these properties. Starbursts in the main sequence appear to be the extreme cases of these trends. We discuss possible scenarios of galaxy evolution to explain the results drawn from our galaxy sample. Our findings suggest that the star formation rate is sustained in SFGs by gas and star formation compression, keeping them within the main sequence even when their gas fractions are low and they are presumably on the way to quiescence.

astro-ph.GA↗

arXiv2021

Magnetically tunable Shubnikov-de Hass oscillations in MnBi2Te4

Shubnikov-de Hass oscillations are directly observed in undoped antiferromagnetic topological insulator MnBi2Te4. With increasing magnetic fields, the oscillation period decreases gradually in the magnetic transition from canted antiferromagnetism to ferromagnetism and then saturates in high magnetic fields, indicating the field-induced evolution of the band structure. From the analysis of the high-field oscillations, a nontrivial Berry phase and a small effective mass are extracted, in agreement with the predicted Weyl semimetal phase in ferromagnetic MnBi2Te4. Furthermore, rotating the magnetization of MnBi2Te4 can lead to a splitting of the high-field oscillations, which suggests the enhanced asymmetry of the Weyl cones in tilted fields. Therefore, the observation of these magnetically tunable quantum oscillations clearly demonstrates the indispensable role of field in tuning the band structure or physical properties of MnBi2Te4.

cond-mat.mtrl-sci↗

arXiv2021

GOODS-ALMA 2.0: Source catalog, number counts, and prevailing compact sizes in 1.1 mm galaxies

Submillimeter/millimeter observations of dusty star-forming galaxies with the Atacama Large Millimeter/submillimeter Array (ALMA) have shown that dust continuum emission generally occurs in compact regions smaller than the stellar distribution. However, it remains to be understood how systematic these findings are. Studies often lack homogeneity in the sample selection, target discontinuous areas with inhomogeneous sensitivities, and suffer from modest $uv$ coverage coming from single array configurations. GOODS-ALMA is a 1.1mm galaxy survey over a continuous area of 72.42arcmin$^2$ at a homogeneous sensitivity. In this version 2.0, we present a new low resolution dataset and its combination with the previous high resolution dataset from the survey, improving the $uv$ coverage and sensitivity reaching an average of $\sigma = 68.4\mu$Jy beam$^{-1}$. A total of 88 galaxies are detected in a blind search (compared to 35 in the high resolution dataset alone), 50% at $S/N_{peak} \geq 5$ and 50% at $3.5 \leq S/N_{peak} \leq 5$ aided by priors. Among them, 13 out of the 88 are optically dark or faint sources ($H$- or $K$-band dropouts). The sample dust continuum sizes at 1.1mm are generally compact, with a median effective radius of $R_{e} = 0"10 \pm 0"05$ (a physical size of $R_{e} = 0.73 \pm 0.29$kpc at the redshift of each source). Dust continuum sizes evolve with redshift and stellar mass resembling the trends of the stellar sizes measured at optical wavelengths, albeit a lower normalization compared to those of late-type galaxies. We conclude that for sources with flux densities $S_{1.1mm} > 1$mJy, compact dust continuum emission at 1.1mm prevails, and sizes as extended as typical star-forming stellar disks are rare. The $S_{1.1mm} < 1$mJy sources appear slightly more extended at 1.1mm, although they are still generally compact below the sizes of typical star-forming stellar disks.

astro-ph.GA↗

arXiv2021

Optical hyperpolarization of heteronuclear spin singlet order in liquids

The nuclear spin singlet order involving coupled pairs of spins-1/2 may be used to store nuclear spin hyperpolarization in a room temperature liquid for a time much longer than the spin-lattice relaxation time $T_1$. There both are observations of long-lived homonuclear and heteronuclear spin-singlet order. Although hyperpolarized singlet order of the same species are accessible, hyperpolarized heteronuclear spin-singlet order has not been presented yet. Here we show hyperpolarized singlet order is achievable in the sample of $^{13}$C-labeled formic acid solution at room temperature by using optically polarized nitrogen vacancy (NV) center spins in nanodiamonds.

quant-ph↗

arXiv2020

Tunneling-tip-induced collapse of the charge gap in the excitonic insulator Ta$_2$NiSe$_5$

Tuning many-body electronic phases by an external handle is of both fundamental and practical importance in condensed matter science. The tunability mirrors the underlying interactions, and gigantic electric, optical and magnetic responses to minute external stimuli can be anticipated in the critical region of phase change. The excitonic insulator is one of the exotic phases of interacting electrons, produced by the Coulomb attraction between a small and equal number of electrons and holes, leading to the spontaneous formation of exciton pairs in narrow-gap semiconductors/semimetals. The layered chalcogenide Ta$_2$NiSe$_5$ has been recently discussed as such an excitonic insulator with an excitation gap of ~250 meV below $T_c$ = 328 K. Here, we demonstrate a drastic collapse of the excitation gap in Ta$_2$NiSe$_5$ and the realization of a zero-gap state by moving the tip of a cryogenic scanning tunneling microscope towards the sample surface by a few angstroms. The collapse strongly suggests the many-body nature of the gap in the insulating state of Ta$_2$NiSe$_5$, consistent with the formation of an excitonic state. We argue that the collapse of the gap is driven predominantly by the electrostatic charge accumulation at the surface induced by the proximity of the tip and the resultant carrier doping of the excitonic insulator. Our results establish a novel phase-change function based on excitonic insulators.

cond-mat.str-el↗

arXiv2020

GOODS-ALMA: Using IRAC and VLA to probe fainter millimeter galaxies

In this paper, we extend the source detection in the GOODS-ALMA field (69 arcmin$^2$, rms sensitivity $σ$ $\simeq$ 0.18 mJy.beam$^{-1}$), to deeper levels than presented in Franco et al. (2018). Using positional information at 3.6 and 4.5 $μ$m (from Spitzer-IRAC), we explore the presence of galaxies detected at 1.1 mm with ALMA below our original blind detection limit of 4.8-$σ$ at which the number of spurious sources starts to dominate over that of real sources. In this Supplementary Catalog, we find a total of 16 galaxies, including 2 galaxies with no counterpart in HST images (also known as optically-dark galaxies) down to a 5$σ$ limiting depth of H = 28.2 AB (HST/WFC3 F160W). This brings the total sample of GOODS-ALMA 1.1 mm sources to 35 galaxies. Galaxies in the new sample cover a wider dynamic range in redshift ($z$ = 0.65 - 4.73), are on average twice as large (1.3 vs 0.65 kpc) and and have lower stellar mass (M$_{\star}^{\rm SC}$ = 7.6$\times$10$^{10}$M$_\odot$ vs M$_{\star}^{\rm MC}$ = 1.2$\times$10$^{11}$M$_\odot$). Although exhibiting larger physical sizes, these galaxies have still far-infrared sizes significantly more compact than inferred from their optical emission. We show that the astrometry of the HST image does not only suffer from a global astrometric shift, as already discussed in previous papers, but also from local shifts. These distortions were artificially introduced in the process of building the mosaic of the GOODS-South HST image. By comparing the positions of almost 400 galaxies detected by HST, Pan-STARRS and ALMA, we create a distortion map which can be used to correct for these astrometric issues.

astro-ph.GA↗