arXiv subjects

G. Xu

Publications and source records attributed to G. Xu.

At least 19 recordsLinked to original sources

arXiv2025

Mode locking via delayed orthogonal-polarization reinjection in semiconductor VCSELs

We demonstrate harmonic mode-locking in a semiconductor VCSEL using polarization-controlled delayed feedback. By integrating a rotatable $\lambda$/2-plate within an external cavity, we achieve precise control over pulse multiplicity and repetition rates in TE and TM modes. For the TE mode, increasing the $\lambda$/2-plate angle ($\theta$) transitions the system from disordered quasi-periodic states to stable fundamental (single-pulse) and harmonic dual-pulse mode-locking. Polarization-resolved measurements and cross-correlation analyses reveal coherent pulse alignment at half the cavity roundtrip time, enabled by polarization-mediated nonlinear dynamics. This work establishes cross-polarization feedback as a fundamental mechanism for ultrafast pulse engineering, advancing the understanding of polarization-mediated nonlinear dynamics in laser physics.

physics.optics↗

arXiv2025

Zeeman split Kramers doublets in spin-supersolid candidate Na$_{2}$BaCo(PO$_{4}$)$_{2}$

Na$_{2}$BaCo(PO$_{4}$)$_{2}$ is a triangular antiferromagnet that displays highly efficient adiabatic demagnetization cooling (J. Xiang $\textit{et al.}$ Nature ${\bf{625}}$, 270 (2024)) near a quantum critical point at $\mu_{0}H_{c}\sim 1.6$ T, separating a low-field magnetically disordered from a high-field fully polarized ferromagnetic phase. We apply high resolution backscattering neutron spectroscopy in an applied field to study the magnetic excitations near $\mu_{0}H_{c}$. At large fields we observe ferromagnetic fluctuations that gradually transition to being overdamped in energy below $\mu_{0}H_{c}$ where the magnetism is spatially disordered. We parameterize the excitations in the high field polarized phase in terms of coupled Zeeman split Kramers doublets originating from the presence of spin-orbit coupling. On reducing the field, the splitting between the Kramers doublets is reduced and if done adiabatically, provides a mechanism for reducing temperature. On lowering the applied field through the $\mu_{0}H_{c}$ the excitations characterize a textured phase that we suggest is inefficient for cooling. Low temperature disordered frustrated magnets built on Kramers doublets with nearby quantum critical points provide a route for efficient magnetocalorics.

cond-mat.str-el↗

arXiv2025

A space-resolved visible spectrometer system using compact endoscopic optics for full vertical profile measurement of impurity line emissions in superconducting EAST tokamak

In Experimental Advanced Superconducting Tokamak (EAST tokamak) with tungsten divertors and molybdenum first wall, lithiumization and boronization have been frequently carried out to improve the plasma performance, in particular, in long pulse discharges. A study on impurity behaviors of lithium, boron and tungsten atoms/ions in the edge plasma is then crucially important. For the purpose, a space-resolved visible spectrometer system has been newly developed to observe full vertical profiles over a length of 1.7m of impurity line emissions in wavelength range of 320-800nm. For the full vertical profile measurement compact endoscopic optics is employed with an optical fiber bundle for the system, which can be inserted into a 1.5m long extension tube called 'long nose', because the distance between the diagnostic port and plasma center is considerably long. Therefore, a quartz glass window mounted from the vacuum vessel side is designed to withstand the reverse pressure. A mechanical shutter is also designed to open at a large angle of 235 degree so that the viewing angle of nearby ports is not blocked. Two sets of the fiber bundle, 60-channel linear array and 11*10 channel planar array , with a length of 30m are attached to two sets of Czerny-Turner visible spectrometers for one-dimensional (1D) vertical profile measurement of core plasma and two-dimensional (2D) spectroscopy of divertor plasma, respectively. A complementary metal oxide semiconductor (CMOS) detector with 2048*2048 pixels is used for the visible spectrometers. A preliminary result on the full vertical profile is obtained for BII line emission at 703.19nm in the 1D system

physics.plasm-ph↗

arXiv2024

Magnetic skin effect in Pb(Fe$_{1/2}$Nb$_{1/2}$)O$_3$

Relaxor-ferroelectrics display exceptional dielectric properties resulting from the underlying random dipolar fields induced by strong chemical inhomogeneity. An unusual structural aspect of relaxors is a skin-effect where the near-surface region in single crystals exhibit structures and critical phenomena that differ from the bulk. Relaxors are unique in that this skin effect extends over a macroscopic lengthscale of $\sim$ 100$\mu$m whereas usual surface layers only extend over a few unit cells (or $\sim$ nm). We present a muon spectroscopy study of Pb(Fe$_{1/2}$Nb$_{1/2}$)O$_{3}$ (PFN) which displays ferroelectric order, including many relaxor-like dielectric properties such as a frequency broadened dielectric response, and antiferromagnetism with spatially short-range polar correlations and hence can be termed a multiferroic. In terms of the magnetic behavior determined by the Fe$^{3+}$ ($S=5/2$, $L\approx0$) ions, PFN has been characterized as a unique example of a "cluster spin-glass". We use variable momentum muon spectroscopy to study the depth dependence of the slow magnetic relaxations in a large 1 cm$^{3}$ crystal of PFN. Zero-field positive muon spin relaxation is parameterized using a stretched exponential, indicative of a distribution of relaxation rates of the Fe$^{3+}$ spins. This bandwidth of frequencies changes as a function of muon momentum, indicative of a change in the Fe$^{3+}$ relaxation rates as a function of muon implantation depth in our single crystal. Using negative muon elemental analysis, we find small-to-no measurable change in the Fe$^{3+}$/Nb$^{5+}$ concentration with depth implying that chemical concentration alone cannot account for the change in the relaxational dynamics. PFN displays an analogous magnetic skin effect reported to exist in the structural properties of relaxor-ferroelectrics.

cond-mat.str-el↗

arXiv2024

XGBoostPP: Tree-based Estimation of Point Process Intensity Functions

We propose a novel tree-based ensemble method, named XGBoostPP, to nonparametrically estimate the intensity of a point process as a function of covariates. It extends the use of gradient-boosted regression trees (Chen & Guestrin, 2016) to the point process literature via two carefully designed loss functions. The first loss is based on the Poisson likelihood, working for general point processes. The second loss is based on the weighted Poisson likelihood, where spatially dependent weights are introduced to further improve the estimation efficiency for clustered processes. An efficient greedy search algorithm is developed for model estimation, and the effectiveness of the proposed method is demonstrated through extensive simulation studies and two real data analyses. In particular, we report that XGBoostPP achieves superior performance to existing approaches when the dimension of the covariate space is high, revealing the advantages of tree-based ensemble methods in estimating complex intensity functions.

stat.ME↗

arXiv2023

Charge order above room-temperature in a prototypical kagome superconductor La(Ru$_{1-x}$Fe$_{x}$)$_{3}$Si$_{2}$

The kagome lattice is an intriguing and rich platform for discovering, tuning and understanding the diverse phases of quantum matter, which is a necessary premise for utilizing quantum materials in all areas of modern and future electronics in a controlled and optimal way. The system LaRu$_{3}$Si$_{2}$ was shown to exhibit typical kagome band structure features near the Fermi energy formed by the Ru-$dz^{2}$ orbitals and the highest superconducting transition temperature $T_{\rm c}$ ${\simeq}$ 7K among the kagome-lattice materials. However, the effect of electronic correlations on the normal state properties remains elusive. Here, we report the discovery of charge order in La(Ru$_{1-x}$Fe$_{x}$)$_{3}$Si$_{2}$ ($x$ = 0, 0.01, 0.05) beyond room-temperature. Namely, single crystal X-ray diffraction reveals charge order with a propagation vector of ($\frac{1}{4}$,0,0) below $T_{\rm CO-I}$ ${\simeq}$ 400K in all three compounds. At lower temperatures, we see the appearance of a second set of charge order peaks with a propagation vector of ($\frac{1}{6}$,0,0). The introduction of Fe, which is known to quickly suppress superconductivity, does not drastically alter the onset temperature for charge order. Instead, it broadens the scattered intensity such that diffuse scattering appears at the same onset temperature, however does not coalesce into sharp Bragg diffraction peaks until much lower in temperature. Our results present the first example of a charge ordered state at or above room temperature in the correlated kagome lattice with bulk superconductivity.

cond-mat.supr-con↗

arXiv2022

Fast broadband cluster spin-glass dynamics in PbFe$_{1/2}$Nb$_{1/2}$O$_{3}$

PbFe$_{1/2}$Nb$_{1/2}$O$_{3}$ (PFN) is a relaxor ferroelectric (T$_{c}$ $\sim$ 400 K) consisting of disordered magnetic Fe$^{3+}$ (S=${5\over2}$, L$\approx$0) ions resulting in a low temperature ``cluster glass" phase (W. Kleemann $\textit{et al.}$ Phys. Rev. Lett. ${\bf{105}}$, 257202 (2010)). We apply neutron scattering to investigate the dynamic magnetism of this phase in a large single crystal which displays a low temperature spin glass transition (T$_{g} \sim$ 15 K), but no observable spatially long-range antiferromagnetic order. The static response in the cluster glass phase (sampled on the timescale set by our resolution) is found to be characterized by an average magnetic spin direction that lacks any preferred direction. The dynamics that drive this phase are defined by a magnetic correlation length that gradually increases with decreasing temperature. However, below $\sim$ 50 K the spatial correlations gradually becoming more short range indicative of increasing disorder on cooling, thereby unravelling magnetism, until the low temperature glass phase sets in at T$_{g}$ $\sim$ 15 K. Neutron spectroscopy is used to characterize the spin fluctuations in the cluster glass phase and are found to be defined by a broadband of frequencies on the scale of $\sim$ THz, termed here ``fast" fluctuations. The frequency bandwidth driving the magnetic fluctuations mimics the correlation length and decreases until $\sim$ 50 K, and then increases again until the glass transition. Through investigating the low-energy acoustic phonons we find evidence of multiple distinct structural regions which form the basis of the clusters, generating a significant amount of local disorder. We suggest that random molecular fields originating from conflicting interactions between clusters is important for the destruction of magnetic order and the eventual formation of the cluster glass in PFN.

cond-mat.mtrl-sci↗

arXiv2022

An auto-locking ultra-stable laser with sub-hertz linewidth

We report in detail the design process and performance of an auto-locking ultra-stable laser with sub-hertz linewidth at the first time. The laser frequency is automatically stabilized to an optical reference cavity with a home-made controller, which is based on a combination of digital circuit and analog circuit. The digital circuit is used for diagnosing and manipulating the state of the ultra-stable laser, and the analog circuit is used for demodulating the discriminate signal and servo control. A method of searching the transmission signal in the closed-loop state instead of the open-loop state is proposed to reduce the locking time and improve the reliable of the auto-locking ultra-stable laser. The median time of 16.6s is obtained after 157 times of relocking, and the probability of less than 20 s is more than 86%. The median linewidth of 1.08 Hz is obtained, and the fractional frequency instability is less than 3.4E-15 at integration time between 0.1 and 100 s. The performance of this system demonstrates that will be used as an important subsystem to transfer the optical clock signal.

physics.ins-det↗

arXiv2021

$E_8$ Spectra of Quasi-one-dimensional Antiferromagnet BaCo$_2$V$_2$O$_8$ under Transverse Field

We report $^{51}$V nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS) measurements on a quasi-1D antiferromagnet BaCo$_2$V$_2$O$_8$ under transverse field along the [010] direction. The scaling behavior of the spin-lattice relaxation rate above the Néel temperatures unveils a 1D quantum critical point (QCP) at $H_c^{1D}\approx 4.7$ T, which is masked by the 3D magnetic order. With the aid of accurate analytical analysis and numerical calculations, we show that the zone center INS spectrum at $H_c^{1D}$ is precisely described by the pattern of the 1D quantum Ising model in a magnetic field, a class of universality described in terms of the exceptional $E_8$ Lie algebra. These excitations keep to be non-diffusive over a certain field range when the system is away from the 1D QCP. Our results provide an unambiguous experimental realization of the massive $E_8$ phase in the compound, and open new experimental route for exploring the dynamics of quantum integrable systems as well as physics beyond integrability.

cond-mat.str-el↗

arXiv2021

Two-dimensional ferromagnetic spin-orbital excitations in the honeycomb VI$_{3}$

VI$_{3}$ is a ferromagnet with planar honeycomb sheets of bonded V$^{3+}$ ions held together by van der Waals forces. We apply neutron spectroscopy to measure the two dimensional ($J/J_{c} \approx 17$) magnetic excitations in the ferromagnetic phase, finding two energetically gapped ($Δ\approx k_{B} T_{c} \approx$ 55 K) and dispersive excitations. We apply a multi-level spin wave formalism to describe the spectra in terms of two coexisting domains hosting differing V$^{3+}$ orbital ground states built from contrasting distorted octahedral environments. This analysis fits a common nearest neighbor in-plane exchange coupling ($J$=-8.6 $\pm$ 0.3 meV) between V$^{3+}$ sites. The distorted local crystalline electric field combined with spin-orbit coupling provides the needed magnetic anisotropy for spatially long-ranged two-dimensional ferromagnetism in VI$_{3}$.

cond-mat.str-el↗

arXiv2021

Observation of the topological Dirac fermions and surface states in superconducting BaSn3

The interplay between topological electronic structure and superconductivity has attracted tremendous research interests recently as they could induce topological superconductivity (TSCs) which may be used to realize topological qubits for quantum computation. Among various TSC candidates, superconducting BaSn3 (Tc ~ 4.4 K) has been predicted to be a topological Dirac semimetal (TDS) hosting two pairs of Dirac points along the G - A direction. Here, by combining the use of angle-resolved photoemission spectroscopy and ab initio calculations, we identified the predicted topological Dirac fermions and confirmed the TDS nature of the compound. In addition, we observed surface states connecting the Dirac points. Our observations demonstrate BaSn3 as a superconductor with nontrivial topological electronic structures.

cond-mat.supr-con↗

arXiv2020

Strong coupling nature of kagome superconductivity in LaRu$_3$Si$_2$

We report muon spin rotation ($μ$SR) experiments together with first-principles calculations on microscopic properties of superconductivity in the kagome superconductor LaRu$_3$Si$_2$ with $T_{\rm c}$ ${\simeq}$ 7K. We find that the calculated normal state band structure features a kagome flat band and Dirac as well as van Hove points formed by the Ru-$dz^2$ orbitals near the Fermi level. Below $T_{\rm c}$, $μ$SR reveals isotropic type-II superconductivity, which is robust against hydrostatic pressure up to 2 GPa. Intriguingly, the ratio 2$Δ/k_{\rm B}T_{\rm c}$ ${\simeq}$ 4.3 (where $Δ$ is the superconducting energy gap) is in the strong coupling limit, and $T_{\rm c}$/$λ_{eff}^{-2}$ (where $λ$ is the penetration depth) is comparable to that of high-temperature unconventional superconductors. We also find that electron-phonon coupling alone can only reproduce small fraction of $T_{\rm c}$ from calculations, which suggests other factors in enhancing $T_{\rm c}$ such as the correlation effect from the kagome flat band, the van Hove point on the kagome lattice, and high density of states from narrow kagome bands. Our experiments and calculations taken together point to strong coupling and the unconventional nature of kagome superconductivity in LaRu$_3$Si$_2$.

cond-mat.supr-con↗

arXiv2020

Magnetic order and instability in newly synthesized CoSeAs marcasite

Marcasite class of compounds provide facile platform to explore novel phenomena of fundamental and technological importance, such as unconventional superconductivity or high performance electrocatalyst. We report the synthesis and experimental investigation of a new marcasite CoSeAs in this letter. Experimental investigation of the new material using neutron scattering measurements reveal weak magnetic correlation of cobalt ions below $T$ = 36.2 K. The modest isotropic exchange interaction between cobalt moments, inferred from random phase approximation analysis, hints of magnetically unstable environment. It is a desirable characteristic to induce unconventional superconductivity via chemical pressure application

cond-mat.str-el↗

arXiv2020

Broadband critical dynamics in disordered lead-based perovskites

Materials based on the cubic perovskite unit cell continue to provide the basis for technologically important materials with two notable recent examples being lead-based relaxor piezoelectrics and lead-based organic-inorganic halide photovoltaics. These materials carry considerable disorder, arising from site substitution in relaxors and molecular vibrations in the organic-inorganics, yet much of our understanding of these systems derives from the initial classic work of Prof. Roger A. Cowley, who applied both theory and neutron scattering methods while at Chalk River Laboratories to the study of lattice vibrations in SrTiO$_{3}$. Neutron scattering continues to play a vital role in characterizing lattice vibrations in perovskites owing to the simple cross section and the wide range of energy resolutions achievable with current neutron instrumentation. We discuss the dynamics that drive the phase transitions in the relaxors and organic-inorganic lead-halides in terms of neutron scattering and compare them to those in phase transitions associated with a ``central peak" and also a soft mode. We review some of the past experimental work on these materials and present new data from high-resolution time-of-flight backscattering spectroscopy taken on organic-inorganic perovskites. We will show that the structural transitions in disordered lead-based perovskites are driven by a broad frequency band of excitations.

cond-mat.mtrl-sci↗

arXiv2019

Universal gapless Dirac cone and tunable topological states in (MnBi$_2$Te$_4$)$_m$(Bi$_2$Te$_3$)$_n$ heterostructures

In the newly discovered magnetic topological insulator MnBi$_2$Te$_4$, both axion insulator state and quantized anomalous Hall effect (QAHE) have been observed by tuning the magnetic structure. The related (MnBi$_2$Te$_4$)$_m$(Bi$_2$Te$_3$)$_n$ heterostructures with increased tuning knobs, are predicted to be a more versatile platform for exotic topological states. Here, we report angle-resolved photoemission spectroscopy (ARPES) studies on a series of the heterostructures (MnBi$_2$Te$_4$, MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$). A universal gapless Dirac cone is observed at the MnBi$_2$Te$_4$ terminated (0001) surfaces in all systems. This is in sharp contrast to the expected gap from the original antiferromagnetic ground state, indicating an altered magnetic structure near the surface, possibly due to the surface termination. In the meantime, the electron band dispersion of the surface states, presumably dominated by the top surface, is found to be sensitive to different stackings of the underlying MnBi$_2$Te$_4$ and Bi$_2$Te$_3$ layers. Our results suggest the high tunability of both magnetic and electronic structures of the topological surface states in (MnBi$_2$Te$_4$)$_m$(Bi$_2$Te$_3$)$_n$ heterostructures, which is essential in realizing various novel topological states.

cond-mat.mtrl-sci↗

arXiv2019

Spin fluctuation induced Weyl semimetal state in the paramagnetic phase of EuCd$_2$As$_2$

Weyl fermions as emergent quasiparticles can arise in Weyl semimetals (WSMs) in which the energy bands are nondegenerate, resulting from inversion or time-reversal symmetry breaking. Nevertheless, experimental evidence for magnetically induced WSMs is scarce. Here, using photoemission spectroscopy, we observe that the degeneracy of Bloch bands is already lifted in the paramagnetic phase of EuCd$_2$As$_2$. We attribute this effect to the itinerant electrons experiencing quasistatic and quasi-long-range ferromagnetic fluctuations. Moreover, the spin nondegenerate band structure harbors a pair of ideal Weyl nodes near the Fermi level. Hence, we show that long-range magnetic order and the spontaneous breaking of time-reversal symmetry are not an essential requirement for WSM states in centrosymmetric systems, and that WSM states can emerge in a wider range of condensed-matter systems than previously thought.

cond-mat.str-el↗

arXiv2019

Layered semiconductor EuTe 4 with charge density wave order in square tellurium sheets

We report a novel quasi-two dimensional compound of EuTe4 hosting charge density waves (CDW) instability. The compound has a crystallographic structure in a orthorhombic space group Pmmn (No.59) with cell parameters a = 4.6347(2)Å, b = 4.5119(2)Å, c = 15.6747(10)Å at room temperature. The pristine structure contains consecutive near-square Te sheets separated by corrugated Eu-Te slabs. Upon cooling, the compound experiences a phase transition near 255 K. X-ray crystallographic analysis and transmission electron microscopy (TEM) measurements reveal strong structural distortions in the low temperature phase, showing a superstructure with a periodic formation of Te-trimers in the monolayer Te sheets, yielding evidence for the formation of CDW order. The charge transport properties show a semiconducting behavior in the CDW state. Density functional theory calculations reveals a Fermi surface nesting driven instability with a nesting vector in good agreement with the one observed experimentally. Our finding provides a promising system for the study of CDW driven 2D semiconducting mechanisms, which would shed a new light on exploring novel 2D semiconductors with collective electronic states.

cond-mat.str-el↗

arXiv2019

Giant optical nonlinearity cancellation in quantum wells

Second-order optical nonlinearities can be greatly enhanced by orders of magnitude in resonantly excited nanostructures, theoretically predicted and experimentally investigated in a variety of semiconductor systems. These resonant nonlinearities continually attract attention, particularly in newly discovered materials, but tend not to be as efficient as currently predicted. This limits their exploitation in frequency conversion. Here, we present a clear-cut theoretical and experimental demonstration that the second-order nonlinear susceptibility can vary by orders of magnitude as a result of giant cancellation effects in systems with many confined quantum states. Using terahertz quantum cascade lasers as a model source to investigate interband and intersubband resonant nonlinearities, we show that these giant cancellations are a result of interfering second-order nonlinear contributions of light and heavy hole states. As well as of importance to understand and engineer the resonant optical properties of materials, this work can be employed as a new, extremely sensitive tool to elucidate the bandstructure properties of complex quantum well systems.

physics.optics↗