arXiv subjects

Kehao Zhang

Publications and source records attributed to Kehao Zhang.

12 recordsLinked to original sources

arXiv2026

Spin Femtoscopy: A Framework for Revealing Genuine Spin Correlations

Spin correlations are among the most fundamental quantum observables in many-body systems, yet they remain difficult to access experimentally in relativistic heavy-ion collisions. Existing spin measurements, including hyperon polarization and vector-meson spin alignment, have revealed important single-particle spin phenomena, but genuine two-particle spin correlations in the produced hadronic system remain largely unexplored. Here we propose spin femtoscopy, a framework for accessing genuine two-particle spin correlations through spin-resolved femtoscopic measurements. The key principle is that different two-particle spin configurations can give rise to different femtoscopic correlation functions because of quantum statistics, spin-dependent final-state interactions. Using $\Lambda\Lambda$ pairs as a proof of principle, we exploit the self-analyzing weak decay of $\Lambda$ hyperons to construct spin-sensitive femtoscopic correlation functions with different singlet and triplet admixtures. We show that these observables provide experimental access to the spin-state populations of the pair and allow genuine spin correlations to be separated from spin-dependent femtoscopic mixing caused by quantum statistics and final-state interactions. This work extends femtoscopy from a probe of source geometry and final-state interactions to a framework for revealing the quantum spin structure of strongly interacting matter.

nucl-th↗

arXiv2026

Learning to Remember: End-to-End Training of Memory Agents for Long-Context Reasoning

Long-context LLMs and Retrieval-Augmented Generation defer state tracking and evidence consolidation to query time, which is brittle when facts evolve and answers depend on latent states. We introduce Unified Memory Agent (UMA) for a one-to-many setting: query-agnostic external memory is constructed once from a stream and reused across multiple future QA sessions. A single policy maintains a structured Memory Bank through CRUD operations and answers using both the Memory Bank and raw context. Task-Stratified GRPO uses the mean reward of QA trajectories branching from each sampled memory state to supervise memory maintenance, while normalizing memory and per-question QA groups separately. We also introduce Ledger-QA, a diagnostic benchmark for long-horizon state tracking over accumulated updates. At the 16k budget, UMA-Generalist achieves the highest average score among compared methods across the test-time-learning and accurate-retrieval benchmarks and transfers to Ledger-QA without task-specific training; UMA-Specialist further improves long-horizon tracking after task adaptation. These results support learned proactive memory management for long-context reasoning.

cs.LG↗

arXiv2025

Search for the Strange Dibaryons with Baryon Correlations in Isobar Collisions at STAR

Dibaryons provide insight into the strong interaction beyond conventional hadrons. Strange dibaryons, containing strange quarks, are especially valuable for probing hyperon-nucleon (YN) and hyperon-hyperon (YY) interactions. We report measurements of $p$-$\Xi^-$ and $p$-$\Omega^-$ correlation functions in Ru+Ru and Zr+Zr collisions at $\sqrt{s_{NN}} = 200$ GeV. The analysis, using the Lednicky-Lyuboshitz formalism, yields scattering parameters that offer key implications for the possible formation of strange dibaryon bound states, particularly $H$ ($S = -2$) and $N\Omega$ ($S = -3$).

nucl-ex↗

arXiv2025

PSO-Merging: Merging Models Based on Particle Swarm Optimization

Model merging has emerged as an efficient strategy for constructing multitask models by integrating the strengths of multiple available expert models, thereby reducing the need to fine-tune a pre-trained model for all the tasks from scratch. Existing data-independent methods struggle with performance limitations due to the lack of data-driven guidance. Data-driven approaches also face key challenges: gradient-based methods are computationally expensive, limiting their practicality for merging large expert models, whereas existing gradient-free methods often fail to achieve satisfactory results within a limited number of optimization steps. To address these limitations, this paper introduces PSO-Merging, a novel data-driven merging method based on the Particle Swarm Optimization (PSO). In this approach, we initialize the particle swarm with a pre-trained model, expert models, and sparsified expert models. We then perform multiple iterations, with the final global best particle serving as the merged model. Experimental results on different language models show that PSO-Merging generally outperforms baseline merging methods, offering a more efficient and scalable solution for model merging.

cs.LG↗

arXiv2024

BayLing 2: A Multilingual Large Language Model with Efficient Language Alignment

Large language models (LLMs), with their powerful generative capabilities and vast knowledge, empower various tasks in everyday life. However, these abilities are primarily concentrated in high-resource languages, leaving low-resource languages with weaker generative capabilities and relatively limited knowledge. Enhancing the multilingual capabilities of LLMs is therefore crucial for serving over 100 linguistic communities worldwide. An intuitive approach to enhance the multilingual capabilities would be to construct instruction data for various languages, but constructing instruction data for over 100 languages is prohibitively costly. In this paper, we introduce BayLing 2, which efficiently transfers generative capabilities and knowledge from high-resource languages to low-resource languages through language alignment. To achieve this, we constructed a dataset of 3.2 million instructions, comprising high-resource language instructions (Chinese and English) and cross-lingual instructions for 100+ languages and performed instruction tuning based on the dataset to facilitate the capability transfer between languages. Using Llama as the foundation model, we developed BayLing-2-7B, BayLing-2-13B, and BayLing-2-8B, and conducted a comprehensive evaluation of BayLing. For multilingual translation across 100+ languages, BayLing shows superior performance compared to open-source models of similar scale. For multilingual knowledge and understanding benchmarks, BayLing achieves significant improvements across over 20 low-resource languages, demonstrating its capability of effective knowledge transfer from high-resource to low-resource languages. Furthermore, results on English benchmarks indicate that BayLing maintains high performance in highresource languages while enhancing the performance in low-resource languages. Demo, homepage, code and models of BayLing are available.

cs.CL↗

arXiv2020

Acquisition and Analysis of Scanning Tunneling Spectroscopy Data $-$ WSe$_2$ Monolayer

Acquisition and analysis is described for scanning tunneling spectroscopy data acquired from a monolayer of WSe$_2$ grown on epitaxial graphene on SiC. Curve fitting of the data is performed, in order to deduce band-edge energies. In addition to describing the details of the theoretical curves used for the fitting, the acquisition and analysis methods are also discussed within the larger context of the historical development of scanning tunneling spectroscopy techniques.

cond-mat.mes-hall↗

arXiv2020

Flatbands and Mechanical Deformation Effects in the Moiré Superlattice of MoS$_2$-WSe$_2$ Heterobilayers

It has recently been shown that quantum-confined states can appear in epitaxially grown van der Waals material heterobilayers without a rotational misalignment ($θ=0^\circ$), associated with flat bands in the Brillouin zone of the moiré pattern formed due to the lattice mismatch of the two layers. Peaks in the local density of states and confinement in a MoS$_2$/WSe$_2$ system was qualitatively described only considering local stacking arrangements, which cause band edge energies to vary spatially. In this work, we report the presence of large in-plane strain variation across the moiré unit cell of a $θ=0^\circ$ MoS$_2$/WSe$_2$ heterobilayer, and show that inclusion of strain variation and out-of-plane displacement in density functional theory calculations greatly improves their agreement with the experimental data. We further explore the role of twist-angle by showing experimental data for a twisted MoS$_2$/WSe$_2$ heterobilayer structure with twist angle of $θ=15^\circ$, that exhibits a moiré pattern but no confinement.

cond-mat.mtrl-sci↗

arXiv2019

Contactless photo-induced carrier density control in nanocrystal MoS2 hybrids

The ultrathin nature of two-dimensional monolayer semiconductors yields optoelectronic properties which are highly responsive to changes in free-carrier density, making it imperative to masterfully control their doping levels. We report a new photo-doping scheme that quasi-permanently dopes the monolayer MoS2 to extents competing with electrostatic gating. The photo-doping is achieved by coupling monolayer MoS2 with indium tin oxide nanocrystals that can store multiple electrons per nanocrystal after UV illumination. In the hybrid structure, the photo-generated valence band holes in the nanocrystals are filled by MoS2 electrons, photo-doping the MoS2 with holes. Reductions in carrier density by ~6x10^12 cm^-2 are observed, equivalent to the storage of ~40 electrons per nanocrystal. Long-range changes proliferating up to 40 micrometers away from the localized photodoping result from local bandstructure variations in MoS2. These studies reveal novel all-optical carrier density control in monolayer semiconductors, enabling remote-control of local charge density and innovative energy storage technologies.

cond-mat.mes-hall↗

arXiv2018

Considerations for utilizing sodium chloride in epitaxial molybdenum disulfide

The utilization of alkali salts, such as NaCl and KI, have enabled the successful growth of large single domain and fully coalesced polycrystalline two-dimensional (2D) transition metal dichalcogenide layers. However, the impact of alkali salts on photonic and electronic properties are not fully established. In this work, we report alkali-free epitaxy of MoS2 on sapphire and benchmark the properties against alkali-assisted growth of MoS2. This study demonstrates that although NaCl can dramatically increase the domain size of monolayer MoS2 by 20 times, it can also induce strong optical and electronic heterogeneities in as-grown large-scale films. This work elucidates that utilization of NaCl can lead to variation in growth rates, loss of epitaxy, and a high density of nanoscale MoS2 particles (4/μm2). Such phenomena suggest that alkali atoms play an important role in Mo and S adatom mobility and strongly influence the 2D/sapphire interface during growth. Compared to alkali-free synthesis under the same growth conditions, MoS2 growth assisted by NaCl results in >1% tensile strain in as-grown domains, which reduces photoluminescence by ~20x and degrades transistor performance.

cond-mat.mtrl-sci↗

arXiv2018

Dynamics of liquid crystal on hexagonal lattice

Nematic liquid crystal (LC) molecules adsorbed on two dimensional materials are aligned along the crystal directions of the hexagonal lattice. It was demonstrated that short electric pulses can reorient the aligned LC molecules in the preferred armchair direction of hexagonal boron nitride (h-BN). Several states with a variety of colors were obtained by changing the direction and strength of the electric pulses. The ab initio calculations based on density functional theory was carried out to determine the favorable adsorption configurations of the LC molecules on the h-BN surface. A non-volatile display, in which pixel resolution can be determined by grains of hexagonal surface, is proposed, which can offer a pathway towards dynamic high-quality pixels with low power consumption, and could define a new paradigm for all non-volatile display applications.

cond-mat.soft↗

arXiv2018

Quantum-Confined Electronic States arising from Moiré Pattern of MoS2-WSe2 Hetero-bilayers

A two-dimensional (2D) hetero-bilayer system consisting of MoS2 on WSe2, deposited on epitaxial graphene, is studied by scanning tunneling microscopy and spectroscopy at temperatures of 5 and 80 K. A moiré pattern is observed, arising from lattice mismatch of 3.7% between the MoS2 and WSe2. Significant energy shifts are observed in tunneling spectra observed at the maxima of the moiré corrugation, as compared with spectra obtained at corrugation minima, consistent with prior work. Furthermore, at the minima of the moiré corrugation, sharp peaks in the spectra at energies near the band edges are observed, for spectra acquired at 5 K. The peaks correspond to discrete states that are confined within the moiré unit cells. Conductance mapping is employed to reveal the detailed structure of the wave functions of the states. For measurements at 80 K, the sharp peaks in the spectra are absent, and conductance maps of the band edges reveal little structure.

cond-mat.mes-hall↗

arXiv2016

Formation of hexagonal Boron Nitride on Graphene-covered Copper Surfaces

Graphene-covered copper surfaces have been exposed to borazine, (BH)3(NH)3, with the resulting surfaces characterized by low-energy electron microscopy. Although the intent of the experiment was to form hexagonal boron nitride (h-BN) on top of the graphene, such layers were not obtained. Rather, in isolated surface areas, h-BN is found to form micrometer-size islands that substitute for the graphene. Additionally, over nearly the entire surface, the properties of the layer that was originally graphene is observed to change in a manner that is consistent with the formation of a mixed h-BN/graphene alloy, i.e. h-BNC alloy. Furthermore, following the deposition of the borazine, a small fraction of the surface is found to consist of bare copper, indicating etching of the overlying graphene. The inability to form h-BN layers on top of graphene is discussed in terms of the catalytic behavior of the underlying copper surface and the decomposition of the borazine on top of the graphene.

cond-mat.mtrl-sci↗