arXiv subjects

Kang Xia

Publications and source records attributed to Kang Xia.

10 recordsLinked to original sources

arXiv2026

MemPoison: Uncovering Persistent Memory Threats and Structural Blind Spots in LLM Agents

Persistent external memory enhances agent continuity but introduces persistent security vulnerabilities: adversarial content can be injected via standard interaction channels, retained across turns, and later distort downstream behavior. To address this challenge, we propose MemPoison, a comprehensive benchmark and analysis framework featuring 1227 hand-validated cases across four attack types, three injection channels, and three representative memory substrates, evaluated on seven open-weight and three closed-weight model families. We introduce a three-tier taxonomy: (L1) direct single-record corruption, (L2) compositional multi-record corruption and (L3) context-triggered dormant corruption. Our evaluations reveal a distinct defense frontier: while baseline write-time defenses, such as consistency checks, substantially suppress direct L1 attacks, they fail to reliably suppress L2 and L3 attacks. Through mechanistic influence decomposition (MID), we demonstrate structural blind spots in write-time defenses, which admit seemingly benign records that later become harmful through joint retrieval composition or trigger-conditioned activation. Our findings advocate for shifting from static filtering to adaptive, context-sensitive memory defense strategies.

cs.CR↗

arXiv2019

Growth strategy of microbes on mixed carbon sources

A classic problem in microbiology is that bacteria display two types of growth behavior when cultured on a mixture of two carbon sources: the two sources are sequentially consumed one after another (diauxie) or they are simultaneously consumed (co-utilization). The search for the molecular mechanism of diauxie led to the discovery of the lac operon. However, questions remain as why microbes would bother to have different strategies of taking up nutrients. Here we show that diauxie versus co-utilization can be understood from the topological features of the metabolic network. A model of optimal allocation of protein resources quantitatively explains why and how the cell makes the choice. In case of co-utilization, the model predicts the percentage of each carbon source in supplying the amino acid pools, which is quantitatively verified by experiments. Our work solves a long-standing puzzle and provides a quantitative framework for the carbon source utilization of microbes.

physics.bio-ph↗

arXiv2018

Graphene and Carbon Nanotube Hybrid Structure: A Review

Graphene has been reported with record-breaking properties which have opened up huge potential applications. Considerable amount of researches have been devoted to manipulating or modify the properties of graphene to target a more smart nanoscale device. Graphene and carbon nanotube hybrid structure (GNHS) is one of the promising graphene derivate. The synthesis process and the mechanical properties are essential for the GNHS based devices. Therefore, this review will summarise the recent progress of the highly ordered GNHS synthesis/assembly, and discuss the mechanical properties of GNHS under various conditions as obtained from molecular dynamics simulations.

cond-mat.mtrl-sci↗

arXiv2018

Failure mechanism of monolayer graphene under hypervelocity impact of spherical projectile

The excellent mechanical properties of graphene have enabled it as appealing candidate in the field of impact protection or protective shield. By considering a monolayer graphene membrane, in this work, we assessed its deformation mechanisms under hypervelocity impact (from 2 to 6 km/s), based on a serial of in silico studies. It is found that the cracks are formed preferentially in the zigzag directions which are consistent with that observed from tensile deformation. Specifically, the boundary condition is found to exert an obvious influence on the stress distribution and transmission during the impact process, which eventually influences the penetration energy and crack growth. For similar sample size, the circular shape graphene possesses the best impact resistance, followed by hexagonal graphene membrane. Moreover, it is found the failure shape of graphene membrane has a strong relationship with the initial kinetic energy of the projectile. The higher kinetic energy, the more number the cracks. This study provides a fundamental understanding of the deformation mechanisms of monolayer graphene under impact, which is crucial in order to facilitate their emerging future applications for impact protection, such as protective shield from orbital debris for spacecraft.

cond-mat.mtrl-sci↗

arXiv2018

A novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure searching

Transition metal nitrides have been suggested to have both high hardness and good thermal stability with large potential application value, but so far stable superhard transition metal nitrides have not been synthesized. Here, with our newly developed machine-learning accelerated crystal structure searching method, we designed a superhard tungsten nitride, h-WN6, which can be synthesized at pressure around 65 GPa and quenchable to ambient pressure. This h-WN6 is constructed with single-bonded N6 rings and presents ionic-like features, which can be formulated as W2.4+N62.4-. It has a band gap of 1.6 eV at 0 GPa and exhibits an abnormal gap broadening behavior under pressure. Excitingly, this h-WN6 is found to be the hardest among transition metal nitrides known so far (Vickers hardness around 57 GPa) and also has a very high melting temperature (around 1900 K). These predictions support the designing rules and may stimulate future experiments to synthesize superhard material.

cond-mat.mtrl-sci↗

arXiv2017

Superhard and Superconducting B6C

Crystal structure searching and ab initio calculations have been used here to explore low-energy structures of boron carbides under high pressure. Under pressures of 85-110 GPa, a metastable B6C with R-3m symmetry is found to be energetically more stable than the mixture of previous B4C and elemental boron. This B6C is a rhombohedral structure and contains mooncake-like B24 clusters with stuffing of C2 pairs. The mechanical and dynamical stabilities of this structure at ambient pressure are confirmed by its elastic constants and phonon dispersions. The bulk modulus and shear modulus of this newly predicted B6C at ambient pressure reach high values of 291 GPa and 272 GPa, respectively. The Vickers hardness is calculated to be around 48 GPa, and its melting temperature at ambient pressure is estimated to be around 2500 K, indicating that this metastable B6C is a potential superhard material with very good thermal stability. Interestingly, this superhard B6C structure is also found to be superconducting and its superconducting critical temperature (Tc) is evaluated to be around 12.5 K at ambient pressure by electron-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2017

Silicon clathrates for photovoltaics predicted by a two-step crystal structure search

Silicon in a cubic diamond structure currently plays a significant role in the photovoltaic industry. However, the intrinsic band structures of crystalline silicon restrict its sunlight conversion efficiency. Recently, a clathrate-like Si-24 has been successfully synthesized, which has a quasi-direct bandgap and sheds light on silicon-based photovoltaics. Here, we proposed a two-step crystal structure search method based on first-principles calculations and explored silicon clathrate structures extensively. First, the guest-host compounds were searched at high pressure, and then, the porous guest-free silicon clathrates were obtained by removing the guest atoms. Using potassium as the guest atom, we identified four metastable silicon clathrate structures, and some of them have bandgaps close to the optimal range of the Shockley-Queisser limit and have a better absorption rate than the cubic diamond silicon. These silicon clathrates may have promising value in photovoltaic applications.

cond-mat.mtrl-sci↗

arXiv2017

High temperature superconducting phase of HBr under pressure predicted by first-principles calculations

The high pressure phases of HBr are explored with an ab initio crystal structure search. By taking into account the contribution of zero-point energy (ZPE), we find that the P4/nmm phase of HBr is thermodynamically stable in the pressure range from 150 to 200 GPa. The superconducting critical temperature (Tc) of P4/nmm HBr is evaluated to be around 73 K at 170 GPa, which is the highest record so far among binary halogen hydrides. Its Tc can be further raised to around 95K under 170 GPa if half of the bromine atoms in the P4/nmm HBr are substituted by the lighter chlorine atoms. Our study shows that, in addition to lower mass, higher coordination number, shorter bonds, and more highly symmetric environment for the hydrogen atoms are important factors to enhance the superconductivity in hydrides.

cond-mat.supr-con↗

arXiv2016

Ground state structure of polymeric carbon monoxide with high energy density

Crystal structure prediction methods and first-principles calculations have been used to explore low-energy structures of carbon monoxide (CO). Contrary to the standard wisdom, the most stable structure of CO at ambient pressure was found to be a polymeric structure of Pna21 symmetry rather than a molecular solid. This phase is formed from six-membered (4 Carbon + 2 Oxygen) rings connected by C=C double bonds with two double-bonded oxygen atoms attached to each ring. Interestingly, the polymeric Pna21 phase of CO has a much higher energy density than trinitrotoluene (TNT). On compression to about 7 GPa, Pna21 is found to transform into another chain-like phase of Cc symmetry which has similar ring units to Pna21. On compression to 100 GPa it is energetically favorable for CO to polymerize to form a single-bonded Cmcm phase from another structure of Cmca symmetry composed of units similar to those found in the single-bonded I212121 structure. Thermodynamic stability of these structures was verified using calculations with different density functionals, including hybrid and van der Waals corrected functionals.

cond-mat.mtrl-sci↗

arXiv2015

Two-dimensional Graphene Heterojunctions: the Tunable Mechanical Properties

We report the mechanical properties of different two-dimensional carbon heterojunctions (HJs) made from graphene and various stable graphene allotropes, including α-, β-, γ- and 6612-graphyne (GY), and graphdiyne (GDY). It is found that all HJs exhibit a brittle behaviour except the one with α-GY, which however shows a hardening process due to the formation of triple carbon rings. Such hardening process has greatly deferred the failure of the structure. The yielding of the HJs is usually initiated at the interface between graphene and graphene allotropes, and monoatomic carbon rings are normally formed after yielding. By varying the locations of graphene (either in the middle or at the two ends of the HJs), similar mechanical properties have been obtained, suggesting insignificant impacts from location of graphene allotropes. Whereas, changing the types and percentages of the graphene allotropes, the HJs exhibit vastly different mechanical properties. In general, with the increasing graphene percentage, the yield strain decreases and the effective Young's modulus increases. Meanwhile, the yield stress appears irrelevant with the graphene percentage. This study provides a fundamental understanding of the tensile properties of the heterojunctions that are crucial for the design and engineering of their mechanical properties, in order to facilitate their emerging future applications in nanoscale devices, such as flexible/stretchable electronics.

cond-mat.mtrl-sci↗