Showing 1–50 of 347 results for author: Sun, P

Sensitive dependence of Poor Man's Majorana modes on the length of superconductor

Authors: Zhi-Lei Zhang, Xin Yue, Guo-Jian Qiao, C. P. Sun

Abstract: In a hybrid system where two quantum dots (QDs) are coupled to a conventional $s$-wave superconductor, Poor Man's Majorana modes (PMMs) have been proposed. Existing theories often idealize the superconductor (SC) as a bulk system or an infinitely long chain, or treat it as another quantum dot with proximity-induced superconductivity, while experiments employ superconducting segments of finite leng… ▽ More In a hybrid system where two quantum dots (QDs) are coupled to a conventional $s$-wave superconductor, Poor Man's Majorana modes (PMMs) have been proposed. Existing theories often idealize the superconductor (SC) as a bulk system or an infinitely long chain, or treat it as another quantum dot with proximity-induced superconductivity, while experiments employ superconducting segments of finite length. Here, we model the SC as a finite-length 1D chain and treat the QDs and SC on equal footing. We obtain the conditions for the existence of PMMs, valid for arbitrary SC length and applicable to arbitrary tunneling strengths and magnetic fields. We find that the number of PMMs is highly sensitive to the SC length: it oscillates between zero and two with a period set by the Fermi wavelength ($\sim1\,\textÅ$), while four PMMs appear in the long-SC limit where the effective coupling between the two QDs becomes negligible. We further demonstrate that the PMMs that are separately localized at the two ends of the hybrid system do not exist in the finite-length case. Consequently, only nearly localized PMMs can be identified when the magnetic field is strong enough. In this way, the generalized `sweet spot' of the practical system can be found. △ Less

Submitted 14 April, 2026; originally announced April 2026.

Electrostatic transfer of sub-micron magnetic particles onto cantilevers using a focused ion beam system

Authors: Peter Sun, George R. Du Laney, Tim M. Fuchs, Tjerk H. Oosterkamp, Malcolm G. Thomas, John A. Marohn

Abstract: In this paper, we present a focused-ion-beam-assisted method for preparing magnet tips for magnetic resonance force microscopy measurements. The method electrostatically transfers prefabricated magnetic nanoparticles to microcantilevers, achieving precise control over the magnet overhang past the cantilever leading edge while minimizing the fabrication damage to the leading edge of the tip magnet.… ▽ More In this paper, we present a focused-ion-beam-assisted method for preparing magnet tips for magnetic resonance force microscopy measurements. The method electrostatically transfers prefabricated magnetic nanoparticles to microcantilevers, achieving precise control over the magnet overhang past the cantilever leading edge while minimizing the fabrication damage to the leading edge of the tip magnet. We demonstrate successful fabrication of magnets ranging in size from 460 nm to 2.8 um. These magnets were affixed to two types of cantilevers: silicon cantilevers with a spring constant of 800 uN/m, and single-crystal silicon cantilevers with a spring constant of 30 uN/m. We show that the electrostatic transfer method enables a wide variety of tip shapes, sizes, and materials that were previously not possible with conventional fabrication methods. The transfer procedure allows us to prefabricate the desired particle geometry with minimal ion-beam damage, as confirmed by Monte Carlo simulations. We show that the technique is versatile and can be used to fabricate custom-tipped cantilevers for a broader range of scanning probe techniques. △ Less

Submitted 1 April, 2026; originally announced April 2026.

In-plane and out-of-plane electric dipoles and phase transitions in 2D-layered TlGaS2

Authors: A. D. Molchanova, L. H. Yin, L. P. Gao, W. H. Song, Y. P. Sun, K. R. Allahverdiyev, M. N. Popova

Abstract: Out-of-plane and in-plane electric polarization, which rarely coexist in a two-dimensional (2D) ferroelectric material, offer different advantages in ferroelectricity-based devices. Here, we report the coexistence of in-plane and out-of-plane electric dipoles, along with various phase transitions, in 2D van der Waals layered TlGaS2 single crystal. Quantum paraelectricity was observed along both in… ▽ More Out-of-plane and in-plane electric polarization, which rarely coexist in a two-dimensional (2D) ferroelectric material, offer different advantages in ferroelectricity-based devices. Here, we report the coexistence of in-plane and out-of-plane electric dipoles, along with various phase transitions, in 2D van der Waals layered TlGaS2 single crystal. Quantum paraelectricity was observed along both in-plane and out-of-plane directions of the TlGaS2 crystal. Detailed investigation of the quantum paraelectric soft-mode behavior reveals a close correlation between the electric dipoles and the off-center displacement of Tl1+ ions with 6s2 lone pairs in TlGaS2. Anomalies near temperatures of about 120 K and 60-75 K in dielectric and/or infrared spectra indicate the existence of local or weak long-range structural transitions in TlGaS2. Our results provide important experimental evidence for elucidating the phase transitions and coexistence of in-plane and out-of-plane electric dipoles in 2D layered TlGaS2. △ Less

Submitted 23 March, 2026; originally announced March 2026.

Comments: 23 pages, 10 figures

Efficient photo-Nernst terahertz emission in single heavy-metal films

Authors: Lei Wang, Linxuan Song, Elbert E. M. Chia, Peijie Sun, Jianlin Luo, Rongyan Chen, Yong-Chang Lau, Xinbo Wang

Abstract: State-of-the-art metallic terahertz (THz) emitters rely predominantly on spintronic heterostructures, where heavy metals serve as passive spin-to-charge converters. Here, we demonstrate efficient THz radiation from standalone Pt nanofilms at cryogenic temperatures and under external magnetic fields. The governing mechanism is identified as the ultrafast photo-Nernst effect, wherein a transient the… ▽ More State-of-the-art metallic terahertz (THz) emitters rely predominantly on spintronic heterostructures, where heavy metals serve as passive spin-to-charge converters. Here, we demonstrate efficient THz radiation from standalone Pt nanofilms at cryogenic temperatures and under external magnetic fields. The governing mechanism is identified as the ultrafast photo-Nernst effect, wherein a transient thermal gradient drives a transverse charge current. The THz emission polarity is directly dictated by the sign of the Nernst coefficient, as verified by the phase reversal observed between Pt and W or Ta. Remarkably, both thickness scaling and alloying-induced suppression of thermal conductivity independently amplify the single-layer emission to levels comparable with benchmark spintronic bilayers. These findings redefine the established role of heavy metals from passive spin-sinks to active THz emitters, uncovering a universal emission paradigm applicable across diverse spintronic and quantum materials. △ Less

Submitted 23 March, 2026; originally announced March 2026.

Comments: 7 pages, 4 figures

Observation of Kondo hybridization wave in UTe2

Authors: Xin Yu, Shuikang Yu, Zheyu Wu, Alexander G. Eaton, Andrej Cabala, Michal Vališka, Jun Li, Rui Zhou, Yi-feng Yang, Zhenyu Wang, Peijie Sun, Rui Wu

Abstract: Condensed matter systems with strong electronic correlations often manifest a variety of intertwined ordered phases of charge, spin, orbital and other degrees of freedom. As a prototypical strongly correlated electronic system, the Kondo lattice provides fertile soil for many fascinating quantum states, including quantum criticality, unconventional superconductivity, hidden order and topological K… ▽ More Condensed matter systems with strong electronic correlations often manifest a variety of intertwined ordered phases of charge, spin, orbital and other degrees of freedom. As a prototypical strongly correlated electronic system, the Kondo lattice provides fertile soil for many fascinating quantum states, including quantum criticality, unconventional superconductivity, hidden order and topological Kondo insulator/semimetal. The foundation of Kondo physics lies in the hybridization between localized moments and itinerant electrons. Generally, the evolution of Kondo hybridization is characterized as a broad crossover rather than a phase transition. Thus far, an ordered hybridization phase has not been observed. Here, we use scanning tunneling microscopy (STM) to identify a translational-symmetry-breaking order of Kondo hybridization wave(KHW) for the first time on the surface of the spin-triplet heavy-fermion superconductor UTe2. The unprecedented phase of KHW manifests as a periodically modulated Fano lattice, accompanied by a commensurate charge density wave (CDW) and a pronounced energy gap opening near the Fermi level. This KHW-imprinted CDW has an intriguing real-space texture of complementary occupation of the heavy f and conduction charges, thereby forming a Kondo superlattice. The KHW is coexistent with superconductivity in UTe2, which may provide valuable insight into its controversial spin-triplet pairing symmetry and the underlying mechanism. Our first experimental evidence for an ordered hybridization state potentially sheds new light on the strong correlation physics of Kondo lattice system. △ Less

Submitted 11 March, 2026; originally announced March 2026.

Comments: 23 pages, 11 figures

Giant Magnetocaloric Effect in a High-Spin Shastry-Sutherland Dipolar Magnet

Authors: Jianjian Gong, Junsen Wang, Junsen Xiang, Zhaojun Mo, Lei Zhang, Xinyang Liu, Xuetong He, Lu Tian, Zhixing Ye, Huicai Xie, Xucai Kan, Xinqiang Gao, Zhenxing Li, Peijie Sun, Shouguo Wang, Wei Li, Baogen Shen, Jun Shen

Abstract: The Shastry-Sutherland lattice is a prototypical frustrated quantum magnet. It is notable for its exactly solvable dimer-singlet ground state and hosts a wealth of magnetic phenomena under external fields. Here, this work investigates the high-spin (S = 7/2) Eu-based magnet Eu2MgSi2O7 (EMSO) using low-temperature magnetothermal measurements and Monte Carlo simulations, revealing a giant magnetocal… ▽ More The Shastry-Sutherland lattice is a prototypical frustrated quantum magnet. It is notable for its exactly solvable dimer-singlet ground state and hosts a wealth of magnetic phenomena under external fields. Here, this work investigates the high-spin (S = 7/2) Eu-based magnet Eu2MgSi2O7 (EMSO) using low-temperature magnetothermal measurements and Monte Carlo simulations, revealing a giant magnetocaloric effect (MCE) in this Shastry-Sutherland compound. The entropy change peak value is found to be 55.0 J kg-1 K-1 under a field change of B = 0-4 T, approximately 1.5 times larger than the commercial Gd3Ga5O12 (GGG). Adiabatic demagnetization refrigeration achieves a lowest temperature of 151 mK, deeply into the sub-Kelvin regime. Furthermore, a distinctive cooling effect persists below about 1 T, a characteristic absent for conventional magnetic coolants. A dipolar Shastry-Sutherland model is introduced as a minimal model to describe this system; in particular, the experimentally revealed 1/3 magnetization pseudo-plateau can be ascribed to the presence of dipolar couplings between Eu2+ ions, further stabilized by the thermal fluctuations, explaining the persistent cooling effect. This work establishes EMSO as a novel platform for exploring the dipolar Shastry-Sutherland system and for sub-Kelvin adiabatic demagnetization refrigeration. △ Less

Submitted 9 February, 2026; originally announced February 2026.

From Literature to Lab: Closed-Loop Advancement of Perovskite Solar Cells via Domain Knowledge Guided LLM

Authors: Penglei Sun, Shuyan Chen, Xiang Liu, Longhan Zhang, Huajie You, Chang Yan, Yongqi Zhang, Xiaowen Chu, Tong-yi Zhang

Abstract: Perovskite solar cells (PSCs) have been considered as a next-generation disruptive photovoltaic technology, yet their advancement is constrained by the complexity of perovskite recipe with high-dimensional material and process design space. Despite the impressive general reasoning of Large Language Models (LLMs), they struggle with two limitations for application in PSCs: an inability to align gen… ▽ More Perovskite solar cells (PSCs) have been considered as a next-generation disruptive photovoltaic technology, yet their advancement is constrained by the complexity of perovskite recipe with high-dimensional material and process design space. Despite the impressive general reasoning of Large Language Models (LLMs), they struggle with two limitations for application in PSCs: an inability to align general semantics with the perovskite domain knowledge, and an inefficiency in navigating high-dimensional perovskite material and recipe design spaces. To address these limitations, we introduce a domain-knowledge-guided framework PVK-LLM, a specialized model to serve as an expert to bridge general semantics with perovskite domain knowledge. By integrating this domain knowledge into a hierarchical Bayesian Optimization workflow, our approach efficiently navigates the high-dimension design space on a solar cell simulator platform. The domain knowledge resolves cold-start problems while dynamically adapting to simulator feedback. Moreover, in an individual wet-lab experiment aimed at maximizing power conversion efficiency (PCE), our framework autonomously proposes a novel synergistic four-component recipe comprising specialized organic passivation recipe (3MTPAI, PDAI2, EDAI2, and PipDI) which has not been reported in existing literature. This AI-designed recipe effectively achieves a champion PCE value of over 26.0 %, approaching world records achieved through extensive expert trial-and-error. Our approach can effectively enable LLM comprehend the domain knowledge, which can efficiently navigate in a high-dimensional, capable to accelerate the advancement in real-world perovskite as well as other material science development. △ Less

Submitted 3 February, 2026; originally announced February 2026.

Ising Supercriticality and Universal Magnetocalorics in Spiral Antiferromagnet Nd$_3$BWO$_9$

Authors: Xinyang Liu, Enze Lv, Xueling Cui, Han Ge, Fangyuan Song, Zhaoming Tian, Gang Su, Kan Zhao, Junsen Xiang, Peijie Sun, Wei Li

Abstract: The celebrated analogy between the pressure-temperature phase diagram of a liquid-gas system and the field-temperature phase diagram of ferromagnet has long been a cornerstone for understanding universality of phase transitions and critical phenomena. Here we extend this analogy to a highly frustrated antiferromagnet, the spiral Ising compound Nd$_3$BWO$_9$ with kagome layers. In its phase diagram… ▽ More The celebrated analogy between the pressure-temperature phase diagram of a liquid-gas system and the field-temperature phase diagram of ferromagnet has long been a cornerstone for understanding universality of phase transitions and critical phenomena. Here we extend this analogy to a highly frustrated antiferromagnet, the spiral Ising compound Nd$_3$BWO$_9$ with kagome layers. In its phase diagram, we identify a metamagnetic transition line with a critical endpoint (CEP) located at $μ_0H_{\mathrm{c}} \simeq 1.04$ T and $T_{\mathrm{c}} \simeq 0.3$ K. Above the CEP, an Ising supercritical regime} emerges with supercritical crossover lines that adhere to a universal scaling law, as evidenced by the specific heat, magnetic susceptibility, and magnetocaloric measurements. Remarkably, we observe a highly sensitive field dependence in the magnetic cooling near the emergent CEP, characterized by a divergent magnetic Grüneisen ratio $Γ_H \propto 1/t^{β+γ-1}$, with $β+ γ\simeq 1.563$ the critical exponents of 3D Ising universality class and $t \equiv (T-T_{\rm c})/T_{\rm c}$ the reduced temperature. Our adiabatic demagnetization measurements on Nd$_3$BWO$_9$ reveal a lowest temperature of 195~mK, achieved from the initial condition of 2 K and 4 T. Our findings open a new avenue for studying supercritical phenomena and magnetic cooling in rare-earth RE$_3$BWO$_9$ family and, more broadly, in Ising-anisotropic magnets such as spin ices. △ Less

Submitted 10 April, 2026; v1 submitted 12 January, 2026; originally announced January 2026.

Comments: 5 pages, 4 figures

Kinetic Catalysis of Spontaneous Knotting: How Free Particles Modulate Filament Entanglement

Authors: Peimo Sun, Yuhan Qin, Zheng Li

Abstract: Entangled knots form spontaneously in flexible filaments, yet the influence of the surrounding environment on this process is poorly understood. Here we demonstrate that free-moving particles act as kinetic catalysts for spontaneous knotting. Through controlled agitation experiments, we find that a small number of inert beads substantially enhance the probability and accelerate the rate of knot fo… ▽ More Entangled knots form spontaneously in flexible filaments, yet the influence of the surrounding environment on this process is poorly understood. Here we demonstrate that free-moving particles act as kinetic catalysts for spontaneous knotting. Through controlled agitation experiments, we find that a small number of inert beads substantially enhance the probability and accelerate the rate of knot formation. This catalytic effect is non-monotonic: an optimal particle size and concentration that maximizes entanglement, while an excess of particles suppresses knotting by impeding the filament's dynamics. We develop a stochastic model that quantitatively reproduces this behavior, attributing it to a competition between entanglement-promoting collisions and motion-suppressing drag. Our findings reveal a mechanism for tuning topological complexity, whereby adjusting these environmental agitators can either promote rapid self-assembly or inhibit unwanted entanglement. This work suggests new strategies for controlling filament topology in settings ranging from crowded biological environments to advanced materials processing. △ Less

Submitted 29 December, 2025; originally announced December 2025.

Comments: 13 pages, 4 figures

Coexistence of near-EF van Hove singularity and in-gap topological Dirac surface states in superconducting electrides

Authors: Yin Yang, Peihan Sun, Ye Shen, Zhijun Tu, Pengcheng Ma, Hongrun Zhen, Tianqi Wang, Longli Tian, Tian Cui, Hechang Lei, Kai Liu, Zhonghao Liu

Abstract: Superconducting electrides have attracted growing attention for their potential to achieve high superconducting transition temperatures (TC) under pressure. However, many known electrides are chemically reactive and unstable, making high-quality single-crystal growth, characterization, and measurements difficult, and most do not exhibit superconductivity at ambient pressure. In contrast, La3In sta… ▽ More Superconducting electrides have attracted growing attention for their potential to achieve high superconducting transition temperatures (TC) under pressure. However, many known electrides are chemically reactive and unstable, making high-quality single-crystal growth, characterization, and measurements difficult, and most do not exhibit superconductivity at ambient pressure. In contrast, La3In stands out for its ambient-pressure superconductivity (TC ~ 9.4 K) and the availability of high-quality single crystals. Here, we investigate its low-energy electronic structure using angle-resolved photoemission spectroscopy and first-principles calculations. The bands near the Fermi energy are mainly derived from La 5d and In 5p orbitals. A saddle point is directly observed at the Brillouin zone (BZ) boundary, while a three-dimensional van Hove singularity crosses EF at the BZ corner. First-principles calculations further reveal topological Dirac surface states within the bulk energy gap above EF. The coexistence of a high density of states and in-gap topological surface states near EF suggests that La3In offers a promising platform for tuning superconductivity and exploring possible topological superconducting phases through doping or external pressure. △ Less

Submitted 28 November, 2025; originally announced November 2025.

Comments: See also our related works on the formation mechanisms of electrides and their connection to SC and unconventional metallic behavior: "Evidence for Anion-Electron Duality and Enhanced Superconducting Role of Interstitial Anionic Electrons in Electrides" and "Interstitial Anionic Electrons-Involved Superconductivity and T-Linear Resistivity Behavior in Electride"

Size optimization for observeing Majorana fermions

Authors: Guo-Jian Qiao, Zhi-Lei Zhang, Xin Yue, C. P. Sun

Abstract: Majorana fermions (zero modes) are predicted to emerge in nanowire-superconductor heterostructures. This theoretical prediction typically relies on an oversimplified model, where both the nanowire and the superconductor are idealized as one-dimensional systems. In reality, heterostructures have finite sizes that deviate from this idealization-and as a result, smoking-gun evidence confirming the ex… ▽ More Majorana fermions (zero modes) are predicted to emerge in nanowire-superconductor heterostructures. This theoretical prediction typically relies on an oversimplified model, where both the nanowire and the superconductor are idealized as one-dimensional systems. In reality, heterostructures have finite sizes that deviate from this idealization-and as a result, smoking-gun evidence confirming the existence of these zero modes remains elusive. Here, we investigate the finite-size effects of both the nanowire and the superconductor, and optimize their sizes to ensure that only one Majorana fermion exists at each end of the heterostructure. It is discovered that the optimal transverse sizes of the nanowire are less than 100nm in width and approximately 1nm in thickness. For the superconductor layer, its optimal thickness (a key aspect of its size) must exceed its coherence length. We also present the optimal sizes of the two types of materials used in the experiment in a quantitative manner. Notably, the identified optimal thickness of the superconductor (Al films, $\sim$1000nm)--a critical size parameter--is two orders of magnitude larger than the thickness of Al films currently utilized in experimental devices (e.g., InSb-Al and InAs-Al heterostructures). Our findings could explain why Majorana fermions have not been observed in current experiments, and offer guidance for the size selection of heterostructures to implement Majorana fermions in future studies. △ Less

Submitted 26 November, 2025; originally announced November 2025.

Comments: 12 pages, 1 table, 2 figures

Quantum fluctuations associated with first-order magnetic transition in a frustrated kagome lattice antiferromagnet

Authors: Zhongchen Xu, Xinyang Liu, Cuiwei Zhang, Shuai Zhang, Feng Jin, Junsen Xiang, Quansheng Wu, Xianmin Zhang, Peijie Sun, Youguo Shi

Abstract: Intense quantum fluctuations arising from geometrical frustrations in kagome-lattice magnets provide a feasible approach to exotic quantum states. Here, we document an unexpected isosymmetric first-order magnetic transition in the recently synthesized frustrated kagome-lattice antiferromagnet Nd3ScBi5, which is characterized by significant latent heat and a pronounced magnetocaloric effect, as wel… ▽ More Intense quantum fluctuations arising from geometrical frustrations in kagome-lattice magnets provide a feasible approach to exotic quantum states. Here, we document an unexpected isosymmetric first-order magnetic transition in the recently synthesized frustrated kagome-lattice antiferromagnet Nd3ScBi5, which is characterized by significant latent heat and a pronounced magnetocaloric effect, as well as discontinuous Raman shifts and negligible hysteresis. Employing the magnetocaloric effect as a detection method, in conjunction with systematical field-dependent physical properties, we uncover a distinctive 1/2 magnetization plateau phase with significant quantum fluctuations. Our study unveils Nd3ScBi5 as a prototypical model with an emerging phase of enhanced quantum fluctuations triggered by first-order magnetic transitions. △ Less

Submitted 12 November, 2025; originally announced November 2025.

Comments: 13+20 pages, 6+12 figures. Submitted to Physical Review B

Superconductivity in cubic La3Al with interstitial anionic electrons

Authors: Zhijun Tu, Peihan Sun, Donghan Jia, Huiyang Gou, Kai Liu, Hechang Lei

Abstract: We report the observation of superconductivity in cubic La3Al single crystal. It shows a metallic behavior at a normal state without observable structural transition and enters the superconducting state below Tc ~ 6.32 K. Detailed characterizations and analysis indicate that cubic La3Al is a bulk type-II BCS superconductor. Moreover, theoretical calculations show that it can host interstitial anio… ▽ More We report the observation of superconductivity in cubic La3Al single crystal. It shows a metallic behavior at a normal state without observable structural transition and enters the superconducting state below Tc ~ 6.32 K. Detailed characterizations and analysis indicate that cubic La3Al is a bulk type-II BCS superconductor. Moreover, theoretical calculations show that it can host interstitial anionic electrons, which are located at the body center of cubic unit cell, and confirm the electron-phonon coupling as the superconducting mechamism. Thus, cubic La3Al can be regarded as an novel electride superconductor. △ Less

Submitted 26 September, 2025; originally announced September 2025.

Comments: 7 page, 4 figure, 1 table

Journal ref: Chin. Phys. Lett. 42, 027302 (2025)

Realization of large magnetocaloric effect in the Kagome antiferromagnet Gd3BWO9 for Sub-Kelvin cryogenic refrigeration

Authors: Fangyuan Song, Xinyang Liu, Chao Dong, Jin Zhou, Xinlong Shi, Yuyan Han, Langsheng Ling, Huifen Ren, Songliu Yuan, Shun Wang, Junsen Xiang, Peijie Sun, Zhaoming Tian

Abstract: Rare-earth (RE) based frustrated magnets have attracted great attention as excellent candidates for magnetic refrigeration at sub-Kelvin temperatures, while the experimental identification on systems exhibiting both large volumetric cooling capacity and reduced working temperatures far below 1 K remain to be a challenge. Here, through the ultra-low temperature magnetism and thermodynamic character… ▽ More Rare-earth (RE) based frustrated magnets have attracted great attention as excellent candidates for magnetic refrigeration at sub-Kelvin temperatures, while the experimental identification on systems exhibiting both large volumetric cooling capacity and reduced working temperatures far below 1 K remain to be a challenge. Here, through the ultra-low temperature magnetism and thermodynamic characterizations, we unveil the large magnetocaloric effect (MCE) realized at sub-Kelvin temperatures in the frustrated Kagome antiferromagnet Gd3BWO9 with TN~1.0 K. The isothermal magnetization curves indicate the existence of field (B) induced anisotropic magnetic phase diagrams, where four distinct magnetic phases for B // c-axis and five magnetic phases for B // ab-plane are identified at T< TN. The analysis of magnetic entropy S(B, T) data and direct adiabatic demagnetization tests reveal a remarkable cooling performance at sub-Kelvin temperatures featured by a large volumetric entropy density 502.2 mJ/K/cm3 and a low attainable minimal temperature Tmin~168 mK from the initial cooling condition of 2 K and 6 T, surpassing most of Gd-based refrigerants previously documented in temperature ranges of 0.25-4 K. The realized Tmin~168 mK far below TN ~ 1.0 K in Gd3BWO9 is related to the combined effects of magnetic frustration and criticality-enhanced MCE, which together leave a substantial magnetic entropy at reduced temperatures by enhancing spin fluctuations. △ Less

Submitted 14 September, 2025; originally announced September 2025.

Exact bound of power-efficiency trade-off in finite-time thermodynamic cycles

Authors: R. X. Zhai, Xin Yue, C. P. Sun

Abstract: Power and efficiency are fundamental criteria for evaluating the performance of thermodynamic cycles. However, it is generally impossible to maximize both simultaneously. In particular, achieving maximum efficiency inevitably leads to vanishing power as the cycle duration approaches infinity. A quantitative characterization of this trade-off yields significant theoretical and practical implication… ▽ More Power and efficiency are fundamental criteria for evaluating the performance of thermodynamic cycles. However, it is generally impossible to maximize both simultaneously. In particular, achieving maximum efficiency inevitably leads to vanishing power as the cycle duration approaches infinity. A quantitative characterization of this trade-off yields significant theoretical and practical implications. In this letter, we analytically derive an exact bound constraining power and efficiency in low-dissipation finite-time heat engines. This bound specifies the maximum power attainable at any prescribed efficiency, thereby providing a benchmarking for evaluating the performance of heat engines. △ Less

Submitted 16 September, 2025; v1 submitted 9 September, 2025; originally announced September 2025.

Comments: 4 pages 1 figure and 1 table

Anomalous Nernst Effect and Its Implications for Time-Reversal Symmetry Breaking in Kagome Metal ScV6Sn6

Authors: Yazhou Li, Saizheng Cao, Jiaxing Liao, Jiajun Ma, Yuwei Zhang, Tao Li, Jialu Wang, Chenchao Xu, Jianhui Dai, Chao Cao, Yu Song, Peijie Sun, Yuke Li

Abstract: The nonmagnetic kagome metal ScV6Sn6 displays an unconventional charge order (CO) accompanied by signatures of an anomalous Hall effect, hidden magnetism, and multiple lattice instabilities. In this study, we report the observation of unconventional anomalous thermoelectric properties. Notably, unexpected anomalous transverse Nernst signals reach a peak value of ~4 μV/K near the TCDW ~92 K in ScV6… ▽ More The nonmagnetic kagome metal ScV6Sn6 displays an unconventional charge order (CO) accompanied by signatures of an anomalous Hall effect, hidden magnetism, and multiple lattice instabilities. In this study, we report the observation of unconventional anomalous thermoelectric properties. Notably, unexpected anomalous transverse Nernst signals reach a peak value of ~4 μV/K near the TCDW ~92 K in ScV6Sn6, and these signals persist in the charge-ordered state as the temperature decreases to 10 K. Furthermore, both thermopower and thermal conductivity exhibit significant changes under magnetic fields, even in the nonmagnetic ground state. These observations strongly suggest the emergence of time-reversal symmetry breaking in ScV6Sn6, as supported by muon spin relaxation (μSR) measurements. While hidden magnetism represents the most plausible origin, alternative mechanisms involving orbital currents and chiral charge order remain possible. △ Less

Submitted 18 August, 2025; originally announced August 2025.

Comments: 14 pages, 4 figures, to appear in SCIENCE CHINA Physics, Mechanics and Astronomy

Report number: 2508.12667

Journal ref: SCIENCE CHINA Physics, Mechanics & Astronomy 69, 217411 (2026)

Universal Magnetocaloric Effect near Quantum Critical Point of Magnon Bose-Einstein Condensation

Authors: Junsen Xiang, Enze Lv, Qinxin Shen, Cheng Su, Xuetong He, Yinghao Zhu, Yuan Gao, Xin-Yang Liu, Dai-Wei Qu, Xinlei Wang, Xi Chen, Qian Zhao, Haifeng Li, Shuo Li, Jie Yang, Jun Luo, Peijie Sun, Wentao Jin, Yang Qi, Rui Zhou, Wei Li, Gang Su

Abstract: Bose-Einstein condensation (BEC), a macroscopic quantum phenomenon arising from phase coherence and bosonic statistics, has been realized in quantum magnets. Here, we report the observation of a universal magnetocaloric effect (MCE) near a BEC quantum critical point (QCP) in copper sulfate crystal ($CuSO_4 \cdot 5H_2O$). By conducting magnetocaloric and nuclear magnetic resonance measurements, we… ▽ More Bose-Einstein condensation (BEC), a macroscopic quantum phenomenon arising from phase coherence and bosonic statistics, has been realized in quantum magnets. Here, we report the observation of a universal magnetocaloric effect (MCE) near a BEC quantum critical point (QCP) in copper sulfate crystal ($CuSO_4 \cdot 5H_2O$). By conducting magnetocaloric and nuclear magnetic resonance measurements, we uncover a field-driven BEC QCP, evidenced by the universal scaling law $T_c \propto (B_c - B)^{2/3}$ and the perfect data collapse of the magnetic Grüneisen ratio. Thermal excitation triggers a dimensional crossover to a 1D quantum-critical regime, where the MCE scaling strictly matches the universality class of 1D Fermi gases. Notably, the quantum-critical MCE enables cooling down to 12.8 mK without helium-3, with very fast thermal relaxation rate that is critical for high cooling power. This work demonstrates the universal MCE in magnon BEC systems, using a common copper sulfate compound as a paradigmatic example, and paves the way for next-generation sub-Kelvin cooling. △ Less

Submitted 7 August, 2025; originally announced August 2025.

Comments: 20 pages, 15 figures

Laser-induced ultrafast structural transformations in thin Fe layer revealed by time-resolved X-ray diffraction

Authors: O. Liubchenko, J. Antonowicz, K. Sokolowski-Tinten, P. Zalden, R. Minikayev, I. Milov, T. J. Albert, C. Bressler, M. Chojnacki, P. Dłużewski, P. Dzięgielewski, A. Rodriguez-Fernandez, K. Fronc, W. Gawelda, K. Georgarakis, A. L. Greer, I. Jacyna, R. W. E. van de Kruijs, R. Kamiński, D. Khakhulin, D. Klinger, K. Kosyl, K. Kubicek, A. Olczak, N. T. Panagiotopoulos , et al. (5 additional authors not shown)

Abstract: The ultrafast structural response of a thin iron film to sub-ps pulsed laser-induced heating has been investigated using time-resolved X-ray diffraction in the partial melting regime. A tetragonal distortion of the bcc-phase emerges at ~6 ps. Its formation is delayed relative to the initial heating (1-2 ps) and partial melting (2-5 ps) of the material, and controlled by the stress release in the q… ▽ More The ultrafast structural response of a thin iron film to sub-ps pulsed laser-induced heating has been investigated using time-resolved X-ray diffraction in the partial melting regime. A tetragonal distortion of the bcc-phase emerges at ~6 ps. Its formation is delayed relative to the initial heating (1-2 ps) and partial melting (2-5 ps) of the material, and controlled by the stress release in the quasi-instantaneously pressurized film. The distortion persists for at least 60 ps indicating the formation of a metastable bct phase between the equilibrium bcc and fcc phases. △ Less

Submitted 8 August, 2025; v1 submitted 23 June, 2025; originally announced June 2025.

Comments: The revised manuscript features an upgraded discussion and conclusion parts, with particular emphasis on the role of stress-release mechanisms. The text has been substantially shortened and refined, with careful language improvements to ensure clarity and precision. Additionally, the manuscript has been formatted in accordance with PRL submission guidelines

Finite Thickness Effects on Metallization Vs. Chiral Majorana Fermions

Authors: Xin Yue, Guo-Jian Qiao, C. P. Sun

Abstract: The search for chiral Majorana fermions in quantum anomalous Hall insulator/\textit{s}-wave superconductor heterostructures has attracted intense interest, yet remains controversial due to the lack of conclusive evidence. A key issue is that the heterostructure's metallization can produce half-integer conductance signatures resembling those of chiral Majorana fermions, thereby complicating their i… ▽ More The search for chiral Majorana fermions in quantum anomalous Hall insulator/\textit{s}-wave superconductor heterostructures has attracted intense interest, yet remains controversial due to the lack of conclusive evidence. A key issue is that the heterostructure's metallization can produce half-integer conductance signatures resembling those of chiral Majorana fermions, thereby complicating their identification. In this Letter, we investigate how the competition between metallization and chiral Majorana fermions depends on superconductor thickness, revealing its critical role through three distinct regimes: (i) For thin superconductors ($\sim$10 nm), metallization shows periodic oscillations with thickness, matching the Fermi wavelength. (ii) Intermediate thicknesses ($\sim$100 nm) exhibit periodic windows for observing chiral Majorana fermions. (iii) Thick superconductors ($\sim$1000 nm) sustain stable chiral Majorana fermions that are insensitive to thickness variations. These results suggest that superconductor thickness is a key control parameter for advancing efforts to conclusively identify chiral Majorana fermions. △ Less

Submitted 8 January, 2026; v1 submitted 19 June, 2025; originally announced June 2025.

Journal ref: Phys. Rev. B 113, 115416 (2026)

Poor Man's Majoranon in Two Quantum Dots Dressed by Superconducting Quasi-Excitations

Authors: Zhi-Lei Zhang, Guo-Jian Qiao, C. P. Sun

Abstract: In a hybrid system consisting of two quantum dots (QDs) coupled to a superconductor (SC), zero-bias peaks in the differential conductance spectrum have been reported as potential signatures of Majorana fermions (MFs). However, such signatures typically appear only at specific parameter values of the QDs--so-called `sweet spots'--and are referred to as the Poor Man's Majorana (PMM). To investigate… ▽ More In a hybrid system consisting of two quantum dots (QDs) coupled to a superconductor (SC), zero-bias peaks in the differential conductance spectrum have been reported as potential signatures of Majorana fermions (MFs). However, such signatures typically appear only at specific parameter values of the QDs--so-called `sweet spots'--and are referred to as the Poor Man's Majorana (PMM). To investigate whether these signatures can be conclusively attributed to genuine MFs emerging over a continuous parameter range, we present an alternative approach that microscopically incorporates the superconducting effects into the QDs, rather than simply attribute them into two phenomenological parameters of QDs. This forms the dressed Majorana fermions (DMFs), which can be viewed as superpositions of quasi-excitations from both the QDs and the SC. We show that DMFs can localize at one end of a one-dimensional SC and persist across a continuous parameter range, thereby enhancing the feasibility of experimental detection. Our results provide a more accurate description of the PMM in such hybrid systems and offer practical guidance for observing end-localized PMM modes in continuous one-dimensional SC. △ Less

Submitted 13 June, 2025; v1 submitted 12 June, 2025; originally announced June 2025.

Optimization of target film materials and protective coatings for sealed neutron generator

Authors: Yingying Cao, Sijia Zhou, Pingwei Sun, Jiayu Li, Shangrui Jiang, Shiwei Jing

Abstract: Magnesium target film has better thermal stability and neutron yield than titanium target, making it a potential neutron generator target film material. The radiation resistance of elemental magnesium targets is relatively weak, and their radiation resistance can be improved by alloying magnesium target films. The irradiation damage of pure magnesium targets and magnesium alloy target films was st… ▽ More Magnesium target film has better thermal stability and neutron yield than titanium target, making it a potential neutron generator target film material. The radiation resistance of elemental magnesium targets is relatively weak, and their radiation resistance can be improved by alloying magnesium target films. The irradiation damage of pure magnesium targets and magnesium alloy target films was studied using SRIM. The results indicate that the irradiation damage of magnesium alloy target films (magnesium-niobium, magnesium-zirconium alloys) is lower than that of pure magnesium targets. In addition, under the same alloy ratio, the radiation resistance of magnesium-niobium alloy target film is better than that of magnesium-zirconium alloy. In order to further in-vestigate the performance of magnesium alloy target films, the incident ion energy, protective coatings (nickel oxide, aluminum oxide, palladium oxide), magnesium alloy target films, and alloy doping ratios (0.2, 0.4, 0.6, 0.8, 1.0) were changed. After calculating the effects of the above conditions on the neutron generator yield, sputtering yield, and considering irradiation damage, it was determined that a magnesium-zirconium alloy with a doping rate of 0.2 and a nickel oxide protective coating with a thickness of 7.5 nm are potential target film materials for the neutron generator. △ Less

Submitted 5 June, 2025; originally announced June 2025.

Giant Magnetocaloric Effect in Spin Supersolid Candidate Na$_2$BaCo(PO$_4$)$_2$

Authors: Junsen Xiang, Chuandi Zhang, Yuan Gao, Wolfang Schmidt, Karin Schmalzl, Chin-Wei Wang, Bo Li, Ning Xi, Xin-Yang Liu, Hai Jin, Gang Li, Jun Shen, Ziyu Chen, Yang Qi, Yuan Wan, Wentao Jin, Wei Li, Peijie Sun, Gang Su

Abstract: Supersolid, an exotic quantum state of matter that consists of particles forming an incompressible solid structure while simultaneously showing superfluidity of zero viscosity [1], is one of the long-standing pursuits in fundamental research [2, 3]. Although the initial report of $^4$He supersolid turned out to be an artifact [4], this intriguing quantum matter has inspired enthusiastic investigat… ▽ More Supersolid, an exotic quantum state of matter that consists of particles forming an incompressible solid structure while simultaneously showing superfluidity of zero viscosity [1], is one of the long-standing pursuits in fundamental research [2, 3]. Although the initial report of $^4$He supersolid turned out to be an artifact [4], this intriguing quantum matter has inspired enthusiastic investigations into ultracold quantum gases [5-8]. Nevertheless, the realization of supersolidity in condensed matter remains elusive. Here we find evidence for a quantum magnetic analogue of supersolid -- the spin supersolid -- in the recently synthesized triangular-lattice antiferromagnet Na$_2$BaCo(PO$_4$)$_2$ [9]. Notably, a giant magnetocaloric effect related to the spin supersolidity is observed in the demagnetization cooling process, manifesting itself as two prominent valley-like regimes, with the lowest temperature attaining below 100 mK. Not only is there an experimentally determined series of critical fields but the demagnetization cooling profile also shows excellent agreement with the theoretical simulations with an easy-axis Heisenberg model. Neutron diffractions also successfully locate the proposed spin supersolid phases by revealing the coexistence of three-sublattice spin solid order and interlayer incommensurability indicative of the spin superfluidity. Thus, our results indicate a strong entropic effect of the spin supersolid phase in a frustrated quantum magnet and open up a viable and promising avenue for applications in sub-Kelvin refrigeration, especially in the context of persistent concerns about helium shortages [10, 11]. △ Less

Submitted 15 April, 2025; originally announced April 2025.

Comments: 19 pages, 13 figures

Journal ref: Nature volume 625, pages 270-275 (2024)

Weyl Fermion Manipulation through Magnetic Transitions in the Ferromagnetic Non-Centrosymmetric Weyl semimetal PrAlSi

Authors: K. P. Wang, W. J. Shi, W. Z. Cao, X. T. Yang, Z. Y. Lv, C. Peng, C. Chen, D. F. Liu, H. F. Yang, L. X. Yang, M. Lyu, P. J. Sun, E. K. Liu, M. Ye, Y. L. Chen, Y. Sun, Y. P. Qi, Z. K. Liu

Abstract: PrAlSi, a non-centrosymmetric ferromagnetic Weyl semimetal candidate with a Curie temperature of 17.8K, offers a unique platform for exploring the interplay of symmetry breaking and topological electronic structures. Up to now, the Weyl fermion distribution as well as their evolution across the ferromagnetic to paramagnetic phase transition in PrAlSi has not been explored. Here, we uncover the pre… ▽ More PrAlSi, a non-centrosymmetric ferromagnetic Weyl semimetal candidate with a Curie temperature of 17.8K, offers a unique platform for exploring the interplay of symmetry breaking and topological electronic structures. Up to now, the Weyl fermion distribution as well as their evolution across the ferromagnetic to paramagnetic phase transition in PrAlSi has not been explored. Here, we uncover the presence of Weyl fermions in PrAlSi and demonstrate they could be manipulated through the magnetic phase transition. Our ab-initio calculations indicate a shift in the momentum and energy positions of Weyl fermions, alongside an increase in Weyl point numbers due to band splitting. The predicted band splitting and shifting of Weyl fermions are corroborated by our angle-resolved photoemission spectroscopy experiments. Such manipulation of Weyl fermions leads to the appearance of a net chirality charge and a significant modulation in optical conductivity, as proposed by our calculations. Our research presents a novel method for adjusting the properties of Weyl semimetals by controlling Weyl fermions through magnetic phase transitions, positioning PrAlSi as a model system. △ Less

Submitted 17 March, 2025; originally announced March 2025.

Comments: 21 pages, 4 figures

Journal ref: Advanced Electronic Materials (2025)

A low-background setup for in-situ X-ray total scattering combined with fast scanning calorimetry

Authors: Peihao Sun, Jacopo Baglioni, Beatrice Baraldi, Weilong Chen, Daniele Lideo, Lara Piemontese, Francesco Dallari, Marco Di Michiel, Giulio Monaco

Abstract: We demonstrate a setup combining fast scanning calorimetry with X-ray total scattering at a synchrotron beamline, allowing for \emph{in-situ} characterizations of the nano-scale structure of samples during and after temperature scans. The setup features a portable vacuum chamber giving high signal-to-background ratio even on amorphous samples, which enables the observation of detailed structural c… ▽ More We demonstrate a setup combining fast scanning calorimetry with X-ray total scattering at a synchrotron beamline, allowing for \emph{in-situ} characterizations of the nano-scale structure of samples during and after temperature scans. The setup features a portable vacuum chamber giving high signal-to-background ratio even on amorphous samples, which enables the observation of detailed structural changes between different sample states. We show three use cases, including one which leverages the high cooling rate of 10$^4$\,K/s achievable by this setup. Our demonstration opens the door to various applications in materials science where it is important to understand the interplay between structure and thermodynamics. △ Less

Submitted 16 July, 2025; v1 submitted 12 March, 2025; originally announced March 2025.

Comments: 17 pages, 7 figures

Journal ref: J. Synchrotron Rad. (2025). 32, 1228-1234

Strong superconducting pairing strength and pseudogap features in a putative multiphase heavy-fermion superconductor CeRh2As2 by soft point-contact spectroscopy

Authors: Qingxin Dong, Tong Shi, Pengtao Yang, Xinyang Liu, Xiaofan Shi, Lei Wang, Junsen Xiang, Hanming Ma, Zhaoming Tian, Jianping Sun, Yoshiya Uwatoko, Genfu Chen, Xinbo Wang, Jie Shen, Rui Wu, Xin Lu, Peijie Sun, Grzegorz Chajewski, Dariusz Kaczorowski, Bosen Wang, Jinguang Cheng

Abstract: CeRh2As2 is a newly discovered candidate of multiphase heavy-fermion superconductor (Tc=0.3 K) with intriguing physical properties. Here, we employ soft point-contact spectroscopy to investigate its energy gap behaviors in both the normal and superconducting states. The differential conductance below Tc reveals an estimated superconducting energy gap of 2ΔSC=0.24 meV and thus an extremely strong s… ▽ More CeRh2As2 is a newly discovered candidate of multiphase heavy-fermion superconductor (Tc=0.3 K) with intriguing physical properties. Here, we employ soft point-contact spectroscopy to investigate its energy gap behaviors in both the normal and superconducting states. The differential conductance below Tc reveals an estimated superconducting energy gap of 2ΔSC=0.24 meV and thus an extremely strong superconducting pairing strength 2ΔSC/kBTc=8.8, which is comparable to those of cuprates and iron-based high-Tc superconductors as well as infinite-layer nickelates. Above Tc, a well-defined pseudogap feature is manifested as a V-shaped dip in the differential conductance spanning an energy scale of 2Δg=0.95-3.0 meV. The pseudogap feature persists to the highest characteristic temperature of Tg=8-9 K and is gradually suppressed by magnetic field of Bg=9.0T regardless of its direction relative to the crystallographic axes. The observation of pseudogap features prior to the superconducting phase transition enriches the phase diagram of CeRh2As2 and provides a novel platform to study the interplay of unconventional superconductivity and pseudogap phenomena. △ Less

Submitted 4 January, 2025; originally announced January 2025.

Comments: 15 pages, 4 figures

Intrinsic pinning of FeSe$_1$$_-$$_x$S$_x$ single crystals probed by torque magnetometry

Authors: Nan Zhou, Yue Sun, Q. Hou, T. Sakakibara, X. Z. Xing, C. Q. Xu, C. Y. Xi, Z. S. Wang, Y. F. Zhang, Y. Q. Pan, B. Chen, X. Luo, Y. P. Sun, Xiaofeng Xu, T. Tamegai, Mingxiang Xu, Zhixiang Shi

Abstract: Intrinsic pinning is caused by natural pinning centers that occur because of the modulation of the order parameter or weak superconducting layers. Early work has shown that intrinsic pinning generates a high pinning force and critical current density in some layered oxide superconductors. Studying the intrinsic pinning of superconductors is crucial for both fundamental studies and potential applic… ▽ More Intrinsic pinning is caused by natural pinning centers that occur because of the modulation of the order parameter or weak superconducting layers. Early work has shown that intrinsic pinning generates a high pinning force and critical current density in some layered oxide superconductors. Studying the intrinsic pinning of superconductors is crucial for both fundamental studies and potential applications. Herein, we use torque magnetometry to study angle-resolved in-plane and out-of-plane magnetic torque for a series of high-quality FeSe$_1$$_-$$_x$S$_x$ single crystals. A fourfold torque signal was observed when the magnetic field was within the \textit{ab} plane. We interpret that this fourfold in-plane irreversible torque is from the intrinsic pinning due to combined effects of gap nodes/minimum and twin domains. Additionally, we attributed the observed out-of-plane torque peaks to intrinsic pinning due to the layered structure. △ Less

Submitted 6 December, 2024; originally announced December 2024.

Comments: 10 pages, 7 figures

Journal ref: Materials Today Physics 37 (2023) 101195

Deep Learning Models for Colloidal Nanocrystal Synthesis

Authors: Kai Gu, Yingping Liang, Jiaming Su, Peihan Sun, Jia Peng, Naihua Miao, Zhimei Sun, Ying Fu, Haizheng Zhong, Jun Zhang

Abstract: Colloidal synthesis of nanocrystals usually includes complex chemical reactions and multi-step crystallization processes. Despite the great success in the past 30 years, it remains challenging to clarify the correlations between synthetic parameters of chemical reaction and physical properties of nanocrystals. Here, we developed a deep learning-based nanocrystal synthesis model that correlates syn… ▽ More Colloidal synthesis of nanocrystals usually includes complex chemical reactions and multi-step crystallization processes. Despite the great success in the past 30 years, it remains challenging to clarify the correlations between synthetic parameters of chemical reaction and physical properties of nanocrystals. Here, we developed a deep learning-based nanocrystal synthesis model that correlates synthetic parameters with the final size and shape of target nanocrystals, using a dataset of 3500 recipes covering 348 distinct nanocrystal compositions. The size and shape labels were obtained from transmission electron microscope images using a segmentation model trained with a semi-supervised algorithm on a dataset comprising 1.2 million nanocrystals. By applying the reaction intermediate-based data augmentation method and elaborated descriptors, the synthesis model was able to predict nanocrystal's size with a mean absolute error of 1.39 nm, while reaching an 89% average accuracy for shape classification. The synthesis model shows knowledge transfer capabilities across different nanocrystals with inputs of new recipes. With that, the influence of chemicals on the final size of nanocrystals was further evaluated, revealing the importance order of nanocrystal composition, precursor or ligand, and solvent. Overall, the deep learning-based nanocrystal synthesis model offers a powerful tool to expedite the development of high-quality nanocrystals. △ Less

Submitted 14 December, 2024; originally announced December 2024.

YbNi$_4$Mg: Superheavy fermion with enhanced Wilson ratio and magnetocaloric effect

Authors: Xiaoci Zhang, Te Zhang, Zhaotong Zhuang, Zixuan Leng, Zixuan Wei, Xinyang Liu, Junsen Xiang, Shuai Zhang, Peijie Sun

Abstract: A comprehensive study of the low-temperature properties of YbNi$_4$Mg has revealed evidence of a superheavy-fermion state, characterized by a large electronic specific-heat coefficient $γ_0$ $\approx$ 5.65 J mol$^{-1}$ K$^{-2}$ and an elevated Wilson ratio $R_W$ = 32.1. No magnetic ordering was observed down to 70 mK; however, a broad maximum appears in the specific heat at $T^*$ = 0.3 K, along wi… ▽ More A comprehensive study of the low-temperature properties of YbNi$_4$Mg has revealed evidence of a superheavy-fermion state, characterized by a large electronic specific-heat coefficient $γ_0$ $\approx$ 5.65 J mol$^{-1}$ K$^{-2}$ and an elevated Wilson ratio $R_W$ = 32.1. No magnetic ordering was observed down to 70 mK; however, a broad maximum appears in the specific heat at $T^*$ = 0.3 K, along with a shoulder in the derivative of susceptibility d$χ$/d$T$ and resistivity d$ρ$/d$T$. These features indicate a cooperative yet short-ranged magnetism entwined with the superheavy Fermi liquid. The large Wilson ratio, which is also detected in other superheavy-fermion compounds lacking long-range order, might be a signature of residual spin fluctuations. Applying a weak magnetic field of $\sim$0.1 T induces a metamagnetic-like crossover, as demonstrated by the quasi-adiabatic demagnetization measurements showing a broad minimum in the temperature-field trace. Here, an enhanced magnetocaloric cooling effect stemming from the field-sensitive superheavy-fermion state is observed, rivaling that of the well-established insulating magnetic coolants like the rare-earth garnet Gd$_3$Ga$_5$O$_{12}$. △ Less

Submitted 10 December, 2024; originally announced December 2024.

Successive magnetic transitions in the spin-5/2 easy-axis triangular-lattice antiferromagnet Na$_2$BaMn(PO$_4$)$_2$: A neutron diffraction study

Authors: Chuandi Zhang, Junsen Xiang, Cheng Su, Denis Sheptyakov, Xinyang Liu, Yuan Gao, Peijie Sun, Wei Li, Gang Su, Wentao Jin

Abstract: Motivated by the recent observations of various exotic quantum states in the equilateral triangular-lattice phosphates Na$_2$BaCo(PO$_4$)$_2$ with $J\rm_{eff}$ = 1/2 and Na$_2$BaNi(PO$_4$)$_2$ with $S$ = 1, the magnetic properties of spin-5/2 antiferromagnet Na$_2$BaMn(PO$_4$)$_2$, their classical counterpart, are comprehensively investigated experimentally. DC magnetization and specific heat meas… ▽ More Motivated by the recent observations of various exotic quantum states in the equilateral triangular-lattice phosphates Na$_2$BaCo(PO$_4$)$_2$ with $J\rm_{eff}$ = 1/2 and Na$_2$BaNi(PO$_4$)$_2$ with $S$ = 1, the magnetic properties of spin-5/2 antiferromagnet Na$_2$BaMn(PO$_4$)$_2$, their classical counterpart, are comprehensively investigated experimentally. DC magnetization and specific heat measurements on polycrystalline samples indicate two successive magnetic transitions at $T\rm_{N1}$ $\approx$ 1.13 K and $T\rm_{N2}$ $\approx$ 1.28 K, respectively. Zero-field neutron powder diffraction measurement at 67 mK reveals a Y-like spin configuration as its ground-state magnetic structure, with both the $ab$-plane and $c$-axis components of the Mn$^{2+}$ moments long-range ordered. The incommensurate magnetic propagation vector $k$ shows a dramatic change for the intermediate phase between $T\rm_{N1}$ and $T\rm_{N2}$, in which the spin state is speculated to change into a collinear structure with only the $c$-axis moments ordered, as stabilized by thermal fluctuations. The successive magnetic transitions observed in Na$_2$BaMn(PO$_4$)$_2$ are in line with the expectation for a triangle-lattice antiferromagnet with an easy-axis magnetic anisotropy. △ Less

Submitted 4 December, 2024; originally announced December 2024.

Comments: 8 Pages, 6 figures

Journal ref: Physical Review B 110, 214405 (2024)

Emerging quantum critical phase in a cluster spin-glass

Authors: Fang Zhang, Tao Feng, Yurong Ruan, Xiaoyuan Ye, Bing Wen, Liang Zhou, Minglin He, Zhaotong Zhuang, Liusuo Wu, Hongtao He, Peijie Sun, Zhiyang Yu, Weishu Liu, Wenqing Zhang

Abstract: Magnetic frustration has been recognized as pivotal to investigating new phases of matter in correlation-driven Kondo breakdown quantum phase transitions that are not clearly associated with broken symmetry. The nature of these new phases, however, remains underexplored. Here, we report quantum criticalities emerging from a cluster spin-glass in the heavy-fermion metal TiFe$_x$Cu$_{2x-1}$Sb, where… ▽ More Magnetic frustration has been recognized as pivotal to investigating new phases of matter in correlation-driven Kondo breakdown quantum phase transitions that are not clearly associated with broken symmetry. The nature of these new phases, however, remains underexplored. Here, we report quantum criticalities emerging from a cluster spin-glass in the heavy-fermion metal TiFe$_x$Cu$_{2x-1}$Sb, where frustration originates from intrinsic disorder. Specific heat and magnetic Grüneisen parameter measurements under varying magnetic fields exhibit quantum critical scaling, indicating a quantum critical point near 0.13 Tesla. As the magnetic field increases, the cluster spin-glass phase is progressively suppressed. Upon crossing the quantum critical point, resistivity and Hall effect measurements reveal enhanced screening of local moments and an expanding Fermi surface, consistent with the Kondo breakdown scenario. △ Less

Submitted 19 October, 2024; originally announced October 2024.

Comments: 18 pages, 4 figures, with Supplementary Information

Interstitial anionic electrons involved superconductivity and T-linear resistivity behavior in electride La3In

Authors: Zhijun Tu, Peihan Sun, Pengcheng Ma, Hongrun Zhen, Shangjie Tian, Shouguo Wang, Tian Cui, Zhonghao Liu, Kai Liu, Hechang Lei

Abstract: Electrides are unique materials because of the existence of interstitial anionic electrons (IAEs). Due to these loosely bound IAEs and their strong interaction with the framework of cations, electrides can host superconductivity with rather high Tc, especially under high pressure, as predicted in theory. However, the experimental observations of superconductivity in electrides are very rare, let a… ▽ More Electrides are unique materials because of the existence of interstitial anionic electrons (IAEs). Due to these loosely bound IAEs and their strong interaction with the framework of cations, electrides can host superconductivity with rather high Tc, especially under high pressure, as predicted in theory. However, the experimental observations of superconductivity in electrides are very rare, let alone the detailed studies on intrinsic properties of single crystals. Here, we report the superconducting and normal-state properties of electride La3In single crystals. La3In shows a type-II superconductivity with Tc ~ 9.4 K and a T-linear resistivity in a wide temperature range. Experimental measurements and theoretical calculations suggest that the relatively high Tc could be ascribed to the high density of states around the Fermi level caused by short flat bands along R-M direction and the strong electron-phonon coupling, partially derived from the IAEs. Meanwhile, the T-linear resistivity may reflect the significant electronic correlation effect in this material. These findings will shed light on understanding the role of IAEs in superconductivity and open a promising way to explore high-temperature superconductors in electrides. △ Less

Submitted 14 October, 2024; originally announced October 2024.

Comments: 7 pages, 4 figures

High proton conductivity through angstrom-porous titania

Authors: Y. Ji, G. -P. Hao, Y. -T. Tan, W. Q. Xiong, Y. Liu, W. Z. Zhou, D. -M. Tang, R. Z. Ma, S. J. Yuan, T. Sasaki, M. Lozada-Hidalgo, A. K. Geim, Pengzhan Sun

Abstract: Two dimensional (2D) crystals have attracted strong interest as a new class of proton conducting materials that can block atoms, molecules and ions while allowing proton transport through the atomically thin basal planes. Although 2D materials exhibit this perfect selectivity, the reported proton conductivities have been relatively low. Here we show that vacancy-rich titania monolayers are highly… ▽ More Two dimensional (2D) crystals have attracted strong interest as a new class of proton conducting materials that can block atoms, molecules and ions while allowing proton transport through the atomically thin basal planes. Although 2D materials exhibit this perfect selectivity, the reported proton conductivities have been relatively low. Here we show that vacancy-rich titania monolayers are highly permeable to protons while remaining impermeable to helium with proton conductivity exceeding 100 S cm-2 at 200 C and surpassing targets set by industry roadmaps. The fast and selective proton transport is attributed to an extremely high density of titanium-atom vacancies (one per square nm), which effectively turns titania monolayers into angstrom-scale sieves. Our findings highlight the potential of 2D oxides as membrane materials for hydrogen-based technologies. △ Less

Submitted 8 October, 2024; originally announced October 2024.

Journal ref: Nature Communications 15, 10546 (2024)

Mechanical properties and deformation mechanisms of the C14 Laves and μ-phase in the ternary Ta-Fe(-Al) system

Authors: Christina Gasper, Elif Merve Soysal, Nisa Ulumuddin, Tobias Stollenwerk, Tom Reclik, Pei-Ling Sun, Sandra Korte-Kerzel

Abstract: As structural and functional materials, topologically close-packed (TCP) phases of transition metal compounds offer a wide range of attractive properties. Due to their complex crystal structure and the resulting brittleness, the knowledge on their mechanical behaviour is still very limited, especially below the brittle-to-ductile transition temperature. In this study, we systematically analyse the… ▽ More As structural and functional materials, topologically close-packed (TCP) phases of transition metal compounds offer a wide range of attractive properties. Due to their complex crystal structure and the resulting brittleness, the knowledge on their mechanical behaviour is still very limited, especially below the brittle-to-ductile transition temperature. In this study, we systematically analyse the influence of composition and crystal structure on the mechanical properties and deformation mechanisms in the binary Ta-Fe system as well as in the ternary Ta-Fe-Al system as both systems contain a hexagonal C14 Laves and a mu-phase. We use nanoindentation, slip trace analysis and transmission electron microscopy to study the influence of crystal structure, composition and crystal orientation. The composition strongly influences the indentation modulus in the binary Ta-Fe system, showing a decreasing trend with increasing Ta content. The addition of Al, however, does not lead to a significant change of the mechanical properties of the ternary TCP phases. The investigation of the deformation mechanisms revealed that the Laves phase primarily deforms via non-basal slip while the basal plane is the favoured slip plane in the mu-phase. By partly replacing Fe with Al, the plasticity is not affected strongly, but the proportion of non-basal slip slightly increases for both ternary TCP phases compared to the binary ones. △ Less

Submitted 30 September, 2024; originally announced September 2024.

Fluid-network relations: decay laws meet with spatial self-similarity, scale-invariance, and control scaling

Authors: Yang Tian, Pei Sun, Yizhou Xu

Abstract: Diverse implicit structures of fluids are discovered lately, providing opportunities to study the physics of fluids applying network analysis. Although considerable works devote to identifying informative network structures of fluids, we have limited understanding about the information these networks convey about fluids. To analyze how fluid mechanics is embodied in network topology or vice versa,… ▽ More Diverse implicit structures of fluids are discovered lately, providing opportunities to study the physics of fluids applying network analysis. Although considerable works devote to identifying informative network structures of fluids, we have limited understanding about the information these networks convey about fluids. To analyze how fluid mechanics is embodied in network topology or vice versa, we reveal a set of fluid-network relations that quantify the interactions between fundamental fluid properties (e.g., kinetic energy and enstrophy decay laws) and defining network characteristics (e.g., spatial self-similarity, scale-invariance, and control scaling). By analyzing spatial self-similarity in classic and generalized contexts, we first assess the self-similarity of vortical interactions in fluid flows. Deviations from self-similarity in networks exhibit power-law scaling behaviors with respect to fluid properties, suggesting the diversity among vortex as essential to self-similar fluid flows. Then, the same paradigm is adopted to investigate scale-invariance using renormalization groups, which reveals that the breaking extents of scale-invariance in networks, similar to those of spatial self-similarity, also scale with fluid properties in power-law manners. Furthermore, we define a control problem on networks to study the propagation of perturbations through vortical interactions over different ranges. The minimum cost of controlling vortical networks exponentially scales with range diameters (i.e., control distances), whose growth rates experiences temporal decays. We show that this temporal decay speed is fully determined by fluid properties in power-law scaling behaviours. In summary, these fluid-network relations enable a deeper understanding of implicit fluid structures and their interactions with fluid dynamics. △ Less

Submitted 23 August, 2024; originally announced August 2024.

Dynamics of Nanoscale Phase Decomposition in Laser Ablation

Authors: Yanwen Sun, Chaobo Chen, Thies J. Albert, Haoyuan Li, Mikhail I. Arefev, Ying Chen, Mike Dunne, James M. Glownia, Matthias Hoffmann, Matthew J. Hurley, Mianzhen Mo, Quynh L. Nguyen, Takahiro Sato, Sanghoon Song, Peihao Sun, Mark Sutton, Samuel Teitelbaum, Antonios S. Valavanis, Nan Wang, Diling Zhu, Leonid V. Zhigilei, Klaus Sokolowski-Tinten

Abstract: Femtosecond laser ablation is a process that bears both fundamental physics interest and has wide industrial applications. For decades, the lack of probes on the relevant time and length scales has prevented access to the highly nonequilibrium phase decomposition processes triggered by laser excitation. Enabled by the unprecedented intense femtosecond X-ray pulses delivered by an X-ray free electr… ▽ More Femtosecond laser ablation is a process that bears both fundamental physics interest and has wide industrial applications. For decades, the lack of probes on the relevant time and length scales has prevented access to the highly nonequilibrium phase decomposition processes triggered by laser excitation. Enabled by the unprecedented intense femtosecond X-ray pulses delivered by an X-ray free electron laser, we report here results of time-resolved small angle scattering measurements on the dynamics of nanoscale phase decomposition in thin gold films upon femtosecond laser-induced ablation. By analyzing the features imprinted onto the small angle diffraction patterns, the transient heterogeneous density distributions within the ablation plume as obtained from molecular dynamics simulations get direct experimental confirmation. △ Less

Submitted 15 July, 2024; originally announced July 2024.

Comments: Main manuscript with 32 pages incl. 9 figures + supplementary materials with 16 pages incl. 5 figures

Quantum Analog of Vicsek Model for Active Matter

Authors: Hong Yuan, L. X. Cui, L. T. Chen, C. P. Sun

Abstract: We propose a quantum model consisting of an ensemble of overdamped spin$-1/2$ particles with ferromagnetic couplings, driven by a radially homogeneous magnetic field. The spontaneous magnetization of the spin components breaks the $SO(3)$ (or $SO(2)$) symmetry, inducing an ordered phase of flocking. Our model converges to the Vicsek model in the classical limit and corresponds to the Toner-Tu mode… ▽ More We propose a quantum model consisting of an ensemble of overdamped spin$-1/2$ particles with ferromagnetic couplings, driven by a radially homogeneous magnetic field. The spontaneous magnetization of the spin components breaks the $SO(3)$ (or $SO(2)$) symmetry, inducing an ordered phase of flocking. Our model converges to the Vicsek model in the classical limit and corresponds to the Toner-Tu model in the continuous limit. Our investigation not only elucidates the intrinsic connection between these two models, but also introduces new opportunities for exploring the mechanisms underlying flocking order and correlations at the quantum level, which maybe pave the way for a new field of research -- the quantum active matter. △ Less

Submitted 15 January, 2026; v1 submitted 13 July, 2024; originally announced July 2024.

Quantum Thermodynamic Integrability for Canonical and non-Canonical Statistics

Authors: Ruo-Xun Zhai, C. P. Sun

Abstract: We extend the Carathéodory principle of the Second Law to quantum thermodynamics with energy levels depending on macroscopic variables, such as volume and magnetic field. This extension introduces the concept of Quantum Thermodynamic Integrability (QTI), offering an alternative foundation for statistical mechanics. QTI is characterized by the path-independence of work and heat within the thermodyn… ▽ More We extend the Carathéodory principle of the Second Law to quantum thermodynamics with energy levels depending on macroscopic variables, such as volume and magnetic field. This extension introduces the concept of Quantum Thermodynamic Integrability (QTI), offering an alternative foundation for statistical mechanics. QTI is characterized by the path-independence of work and heat within the thermodynamic manifold, which is locally described by energy levels and specific thermodynamic parameters. Within this framework, temperature naturally emerges as an integrating factor, allowing for the derivation of both canonical and non-canonical states from the Entropy Integrable Equations (EIE) based on QTI. Notably, non-canonical states, which become particularly significant outside the thermodynamic limit, reveal the existence of informational correlations in finite-size thermodynamic systems. △ Less

Submitted 8 September, 2025; v1 submitted 11 July, 2024; originally announced July 2024.

Comments: 10 pages, 3 figures

Feynman Paradox about the Josephson effect and a sawtooth current in the double junction

Authors: Zhi-Lei Zhang, Guo-Jian Qiao, C. P. Sun

Abstract: We revisit the Feynman approach to the Josephson effect, which employs a pair of linear coupling equations for its modeling. It is found that while the exact solutions can account for the AC Josephson effect when the coupling strength is significantly less than the voltage, they fail to produce the DC Josephson effect in any practical scenario. To address this fundamental discrepancy, we derive th… ▽ More We revisit the Feynman approach to the Josephson effect, which employs a pair of linear coupling equations for its modeling. It is found that while the exact solutions can account for the AC Josephson effect when the coupling strength is significantly less than the voltage, they fail to produce the DC Josephson effect in any practical scenario. To address this fundamental discrepancy, we derive the coupled Ginzburg-Landau (GL) equations for two interconnected superconductors based on BCS theory. These equations reveal that the nonlinear coupling, which is overlooked in the Feynman method, is crucial in describing the spontaneous symmetry breaking in superconductors, a critical factor for achieving the DC Josephson effect. When the coupled GL equations are applied to a double junction, a sawtooth current pattern emerges, a result unattainable via the Feynman approach. △ Less

Submitted 18 May, 2024; v1 submitted 13 May, 2024; originally announced May 2024.

Comments: 9 pages, 3 figures

Double Magnon-Roton Excitations in the Triangular-Lattice Spin Supersolid

Authors: Yuan Gao, Chuandi Zhang, Junsen Xiang, Dehong Yu, Xingye Lu, Peijie Sun, Wentao Jin, Gang Su, Wei Li

Abstract: Supersolid is an exotic quantum state of matter that spontaneously hosts the features of both solid and superfluid, which breaks the translation and U(1) gauge symmetries. Here we study the spin dynamics in the triangular-lattice compound Na$_2$BaCo(PO$_4$)$_2$, which is revealed in [Xiang et al., Nature 625, 270-275 (2024)] as a quantum magnetic analog of supersolid. We simulate the easy-axis Hei… ▽ More Supersolid is an exotic quantum state of matter that spontaneously hosts the features of both solid and superfluid, which breaks the translation and U(1) gauge symmetries. Here we study the spin dynamics in the triangular-lattice compound Na$_2$BaCo(PO$_4$)$_2$, which is revealed in [Xiang et al., Nature 625, 270-275 (2024)] as a quantum magnetic analog of supersolid. We simulate the easy-axis Heisenberg model with tensor network approach and uncover unique dynamic traits. These features are manifested in two branches of excitations that can be associated with the spin solidity and superfluidity, respectively. One branch contains the U(1) Goldstone and roton modes, while the other comprises pseudo-Goldstone and roton modes. The gapless Goldstone modes of the in-plane superfluid order are confirmed by our inelastic neutron scattering measurements. Together with the evident out-of-plane solid order indicated by the magnetic Bragg peaks, our findings provide spectroscopic evidence for spin supersolidity in this easy-axis antiferromagnet. Akin to the role of phonon-roton modes -- Landau elementary excitations -- in shaping the helium superfluid thermodynamics, the intriguing double magnon-roton dispersion here determines the low-temperature thermodynamics of spin supersolid down to sub-Kelvin regime, explaining the recently observed giant magnetocaloric effect in Na$_2$BaCo(PO$_4$)$_2$. △ Less

Submitted 5 December, 2024; v1 submitted 24 April, 2024; originally announced April 2024.

Comments: 8+9 pages, 4+9 figures

Journal ref: Physical Review B 110, 214408 (2024)

Dressed Majorana fermion in a hybrid nanowire

Authors: Guo-Jian Qiao, Xin Yue, C. P. Sun

Abstract: The low-energy theory of hybrid nanowire systems fails to define Majorana fermion (MF) in the strong tunneling and magnetic field strength. To address this limitation, we propose a holistic approach to define MF in which the quasi-excitation in nanowire and superconductor constitutes together its own ``antiparticles''. This definition is general, beyond the constraint presented in the low-energy t… ▽ More The low-energy theory of hybrid nanowire systems fails to define Majorana fermion (MF) in the strong tunneling and magnetic field strength. To address this limitation, we propose a holistic approach to define MF in which the quasi-excitation in nanowire and superconductor constitutes together its own ``antiparticles''. This definition is general, beyond the constraint presented in the low-energy theory. It reveals that the Majorana phase depends not only on the chemical potential and Zeeman energy in nanowire but also on those of superconductor, and that the mismatch of chemical potential leads not to observe MF. Such a broader perspective provides more specific experimental guidance under various conditions △ Less

Submitted 8 April, 2024; originally announced April 2024.

Journal ref: Physical Review Letters 133, 266605 (2024)

Structural, magnetic and magnetocaloric properties of triangular-lattice transition-metal phosphates

Authors: Chuandi Zhang, Junsen Xiang, Quanliang Zhu, Longfei Wu, Shanfeng Zhang, Juping Xu, Wen Yin, Peijie Sun, Wei Li, Gang Su, Wentao Jin

Abstract: The recent discovery of the spin supersolid candidate Na$_2$BaCo(PO$_4$)$_2$ stimulates numerous research interest on the triangular-lattice transition-metal phosphates. Here we report a comprehensive study on the structural, magnetic and magnetocaloric properties of polycrystalline Na$_2$$A$$T$(PO$_4$)$_2$ ($A$ = Ba, Sr; $T$ = Co, Ni, Mn). X-ray and neutron diffraction measurements confirm that N… ▽ More The recent discovery of the spin supersolid candidate Na$_2$BaCo(PO$_4$)$_2$ stimulates numerous research interest on the triangular-lattice transition-metal phosphates. Here we report a comprehensive study on the structural, magnetic and magnetocaloric properties of polycrystalline Na$_2$$A$$T$(PO$_4$)$_2$ ($A$ = Ba, Sr; $T$ = Co, Ni, Mn). X-ray and neutron diffraction measurements confirm that Na$_2$Ba$T$(PO$_4$)$_2$ (NB$T$P) crystallizes in a trigonal structure, while Na$_2$Sr$T$(PO$_4$)$_2$ (NS$T$P) forms a monoclinic structure with a slight distortion of the triangular network of $T^{2+}$ ions. The dc magnetization data show that all six compounds order antiferromagnetically below 2 K, and the Néel temperatures of NS$T$P are consistently higher than those of NB$T$P for $T$ = Co, Ni, and Mn, due to the release of geometrical frustration by monoclinic distortions. Further magnetocaloric measurements show that trigonal NB$T$P can reach a lower temperature in the quasi-adiabatic demagnetization process and thus shows a better performance in the magnetic refrigeration, compared with monoclinic NS$T$P. Our findings highlight the outstanding magnetocaloric performances of the trigonal transition-metal phosphates, and disclose two necessary ingredients for a superior magnetic coolant that can reach an ultra-low temperature, including a perfect geometrically frustrated lattice and a small effective spin number associated with the magnetic ions. △ Less

Submitted 1 April, 2024; originally announced April 2024.

Comments: 10 Pages, 6 figures, accepted for publication in Physical Review Materials

Journal ref: Physical Review Materials 8, 044409 (2024)

Influence of chemical composition on the room temperature plas-ticity of C15 Ca-Al-Mg Laves phases

Authors: Martina Freund, Zhuocheng Xie, Pei-Ling Sun, Lukas Berners, Joshua Spille, Hexin Wan, Carsten Thomas, Michael Feuerbacher, Marta Lipinska-Chwalek, Joachim Mayer, Sandra Korte-Kerzel

Abstract: The influence of chemical composition changes on the room temperature mechanical proper-ties in the C15 CaAl2 Laves phase were investigated in two off-stoichiometric compositions with 5.7 at.-% Mg addition (Ca33Al61Mg6) and 10.8 at.-% Mg and 3.0 at.-% Ca addition (Ca36Al53Mg11) and compared to the stoichiometric (Ca33Al67) composition. Cubic Ca-Al-Mg Laves phases with multiple crystallographic ori… ▽ More The influence of chemical composition changes on the room temperature mechanical proper-ties in the C15 CaAl2 Laves phase were investigated in two off-stoichiometric compositions with 5.7 at.-% Mg addition (Ca33Al61Mg6) and 10.8 at.-% Mg and 3.0 at.-% Ca addition (Ca36Al53Mg11) and compared to the stoichiometric (Ca33Al67) composition. Cubic Ca-Al-Mg Laves phases with multiple crystallographic orientations were characterised and deformed using nanoindentation. The hardness and indentation modulus were measured to be 4.1 +- 0.3 GPa and 71.3 +- 1.5 GPa for Ca36Al53Mg11, 4.6 +- 0.2 GPa and 80.4 +- 3.8 GPa for Ca33Al61Mg6 and 4.9 +- 0.3 GPa and 85.5 +- 4.0 GPa for Ca33Al67, respectively. The resulting surface traces as well as slip and crack planes, were distinguished on the indentation surfac-es, revealing the activation of several different {11n} slip systems, as further confirmed by conventional transmission electron microscopic observations. Additionally, the deformation mechanisms and corresponding energy barriers of activated slip systems were evaluated by atomistic simulations. △ Less

Submitted 20 March, 2024; originally announced March 2024.

Nanoscale brittle-to-ductile transition of the C15 CaAl$_2$ Laves phase

Authors: Anwesha Kanjilal, Ali Ahmadian, Martina Freund, Pei-Ling Sun, Sandra Korte-Kerzel, Gerhard Dehm, James P. Best

Abstract: The influence of temperature on the deformation behaviour of the C15 CaAl$_2$ Laves phase, a key constituent for enhancing the mechanical properties of Mg alloys up to service temperatures of 200 °C, remains largely unexplored. This study presents, for the first time, the nanoscale brittle-to-ductile transition (BDT) of this intermetallic phase through in situ testing including nanoindentation, sc… ▽ More The influence of temperature on the deformation behaviour of the C15 CaAl$_2$ Laves phase, a key constituent for enhancing the mechanical properties of Mg alloys up to service temperatures of 200 °C, remains largely unexplored. This study presents, for the first time, the nanoscale brittle-to-ductile transition (BDT) of this intermetallic phase through in situ testing including nanoindentation, scratch testing, and micropillar splitting conducted at elevated temperatures. By correlating observations from these techniques, changes in deformation of CaAl$_2$ were identified in relation to temperature. High-temperature nanoindentation quantitatively determined the temperature range for the BDT, and revealed that CaAl$_2$ undergoes a BDT at ~0.55T$_m$, exhibiting an intermediate region of microplasticity. A noticeable decrease in nanoindentation hardness was observed at ~450-500 °C, accompanied by an increase in residual indent size, while indentation cracking was not observed above 300 °C. Results from high-temperature micropillar splitting revealed cracking and brittle pillar splitting up to 300 °C, with an increase in apparent fracture toughness from 0.9 $\pm$ 0.1 MPa$\cdot\sqrt m$ to 2.8 $\pm$ 0.3 MPa$\cdot\sqrt m$, and subsequent crack-free plastic deformation from 400 °C. Transmission electron microscopy analysis of the deformed material from nanoindentation revealed that the BDT of CaAl$_2$ may be attributed to enhanced dislocation plasticity with increasing temperature. △ Less

Submitted 19 March, 2024; originally announced March 2024.

Fast renormalizing the structures and dynamics of ultra-large systems via random renormalization group

Authors: Yang Tian, Yizhou Xu, Pei Sun

Abstract: Criticality and symmetry, studied by the renormalization groups, lie at the heart of modern physics theories of matters and complex systems. However, surveying these properties with massive experimental data is bottlenecked by the intolerable costs of computing renormalization groups on real systems. Here, we develop a time- and memory-efficient framework, termed as the random renormalization grou… ▽ More Criticality and symmetry, studied by the renormalization groups, lie at the heart of modern physics theories of matters and complex systems. However, surveying these properties with massive experimental data is bottlenecked by the intolerable costs of computing renormalization groups on real systems. Here, we develop a time- and memory-efficient framework, termed as the random renormalization group, for renormalizing ultra-large systems (e.g., with millions of units) within minutes. This framework is based on random projections, hashing techniques, and kernel representations, which support the renormalization governed by linear and non-linear correlations. For system structures, it exploits the correlations among local topology in kernel spaces to unfold the connectivity of units, identify intrinsic system scales, and verify the existences of symmetries under scale transformation. For system dynamics, it renormalizes units into correlated clusters to analyze scaling behaviours, validate scaling relations, and investigate potential criticality. Benefiting from hashing-function-based designs, our framework significantly reduces computational complexity compared with classic renormalization groups, realizing a single-step acceleration of two orders of magnitude. Meanwhile, the efficient representation of different kinds of correlations in kernel spaces realized by random projections ensures the capacity of our framework to capture diverse unit relations. As shown by our experiments, the random renormalization group helps identify non-equilibrium phase transitions, criticality, and symmetry in diverse large-scale genetic, neural, material, social, and cosmological systems. △ Less

Submitted 29 January, 2024; originally announced January 2024.

Hard X-ray Generation and Detection of Nanometer-Scale Localized Coherent Acoustic Wave Packets in SrTiO$_3$ and KTaO$_3$

Authors: Yijing Huang, Peihao Sun, Samuel W. Teitelbaum, Haoyuan Li, Yanwen Sun, Nan Wang, Sanghoon Song, Takahiro Sato, Matthieu Chollet, Taito Osaka, Ichiro Inoue, Ryan A. Duncan, Hyun D. Shin, Johann Haber, Jinjian Zhou, Marco Bernardi, Mingqiang Gu, James M. Rondinelli, Mariano Trigo, Makina Yabashi, Alexei A. Maznev, Keith A. Nelson, Diling Zhu, David A. Reis

Abstract: We demonstrate that the absorption of femtosecond x-ray pulses can excite quasi-spherical high-wavevector coherent acoustic phonon wavepackets using an all x-ray pump and probe scattering experiment. The time- and momentum-resolved diffuse scattering signal is consistent with strain pulses induced by the rapid electron cascade dynamics following photoionization at uncorrelated excitation centers.… ▽ More We demonstrate that the absorption of femtosecond x-ray pulses can excite quasi-spherical high-wavevector coherent acoustic phonon wavepackets using an all x-ray pump and probe scattering experiment. The time- and momentum-resolved diffuse scattering signal is consistent with strain pulses induced by the rapid electron cascade dynamics following photoionization at uncorrelated excitation centers. We quantify key parameters of this process, including the localization size of the strain wavepacket and the energy absorption efficiency, which are determined by the photoelectron and Auger electron cascade dynamics, as well as the electron-phonon interaction. In particular, we obtain the localization size of the observed strain wave packet to be 1.5 and 2.5 nm for bulk SrTiO$_3$ and KTaO$_3$ single crystals, even though there are no nanoscale structures or light-intensity patterns that would ordinarily be required to generate acoustic waves of wavelengths much shorter than the penetration depth. Whereas in GaAs and GaP we do not observe a signal above background. The results provide crucial information on x-ray matter interactions, which sheds light on the mechanism of x-ray energy deposition, and the study of high wavevector acoustic phonons and thermal transport at the nanoscale. △ Less

Submitted 2 January, 2024; v1 submitted 27 December, 2023; originally announced December 2023.

Directly observing atomic-scale relaxations of a glass forming liquid using femtosecond X-ray photon correlation spectroscopy

Authors: Tomoki Fujita, Yanwen Sun, Haoyuan Li, Thies J. Albert, Sanghoon Song, Takahiro Sato, Jens Moesgaard, Antoine Cornet, Peihao Sun, Ying Chen, Mianzhen Mo, Narges Amini, Fan Yang, Arune Makareviciute, Garrett Coleman, Pierre Lucas, Jan Peter Embs, Vincent Esposito, Joan Vila-Comamala, Nan Wang, Talgat Mamyrbayev, Christian David, Jerome Hastings, Beatrice Ruta, Paul Fuoss , et al. (3 additional authors not shown)

Abstract: Glass forming liquids exhibit structural relaxation behaviors, reflecting underlying atomic rearrangements on a wide range of timescales. These behaviors play a crucial role in determining many material properties. However, the relaxation processes on the atomic scale are not well understood due to the experimental difficulties in directly characterizing the evolving correlations of atomic order i… ▽ More Glass forming liquids exhibit structural relaxation behaviors, reflecting underlying atomic rearrangements on a wide range of timescales. These behaviors play a crucial role in determining many material properties. However, the relaxation processes on the atomic scale are not well understood due to the experimental difficulties in directly characterizing the evolving correlations of atomic order in disordered systems. Here, taking the model system Ge15Te85, we demonstrate an experimental approach that probes the relaxation dynamics by scattering the coherent X-ray pulses with femtosecond duration produced by X-ray free electron lasers (XFELs). By collecting the summed speckle patterns from two rapidly successive, nearly identical X-ray pulses generated using a split-delay system, we can extract the contrast decay of speckle patterns originating from sample dynamics and observe the full decorrelation of local order on the sub-picosecond timescale. This provides the direct atomic-level evidence of fragile liquid behavior of Ge15Te85. Our results demonstrate the strategy for XFEL-based X-ray photon correlation spectroscopy (XPCS), attaining femtosecond temporal and atomic-scale spatial resolutions. This twelve orders of magnitude extension from the millisecond regime of synchrotron-based XPCS opens a new avenue of experimental studies of relaxation dynamics in liquids, glasses, and other highly disordered systems. △ Less

Submitted 8 June, 2024; v1 submitted 13 December, 2023; originally announced December 2023.

Proton and molecular permeation through the basal plane of monolayer graphene oxide

Authors: Z. F. Wu, P. Z. Sun, O. J. Wahab, Y. -T. Tao, D. Barry, D. Periyanagounder, P. B. Pillai, Q. Dai, W. Q. Xiong, L. F. Vega, K. Lulla, S. J. Yuan, R. R. Nair, E. Daviddi, P. R. Unwin, A. K. Geim, M. Lozada-Hidalgo

Abstract: Two-dimensional (2D) materials offer a prospect of membranes that combine negligible gas permeability with high proton conductivity and could outperform the existing proton exchange membranes used in various applications including fuel cells. Graphene oxide (GO), a well-known 2D material, facilitates rapid proton transport along its basal plane but proton conductivity across it remains unknown. It… ▽ More Two-dimensional (2D) materials offer a prospect of membranes that combine negligible gas permeability with high proton conductivity and could outperform the existing proton exchange membranes used in various applications including fuel cells. Graphene oxide (GO), a well-known 2D material, facilitates rapid proton transport along its basal plane but proton conductivity across it remains unknown. It is also often presumed that individual GO monolayers contain a large density of nanoscale pinholes that lead to considerable gas leakage across the GO basal plane. Here we show that relatively large, micrometer-scale areas of monolayer GO are impermeable to gases, including helium, while exhibiting proton conductivity through the basal plane which is nearly two orders of magnitude higher than that of graphene. These findings provide insights into the key properties of GO and demonstrate that chemical functionalization of 2D crystals can be utilized to enhance their proton transparency without compromising gas impermeability. △ Less

Submitted 25 October, 2023; originally announced October 2023.

Report number: 14, 7756 (2023)

Journal ref: Nature Communications 2023

X-type stacking in cross-chain antiferromagnets

Authors: Shui-Sen Zhang, Zi-An Wang, Bo Li, Yuan-Yuan Jiang, Shu-Hui Zhang, Rui-Chun Xiao, Lan-Xin Liu, X. Luo, W. J. Lu, Mingliang Tian, Y. P. Sun, Evgeny Y. Tsymbal, Haifeng Du, Ding-Fu Shao

Abstract: Physical phenomena in condensed matter normally arise from the collective effect of all atoms, while selectively addressing a lone atomic sublattice by external stimulus is elusive. The later functionality may, however, benefit various applications, as the response may differ when the external stimulus affects only a specific sublattice rather than the entire solid. Here, we introduce cross-chain… ▽ More Physical phenomena in condensed matter normally arise from the collective effect of all atoms, while selectively addressing a lone atomic sublattice by external stimulus is elusive. The later functionality may, however, benefit various applications, as the response may differ when the external stimulus affects only a specific sublattice rather than the entire solid. Here, we introduce cross-chain antiferromagnets, where the stacking of two magnetic sublattices forms a pattern of intersecting atomic chains, allowing for the sublattice selectivity. We dub this antiferromagnetic (AFM) stacking X-type and demonstrate that it exhibits unique spin-dependent transport properties not present in conventional magnets. Through high-throughput analyses and computations, we unveil three prototypes of X-type AFM stacking and identify 15 candidate candidates. Using $β$-Fe$_{2}$PO$_{5}$ as a representative X-type antiferromagnet, we predict sublattice-selective spin-polarized transport driven by the X-type stacking, where one magnetic sublattice conducts, while the other does not. Consequently, a spin torque can be exerted solely on a single sublattice, leading to unconventional ultrafast dynamics of the Nèel vector capable of deterministic switching of the AFM domains. Our work uncovers a previously overlooked type of magnetic moment stacking and reveals sublattice-selective physical properties promising for high-performance spintronic applications. △ Less

Submitted 14 April, 2025; v1 submitted 20 October, 2023; originally announced October 2023.

Comments: Published version

Journal ref: Newton 1, 100068 (2025)

Structural pathways for ultrafast melting of optically excited thin polycrystalline Palladium films

Authors: Jerzy Antonowicz, Adam Olczak, Klaus Sokolowski-Tinten, Peter Zalden, Igor Milov, Przemysław Dzięgielewski, Christian Bressler, Henry N. Chapman, Michał Chojnacki, Piotr Dłużewski, Angel Rodriguez-Fernandez, Krzysztof Fronc, Wojciech Gawełda, Konstantinos Georgarakis, Alan L. Greer, Iwanna Jacyna, Robbert W. E. van de Kruijs, Radosław Kamiński, Dmitry Khakhulin, Dorota Klinger, Katarzyna M. Kosyl, Katharina Kubicek, Kirill P. Migdal, Roman Minikayev, Nikolaos T. Panagiotopoulos , et al. (6 additional authors not shown)

Abstract: Due to its extremely short timescale, the non-equilibrium melting of metals is exceptionally difficult to probe experimentally. The knowledge of melting mechanisms is thus based mainly on the results of theoretical predictions. This work reports on the investigation of ultrafast melting of thin polycrystalline Pd films studied by optical laser pump - X-ray free-electron laser probe experiments and… ▽ More Due to its extremely short timescale, the non-equilibrium melting of metals is exceptionally difficult to probe experimentally. The knowledge of melting mechanisms is thus based mainly on the results of theoretical predictions. This work reports on the investigation of ultrafast melting of thin polycrystalline Pd films studied by optical laser pump - X-ray free-electron laser probe experiments and molecular-dynamics simulations. By acquiring X-ray diffraction snapshots with sub-picosecond resolution, we capture the sample's atomic structure during its transition from the crystalline to the liquid state. Bridging the timescales of experiments and simulations allows us to formulate a realistic microscopic picture of melting. We demonstrate that the existing models of strongly non-equilibrium melting, developed for systems with relatively weak electron-phonon coupling, remain valid even for ultrafast heating rates achieved in femtosecond laser-excited Pd. Furthermore, we highlight the role of pre-existing and transiently generated crystal defects in the transition to the liquid state. △ Less

Submitted 19 September, 2023; originally announced September 2023.

Comments: main manuscript 33 pages, 9 figures; supplemental material 19 pages, 13 figures - all in one file

Prediction of Giant Tunneling Magnetoresistance in RuO$_{2}$/TiO$_{2}$/RuO$_{2}$ (110) Antiferromagnetic Tunnel Junctions

Authors: Yuan-Yuan Jiang, Zi-An Wang, Kartik Samanta, Shu-Hui Zhang, Rui-Chun Xiao, W. J. Lu, Y. P. Sun, Evgeny Y. Tsymbal, Ding-Fu Shao

Abstract: Using first-principles quantum-transport calculations, we investigate spin-dependent electronic and transport properties of antiferromagnetic tunnel junctions (AFMTJs) that consist of (110)-oriented antiferromagnetic (AFM) metal RuO$_{2}$ electrodes and an insulating TiO$_{2}$ tunneling barrier. We predict the emergence of a giant tunneling magnetoresistance (TMR) effect in a wide energy window, a… ▽ More Using first-principles quantum-transport calculations, we investigate spin-dependent electronic and transport properties of antiferromagnetic tunnel junctions (AFMTJs) that consist of (110)-oriented antiferromagnetic (AFM) metal RuO$_{2}$ electrodes and an insulating TiO$_{2}$ tunneling barrier. We predict the emergence of a giant tunneling magnetoresistance (TMR) effect in a wide energy window, a series of barrier layer thicknesses, and different interface terminations, indicating the robustness of this effect. We show that the predicted TMR cannot be explained in terms of the global transport spin-polarization of RuO$_{2}$ (110) but is well understood based on matching the momentum-dependent spin-polarized conduction channels of the two RuO$_{2}$ (110) electrodes. We predict oscillations of TMR with increasing barrier thickness, indicating a non-negligible contribution from the perfectly epitaxial interfaces. Our work helps the understanding of the physics of TMR in AFMTJs and aids in realizing efficient AFM spintronic devices. △ Less

Submitted 8 November, 2023; v1 submitted 5 September, 2023; originally announced September 2023.

Comments: The colorbar of originl Fig. 2a was not set properly, which is corrected in this version. To be published in PRB