Temperature Gradient Research Articles

Microstructure and properties of electroless Ni–P/Sn2.5Ag0.7Cu0.1RE micro-joints during thermomigration

The Ni–P stratum was fabricated upon the Cu substrate via an electroless plating technique, and the microstructure and properties of electroless Ni–P/Sn2.5Ag0.7Cu0.1RE micro-joints under temperature gradient was studied. Research indicates that in the initial stage of thermomigration in micro-solder joints, the intermetallic compound (IMC) in the Ni–P/soldering seam transition area appears as both “needle-shaped” and “block-shaped” (Ni, Cu)3Sn4, with an average thickness of 1.1–1.5 μm. Additionally, between Cu and (Ni, Cu)3Sn4 IMC, there exists a 0.8 μm thick “layered” Ni3P within the Ni–P layer. The temperature gradient causes the asymmetric growth of (Ni, Cu)3Sn4 IMC and the asymmetric evolution of the Ni–P layer at the hot and cold ends of the micro-solder joint. The Ni–P layer evolution is divided into two stages: Ni–P → Ni3P + Ni and Ni3P + Sn → Ni–Sn–P, and the cold end structure evolves faster than the hot end. After 60 h under the temperature gradient condition of 550 °C/cm, the shear fracture position of the micro-solder joint shifts from the soldering seam to the Ni–Sn–P/IMC layer junction, and fracture mode changes from ductile fracture dominated by dimples to brittle fracture dominated by cleavage and slip steps, corresponding to a decrease of 21.8% in micro-solder joint pushing shear force from 16N.

Full-Scale Simulation and Experimental Study of Heat Transfer in Landing Gear Brake Discs for Medium-Sized Passenger Aircraft

Aircraft brake discs undergo rapid temperature rises during braking, critically impacting safety, lifespan, and adjacent components. Therefore, it is particularly important to study the heat transfer mechanism during the braking process and predict the temperature distribution of the brake disc. To address challenges in experimental studies (e.g., high costs and extreme conditions), this study employs full-scale numerical simulations to investigate heat transfer behaviours in medium-sized passenger aircraft brake discs. In the numerical simulation process, a coupling model that comprehensively considers the friction heat generation of the brake disc, the solid heat transfer between the discs, and the heat dissipation of the outer surface of the disc and the surrounding air is constructed to accurately describe the heat transfer characteristics of the brake disc under dynamic conditions. The study shows that the surface temperature of the brake disc rises sharply during the braking process, resulting in a significant increase in the temperature gradient; at the same time, the surrounding air flow state significantly affects the heat dissipation efficiency of the brake disc and affects its temperature distribution. Finally, the effectiveness of the numerical simulation was verified by experiments, and the maximum relative error between the experimental results and the simulation results was about 4.5%. This study provides a research basis for optimizing the structural design of the brake disc, improving its heat dissipation performance and operating safety.

Efficient Photothermoelectric Conversion of CSS@BP/Bi2Te3 Array for Innovative Aircraft Attitude Recognition.

The realization of fast, simple and efficient flight attitude recognition is crucial for flight safety and control stability, but still faces challenges in new materials and technologies. Herein, a chloroplast-like selenium-doped copper sulfide@black phosphorus (CSS@BP) composite material is prepared by ultrasonic chemical synthesis using BP nanosheets to effectively absorb light energy and disperse CSS layers to promote rapid photothermal conversion, which shows the temperature change more than ≈40°C and an excellent photothermal conversion efficiency of 68.9% at 405nm, corresponding to the theoretical calculation results. Moreover, the CSS@BP/Bi2Te3 photothermoelectric conversion array prepared by pulsed laser deposition coated Bi2Te3 thermoelectric layer and laminated porous insulating polyimide film can generate rapid thermal current changes through irradiated/non-irradiated thermal gradients. Hence, a portable attitude recognition box (ARB) is assembled with a based CSS@BP/Bi2Te3 array with a self-balancing laser and a current measurement chip that enables accurate attitude recognition through the bidirectional current generated by changes of irradiated area. Excitably, the ARB demonstrates over 86.47% accuracy without complex algorithms, showing excellent stability and robustness. Thus, this work offers an innovative solution for advancing photothermal materials and low-cost high-precision flight attitude sensing technologies.

Effect of Laser Energy Density on Temperature Field, Forming Quality, and Performance of LPBF-Fabricated 1 Nickel-Based Superalloy Composites

Abstract Laser energy density plays a crucial role in determining the forming quality, microstructure, and mechanical properties of components fabricated by Laser Powder Bed Fusion (LPBF). 0.2 wt.% hexagonal boron nitride (h-BN) reinforced Hastelloy X (HX) composites have been proven to eliminate cracks by regulating laser absorption behavior and the temperature field, thereby reducing the temperature gradient and carbide segregation. This approach synergistically enhanced both the strength and elongation of HX formed via LPBF. However, the lack of process optimization studies for this system has hindered its broader industrial application. This study investigated the effects of varying laser energy densities on 0.2 wt.% h-BN/HX composites and identified the optimal laser processing parameter. Experimental results showed that an optimal laser energy density of 41.67 J/mm3 resulted in the best microstructure, characterized by fine grains (10.97 µm), high densification (99.81%), and low surface roughness (Sa = 4.68 µm). The mechanical properties, including ultimate tensile strength (1259.18 MPa) and elongation (18.26%), were also maximized at this energy density. Additionally, the optimal energy density improved microstructural uniformity, dislocation density (KAM = 0.64), and texture strength (Taylor factor = 3.18). This study provides valuable insights into the influence of laser energy density on thermal behavior, forming quality, and mechanical performance of LPBF-manufactured 0.2 wt.% h-BN/HX, offering guidance for optimizing LPBF processing parameters and enhancing the performance of nickel-based composites in advanced engineering applications.

Low frequency m = 1 modes during standard and improved confinement scenarios in W7-X

Abstract In the Wendelstein 7-X stellarator (W7-X), two related global low-frequency electromagnetic oscillations are observed on many diagnostic systems (e.g. plasma stored energy, Mirnov coils, Langmuir probes, etc). The activity is related to the presence of magnetic islands at certain radii. However, in contrast to initial analyses in earlier works, the mode activity is not localized at the magnetic islands, but rather of an m = 1 type in the plasma outer-core. We observe a strong cross-correlation between electron temperature fluctuations in the confined plasma region, measured with electron cyclotron emission radiometer, and scrape-off layer fluctuations measured with electric probes. Two limiting cases of the mode activity can be observed based on magnetic configuration. A quasi-continuous (QC) mode case, with frequencies of typically some 100 Hz , is found in configurations that have a 5 / 5 magnetic island chain at the last closed flux surface (LCFS). The mode frequency is found to increase linearly with the core electron temperature and decrease with increasing electron density. In contrast, for magnetic configurations with the 5 / 5 magnetic island chain just inside the LCFS, the mode activity becomes bursty and intermittent, exhibiting a crash-event like signature. Although these intermittent events are less frequent, more plasma stored energy is lost per event compared to a period of the QC oscillations. The intermittent oscillations are observable as a broadband modulation in turbulence spectra on most diagnostic systems. The presence of these intermittent bursts goes along with an improved energy confinement in the plasma, superficially resembling H-mode and Edge Localized Modes in tokamaks. A self-limiting process is proposed as an explanation of the intermittent oscillations based on observed ejection of particles during each crash event, which leads to steepening of density profiles and temporary suppression of ion temperature gradient turbulence and in turn suppression of turbulence driven m = 1 mode causing the crash.

Role of Particle Size Distribution on Microstructure, Defects, and Mechanical Properties in Laser-Based Powder Bed Fusion of Scalmalloy®

Abstract This study investigates the impact of particle size distribution (PSD) in Scalmalloy® powder feedstocks on the geometrical accuracy, microstructure, defect formation, and mechanical properties of test artifacts fabricated via laser-based powder bed fusion of metals (PBF-LB/M). Two powder feedstocks were analyzed: Powder-1, with a mean particle size of 29.30–29.72 µm and a narrower, more uniform PSD, and Powder-2, with a mean particle size of 40.80–41.53 µm and a broader, more heterogeneous PSD. Powder-2 produced artifacts with improved geometrical accuracy and surface finish but exhibited cracking in both lattice (small features) and bulk regions. Scanning electron microscopy revealed Si precipitation near cracks in the lattice regions, attributed to segregation caused by heat accumulation in thin features and the coarser, more heterogeneous PSD of Powder-2. Powder-2 samples exhibited higher strain at a break during tensile testing, likely due to its lower porosity compared to Powder-1. Single-line and hatch scan results indicated no cracking for either powder, highlighting the susceptibility of small features to thermal gradients, segregation, and geometric stress concentrators. The observation of cracking exclusively in the thin features, and not in the bulk regions, underscores the artifact's utility in investigating how geometrical features influence thermal fields, microstructural evolution, and ultimately, performance in PBF-LB/M processes.

Growth Potential of Bipolaris oryzae under Temperature, Storage and Nutrient Conditions

This study evaluates the growth potential of Bipolaris oryzae under varying temperature conditions, nutrient composition, and storage duration. Experiments were conducted to assess the development of fungal colonies across a temperature gradient from 10°C to 40°C, with optimal growth observed between 20°C and 30°C. Additionally, the fungus was cultured on various artificial media, with Potato Glucose Agar (PGA) and Malt Extract Agar (MEA) proving most favorable for growth and sporulation. The study also investigated the effect of seed storage on fungal persistence, showing consistent infection rates over a nine-month period. These results highlight the crucial role of temperature, nutrient availability, and storage conditions in managing B. oryzae in agriculture. The findings reinforce the importance of avoiding infected fields for seed production and implementing proper storage practices to minimize fungal spread.

Robust Bi2(Te,Se)3 Thermoelectrics From Large Shrinkage Ratio Extrusion Drives Advanced Peltier Microcooler and Power Generator

AbstractThe “Barrel Effect” of n‐type Bi2Te3 materials have always been the biggest obstacle to expanding thermoelectric commercial applications. This work develops an industrial‐scale large shrinkage ratio extrusion method that synchronously coordinates appropriate texturing and modifies atomic defects, as well as micro and nanostructures, prompting simultaneous gains in rods with a diameter of 30 mm increases to ≈1.20 at 325 kelvin, with flexural and compressive strengths significantly improve to 77.7 and 192.4 MPa, respectively. Successfully fabricated Peltier microcoolers (1 × 3 mm2 cross‐section) exhibit a competitive maximum cooling temperature difference of 90.6 kelvin at a hot‐side temperature of 348 kelvin. A designed and integrated 17‐by‐17 power generator demonstrates a high conversion efficiency of 7% under a 200‐kelvin temperature gradient and highlights operational stability. These achievements hold great potential for advancing applications in compact solid‐state cooling and low‐grade waste heat recovery.

Controlled Multi-Stage Evaluation of Growth and Physiochemical Traits Between Low- and Normal-Temperature Strains of Scylla paramamosain

The mud crab Scylla paramamosain is a key economic crab species along the southern coastal regions of China. This study systematically compared the physiological and biochemical characteristics of low-temperature (LT) and normal-temperature (NT) strains of S. paramamosain at different life stages (juveniles and adults), integrating temperature gradient experiments with conventional aquaculture evaluations. The experimental results revealed the following: (1) Growth superiority: LT-strain crabs exhibited significantly greater final weight, survival rate, hepatopancreatic index, and gonadal index than their NT counterparts (p < 0.05). Moreover, LT individuals displayed an enhanced nutritional profile, with 16.56% higher muscle crude fat and a 23.80% increase in ovarian ash content. (2) Immune competence: Juvenile LT crabs exhibited greater antioxidant capacity at 18–21 °C, with significantly higher total antioxidant capacity (T-AOC) and superoxide dismutase (SOD) activity than NT crabs (p < 0.05). In adults, immune enzyme activity remained superior, particularly in serum acid phosphatase (ACP). (3) Nutritional advantage: LT mature females exhibited higher accumulation of essential amino acids (e.g., lysine, threonine) and polyunsaturated fatty acids (C18:2n-6, C20:2n-6) in the hepatopancreas and gonads (p < 0.05). These findings confirm the LT strain’s superior cold resilience and aquaculture potential, offering practical insights for S. paramamosain selective breeding programs and sustainable aquaculture development.

Improved Wire Quality of Twinning-Induced Plasticity Steel During Wire Drawing Through Temperature Gradient with Warm Die

The drawability and microstructural homogeneity of twinning-induced plasticity (TWIP) steel were improved during the wire drawing process by utilizing a temperature gradient along the wire’s radial direction. The surface temperature of the wire increased by applying heat to the die during the drawing process, thereby creating a temperature gradient across the wire during wire drawing. The drawability of the wire subjected to the temperature gradient with warm die (WD) increased by approximately 33% compared to that of conventional wire drawing with cold die (CD). The higher temperature of about 300 °C at the surface region of the wire with the WD suppressed the twinning rate at the surface region owing to the increase in the stacking fault energy (SFE) from 34 to 55 mJ/m2, leading to a uniform twinning rate along the wire’s radial direction compared with the CD wire, finally resulting in the improvement of the homogeneity in the microstructure and mechanical properties of TWIP steel. As a result, the drawability of the TWIP steel improved. Therefore, the general conclusion was derived that controlling the SFE within the area of the workpiece by tailoring the temperature can improve the formability in TWIP steels during the plastic forming process.

Modelling of wax deposition for waxy crude oil by ANSYS fluent

Wax deposition is a major flow assurance issue arises during pipeline transportation and wax gelation phenomenon, a fundamental characteristic of this deposition process, necessitates detailed investigation to elucidate the underlying mechanisms. The modelling approach helps in understanding the phenomenon of wax deposition and help in deciding appropriate mitigation technique during pipeline transportation of crude oil. Current study however aims to utilize Analysis of Systems (ANSYS) fluent software to model wax deposition phenomena using field data. The crude oil utilized for the study contains 38% wax and has pour point of 36 °C. The fluid flow was 1 GPM and the inlet temperature was 333.15 K which decreases up to 298.15 K. A Computational Fluid Dynamics (CFD) model was created to predict the wax gelation as a function of time, temperature and pipeline length for Indian crude oil using field data. It includes heat and mass transfer calculations, and the molecular diffusion process is considered as the mechanism for wax deposition. The CFD model in this work uses enthalpy porosity method where waxy oil is treated as the solid-liquid region with a porosity which equal to the liquid fraction. Moreover, the efficacy of the chemical additive was also evaluated in terms of liquid fraction by modelling approach in crude oil during pipeline transportation to evaluate its effect during field implementation. The results indicate that the wax deposition process is time dependent as it subsequently decreases with time. Furthermore, the temperature gradient between wall temperature and bulk fluid temperature also has a significant impact on wax deposition. Increase in the temperature gradient between wall and ambient temperature leads to increase the wax deposition and gelling formation. The use of Nanohybrid polymer (NHP) as an additive significantly improves the flow properties by reducing the wax deposition and increases the liquid fraction up to 75%. The study of CFD to model the wax-gelling process, with the wax gelation being monitored by analysing the variations in the liquid fractions of the crude oil aims to offer a thorough understanding of wax deposition and gelation phenomena in waxy oils, thereby facilitating the design of pipelines for waxy oils with suitable interventions, particularly under subsea conditions.

Entropy Generation Optimization for Casson Hybrid Nanofluid Flow Along a Curved Surface With Bioconvection Mechanism and Exothermic/Endothermic Catalytic Reaction

AbstractThis article deals with the heat and mass transfer analysis of Casson hybrid nanofluid flow over a curved Riga surface with slip conditions in the presence of gyrotactic microorganisms. The mechanism of Soret and Dufour effects, exothermic/endothermic catalytic reaction, and an exponential heat source are also investigated. The mixture of aluminum oxide and multi‐walled carbon nanotubes with Therminol‐VPI fluid is assumed as the hybrid nanofluid. Boundary layer assumptions are taken in the mathematical modeling of governing equations. Transformation variables are introduced to get the dimensionless governing equations. Numerical simulation of the transformed equations is done with the help of the Matlab computational tool using the Cash and Carp numerical method. Numerical results corresponding to the influential factors are plotted in graphs for velocity profile, temperature profile, concentration profile, drag coefficient, Nusselt number, Sherwood number, and entropy generation. It is observed that the fluid velocity diminishes with an enhancement in the curvature parameter, and fluid velocity enhances with an improvement in the suction parameter. Thermal profile improves for enhancing modified magnetic field parameter and drops with an increase in exponential index parameter. The microorganisms respond to temperature and concentration gradients, affecting the overall heat and mass transfer dynamics. This research aims to reveal the coupled effects of heat transfer, diffusion, and microorganism behavior in computational simulations, which have various applications in different sectors like electronics, chemical engineering, and material science.

Convection, but How Fast Does Fluid Mix in Hydrothermal Systems?

AbstractThe destabilizing thermal gradient across the Earth's lithosphere drives convection in superconfined hydrothermal environments at mid‐ocean ridges and geothermal reservoirs in the continental crust. Deep, hot waters rise in these regions and meet cold surface water that percolates through open fractures, creating complex and poorly understood mixing dynamics. This Letter explores the relationship between energy, convection, and mixing in analog hydrothermal systems. Leveraging energetics theory and lab‐scale experiments, we present a scaling formulation for estimating the irreversible mixing boosted by convective flows occurring within faulted and fractured hydrothermal environments. These findings bear relevance to natural Earth processes and human‐engineered applications, such as geothermal energy harvesting and geologic sequestration.

The Transport Properties of Water and Ions Confined in the Highly Aligned Graphene Aerogels: A Molecular Dynamics Simulation.

Highly aligned graphene aerogels (HAGA) with three-dimensional (3D) porous structures, excellent photothermal conversion ability and spectral absorption rate are considered to be a potential material to develop efficient and clean water production by utilizing solar energy solar energy. In this study, we employed molecular dynamics (MD) simulations to investigate the mechanisms of water and salt ion transport within HAGA. We also explored how the nanopore size of the network structure affects the movement behavior of water and salt ions. Improved water transport and salt ion blocking abilities were observed when the nanopore size of HAGA was smaller. Specifically, when the nanopore size was 0.83 nm, both the mobility of water and salt ions were significantly enhanced due to the single-chain phenomenon. In addition, the effects of the external temperature field on the transport process of water and salt ions within the nanoscale HAGA are also considered. It is found that the abilities of water and salt ions transport became drastic with the increase of temperature. Under the same temperature gradient, the water molecules flowed toward the heat temperature direction, however, the salt ions moved toward the cold temperature direction. These special phenomena can be explained by the thermal creep effect and the thermophoretic effect, respectively. Overall, these findings provide a more thorough understanding of the water and salt ions transport mechanisms of HAGA, which are significant for providing useful guidelines of HAGA design.

High Radiation Resistance in the Binary W-Ta System Through Small V Additions: A New Paradigm for Nuclear Fusion Materials.

Refractory High-Entropy Alloys (RHEAs) are promising candidates for structural materials in nuclear fusion reactors, where W-based alloys are currently leading. Fusion materials must withstand extreme conditions, including i) severe radiation damage from energetic neutrons, ii) embrittlement due to H and He ion implantation, and iii) exposure to high temperatures and thermal gradients. Recent RHEAs, such as WTaCrV and WTaCrVHf, have shown superior radiation tolerance and microstructural stability compared to pure W, but their multi-element compositions complicate bulk fabrication and limit practical use. In this study, it is demonstrated that reducing alloying elements in RHEAs is feasible without compromising radiation tolerance. Herein, two Highly Concentrated Refractory Alloys (HCRAs) - W53Ta44V3 and W53Ta42V5 (at.%) -were synthesized and investigated. Wefound that small V additions significantly influence the radiation response of the binary W-Ta system. Experimental results, supported by ab-initio Monte Carlo simulations and machine-learning-driven molecular dynamics, reveal that minor variations in V content enhance Ta-V chemical short-range order (CSRO), improving radiation resistance in the W53Ta42V5 HCRA. By focusing on reducing chemical complexity and the number of alloying elements, the conventional high-entropy alloy paradigm is challenged, suggesting a new approach to designing simplified multi-component alloys with refractory properties for thermonuclear fusion applications.

Gradually Disappearing Cross‐Seasonal Teleconnection Between ENSO and Tibetan Plateau Upper‐Level Westerlies

AbstractChanges in the Tibetan Plateau upper‐level westerlies (TPUW) and El Niño‐Southern Oscillation (ENSO) are crucial in regulating local hydrological cycles and large‐range climate. Here we report a gradual disappearance of the cross‐seasonal ENSO‐TPUW teleconnection since the early‐2000s, with correlation decreasing from 0.75 to near zero. Before the early‐2000s, the anomalous western North Pacific (WNP) anticyclone (WNPAC) induced by ENSO can last from winter to summer under the maintenance of Indo‐western Pacific surface seawater temperature gradient (IWPSSTG), prompting the WNP subtropical high to expand and strengthen toward South Asia, attracting the Tibet Plateau subtropical high southward, thereby causing the subtropical westerly jet to move southward and forming a cross‐seasonal ENSO‐TPUW teleconnection. However, given the prominent tropical WNP warming since the early‐2000s, the early‐summer outage of IWPSSTG and WNPAC disrupts the ENSO‐TPUW chain. This discontinuity poses severe challenges to the improvement of short‐term climate predictions in the Tibetan Plateau and surrounding areas.

Effect of Sunshine Temperature Difference on Multi-span Continuous Rigid Box Girder Bridge

Abstract The deformation caused by the temperature gradient, when constrained, generates thermal stress, and solar radiation-induced thermal stress is a significant cause of cracking in many bridges. To investigate the influence of solar radiation on multi-span continuous rigid box girder bridges and assess the applicability of international bridge codes temperature gradient models in southwestern China, this study employs numerical simulations to analyze temperature and stress distributions in box girders. The maximum daily temperature difference in summer and winter was selected as the calculation case, and the influence of the pavement on the temperature gradient of the box girder section was analyzed in detail. The results indicate that the temperature difference on the top plate can reach up to 14°C under a maximum daily temperature difference of 16°C in summer, representing the most unfavorable vertical temperature gradient. The temperature gradient model specified in the Chinese bridge code most closely matches the calculation results in this study, followed by the British bridge code. If the shading effect of the flange plate on the web is ignored, the web will exhibit a transverse temperature gradient of 21.6°C in winter. When the pavement is asphalt, the calculated value in this study closely matches the reference value from the Chinese bridge code. However, when the pavement is concrete, the calculated value exceeds the reference value by 14%. This suggests that applying the temperature gradient provided by the Chinese bridge code directly to southwest may be unsafe. The results of this study offer recommendations for improving the temperature gradient models in Chinese bridge code.

In‐Plane versus Through‐Plane Thermal Gradients during Cyclic Aging of Lithium‐Ion Batteries: An Experimental Study

During application, lithium‐ion battery cells can be exposed to inhomogeneous temperature distributions which has been argued to accelerate aging. A through‐plane thermal gradient with respect to the cell stack is suggested to accelerate aging even further. In this work, in‐plane and through‐plane inhomogeneous temperature boundary conditions on a 20 Ah pouch cell are applied during cyclic aging and the capacity loss is compared to a reference scenario. Both directions of temperature gradients from 10 to 40 °C accelerate the linear capacity loss trajectory by 25% ± 4% in comparison to an equivalent homogeneous aging temperature of 25 °C. Postmortem analyses reveal locally increased solid‐electrolyte interface growth as a probable reason for the globally observed aging acceleration. This significant aging acceleration is still 6 times lower than by increasing the homogeneous aging temperature from 25 to 40 °C for the investigated cell. An increase in average temperature with thermal gradient leads to a higher capacity loss acceleration for through‐plane than an in‐plane thermal gradient. Additional aging mechanisms are found for through‐plane thermal gradients whose origins are open for future research.

Controlling Self-Oscillation of a Single-Layer Liquid Crystal Elastomer at the Air-Water Interface via Light Programming for Water Strider-Inspired Aquatic Robots.

Biomimicking aquatic organisms offers many opportunities for designing intelligent robots that can freely move on water. However, most works were focused on multilayered materials or assembled structures and faced limitations in stability, versatility, and motion navigation. Here, we develop an assembly-free water-strider-like aquatic robot using a single layer of light-programmable liquid-crystal elastomer (LCE) that could be used to create asymmetric structures. The LCE strider mimics both the shape and functions of natural water striders; it is designed with four legs, with the fore and hind legs being programmed respectively via light. Consequently, the LCE strider shows self-oscillation and self-propulsion behaviors on low-grade thermal water with a temperature gradient at the air-water interface, owing to unbalanced changes in the contact areas and tensions between the legs and water. Furthermore, the trajectories of the LCE strider are manipulated by NIR light after selectively depositing polydopamine with photothermal conversion. In this way, path navigation is realized, that is, moving straight and on-demand turning, similar to the movement of natural water striders. This study should inspire the development of soft intelligent robots using shape-morphing materials.

Spin Seebeck Effect as a Probe for Majorana Fermions in Kitaev Spin Liquids

Quantum entanglement in strongly correlated electron systems often leads to exotic elementary excitations. Quantum spin liquids provide a paradigmatic example, where the elementary excitations are described by fractional quasiparticles such as spinons. However, such fractional quasiparticles behave differently from electrons, making their experimental identification challenging. Here, we theoretically investigate the spin Seebeck effect, which is a thermoelectric response via a spin current, as an efficient probe of the fractional quasiparticles in quantum spin liquids, focusing on the Kitaev honeycomb model. By comprehensive studies using real-time dynamics, perturbation theory, and linear spin-wave theory based on the tunnel spin-current theory, we find that the spin current is induced by thermal gradient in the Kitaev spin liquid via the low-energy fractional Majorana excitations. This identification underscores the ability of Majorana fermions to carry spin current, despite lacking spin angular momentum. Furthermore, we find that the induced spin current changes its sign depending on the sign of the Kitaev interaction, indicating that the Majorana fermions contribute to the spin current with (up-) down-spin-like nature when the exchange coupling is (anti)ferromagnetic. Thus, in contrast to the negative spin current already found in a one-dimensional quantum spin liquid, our calculation reveals that the spin Seebeck effect can exhibit either positive or negative signals, contingent upon the nature of fractional excitations in the quantum spin liquids. We also clarify contrasting field-angle dependence between the Kitaev spin liquid in the low-field limit and the high-field ferromagnetic state, which is useful for the experimental identification. Our finding suggests that the spin Seebeck effect could be used not only to detect fractional quasiparticles emerging in quantum spin liquids but also to generate and control them. Published by the American Physical Society 2025