arXiv subjects

Hossein Sepehri-Amin

Publications and source records attributed to Hossein Sepehri-Amin.

At least 19 recordsLinked to original sources

arXiv2026

Observation of the orbital Nernst effect

The Nernst effect, which converts a temperature gradient into a transverse charge current, is fundamental to thermoelectrics. Its spin analogue, the spin Nernst effect, enables thermal generation of transverse spin currents and is central to spin caloritronics. Recently, the discovery of orbital currents, the orbital counterpart of spin currents, has extended angular-momentum transport beyond spin, leading to the prediction of the orbital Nernst effect, in which a temperature gradient drives a transverse orbital current. However, experimental evidence for this effect has been lacking. Here, we report the observation of the orbital Nernst effect in Ti. Using Ni electrodes on Ti, we detect a thermally induced voltage that depends on the magnetization direction and scales linearly with the temperature gradient. This voltage is strongly suppressed both when Ni is replaced with Ni$_{81}$Fe$_{19}$ and when Ti is replaced with Cr, providing strong evidence that the signal originates from the orbital Nernst effect rather than the anomalous Nernst or spin Nernst effect. These results establish thermally driven orbital transport, opening a pathway toward orbital caloritronics.

cond-mat.mes-hall↗

arXiv2026

Observation of the Nernst effect driven by longitudinal spin fluctuations

The Nernst effect, which converts a heat current into a charge current in the orthogonal direction, is generally classified into the ordinary effect due to an external magnetic field applied to conductors and the anomalous effect due to static magnetization in magnetic materials. Thus, one expects that the anomalous Nernst effect disappears at the Curie temperature following the magnetization. Here, we observe the Nernst effect which rather manifests around the Curie temperature. In a series of ferromagnetic (Mn,Cr)Sb samples, the Nernst coefficients exhibit finite values even across the Curie temperatures, whereas the anomalous Hall resistivity disappears, suggesting the breakdown of the Mott relation. These surviving Nernst and vanishing Hall effects result in a peak behavior of the transverse thermoelectric conductivity around the Curie temperature, which we theoretically reproduce by introducing longitudinal fluctuations of spins without accounting for an exotic spin texture, a non-trivial electronic structure, or magnon- and phonon-drag effects. The Nernst effect driven by the longitudinal spin fluctuations reveals that not only the static magnetic order but also the spin fluctuations can be a driving force of the Nernst effect in magnetic materials, opening another way for boosting transverse thermoelectric conversion.

cond-mat.mtrl-sci↗

arXiv2026

Observation of nonlocal ferron-drag thermoelectricity

The Peltier effect induces a heat current when a charge current passes through a conductor. Since a charge current is conserved at the junction between two different conductors, the difference between the heat flowing in both conductors for the same charge current leads to heating or cooling of the interface, providing an operating mechanism of solid-state heat pumps. Here, we report observation of heat absorption and release signals even in a junction-free, homogeneous metal when placed in proximity to a ferroelectric insulator. Our experiments using active thermographic imaging techniques confirm the prediction of the ferron-drag effect, i.e., the nonlocal excitation of ferrons, the collective excitation of the ferroelectric order, by conduction electrons in the adjacent metal. We reveal the electric-polarization-direction dependence of the temperature change signals and their unexpected increase with the metal thickness beyond the charge screening length, uncovering additional electron-phonon-ferron interactions in the metal/ferroelectric hybrid structure. The discovery of crosstalk between metals and ferroelectrics via remote ferrons could become both a nuisance and an opportunity for highly integrated circuits with ferroelectric barrier materials and revolutionize the design architecture of thermoelectric devices.

cond-mat.mes-hall↗

arXiv2026

Reconstruction-Dependent Imaging, Reactivity and Local Reduction of the CeO$_2$(100) surface

The possibility of mapping the local reactivity and reduction state to the atomic structure of chemically active oxide surfaces opens new avenues for further understanding of catalysis. Here, we combine scanning tunnelling (STM) and atomic force microscopy (AFM) with first-principles modelling to explore this possibility on the CeO2(100) surface. While STM reveals the periodicity of cerium-terminated and oxygen-terminated CeO$_2$(100) reconstructions coexisting on the same surface, AFM imaging and force spectroscopy provide direct identification of the exposed atomic species and their reactivity as the chemical interaction with the probe. Density functional theory based STM and AFM simulations reproduce the main experimental observations and show that STM contrast cannot be in general assigned to the atomic positions of certain chemical species, as traditionally assumed from previous studies. Simulated STM contrast of the two reconstructions across different reduction states associated with the removal of oxygen atoms in deeper layers, evidence that STM alone does not offer a robust fingerprint of the local reduction state for the cerium-terminated reconstruction, but it is sensitive to the reduced state in the case of the oxygen-terminated one, being able to provide information on a mixed distribution of Ce$^{3+}$ and Ce$^{4+}$ ions on the first sub-surface Ce layer.

cond-mat.mtrl-sci↗

arXiv2026

Trans-scale spin Seebeck effect in nanostructured bulk composites based on magnetic insulator

The spin Seebeck effect (SSE) enables thermoelectric conversion through thermally generated spin currents in magnetic materials, offering a promising transverse geometry for scalable devices. However, conventional SSE devices are confined to nanoscale thin-film architectures, with significantly restricted output power due to the intrinsic constraints of spin and magnon diffusion lengths. Here, we demonstrate a trans-scale SSE using nano-structured bulk composites composed of Pt-coated yttrium iron garnet powders fabricated via dynamic powder sputtering and low-temperature sintering. The resulting three-dimensional composites exhibit continuous Pt channels and robust mechanical integrity. Transverse thermoelectric measurements confirm isotropic SSE signals at the bulk scale. Power analysis indicates that the three-dimensional architecture enables scalable volumetric thermoelectric power generation beyond diffusion-limited thin-film SSE geometries. This work establishes a scalable platform for bulk SSE-based thermoelectrics, bridging nanoscale spin caloritronics with macroscopic device integration.

cond-mat.mtrl-sci↗

arXiv2026

Observation of giant nonvolatile magneto-thermal switching in superconductor-ferromagnet hybrids

Magneto-thermal switch is a crucial thermal component which enables heat transfer control by the application of an external magnetic field. Recently, a nonvolatile behavior in magneto-thermal conductivity at zero magnetic field was observed in type-II and phase-separated superconductors owing to magnetic flux pinning nature, leading to an energy-efficient thermal control technology. However, the nonvolatile magneto-thermal switching ratio has been much lower than the volatile one in conventional materials. Here, we demonstrate a giant nonvolatile magneto-thermal switching in ferromagnetic Fe-superconducting Pb hybrids. The dispersion of pure Fe particles realizes increased electron and decreased phonon contributions in the thermal conductivity, which enhances the magneto-thermal switching ratio at the superconducting-to-normal conducting phase transition. Furthermore, in concert with trapped magnetic flux by supercurrent, ferromagnetic moment of Fe breaks the superconductivity of Pb matrix at zero magnetic field, enabling a significantly large nonvolatility even with a slight amount of Fe inclusions. Consequently, the nonvolatile magneto-thermal switching ratio reaches 719% in maximum at the Fe ratio of 8.7 vol%, which is more than twice the previous record value observed in Pb-Sn composites and the volatile one in pure Pb. This work broadens the exploration space and strategy for giant nonvolatile magneto-thermal switching materials.

cond-mat.supr-con↗

arXiv2026

Revealing nonvolatile behaviors in magneto-thermal switching using microstructure-controlled superconducting composites

Thermal conductivity in a conductor changes by the application of an external magnetic field, which functions as a magneto-thermal switch. For superconductors, a large magneto-thermal switching can occur through a superconducting-to-normal conducting phase transition due to the change in the electron contribution in thermal conductivity. Arima et al. recently reported a nonvolatile nature of the magneto-thermal switching for superconducting solders, which consist of phase-separated Sn and Pb domains. Although they clarified that magnetic flux trapping is required to induce the nonvolatile magneto-thermal switching, a rule for such material design is still unclear. Here, we investigate the microstructure dependence of magneto-thermal switching in superconducting Sn/Pb multilayered composites, which are created by an accumulative roll bonding method. The thickness of each layer, that is the scale of microstructure, can be systematically controlled by the repetition number of roll bonding while the whole sample size and average composition are unchanged. We find that, as the formation of micro-scaled Sn domains proceeds by increasing the repetition number, a nonvolatility in the magneto-thermal conductivity gradually appears in correlation with the remanent magnetization. This study directly confirms that the inclusions with a size comparable to or less than the magnetic vortex in superconducting matrix is essential for magnetic flux trapping, enabling the nonvolatile magneto-thermal switching in superconducting composites.

cond-mat.supr-con↗

arXiv2026

Structural heterogeneity-induced enhancement of transverse magneto-thermoelectric conversion revealed by thermoelectric imaging in functionally graded materials

Functionally graded materials (FGMs) exhibit continuous property variations that enable unique functionalities and provide efficient platforms for systematic property optimization. Here, we report the fabrication of FGMs with graded structural heterogeneity by annealing an amorphous metal under a one-dimensional temperature gradient. Using lock-in thermography (LIT), we spatially mapped transverse thermoelectric conversion with high spatial and temperature resolution. A pronounced non-monotonic response was observed, with the maximum anomalous Ettingshausen effect, transverse charge-to-heat conversion in magnetic materials, appearing in the atomic-heterogeneity regime well before crystallization. This enhancement was not captured by conventional structural or longitudinal transport measurements, highlighting the exceptional sensitivity of transverse thermoelectric phenomena to subtle structural heterogeneity. Structural analyses using scanning transmission electron microscopy and atom probe tomography revealed Fe-based crystalline alloys and Cu nanoclusters embedded in the amorphous matrix, whose heterogeneity accounts for the enhanced response. These findings establish temperature-gradient-annealed FGMs, combined with LIT, as a powerful methodology for probing structural-heterogeneity-driven transverse electron transport and designing high-performance flexible materials.

cond-mat.mtrl-sci↗

arXiv2025

Control of Covalent Bond Enables Efficient Magnetic Cooling

Magnetic cooling, harnessing the temperature change in matter when exposed to a magnetic field, presents an energy-efficient and climate-friendly alternative to traditional vapor-compression refrigeration systems, with a significantly lower global warming potential. The advancement of this technology would be accelerated if irreversible losses arising from hysteresis in magnetocaloric materials were minimized. Despite extensive efforts to manipulate crystal lattice constants at the unit-cell level, mitigating hysteresis often compromises cooling performance. Herein, we address this persistent challenge by forming Sn(Ge)3/Sn(Ge)3 bonds within the unit cell of the Gd5Ge4 compound. Our approach enables an energetically favorable phase transition, leading to the elimination of thermal hysteresis. Consequently, we achieve a synergistic improvement of two key magnetocaloric figures of merit: a larger magnetic entropy change and a twofold increase in the reversible adiabatic temperature change (from 3.8 to 8 K) in the Gd5Sn2Ge2 compound. Such synergies can be extended over a wide temperature range. This study demonstrates a paradigm shift in mastering hysteresis toward simultaneously achieving exceptional magnetocaloric metrics and opens up promising avenues for gas liquefaction applications in the longstanding pursuit of sustainable energy solutions.

cond-mat.mtrl-sci↗

arXiv2025

Simultaneous achievement of large anomalous Nernst effect and reduced thermal conductivity in sintered polycrystalline topological Heusler ferromagnets

This study reports the observation of the large anomalous Nernst effect in polycrystalline ferromagnetic Co$_{2}$MnGa (CMG) slabs prepared by a spark plasma sintering method. By optimizing the sintering conditions, the anomalous Nernst coefficient reaches ~7.5 $μ$V K$^{-1}$ at room temperature, comparable to the highest value reported in the single-crystalline CMG slabs. Owing to the sizable anomalous Nernst coefficient and reduced thermal conductivity, the dimensionless figure of merit in our optimized CMG slab shows the record-high value of ~8$\times$10$^{-4}$ at room temperature. With the aid of the nano/microstructure characterization and first-principles phonon calculation, this study discusses the dependence of the transport properties on the degree of crystalline ordering and morphology of crystal-domain boundaries in the sintered CMG slabs. The results reveal a potential of polycrystalline topological materials for transverse thermoelectric applications, enabling the construction of large-scale modules.

cond-mat.mtrl-sci↗

arXiv2025

Near-surface Defects Break Symmetry in Water Adsorption on CeO$_{2-x}$(111)

Water interactions with oxygen-deficient cerium dioxide (CeO$_2$) surfaces are central to hydrogen production and catalytic redox reactions, but the atomic-scale details of how defects influence adsorption and reactivity remain elusive. Here, we unveil how water adsorbs on partially reduced CeO$_{2-x}$(111) using atomic force microscopy (AFM) with chemically sensitive, oxygen-terminated probes, combined with first-principles calculations. Our AFM imaging reveals water molecules as sharp, asymmetric boomerang-like features radically departing from the symmetric triangular motifs previously attributed to molecular water. Strikingly, these features localize near subsurface defects. While the experiments are carried out at cryogenic temperature, water was dosed at room temperature, capturing configurations relevant to initial adsorption events in catalytic processes. Density functional theory identifies Ce$^{3+}$ sites adjacent to subsurface vacancies as the thermodynamically favored adsorption sites, where defect-induced symmetry breaking governs water orientation. Force spectroscopy and simulations further distinguish Ce$^{3+}$ from Ce$^{4+}$ centers through their unique interaction signatures. By resolving how subsurface defects control water adsorption at the atomic scale, this work demonstrates the power of chemically selective AFM for probing site-specific reactivity in oxide catalysts, laying the groundwork for direct investigations of complex systems such as single-atom catalysts, metal-support interfaces, and defect-engineered oxides.

cond-mat.mtrl-sci↗

arXiv2025

High-throughput development of flexible amorphous materials showing large anomalous Nernst effect via automatic annealing and thermoelectric imaging

This work demonstrates high-throughput screening of flexible magnetic materials for efficient transverse thermoelectric conversion based on the anomalous Nernst effect (ANE). The approach integrates automated annealing and contactless measurement of transport properties using lock-in thermography. We screen 151 Fe-based alloy ribbons with varying compositions and annealing conditions. Seven high-performance candidates with mechanical flexibility are identified, exhibiting anomalous Nernst coefficients of up to 4.8 uV/K, the highest value reported for flexible materials. Structural analysis reveals that ANE enhancement occurs universally near the first crystallization temperature of the Fe-based ribbons, without strong correlation with composition. Notably, the enhancement is also observed in samples without Cu or Fe nanoclusters, indicating that short-range atomic order in the amorphous matrix may play a role in ANE. These findings demonstrate the effectiveness of high-throughput methodologies for discovering advanced ANE materials and provide new insights into thermoelectric conversion in disordered systems where conventional design principles fall short.

cond-mat.mtrl-sci↗

arXiv2025

Multifunctional composite magnet realizing record-high transverse thermoelectric generation

Permanent magnets are used in various products and essential for human society. If omnipresent permanent magnets could directly convert heat into electricity, they would lead to innovative energy harvesting and thermal management technologies. However, achieving such "multifunctionality" has been difficult due to poor thermoelectric performance of conventional magnets. In this work, we develop a multifunctional composite magnet (MCM) that enables giant transverse thermoelectric conversion while possessing permanent magnet features. MCM comprising alternately and obliquely stacked SmCo$_{5}$/Bi$_{0.2}$Sb$_{1.8}$Te$_{3}$ multilayers exhibits an excellent transverse thermoelectric performance at room temperature by optimizing its anisotropic structure. Owing to the extremely low interfacial electrical and thermal resistivities, the experimentally determined figure of merit $z_{\rm xy}T$ reaches 0.20 close to the analytically calculated ideal value. The MCM-based thermopile module generates 204 mW in maximum at a temperature difference of 152 K, whose power density normalized by heat transfer area and temperature gradient is not only record-high among transverse thermoelectric modules but also comparable to that of commercial thermoelectric modules utilizing the Seebeck effect. The multifunctionality of our MCM provides unprecedented opportunities for energy harvesting and thermal management everywhere permanent magnets are currently used.

cond-mat.mtrl-sci↗

arXiv2025

Direct Observation of Vortices and Antivortices Generation in Phase-Separated Superconductor Sn-Pb Solder

Quantized vortices in type-II superconductors provide insights into the mechanisms of superconductivity. However, the generation of antivortices, characterized by magnetization antiparallel to the external magnetic field, remains less understood. In this study, we investigate Sn-Pb solder, a superconductor with phase-separated Sn and Pb phases, and report the observation of both vortices and antivortices. Scanning SQUID (superconducting quantum interference device) microscopy revealed the presence of both vortices and antivortices, while magneto-optical imaging demonstrated flux avalanches. Our results demonstrate that Sn in Sn-Pb solder behaves as a type-II superconductor when magnetic fluxes are trapped, despite bulk Sn being a type-I superconductor with a transition temperature (TcSn) of 3.7 K. Our findings suggest that the size effect and proximity effect with Pb contribute synergistically to induce type-II superconductivity in Sn Notably, vortices were observed at temperatures as high as 5 K, exceeding the bulk TcSn. Furthermore, the interplay between the type-I superconducting Pb phase and the type-II superconducting Sn phase results in the generation of antivortices, providing a mechanism to accommodate excess magnetic flux. This study shed light on new research on composites combining type-I and type-II superconductors.

cond-mat.supr-con↗

arXiv2024

Designing flexible hard magnetic materials for zero-magnetic-field operation of the anomalous Nernst effect

The global shift towards a carbon-neutral society has accelerated the demand for green energy, driving research into efficient technologies for harvesting energy from low-grade waste heat. Recently, transverse thermoelectrics based on the anomalous Nernst effect (ANE) has gained attention due to their simple device structure, scalability, and manufacturing-friendly nature. While topological single crystals and epitaxial films have been focused for enhancing the ANE-driven thermoelectric performance, further improvements in material design are necessary for practical applications. Here, we report an easy-to-implement strategy for designing mechanically flexible and magnetically hard transverse thermoelectric materials by creating amorphous-crystalline heterogeneous composites. We fabricated and optimized the heterogeneous composites through controlled heat treatment, achieving significant enhancements in the coercivity and anomalous Nernst coefficient, while maintaining flexibility. Additionally, using the developed material, we constructed a single-material-based coiled device and demonstrated the zero-field operation of the ANE-based energy harvesting from curved heat sources. These results validate the feasibility of using the ANE-based flexible materials for energy harvesting applications.

cond-mat.mtrl-sci↗

arXiv2024

Hybridizing anomalous Nernst effect in artificially tilted multilayer based on magnetic topological material

Transverse thermoelectric conversion holds significant potential in addressing complex challenges faced by classical Seebeck/Peltier modules. A promising transverse thermoelectric phenomenon is the anomalous Nernst effect (ANE) originating from nontrivial band structures in magnetic topological materials. However, the currently reported performance for ANE in topological materials, e.g., Co$_2$MnGa, remains insufficient for practical thermoelectric applications. Here, we unveil a new availability for ANE by integrating magnetic topological materials into artificially tilted multilayers (ATMLs), known to exhibit the structure-induced transverse thermoelectric conversion due to the off-diagonal Seebeck effect. Our experiments reveal that the transverse thermoelectric performance in Co$_2$MnGa-based ATMLs is improved through the hybrid action of ANE and the off-diagonal Seebeck effect, with the modulation of performance dependent on magnetization being significantly greater than the performance achievable with ANE alone. This unconventional synergy underscores the importance of hybrid transverse thermoelectric conversion and paves a way for advancing thermoelectric applications using magnetic materials.

cond-mat.mtrl-sci↗

arXiv2024

Nonvolatile magneto-thermal switching driven by vortex trapping in commercial In-Sn solder

Magneto-thermal switching (MTS) is a key technology for efficient thermal management. Recently, large MTS with nonvolatility has been observed in Sn-Pb solders [H. Arima et al., Commun. Mater. 5, 34 (2024)] where phase separation, different superconducting transition temperature (Tc) of Sn and Pb, and magnetic-flux trapping are the causes of the nonvolatile MTS. To further understand the mechanism and to obtain the strategy for enhancing switching ratio, exploration of new phase-separated superconductors with nonvolatile MTS is needed. Here, we show that the In52-Sn48 commercial solder is a phase-separated superconducting composite with two Tc and traps vortices after field cooling. A clear signature of nonvolatile MTS was observed at T = 2.5 K. From specific heat analyses, we conclude that the vortices are mainly trapped in the lower-Tc phase (γ-phase) after field cooling, which is the evidence that vortex trapping also works on achieving nonvolatile MTS in phase-separated superconducting composites.

cond-mat.supr-con↗

arXiv2023

Creation of flexible spin-caloritronic material with giant transverse thermoelectric conversion by nanostructure engineering

Functional materials such as magnetic, thermoelectric, and battery materials have been revolutionized through nanostructure engineering. However, spin caloritronics, an advancing field based on spintronics and thermoelectrics with fundamental physics studies, has focused only on uniform materials without complex microstructures. Here, we show how nanostructure engineering enables transforming simple magnetic alloys into spin-caloritronic materials displaying significantly large transverse thermoelectric conversion properties. The anomalous Nernst effect (ANE), a promising transverse thermoelectric phenomenon for energy harvesting and heat sensing, has been challenging to utilize due to the scarcity of materials with large anomalous Nernst coefficients. We demonstrate a remarkable improvement in the anomalous Nernst coefficients in flexible Fe-based amorphous materials through nanostructural engineering, without altering their composition. This surpasses all reported amorphous alloys and is comparable to single crystals showing large ANE. The enhancement is attributed to Cu nano-clustering, facilitating efficient transverse thermoelectric conversion. This discovery advances the materials science of spin caloritronics, opening new avenues for designing high-performance transverse thermoelectric devices for practical applications.

cond-mat.mtrl-sci↗