An official website of the United States government
Local thermal magnetization fluctuations in Li-doped MnTe are found to increase its thermopower α strongly at temperatures up to 900 K. Below the Néel temperature ( T N ~ 307 K), MnTe is antiferromagnetic, and magnon drag contributes α md to the thermopower, which scales as ~ T 3 . Magnon drag persists into the paramagnetic state up to >3 × T N because of long-lived, short-range antiferromagnet-like fluctuations (paramagnons) shown by neutron spectroscopy to exist in the paramagnetic state. The paramagnon lifetime is longer than the charge carrier–magnon interaction time; its spin-spin spatial correlation length is larger than the free-carrier effective Bohr radius and de Broglie wavelength. Thus, to itinerant carriers, paramagnons look like magnons and give a paramagnon-drag thermopower. This contribution results in an optimally doped material having a thermoelectric figure of merit ZT > 1 at T > ~900 K, the first material with a technologically meaningful thermoelectric energy conversion efficiency from a spin-caloritronic effect.
more »
« less
Anomalous Thermoelectric Transport Properties of Fe‐Rich Magnetic FeTe
The interplay between magnetism and quantum effects has motivated several thermoelectric studies on iron‐telluride yet with little insight on the anomalous features in transport properties near magnetostructural transition temperature (≈70 K). A detailed investigation is carried out on Fe1.1Te by characterizing magnetic, heat capacity, galvanomagnetic, and thermoelectric transport properties to understand the electronic, magnetic, and structural origin of those anomalies. The magnetic susceptibility indicates a bicollinear stripe and short‐range ordering in the antiferromagnetic and paramagnetic domains, respectively. Hall conductivity and transverse magnetoresistance reveal a multicarrier transport impacted by spin fluctuations and magnons. Contributions from phonon‐drag and magnon‐drag are evaluated to understand the origin of the broad peak in antiferromagnetic thermopower. The peak at ≈50 K and the insignificant entropy contribution from the magnonic heat capacity support the phonon‐drag as the origin. The field‐dependent enhancement of thermal conductivity must be associated with field‐dependent spin‐phonon coupling modification. The field‐induced thermopower reduction can be attributed to the suppression of magnons or paramagnons, as evidenced by the magnetic susceptibility data. Above 70 K, the thermal conductivity drops sharply due to the structural change modifying phonon modes. Understanding these properties originated from the spin, and quantum effects are instrumental for designing high‐performance spin‐driven thermoelectrics.
more »
« less
- PAR ID:
- 10365598
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- physica status solidi (RRL) – Rapid Research Letters
- Volume:
- 15
- Issue:
- 10
- ISSN:
- 1862-6254
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Thermoelectric materials, capable of converting temperature gradients into electrical power, have been traditionally limited by a trade‐off between thermopower and electrical conductivity. This study introduces a novel, broadly applicable approach that enhances both the spin‐driven thermopower and the thermoelectric figure‐of‐merit (zT) without compromising electrical conductivity, using temperature‐driven spin crossover. Our approach, supported by both theoretical and experimental evidence, is demonstrated through a case study of chromium doped‐manganese telluride, but is not confined to this material and can be extended to other magnetic materials. By introducing dopants to create a high crystal field and exploiting the entropy changes associated with temperature‐driven spin crossover, we achieved a significant increase in thermopower, by approximately 136 μV K−1, representing more than a 200% enhancement at elevated temperatures within the paramagnetic domain. Our exploration of the bipolar semiconducting nature of these materials reveals that suppressing bipolar magnon/paramagnon‐drag thermopower is key to understanding and utilizing spin crossover‐driven thermopower. These findings, validated by inelastic neutron scattering, X‐ray photoemission spectroscopy, thermal transport, and energy conversion measurements, shed light on crucial material design parameters. We provide a comprehensive framework that analyzes the interplay between spin entropy, hopping transport, and magnon/paramagnon lifetimes, paving the way for the development of high‐performance spin‐driven thermoelectric materials.more » « less
-
Abstract Heat transport in nanoscale carbon materials such as carbon nanotubes and graphene is normally dominated by phonons. Here, measurements of in‐plane thermal conductivity, electrical conductivity, and thermopower are presented from 77–350 K on two films with thickness <100 nm formed from semiconducting single‐walled carbon nanotubes. These measurements are made with silicon–nitride membrane thermal isolation platforms. The two films, formed from disordered networks of tubes with differing tube and bundle size, have very different thermal conductivity. One film matches a simple model of heat conduction assuming constant phonon velocity and mean free path, and 3D Debye heat capacity with a Debye temperature of 770 K. The second film shows a more complicated temperature dependence, with a dramatic drop in a relatively narrow window near 200 K where phonon contributions to thermal conductivity essentially vanish. This causes a corresponding large increase in thermoelectric figure‐of‐merit at the same temperature. A better understanding of this behavior can allow significant improvement in thermoelectric efficiency of these low‐cost earth‐abundant, organic electronic materials. Heat and charge conductivity near room temperature is also presented as a function of doping, which provides further information on the interaction of dopant molecules and phonon transport in disordered nanotube films.more » « less
-
Abstract Spin excitations, including magnons and spinons, can carry thermal energy and spin information. Studying spin‐mediated thermal transport is crucial for spin caloritronics, enabling efficient heat dissipation in microelectronics and advanced thermoelectric applications. However, designing quantum materials with controllable spin transport is challenging. Here, highly textured spin‐chain compound Ca2CuO3is synthesized using a solvent‐cast cold pressing technique, aligning 2D nanostructures with spin chains perpendicular to the pressing direction. The sample exhibits high thermal conductivity anisotropy and an excellent room‐temperature thermal conductivity of 12 ± 0.7 W m−1K−1, surpassing all polycrystalline quantum magnets. Such a high value is attributed to the significant spin‐mediated thermal conductivity of 10 ± 1 W m−1K−1, the highest reported among all polycrystalline quantum materials. Analysis through a 1D kinetic model suggests that near room‐temperature, spinon thermal transport is dominated by coupling with high‐frequency phonons, while extrinsic spinon‐defect scattering is negligible. Additionally, this method is used to prepare textured La2CuO4, exhibiting highly anisotropic magnon thermal transport and demonstrating its broad applicability. A distinct role of defect scattering in spin‐mediated thermal transport is observed in two spin systems. These findings open new avenues for designing quantum materials with controlled spin transport for thermal management and energy conversion.more » « less
-
Single crystals of the quaternary chalcogenide BaCuGdTe 3 were obtained by direct reaction of elements allowing for a complete investigation of the intrinsic electrical and thermal properties of this previously uninvestigated material. The structure was investigated by high-resolution single-crystal synchrotron X-ray diffraction, revealing an orthorhombic crystal structure with the space group Cmcm. Although recently identified as a semiconductor suitable for thermoelectric applications from theoretical analyses, our electrical resistivity and Seebeck coefficient measurements show metallic conduction, the latter revealing strong phonon-drag. Temperature dependent hole mobility reveals dominant acoustic phonon scattering. Heat capacity data reveal a Debye temperature of 183 K and a very high density of states at the Fermi level, the latter confirming the metallic nature of this composition. Thermal conductivity is relatively high with Umklapp processes dominating thermal transport above the Debye temperature. The findings in this work lay the foundation for a more detailed understanding of the physical properties of this and similar multinary chalcogenide materials, and is part of the continuing effort in investigating quaternary chalcogenide materials and their suitability for use in technological applications.more » « less
