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Resilience of the physicochemical properties of graphene-based materials for applications in harsh radiation environments
Authors:
Marcilei A. Guazzelli,
Saulo G. Alberton,
Nemitala Added,
Vitor A. P. Aguiar,
Koiti Araki,
Luis H. Avanzi,
Francesco Cappuzzello,
Manuela Cavallaro,
Eliane F. Chinaglia,
Marcia T. Escote,
Fabio F. Ferreira,
Mauro Giovannini,
Renato F. Jardim,
Sueli H. Masunaga,
Nilberto H. Medina,
Marcelo Nakamura,
José R. B. Oliveira,
Roberto B. B. Santos,
Alexis C. Villas-Bôas
Abstract:
The development of radiation-tolerant materials capable of maintaining structural, electrical, and thermal stability in extreme, radiation-rich environments remains a critical challenge in materials science. In this work, the effects of 60 MeV 35Cl ion irradiation on highly oriented pyrolytic graphite (HOPG) and multilayer reduced graphene oxide (ML-rGO) were investigated. The samples were exposed…
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The development of radiation-tolerant materials capable of maintaining structural, electrical, and thermal stability in extreme, radiation-rich environments remains a critical challenge in materials science. In this work, the effects of 60 MeV 35Cl ion irradiation on highly oriented pyrolytic graphite (HOPG) and multilayer reduced graphene oxide (ML-rGO) were investigated. The samples were exposed to fluences of 5.11 x 10^9 and 1.3 x 10^10 ions/cm^2 and characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and electrical transport measurements. The results show that the irradiation response is strongly influenced by the initial structural organization of the material. In HOPG, ion exposure leads to a progressive loss of crystalline order, evidenced by XRD peak broadening and an increase in the Raman ID/IG ratio, accompanied by a reduction in electrical transport performance. In contrast, ML-rGO exhibits distinct behavior at higher fluences, suggesting partial structural reorganization. The appearance of more defined graphitic features in XRD and Raman analyses, along with changes in surface morphology and electrical response, suggests the formation of more ordered sp2 domains. These findings indicate that irradiation effects vary with the initial degree of order, providing useful insights for selecting carbon-based materials for devices operating under severe radiation conditions.
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Submitted 25 May, 2026;
originally announced May 2026.
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Search for magnetoacoustic quantum oscillations in the insulating phase of YbB$_{12}$
Authors:
Ryosuke Kurihara,
Atsuhiko Miyata,
Koji Araki,
Shusaku Imajo,
Ruo Hibino,
Atsushi Miyake,
Sergei Zherlitsyn,
Joachim Wosnitza,
Hiroshi Yaguchi,
Fumitoshi Iga,
Masashi Tokunaga,
Yasuhiro H. Matsuda
Abstract:
A highly exotic phenomenon in solid-state physics is the observation of magnetic quantum oscillations in insulators. For instance, in the Kondo insulator YbB$_{12}$ various groups reported the observation of such oscillations seemingly originating from Fermi surfaces, though this contradicts the concept of an insulator having no charged quasiparticles. In this study, we searched for quantum oscill…
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A highly exotic phenomenon in solid-state physics is the observation of magnetic quantum oscillations in insulators. For instance, in the Kondo insulator YbB$_{12}$ various groups reported the observation of such oscillations seemingly originating from Fermi surfaces, though this contradicts the concept of an insulator having no charged quasiparticles. In this study, we searched for quantum oscillations in YbB$_{12}$ by using bulk-sensitive ultrasonic experiments in high magnetic fields up to 65 T and down to 485 mK. For that, we utilized an YbB$_{12}$ single crystal that, in previous experiments, revealed oscillations in the magnetoresistance in the insulating state. We confirmed oscillation-like behavior of the magnetoresistance as well as field-dependent oscillations in the magnetocaloric effect. However, we could not observe magnetoacoustic quantum oscillations in the insulating state, only in the field-induced metallic state. In the insulating state, we found some anomalies in our ultrasound data, the origin of which remains elusive. Our findings provide further information on the puzzling behavior of the insulating state of YbB$_{12}$.
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Submitted 5 May, 2026;
originally announced May 2026.
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Realization of a triangular spin necklace in a verdazyl-based Ni complex
Authors:
Itsuki Shimamura,
Risa Yagura,
Takanori Kida,
Masayuki Hagiwara,
Koji Araki,
Yoshiki Iwasaki,
Yuko Hosokoshi,
Kenta Kimura,
Hironori Yamaguchi
Abstract:
We successfully synthesized a verdazyl-based complex, ($m$-Py-V)$_3$[Ni(NO$_3$)$_2$], in which Ni$^{2+}$ ions and verdazyl radicals form a one-dimensional, triangular spin necklace consisting of spin-1/2 and spin-1 units. Molecular orbital calculations reveal strong antiferromagnetic (AF) interactions between inversion-related radical pairs that form spin-1/2 singlet dimers. The remaining verdazyl…
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We successfully synthesized a verdazyl-based complex, ($m$-Py-V)$_3$[Ni(NO$_3$)$_2$], in which Ni$^{2+}$ ions and verdazyl radicals form a one-dimensional, triangular spin necklace consisting of spin-1/2 and spin-1 units. Molecular orbital calculations reveal strong antiferromagnetic (AF) interactions between inversion-related radical pairs that form spin-1/2 singlet dimers. The remaining verdazyl and Ni$^{2+}$ spins form frustrated triangular units, creating a distinctive spin network. Magnetic susceptibility and specific heat measurements identify a phase transition to an AF order. The application of magnetic fields suppresses the phase transition signal, suggesting field-induced decoupling of the spin-1 moments. Electron spin resonance measurements are used to evaluate the easy-axis anisotropy of spin-1, which may promote the AF order. This work provides a rare example of a geometrically frustrated quantum spin chain realized via molecular design, thereby offering a platform for exploring frustration-driven quantum phases in low-dimensional materials.
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Submitted 22 January, 2026;
originally announced January 2026.
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Emergence of Kondo-assisted Néel order in a Kondo necklace model
Authors:
Hironori Yamaguchi,
Shunsuke C. Furuya,
Yu Tominaga,
Takanori Kida,
Koji Araki,
Masayuki Hagiwara
Abstract:
The interplay between Kondo screening and magnetic order has long been a central issue in the physics of strongly correlated systems. While the Kondo effect has traditionally been understood to suppress magnetism through the formation of local singlets, recent studies suggest that Kondo interactions may enhance magnetic order under certain conditions. However, these scenarios often rely on complex…
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The interplay between Kondo screening and magnetic order has long been a central issue in the physics of strongly correlated systems. While the Kondo effect has traditionally been understood to suppress magnetism through the formation of local singlets, recent studies suggest that Kondo interactions may enhance magnetic order under certain conditions. However, these scenarios often rely on complex electronic structures, including orbital and charge degrees of freedom, making the essential mechanisms difficult to isolate. Here we report the realization of a spin-(1/2,1) Kondo necklace model in a Ni-based complex-a minimal spin-only analog of the Kondo lattice that isolates quantum spin correlations by eliminating charge degrees of freedom. Thermodynamic measurements identify a magnetic phase transition and a field-induced quantum phase transition. Perturbative analysis reveals that the Kondo coupling mediates effective antiferromagnetic interactions between the spin-1 sites, stabilizing the Néel order across the entire chain. Our results establish a universal boundary in Kondo physics, where coupling to spin-1/2 moments yields singlets, but to spin-1 and higher stabilizes magnetic order.
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Submitted 22 January, 2026;
originally announced January 2026.
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Experimental benchmark of the quantum-classical crossover in a spin ladder
Authors:
Hironori Yamaguchi,
Itsuki Shimamura,
Akira Matsuo,
Koichi Kindo,
Koji Araki,
Yoshiki Iwasaki,
Masayuki Hagiwara
Abstract:
We report a spin-(1/2, 5/2) three-leg ladder realized in a radical-Mn polymer, exhibiting an antiferromagnetic transition and magnetization curves accurately described by classical mean-field theory. Although the underlying spin model intrinsically supports strong quantum fluctuations, as confirmed by quantum Monte Carlo simulations, the real system shows an anomalously complete suppression of qua…
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We report a spin-(1/2, 5/2) three-leg ladder realized in a radical-Mn polymer, exhibiting an antiferromagnetic transition and magnetization curves accurately described by classical mean-field theory. Although the underlying spin model intrinsically supports strong quantum fluctuations, as confirmed by quantum Monte Carlo simulations, the real system shows an anomalously complete suppression of quantum behavior. These findings provide a key experimental benchmark for the quantum-classical crossover and suggest that lattice topology can play a crucial role in tuning the balance between quantum and classical physics in strongly correlated systems.
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Submitted 13 December, 2025;
originally announced December 2025.
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Realization of a Spin-1/2 Hexagonal-Plaquette Chain with Ising-Like Anisotropy
Authors:
Hironori Yamaguchi,
Shunsuke C. Furuya,
Yu Tominaga,
Koji Araki,
Masayuki Hagiwara
Abstract:
We present the realization of a spin-1/2 hexagonal-plaquette chain with Ising anisotropy, an unexplored quantum spin model that serves as a platform for investigating anisotropic quantum magnetism. Specific heat at zero field reveals a sharp peak at $T_{\rm{N}}$ = 1.0 K, indicating a phase transition to a N$\acute{\rm{e}}$el order stabilized by interchain couplings. A perturbative analysis maps th…
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We present the realization of a spin-1/2 hexagonal-plaquette chain with Ising anisotropy, an unexplored quantum spin model that serves as a platform for investigating anisotropic quantum magnetism. Specific heat at zero field reveals a sharp peak at $T_{\rm{N}}$ = 1.0 K, indicating a phase transition to a N$\acute{\rm{e}}$el order stabilized by interchain couplings. A perturbative analysis maps the system onto an effective spin-1/2 Ising-like chain, supporting the presence of an anisotropy-induced excitation gap. Furthermore, the interchain interactions may induce discrete excitations in the spinon continuum, reminiscent of Zeeman ladder physics observed in related 1D Ising-like systems. These results establish a well-defined model system for correlated spin phenomena in anisotropic magnets and highlight a route for engineering Ising-like quantum states in molecular-based frameworks.
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Submitted 10 September, 2025;
originally announced September 2025.
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Tunable Coatings on Various Substrates for Self-Adaptive Energy Harvesting with Daytime Solar Heating and Nighttime Radiative Cooling
Authors:
Ken Araki,
Vishwa Krishna Rajan,
Liping Wang
Abstract:
In this work, tunable vanadium dioxide (VO2) metafilms on different substrate materials fabricated via low-oxygen furnace oxidation are demonstrated for self-adaptive daytime solar heating and nighttime radiative cooling. Because of its thermally-driven insulator-to-metal phase transition behavior, the VO2 metafilms work as spectrally-selective solar absorber with a high solar absorptance of 0.86…
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In this work, tunable vanadium dioxide (VO2) metafilms on different substrate materials fabricated via low-oxygen furnace oxidation are demonstrated for self-adaptive daytime solar heating and nighttime radiative cooling. Because of its thermally-driven insulator-to-metal phase transition behavior, the VO2 metafilms work as spectrally-selective solar absorber with a high solar absorptance of 0.86 and a low infrared emissivity of ~0.2 at daytime, while they behave as selective cooler at nighttime to dissipate heat effectively through the atmospheric transparency window with a high emissivity of ~0.76 to cold outer space. From the outdoor vacuum tests, a significant temperature rise up to 169 K upon solar heating and a temperature drop of 17 K at night are experimentally observed from these tunable VO2 metafilms. With the atmosphere temperature fitted in-situ, the accurate heat transfer model shows excellent agreement with the stagnation temperature measurement, and indicates a high heating power of ~400 W/m2 at 80°C sample temperature in the middle of the day, and a cooling power of ~60 W/m2 at 30°C in equilibrium with ambient at night. This work would facilitate the development of self-adaptive coatings with cost-effective and scalable fabrication approaches for all-day energy harvesting.
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Submitted 2 September, 2025;
originally announced September 2025.
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High-quality Tungsten-doped Vanadium Dioxide Thin Films Fabricated in an Extremely Low-oxygen Furnace Environment
Authors:
Vishwa Krishna Rajan,
Ken Araki,
Robert Y. Wang,
Liping Wang
Abstract:
This work reports the fabrication and characterization of high-quality tungsten-doped vanadium dioxide (WxV1-xO2, x = 0~3 at. %) by thermal oxidation of sputtered tungsten-vanadium alloyed thin films with different atomic percentages and high-temperature annealing in an extremely low oxygen atmosphere (5 to 20 ppm) along with reduction of surface over-oxides in high vacuum (1 mPa). Oxidation param…
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This work reports the fabrication and characterization of high-quality tungsten-doped vanadium dioxide (WxV1-xO2, x = 0~3 at. %) by thermal oxidation of sputtered tungsten-vanadium alloyed thin films with different atomic percentages and high-temperature annealing in an extremely low oxygen atmosphere (5 to 20 ppm) along with reduction of surface over-oxides in high vacuum (1 mPa). Oxidation parameters such as temperature, time and nitrogen purging rate are first optimized for obtaining high quality undoped VO2 thin film. Insulator-to-metal (IMT) phase transition behavior of VO2 thin films fabricated in a low-O2 environment is characterized with temperature dependent spectral infrared transmittance and electrical resistivity measurements, where there is 15% higher infrared transmittance change and additional 1 order change in resistivity in comparison with VO2 thin films fabricated in a O2-rich environment. Grazing angle X-ray diffraction scan confirms no presence of higher oxides in the VO2 oxidized in low-O2 environment, which improves its quality significantly. Comprehensive studies on thermal annealing and vacuum reduction for tungsten doped VO2 thin films are also carried out to find the optimal fabrication conditions. With the tungsten at. % measured by X-ray photoelectron spectroscopy, the optimal WVO2 thin films fabricated through this streamlined oxidation, annealing and reduction processes in extremely low-O2 furnace environment exhibit lowered IMT temperature at -23°C per at.% of tungsten dopants from 68°C without doping. This low-cost and scalable fabrication method could facilitate the wide development of tunable WVO2 coatings in thermal and energy applications.
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Submitted 2 September, 2025;
originally announced September 2025.
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Maximizing Infrared Transmission Contrast Upon Phase Transition of Thermally Grown Vanadium Dioxide Thin Films by Rapid Thermal Processing
Authors:
Ken Araki,
Vishwa Krishna Rajan,
Liping Wang
Abstract:
Pristine vanadium dioxide (VO2), an insulator-to-metal transition (IMT) material, is grown via furnace oxidation followed by rapid thermal annealing with forming gas (5%H2/95%N2) which reduces surface over-oxides such as V2O5 formed during the oxidation. The evolutional IMT behaviors of the thermochromic film and vanadium oxide states over different reduction time are systematically studied with t…
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Pristine vanadium dioxide (VO2), an insulator-to-metal transition (IMT) material, is grown via furnace oxidation followed by rapid thermal annealing with forming gas (5%H2/95%N2) which reduces surface over-oxides such as V2O5 formed during the oxidation. The evolutional IMT behaviors of the thermochromic film and vanadium oxide states over different reduction time are systematically studied with temperature-dependent infrared spectrometry, electrical resistivity, and X-ray diffraction measurements. After optimally reducing surface over-oxides to VO2, infrared transmission contrast upon phase transition is enhanced to 46% (at 9 um wavelength) compared to 23% from fully oxidation without any reduction. Moreover, pristine VO2 thin film obtained from thermal oxidation and optimal reduction processes exhibits sharp phase transition and narrow thermal hysteresis within 2~4°C in both infrared transmission and electrical resistivity, which are comparable to the VO2 of best quality prepared by other sophisticated fabrication techniques. The thermally grown method presented here would facilitate the scalable fabrication of high-quality VO2 thin films and tunable radiative coatings for high-performance thermal control applications.
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Submitted 28 April, 2025;
originally announced April 2025.
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Gapless behavior in a two-leg spin ladder with bond randomness
Authors:
Yu Tominaga,
Itsuki Shimamura,
Takanori Kida,
Masayuki Hagiwara,
Koji Araki,
Yuko Hosokoshi,
Yoshiki Iwasaki,
Hironori Yamaguchi
Abstract:
We successfully synthesized [Cu$_2$(AcO)$_4$($p$-Py-V-$p$-F)$_2$]$\cdot$4CHCl$_3$, a verdazyl-based complex with a paddlewheel structure comprising two Cu atoms, which induces strong antiferromagnetic (AF) exchange interactions between Cu spins, generating a nonmagnetic singlet state at low temperatures. Two primary exchange interactions between radical spins generate a spin-1/2 AF two-leg ladder.…
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We successfully synthesized [Cu$_2$(AcO)$_4$($p$-Py-V-$p$-F)$_2$]$\cdot$4CHCl$_3$, a verdazyl-based complex with a paddlewheel structure comprising two Cu atoms, which induces strong antiferromagnetic (AF) exchange interactions between Cu spins, generating a nonmagnetic singlet state at low temperatures. Two primary exchange interactions between radical spins generate a spin-1/2 AF two-leg ladder. In addition, two possible positional configurations of the F atom in the complex create four different overlap patterns of molecular orbitals, introducing bond randomness in the spin ladder. The observed experimental behaviors, such as the Curie tail in the magnetic susceptibility and the gapless gradual increase in the magnetization curve, are attributed to a broad distribution of excitation energies and a few orphan spins in the random-singlet (RS) state that are stabilized by bond randomness. The low-temperature specific heat exhibits a temperature dependence with $\propto 1/|{\rm{ln}}T|^3$, demonstrating the formation of the RS state in unfrustrated systems. We also consider the effect of restricted patterns of exchange interactions and one-dimensional nature of the system on the RS state.
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Submitted 21 April, 2025;
originally announced April 2025.
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Effects of Neutron Radiation on the Thermal Conductivity of Highly Oriented Pyrolitic Graphite
Authors:
Marcilei A. Guazzelli,
Luis H. Avanzi,
Vitor A. P. Aguiar,
Alexis C. Vilas-Bôas,
Saulo G. Alberton,
Sueli H. Masunaga,
Eliane F. Chinaglia,
Koiti Araki,
Marcelo Nakamura,
Marcos M. Toyama,
Fabio F. Ferreira,
Marcia T. Escote,
Roberto B. B. Santos,
Nilberto H. Medina,
José Roberto B. Oliveira,
Francesco Cappuzzello,
Manuela Cavallaro
Abstract:
Highly Ordered Pyrolytic Graphite (HOPG) has been extensively researched due to its chemical and physical properties that make it suitable for applications in several technologies. Its high thermal conductivity makes HOPG an excellent heat sink, a crucial characteristic for manufacturing targets used in nuclear reactions, such as those proposed by the NUMEN project. However, when subjected to diff…
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Highly Ordered Pyrolytic Graphite (HOPG) has been extensively researched due to its chemical and physical properties that make it suitable for applications in several technologies. Its high thermal conductivity makes HOPG an excellent heat sink, a crucial characteristic for manufacturing targets used in nuclear reactions, such as those proposed by the NUMEN project. However, when subjected to different radiation sources, this material undergoes changes in its crystalline structure, which alters its intended functionality. This study examined HOPG sheets before and after exposure to a 14 MeV neutron beam. Morphological and crystallographic analyses reveal that even minor disruptions in the high atomic ordering result in modifications to its thermal properties. The results of this study are essential to establish the survival time of the HOPG used as thermal interface material to improve heat dissipation of a nuclear target to be bombarded by an intense high-energy heavy-ion beam.
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Submitted 19 November, 2024;
originally announced November 2024.
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Quantum gapped state in a spin-1/2 distorted honeycomb-based lattice with frustration
Authors:
Hironori Yamaguchi,
Satoshi Morota,
Takanori Kida,
Seiya Shimono,
Koji Araki,
Yoshiki Iwasaki,
Yuko Hosokoshi,
Masayuki Hagiwara
Abstract:
We successfully synthesized ($p$-Py-V)[Cu(hfac)$_2$], a verdazyl-based complex. Molecular orbital calculations revealed five types of intermolecular interactions between the radical spins and two types of intramolecular interactions between the radical and the Cu spins, resulting in a spin-1/2 distorted honeycomb-based lattice. Additionally, competing ferromagnetic and antiferromagnetic (AF) inter…
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We successfully synthesized ($p$-Py-V)[Cu(hfac)$_2$], a verdazyl-based complex. Molecular orbital calculations revealed five types of intermolecular interactions between the radical spins and two types of intramolecular interactions between the radical and the Cu spins, resulting in a spin-1/2 distorted honeycomb-based lattice. Additionally, competing ferromagnetic and antiferromagnetic (AF) interactions induce frustration. The magnetization curve displayed a multistage increase, including a zero-field energy gap. Considering the stronger AF interactions that form dimers and tetramers, the magnetic susceptibility and magnetization curves were qualitatively explained. These findings demonstrated that the quantum state, based on the dominant AF interactions, was stabilized due to the effects of frustration in the lattice. Hence, the exchange interactions forming two-dimensional couplings decoupled, reducing energy loss caused by frustration and leading to frustration-induced dimensional reduction.
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Submitted 23 October, 2023;
originally announced October 2023.
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Field-induced quantum phase in a frustrated zigzag-square lattice
Authors:
Hironori Yamaguchi,
Kazutoshi Shimamura,
Yasuo Yoshida,
Akira Matsuo,
Koichi Kindo,
Kiichi Nakano,
Satoshi Morota,
Yuko Hosokoshi,
Takanori Kida,
Yoshiki Iwasaki,
Seiya Shimono,
Koji Araki,
Masayuki Hagiwara
Abstract:
This study presents the experimental realization of a spin-1/2 zigzag-square lattice in a verdazyl-based complex, namely ($m$-Py-V-2,6-F$_2$)$[$Cu(hfac)$_2]$. Molecular orbital calculations suggest the presence of five types of frustrated exchange couplings. Our observations reveal an incremental increase in the magnetization curve beyond a critical field, signifying a phase transition from the an…
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This study presents the experimental realization of a spin-1/2 zigzag-square lattice in a verdazyl-based complex, namely ($m$-Py-V-2,6-F$_2$)$[$Cu(hfac)$_2]$. Molecular orbital calculations suggest the presence of five types of frustrated exchange couplings. Our observations reveal an incremental increase in the magnetization curve beyond a critical field, signifying a phase transition from the antiferromagnetic ordered state to a quantum state characterized by a 1/2 plateau. This intriguing behavior arises from the effective stabilization of a zigzag chain by the external fields. These results provide evidence for field-induced dimensional reduction in a zigzag-square lattice attributed to the effects of frustration.
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Submitted 25 September, 2023;
originally announced September 2023.
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Anomalous Shubnikov-de Haas effect and observation of the Bloch-Grüneisen temperature in the Dirac semimetal ZrTe5
Authors:
S. Galeski,
K. Araki,
O. K. Forslund,
R. Wawrzynczak,
H. F. Legg,
P. K. Sivakumar,
U. Miniotaite,
F. Elson,
M. Månsson,
C. Witteveen,
F. O. von Rohr,
A. Q. R. Baron,
D. Ishikawa,
Q. Li,
G. Gu,
L. X. Zhao,
W. L. Zhu,
G. F. Chen,
Y. Wang,
S. S. P. Parkin,
D. Gorbunov,
S. Zherlitsyn,
B. Vlaar,
D. H. Nguyen,
S. Paschen
, et al. (7 additional authors not shown)
Abstract:
Appearance of quantum oscillations (QO) in both thermodynamic and transport properties of metals at low temperatures is the most striking experimental consequence of the existence of a Fermi surface (FS). The frequency of these oscillations and the temperature dependence of their amplitude provides essential information about the FS topology and fermionic quasiparticle properties. Here, we report…
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Appearance of quantum oscillations (QO) in both thermodynamic and transport properties of metals at low temperatures is the most striking experimental consequence of the existence of a Fermi surface (FS). The frequency of these oscillations and the temperature dependence of their amplitude provides essential information about the FS topology and fermionic quasiparticle properties. Here, we report the observation of an anomalous suppression of the QO amplitude seen in resistivity (Shubnikov de-Haas effect) at sub-kelvin temperatures in ZrTe5 samples with a single small FS sheet comprising less than 5% of the first Brillouin zone. By comparing these results with measurements of the magneto-acoustic QO and the recovery of the usual Lifshitz-Kosevich behavior of the Shubnikov de-Haas (SdH) effect in ZrTe$_5$ samples with a multi-sheet FS, we show that the suppression of the SdH effect originates from a decoupling of the electron liquid from the lattice. On crossing the so-called Bloch-Grüneisen temperature, T$_BG$, electron-phonon scattering becomes strongly suppressed and in the absence of Umklapp scattering the electronic liquid regains Galilean invariance. In addition, we show, using a combination of zero-field electrical conductivity and ultrasonic-absorption measurements, that entering this regime leads to an abrupt increase of electronic viscosity.
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Submitted 31 January, 2024; v1 submitted 19 September, 2023;
originally announced September 2023.
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Rhombic Fermi surfaces in a ferromagnetic MnGa thin film with perpendicular magnetic anisotropy
Authors:
M. Kobayashi,
N. H. D. Khang,
T. Takeda,
K. Araki,
R. Okano,
M. Suzuki,
K. Kuroda,
K. Yaji,
K. Sugawara,
S. Souma,
K. Nakayama,
K. Yamauchi,
M. Kitamura,
K. Horiba,
A. Fujimori,
T. Sato,
S. Shin,
M. Tanaka,
P. N. Hai
Abstract:
Mn$_{1-x}$Ga$_x$ (MnGa) with the $L1_0$ structure is a ferromagnetic material with strong perpendicular magneto-crystalline anisotropy. Although MnGa thin films have been successfully grown epitaxially and studied for various spintronics devices, fundamental understandings of its electronic structure are still lacking. To address this issue, we have investigated $L1_0$-MnGa thin films using angle-…
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Mn$_{1-x}$Ga$_x$ (MnGa) with the $L1_0$ structure is a ferromagnetic material with strong perpendicular magneto-crystalline anisotropy. Although MnGa thin films have been successfully grown epitaxially and studied for various spintronics devices, fundamental understandings of its electronic structure are still lacking. To address this issue, we have investigated $L1_0$-MnGa thin films using angle-resolved photoemission spectroscopy (ARPES). We have observed a large Fermi surface with a rhombic shape in the $k_x$-$k_y$ plane overlapping neighboring Fermi surfaces. The $k_z$ dependence of the band structure suggests that the band dispersion observed by ARPES comes from the three-dimensional band structure of MnGa folded by a $\sqrt{2} \times \sqrt{2}$ reconstruction. The band dispersion across the corner of the rhombic Fermi surface forms an electron pocket with a weak $k_z$ dependence. The effective mass and the mobility of the bands crossing the Fermi level near the corner are estimated from the ARPES images. Based on the experimental findings, the relationship between the observed band structure and the spin-dependent properties in MnGa-based heterostructures is discussed.
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Submitted 22 March, 2022;
originally announced March 2022.
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Ferromagnetism induced by hybridization of Fe 3d orbitals with ligand InSb bands in n-type ferromagnetic semiconductor (In,Fe)Sb
Authors:
Ryo Okano,
Tomoki Hotta,
Takahito Takeda,
Kohsei Araki,
Kengo Takase,
Le Duc Anh,
Shoya Sakamoto,
Yukiharu Takeda,
Atsushi Fujimori,
Masaaki Tanaka,
Masaki Kobayashi
Abstract:
Fe-doped III-V ferromagnetic semiconductor (FMS) (In,Fe)Sb is a promising material for spintronic device applications because of the n-type carrier conduction and the ferromagnetism with high Curie temperature (TC > 300 K). To clarify the mechanism of the high-TC ferromagnetism, we have investigated the electronic structure and magnetic properties of an (In,Fe)Sb thin film by performing x-ray abso…
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Fe-doped III-V ferromagnetic semiconductor (FMS) (In,Fe)Sb is a promising material for spintronic device applications because of the n-type carrier conduction and the ferromagnetism with high Curie temperature (TC > 300 K). To clarify the mechanism of the high-TC ferromagnetism, we have investigated the electronic structure and magnetic properties of an (In,Fe)Sb thin film by performing x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe L2,3 edges. The magnetic-field dependence of the XMCD spectra reveals that there are ferromagnetic-like Fe and paramagnetic-like Fe components in the (In,Fe)Sb thin film. The XAS and XMCD spectra of the ferromagnetic-like and paramagnetic-like Fe components resemble those of other Fe-doped FMSs and extrinsic oxides, respectively. The finite value of the ratio between the orbital and spin magnetic moments estimated by applying the XMCD sum rules indicates that the valence state of the Fe ions substituting for the In sites in (In,Fe)Sb is not purely ionic Fe3+, but intermediate between Fe3+ and Fe2+. The qualitative correspondence between the magnetic-field dependence of the visible-light magnetic circular dichroism intensity and that of the XMCD intensity demonstrates that the Zeeman splitting of the InSb band is proportional to the net magnetization of the doped Fe. These results suggest that the ferromagnetism of (In,Fe)Sb originates from the Fe 3d orbitals hybridized with the host InSb bands.
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Submitted 9 February, 2022;
originally announced February 2022.
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Itinerant ferromagnetism mediated by giant spin polarization of metallic ligand band in van der Waals magnet Fe5GeTe2
Authors:
K. Yamagami,
Y. Fujisawa,
B. Driesen,
C. H. Hsu,
K. Kawaguchi,
H. Tanaka,
T. Kondo,
Y. Zhang,
H. Wadati,
K. Araki,
T. Takeda,
Y. Takeda,
T. Muro,
F. C. Chuang,
Y. Niimi,
K. Kuroda,
M. Kobayashi,
Y. Okada
Abstract:
We investigate near-Fermi-energy (EF) element-specific electronic and spin states of ferromagnetic van der Waals (vdW) metal Fe5GeTe2. The soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurement provides spectroscopic evidence of localized Fe 3d band. We also find prominent hybridization between the localized Fe 3d band and the delocalized Ge/Te p bands. This picture is strongl…
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We investigate near-Fermi-energy (EF) element-specific electronic and spin states of ferromagnetic van der Waals (vdW) metal Fe5GeTe2. The soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurement provides spectroscopic evidence of localized Fe 3d band. We also find prominent hybridization between the localized Fe 3d band and the delocalized Ge/Te p bands. This picture is strongly supported from direct observation of the remarkable spin polarization of the ligand p bands near EF, using x-ray magnetic circular dichroism (XMCD) measurements. The strength of XMCD signal from ligand element Te shows the highest value, as far as we recognize, among literature reporting finite XMCD signal for none-magnetic element in any systems. Combining SX-ARPES and elemental selective XMCD measurements, we collectively point an important role of giant spin polarization of the delocalized ligand Te states for realizing itinerant long-range ferromagnetism in Fe5GeTe2. Our finding provides a fundamental elemental selective view-point for understanding mechanism of itinerant ferromagnetism in low dimensional compounds, which also leads insight for designing exotic magnetic states by interfacial band engineering in heterostructures.
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Submitted 4 January, 2021;
originally announced January 2021.
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Evolution of the Fe-3$d$ impurity band state as the origin of high Curie temperature in p-type ferromagnetic semiconductor (Ga,Fe)Sb
Authors:
Takahito Takeda,
Shoya Sakamoto,
Kohsei Araki,
Yuita Fujisawa,
Le Duc Anh,
Nguyen Thanh Tu,
Yukiharu Takeda,
Shin-ichi Fujimori,
Atsushi Fujimori,
Masaaki Tanaka,
Masaki Kobayashi
Abstract:
(Ga$_{1-x}$,Fe$_x$)Sb is one of the promising ferromagnetic semiconductors for spintronic device applications because its Curie temperature ($T_{\rm C}$) is above 300 K when the Fe concentration $x$ is equal to or higher than ~0.20. However, the origin of the high $T_{\rm C}$ in (Ga,Fe)Sb remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and first-p…
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(Ga$_{1-x}$,Fe$_x$)Sb is one of the promising ferromagnetic semiconductors for spintronic device applications because its Curie temperature ($T_{\rm C}$) is above 300 K when the Fe concentration $x$ is equal to or higher than ~0.20. However, the origin of the high $T_{\rm C}$ in (Ga,Fe)Sb remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and first-principles calculations to investigate the $x$ dependence of the Fe 3$d$ states in (Ga$_{1-x}$,Fe$_x$)Sb ($x$ = 0.05, 0.15, and 0.25) thin films. The observed Fe 2$p$-3$d$ RPES spectra reveal that the Fe-3$d$ impurity band (IB) crossing the Fermi level becomes broader with increasing $x$, which is qualitatively consistent with the picture of double-exchange interaction. Comparison between the obtained Fe-3$d$ partial density of states and the first-principles calculations suggests that the Fe-3$d$ IB originates from the minority-spin ($\downarrow$) $e$ states. The results indicate that enhancement of the interaction between $e_\downarrow$ electrons with increasing $x$ is the origin of the high $T_{\rm C}$ in (Ga,Fe)Sb.
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Submitted 18 August, 2020;
originally announced August 2020.
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Tailoring Magnetism in Self-intercalated Cr1+δTe2 Epitaxial Films
Authors:
Y. Fujisawa,
M. Pardo-Almanza,
J. Garland,
K. Yamagami,
X. Zhu,
X. Chen,
K. Araki,
T. Takeda,
M. Kobayashi,
Y. Takeda,
C. H. Hsu,
F. C. Chuang,
R. Laskowski,
K. H. Khoo,
A. Soumyanarayanan,
Y. Okada
Abstract:
Magnetic transition metal dichalcogenide (TMD) films have recently emerged as promising candidates to host novel magnetic phases relevant to next-generation spintronic devices. However, systematic control of the magnetization orientation, or anisotropy, and its thermal stability, characterized by Curie temperature (Tc) remains to be achieved in such films. Here we present self-intercalated epitaxi…
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Magnetic transition metal dichalcogenide (TMD) films have recently emerged as promising candidates to host novel magnetic phases relevant to next-generation spintronic devices. However, systematic control of the magnetization orientation, or anisotropy, and its thermal stability, characterized by Curie temperature (Tc) remains to be achieved in such films. Here we present self-intercalated epitaxial Cr1+δTe2 films as a platform for achieving systematic/smooth magnetic tailoring in TMD films. Using a molecular beam epitaxy (MBE) based technique, we have realized epitaxial Cr1+δTe2 films with smoothly tunable over a wide range (0.33-0.82), while maintaining NiAs-type crystal structure. With increasing δ, we found monotonic enhancement of Tc from 160 to 350 K, and the rotation of magnetic anisotropy from out-of-plane to in-plane easy axis configuration for fixed film thickness. Contributions from conventional dipolar and orbital moment terms are insufficient to explain the observed evolution of magnetic behavior with δ. Instead, ab initio calculations suggest that the emergence of antiferromagnetic interactions with δ, and its interplay with conventional ferromagnetism, may play a key role in the observed trends. To our knowledge, this constitutes the first demonstration of tunable Tc and magnetic anisotropy across room temperature in TMD films, and paves the way for engineering novel magnetic phases for spintronic applications.
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Submitted 1 August, 2020;
originally announced August 2020.
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Magnetic Phases of Frustrated Ferromagnetic Spin-Trimer System Gd_3_Ru_4_Al_12_ With a Distorted Kagome Lattice Structure
Authors:
S. Nakamura,
N. Kabeya,
M. Kobayashi,
K. Araki,
K. Katoh,
A. Ochiai
Abstract:
The magnetization and specific heat measurements have been performed on single-crystalline Gd_3_Ru_4_Al_12_ with a distorted Kagome lattice structure. This spin system is regarded as an antiferromagnetic triangular lattice of XY like Heisenberg model at low temperatures. The magnetic phase diagrams indicate the existence of frustration and Z_2_ degeneracy. The magnetization and specific heat imply…
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The magnetization and specific heat measurements have been performed on single-crystalline Gd_3_Ru_4_Al_12_ with a distorted Kagome lattice structure. This spin system is regarded as an antiferromagnetic triangular lattice of XY like Heisenberg model at low temperatures. The magnetic phase diagrams indicate the existence of frustration and Z_2_ degeneracy. The magnetization and specific heat imply the successive phase transitions with partial disorder and a T-shaped spin structure in the ground state.
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Submitted 30 November, 2018;
originally announced November 2018.
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Giant Anisotropic Magnetoresistance due to Purely Orbital Rearrangement in the Quadrupolar Heavy Fermion Superconductor PrV$_2$Al$_{20}$
Authors:
Yasuyuki Shimura,
Qiu Zhang,
Bin Zeng,
Daniel Rhodes,
Rico Uwe Schonemann,
Masaki Tsujimoto,
Yosuke Matsumoto,
Akito Sakai,
Toshiro Sakakibara,
Koji Araki,
Wenkai Zheng,
Qiong Zhou,
Luis Balicas,
Satoru Nakatsuji
Abstract:
We report the discovery of giant and anisotropic magnetoresistance due to the orbital rearrangement in a non-magnetic correlated metal. In particular, we measured the magnetoresistance under fields up to 31.4 T in the cubic Pr-based heavy fermion superconductor PrV$_2$Al$_{20}$ with a non-magnetic $Γ_3$ doublet ground state, exhibiting antiferro-quadrupole ordering below 0.7 K. For the [100] direc…
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We report the discovery of giant and anisotropic magnetoresistance due to the orbital rearrangement in a non-magnetic correlated metal. In particular, we measured the magnetoresistance under fields up to 31.4 T in the cubic Pr-based heavy fermion superconductor PrV$_2$Al$_{20}$ with a non-magnetic $Γ_3$ doublet ground state, exhibiting antiferro-quadrupole ordering below 0.7 K. For the [100] direction, we find that the high-field phase appears between 12 T and 25 T, accompanied by a large jump at 12 T in the magnetoresistance ($ΔMR \sim $ 100 $\% $) and in the anisotropic magnetoresistivity (AMR) ratio by $\sim $ 20 $\% $. These observations indicate that the strong hybridization between the conduction electrons and anisotropic quadrupole moments leads to the Fermi surface reconstruction upon crossing the field-induced antiferro-quadrupole (orbital) rearrangement.
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Submitted 10 May, 2018;
originally announced May 2018.
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Quadrupole-Driven Non-Fermi Liquid and Magnetic-Field Induced Heavy Fermion States in a Non-Kramers Doublet System
Authors:
T. Onimaru,
K. Izawa,
K. T. Matsumoto,
T. Yoshida,
Y. Machida,
T. Ikeura,
K. Wakiya,
K. Umeo,
S. Kittaka,
K. Araki,
T. Sakakibara,
T. Takabatake
Abstract:
Orbital degrees of freedom in condensed matters could play important roles in forming a variety of exotic electronic states by interacting with conduction electrons. In 4f electron systems, because of strong intra-atomic spin-orbit coupling, an orbitally degenerate state inherently carries quadrupolar degrees of freedom. The present work has focussed on a purely quadrupole-active system PrIr2Zn20…
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Orbital degrees of freedom in condensed matters could play important roles in forming a variety of exotic electronic states by interacting with conduction electrons. In 4f electron systems, because of strong intra-atomic spin-orbit coupling, an orbitally degenerate state inherently carries quadrupolar degrees of freedom. The present work has focussed on a purely quadrupole-active system PrIr2Zn20 showing superconductivity in the presence of an antiferroquadrupole order at TQ = 0.11 K. We observed non-Fermi liquid (NFL) behaviors emerging in the electrical resistivity and the 4f contribution to the specific heat, C_4f, in the paramagnetic state at T > TQ. Moreover, in magnetic fields below 6 T, all data set of the electrical resistivity and C_4f(T) are well scaled with characteristic temperatures T0's. This is the first observation of the NFL state in the nonmagnetic quadrupole-active system, whose origin is intrinsically different from that observed in the vicinity of the conventional quantum critical point. It implies possible formation of a quadrupole Kondo lattice resulting from hybridization between the quadrupoles and the conduction electrons. Below 0.13 K, the electrical resistivity and C_4f(T) exhibit anomalies as B approaches 5 T. This is the manifestation of a field-induced crossover toward a Fermi-liquid ground state in the quadrupole Kondo lattice.
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Submitted 30 June, 2016;
originally announced June 2016.
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Assessing Material Qualities and Efficiency Limits of III-V on Silicon Solar Cells Using External Radiative Efficiency
Authors:
Kan-Hua Lee,
Kenji Araki,
Li Wang,
Nobuaki Kojima,
Yoshio Ohshita,
Masafumi Yamaguchi
Abstract:
The paper presents a quantitative approach to the investigation and comparison of the material qualities of III-V on silicon (III-V/Si) solar cells by using external radiative efficiencies. We use this analysis to predict the limiting efficiencies and evaluate the criteria of material quality in order to achieve high efficiency III-V/Si solar cells. This result yields several implications for the…
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The paper presents a quantitative approach to the investigation and comparison of the material qualities of III-V on silicon (III-V/Si) solar cells by using external radiative efficiencies. We use this analysis to predict the limiting efficiencies and evaluate the criteria of material quality in order to achieve high efficiency III-V/Si solar cells. This result yields several implications for the design of high efficiency III-V/Si solar cells.
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Submitted 2 June, 2016; v1 submitted 7 December, 2015;
originally announced December 2015.
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Field-induced incommensurate phase in the strong-rung spin ladder with ferromagnetic legs
Authors:
H. Yamaguchi,
H. Miyagai,
M. Yoshida,
M. Takigawa,
K. Iwase,
T. Ono,
N. Kase,
K. Araki,
S. Kittaka,
T. Sakakibara,
T. Shimokawa,
T. Okubo,
K. Okunishi,
A. Matsuo,
Y. Hosokoshi
Abstract:
We report magnetization, specific heat, and NMR measurements of 3-Br-4-F-V [=3-(3-bromo-4-fluorophenyl)-1,5-diphenylverdazyl], a strong-rung S=1/2 Heisenberg spin ladder with ferromagnetic leg interactions. We explain the magnetic and thermodynamic properties based on the strong-rung regime. Furthermore, we find a field-induced successive phase transition in the specific heat and the nuclear spin-…
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We report magnetization, specific heat, and NMR measurements of 3-Br-4-F-V [=3-(3-bromo-4-fluorophenyl)-1,5-diphenylverdazyl], a strong-rung S=1/2 Heisenberg spin ladder with ferromagnetic leg interactions. We explain the magnetic and thermodynamic properties based on the strong-rung regime. Furthermore, we find a field-induced successive phase transition in the specific heat and the nuclear spin-lattice relaxation rate 1/T1. 19F-NMR spectra for higher- and lower-temperature phases indicate partial magnetic order and incommensurate long-range order, respectively, evidencing the presence of frustration due to weak interladder couplings.
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Submitted 9 June, 2014;
originally announced June 2014.
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Fine-Tuning of Magnetic Interactions in Organic Spin Ladders
Authors:
Hironori Yamaguchi,
Hirotsugu Miyagai,
Tokuro Shimokawa,
Kenji Iwase,
Toshio Ono,
Yohei Kono,
Naoki Kase,
Koji Araki,
Shunichiro Kittaka,
Toshiro Sakakibara,
Takashi Kawakami,
Kouichi Okunishi,
Yuko Hosokoshi
Abstract:
We have succeeded in synthesizing two types of new organic radical crystals 3-I-V [= 3- (3-iodophenyl)-1,5-diphenylverdazyl] and 3-Br-4-F-V [= 3-(3-bromo-4-fluorophenyl)-1,5- diphenylverdazyl]. Their crystal strucutures are found to be isomorphous to that of previously reported spin ladder 3-Cl-4-F-V. Through the quantitative analysis of their molecular arrangements and magnetic properties, we con…
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We have succeeded in synthesizing two types of new organic radical crystals 3-I-V [= 3- (3-iodophenyl)-1,5-diphenylverdazyl] and 3-Br-4-F-V [= 3-(3-bromo-4-fluorophenyl)-1,5- diphenylverdazyl]. Their crystal strucutures are found to be isomorphous to that of previously reported spin ladder 3-Cl-4-F-V. Through the quantitative analysis of their molecular arrangements and magnetic properties, we confirm that these materials form ferromagnetic chain-based spin ladders with slightly modulated magnetic interactions. These results present the first quantitative demonstration of the fine-tuning of magnetic interactions in the molecular- based materials.
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Submitted 12 February, 2014;
originally announced February 2014.
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Magneto-elastic Effects in $\text{Tb}_{3}\text{Ga}_{5}\text{O}_{12}$
Authors:
U. Löw,
S. Zherlitsyn,
K. Araki,
M. Akatsu,
Y. Nemoto,
T. Goto,
U. Zeitler,
B. Lüthi
Abstract:
We report new results for the elastic constants studied in Faraday and Cotton-Mouton geometry in Tb$_3$Ga$_5$O$_{12}$ (TGG), a frustrated magnetic substance with strong spin-phonon interaction and remarkable crystal-electric-field (CEF) effects. We analyze the data in the framework of CEF theory taking into account the individual surroundings of the six inequivalent Tb$^{3+}$-ion positions. This t…
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We report new results for the elastic constants studied in Faraday and Cotton-Mouton geometry in Tb$_3$Ga$_5$O$_{12}$ (TGG), a frustrated magnetic substance with strong spin-phonon interaction and remarkable crystal-electric-field (CEF) effects. We analyze the data in the framework of CEF theory taking into account the individual surroundings of the six inequivalent Tb$^{3+}$-ion positions. This theory describes both, elastic constants in the magnetic field and as a function of temperature. Moreover we present sound-attenuation data for the acoustic Cotton-Mouton effect in TGG.
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Submitted 13 December, 2013;
originally announced December 2013.
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Magnetic-Field-Independent Ultrasonic Dispersions in the Magnetically Robust Heavy Fermion System SmOs4Sb12
Authors:
Tatsuya Yanagisawa,
Hitoshi Saito,
Hiroyuki Hidaka,
Hiroshi Amitsuka,
Koji Araki,
Mitsuhiro Akatsu,
Yuichi Nemoto,
Terutaka Goto,
Pei-Chun Ho,
Ryan E. Baumbach,
M. Brian Maple
Abstract:
Elastic properties of the filled skutterudite compound SmOs$_4$Sb$_{12}$ have been investigated by ultrasonic measurements. The elastic constant $C_{11}(ω)$ shows two ultrasonic dispersions at $\sim$15 K and $\sim$53 K for frequencies $ω$ between 33 and 316 MHz, which follow a Debye-type formula with Arrhenius-type temperature-dependent relaxation times, and remain unchanged even with applied magn…
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Elastic properties of the filled skutterudite compound SmOs$_4$Sb$_{12}$ have been investigated by ultrasonic measurements. The elastic constant $C_{11}(ω)$ shows two ultrasonic dispersions at $\sim$15 K and $\sim$53 K for frequencies $ω$ between 33 and 316 MHz, which follow a Debye-type formula with Arrhenius-type temperature-dependent relaxation times, and remain unchanged even with applied magnetic fields up to 10 T. The corresponding activation energies were estimated to be $E_2$ = 105 K and $E_1$ = 409 K, respectively. The latter, $E_1$, is the highest value reported so far in the Sb-based filled skutterudites. The presence of magnetically robust ultrasonic dispersions in SmOs$_4$Sb$_{12}$ implies a possibility that an emergence of a magnetically insensitive heavy fermion state in this system is associated with a novel local charge degree of freedom which causes the ultrasonic dispersion.
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Submitted 16 February, 2011; v1 submitted 7 October, 2010;
originally announced October 2010.